SMEs refers to the ability of the material to memorise a shape and materials that possess these properties have a multitude of exciting technical and medical applications (1–14). For materials such as alloys this is commonly in a one-way SME (7, 15), however, there are a variety of materials that are capable of reverting to their permanent shape or original state upon exposure to a stimulus (such as a temperature change) or indeed multiple stimuli (16). SMP-based materials have been widely investigated since the 1980s because of the abundance of potential applications imparted by their interesting properties (for instance, stimuli-responsiveness and ability to change shape), which can lead to technological innovation and the generation of new high value products for technical and medical applications (1, 17–19).

The reversible transformation of SMPs functions by primary crosslinking net points (hard segments) memorising and determining the permanent shape, and secondary switching segments (soft segments) with a transition (T_trans) to reduce strain stress and hold the temporary shape. Below the T_trans, the material will be in its permanent shape and be stiffer than when T_trans is reached and the SMPs are more malleable and can be deformed into a desired shape (usually through application of an external force). The deformed state is maintained once the external force has been removed and the system is no longer at or above T_trans. SMPs revert to their original state once the T_trans conditions are met. This process describes the SME pathway of SMP-based materials that are thermally-induced (albeit not for some light or chemical-induced systems).

While most SMP-based materials hold a single permanent shape and a single temporary shape, recent advances in SMP technology have allowed the generation of multiple-shaped-memory materials with different stimuli responses (light or chemical) (16, 20, 21). An interesting example of this is a triple shaped-memory material generated by combining two dual SMPs with different glass transition temperatures (T_g) (22, 23), where the SMPs switch from one temporary shape to another at the first T_trans, and then back to the permanent shape at another, higher activation temperature (22).

SMPs have a large range of properties from stable to biodegradable and transient, elastic to rigid or soft to hard, depending on the structural units that constitute the SMP. Consequently, SMPs not only respond to temperature (24) and magnetism (25) like shape-memory alloys (SMAs) (26), but also to moisture (27), electricity (28), light (29) and chemical stimuli (such as a pH change) (30). Moreover, there are other principles of SME; for instance, a thermal-responsive SMP can proceed via a Diels-Alder reaction (chemical crosslinking/reversible covalent bonds) (31). SMPs tend to have much milder processing conditions than SMAs (<200°C, low pressure), have a greater extent of deformation (strain more than 200% for most materials) and tend to be based on cheap starting materials with simple synthetic procedures (12, 32). After the term ‘shape-memory’ was first proposed by Vernon in 1941 (32), the significance of SMPs was not fully realised until the 1960s, when crosslinked polyethylene (PE) was used to make heat-shrinkable tubes and films (33). Significant investment in the development of SMPs began in the 1980s (34) with rapid progress realised in the last decade, particularly with a view to the generation of shape-memory materials with exciting and versatile features.

Two important quantities used to describe SMEs are the strain recovery rate (R_r) and the strain fixity rate (R_f). R_r describes the ability of a material to memorise its permanent shape, while R_f describes the ability of switching segments to fix the mechanical deformation. R_r is calculated using Equation (i):

(i)

where N is the cycle number, ε_m is the maximum strain imposed on the material and ε_p is the strain of the sample after recovery. R_f is calculated using Equation (ii):

(ii)

where ε_u is the strain in the fixed temporary shape. SMPs respond to specific stimuli through changes in their macroscopic properties (for example, shape) (26). The polymer network underlying active movement involves a dual system, one that is highly elastic and another that can reduce the stiffness upon application of a certain stimulus. The latter system incorporates either molecular switches or stimulus sensitive domains (35). Their shape-memory feature is a result of the combination of the polymer’s architecture, and a programming procedure that enables the formation of a temporary shape. Net points consist of covalent bonds or intermolecular interactions and the SMP’s hard segments form the net points that link the soft segments (acting as a fixed phase), whereas the soft segments work as the molecular switches (acting as a reversible phase). The fixed phase prevents free flow of the surrounding polymer chains upon the application of stress. The reversible phase, on the other hand, undergoes deformation in a shape-memory cycle and is responsible for elasticity. For example, if the T_trans is T_g, the micro-Brownian motion of the network chains is fixed at low temperature (below T_g) and will be switched back on at high temperature (above T_g), recovering its original state. When T_trans is the crystal melting temperature (T_m), the switching segments crystallise at low temperature (below T_m), and then recover their original state at high temperature (above T_m). In addition, T_g normally extends over a broader temperature range compared to T_m, which tends to have relatively sharper transitions in most cases (26). Moreover, after the exposure to a specific stimulus and the T_trans is achieved, the strain energy in the deformed state is released, resulting in the shape recovery phenomenon. The general process of this SME for SMPs is depicted in Figure 1, wherein the polymer network structure is either chemically or physically crosslinked and the switching units are made from a semi-crystalline or amorphous phase.

Fig. 1.

Shape-memory behaviour can be demonstrated in various polymer systems that are significantly different in molecular structure and morphology. SME mechanisms differ according to the specific SMP(s); for instance, the SME mechanism of the chemically crosslinked semi-crystalline PE SMP. The crystalline phase, with a T_trans being T_m, is used as the molecular switching unit providing shape fixity. The chemically crosslinked PE network memorises the permanent shape after deformation upon heating (12, 36, 37), and the mechanism of the thermally-induced shape-memory PE (SMPE) is depicted in Figure 2.

Fig. 2.

The associated modulus of elasticity is dictated by configurational entropy reduction that occurs with deformation of the constituent chains and is therefore often termed entropy elasticity. For T>T_trans (T_g, T_m or other), polymer networks exhibit super-elasticity wherein the polymer chain segments between crosslink points can deform quite freely and are prone to being twisted randomly via rotations about backbone bonds, maintaining a maximum entropy and minimum internal energy as macroscopic deformation occurs (12). The classic prediction from rubber elastic theory is that the resulting elastic shear modulus (G) is proportional to both crosslink density and temperature (Equation (iii)):

(iii)

where ν is the number density of network chains, p the mass density, R the universal gas constant and M_C the molecular weight between crosslinks. From a macroscopic viewpoint, the SME in SMPs can be graphically represented in three-dimensions (3D). Tensile strain vs. temperature and tensile stress (for example, elongation) is depicted in Figure 3.

Fig. 3.

Using the shape-memory strain-temperature-stress relationship description in Figure 3, the features of SMPs that allow for good shape-memory behaviour include: a sharp transition that can be used to quickly fix the temporary shape at low temperature, and the ability to trigger shape recovery at high temperature; super-elasticity above T_trans that leads to the eventual shape recovery and avoids residual strain (permanent deformation); and complete and rapid fixing of the temporary shape by immobilising the polymeric chains without creep thereafter (12, 37). Thus far, the SME models describing how SMPs recover their original state prominently involve thermo-responsive SMPs. However, careful design of the polymers allows the opportunity for SMPs to possess different stimuli responses and applications.

A multitude of different triggers for SMEs and SMPs exist. However, an in-depth review is outside the scope of this mini-review, and therefore a few examples are highlighted below.

It is possible to generate thermally-induced SMEs in a variety of materials (18–20, 38–40), however a comprehensive overview is outside the scope of this mini-review. As previously discussed, the SME of SMPs can be thermally-induced, and these SMPs are the most common (26). Figure 1 depicts a general overview of the SME mechanism of SMPs, with a schematic of the SME mechanism for thermally-induced SMPs with T_g (amorphous cases) and T_m (crystalline cases). Figure 2 presents a specific example of the SME mechanism for SMPE with the T_trans being T_m. In addition, advanced thermomechanical constitutive models have been used to study the materials’ behaviour (for example strain-temperature-stress development with time) in a very accurate way (41). By applying these models to SME mechanistic studies and the detailed characterisation of the SMPs (crosslinks, intermolecular and intramolecular interactions involving the SMPs) (12), a deeper understanding of the SME of SMPs can be achieved, which has proven beneficial for the development of new SMPs and their proposed applications (31). For example, poly(ε-caprolactone) (PCL), typically a biodegradable polymer, has been reported to possess high shape fixity and recovery. This was achieved by integrating reversible bonds within the PCL polymer network via the Diels-Alder addition of 1,2,4-triazoline-3,5-dione (TAD)-anthracene and Alder-ene addition of TAD-indole (42). These PCL SMPs were reported to attain recovery ratios greater than 99% (43). Furthermore, a dual-functional (self-healing and shape-memory) polymer network was achieved by crosslinking a polydimethylsiloxane (PDMS) polymer containing dense carboxylate groups (100% mol) (PDMS-COOH) with small amount of poly(ethylene glycol) diglycidyl ether (PEGDGE) (44). This SMP (PDMS‐COO-E) actuates at body temperature (37°C) with possible strain ca. 200% and shape recovery ratios at 98.06%. In addition, a 25 mm × 4 mm × 1 mm sample cut into two separate pieces healed (the two pieces become one whole piece with no evidence of a cut) when the two cut surfaces were brought into contact after 6 h at 25°C. Thus, the unique material, PDMS‐COO-E, may have a wide range of applications in many fields, including wearable electronics, biomedical devices and four‐dimensional (4D) printing (1, 19). Interestingly, the material was also reported to possess a greater than 85% light transmittance (425 nm to 700 nm) (44), therefore PDMS-COO-E has potential applications in transparent electronic devices. Figure 4 illustrates the possible SME mechanism of PDMS-COO-E. The short PDMS linear chains are crosslinked by chemical covalent interactions and abundant hydrogen bonds into a 3D network. The covalent crosslinked networks of PDMS-COO-E maintain the permanent shape and resilience, whereas, at ca. 37°C the weak hydrogen bonds are broken, and the dynamics of polymer chains increase, resulting in recovering the permanent shape. Meanwhile, a large number of hydrogen bonds enable the samples to heal at temperature without external stimulus (44).

Fig. 4.

It is possible to generate light-induced SMEs in a variety of materials (18–20, 38, 40, 45), however a comprehensive overview is outside the scope of this mini-review. Light-activated SMPs (LASMPs) (46) typically use photothermal or photochemical (photocrosslinking or photocleavage) triggers for SMEs. For instance, photothermal LASMPs typically employ photo-absorber molecules and particles that convert light to heat, thereby increasing the temperature at the desired region within the LASMP. Photochemical LASMPs incorporate photosensitive molecules to create or cleave bonds during irradiation with light, imparting potentially very swift SMEs (47, 48). It is possible to improve the response time of SMPs by increasing the thermal conductivity with various conductive additives (49). However, the heating and cooling of materials with substantial thickness takes time, which can be minimised by using light to trigger transitions in LASMPs (46). It is also possible to generate multistimuli-responsive materials using components of the materials that respond to different wavelengths of light (for example, one wavelength of light to induce photocrosslinking, while a second wavelength of light cleaves bonds). It is possible to produce materials that can be reversibly switched between an elastomer and a rigid polymer employing polymers containing cinnamic groups (48) that can be fixed into pre‐determined shapes utilising ultraviolet (UV) light illumination (>260 nm), and then recovered their original state when exposed to UV light at a different wavelength (<260 nm) (49). Figure 5 depicts one example of the process of LASMPs shape recoverability.

Fig. 5.

It is possible to generate electrically-induced SMEs in a variety of materials (18, 20, 50–55), however a comprehensive overview is outside the scope of this mini-review. A variety of electrically conductive materials including organic electronic materials (including conductive polymers such as polypyrrole (PPy) (28, 56–58) and carbon nanotubes (CNTs) (59, 60)) and inorganic electronic materials (such as alloys, metals (61) and silver nanowires (NWs)), have been incorporated in materials displaying SMEs to impart swift triggers to the SMEs, enabling a variety of interesting applications.

Highlighting some of the potential of electrically-induced SMEs, electrically-induced SMP composites incorporating shape-memory polyurethane (SMPU) and Ag NWs in a bilayer structure exhibits flexibility and electrical conductivity (62–64), which may find applications as capacitive sensors, healable transparent conductors and wearable electronics (65). In such materials the Ag NWs are randomly distributed on the surface layer of the composite to form a conductive percolating network that retains conductivity (200 Ω sq⁻¹) after a 12% elongation. However, continual increase in elongation causes a dramatic increase to the composites’ resistance value and the eventual loss of electrical conductivity (66). When the material (deformed or in its original state), is connected to a typical circuit, a low voltage of 1.5 V was enough to activate a light-emitting diode (LED) (65). The composites possessing a higher Ag NW content exhibited a higher recovery ratio and reached the maximum recovery speed quicker (66). It was assumed that all the heat from electrical (Joule) heating was absorbed by the sample, i.e. no convective loss (67). Therefore, the composites with higher Ag NW content had a lower resistance value and the heating effectiveness was promoted. Heat initiates the thermal T_trans of the SMPU leading to an improved shape recovery, and voltages as low as 5 V reverted bent composites to their original state within 3 s (66). This represents a good example of a multifunctional SMP and demonstrates the potential of SMP designs driving technological innovation. A schematic of the composite is shown in Figure 6.

Fig. 6.

Polymeric blend SMPs can be constructed from two immiscible polymeric matrices. The shape-recovery of these systems can be controlled with relative ease by varying the ratio of the polymer blends (68). However, this process may have adverse effects on shape-memory characteristics and diminish the material’s performance, thereby limiting potential applications. On the other hand, SMP functionality may also be enhanced with other capabilities. For instance, it was recently reported that a new hybrid SMP was developed by combining single-walled CNTs (SWCNT) into a poly(lactic acid) (PLA) and thermoplastic polyurethane (TPU) SMP system, containing poly(ethylene glycol) (PEG) plasticiser (68). By incorporating PEG, the hybrid SMP composite achieved a lower temperature T_g (for example, 10 wt% of PEG lowered T_g of the PLA/TPU sample from 60°C to 40°C), meanwhile enhancing the dispersion of SWCNT (for instance, even at 4 wt% of SWCNT loading, 100% SMP tensile strain was possible, much greater than previously reported electrically-induced SMP studies, i.e. 12% discussed previously). In addition, the presence of the SWCNT can stabilise the SMP system and enhance its shape-fixity after deformations at room temperature conditions (68). Furthermore, the material was capable of a conductivity above 10⁻⁷ S cm⁻¹, which can be considered conductive, as documented (68). The PLA/TPU SMP composite (2 wt% SWCNT and 10 wt% PEG) also achieved shape-recovery, via Joule heating derived from electricity, in 80 s when currents of 125 mA were applied. The high stiffness of SWCNT filler results in decreasing shape-recovery performance because of the hindrance on the polymer chain movements (68). As a result, under room temperature stretching, the R_f and R_r values obtained were ca. 80% and 65%, respectively. Therefore, when its shape-recoverability is compared to other SMPs (shape-recovery ratios being upwards of 98%), the material is lacking. However, the hybrid SMP composite does possess electroactive ability, thus a trade-off relationship between shape-memory/recovery and electroactive ability needs to be carefully considered when designing similar materials.

It is possible to generate water-induced SMEs in a variety of materials (18, 20, 38, 39, 69–72), however a comprehensive overview is outside the scope of this mini-review. Water is an important stimulus due to the fact it is abundant in a multitude of different environments, non-toxic and safe for a variety of applications.

An interesting example highlighting the potential of such materials is based on strong and flexible composite films (73) utilising the combination of a flexible interpenetrating polyol-borate network (74) and electroactive PPy (75, 76) that exchange water with the environment resulting in film expansion or contraction. The free-standing multi-functional SMP films were prepared by electropolymerisation of pyrrole in the presence of the polyol-borate complex (composed of pentaerythritol ethoxylate (PEE) coordinated to boron(III)) (74), wherein the interpenetrating network enables water-gradient-induced displacement, converting chemical potential energy in water gradients to mechanical work (73), and results in adaptation of the architecture in response to an environmental condition change (i.e. sorption and desorption of water which drives the SME process, as depicted in Figure 7). The design of the water-responsive PPy-PEE composites was creatively applied to prepare actuators and generators driven by water gradients. The film actuator can generate contractile stress up to 27 MPa, lift objects 380 times heavier than itself and transport cargo 10 times heavier than itself (73). An assembled generator associating the actuator with a piezoelectric element driven by water gradients, outputs alternating electricity at ca. 0.3 Hz, with a peak voltage of ca. 1 V (73). The electrical energy can be stored in capacitors that could power micro and nanoelectronic devices (73). The SME mechanism for this SMP differs to that of Figure 1 and Figure 2, utilising water as the shape-memory trigger for T_trans, and the original and deformed state interchange automatically via water sorption and desorption states. However, the shape-memory phenomenon remains the same, further demonstrating the potential of SMP designs driving technological innovation.

Fig. 7.

It is possible to generate pH-induced SMEs in a variety of materials (18, 20, 38, 77–80), however a comprehensive overview is outside the scope of this mini-review. An example of the interesting properties of such pH-responsive SMPs and their composites is produced by blending poly(ethylene glycol)-poly(ε-caprolactone)-based polyurethane (PECU) with functionalised cellulose nanocrystals (CNCs) displaying pH responsive pyridine moieties (CNC-C₆H₄NO₂) (81, 82). At high pH values the pyridine is deprotonated, facilitating hydrogen bonding interactions between the pyridine groups and hydroxyl moieties on the cellulose, whereas at low pH values, the protonation of the pyridine moieties diminishes these interactions. By comparison, carboxylic acid functionalised cellulose nanocrystals (CNC-CO₂H) responded to pH variation in the opposite manner (83–85). When the functionalised CNCs were combined with PECU polymer matrix to form a nanocomposite network, the mechanical properties of PECU were improved along with the pH-responsiveness of CNCs (85). The percolated network of pH-sensitive CNC in the polymer matrix served as the switching units for the shape-memory composite, the SME process of this material is depicted in Figure 8 (81, 82). The CNC serves as the switching unit of the SMP composite within the matrix of PECU which is physically crosslinked and microphase separated to yield the net points. Such pH-responsive shape-memory nanocomposites have promise in the design of biomaterials for biomedical applications (for example, SMP-based drug delivery systems triggered by transition along the digestive tract) (83).

Fig. 8.

It is possible to generate magnetically-induced SMEs in a variety of materials (18, 20, 38, 86–88), however a comprehensive overview is outside the scope of this mini-review. The SMP devices discussed thus far are being researched with potential application into wearable electronics, nanoelectronics (such as actuators), biomaterials and biomedical devices (1, 18, 19). However, in some instances (such as medical devices) a key challenge is the design and implementation of a safe and effective method of actuating a variety of device geometries in vivo. As previously discussed, a pH‐triggered SMP design can be potentially effective when utilised as drug delivery devices, when the target environment has a substantial pH difference (for instance, the digestive system) (83). However, the development of electrically and thermally-triggered devices that safely operate in vivo is difficult due to the (generally) high temperatures these SMPs can reach (relative to biological systems). For instance, the electroactive PLA-TPU SMP composite (2 wt% SWCNT and 10 wt% PEG) reaches temperatures greater than 70°C in 80 s as shape-recovery is achieved (68).

An alternative method of achieving actuation is inductive heating by loading ferromagnetic particles into an SMP system and exposing the doped device to an alternating electromagnetic field (89), benefiting from the innate thermoregulation offered by a ferromagnetic material’s Curie temperature (T_c, at which a ferromagnetic material becomes paramagnetic, losing its ability to generate heat via a hysteresis loss mechanism) (90). By using particle sizes and materials that will heat mainly via a magnetic hysteresis loss mechanism over an eddy current mechanism, it is possible to have an innate thermoregulation mechanism that limits the maximum achievable temperature to T_c (89). Therefore, by selecting ferromagnetic particle materials with a T_c within safe medical limits, Curie thermoregulation eliminates the danger of overheating and the need for a feedback system to monitor implanted device temperatures (89). However, this technology is not only useful when applied to medical devices. Other useful applications include remote activation in which wires or connections to SMP devices could be eliminated, simplifying the design and reducing possible points of failure. An example of this method of actuation involves the incorporation of 10% by volume nickel zinc ferrites (for example C2050 (Ceramic Magnetics Inc, USA) and CMD5005 (Ceramic Magnetics Inc), particle sizes ca. 50 μm with spherical shapes) with an ester-based thermoset polyurethane (PU) SMP, MP5510 (SMP Technologies Inc, Japan) (T_g of 55°C) (89). The magnetic field utilised to achieve shape-recovery was a copper-wound solenoid coil with a 2.54 cm diameter, 7.62 cm length and with a total of 7.5 turns. The unit possessed an adjustable power setting capable of outputting 27 W to 1500 W at between 10 MHz and 15 MHz frequency (note: this high frequency may induce eddy currents in the tissue, causing undesirable direct heating of the human body in medical applications) (91). However, an alternating magnetic field of 12.2 MHz and approx. 400 A m⁻¹ (centre of the inductive coil) at room temperature was used for actuation to demonstrate proof of concept for the device. It was also reported that clinically useable frequencies (50 kHz to 100 kHz) (92) should still be effective (89), albeit this could result at a different quantitative level (i.e. shape-recovery and memory performance may be reduced). Furthermore, C2050 and CMD5005 possess a T_c of 340°C and 130°C, respectively. These temperatures exceed physiological limits and are therefore not practical for medical devices currently, however, these doped SMP composites did not exceed temperatures above the respective Ni Zn particle T_c values, signifying a thermoregulation characteristic. In addition, it was stated that the 10% volume of Ni Zn particles did not impact the SMPs shape-memory properties significantly (89). The T_g increased from 55°C to 61.4°C and the shape-recovery of a flower and foam-based device was achieved within 15 s to 25 s, at a temperature range of 23°C to 78.6°C. The potential applications for this device are illustrated in Figure 9. Optimisation of this device/design is still required before it can be considered clinically viable, however, this SMP composite highlights very interesting characteristics, remote activation (via magnetic fields inducing thermally-triggered actuation) and thermoregulation (via T_c temperature of the material being employed).

Fig. 9.

As highlighted above, SMP materials are diverse and respond to many different external stimuli (including temperature, light, electricity, water, pH and electromagnetic fields) by a variety of mechanisms. Although SMPs can be classified based on their composition and structure, stimulus and shape-memory function, their classification can be difficult, as organising these polymeric smart materials into one or two simple categories is an over-simplification of their abilities and characteristics (93).

SMPs are considered to consist of net points and molecular switches or stimuli sensitive domains. These net points can be achieved by covalent bonds (chemically crosslinked) or intermolecular interactions (physically crosslinked). Chemically crosslinked SMPs involve suitable crosslinking chemistry and are referred to as thermosets (94, 95). Physically crosslinked SMPs involve a polymer morphology consisting of at least two segregated domains and are referred to as thermoplastics (96). The network chains of the SMP can be either amorphous or crystalline and therefore, the T_trans is either a T_g or T_m. The network architectures are thought to be constructed through crosslinking net points, with polymer segments connecting adjacent net points. The strongly crosslinked architectures ensure the polymer can maintain a stable shape on the macroscopic level (93). Thermoplastic polymers exhibit a more reversible nature (97), meaning the physical crosslinked net points can be disrupted and reformed with relative ease. The interconnection of the individual polymer chains in a physically crosslinked network is achieved by the formation of crystalline or glassy phases. For thermoset polymers, the individual polymer chains are connected by covalent bonds and are therefore more stable than physically crosslinking networks and show an irreversible nature (98–100).

Regarding thermo-responsive SMPs, they can be classified according to the nature of their permanent net points and the T_trans related to the switching domains into four different categories: (a) physically crosslinked thermoplastics, T_trans = T_g; (b) physically crosslinked thermoplastics, T_trans = T_m; (c) chemically crosslinked amorphous polymers, T_trans = T_g; (d) chemically crosslinked semi-crystalline polymer networks T_trans = T_m (93).

For the physically crosslinked SMPs, the formation of a phase-segregated morphology is the fundamental mechanism behind the thermally-induced SME of these materials (93, 99). One phase provides the physical crosslinks while the other acts as a molecular switch. They can be further classified into linear polymers, branched polymers or a polymer complex. Linear SMPs may consist of block copolymers and high molecular weight polymers, the typical physically crosslinked SMP is linear block copolymers, such as PU. In polyesterurethanes (PEU), oligourethane segments are the hard-elastic segments, while polyester serves as the switching segment (99).

For chemically crosslinked SMPs, two methods are commonly used to synthesise covalently crosslinked networks (36, 41). The first method relies on addition of a multi-functional crosslinker during polymerisation (41), whereas the second method relies on the subsequent crosslinking of a linear or branched polymer (36). The networks are formed based on many different polymer backbones. Covalently crosslinked SMPs possess chemically interconnected structures determining the original macroscopic shape. The switching segments of these materials are generally the network chains between net points, and a T_trans of the polymer segments is used as the shape-memory switch. The chemical, thermal, mechanical and shape-memory properties are determined by the reaction conditions, curing times, the type and length of the network chains and the crosslinking density (35). Comparing physically crosslinked SMPs with chemically crosslinked SMPs, the chemically crosslinked SMPs often show less creep, thus, any irreversible deformation of the polymer during shape recovery is less. This is because covalent crosslinked networks are more stable than physical crosslinked networks. As a result, chemically crosslinked SMPs usually show better chemical, thermal, mechanical and shape-memory properties than physically crosslinked SMPs (96). For example, the shape recovery ratio of thermoplastic SMPU is usually in the range of 90% to 95% within 20 shape recovery cycles, and the elastic modulus is between 0.5 GPa and 2.5 GPa at room temperature (26). Additionally, when exposed to air, it is sensitive to moisture and therefore possesses unstable mechanical properties. In contrast, an epoxy SMP shows better overall performance as a shape-memory material. The shape recovery ratio typically reaches 98–100%, the elastic modulus between 2 GPa and 4.5 GPa, and it is generally stable in the presence of moisture (26). Thermoplastic SMPs (such as SMPU) are mostly researched and used as functional materials at a small scale, such as for biomaterials (30, 97). However, thermosetting SMPs (for example styrene-based SMP (SSMP) and epoxy SMPs) are generally used for structural materials, such as space deployable structures and automobile actuators (97, 98).

The approaches to designing different shape-memory functions become more abundant as scientists and engineers better understand the SME mechanism of SMPs. For instance, discussed thus far are examples of SMPs with polymeric blends, addition of crosslinking species, incorporation of electroactive and ferromagnetic substances. All of which enhances an SMP device functionality, enabling unique and interesting characteristics which can be tailored to a plethora of applications (for example, self-healing and wearable electronics, drug delivery and implantable medical devices) (101–110). Further still, one-way SMEs, two-way SMEs (such as dual shape PPy-PEE, discussed previously), triple SMEs, multiple SMEs and even temperature-memory effects (TMEs) have been widely investigated in SMPs (34). As the types of SMP materials increasingly diversify, two and even three different types of shape-memory functions can be achieved simultaneously in the same SMP material (34, 111). These types of materials can usually be achieved when combining different SMPs possessing different properties. A schematic of one-way, two-way, dual shape and triple shape functionality SMPs is shown in Figure 10, and an integrated insight into the classification of SMPs is shown in Figure 11.

Fig. 10.

Fig. 11.

An example of a selective triple shape multicomposite SMP was documented to incorporate a neat SSMP (112) and two SSMP composites (113). One incorporated iron(II, III) oxide nanoparticles while the other CNT nanoparticles. This unique SMP composite successfully possessed three different regions within the sample: neat SSMP, SSMP-Fe₃O₄ and SSMP-CNT. Because of this, the material also possessed distinct shape-memory capabilities with different triggers. For instance, the material was documented undergoing a three-step shape-memory recovery process, subjected to an alternating magnetic field of 30 kHz, a radio frequency (RF) field of 13.56 MHz and direct oven heating at 130°C (113). Furthermore, the R_f and R_r for the original shape to the first temporary shape (and back to the original shape) was reported at 93% and 93%, respectively. Meanwhile, the R_f and R_r for the first temporary shape to the second temporary shape (and back to the first temporary shape) was at 95% and 99%, respectively (113). The SME mechanism for this multicomposite is represented in Figure 12 and it was concluded that this unique material has promising characteristics to be used in biomimetic materials. Examples of applications of SMP-based materials and their composites are highlighted in Table I.

Fig. 12.

As the understanding of SMPs continually develops among the academic and industrial communities, the generation of new and potentially innovative SMPs will be more rapid while we realise the full potential of these materials. SMPs are one of the most interesting of polymer classes within the field of functional polymers. In addition, SMP composites can enhance the already impressive capabilities of SMPs by imparting new functional characteristics, broadening the potential applications of these materials and enabling a multipurpose material. SMPs and their composites are capable of industrially important applications (examples of which include: self-healing (101–104), generators driven by water gradients (73), sensors (72), task-specific medical devices (18, 105) and wearable electronics (106–110), a few examples of which are highlighted in Table I. The literature published to date de-risks investment from governments and industry to raise the technology readiness levels towards products on the market.

The massive discharge of volatile organic compounds (VOCs) has a great negative impact on the environment (1). Toluene is a common pollutant in VOCs and is produced in a large number of industrial activities, such as chemical refining and dye processing. Toluene stimulates skin and mucosa, and when it reaches a certain high concentration, it also causes paralysis of the human nervous system. Compared with photocatalysis and chemical oxidation, using a biofilter to remove VOCs is more economical and environmentally friendly (2). More important is that it does not produce secondary pollution. The key element to ensure the removal capacity of the biofilter is the preparation of the filler. As a carrier for the transfer of pollutants, the filler can provide a suitable growth environment for microorganisms (3).

Micro-embedding technology is a method which uses physical or chemical methods to keep microorganisms in a defined space, ensuring microorganisms with high activity. The principle of using micro-embedding technology to degrade VOCs is to use a hollow porous membrane to intercept microorganisms inside the filler. The pore size of the hollow porous membrane is smaller than that of microbial cells, so that microorganisms can be embedded. The VOCs can enter the interior of the embedded carrier freely due to the small particle size, and the degradation products can flow out of the carrier through the pore size (4, 5).

A large number of studies have been carried out on different types of fillers. Chen et al. (6) used a two-layer biofilter filled with new mixed packing materials to remove hydrogen sulfide gas. Dumont et al. (7) prepared a nutritional slow-release filler (UP20) to biodegrade H₂S. In the above studies, the fillers were not embedded with microorganisms. The concentration of microorganisms in the filler was small, and the removal efficiency of the biofilter was low in the start-up phase, resulting in a longer start-up period. Zhu et al. (8) used a composite packing material with functional microorganisms to remove H₂S. However, toluene does not biodegrade easily due to the presence of a benzene ring. Zuo et al. (9) found that engineered P. putida could simultaneously degrade organophosphates, pyrethroids and carbamates. Muñoz et al. (10) studied the long-term performance and stability of P. putida in a toluene removal bioreactor. The above studies have found that P. putida is highly effective in degrading organics containing benzene rings.

However, there is a lack of studies on filler micro-embedded P. putida for toluene biodegradation. Existing problems with biofilters packed with fillers include bed clogging, low biomass concentration and pressure drops. These problems become more prominent when the biofilter is operated under high VOC loading rates or long-term operation (11). For example, Ryu et al. (12) found that the benzene removal efficiency of a well-designed biofilter decreased from greater than 90% to approximately 75% after 27 days of operation due to clogging caused by the excess growth of biomass.

The main objective of this study was to evaluate the performance of a self-developed filler micro-embedded with P. putida for toluene removal under various inlet loading rates. The variations in start-up period, pressure drop, biomass concentration and tolerance to transient shock loading were monitored throughout the experiments. Special attention was paid to the analysis of the microbial community attached to these fillers and to monitoring the evolution of the microbial community in various periods.

2.1 Preparation of Filler

2.2 Experimental Setup

2.3 Toluene Concentration Analysis

The determination of toluene concentration was carried out by adsorption of activated carbon and desorption of carbon disulfide, and then the toluene gas was injected into a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with a packed column (free fatty acid phase (FFAP) capillary column, 30 m × 0.25 mm × 0.25 μm) and a flame ionisation detector (FID). The gas chromatography nitrogen was used as the carrier gas with a flow rate of 1 ml min⁻¹. Temperatures of the injection port, column and detection port were set to 150°C, 65°C and 150°C, respectively. Gas samples were collected from the inlet and outlet of the biofilter with a gas-tight syringe and injected into the GC daily (14). Data were obtained from the workstation by automatic comparison of the peak area of the inlet and outlet samples with the baseline of toluene. The performance of the biofilter was evaluated in terms of (%) RE and the elimination capacity (EC) as a function of toluene loading. The RE and EC were calculated as in Equations (i)–(iii):

(i)

(ii)

(iii)

where the C_in and C_out are the inlet and outlet toluene concentration (mg m⁻³), the V is the volume of the whole biofilter (l) and Q is the gas flow rate (l min⁻¹).

2.4 Physical and Chemical Property Analysis

2.5 DNA Extraction and Sequencing

3.1 Physicochemical Properties of the Filler

3.2 Start-up Performance

3.3 Continuous Biodegradation Performance

3.4 Tolerance for Transient Shock Loading

3.5 Biomass Concentration and Pressure Drop in the Biofilter

To explore the bacterial communities in the biomass attached to BF1, genetic sequencing analyses were carried out. Sequencing of 16S rRNA genes amplified from the active bacterial communities during the operational stages revealed 21 phyla, 41 classes, 96 orders, 184 families and 347 genera (28, 29). The community analysis at phylum level of the fillers is shown in Figure 7. The four operational stages were sampled at the 25th day, the 65th day, the 95th day and the 145th day, where the 25th day, the 65th day and the 95th day had a different EBRT and the same inlet toluene concentration, and the 145th day was after two interference-shutdown experiments. The dominant phyla were Firmicutes (63.4 ± 8.7%), followed by Actinobacteria (14.6 ± 3.9%) and Proteobacteria (10.1 ± 4.2%). With decreased EBRT, the abundance of Firmicutes remained high, but the abundance of Actinobacteria decreased, and the abundance of Proteobacteria increased. This is mainly due to a reduction in residence time leading to the inability of microorganisms to fully utilise toluene, and a reduction in the carbon source leading to a change in the proportion of microorganisms (30). After two interference-shutdown experiments, the abundance of Bacteroidetes increased and the normal microecological balance was broken, which indicated that Bacteroidetes is a sensitive biological indicator, similar to the results found by Wolińska (31). Using this indicator (the increase in Bacteroidetes), it can be judged whether the biofilter is in an unstable state, which would provide some guidance for practical engineering applications.

Fig. 7.

In the four operational periods, few Pseudomonas (abundance less than 1%, as shown in Figure S3) were found in the sampling of the above four periods. As the inlet loading rate increased, the abundance of Pseudomonas genus increased from 4.7 × 10⁻⁴ to 1.9 × 10⁻³. After two intervention-shutdown experiments, the abundance of Pseudomonas genus decreased to 8.5 × 10⁻⁵, which indicates that the biofilter was not in a sterile environment and that there are other microorganisms competing with the P. putida added to the filler. When the environmental conditions and the nutrients in the biofilter became unsuitable for the added microorganisms and were suitable for other microorganisms, the other microorganisms were activated and enriched (32). However, in the start-up phase, the biofilter embedded with P. putida started quickly, and the removal efficiency of toluene remained high, which indicated that the added P. putida contributed to the efficient operation of the biofilter (33). These results indicated that the biomass could maintain itself by microbial community changes, and the rapid re-adaptation of the biofilter could contribute to the activity retention of its biomass during the starvation period.

A composite filler micro-embedded with P. putida was prepared and evaluated for the biodegradation of toluene. The biofilter packed with the fillers could start up quickly with 85% RE on the eighth day, and tolerate substantial transient shock loadings. The RE of the biofilter remained above 90% when the EBRT was 18 s and the intake load was not higher than 41.4 g m⁻³ h⁻¹. In the experimental period of 145 days, no filter plugging phenomenon was observed. Moreover, the high removal efficiency and elimination capacity contributed to rich bacterial communities for the efficient biodegradation of toluene. The communities mainly included Firmicutes, Actinobacteria and Proteobacteria, and the abundance of Bacteroidetes increased significantly during the recovery period. The composite filler exhibited favourable physicochemical properties in this experiment and its practicability in industrial engineering should be further investigated.

Acknowledgments

The authors would like to acknowledge the support of the National Natural Science Foundation of China (No. U1304216), the Science and Technology Plan of He’nan Province, China (No. 122102310366), the University Key Research Project of He’nan Province, China (No. 19A610002 and 19A150010), and the China Postdoctoral Science Foundation (No. 2018M632794).

As humans, we seem to desire structure, relationships and laws to understand the universe. Through increased understanding, we can solve the problems and challenges that we perceive. This method and the output are given the label of science. At its best, science provides exquisite understanding, life-changing solutions or sometimes both.

The downside of the structures and rules we impose is that they can create inertia. Because the structure or rule served a purpose in the past, we can be more willing to stand by it blindly than openly seek the understanding or solutions we truly desire; a dynamic seen in the natural and social sciences alike and revealing more about human nature than the universe. One such structure is that of the disciplines within science. We should challenge ourselves to be very clear on the purpose of any structures we adhere to and be ready to remove barriers that get in the way of progress. One such example is uncovering the fertile ground of interdisciplinary research. In recent years interdisciplinary research has been of increasing importance across the sciences. Volume 64 of the Johnson Matthey Technology Review started a celebration of interdisciplinary science by looking at when chemistry collaborates with physics (1) and in this issue, we will celebrate the cross-disciplinary contributions of biology with other fields.

This wide-ranging issue explores topics such as: what we can continue to learn from organisms in unusual environments; how we might leverage biology in artificial situations; and even how we manage the interface between human-made, controlled systems and the outside world. In particular, the diversity of industrial applications is striking. Some are familiar to Johnson Matthey and this journal such as fine chemical synthesis, while others, such as hides and textiles, show that as boundaries within science are removed, previously distant industries will have much to learn from one another.

I have reflected on three themes as this issue has come together. There are numerous examples on each theme and I would challenge the reader to think “what next?” for each:

This third theme is perhaps the most important as it turns niche expertise into something accessible to scientists across fields. Understanding the technology may be beneficial but is not a prerequisite to accessing it. Biology follows favourably in the footsteps of computing in producing such platform technologies and it is an attribute that perhaps we should value and prioritise more in other fields. To expand on this theme, it is exciting to look both backwards and forwards to the contributions made possible by platform technologies from the field of biology. Often these point back to unlocking our understanding of the structure and function of DNA at a molecular level and have resulted in some of the most impactful scientific contributions of the last 50 years or so. Our health has been a significant beneficiary of these advances with cancer drugs providing an illustrative case study. Looking back, we can see recent classes of therapeutics that were significantly enabled by this flow of understanding and platform technologies such as tyrosine kinase inhibitors and antibody-based therapies (2). Most importantly, patient outcomes have improved substantially in part, thanks to these therapies (3).

Looking to the future, gene and cell therapies appear to be following a similar pattern and will hopefully deliver similar patient benefits. Outside of cancer treatments and healthcare, we can see many industries set to benefit from being able to access biological understanding and technologies. This is particularly as we seek to learn from biology and reduce our impact on the planet by using materials and energy in keeping with what Earth can sustain.

As you read through this issue, I hope you enjoy reading something outside of your current field. I would take you back to my earlier challenge and see if you can gain any greater insights by not seeing the separation between your field and those of the authors. Rather, question what you can leverage, what you can learn and what next?

The use of hydrogen is not new. Fuel cells were invented over 150 years ago and have been providing on-board power to space missions for over 50 years. Industry makes millions of tonnes of hydrogen every year, with its main uses (in pure and mixed forms) being: oil refining (33%), ammonia production (27%), methanol production (11%) and steel production via the direct reduction of iron ore (3%). Hydrogen is manufactured primarily from the conversion of natural gas (~75%) and coal (~20%), with 2% from electrolysis. The associated CO₂ emissions are of the same magnitude as those of the UK and Indonesia combined (1).

The urgent need to minimise and then eliminate CO₂ and other GHG emissions to avoid a climate catastrophe is driving new dialogue around ways to achieve this, and hydrogen is moving to the centre in many of these discussions. For example, the Committee on Climate Change (CCC), the UK Government’s independent advisor on climate change, said in its net zero policy document that moving from the previous target of 80% GHG emissions reduction to the net zero target “changes hydrogen from being an option to an integral part of the strategy” (2).

This article will present an overview of some ongoing and planned demonstration projects, outlining the importance of such activities in providing confidence that the hydrogen approach is the right one for multiple geographies around the world and that there are technologies that are ready to be deployed today.

The IPCC reported in November 2018 that global warming should be limited to 1.5°C (3), and they showed that this requires net CO₂ and GHG emissions to become zero by 2050. Achieving net zero by 2050 is going to be very challenging, both at the country and the worldwide level. While CO₂ emissions in the developed economies have generally either stabilised or started to drop, those in rapidly developing countries such as China and India are increasing significantly, as shown in Figure 1.

Fig. 1

The global requirement for energy to drive industry, transportation, heating and cooking is also rising, placing further stress on efforts to limit global warming (5). Nevertheless, several national governments have set net zero targets, and some have already enshrined them in legislation (6, 7, 8). In the UK, the Department of Business, Energy and Industrial Strategy (BEIS) responded to the IPCC report by commissioning the CCC to review the implications of a net zero target, and to assess whether there was a credible pathway to achieve zero GHG emissions. The CCC’s ground-breaking work outlined a bottom up approach to a total energy system decarbonisation, achieving net zero. On the back of this, the UK was the first of the G20 major global economies to legislate a net zero emissions target by 2050 when it updated the Climate Change Act early in 2020 (6). 15 other countries have now set net zero targets, including Sweden (2045), Denmark, France and New Zealand (all 2050) and several others (including Chile, Spain and the EU27, through the European Commission) are discussing the target and its timeline (9).

The implications of net zero are marked. In the past, those emissions most costly and difficult to abate could be left. However, net zero means that most sectors will have to become completely emission free. Furthermore, processes which offer negative emissions will become extremely important to offset areas such as aviation where a zero emission pathway will be extremely challenging for the foreseeable future. For example, the combustion of biomass with the capture and storage of the CO₂ generated is one route to negative emissions, as is the more well-known example of planting trees.

Hydrogen is regarded as a flexible energy vector, and this section discusses its potential application in a number of key sectors: power generation (including energy storage), transportation, industrial and chemical processes and heating buildings (10). There are many divergent forecasts, as the appreciation of the role that hydrogen could play in reducing global emissions develops (2, 10, 11). However, many proposals require at least a tenfold increase in production of low carbon hydrogen over the fossil fuelled production today. As an example in 2017 the Hydrogen Council produced a report which described the scaling up of hydrogen out to 2050. The analysis showed a requirement for 78 exajoules (EJ) of low carbon hydrogen versus 10 EJ of fossil derived hydrogen today. The split proposed between different sectors was 9 EJ for power generation, 22 EJ for surface transport, 16 EJ for industrial energy, 11 EJ for building heat and power, 9 EJ of new process feedstocks and 10 EJ to convert existing feedstocks (10) to low carbon hydrogen.

3.1 Power Generation

3.2 Transportation

3.3 Industrial Heat and Feedstock for Chemical Processes

3.4 Heating Buildings

While hydrogen can be produced through the electrolysis of water, most of the hydrogen produced today is manufactured by steam methane reforming (SMR), in which, at high temperatures, natural gas is converted to hydrogen and CO₂. As identified by the CCC, production of bulk low-cost, low carbon hydrogen from fossil resources is an integral part of meeting the UK’s net zero obligations (and net zero targets around the world). It can also make a significant and important contribution to the UK’s pressing 4th and 5th carbon budget shortfalls. The low cost aspect is important: at present the cost of manufacturing hydrogen by advanced gas reforming incorporating downstream CCUS (to ensure the hydrogen has a low carbon footprint) is around US$1.50–2.80 kg⁻¹, while the cost of hydrogen from renewables is much higher, falling between US$3.00–7.50 kg⁻¹ (1). Hydrogen made from electrolysis using renewable electricity is regarded as zero carbon and is referred to as ‘green’ hydrogen, while that made via methane reforming with CCUS is regarded as low carbon and referred to as ‘blue’ hydrogen. While the end-point in a fully decarbonised ecosystem will be green hydrogen, the most cost effective way to integrate hydrogen broadly into a wide range of applications today (and for the foreseeable future in many parts of the world) is to use blue hydrogen. For example, the CCC’s Net Zero report and roadmap predicts that the UK will require approximately 270 TWh of hydrogen in 2050 (up from around 15 TWh today), and they estimate that around 80% of this will be blue hydrogen, with the remaining 20% being green, as shown in Figure 5 (2).

Fig. 5

Before we discuss the routes to blue hydrogen, electrolysis will be outlined. Electrolysis uses electricity to split water into hydrogen and oxygen. This reaction takes place in an electrolyser, which like fuel cells consists of an anode and a cathode separated by an electrolyte. There are two commercially available technologies:

There are two other types of electrolyser at earlier technology readiness levels:

The topic of electrolysis will be revisited in the future as there are important advances required to enable large scale deployment. In the near term, as mentioned above, the consensus is that blue hydrogen will be key. Johnson Matthey, UK, has developed a process known as Low Carbon Hydrogen (LCH^TM), which has a gas heated reformer and autothermal reformer at its core to generate blue hydrogen from natural gas, shown in Figure 6 (40). This approach gives a higher hydrogen yield and is more energy efficient than existing SMR technologies. And, crucially, this process is easier and cheaper to decarbonise through CCUS than an SMR. The process delivers a high CO₂ capture rate, high efficiency and low-cost solution, providing significant benefits compared with SMR and alternative autothermal reforming (ATR) technologies. The approach is based on established chemical process engineering, designed to operate at scale, enabling carbon reduction for industry, dispatchable power, domestic heating and transport.

Fig. 6

The main benefits of the LCH^TM technology compared to the current SMR technology with >95% CO₂ capture rates are:

A comparison of the process performance for LCH^TMversus an SMR is shown in Table I, where the hydrogen production rate has been fixed and a minimum CO₂ capture rate of 95% has been required.

Overall, the LCH^TM technology offers the UK and other countries a ‘low regrets’ way of moving towards a net zero scenario as all of the unit operations have been deployed at scale in other areas, such as in production of methanol and ammonia. Design work has confirmed that a single train is capable of producing 300 MW (lower heating value) of high purity hydrogen. Furthermore, work has been conducted that indicates that a 1.5 GW hydrogen plant could be built in a single train with a number of equipment items in parallel.

One of the major barriers to hydrogen deployment versus other renewable technologies has been the requirement to build new infrastructure immediately, particularly for generation and distribution to the various customers. Today much of the hydrogen market is dominated by captive supply where generation is next to use, for example hydrogen production for use at a refinery for upgrading transport fuels.

The view that hydrogen can be crucial to decarbonise multiple market sectors means that hydrogen production at scale will be required. It is envisaged that a hub and spoke model will work best, with centralised production facilities bearing the brunt of the load, supplemented by smaller production facilities operating away from large emissions centres. The clustering of existing industry, gas facilities (liquified natural gas, gas turbines), ports, major pipelines and intersections with hydrogen production and CCUS facilities represents the lowest cost route to net zero. The additional ability to reuse existing gas distribution networks in some countries will play a large role in reducing transport costs.

Before returning to examples of key UK projects it is worth discussing how energy is moved as this is one of the key infrastructure challenges to make a dramatic energy transition. Transportation and storage are costly elements of the value chain. At small scale distributed production will rely on local storage and distribution, for example tube trailers. At large scale the reuse of gas pipelines will allow hydrogen to be moved around cost effectively and there are known and available solutions for storing hydrogen such as salt caverns. More capacity will be required to deal with the volumes of gas required, but this is not seen as a barrier for deployment.

There is another opportunity that hydrogen offers, which is to move renewable energy from where it can be generated at very low cost to where it can be monetised. There are areas of the world which have very good utilisation factors for renewables, but they are not near demand centres and the cost and practicality of a transmission system would not be viable. The focus has been on using hydrogen to transport the energy in a chemical bond. Different strategies are being considered, such as liquefication of hydrogen, synthesis of a hydrogen containing molecule (ammonia or methanol) that can be converted back to hydrogen or use of a carrier (liquid organic hydrogen carriers) where an organic molecule is hydrogenated and dehydrogenated. The main considerations are process efficiency, energy density, safety and whether there is existing infrastructure (41).

Extensive studies have been carried out and large-scale projects are now being initiated to demonstrate how low and zero carbon hydrogen can be manufactured at scale and integrated at a city-wide and regional level (42–45). In the UK, BEIS are currently engaged in supporting a number of studies covering the whole value chain to understand the current technology options and potential lowest cost solutions. The strategy is being developed as part of the Clean Growth Plan. In addition, since the announcement of the UK’s Net Zero legislation there have been further funding streams announced, which are either live (Industrial Strategy Challenge Fund), under consultation (Industrial Energy Transformation Fund) or will be consulted on in 2020 (Low Carbon Hydrogen). However, this should only be considered as the tip of the iceberg. Of critical importance to the sustained roll out of low carbon hydrogen will be the business models that allow private investment, which improves the supply chain and increases scale ultimately driving down costs to the consumer.

Whilst no definitive policy changes have been made to date in the UK there has been much more focus on how the UK can lead in low carbon technologies and embed this at the heart of plans for clean growth. BEIS has responsibility for both the Clean Growth Plan and Industrial Strategy. It has recently been much more active in the hydrogen and CCUS space, considering production, transport and use. Another £33 million has been made available under the Hydrogen Supply Competition (HSC) focused on production (46).

The HyNet project comprises the development and deployment of a 100 kNm³ h⁻¹ (equivalent to 300 MW of hydrogen, lower heating value) hydrogen production and supply facility to be sited at Essar Oil’s Stanlow refinery utilising Johnson Matthey’s LCH^TM technology. It could represent one of the first deployments of a technology proven in other sectors to the production of clean hydrogen and will achieve this at scale, at higher efficiency than other reforming technologies and with a very high carbon capture rate. It therefore will deliver low cost, low carbon bulk hydrogen.

This plant is core to the North West HyNet project. It is not a theoretical plant design but one that meets the specific regional demands, delivered on a specific project site. It will provide a foundation reference design for replication through multiple lines in the North West, elsewhere in the UK and internationally. When associated with the HyNet CO₂ transport and storage infrastructure, this delivers low cost, low carbon hydrogen for key industrials alongside non-disruptive blending to over two million households as part of delivering a net zero industrial cluster in the region. A schematic of the concept is shown in Figure 7.

Fig. 7

Having completed prefeasibility work under Phase 1 (47) of the BEIS HSC, the full front-end engineering design and wider operational, delivery, contracting and consenting programme is underway as part of Phase 2 of the HSC, which will deliver a shovel-ready project, providing the basis for a final investment decision. The project is being developed by a consortium of Johnson Matthey, as technology provider, SNC-Lavalin, Canada, as project delivery specialists, Essar Oil which owns the land, and led by project developer Progressive Energy, UK.

The Acorn Hydrogen Project, in North East Scotland (Figure 8) places advanced reforming technology at its core. The project will deliver a replicable process for cost-efficient hydrogen production based around natural gas, whilst capturing and sequestering climate changing CO₂ emissions.

Fig. 8

By 2025, the plant could be the first operational clean hydrogen plant in Europe, enabled for early development by the Acorn CCUS Project which is under development at the same location. North East Scotland is home to the oil, gas and renewables supply chain, which has the capability, capacity, technology and assets to diversify into a future hydrogen supply chain, creating economic value and jobs for the region and supporting a just transition to a low carbon economy. Clean hydrogen can be blended into the National Transmission System (NTS) and used in the region for decarbonising heat, industry and transport.

Phase 1 of the HSC delivered a feasibility study for an advanced reforming process at St Fergus (48). The basis of the study was export of hydrogen at a 2% by volume blend into the NTS. No technical issues were identified. Crucially, the work has also strengthened the partnering and route to market aspects of the Acorn Hydrogen Project.

The Acorn Hydrogen Project is led by Pale Blue Dot Energy, UK, and benefits from strong industry study partners in Shell, The Netherlands, Chrysaor, UK and Total, France, while Johnson Matthey will play a significant role in providing a hydrogen technology option for the project. Acorn Hydrogen offers Scotland and the UK the opportunity to capture up to 19 million tonnes CO₂ equivalents of CO₂ per year through the build-out, enabling the UK to reach its net zero obligations by 2050 and Scotland by 2045.

These are not the only projects that are being discussed in the UK. Recently announced, the Zero Carbon Humber (49) project brings together Equinor, Norway, Drax, UK and National Grid, UK with a vision to cut the emissions from the largest UK hotspot and again has hydrogen at the core. Johnson Matthey is also involved in a project called Cavendish (50) looking to produce low carbon hydrogen at the Isle of Grain, which would provide decarbonised dispatchable power to service London as well as providing a decarbonised gas for domestic heating.

It should be noted that this is not purely a UK opportunity as shown by the projects being discussed in The Netherlands, H-Vision project (51) at the Port of Rotterdam as well as Magnum (52), which is the conversion of a natural gas combined cycle gas turbine (CCGT) to hydrogen. The recently published US Hydrogen Roadmap (53) also discusses routes to hydrogen and sees a role for low carbon hydrogen production from natural gas.

Low carbon hydrogen has the potential to play a large role in supporting the journey to net zero. Projects should be deployed in the next 10 years to learn the real costs of operation and stimulate the supply chain. It will take time to build the volume of hydrogen production and the infrastructure for hydrogen use in all the sectors discussed above. There is always the question of balancing supply and demand, but with the many potential use cases building capacity will be a key starting point. Hydrogen produced by electrolysis powered by renewables is the ultimate answer and efforts need to be developed and scaled up, but it will struggle to deploy at the scale required in many locations in the near term. Both routes to low carbon hydrogen will be needed and they should be seen as complimentary with a transition happening over time.

Decarbonisation of transport relates to the structure of energy consumption in the transport sector, unless vehicle-mounted carbon capturing devices are considered. The structure of primary energy consumption in the world and final energy demand for transportation services in 2016 are shown in Figure 1. Obviously, the main pattern for transport decarbonisation is associated with substitution of petroleum for other energy carriers with lower carbon footprints. Such energy carriers are gas (primarily methane), electricity and hydrogen. Vehicles utilising the last two types of energy as input, provided that hydrogen is used as fuel for fuel cells (FCs), are called zero emission vehicles (ZEVs). However, it is necessary to take into account the origin of these energy carriers, since their source could be coal, oil or natural gas. The ultimate solution to the issue of transport decarbonisation is complete electrification of transport, including the use of so-called ‘green’ electricity and hydrogen, i.e. those originated from renewable or nuclear energy. The fact that water vapour has global warming potential is beyond the scope of the topic under discussion.

Fig. 1.

Transport decarbonisation patterns have several aspects: social, economic, technological and institutional. The social aspect is affected by fears of future crude oil supply exhaustion and anthropogenic impact on the environment. The economic drivers are profit-making for vehicle manufacturing and energy supply businesses and value-added ambitions for national governments, including substitution of energy import by establishing domestic innovative energy technology chains. The technological aspect relates to maturing and commercialisation of technologies for more effective utilisation of traditional fuels and the ‘green’ production, transportation and storage of electricity and hydrogen. The institutional factor refers to the regulatory mechanisms to reduce greenhouse gas (GHG) emissions associated with passenger and cargo traffic by all transportation modes, both at national and international levels. Other issues of technological and comparative socio-economic assessments of transport systems involving the shift from petroleum to gas fuel, improvements in vehicles’ energy efficiency, introduction of biofuels, carbon capturing systems and rationalisation of transport services remain outside the scope of this article. Aspects of transport decarbonisation, related to the creation and development of hydrogen technologies in the industrialised economies of East Asia in recent years, will be considered further.

The East Asian economies of Japan, South Korea, China and Taiwan are among the global leaders in a number high-technology industries. More than half of cars, buses, trucks and more than 90% of newbuild ships in the world are produced in these economies (Figure 2 and Figure 3), and they hold significant share of the world’s electric vehicles stock and sales, including infrastructure for charging battery electric vehicles (BEVs), see Table I and Table II. The industrial might of East Asian countries combined with energy resource shortage has led to their overwhelming dependence on coal, oil and gas imports. Taiwan, Japan and South Korea are characterised by extreme dependency on energy imports (Figure 4), while China is the world’s largest energy importer (Figure 5).

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

The carbon footprint of transportation systems is usually measured from ‘tank to wheel’, i.e. GHG emissions from fuel and energy stored on-board the vehicle. Following this approach a BEV is considered a ZEV even in cases where the electricity stored in its battery is produced from coal or gas. However, ‘tank-to-wheel’ GHG emission is an important metric when strictly defined common transportation systems like roads, aviation, water and railways are considered. The respective share of these modes within the total final energy consumption for transport in four East Asian economies in 2016 and their share by energy consumed are shown in Table III.

The per capita GHG emissions from domestic transportation, and particularly those of road vehicles in South Korea, Taiwan and Japan are significantly higher than the world’s average GHG emissions (Figure 6). As the International Energy Agency (IEA)’s report shows (7), the GHG emissions due to international bunkering are relatively small in comparison to domestic transportation emissions. However, it seems that a significant part of such emissions, induced by international marine and aviation traffic originated in East Asia, is attributed to the countries proportionally to the traffic within their economic zones, not by the site of actual fuel bunkering. This shows a strong link between international initiatives for GHG emissions reduction and energy policy drivers for the development of low-carbon technologies for mobile energy systems in East Asian economies.

Fig. 6.

It is clear that shifting from petroleum to natural gas and electricity will lead to lower carbon footprints. Electrification eventually will end up in zero ‘tank-to-wheel’ GHG emissions. Importantly, vehicle electrification could be based on two approaches: (a) electricity generated outside the vehicle; and (b) electricity generated on board. The latter implies existence of fuel storage, electricity generator and power transmission within a single vehicle. If such a transport vehicle (ship, aircraft, locomotive, road or off-road vehicle) is fuelled by ‘green’ hydrogen or electricity, it is a true ZEV under the ‘well-to-wheel’ terms.

East Asia is already a leader in FC electric vehicle (FCEV) production. Currently, there are more than 11,000 FCEVs in the world, and while most FCEVs are used in the USA, up to 85% of them have been produced in East Asia (8). East Asian economies are characterised by the widespread use of light vehicles for individual movement, such as mono-, bi- and tricycles, mopeds and motorcycles, thus different types of battery and FC scooters are under development (9).

Despite extensive railway electrification in East Asia, FC locomotives are being designed in Japan and South Korea. International aviation will not be a priority for the implementation of hydrogen technologies, while electric propeller aircrafts and drones could be powered by FC. Recent advances in liquefied natural gas (LNG) fuelled ships and its combination with FC technologies will bring new impetus to low-carbon powertrain development for water transport systems.

Electrification is the main option for ultimate decarbonisation for all types of transportation systems. The trends of transport electrification are determined by advances in storage of electricity and hydrogen, and by improvements in onboard powertrain (the efficiency of the transformation of stored energy into mechanical work) for these types of energy carriers.

The Johnson Matthey Technology Review provides significant contribution to the FC and car batteries technologies development, which is recorded in the issue on the occasion of the 200th anniversary of the journal (10, 11).

Road vehicles are ideal for the development of hydrogen and electric battery technologies because:

The main advantages of hydrogen technologies over those based on batteries are higher gravimetric density of onboard energy storage and the speed of vehicle refuelling (Table IV). Similar to battery-based transportation systems, progress in FC technologies needs intensive hydrogen infrastructure development.

The commercialisation of FCEVs and introduction of hydrogen infrastructure will lead to the creation of hydrogen mobility energy systems, the ultimate stage for all carbonless non-catenary electrified transportation modes. It is the start of the process of transitioning energy systems to full independence from fossil fuels.

The most worked out concept for a sustainable circular society within East Asian economies has been developed in Japan (Figure 7) (15). Hydrogen technologies are an integral part of this concept, which introduces hydrogen as a new energy carrier ‘electrofuel’ (8), fungible to electricity.

Fig. 7.

Japan acts as an international icebreaker, capturing leadership positions in hydrogen energy systems development at a national level. This East Asian economy provides an example of energy institutions’ reformation to decarbonise transportation by substituting petroleum for hydrogen. At the summit of the G20 leaders in Osaka in June 2019, the report “The Future of Hydrogen: Seizing Today’s Opportunities” was presented (8). The report was prepared by the IEA on behalf of the Government of Japan.

The energy supply framework and policy drivers to reduce carbon intensity in the transport sector for China, Japan, South Korea and Taiwan will now be reviewed. Since the topic for discussion on technological and institutional options to reduce carbon footprints of transport services in East Asia is very broad, the study will focus on programmes for hydrogen technologies and related institutional developments.

3.1 Energy Consumption and Transportation Sector

3.2 Institutions

3.3 Major Recent Developments

4.1 Energy Consumption and Transportation Sector

4.2 Institutions

4.3 Major Recent Developments

5.1 Energy Consumption and the Transportation Sector

5.2 Institutions

5.3 Major Recent Developments

6.1 Energy Consumption and the Transportation Sector

6.2 Institutions

Improving energy security and reducing anthropogenic environmental impacts are strategic issues for the energy policies of industrial economies in East Asia: China, Japan, South Korea and Taiwan. The transport sector is of particular importance, since it is pivotal in efforts to relieve peaking oil demand, and is instrumental in decarbonising final energy consumers.

The East Asian economies’ thirst for energy security is the most important driver for transport decarbonisation. The next driver is a commitment to combat climate change, as a number of binding regulations and government programmes aimed at reducing GHG emissions have been adopted. Additional policy drivers are the role of China, Japan and South Korea in the world’s vehicle manufacturing and shipbuilding; as well as the size of the international ship and aircraft bunkering business in East Asia for passenger and cargo traffic.

There are considerable efforts within East Asian economies to develop policy towards low carbon energy supply infrastructure in general, and low carbon transportation systems in particular. The general trend is fuel substitution (petroleum to gas, internal combustion engine to more energy-efficient combinations of motor and powertrain) and electrification of transport vehicles, including advances in mobile energy systems, like hybrid and FC powertrains. The ‘hydrogen society’ concept combines renewable energy for green hydrogen production and its utilisation as the ultimate non-carbon fuel. While key hydrogen technologies have a wide range of applications in transportation, from tankers, locomotives and aircraft to hydrogen-driven monocycles, road transport applications are important at the commercialisation stage for a number of economic and technological reasons.

A scramble for capturing leading positions in the global ZEV market has become a distinctive feature of BEV, FCEV and hydrogen technologies development in the East Asian economies. They are at the forefront of the course to introduce hydrogen as new energy carrier, and it can be seen as the starting (icebreaking) position for transition of a petroleum-based transportation system into one ultimately independent from fossil energy. Japan, China and South Korea are already implementing regulation, energy institute transformation and transition from the pilot stage to the practical development of carbon-free mobility systems at a national level. Currently, the fundamentals for the competitive development of all low-carbon technologies have been created in East Asian economies in order to reduce the transport system’s carbon footprint.

The book titled “Graphene-Based Nanotechnologies for Energy and Environmental Applications”, edited by Mohammad Jawaid, Akil Ahmad and David Lokhat, focuses on recent developments in graphene-based materials, composites and devices for a variety of applications in storage devices, supercapacitors, water treatment, ion-separation, photocatalysts and antimicrobial applications. It is part of the series Micro and Nano Technologies published by Elsevier. The first editor of the book, Mohammad Jawaid from the Universiti Putra Malaysia, has expertise in nanomaterials (particularly graphene materials) and their composites and has significant research output with a h-index of 53. The second editor, Akil Ahmad, currently a postdoctoral researcher at the University of KwaZulu-Natal, South Africa, has worked on nanomaterials synthesis and applications of nanomaterials in wastewater treatment. David Lokhat, the third editor of the book from the University of KwaZulu-Natal, has been working on reactor and extraction technologies.

The book is divided into three major parts: Introduction, Energy and Environment. Each category has many chapters written by diverse authors. A total of 59 authors from different affiliations contributed to the different chapters. Firstly, the introduction covers basic terminologies and definitions of nanotechnology, nanomaterials and provides specific literature background on graphene-based materials and their composites for energy and environmental applications. Ahmad et al. collected literature on graphene-based nanotechnologies, which covers the latest developments in graphene research around the world, and David Lokhat contributed towards energy and environmental applications leveraged by graphene derivatives along with publication statistics. Production methods, characterisation methods and properties of graphene and its applications in different areas are covered. The literature and data were collected and compiled from over 350 publications. Every chapter has a conclusion or concise summary with potential prospects for the future in each research or subject area.

Mamvura et al. (University of South Africa) have written a chapter on renewable energy systems using graphene derivatives. The chapter covers applications of graphene in battery-powered vehicles, fuel cells, solar cells and energy storage devices. Mohamed I. Fadlalla and Sundaram Ganesh Babu (University of Cape Town, South Africa) presented a chapter on graphene materials in photocatalytic water splitting for hydrogen production. Topics such as mono- or bi-semiconducting catalyst and metal and non-metal doped graphene-based photocatalysts for water splitting applications are covered. Umar et al. (Universiti Sains Malaysia) included topics on metal decorated graphene nanocomposites for energy storage applications. Their chapter is mainly focused on metal-based composites, solar and fuel cells, supercapacitors and lithium-ion batteries and mechanisms of energy conversions are covered in detail. A chapter on graphene oxide (GO) for hydrogen storage applications was written by Azim et al. (University of KwaZulu-Natal). Composites of GO and reduced GO with metal oxides, carbon nanotubes and organic materials and relevant fabrication methods are well elaborated in this chapter. Professor Mohammad et al. (King Saud University, Saudi Arabia) have contributed a chapter towards graphene-derived nanocomposites as supercapacitors and electrochemical cells. This chapter includes the synthesis (Figure 1) and physical properties of graphene nanosheets, a section on biosensors and a short note on supercapacitors produced from graphene nanocomposites. Jean Mulopo and Jibril Abdulsalam (University of the Witwatersrand, South Africa) have provided a chapter on graphene-based energy storage applications (capacitors, batteries, fuel cells and solar cells) with an emphasis on electrical and thermal conductivity, specific surface area and specific heat properties.

Fig. 1

Overall, this section of the book with six chapters covers a wide range of electronic devices incorporating graphene and its derivatives. In-depth analysis and data have been included from a significant number of publications and research works. Topics on graphene composite as air filters, gas sensors, volatile sensors, liquid sensors, radiation sensors and pollutant sensors are adequately discussed in these chapters.

The section of the book begins with a chapter on graphene-based sensors for the detection of volatile organic compounds (VOCs) written by Ansari et al. (Aligarh Muslim University, India). Graphene with metal additives as sensors and their functioning mechanisms have been well discussed in this chapter. Haseen et al. (Aligarh Muslim University) concentrated on the application of magnetite-GO composite for wastewater treatment. This chapter covers magnetite-GO for specific dispersive solid-phase extraction. Mohamma Laskar and Sana Siddiqui (Jazan University, Saudi Arabia) focused on GO-based filters for solid-phase extractions, including nascent GO, chelates adsorbed GO, functionalised GO with external molecules and specific GO nanocomposites. GO functionalised with magnetic molecules and their composites with polymer or metal matrices have been extensively studied. Reduced GO (rGO) derivatives for such applications are also included.

A chapter by Kumar et al. (King Abdulaziz University, Saudi Arabia) covers graphene-metal oxide composite photocatalyst for degrading water pollution. Structure and property (chemical and physical) relationships and the effect of graphene’s bandgap on photocatalytic decomposition are interpreted. The mechanism of photocatalysis for relevant graphene materials and metal-GO and rGO composites are included. Hussain et al. (Jubail Industrial College, Saudi Arabia) collated information on a new generation of GO for removal of polycyclic aromatic hydrocarbons from a wide range of literature and new results. The chapter covers several properties of graphene, such as mechanical, electrical and thermal properties and their influence on the interaction of polycyclic aromatic hydrocarbons as well as the role of GO as an adsorbent for such hydrocarbons. A chapter by Ng et al. (UCSI University, Malaysia) is dedicated to graphene-based membranes for separating hazardous contaminants in wastewater. This is probably the only chapter that gives importance to both polymer-based and metal-based graphene composites for the targeted application. Traditional thermoplastics (polystyrene, polyvinylidene fluoride, polyamide-imide, polyacrylonitrile and polyethersulfone) composites and conducting polymer (polyaniline)-based graphene composites are organised with their fabrication process and efficiency as a membrane in a descriptive manner.

Hossain et al. (Universiti Sains Malaysia) focused on antimicrobial activity of graphene-based materials. The antimicrobial mechanism of major graphene derivatives (GO, rGO and graphene) are discussed along with the performances of their composites with hydrogel and polymer dispersions. The effect of toxicity of graphene materials on antimicrobial activity adds to the value of this chapter. Graphene-metal oxide hybrid composites for treating textile dyes are discussed in a chapter by Shahadat et al. (Indian Institute of Technology, Delhi). This short chapter attempts to add to the knowledge of graphene-metal synthesis for removal of industrial dyes and provides details of the effects of functional groups (hydroxyl, carboxyl and oxygen) present in the composite systems on their performance. Reddy (Universiti Teknologi PETRONAS, Malaysia) and co-authors emphasised graphene nanomaterials for removal of pharmaceutical compounds in drinking water. The impacts of surface functional groups, sorption kinetics, pH and temperature on absorption stability of graphene-based materials and nanocomposites are discussed in detail. Research data on polymer-based, ceramics-based and metal-based composites are also covered in this chapter. Two chapters, by Yadav et al. (Shree Velagapudi Ramakrishna Memorial College, India) and Abbas et al. (Universiti Sains Malaysia), focus on the application of graphene composites in air quality and wastewater treatment. Figure 2 depicts different applications in which graphene nanocomposites can be utilised.

Fig. 2

Each chapter provides solid knowledge in its prescribed subject matter, and they read and flow well. However, looking collectively, there are several duplications and repetitions found in the book, especially the synthesis of graphene and applications such as storage devices and water treatment. These chapters are written using different language, and the knowledge is not very diverse. Another major flaw of the book is that it has missed out on the latest developments in graphene-based polymer composites and their multifunctional applications in energy and environment, which is a significant subject area that is expected to be covered in a book like this. There is only one chapter (Chapter 15) that covers sufficient polymer-graphene composites in the removal of hazardous contaminants from wastewater. Other application areas related to energy and environment are completely neglected. Furthermore, while most chapters have excellent illustrative figures, a few chapters do not have a single figure. It is always better and more attractive to have figures to effectively convey scientific concepts and processes. The summary in each chapter is concise, and future prospects are given appropriately. The front cover, preface, table of contents, index and back cover are suitable and sufficient.

Summing up, this book provides useful knowledge predominantly in graphene-based materials for storage cells, sensory and wastewater treatment applications.

Global car and truck manufacturers, along with their supply chains, have made huge steps to minimise vehicular emissions since the advent of the internal combustion engine (ICE). Of particular note are the criteria pollutant emissions regulations, which have focused on reducing the tailpipe carbon monoxide, hydrocarbons, nitrogen oxides (NOx) and particulate matter (PM) emissions from the global vehicle fleet. For example, since the first European regulations were introduced in the early 1990s, the permitted NOx emissions of cars have dropped by almost a factor of 15, which has been enabled by close collaboration between the car manufacturers and the substrate and catalyst suppliers. More recently, the focus has shifted to CO₂ emissions, as governments and regulators work towards the implementation of measures to enable the Paris Agreement climate change commitments to be met (1). Indeed, the latest view of the Intergovernmental Panel on Climate Change (IPCC) is that there are significant benefits in targeting a maximum temperature increase of 1.5°C over pre-industrial levels, rather than the 2°C Paris Agreement target, and this 1.5°C target essentially means that CO₂ emissions need to reduce to net zero globally by 2050 (2). This is an extremely challenging target, with massive implications for all energy-hungry sectors such as transportation, which currently accounts for around 24% of global CO₂ emissions (3). Moving the transport segment to net zero by 2050 means that only vehicles with zero CO₂ emissions can be sold from 2040 or earlier, to avoid legacy fleet emissions, since cars, buses and trucks typically stay in use for 10 years or more. Indeed, the recommendation of the Committee on Climate Change (CCC), the UK Government’s independent advisor on climate change, is that introduction date of the ban on vehicles powered by ICE should be brought forward from 2040 (which was the original plan) to 2035 “at the latest” or, more preferably, 2030 (4). Currently, around 1% of new passenger car sales globally do not have any tailpipe CO₂ emissions (that is, they are regarded as zero emission vehicles (ZEVs)) with almost all of these being battery electric vehicles (BEVs). There are far fewer zero emission commercial vehicle sales, so the automotive industry has a long way to go on its journey to net zero.

The two principal ZEV ground transportation options are the BEV and the fuel cell electric vehicle (FCEV). BEVs use electricity to charge the battery to provide motive power, while FCEVs use an electrochemical cell to convert the chemical energy of hydrogen (supplied from an on-board tank) and oxygen (from the air) into electricity. There are several types of fuel cell, but the one most applicable to transport applications is the polymer or proton electrolyte membrane (PEM) fuel cell (also called proton exchange membrane fuel cell), which starts up rapidly, operates at low temperature, and delivers high power density at low weight or volume compared to other fuel cells.

In some geographies there are discussions on whether biofuels or power-to-liquids, also known as e-fuels, can play a major role in the decarbonisation of the transport sector. In this context, a biofuel is a liquid or gaseous fuel produced from biomass or waste, and an e-fuel is a fuel generated through the use of renewable electricity to generate hydrogen which is then attached to carbon from CO₂ for subsequent conversion into hydrocarbon fuels similar to those used today.

Biofuels can be ethanol, methanol, fatty acid methyl ester (FAME), hydrotreated vegetable oil (HVO), biomethane (either compressed or liquefied) or advanced biofuel such as biodiesel or bio jet fuel. These biofuels can be split into generations of biofuels:

Biofuels were first introduced in the hope of reducing carbon intensity of fuel, since in a simplistic sense the CO₂ generated by combustion is absorbed by the regeneration of the crops used to make it, and because they can be blended into fossil fuels without the need to modify engine technology.

First generation biofuels do not represent good decarbonisation options since when both direct and indirect emissions are taken into account (for example, from changes in land use), such biofuels are often only a marginal GHG emission improvement over fossil fuels, and in some cases actually have higher emissions (5). Sustainable advanced biofuels are based on wastes and residues, so their potential contribution to fuel requirements is finite. The industries that typically contribute the most to advanced biofuels are agriculture and the food industry (through residues such as organic waste sludges, manure or straw) and forestry industries, especially in the Nordics (from saw and pulp mills). Biomass resources are also already well utilised, so if the current consumption in other areas (for example, paper production) is assumed to continue, the maximum biofuel production would be able to supply around 8.5% of all road transport in the EU (5). However, the aviation and marine sectors are already making their case to use these biofuels, so while they may make a contribution to reducing road transport CO₂ in the short term (via blending into current hydrocarbon fuels), it is not expected that they will make a major contribution in the medium term and beyond.

For e-fuels, the first step in their production is to generate hydrogen via water electrolysis using renewable electricity. Hydrogen is then combined with CO₂ to form hydrocarbon fuels, with the CO₂ coming from, for example, industrial or biogenic sources, or from the direct capture of CO₂ from the air (direct air capture (DAC)). At the present time, industrial CO₂ emissions are regarded as ‘waste’, but capturing this CO₂, converting it into a hydrocarbon fuel, and then combusting it still leads to its release into the environment, and in the medium to long term it is expected that the CO₂ sources will either need to be from DAC or from ‘green’ sources such as biomass combustion.

The technologies to convert CO₂ and H₂ into both synthetic natural gas (SNG) and methanol at scale are known, and it can be expected that processes at early technology readiness level (TRL) currently under development (for example, direct electrochemical synthesis) will get more attention should there be a market. To produce e-fuels the SNG and methanol can undergo further conversion, for example to dimethyl ether or via the methanol-to-gasoline process to gasoline. However, not all pathways to the higher value e-fuels are commercially viable, and indeed the most attractive product to make, kerosene, is not accessible today as there is no large scale implementation of the reverse water gas shift reaction, which converts CO₂ into CO, which is the active carbon species in the Fischer-Tropsch reaction. Today the process is used to convert gas and coal to liquid fuels, but there are several projects focusing on the conversion of biomass and waste to liquid fuels.

The attraction of e-fuels in the form of the hydrocarbon fuels used today is that they can be a direct drop-in for current fuels, using the same distribution network and being burned in the same kind of engines that we have today. In some applications, such as aviation and marine, their use seems likely, as discussed elsewhere in this edition of Johnson Matthey Technology Review, since liquid fuels are expected to be required for a substantial period of time in these areas due to challenges with the use of battery or hydrogen fuel cells in such applications. For ground transportation, however, their widespread use seems less likely for several reasons, including:

So, while it is expected that biofuels and e-fuels will play a significant role in the aviation and marine areas, the focus of this article is on ground transportation, where BEVs and FCEVs are expected to be the major technologies. This is consistent with, for example, the views of Martin Daum, Member of the Board of Management of Daimler AG, responsible for trucks and buses:

“Truly CO₂-neutral transport only works with battery-electric or hydrogen-based drive” (7).

The regulations in the passenger car and commercial vehicle sectors focus on emissions from the tailpipe, and historically the main focus has been on criteria pollutants, which have enabled major improvements in urban air quality to be made. The focus is now shifting to CO₂, and Figure 1 shows the current and incoming CO₂ regulations for cars in various countries and regions around the world (8), illustrating the substantial reductions required going forward.

Fig. 1.

The European regulations for 2025 and 2030 require reductions of 15% and 37.5% respectively over the 2021 legislation. These regulations are intended to continue to drive the decarbonisation of the automotive industry, and the fleet average CO₂ emissions required in 2030 will require extensive electrification of the fleet. Indeed, Herbert Diess, CEO of the VW Group, has stated that these 2030 regulations will require at least 40% of VW’s European sales to be electric vehicles (BEV and plug-in hybrid electric vehicles (PHEV)) in 2030 (9).

Legislation on CO₂ and GHG is also tightening in the commercial vehicle sector, with the next set of European regulations requiring a 15% drop in CO₂ emissions from today by 2025, and a 30% reduction from today in 2030. This 2030 target is expected to lead to significant hybridisation of the commercial vehicle fleet, along with some completely electrified vehicle sales. Trucks, buses and coaches are responsible for about a quarter of CO₂ emissions from road transport in the EU and for some 6% of total EU emissions (10), so introducing low and zero emission vehicles in this sector is critical to support global moves towards net zero.

The electrification of the bus market is already underway, with over 400,000 battery electric buses in use in China today (out of around 425,000 BEV buses worldwide). Some Chinese cities, such as Shenzhen, have completely transitioned to battery-powered buses, with around 16,500 such vehicles on the road. Many other cities worldwide are committed to moving away from diesel and towards zero emission buses in the coming years. For example, 13 cities have signed the C40 Fossil Fuel-Free Streets Declaration (11), and will procure only zero emission buses from 2025. These are: Auckland, Barcelona, Cape Town, Copenhagen, London, Los Angeles, Mexico City, Milan, Paris, Quito, Rome, Seattle and Vancouver. London has committed to increase its BEV fleet from 120 to 300 by 2020, and in Paris 80% of the fleet will be e-buses by 2025. Oslo has gone further, and will have fossil fuel-free public transport by 2020, while in the Netherlands all new buses will be zero emission by 2025, with the whole fleet being all‐electric by 2030. These commitments will lead to improved urban air quality and a reduced CO₂ footprint, as long as the electricity used to charge the buses is from low carbon sources, as discussed later.

California often takes a mandate-based approach to regulations, in order to drive the development and initial implementation of new technologies. Within the commercial vehicle sector they are proposing an Advanced Clean Trucks mandate, which will require original equipment manufacturers (OEMs) with more than 500 truck sales in California to sell an increasing proportion of zero emission trucks, starting in 2024, per the schedule outlined in Table I. The intention of the California Air Resources Board (CARB) is to accelerate the first wave of zero-emission trucks, which are seen as essential if net zero targets are to be met, particularly since the commercial vehicle market is widely regarded as being significantly more difficult to decarbonise than the passenger car fleet. The schedule outlined in Table I will lead to a ZEV truck fleet of around 100,000 vehicles on California’s roads in 2030, rising to around 300,000 in 2035.

Of course, tailpipe emissions are only part of the CO₂ story, since the emissions associated with the manufacture of the vehicle and the fuel also need to be considered in any holistic analysis. For BEVs the manufacture of the battery generates significant levels of CO₂, estimated to be around 175 kg CO₂ kWh^–1 of battery capacity (12), and with vehicle batteries typically having between 30 kWh and 100 kWh of stored energy, this leads to upstream emissions of 5–17.5 tonnes of CO₂ per battery pack. In addition, the electricity used to charge the battery has associated CO₂ emissions, unless it is generated from renewable sources such as wind or solar power. For example, UK electricity currently has a carbon intensity of around 200 g CO₂ kWh^‐1 (13), which is below the average of European Union countries, while Norway (extensive use of renewable hydroelectric power) and France (predominantly nuclear power) have much lower carbon signatures.

The hydrogen used to power FCEVs is typically generated in one of two ways: either through electrolysis (in which an electric current is used to split water into hydrogen and oxygen) or the steam reforming of methane. The former route is, therefore, subject to the same CO₂ emission challenges (and opportunities) as the BEV, while the latter route generates relatively high levels of CO₂ which in future will need to be abated using carbon capture utilisation and storage (CCUS) technology, in which the CO₂ generated by the process is captured with high efficiency (which can be around 95%) and then either stored (for example, in depleted oil and gas fields) or used for other purposes (for example, to make chemicals).

Therefore, a full CO₂ life cycle analysis (LCA) of BEVs and FCEVs is required to paint a true picture of the carbon intensity of these vehicles. Some LCA studies are now being published and one of the most thorough is the one carried out by the International Council on Clean Transportation (12) who calculated the g km^–1 CO₂ emissions over the life of the Nissan Leaf BEV (with 30 kWh battery pack, lasting for the life of the vehicle), which they assumed would cover 150,000 km in its lifetime, and compared it to the average and the lowest emitting European cars powered by ICEs. In addition, Toyota have analysed the LCA of a FCEV (14), and these values have been updated for this paper based on Johnson Matthey’s knowledge of the CO₂ emissions when making hydrogen from CH₄ (with and without carbon capture and storage (CCS)).

Figure 2 shows the LCA CO₂ from the European car with average CO₂ emissions in 2017, along with the most fuel efficient ICE-based car in that year (which was in fact a hybrid), together with a BEV being operated on electricity with the EU average CO₂ footprint (the UK’s level is a little lower than this average), and that in Norway, whose extensive use of hydroelectric power reduces the CO₂ emissions during electricity generation to zero. The FCEV LCA is also shown, based on hydrogen generated from steam methane reforming (SMR) with and without CCUS, and based on electrolysis using Norwegian electricity. It is clear that BEVs and FCEVs have significantly lower CO₂ LCA than ICE-based cars today, and this gap will increase further as the carbon footprint of electricity generation continues to drop (see Section 4). (Note that this analysis does not include the recycling or disposal of the vehicles and their components).

Fig. 2.

A very recent BEV vs. ICE life cycle analysis subdivided passenger car GHG emissions into use-phase emissions (from driving the car), and production of all components (including, for example, emissions during mining of raw materials) and end-of-life emissions (15). This study concluded that driving a BEV is already lower in life cycle CO₂ emissions than petrol cars in 95% of the world. The only exceptions are countries such as Poland, where the electricity network is still mostly based on coal-fired power generation. In countries with a heavily decarbonised power system, such as Sweden and France which have large amounts of renewable and nuclear generating capacity, the average lifetime emissions from BEVs are up to 70% lower than petrol cars. In the UK, which is rapidly phasing out coal but still has a reasonable amount of gas-fired power plants, emissions are around 30% lower. The authors also point out that the advantages of BEVs will continue to grow, as power systems around the world become less carbon-intensive. The study projected that by 2050 half of the cars on the roads could be BEVs, leading to a reduction in global CO₂ emissions of up to 1.5 billion tonnes per year, which is the same as the total current CO₂ emissions of Russia.

This focus on LCA is already having a profound impact in the automotive sector. For example, the incoming VW ID.3 BEV is the first vehicle in the company’s history to be built with a CO₂ neutral balance sheet, covering the supply chain (for example, only green energy is used in the production of the battery cells), production (using only green energy at the Zwickau, Germany, manufacturing plant), use phase and recycling, with any currently unavoidable CO₂ emissions being offset by investments in climate protection projects (16).

Governments, states and regions are proposing, and in some cases (such as the UK) committing to, net zero GHG or CO₂ emission targets over the coming years. Indeed, at the time of writing two countries (Bhutan and Suriname) are already carbon neutral, 15 countries have set defined dates to become net zero, and other countries and regions, such as Germany and the EU, are discussing when to implement such a target. Within Europe, Norway plans to become net zero by 2030, Sweden by 2045 and Denmark, France and the UK by 2050. The implications of this are clear: road transport needs to decarbonise rapidly. As outlined above, a 2050 net zero target means that sales of new ICE powered vehicles need to stop by 2040 at the very latest, and preferably at some point during the 2030s, since cars and trucks are often on the road for 10–15 years or more before being scrapped.

This will be a substantial undertaking, requiring all new cars, trucks and buses to be powered by either batteries or hydrogen fuel cells on this timescale. As discussed above, this move to zero (tailpipe) CO₂ or GHG vehicles is only part of the challenge. The electricity used to charge the batteries, and the hydrogen used in the fuel cell vehicles, must also be generated in a very low or zero carbon manner, such as through renewable electricity or advanced CH₄ reforming with CCUS.

Many countries are driving down the CO₂ emissions from power generation. For example, the UK almost halved the carbon footprint of its electricity generation between 2013 and 2017, and one future projected UK pathway to 2050 is shown in Figure 3, from analysis for the National Grid’s Future Energy Scenarios 2019 document (17). This “Two Degrees” scenario foresees significant increases in renewable use, along with a large reduction in natural gas use and the cessation of coal-fired power generation, leading to a reduction in carbon intensity from 120 g CO₂ kWh^–1 in 2019, to just 14 g CO₂ kWh^–1 in 2050. This scenario is consistent with the UK achieving the 2050 decarbonisation target with large-scale centralised solutions.

Fig. 3.

Net zero targets will demand the decarbonisation of road transport (and other forms of transport), and will require strong governmental and regional policies to drive and support the uptake of zero emission vehicles. Extensive public charging and hydrogen refuelling infrastructure will be necessary, and the vehicles must be attractive and affordable options, with features that suit today’s and tomorrow’s lifestyles and transport needs.

The passenger car sector is largely driven by price, convenience and lifestyle: will my vehicle get me comfortably from A to B; can it carry the things I need to take with me; is it a sensible financial choice, in terms of purchase price, fuel price and overall cost of ownership (including likely resale value); and can I easily and conveniently refuel the car after driving the kind of distances that matter to me?

The main questions asked in the commercial vehicle market relate to how this purchase will help the business. The total cost of ownership (TCO) is a critical make-or-break calculation in this sector, as is the requirement for a very high level of vehicle uptime; so a long driving range and rapid refuelling are important here, as is the total load that can be carried by the vehicle.

Given the very different requirements in the two segments, they are considered separately in the subsequent analysis of critical drivers.

5.1 Customer Pull

5.2 Vehicle Price, Ownership Cost, Range and Fuelling Infrastructure

Another important comparison between BEVs and FCEVs is their respective refuelling rate, as shown in Table IV. FCEVs can refuel in around five minutes, corresponding to an energy input per second of around 4 MW which, while slower than the 20 MW typical of gasoline and diesel fuelling, is much faster than that of BEVs, where even Tesla superchargers can only deliver a maximum rate of 0.12 MW. The introduction of ultra-fast chargers is just starting in Europe and North America, with a maximum refuelling rate of 0.35 MW, still a factor of 10 slower than fuelling a FCEV with hydrogen. These differences in fuelling rate are particularly important for some applications, for example high utilisation fleet vehicles and vehicles which do a lot of long distance driving (and heavy commercial vehicles).

There are challenges in both the BEV and FCEV supply chains. For BEVs there are some concerns around Co availability and the ethics around some mines in the Democratic Republic of Congo, where around 50% of the world’s Co is mined. In addition, the projected increases in BEV penetration will likely lead to supply chain pressure on commodities such as Ni (which is a key component in the battery itself) and copper (which is used to move electrons around on the vehicle, and throughout the charging infrastructure), which will put upward price pressure on BEVs. The FCEV supply chain is not well developed today because vehicle volumes are so low, so there is work to do to build the volumes required to support this technology going forward, for example around the fluoropolymer and hydrogen tank components. However, one of the most expensive fuel cell constituents, platinum, already has a highly developed supply chain, and there is plenty of Pt above ground that will be accessible via autocatalyst recycling.

On recycling, the importance of developing cradle‐to-cradle supply chains for future technology has never been greater. There is a legal imperative for vehicle OEMs to ensure their vehicles are extensively recycled, and there are components of high value (and relative scarcity) in both FCEV membrane electrode assemblies (MEAs) and BEV batteries, so it is essential that effective recycling loops are set up going forward. Neither FCEV MEAs nor BEV batteries are recycled to a large extent today, and optimised processes do not exist for either option, but work is ongoing to develop such processes.

A number of factors will determine the proportion of BEVs and FCEVs in the future powertrain mix, with different countries, regions, OEMs and consumers making different choices. There is broad consensus, however, that BEVs are likely to dominate the passenger car ZEV sector, based on their relatively low cost and TCO, the rapidly growing charging infrastructure, the increased range of incoming BEVs and the fact that all major OEMs are bringing attractive BEVs to market over the coming years (and most OEMs also have fuel cell vehicles in small scale production (Honda, Hyundai and Toyota) or have fuel cell programmes in advanced stages of development). FCEVs are likely to play a role in the high mileage, high utilisation end of the passenger car and light commercial vehicle sectors, where their range and refuelling time advantages over BEVs are attractive.

There are many views of the rate at which the global powertrain will shift from the ICE to electrification (BEV and FCEV). LMC, an automotive global forecasting and market intelligence provider, has recently published its view of the evolution of the global passenger car powertrain out to 2050. Their base case scenario (Figure 10(a)) reflects their current “most likely” view of progress in technology, policy and cost, and they see BEVs with the major share in the ZEV space with over 40% of global car sales by 2050. FCEVs, helped by major growth in renewable electricity generation, become significant by 2035, exceeding 20% of global sales by 2050. Hybrids, though squeezed by ZEVs, remain important in some markets, including Japan, and make up around 20% of global vehicle sales in 2050, with the 2050 ICE sales dominated by India.

Fig. 10.

LMC’s “Progressive” scenario (Figure 10(b)) is based on increases in public and political pressure to get the world to act more rapidly to mitigate climate change, leading to more aggressive decarbonisation policies and faster adoption of BEVs and FCEVs. Within this scenario, ICE-only sales cease in the mid-2040s and ZEV sales reach over 90% of demand by 2050, with BEVs accounting for over 60% of global sales, and FCEVs around 30%.

Even this “Progressive” case does not represent a net zero scenario globally, since this would require sales of ICEs, HEVs and PHEVs to stop before 2040, and this scenario still has ICE-containing powertrains making up over 40% of global sales in 2040. However, it could be consistent with a scenario in which Europe moves to net zero in 2050 with the rest of the world following behind and achieving net zero just after 2060.

As outlined above, CO₂ legislation for commercial vehicles is becoming stricter in all the major economies, and on top of the CO₂ regulations, California is planning to introduce a zero emission vehicle mandate as part of its Advanced Clean Trucks regulatory package. By 2030, this will require 15% of Class 7 and 8 trucks (i.e. vehicles over 11.8 tonnes) sold in the state to be zero emission. While batteries are expected to be the technology of choice in the lighter segments, fuel cells are becoming seen as the most likely solution to decarbonise the larger trucks. As in the passenger car sector, governments planning net zero commitments will need to transition their commercial vehicle fleets from diesel to electricity and hydrogen as they move through the 2030s, to ensure a zero emission fleet by 2050.

OEMs are beginning to position themselves for this new reality; for example, Daimler Trucks recently announced that they plan for all new trucks and buses in the triad markets of Europe, Japan and the North American Free Trade Agreement (NAFTA) to be CO₂-neutral when driving by 2039 (i.e. tank to wheels) (29). They plan to achieve this using a combination of BEV and FCEV, with battery electrics in series production by 2022 in all core regions and hydrogen fuel cell-based series production vehicles by the end of the 2020s. Daimler Truck AG and the Volvo Group, two leading companies in the commercial vehicle industry, have signed a preliminary non-binding agreement to establish a new joint venture to develop, produce and commercialise fuel cell systems for heavy-duty vehicle applications and other use cases in the second half of the 2020s. Daimler will consolidate its current fuel cell activities in the joint venture and the Volvo Group will acquire 50% in the joint venture for approximately €600 million (30).

Cummins are also investing in both battery‐based powertrain technology and hydrogen and fuel cells, including acquiring a US$290 million controlling stake in Hydrogenics, a leading fuel cell and hydrogen production technologies provider (31). CNH Industrial have entered into a US$250 million strategic and exclusive heavy-duty truck partnership with Nikola Corporation (32), pioneers in the introduction of zero emission heavy duty trucks powered by hydrogen fuel cell and battery technology. The deal with CNH gives Nikola access to the European commercial vehicle market, as well as to IVECO’s global manufacturing and sales network. In addition, Nikola now has Nel (electrolysis) and Hanwha (solar energy) on board to develop a clean H₂ infrastructure to power these fuel cell vehicles, where conditions allow, supporting the moves towards net zero.

TCO is the critical factor in the long-haul truck sector. Recent analyses by Cummins (33) and AVL (34) have shown that BEV trucks are not viable for this sector due to the high cost, size and weight of batteries for the required range (their weight would reduce payload) and the relatively long recharging time for such large batteries (which would reduce vehicle utilisation significantly). These studies show that the FCEV solution is strongly preferred due to the long range, rapid refueling times and overall TCO. A hydrogen price of €3.50–5.00 kg^–1 is estimated to lead to TCO parity even with today’s diesel-based trucks once such FC trucks are made in significant volumes (100,000 or so); as outlined earlier, the hydrogen price is expected to drop to around €4 kg^–1 by 2030.

In the medium duty distribution truck sector, where driving ranges are lower than in the long‐haul space, BEVs are expected to play a significant role, and for some such distribution applications BEVs already have a lower TCO than current diesel trucks. CARB estimates that the TCO of battery trucks will be lower than diesel trucks by 2024 for many local truck applications (35). They also project that FCEVs will approach the TCO of diesel by 2030.

The development of the fuelling infrastructure for zero emission commercial vehicles is generally regarded as an easier proposition than that for passenger cars, since many commercial vehicles (especially buses and distribution trucks) return to a depot overnight. For BEV-based vehicles this requires charging infrastructure at their home depots and along the parts of the strategic road network along which they operate, for cases where top-up charging away from the depot is required.

For longer distance buses, coaches, and medium and heavy commercial vehicles, the fuel cell powertrain is expected to be widely employed, requiring HRSs at home depots and along the strategic road network. Depot-based HRSs for centralised refuelling have the advantage of increased utilisation, reducing the cost of the hydrogen delivered. A recent report from the Hydrogen Council projects that the cost of hydrogen refuelling infrastructure per vehicle should ultimately drop to below the cost of the BEV recharging infrastructure due to the significant economies of scale available from increasing the size of the distribution network and the introduction of larger retail stations (36). Their analysis led them to conclude that the cost of investment per kilogram of pumping capacity from a HRS will decline roughly 70% over the next 10 years, from about US$6000 for a small station in 2020 to an estimated US$2000 for a large station in 2030. Such a cost trajectory further increases the attractiveness of the hydrogen fuel cell solution for large and longer distance commercial vehicles, since it significantly reduces the TCO of these vehicles.

There are far fewer projections of the future uptake of zero emission commercial vehicles than there are for passenger cars, but it is clear that the commercial vehicle sector needs to develop and implement zero emission vehicles rapidly to support broader decarbonisation initiatives and, particularly, moves to net zero. KGP, a consultancy that provides services to the automotive and related industries worldwide, has developed a “2°C Scenario” for the commercial vehicle market (Figure 11) (37), projecting that the sales of “Electric” commercial vehicles, i.e. those powered by BEV and FCEV, would need to increase from around 87,000 in 2019 to over one million per year by 2030, to be on a trajectory to enable the GHG emissions from the commercial vehicle sector to be aligned with the Paris Agreement’s aim of limiting the global temperature increase due to GHG emissions to 2°C above pre-industrial levels. This level would correspond to around 25% of new sales of commercial vehicles globally.

Fig. 11.

The recent report from the IPCC (2) recommends that the target temperature increase should be at or below 1.5°C, implying that a faster rate of uptake of zero emission commercial vehicles will be required. Approaches to increase ZEV penetration include increasing the stringency of CO₂ tailpipe regulations, and introducing mandates for ZEV fleet levels (such as those proposed within the Advanced Clean Trucks rule by CARB). Interestingly, 30 businesses including Nestle and Unilever recently signed a letter to the new European Commission president Ursula von der Leyen and new EU climate chief Frans Timmermans, calling for legally binding zero-emission truck and van sales targets for 2025 and 2030 (38). They pointed out that these sales targets need to be ambitious, to drive a huge increase in the supply of zero-emission vehicles compared to a business-as-usual scenario, and to put Europe on track to meeting its 2030 climate targets. The businesses believe that binding sales targets will accelerate the uptake of zero-emission vehicles, make air in cities cleaner, put European vehicle-makers at the forefront of innovation while at the same time making Europe less dependent on oil imports. The signatories also say that the EU’s 2030 emissions reduction target must be increased to 55% and the bloc should go climate-neutral by 2050; the latter is the target already proposed by the European Commission.

Overall, therefore, the commercial vehicle sector is becoming increasingly aware of its need to develop zero emission vehicles to play a major role in the decarbonisation of road transport, and extensive work is underway to develop and bring such vehicles to market. Governments and regulators have a significant role to play in creating the right policy framework to drive the initial introduction of such vehicles into the marketplace, and then to encourage their further uptake to enable net zero and air quality targets to be achieved.

The demand for cleaner urban air and massive reductions in CO₂ and other GHG emissions is increasing both from the public and from regulators and governments in many countries and regions. Net zero GHG targets have been set and legislated in several geographies, and more are clearly going to follow in the coming months and years. Transportation is currently a major emitter of criteria pollutants (including CO, hydrocarbons, NOx and PM) and of CO₂, and the decarbonisation of this sector requires the transition from ICE‐powered vehicles to battery electric and fuel cell electric zero emission powertrains. Of course, the minimisation of the carbon footprint of such vehicles is contingent on the electricity and hydrogen used to fuel them being low or zero carbon. In the case of BEVs this means the electricity grid needs to be decarbonised, and this is occurring at good pace in many countries, accelerated by the ongoing reductions in the cost of renewable energy derived from, for example, solar and wind. For FCEVs it means decarbonised electricity to generate green hydrogen by the electrolysis of water, and the addition of CCUS technology to advanced reforming plants, to convert CH₄ into blue (low carbon) hydrogen. The low carbon hydrogen infrastructure and distribution network will constitute part of the transition towards a broader hydrogen economy in many countries, supporting moves to net zero across industry, power generation (including seasonal energy storage to enable increased renewable power generation) and heating for buildings, as well as transportation.

The decarbonisation of the passenger car sector will be driven by rapid uptake of BEVs, which will occur as their purchase costs continue to fall, their driving range continues to increase, and the required charging infrastructure is rolled out worldwide. BEVs are also expected to play a significant role in urban bus and distribution truck applications. FCEVs are expected to dominate the long haul trucking segment as it decarbonises, due to their cost, weight, range and charging time advantages over battery-based technology. They will also likely play a role in inter-city buses and distribution trucks, and in larger passenger cars and sport utility vehicles (SUVs) for applications and customers requiring a long driving range and rapid refuelling.

There is no doubt that net zero targets at the state, country and regional level will be challenging to meet on the 2050 timeframe recommended by the IPCC, but the surface transportation sector is developing and introducing the technologies to enable this. As long as governments and regulators put in place an appropriate set of policy measures and incentives to encourage the early implementation and subsequent mass uptake of zero emission vehicles, the car, van, bus and truck segments will make a huge contribution to global moves towards decarbonisation and the development of net zero economies worldwide.

Outdoor air pollution is linked to an estimated 4.2 million deaths each year worldwide (1). Tailpipe emissions from conventional internal combustion engine (ICE) vehicles are a major contributor to urban air pollution, and as such have been subject to ever tighter legislation for decades, requiring increasingly innovative improvements and catalytic emissions controls. We have now reached the point where a move away from the ICE is required to continue air quality improvements, with several countries going so far as banning new purely ICE vehicles in the coming years. This is where EVs will play their part – both pure EV and hybrid systems powered by LIB technologies, as well as fuel cell technologies, are set to see increased uptake and demand as we strive for cleaner air. In this article, we will add to the automotive-focused literature (2–4) and review what technologies are required to drive the uptake of pure EVs, and what the industry is doing now to respond to consumer requirements as this market rapidly grows.

There are several characteristic battery parameters that it is important to consider and contrast with consumer behaviours and expectations for automotive applications: perhaps most significant, the energy or capacity of the cell equates to the ‘miles in your tank’, and is an area where EVs have lagged behind the ICE in previous years. This is evolving, with the most successful EVs on the market now having an average range of 350 km (5). Range anxiety, equating to energy density, is a major theme for the battery materials industry, with contributions from and innovations required in three areas: the cathode, anode and electrolyte. Cost is also an important factor; as well as the material costs for the active components, analysis has shown that the electrode thickness within the cell is a major contributor to automotive cell costs (6) – materials with increased volumetric energy density are therefore additionally attractive from this perspective. There is also the practical cost benefit afforded by developing systems that can operate at higher voltage cut-offs (7), owing to the usable advantages, towards which multiple cell components can be developed and optimised. Herein, we review one topic of significant industry focus from each area: high-Ni cathode materials, with lithium nickel manganese cobalt oxide (NMC) 811 and beyond being commercialised within the next three years; high energy silicon anode technologies, expected to be at commercial scale in the next three to five years; and solid-state electrolytes, with significant progress expected from the next five years and beyond.

Whilst the cathode active material technology landscape remains diverse, with no one material that will meet all EV requirements, the general trend for passenger EVs is using high-Ni NMC, and lithium nickel cobalt aluminium oxide (NCA) materials. The layered Li Ni oxide (LNO), has been studied for the past 25 years, ever since the commercial application of the isostructural Li Co oxide (LCO) by Sony, Japan, in 1991; the relative low cost of Ni compared to Co was an initial driver for this work – and continues to be a factor today (8–12). Until relatively recently, automotive industry uptake was focused on lower Ni NMC variants, such as LiNi_1/3Mn_1/3Co_1/3O₂ (NMC 111), and lower energy chemistries such as Li Mn oxide (LMO), and Li iron phosphate (LFP). Tesla, USA, bucked the trend; as an early adopter of higher‐Ni NCA materials, it was ahead in the EV mileage stakes. Now, driven by consumer demand for more range, high-Ni is in vogue – the key for research and industry alike is to innovate-out the technical problems associated with LNO regarding its stability.

LNO tends towards non-stoichiometry, owing to the relative instability of Ni³⁺ compared to Ni²⁺, and the similar ionic radii of Ni²⁺ (0.69 Å) and Li⁺ (0.73 Å) (12, 13). It has been shown that synthesis conditions are key to prevent the formation of Ni²⁺ anti-site defects, with near-stoichiometric LNO requiring control of calcination temperature, atmosphere and Li content (12, 14). LNO is also known to undergo several phase transformations on electrochemical cycling; whilst a capacity of over 200 mAh g^–1 can be achieved, these transformations lead to significant capacity fade over the first cycles (15). Early research showed the benefits of incorporating relatively small amounts of other metals, most notably Co, Al and Mn, into the structure to impart stability and significantly improve capacity retention. Owing to the isostructural nature of its end members, all compositions in the series LiNi_1–x Co_x O₂ (x = 0–1) can be formed; Co³⁺ imparts stability by hindering the formation of Ni²⁺ anti-site defects (16). Conversely, doping Mn into the LNO structure has been shown to detrimentally effect the reversible capacity but to impart thermal stability benefits – a key property for battery safety (17, 18). The beneficial effect of Al substitution at low levels is two-fold: an improvement in capacity retention by minimising detrimental phase transformations and an increase in thermal stability (18, 19). There is, however, a limitation to the amount of Al that can be usefully incorporated into the structure; the addition of high-levels of an electrochemically inactive dopant will result in a reduction in capacity, and Al³⁺ has been shown to segregate and create localised defects within the lattice, due to the different ionicity of Al–O and Ni–O bonds (20).

This combined work has ultimately led to continued focus on the multiple metal dopant strategies found in NCA and NMC, where greater benefits are observed than in single dopant systems. Whilst not as catastrophic as those in LNO, NCA and high-Ni NMC materials (such as NMC 811) undergo significant structural changes on cycling, which their lower Ni counterparts (for example NMC 622, NMC 111) do not (Figure 1): at high states of charge, a transformation from the second hexagonal phase (H2) to the third hexagonal phase (H3) occurs in high-Ni materials that is associated with c lattice contraction and capacity fade (21–23). The addition of dopants to the bulk structure of LNO such as cobalt, manganese, aluminium, magnesium, titanium and combinations thereof has been shown to influence stability by affecting the volume change on cycling associated with the H2/H3 phase transformation (24–26).

Fig. 1.

Coating strategies have been employed to high‐Ni NCA and NMC systems, providing benefits in two key areas: handling and performance. The handling and processability of high-Ni materials is a well-known challenge, with surface reactivity towards the ambient resulting in the formation of Li hydroxide and Li carbonate impurities, and the resultant propensity of electrode slurries to gel: this creates obvious challenges before materials have even reached the cell (27–29). Once in the cell, these surface impurities contribute to resistance growth and side reactions resulting in gassing (30, 31). Moreover, the high-Ni surface itself is known to undergo phase changes upon cycling, with the formation of the rock salt phase Ni oxide also contributing to instability and capacity fade (32, 33). In its simplest sense, the application of an inactive coating such as Al oxide passivates the surface with respect to these undesirable side reactions, creating more benign materials that are easier to handle; but only so much of this type of coating can be applied before either significant capacity loss or resistance gains are observed (34). As such, the move toward active coatings, where the removal of an inherent risk of capacity loss does not limit the amount or depth of coating that can be applied, is very attractive. A notable example in this area is the extensive work by the Sun group, who have developed several generations of active coatings and complex morphologies for high-Ni materials (Figure 2): starting with a core@shell strategy, a low-Ni NMC was applied to the surface of a high-Ni NMC, creating a system that combined a high-energy core with a high-stability surface and building a system that was electrochemically active throughout (35). The drawback of this system was the observation that the shell layer broke away from the core on cycling, due to the mismatched volume changes within the core and shell NMC layers. To counteract this, the group developed a gradient coating strategy, whereby a lattice expansion or contraction mismatch was avoided by creating a continuous region of gradual compositional change, thus removing a core@shell interface (38). The Sun group further extended this work to look at deeper and multi-component gradients and their potential benefits (36, 37, 39). Such gradient systems can be viewed as a sophisticated hybrid between bulk doping and surface coating strategies, helping to mitigate the trade-offs associated with each strategy alone.

Fig. 2.

These gradient systems demonstrate the importance of considering morphology and process alongside composition in materials engineering. Another area of interest is the mitigation of microcrack formation through the control of primary particle shape, size and interfaces; fewer cracks means a more stable cathode electrolyte interface (CEI) layer, alleviating resistance growth and gas-generating side reactions (33, 40, 41). Most recently, this has led to particular interest in single crystalline morphologies, which promise greater long-term cycling stability compared to their polycrystalline counterparts by minimising the number of interfaces where microcracks can occur. The majority of published research in this area has focused on lower-Ni NMCs (i.e. NMC 622 or less), where reduction in gassing has been observed compared to polycrystalline counterparts, albeit at the cost of rate capability (42, 43). This lower Ni focus is in part due to the challenging nature of high Ni synthesis at the typically elevated temperatures required to form single crystalline materials compared to those used to generate polycrystalline materials. There are examples demonstrating similar advantages for a single crystalline morphology with up to 80% Ni content and efforts are clearly growing in this area: single crystalline NMC 811 has been shown to exhibit less gassing than its polycrystalline counterpart during high temperature storage (30). Zhu et al. undertook a broad study looking at NMCs from NMC 111 to NMC 811 prepared by multiple approaches and demonstrated the need to tune synthesis conditions to Ni content (44).

The engineering opportunities to overcome the challenges presented by high Ni materials continue to grow. As the automotive industry strives for higher energy, the drive to increase the Ni content of NCA and NMC type materials is clear – the common theme across the industry is to move from NMC 622 to NMC 811 and toward 90% Ni content to meet energy requirements, but also to reduce the Co content required, due to sourcing and cost challenges. Ultimately, a combination of the strategies reviewed above are required to develop and commercialise materials with a Ni content of 80% and above to meet the energy and stability requirements of the automotive industry.

Aligned with the drive toward higher energy cathode materials, there is a requirement to enhance and optimise LIB anode materials toward greater energy density, improved cycle life, lower cost per kilowatt hour and improved gravimetric and volumetric densities (3, 46). In particular, the use of higher energy cathode materials allows increased ampere hour per geometric area and volume of active cathode which is important to retain realistic active material loadings and thicknesses and achieve battery EV (BEV) cell and pack targets. A commensurate improvement in storable energy per area and volume of anode electrode is therefore also required. Cell manufacturers and original equipment manufacturers (OEMs) are increasingly moving beyond todays natural and synthetic graphite materials (or combinations of these) toward blending graphite with a higher energy density Si or Si oxide component to enhance cell level energy gravimetric and volumetric density (47). Table I illustrates examples of such Si containing materials (48).

The high natural abundance of Si and low operating voltage (0.2 V discharging potential compared to Li/Li⁺) single out Si as a highly promising anode material for LIBs (49). However, Si containing materials as battery anodes exhibit a number of challenges, with the greatest of these being significant volume expansion during the lithiation process (see Table I). Particle cracking or fragmentation, loss of electrical contact, ongoing parasitic reactions between electrolyte and ‘fresh’ surfaces, cell swelling and gassing all contribute to cycle life issues (see Figure 3 and Figure 4) (46). Various approaches can be deployed to address the volume change issue for pure Si anodes, including nano-engineering of the Si electrode structure (nanowires and nanoparticles, formation of secondary agglomerates) along with advanced binder combinations to create a flexible electrode structure (46, 50, 51). The addition of carbon dioxide into pouch cells has also been trialled to limit parasitic reactions (52). Formation of nanocomposites of Si–C via mechanical or chemical deposition processes, addition of other alloying components or the choice of a SiO_x material (where first cycle lithiation allows an irreversible reaction creating stabilising LiO_x and Li silicate components within the structure) can all bring improvements (50, 53). Incorporation of conductive carbon also addresses the challenge posed by the intrinsic low conductivity of Si containing materials (54).

Fig. 3.

Fig. 4.

A strategy of blending Si containing materials with existing graphite types is already in progress to achieve moderate capacity increase and lessen volume change, as illustrated by cell level calculations for this approach (for example Si:C 1:3 with capacity of 1100 mAh g^–1 by Andre et al.) (3, 47). Table I illustrates an additional challenge present in Si containing anodes in the form of lower first cycle efficiency (FCE) vs. graphite, related to reactions consuming Li between the electrolyte and anode, the formation of the SEI and associated reduction in useful Li inventory in the working cell, reducing effective watt hour per kilogram. Pre-lithiation approaches, where sacrificial Li containing materials are added to the Si anode during electrode fabrication or strategies such as electrochemical pre-lithiation of formed electrodes ahead of cell assembly are possible (55, 56) along with chemical pretreatments ‘artificial SEI formation’ (57, 58). However, these all represent additional steps and cost in a cell manufacturing process, also pre-lithiated materials and electrodes and Si nanoparticles require careful handling due to the reactivity of the materials with moisture and air (48).

Careful optimisation of the liquid electrolyte additives is also crucial to achieve prolonged cycle life and good FCE, with fluorinated additives, especially fluoroethylene carbonate (FEC), showing benefit (59). The discharge and charge voltage profile of Si containing anodes is slightly different to graphite-only examples, leading to reduced chance of Li plating during charging in Si anodes, but typically slightly lower discharge voltage with graphite, thus adjustments to cell balancing and understanding of the operational state of charge window in the usable voltage range are important for full cell (60).

Assessment of the sustainability of changing to Si containing anode components and advanced higher energy cell chemistries is also vital as electrification of the power train advances worldwide (61).

A further driver to increase the energy density of cells is to replace existing anode materials with metallic Li. Li metal was used as the first anode material in rechargeable Li-ion cells due to its very high energy density (3860 mAh g^–1) and low electrochemical potential (–3.040 V vs. the standard hydrogen electrode). However, numerous challenges prevented its widespread adoption, including low cycle life predominating from issues such as the formation of dendrites and unstable solid-electrolyte interfaces. Recently, there has been increasing investigations into using solid-state electrolytes to mitigate the challenges of using metal anodes, whilst maintaining their advantages.

In addition to potentially enabling the use of Li metal anodes, the evolution to solid state batteries has other advantages to conventional Li-ion cells (62). The primary reason is the displacement of the highly flammable cocktail of organic electrolytes that is used currently. This both reduces the risk of unwanted thermal events in the instance of cell misuse or damage, but it also results in a simpler packaging, further increasing the energy density (63) (Figure 5). In addition, solid state materials could offer increased electrochemical stability windows in comparison to existing organic electrolytes; potentially enabling alternative materials, such as higher voltage cathode materials, to be deployed.

Fig. 5.

The use of polymers as electrolytes in batteries was first pioneered in the 1970s (64, 66). This enables cells with high degrees of safety to be manufactured in various form factors. Polymer-based systems such as polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polyacrylonitrile and polymethyl methacrylate (PMMA) based electrodes have all been widely studied as polymer electrolytes (67). PEO-based polymer electrolytes have been studied the most due to their advantageous properties including lower cost, ability to solvate a wide variety of ions, relatively high chemical stability and the use of their moderate mechanical strength (~10⁶ Pa) to supress the growth of dendrites (68, 69). However, the low conductivity (~10^–7 S cm^–1) of the electrolyte systems, due to the crystallinity of the polymer chains, has been a limitation (70). Overall, the general uptake of polymer gel cells has been restricted by their lower energy densities and poor electrochemical stability compared to liquid electrolytes.

More recently, researchers have explored a range of solid inorganic materials, which allow ionic mobility through the solid. Numerous classes of these are currently being explored, all possessing different advantages and disadvantages (63, 71, 72). A summary of these are highlighted in Table II.

Researchers have looked to examine inorganic electrolyte materials with high ionic conductivities, such as Li₁₀GeP₂S₁₂, which exhibits high conductivity at RT (73). However, sulfide-based solid electrolytes are generally expensive, more challenging to synthesise and are sensitive to moisture, potentially releasing toxic gases. This brings challenges in their handling and subsequent fabrication.

Although most solid electrolytes have been shown to react with Li metal, garnet materials (such as Li₇La₃Zr₂O₁₂ (LLZO)), have shown the greatest stability (74, 75). In addition, they have relatively low costs and a wide electrochemical window (~6 V vs. Li metal) potentially enabling the use of higher voltage cathode materials; and are therefore attracting increasing investigations (74). The cubic phase of LLZO is found to offer greater ionic conductivity than the tetragonal phase. A typical strategy to promote this is to dope elements such as Al, tantalum and gallium into the structure thus stabilising the highly conductive cubic phase at RT (76).

Despite these advantages, a challenge in using LLZO remains its instability in the ambient atmosphere, due to CO₂ and moisture (77). This results in increased complexity upon subsequent material handling and processing. Further challenges include poor interfacial compatibility of LLZO with electrodes. To overcome this, methods to increase the wettability of the electrolyte have been explored, such as the atomic layer deposition of Al₂O₃ to reduce interfacial resistance by the formation of a desirable Li-Al-O layer (73); or alloying Li with other elements (such as Si, Al, Ge) to increase compatibility (72).

In addition to the preparation of materials capable of high levels of Li-ion conductivity, it is vital that these materials can be manufactured at an industrial scale at a reasonable cost. While there has been considerable interest in the use of oxides for an all solid electrolyte, their brittleness and fragility impose new challenges for mass production (78, 80). As a result the scale up of such activities is being explored using a variety of different processing technologies (Figure 6). Mature slurry-based technologies have been shown to provide dense layers using high throughput techniques. However, subsequent high temperature sintering inhibits the co-firing of solid electrolytes and cathode particles.

Fig. 6.

When using Li metal as an anode material it is vitally important to prepare dense electrolyte layers in the absences of holes. It has been suggested that a critical relative density of >93% are required to eliminate the formation of dendrites in LLZO electrolytes (79); with short circuits believed to propagate through voids and grain boundaries (81). To obtain highly sintered garnet-based solid electrolytes by conventional sintering techniques, generally high temperatures (>1200°C) and long sintering times (>30 h) are required. Such conditions can result in the decomposition of the solid electrolytes and loss of Li from the structure.

To overcome these challenges, alternative processes such as hot pressing, field-assisted sintering and spark plasma synthesis have been investigated to fabricate the optimal dense ceramic layer (82–85). To that end further evaluation of deposition and sintering technologies will be required to provide an economically viable solution.

There are also multiple technologies (such as Li‐sulfur and Li-air chemistry) that have the potential to deliver significant advances in performance, such as increased energy density (86). For example, Li-S chemistry benefits from the low cost and high abundance of S and an energy density significantly higher than current Li-ion cells (~2500 Wh kg^–1) (87, 88). However, these technologies currently suffer from technical challenges that limit their uptake. To fully maximise the benefit of these technologies, it is necessary to overcome the challenges of working with a Li metal anode. The use of solid-state electrolytes is a recent area where people have been exploring with the aim of enabling the technology via anode protection.

The demand for cleaner air is accelerating and this is giving rise to increased electrification in the automotive drivetrain. There is also a growing acceptance of vehicles with varying degrees of electrification, and this trend looks set to continue. Current concerns for increased energy density to counter consumer’s ‘range anxiety’ are leading to material developments to meet this. In particular, the careful design and manufacturing of cathode materials with high amounts of Ni and anode materials with increasing Si content are steadily improving these key parameters. Furthermore, significant exploration into next generation technologies, such as solid-state electrolytes, opens the possibility of redesigning the cell. While options to the type of material used and their processing remain; the replacement of conventional liquid electrolytes promises to deliver further improvements in energy density as well as other benefits, such as safety performance. These three examples highlight the major trends being investigated and introduced into automotive cells to meet the demands of society.

The tremendous growth of the global economy is directly related to increased energy demand and (currently) high greenhouse gas emissions. Substantial reduction in global emissions is required to minimise environmental hazard and ongoing climate change. Legislations are pushing for energy transition, replacing fossil fuels with alternatives for reduced emissions. Wind, solar and biomass are key-players for the energy future, as depicted in the latest statistics and forecast (1, 2). According to one of the possible energy transition scenarios, to accommodate the increasing energy demands with the least environmental impact, renewable sources will rapidly grow their share in the energy mix, while natural gas is foreseen to maintain a key role during the transition phase (1, 2). However, natural gas contributes to CO₂ emissions, with approximately 7 billion tonnes of CO₂ being produced on a yearly basis, with approximately 5% of this amount attributed to flaring (Figure 1). This percentage adds to both the environmental problem and to the waste of an important resource, methane (3–5).

Fig. 1.

ADREM (EU project Horizon 2020 No. 636820), focused on the development of novel reactor concepts that are capable of converting methane to higher chemicals with a compact, modular and flexible process design. The University of Zaragoza (UniZar), Spain; Delft University of Technology (TU Delft), The Netherlands; and SAIREM, Décines-Charpieu, France, investigated microwave reactor technology for methane non-oxidative coupling (MNOC). Katholieke Universiteit Leuven (KU Leuven), Belgium and Kemijski inštitut in Ljubljana, Slovenia, worked with plasma technology for methane non-oxidative coupling and dry reforming respectively. Ghent University, Belgium, investigated the gas solid vortex reactor (GSVR) for oxidative methane coupling (OCM). In the present paper, we give an overview of the technologies that were developed, the status, the main bottlenecks and the path forward.

2.1. Microwave Non-Oxidative Methane Coupling with Both a Multistage Monomodal Reactor and with a Travelling Wave Reactor

2.2. Plasma Non-Oxidative Coupling of Methane

2.3. Oxidative Coupling of Methane with a Gas-Solid Vortex Reactor

2.4. Plasma Dry Reforming

To assess the potential of the reactors that were developed in ADREM, a case study of valorising associated (flared) gas has been simulated. The feed is rich in methane (>95% vol) with a flowrate of 1000 Nm³ h^–1. All the cases include pretreatment for sulfur and CO₂ removal, while for comparison purposes, the downstream processing follows the conventional approach, with either cryogenic separation (for C2+ hydrocarbons) or methanol loop (for syngas to methanol conversion). The end product consists either of mixtures of products (i.e. ethane/ethylene) or product at low purity (for example, raw methanol). Further purification in centralised units is necessary to reach the required quality.

The specific energy (Table I) of each technology consists of the reactors’ energy demands and the downstream processing (DSP) intensity (the latter being directly related to methane conversion and productivity). The microwave and GSVR technologies have the lowest specific energy consumption, as a result of the upscaled microwave reactor design of SAIREM and the exothermic OCM reaction respectively. The plasma technology is more energy intensive predominantly due to numbering up of the modules in order to accommodate the required flow. The technologies that produce BTX and ethylene would obviously benefit from replacement of the cryogenic separation by energy-efficient and modular alternatives (for example, ethane/ethylene membranes (14) or adsorption based technology) to decrease the energy demand. For the plasma dry reforming, the product syngas enables alternative downstream processing (for example, a methanol reactor), but the high operational pressure of such a design still adds to the overall energy efficiency and complicates the modularity of the plant. However, the modular methanol reactor is already available in commercial scale (3).

The capital intensity (Table I) is a function of the conversion and selectivity and the ease of upscale. On one hand, low conversion results in a large recycle flow (due to unconverted methane), and more energy-demanding units. On the other hand the numbering up strategy to accommodate the required throughput implies high capital requirements for all the technologies. The MW reactor with the realised upscaled concept and the GSVR that can accommodate high flowrate, appear to be the most cost-competitive at the present development stage. Collectively, the first step of further development for the ADREM reactors is to improve the reactor performance in terms of conversion and selectivity.

Compared to flaring, for all the technologies the CO₂ emissions are low (25–80% decrease, depending on the technology), with the highest CO₂ emissions coming from the GSVR reactor (where CO₂ is a product) and the lowest emissions coming from plasma dry reforming (where CO₂ is the reactant). Applying the ADREM technologies in situations associated with gas flaring in remote locations will have a huge environmental benefit when renewable electricity is available in abundance.

During the project, partners have been developing new small scale gas-to-liquids (GTL) technology, where methane is valorised to chemicals. Two of the reactor technologies have been successfully demonstrated in TRL 5 (microwave and plasma). With tighter regulation on greenhouse gas emissions and flaring, there are clear opportunities for the ADREM technologies to find applications. The UniZar reactor has efficiently been upscaled (32x) and the GSVR reactor is designed in such a way that it can accommodate relatively high flowrate. The plasma reactors (both NPD and dry reforming) showed the highest conversions and selectivities, but they still need to improve the upscale strategy.

For further upscaling and demonstration of the technologies, it is required to improve productivity, conversion and mitigation of carbon formation. Different operating conditions (in terms of pressure, temperature, catalysis or reactor geometry) or in situ product separation could potentially enable higher conversions and selectivity and are planned for the next steps of development. Improving the reactor performance will decrease the unit size for each technology and simplify the downstream processing. Downstream processing is an essential point that should be developed and optimised once the selectivity and conversion are improved.