Challenges and Emerging Trends in Hydrogen Energy Industrialization: From Hydrogen Evolution Reaction to Storage, Transportation, and Utilization

2.1.1 Cost-Efficiency of HER Electrocatalysts

To date, the efficient catalysis of the HER typically relies on platinum-group metals, such as Pt and Ru, due to their ideal metal–H bond strengths of ≈62 and 65 kcal mol⁻¹ for Pt–H and Ru–H, respectively. These strengths facilitate a balance between hydrogen adsorption and desorption, enabling a kinetically rapid HER.^{[17, 18]} However, as indicated in Figure 4a,b and reported by the National Minerals Information Center, the average price of Pt over the last 20 years has exceeded that of Au and is 71.3 times higher than that of Ag. Compared to more common metals like Cu, Fe, Co, and Ni, elements of Pt and Ru entail significantly higher costs, which poses a strong financial burden on the development of HER electrocatalysis.^[19-25] Therefore, in the mass production and development of industrial-scale electrocatalysts, strategies to reduce or replace platinum-group metals are crucial to lower the costs associated with H₂ production.

2.1.2 Scalable and Stable H₂ Production

Undoubtedly, in addition to HER activity, the durability of an electrocatalyst that can operate under industrial-scale current densities (higher than 500 mA cm⁻²) is essential for the industrialization of HER. While the performance of HER electrocatalysts can initially be evaluated by the strength of the metal–H bond at low current densities, at higher currents, factors such as interfacial charge transfer resistance, coverage rate of intermediates, electrocatalytic durability, and H₂ bubble release kinetics must also be considered.^[26-29] For example, sluggish electron transfer often occurs on layered transition metal dichalcogenides (TMDs) due to large interlayer potential barriers,^[30] or increased electrocatalytic impedance can arise in assembled electrocatalysts that require post-coating with binders and conductive agents.^[31] Typically, advanced electrocatalysts exhibit Tafel slopes of ≈30 mV dec⁻¹ at low current densities; however, these slopes often rise above 120 mV dec⁻¹ at higher densities, limited by real-world proton diffusion and adsorption characteristics.^{[14, 32]} Additionally, subsequent inefficient H₂ desorption at these higher current densities can block electrocatalytic active sites, impeding electron and mass transfer during HER.^{[33, 34]} Furthermore, under high HER current densities, the elevated reaction potentials and rapid H₂ bubble formation can cause the electrocatalyst layers to detach from their substrates due to weak adhesion, a common issue in conventionally assembled electrocatalysts.^{[35, 36]} Overall, developing an electrocatalyst capable of withstanding the demanding conditions of industrial-scale H₂ production remains a significant challenge.

2.2.1 Cost-Efficient Electrocatalysts

Considering the superior activity of Pt-based electrocatalysts, efforts to reduce the cost of HER have initially focused on the development of Pt nanocluster or single-atom electrocatalysts. These strategies aim to achieve high HER performance by maximizing active surface areas while minimizing the quantity of Pt used. For instance, Han et al. developed a novel Pt single atom electrocatalyst supported on N-doped porous carbon nanofibers (Pt-SA/pCNFs), employing electrospinning followed by carbonization to form the nanofibers, and impregnation followed by pyrolysis to synthesize Pt-SA/pCNFs, as depicted in Figure 5a.^[37] In the Pt-SA/pCNFs, the porous structure of the N-doped carbon nanofibers, shown in the transmission electron microscope (TEM) image in Figure 5b, imparted significant hydrophilicity, enhancing the HER electrocatalysis. As presented in Figure 5c, the high-angle annual dark-field scanning transmission electron microscopy (HAADF-STEM) image illustrated that the Pt single atoms were anchored through strong interactions from tailored N and C, forming a stable Pt─N₂C₂ configuration, which led to exceptional Pt atomic utilization and electrocatalytic activity. Additionally, the density functional theory (DFT) results depicted in Figure 5d,e highlight a small negative binding energy at the Pt─N₂C₂ site and near-zero Gibbs free energy of hydrogen intermediate (ΔG_{H ⃰}) for the Pt-SA/pCNFs, indicating robust structural stability and HER activity. Remarkably, in a 0.5 m H₂SO₄ electrolyte, the Pt-SA/pCNFs electrocatalyst achieved an exceptionally low overpotential of 64 mV at a high current density of 500 mA cm⁻², maintaining stability with minimal decay over 200-h at 10 mA cm⁻² and 10-h at 500 mA cm⁻² (Figure 5f,g). These results suggested a promising potential for large-scale H₂ production using the Pt-SA/pCNFs electrocatalyst.

According to the statistical analysis previously presented in Figure 4, the cost of Pt significantly exceeds that of other metals such as Ru, Cu, Fe, Co, and Ni. Consequently, the long-term development of HER technologies based on Pt is not sustainable. In response, researchers have explored numerous transition metal-based HER electrocatalysts as alternatives to Pt, benefiting from their lower cost and adjustable HER activity through doping or alloying processes.

In terms of single transition metal-based HER electrocatalysts, as presented in Figure 6a–f, Hu et al. designed the Ru nanocrystals ranging from nanoparticles to sub-nanoclusters and single atoms through a calcination process in H₂/Ar flow.^[38] In this work, the diameter of Ru directly influenced the d-band center of fabricated Ru nanocrystals, with the smallest Ru clusters exhibiting an upshifted d-band center that provided enhanced water dissociation capability compared to Ru nanoparticles and single atoms, thereby improving HER activity, especially under proton-deficient conditions (Figure 6g). Additionally, the cluster morphology of Ru increased the specific surface area and significantly boosted the number of active HER sites. Consequently, Ru clusters demonstrated superior alkaline HER activity, achieving a very low overpotential of 13 mV at a current density of 10 mA cm⁻² and a high turnover frequency (TOF) of 43.3 s⁻¹ at a potential of 100 mV (Figure 6h,j). Remarkably, as shown in Figure 6i,k, Ru clusters could achieve a large current density of 1000 mA cm⁻² at just overpotential of 196 mV when a mass of Ru loading on electrode reached to 1 mg cm⁻², along with maintaining stability over 100-h at this high current density without significant degradation. These exceptional HER performances demonstrated the potential for industrial-scale H₂ production using cost-effective Ru-based electrocatalysts.

To further enhance the cost-efficiency of HER using transition metals, TMDs and transition metal phosphides (TMPs) have gained tremendous attention recently. The layered structure of TMDs, with abundant exposed edge sites, is typically considered rich in active sites for HER electrocatalysis.^[39] Concurrently, the more electrically conductive TMPs feature numerous triangular prism structures that can expose additional crystal planes and potential HER active sites.^[40] Based on this understanding, through a colloidal reaction, Kwak et al. synthesized the Nb_1–xV_xSe₂ nanosheets across the full compositional range of x.^[41] At x = 0.3, the Nb_1–xV_xSe₂ nanosheets exhibited the lowest overpotentials of 236 and 298 mV at current densities of 10 and 100 mA cm⁻² respectively, with a Tafel slope of 72 mV dec⁻¹. The chronoamperometry stability test at current densities of 10 and 20 mA cm⁻² showed no noticeable current reduction after the long-term stability test, and even after 5000 linear sweep voltammetry (LSV) cycles, the overpotentials of Nb_1 − xV_xSe₂ nanosheets remained at 238 and 302 mV at current densities of 10 and 100 mA cm⁻² respectively, with only negligible decay.

2.2.2 Self-Supported Electrocatalysts

To facilitate industrial-scale H₂ production with robust durability, managing disruptions from rapid H₂ bubble evolution is crucially dependent on the adhesive strength of electrocatalysts applied to electrodes. However, in cases involving large current densities and extensive H₂ bubble formation, the electrocatalyst layer, often combined with a binder and conducting agent, can easily detach due to insufficient adhesion. This detachment not only buries active sites but also leads to increased impedance in HER.^[31] To address this challenge, researchers have developed various self-supported electrocatalysts, designed to prevent peeling and secondary aggregation through the in situ growth of electrocatalysts on substrates. These substrates, equipped with active electrocatalysts, can then be directly utilized as working electrodes in HER electrocatalysis.

As demonstrated in Figure 7a, Ni et al. successfully in situ constructed a hierarchically self-supported HER electrode, through the integration of Cu nanowires, NiFe nanosheets, and Pt₃Ir alloy nanoparticles (Cu NWs@NiFe-Pt₃Ir).^[42] DFT calculations confirmed that the coupling between NiFe and Pt₃Ir significantly enhanced the HER kinetics by facilitating electron consumption at the electrode–electrolyte interface (Figure 7b–d). Additionally, as shown in Figure 7e,f, incorporating Ir atoms into the Pt₃Ir structure lowered the d-band center energy level of the Pt 5d orbital, optimizing H* adsorption and desorption during HER electrocatalysis. Leveraging its hierarchical structure with numerous active sites and the in situ contact structure, the Cu NWs@NiFe-Pt₃Ir electrode achieved exceptionally low HER overpotentials of only 210 and 239 mV to reach industrial-scale current densities of 500 and 1000 mA cm⁻² in 1 m KOH, as evidenced by the corresponding TEM images and HER activities displayed in Figure 7g,h. In terms of electrochemical stability, as shown in Figure 7i, the Cu NWs@NiFe-Pt₃Ir self-supported electrocatalyst consistently produced H₂ for 7 days, even under a substantial current density of 500 mA cm⁻², with an inappreciable potential increase of only 8 mV. This expertly designed self-supported electrode demonstrated outstanding HER activity and durability at an industrial scale.

Furthermore, through a straightforward electrospinning strategy optimized with pyrolysis temperature and precursor mass ratios, Ouyang et al. effectively developed in situ grown and self-supported Mo₂C@carbon nanofibers (Mo₂C@C NF) film demonstrating exceptional HER electrocatalytic performance.^[43] This self-supported electrocatalyst avoided the occlusion of active sites and enhanced the diffusion of electrons and reaction intermediates during HER electrocatalysis. Consequently, the Mo₂C@C NF achieved a low overpotential of 86 mV at a current density of 10 mA cm⁻² and a minimal Tafel slope of 43 mV dec⁻¹ in 1 m KOH. Thanks to its robust construction from in situ growth, the Mo₂C@C NF displayed outstanding electrocatalytic stability in alkaline HER, maintaining stability for 340-h and 100-h at current densities of 20 and 50 mA cm⁻², respectively. Similarly, Liu et al. developed a mechanically stable, all-metal, and highly efficient CuMo₆S₈/Cu electrode through an in situ reaction between MoS₂ and Cu.^[44] The resulting electrocatalyst demonstrated robust interfacial binding between the active CuMo₆S₈ and support Cu while exhibiting weak adhesion between the CuMo₆S₈/Cu interface and H₂ bubbles. This unique combination effectively prevented electrocatalyst detachment and enhanced reaction kinetics at industrial-scale current densities. As a result, in a 1 m KOH electrolyte, the self-supported CuMo₆S₈/Cu electrocatalyst achieved a remarkably low overpotential of 334 mV and demonstrated stable operation for over 100 h at a high current density of 2500 mA cm⁻².

2.2.3 Hydrophilic Electrocatalysts

Obviously, a well-designed self-supported electrocatalyst with robust durability can effectively resist the bubble disturbance to maintain steady H₂ generation. However, during industrial-scale HER with rapid H₂ bubble generation, the diffusion and adsorption of intermediates (e.g., protons, H*, and H₂O*) are also significantly impacted by the kinetics of H₂ bubble release during the HER processes. In practice, when H₂ bubbles extensively cover the surface of the electrocatalyst, the occupied air zones seriously obstruct the diffusion of water molecules and protons to the electrocatalytic active sites at the interface between the electrocatalyst surface and the electrolyte.^[45] Additionally, these occupied air zones can cause an inhomogeneous current distribution on the electrocatalyst surface, increase the HER overpotentials in areas not covered by H₂ bubbles, and ultimately lead to premature degradation of the electrocatalyst durability.^[46] Therefore, to facilitate the rapid release of H₂ bubbles, designing HER electrocatalysts with more hydrophilic (i.e., aerophobic) surfaces is advantageous. This design ensures that the electrolyte, rather than the H₂ bubbles, preferentially contacts the electrocatalyst surface, thereby enhancing the exchange efficiency of intermediates during HER electrocatalysis.

The highlighted work from He et al. introduced a free-standing heterostructure electrocatalyst,^[47] consisting of microporous Ni(OH)_x/Ni₃S₂ on Ni foam (Ni(OH)_x/Ni₃S₂/NF presented in the scanning electron microscopy (SEM) image in Figure 8a. As demonstrated in Figure 8b,c, compared to the pristine Ni₃S₂/NF (water contact angle of 146°), this well-designed heterostructure electrocatalyst exhibited a water contact angle of 0°, indicating an absolute hydrophilic property. This exceptional wettability ensured a high utilization rate of active sites on the Ni(OH)_x/Ni₃S₂/NF electrode and supported continuous H₂ production even under long-term, high-current-density HER electrocatalysis. Leveraging its microporous and self-supported structures, the Ni(OH)_x/Ni₃S₂/NF exhibited remarkable HER activity and stability in 1 m KOH electrolyte, achieving overpotentials of just 126, 193, and 238 mV to reach substantial current densities of 100, 500, and 1000 mA cm⁻², respectively (Figure 8d). These impressive HER activities were further corroborated by DFT calculations (Figure 8e), which indicated that easy H₂O dissociation and reduced H* adsorption significantly enhance HER kinetics. Additionally, a 1000-h long-term durability test conducted under a high current density of ≈320 mA cm⁻² showed no significant decay (Figure 8f). These results surpassed those of commercial Pt/C electrocatalysts, positioning this as a leading HER performer among recently reported state-of-the-art non-noble metal electrocatalysts.

Further study focusing on the hydrophilic modification of HER electrodes is highlighted in Sun's study.^[48] Through a combination of hydrothermal and chemical etching processes, crystalline nickel tellurium nanorods enclosed by amorphous rhodium hydroxide (a-Rh(OH)₃/NiTe) were produced. Notably, the resulting a-Rh(OH)₃/NiTe demonstrated a significantly enhanced hydrophilic property, exhibiting a water contact angle of ≈0°. This superhydrophilic surface of a-Rh(OH)₃/NiTe ensured close contact between the electrocatalyst and the electrolyte during HER processes, facilitating accelerated electrocatalytic kinetics through rapid electron and mass exchange. Attributed to these superiorities, the a-Rh(OH)₃/NiTe achieved exceptionally low HER overpotentials of only 51, 109, and 64 mV in 1 m KOH, 1 m PBS, and 0.5 m H₂SO₄ to reach a current density of 100 mA cm⁻², alongside robust electrochemical stability. Moreover, Luo et al. synthesized molybdenum disulfide microspheres modified with molybdenum carbide nanoparticles (MoS₂/Mo₂C) through a combination of hydrothermal and chemical vapor deposition processes.^[49] The unique structure of the microspheres, composed of numerous aligned MoS₂ nanosheets, generates a strong capillary force that efficiently pumps the electrolyte onto the MoS₂/Mo₂C surface. This significantly reduces gas–solid adhesion and facilitates the rapid detachment of H₂ bubbles, as evidenced by the nearly zero contact angle when using a 1 m KOH droplet. Furthermore, the presence of a large number of exposed active Mo₂C nanoparticles at the edges of the MoS2 nanosheets enhances catalytic performance. The resulting MoS₂/Mo₂C electrocatalyst demonstrates remarkably low overpotentials of 227 and 220 mV at a high current density of 1000 mA cm⁻² in acidic and alkaline conditions, respectively. Additionally, it exhibits excellent electrochemical stability in both catalytic media.

2.2.4 Mechanically Stable Electrocatalysts

During industrial-scale HER, the rapid formation and detachment of large volumes of H₂ bubbles typically exert significant force on the electrocatalysts, often leading to the deformation of electrocatalysts and degradation of HER performance. In such cases, the flexibility and adaptability of the electrocatalysts become crucial in alleviating the tension and vibration generated by the release of H₂ bubbles.^[50]

Zhang et al. designed a 2D CoOOH sheet-encapsulated Ni₂P (Ni₂P–CoOOH) tubular array electrocatalytic system,^[51] where the 2D stacked structure acted as an adaptive material to buffer the impact of H₂ bubble rupture and evolution by releasing stress (Figure 9a). As shown in Figure 9b, during the bending deformation and restoration processes, the single nanotube of Ni₂P–CoOOH demonstrated remarkable flexibility, withstanding a maximum bending angle of up to 27.7° and fully recovering once the external force was removed. Moreover, the porous structure of Ni₂P–CoOOH, as evidenced by the contact angle results in Figure 9c, enhanced electrolyte contact with the electrode surface and facilitated H₂ bubble detachment. As a result, the well-engineered Ni₂P–CoOOH system demonstrated outstanding long-term HER stability, operating for over 100 h at current densities of 2000, 1200, and 100 mA cm⁻² in alkaline, neutral water, and seawater, respectively (Figure 9d,e). These results signify a promising transition from laboratory-scale to industrial-scale water electrolysis. Similarly, Wang et al. employed dealloying and potentiostatic polarization techniques to develop a flexible, nanostructured Zr-based amorphous HER electrocatalyst.^[52] During the dealloying process, sharp cracks were mitigated through potentiostatic polarization, resulting in low-stress concentration and high flexibility. The porous structure of the synthesized electrocatalyst delivered a low HER overpotential of 53 mV at a current density of 10 mA cm⁻² in 1 m KOH, along with long-term stability under an industrial-scale current density of 300 mA cm⁻² for 100 h.

2.2.5 Intrinsically Electrochemically Stable Electrocatalysts

By implementing in situ growth and optimizing surface morphology to facilitate rapid H₂ bubble detachment, improved HER activity and stability have been achieved through stable electrocatalyst structures. These advancements can be extended to a wide range of electrocatalytic materials. Besides, a certain group of electrocatalysts possess inherent properties that withstand against electrochemical decay, such as electrocatalytically active materials protected by another fence layer, in situ self-reconstruction at the electrocatalyst surface, and dealloying processes during HER electrocatalysis.

Utilizing this concept of protective fence engineering for efficient and steady alkaline HER, Huang et al. successfully shielded highly active cobalt-doped MoS₂ with a molecule-selective CoS₂ barrier fence (Co─MoS₂@CoS₂).^[53] DFT studies showed that the CoS₂ barrier prevented the penetration of O₂ and OH⁻ ions species while allowing reactant H₂O to pass through, thereby preventing surface poisoning of the Co─MoS₂ catalyst during alkaline HER electrocatalysis. Simultaneously, the CoS₂ barrier also helped to prevent the oxidation or dissolution of active Co─MoS₂ into the electrolyte in the form of MoO₄²⁻ and SO₃²⁻. These engineered modifications to Co─MoS₂@CoS₂ enhanced both HER activity and stability without compromise. In terms of electrochemical stability, Co─MoS₂@CoS₂ maintained a long-term 120-h test with negligible activity decay at a constant potential of 100 mV. These enhanced HER performances after the protective fence engineering undoubtedly provided a viable blueprint for developing highly active and long-term stable HER electrocatalysts.

Similarly, in terms of the electrochemical stability enhancements, Zhang's work demonstrated a notable enhancement in electrochemical stability through an in situ self-reconstruction phenomenon on the surface of the designed electrocatalyst.^[29] Investigating Sr₂RuO₄ bulk single crystals during a high current density HER test revealed the in situ formation of ferromagnetic Ru clusters on the surface of Sr₂RuO₄, evidenced by numerous micro-scale Ru islands as shown in Figure 10a,b, with the formation of Ru clusters confirmed by using X-ray photoelectron spectroscopy (XPS) test (Figure 10c). This in situ exsolution of Ru from Sr₂RuO₄, subsequently re-incorporated into the Sr₂RuO₄ bulk crystal, not only improved the cohesion between the Ru clusters and the Sr₂RuO₄ substrate but also ensured overall electrocatalyst stability and uniform distribution of Ru clusters, which is advantageous for HER activity due to accelerated kinetics in the electron mobility. The corresponding electron redistribution and improved electrocatalytic activity were validated via DFT calculations, as depicted in Figure 10d. Here, the Ru atoms at the top of the Ru₆ clusters are identified as HER active sites, exhibiting a ΔG_{H ⃰} value comparable to that of a Pt catalyst (Figure 10e), and significantly lower than that of the original Sr₂RuO₄. Remarkably, the in situ self-reconstructed Ru clusters on Sr₂RuO₄ also formed highly porous and hierarchical structures that facilitated the growth and detachment of H₂ bubbles (Figure 10f), simultaneously enhancing mass transfer efficiency and the mechanical stability of the electrocatalyst, particularly at industrial-scale current densities. Ultimately, as demonstrated in Figure 10g,h, the reconstructed Ru₆/Sr₂RuO₄ achieved ultra-low overpotentials of 182 and 278 mV to reach exceptionally high current densities of 1000 mA cm⁻² in 0.5 m H₂SO₄ and 1 m KOH, respectively. Coupled with robust electrochemical stability, continuous testing over 56 days at initial temperatures of 25 °C followed by 70 °C, at a high current density of 1000 mA cm⁻², demonstrated the impressive HER performance of this well-reconstructed Ru₆/Sr₂RuO₄ electrocatalyst. These remarkable results confirmed the viability of using this advanced electrocatalyst for industrial-scale H₂ production through HER electrocatalysis.

Furthermore, leveraging the dealloying processes as illustrated in Figure 11a, Ji et al. developed optimal HER electrocatalysts using ScRu_0.25Co_1.75, characterized by zeolite-like metal frameworks.^[54] Initially, the minor presence of Ru in ScRu_0.25Co_1.75 did not alter the dealloying behavior, which involved stripping Sc metal from the center of tunnel structures, thereby forming voids or vacancies in the ScRu_0.25Co_1.75 structure and significantly increasing the electrocatalytically active surface areas. Further, as shown in Figure 11b, the total density of state (TDOS) indicated a slight downward shift in Fermi level (E_Fermi) for Sc_0.875Co₂ (partially Sc stripped) compared to pristine ScCo₂. Yet, combined with the crystal orbital Hamilton population (COHP) results presented in Figure 11b, this downshift led to a depopulation of the antibonding states of the Co─Co bond, thereby strengthening the Co–Co connection in Sc_0.875Co₂. These structural changes, brought about by dealloying engineering, significantly enhanced the electrocatalytic stability of ScRu_0.25Co_1.75. Additionally, the depopulation of the antibonding states of the Co─Co bond improved the Co─H bond in Sc_0.875Co₂, potentially increasing H* adsorption following Sc stripping to enhance HER electrocatalysis (Figure 11c). As a result, under industrial HER conditions in 6 m KOH at 60 °C, ScRu_0.25Co_1.75 achieved an overpotential of only 132 mV to reach a high current density of 500 mA cm⁻² (Figure 11d) and demonstrated 1000-h long-term stable H₂ production (Figure 11e). These enhanced properties of ScRu_0.25Co_1.75, derived from the dealloying processes during HER electrocatalysis, demonstrated its potential for industrial-scale H₂ production.

2.2.6 Large-Scale Synthesis of Electrocatalysts

For industrial-scale H₂ production, an ideal HER electrocatalyst should be easily manufactured in large quantities, avoiding complex or low-yield initial laboratory synthesis methods such as high-temperature annealing in various atmospheres.^[55] In recent years, large-scale synthesis of diverse electrocatalysts in ambient conditions has remained popular, utilizing representative methods such as room-temperature immersion synthesis, centrifugal spinning fabrication, and ball-milling manufacturing.

The classic room temperature immersion synthesis by Yang et al. developed a well-designed NiCoRu hydroxide/sulfide heterostructure on Ni foam (i.e., (NiCoRu)OH/S).^[56] As illustrated in Figure 12a, the (NiCoRu)OH/S electrocatalyst can be readily produced through industrial-scalable room temperature soaking for in situ growth on Ni foam. According to the characterized results shown in Figure 12b, after four days of room temperature immersion, abundant (NiCoRu)OH/S nanoparticles were distributed on the Ni foam substrate, significantly increasing the specific surface areas and enhancing electron and mass transfer during HER electrocatalysis to accelerate the final HER kinetics. Additionally, the coexistence of crystalline and amorphous planes in (NiCoRu)OH/S exposed numerous active sites (Figure 12c), further facilitating electron transfer. Coupled with the confirmed superhydrophilic properties shown in Figure 12d and intrinsic HER activity from the multiple chemical states of the trimetallic compound, the (NiCoRu)OH/S-2 sample treated with optimal immersion time demonstrated remarkable HER activity with a low overpotential of 78 mV at a current density of 10 mA cm⁻² and maintained 72-h robust electrochemical durability tested at a constant current density of 100 mA cm⁻² in 1 m KOH (Figure 12e,f). Even under industrial operation conditions in 6 m KOH at 60 °C, (NiCoRu)OH/S-2 required overpotentials of only 148 and 250 mV to reach high current densities of 100 and 1200 mA cm⁻², respectively, maintaining 72-h robust durability at a constant current density of 100 mA cm⁻² (Figure 12g,h). Beyond excellent HER performance, (NiCoRu)OH/S-2 also served as an effective OER electrocatalyst, showing outstanding OER activity and stability even under industrial conditions. This sophisticated heterostructure electrocatalyst, (NiCoRu)OH/S, holds promise as a ready-to-use bifunctional electrode, poised to advance the industrialization of HER technologies.

Compared to electrospinning, which has low throughputs and requires high voltage, centrifugal spinning technology generally achieves higher throughputs. Typical centrifugal spinning equipment with two nozzles can produce ≈50 g h⁻¹, a rate nearly two orders of magnitude higher than that of laboratory-scale electrospinning. Utilizing an in-house developed centrifugal spinning setup, Mukkavilli et al. produced TaO_xN_1 − x fibers through ammonolysis of as-spun ([Ta(OEt)₅]₂)/polyvinylpyrrolidone (PVP) fiber mats.^[57] Consequently, the TaO_xN_1 − x fibers demonstrated superior HER activity compared to Ta₂O₅. This centrifugal spinning method undoubtedly provides a universal scalable approach for producing other up-scaled electrocatalysts in the future. Moreover, as a prominent solvent-free mechanochemical technique, the high-energy ball milling approach is straightforward and highly scalable, suitable for producing large quantities of electrocatalysts. In this context, Hu et al. utilized graphene quantum dots (GQDs) as exfoliation agents to synthesize and functionalize nearly atom-layered MoS₂ nanosheets (ALMS).^[58] Impressively, the produced ALMS measured ≈4 nm and could be uniformly dispersed in isopropanol, forming a stable colloidal solution due to the sulfonation process facilitated by GQDs, demonstrating that the scaled-up-fabricated ALMS can be reliably stored. Without requiring organic solvents, catalysts, substrates, or the harsh conditions of vacuum systems, the synthesis yield of ALMS reached 63% with efficient recovery and recycling of GQDs as an exfoliation agent, ensuring a reliable and scalable production of electrocatalysts for industrial HER needs. Finally, the produced ALMS exhibited considerable HER activity and outstanding electrochemical durability even after a 200-h long-term stability test in 0.5 m H₂SO₄ solution. The development of this stable 2D nanomaterial electrocatalyst provides a feasible route for producing electrocatalysts on a large scale, aiming to meet the increasing demands for HER industrialization.

To provide a clear understanding of the current state of green H₂ production, we have summarized the advanced designs of HER electrocatalysts in Table 1 with taking into account various key properties of the reviewed electrocatalysts, such as cost-effectiveness, self-supporting structures, hydrophilicity, mechanical stability, intrinsic electrochemical stability, and scalable designs to satisfy the high demands of HER electrocatalyst for enabling industrial-scale H₂ production. In addition, we further analyzed hundreds of studies closely related to the topic of high-performance HER in recent three years. Specifically, an industrial-scale current density normally should exceed 500 mA cm⁻², also along with a long-term stable utilization. However, as shown in Figure 13a and Table S1 (Supporting Information), although most HER studies have demonstrated progressively lower overpotentials even at a current density of 100 mA cm⁻², electrochemical stability remains largely limited to low current densities and short testing durations (Figure 13b). Additionally, the analyzed degradation rate of voltage presented in Figure 13c still falls significantly short of the DOE 2026 target, both in terms of operating current density and the average voltage degradation rate. These results clearly indicate that the development of scalable and stable electrocatalysts is an essential future direction for generating green H₂. Furthermore, to achieve scalable and stable H₂ production, it is essential to integrate the key qualities discussed in this chapter—such as cost efficiency, self-supporting architecture, hydrophilicity, mechanical robustness, and intrinsic electrochemical stability—into a single, all-in-one electrocatalyst for ensuring efficient and long-term stable water electrolysis. Ultimately, this well-designed electrocatalyst should be a scalable synthesis to meet the large demands of industrial H₂ production.

3.1.1 Precipitations of Seawater HER Electrocatalysts

Admittedly, sediments, microplastics, and microorganisms present in seawater can be readily filtered out prior to seawater HER. However, the removal of many ions (e.g., F⁻, Cl⁻, Br⁻, Na⁺, Mg²⁺, K⁺, Ca²⁺, Cu²⁺, Sr²⁺, Cd²⁺, HCO₃⁻, CO₃²⁻ and SO₄²⁻, with concentrations detailed in Table 2) poses a challenge, as they cannot be easily eliminated through simple filtration.^[75] On one hand, during seawater, HER, the concentration of OH⁻ increases (manifested as a pH increase) alongside H₂ evolution at the cathode. This results in the precipitation of Ca(OH)₂ and Mg(OH)₂, which can cover the electrocatalyst surface, obstructing the active HER sites and ultimately reducing the durability of seawater electrocatalysts. On the other hand, due to the reduction potential at the cathode, ions such as Cu²⁺ and Cd²⁺ can undergo reduction to metallic forms under specific potentials instead of contributing to HER. These competitive reactions can similarly block the HER active sites and affect the stability of electrocatalysts during seawater HER.^{[77, 78]} Currently, the addition of buffer electrolytes to stabilize the pH of seawater is a popular method to mitigate these issues.^[79] However, this approach also increases costs, hindering the scalability of industrial-scale seawater HER. Consequently, developing seawater electrolyzers that can effectively separate sediments from the cathode surface represents a promising direction. This innovation aims to enable the direct use of seawater for H₂ production without additional concerns.

3.1.2 Competitive Anodic Chlorine Evolution

These results indicate that ClER can be circumvented by using a high-performance electrocatalyst that achieves an OER overpotential of less than ≈180 mV in acidic conditions, or less than ≈480 mV in alkaline conditions. Obviously, the latter case is generally more feasible to implement. Nonetheless, achieving industrial-scale current densities at an overpotential under 480 mV presents a significant challenge. Extensive research is needed to enhance activity and simultaneously stabilize the pH to prevent the precipitation of Ca(OH)₂ and Mg(OH)₂ during the electrocatalytic production of H₂ from seawater.

3.1.3 Conventional Electrolyzers for Seawater HER

Given the shift toward using readily available and inexpensive seawater in industrial HER electrocatalysis, conventional water electrolyzers must be adapted to address the specific challenges posed by seawater splitting. Briefly, in this sub-section, we provide an overview of typical traditional electrolyzers, including alkaline water electrolyzers (AWE), proton exchange membrane water electrolyzers (PEMWE), and anion exchange membrane water electrolyzers (AEMWE). The discussion aims to clarify which advantages of these systems should be preserved and which shortcomings or unsuitable aspects need modification for effective seawater electrolysis.

As depicted in Figure 18a, the AWE is a well-established electrolyzer used for water splitting, utilizing a porous diaphragm to separate the cathode and anode. This diaphragm allows the conduction of OH⁻ ions from the cathode to the anode while keeping the produced H₂ and O₂ gases separate. Notably, when operating an alkaline seawater splitting, non-noble metal-based electrocatalysts can be used, which offer considerable overpotential and stability compared to acidic conditions.^[86] As previously mentioned, in OH⁻-rich environments typical of seawater electrolysis, competitive ClER can be effectively suppressed, enhancing OER performance while maintaining the overpotential below 480 mV. Additionally, the alkaline environment in AWE can precipitate Ca²⁺ and Mg²⁺ ions, reducing sediment coverage of Ca(OH)₂ and Mg(OH)₂ on the cathodic electrode surface, making AWE technology suitable for large-scale seawater electrolysis. However, the porous diaphragm often leads to higher ohmic losses with low working current densities (below 400 mA cm⁻²) and the inevitable risk of H₂ and O₂ crossover, which increases the potential for explosions during H₂ production.^{[87, 88]} Moreover, the diaphragm can become clogged by ions and impurities in seawater, reducing the efficiency of seawater splitting. Maintaining concentrated alkaline conditions in the AWE system also incurs additional costs for H₂ production. The strengths and weaknesses of AWE are detailed and compared with PEMWE and AEMWE in Table 3.

The PEMWE similarly consists of cathode and anode electrocatalysts (predominantly Ir and Pt-based materials) and employs an acidic membrane (i.e., PEM) made from materials like Nafion as a solid electrolyte. This configuration enables the migration of protons from the anode to the cathode, leading to the generation of H₂.^[89] In PEMWE, seawater is exclusively introduced at the anode, effectively preventing the formation of Ca(OH)₂ and Mg(OH)₂ precipitates on the cathode surface (Figure 18b). The electrolysis process in PEMWE features faster kinetics compared to AWE due to the high proton conductivity and reduced ohmic loss in the PEM, allowing PEMWE to achieve current densities exceeding 2000 mA cm⁻², thus lowering energy consumption for H₂ production. Additionally, thanks to minimal H₂ and O₂ crossover, the purity of the H₂ produced (over 99.999%) is significantly higher than that from AWE, also reducing explosion risks during seawater splitting.^[90] However, the acidic environment within the PEMWE system makes the ClER more competitive than the OER at the anode. The resultant Cl₂ and oxychlorides can severely damage the PEM, electrodes, and bipolar plates in the PEMWE system, degrading overall seawater electrolysis performance and shortening the lifetime of the PEMWE. Furthermore, the high concentration of Na⁺ in seawater often competes with H⁺ for passage through the PEM, leading to H⁺ accumulation at the anode, increasing the Nernst overpotential, and resulting in unstable PEMWE operation. Besides, the use of noble metal-based electrocatalysts and anti-corrosive Ti-based bipolar plates also significantly elevates the fabrication and operational costs of PEMWE (Table 3).^[91]

The AEMWE combines elements of both AWE and PEMWE, incorporating cathode and anode electrocatalysts separated by a solid electrolyte of nonporous polymer membrane, known as AEM (Figure 18c). Similar to AWE, AEMWE operates in an alkaline environment, with the AEM facilitating the movement of OH⁻ from the cathode to the anode. Compared to the Nafion-based PEM used in PEMWE, AEM is generally more cost-effective. Introducing seawater exclusively at the cathode reduces the risk of ClER at the anode, which is enriched with OH⁻ rather than Cl⁻. This makes it possible to replace the typically expensive anti-corrosive Ti-based bipolar plates with more affordable stainless steel. Furthermore, the alkaline environment enables the use of non-noble metal-based electrocatalysts, significantly lowering the overall cost of AEMWE.^[92] However, despite integrating some advantages of AWE and PEMWE, AEMWE faces challenges. The electrochemical stability of the AEM against Cl⁻ corrosion is suboptimal, and its transport of OH⁻ is slower compared to H⁺ transport in PEM, resulting in much lower efficiency during seawater electrolysis than PEMWE.^{[93, 94]} Simultaneously, due to the direct supply of seawater to the cathode, the precipitation issue of Ca(OH)₂ and Mg(OH)₂ during cathodic HER is serious, which covers the cathodic electrode, blocks active sites on the electrocatalyst, and ultimately impedes overall seawater splitting efficiency in AEMWE (Table 3). Meanwhile, in terms of economic competitiveness, the well-established AWE technology is projected to grow at a compound annual growth rate of 6.5% from 2021 to 2028, with the market size expected to reach 180.03 million dollars by 2028 and an H₂ production capacity exceeding 3880 Nm³ h⁻¹.^[95] However, one of the primary obstacles to further commercialization of AWE remains its high capital expenditure. On the other hand, AEMWE, as an emerging H₂ production technology, shows the potential in overcoming the cost-effectiveness limitations of AWE. Although its current H₂ production capacity is ≈210 Nm³ h⁻¹, the investment cost remains relatively high at over 2000 USD kW⁻¹, even surpassing that of AWE, which ranges from 500 to 1700 USD kW⁻¹.^[96] In brief, the cost-efficiency and investment trends of AEMWE indicate a promising direction for future research and development.

3.2.1 Precipitation Resistant Seawater HER Electrocatalysts

During seawater HER electrocatalysis, the consumption of protons from H₂O increases the local concentration of OH⁻ near the cathode, leading to a rise in pH and the formation of precipitates such as Ca(OH)₂ and Mg(OH)₂ on the active surface of the electrocatalyst. To address this issue, designs for cathodic seawater HER electrocatalysts should focus on self-cleaning of sediments, blocking Ca²⁺/Mg²⁺ ions, or maintaining local seawater pH. Based on this understanding, as schematically illustrated in Figure 19a, researchers are developing electrocatalysts with porous or channel-incorporated structures to minimize the retention of precipitates on the surface, allowing precipitates to be carried away with the electrolyte flow. Additionally, Figure 19b shows an HER electrocatalyst coated with an ion-sieving layer that blocks Ca²⁺ and Mg²⁺ ions from seawater while permitting the penetration of H₂O and OH⁻, thereby facilitating the seawater HER process. This approach fundamentally reduces the impact of sedimentary corrosion through physical resistance. Moreover, as depicted in Figure 19c, the application of a Lewis acid layer on the electrocatalyst surface is becoming popular. This layer captures the produced OH⁻ from water dissociation, preventing its diffusion into the bulk seawater and the formation of insoluble precipitates. Although these strategies are theoretically sound, practical applications to fully realize these concepts are still in developmental stages.

Drawing inspiration from the natural channel structure in plants, Li et al. developed a Mo₂C composite electrocatalyst in situ grown on corn straw biochar using a cold isostatic pressure method (Mo₂C/B (CIP)), as illustrated in Figure 20a.^[97] The schematic in Figure 20b shows that the Mo₂C/B (CIP) benefited from an abundance of mesoporous and vertically wide channels, facilitating timely electron exchange and the release of H₂ bubbles. Although the formation of precipitates such as Ca(OH)₂ and Mg(OH)₂ was still inevitable, the natural porous and channel structures in Mo₂C/B (CIP) effectively alleviated congestion caused by these precipitates, enabling rapid mass transfer of H₂ bubbles and seawater electrolyte through its low-tortuosity channels. As shown in Figure 20c,d, this well-designed seawater HER electrocatalyst demonstrated a notably low overpotential of 251 mV to achieve a current density of 10 mA cm⁻² in simulated seawater. Remarkably stable performance was also maintained even after the 12-h seawater HER test, with the specific surface area of Mo₂C/B (CIP) maintained at 206.7 m² g⁻¹, demonstrating its resistance to Ca(OH)₂ and Mg(OH)₂ precipitations during seawater HER electrocatalysis, as depicted in Figure 20e,f.

In a similar pursuit for sediment resistance during seawater HER, Liu et al. developed a novel Ni foam electrode coated with a Ni(OH)₂ nanofiltration membrane (Ni(OH)₂–NF), as detailed in Figure 21a.^[98] This positively charged Ni(OH)₂ membrane with nanometer-scale cracks effectively prevented Ca²⁺ or Mg²⁺ from reaching the HER electrocatalyst surface, while permitting the passage of reactants or intermediates such as H₂O and OH⁻ during seawater HER processes, as shown in Figure 21b,c. When compared to the bare Ni foam electrode, the Ni(OH)₂–NF electrode achieved a dramatic reduction in precipitations by 98.3%, maintaining stable operation for 100-h in a Mg(OH)₂-saturated seawater solution, as shown in Figure 21d,e. In the context of a flow-type direct seawater electrolyzer, as depicted in Figure 21f, the system utilizing the Ni(OH)₂–Pt–NF electrode (cathodic HER) alongside a Cr₂O₃–IrRuO₂–Ti electrode (anodic OER) exhibited superior resistance to seawater HER precipitations compared to a conventional water splitting system with a Pt–NF cathodic electrode. After a 100-h seawater HER stability test, the sediment accumulation on the Ni(OH)₂–Pt–NF electrode was just 0.82 mg cm⁻², significantly lower than the 10.06 mg cm⁻² on the Pt–NF electrode without the Ni(OH)₂ nanofiltration coating, as presented in Figure 21g.

In a notable study aimed at mitigating corrosion from the precipitation of Ca(OH)₂ and Mg(OH)₂ during seawater HER, Guo et al. implemented a Lewis acid layer on their electrocatalyst design.^[99] This novel seawater HER electrocatalyst featured CoO_x grown on carbon fibers and encapsulated with a durable Lewis acid layer of Cr₂O₃, chosen for its electrochemical stability and extremely low pKa value of 2.05, as shown in Figure 22a. This layer effectively captured OH^– ions generated during water dissociation in the seawater HER process, thus preventing their diffusion into the seawater and inhibiting the formation of Ca(OH)₂ and Mg(OH)₂ precipitations on the electrode surface, as illustrated in Figure 22b. Performance results detailed in Figure 22c–e demonstrate that the Cr₂O₃–CoO_x electrocatalyst achieved a low operational cell voltage of only 1.87 V to reach an industrial-scale current density of 1000 mA cm⁻² at 60 °C. It also maintained stable performance at a current density of 500 mA cm⁻² over 100 h, with high Faradaic efficiencies for H₂ (≈93%) and O₂ (≈92%) generation in a flow-type seawater electrolyzer using real seawater. This sophisticated design not only effectively supported direct seawater electrolysis but also adapted to industrial-scale current densities and stability requirements.

3.2.2 Highly Selective Anodic Electrocatalysts

Chloride (Cl⁻), the predominant anion in seawater (ranging from 19 500 to 22 000 ppm as detailed in Table 2), often undergoes competitive ClER processes at the anode, typically generating corrosive Cl₂ gas. While the strategic design of anodic OER electrocatalysts to circumvent ClER indirectly benefits seawater HER activities, minimizing Cl₂ damage is crucial for extending the operational life of industrial seawater electrolyzers. As mentioned previously, to achieve industrial-scale current densities in acidic conditions without Cl₂ evolution, the applied overpotential must remain below 180 mV, a challenging target seldom met in current research. However, in alkaline conditions, an overpotential up to 480 mV is permissible, which significantly reduces the concern on the competitive ClER. Thus, in view of these obstacles and potential opportunities, and also essential for a comprehensive evaluation of seawater HER progress, recent studies have focused on developing functional seawater OER electrocatalysts, such as those depicted in Figure 23a,b. These include developing an OER-selective layer that blocks Cl⁻ while allowing H₂O and H⁺ to pass through during seawater OER or establishing a localized alkaline environment using a Lewis acid layer on the electrocatalyst surface. This setup creates a stable electrical double layer rich in OH⁻ and H₃O⁺, which effectively prevents localized Cl⁻ migration from the bulk seawater to the electrocatalyst surface, thereby inhibiting competitive ClER at the anodic side of seawater splitting.

In a noteworthy development, Tan et al. engineered an in situ sulfate passivating layer on a Ni foam-supported Ni₂Fe-LDH/FeNi₂S₄ heterostructure (Ni₂Fe-LDH/FeNi₂S₄/NF) during repeated CV scans for OER electrocatalysis.^[100] This heterostructure, featuring numerous bimetallic hydroxide/sulfide interfaces, enhanced the adsorption of oxygen intermediates, thereby accelerating OER. Intriguingly, the Cl⁻-repellent sulfate layer formed in situ on the electrocatalyst surface further extended its application in seawater OER. Following the establishment of this sulfate film, the Ni₂Fe-LDH/FeNi₂S₄/NF demonstrated exceptional performance in alkaline simulated seawater electrolyte, achieving an ultralow OER overpotential of 250 mV at a current density of 100 mA cm⁻². The enhanced long-term stability observed post-film formation surpassed the performance of the uncoated Ni₂Fe-LDH/FeNi₂S₄/NF, demonstrating highly increased resistance to Cl⁻ thanks to the post-formed sulfate passivating layer, effectively inhibiting Cl₂ evolution in seawater splitting. Similarly, using a two-step approach involving hydrothermal synthesis and electrochemical activation, Kang et al. enhanced the performance of the RuMoNi electrocatalyst for seawater OER.^[101] During electrochemical reconstruction, in situ generated MoO₄²⁻, formed through the leaching of Mo from the RuMoNi electrocatalyst, effectively repels Cl⁻ from the electrocatalyst surface. This reversible dissolution and precipitation of NiMoO₄ results in the formation of a selective MoO₄²⁻ layer, stabilizing the RuMoNi electrocatalyst for at least 3000 h, even under an industrial-scale current density of 500 mA cm⁻², with a minimal voltage degradation rate of 0.64 µV h⁻¹. Remarkably, the system exhibits ≈100% selectivity for alkaline seawater OER. Furthermore, the AEM electrolyzer employing the RuMoNi electrocatalyst achieved seawater electrolysis at a high current density of 1000 mA cm⁻² with an applied voltage of just 1.72 V, delivering an energy conversion efficiency of 77.9%. As a result, the calculated cost per gallon of gasoline-equivalent for the produced hydrogen is 0.85 dollars, significantly lower than the DOE 2026 target of 2 dollars. Fan et al. developed the CoFe-Ci@GQD seawater electrolysis electrocatalyst by strategically combining several innovative features.^[102] They constructed CoFe-based host layers to enhance OER activity, introduced CO₃²⁻ into the interlayer to create narrow interlayer spacing and repel Cl⁻ ions, and anchored graphene quantum dots (GQDs) on the catalyst surface to generate Coulomb repulsion, preventing Cl⁻ from accessing surface active sites. Benefiting from these superior characteristics, CoFe-Ci@GQD demonstrated remarkable durability, achieving over 2800 h of alkaline seawater OER operation even at an industrial-scale current density of 1250 mA cm⁻². Additionally, the CoFe-Ci@GQD electrocatalyst was integrated as both HER and OER components in a photovoltaic–electrolysis device, achieving a record solar-to-hydrogen efficiency of 18.1% for overall seawater splitting, along with excellent stability for up to 200 h under a high current of 440 mA. These outstanding performances undoubtedly reinforce the potential of green H₂ as a sustainable energy solution for the future.

As previously discussed, Guo et al. developed a Cr₂O₃–CoO_x electrocatalyst featuring a Lewis acid layer, which was initially designed to prevent precipitation of Ca(OH)₂ and Mg(OH)₂ during cathodic HER electrocatalysis.^[99] Surprisingly, this same Lewis acid layer proved effective in anodic OER settings, where it helped suppress competitive ClER. As illustrated in Figure 23b and Figure 24a, during seawater OER, H⁺ ions generated from water dissociation combined with H₂O to form H₃O⁺, establishing a stable electrical double layer with excess OH⁻ (partial amount of OH* was consumed for O₂ generation), which served as a barrier preventing Cl⁻ from reaching the electrocatalyst surface. As a result, when the Cr₂O₃–CoO_x electrocatalyst was deployed for anodic seawater OER, it demonstrated remarkable Cl⁻ resistance, achieving a 98% suppression rate, as evidenced by the OH⁻ concentration and Cl 2p spectra results shown in Figure 24b,c. By applying an anodic potential of 1.8 V versus RHE, the electrocatalyst maintained over 200-h of long-term stability at large-scale current densities (Figure 24d), confirming its superior Cl⁻ resistance. Consistent with the results presented in Figure 22c–e, in a flow-type seawater electrolyzer using actual seawater, the Cr₂O₃–CoO_x electrocatalyst achieved a low cell voltage of only 1.87 V to support an industrial-scale current density of 1000 mA cm⁻² at 60 °C, demonstrating high Faradaic efficiencies for both H₂ and O₂ production and sustained operational stability.

3.2.3 Developments of Seawater Electrolyzers

In the previous section, we comprehensively reviewed the advantages and limitations of traditional AWE, PEMWE, and AEMWE used in seawater electrolysis. Building on this foundation, this section aims to retain the strengths of these conventional electrolyzers while proposing novel ideas and designs to address their existing shortcomings and adaptability issues. These enhancements are intended to improve both the activity and stability of seawater electrolysis. Certainly, the main shortcomings of conventional electrolyzers include issues with membrane conductivity and stability, along with precipitate accumulation on both the membrane and electrodes. These challenges lead to irreversible declines in activity and stability during seawater electrolysis, as noted in previous research.^[103] In response, innovations focused on electrolyzer membranes have become a focal area of study. New concepts such as the waterproof breathable membrane combined with self-dampening electrolyte water electrolyzer (WBM-SDEWE), asymmetric water electrolyzer (ASWE), and bipolar membranes water electrolyzer (BPMWE) are emerging (Figure 25a–c).

As depicted in Figure 25a, the WBM-SDEWE modifies the conventional AWE by incorporating a hydrophobic porous polytetrafluoroethylene (PTFE)-based WBM to separate seawater from concentrated KOH solution. This system utilizes the KOH solution directly in contact with both the cathode and anode. Due to varying water vapor pressures between the seawater and KOH solution, water vapor can migrate from seawater to KOH solution through the PTFE-based WBM via a liquid–gas–liquid phase transition, while effectively isolating impurity ions such as Cl⁻, Ca²⁺, and Mg²⁺. This innovative design (Figure 26a), inspired by self-driven phase transition processes, was successfully demonstrated by Xie et al. in their symmetrical seawater electrolysis system.^[104] The micrometer-scale gas diffusion path within the PTFE-based WBM not only ensured a high rate of water migration to support H₂ and O₂ production but also effectively blocked impurities, preventing the precipitation of Ca(OH)₂ and Mg(OH)₂ and corrosion from Cl₂ and oxychlorides. As shown in Figure 26b,c, this symmetrical seawater electrolysis system operated steadily for over 72-h with ≈100% Faradaic efficiency. It achieved an impressive H₂ production rate of 386 L h⁻¹ at a current density of 250 mA cm⁻² for more than 3200 h, with exceptionally low energy consumption of 5 kWh Nm⁻³_H2 (Figure 26d,e). After extensive use, minimal corrosion was observed on the electrocatalyst, and the PTFE-based WBM remained effectively dry. Therefore, this well-designed WBM-SDEWE system holds significant potential for broad application in seawater electrolysis and other complex wastewater electrolysis cases.

Additionally, building on the traditional architecture of AEMWE, asymmetric electrolytes are provided in the ASWE. As detailed in Figure 25b, real seawater and NaOH solution are independently supplied to the cathode and anode, respectively. A Na⁺ exchange membrane is utilized instead of AEM, facilitating the transfer of Na⁺ from the anode to the cathode while preventing the reverse movement of Cl⁻. This configuration effectively eliminates the competitive ClER during the anodic OER reaction. On the cathode side, by employing neutral real seawater, the formation of highly alkaline conditions and the consequent serious precipitations of Ca(OH)₂ and Mg(OH)₂ are successfully avoided. Inspired from this idea, Shi et al. employed this ASWE system,^[105] integrating it with an electrocatalyst composed of Ni-Fe-P nanowires anchored with atomic Pt (Figure 27a). The system achieved a current density of 100 mA cm⁻² at cell voltages of 1.46 and 1.5 V in simulated and real seawater at 25 °C, respectively. Remarkably, even at 80 °C in simulated seawater, the system required only a low cell voltage of 1.66 V to achieve a high current density of 400 mA cm⁻² (Figure 27b). Coupled with its excellent 120-h long-term stability at a current density of 100 mA cm⁻² (Figure 27c,d), this well-constructed ASWE demonstrated superior seawater electrolysis activity and stability compared to other studies. Ultimately, the calculated energy consumption led to a hydrogen cost of only 1.36 USD per kilogram, lower than that of the “Green H₂” targets set by the US DOE.^[67]

To leverage the advantages of both PEMWE and AEMWE, the integration of PEM and AEM into a BPMWE is also explored. As depicted in Figure 25c, PEM and AEM are situated respectively at the cathode and anode. When seawater is introduced, water migrates through the PEM and AEM to the water dissociation layer (WDL), where it dissociates into H⁺ and OH⁻. These ions are then selectively transported to the cathode and anode, respectively, facilitating HER and OER electrocatalysis. Notably, the accumulation of H⁺ at the cathode induces a local acidic environment that inhibits the precipitation of Ca(OH)₂ and Mg(OH)₂, while the accumulation of OH⁻ at the anode incurs a local alkaline environment conducive to OER rather than ClER, ensuring prolonged active and stable operation of seawater electrolysis. In this case, the thickness of the PEM and AEM is critical for effective water transport to the WDL. A thinner AEM can aid the mobility of OH⁻ to better match the H⁺ transfer velocity through the PEM, which typically occurs several times faster. As shown in Figure 28a–c, Mayerhöfer et al. utilized a BPMWE with a thinner AEM,^[106] achieving an industrial-scale current density of 450 mA cm⁻² at a cell voltage of only 2.2 V in pure water, along with 100-h long-term stability at a high current density of 2000 mA cm⁻² at 80 °C (Figure 28d,e). This expertly designed BPMWE holds promise for both pure and seawater electrolysis, positioning the half-reactions of HER and OER in optimally suited pH environments to potentially reduce overall maintenance costs in practical seawater electrolysis applications.

Enhancing membrane performance is a crucial strategy to improve the activity and stability of seawater electrolyzers. Conversely, eliminating membranes in seawater electrolysis can also effectively reduce associated drawbacks such as ohmic losses while decreasing costs. A notable design, as depicted in Figure 25d, utilizes the Segré–Silberberg effect in a microfluid water electrolyzer (MWE) that operates without membranes. This method efficiently separates and collects generated H₂ and O₂ gases. To be specific, a seawater velocity gradient across the generated H₂ or O₂ molecules exerts a net inertial lift force on them toward the wall and then is counterbalanced by a force generated between the H₂ or O₂ molecules and the channel wall, finally stabilizing H₂ and O₂ gases near the cathode and anode, respectively. According to research by Hashemi's group,^[107] this well-designed MWE operated effectively under high seawater flow rates, achieving water splitting at a current density of 300 mA cm⁻², with a conversion efficiency exceeding 42%, and maintaining an exceptionally low H₂ and O₂ crossover rate of 0.4%. This MWE design ensured that precipitates at the cathode and Cl₂ at the anode are swiftly removed from the device, preventing electrode coverage or corrosion, while still capturing the produced H₂. Remarkably, the MWE can be manufactured using straightforward techniques, such as high-resolution 3D printing or injection molding. Therefore, when configured in cascade arrays of multiple rows and columns, this approach can readily facilitate scalable H₂ production, potentially meeting industrial-scale demands.