2D CoOOH Sheet-Encapsulated Ni2P into Tubular Arrays Realizing 1000 mA cm-2-Level-Current-Density Hydrogen Evolution Over 100 h in Neutral Water

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

Green hydrogen (H2) emerges as a sustainable alternative to fossil fuels, offering a clean method to store renewable energy through water electrolysis with high energy content and zero carbon emissions. While research largely focuses on specific aspects such as hydrogen evolution reaction (HER), seawater HER electrocatalysts, and electrolyzer development, these studies often overlook the broader hydrogen economy from an integrated industry chain perspective. This review bridges that gap by providing a comprehensive analysis of hydrogen energy industrialization, covering advancements in HER, seawater HER, and electrolyzers, all aim at enabling industrial-scale H2 production. It further explores innovations and challenges in hydrogen storage and transportation, as well as real-world projects spanning the green hydrogen supply chain. Additionally, life cycle assessment studies validate the environmental benefits of using renewable energy sources for green H2 production. Furthermore, this review highlights advancements in counter-oxygen evolution reactions and organic oxidation reactions, alongside strategies to mitigate competing chlorine evolution reactions. Through this comprehensive examination, this review aims to inform readers of the latest developments in hydrogen energy industrialization, explore its growth potential, and provide new insights to propel the hydrogen economy forward.

Anchoring platinum clusters in CoP@CoNi layered double hydroxide to prepare high-performance and stable electrodes for efficient water splitting at high current density

Hydrogen production via electrocatalytic water splitting has garnered significant attention, due to the growing demand for clean and renewable energy. However, achieving low overpotential and long-term stability of water splitting catalysts at high current densities remains a major challenge. Herein, a CoP@CoNi layered double hydroxide (LDH) electrode was synthesized via a two-step electrodeposition process, demonstrating oxygen evolution reaction, with an overpotential (ƞ400) of 373 mV and a Tafel slope of 64.2 mV/dec. Subsequently, a superhydrophilic CoP@CoNi LDH-Pt electrode was prepared using an in situ oxidation method, with Pt clusters anchored within the CoP@CoNi LDH for the hydrogen evolution reaction, with an overpotential (ƞ400) of 119 mV and a Tafel slope of 33.6 mV/dec. The CoP@CoNi LDH-Pt||CoP@CoNi LDH system achieved a cell voltage of 1.96 V at 1000 mA/cm2, maintaining stable performance after water electrolysis for 160 h. These remarkable outcomes stem from the fast charge transfer, improved mass transfer, and superior stability of the catalysts. Heterostructure electrodes were constructed to optimize the electrocatalytic kinetics through strong interfacial electronic interactions, while superhydrophilic electrodes were fabricated to improve the mass transfer process at a high current density. Anchoring high-activity components in catalysts prevents their detachment during water electrolysis, enhancing the structural stability of the electrodes. This study provides a new approach for large-scale preparation of efficient water electrolysis catalysts and offers significant potential for industrial hydrogen production applications.

Recent Progress and Perspectives on Functional Materials and Technologies for Renewable Hydrogen Production

Scientists worldwide have been inspecting hydrogen production routes and showing the importance of developing new functional materials in this domain. Numerous research articles have been published in the past few years, which require records and analysis for a comprehensive bibliometric and bibliographic review of low-carbon hydrogen production. Hence, a data set of 297 publications was selected after filtering journal papers published since 2010. The search keywords in the Scopus Database were "green hydrogen" and "low carbon hydrogen production and materials". The data were analyzed using the R Bibliometrix package. This analysis made it possible to determine the total annual publication rate and to segregate it by country, author, journal, and research institution. With a general upward trend in the total number of publications, China was identified as the leading country in research on the subject, followed by Germany and Korea. Keyword analysis and the chronological evolution of several important publications related to the topic showed the focus was on water splitting for low-carbon H2 production. Finally, this review provides future directions for technologies and functional materials for low-carbon hydrogen production.

Oxyanions Enhancing Crystallinity of Reconstructed Phase for Oxygen Evolution Reaction

The catalysts were always undergoing continuous amorphization and dissolution of active structure in operating condition, hindering the compatibility between stability and activity for oxygen evolution reaction (OER). Herein, we propose the selective adsorption of leached NO3 - to strengthen the crystallinity and activity of surface reconstructed layer with amorphous and crystalline (a-c) heterojunction. Taking a-c Ni doped Fe2O(OH)3NO3 ⋅ H2O (Ni-FeNH) as a model precatalyst, we uncover that the leached NO3 - are readily adsorbs on the crystalline phase in the formed a-c Fe(Ni)OOHduring OER, lowering the disorder degree and further activating Ni and Fe ion of the crystalline Fe(Ni)OOH on a-c heterojunctions. Accordingly, Ni-FeNH deliver a low overpotential of 303 mV and high durability of 500 hours at 500 mA cm-2 for OER. Particularly, constructing industrial water electrolysis equipment exhibits high stability of 100 hours under a high operating current of 8000 mA.

Vertical Conductive Metal-Organic Framework Single-Crystalline Nanowire Arrays for Efficient Electrocatalytic Hydrogen Evolution

The construction of crystalline metal-organic frameworks with regular architectures supportive of enhanced mass transport and bubble diffusion is imperative for electrocatalytic applications; however, this poses a formidable challenge. Here, a method is presented that confines the growth of nano-architectures to the liquid-liquid interface. Using this method, vertically oriented single crystalline nanowire arrays of an Ag-benzenehexathiol (BHT) conductive metal-organic framework (MOF) are fabricated via an "in-plane self-limiting and out-of-plane epitaxial growth" mechanism. This material has excellent electrocatalytic features, including highly exposed active sites, intrinsically high electrical conductivity, and superhydrophilic and superaerophobic properties. Leveraging these advantages, the carefully designed material demonstrates superior electrocatalytic hydrogen evolution activity, resulting in a low Tafel slope of 66 mV dec-1 and a low overpotential of 275 mV at a high current density of 1 A cm-2. Finite element analysis (FEA) and in situ microscopic verification indicates that the nanowire array structure significantly enhances the electrolyte transport kinetics and promotes the rapid release of gas bubbles. The findings highlight the potential of using MOF-based ordered nanoarray structures for advanced electrocatalytic applications.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Progress in Anode Stability Improvement for Seawater Electrolysis to Produce Hydrogen

Seawater electrolysis for hydrogen production is a sustainable and economical approach that can mitigate the energy crisis and global warming issues. Although various catalysts/electrodes with excellent activities have been developed for high-efficiency seawater electrolysis, their unsatisfactory durability, especially for anodes, severely impedes their industrial applications. In this review, attention is paid to the factors that affect the stability of anodes and the corresponding strategies for designing catalytic materials to prolong the anode's lifetime. In addition, two important aspects-electrolyte optimization and electrolyzer design-with respect to anode stability improvement are summarized. Furthermore, several methods for rapid stability assessment are proposed for the fast screening of both highly active and stable catalysts/electrodes. Finally, perspectives on future investigations aimed at improving the stability of seawater electrolysis systems are outlined.

Electron Manipulation and Surface Reconstruction of Bimetallic Iron-Nickel Phosphide Nanotubes for Enhanced Alkaline Water Electrolysis

Developing high-efficiency and stable bifunctional electrocatalysts for water splitting remains a great challenge. Herein, NiMoO4 nanowires as sacrificial templates to synthesize Mo-doped NiFe Prussian blue analogs are employed, which can be easily phosphorized to Mo-doped Fe2xNi2(1-x)P nanotubes (Mo-FeNiP NTs). This synthesis method enables the controlled etching of NiMoO4 nanowires that results in a unique hollow nanotube architecture. As a bifunctional catalyst, the Mo-FeNiP NTs present lower overpotential and Tafel slope of 151.3 (232.6) mV at 100 mA cm-2 and 76.2 (64.7) mV dec-1 for HER (OER), respectively. Additionally, it only requires an ultralow cell voltage of 1.47 V to achieve 10 mA cm-2 for overall water splitting and can steadily operate for 200 h at 100 mA cm-2. First-principles calculations demonstrate that Mo doping can effectively adjust the electron redistribution of the Ni hollow sites to optimize the hydrogen adsorption-free energy for HER. Besides, in situ Raman characterization reveals the dissolving of doped Mo can promote a rapid surface reconstruction on Mo-FeNiP NTs to dynamically stable (Fe)Ni-oxyhydroxide layers, serving as the actual active species for OER. The work proposes a rational approach addressed by electron manipulation and surface reconstruction of bimetallic phosphides to regulate both the HER and OER activity.

High-Throughput Designed and Laser-Etched NiFeCrVTi High-Entropy Alloys with High Catalytic Activities and Corrosion Resistance for Hydrogen Evolution in Seawater

Electrocatalytic hydrogen evolution from seawater through wind or solar energy is a cost-effective way to produce green hydrogen fuel. However, the lack of highly active and anti-corrosive electrocatalysts in seawater severely hinders the industrial application. Herein, a novel Ni1.1FeCr0.4V0.3Ti0.3 high-entropy alloy (HEA) is designed through high throughput computing and prepared via powder metallurgy with the surface treated by laser etching under different laser power. The laser-etched NiFeCrVTi high-entropy alloys exhibit a unique periodically ordered structure with multiple active centers and high porosity. The Ni-HEA-30 displays remarkable hydrogen evolution reaction (HER) performance with an overpotential of 55.9 mV and a Tafel slope of 47.3 mV dec-1 in seawater. Density functional theory (DFT) calculations are applied to identify the real active sites for HER on the HEA surface as the key factor for both proton and intermediate transformation, which also reveals that the Cr atom promotes the adsorption energy of water molecules, and the modulation of the electronic structure plays a crucial role in optimizing the hydrogen binding capabilities of the Ni atoms within the alloy. Additionally, the electrocatalyst displays high corrosion resistance in seawater, contributing to the good durability for hydrogen production. This work uncovers a new paradigm to develop novel electrocatalysts with superior reaction activity in seawater.

Design Strategies towards Advanced Hydrogen Evolution Reaction Electrocatalysts at Large Current Densities

Hydrogen (H2), produced by water electrolysis with the electricity from renewable sources, is an ideal energy carrier for achieving a carbon-neutral and sustainable society. Hydrogen evolution reaction (HER) is the cathodic half-reaction of water electrolysis, which requires active and robust electrocatalysts to reduce the energy consumption for H2 generation. Despite numerous electrocatalysts have been reported by the academia for HER, most of them were only tested under relatively small current densities for a short period, which cannot meet the requirements for industrial water electrolysis. To bridge the gap between academia and industry, it is crucial to develop highly active HER electrocatalysts which can operate at large current densities for a long time. In this review, the mechanisms of HER in acidic and alkaline electrolytes are firstly introduced. Then, design strategies towards high-performance large-current-density HER electrocatalysts from five aspects including number of active sites, intrinsic activity of each site, charge transfer, mass transfer, and stability are discussed via featured examples. Finally, our own insights about the challenges and future opportunities in this emerging field are presented.

Hierarchical Ohmic Contact Interface Engineering for Efficient Hydrazine-Assisted Hydrogen Evolution Reaction

Hydrazine oxidation reaction coupled with hydrogen evolution reaction (HER) is an effective strategy to achieve low energy water splitting for hydrogen production. In order to realize the application of hydrazine-assisted HER system, researchers have been focusing on the development of electrocatalysts with integrated dual active sites, while the performance under high current density is still unsatisfying. In this work, hierarchical Ohmic contact interface engineering is designed and used as a bridge between the NiMo and Ni2 P heterojunction toward industrial current density applications, with the charge transfer impedance greatly eliminated via such a pathway with low energy barrier. As a proof-of-concept, the importance of charge redistribution and energy barrier at the Ohmic contact interface is investigated by significantly reducing the voltage of overall hydrazine splitting (OHzS) at high current density. Intriguingly, the NiMo/Ni2 P hierarchical Ohmic contact heterojunction can drive current densities of 100 and 500 mA cm-2 with only 181 and 343 mV cell voltage in the OHzS electrolyzer with high electrocatalytic stability. The proposed hierarchical Ohmic contact interface engineering paves new avenue for hydrogen production with low energy consumption.

Deformable Catalytic Material Derived from Mechanical Flexibility for Hydrogen Evolution Reaction

Deformable catalytic material with excellent flexible structure is a new type of catalyst that has been applied in various chemical reactions, especially electrocatalytic hydrogen evolution reaction (HER). In recent years, deformable catalysts for HER have made great progress and would become a research hotspot. The catalytic activities of deformable catalysts could be adjustable by the strain engineering and surface reconfiguration. The surface curvature of flexible catalytic materials is closely related to the electrocatalytic HER properties. Here, firstly, we systematically summarized self-adaptive catalytic performance of deformable catalysts and various micro-nanostructures evolution in catalytic HER process. Secondly, a series of strategies to design highly active catalysts based on the mechanical flexibility of low-dimensional nanomaterials were summarized. Last but not least, we presented the challenges and prospects of the study of flexible and deformable micro-nanostructures of electrocatalysts, which would further deepen the understanding of catalytic mechanisms of deformable HER catalyst.

A Strongly Coupled Ag(S)@NiO/Nickel Foam Electrode Induced by Laser Direct Writing for Hydrogen Evolution at Ultrahigh Current Densities with Long-Term Durability

Highly active, durable, and cost-effective electrodes for hydrogen evolution reaction (HER) at ultrahigh current densities (≥1 A cm-2 ) are extremely demanded for industrial high-rate hydrogen production, but challenging. Here, a robust strongly coupled Ag(S)@NiO/nickel foam (NF) electrode is reported. Taking advantage of millisecond laser direct writing in liquid nitrogen technique, lattice-matched and coherent interfaces are formed between Ag nanoparticles with stacking faults (denoted by Ag(S)) and NiO nanosheets, leading to strong interfacial electronic coupling, not only promoting H2 O adsorption and dissociation on Ni2+ but also enhancing H* adsorption on intrinsically inactive but most electrically conductive Ag. Strong chemical bonding is established at NiO/NF interface, guaranteeing rapid electron transfer and excellent mechanical durability under high-rate hydrogen evolution. The physicochemically stable electrode achieves record-low alkaline HER overpotential of 167 and 180 mV at 1 and 1.5 A cm-2 , respectively, along with negligible activity decay after 120 h test at ≈1.5 A cm-2 , surpassing reported non-platinum group metal electrocatalysts.

Hierarchical Interconnected NiMoN with Large Specific Surface Area and High Mechanical Strength for Efficient and Stable Alkaline Water/Seawater Hydrogen Evolution

NiMo-based nanostructures are among the most active hydrogen evolution reaction (HER) catalysts under an alkaline environment due to their strong water dissociation ability. However, these nanostructures are vulnerable to the destructive effects of H2 production, especially at industry-standard current densities. Therefore, developing a strategy to improve their mechanical strength while maintaining or even further increasing the activity of these nanocatalysts is of great interest to both the research and industrial communities. Here, a hierarchical interconnected NiMoN (HW-NiMoN-2h) with a nanorod-nanowire morphology was synthesized based on a rational combination of hydrothermal and water bath processes. HW-NiMoN-2h is found to exhibit excellent HER activity due to the accomodation of abundant active sites on its hierarchical morphology, in which nanowires connect free-standing nanorods, concurrently strengthening its structural stability to withstand H2 production at 1 A cm-2. Seawater is an attractive feedstock for water electrolysis since H2 generation and water desalination can be addressed simultaneously in a single process. The HER performance of HW-NiMoN-2h in alkaline seawater suggests that the presence of Na+ ions interferes with the reation kinetics, thus lowering its activity slightly. However, benefiting from its hierarchical and interconnected characteristics, HW-NiMoN-2h is found to deliver outstanding HER activity of 1 A cm-2 at 130 mV overpotential and to exhibit excellent stability at 1 A cm-2 over 70 h in 1 M KOH seawater.

Multistage interfacial engineering of 3D carbonaceous Ni2P nanospheres/nanoflowers derived from Ni-BTC metal-organic frameworks for overall water splitting

The regulation of the multi-dimensional interface plays an important role in optimizing the electron transport and gas mass transfer during catalysis, which is conducive to promoting the electrocatalytic process. Herein, a self-supporting electrode has been developed with the multistage interface within 3D Ni₂P@C nanospheres/nanoflowers arrays derived from metal-organic frameworks (MOFs) as template skeletons and precursors. The constructed nanosphere interface protrudes outward to optimize the contact with the electrolyte while the nanoflower lamellar connection promotes rapid electron transfer and exposes more active sites, and accelerates the gas diffusion with the abundant interspace channels. According to theoretical calculation, the synergistic effect between Ni₂P and C is conducive to the optimal adsorption and desorption of H*, thus contributing to the improvement of catalytic kinetics. With the optimized growth times assembled onto nickel foam substrates, the Ni₂P@C-12 h requires overpotentials of only 69 mV and 205 mV to drive the current density of 10 mA cm^-2 towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. And it reveals an ultralow cell voltage of 1.55 V at 10 mA cm^-2 to achieve overall water splitting (OWS). In addition, the stability of the Ni₂P@C/NF electrocatalyst emerges as prominent long-term stability, which is attributed to the carbonaceous nanosphere anchors on the substrate to minimize the possibility of oxidation of the catalyst surface. This strategy of in situ growth of MOF-derived phosphates provides a general idea for interfacial engineering modification of OWS electrode materials.

A General Strategy for Synthesis of Binary Transition Metal Phosphides Hollow Sandwich Heterostructures

The hollow sandwich core-shell micro-nanomaterials are widely used in materials, chemistry, and medicine, but their fabrication, particularly for transition metal phosphides (TMPs), remains a great challenge. Herein, a general synthesis strategy is presented for binary TMPs hollow sandwich heterostructures with vertically interconnected nanosheets on the inside and outside surfaces of polyhedron FeCoP_x /C, demonstrated by a variety of transition metals (including Co, Fe, Cd, Mn, Cu, Cr, and Ni). Density functional theory (DFT) calculation reveals the process and universal mechanism of layered double hydroxide (LDH) growth on Prussian blue analog (PBA) surface in detail for the first time, which provides the theoretical foundations for feasibility and rationality of the synthesis strategy. This unique structure exhibits a vertical nanosheet-shell-vertical nanosheet configuration combining the advantages of sandwich, hollow and vertical heterostructures, effectively achieving their synergistic effect. As a proof-of-concept of their applications, the CoNiP_x @FeCoP_x /C@CoNiP_x hollow sandwich polyhedron architectures (representative samples) show excellent catalytic performance for the oxygen evolution reaction (OER) in alkaline electrolytes. This work provides a general method for constructing hollow-sandwich micro-nanostructures, which provides more ideas and directions for design of micro-nano materials with special geometric topology.

Heterostructured Ultrathin Two-Dimensional Co-FeOOH Nanosheets@1D Ir-Co(OH)F Nanorods for Efficient Electrocatalytic Water Splitting

It is highly desirable to develop high-performance and robust electrocatalysts for overall water splitting, as the existing electrocatalysts exhibit poor catalytic performance toward hydrogen and oxygen evolution reactions (HER and OER) in the same electrolytes, resulting in high cost, low energy conversion efficiency, and complicated operating procedures. Herein, a heterostructured electrocatalyst is realized by growing Co-ZIF-67-derived 2D Co-doped FeOOH on 1D Ir-doped Co(OH)F nanorods, denoted as Co-FeOOH@Ir-Co(OH)F. The Ir-doping couples with the synergy between Co-FeOOH and Ir-Co(OH)F effectively modulate the electronic structures and induce defect-enriched interfaces. This bestows Co-FeOOH@Ir-Co(OH)F with abundant exposed active sites, accelerated reaction kinetics, improved charge transfer abilities, and optimized adsorption energies of reaction intermediates, which ultimately boost the bifunctional catalytic activity. Consequently, Co-FeOOH@Ir-Co(OH)F exhibits low overpotentials of 192/231/251 and 38/83/111 mV at current densities of 10/100/250 mA cm^-2 toward the OER and HER in a 1.0 M KOH electrolyte, respectively. When Co-FeOOH@Ir-Co(OH)F is used for overall water splitting, cell voltages of 1.48/1.60/1.67 V are required at current densities of 10/100/250 mA cm^-2. Furthermore, it possesses outstanding long-term stability for OER, HER, and overall water splitting. Our study provides a promising way to prepare advanced heterostructured bifunctional electrocatalysts for overall alkaline water splitting.

Rational Synthesis of Core-Shell-Structured Nickel Sulfide-Based Nanostructures for Efficient Seawater Electrolysis

Versatile electrocatalysis at higher current densities for natural seawater splitting to produce hydrogen demands active and robust catalysts to overcome the severe chloride corrosion, competing chlorine evolution, and catalyst poisoning. Hereto, the core-shell-structured heterostructures composed of amorphous NiFe hydroxide layer capped Ni₃ S₂ nanopyramids which are directly grown on nickel foam skeleton (NiS@LDH/NF) are rationally prepared to regulate cooperatively electronic structure and mass transport for boosting oxygen evolution reaction (OER) performance at larger current densities. The prepared NiS@LDH/NF delivers the anodic current density of 1000 mA cm^-2 at the overpotential of 341 mV in 1.0 m KOH seawater. The feasible surface reconstruction of Ni₃ S₂ -FeNi LDH interfaces improves the chemical stability and corrosion resistance, ensuring the robust electrocatalytic activity in seawater electrolytes for continuous and stable oxygen evolution without any hypochlorite production. Meanwhile, the designed Ni₃ S₂ nanopyramids coated with FeNi₂ P layer (NiS@FeNiP/NF) still exhibit the improved hydrogen evolution reaction (HER) activity in 1.0 m KOH seawater. Furthermore, the NiS@FeNiP/NF||NiS@LDH/NF pair requires cell voltage of 1.636 V to attain 100 mA cm^-2 with a 100% Faradaic efficiency, exhibiting tremendous potential for hydrogen production from seawater.

Tube-Sponge-Inspired Hierarchical Electrocatalysts with Boosted Mass and Electron Transfer for Efficient Oxygen Evolution

Hindered gas bubble release and limited electron conducting process represent the major bottlenecks for large-scale electrochemical water splitting. Both the desorption of bubbles and continuous electron transport are achievable on the surfaces of biomimetic catalytic materials by designing multiscale structural hierarchy. Inspired by the tubular structures of the deep-sea sponges, an exceptionally active and binder-free porous nickel tube arrays (PNTA) decorated with NiFe-Zn2+ -pore nanosheets (NiFe-PZn ) are fabricated. The PNTA facilitate removal of bubbles and electron transfer in the oxygen evolution reaction by reproducing trunks of the sponges, and simultaneously, the NiFe-PZn increase the number of catalytic active sites by simulating the sponge epidermis. With improved external mass transfer and interior electron transfer, the hierarchical NiFe-PZn @PNTA electrode exhibits superior oxygen evolution reaction performance with an overpotential of 172 mV at 10 mA cm-2 (with a Tafel slope of 50 mV dec-1 ). Furthermore, this electrocatalytic system recorded excellent reaction stability over 360 h with a constant current density of 100 mA cm-2 at the potential of 1.52 V (versus RHE). This work provides a new strategy of designing hierarchical electrocatalysts for highly efficient water splitting.

Heterostructured ZnCo2O4-CoOOH nanosheets on Ni foam for a high performance bifunctional alkaline water splitting catalyst

It is of utmost importance to explore bifunctional electrocatalysts for water splitting. Herein, unique ZnCo₂O₄-CoOOH heterostructured ultrathin nanosheets on Ni foam are reported that combines a two-step hydrothermal method. This catalyst exhibits excellent catalytic performances to achieve a current density of 10 mA cm^-2 with an ultralow overpotential of 115 mV for HER, attaining an overpotential of 238 mV at 20 mA cm^-2 for OER. Remarkably, ZnCo₂O₄-CoOOH/Ni shows a voltage of 1.494 V to drive a current density of 10 mA cm^-2. Such performances are due to the inter-penetrative pores present in the ultrathin nanosheets that provide large surface areas and expose massive active sites to enhance activities. In addition, the unique nanosheet structure and the 3D Ni foam substrate possess large specific surface areas, which can facilitate mass diffusion. This excellent performance is ascribed to the ZnCo₂O₄-CoOOH heterostructure that manipulates strong synergy to improve the electrochemical activity. This study offers new insight on an innovative approach for the exploitation of effective bifunctional electrocatalysts with a heterostructure.

Design Strategies for Large Current Density Hydrogen Evolution Reaction

Hydrogen energy is considered one of the cleanest and most promising alternatives to fossil fuel because the only combustion product is water. The development of water splitting electrocatalysts with Earth abundance, cost-efficiency, and high performance for large current density industrial applications is vital for H2 production. However, most of the reported catalysts are usually tested within relatively small current densities (< 100 mA cm-2), which is far from satisfactory for industrial applications. In this minireview, we summarize the latest progress of effective non-noble electrocatalysts for large current density hydrogen evolution reaction (HER), whose performance is comparable to that of noble metal-based catalysts. Then the design strategy of intrinsic activities and architecture design are discussed, including self-supporting electrodes to avoid the detachment of active materials, the superaerophobicity and superhydrophilicity to release H2 bubble in time, and the mechanical properties to resist destructive stress. Finally, some views on the further development of high current density HER electrocatalysts are proposed, such as scale up of the synthesis process, in situ characterization to reveal the micro mechanism, and the implementation of catalysts into practical electrolyzers for the commercial application of as-developed catalysts. This review aimed to guide HER catalyst design and make large-scale hydrogen production one step further.

Recent Advances in Design of Electrocatalysts for High-Current-Density Water Splitting

Electrochemical water splitting technology for producing "green hydrogen" is important for the global mission of carbon neutrality. Electrocatalysts with decent performance at high current densities play a central role in the industrial implementation of this technology. This field has advanced immensely in recent years, as witnessed by many types of catalysts designed and synthesized toward industriallyrelevant current densities (>200 mA cm^-2 ). By discussing recent advances in this field, several key aspects are summarized that affect the catalytic performance for high-current-density electrocatalysis, including dimensionality of catalysts, surface chemistry, electron transport path, morphology, and catalyst-electrolyte interplay. The multiscale design strategy that considers these aspects comprehensively for developing high-current-density electrocatalysts are highlighted. The perspectives on the future directions in this emerging field are also put forward.

Nanoscale Heterogeneities of Non-Noble Iron-Based Metallic Glasses toward Efficient Water Oxidation at Industrial-Level Current Densities

Scaling up the production of cost-effective electrocatalysts for efficient water splitting at the industrial level is critically important to achieve carbon neutrality in our society. While noble-metal-based materials represent a high-performance benchmark with superb activities for hydrogen and oxygen evolution reactions, their high cost, poor scalability, and scarcity are major impediments to achieve widespread commercialization. Herein, a flexible freestanding Fe-based metallic glass (MG) with an atomic composition of Fe₅₀Ni₃₀P₁₃C₇ was prepared by a large-scale metallurgical technique that can be employed directly as a bifunctional electrode for water splitting. The surface hydroxylation process created unique structural and chemical heterogeneities in the presence of amorphous FeOOH and Ni₂P as well as nanocrystalline Ni₂P that offered various active sites to optimize each rate-determining step for water oxidation. The achieved overpotentials for the oxygen evolution reaction were 327 and 382 mV at high current densities of 100 and 500 mA cm^-2 in alkaline media, respectively, and a cell voltage of 1.59 V was obtained when using the MG as both the anode and the cathode for overall water splitting at a current density of 10 mA cm^-2. Theoretical calculations unveiled that amorphous FeOOH makes a significant contribution to water molecule adsorption and oxygen evolution processes, while the amorphous and nanocrystalline Ni₂P stabilize the free energy of hydrogen protons (ΔG_H*) in the hydrogen evolution process. This MG alloy design concept is expected to stimulate the discovery of many more high-performance catalytic materials that can be produced at an industrial scale with customized properties in the near future.

Oxygen Vacancy and Core-Shell Heterojunction Engineering of Anemone-Like CoP@CoOOH Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Constructing cost-efficient and robust bifunctional electrocatalysts for both neutral and alkaline water splitting is highly desired, but still affords a great challenge, due to sluggish hydrogen/oxygen evolution reaction (HER/OER) kinetics. Herein, an in situ integration engineering strategy of oxygen-vacancy and core-shell heterojunction to fabricate an anemone-like CoP@CoOOH core-shell heterojunction with rich oxygen-vacancies supported on carbon paper (CoP@CoOOH/CP), is described. Benefiting from the synergy of CoP core and oxygen-vacancy-rich CoOOH shell, the as-obtained CoP@CoOOH/CP catalyst displays low overpotentials at 10 mA cm^-2 for HER (89.6 mV/81.7 mV) and OER (318 mV/200 mV) in neutral and alkaline media, respectively. Notably, a two-electrode electrolyzer, using CoP@CoOOH/CP as bifunctional catalyst to achieve 10 mA cm^-2 , only needs low-cell voltages in neutral (1.65 V) and alkaline (1.52 V) electrolyte. Besides, systematically experimental and theoretical results reveal that the core-shell heterojunction efficiently accelerates the catalytic kinetics and strengthens the structural stability, while rich oxygen-vacancies efficiently decrease the kinetic barrier and activation energy, and reduce the energy barrier of the rate-determining-step for OER intermediates, thus intrinsically boosting OER performance. This work clearly demonstrates that oxygen-vacancy and core-shell heterojunction engineering provide an effective strategy to design highly-efficient non-precious, bi-functional electrocatalysts for pH-universal water splitting.

Engineering transition metal catalysts for large-current-density water splitting

Electrochemical water splitting plays a crucial role in transferring electricity to hydrogen fuel and appropriate electrocatalysts are crucial to satisfy the strict industrial demand. However, the successfully developed non-noble metal catalysts have a small tested range and the current density is usually less than 100 mA cm^-2, which is still far away from the practical application standards. Aiming to provide guidance for the fabrication of more advanced electrocatalysts with a large current density, we herein systematically summarize the recent progress achieved in the field of cost-efficient and large-current-density electrocatalyst design. Beginning by illustrating the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) mechanisms, we elaborate on the concurrent issues of non-noble metal catalysts that are required to be addressed. In view of large-current-density operating conditions, some distinctive features with regard to good electrical conductivity, high intrinsic activity, rich active sites, and porous architecture are also summarized. Next, some representative large-current-density electrocatalysts are classified. Finally, we discuss the challenges associated with large-current-density water electrolysis and future pathways in the hope of guiding the future development of more efficient non-noble-metal catalysts to boost large-scale hydrogen production with less electricity consumption.

Single-Atomic Ruthenium Active Sites on Ti3 C2 MXene with Oxygen-Terminated Surface Synchronize Enhanced Activity and Selectivity for Electrocatalytic Nitrogen Reduction to Ammonia

Downsizing the catalyst to atom scale offers an effective way to maximize the atom utilization efficiency for electrocatalytic nitrogen reduction reaction (NRR). Herein, single-atomic ruthenium (Ru) anchored on a chemically activated Ti₃ C₂ with O-terminated groups (Ti₃ C₂ O) was designed to catalyze the NRR process. The catalyst achieved a superior activity and selectivity with ammonia yield rate of 27.56 μg h^-1  mg^-1 and faradaic efficiency of 23.3 % at a low potential of -0.20 V versus the reversible hydrogen electrode. According to the atomic resolution images from aberration-corrected scanning transmission electron microscopy, Ru sites on Ti₃ C₂ O achieved good dispersion on atomic scale. X-ray photoelectron spectroscopy analysis further demonstrated that the O-termination groups were successfully activated. Density functional theory calculations combined with experiments revealed that single Ru sites binding to four oxygen were the main reaction centers that permitted the hydrogenation of *NNH₂ to *NHNH₂ in a novel distal/alternating hybrid path while reducing the energy barrier of the potential-limiting step to 0.78 eV from 0.96 eV in the distal path alone or 1.18 eV in the alternating path alone, thereby significantly promoting the NRR dynamics.

Vanadium doped FeP nanoflower with optimized electronic structure for efficient hydrogen evolution

Reasonable design of hydrogen evolution reaction (HER) electrocatalyst from the perspective of electronic structure is a vital way to optimize the catalytic activity. Mono-metallic iron-based phosphates have been shown to be active toward HER, but their performance remains unsatisfactory despite their abundant reserves and low preparation cost. Here, guided by the d-band center and band structure theories, V-doped FeP nanoflower grown directly on iron foam are constructed. Combining the density functional theory (DFT) simulations with physical characterizations reveal that the enhanced HER activity is mainly attributed to the lowed d-band central position, increased water dissociation capacity, decreased hydrogen formation energy barrier and reduced charge transfer impedance. As a HER catalyst in 1 M KOH, the obtained V-FeP shows low overpotentials of ∼149, ∼246 and ∼290 mV to deliver the current densities of 100, 500 and 1000 mA cm^-2 with at least 24 h. When coupled with other highly active oxygen evolution reaction (OER) catalyst (NiFe-LDH/IF), the NiFe-LDH/IF⁽⁺⁾ || V-FeP/IF^(-) pair also performs a low cell voltage and over 100-h stability at high current density of 1000 mA cm^-2, which endows it a large potential in the practical electrolytic water industry. Our work may provide a reference for the enhancement of inert and low-cost HER-active iron phosphide.

Recent Progress on Polyvinyl Alcohol-Based Materials for Energy Conversion

Electrocatalytic energy conversion shows a promising “bridge” to mitigate energy shortage issues and minimizes the ecological implications by synergy with the sustainable energy sources, which calls for low-cost, highly active,...

Ultrahigh-Current-Density and Long-Term-Durability Electrocatalysts for Water Splitting

Hydrogen economy is imagined where excess electric energy from renewable sources stored directly by electrochemical water splitting into hydrogen is later used as clean hydrogen fuel. Electrocatalysts with the superhigh current density (1000 mA cm^-2 -level) and long-term durability (over 1000 h), especially at low overpotentials (<300 mV), seem extremely critical for green hydrogen from experiment to industrialization. Along the way, numerous innovative ideas are proposed to design high efficiency electrocatalysts in line with industrial requirements, which also stimulates the understanding of the mass/charge transfer and mechanical stability during the electrochemical process. It is of great necessity to summarize and sort out the accumulating knowledge in time for the development of laboratory to commercial use in this promising field. This review begins with examining the theoretical principles of achieving high-efficiency electrocatalysts with high current densities and excellent durability. Special attention is paid to acquaint efficient strategies to design perfect electrocatalysts including atomic structure regulation for electrical conductivity and reaction energy barrier, array configuration constructing for mass transfer process, and multiscale coupling for high mechanical strength. Finally, the importance and the personal perspective on future opportunities and challenges, is highlighted.

Environmentally benign general synthesis of nonconsecutive carbon-coated RuP2 porous microsheets as efficient bifunctional electrocatalysts under neutral conditions for energy-saving H2 production in hybrid water electrolysis

A nonconsecutive carbon-coated RuP2 porous microsheet (RuP2@InC-MS) with bifunctionality for HzOR and HER is realized. DFT calculations evidence that C is more thermoneutral for HER while Ru boosts the dehydrogenation kinetics during HzOR process.

Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition

Constructing heterojunction is an effective strategy to develop high-performance non-precious-metal-based catalysts for electrochemical water splitting (WS). Herein, we design and prepare an N-doped-carbon-encapsulated Ni/MoO2 nano-needle with three-phase heterojunction (Ni/MoO2@CN) for accelerating the WS under industrial alkaline condition. Density functional theory calculations reveal that the electrons are redistributed at the three-phase heterojunction interface, which optimizes the adsorption energy of H- and O-containing intermediates to obtain the best ΔGH* for hydrogen evolution reaction (HER) and decrease the ΔG value of rate-determining step for oxygen evolution reaction (OER), thus enhancing the HER/OER catalytic activity. Electrochemical results confirm that Ni/MoO2@CN exhibits good activity for HER (ƞ-10 = 33 mV, ƞ-1000 = 267 mV) and OER (ƞ10 = 250 mV, ƞ1000 = 420 mV). It shows a low potential of 1.86 V at 1000 mA cm-2 for WS in 6.0 M KOH solution at 60 °C and can steadily operate for 330 h. This good HER/OER performance can be attributed to the three-phase heterojunction with high intrinsic activity and the self-supporting nano-needle with more active sites, faster mass diffusion, and bubbles release. This work provides a unique idea for designing high efficiency catalytic materials for WS.

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm^-2 needs a cell voltage range of 1.8-2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Single WTe2 Sheet-Based Electrocatalytic Microdevice for Directly Detecting Enhanced Activity of Doped Electronegative Anions

The high electrical conductivity of 1T'-WTe₂ deserves particular attention and may show a high potential for hydrogen evolution reaction (HER) catalysis. However, the actual activity certainly does not match expectations, and the inferior HER activity is actually still ambiguous at the atomic level. Unraveling the underlying HER behaviors of 1T'-WTe₂ will give rise to a new family of HER catalysts. Our structural analysis reveals that the inferior activity could result from insufficient charge density around the Te site and blocked adsorption channel at the W site, which cause too weak hydrogen adsorption. Herein, we fabricated a single WTe₂ sheet-based electrocatalytic microdevice for directly extracting enhanced HER activity of doped electronegative F atoms. The overpotential at -10 mA cm^-2 reduced to 0.27 V after F doping compared to 0.45 V for the original state. In situ electrochemical measurement and electrical tests on a single sheet indicate that doped F can regulate surface charge and hydrogen adsorption behavior. Furthermore, the theory simulation uncovers that the smaller atomic radius of F contributes to an empty coordination environment; meanwhile, strong electronegativity induces hydrogen adsorption. Thus, the ΔG_H^* at W sites around the doped F is as low as 0.18 eV. Synergistically modulating the charge properties and opening steric hindrance provides a new pathway to rationally construct electrocatalysts and beyond.

N-Doped Graphene-Decorated NiCo Alloy Coupled with Mesoporous NiCoMoO Nano-sheet Heterojunction for Enhanced Water Electrolysis Activity at High Current Density

Developing highly effective and stable non-noble metal-based bifunctional catalyst working at high current density is an urgent issue for water electrolysis (WE). Herein, we prepare the N-doped graphene-decorated NiCo alloy coupled with mesoporous NiCoMoO nano-sheet grown on 3D nickel foam (NiCo@C-NiCoMoO/NF) for water splitting. NiCo@C-NiCoMoO/NF exhibits outstanding activity with low overpotentials for hydrogen and oxygen evolution reaction (HER: 39/266 mV; OER: 260/390 mV) at ± 10 and ± 1000 mA cm-2. More importantly, in 6.0 M KOH solution at 60 °C for WE, it only requires 1.90 V to reach 1000 mA cm-2 and shows excellent stability for 43 h, exhibiting the potential for actual application. The good performance can be assigned to N-doped graphene-decorated NiCo alloy and mesoporous NiCoMoO nano-sheet, which not only increase the intrinsic activity and expose abundant catalytic activity sites, but also enhance its chemical and mechanical stability. This work thus could provide a promising material for industrial hydrogen production.

Ni2P nanoflakes for the high-performing urea oxidation reaction: linking active sites to a UOR mechanism

Urea electrolysis is regarded as an effective method for addressing both energy and environment issues. Herein, we successfully synthesized Ni2P nanoflakes for catalyzing the urea oxidation reaction (UOR). Due to the higher electrical conductivity as well as the prevailing tendency in triggering the UOR via a direct electro-oxidation mechanism, Ni2P nanoflakes exhibit comparable UOR activity (1.33 V vs. RHE for onset-potential, and 95.47 mA·cm-2 at 1.6 V vs. RHE) to the most active state-of-the-art catalysts, rendering them an effective alternative to precious metals such as Pt and Rh. The accelerated proton-coupled electron transfer (PCET) process caused by PO43- facilitates the in situ generation of NiOOH; thus, the UOR process is initiated at a lower onset-potential on Ni2P nanoflakes than on β-Ni(OH)2 nanoflakes. The in situ generated NiOOH instead of the Ni2P phase in Ni2P nanoflakes functions as an active site during the UOR process, while both NiOOH and the Ni2P phase serve as active sites in the OER process. This work provides insights into the understanding of the UOR mechanism and opens a new avenue to design low-cost Ni-based phosphide UOR catalysts.

A “Superaerophobic” Se-Doped CoS2 Porous Nanowires Array for Cost-Saving Hydrogen Evolution

The pursuit of low-cost and high-efficiency catalyst is imperative for the development and utilization of hydrogen energy. Heteroatomic doping which is conducive to the redistribution of electric density is one of the promising strategies to improve catalytic activity. Herein, the Se-doped CoS2 porous nanowires array with a superaerophobic surface was constructed on carbon fiber. Due to the electronic modulation and the unique superaerophobic structure, it showed improved hydrogen evolution activity and stability in urea-containing electrolyte. At a current density of 10 mA cm−2, the overpotentials are 188 mV for hydrogen evolution reaction (HER) and 1.46 V for urea oxidation reaction (UOR). When it was set as a cell, the voltage is low as 1.44 V. Meanwhile, the current densities of HER and UOR, as well as of cell remained basically unchanged after a continuous operation for 48 h. This work opens up a new idea for designing of cost-saving hydrogen evolution electrocatalysts.

One-step synthesis of amorphous nickel iron phosphide hierarchical nanostructures for water electrolysis with superb stability at high current density

The development of noble-metal-free high-performance bifunctional catalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential but challenging for hydrogen production from water electrolysis. Herein, amorphous bimetallic nickel-iron phosphide hierarchical nanostructures enrooted on nickel-iron alloy foam (NiFeP/NFF) are facilely fabricated via direct phosphidation of NFF at low temperature and developed as an efficient self-supporting bifunctional electrocatalyst to catalyze both the OER and HER with high activity, fast kinetics and excellent stability. Moreover, an alkaline water electrolyzer simultaneously utilizing NiFeP/NFF as the cathode and anode only needs a cell voltage of 1.58 V to afford a current density of 10 mA cm-2, overpassing most of the reported bifunctional electrocatalysts and comparable to noble metal-based ones. Impressively, the NiFeP/NFF-based symmetric electrolyzer can work well without appreciable performance degradation at a high current density of 500 mA cm-2 for over 1000 h for continuous hydrogen production with 100% faradaic efficiency, showing superb durability and great promise for industrial application.