Polymorphism-Interface-Induced Work Function Regulating on Ru Nanocatalyst for Enhanced Alkaline Hydrogen Oxidation Reaction

Exploring high-performance Pt-free electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline media is highly imperative for the development of alkaline polymer electrolyte fuel cells. Phase engineering is an effective strategy for boosting the catalytic performance of electrocatalysts; however, the fabrication of unconventional polymorphism-interfaced metal catalysts remains a significant challenge. In this study, a polymorphism-interfaced Ru nanocatalyst with a stable hexagonal close-packed (hcp) phase and a metastableface-centered-cubic (fcc) phase is successfully prepared. Owing to the built-in electric field and stacking fault on the unique polymorphic interface, the fcc-hcp-Ru catalyst exhibits outstanding alkaline HOR performance with a mass activity of 1016 A g_PGM^-1, which is six and three times higher than that of conventional hcp-Ru andcommercial Pt/C, respectively. The regulated electron distribution at the polymorphic interface is attributed to the discrepant work functions, which not only optimize the adsorption energy of hydrogen but also facilitate the water formation step to promote the alkaline HOR process. This study demonstrates that unconventional polymorphism-interfaced engineering is an efficient strategy to regulate the electronic structure of metal catalysts and identifies the prominent role of the work function in alkaline HORs, providing a new avenue for the rational design of highly efficient materials for electrocatalysis.     

Iron‐Doped Cobalt Monophosphide Nanosheet/Carbon Nanotube Hybrids as Active and Stable Electrocatalysts for Water Splitting

Developing earth‐abundant, active, and stable electrocatalysts for water splitting is a vital but challenging step for realizing efficient conversion and storage of sustainable energy. Here, a multiscale structure‐engineering approach to construct iron (Fe) doped cobalt monophosphide (CoP) hybrids for efficient electrocatalysis of water splitting is reported. A two‐step method is developed to synthesize CoP nanosheets with uniform Fe doping and hybridization with carbon nanotubes (CNTs). The nanostructuring, uniform doping, and hybridization with CNT afford efficient electrocatalysts comparable to Pt/C for hydrogen evolution reactions in acidic, neutral, and alkaline electrolytes. It is found that the Fe doping level has different effects on catalytic activities in different electrolytes. Furthermore, after in situ oxidization/hydrolysis of the phosphides to corresponding oxyhydroxides, the hybrid electrocatalysts exhibit better performances than the benchmark commercial Ir/C for catalyzing the oxygen evolution reaction. A two‐electrode alkaline water electrolyzer constructed with these hybrid electrocatalysts can afford a current density of 10 mA cm^?2 at a voltage of 1.5 V.     

Rational Design of Nanoarray Architectures for Electrocatalytic Water Splitting

Electrochemical water splitting is recognized as a practical strategy for impelling the transformation of sustainable energy sources such as solar energy from electricity to clean hydrogen fuel. To actualize the large‐scale hydrogen production, it is paramount to develop low‐cost, earth‐abundant, efficient, and stable electrocatalysts. Among those electrocatalysts, alternative architectural arrays grown on conductive substrates have been proven to be highly efficient toward water splitting due to large surface area, abundant active sites, and synergistic effects between the electrocatalysts and the substrates. Herein, the advancement of nanoarray architectures in electrocatalytic applications is reviewed. The categories of different nanoarrays and the reliable and versatile synthetic approaches of electrocatalysts are summarized. A unique emphasis is highlighted on the promising strategies to enhance the electrocatalytic activities and stability of architectural arrays by component manipulation, heterostructure regulation, and vacancy engineering. The intrinsic mechanism analysis of electronic structure optimization, intermediates' adsorption facilitation, and coordination environments' amelioration is also discussed with regard to theoretical simulation and in situ identification. Finally, the challenges and opportunities on the valuable directions and promising pathways of architectural arrays toward outstanding electrocatalytic performance are provided in the energy conversion field, facilitating the development of promising water splitting systems.     

面向电解水的激光制备微纳米结构催化电极

相对于使用化石燃料的制氢方式,电解水不存在碳排放,是一种真正绿色环保的制氢技术,对发展氢能源具有重要意义。电解水的能耗和成本都较大,需要使用高效稳定的非贵金属催化剂,以降低过电压。激光具有高效、灵活、非接触、高度可控等优点,近年来已成为制备电解水催化剂的有效工具,但在一体化微纳米结构催化电极的制备方面存在不足之处。本文基于激光微纳制备方法,总结了激光液相合成粉末催化剂和激光制备自支撑微纳米结构催化电极的最新研究进展,并讨论了该领域未来的研究方向。     

Dual‐Phase Engineering of Nickel Boride‐Hydroxide Nanoparticles toward High‐Performance Water Oxidation Electrocatalysts

The development of earth‐abundant and efficient oxygen evolution reaction (OER) electrocatalysts is necessary for green hydrogen production. The preparation of efficient OER electrocatalysts requires both the adsorption sites and charge transfer on the catalyst surface to be suitably engineered. Herein, the design of an electrocatalyst is reported with significantly enhanced water oxidation performance via dual‐phase engineering, which displays a high number of adsorption sites and facile charge transfer. More importantly, a simple chemical etching process enables the formation of a highly metallic transition boride phase in conjunction with the transition metal hydroxide phase with abundant adsorption sites available for the intermediates formed in the OER. In addition, computational simulations are carried out to demonstrate the water oxidation mechanism and the real active sites in this engineered material. This research provides a new material design strategy for the preparation of high‐performance OER electrocatalysts.     

Bifunctional Heterostructured Transition Metal Phosphides for Efficient Electrochemical Water Splitting

Reducing green hydrogen production costs is essential for developing a hydrogen economy. Developing cost‐effective electrocatalysts for water electrolysis is thus of great research interest. Among various material candidates, transition metal phosphides (TMP) have emerged as robust bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to their various phases and tunable electronic structure. Recently, heterostructured catalysts have exhibited significantly enhanced activities toward HER/OER. The enhancement can be attributed to the increased amount of accessible active sites, accelerated mass/charge transfer, and optimized adsorption of intermediates, which arise from the synergistic effects of the heterostructure. Herein, a comprehensive overview of the recent progress of bifunctional TMP‐based heterostructure is introduced to provide an insight into their preparation and corresponding reaction mechanisms. It starts with summarizing general fundamental aspects of HER/OER and the synergistic effect of heterostructures for enhanced catalytic activity. Next, the innovational strategies to design and construct bifunctional TMP‐based heterostructures with enhanced overall water splitting activity, as well as the related mechanisms, are discussed in detail. Finally, a summary and perspective for further opportunities and challenges are highlighted for the further development of bifunctional TMP‐based heterostructures from the points of practical application and mechanistic studies.     

Cobalt Boron Imidazolate Framework Derived Cobalt Nanoparticles Encapsulated in B/N Codoped Nanocarbon as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

The development of efficient and low‐cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable for electrochemical energy conversion. Herein, this study puts forward a new Co decorated N,B‐codoped interconnected graphitic carbon and carbon nanotube materials (Co/NBC) synthesized by direct carbonization of a cobalt‐based boron imidazolate framework. It is demonstrated that the carbonization temperature can tune the surface structure and component of the resultant materials and optimize the electrochemically active surface area to expose more accessible active sites, effectively boosting the electrocatalytic activity. As a result, the optimized Co/NBC shows superior bifunctional catalytic activity and stability toward OER and HER in 1.0 m KOH solution. Furthermore, the catalyst can serve as both the anode and cathode for water splitting to achieve a current density of 10 mA cm^?2 at a cell voltage of 1.68 V. Experimental results and theoretical calculations indicate that the excellent activity of Co/NBC catalyst benefits from the synergistic effect of partial oxidation of metallic cobalt, conductive N,B‐codoped graphitic carbon and carbon nanotube, and the coupled interactions among these components. This work opens a promising avenue toward the exploration of boron imidazolate frameworks as efficient heteroatom‐doped catalysts for electrocatalysis.     

Graphene Paper Doped with Chemically Compatible Prussian Blue Nanoparticles as Nanohybrid Electrocatalyst

Along with reduced graphene oxide (RGO), water soluble Prussian blue nanoparticles (PBNPs, around 6 nm) are synthesized and broadly characterized. These two types of highly stable, low‐cost and chemically compatible nanomaterials are exploited as building ingredients to prepare electrically enhanced and functionally endorsed nanohybrid electrocatalysts, which are further transformed into free‐standing graphene papers. PBNPs doped graphene papers show highly efficient electrocatalysis towards reduction of hydrogen peroxide and can be used alone as flexible chemical sensors for potential applications in detection of hydrogen peroxide or/and other organic peroxides. The as‐prepared PBNPs–RGO papers are further capable of biocompatible accommodation of enzymes for development of free‐standing enzyme based biosensors. In this regard, glucose oxidase is used as an example for electrocatalytic oxidation and detection of glucose. The present work demonstrates a facile and highly reproducible way to construct free‐standing and flexible graphene paper doped with electroactive catalyst. Thanks to high stability, low‐cost and efficient electrocatalytic characteristics, this kind of nanohybrid material has potential to be produced on a large scale, and offers a broad range of possible applications, particularly in the fabrication of flexible sensing devices and as a platform for electrocatalytic energy conversion.     

Fully Solar‐Powered Uninterrupted Overall Water‐Splitting Systems

Extensive research efforts have been recently devoted to the development of self‐driven electrocatalytic water‐splitting systems to generate clean hydrogen chemical fuels. Currently, self‐driven electrocatalytic water‐splitting devices are powered by solar cells, which operate intermittently, or by aqueous batteries, which deliver stored electric power, leading to high operating costs and environmental pollution. Thus, a fully solar‐powered uninterrupted overall water‐splitting system is greatly desirable. Here, the solar cells, stable output voltage of 1.75 V Ni–Zn batteries, and high efficiency zinc–nickel–cobalt phosphide electrocatalysts are successfully assembled together to create a 24 h overall water‐splitting system. Specifically, the silicon‐based solar cells enable the charging of aqueous Ni–Zn batteries for energy storage as well as providing sufficient energy for electrocatalysis throughout the day; in addition, the high‐capacity Ni–Zn batteries offer a steady output voltage for overall water‐splitting at night. Such an uninterrupted solar‐to‐hydrogen system opens up exciting opportunities for the development and applications of renewable energy.     

Recent Progress in MXene‐Based Materials: Potential High‐Performance Electrocatalysts

The family of transition metal carbides, nitrides, and carbonitrides (collectively called MXenes) has been a thriving field since the first invention of Ti₃C₂T_x (MXene) in 2011. MXene is a new type of nanometer 2D sheet material, which exhibits great application potentials in various fields due to its multiple advantages such as high specific surface area, good electrical conductivity, and high mechanical strength. Electrocatalysis is regarded as the core of future clean energy conversion technologies, and MXene‐based materials provide inspiration for the design and preparation of electrocatalysts with high activity, high selectivity, and long loading life time. The applications of MXene‐based materials in electrocatalysis, including hydrogen evolution reaction, nitrogen reduction reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and methanol oxidation reaction are summarized in this review. As a crucial session regarding experiments, the current safer and more environmentally friendly preparation methods of MXene are also discussed. Focusing on the materials design and enhancement methods, the key challenges and opportunities for MXene‐based materials as a next‐generation platform in both fundamental research and practical electrocatalysis applications are presented. This account serves to promote future efforts toward the development of MXenes and related materials in the electrocatalysis applications.     

Bimetal Schottky Heterojunction Boosting Energy‐Saving Hydrogen Production from Alkaline Water via Urea Electrocatalysis

Hydrogen production via water electrocatalysis is limited by the sluggish anodic oxygen evolution reaction (OER) that requires a high overpotential. In response, a urea‐assisted energy‐saving alkaline hydrogen‐production system has been investigated by replacing OER with a more oxidizable urea oxidation reaction (UOR). A bimetal heterostructure CoMn/CoMn₂O₄ as a bifunctional catalyst is constructed in an alkaline system for both urea oxidation and hydrogen evolution reaction (HER). Based on the Schottky heterojunction structure, CoMn/CoMn₂O₄ induces self‐driven charge transfer at the interface, which facilitates the absorption of reactant molecules and the fracture of chemical bonds, therefore triggering the decomposition of water and urea. As a result, the heterostructured electrode exhibits ultralow potentials of ?0.069 and 1.32 V (vs reversible hydrogen electrode) to reach 10 mA cm^?2 for HER and UOR, respectively, in alkaline solution, and the full urea electrolysis driven by CoMn/CoMn₂O₄ delivers 10 mA cm^?2 at a relatively low potential of 1.51 V and performs stably for more than 15 h. This represents a novel strategy of Mott–Schottky hybrids in electrocatalysts and should inspire the development of sustainable energy conversion by combining hydrogen production and sewage treatment.     

Recent Advances in Non‐Noble Bifunctional Oxygen Electrocatalysts toward Large‐Scale Production

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial reactions in energy conversion and storage systems including fuel cells, metal–air batteries, and electrolyzers. Developing low‐cost, high‐efficiency, and durable non‐noble bifunctional oxygen electrocatalysts is the key to the commercialization of these devices. Here, based on an in‐depth understanding of ORR/OER reaction mechanisms, recent advances in the development of non‐noble electrocatalysts for ORR/OER are reviewed. In particular, rational design for enhancing the activity and stability and scalable synthesis toward the large‐scale production of bifunctional electrocatalysts are highlighted. Prospects and future challenges in the field of oxygen electrocatalysis are presented.     

Advanced Oxygen Electrocatalysis in Energy Conversion and Storage

Oxygen electrocatalysis is of great significance in electrochemical energy conversion and storage. Many strategies have been adopted for developing advanced oxygen electrocatalysts to promote these technologies. In this invited contribution, recent progress in understanding the oxygen electrochemistry from theoretical and experimental aspects is summarized. The major categories of oxygen electrocatalysts, namely, noble-metal-based compounds, transition-metal-based composites, and nanocarbons, are successively discussed for oxygen reduction and evolution. Design strategies of various oxygen electrocatalysts and their relationship on the structure–activity–performance are comprehensively addressed with the perspectives. Finally, the challenge and outlook for advanced oxygen electrocatalysts are discussed toward energy conversion and storage technologies.     

Advanced Characterization Techniques for Identifying the Key Active Sites of Gas‐Involved Electrocatalysts

Highly efficient electrocatalysts play an integral part in developing renewable energy conversion and storage technologies. Despite considerable efforts devoted to synthesizing electrocatalysts with superior performance, the identification of active moieties and understanding of reaction mechanisms under practical conditions still remain elusive. Herein, the substantial progresses in unraveling the local electronic and atomic structure optimizations of nanocatalysts for gas‐involved electrocatalysis, disclosing real active sites, and clarifying their relationships with intrinsic activities by combining advanced characterization techniques with computational simulations are summarized. The continuous development of in situ and ex situ characterization tools, particularly at multi‐scale resolution, to monitor or even directly observe the active center structure is systematically discussed, which is divided into four main categories based on the type of active sites: atomically dispersed active sites, vacancies, heteroatom doping sites, and edge sites. Current challenges and perspectives in both fundamental area and industrial application are finally proposed for the future research direction of next‐generation electrode materials. The aim of this review is to provide mechanistic insights into the real catalytically active structure with the assistance of newly developed characterization techniques, guiding the rational design and structure engineering of advanced functional materials with outstanding activity, selectivity, and durability.     

Surface Reconstruction of Ni–Fe Layered Double Hydroxide Inducing Chloride Ion Blocking Materials for Outstanding Overall Seawater Splitting

Generation of hydrogen fuel via electrochemical water splitting powered by sustainable energy, such as wind or solar energy, is an attractive path toward the future renewable energy landscape. However, current water electrolysis requires desalinated water resources, eventually leading to energy costs and water scarcity. The development of cost-effective electrocatalysts capable of splitting saline water feeds directly can be an evident solution. Herein, a surface reconstructed nickel-iron layered double hydroxide (NF-LDH) is reported as an exceptionally active and durable bifunctional electrocatalyst for saline water splitting without chloride corrosion. The surface reconstructed NF-LDH consists of Ni₃Fe alloy phase interconnected in a 2D network in which an ultrathin (≈2 nm) and low-crystalline NiFe (oxy)hydroxide phase are formed on the surface. The NiFe (oxy)hydroxide phase draws large anodic current densities, satisfying the level of practical application, while the Ni₃Fe alloy phase is simultaneously responsible for the high catalytic activity for cathodic reactions and superior corrosion resistance. The surface reconstructed NF-LDH electrode can be easily fabricated in a large electrode area (up to 25 cm²) and can successfully produce hydrogen fuels from saline water powered by the laboratory-made low-intensity photovoltaic cell.     

A Type of 1 nm Molybdenum Carbide Confined within Carbon Nanomesh as Highly Efficient Bifunctional Electrocatalyst

Highly efficient platinum‐alternative bifunctional catalysts by using abundant non‐noble metal species are of critical importance to the future sustainable energy reserves. Unfortunately, current electrocatalysts toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are far from satisfactory because of lacking reasonable design and assembly protocols. A type of 1‐nm molybdenum carbide nanoparticles confined in mesh‐like nitrogen‐doped carbon (Mo₂C@NC nanomesh) with high specific surface area is reported here. In addition to the superior ORR performance comparable to platinum, the catalyst offers a high HER activity with small Tafel slope of 33.7 mV dec^?1 and low overpotential of 36 mV to reach ?10 mA cm^?2. Theoretical calculations indicate that the active sites of the catalyst are mainly located at Mo atoms adjacent to the N‐doped carbon layer, which contributes the high HER activity. These findings show the great potential of Mo₂C species in wide electrocatalysis applications.     

Designing High-Valence Metal Sites for Electrochemical Water Splitting

Electrochemical water splitting is a critical energy conversion process for producing clean and sustainable hydrogen; this process relies on low-cost, highly active, and durable oxygen evolution reaction/hydrogen evolution reaction electrocatalysts. Metal cations (including transition metal and noble metal cations), particularly high-valence metal cations that show high catalytic activity and can serve as the main active sites in electrochemical processes, have received special attention for developing advanced electrocatalysts. In this review, heterogenous electrocatalyst design strategies based on high-valence metal sites are presented, and associated materials designed for water splitting are summarized. In the discussion, emphasis is given to high-valence metal sites combined with the modulation of the phase/electronic/defect structure and strategies of performance improvement. Specifically, the importance of using advanced in situ and operando techniques to track the real high-valence metal-based active sites during the electrochemical process is highlighted. Remaining challenges and future research directions are also proposed. It is expected that this comprehensive discussion of electrocatalysts containing high-valence metal sites can be instructive to further explore advanced electrocatalysts for water splitting and other energy-related reactions.     

MnO Enabling Highly Efficient and Stable Co-Nx/C for Oxygen Reduction Reaction in both Acidic and Alkaline Media

In situ growing transition metals on N-doped carbon by atomic doping produces a class of promising alternatives to replace Pt-based catalysts for redox reactions, yet still suffer from unsatisfactory activity and durability in acidic and basic media. Herein, a simple synthetic strategy to fabricate an MnO modifying Co-N_x/C catalyst with high activity and robust durability is presented. The interphase engineering well controls the Co and N species in the carbon matrix, affording the material with more pyridine N and graphite N; the interaction between Co-N_x and MnO phase is also well discussed. Accordingly, the obtained Co-N_x/C-MnO catalyst exhibits excellent electrocatalytic properties towards oxygen reduction reaction, achieving a half-wave potential of 0.87 and 0.66 V versus reversible hydrogen electrode in 0.1 m KOH and 0.1 m HClO₄ solutions, as well as excellent durability with only −16.9 and −12.2 mV shift after 1000 cycles, respectively. This study provides insights into the design of noble-metal-free electrocatalysts from the perspective of active sites and catalyst carriers.     

Orienting Active Crystal Planes of New Class Lacunaris Fe2PO5 Polyhedrons for Robust Water Oxidation in Alkaline and Neutral Media

Developing efficient and stable oxygen evolution reaction (OER) electrocatalysts is essential for realizing sustainable energy conversion, such as solar fuels. Although modulating active sites and electron transfer is of great significance to boost electrocatalysis activity, it still remains a big challenge to desirably actualize this goal. Herein, engineering of active sites and electronic framework is implemented via oriented modulation of crystal planes and construction of lacunaris architecture supported by ammonification‐elicited simultaneous incorporation of nitrogen and oxygen‐defect strategy. The new class porous nitrogen‐incorporated Fe₂PO₅ with oxygen‐defect (N‐Fe₂PO_5–x) polyhedron with dominantly exposed {110} reactive facets exhibits superior performance toward water oxidation, achieving current densities of 10 mA cm^?2 at quite low overpotentials of 235 and 315 mV in alkaline and neutral media, respectively. Furthermore, density functional theoretical calculations reveal the energetically favorable {110} planes of lower absorption energy of intermediates and remolding of electronic density framework arising from the ammoniated elicitation process, contributing to excellent OER performance of lacunaris N‐Fe₂PO_5–x polyhedrons. This work may offer a feasible guideline for regulating active sites and electron transfer to develop low‐cost and highly efficient OER electrocatalysts in energy conversion systems.     

Non-Platinum Group Metal Electrocatalysts toward Efficient Hydrogen Oxidation Reaction

Developing non-platinum group metal (non-PGM) electrocatalysts for the hydrogen oxidation reaction (HOR) represents the efforts towards the more economical use of hydrogen fuel cells and hydrogen energy, which has attracted tremendous attention recently. However, non-PGM electrocatalysts for the HOR are still in their early development stages as compared with the significant advances in those for the oxygen reduction reaction and hydrogen evolution reaction. Herein, this paper summarizes the recent progresses and highlights the key challenges for the rational design of non-PGM electrocatalysts, aiming to promote the development of non-PGM HOR electrocatalysts. Fundamental understandings of the HOR mechanism are firstly reviewed, where theoretical interpretations on the low HOR kinetics in alkaline media, including the hydrogen binding energy theory, the bifunctional mechanism, and the water molecule reorganization, are particularly discussed. Subsequently, progresses of typical non-PGM HOR electrocatalysts in acid and alkaline media are summarized separately. For the HOR under alkaline conditions, the superiorities and challenges of Ni-based catalysts are discussed with a particular focus as they are the most promising non-PGM electrocatalysts. Finally, this paper highlights the challenges and provide perspectives on the future development directions of non-PGM HOR electrocatalysts.