Atomic Ensemble Effects in Electrocatalysis: The Site‐Knockout Strategy

During an electrocatalytic reaction bonds are broken and formed, and this requires that the reactants, the intermediates formed at the elementary reaction steps, and the products interact with a given number of surface atoms of the catalyst. Modifying the number of groups with an adequate number of surface atoms in a suitable geometric arrangement for a determined reaction step to proceed may affect the activity and/or selectivity of the catalyst. Although separating purely geometric atomic ensemble effects from electronic effects is not straightforward, the insights extracted from a detailed investigation of atomic ensemble effects can have a profound impact in the determination of electrocatalytic reaction mechanisms and in the design of more active and more selective electrocatalysts. This Minireview illustrates, using cyanide‐modified Pt(111) electrodes as an archetype, how eliminating only one kind of site from the surface (the site‐knockout strategy) by means of a regular array of inert adsorbates can be used to successfully study atomic ensemble effects in electrocatalysis. The possible consequences for the design of more efficient and more selective electrocatalysts are also commented on.     

异原子配位结构碳基单原子电催化剂结构与性能相关性的研究进展

单原子催化剂是一类以相互孤立的单个金属原子作为催化活性中心的、 具有高原子经济性及高活性的负载型催化剂, 被广泛应用于能源电催化领域. 近年来, 通过使用两种或两种以上原子与活性中心金属原子配位, 构建具有异原子配位结构的单原子材料, 展现了优异的电催化性能. 研究发现, 这种不对称的配位结构有效调控了中心金属原子的电子结构, 优化了催化反应的吸附和脱附能量, 提高了电催化的性能. 本文综合评述了具有异原子配位结构碳基单原子电催化剂的合成策略、 表征技术与方法, 以及在前沿能源电催化应用中的催化剂性能与结构之间的构效关系, 并展望了异原子配位结构碳基单原子电催化剂的研究前景.     

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.     

Oxygen Evolution and Reduction Reaction Activity Investigations on Fe,Co or Ni embedded Tetragonal Graphene by A Thermodynamical Full-Landscape Searching Scheme

Single transition metal (TM) atoms such as Fe, Co and Ni occupying a carbon divacancy in tetragonal graphene (TG) and bonded with four nitrogen atoms (TM@N₄TG) as electrocatalysts are investigated by means of first-principles calculations. To consider the effect of solvent species on the local configuration of the active single metal, a thermodynamical full-landscape searching (TFLS) scheme is employed. The calculated thermodynamic overpotentials (η_td) from our TFLS indicate that Co@N₄TG displays high catalytic activity toward both oxygen evolution reaction (OER) and reduction reaction (ORR), with η_td^OER and η_td^ORR as 0.397 and 0.357 V, respectively. Its OER potential cannot be captured if only one four electron reaction loop (FERL) is considered. The actual active pathways do not always turn out to be the reactions starting from the bare site. Our findings demonstrate that TG is a promising support and TM confined TD can be used to design effective and cheap multifunctional electrocatalysts.     

Single‐Atom Electrocatalysts

Recent years have witnessed a dramatic increase in the production of sustainable and renewable energy. However, the electrochemical performances of the various systems are limited, and there is an intensive search for highly efficient electrocatalysts by more rational control over the size, shape, composition, and structure. Of particular interest are the studies on single‐atom catalysts (SACs), which have sparked new interests in electrocatalysis because of their high catalytic activity, stability, selectivity, and 100 % atom utilization. In this Review, we introduce innovative syntheses and characterization techniques for SACs, with a focus on their electrochemical applications in the oxygen reduction/evolution reaction, hydrogen evolution reaction, and hydrocarbon conversion reactions for fuel cells (electrooxidation of methanol, ethanol, and formic acid). The electrocatalytic performance is further considered at an atomic level and the underlying mechanisms are discussed. The ultimate goal is the tailoring of single atoms for electrochemical applications.     

Several Key Factors for Efficient Electrocatalytic Water Splitting: Active Site Coordination Environment,Morphology Changes and Intermediates Identification

Water-splitting has emerged as a promising alternative strategy to produce clean hydrogen fuel. However, current electrocatalytic water splitting suffers from sluggish kinetics, thus developing efficient electrocatalysts is crucial. Identifying reaction centers discloses the reaction mechanism and will undoubtedly facilitate the design and optimization of efficient water splitting electrocatalysts. This review summarizes several advances involving the identification of the actual active sites and intermediates capture on the catalytic surface. The morphology and valence states change on 2D materials are chose to illustrate how structural evolution affect catalytic activity. Specifically, in situ/ex situ electron microscopy techniques that used for the characterization of catalytic sites, and spectroscopy techniques that used to detect active intermediates at the molecular level are highlighted. In addition, several perspectives, such as the development of new in situ techniques and electrokinetic analysis methods, are emphasized to shed light on future research.     

Understanding electrocatalysis at nanoscale electrodes and single atoms with operando vibrational spectroscopy

Electrocatalysis presents an environmentally benign route to carry out energy conversion reactions and organic transformations. The use of nanomaterials and single atoms as catalysts in electrochemical reactions has led to improvements in activity and selectivity over bulk electrocatalysts, however, optimal design of electrocatalytic reactions requires an in-depth understanding of the dynamic solid–liquid interface, reaction intermediates, and catalyst structural changes. Tracking bond breaking and formation events in real time is an important complement to catalyst characterization. This short review highlights advancements in fundamental understanding of electrocatalysis at nanoscale and single atom electrocatalysts obtained using operando vibrational spectroscopies. To push the boundaries of our fundamental understanding in electrocatalysis, we also emphasize a need to further develop operando vibrational techniques to reduce ensemble averaging with improvements in spatial and temporal resolution.     

A comparison study on single metal atoms (Fe,Co, Ni) within nitrogen-doped graphene for oxygen electrocatalysis and rechargeable Zn-air batteries

Single atom catalysts (SACs) with atomically dispersed transition metals on nitrogen-doped carbon supports have recently emerged as highly active non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), showing great application potential in Zn-air batteries. However, because of the complex structure-performance relationships of carbon-based SACs in the oxygen electrocatalytic reactions, the contribution of different metal atoms to the catalytic activity of SACs in Zn-air batteries still remains ambiguous. In this study, SACs with atomically dispersed transition metals on nitrogen-doped graphene sheets (M-N@Gs, M = Co, Fe and Ni), featured with similar physicochemical properties and M-N@C configurations, are obtained. By comparing the on-set potentials and the maximum current, we observed that the ORR activity is in the order of Co-N@G > Fe-N@G > Ni-N@G, while the OER activity is in the order of Co-N@G > Ni-N@G > Fe-N@G. The Zn-air batteries with Co-N@G as the air cathode catalysts outperform those with the Fe-N@G and Ni-N@G. This is due to the accelerated charge transfer between Co-N@C active sites and the oxygen-containing reactants. This study could improve our understanding of the design of more efficient bifunctional electrocatalysts for Zn-air batteries at the atomic level.     

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal–air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M–N–C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal–nitrogen coordination sites (M–N_x) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

This Perspective summarizes and highlights the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the electrocatalytic oxygen reduction reaction activity.     

Recent Progress on the Synthesis and Modified Strategies of Zeolitic-Imidazole Framework-67 Towards Electrocatalytic Oxygen Evolution Reaction

As a class of metal-organic framework, the zeolitic-imidazole framework-67 is constructed from bridging cobalt ions and 2-methylimidazole. The high content of abundant active cobalt species, uniform structure, ultrahigh porosity, and large surface area show the potential for multiple catalytic applications, especially electrocatalytic oxygen evolution reaction (OER). The design and synthetic strategies of catalyst-based ZIF-67 that approach the maximized catalytic performance are still challenging in further development. Herein, the current progress strategy on the structural design, synthetic route, and functionalization of electrocatalysts based on ZIF-67 to boost the catalytic performance of OER is reviewed. Besides, the structurally designed catalyst from various fabricated strategies corresponding to enhancing catalytic activity is discussed. The emphasized review for understanding design and synthetic structure with catalytic performance could guide researchers in further developing catalyst-based ZIF-67 for improving the efficient electrocatalytic OER.     

精确可控串联策略触发高效的氧还原活性

探索高效、经济的非金属氧还原(ORR)电催化剂已成为电化学能源体系的关键.科学界最具挑战性的目标之一是通过合理地验证和精确地调节活性位点来设计结构明确、性能优异的催化剂材料.本文提出一种精确和可控的串联协同作用的活性位点策略,以提高MFCOFs的ORR催化活性.以亚胺-N、噻吩-S和三嗪-N等作为结构单元,通过精确的串...     

Design of Complex Solid‐Solution Electrocatalysts by Correlating Configuration,Adsorption Energy Distribution Patterns,and Activity Curves

Complex solid‐solution electrocatalysts (also referred to as high‐entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single‐phase solid solutions from five or more constituents, they offer unique capabilities of fine‐tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media.     

Low-Dimensional High-Entropy Alloys for Advanced Electrocatalytic Reactions

Low-dimensional high-entropy alloy (HEA) nanomaterials are widely employed as electrocatalysts for energy conversion reactions, due to their inherent advantages, including high electron mobility, rich catalytically active site, optimal electronic structure. Moreover, the high-entropy, lattice distortion, and sluggish diffusion effects also enable them to be promising electrocatalysts. A thorough understanding on the structure-activity relationships of low-dimensional HEA catalyst play a huge role in the future pursuit of more efficient electrocatalysts. In this review, we summarize the recent progress of low-dimensional HEA nanomaterials for efficient catalytic energy conversion. By systematically discussing the fundamentals of HEA and properties of low-dimensional nanostructures, we highlight the advantages of low-dimensional HEAs. Subsequently, we also present many low-dimensional HEA catalysts for electrocatalytic reactions, aiming to gain a better understanding on the structure-activity relationship. Finally, a series of upcoming challenges and issues are also thoroughly proposed as well as their future directions.     

Evaluating the electro-sensing behaviors of single-atom catalysts based on mechanistic insights

Single-atom catalysts (SACs) have been widely applied as electrocatalysts due to their excellent catalytic ability, selectivity, and stability, which are also key aspects considered for electro-sensing interfaces. Typical SACs are catalytic single atoms dispersed over oxide-, sulfide-, or carbon-based material supports. In this review, the electrocatalytic mechanisms of SACs are briefly summarized. SACs can increase the reactivity and modulate the reaction pathway via redox mediating, adsorbing to the preferred reactant/intermediate, and cooperating with other active sites either from the substrate or from a nearby heteroatom. Based on the mechanistic insights of SACs, this review aims to provide some inspirations for future applications of SACs in the design of efficient electro-sensing interfaces.     

通过表面钨掺杂大幅提升钯纳米立方体的燃料电池催化性能（英文）

发展兼具高活性和高稳定性的规整非铂电化学催化剂无论对于燃料电池的推广应用还是基础研究都具有重要意义.我们将钯纳米立方体(Pd nanocubes)作为晶种,使用表面掺杂的手段制备了一种表面结构规整的钨掺杂钯纳米立方体(W-doped Pd nanocubes).通过改变合成过程中所加入羰基钨前驱体的量以调控表面钨的原子比例,继而获得了钨原子比例分别为0%,0.8%,1.2%,1.5%的纳米立方体.所制W-doped Pd nanocubes/C催化剂在碱性条件下的氧还原反应中表现出优异性能,其中1.2%W-doped Pd nanocubes/C催化剂性能最佳,在0.9 VRHE时比活性达1.18 mA cm~(-2),质量活性达0.25 A mg~(-1)Pd,分别是商业Pt/C催化剂的4.7倍和2.5倍.研究表明,随着钨的掺杂量从0%增至1.5%,钨掺杂钯纳米立方体的d带中心从-2.49 eV逐渐降至-3.08 eV.同时,光电子能谱结果表明,随着钨掺杂量的增加,钯的3d峰位向低能逐渐偏移,说明了钨掺杂导致了电荷由钨转向钯.而d带中心的下移能够将更多的反键态拉下费米能级,继而导致反应中间体的吸附减弱.因此,由钨到钯的电荷转移导致的d带中心的下移,继而引起的反应中间体对催化剂的吸附作用变弱是氧还原催化活性增强的原因.而过高的W掺杂(1.5%)导致活性的降低也可以用Sabatier规则解释.在循环测试10000圈之后,1.2%W-doped Pd nanocubes/C催化剂的质量活性仅仅减少了14.8%,而商业Pt/C催化剂减少了40%,可见其具有极佳的稳定性.而且循环测试之后的透射电镜表征显示,相比于团聚严重的商业Pt/C催化剂,1.2%W-doped Pd nanocubes/C催化剂仍然分散良好,其形貌也几乎没有发生变化.此外,该催化剂对乙醇氧化反应也表现出优异的性能.在1.0 mol L~(-1)氢氧化钾和1.0 mol L~(-1)乙醇混合溶液中,测试峰电流达6.6 A mg~(-1)Pd,是Pd nanocubes/C催化剂的2.2倍,商业Pd/C催化剂的5.1倍.这同样得益于适量钨掺杂所导致的催化剂d带中心—下移引起的含碳中间体吸附的削弱.经过1000 s的稳定性测试,1.2%W-doped Pd nanocubes/C同样表现出高于商业Pd/C催化剂的稳定性.优异的氧还原和乙醇氧化性能表明所制1.2%W-doped Pd nanocubes/C是一种极具潜力的双功能燃料电池催化剂.     

Polyoxometalate-Based Compounds for Photo- and Electrocatalytic Applications

Photo/electrocatalysis of water (H₂O) splitting and CO₂ reduction reactions is a promising strategy to alleviate the energy crisis and excessive CO₂ emissions. For the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO₂ reduction reaction (CO₂RR) involved, the development of effective photo/electrocatalysts is critical to reduce the activation energy and accelerate the sluggish dynamics. Polyoxometalate (POM)-based compounds with tunable compositions and diverse structures are emerging as unique photo/electrocatalysts for these reactions as they offer unparalleled advantages such as outstanding solution and redox stability, quasi-semiconductor behaviour, etc. This Minireview provides a basic introduction related to photo/electrocatalytic HER, OER and CO₂RR, followed by the classification of pristine POM-based compounds toward different catalytic reactions. Recent breakthroughs in engineering POM-based compounds as efficient photo/electrocatalysts are highlighted. Finally, the advantages, challenges, strategies and outlooks of POM-based compounds on improving photo/electrocatalytic performance are discussed.     

多金属氧酸盐材料在二氧化碳电催化还原领域中的研究进展

随着工业发展和全球人口的持续增长,人类对化石燃料的消耗日益增加,从而导致大气中二氧化碳含量的显著增加以及与之相伴的一系列环境问题.电化学还原二氧化碳制备高附加值的燃料和化学品具有稳定的效率和较高的经济可行性等特点,目前已成为一种有前景的策略来缓解当前全球面临的能源短缺和气候变暖问题.然而,电催化二氧化碳还原过程存在反应能垒高和复杂的多电子/质子耦合过程等不足,因此,合理有效的电催化剂设计成为该领域的关键问题.近年,理解和明确电化学二氧化碳还原反应过程的活性起源、选择性调控机制和催化反应机理已成为高效电催化剂设计过程中的重要指导原则.作为一类独特的纳米尺度的金属氧簇,多金属氧酸盐(多酸)已成为二氧化碳还原领域的热点材料.尤其是,多酸明确的结构、优越的电子/质子存储转移能力和二氧化碳吸附活化能力有助于探究二氧化碳还原反应过程中的活性起源和构效机制.因此,利用多酸阐明电化学二氧化碳还原反应中的这些关键问题对于开发高效、可实用化的电催化剂意义重大.本文综述了近年多酸在电催化二氧化碳还原反应中取得的进展,重点介绍了多酸阴离子均相分子催化剂、多酸基无机-有机杂化材料催化剂、多酸电解质溶液、多酸-纳米复合材料在电催化二氧化碳还原反应中的应用.利用密度泛函理论结合原位实验证据推测了可能的反应机理,探讨了多酸对电催化活性和产物选择性的影响,揭示了电子/质子存储-转移过程和多酸表面修饰工程在电催化二氧化碳还原过程中的重要作用.最后,本文还分析了多酸基材料存在的问题与面临的挑战,并对多酸基材料在二氧化碳还原领域的未来发展进行了展望,这对理解电催化二氧化碳还原反应中的关键步骤和开发新型高效的电催化二氧化碳还原电催化剂具有启发意义.     

Electrocatalysis in confined space

The complex interplay of restricted mass transport leading to local accumulation or depletion of educts, intermediates, products, counterions and co-ions influences the reactions at the active sites of electrocatalysts when electrodes are rough, three-dimensionally mesoporous or nanoporous. This influence is important with regard to activity, and even more to selectivity, of electrocatalytic reactions. The underlying principles are discussed based on the growing awareness of these considerations over recent years.     

高熵纳米合金在电化学催化中的应用

The implementation of clean energy techniques, including clean hydrogen generation, use of solar-driven photovoltaic hybrid systems, photochemical heat generation as well as thermoelectric conversion, is crucial for the sustainable development of our society. Among these promising techniques, electrocatalysis has received significant attention for its ability to facilitate clean energy conversion because it promotes a higher rate of reaction and efficiency for the associated chemical transformations. Noble-metal-based electrocatalysts typically show high activity for electrochemical conversion processes. However, their scarcity and high cost limit their applications in electrocatalytic devices. To overcome this limitation, binary catalysts prepared by alloying with transition metals can be used. However, optimization of the activity of the binary catalysts is considerably limited because of the presence of the miscibility gap in the phase diagram of binary alloys. The activity of binary electrocatalysts can be attributed to the adsorption energy of molecules and intermediates on the surface. High-entropy alloys (HEAs), which consist of diverse elements in a single NP, typically exhibit better physical and/or chemical properties than their single-element counterparts, because of their tunable composition and inherent surface complexity. Further, HEAs can improve the performance of binary electrocatalysts because they exhibit a near-continuous distribution of adsorption energy. Recently, HEAs have gained considerable attention for their application in electrocatalytic reactions. This review summarizes recent research advances in HEA nanostructures and their application in the field of electrocatalysis. First, we introduce the concept, structure, and four core effects of HEAs. We believe that this part will provide the basic information about HEAs. Next, we discuss the reported top-down and bottom-up synthesis strategies, emphasizing on the carbothermal shock method, nanodroplet-mediated electrodeposition, fast moving bed pyrolysis, polyol process, and dealloying. Other methods such as combinatorial co-sputtering, ultrashort-pulsed laser ablation, ultrasonication-assisted wet chemistry, and scanning-probe block copolymer lithography are also highlighted. Among these methods, wet chemistry has been reported to be effective for the formation of nano-scale HEAs because it facilitates the concurrent reduction of all metal precursors to form solid-solution alloys. Next, we present the theoretical investigation of HEA nanocatalysts, including their thermodynamics, kinetic stability, and adsorption energy tuning for optimizing their catalytic activity and selectivity. To elucidate the structure–property relationship in HEAs, we summarize the research progress related to electrocatalytic reactions promoted by HEA nanocatalysts, including the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, methanol oxidation reaction, and CO₂ reduction reaction. Finally, we discuss the challenges and various strategies toward the development of HEAs.     

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N₄ moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O₄ configuration are transformed into the ones with Fe-N₄ configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N₄. Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.