Impact of the Coordination Environment on Atomically Dispersed Pt Catalysts for Oxygen Reduction Reaction

The promise of chiral electrocatalysis for efficient and sustainable energy conversion and storage: a comprehensive review of the CISS effect and future directions

The integration of chirality, specifically through the chirality-induced spin selectivity (CISS) effect, into electrocatalytic processes represents a pioneering approach for enhancing the efficiency of energy conversion and storage systems. This review delves into the burgeoning field of chiral electrocatalysis, elucidating the fundamental principles, historical development, theoretical underpinnings, and practical applications of the CISS effect across a spectrum of electrocatalytic reactions, including the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). We explore the methodological advancements in inducing the CISS effect through structural and surface engineering and discuss various techniques for its measurement, from magnetic conductive atomic force microscopy (mc-AFM) to hydrogen peroxide titration. Furthermore, this review highlights the transformative potential of the CISS effect in addressing the key challenges of the NRR and CO2RR processes and in mitigating singlet oxygen formation in metal-air batteries, thereby improving their performance and durability. Through this comprehensive overview, we aim to underscore the significant role of incorporating chirality and spin polarization in advancing electrocatalytic technologies for sustainable energy applications.

Second-Shell N Dopants Regulate Acidic O2 Reduction Pathways on Isolated Pt Sites

Pt is a well-known benchmark catalyst in the acidic oxygen reduction reaction (ORR) that drives electrochemical O2-to-H2O conversion with maximum chemical energy-to-electricity efficiency. Once dispersing bulk Pt into isolated single atoms, however, the preferential ORR pathway remains a long-standing controversy due to their complex local coordination environment and diverse site density over substrates. Herein, using a set of carbon nanotube supported Pt-N-C single-atom catalysts, we demonstrate how the neighboring N dopants regulate the electronic structure of the Pt central atom and thus steer the ORR selectivity; that is, the O2-to-H2O2 conversion selectivity can be tailored from 10% to 85% at 0.3 V versus reversible hydrogen electrode. Moreover, via a comprehensive X-ray-radiated spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy analysis coupled with theoretical modeling, we reveal that a dominant pyridinic- and pyrrolic-N coordination within the first shell of Pt-N-C motifs favors the 4e- ORR, whereas the introduction of a second-shell graphitic-N dopant weakens *OOH binding on neighboring Pt sites and gives rise to a dominant 2e- ORR. These findings underscore the importance of the chemical environment effect for steering the electrochemical performance of single-atom catalysts.

An improvement on the electrocatalytic performance of ZIF-67 byin situself-growing CNTs on surface

Efficient and robust oxygen reduction reaction (ORR) catalysts are essential for the development of high-performance anion-exchange membrane fuel cells (AEMFC). To enhance the electrochemical performance of metal-organic frameworks of cobalt-based zeolite imidazolium skeleton (ZIF-67), this study reported a novel ZIF-67-4@CNT byin situgrowing carbon nanotubes (CNTs) on the surface of ZIF-67 via a mild two-step pyrolysis/oxidation treatment. The electrochemical results showed that the as-prepared ZIF-67-4@CNT after CTAB modification exhibited excellent catalytic activity with good stability, with Eonset, E1/2, and Ilimit, respectively were 0.98 V (versus RHE), 0.87 V (versus RHE) and 6.04 mA cm-2@1600 rpm, and a current retention rate of about 94.21% after polarized at 0.80 V for 10 000 s, which were all superior to that of the commercial 20 wt% Pt/C. The excellent ORR catalytic performance was mainly attributed to the large amount of thein situgrowing CNTs on the surface, encapsulated with a wide range of valence states of metallic cobalt.

Atomically-Thin Holey 2D Nanosheets of Defect-Engineered MoN-Mo5 N6 Composites as Effective Hybridization Matrices

The defect engineering of inorganic solids has received significant attention because of its high efficacy in optimizing energy-related functionalities. Consequently, this approach is effectively leveraged in the present study to synthesize atomically-thin holey 2D nanosheets of a MoN-Mo5 N6 composite. This is achieved by controlled nitridation of assembled MoS2 monolayers, which induced sequential cation/anion migration and a gradual decrease in the Mo valency. Precise control of the interlayer distance of the MoS2 monolayers via assembly with various tetraalkylammonium ions is found to be crucial for synthesizing sub-nanometer-thick holey MoN-Mo5 N6 nanosheets with a tunable anion/cation vacancy content. The holey MoN-Mo5 N6 nanosheets are employed as efficient immobilization matrices for Pt single atoms to achieve high electrocatalytic mass activity, decent durability, and low overpotential for the hydrogen evolution reaction (HER). In situ/ex situ spectroscopy and density functional theory (DFT) calculations reveal that the presence of cation-deficient Mo5 N6 domain is crucial for enhancing the interfacial interactions between the conductive molybdenum nitride substrate and Pt single atoms, leading to enhanced electron injection efficiency and electrochemical stability. The beneficial effects of the Pt-immobilizing holey MoN-Mo5 N6 nanosheets are associated with enhanced electronic coupling, resulting in improvements in HER kinetics and interfacial charge transfer.

Advancements in Microwave Absorption Motivated by Interdisciplinary Research

Microwave absorption materials (MAMs) are originally developed for military purposes, but have since evolved into versatile materials with promising applications in modern technologies, including household use. Despite significant progress in bench-side research over the past decade, MAMs remain limited in their scope and have yet to be widely adopted. This review explores the history of MAMs from first-generation coatings to second-generation functional absorbers, identifies bottlenecks hindering their maturation. It also presents potential solutions such as exploring broader spatial scales, advanced characterization, introducing liquid media, utilizing novel toolbox (machine learning, ML), and proximity of lab to end-user. Additionally, it meticulously presents compelling applications of MAMs in medicine, mechanics, energy, optics, and sensing, which go beyond absorption efficiency, along with their current development status and prospects. This interdisciplinary research direction differs from previous research which primarily focused on meeting traditional requirements (i.e., thin, lightweight, wide, and strong), and can be defined as the next generation of smart absorbers. Ultimately, the effective utilization of ubiquitous electromagnetic (EM) waves, aided by third-generation MAMs, should be better aligned with future expectations.

Boosting Alkaline Hydrogen Oxidation Activity of Ru Single-Atom Through Promoting Hydroxyl Adsorption on Ru/WC1- x Interfaces

The sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline conditions continue to pose a significant challenge for the practical implementation of anion-exchange membrane fuel cells. Developing single-atom catalysts can accelerate the pace of new HOR catalyst discovery for highly cost-effective and active HOR performance. However, single-atom catalysts (SACs) for the alkaline HOR have rarely been reported, and fundamental studies on the rational design of SACs are still required. Herein, the design of Ru SAC supported on the tungsten carbide (Ru SA/WC1- x ) via in situ high-temperature annealing strategy is reported. The resulting Ru SA/WC1- x catalyst exhibits remarkably enhanced HOR performance in alkaline media, a level of activity that can not be achieved with carbon-supported Ru SAC. Electrochemical results and density functional theory demonstrate that promoting the hydroxyl adsorption on Ru SA/WC1- x interfaces, which is derived from the low potential of zero charge of WC1- x support, has a significant effect on enhancing the HOR performance of SACs. This enhancement leads to 5.8 and 60.1 times higher Ru mass activity of Ru SA/WC1- x than Ru nanoparticles on carbon and Ru single-atom on N-doped carbon, respectively. This work provides new insights into the design of highly active SACs for alkaline HOR.

Bimetallic-MOF Derived Carbon with Single Pt Anchored C4 Atomic Group Constructing Super Fuel Cell with Ultrahigh Power Density And Self-Change Ability

Pursuing high power density with low platinum catalysts loading is a huge challenge for developing high-performance fuel cells (FCs). Herein, a new super fuel cell (SFC) is proposed with ultrahigh output power via specific electric double-layer capacitance (EDLC) + oxygen reduction reaction (ORR) parallel discharge, which is achieved using the newly prepared catalyst, single-atomic platinum on bimetallic metal-organic framework (MOF)-derived hollow porous carbon nanorods (PtSA /HPCNR). The PtSA-1.74 /HPCNR-based SFC has a 3.4-time higher transient specific power density and 13.3-time longer discharge time with unique in situ self-charge and energy storage ability than 20% Pt/C-based FCs. X-ray absorption fine structure, aberration-corrected high-angle annular dark-field scanning transmission electron microscope, and density functional theory calculations demonstrate that the synergistic effect of Pt single-atoms anchored on carbon defects significantly boosts its electron transfer, ORR catalytic activity, durability, and rate performance, realizing rapid " ORR+EDLC" parallel discharge mechanism to overcome the sluggish ORR process of traditional FCs. The promising SFC leads to a new pathway to boost the power density of FCs with extra-low Pt loading.

Ultrafine Pt Nanoparticles Anchored on 2D Metal-Organic Frameworks as Multifunctional Electrocatalysts for Water Electrolysis and Zinc-Air Batteries

Multifunctional electrocatalysts are crucial to cost-effective electrochemical energy conversion and storage systems requiring mutual enhancement of disparate reactions. Embedding noble metal nanoparticles in 2D metal-organic frameworks (MOFs) are proposed as an effective strategy, however, the hybrids usually suffer from poor electrochemical performance and electrical conductivity in operating conditions. Herein, ultrafine Pt nanoparticles strongly anchored on thiophenedicarboxylate acid based 2D Fe-MOF nanobelt arrays (Pt@Fe-MOF) are fabricated, allowing sufficient exposure of active sites with superior trifunctional electrocatalytic activity for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. The interfacial Fe─O─Pt bonds can induce the charge redistribution of metal centers, leading to the optimization of adsorption energy for reaction intermediates, while the dispersibility of ultrafine Pt nanoparticles contributes to the high mass activity. When Pt@Fe-MOF is used as bifunctional catalysts for water-splitting, a low voltage of 1.65 V is required at 100 mA cm-2 with long-term stability for 20 h at temperatures (65 °C) relevant for industrial applications, outperforming commercial benchmarks. Furthermore, liquid Zn-air batteries with Pt@Fe-MOF in cathodes deliver high open-circuit voltages (1.397 V) and decent cycling stability, which motivates the fabrication of flexible quasisolid-state rechargeable Zn-air batteries with remarkable performance.

Pt nanoparticles-decorated molybdenum nitrides for efficient hydrogen evolution reaction

Exploring low cost and high efficiency catalysts for hydrogen production from electrochemical water splitting is preferable and remains a significant challenge. As an alternative to Pt-based catalysts, molybdenum nitrides have attracted more attention for their hydrogen evolution reaction (HER). However, their performance is restricted due to the strong bonding of Mo-H. Herein, molybdenum nitrides with Pt-doping are fabricated to enhance the catalytic activity for HER in acidic solution. As expected, Pt (5 wt%)-MoNx delivers a low overpotential of 47 mV at a current density of 10 mA cm-2 with a high exchange current density (j0 = 0.98 mA cm-2). The superior performance is attributed to the modified electronic structure of Mo with Pt incorporation.

Performance Regulation of Single-Atom Catalyst by Modulating the Microenvironment of Metal Sites

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

Pt Single Atoms on CrN Nanoparticles Deliver Outstanding Activity and CO Tolerance in the Hydrogen Oxidation Reaction

The large-scale application of proton exchange membrane fuel cells is currently hampered by high cost of commercial Pt catalysts and their susceptibility to poisoning by CO impurities in H₂ feed. In this context, the development of CO-tolerant electrocatalysts with high Pt atom utilization efficiency for hydrogen oxidation reaction (HOR) is of critical importance. Herein, Pt single atoms are successfully immobilized on chromium nitride nanoparticles by atomic layer deposition method, denoted as Pt SACs/CrN. Electrochemical tests establish Pt SACs/CrN to be a very efficient HOR catalyst, with a mass activity that is 5.7 times higher than commercial PtRu/C. Strikingly, the excellent performance of Pt SACs/CrN is maintained after introducing 1000 ppm of CO in H₂ feed. The excellent CO-tolerance of Pt SACs/CrN is related to weaker CO adsorption on Pt single atoms. This work provides guidelines for the design and construction of active and CO-tolerant catalysts for HOR.

Magnetic Field-Assisted Construction and Enhancement of Electrocatalysts

Driven by the energy crisis and environmental pollution, developing sustainable clean energy is an effective strategy to realize carbon neutrality. Electrocatalytic reactions are crucial to sustainable energy conversion and storage technologies, and advanced electrocatalysts are required to improve the sluggish electrocatalytic reactions. The magnetic field, as a thermodynamic parameter independent of temperature and pressure, is vital in the construction of electrocatalysts and enhancement of electrocatalysis. In this Review, the recent progress of magnetic field-assisted construction of electrocatalysts and enhancement of electrocatalysis is comprehensively summarized. Originating from the structure-activity-performance relationship of electrocatalysts, the fundamentals of the magnetic field-induced construction of electrocatalysts, including the magnetocaloric effect, nucleation and growth, and phase regulation, have been illustrated. In addition, the magnetic effect on the electrocatalytic reaction, namely, the magnetothermal, magnetohydrodynamic and micro magnetohydrodynamic, Maxwell stress, Kelvin force, and spin selection effects, are discussed. Finally, the perspective and challenges for magnetic field-assisted construction of electrocatalysts and enhancement of electrocatalysis are proposed.

Robust Oxygen Reduction Electrocatalysis Enabled by Platinum Rooted on Molybdenum Nitride Microrods

Robust oxygen reduction electrocatalysis is central to renewable fuel cells and metal-air batteries. Herein, Pt nanoparticles (NPs) rooted on porous molybdenum nitride microrods (Pt/Mo₂N MRs) are rationally constructed toward the oxygen reduction reaction (ORR). Owing to the desired composition with strong electronic metal-support interactions (EMSIs) and a porous one-dimensional structure supporting ultrafine NPs, the developed Pt/Mo₂N MRs possess much higher ORR mass and specific activities than commercial Pt/C. In situ Raman and density functional theory calculations reveal that the EMSI weakens the adsorption of intermediates over Pt/Mo₂N MRs via an associative mechanism. Moreover, the porous Mo₂N support stabilizes these high activities. Impressively, a homemade zinc-air battery driven by Pt/Mo₂N MRs delivers excellent performance including a peak power density of 167 mW cm^-2 and a high rate capability that ranged from 5 to 50 mA cm^-2. This work highlights the role of EMSI in promoting robust ORR electrocatalysis, thus providing a promising approach for efficient and robust cathode materials for advanced metal-air batteries.

Self-Strained Platinum Clusters with Finite Size: High-Performance Catalysts with CO Tolerance for PEMFCs

Strained platinum-based materials with high performance have been regarded as the most promising electrocatalysts for proton exchange membrane fuel cells (PEMFCs) recently. Herein, self-strained platinum clusters with finite size (about 1 nm) are prepared by a combining liquid- and solid-phase UV irradiation cycle strategy. It started with a fresh H₂PtCl₆ solution irradiated by UV light and then mixed with a graphitized carbon, followed by the dried mixture being subjected to UV light to generate monodispersed Pt clusters on the carbon surface. The obtained platinum clusters feature narrower size distribution and higher loading on carbon, exhibiting significantly improved activity and durability, much higher than that of the-state-of-art commercial Pt/C for the oxygen reduction reaction. More importantly, the self-strained Pt clusters display a surprising CO tolerance, which can be attributed to the unique adaptive lattice compressive strain that triggers an electron enrichment phenomenon for the Pt clusters. Therefore, this stepwise UV irradiation method solves the long-standing problem of both wide size distribution and low loading of metal clusters fabricated by one-step photochemical reduction, providing a potential route for the synthesis of other metal clusters with strained structures.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

The hydrogen peroxide (H₂ O₂ ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H₂ and O₂ . It enables on-site and decentralized H₂ O₂ production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H₂ O₂ production via the 2e^- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O₂ to H₂ O₂ reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e^- ORR process is aimed to provide. Due to the oxidative nature of H₂ O₂ and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H₂ O₂ are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

Highly Durable Heterogeneous Atomic Catalysts

ConspectusSingle-atom catalysts (SACs), in which surface metal atoms are isolated on the surface of a support, have received a tremendous amount of attention recently because this structure would utilize precious metals fully, without occluding atoms inside nanoparticles, and enable unique surface reactions which typical nanoparticle catalysts cannot induce. Various synthesis methods and characterization techniques have been reported that yield enhanced activity and selectivity. The single-atom structures were realized on various supports such as metal oxide/carbide/nitride, porous materials derived from zeolite or metal-organic frameworks, and carbon-based materials. Additionally, when the metal atoms are isolated on other metal nanoparticles, this material is denoted as a single-atom alloy (SAA). The single-atom structure, however, cannot catalyze the surface reaction that necessitates ensemble sites, where several metal atoms are located nearby. Very recently, ensemble catalysts, in which all of the metal atoms are exposed at the surface with neighboring metal atoms, have been reported, overcoming the limitation of single-atom catalysts. We call all of these materials (SACs, SAAs, and ensemble catalyst) heterogeneous atomic catalysts, indicating that the surface metal atomic structure is intentionally controlled. To use these atomic catalysts for practical applications, high durability should be guaranteed, which has received relatively less attention.In this Account, we discuss recent examples of heterogeneous atomic catalysts with high durability. Structural stability, indicating whether the surface atomic structure is thermodynamically stable, should be carefully considered. Typically, metal atoms are immobilized on a highly defective support, stabilizing both the metal atom and the support. The surface metal atoms might become destabilized upon the adsorption of chemical intermediates. This transient behavior should be carefully monitored; density functional theory (DFT) calculations are particularly useful in estimating this stability. Aside from structural stability, the catalyst performance can be degraded significantly by poisoning with impurities. If the single-atom sites are susceptible to impurities with stronger adsorption, the surface reaction would not occur efficiently, leading to a decrease in activity without structure degradation. A long-term durability test should be performed for target reactions. Heterogeneous atomic catalysts have been used for various electrochemical, photocatalytic, and thermal reactions. Although electricity, light, and heat are just different forms of energy, the specific conditions which the catalyst should satisfy are different. Whereas precious metal atoms are mostly used as surface-active sites, the properties of the support are different depending on the type of reaction. For example, the support should have high conductivity for electrochemical reactions, it should be able to absorb light for photocatalytic reactions, and it should be durable at high temperature in the presence of steam for thermal reactions. Highly durable heterogeneous atomic catalysts are certainly possible with a great potential for practical applications. These new catalysts can accelerate the current paradigm shift toward more sustainable chemical production.

Selectively anchoring single atoms on specific sites of supports for improved oxygen evolution

The homogeneity of single-atom catalysts is only to the first-order approximation when all isolated metal centers interact identically with the support. Since the realistic support with various topologies or defects offers diverse coordination environments, realizing real homogeneity requires precise control over the anchoring sites. In this work, we selectively anchor Ir single atoms onto the three-fold hollow sites (Ir₁/T_O–CoOOH) and oxygen vacancies (Ir₁/V_O–CoOOH) on defective CoOOH surface to investigate how the anchoring sites modulate catalytic performance. The oxygen evolution activities of Ir₁/T_O–CoOOH and Ir₁/V_O–CoOOH are improved relative to CoOOH through different mechanisms. For Ir₁/T_O–CoOOH, the strong electronic interaction between single-atom Ir and the support modifies the electronic structure of the active center for stronger electronic affinity to intermediates. For Ir₁/V_O–CoOOH, a hydrogen bonding is formed between the coordinated oxygen of single-atom Ir center and the oxygenated intermediates, which stabilizes the intermediates and lowers the energy barrier of the rate-determining step.

While single-atom catalysts offer well-defined structures, the homogeneity of the active sites is determined by localized coordination environments. Here, authors anchor Ir single atoms onto different sites on CoOOH and show how their distinct coordinations activate oxygen-evolving electrocatalysis

Anchoring Highly Dispersed Pt Electrocatalysts on TiOx with Strong Metal-Support Interactions via an Oxygen Vacancy-Assisted Strategy as Durable Catalysts for the Oxygen Reduction Reaction

Pt electrocatalysts with high activity and durability have still crucial issues for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). In this study, a novel catalyst consisting of Pt nanoparticles (NPs) on TiO_x/C composites (TiO_x-V_o-H/C) with abundant oxygen vacancies (V_o) is proposed, which is abbreviated as PTO-V_o-H/C. The introduction of V_o helps anchor highly dispersed Pt NPs with low loading and strengthen the strong metal-support interaction (SMSI), which benefits to the enhanced ORR catalytic activity. Moreover, the accelerated durability test (ADT) demonstrates the higher retention of ORR activity for PTO-V_o-H/C. Experimental and theoretical analyses reveal that electronic interactions between Pt NPs and TiO_x/C composite support give rise to an electron-rich Pt NPs and strong SMSI effect, which is favorable for the electron transfer and stabilization of Pt NPs. More importantly, the assembled PEMFC with PTO-V_o-H/C shows only 6.9% of decay on maximum power density after 3000 ADT cycles while the performance of Pt/C sharply decreased. This work provides a new insight into the unique vacancy regulation of dispersive Pt on metal oxides for superior ORR performance.

Interactions between atomically dispersed copper and phosphorous species are key for the hydrochlorination of acetylene

Vinyl chloride, the monomer of polyvinyl chloride (PVC), is industrially synthesized via acetylene hydrochlorination. Thereby, easy to sublimate but toxic mercury chloride catalysts are widely used. It is imperative to find environmentally friendly non-mercury catalysts to promote the green production of PVC. Low-cost copper-based catalysts are promising candidates. In this study, phosphorus-doped Cu-based catalysts are prepared. It is shown that the type of phosphorus configuration and the distribution on the surface of the carrier can be adjusted by changing the calcination temperature. Among the different phosphorus species, the formed P-C bond plays a key role. The coordination structure formed by the interaction between P-C bonds and atomically dispersed Cu²⁺ species results in effective and stable active sites. Insights on how P-C bonds activate the substrate may provide ideas for the design and optimization of phosphorus-doped catalysts for acetylene hydrochlorination.

Crystalline Phase Induced Raman Enhancement on Molybdenum Carbide

Crystalline phase can greatly influence the Raman enhancement on semiconductor materials. Here, we demonstrate the crystalline phase induced Raman enhancement on molybdenum carbide materials (β-Mo2C and α-MoC). From all the...

Support Effects in Electrocatalysis and Their Synchrotron Radiation-Based Characterizations

Electrocatalysis is recognized as a significant process for energy conversion. In fact, numerous factors, including the variable electronic structure of electrocatalysts, rich intermediates, and mutable active phases, have important but complex influences on the catalytic process. In addition, the support of electrocatalysts is considered as one of key factors that correlate to the final catalytic performance. In this Perspective, some recent advances regarding the support effects in electrocatalysis are briefly summarized. Synchrotron radiation-based characterizations are introduced to reveal the support-induced modulation in electrocatalysts. Recent in situ/operando studies are emphasized for better understanding of the real interaction between catalysts and support, together with visualizing the dynamic catalytic process. Some perspectives are proposed that may accelerate more attention being given to the support's optimization for future practical applications.

Engineering Active Sites of Gold-Cuprous Oxide Catalysts for Electrocatalytic Oxygen Reduction Reaction

Understanding how the catalyst morphology influences surface sites is crucial for designing active and stable catalysts and electrocatalysts. We here report a new approach to this understanding by decorating gold (Au) nanoparticles on the surface of cuprous oxides (Cu₂O) with three different shape morphologies (spheres, cubes, and petals). The Au-Cu₂O particles are dispersed onto carbon nanotube (CNT) matrix with high surface area, stability, and conductivity for oxygen reduction reaction. A clear morphology-dependent enhancement of the electrocatalytic activity is revealed. Oxygenated gold species (AuO^-) are found to coexist with Au⁰ on the cube and petal catalysts, whereas only Au⁰ species are present on the sphere catalyst. The AuO^- species function effectively as active sites, resulting in the improved catalytic performance by changing the reaction mechanism. The enhanced catalytic performance of the petal-shaped catalyst in terms of onset potential, half-wave potential, diffusion-limited current density, and stability is closely associated with the presence of the most abundant AuO^- species on its surface. Highly active AuO^- species are identified on the surface of the catalysts as a result of the unique structural characteristics, which is attributed to the structural origin of high activity and stability. This insight constitutes the basis for assessing the detailed correlation between the morphology and the electrocatalytic properties of the nanocomposite catalysts, which has implications for the design of surface-active sites on metal/metal oxide electrocatalysts.

Anchoring Sites Engineering in Single-Atom Catalysts for Highly Efficient Electrochemical Energy Conversion Reactions

Single-atom catalysts (SACs) have been at the frontier of research field in catalysis owing to the maximized atomic utilization, unique structures and properties. The atomically dispersed and catalytically active metal atoms are necessarily anchored by surrounding atoms. As such, the structure and composition of anchoring sites significantly influence the catalytic performance of SACs even with the same metal element. Significant progress has been made to understand structure-activity relationships at an atomic level, but in-depth understanding in precisely designing highly efficient SACs for the targeted reactions is still required. In this review, various anchoring sites in SACs are summarized and classified into five different types (doped heteroatoms, defect sites, surface atoms, metal sites, and cavity sites). Then, their impacts on catalytic performance are elucidated for electrochemical reactions based on their distance from the metal center (first coordination shell and beyond). Further, SACs anchored on two typical types of hosts, carbon- and metal-based materials, are highlighted, and the effects of anchoring points on achieving the desirable atomic structure, catalytic performance, and reaction pathways are elaborated. At last, insights and outlook to the SAC field based on current achievements and challenges are presented.

Single-atom catalyst for high-performance methanol oxidation

Single-atom catalysts have been widely investigated for several electrocatalytic reactions except electrochemical alcohol oxidation. Herein, we synthesize atomically dispersed platinum on ruthenium oxide (Pt₁/RuO₂) using a simple impregnation-adsorption method. We find that Pt₁/RuO₂ has good electrocatalytic activity towards methanol oxidation in an alkaline media with a mass activity that is 15.3-times higher than that of commercial Pt/C (6766 vs. 441 mA mg^‒1_Pt). In contrast, single atom Pt on carbon black is inert. Further, the mass activity of Pt₁/RuO₂ is superior to that of most Pt-based catalysts previously developed. Moreover, Pt₁/RuO₂ has a high tolerance towards CO poisoning, resulting in excellent catalytic stability. Ab initio simulations and experiments reveal that the presence of Pt‒O_3f (3-fold coordinatively bonded O)‒Ru_cus (coordinatively unsaturated Ru) bonds with the undercoordinated bridging O in Pt₁/RuO₂ favors the electrochemical dehydrogenation of methanol with lower energy barriers and onset potential than those encountered for Pt‒C and Pt‒Ru.

It is still challenging to engineer single-atom catalysts for electrocatalytic methanol oxidation. Here, the authors design Pt single atom supported on RuO2 for highly active methanol oxidation in contrast to the inert Pt single atom supported on carbon.

High-Performance MoC Electrocatalyst for Hydrogen Evolution Reaction Enabled by Surface Sulfur Substitution

Molybdenum carbides have been expected to be one of the promising catalysts for the hydrogen evolution reaction (HER) due to their similar d-band electronic structures to the Pt-group metals. However, the weaker hydrogen-adsorption ability of MoC severely hinders its applications. Guided by density functional theory calculations, we put forward a strategy to design the novel MoC-based electrocatalyst with surface reconstruction through sulfur doping. The incorporation of minor sulfur not only greatly increases the number of active sites and intrinsic activity but also optimizes the electronic structure to improve the electron transfer efficiency. As a result, the as-prepared sulfur-substituted MoC tackles the limitation of the Volmer step and exhibits superior HER performance with a small Tafel slope of 48 mV dec^-1. Theoretical investigations demonstrate that the terminal sulfur plays a critical role in facilitating a close to zero hydrogen adsorption energy (ΔG_H*) and a lower hydrogen release barrier.

Quasi-Paired Pt Atomic Sites on Mo2 C Promoting Selective Four-Electron Oxygen Reduction

Atomically dispersed Pt species are advocated as a promising electrocatalyst for the oxygen reduction reaction (ORR) to boost noble metal utilization efficiency. However, when assembled on various substrates, isolated Pt single atoms are often demonstrated to proceed through the two-electron ORR pathway due to the unfavorable O─O bond cleavage thermodynamics in the absence of catalytic ensemble sites. In addition, although their distinct local coordination environments at the exact single active sites are intensively explored, the interactions and synergy between closely neighboring single atom sites remain elusive. Herein, atomically dispersed Pt monomers strongly interacting on a Mo₂ C support is demonstrated as a model catalyst in the four-electron ORR, and the beneficial interactions between two closely neighboring and yet non-contiguous Pt single atom sites (named as quasi-paired Pt single atoms) are shown. Compared to isolated Pt single atom sites, the quasi-paired Pt single atoms deliver a superior mass activity of 0.224 A mg^-1 _Pt and near-100% selectivity toward four-electron ORR due to the synergistic interaction from the two quasi-paired Pt atom sites in modulating the binding mode of reaction intermediates. Our first-principles calculations reveal a unique mechanism of such quasi-paired configuration for promoting four-electron ORR.

Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of the Oxygen Reduction Reaction

Rational control of the coordination environment of atomically dispersed catalysts is pivotal to achieve desirable catalytic reactivity. We report the reversible control of coordination structure in atomically dispersed electrocatalysts via ligand exchange reactions to reversibly modulate their reactivity for oxygen reduction reaction (ORR). The CO-ligated atomically dispersed Rh catalyst exhibited ca. 30-fold higher ORR activity than the NH_x -ligated catalyst, whereas the latter showed three times higher H₂ O₂ selectivity than the former. Post-treatments of the catalysts with CO or NH₃ allowed the reversible exchange of CO and NH_x ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. DFT calculations revealed that more reduced oxidation state of CO-ligated Rh site could further stabilize the *OOH intermediate, facilitating the two- and four-electron pathway ORR. The reversible ligand exchange reactions were generalized to Ir- and Pt-based catalysts.

Construction of Dual-Site Atomically Dispersed Electrocatalysts with Ru-C5 Single Atoms and Ru-O4 Nanoclusters for Accelerated Alkali Hydrogen Evolution

Rationally integrating multi-active sites into one ideal catalyst is an effective approach to accelerate multistep reactions by synergic catalysis. Herein, a universal and facile room temperature impregnation strategy is developed to construct Ru atomically dispersed catalyst (Ru ADC) with Ru-C₅ single atoms and Ru oxide nanoclusters (≈1.5 nm), which can also be extended to prepare Ir, Rh, Pt, Au, and Mo atomically dispersed catalysts (ADCs). It is found that the obtained Ru ADC largely boosts alkali hydrogen evolution by concerted catalysis between single atoms and sub-nanoclusters, which only needs an overpotential of 18 mV at 10 mA cm^-2 . Further mechanistic studies reveal that Ru-C₅ single atoms and Ru oxide nanoclusters with Ru-O₄ configuration in one catalyst can synergically boost water molecule capture, water dissociation, and hydrogen release. This study opens up a simple method to synthesize dual-site metal ADCs for synergic catalysis of typical multistep reactions.

Engineering the Coordination Sphere of Isolated Active Sites to Explore the Intrinsic Activity in Single-Atom Catalysts

Reducing the dimensions of metallic nanoparticles to isolated, single atom has attracted considerable attention in heterogeneous catalysis, because it significantly improves atomic utilization and often leads to distinct catalytic performance. Through extensive research, it has been recognized that the local coordination environment of single atoms has an important influence on their electronic structures and catalytic behaviors. In this review, we summarize a series of representative systems of single-atom catalysts, discussing their preparation, characterization, and structure-property relationship, with an emphasis on the correlation between the coordination spheres of isolated reactive centers and their intrinsic catalytic activities. We also share our perspectives on the current challenges and future research promises in the development of single-atom catalysis. With this article, we aim to highlight the possibility of finely tuning the catalytic performances by engineering the coordination spheres of single-atom sites and provide new insights into the further development for this emerging research field.

Structural Evolution and Underlying Mechanism of Single-Atom Centers on Mo2C(100) Support during Oxygen Reduction Reaction

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using Mo₂C(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the Mo₂C(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the Mo₂C(100) surface (TM@Mo₂C(100)_O* term for SAC). While the intermediate in 2 e^- ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e^- ORR can damage Rh@ and Cu@Mo₂C(100)_O*. Furthermore, on the basis of these findings, we propose Pt@Mo₂C(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e^- ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.

Prototype Atomically Dispersed Supported Metal Catalysts: Iridium and Platinum

When metal nanoparticles on supports are made smaller and smaller-to the limit of atomic dispersion-they become cationic and take on new catalytic properties that are only recently being discovered. The synthesis of these materials is reviewed, including their structure characterization-especially by atomic-resolution electron microscopy and X-ray absorption and infrared spectroscopies-and relationships between structure and catalyst performance, for reactions including hydrogenations, oxidations, and the water gas shift. Structure determination is challenging because of the intrinsic nonuniformity of the support surfaces-and therefore the structures on them-but fundamental understanding has advanced rapidly, benefiting from nearly uniform catalysts consisting of metals on well-defined-crystalline-supports and their characterization by spectroscopy and microscopy. Recent advances in atomic-resolution electron microscopy have spurred the field, providing stunning images and deep insights into structure. The iridium catalysts have typically been made from organoiridium precursors, opening the way to understanding and control of the metal-support bonding and ligands on the metal, including catalytic reaction intermediates. Platinum catalysts are usually made with less precision, from salt precursors, but they catalyze a wider array of reactions than the iridium, typically being stable at higher temperatures and seemingly offering rich prospect for discovery of new catalysts.

Synergistic Modulation of Carbon-Based, Precious-Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Developing alternatives to noble-metal-based catalysts toward the oxygen reduction reaction (ORR) process plays a key role in the application of low-temperature fuel cells. Carbon-based, precious-metal-free electrocatalysts are of great interest due to their low cost, abundant sources, active catalytic performance, and long-term stability. They are also supposed to feature intrinsically high activity and highly dense catalytic sites along with their sufficient exposure, high conductivity, and high chemical stability, as well as effective mass transfer pathways. In this Review, we focus on carbon-based, precious-metal-free nanocatalysts with synergistic modulation of active-site species and their exposure, mass transfer, and charge transport during the electrochemical process. With this knowledge, perspectives on synergistic modulation strategies are proposed to push forward the development of Pt-free ORR catalysts and the wide application of fuel cells.

Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis

Plasmonic nanomaterials coupled with catalytically active surfaces can provide unique opportunities for various catalysis applications, where surface plasmons produced upon proper light excitation can be adopted to drive and/or facilitate various chemical reactions. A brief introduction to the localized surface plasmon resonance and recent design and fabrication of highly efficient plasmonic nanostructures, including plasmonic metal nanostructures and metal/semiconductor heterostructures is given. Taking advantage of these plasmonic nanostructures, the following highlights summarize recent advances in plasmon-driven photochemical reactions (coupling reactions, O₂ dissociation and oxidation reactions, H₂ dissociation and hydrogenation reactions, N₂ fixation and NH₃ decomposition, and CO₂ reduction) and plasmon-enhanced electrocatalytic reactions (hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, alcohol oxidation reaction, and CO₂ reduction). Theoretical and experimental approaches for understanding the underlying mechanism of surface plasmon are discussed. A proper discussion and perspective of the remaining challenges and future opportunities for plasmonic nanomaterials and plasmon-related chemistry in the field of energy conversion and storage is given in conclusion.

Advanced Platinum-Based Oxygen Reduction Electrocatalysts for Fuel Cells

ConspectusFuel cells are among the cutting-edge energy technologies. Their commercial development is still hindered by noble platinum (Pt) catalysts for the oxygen reduction reaction (ORR) at the cathode, which not only determine the energy conversion efficiency and service life but also are closely related to the cost and broad application of fuel cells. Given the bright and enormous future of fuel cells, ORR catalysts should possess highly efficient performance yet meet the acceptable Pt costs for large-scale application. Extensive efforts are concentrated on the optimization of Pt-based nanostructures and upgradation of functional carriers to achieve the low-cost and high-activity Pt-based catalysts. By improving the Pt utilization and accessible surface, reducing Pt consumption and catalyst costs, accelerating mass exchange and electron transfer, alleviating the corrosion and agglomeration of carriers and Pt, accompanying with the assistance of robust yet effective functional supports, the service level and life of Pt-based electrocatalysts would be significantly improved and fuel cells could get into commercial market covering broader applications.In this Account, we focus on the recent development of Pt-based catalysts to figure out the problems associated with ORR catalysts in fuel cells. Recent development of Pt-based catalysts is discussed in different stages: (1) multiscale development of Pt-based nanostructures; (2) multielement regulation over Pt-based alloy composition; (3) upgradation of carbon and noncarbon support architectures; (4) development of integrated Pt-based catalysts for fuel cells. Finally, we propose some future issues (such as reaction mechanism, dynamic evolutions, and structure-activity relationship) for Pt-based catalysts, which mainly involve the preparation strategy of Pt-integrated catalysts (combination of Pt nanostructures with nanocarbons), performance evaluation (standard measurement protocols, laboratory-level rotating disk electrode (RDE) measurements, application-level membrane electrode assembly (MEA) service test), advanced interpretation techniques (spectroscopy, electron microscopy, and in situ monitoring), and cutting-edge simulation/calculations and artificial intelligence (simulation, calculations, machine learning, big data screening). This Account calls for the comprehensive development of multiscale, multicomponent, and high-entropy Pt-based alloy nanostructures, and novel and stable carriers, which provide more available options for rational design of low-cost and high-performance Pt-integrated ORR catalysts. More importantly, it will give an in-depth understanding of the reaction mechanism, dynamic development, and structure-performance relationship for Pt-based catalysts in fuel cells and related energy technologies.