Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

Fuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.     

Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution,Oxygen Evolution Reactions,and Full Water Splitting

The sustainable and scalable production of hydrogen through hydrogen evolution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state‐of‐the‐art electrocatalysts of Pt and IrO₂/RuO₂ exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. Therefore, the development of alternative effective electrocatalysts for water splitting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster‐based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed.     

Ruthenium‐Based Single‐Atom Alloy with High Electrocatalytic Activity for Hydrogen Evolution

Highly efficient and stable catalysts for the hydrogen evolution reaction, especially in alkaline conditions are crucial for the practical demands of electrochemical water splitting. Here, the synthesis of a novel RuAu single‐atom alloy (SAA) by laser ablation in liquid is reported. The SAA exhibits a high stability and a low overpotential, 24 mV@10 mA cm^?2, which is much lower than that of a Pt/C catalyst (46 mV) in alkaline media. Moreover, the turnover frequency of RuAu SAA is three times that of Pt/C catalyst. Density functional theory computation indicates the excellent catalytic activity of RuAu SAAs originates from the relay catalysis of Ru and Au active sites. This work opens a new avenue toward high‐performance SAAs via fast quenching of immiscible metals.     

Sequential Synthesis and Active‐Site Coordination Principle of Precious Metal Single‐Atom Catalysts for Oxygen Reduction Reaction and PEM Fuel Cells

Carbon‐supported precious metal single‐atom catalysts (PM SACs) have shown promising application in proton exchange membrane fuel cells (PEMFCs). However, the coordination principle of the active site, consisting of one PM atom and several coordinating anions, is still unclear for PM SACs. Here, a sequential coordination method is developed to dope a large amount of PM atoms (Ir, Rh, Pt and Pd) into a zeolite imidazolate framework (ZIF), which are further pyrolyzed into nitrogen‐coordinated PM SACs. The PM loadings are as high as 1.2–4.5 wt%, achieving the highest PM loadings in ZIF‐derived SACs to date. In the acidic half‐cell, Ir₁‐N/C and Rh₁‐N/C exhibit much higher oxygen reduction reaction (ORR) activities than nanoparticle catalysts Ir/C and Rh/C. In the contrast, the activities of Pd₁‐N/C and Pt₁‐N/C are considerably lower than Pd/C and Pt/C. Density function theory (DFT) calculations reveal that the ORR activity of PM SAC depends on the match between the OH* adsorption on PM and the electronegativity of coordinating anions, and the stronger OH* adsorption is, the higher electronegativity is needed for the coordinating anions. PEMFC tests confirm the active‐site coordination principle and show the extremely high atomic efficiency of Ir₁‐N/C. The revealed principle provides guidance for designing future PM SACs for PEMFCs.     

Tailoring Active Sites in Mesoporous Defect‐Rich NC/Vo‐WON Heterostructure Array for Superior Electrocatalytic Hydrogen Evolution

Tailoring active sites in earth‐abundant non‐noble metal electrocatalysts are required toward widespread applications in sustainable energy fields. Herein, an integrated mesoporous heterostructure array is reported by a hydrogenation/nitridation‐induced in situ growth strategy. Highly conductive oxygen‐vacancies‐rich tungsten oxynitride (V_o‐WON) nanorod array acts as the backbone encapsulated by ultrathin nitrogen‐doped carbon (NC) nanolayers, forming high‐quality shell/core NC/V_o‐WON heterostructures. Density functional theory calculations reveal that defect‐rich heterostructure arrays not only enhance the conductivity and modulate electronic structure but also promote the adsorption and dissociation of reactants and offer substantial potential sites. As expected, porous NC/V_o‐WON array exhibits a small overpotential of 16 mV at the current density of 10 mA cm^?2 and a low Tafel slope of 33 mV per decade in alkaline media, accompanied by negligible loss upon a large current density over 100 h. Benefiting from outstanding electrocatalytic hydrogen evolution reaction performance and stability, this defective heterostructure could serve as a prominent alternative electrocatalyst for renewable energy applications.     

Mesoporous MoO3–x Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions

A unique approach for the synthesis of nonstoichiometric, mesoporous molybdenum oxide (MoO_3–x) with nanosized crystalline walls by using a soft template (PEO‐b‐PS) synthesis method is introduced. The as‐synthesized mesoporous MoO_3–x is very active and stable (durability > 12 h) for the electrochemical hydrogen evolution reaction (HER) under both acidic and alkaline conditions. The intrinsic MoO₃ serves as an HER electrocatalyst without the assistance of carbon materials, noble metals, or MoS₂ materials. The results from transmission electron microscopy and N₂ sorption techniques show that the as‐synthesized mesoporous MoO_3–x has large accessible pores (20–40 nm), which are able to facilitate mass transport and charge transfer during HER. In terms of X‐ray diffraction, X‐ray photoelectron spectroscopy, temperature‐programmed oxidation, and diffusive reflectance UV–vis spectroscopy, the mesoporous MoO_3–x exhibits mixed oxidation states (Mo⁵⁺, Mo⁶⁺) and an oxygen‐deficient structure. The as‐synthesized MoO_3–x only requires a low overpotential (≈0.14 V) to achieve a 10 mA cm^?2 current density in 0.1 m KOH and the Tafel slope is as low as 56 mV dec^?1. Density functional theory calculations demonstrate a change of electronic structure and the possible reaction pathway of HER. Oxygen vacancies and mesoporosity serve as key factors for excellent performance.     

A Review of Phosphide‐Based Materials for Electrocatalytic Hydrogen Evolution

Hydrogen evolution by means of electrocatalytic water‐splitting is pivotal for efficient and economical production of hydrogen, which relies on the development of inexpensive, highly active catalysts. In addition to sulfides, the search for non‐noble metal catalysts has been mainly directed at phosphides due to the superb activity of phosphides for hydrogen evolution reaction (HER) and their low‐cost considering the abundance of the non‐noble constituents of phosphides. Here, recent research focusing on phosphides is summarized based on their synthetic methodology. A comparative study of the catalytic activity of different phosphides towards HER is then conducted. The catalytic activity is evaluated by overpotentials at fixed current density, Tafel slope, turnover frequency, and the Gibbs free energy of hydrogen adsorption. Based on the methods discussed, perspectives for the various methods of phosphides synthesis are given, and the origins of the high activity and the role of phosphorus on the improved activity towards HER are discussed.     

Accelerating Atomic Catalyst Discovery by Theoretical Calculations‐Machine Learning Strategy

Atomic catalysts (AC) are emerging as a highly attractive research topic, especially in sustainable energy fields. Lack of a full picture of the hydrogen evolution reaction (HER) impedes the future development of potential electrocatalysts. In this work, the systematic investigation of the HER process in graphdyine (GDY) based AC is presented in terms of the adsorption energies, adsorption trend, electronic structures, reaction pathway, and active sites. This comprehensive work innovatively reveals GDY based AC for HER covering all the transition metals (TM) and lanthanide (Ln) metals, enabling the screening of potential catalysts. The density functional theory (DFT) calculations carefully explore the HER performance beyond the comparison of sole H adsorption. Therefore, the screened catalysts candidates not only match with experimental results but also provide significant references for novel catalysts. Moreover, the machine learning (ML) technique bag‐tree approach is innovatively utilized based on the fuzzy model for data separation and converse prediction of the HER performance, which indicates a similar result to the theoretical calculations. From two independent theoretical perspectives (DFT and ML), this work proposes pivotal guidelines for experimental catalyst design and synthesis. The proposed advanced research strategy shows great potential as a general approach in other energy‐related areas.     

High‐Performance Hydrogen Evolution by Ru Single Atoms and Nitrided‐Ru Nanoparticles Implanted on N‐Doped Graphitic Sheet

The most efficient electrocatalyst for the hydrogen evolution reaction (HER) is a Pt‐based catalyst, but its high cost and nonperfect efficiency hinder wide‐ranging industrial/technological applications. Here, an electrocatalyst of both ruthenium (Ru) single atoms (SAs) and N‐doped‐graphitic(G_N)‐shell‐covered nitrided‐Ru nanoparticles (NPs) (having a Ru‐N_x shell) embedded on melamine‐derived G_N matrix { 1 : [Ru(SA)+Ru(NP)@RuN_x@G_N]/G_N}, which exhibits superior HER activity in both acidic and basic media, is presented. In 0.5 m H₂SO₄/1 m KOH solutions, 1 shows diminutive “negative overpotentials” (?η = |η| = 10/7 mV at 10 mA cm^?2, lowest ever) and high exchange current densities (4.70/1.96 mA cm^?2). The remarkable HER performance is attributed to the near‐zero free energies for hydrogen adsorption/desorption on Ru(SAs) and the increased conductivity of melamine‐derived G_N sheets by the presence of nitrided‐Ru(NPs). The nitridation process forming nitrided‐Ru(NPs), which are imperfectly covered by a G_N shell, allows superb long‐term operation durability. The catalyst splits water into molecular oxygen and hydrogen at 1.50/1.40 V (in 0.1 m HClO₄/1 m KOH), demonstrating its potential as a ready‐to‐use, highly effective energy device for industrial applications.     

A New Platinum‐Like Efficient Electrocatalyst for Hydrogen Evolution Reaction at All pH: Single‐Crystal Metallic Interweaved V8C7 Networks

Exploring low‐cost hydrogen evolution reaction (HER) catalysts with remarkable activity over wide pH range (0–14) still remains an enormous challenge. Herein, for the first time, a novel platinum‐like, double‐deck carbon coated V₈C₇ networks with the highly active (110) facet exposed as a new efficient HER electrocatalyst is reported. The single‐crystal interweaved V₈C₇ networks are designed and fabricated based on a low crystal‐mismatch strategy and confinement effect of double‐deck carbon coating. In addition, electrochemical tests and theoretical simulation indicate that the metallic character of V₈C₇, high‐activity of exposed facet, and low barrier energy for water dissociation can contribute to highly catalytic activity of HER. Impressively, the HER performances of the interweaved V₈C₇ networks can be comparable to those of Pt at an all‐pH environment, with Tafel slopes of 44, 64, and 34.5 mV dec^?1and overpotential of 47, 77, and 38 mV at ?10 mA cm^?2 in 1 m KOH, 0.1 m phosphate buffer, and 0.5 m H₂SO₄, respectively. This work provides a blueprint for exploring new‐type platinum‐like catalysts for various energy conversion systems.     

Experimental and Theoretical Assessment of Ni‐Based Binary Compounds for the Hydrogen Evolution Reaction

Metallic binary compounds have emerged in recent years as highly active and stable electrocatalysts toward the hydrogen evolution reaction. In this work, the origin of their high activity from a theoretical and experimental point of view is elucidated. Here, different metallic ceramics as Ni₃S₂, Ni₃N, or Ni₅P₄ are grown directly on Ni support in order to avoid any contaminations. The correlation of theoretical calculations with detailed material characterization and electrochemical testing paves the way to a deeper understanding of possible active adsorption sites for each material and the observed catalytic activity. It is shown that heteroatoms as P, S, and N actively take part in the reaction and do not act as simple spectator. Due to the anisotropic nature of the materials, a variety of adsorption sites with highly coverage‐dependent properties exists, leading to a general shift in hydrogen adsorption free energies ΔG _H close to zero. Extending the knowledge gained about the here described materials, a new catalyst is prepared by modifying a high surface Ni foam, for which current densities up to 100 mA cm^?2 at around 0.15 V (for Ni₃N) are obtained.     

Wrinkled Rh2P Nanosheets as Superior pH‐Universal Electrocatalysts for Hydrogen Evolution Catalysis

Searching for highly efficient and durable electrocatalysts for the hydrogen evolution reaction (HER) that function effectively at all pHs is of great interest to the scientific community, however it is still a grand challenge, because the HER kinetics of Pt in alkaline solutions are approximately two to three orders of magnitude lower than that in acidic solution. Herein, a new class of wrinkled, ultrathin Rh₂P nanosheets for enhancing HER catalysis at all pHs is reported. They exhibit a small overpotential of 18.3 mV at 10 mA cm^?2, low Tafel slope of 61.5 mV dec^?1, and good durability in alkaline media, much better than the commercial Pt/C catalyst. Density functional theory calculations reveal that the active open‐shell effect from the P‐3p band not only promotes Rh‐4d for increased proton–electron charge exchange but also provides excellent p–p overlapping to locate the O‐related species as distributary center, which can benefit the HER process in alkaline media. It is also demonstrated that the present wrinkled, ultrathin Rh₂P nanosheets are highly efficient and durable electrocatalysts toward HER in both acid and neutral electrolytes. The present work opens a new material design for ultrathin 2D metal phosphide nanostructures for the purpose of boosting HER performance at all pHs.     

Structural Regulation and Support Coupling Effect of Single‐Atom Catalysts for Heterogeneous Catalysis

Single atom catalysts (SACs) that integrate the merits of homogeneous and heterogeneous catalysts have been attracting considerable attention in recent years. The individual metal atoms of SACs can be stabilized on supports through various unsaturated chemical sites or space confinement for achieving the maximized atom utilization efficiency. Aside from the development of strategies for preparing high loading and high purity SACs, another key challenge in this field is precisely manipulating the geometric and electronic structure of catalytically active single metal sites, thus rendering the catalysts exceptionally reactive, selective, and stabile compared to their bulk counterparts. This review summarizes recent advancements in SACs for heterogeneous catalysis from the perspective of local structural regulation and the synergistic coupling effect between metal species and supports. Special emphasis is placed on the elucidation of the catalytic structure‐performance relationship in terms of coordination environment, valence state and metal‐support interactions by advanced characterization and theoretical studies. Select in situ or operando characterization techniques for tracking the SACs’ structure evolution under realistic conditions are highlighted. Finally, the challenges and opportunities are discussed to offer insight into the rational design of more intriguing SACs with high activity and distinct chemoselectivity.     

Atomic‐Local Environments of Single‐Atom Catalysts: Synthesis,Electronic Structure,and Activity

Single‐atom catalysts (SACs) hold great promise for maximizing atomic efficiency of supported metals via the ultimate utilization of every single atom. The foreign isolated substitutions anchored on different supports build varieties of local structural centers, changing the physical and chemical properties. Thus, distinct atomic local environments for single‐atom catalysts are essential for determining superior catalytic performance for a wide variety of chemical reactions. The examples of synthesizing single atoms on various supports presented here deepen the understanding of the different structural and electronic properties of SACs, in which the metal single atom does not bind with any other atoms of this metal, but substantially interacts with the support ions. Due to the strong support effects, the ubiquitous aggregation of metal single atoms can be addressed, achieving highly stable SACs. This review discusses recent progress in theoretical electronic effects between atomic dopants and supports, which reveal the electronic structures of various SACs and offers guidance for rational prediction and design of highly stable and reactive SACs. It is argued that tuning this interaction by the selection of the supports toward favorable atomic and electronic structures on the surface should be taken into consideration for the development of more efficient SACs.     

Single‐Atom Catalysts: Emerging Multifunctional Materials in Heterogeneous Catalysis

Supported metal nanoparticles are the most widely investigated heterogeneous catalysts in catalysis community. The size of metal nanostructures is an important parameter in influencing the activity of constructed catalysts. Especially, as coordination unsaturated metal atoms always work as the catalytically active centers, decreasing the particle size of the catalyst can greatly boost the specific activity per metal atom. Single‐atom catalysts (SACs), containing single metal atoms anchored on supports, represent the utmost utilization of metallic catalysts and thus maximize the usage efficiency of metal atom. However, with the decreasing of particle size, the surface free energy increases obviously, and tends to aggregate into clusters or particles. Selection of an appropriate support is necessary to interact with isolated atoms strongly, and thus prevents the movement and aggregation of isolated atoms, creating stable, finely dispersed active sites. Furthermore, with uniform single‐atom dispersion and well‐defined configuration, SACs afford great space for optimizing high selectivity and activity. In this review, a detailed discussion of preparing, characterizing, and catalytically testing within this family is provided, including the theoretical understanding of key aspects of SACs materials. The main advantages of SACs as catalysts and the challenges faced for further improving catalytic performance are also highlighted.     

A Multisite Strategy for Enhancing the Hydrogen Evolution Reaction on a Nano‐Pd Surface in Alkaline Media

The hydrogen evolution reaction (HER) on a noble metal surface in alkaline media is more sluggish than that in acidic media due to the limited proton supply. To promote the reaction, it is necessary to transform the alkaline HER mechanism via a multisite catalyst, which has additional water dissociation sites to improve the proton supply to an optimal level. Here, this study reports a top‐down strategy to create a multisite HER catalyst on a nano‐Pd surface and how to further fine‐tune the areal ratio of the water dissociation component to the noble metal surface in core/shell‐structured nanoparticles (NPs). Starting with Pd/Fe₃O₄ core/shell NPs, electrochemical cycling is used to tune the coverage of iron (oxy)hydroxide on a Pd surface. The alkaline HER activity of the core/sell Pd/FeO_x (OH)_2?2x NPs exhibits a volcano‐shaped correlation with the surface Fe species coverage. This indicates an optimum coverage level where the rates of both the water dissociation step and the hydrogen formation step are balanced to achieve the highest efficiency. This multisite strategy assigns multiple reaction steps to different catalytic sites, and should also be extendable to other core/shell NPs to optimize their HER activity in alkaline media.