An Isolated Zinc–Cobalt Atomic Pair for Highly Active and Durable Oxygen Reduction

Enhanced oxygen evolution and power density of Co/Zn@NC@MWCNTs for the application of zinc-air batteries

Rechargeable zinc-air batteries (ZABs) are viewed as a promising solution for electric vehicles due to their potential to provide a clean, cost-effective, and sustainable energy storage system for the next generation. Nevertheless, sluggish kinetics of the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR) at the air electrode, and low power density are significant challenges that hinder the practical application of ZABs. The key to resolving the development of ZABs is developing an affordable, efficient, and stable catalyst with bifunctional catalytic. In this study, we present a series of bifunctional catalysts composed of Co/Zn nanoparticles uniformly embedded in nitrogen-doped carbon (NC) and multi-walled carbon nanotubes (MWCNTs) denoted as Co/Zn@NC@MWCNTs. The incorporation of MWCNTs using a facile and non-toxic method significantly decreased the overpotential of the OER from 570 to 430 mV at 10 mA cm-2 and the peak power density from 226 to 263 mW cm-2. Besides, the electrochemical surface area measurements and electrochemical impedance spectroscopy indicate that the three-dimensional (3D) network structure of MWCNTs facilitates mass transport for ORR and reduces electron transfer resistance during OER, leading to a small potential gap of 0.86 V between OER and ORR, high electron transfer number (3.92-3.98) of the ORR, and lowest Tafel slope (47.8 mV dec-1) of the OER in aqueous ZABs. In addition, in-situ Raman spectroscopy revealed a notable decrease in the ID/IG ratio for the optimally configured Co/Zn@NC@MWCNTs (75:25), indicating a reduction in defect density and improved structural ordering during the electrochemical process, which directly contributes to enhanced ORR activity. Hence, this study provides an excellent strategy for constructing a bifunctional catalyst material with a 3D MWCNTs conductive network for the development of advanced ZAB systems for sustainable energy applications.

A General Metal Ion Recognition Strategy to Mediate Dual-Atomic-Site Catalysts

Heterogeneous dual-atomic-site catalysts (DACs) hold great potential for diverse applications. However, to date, the synthesis of DACs primarily relies on different atoms freely colliding on the support during synthesis, principally leading to low yields. Herein, we report a general metal ion recognition (MIR) strategy for constructing a series of DACs, including but not limited to Fe1Sn1, Fe1Co1, Fe1Ni1, Fe1Cu1, Fe1Mn1, Co1Ni1, Co1Cu1, Co2, and Cu2. This strategy is achieved by coupling target inorganometallic cations and anions as ion pairs, which are sequentially adsorbed onto a nitrogen-doped carbon substrate as the precursor. Taking the oxygen reduction reaction as an example, we demonstrated that the Fe1Sn1-DAC synthesized through this strategy delivers a record peak power density of 1.218 W cm-2 under 2.0 bar H2-O2 conditions and enhanced stability compared to the single-atom-site FeN4. Further study revealed that the superior performance arises from the synergistic effect of Fe1Sn1 dual vicinal sites, which effectively optimizes the adsorption of *OH and alleviates the troublesome Fenton-like reaction.

Synergistic Ni-W Dimer Sites Induced Stable Compressive Strain for Boosting the Performance of Pt as Electrocatalyst for the Oxygen Reduction Reaction

Alloying Pt catalysts with transition metal elements is an effective pathway to enhance the performance of oxygen reduction reaction (ORR), but often accompanied with severe metal dissolution issue, resulting in poor stability of alloy catalysts. Here, instead of forming traditional alloy structure, we modify Pt surface with a novel Ni-W dimer structure by the atomic layer deposition (ALD) technique. The obtained NiW@PtC catalyst exhibits superior ORR performance both in liquid half-cell and practical fuel cell compared with initial Pt/C. It is discovered that strong synergistic Ni-W dimer structure arising from short atomic distance induced a stable compressive strain on the Pt surface, thus boosting Pt catalytic performance. This surface modification by synergistic dimer sites offers an effective strategy in tailoring Pt with excellent activity and stability, which provides a significant perspective in boosting the performance of commercial Pt catalyst modified with polymetallic atom sites.

2D nanocomposite materials for HER electrocatalysts - a review

Hydrogen energy has the potential to be a cost-effective and strong technology for brighter development. Hydrogen fuel production by water electrolyzers has attracted attention. 2D nanocomposites with distinctive properties have been extensively explored for various applications from hydrogen evolution reactions to improving the efficiency of water electrolyzer, which is the most eco-friendly, and high-performance for hydrogen production. Recently, typical 2D nanocomposites such as Metal-Free 2D, TMDs, Mxene, LDH, organic composites, and Heterostructure have recently been thoroughly researched for use in the HER. We discuss effective ways for increasing the HER efficiency of 2D catalysts in this paper, And the unique advantages and mechanisms for specific applications are highlighted. Several essential regulating strategies for developing 2D nanocomposite-based HER electrocatalysts are included such as interface engineering, defect engineering, heteroatom doping, strain & phase engineering, and hybridizing which improve HER kinetics, the electrical conductivity, accessibility to catalytic active sites, and reaction energy barrier can be optimized. Finally, the future prospects for 2D nanocomposites in HER are discussed, as well as a thorough overview of a variety of methodologies for designing 2D nanocomposites as HER electrocatalysts with excellent catalytic performance. We expect that this review will provide a thorough overview of 2D nanocatalysts for hydrogen production.

Transient and general synthesis of high-density and ultrasmall nanoparticles on two-dimensional porous carbon via coordinated carbothermal shock

Carbon-supported nanoparticles are indispensable to enabling new energy technologies such as metal-air batteries and catalytic water splitting. However, achieving ultrasmall and high-density nanoparticles (optimal catalysts) faces fundamental challenges of their strong tendency toward coarsening and agglomeration. Herein, we report a general and efficient synthesis of high-density and ultrasmall nanoparticles uniformly dispersed on two-dimensional porous carbon. This is achieved through direct carbothermal shock pyrolysis of metal-ligand precursors in just ~100 ms, the fastest among reported syntheses. Our results show that the in situ metal-ligand coordination (e.g., N → Co2+) and local ordering during millisecond-scale pyrolysis play a crucial role in kinetically dominated fabrication and stabilization of high-density nanoparticles on two-dimensional porous carbon films. The as-obtained samples exhibit excellent activity and stability as bifunctional catalysts in oxygen redox reactions. Considering the huge flexibility in coordinated precursors design, diversified single and multielement nanoparticles (M = Fe, Co, Ni, Cu, Cr, Mn, Ag, etc) were generally fabricated, even in systems well beyond traditional crystalline coordination chemistry. Our method allows for the transient and general synthesis of well-dispersed nanoparticles with great simplicity and versatility for various application schemes.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

General Synthesis of a Diatomic Catalyst Library via a Macrocyclic Precursor-Mediated Approach

Heterogeneous catalysts containing diatomic sites are often hypothesized to have distinctive reactivity due to synergistic effects, but there are limited approaches that enable the convenient production of diatomic catalysts (DACs) with diverse metal combinations. Here, we present a general synthetic strategy for constructing a DAC library across a wide spectrum of homonuclear (Fe₂, Co₂, Ni₂, Cu₂, Mn₂, and Pd₂) and heteronuclear (Fe-Cu, Fe-Ni, Cu-Mn, and Cu-Co) bimetal centers. This strategy is based on an encapsulation-pyrolysis approach, wherein a porous material-encapsulated macrocyclic complex mediates the structure of DACs by preserving the main body of the molecular framework during pyrolysis. We take the oxygen reduction reaction (ORR) as an example to show that this DAC library can provide great opportunities for electrocatalyst development by unlocking an unconventional reaction pathway. Among all investigated sites, Fe-Cu diatomic sites possess exceptional high durability for ORR because the Fe-Cu pairs can steer elementary steps in the catalytic cycle and suppress the troublesome Fenton-like reactions.

Atomically Dispersed Zn-Pyrrolic-N4 Cathode Catalysts for Hydrogen Fuel Cells

To achieve practical application of fuel cell, it is vital to develop highly efficient and durable Pt-free catalysts. Herein, we prepare atomically dispersed ZnNC catalysts with Zn-Pyrrolic-N₄ moieties and abundant mesoporous structure. The ZnNC-based anion-exchange membrane fuel cell (AEMFC) presents an ultrahigh peak power density of 1.63 and 0.83 W cm^-2 in H₂ -O₂ and H₂ -air (CO₂ -free), and also exhibits long-term stability with more than 120 and 100 h for H₂ -air (CO₂ -free) and H₂ -O₂ , respectively. Density functional calculations further unveil that the Zn-Pyrrolic-N₄ structure is the origin of high activity of as-synthesized ZnNC catalyst, while the Zn-Pyridinic-N₄ moiety is inactive for oxygen reduction reaction (ORR), which successfully explain the puzzle why most Zn-metal-organic framework -derived ZnNC catalysts in previous reports did not present good ORR activity because of their Zn-Pyridinic-N₄ moieties. This work offers a new route for speeding up development of AEMFCs.

Effective Approaches for Designing Stable M-Nx /C Oxygen-Reduction Catalysts for Proton-Exchange-Membrane Fuel Cells

The large-scale commercialization of proton-exchange-membrane fuel cells (PEMFCs) is extremely limited by their costly platinum-group metals (PGMs) catalysts, which are used for catalyzing the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Among the reported PGM-free catalysts so far, metal-nitrogen-carbon (M-N_x /C) catalysts hold a great potential to replace PGMs catalysts for the ORR due to their excellent initial activity and low cost. However, despite tremendous progress in this field in the past decade, their further applications are restricted by fast degradation under practical conditions. Herein, the theoretical fundamentals of the stability of the M-N_x /C catalysts are first introduced in terms of thermodynamics and kinetics. The primary degradation mechanisms of M-N_x /C catalysts and the corresponding mitigating strategies are discussed in detail. Finally, the current challenges and the prospects for designing highly stable M-N_x /C catalysts are outlined.

The Dynamic Formation from Metal-Organic Frameworks of High-Density Platinum Single-Atom Catalysts with Metal-Metal Interactions

Single-atom catalysts (SACs) hold great promise for highly efficient heterogeneous catalysis, yet the practical applications require the development of high-density active sites with flexible geometric structures. The lack of understanding in the dynamic formation process of single atoms in the host framework has been plaguing the controllable synthesis of next generation SACs. Here using Co-based metal-organic frameworks (MOFs) as a starting substrate, we fully elucidated the formation of high-density Pt single atoms with inter-site interactions in derived Co₃ O₄ host. The cation exchange process and dynamic evolution of Pt-Pt interactions, organic ligand cleavage and Pt-oxygen coordination formation during the pyrolysis process have been unambiguously interpreted by a series of in situ/ex situ spectroscopic measurements and theoretical computation. These findings would direct the synthesis of high-density SACs with metal-metal interactions, which demonstrate significantly enhanced structural flexibility and catalytic properties.

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal-organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.

Synergistic effect of atomically dispersed Fe-Ni pair sites for electrocatalytic reactions to remove chlorinated organic compounds

Electrocatalysis is a promising and environmentally friendly technology for the removal of refractory organics. Diatomic catalysts with an increased number of active sites have emerged with further expansion of the field of atomic catalysts. Here, a metal diatomic FeNi supported graphene (FeNi/N-rGO) catalyst is successfully synthesized. The atomically dispersed Fe and Ni species on graphene is verified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy. The pollutant degradation efficiencies for the cathode and anode are found to reach 97.6% and 95.8%, respectively, within 90 min in the diatomic catalytic system. According to DFT theoretical calculations, FeNi diatomic catalysts have a lower free energy (ΔG = -0.2 eV), and the higher adsorption energy for the active substance H* is -0.412 eV. This work presents a method for the preparation of high-performance diatomic catalysts and promotes their application in the electrochemical degradation of chlorinated organic pollutants.

Synergetic Dual-Atom Catalysts: The Next Boom of Atomic Catalysts

Dual-atom catalysts (DACs) are an important branch of single-atom catalysts (SACs), in which the former can effectively break the dilemma faced by the traditional SACs. The synergetic effects between bimetallic atoms provide many active sites, promising to improve catalytic performance and even catalyze more complex reactions. This paper reviews the recent research progresses of two kinds of DACs, including homonuclear and heteronuclear DACs, and their applications in oxygen reduction, carbon dioxide reduction, hydrogen evolution, oxygen evolution, Zn-air batteries, tandem catalytic reactions, and so on. In addition, in order to promote the further development of DACs, the challenges and perspectives of DACs are put forward.

Quasi-Covalently Coupled Ni-Cu Atomic Pair for Synergistic Electroreduction of CO2

Developing highly active, selective, and stable electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) is crucial to establish a CO₂ conversion system for industrial implementation and, therefore, to realize an artificially closed carbon loop. This can only be achieved through the rational material design based upon the knowledge of the operational active site at the molecular scale. Enlightened by theoretical screening, herein, we for the first time manipulate a novel Ni-Cu atomic pair configuration toward improved CO₂RR performance. Systematic characterizations and theoretical modeling reveal that the secondary Cu metal incorporation positively shifts the Ni 3d orbital energy to the Fermi level and thus accelerates the rate-determining step, *COOH formation. In addition, the intrinsic inactivity of Cu toward the competing hydrogen evolution reaction causes a considerable reaction barrier for water dissociation on the Ni-Cu moiety. Due to these attributes, the as-developed Ni/Cu-N-C catalyst exhibits excellent catalytic activity and selectivity, with a record-high turnover frequency of 20,695 h^-1 at -0.6 V (vs RHE) and a maximum Faradaic efficiency of 97.7% for CO production. Furthermore, the dynamic structure evolution monitored by operando X-ray absorption fine-structure spectroscopy unveils the interaction between the Ni center and CO₂ molecules and the synergistic effect of the Ni-Cu atomic pair on CO₂RR activity.

Electronic Structure Engineering of Single-Atom Ru Sites via Co-N4 Sites for Bifunctional pH-Universal Water Splitting

The development of bifunctional water-splitting electrocatalysts that are efficient and stable over a wide range of pH is of great significance but challenging. Here, an atomically dispersed Ru/Co dual-sites catalyst is reported anchored on N-doped carbon (Ru/Co-N-C) for outstanding oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes. The Ru/Co-N-C catalyst requires the overpotential of only 13 and 23 mV for HER, 232 and 247 mV for OER to deliver a current density of 10 mA cm_geo ^-2 in 0.5 m H₂ SO₄ and 1 m KOH, respectively, outperforming benchmark catalysts Pt/C and RuO₂ . Theoretical calculations reveal that the introduction of Co-N4 sites into Ru/Co-N-C efficiently modify the electronic structure of Ru by enlarging Ru-O covalency and increasing Ru electron density, which in turn optimize the bonding strength between oxygen/hydrogen intermediate species with Ru sites, thereby enhancing OER and HER performance. Furthermore, the incorporation of Co-N4 sites induces electron redistribution around Ru-N4, thus enhancing corrosion-resistance of Ru/Co-N-C during acid and alkaline electrolysis. The Ru/Co-N-C has been applied in a proton exchange membrane water electrolyzer and steady operation is demonstrated at a high current density of 450 mA cm_geo ^-2 for 330 h.

Single Atom Catalysts: An Overview of the Coordination and Interactions with Metallic Supports

Catalyst utilization is a key economic factor in heterogeneous catalysis, particularly, when noble metals are used as the active phase. A huge saving on catalyst cost can be achieved with developing a single atomic layer of the active catalyst on a given cheap support. Besides the economic benefit, single atom catalysts (SACs) have also shown superior activity and selectivity relative to catalytic particles or nanoparticles; yet they are prone to aggregation and deactivation. The development of effective, stable, and commercially viable SACs is still a huge challenge. One of the remaining key obstacles is the ability to easily and effectively tune SACs-support interactions and coordination in a way that enables the production of robust, stable, and versatile SACs. Accordingly, the coordination and interactions between metallic supports and SACs and their impacts on SACs stability and activity are reviewed in this article.

Multi-atom cluster catalysts for efficient electrocatalysis

This review presents recent developments in the synthesis, modulation and characterization of multi-atom cluster catalysts for electrochemical energy applications.

Moving beyond bimetallic-alloy to single-atom dimer atomic-interface for all-pH hydrogen evolution

Single-atom-catalysts (SACs) afford a fascinating activity with respect to other nanomaterials for hydrogen evolution reaction (HER), yet the simplicity of single-atom center limits its further modification and utilization. Obtaining bimetallic single-atom-dimer (SAD) structures can reform the electronic structure of SACs with added atomic-level synergistic effect, further improving HER kinetics beyond SACs. However, the synthesis and identification of such SAD structure remains conceptually challenging. Herein, systematic first-principle screening reveals that the synergistic interaction at the NiCo-SAD atomic interface can upshift the d-band center, thereby, facilitate rapid water-dissociation and optimal proton adsorption, accelerating alkaline/acidic HER kinetics. Inspired by theoretical predictions, we develop a facile strategy to obtain NiCo-SAD on N-doped carbon (NiCo-SAD-NC) via in-situ trapping of metal ions followed by pyrolysis with precisely controlled N-moieties. X-ray absorption spectroscopy indicates the emergence of Ni-Co coordination at the atomic-level. The obtained NiCo-SAD-NC exhibits exceptional pH-universal HER-activity, demanding only 54.7 and 61 mV overpotentials at −10 mA cm⁻² in acidic and alkaline media, respectively. This work provides a facile synthetic strategy for SAD catalysts and sheds light on the fundamentals of structure-activity relationships for future applications.

While single, dispersed atoms enable efficient atomic utilization, controllably preparing single-atom dimers remains challenging. Here, authors prepare nickel-cobalt single-atom dimers as high-performance pH-universal H₂ evolution electrocatalysts.

Synergistic Effect of Active Sites of Double-Atom Catalysts for Nitrogen Reduction Reaction

Nitrogen fixation to produce ammonia is a vital process since nitrogen is an essential element for the human body. Industrial nitrogen fixation mainly relies on the Haber-Bosch process. However, this process requires huge energy consumption and leads to pollution emission. In this study, the behaviors of intermediates in the nitrogen reduction reaction (NRR) are investigated for fifteen double-atom catalysts (DACs) through density functional theory calculations, revealing that under the synergistic effect of active sites on appropriate DACs, intermediates can be adsorbed through different configurations according to the activity improvement needs. VFe-N-C shows the best catalytic activity for electrochemical NRR with a limiting potential of -0.36 V vs. the reversible hydrogen electrode. The proposed synergistic effect of active sites on DACs for NRR could provide a new method for design of NRR catalysts.

Flash Bottom-Up Arc Synthesis of Nanocarbons as a Universal Route for Fabricating Single-Atom Electrocatalysts

Despite considerable development in the field of single-atom catalysts (SACs) on carbon-based materials, the reported strategies for synthesizing SACs generally rely on top-down approaches, which hinder achieving both simple and universal synthesis routes that are simultaneously applicable to various metals and nanocarbons. Here, a universal strategy for fabricating nanocarbon based-SACs using a flash bottom-up arc discharge method to mitigate these issues is reported. The ionization of elements and their recombination process during arc discharge allows the simultaneous incorporation of single metal atoms (Mn, Fe, Co, Ni, and Pt) into the crystalline carbon lattice during the formation of carbon nanohorns (CNHs) and N-doped arc graphene. The coordination environment around the Co atoms of Co₁ /CNH can be modulated by a mild post-treatment with NH₃ . As a result, Co₁ /CNH exhibits good oxygen reduction reaction activity, showing a 1.92 times higher kinetic current density value than the commercial Pt/C catalyst in alkaline media. In a single cell experiment, Co₁ /CNH exhibits the highest maximum power density of 472 mW cm^-2 compared to previously reported nonprecious metal-based SACs.

Intrinsic ORR Activity Enhancement of Pt Atomic Sites by Engineering the d-Band Center via Local Coordination Tuning

A considerable amount of platinum (Pt) is required to ensure an adequate rate for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Thus, the implementation of atomic Pt catalysts holds promise for minimizing the Pt content. In this contribution, atomic Pt sites with nitrogen (N) and phosphorus (P) co-coordination on a carbon matrix (PtNPC) are conceptually predicted and experimentally developed to alter the d-band center of Pt, thereby promoting the intrinsic ORR activity. PtNPC with a record-low Pt content (≈0.026 wt %) consequently shows a benchmark-comparable activity for ORR with an onset of 1.0 V_RHE and half-wave potential of 0.85 V_RHE . It also features a high stability in 15 000-cycle tests and a superior turnover frequency of 6.80 s^-1 at 0.9 V_RHE . Damjanovic kinetics analysis reveals a tuned ORR kinetics of PtNPC from a mixed 2/4-electron to a predominately 4-electron route. It is discovered that coordinated P species significantly shifts d-band center of Pt atoms, accounting for the exceptional performance of PtNPC.

Machine-Learning-Accelerated Catalytic Activity Predictions of Transition Metal Phthalocyanine Dual-Metal-Site Catalysts for CO2 Reduction

The highly active and selective carbon dioxide reduction reaction (CO₂RR) can generate valuable products such as fuels and chemicals and reduce the emission of greenhouse gases. Single-atom catalysts (SACs) and dual-metal-sites catalysts (DMSCs) with high activity and selectivity are superior electrocatalysts for the CO₂RR as they have higher active site utilization and lower cost than traditional noble metals. Herein, we explore a rational and creative density-functional-theory-based, machine-learning-accelerated (DFT-ML) method to investigate the CO₂RR catalytic activity of hundreds of transition metal phthalocyanine (Pc) DMSCs. The gradient boosting regression (GBR) algorithm is verified to be the most desirable ML model and is used to construct catalytic activity prediction, with a root-mean-square error of only 0.08 eV. The results of ML prediction demonstrate Ag-MoPc as a promising CO₂RR electrocatalyst with the limiting potential of only -0.33 V. The DFT-ML hybrid scheme accelerates the efficiency 6.87 times, while the prediction error is only 0.02 V, and it sheds light on the path to accelerate the rational design of efficient catalysts for energy conversion and conservation.

Single-Atom and Dual-Atom Electrocatalysts Derived from Metal Organic Frameworks: Current Progress and Perspectives

Single-atom catalysts (SACs) have attracted increasing research interests owing to their unique electronic structures, quantum size effects and maximum utilization rate of atoms. Metal organic frameworks (MOFs) are good candidates to prepare SACs owing to the atomically dispersed metal nodes in MOFs and abundant N and C species to stabilize the single atoms. In addition, the distance of adjacent metal atoms can be turned by adjusting the size of ligands and adding volatile metal centers to promote the formation of isolated metal atoms. Moreover, the diverse metal centers in MOFs can promote the preparation of dual-atom catalysts (DACs) to improve the metal loading and optimize the electronic structures of the catalysts. The applications of MOFs derived SACs and DACs for electrocatalysis, including oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction and nitrogen reduction reaction are systematically summarized in this Review. The corresponding synthesis strategies, atomic structures and electrocatalytic performances of the catalysts are discussed to provide a deep understanding of MOFs-based atomic electrocatalysts. The catalytic mechanisms of the catalysts are presented, and the crucial challenges and perspectives are proposed to promote further design and applications of atomic electrocatalysts.

Highly Efficient Photo-/Electrocatalytic Reduction of Nitrogen into Ammonia by Dual-Metal Sites

The photo-/electrocatalytic nitrogen reduction reaction (NRR) is an up and coming method for sustainable NH3 production; however, its practical application is impeded by poor Faradaic efficiency originating from the competing hydrogen evolution reaction (HER) and the inert N≡N triple bond activation. In this work, we put forth a method to boost NRR through construction of donor-acceptor couples of dual-metal sites. The synergistic effect of dual active sites can potentially break the metal-based activity benchmark toward efficient NRR. By systematically evaluating the stability, activity, and selectivity of 28 heteronuclear dual-atom catalysts (DACs) of M1M2/g-C3N4 candidates, FeMo/g-C3N4 is screened out as an effective electrocatalyst for NRR with a particularly low limiting potential of -0.23 V for NRR and a rather high potential of -0.79 V for HER. Meanwhile, TiMo/g-C3N4, NiMo/g-C3N4, and MoW/g-C3N4 with suitable band edge positions and visible light absorption can be applied to NRR as photocatalysts. The excellent catalytic activity is attributed to the tunable composition of metal dimers, which play an important role in modulating the binding strength of the target intermediates. This work may pave a new way for the rational design of heteronuclear DACs with high activity and stability for NRR, which may also apply to other reactions.

Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy Efficiency of CO2 Reduction

Simultaneously improving energy efficiency (EE) and material stability in electrochemical CO₂ conversion remains an unsolved challenge. Among a series of ternary Sn-Ti-O electrocatalysts, 3D ordered mesoporous (3DOM) Sn_0.3 Ti_0.7 O₂ achieves a trade-off between active-site exposure and structural stability, demonstrating up to 71.5 % half-cell EE over 200 hours, and a 94.5 % Faradaic efficiency for CO at an overpotential as low as 430 mV. DFT and X-ray absorption fine structure analyses reveal an electron density reconfiguration in the Sn-Ti-O system. A downshift of the orbital band center of Sn and a charge depletion of Ti collectively facilitate the dissociative adsorption of the desired intermediate COOH* for CO formation. It is also beneficial in maintaining a local alkaline environment to suppress H₂ and formate formation, and in stabilizing oxygen atoms to prolong durability. These findings provide a new strategy in materials design for efficient CO₂ conversion and beyond.

Single atom catalysts: a surface heterocompound perspective

The concept of single atom catalysts (SACs) originated from reducing the amount of noble metals used, by steadily refining the particle size loaded on a substrate surface. It has been rapidly moving to non-noble elements and their compounds in recent years, notably transition metals and even non-metals. They are of heterogeneous types, where the active species are refined to atomic dispersion scales on the surfaces/sub-surfaces of the solid support. The catalytic performance is governed by both the type and population of accessible active sites, and their bond and coordination environment, largely as a result of the interactions with the substrate surface. Unlike the internal structure within a crystalline solid, there is a large spatial variation in the bond and coordination environment of different atoms on the solid surface across different length scales, and in particular with the unsaturated surface, where there are various defects. They can also be dramatically altered during both the catalyst synthesis and actual catalysis process. In a way, they form a "surface heterocompound", where the local bonds for each metal atom are of a compound type, while there can be a large variation from one to another. Herein, we will look into the evolution from traditional heterogeneous catalysts to SACs, from the surface heterocompound perspective. Discussion will then be made on the on-going strategies and challenges in manipulating and identifying the local bond and coordination environment on the hetero-surfaces, in an attempt to develop efficient catalysts for the targeted applications, where both synthesis techniques and analytical tools are critically important, and computational studies can provide the key guiding principles. With selected paradigm studies, we will briefly examine the future perspectives for this newly emerging catalysis frontier.

Activity Origin and Design Principles for Oxygen Reduction on Dual-Metal-Site Catalysts: A Combined Density Functional Theory and Machine Learning Study

Dual-metal-site catalysts (DMSCs) are emerging as a new frontier in the field of oxygen reduction reaction (ORR). However, there is a lack of design principles to provide a universal description of the relationship between intrinsic properties of DMSCs and the catalytic activity. Here, we identify the origin of ORR activity and unveil design principles for graphene-based DMSCs by means of density functional theory computations and machine learning (ML). Our results indicate that several experimentally unexplored DMSCs can show outstanding ORR activity surpassing that of platinum. Remarkably, our ML study reveals that the ORR activity of DMSCs is intrinsically governed by some fundamental factors, such as electron affinity, electronegativity, and radii of the embedded metal atoms. More importantly, we propose predictor equations with acceptable accuracy to quantitatively describe the ORR activity of DMSCs. Our work will accelerate the search for highly active DMSCs for ORR and other electrochemical reactions.

Engineering Energy Level of Metal Center: Ru Single-Atom Site for Efficient and Durable Oxygen Reduction Catalysis

Emerging as a new frontier in heterogeneous catalysis, single-atom site catalysts (SSCs) have sparked enormous attention and bring about new opportunities to oxygen reduction electrocatalysis. Despite considerable progress achieved recently, most of the reported SSCs suffer from either insufficient activity or unsatisfactory stability, which severely retards their practical application. Here, we demonstrate a novel Ru-SSC with appropriate adsorption free energy of OH* (ΔG_OH*) to confer excellent activity and low Fenton reactivity to maintain long-term stability. The as-developed Ru-SSC exhibits encouraging oxygen reduction reaction turnover frequency of 4.99 e^- s^-1 sites^-1, far exceeding the state-of-the-art Fe-SSC counterpart (0.816 e^- s^-1 sites^-1), as a result of Ru energy level regulation via spontaneous OH binding. Furthermore, Ru-SSC exhibits greatly suppressed Fenton reactivity, with restrained generation of reactive oxygen species directly observed, thus endowing the Ru-SSC with much more superior stability (only 17 mV negative shift after 20 000 cycles) than the Fe-SSC counterpart (31 mV). The practical application of Ru-SSC is further validated by its excellent activity and stability in a real fuel cell device.

Atomically Dispersed Binary Co-Ni Sites in Nitrogen-Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution

With the inspiration of developing bifunctional electrode materials for reversible oxygen electrocatalysis, one strategy of heteroatom doping is proposed to fabricate dual metal single-atom catalysts. However, the identification and mechanism functions of polynary single-atom structures remain elusive. Atomically dispersed binary Co-Ni sites embedded in N-doped hollow carbon nanocubes (denoted as CoNi-SAs/NC) are synthesized via proposed pyrolysis of dopamine-coated metal-organic frameworks. The atomically isolated bimetallic configuration in CoNi-SAs/NC is identified by combining microscopic and spectroscopic techniques. When employing as oxygen electrocatalysts in alkaline medium, the resultant CoNi-SAs/NC hybrid manifests outstanding catalytic performance for bifunctional oxygen reduction/evolution reactions, boosting the realistic rechargeable zinc-air batteries with high efficiency, low overpotential, and robust reversibility, superior to other counterparts and state-of-the-art precious-metal catalysts. Theoretical computations based on density functional theory demonstrate that the homogenously dispersed single atoms and the synergistic effect of neighboring Co-Ni dual metal center can optimize the adsorption/desorption features and decrease the overall reaction barriers, eventually promoting the reversible oxygen electrocatalysis. This work not only sheds light on the controlled synthesis of atomically isolated advanced materials, but also provides deeper understanding on the structure-performance relationships of nanocatalysts with multiple active sites for various catalytic applications.

Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe Catalysts via Integration with Fe Nanoclusters

Single-atom catalysts (SACs) have emerged as one of the most promising alternatives to noble metal-based catalysts for highly efficient oxygen reduction reaction (ORR). While SACs can offer notable benefits in terms of lowering overall catalyst cost, there is still room for improvement regarding catalyst activity. To this end, we designed and successfully fabricated an ORR electrocatalyst in which atomic clusters are embedded in an atomically dispersed Fe-N-C matrix (Fe_AC@Fe_SA-N-C), as shown by comprehensive measurements using aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy (XAS). The half-wave potential of Fe_AC@Fe_SA-N-C is 0.912 V (versus reversible hydrogen electrode (RHE)), exceeding that of commercial Pt/C (0.897 V), Fe_SA-N-C (0.844 V), as well as the half-wave potentials of most reported non-platinum-group metal catalysts. The ORR activity of the designed catalyst stems from single-atom active centers but is markedly enhanced by the presence of Fe nanoclusters, as confirmed by both experimental measurements and theoretical calculations.

Recent Advances in Isolated Single-Atom Catalysts for Zinc Air Batteries: A Focus Review

Recently, zinc–air batteries (ZABs) have been receiving attention due to their theoretically high energy density, excellent safety, and the abundance of zinc resources. Typically, the performance of the zinc air batteries is determined by two catalytic reactions on the cathode—the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Therefore, intensive effort has been devoted to explore high performance electrocatalysts with desired morphology, size, and composition. Among them, single-atom catalysts (SACs) have emerged as attractive and unique systems because of their high electrocatalytic activity, good durability, and 100% active atom utilization. In this review, we mainly focus on the advance application of SACs in zinc air batteries in recent years. Firstly, SACs are briefly compared with catalysts in other scales (i.e., micro- and nano-materials). A main emphasis is then focused on synthesis and electrocatalytic activity as well as the underlying mechanisms for mono- and dual-metal-based SACs in zinc air batteries catalysis. Finally, a prospect is provided that is expected to guide the rational design and synthesis of SACs for zinc air batteries.