Single-atom catalysts for the electrochemical reduction of carbon dioxide into hydrocarbons and oxygenates

The electrochemical reduction of carbon dioxide offers a sound and economically viable technology for the electrification and decarbonization of the chemical and fuel industries.In this technology,an electrocatalytic material and renewable energy-generated electricity drive the conversion of carbon dioxide into high-value chemicals and carbon-neutral fuels.Over the past few years,single-atom catalysts have been intensively studied as they could provide near-unity atom utilization and unique catalytic performance.Single-atom catalysts have become one of the state-of-the-art catalyst materials for the electrochemical reduction of carbon dioxide into carbon monoxide.However,it remains a challenge for single-atom catalysts to facilitate the efficient conversion of carbon dioxide into products beyond carbon monoxide.In this review,we summarize and present important findings and critical insights from studies on the electrochemical carbon dioxide reduction reaction into hydrocarbons and oxygenates using single-atom catalysts.It is hoped that this review gives a thorough recapitulation and analysis of the science behind the catalysis of carbon dioxide into more reduced products through singleatom catalysts so that it can be a guide for future research and development on catalysts with industry-ready performance for the electrochemical reduction of carbon dioxide into high-value chemicals and carbon-neutral fuels.

Progress and perspective of single-atom catalysts for membrane electrode assembly of fuel cells

A fuel cell is an energy conversion device that can continuously input fuel and oxidant into the device through an electrochemical reaction to release electrical energy.Although noble metals show good activity in fuel cell-related electrochemical reactions,their ever-increasing price considerably hinders their industrial application.Improvement of atom utilization efficiency is considered one of the most effective strategies to improve the mass activity of catalysts,and this allows for the use of fewer catalysts,saving greatly on the cost.Thus,single-atom catalysts(SACs)with an atom utilization efficiency of 100%have been widely developed,which show remarkable performance in fuel cells.In this review,we will describe recent progress on the development of SACs for membrane electrode assembly of fuel cell applications.First,we will introduce several effective routes for the synthesis of SACs.The reaction mechanism of the involved reactions will also be introduced as it is highly determinant of the final activity.Then,we will systematically summarize the application of Pt group metal(PGM)and nonprecious group metal(non-PGM)catalysts in membrane electrode assembly of fuel cells.This review will offer numerous experiences for developing potential industrialized fuel cell catalysts in the future.

Defect engineering of high-loading single-atom catalysts for electrochemical carbon dioxide reduction

Electrochemical carbon dioxide reduction reaction(CO_(2)RR)provides an attractive approach to carbon capture and utilization for the production high-value-added products.However,CO_(2)RR still suffers from poor selectivity and low current density due to its sluggish kinetics and multitudinous reaction pathways.Single-atom catalysts(SACs)demonstrate outstanding activity,excellent selectivity,and remarkable atom utilization efficiency,which give impetus to the search for electrocatalytic processes aiming at high selectivity.There appears significant activity in the development of efficient SACs for CO_(2)RR,while the density of the atomic sites remains a considerable barrier to be overcome.To construct high-metal-loading SACs,aggregation must be prevented,and thus novel strategies are required.The key to creating high-density atomically dispersed sites is designing enough anchoring sites,normally defects,to stabilize the highly mobile separated metal atoms.In this review,we summarized the advances in developing high-loading SACs through defect engineering,with a focus on the synthesis strategies to achieve high atomic site loading.Finally,the future opportunities and challenges for CO_(2)RR in the area of high-loading single-atom electrocatalysts are also discussed.

Regulating the coordination environment of Ru single-atom catalysts and unravelling the reaction path of acetylene hydrochlorination

In this work,DFT calculations were used firstly to simulate the nitrogen coordinated metal single-atom catalysts(M-N_(x)SACs,M=Hg,Cu,Au,and Ru) to predict their catalytic activities in acetylene hydrochlorination.The DFT results showed that Ru-N_(x)SACs had the best catalytic performance among the four catalysts,and Ru-N_(x)SACs could effectively inhibit the reduction of ruthenium cation.To verify the DFT results,Ru-N_(x)SACs were fabricated by pyrolyzing MOFs in-situ spatially confined metal precursors.The N coordination environment could be controlled by changing the pyrolysis temperature.Catalytic performance tests indicated that low N coordination number(Ru-N_(2),Ru-N_(3))exhibited excellent catalytic activity and stability compared to RuCl_(3)catalyst.DFT calculations further revealed that Ru-N_(2)and Ru-N_(3)had a tendency to activate HCl at the first step of reaction,whereas Ru-N4tended to activate C_(2)H_(2).These findings will serve as a reference for the design and control of metal active sites.

1+1>2: Learning from the interfacial modulation on single-atom electrocatalysts to design dual-atom electrocatalysts for dinitrogen reduction

Developing efficient electrocatalysts for converting dinitrogen to ammonia through electrocatalysis is of significance to the decentralized ammonia production. Here, through high-throughput density functional theory calculations, we demonstrated that the interfacial modulation of hexagonal boron nitride/graphene(hBN-graphene) could sufficiently improve the catalytic activity of the single transition metal atom catalysts for nitrogen reduction reaction(NRR). It was revealed that Re@hBN-graphene and Os@hBN-graphene possessed remarkable NRR catalytic activity with low limiting potentials of 0.29 V and 0.33 V, respectively. Furthermore, the mechanism of the enhanced catalytic activity was investigated based on various descriptors of the adsorption energies of intermediates, where the synergistic effect of hBN and graphene in the hybrid substrate was found to play a key role. Motivated by the synergistic effect of hybrid substrate in single-atom catalysts, a novel strategy was proposed to efficiently design dual-atom catalysts by integrating the merits of both metal components. The as-designed dual-atom catalyst Fe-Mo@hBN exhibited more excellent NRR catalytic performance with a limiting potential of 0.17 V, manifesting the solidity of the design strategy. Our findings open new avenues for the search of heterostructure substrates for single-atom catalysts and the efficient design of dualatom catalysts for NRR.

Metal-organic frameworks based single-atom catalysts for advanced fuel cells and rechargeable batteries

The next-generation energy storage systems such as fuel cells,metal-air batteries,and alkali metal(Li,Na)-chalcogen(S,Se)batteries have received increasing attention owing to their high energy density and low cost.However,one of the main obstacles of these systems is the poor reaction kinetics in the involved chemical reactions.Therefore,it is essential to incorporate suitable and efficient catalysts into the cell.These years,single-atom catalysts(SACs)are emerging as a frontier in catalysis due to their maximum atom efficiency and unique reaction selectivity.For SACs fabrication,metal-organic frameworks(MOFs)have been confirmed as promising templates or precursors due to their high metal loadings,structural adjustability,porosity,and tailorable catalytic site.In this review,we summarize effective strategies for fabricating SACs by MOFs with corresponding advanced characterization techniques and illustrate the key role of MOFs-based SACs in these batteries by explaining their reaction mechanisms and challenges.Finally,current applications,prospects,and opportunities for MOFs-based SACs in energy storage systems are discussed.

Transition metal-based single-atom catalysts(TM-SACs);rising materials for electrochemical CO_(2) reduction

The continuous increase of global atmospheric CO_(2) concentrations brutally damages our environment. A series of methods have been developed to convert CO_(2) to valuable fuels and value-added chemicals to maintain the equilibrium of carbon cycles. The electrochemical CO_(2) reduction reaction(CO_(2)RR) is one of the promising methods to produce fuels and chemicals, and it could offer sustainable paths to decrease carbon intensity and support renewable energy. Thus, significant research efforts and highly efficient catalysts are essential for converting CO_(2) into other valuable chemicals and fuels. Transition metal-based single atoms catalysts(TM-SACs) have recently received much attention and offer outstanding electrochemical applications with high activity and selectivity opportunities. By taking advantage of both heterogeneous and homogeneous catalysts, TM-SACs are the new rising star for electrochemical conversion of CO_(2) to the value-added product with high selectivity. In recent years, enormous research effort has been made to synthesize different TM-SACs with different M–Nxsites and study the electrochemical conversion of CO_(2) to CO. This review has discussed the development and characterization of different TMSACs with various catalytic sites, fundamental understanding of the electrochemical process in CO_(2) RR,intrinsic catalytic activity, and molecular strategics of SACs responsible for CO_(2)RR. Furthermore, we extensively review previous studies on 1 st-row transition metals TM-SACs(Ni, Co, Fe, Cu, Zn, Sn) and dual-atom catalysts(DACs) utilized for electrochemical CO_(2) conversions and highlight the opportunities and challenges.

General approach for atomically dispersed precious metal catalysts toward hydrogen reaction

As a carbon-free energy carrier,hydrogen has become the pivot for future clean energy,while efficient hydrogen production and combustion still require precious metal-based catalysts.Single-atom catalysts(SACs)with high atomic utilization open up a desirable perspective for the scale applications of precious metals,but the general and facile preparation of various precious metal-based SACs remains challenging.Herein,a general movable printing method has been developed to synthesize various precious metal-based SACs,such as Pd,Pt,Rh,Ir,and Ru,and the features of highly dispersed single atoms with nitrogen coordination have been identified by comprehensive characterizations.More importantly,the synthesized Pt-and Ru-based SACs exhibit much higher activities than their corresponding nanoparticle counterparts for hydrogen oxidation reaction and hydrogen evolution reaction(HER).In addition,the Pd-based SAC delivers an excellent activity for photocatalytic hydrogen evolution.Especially for the superior mass activity of Ru-based SACs toward HER,density functional theory calculations confirmed that the adsorption of the hydrogen atom has a significant effect on the spin state and electronic structure of the catalysts.

Long-range electron synergy over Pt_(1)-Co_(1)/CN bimetallic single-atom catalyst in enhancing charge separation for photocatalytic hydrogen production

The development of novel single-atom catalysts with optimal electron configuration and economical noble-metal cocatalyst for efficient photocatalytic hydrogen production is of great importance,but still challenging.Herein,we fabricate Pt and Co single-atom sites successively on polymeric carbon nitride(CN).In this Pt_(1)-Co_(1)/CN bimetallic single-atom catalyst,the noble-metal active sites are maximized,and the single-atomic Co_(1)N_4sites are tuned to Co_(1)N_3sites by photogenerated electrons arising from the introduced single-atomic Pt_(1)N_4sites.Mechanism studies and density functional theory(DFT)calculations reveal that the 3d orbitals of Co_(1)N_3single sites are filled with unpaired d-electrons,which lead to the improved visible-light response,carrier separation and charge migration for CN photocatalysts.Thereafter,the protons adsorption and activation are promoted.Taking this advantage of long-range electron synergy in bimetallic single atomic sites,the photocatalytic hydrogen evolution activity over Pt_(1)-Co_(1)/CN achieves 915.8 mmol g^(-1)Pt h^(-1),which is 19.8 times higher than Co_(1)/CN and 3.5 times higher to Pt_(1)/CN.While this electron-synergistic effect is not so efficient for Pt nanoclusters.These results demonstrate the synergistic effect at electron-level and provide electron-level guidance for the design of efficient photocatalysts.

Secondary-Atom-Doping Enables Robust Fe-N-C Single-Atom Catalysts with Enhanced Oxygen Reduction Reaction

Single-atom catalysts(SACs) with nitrogen-coordinated nonprecious metal sites have exhibited inimitable advantages in electrocatalysis.However,a large room for improving their activity and durability remains.Herein,we construct atomically dispersed Fe sites in N-doped carbon supports by secondary-atom-doped strategy.Upon the secondary doping,the density and coordination environment of active sites can be efficiently tuned,enabling the simultaneous improvement in the number and reactivity of the active site.Besides,structure optimizations in terms of the enlarged surface area and improved hydrophilicity can be achieved simultaneously.Due to the beneficial microstructure and abundant highly active FeN_5 moieties resulting from the secondary doping,the resultant catalyst exhibits an admirable half-wave potential of 0.81 V versus 0.83 V for Pt/C and much better stability than Pt/C in acidic media.This work would offer a general strategy for the design and preparation of highly active SACs for electrochemical energy devices.

Single-Atom Catalysts for Electrochemical Hydrogen Evolution Reaction: Recent Advances and Future Perspectives

Hydrogen,a renewable and outstanding energy carrier with zero carbon dioxide emission,is regarded as the best alternative to fossil fuels.The most preferred route to large-scale production of hydrogen is by water electrolysis from the intermittent sources(e.g.,wind,solar,hydro,and tidal energy).However,the efficiency of water electrolysis is very much dependent on the activity of electrocatalysts.Thus,designing high-effective,stable,and cheap materials for hydrogen evolution reaction(HER)could have a substantial impact on renewable energy technologies.Recently,single-atom catalysts(SACs)have emerged as a new frontier in catalysis science,because SACs have maximum atom-utilization efficiency and excellent catalytic reaction activity.Various synthesis methods and analytical techniques have been adopted to prepare and characterize these SACs.In this review,we discuss recent progress on SACs synthesis,characterization methods,and their catalytic applications.Particularly,we highlight their unique electrochemical characteristics toward HER.Finally,the current key challenges in SACs for HER are pointed out and some potential directions are proposed as well.

Non-noble metal single-atom catalysts prepared by wet chemical method and their applications in electrochemical water splitting

Water splitting by electrolysis is an appealing pathway for sustainable hydrogen production. The practical performance of water splitting is highly dependent on the efficiency of electrocatalysts, which can promote the anodic oxygen evolution reaction(OER) or cathodic hydrogen evolution reaction(HER). Downsizing the metal nanostructures to atomic level to construct single-atom catalysts(SACs) has attracted enormous attention due to its distinct advantages in maximizing the efficiency of metal atom utilization and enhancing activity over corresponding metal nanoparticles. Research on SACs towards electrochemical water splitting application is an emerging field and intensive investigations have been focused on their rational syntheses and applications in HER/OER. In this review, we focus on the wet chemical method developed to prepare non-noble metal based SACs with an emphasis on the synthetic strategies and structure-activity relationship between single metal atoms and catalytic activity. Finally, the challenges and future opportunities for application of single-atom catalysts in water splitting are briefly addressed.

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.

Advances on Axial Coordination Design of Single‑Atom Catalysts for Energy Electrocatalysis:A Review

Single-atom catalysts(SACs)have garnered increasingly growing attention in renewable energy scenarios,especially in electrocatalysis due to their unique high efficiency of atom utilization and flexible electronic structure adjustability.The intensive efforts towards the rational design and synthesis of SACs with versatile local configurations have significantly accelerated the development of efficient and sustainable electrocatalysts for a wide range of electrochemical applications.As an emergent coordination avenue,intentionally breaking the planar symmetry of SACs by adding ligands in the axial direction of metal single atoms offers a novel approach for the tuning of both geometric and electronic structures,thereby enhancing electrocatalytic performance at active sites.In this review,we briefly outline the burgeoning research topic of axially coordinated SACs and provide a comprehensive summary of the recent advances in their synthetic strategies and electrocatalytic applications.Besides,the challenges and outlooks in this research field have also been emphasized.The present review provides an in-depth and comprehensive understanding of the axial coordination design of SACs,which could bring new perspectives and solutions for fine regulation of the electronic structures of SACs catering to high-performing energy electrocatalysis.

Dual atomic catalysts from COF-derived carbon for CO_(2)RR by suppressing HER through synergistic effects

The electrochemical carbon dioxide reduction reaction(CO_(2)RR)for highvalue-added products is a promising strategy to tackle excessive CO_(2) emissions.However,the activity of and selectivity for catalysts for CO_(2)RR still need to be improved because of the competing reaction(hydrogen evolution reaction).In this study,for the first time,we have demonstrated dual atomic catalytic sites for CO_(2)RR from a core-shell hybrid of the covalent-organic framework and the metal-organic framework.Due to abundant dual atomic sites(with CoN_(4)O and ZnN_(4) of 2.47 and 11.05 wt.%,respectively)on hollow carbon,the catalyst promoted catalysis of CO_(2)RR,with the highest Faradic efficiency for CO of 92.6%at-0.8 V and a turnover frequency value of 1370.24 h^(-1) at-1.0 V.More importantly,the activity and selectivity of the catalyst were well retained for 30 h.The theoretical calculation further revealed that CoN_(4)O was the main site for CO_(2)RR,and the activity of and selectivity for Zn sites were also improved because of the synergetic roles.

Atom-level interfacial synergy of single-atom site catalysts for electrocatalysis

Single-atom site catalysts(SACs)have made great achievements due to their nearly 100%atomic utilization and uniform active sites.Regulating the surrounding environment of active sites,including electron structure and coordination environment via atom-level interface regulation,to design and construct an advanced SACs is of great significance for boosting electrocatalytic reactions.In this review,we systemically summarized the fundamental understandings and intrinsic mechanisms of SACs for electrocatalytic applications based on the interface site regulations.We elaborated the several different regulation strategies of SACs to demonstrate their ascendancy in electrocatalytic applications.Firstly,the interfacial electronic interaction was presented to reveal the electron transfer behavior of active sites.Secondly,the different coordination structures of metal active center coordinated with two or three non-metal elements were also summarized.In addition,other atom-level interfaces of SACs,including metal atom–atom interface,metal atom-X-atom interface(X:non-metal element),metal atom-particle interface,were highlighted and the corresponding promoting effect towards electrocatalysis was disclosed.Finally,we outlooked the limitations,perspectives and challenges of SACs based on atomic interface regulation.

Recent advances and strategies in the stabilization of single-atom catalysts for electrochemical applications

Owing to the rapidly increasing consumption of fossil fuels,finding clean and reliable new energy sources is of the utmost importance.Thus,developing highly efficient and low-cost catalysts for electrochemical reactions in energy conversion devices is crucial.Single-atom catalysts(SACs)with maximum metal atom utilization efficiency and superior catalytic performance have attracted significant attention,especially for electrochemical reactions.However,because of the highly unsaturated coordination environment,the stability of SACs can be a challenge for practical applications.In this review,we will summarize the strategies to increase the stability of SACs and synthesizing stable SACs,as well as the application of SACs in electrochemical reactions.Finally,we offer a perspective on the development of advanced SACs through rational design and a deeper understanding of SACs with the help of in situ or operando techniques in electrochemical reactions.

Isolated Co Atoms Anchored on Defective Nitrogen-doped Carbon Graphene as Efficient Oxygen Reduction Reaction Electrocatalysts

Oxygen reduction reaction(ORR)is the heart of many new energy conversions and storage devices,such as metal-air batteries and fuel cells.However,ORR is currently facing the dilemma of sluggish intrinsic kinetics and the noble electrocatalysts of high price and low reserves.In this work,isolated Co atoms anchored on defective nitrogen-doped carbon graphene single-atom catalyst(Co-SAC/NC)are synthesized via the proposed movable type printing method.The prepared Co-SAC/NC catalyst demonstrates admirable ORR performance,with a high half-wave potential of 0.884 V in alkaline electrolytes and outstanding durability.In addition,an assembled zinc–air battery with prepared Co-SAC/NC as air-cathode catalyst displays a high-peak power density of 179 mW cm^(-2)and a high-specific capacity(757 mAh g^(-1)).Density functional theory calculations confirm that the true active sites of the prepared catalyst are Co-N_(4)moieties,and further reveal a significantly electronic structure evolution of Co sites in the ORR process,in which the project density of states and local magnetic moment of Co atom varies during its whole reaction process.This work not only paves a new avenue for synthesizing SACs as robust electrocatalysts,but also provides an electronic-level insight into the evolution of the electronic structure of single-atom catalysts.

Single-atom catalysts for metal-sulfur batteries:Current progress and future perspectives

Metal-sulfur batteries are recognized as a promising candidate for next generation electrochemical energy storage systems owing to their high theoretical energy density,low cost and environmental friendliness.However,sluggish redox kinetics of sulfur species and the shuttle effect lead to large polarization and severe capacity decay.Numerous approaches from physical barrier,chemical adsorption strategies to electrocatalysts have been tried to solve these issues and pushed the rate and cycle performance of sulfur electrodes to higher levels.Most recently,single-atom catalysts(SACs)with high catalytic efficiency have been introduced into metal-sulfur systems to achieve fast redox kinetics of sulfur conversion.In this review,we systematically summarize the current progress on SACs for sulfur electrodes from aspects of synthesis,characterization and electrochemical performance.Challenges and potential solutions for designing SACs for high-performance sulfur electrodes are discussed.

Single-atom catalysts for CO oxidation,CO_(2) reduction,and O_(2) electrochemistry

CO_(x)(x=1,2)and O_(2) chemistry play key roles in tackling global severe environmental challenges and energy issues.To date,the efficient selective electrocatalytic transformations of COx-carbon chemicals,and O_(2)-hydrogenated products are still huge challenges.Single-atom catalysts(SACs)as atomic-scale novel catalysts in which only isolated metal atoms are dispersed on supports shed new insights in overcome these obstacles in CO_(x) and O_(2) chemistry,including CO oxidation,CO_(2) reduction reaction(CO_(2)RR),oxygen reduction reaction(ORR),and oxygen evolution reaction(OER).In this review,the unique features and advanced synthesis strategies of SACs from a viewpoint of fundamental synthesis design are first highlighted to guide future strategy design for controllable SAC synthesis.Then,the to-date reported CO_(2)RR,CO oxidation,OER,and ORR mechanism are included and summarized.More importantly,the design principles and design strategies of improving the intrinsic activity,selectivity,and stability are extensively discussed and the engineering strategy is classified as neighbor coordination engineering,metal-atom engineering,and substrate engineering.Via the comprehensive review and summary of state-of-the-art SACs,the synthesis–structure–property–mechanism–design principle relation can be revealed to shed lights into the structural construction of SACs.Finally,we present an outlook on current challenges and future directions for SACs in CO_(x) and O_(2) chemistry.