Ten years of carbon-based metal-free electrocatalysts

Since the discovery of the first carbon-based metal-free electrocatalysts(C-MFECs,i.e.,N-doped carbon nanotubes)for the oxygen reduction reaction in 2009,the field of C-MFECs has grown enormously over the last 10 years.C-MFECs,as alternatives to nonprecious transition metals and/or precious noble metal-based electrocatalysts,have been consistently demonstrated as efficient catalysts for oxygen reduction,oxygen evolution,hydrogen evolution,carbon dioxide reduction,nitrogen reduction,and many other(electro-)chemical reactions.Recent research and development of C-MFECs have indicated their potential applications in fuel cells,metal-air batteries,and hydrogen generation through water oxidation as well as electrochemical production of various commodity chemicals,such as ammonia,alcohols,hydrogen peroxide,and other useful hydrocarbons.Further research and development of C-MFECs would surely revolutionize traditional energy conversion and storage technologies with minimal environmental impact.In this short review article,we summarize the journey of C-MFECs over the past 10 years with an emphasis on materials development and their structure-property characterization for applications in fuel cells and metal-air batteries.Current challenges and future prospects of this emerging field are also discussed.

Nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions

The oxygen reduction/evolution reactions(ORR/OER) are a key electrode process in the development of electrochemical energy conversion and storage devices,such as metal-air batteries and reversible fuel cells.The search for low-cost high-performance nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER alternatives to the widely-used noble metal-based catalysts is a research focus.This review aims to outline the opportunities and available options for these nanocarbon-based bifunctional electrocatalysts.Through discussion of some current scientific issues,we summarize the development and breakthroughs of these electrocatalysts.Then we provide our perspectives on these issues and suggestions for some areas in the further work.We hope that this review can improve the interest in nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER.

Fullerenes and derivatives as electrocatalysts: Promises and challenges

Carbon-based metal-free nanomaterials are promising alternatives to precious metals as electrocatalysts of key energy storage and conversion technologies.Of paramount significance are the establishment of design principles by understanding the catalytic mechanisms and identifying the active sites.Distinct from sp2-conjugated graphene and carbon nanotube,fullerene possesses unique characteristics that are growingly being discovered and exploited by the electrocatalysis community.For instance,the well-defined atomic and molecular structures,the good electron affinity to tune the electronic structures of other substances,the intermolecular self-assembly into superlattices,and the on-demand chemical modification have endowed fullerene with incomparable advantages as electrocatalysts that are otherwise not applicable to other carbon ma-terials.As increasing studies are being reported on this intriguing topic,it is necessary to provide a state-of-the-art overview of the recent progress.This review takes such an initiative by summarizing the promises and challenges in the electrocatalytic applications of fullerene and its derivatives.The content is structured according to the composition and structure of fullerene,including intact fullerene(e.g.,fullerene composite and superlattices)and fullerene derivatives(e.g.,doped,endohedral,and disintegrated fullerene).The synthesis,characterization,catalytic mechanisms,and deficiencies of these fullerene-based materials are explicitly elaborated.We conclude it by sharing our perspectives on the key aspects that future efforts shall consider.

Surface/interface engineering of high-efficiency noble metal-free electrocatalysts for energy-related electrochemical reactions

To date,much efforts have been devoted to the high-efficiency noble metal-free electrocatalysts for hydrogen-and oxygen-involving energy conversion reactions,due to their abundance,low cost and nultifunctionally.Surface/interface engineering is found to be effective in achieving novel physicochemical properties and synergistic effects in nanomaterials for electrocatalysis.Among various engineering strategies,heteroatom-doping has been regarded as a most promising method to improve the electrocatalytic performance via the regulation of electronic structure of catalysts,and numerous works were reported on the synthesis method and mechanism investigation of heteroatom-doping electrocatalysts,though the heteroatom-doping can only provide limited active sites.Engineering of other defects such as vacancies and edge sites and construction of heterostructure have shown to open up a potential avenue for the development of noble metal-free electrocatalysts.In addition,surface functionalization can attach various molecules onto the surface of materials to easily modify their physical or chemical properties,being as a promising complement or substitute for offering materials with catalytic properties.This paper gives the insights into the diverse strategies of surface/interface engineering of the highefficiency noble metal-free electrocatalysts for energy-related electrochemical reactions.The significant advances are summarized.The unique advantages and mechanisms for specific applications are highlighted.The current challenges and outlook of this growing field are also discussed.

Pyrazine-nitrogen-rich exfoliated C4N nanosheets as efficient metal-free polymeric catalysts for oxygen reduction reaction

Herein,we for the first time demonstrate the synthesis of exfoliated C4N nanosheets via a top-down approach and exploit their use as a new class of organic polymeric catalyst for the oxygen reduction reaction(ORR).The obtained C4N nanosheets are semi-conductive with a small band gap of 1.41 eV and contain abundant pyrazine-nitrogen moieties uniformly distributed throughout C4N.Density function theory calculations reveal that the intramolecular charge transfer induced by pyrazine-nitrogen in C4N enables effective charge redistribution to activate the conjugated structure and facilitate the oxygen adsorption,while the exfoliated sheet-like C4N formation renders improved electrochemical active surface area and results in high exposure of active sites.As a result,despite the bulk C4N is not active,the sheet-like C4N yield markedly improved ORR performance,even on a par with the commercial Pt/C catalyst.Our recent findings not only enrich the family members of two-dimensional conjugated polymer nanosheets but also open up new opportunity to explore new metal-free organic polymeric materials for efficient oxygen reduction catalysis and beyond.

Transition metal embedded in nonmetal-doped T-carbon [110]: A superior synergistic trifunctional electrocatalyst for HER, OER and ORR

The design of efficient and low-cost multifunctional electrocatalysts for hydrogen evolution reaction(HER), oxygen evolution reaction(OER) and oxygen reduction reaction(ORR) is critical for the development of clean energy. Two-dimensional(2D) carbon-based nano-materials are becoming more and more popular in heterogeneous catalysis due to their cost-effective and multi-scale tunability as single-atom catalysis(SACs) substrates. In this paper, by using first-principles calculation, we designed and demonstrated a novel macropore T-carbon [110](TC) monolayer as 2D electrocatalyst substrate for HER/OER/ORR, and the synergistic modification of the transition metal and nonmetal atoms(TM-X) were investigated to enhance the multifunctional electrocatalytic performance. We screened out the Co embedded in N-doped TC(Co3@N-TC) from 30 TM@X-TC monolayers as a trifunctional electrocatalysts, which exhibits superior performance for HER/ORR/OER on both thermodynamic and kinetic scales, and with excellent thermal and electrochemical stability. Then, the TC monolayer is naturally macropore with a diameter of 5.36 A and exhibits excellent adsorption capacity for the intermediates and products of the redox reactions. Moreover, we revealed the origin of the electrocatalytic activity using the crystal orbital Hamilton population(COHP) and the molecular orbitals(MOs). The d orbital of Co3@N-TC is significantly hybridized with the p orbital of the intermediates, so that the lone electrons initially occupied in the antibonding state pair up and occupy the downward bonding state, allowing *OH to be appropriately adsorbed onto the TC monolayer. This work not only demonstrates that the TM@X-TC monolayer is a superior synergistic trifunctional electrocatalyst, but also reveals a macropore monolayer material with potential applications in electrocatalysis.

Low-Temperature Working Feasibility of Zinc–Air Batteries with Noble Metal-Free Electrocatalysts

Expanding the application scenario for rechargeable batteries is the key to the terminal utilization of renewable energy.Enabling zinc–air batteries at low temperatures is drawing increasing attention,yet the low-temperature working feasibility of zinc–air batteries with noble metalfree electrocatalysts remains indistinct.In this contribution,the low-temperature performances of zinc–air batteries with noble metal-free electrocatalysts are comprehensively investigated.Armed with a representative noble metal-free bifunctional oxygen electrocatalyst,the zinc–air batteries demonstrate satisfactory yet relatively depressed performance at low temperatures,compared with that at room temperatures.The reduced electrolyte conductivity is identified as one of the limiting factors for the reduced low-temperature performance.Furthermore,electrolyte engineering via solvation structure regulation is performed on the zinc–air batteries with noblemetal-free electrocatalysts,where an improved low-temperature performance is achieved.This work reveals the compatibility between noble metal-free electrocatalysts and low-temperature feasibility/low-temperature performance enhancement strategies for zinc–air batteries and affords new opportunities to satisfy low-cost and efficient energy storage at harsh working conditions.

Fabricating pyridinic N-B sites in porous carbon as efficient metal-free electrocatalyst in conversion CO_(2)into CH_(4)

Electrochemical reduction of CO_(2)(CO_(2)RR)to value-added chemicals is an attractive strategy for greenhouse gas mitigation and carbon recycle.Carbon material is one of most promising electrocatalysts but its product selectivity is limited by few modulating approaches for active sites.Herein,the predominant pyridinic N-B sites(accounting for 80%to all N species)are fabricated in hierarchically porous structure of graphene nanoribbons/amorphous carbon.The graphene nanoribbons and porous structure can accelerate electron and ion/gas transport during CO_(2)RR,respectively.This carbon electrocatalyst exhibits excellent selectivity toward CO_(2)reduction to CH_(4)with the faradaic efficiency of 68%at−0.50 V vs.RHE.As demonstrated by density functional theory,a proper adsorbed energy of∗CO and∗CH_(2)O are generated on the pyridinic N-B site resulting into high CH_(4)selectivity.Therefore,this study provides a novel method to modulate active sites of carbon-based electrocatalyst to obtain high CH_(4)selectivity.

Recent progress in carbon-based electrochemical catalysts:From structure design to potential applications

Advances in research and development of carbon-based metal-free electrocatalysts(C-MFECs)have provided potential alternatives to precious metal catalysts for various reactions important to renewable energy and environmental remediation.This timely but critical review provides an overview of recent breakthroughs(within the past 5 years or so)on C-MFECs in all aspects,including the design and regulation of intrinsic catalytic active sites,design and synthesis of carbon composite and hybrid carbon catalysts,mechanism understanding,and potential applications in clean energy storage and energy/chemical conversion.Current challenges and future opportunities in the field of metal-free carbon electrocatalysis are also discussed to provide forward-looking opportunities for their potential applications in various catalytic processes of practical significance.

Quasi-Three-Dimensional Cyclotriphosphazene-Based Covalent Organic Framework Nanosheet for Efficient Oxygen Reduction

Metal-free carbon-based materials are considered as promising oxygen reduction reaction(ORR)electrocatalysts for clean energy conversion,and their highly dense and exposed carbon active sites are crucial for efficient ORR.In this work,two unique quasi-three-dimensional cyclotriphosphazene-based covalent organic frameworks(Q3CTP-COFs)and their nanosheets were successfully synthesized and applied as ORR electrocatalysts.The abundant electrophilic structure in Q3CTP-COFs induces a high density of carbon active sites,and the unique bilayer stacking of[6+3]imine-linked backbone facilitates the exposure of active carbon sites and accelerates mass diffusion during ORR.In particular,bulk Q3CTP-COFs can be easily exfoliated into thin COF nanosheets(NSs)due to the weak interlayerπ-πinteractions.Q3CTP-COF NSs exhibit highly efficient ORR catalytic activity(half-wave potential of 0.72 V vs.RHE in alkaline electrolyte),which is one of the best COF-based ORR electrocatalysts reported so far.Furthermore,Q3CTP-COF NSs can serve as a promising cathode for Zn-air batteries(delivered power density of 156 mW cm-2 at 300 mA cm^(-2)).This judicious design and accurate synthesis of such COFs with highly dense and exposed active sites and their nanosheets will promote the development of metal-free carbon-based electrocatalysts.

Electronic structure regulation of noble metal-free materials toward alkaline oxygen electrocatalysis

Developing highly efficient,inexpensive catalysts for oxygen electrocatalysis in alkaline electrolytes(i.e.,the oxygen reduction reaction(ORR)and the oxygen evolution reaction(OER))is essential for constructing advanced energy conversion techniques(such as electrolyzers,fuel cells,and metal–air batteries).Recent achievements in efficient noble metal-free ORR and OER catalysts make the replacement of conventional noble metal counterparts a realistic possibility.In particular,various electronic structure regulation strategies have been employed to endow these oxygen catalysts with attractive physicochemical properties and strong synergistic effects,providing significant fundamental understanding to advance in this direction.This review article summarizes recently developed electronic structure regulation strategies for three types of noble metal-free oxygen catalysts:transition metal compounds,single-atom catalysts,and metal-free catalysts.We begin by briefly presenting the basic ORR and OER reaction mechanisms,following this with an analysis of the fundamental relationship between electronic structure and intrinsic electrocatalytic activity for the three categories of catalysts.Subsequently,recent advances in electronic structure regulation strategies for noble metal-free ORR and OER catalysts are systematically dis-cussed.We conclude by summarizing the remaining challenges and presenting our outlook on the future for designing and synthesizing noble metal-free oxygen electrocatalysts.

Carbon-based materials for electrochemical dechlorination

Electrochemical dechlorination reaction(EDR)is a promising,environmentally friendly,and economically profitable technology for treating chlorinated organic pollutants.For efficient environmental protection,electrocatalysts with high stability and low cost are of extremely significance to the development of EDR technology.Carbon-based materials have aroused broad interest as electrocatalysts for many electrochemical reactions due to their characteristics including large specific surface area,controllable structure,good conductivity,and chemical stability.For EDR,the carbon-based materials also show many unique superiorities,like strong adsorption capacity to chlorinated organic compounds(COCs),excellent catalytic activity and stability,and environmental compatibility.This review starts with a detailed summary on the mechanisms of electrochemical dechlorination(direct and indirect electron transfer pathway)and factors affecting the effectiveness of EDR.Then the paper comprehensively overviews the current progresses of carbon-based materials for EDR of COCs,following their two major application scenarios,i.e.,directly as electrocatalysts and as advanced supports for other catalysts.Moreover,the formation of different active sites in carbon-based electrocatalysts and their EDR activities are analyzed.Finally,the current challenges and perspectives in this field are discussed.This review will provide an in-depth understanding for the design of advanced carbon-based materials and promote the development of EDR technology.

Polarized few-layer g-C3N4 as metal-free electrocatalyst for highly efficient reduction of CO2

The greenhouse effect and global warming are serious problems because the increasing global demand for fossil fuels has led to a rapid rise in greenhouse gas exhaust emissions in the atmosphere and disruptive changes in climate. As a major contributor, CO2 has attracted much attention from scientists, who have attempted to convert it into useful products by electrochemical or photoelectrochemical reduction methods. Facile design of efficient but inexpensive and abundant catalysts to convert CO2 into fuels or valuable chemical products is essential for materials chemistry and catalysis in addressing global climate change as well as the energy crisis. Herein, we show that two-dimensional fewlayer graphitic carbon nitride （g-C3N4） can function as an efficient metal-free electrocatalyst for selective reduction of CO2 to CO at low overpotentials with a high Faradaic efficiency of - 80%. The polarized surface of ultrathin g-C3N4 layers （thickness： -1 nm）, with a more reductive conduction band, yields excellent electrochemical activity for CO2 reduction.

Effect of doping order on metal-free heteroatoms dual-doped carbon as oxygen reduction electrocatalyst

Metal-free heteroatoms dual-doped carbon has been recognized as one of the most promising Pt/C-substitutes for oxygen reduction reaction(ORR).Herein,we optimize the preparation process by doping order of metal-free heteroatoms to obtain the best electrocatalytic performance through three types of dual-doped carbon,including XC-N(first X doping then N doping),NC-X(first N doping then X doping) and NXC(N and X doping)(X=P,S and F).XC-N has more defect than the other two indicated by Raman spectra.X-ray photoelectron spectrom(XPS) measurements indicate that N and X have been dual-doped into the carbon matrix with different doping contents and modes,Electrocatalytic results,including the potential of ORR peak(Ep),the half-wave potential,the diffusion-limiting current density mainly follows the order of XC-N>NC-X> NXC,Furthermore,the synergistic effect of second atom doping are also compared with the single doped carbon(NC,PC,SC and FC).The differences in electronegativity and atomic radius of these metal-free heteroatoms can affect the defect degree,the doping content and mode of hete roatoms on carbon matrix,induce polarization effect and space effect to affect O2 adsorption and product desorption,ultimately to the ORR electrocatalytic performance.

Biomass-derived nitrogen self-doped porous activation carbon as an effective bifunctional electrocatalysts

The strategy of adopting cheap precursors or abundant resources,which can be obtained directly from nature,is a simple and excellent method of introducing accessible research into environmentally friendly development.Moreover,this is also an urgent requirement for the sustainable development of green technology.Herein,we introduce a simplistic and expandable method to prepare metal-free biomassderived nitrogen self-doped porous activation carbon(N-PAC) with large specific surface area(S_(BET)=1300.58 m^(2)/g).Moreover,the manufactural electrocatalysts exhibit prominent oxygen reduction reaction(ORR) performance in all PH values.As compared with the commercial Pt/C catalyst,the N-PAC/800 with a positive onset potential at 10 mA/cm^(2)(0.93 V),half-wave potential(0.87 V),and limiting current(6.34 mA/cm^(2)) bring to light excellent catalytic stability,selectivity,and much-enhanced methanol tolerance.Furthermore,the prepared electrocatalysts possess considerable hydrogen evolution reaction(HER) performance with a less onset potential of 0.218 V(acidic medium) and0.271 V(alkaline medium) respectively,which can show similar catalytic activity across the whole pH range.Such bifunctional electrocatalyst,with excellent electrocatalytic properties,resource-rich,low cost,and environmental-friendly,hold a promising application in energy conversion and reserve.

Copper nanoparticles/polyaniline-derived mesoporous carbon electrocatalysts for hydrazine oxidation

Copper nanoparticles-decorated polyaniline- derived mesoporous carbon that can serve as noble metal-free electrocatalyst for the hydrazine oxidation reaction （HzOR） is synthesized via a facile synthetic route. The material exhibits excellent electrocatalytic activity toward HzOR with low overpotential and high current density. The material also remains stable during the electrocatalytic reaction for long time. Its good electro- catalytic performance makes this material a promising alternative to conventional noble metal-based catalysts （e.g., Pt） that are commonly used in HzOR-based fuel cells.

Recent Progress in Carbon-based Materials of Non-Noble Metal Catalysts for ORR in Acidic Environment

Proton exchange membrane fuel cell(PEMFC)has important implications for the success of clean transportation in the future.One of the key factors affecting the cost and performance of PEMFC is the cathode electrocatalyst for the oxygen reduction reaction(ORR)to overcome sluggish kinetics and instability in an acidic environment.As an essential component of the electrocatalyst,the support material largely determines the activity,mass transfer,charge transfer,and durability of the electrocatalyst.Thereby,the support material plays a critical role in the overall performance of the electrocatalyst.Carbonbased materials are widely used as electrocatalyst supports because of their high porosity,conductivity,chemical stability,and tunable morphology.Recently,some new carbon-based materials with excellent structure have been introduced,such as carbon nanotubes,carbon nanowires,graphene,metal-organic framework(MOF)-derived carbon,and biomass-derived carbon materials.Combined with a variety of strategies,such as controllable construction of porous structures and surface defects,proper doping heteroatoms,the ingenious design of model electrocatalysts,and predictive theoretical calculation,a new reliable path was provided for further improving the performance of electrocatalysts and exploring the catalytic mechanism.Based on the topic of carbon-based materials for ORR in acidic medium,this review summarizes the up-to-date progress and breakthroughs,highlights the factors affecting the catalytic activity and stability of ORR electrocatalysts in acids,and discusses their future application and development.

Rational confinement engineering of MOF-derived carbonbased electrocatalysts toward CO_(2)reduction and O_(2)reduction reactions

The goal of global carbon peak and neutrality gives an impetus to the utilization of clean energy(e.g.,fuel cell)and carbon dioxide(CO_(2))at a large scale,where the oxygen reduction reaction(ORR)and CO_(2)reduction reaction(CO_(2)RR)are the key reactions via the sustainable system,respectively.As a main precursor for fabricating affordable carbon-based electrocatalysts with uniformly dispersed active centers and tailorable performances for ORR and CO_(2)RR,metal organic frameworks(MOFs)have captured a surge of interest in recent years.Despite the facilitated development of MOF-derived carbon-based electrocatalysts by many investigations,it is still plagued by high overpotential and unsatisfied life span,which are greatly determined by the efficient and alterable confinement effect on synthesis and performance.In this review,firstly,the confined synthetic strategies(doping engineering,defect engineering,geometric engineering,etc.)of MOF-derived carbon-based electrocatalysts with multi-sized active centers(atom,atomic clusters and nanoparticles(NPs))are systematically summarized;secondly,the confinement effect on the interaction of ORR and CO_(2)RR intermediates,as well as the catalytic durability and activity,was discussed from chemical and physical aspects.In the end,the review discusses the remaining challenges and emerging research topics in the future,including support upgradation and catalyst innovation,high selectivity and effective confinement synthesis,in situ and operando characterization techniques,theoretical investigation,and artificial intelligence(AI)assistant.The new understanding and insights into these aspects will guide the rational confinement concept of MOF-derived carbon-based electrocatalysts for ORR and CO_(2)RR with optimized performances in terms of confinement engineering and are believed to be helpful for filling the existing gaps between scientific communities and practical use.

Boosting the Activity and Stability with Dual-Metal-N Couplings for Li–O_(2)Battery

Electrocatalysts with high efficiency are crucial for improving the storage capacity and electrochemical stability of lithium–oxygen batteries(LOBs).In this work,through a facile hydrothermal method,cobalt–nitrogen-doped carbon nanocubes(Co–N/C),the calcination products of zeolitic imidazolate framework(ZIF–67)are encapsulated by ultrathin C–MoS_(2) nanosheets to obtain Co–N/C@C–MoS_(2) composites which are used as host materials for the oxygen cathode.The synergistic effect between Co–N_(x) active sites and Mo–N coupling centers effectively promotes the formation and decomposition of Li_(2)O_(2) during repeated discharge and charge process.The mesoporous C–MoS_(2) nanosheets with delicately designed morphology facilitate charge transfer and account for improved reaction kinetics and more importantly,suppressed side reactions between the carbon materials and the electrolyte.The oxygen cathode with the Co–N/C@C–MoS_(2)host shows a high initial discharge specific capacity of 21197 mAh g^(-1)and a long operation life of 332 cycles.Theoretical calculation provides in-depth explanation for the reaction mechanism and offers insights for the rational design of electrocatalysts for LOBs.

Synergistic integration of self-supported 1T/2H-WS_(2) and nitrogendoped rGO on carbon cloth for pH-universal electrocatalytic hydrogen evolution

Hydrogen economy based on electrochemical water splitting exemplified one of the most promising means for overcoming the rapid consumption of fossil fuels and the serious deterioration of global climate.The development of earth-abundant,efficient,and durable electrocatalysts for hydrogen evolution reaction(HER)plays a vital role in the commercialization of water electrolysis.Regard,the self-supported electrode with unique nitrogen-doped reduced graphene oxide(N-rGO)nanoflakes and WS_(2) hierarchical nanoflower that were grown directly on carbon cloth(CC)substrate(WS_(2)/N-rGO/CC)was successfully synthesized using a facile dual-step hydrothermal approach.The as-synthesized 50%1T/2H-WS_(2)/N-rGO/CC(WGC),which possessed high metallic 1T phase of 57%not only efficiently exposed more active sites and accelerated mass/charge diffusion,but also endowed excellent structural lustiness,robust stability,and durability at a high current density.As a result,the 50%WGC exhibited lower overpotentials and Tafel slopes of 21.13 mV(29.55 mV∙dec^(-1))and 80.35 mV(137.02 mV∙dec^(-1))as compared to 20%Pt-C/CC,respectively for catalyzing acidic and alkaline hydrogen evolution reactions.Pivotally,the as-synthesized 50%WGC also depicted long-term stability for more than 8 h in the high-current-density regions(100 and 220 mA∙cm^(-2)).In brief,this work reveals a self-supported electrode as an extraordinary alternative to Pt-based catalysts for HER in a wide pH range,while paving a facile strategy to develop advanced electrocatalysts with abundant heterointerfaces for practical applications in energy-saving hydrogen production.