A review of carbon-based catalysts and catalyst supports for simultaneous organic electro-oxidation and hydrogen evolution reactions

WANG Zhi-dong ,XIA Tian ,LI Zhen-hua,2, ,SHAO Ming-fei,2,

（1.Beijing University of Chemical Technology,Beijing 100029, China;2.Quzhou Institute for Innovation in Resource Chemical Engineering,Quzhou 324000, China）

Abstract：Producing organic electro-oxidation and hydrogen evolution reactions (HER) simultaneously in an electrolytic cell is an appealing method for generating valuable chemicals at the anode while also producing H2 at the cathode.Within this framework,the task of designing energy-saving electrocatalysts with high selectivity and stability is a considerable challenge.Carbon-based catalysts,along with their supports,have emerged as promising candidates due to their diverse sources,large specific surface area,high porosity and multidimensional characteristics.This review summarizes progress from 2012 to 2022,in the use of carbon-based catalysts and their supports for organic electrooxidation and HER.It delves into outer-sphere electrooxidation mechanisms involving molecule-mediated oxidation and oxidative radical coupling reactions,as well as inner-sphere electrooxidation mechanisms,encompassing both acidic and alkaline electrolytes.The review also explores prospective research directions within this domain,addressing various aspects such as the design of electrocatalytic materials,the study of the relationship between the structure and properties of electrocatalysts,as well as examining their potential industrial applications.

Key words: Carbon-based materials；Hydrogen；Electrochemical water splitting；Organic oxidation；Electrocatalysis

1 Introduction

Hydrogen (H2) has diverse sources,possesses a high calorific value (1.4×108J kg−1),and generates zero emissions.Consequently,it is regarded as the“ultimate energy” to mitigate the current environmental problems and energy crises[1–4].The electrochemical conversion of water to H2driven by clean energy(e.g.,solar,wind) is one of the most promising green pathways for decarbonization[5–6].However,this process exhibits high overpotential between the anode and cathode (>1.50 V at 10 mA cm−2)[7–10].Traditional electrochemical water splitting involves hydrogen evolution reaction (HER) and oxygen evolution reactions (OER),as shown below (Fig.1a):

Fig.1 (a) Diagrammatic representation of electrolytic H2 production.(b) Scheme forelectrolytic H2 production coupled with organic electrooxidation and (c) Comparison of the HER,ethanol electrooxidation reaction (EOR) and OER

In an acidic or neutral medium:

The HER involves a two-electron transfer,whereas the OER involves a four-electron transfer.The main bottleneck of H2production via water splitting is the sluggish OER,where the generated O2is not valuable and this might lead to the mixing of H2and O2[11–13].

The addition of the oxygen atoms of H2O to organic molecules instead of their evolution as O2is a more atom-economical approach to upgrading the anode for H2production,which is thermodynamically more favorable than OER[14–19](Fig.1b,c).Electrochemical alcohol oxidation reactions (AORs) involving methanol,ethanol,glycerol,glucose,5-hydroxymethylfurfural and aryl alcohols are used in liquid fuel cells or for generating high-value fine chemicals (e.g.,acids,ketones or aldehydes) along with H2production[20–22].Despite remarkable advancements in this field,the ongoing challenge lies in enhancing the efficiency and stability of the corresponding electrocatalysts.For instance,Pt-and Pd-based catalysts exhibit excellent catalytic activity in both organic electrooxidation and HER,but have limited availability and are expensive in nature[23].In addition,their stability is unsatisfactory[24].In this context,carbonbased materials are promising catalysts for organic electrooxidation and HER due to their diverse sources,large specific surface area,high porosity,and multidimensional structure[25–28].

Carbons are typically utilized as conductive supports to load and uniformly disperse catalytically active species,especially precious metals.Carbon supports reduce the dosage of the active species and stabilize them[29].Moreover,heteroatom (N,P and S)-doped carbons can be directly used as catalysts for organic electrooxidation and HER[30–36].

This review summarizes the recent advancements in carbon-based catalysts and conductive supports employed in the context of organic electrooxidation coupled with HER.Section 2 provides an overview of recent findings concerning the utilization of carbon as both electrocatalyst and support material in the electrooxidation of different substances such as methanol,ethanol,glycerol,benzyl alcohol,5-hydroxymethylfurfural and various other chemicals.This section also deals with the outer and inner sphere mechanisms of electrooxidation.Section 3 summarizes the recent reports on the use of carbon as an electrocatalyst and a support material in HER.Finally,the review provides a prospect on potential research directions to commercialize carbon-based materials and supports for bi-functional organic electrooxidation and HER activity.

2 Organic electrooxidation using carbon-based materials

2.1 Direct use of carbon catalysts

Taube[37]and Bard[38]proposed that an electrochemical organic oxidation reaction involves outer and inner sphere reactions.In the inner sphere reaction,the reactants are chemically adsorbed onto the electrode surface (Fig.2a) and involve proton-coupled electron transfer (PCET).This pathway requires the mediation of an active oxygen species.For instance,ethanol oxidation on gold surfaces requires the deprotonation of some ethanol molecules to form ethoxy anions.In an alkaline environment,negatively polarized reactive species,including OH−ions,ethoxy anions,and neutral ethanol molecules with terminal oxygen atoms,are sequentially repelled from the negatively charged electrode.Following this,a considerable number of ethanol molecules undergo electrochemical adsorption onto the gold electrode surface,resulting in the formation of Au(OH)adsspecies.These species react with the OH−ions,chemisorbed onto the gold surface,thus oxidizing ethanol.A higher number of OH−ions on the electrode surface generates a greater number of active sites,thereby accelerating the electrocatalytic reaction (Fig.2b)[39].

Fig.2 (a) Scheme for the outer and inner sphere reactions and (b) reaction mechanism for the oxidation of ethanol to the acetate ion on the surface of a gold film working electrode (WE) in an alkaline solution.Used with authorization from Ref [39].Copyright by 2019 American Chemical Society

In contrast,outer sphere reactions occur near the electrode surface without strong interactions.In this pathway,the electrons necessary for the reaction pass through the solvent layer,facilitating the reaction.Outer sphere electrooxidation has fewer restrictions for the choice of electrode materials.Thus,cheaper carbon-based materials have been preferred for these reactions[40],such as C―H bond oxidation[41]and C―OH bond oxidation[42].However,carbon-based anodes only function as conductors,aiding in the charge collection and transmission.It is essential to note that they do not directly engages in the corresponding chemical reactions.Instead,their main function is to provide a conductive surface for efficient electron transfer.Notably,solvents and electrolytes determine the reaction pathway selectivity,yield,and other factors.

Outer sphere electrooxidation can occur in a molecule-mediated oxidation reaction or an oxidative radical coupling reaction.For molecule-mediated oxidation,soluble redox mediator,such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO),are needed to drive the electrooxidation process.Cha et al.[43]used TEMPO as the mediator to achieve the electrooxidation of biomass-derived 5-hydroxymethylfurfural(HMF) to produce 2,5-furandicarboxylic acid (FDCA)with 100% Faradaic efficiency (FE).FDCA is a valuable feedstock to produce renewable polymers such as polyethylene 2,5-furandicarboxylate (PEF),which is a promising alternative to polyethylene terephthalate(PET)[44–46].Initially,the reaction was conducted with a gold anode,but the addition of TEMPO and HMF substantially decreased the initial potential of the reaction (1.09 V vs.RHE).As this reaction is an outer sphere reaction,carbon-based materials with larger surface areas can theoretically drive the reaction(Fig.3a).A carbon-based anode exhibited an earlier reaction onset potential than that of the gold anode.However,the HMF conversion rate (100%) and the FDCA yield (98.8%) from the carbon-based anode were comparable to those of the gold anode (99.8%FDCA yield).Hence,noble metal-based anodes can be replaced by inexpensive carbon-based anodes in such reactions.

Fig.3 (a) Scheme for TEMPO-mediated HMF electrooxidation.(b) Formation of 4-ethylnonane,a valuable liquid fuel,from the electrooxidation of 2-methylfuran (2-MF).Reproduced with permission from Ref [48].Copyright from 2019 American Chemical Society

Oxidative radical coupling has proven to be an efficient method,to increase the carbon number of organic chemicals through outer sphere electrooxidation,without any need for mediators.In this process,the organic reactant first involves a single-electron transfer to be oxidized to radical cations.Thus coupling of radical cations can afford multi-carbon products[47].For instance,Chen et al.[48]reported the use of graphite electrode to oxidize 2-methylfuran (2-MF).Then,3-(5-methylfuran-2-yl) hexane-2,5-dione was produced via the reaction between 3-hexene-2,5-dione and the initially formed radical cation.Moreover,4-ethylnonane,a valuable liquid fuel,was obtained in 91% yield via further hydrodeoxygenation (Fig.3b).

2.2 Use of carbon as a support material

The process of inner sphere electrooxidation necessitates the utilization of an electrocatalyst capable of precisely regulating key reaction elements.These encompass the reaction rate,selectivity and product yields.However,pure carbon materials (such as amorphous carbon,graphene and carbon nanotubes)usually show a poor catalytic activity.Hence,carbon materials have been used as conductive supports in inner sphere electrooxidation reactions.The prototypical electrocatalyst containing carbon-based support is the Pt/C catalyst,which is widely used in various organic reactions.Over recent years,there has been extensive research on the electrocatalysts grown in situ on self-supported carbon substrates,such as carbon paper.This approach promotes the exposure of reactive sites and facilitates mass transfer,thereby enhancing the reaction rate.This section highlights the recent advancements in the inner sphere electrooxidation reactions of methanol,ethanol,glycerol,benzyl alcohol,HMF and various other chemicals (Table 1)conducted on carbon-based supports.

Table 1 Summary of reported electrocatalysts containing carbon substrates for organic oxidation reactions

Methanol electrooxidation reaction (MOR): This reaction showcases the versatility of methanol as an ideal reactant for alcohol electrooxidation,due to its widespread availability,cost-effectiveness and inherent reactivity.It is reactive and water-soluble owing to its hydroxyl groups and high hydrogen content(12.6%,mass fraction),which enhances its suitability for electrochemical processes[49–50].The MOR mechanism varies with the acidity or alkalinity of the solution.

In an acidic environment,MOR occurs via the following pathway:

Conversely,in an alkaline environment,MOR is diverse,involving the formate and CO2pathways,as shown below:

As the onset potential of MOR lags behind that of OER and CO2,which is produced in an acidic environment,researchers have primarily focused on MOR in an alkaline environment[51–52].Fu et al.[53]synthesized Pt-Co3O4on carbon paper (CP) as a bifunctional catalyst for MOR and HER (Table 1).Pt-Co3O4/CP demonstrated the need for a potential of0.56 V (vs.RHE) to reach a current density of 10 mA cm−2in 1.0 mol L−1NaOH+3.5% NaCl containing 2 mol L−1methanol.The potential for MOR is 102 mV less than that of OER,suggesting that MOR is thermodynamically more advantageous relative to OER.Moreover,formate is the main product of MOR with Faradaic Efficiency (FE) >80%,and the current density exhibited little change after 20 h at 1.0 V (vs.RHE).This was attributed to the reduction of the Pt ions into Pt clusters by the electrons around the oxygen vacancy of Co3O4.The uniformly dispersed Pt clusters on Co3O4facilitate intimate interfacial contact,which is beneficial for long-term electrocatalysis.

In addition,researchers’ interest in the use of non-precious metal catalysts for MOR has enlarged.This interest is drove by concerns about the limited availability and considerable cost associated with precious metal resources.Jin et al.[54]synthesized a NiMn-layered double hydroxide (LDH) electrocatalyst for MOR,which could reach a current density of 10 mA cm−2at 1.33 V (vs.RHE) in 1 mol L−1KOH containing 3 mol L−1MeOH.This potential is less than that required for OER.Furthermore,formate emerges as the primary product of MOR with an FE exceeding 95%.Therefore,advocating for the use of non-precious metals in MOR is warranted.

Ethanol electrooxidation reaction (EOR): Ethanol is a non-hazardous,liquid alcohol at room temperature that is easily available in nature.EOR also occurs via two different pathways in acidic and alkaline environments[55].EOR under acidic conditions produces CO2,whereas EOR under alkaline conditions generates acetate anions,this could further afford high-value chemicals such as acetic acid or ethyl acetate[51].

Precious metal-based catalysts are the preferred anodic EOR catalysts.Vizza et al.[56]used carbon-supported Rh and Pt as anodic EOR and cathodic HER catalysts,this produce acetate and H2,respectively.A high current density (500 mA cm−2) was achieved at 0.7 V (vs.RHE) in 2 mol L−1KOH with 2 mol L−1ethanol.Lucas-Consuegra et al.[57]used PtRh/C as the anodic EOR catalyst to produce acetaldehyde in 1 mol L−1KOH with 6 mol L−1ethanol,where a current density of 250 mA cm−2was achieved at 1.1 V(vs.RHE).Zheng et al.[58]synthesized carbon papersupported Co3O4nanosheets as an EOR catalyst,which demonstrated high FE (98%) for acetate and reached a current density of 10 mA cm−2at 1.445 V(vs.RHE),which is less when compared to OER(1.50 V).The reduced energy requirement was attributed to the abundant presence of Co3+on the (111)surface active sites[59–60].

Notably,transition metal-based catalysts require high reaction potentials compared to the precious metal-based catalysts,which suggests a high energy requirement.Therefore,further research should focus on enhancing the selectivity of the oxidation process and increasing the performance of non-precious metalbased catalysts.

Glycerol electrooxidation reaction (GOR): Glycerol is a simple polyol and mainly obtained as a byproduct during biofuel production.Glycerol can be oxidized into diverse high-value chemicals such as glyceraldehyde,glycerate,glycolate and formate(Fig.4a).The increasing demand for biofuel in recent years has led to an excess supply of glycerol byproducts,thus researchers have increasingly focused on GOR to explore its potential applications[61].

Fig.4 (a) Proposed reaction pathways for glycerol electrooxidation (GOR).(b) Scanning electron microscopy (SEM) images of the Ni-Mo-N.(c) Diagrammatic representation of the concurrent electrolytic H2 and formate production from glycerol aqueous solution and (d) LSV curves of Ni-Mo-N/CFC in 1.0 mol L−1 KOH with or without 0.1 mol L−1 glycerol.Reproduced with permission from Ref [64]

Linares et al.[62]studied the GOR performances of Pt/C electrodes with different Pt mass contents(20%,40% and 60%) in 4.0 mol L−1KOH with 1.0 mol L−1glycerol at 60 °C.The selectivity of the Pt/C catalyst towards the DHA oxidation pathway increased with rise in Pt content.Pd0.5Ni0.5/C,a GOR catalyst,exhibited a peak potential of 0.85 V (vs.RHE),which was less than those of other PdNi/C alloy catalysts under the same conditions (1 mol L−1Na-OH and 0.1 mol L−1glycerol)[63].Additionally,it was observed that the composition and structure of alloy carbon-based catalysts influenced the oxidation pathways of GOR.

However,Pt-based electrodes face challenge,such as the susceptibility to deactivation caused by poisoning from intermediate carbon species (e.g.,formate and CO).Thus,researchers have shown interest in studying the performance of carbon-based catalysts in GOR.For instance,N-doped catalysts demonstrate low electrical resistance and high mechanical stability.Shi et al.[64]synthesized a bi-functional HER and GOR catalyst,Ni-Mo-N,on carbon fiber cloth (CFC),which showed promising bi-functional activity for both HER and GOR.The electrolytic cell with Ni-Mo-N/CFC as both the anode and cathode required a voltage of 1.36 V to reach a current density of 10 mA cm−2in 1 mol L−1KOH with 0.1 mol L−1glycerol;this potential was less compared to OER by 260 mV (Fig.4b-d).The overall FE reached 99.7%with a formate yield of 95%,which not only suggests high catalytic activity but also demonstrates the selective conversion of glycerol to formate,a high-value chemical.Ni-Mo-N/CFC maintained a stable cell voltage at 10 mA cm−2for over 10 h,which confirms its stability and durability in an alkaline environment.Shi et al.also investigated GOR in an acidic environment using carbon paper-supported MnO2as the catalyst,which required a potential of 1.36 V at 10 mA cm−2,which is 270 mV lower than that of acidic OER.MnO2/CP showed a long-term stability of 850 h[65].

Elemental doping of carbon-supported transition metal catalysts is a common approach for varying the charge densities of transition metal materials.This process disrupts their electroneutrality leading to the creation of abundant catalytic active sites.However,the electrocatalysts must be rationally designed to ensure their suitability for the corresponding oxidation reaction.

Benzyl alcohol electrooxidation reaction(BAOR): The products obtained from the oxidation of aromatic alcohols have been subject to detailed investigation due to their potential utility as intermediates or raw materials for the production of fine chemicals[51].

Wang et al.[66]synthesized NC@CuCo2Nx/CF(NC,N-doped carbon;CF,carbon fabric) via pyrolysis under hydrothermal conditions and NH3atmosphere.They used it as the anode and cathode for BAOR and HER.NC@CuCo2Nx/CF exhibited high catalytic activity and selectivity for BAOR,producing benzaldehyde with 95% selectivity.Moreover,NC@CuCo2Nx/CF showed negligible degradation at a stable current density after 60 h,maintaining a benzaldehyde yield of 96% even after 10 BAOR reaction cycles.It was observed that the remarkable electrocatalytic activity was due to hierarchical architecture,which boosts the exposure of catalytic active sites and also facilitates mass transport.This study confirms the probability of simultaneously generating H2and high-value fine chemicals.

HMF electrooxidation reaction (HMFOR): HMF is a crucial platform chemical derived from biomass,specifically cellulose,constituting approximately 40%of lignocellulosic biomass.The oxidation of HMF affords high-value chemicals[67]such as 2,5-diformylfuran(DFF) or 5-hydroxymethyl-2-furancarboxylic acid(HMFCA),both of which can be further converted to FDCA (Fig.5a)[68–69].Li et al.[70]used Pt/C for the electrooxidation of HMF to DFF with 70% selectivity in 0.1 mol L−1KOH with 20 mmol L−1HMF.Yang et al[71].used a Pt/Fe3O4/rGO catalyst for HMFOR and observed a low onset potential (0.30 V vs.Ag/AgCl)in 0.05 mol L−1H2SO4with 0.5 mmol L−1HMF.The HMF conversion rate after 20 h of HMFOR was 7.2%with 94.4% DFF selectivity.Wang et al.[72]synthesized a Ni3N@C catalyst for HMFOR,which required a potential of only 1.55 V vs.RHE to obtain a current density of 50 mA cm−2in 1.0 mol L−1KOH with 10 mmol L−1HMF.This potential was 240 mV less than that of OER,and the FDCA yield was～100%(Fig.5b-e).In-situ electrochemical methods were used to monitor the intermediates during HMFOR under alkaline conditions.The resulting data indicated that the conversion of HMF to FDCA occurred primarily via the HMFCA pathway.The robust interaction between Ni3N and carbon substrates not only modifies the electronic structure of Ni+but also maintains the morphology of the precursor.This is essential for enhancing the charge-transfer rate and the stability of Ni3N@C.The present study demonstrated the feasibility of nonprecious metal carbon-based catalysts for simultaneous HMFOR and HER.

Fig.5 (a) Two possible HMF oxidation pathways.(b) Diagram depicting the synthesis of the Ni3N@C electrocatalyst.(c) SEM image of Ni3N@C.(d) LSV curves of Ni3N@C in 1.0 mol L−1 KOH with or without 10 mmol L−1 HMF and (e) FDCA yield (%) in 6 successive electrolysis cycles with Ni3N@C.Reproduced with permission from Ref [72].Copyright from 2019 Angewandte Chemie International Edition

3 HER using carbon-based materials

HER involves the three steps for the reduction of H+within acidic medium or H2O in alkaline medium,ultimately generating H2on the electrode surface with a minimal applied potential.The initial step in HER involves the Volmer reaction.This process involves the reduction of H+on the surface of the catalyst (M)to generate adsorbed hydrogen atoms (H*).The proton sources in alkaline and acidic electrolytes are H2O and H3O+,respectively (Eq.5).The subsequent H2formation can occur through the Heyrovsky reaction(Eq.6) or the Tafel reaction (Eq.7),or both.In the Heyrovsky step,another proton and electron react with H*to produce H2.In the Tafel step,two H*combine on the electrode surface to produce H2[73].

3.1 Direct use of carbon materials for HER

To date,the direct use of carbon materials as HER catalysts has not been extensively studied due to the catalytic inertness of pure carbon.Kim et al.[74]observed that carbon nanofibers (CNFs) require an overpotential of 442 mV to reach a current density of 10 mA cm−2for HER in a 0.5 mol L−1H2SO4.Mesoporous carbon materials are mostly used in catalysis due to their large specific surface area,adjustable pore size and morphology,excellent conductivity and chemical stability.Additionally,researchers have shown significant interest in elemental doping and composite strategies to obtain mesoporous carbon materials with tunable functionality[75].Zheng et al.[76]doped N,P,and S in porous carbon to fabricate a HER catalysts (NPS-PC) with a homogeneous mesoporous structure,which needed the overpotential of 260 and 250 mV at 10 mA cm−2in 0.5 mol L−1H2SO4solution or 1 mol L−1KOH solution with Tafel slope of 86 mV dec−1and 113 mV dec−1respectively,lower than those of commercial Pt/C catalysts (Fig.6a-d).NPS-PC demonstrated a relatively stable current density for 10 h.This HER performance was the result of a high doping concentration,large surface area,and homogenous porous structure in both acidic and alkaline electrolytes.

Fig.6 (a) Diagrammatic representation of the synthesis of NPS-PC for bi-functional ORR and HER.(b) SEM image of NPS-PC and LSV curves in (c) 0.5 mol L−1 H2SO4 and (d) 1 mol L−1 KOH.Reproduced with permission from Ref [76].Copyright from 2020 Elsevier

Nevertheless,for the direct use of carbon materials for HER,developing carbon-based metal-free electrocatalysts is very crucial.Qiao et al.[77]reported a g-C3N4@NG (N-doped graphene) catalyst that achieved a current density of 10 mA cm−2with an overpotential of 240 mV in 0.5 mol L−1H2SO4solution,and it’s Tafel slope was 51.5 mV dec−1.Subsequently,Qu et al.[78]developed an analogous g-C3N4@G MMs catalyst,which demonstrated a lower overpotential than that of g-C3N4@NG by 21 mV under the same reaction conditions.Cui et al.[79]explored the downsizing of g-C3N4into quantum dots,showing a low onset potential of 0 V and overpotential of 208 mV at 10 mA cm−2in an acidic electrolyte.The Tafel slope was 52 mV dec−1.This downsizing strategy led to the exposure of a greater number of unsaturated C―N centers for efficient H+ion adsorption,while the ultrathin QDs promoted electron transfer,thereby enhancing the HER.

Furthermore,bio-based metal-free carbon materials have been used as HER catalysts.Xie et al.[80]synthesized N-doped Fru/Gu-HTC-1000 catalyst using guanine and glucose using the in-situ method.Their results demonstrated an overpotential of 350 mV at 10 mA cm−2in a 1 mol L−1KOH solution with a Tafel slope of 108 mV dec−1.However,further investigation into the direct use of carbon-based materials in HER is necessary.

3.2 Use of carbon supports in HER

Even though Pt/C is a representative catalyst for HER[81],the use of Pt,which is an expensive metal,must be reduced for widespread practical applications.Wang et al.[82]synthesized a Pt/NPC catalyst in which N and P are co-doped into a carbon network.Pt/NPC exhibited an overpotential of 21.7 mV at 10 mA cm−2in a 0.5 mol L−1H2SO4solution.Notably,the Pt loading in Pt/NPC was only 1.82%,which is significantly less than that in commercial Pt/C (20%).

Carbon-based transition metal catalysts are also good HER catalysts.Bao et al.[83]fabricated a FeCo@NCNTs catalyst by depositing a FeCo alloy onto N-doped carbon nanotubes.FeCo@NCNTs exhibited an overpotential of 240 mV at 10 mA cm−2in a 0.1 mol L−1H2SO4solution with a Tafel slope of 72 mV dec−1.Moreover,various metal oxides and sulfides can be used as active constituents for HER.Hu et al.[84]fabricated a FeS2@RGO catalyst by in-situ deposition of nano-sized FeS2particles onto the reduced graphene oxide.FeS2@RGO demonstrated an overpotential of 139 mV at 10 mA cm−2in 0.5 mol L−1H2SO4with a Tafel slope of 66 mV dec−1.FeS2@RGO demonstrated excellent catalytic activity and stability for 10 h with an overpotential of 170 mV in the same electrolyte.Metal–organic frameworks (MOFs) are made up of inorganic secondary structural elements,such as metal ions or oxygen clusters,and organic linkers.They have organized pore structure,a wide surface area,as well as abundant metal sites[85].Due to presence of these features,MOFs emerge as promising precursors for carbon-based HER catalysts[86].Yun et al.[87]synthesized a Ni-ZIF/NC catalyst (ZIF,zeolitic imidazolate framework;NC,N-doped porous carbon) with a nanotube-like morphology and N-doping.ZIF/NC exhibited low overpotentials of 163.0 mV in 1 mol L−1KOH and 177.4 mV in 0.5 mol L−1H2SO4at 10 mA cm−2,with Tafel slopes of 85.0 and 83.9 mV dec−1,respectively (Fig.7a-b).Ni-ZIF/NC also showed excellent stability (>50 h) in both acidic and alkali environments.In-situ Raman spectroscopy revealed that the presence of carbon defects in ZIF/NC enhance its HER performance by forming Ni-Nxand Cu-Nxactive sites.Ni contained more defects(higherID/IG),which explains its higher HER performance than that of Cu (Fig.7c).Other M-ZIF/NC catalysts were also explored for HER (Fig.7d).The formation of M-Nxsites has been attributed to M-N inter- actions that improve the electronic structure and promote HER electron transfer[88].Hydrogen adsorption at the M-Nxsites is closer to zero than pyridinic-N,pyrrolic-N,and graphitic-N,which indicates HER at the M-Nxsites[89].

Fig.7 (a) Ni-zeolitic imidazolate framework/N-doped porous carbon (Ni-ZIF/NC) electrocatalyst for HER.(b) SEM image of Ni-ZIF/NC.(c) Raman spectra of different catalysts and (d) Defect design in M–ZIF/NC to enhance HER.Reproduced with permission from Ref [87].Copyright from 2021 Elsevier

Semiconductor materials such as TiO2have also been explored for HER[90].However,due to the poor electrical conductivity of semiconductors,combining them with carbon supports is a promising approach to enhance their HER catalytic performance.

4 Summary and outlook

This review summarizes recent developments in bi-functional carbon-based catalysts and catalyst supports for simultaneous organic electrooxidation and HER.Carbon-based catalysts could follow an outer sphere mechanism of molecule-mediated oxidation or oxidative radical coupling reaction.Whereas,carbonsupports based electrocatalysts follow an inner sphere mechanism.Table 2 presents a summary of previously reported carbon-based HER electrocatalysts.Despite the significant progress achieved in this field,further research is required into the following aspects:

Table 2 Summary of carbon-based HER catalysts

(1) Material synthesis.Bi-functional carbonbased catalysts for HER and organic electrooxidation require the use of precious metals.However,thesenoble-metal catalysts are deactivated owing to the accumulation of reaction intermediates or the presence of impurities in the electrolyte during reactions.Researchers have tried the in-situ regeneration of active sites to achieve long-term stable operation[91].Moreover,transition metal-based or metal-free carbon catalysts avoid catalyst deactivation and are less expensive than the noble metal catalysts.Hence,facile methods for the large-scale preparation of carbonbased materials must be developed.

(2) Structure-property relationships and reaction mechanism.The physicochemical properties and electrocatalytic performance of carbon-based materials depend on their structural features.Advanced highthroughput computation studies can predict the properties of new carbon-based materials by establishing structure–property relationships.Nevertheless,the accuracy of these predictions relies on the availability of appropriate computational software and the complexity of the reaction under consideration.Thus,suitable computational research methods must be developed for mechanistic studies of potential carbon-based catalysts for HER and organic electrooxidation reactions.Furthermore,there is a need to develop advanced electrochemical in-situ characterization methods to illustrate the reactive structures of the catalysts and understand the underlying reaction mechanisms,such as advanced in-situ Raman and infrared spectroscopy.

(3) Future applications.There is a considerable gap between academic research on carbon-based electrocatalysts for HER and organic electrooxidation and their industrial applicability.For example,the evaluation of their bifunctional performance is primarily based on three-electrode systems.However,their long-term stability,catalytic activity and selectivity need to be evaluated in a two-electrode flow reactor under industrial conditions.Additionally,the problem of downstream separation and purification need to be addressed.Futhermore,introducing an external field,such as a light field,a magnetic field,elastic strain,external pressure,and a gravity field,is an attractive tactic for improving the mass transfer and changing the reaction kinetics,which could enhance electrocatalytic performance[92].

Acknowledgments

This work was funded by the National Key R&D Program of China (2022YFB4002700),the National Natural Science Foundation of China (22108008,22090031,21991102,22288102),the Young Elite Scientist Sponsorship Program by CAST (2021QNRC001),and the Fundamental Research Funds for the Central Universities (buctrc202011).