過(guò)渡金屬氧化物/硫?qū)倩衔镉糜陔娀瘜W(xué)氧還原制過(guò)氧化氫

關(guān)鍵詞：氧化物；硫?qū)倩衔?；氧還原反應(yīng)；過(guò)氧化氫電合成；活性位點(diǎn)

1 Introduction

Hydrogen peroxide （H2O2）， as an important oxidative reagent，is greatly demanded in paper bleaching 1，2， water purification 3，4，integrated circuit cleaning 5 and various procedures of chemicalengineering 6–9. More than 95% of the mass fabrication of H2O2is based on the conventional anthraquinone process 10， whichrequires noble metal palladium catalysts， centralizedinfrastructures， and intensive energy input 11，12. Thus， a greatdeal of carbon footprint and organic byproducts are inevitablygenerated， leading to severe environmental consequences.Meanwhile， to decrease shipping costs， H2O2 products areusually transported in a high-concentrated form （up to 70 wt%，mass fraction）， consequently posing safety hazards in bothstorage and transportation to customers. To this end， directsynthesis of H2O2 has been gaining attention from academic andindustrial sectors. For instance， persulfates electrolysis andisopropanol multi-step oxidation have ever been considered todirectly produce peroxides. However， both are phased out，owing to their high energy consumption and commercialunprofitability. Besides， the controllable combination ofhydrogen and oxygen also provides a possible way to directlyproduce H2O2 with high atomic utilization and ecofriendliness13–15. Unfortunately， the need of platinum-groupmetallic catalysts and the explosion risks of H2/O2 mixturesprohibit this method from becoming a practical alternative. Also，as an appealing technique， photocatalytic synthesis of H2O2 hasbeen widely explored 16–21， whereas its low production rate andseparation issues require further innovations.

In this regard， electrochemical oxygen reduction via a twoelectronpathway （2e-ORR） has been considered a promisingstrategy for directly producing H2O2. By using oxygen and wateras reactants， peroxides can be selectively formed without anycarbon emissions and byproduct wastes through 2e-ORRprocess. Moreover， compared with the indirect anthraquinonemethod， such O2-to-H2O2 conversion enables on-site H2O2fabrication without complex instruments， eliminating the safetyand cost concerns for end-users. To promote itscommercialization， the main challenge is developing effectiveelectrocatalysts to selectively reduce O2 into H2O2 via 2e-ORRprocess rather than the competitive 4-electron route （4e-ORR）into H2O 22–25. Till now， carbon and carbon derivatives aregenerally regarded as the state-of-the-art catalysts 26. Usually，most of the carbon-based catalysts are endowed with ORRactivity by heteroatom doping 27，28， defect construction 29，30 andsurface modification 31， which all require sophisticatedpreparation procedures and remain active sites unclear 32.Meanwhile， the carbon catalysts tend to present decayed twoelectronselectivity at higher reductive potentials. As a result，pursuing novel 2e-ORR catalyst candidates is still urgent.Recently， transition metal compounds， such as metal oxides（TMOs） and chalcogenides （TMCs） have been widely served aselectrocatalysts in energy-related fields， inclouding sensors 33–35，zinc-air batteries 36， supercapacitors 37，38， hydrogen evolutionreaction 39， and electrochemical energy storage 40， due to theirearth-abundance， structural tunability， and chemical stability.Recently， Yu et al. synthesized Co atom cluster modified porouscarbon carrier catalysts （CoAC/NC） with excellent ORRperformance 41. This catalyst not only outperformed thecommercial Pt/C in terms of half-wave potential and stability，but also offered superior peak power density to the Pt/C in theassembled zinc-air batteries. Further， their substantial potentialin catalyzing 2e-ORR has also been discovered among thetransition metal oxides， sulfides， selenides， and tellurides. Thepresence of unpaired d electrons of transition metal atomselectronically adjusted by different anions can be coupled withlone-paired p electrons of O2， thereby activating the protonationof O―O bonds. Combined with their textural advantages， TMOsand TMCs are becoming another subclass of 2e-ORR catalystsapart from carbon-based materials. So far， the implementation ofTMOs and TMCs in electrochemical O2-to-H2O2 conversion isindeed in its early stages， while the increased development andfundamental research can be foreseen. Therefore， by elucidatingthe catalytic mechanism and utilizing rational material design，exceptional TMOs/TMCs catalysts can be established forpreferential 2e-ORR and are greatly desired for the H2O2-relatedindustry.

Herein， in this review， we present a prospective analysis onsummarizing breakthroughs hitherto in developingTMOs/TMCs catalysts for selective 2e-ORR to produce H2O2.To the best of our knowledge， rare publications have specificallyreviewed the application of TMOs/TMCs in 2e-ORR 42–45. Toaddress this gap， the review first briefly introduces the reactionpathways of ORR， the important parameters of 2e-ORR and themethodology in electrochemical practices. Subsequently， theadvanced progress of TMOs/TMCs for 2e-ORR is discussed，including their preparation routes， identification of active sitesand catalytic performance. The activity trend and effects ofvarious anions are especially emphasized. Then， the factorsaffecting their electrochemical performance are highlighted.Among them， the influence of morphology， phase structure， anddoping/defects engineering is systematically analyzed andsummarized. Based on that， the underlying mechanism andgeneral identification of active sites in TMOs/TMCs aretherefore revealed. Finally， appealing opportunities and outlooksfor the TMOs/TMCs in H2O2 electrochemical synthesis arebriefly provided. The review article prospectively offers ablueprint for TMOs/TMCs in catalyzing 2e-ORR and the H2O2industry. By shedding light on such emerging topic， theunderstanding and design of TMOs and TMCs can be furtherpromoted for broader heterogeneous catalysis.

2 Fundamentals and Methodology

2.1 The fundamentals of ORR.

Generally， electrochemical ORR involves two co-existing andmutually competitive routes， i.e.， 2e-ORR and 4e-ORR pathway.Besides， the product （HO2? in alkaline or H2O2 in acid） of 2e-ORR could be further reduced into OH– or H2O， becoming anindirect （2e + 2e）-ORR. The specific electrochemical reactionsare shown in Table 1. There are differences in the final form ofhydrogen peroxide in acidic （H2O2） and alkaline （HO2?） mediadue to the pH of the electrolyte as well as the self-ionization pKaof hydrogen peroxide.

As a multi-step proton-electron reaction， three keyintermediates are discovered in ORR， including O， OH， andOOH. Specifically， the adsorbed oxygen molecules （O2） oncathodes combine with the first electron and proton， generatingOOH intermediates. Subsequently， the OOH intermediates couldeither be reduced by the second proton-electron， forming 2e-ORR products， or dissociated to become O and OHintermediates， which are then reduced into OH? or H2O （4e-ORR）. Thus， the OOH intermediate is the critical intermediatefor the conversion of O2-to-H2O2 during the 2e-ORR.Appropriate adsorption energy of OOH intermediates on theelectrocatalyst surface is crucial for maintaining O―O bondsand the electrochemical synthesis of H2O2 46–48. The elementaryproton-electron transfer steps in 2e-ORR processes are presentedin Table 2. In addition， the difference in the final product ofintermediate OOH formation results in different bindingenergies for this process， which correspond to differenttheoretical onset potentials.

2.2 Electrochemical measurements for determiningselectivity of 2e-ORR

In typical electrochemical ORR investigations， rotating ringdiskelectrode （RRDE） measurements are usually employed toevaluate the selectivity of 2e-ORR （Fig. 1a）. A conventionalRRDE （Fig. 1b） features a central glassy carbon electrode （i.e.，disk electrode） and a peripheral platinum ring （i.e.， ringelectrode）. During the measurements， oxygen molecules are firstreduced on the disk electrode with drop-casted catalysts andsimultaneously produce H2O2. The generated H2O2 is transferredonto the Pt ring electrode with a constant oxidative potentialunder forced diffusion by high-speed rotations， forming H2O.When ORR occurs， the currents on both disk and ring electrodesare recorded by conducting linear sweep voltammetry （LSV）.The onset potentials are initially evaluated， as compared to theequilibrium potentials （Table 1）. Ideally， less overpotentialsreflect better thermodynamics of the catalysts for 2e-ORR，whereas more positive （gt; 0.8 VRHE） onset potentials may indicatea significant presence of 4e-ORR. In this case， by detecting thecurrents on the ring and disk electrodes， the reaction selectivity（H2O2%） and electron transfer number （n） indicators can also bedetermined by the following calculations：

where， ID and IR represent the disk current and ring current ofORR， respectively. N stands for the collection efficiency of theRRDE. For an ideal 2e-ORR process， the n is expected to be 2and the H2O2% simultaneously to be 100%.

In addition， the selectivity of ORR can also be determined bythe Koutechy-Levich （K-L） equation. The sampling and testingprocedures are same to the above RRDE method， except usingRDE. The currents on the disk electrodes are recorded byperforming LSV at different rotational speed conditions，respectively. Consequently， n can be obtained using thefollowing equations：

where j， jk， and jL are the measured， kinetic-limited， and masstransfer-limited current densities， respectively. D is the diffusioncoefficient of oxygen， ν is the kinematic viscosity of theelectrolyte， F is the Faraday constant， ω is the angular velocity，and C0 is the concentration of oxygen in the electrolyte.However， the K-L method is more applicable to mono-stepreactions. Since ORR is a multi-electron and multi-step process，the determined n values are much less precise than thoseobtained by the RRDE method 49.

Another important descriptor for 2e-ORR is Faradaicefficiency （FE）， which indicates the electron utilization efficacyduring practices. To precisely determine FE， H-type cell setups（Fig. 1c） are commonly used， where the counter and workingelectrodes are separated into two different chambers by an ionexchange membrane. The cathode chamber undergoes ORRwith O2 purging， while the anode chamber usually performsoxidation reactions， such as the oxygen evolution reaction（OER）. The working electrode is commonly apolytetrafluoroethylene （PTFE）-loaded carbon fiber paper withdrop-casted catalysts on the surface， ensuring rapid massdiffusion. By carrying out chronoamperometry tests， cathodiccurrents are recorded and simultaneously H2O2 is accumulatedin the cathode chamber at a constant potential. The actual H2O2production can be obtained by chemical titrations， and then thequantity of consumed electric charges that accounts for H2O2formation can be calculated. Finally， with the recorded totalcharge amount in whole electrochemical process， the FE can becalculated using Eq. （5）. As is often the case， FE varies with thedifferent potentials applied on the cathodes. Thus， the FEmeasurements are usually carried out at several constantpotentials， to identify the best working condition for furtherevaluations. To note， the decomposition， overreduction， andtitration errors of produced H2O2 in the cathode chamber maygenerate inaccuracy in determining FE， which encouragesinnovations in the related measurements. Plus， as anotherimportant descriptor for 2e-ORR， the specific H2O2 yield （γ） canalso be correspondingly acquired to assess the productivity ofthe evaluated catalysts， as determined by Eq. （6） below.

where， c， V， F， Q， t， and m represent the H2O2 concentration（mol?L?1）， the cathode electrolyte volume （L）， the Faradaicconstant （96485 C?mol?1）， the amount of charge （C）， the reactiontime （h）， and the mass of loaded catalysts （g）， respectively.

On the other hand， to examine scaled electrosynthesis ofH2O2， researchers tend to use flow-cell setups （Fig. 1d）， whichcan continuously output synthesized H2O2， enabling variousutilizations （bleaching， decomposition， advanced oxidation， andvalue-added synthesis）. Like H-cell setups， flow-cells are alsoassembled into two? or three-chamber configurations， withnarrowed cavities and flow channels. Compared to H-cells， theaccumulation of fabricated H2O2 is rather restricted， and theformed H2O2 with electrolyte is quickly pumped out for eitherstorage or circulation， while fresh electrolyte and O2 aresimultaneously fed in. Nevertheless， despite the inferioraccuracy of flow-cell tests compared to H-cells， practicalapplication performance can still be demonstrated and evaluated.Finally， long-term stability is critical when evaluating thecatalytic performance of electrocatalysts. Therefore， werecommend that researchers use the above devices to performreasonable stability evaluations， including CV tests over 5000cycles within RRDE setups， and H-cell/flow cell tests over 12 hor above.

3 Progress on metal oxides andchalcogenides for 2e-ORR

As mentioned above， owing to their earth-abundance， lowcostand outstanding catalytic stability under higher bias， TMOsand TMCs have been considered promising alternatives toreplace carbon-based counterparts. A series of both TMOs andTMCs have been developed and identified as activeelectrocatalysts for energy-related conversions， including 4e-ORR 50， OER 51 and hydrogen evolution 52. In the recent decade，their latent potential in conducting 2e-ORR has also beenrecognized. However， the intrinsic ORR activity of pristineTMOs/TMCs is far from industrial requirements. To enhancetheir electrocatalytic performance， several strategies， includingmorphology control， crystalline tuning， heteroatom doping，vacancy constructing， facets/entropy engineering， have beensuccessfully implemented， thereby leading to a family ofadvanced TMOs/TMCs in the O2-to-H2O2 conversion （Table 3）.Therefore， a brief overview of the recent progress for TMOs andTMCs （metal sulfides， metal selenides， and metal tellurides） ishighly necessary.

3.1 Metal oxides

The capability of TMOs for ORR has been recognized bothexperimentally and theoretically. N?rskov and co-workerspredicted the ORR activity trend of TMOs， based on densityfunctional simulations 45. The electric field effect of TMOsenables the break of O―O bonds of adsorbed O2 on the surface.Therefore， transition metals with abundant 3d electrons havebeen preferentially investigated. Moreover， given their diversity，TMOs have received extensive scientific attentions for 2e-ORR，due to their catalytic stability within a wide electrochemicalwindow （around 500 mV）. For example， Wu et al. designed andsynthesized a porous carbon nanosheet-supported amorphousnickel oxide （NiOx） electrocatalyst via a photochemical metalorganic deposition method 53. Within a wide range of 0.15–0.6VRHE， the resulting amorphous NiOx presented high efficiencytoward 2e-ORR， delivering a selectivity of about 91% and anelectron transfer number of ca. 2.2 in alkaline. Likewise， Yanand co-workers introduced oxygen vacancies into Co3O4 andcoupled it with nitrogen-doped carbon nanotubes （CNTs），resulting in a Co/N-CNTs catalyst 54. They observed the glossyvine-like structure of nitrogen-doped CNTs， with diametersranging from 30 to 150 nm. Moreover， the Co/N-CNTs catalystshowed the isolated Co atoms in HAADF-STEM images. In anH-cell setup filled with O2-saturated 0.5 M H2SO4 electrolyte， itexhibited the highest H2O2 production rate of up to 1.6mol?gcat?1?h?1. The oxygen vacancies in Co/N-CNTs werebelieved facilitate the electron transfer on the surface of Co3O4，providing electrons to OOH for its conversion to H2O2. On theother hand， despite the fully occupied 3d orbitals， reducibleindium oxides （In2O3） could also demonstrate capability for 2e-ORR electrocatalytic process. Kang’s group developed In2O3-loaded carbon dots （CDs） catalysts， successfully regulating thekinetic process of electron transfer in electrochemical 2e-ORR 55.The In2O3/CDs achieved a dramatic H2O2 selectivity of nearly100% （Fig. 2b） within a potential range of 0.5–0.7 VRHE. Toassess its practicality in real devices， the developed In2O3/CDswere assembled into a gas diffusion electrode （GDE）， leading toa massive H2O2 productivity of around 4.5 mol?gcat?1?h?1.However， these pioneering TMOs-based catalysts still requirecarbon support as conductive media and/or synergisticcomponents 56–60. To remove the vulnerable carbon from thecatalytic system， a surface modified vanadium oxide （V2O5） wasdeliberately developed 61. The pristine V2O5 underwent a partialreduction through H2 annealing and thereby generated localoxygen vacancies （V2O5-Ov）， improving the adsorption capacityof O2 on the surface. Also， V2O5-Ov was superior in adsorbingO2 in the “end-on” configuration， allowing subsequentprotonation of O2 into H2O2， without the cleavage of O―Obonds. As a result， it exhibited excellent electrochemical 2e-ORR performance with 2e-selectivity of 92.32%， FE of 84%，and H2O2 yield of 1.96 mol?gcat?1?h?1.

In addition to above oxides with mono-metals， theintroduction of secondary metal atoms is believed to enhanceactivation for ORR over pristine oxides by tuning crystallinestructures 62. Therefore， further exploration and research on bi-/multi-metal oxides have also been conducted. Liu et al.prepared NiNb2O6 nanoparticles by a hydrothermal followed bythermal annealing （Fig. 2d） 63. Astonishingly， a H2O2 selectivityof 96% and FE of 92% in a wide potential window from 0.2 to0.6 VRHE could be achieved， indicating that the change in the dbandcenter of Nb atoms promotes the formation of H2O2.Moreover， among the first attempts， a series of Ruddlesden-Popper perovskites （Pr2Ni1?xO4+σ） were successfully fabricatedfor alkaline 2e-ORR 64. Initially， the H2O2 selectivity of the aspreparedPr2Ni1?xO4+σ catalyst could be up to 60% at 0.55 VRHE，illustrating the promising capability of the perovskite towards2e-ORR. Subsequently， its 2e-selectivity was enhanced to 79%after introducing molybdenum atoms， which changed the surfacechemical state and concentration of Ni ions. Furthermore， Qianand co-workers developed a ZnSnO3 perovskite as anelectrocatalyst hydrogen peroxide production through 2e-ORR 65.In alkaline electrolyte， it presented a H2O2 selectivity of 76%，overwhelming that of SnO2 and ZnO. Theoretical calculationsindicated that the presence of Zn species on its surface was moresuitable for adsorbing important OOH intermediates.

Additionally， based on the success of secondary atom dopingstrategies， Kang et al. prepared a novel high-entropy perovskiteoxide ceramic [Pb（NiWMnNbZrTi）1/6O3] via a thermaltriggeredsolid solution method 66. The high entropy of the oxidewas believed to decrease the charge transfer ability and optimizethe surface charge density. Due to its physicochemical virtues，its 2e-ORR selectivity could reach 91% in 0.1 M KOH within awide potential range from 0.1 to 0.7 VRHE， far exceeding that ofPb（ZrTi）1/2O3. Moreover， the H2O2 generated by H-cell for along time could be used for dye degradation （Fig. 2e，f）. On theother hand， given the harsh preparation condition of heteroatomdoping， the combination of different oxides was also proposedto enhance their 2e-ORR performance. In this regard， a spinelcobalt aluminum oxide-cobalt oxide （CoAl2O4/CoO） compositewas prepared via programmed heating bimetallic CoAl-layereddouble hydroxide 67. The synthesized CoAl2O4/CoO catalystfeatured a nanosheet-like morphology as evidenced by SEM andTEM investigations. As a consequence， the catalystdemonstrated up to 85% selectivity for O2-to-H2O2 formation.To assess its practical application in the purification of industrialsewage， Rhodamine B （RhB） was used as pollutant indicator，and the RhB （10 mg?L?1） could be fully decomposed within 110min in 0.1 M KOH.

Owing to the chemical vulnerability of common metal oxidesin acids， most of the reported oxides for 2e-ORR were conductedin alkaline conditions 68. In theoretical studies， Gao and coworkershave developed oxygen-defective α-Fe2O3 singlecrystals with exposed （001） facets by density-functional theory（DFT） calculations 69. The simulation results suggested that theH2O2 selectivity of ORR could reach 90% and 88% in acidic andneutral electrolytes， respectively. Nevertheless， the substantialprogress is strongly encouraged in realizing highly efficient 2e-ORR in non-basic conditions， which is of great industrial andhygiene importance.

3.2 Metal sulfides

With similar chemical properties to oxides， metal sulfides，especially noble metal sulfides， are commonly utilized incatalyzing electrochemical oxygen reduction. For instance， alow-crystalline two-dimensional palladium sulfide （L-PdS-Vs）with isolated Pd sites and sulfur vacancies was synthesized via atwo-step solvothermal preparation 70， as shown in Fig. 3a. Asrevealed by XRD and XPS， the L-PdS-Vs catalysts featured lowcrystallinity and tuned electronic structure of surface Pd atomsby sulfurization， leading to enhanced oxygen adsorption.Consequently， the as-developed L-PdS-Vs exhibited excellentelectrocatalytic ORR performance with high H2O2 selectivity（gt; 90%） in 0.1 M KOH electrolyte （Fig. 3d）. Meanwhile， theaverage H2O2 productivity reached as high as 1.12 mol?gcat?1?h?1，and the corresponding FE was over 90%. Further， unlike oxides，sulfides present considerable robustness in acidic electrolytes.Pd-S nanocrystals （NCs） developed by Huang et al. through asimple wet-chemical method were employed for acidic 2e-ORR 71.The Pd-S NCs exhibited exceptional selectivity gt; 90% for the2e-pathway in 0.05 M H2SO4 electrolyte within a potentialwindow between 0.2 and 0.45 VRHE. As evidenced by the XASresults， the isolated Pd atoms in the Pd-S NCs acted as reactivesites for ORR. Compared with above-discussed TMOs， the metalsites of sulfides tend to serve as ORR active sites rather thanoxygen vacancies of some reported oxides， demonstrating thecritical role of lattice metal sites in preserving O―O bonds for2e-ORR.

Inspired by these findings， researchers have expanded thefamily of sulfide ORR catalysts. Apart from the expensivepalladium-based sulfides， earth-abundant transition metalsulfides can also be able to catalyze O2-to-H2O2 conversion，owing to their lone d electrons/unoccupied d orbitals 72，73. Forinstance， Jin’s group firstly launched computational simulationsand demonstrated that cobalt pyrite （CoS2） was predicted to beactive and selective toward 2e-ORR in acidic solutions， due tothe modest adsorption binding of OOH on the Co sites 74.Experimentally， CoS2 nanowires directly grown on the carbonfiber paper electrodes were consequently synthesized andassembled into an H-type cell， showing a H2O2 selectivity of70% at 0.5 VRHE in 0.05 M H2SO4. The kinetically difficult O―Obond scission facilitated the formation of H2O2， attributed to thelack of Co active sites in the CoS2 crystal structure. Moreover，Liang et al. prepared a nickel sulfide （NiS2） nanosheets via alow-temperature sulfurization reaction strategy 75. The NiS2nanosheets formed oxidized species on their surface， renderingthe catalysts with a high 2e-ORR selectivity of over 90% inacidic medium at a wide potential range of 0.1 to 0.4 VRHE.Additionally， the NiS2 catalyst attained a large FE of 98% at0.456 VRHE and H2O2 yield rate of 109 mg?L?1?h?1 at 0.156 VRHE.

Similar to oxide studies， the strategy of doping secondarymetal atoms has also been applied to develop novel bimetallicand Mult-metal sulfides， such as Mn-CuS 76， Au@Cu2?xS-CNTs 77，and Ti-ZnCoS HSS 78 to acquire better 2e-ORR performance.Among them， a Chevrel phase chalcogenide Ni2Mo6S8 with therhombohedral R3H structure was prepared via a two-step solidstatemethod of high energy mechanical milling and hightemperaturecalcination 79. As shown in Fig. 3f， the resultingNi2Mo6S8 catalyst possessed a 3D framework composed ofinterconnected Mo6S8 clusters and separated Ni sites， where eachcluster contained a Mo6 octahedron surrounded by an S8 cube.Compared with pristine NiS2， such Chevrel phase structureendowed a steric effect on the Ni active sites， effectivelyinhibiting the O―O cleavage and favoring the proton-coupledreduction of O2 to OOH. Therefore， the Ni2Mo6S8 catalystachieved outstanding activity for H2O2 electro-synthesiswith gt; 90% molar selectivity and a high FE of 85% at a broadpotential range of 0 to 0.6 VRHE in 0.1 M KOH. Furthermore， asanother subclass of sulfides， a series of CuCo2?xNixS4 （0 ≤ x ≤ 1.2）thiospinel catalysts were designed and developed for theelectrochemical synthesis of H2O2 in acids 80. It was unveiledthat Ni substitution mainly occurred at octahedral Cu sites of thepristine spinel lattice. Compared to CuCo2S4 control， when x = 0.4，the CuCo2?xNixS4 catalyst presented the highest selectivity for2e-ORR （～80%， at 0.55 VRHE） in 0.05 M H2SO4. Moreinterestingly， the catalysts can be applied to degrade pollutantsin an electro-Fenton system， mainly due to the leaching ofcopper ions from the catalyst under acidic conditions. Due toCu（I） ions， this electro-Fenton process can degrade 40% ofpollutants within 2.5 h， which is close to the observation in anelectro-Fenton process by adding Fe2+ ions.

3.3 Matel selenide

Recently， metal selenides have emerged as promisingcandidates for ORR in H2O2 electro-synthesis. Among earlyinvestigations， noble metal selenides were recognized for theirhigh catalytic activity 81–83. For example， Huang’s groupsynthesized an amorphous PdSe2 particles （a-PdSe2 NPs） withlow-coordinated Pd sites 84. Impressively， the a-PdSe2 NPs/Cdemonstrated a high H2O2 selectivity of over 90% and H2O2productivity of 3245.7， 1725.5， and 2242.1 mmol?gPd?1?h?1 in 0.1M KOH， 0.1 M HClO4， and 0.1 M Na2SO4， respectively （Fig.4a–f）. In a three-phase flow cell reactor， the H2O2 yield couldreach 1081.8 mg?L?1 in 0.1 M Na2SO4 after 2 h. The exceptionalperformance was assigned to low coordination environment ofPd sites in a-PdSe2 NPs， which optimized the adsorption ofoxygenated intermediates and suppressed O―O bond cleavage.Therefore， the a-PdSe2 NPs/C catalyst made itself one of the firstpH-universal catalysts for H2O2 electrochemical synthesis.

Building on the success of PdSe2 materials， there is alsosignificant interest in low-cost transition metal selenides. Todate， most of research have predominantly focused on two typesof transition metal selenides： cobalt selenides and nickelselenides 85–89. As one of the pioneering investigations， Zhang etal. prepared cobalt selenide （CoSe2） nanoparticles supported onnitrogen-doped carbon nanotubes 90. The as-developedorthorhombic CoSe2 exhibited superior catalytic performancefor the 2e-ORR， compared to its cubic phase counterpart. Itturned out that the CoSe2@NCNTs achieved a high H2O2selectivity of 93.2% and a rapid H2O2 yield rate of 172mg?L?1?h?1 along with excellent durability of 24 h in 0.1 MHClO4. The enhanced activity of CoSe2 was attributed to thechange in its crystal structure， which promotes charge transferon the surface of CoSe2. However， the N dopants in carbonmatrix are always believed to be active for 4e-ORR 91–95. In thiscase， to avoid interference from N dopants， atomic-layereddefect-rich CoSe2 nanosheets were designed and grown oncarbon cloth through selenization of CoMoO4 precursornanosheets （NSs）， forming vertically aligned and interconnectedultrathin nanosheets 96. HRTEM results evidenced the richpresence of Co defects on the surface. These defect-richstructures increased the number of exposed atoms and generatedadditional Co active sites， leading to an enhanced H2O2selectivity （92%） and a rapid H2O2 formation rate （1227mg?L?1?h?1）. By taking advantage of their acidic insolubility，theH2O2 production rate in diluted acid （0.05 M H2SO4） reached894 mg?L?1?h?1， superior to most of 2e-ORR catalysts. Differentwith the above， Sheng et al. demonstrated that both theorthogonal and cubic crystal forms of CoSe2 could exhibit highselectivity （gt; 80%） for 2e-ORR in acid （0.05 M H2SO4） 87，attributed to the weak O binding to Se sites.

On the other hand， Wang et al. synthesized a nickel selenide（NiSe2） catalyst with charge polarized Se vacancies （VSe） via asequential phase conversion strategy （Fig. 4g） 97. EPR and XPSresults confirmed the generation of Se vacancies， caused by latticeexpansion during annealing. Compared with NiSe2， the NiSe2-VSecatalyst presented higher selectivity of up to 96% toward 2e-ORRin alkaline media over the wide potential range from 0.25 to 0.55VRHE. Significant achievements have also been made in tin acidicmedia 98. A cubic NiSe2 （c-NiSe2） synthesized by hydrothermalmethod exhibited high selectivity for the formation of H2O2 by 2e-ORR， due to the exposed （100） crystal faces.

Based on the above-mentioned analysis， compared withTMOs and sulfides， metal selenides appear to deliver substantial2e-ORR performance， especially in acidic conditions. Therefore，the underlying roles of anions in TMOs and TMCs requirefurther clarifications. To reveal the influence of anion on ORRcatalytic activity， Sun et al. fabricated a series of nickelcompounds with different anion species （Se2 2?， S2 2?， and O2?） 99.They used Ni（OH）2 grown on hydrophobic carbon paper as theprecursor， and then selenized， sulfurized， or decomposed at alow annealing temperature of 400 °C to obtain the correspondingNiSe2， NiS2， and NiO， respectively. Despite similar roughmorphological structures， NiSe2 showed the lowest bindingenergy of both Ni2+ and Ni3+， indicating higher electron densityof Ni atoms. Therefore， compared to selectivity of NiS2 （70% to40%） and NiO （almost none）， NiSe2 demonstrated the mostoutstanding 2e-ORR performance with high selectivity （90%），long-term durability （12 h）， and large H2O2 yield （988 mg?L?1）.

3.4 Metal tellurides

Although research on metal tellurides as catalysts for 2e-ORRis still in its early stages， these materials have recently garneredscientific interest， due to their promising performance. A fewpioneering studies have reported notable ORR performance withtelluride-based catalysts 100–102. Recently， Sun’s groupdeveloped CoTe nanoparticles embedded in a nitrogen-dopedhollow carbon polyhedron （CoTe@NC） for efficientelectrochemical synthesis H2O2 in acidic medium 103. Within awide potential rang of 0.1 to 0.5 VRHE， the CoTe@NC exhibitedsuperb electrocatalytic activity toward 2e-ORR， achieving highselectivity of gt; 90% （Fig. 5b） and an H2O2 productivity of 297.9mg?L?1?h?1. This exceptional performance was attributed to thehighly active and selective of the CoTe （101） surface forelectrocatalytic H2O2 production. Similarly， Zhao and coworkersfabricated 2H-phase molybdenum telluride （MoTe2）nanoflakes with zigzag edges through ultrasonication-assistedliquid phase exfoliation from bulk powder 104. To obtainhexagonal 2H MoTe2， 2D bulk crystal feedstock was subjectedto ultrasonication （Fig. 5d） to disrupt Van der Waals interactionsand generate few-layered nanoflakes stabilized by Nmethylpyrrolidone.The exposed edges predominantlydemonstrated a Z-shaped pattern and abundant unsaturated Moand Te coordination， providing accessible sites for oxygenadsorption. As expected， the developed MoTe2 nanoflakesshowed excellent activity and selectivity towards 2e-ORR instrong acid （0.5 M H2SO4）， with an onset potential of ～0.56VRHE， a large mass activity of 27 A?g?1 and a high H2O2selectivity of up to ～93%. Moreover， Lee et al. synthesized a Pd-Te nanoparticles via galvanic regrowth crystallization （PdTeGRC） 101. Compared with Pd-Te nanoparticles via galvanicreplacements （PdTe GR）， the PdTe GRC exhibited a maximumH?O? selectivity of 75%， surpassing the 61% selectivity of PdTeGR. The higher selectivity in PdTe GRC was attributed to thegreater amount of surface Pd―Te bonds （0.48）， as compared toPdTe GR （0.25）.

4 Factors towards 2e-ORR performance

The rapid advancement of TMOs and TMCs inelectrochemical 2e-ORR hinges on fundamental studies thatelucidate the relationship between the physicochemicalproperties of electrocatalysts and their ORR performance.Obviously， the electrocatalytic efficacy is strongly governed bythe structure and composition of the catalysts， including theirmorphology， crystalline texture， micro-topology， and activesites. These understandings affect the design and developmentof future TMOs and TMCs catalysts. Unlike extensivelyinvestigated carbon-based ORR catalysts 105–109， the specificimpacts of these factors on TMOs and TMCs， particularly theidentification of active sites for the O2-to-H2O2 conversion，remain subjects of ongoing debate.

4.1 Morphological and crystal phases modifications

As one of the most apparent characterizations， morphologyand its adjustment were considered a main strategy to modify 2e-ORR performance of TMOs and TMCs. As metal compounds，most TMOs and TMCs have ordered and compacted latticestructures， which are kinetically insufficient for gas diffusioncontrolledORR. Therefore， engineering the microstructure ofcatalysts at the nanometer scale is often considered a practicalroute to optimizing the activity and selectivity of the oxygenreduction reaction. For example， a nanocoral-shaped Pd-basedcompound consisting of a mixed phase of Pd17Se15 and Pd3B（Pd-Se-B NC） was designed and fabricated for neutral ORR intoH2O2 81. The porous nanocoral morphology consisted ofprotruding nanorods with an average diameter of approximately20–40 nm. The hierarchical structure offered a larger surfacearea and more active sites， facilitating the diffusion of oxygenand its access to the catalytic sites and ensuring superior 2e-ORRperformance in KPi buffer （pH 8.0， 0.1 M）. Similarly， Liang andcolleagues claimed that a broader plane of the TMCs could alsoreinforce the ORR activity by exposing more metal active sitesto oxygen and thereby developed a NiS2 nanosheets for 2e-ORRin acidic electrolytes （Fig. 6a，b） 75. The mesoporous nanosheetmorphology enabled effective binding of Ni active sites withoxygen and intermediates， resulting the favored 2e-ORRreduction pathway instead of the 4e-ORR reduction pathway.

Inspired by the above research， various types of carbonmaterials were used as substrates for TMOs and TMCs to exposemore catalytic active sites. Zhang et al. embedded CoTenanoparticles， which exhibited the initial dodecahedralmorphology after high-temperature annealing with Te powder，onto a nitrogen-doped hollow carbon polyhedron （CoTe@NC）and utilized as 2e-ORR electrocatalysts 103. The resultingCoTe@NC exhibited uniform hollow characteristics， whichcould enhance mass transfer. Besides， DFT calculations revealedthat the CoTe （101） facets were exposed by the hollowmorphology， boosting the electrocatalytic H2O2 synthesis， as thebinding free energy of the adsorbed OOH species （ΔGOOH） isclose to the theoretical value （3.5 eV） at Te2 sites of CoTe （101）（Fig. 6e）， making the Te2 sites highly active for H2O2 formation.The OOH binds with CoTe （101） via an “end-on” configurationon Te2， which were beneficial in preventing the O―O bondscission and forming O and OH.

Based on that， the catalytic roles of crystalline structure andparticular facets have been further emphasized 110. Someresearchers have achieved high-performance 2e-ORRelectrocatalysts through the manufacturing different crystallattices， such as hematite， perovskite， and spinel. It is believedthat by exposing specific crystal planes and metal cations， moreactive surface sites and thus catalytic activity could be acquired.As a simulation prediction， α-Fe2O3 was established， whichendowed high catalytic activity and selectivity for 2e-ORR viathe engineering of facets and oxygen vacancies 69. It wasindicated that the abundant oxygen vacancies on exposed （001）facets of α-Fe2O3 featured inherent selectivity towards theproduction of H2O2. The oxygen vacancies served as reactivesites for O2 adsorption and protonation， stabilizing OOHintermediates， and thereby preventing O―O bond breakage.Moreover， the high entropy effect can effectively enhance theactivity of electrocatalytic reactions 111. The perovskite[Pb（NiWMnNbZrTi）1/6O3] obtained through entropyenhancement not only induces a polycrystalline phase transitionaccompanied by lattice strain release， but also reduces themigration ability of electrons on the surface， thereby improvingstructural stability and enhancing catalytic activity for theselective production of H2O2 66.

In comparison with ordered crystalline structures， disorderedmicrostructures were also given rise to interesting results. Yuand co-workers synthesized amorphous PdSe2 nanoparticles （a-PdSe2 NPs） with low-coordinated Pd sites via a facile wetchemicalapproach 84. The presence of a disordered lattice in theamorphous structure of a-PdSe2 NPs enriched the electrondensity of Pd sites. Meanwhile， EXAFS results also showed thatthe Pd-Se coordination number of a-PdSe2 NPs/C was ca. 3.5，and Pd in a-PdSe2 NPs/C was subject to a weaker electronwithdrawingeffect of Se， resulting in a lower average chemicalvalence. Therefore， the low-coordinated Pd sites could optimizethe adsorption of oxygen-containing intermediates， therebyinhibiting O―O bond cleavage and significantly improvingH2O2 selectivity and productivity. Likewise， Chen andcolleagues also carried out in-depth research into the localatomic environment properties of amorphous low-coordinationcatalysts in 2e-ORR by in-situ growing three-dimensional，ultrathin， amorphous low-coordination CoSx （CuNW@CoS4）nanosheets on Cu nanowires 72. It was found that when a Coatom was bonded to 4 S atoms in CuNW@CoS4， the Co-S4configuration can effectively optimize the d-band center of Co，making it feasible to carry out the 2e-ORR at a loweroverpotential. Overall， the morphology， crystal structure， andcoordination environment of catalysts play an important role inthe 2e-ORR O2-to-H2O2 process to a certain extent. In the abovework， the researchers achieved high activity and selectivity of2e-ORR catalysts by constructing materials with differentmorphologies， adjusting the crystal structure and exposingspecial crystal surfaces. The core idea is to increase the reactionspecific surface area so that as many active centers as possiblecan participate in the 2e-ORR reaction. Therefore， the study ofthe topology of materials is also an important way to improvethe activity and selectivity of 2e-ORR.

4.2 Doping and defect engineering

According to the modification of the morphology andstructure， it was clearly found that the heteroatom dopants andanion/cation defects pose significant effects on catalyticcapability. Generally， introducing foreign metal atoms intoTMOs and TMCs could create new active sites on the catalysts，thereby endowing the pristine TMOs and TMCs with better ORRactivity. For instance， Mei and colleagues designed a Fe-dopedCeO2 112. Compared to pristine CeO2， Fe doping led to latticedistortion of CeO2 and further formation of many oxygenvacancies and Ce3+. DFT calculations indicated that Fe dopingand oxygen vacancies increased the conductivity of CeO2 andpromoted charge transfer during the electrosynthesis H2O2process （Fig. 7a，b）. Furthermore， Fe doping reduced the bindingenergy between OOH and active sites， allowing the OOH todesorb and form H2O2. In addition to Fe-doped CeO2， Sun?steam had proposed a Mn-doped TiO2 as an efficient two-electronelectroreduction of oxygen catalyst for generating hydrogenperoxide 113. The Ti3+ and VO defects were produced by Mndoping， and the Ti5c3+ site with optimal ΔGOOH was identified asthe most active site to generate H2O2.

Meanwhile， doping heteroatoms could also rearrange thecoordination structure of active sites， leading to either electronsufficientor electron-deficient sites. Such reconfiguration ofelectronic structure could greatly tune the adsorption of ORRintermediates， and consequently promote the activity andselectivity of 2e-ORR. For example， the doping of Mn atomschanges the atomic configuration of CuS， promoting theformation of central active sites by Mn atoms 76. The interactionbetween Mn atoms and adjacent atoms and the enhancement ofelectron delocalization are conducive to the breaking of Mn-OOH bonds. This was hopeful to obtained H2O2. Additionally，Wang et al. induced the formation of Se vacancies duringannealing by doping trace amounts of Fe into CoSe crystals 86.The Fe-induced Se vacancy greatly promotes charge transfer andhas suitable adsorption energy for ΔGOOH， which is beneficial forthe formation of H2O2 in alkaline media.

On the other hand， vacancy engineering is also one of theeffective ways to regulating the catalytic activity ofelectrocatalysts. Among the TMOs and TMCs， anionicvacancies （VO， VS， VSe） have been widely studied as active sitesin 2e-ORR reactions 114，115. Zhang et al. developed a V2O5catalyst with oxygen vacancies through the H2 annealing method 61.The results demonstrated that oxygen vacancies on the （001）facets of V2O5 improved the adsorption of oxygen and optimizedthe 2e-ORR selectivity （Fig. 7d–f）. Besides， Wang andcolleagues discovered that low-crystalline Pd metallene sulfides（L-PdS-VS metallene） with rich sulfur vacancies exhibited highactivity towards 2e-ORR 70. The activity enhancement wasmajorly attributed to the induced formation of abundantamorphous structures and isolated Pd atoms during sulfurization，which was beneficial for the adsorption of O2 and thedissociation of O ― O bonds. Sufficient sulfur vacanciesincreased atomic utilization and exposed more catalytic activesites， thereby promoting the 2e-ORR process. Similar resultswere observed with Se vacancies， for example， the creation ofSe vacancies with charge polarization in the NiSe2 structure wasalso implemented and investigated 97. The as-established NiSe2-VSe catalyst exhibited significant lattice expansion， due tonegatively charged polarized Se vacancies. It endowed thecatalyst with abundant VSe active sites and a stable phasestructure， reducing electron transfer rates， and optimizing theadsorption free energy towards OOH intermediates （ΔGOOH）.

Aside from the anion vacancies， the cation vacancies werealso imposed to transition compounds to regulate the localelectronic configuration of the active sites. Zhang and coworkersproposed a Cu-deficient Au@Cu2?xS-CNTselectrocatalyst 77. They found that Cu-defects could lower thereaction energy barrier of OOH formation to H2O2 in 2e-ORRpathway and accelerate the electron transfer in the process. Incontrast， Ji et al. proposed a CoSe2 nanosheet with Co-vacancieswithout introducing additional elements 96. XPS results indicatedthat CoSe2 nanosheets have a high concentration of Covacancies， which leads to enhanced oxidation of the metalsurface and increased electron density （Fig. 7g–i）. Overall，vacancies are an effective way to enhance the activity of 2e-ORR.

4.3 Active sites and mechanism

Many literatures have elucidated that OOH is a keyintermediate in the two-electron oxygen reduction reaction toform H2O2. Therefore， a suitable adsorption energy for theintermediate OOH is crucial for all 2e-ORR active sites.Currently， studies on the active sites and mechanisms of metaloxides and metal-sulfur compounds can be broadly classifiedinto two types： one believes that active sites are located in theirspecific metal or nonmetal sites; another is the catalyst structureas aiding in exposing the intrinsic active sites or manifestingtheir intrinsic activities.

For reviewed TMOs and TMCs， transition metal sites areusually the key active centers for the efficient formation of H2O2by electrocatalytic 2e-ORR. Platinum group metals （Ir， Rh， Pd，Pt）， as an important class of transition metals， have good activityfor ORR. In the electrocatalytic 2e-ORR process， isolated Pdsites are critical and can be obtained by sulfide induction 70.During the entire reaction process， the isolated Pd sites caneffectively adsorb O2 and inhibit the breaking of O―O bonds 71.The isolated Pd sites have suitable binding energies for OOH andinhibit the 4e-ORR process under the influence of thecoordination environment 84， as well as the atomic compositionand arrangement 71，83. In addition， iron-based metal elements（Fe， Co， Ni） are also good 2e-ORR active centers. By growingCoSe2 nanosheets on carbon cloth to expose Co site defects，these Co site defects could effectively adsorb OOH and showexcellent selectivity for H2O2 96. Meanwhile， the Co active sitesare also affected by the surrounding coordinationenvironment 72， forming low coordination number Co―S4. TheCo atoms are affected by their ligand S atoms， and their d-bandenergy levels can effectively weaken the adsorption of OOH andaccelerate the production of H2O2. Similarly， in Ni2Mo6S8catalysts， its unique active center motif triggered synergisticligand， tethering and spatial effects between Ni sites and Mo6S8，thus providing optimal binding to key reaction intermediates andhaving the ability to inhibit O―O bond breaking 79. Collectively，it is clear that both metal sites and cations can act as activecenters. The metal active sites tend to bind oxygen directly andweaken the O―O bonds for subsequent protonation. Thus， theadsorption of metal sites to ORR intermediates majorly governsthe activity and selectivity of 2e-ORR. Whereas， as for cationsites， such spots， usually presented as vacancies， are involved inORR indirectly in many cases. They contribute to 2e-ORRperformance of the studied catalysts through regulating localelectronic structure of the reactive sites. Therefore， both are ofgreat importance in catalyzing ORR and worth more innovativeefforts.

In the structures of some TMOs and TMCs， electrocatalytic2e-ORR activity can also be modulated by non-metal activesites. Oxygen vacancies were used as active centers， and theformation and concentration of oxygen vacancies can beregulated to obtain highly active and selective site centerseffective for 2e-ORR. In the presence of Mn atoms， TiO2underwent lattice distortion， leading to the generation of oxygenvacancies （Fig. 8e） 113. At the same time， the presence of oxygenvacancies reduced ΔGOOH， which allowed for the efficientproduction of H2O2 （Fig. 8f）. To further modulate oxygenvacancies 61， fine-tuning of the V2O5 surface by H2 caneffectively optimize the reaction pathway of O2 on the catalystsurface， resulting in high selectivity for 2e-ORR. In addition tothe oxygen vacancies as active sites， the selenium vacancies arealso effective for 2e-ORR formation of H2O2. Under theinfluence of selenium vacancies， the free energies of thereaction intermediates in the 2e-ORR could be optimized 97.Meanwhile， the selenium site itself also played a role inORR 87， where O was weakly bound to Se sites， making it highlyactive and selective for the formation of H2O2 from 2e-ORR. Inaddition， Te was also reported as a center for 2e-ORR， with atwo-dimensional monolayer of MoTe2 100， in which the exposedTe sites could effectively bind to OOH and contribute to theformation of H2O2.

Moreover， the synergy between TMOs/TMCs and conductivesupport also contributes to boosting 2e-ORR activity. Asexcellent multifunctional support， carbon-based materials aregenerally used to load TMOs and TMCs active sites. Bydepositing CoSe2 nanoparticles onto nitrogen-doped carbonnanotubes， activities of both towards ORR were enhanced，which facilitated the O2-to-H2O2 conversion efficiency 90.Likewise， embedding CoTe nanoparticles into nitrogen-dopedhollow carbon polyhedra generated the synergistic effectsbetween the Co sites and N dopants， which effectively preventedO―O bonds of OOH intermediates from cleavage 103. Apartfrom that， conductive support and medium can also benefitelectron transportation 55， for instance， electron transfer in In2O3active sites and their selectivity of 2e-ORR were both improvedby introducing carbon dots （CDs）. Furthermore， the texturaldesign of TMOs/TMCs could also adjust the intrinsic ORRcharacteristics. For example， by narrowing the interlayer gap ofcobalt selenide， the coupling between the CoSe2 atomic layerswas enhanced to weaken OOH adsorption and thus promoted theH2O2 production （Fig. 8g） 88. In addition， the construction ofunique zigzag edges is also able to manifest active units 104. Theexfoliated MoTe2 nanosheets exposed zigzag edge active siteswith suitable binding energies for OOH and O. Collectively， bytargeting the OOH intermediates as the critical reaction species，all efforts on the identification and improvement of active sitescould be accordingly carried out. The binding strength of OOHon various active sites governs the activity and selectivity of 2e-ORR， which could technically be realized by tuning the localenvironment of active sites， such as surface chemistry andinterface engineering.

5 Conclusions and perspectives

With increasing attention has been posed onto carbonemissions and carbon footprints in general chemical industrialsectors， traditional chemical synthesis is embracing green andsustainable routes to renew its technology. Among manyalternatives， electrochemical 2e-ORR into H2O2 is becoming anappealing topic in recent years， especially using renewableelectricity as power. Owing to their structural tunability， facilepreparation， and catalytic stability， TMOs and TMCs areemerging as promising alternatives to state-of-the-art noblemetals and carbon-based catalysts. Current research on this topicprimarily focuses on developing novel TMOs/TMCs materialswith different structures， crystalline phases， and compositions.Ultimately， these studies aim to harvesting better activity andhigher 2e selectivity for ORR and have made encouragingprogress. However， 2e-ORR on TMOs/TMCs materials is still inits early stages， and substantial innovations and in-depthfundamental investigations are greatly needed. Herein， wereview recent advances in TMOs/TMCs 2e-ORR catalysts forH2O2 electro-synthesis， discuss the correlation among structurecomposition-activity of catalysts， and highlight the underlyingmechanism of active sites for electrocatalytic H2O2 production.Nevertheless， by analyzing the significant progress inelectrochemical O2-to-H2O2 conversion， many opportunities andchallenges remain for further commercialization. Efforts andacademic attention should be devoted to the following outlines：

1） Expand the TMOs/TMCs catalysts family for efficient andstable 2e-ORR. Till now， the reported TMOs/TMCs catalystswith considerable 2e-ORR performance are majorly focused onseveral transition metals of Group VIII， such as Co， Ni， and Pd.Other kinds of TMOs/TMCs catalysts also show potential forbetter 2e-ORR activity. Therefore， new TMOs/TMCs catalystsshould be developed to enrich the catalyst family. Although theintrinsic ORR activity of these catalysts might be inferior，further modifications could be beneficial to boost theircapability. Specifically， surface coordination of TMOs andTMCs is a practical strategy to boost the intrinsic 2e-ORRactivity of active sites. Meanwhile， engineering proper defectsand suitable morphology of materials would contribute to theaccessibility and binding properties of active sites. Unlike thesimple crystal structure of carbon， TMOs/TMCs can beconstructed into various crystalline phases with lattice strain andexposed facets， adding diversity to the catalyst catalogue.Moreover， based on current research， most TMOs/TMCs haveachieved outstanding 2e-ORR results in alkaline media.However， due to inherent intolerance to acidic media. At present，research， breakthroughs， and achievements in acidic media arelimited. In this regard， TMCs are more promising than TMOsdue to their robustness in acids.

2） Fundamental investigations of active sites in TMOs andTMCs for H2O2 electro-synthesis. Recent research is mainlyconcentrated on the improvement of catalytic 2e-ORRperformance， whereas the identification and elucidation of activesites in materials lack in-depth discovery. Although similarchemical processes have been extensively studied in 4e-ORRaspects 114–117， fewer intermediates （OOH species） are involvedin the 2e-ORR and the maintenance of O―O bonds require moresuitable adsorption active sites. Additionally， unlike H2O （4e-ORR products）， H2O2 products are easily decomposed. Takingthe above into account， mechanism of 2e-ORR is more distinctand complex than 4e-ORR. Moreover， the debate over cation andanion active sites continues， and direct experimental evidencesupporting the attribution of these active sites is still rare.Therefore， advanced characterizations， including in situtechniques with atomic resolution， should be employed to unveilthe genuine active sites for 2e-ORR.

3） Device implementation and industrial trials. Majority of therelevant research on TMOs and TMCs for 2e-ORR is currentlylab-scaled. RRDE and three-electrode systems are the primaryelectrochemical setups used to examine the performance ofdeveloped catalysts. However， these are insufficient to representperformance under actual working conditions. Thus， apart fromcatalyst development， establishing proper gas diffusionelectrodes （GDE） and membrane electrode assembly （MEA） forpractical applications is of great importance. As far as we know，such investigations are very few. Additionally， designing flowcell setups is also encouraged， which now are usually designedfor other three-phase electrochemical reactions， such as CO2reduction. A specific layout for H2O2 electro-synthesis should beprovided. By accomplish that， the catalytic ability of TMOs andTMCs would be kinetically and thermodynamically furthered.More importantly， industrial demonstrations of practicalapplications using such setups should be presented. As currentand flowing liquids reach industrial levels， the underlyingchemistry of these TMOs/TMCs catalysts would be changed andthe electrodes might face several stability issues over the longterm. Feedback from these trials would help to upgrade theTMOs/TMCs catalysts and ultimately fulfill the requirements ofscalable H2O2 electro-synthesis.

4） Electrocatalytic hydrogen peroxide production forapplication. Compared with the anthraquinone process， 2e O2-to-H2O2 conversion enables highly decentralized production ofH2O2， alleviating concerns of storing and transporting hazardoushigh-concentrated H2O2 for many industrial processes. Inchemical synthesis， H2O2 produced by 2e-ORR can realize aslow release of peroxides， avoiding an imbalance in the overallreaction caused by high concentrations of hydrogen peroxide. Inpulp making and textiles， H2O2 produced by electrocatalysis cansubstitute chlorine-containing oxidants， which may bleach rawmaterials with greater eco-friendliness. In disinfection andsterilization， low concentrations （3%） of H2O2 are commonlyrequired and can be easily achieved by 2e-ORR. Suchimplementation could be employed in domestic and refrigeratorsanitizers. Additionally， the oxidative capability of H2O2 couldalso be utilized in pollution decomposition， widely applied inwater treatment and soil remediation. By carrying out 2e-ORRto produce H2O2 rather than the anthraquinone process， masstransportation would be unnecessary， and these decompositionscould be conducted more flexibly and easily. Therefore，electrocatalytic H2O2 production by 2e-ORR is believed to havefruitful prospects.

Author Contributions： Xiaofeng Zhu： fund acquiring，conceptualization， literature investigation， visualization， cowriting-original draft， supervision; Bingbing Xiao： literatureinvestigation， visualization， co-writing-original draft; Jiaxin Su：visualization， writing-original draft; Shuai Wang： visualization，writing; Qingran Zhang： supervision， writing-reviewing andediting; Jun Wang： supervision， fund acquiring， writingreviewingand editing.

Conflict of Interest： The authors declare no conflict of interest.