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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2026 , Vol. 38 >Issue 3: 421 - 442

DOI: https://doi.org/10.7536/PC20251117

Review

Low-Temperature Electrooxidation Catalysts for Methane Conversion

  • Shiyu Jiang 1, 2 ,
  • Jiaxin Jiang 1 ,
  • Haosen Xiong 1 ,
  • Shuyong Shang , 3, * ,
  • Ge He 4 ,
  • Qiang Zhang , 1, *
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  • 1 School of Chemistry and Chemical Engineering，Chongqing University of Technology， Chongqing 400054， China
  • 2 School of Chemistry and Chemical Engineering， Chongqing University， Chongqing 401331， China
  • 3 Sichuan Provincial Engineering Research Center of Resource Utilization for Agricultural and Forestry Wastes， College of Chemistry and Life Sciences， Chengdu Normal University， Chengdu 611130， China
  • 4 School of Chemical Engineering，Sichuan University， Chengdu 610065， China
* （Qiang Zhang）；
（Shuyong Shang）

Received date: 2025-11-20

  Revised date: 2025-12-24

  Online published: 2026-01-08

Supported by

National Natural Science Foundation of China(21902017)

China Postdoctoral Science Foundation(2024M753166)

Natural Science Foundation of Chongqing of China(CSTB2025NSCQ-GPX0960)

Natural Science Foundation of Chongqing of China(CSTB2025TIAD-KPX0017)

Science and Technology Research Program of Chongqing municipal Education Commission of China(KJQN202301121)

Natural Science Foundation of Sichuan of China(2024NSFSC0295)

Natural Science Foundation of Sichuan of China(2024ZYD0157)

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Abstract

This article reviews the challenges and recent advancements in the utilization of methane （CH4） resources via low-temperature electrochemical oxidation （CH4OR） for producing value-added chemicals. Conventional indirect pathways， including methane reforming， are energy-intensive and operate under harsh conditions. In contrast， thermal catalytic partial oxidation frequently results in over-oxidation， thereby limiting its practical applications. In contrast， electrochemical CH4OR represents a promising alternative， facilitating efficient methane conversion under mild conditions， compatible with renewable energy sources， and providing advantages in product separation and transport. This review explores the mechanistic aspects of C—H bond activation during CH4OR， encompassing both direct and radical-mediated indirect pathways.

Contents

1 Introduction

2 The mechanism of low-temperature electrooxidation of methane

2.1 Direct activation mechanism of methane dehydrogenation

2.2 Mechanism of methane dehydrogenation activated by reactive oxygen species

2.3 Kinetic and thermodynamic control in the CH4OR

3 Methane electrooxidation catalyst

3.1 Noble metal catalysts

3.2 Alloy catalysts

3.3 Transition metal oxide catalysts

3.4 MOFs catalysts

3.5 Single atom catalysts

4 Defect engineering： material design strategy for catalytic performance optimization

5 Conclusions and prospects

Key words： methane electrooxidation; oxygen-containing compounds; reaction mechanism; catalyst

Cite this article

Shiyu Jiang , Jiaxin Jiang , Haosen Xiong , Shuyong Shang , Ge He , Qiang Zhang . Low-Temperature Electrooxidation Catalysts for Methane Conversion[J]. Progress in Chemistry, 2026 , 38(3) : 421 -442 . DOI: 10.7536/PC20251117

1 Introduction

Methane （CH4） is an important carbon-based feedstock and fossil fuel， accounting for approximately 21% of the world’s total primary energy supply[1-3]. It is widely found in natural gas， shale gas， coalbed methane， and combustible ice. Due to its abundant sources and low cost， CH4 is becoming a key precursor to replace coal and oil， stimulating the supply of primary hydrocarbon feedstocks. As the main component of natural gas， CH4 is the simplest saturated hydrocarbon with the lowest carbon-to-hydrogen ratio， possessing a high calorific value， with over 90% utilized for heating， cooking， transportation， and electricity generation. Thus， the vast majority of CH4 is currently used as fuel， while its application as a chemical feedstock remains insufficient. On the other hand， CH4 emissions from energy extraction and transportation， agricultural production， and waste are currently the second leading cause of global warming， contributing about 30% to the rise in global temperatures[4]. Although methane persists in the atmosphere for a shorter duration compared to carbon dioxide， its global warming potential （GWP） is up to 25 times that of carbon dioxide equivalent[5]. Therefore， large-scale conversion of CH4 into value-added chemicals， compared to direct combustion or emission， not only helps reduce greenhouse gas emissions but also generates significant economic benefits by providing alternative energy sources. However， due to the stable high-symmetry tetrahedral inert structure of CH4 molecules， which results in a very high C—H bond energy （439.3 kJ/mol）， negligible electron/proton affinity （543.9 kJ/mol）， high ionization energy （≈12.6 eV）， and low polarizability （2.84×10-40 C2 m2/J）， activating CH4 and breaking the CH3―H bond is very challenging. This has led to it being regarded as the “holy grail” in the field of catalysis[6-8]， making the conversion of CH4 into high-value chemicals highly attractive and challenging.
Currently， nearly all commercial processes for synthesizing high-value-added products from CH4 begin with methane reforming， such as dry reforming of methane （DRM）[9-12] and steam methane reforming （SMR）[13-15]. Initially， traditional thermal catalysis is used to convert CH4 into syngas （CO， H2）， which is then transformed into liquid hydrocarbons and methanol （Fig. 1）. However， the standard enthalpy of reactions in processes like DRM and SMR reaches as high as 274.3 and 206.1 kJ/mol， respectively， making them typical energy-intensive reactions. In industrial settings， these processes generally face high temperature （627~1000 ℃） and high pressure （>2.5 bar，1 bar=100 kPa）， accompanied by significant carbon emissions[16]. This results in substantial energy consumption， high equipment investment costs， and environmental damage.
图1 （a） 传统集中式工厂中涉及甲烷重整、甲醇合成及纯化的间接甲醇生产示意图；（b） 分散式中甲烷直接电化学部分氧化制甲醇[17]

Fig.1 （a） Schematic of indirect methanol production involving methane reforming and methanol synthesis and purification in conventional centralized plants. （b） Direct electrochemical partial oxidation of methane to methanol in decentralized locations[17]

To improve these situations， some studies have found that the addition of oxidants， such as oxygen （O2）， ozone （O3）， hydrogen peroxide （H2O2）， and nitrous oxide （N2O）， can facilitate the partial oxidation coupling （POM） of CH4 at lower temperatures （<500 ℃）[18-20]. However， the introduction of oxidants drives the POM process in a thermodynamically favorable direction （CH4 + 1/2O2 → CH3OH， ∆H0298 K=-125.9 kJ/mol）， which is an irreversible process prone to the over-oxidation of methane， inevitably leading to the generation of COx species or carbonaceous materials. Furthermore， the presence of O2 at high temperatures poses potential hazards. This series of stringent conditions greatly limits the industrial application of CH4 as a raw material for chemicals， hindering the development of C1 chemistry[21-24].
Therefore， it is necessary to develop applicable direct CH4 conversion strategies under mild conditions. With the ongoing development of renewable energy sources such as solar power， wind power， and nuclear energy， and the continuous reduction in the cost of renewable energy electricity， electrochemical oxidation has gradually emerged as a highly promising alternative technology for methane oxidation （CH4OR） to generate oxygenated compounds[25-27]. The application of an external potential can optimize the reaction kinetics of CH4OR， rapidly activate C―H bonds to improve the inherent chemical inertness of CH4[28-31]. At the same time， electrochemical processes demonstrate excellent efficiency in utilizing intermittent renewable energy， significantly reducing the release of chemical bond energy， and enabling the storage of intermittent energy in the form of chemical bonds for daily and seasonal energy reserves. Additionally， owing to the mild temperature conditions and the introduction of oxygen atoms， the products of CH4OR （intermolecular hydrogen bonding） are generally in the liquid phase. Consequently， they can be easily transported using existing infrastructure， significantly lowering the costs associated with directly transporting and storing CH4 or syngas （such as those from liquefaction）[32-33].
Moreover， CH4OR can achieve sufficient yields at relatively low costs and operate under low-temperature conditions （100 ℃）. At the same time， electro-catalytic devices are characterized by miniaturization， integration， and ease of scaling， making them suitable for various sizes of CH4OR devices[34]. The electrochemical process of CH4OR is a kinetics-controlled process； therefore， the rate and selectivity of product formation are closely related to the applied electrode potential[35-37]. CH4OR involves a gas-solid-liquid three-phase reaction， typically where methane is introduced to form a methane-saturated electrolyte， followed by using an external electric field to provide favorable electron transfer to the catalyst， thereby effectively activating C—H bonds[38]. This process is generally divided into direct activation and indirect activation. Direct activation involves CH4 directly adsorbing onto active sites of the catalyst to form activated methane （*CH4）， while indirect activation utilizes generated hydroxyl radicals （·OH） and superoxide radicals （·OOH） in the interfacial region or solution to activate C―H bonds through strong oxidative effects. However， due to the very close electrode potentials of various products， there is a tendency to undergo oxidation to CO2. However， due to the close electrode potentials of various products， there is a tendency to undergo oxidation to CO2 （Table 1）. And， from the perspective of standard electrode potentials （Eθ）， the theoretical equilibrium potential for OER is +1.23 V vs. RHE， whereas the standard potential for CH4OR （taking complete oxidation to CH3OH） is approximately +0.58 V， indicating that CH4OR is thermodynamically more favourable. However， under practical reaction conditions， the competition outcome between the two is primarily determined by kinetic factors. OER generally requires a high overpotential （η） to overcome the energy barrier of its sluggish multi-step electron transfer process， whereas CH4OR is also constrained by the high strength of the C―H bond in methane and the complexity of subsequent reaction steps. Therefore， the reported electrochemical activity of CH4OR （such as current density， selectivity， and stability） remains low， presenting significant commercial challenges （such as low reaction efficiency and high separation costs）， greatly hindering the large-scale development of CH4OR.
表1 甲烷氧化反应的标准电极电位[53]

Table 1 Standard electrode potentials for CH4 oxidation reactions[53]

Reaction E/V vs RHE
$\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+{\mathrm{H}}_{2}\mathrm{O}\mathrm{ }\left(\mathrm{l}\right)\to \mathrm{C}{\mathrm{H}}_{3}\mathrm{O}\mathrm{H}+2{\mathrm{H}}^{+}\left(\mathrm{a}\right)+2{\mathrm{e}}^{-}$ 0.58
$\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+{\mathrm{H}}_{2}\mathrm{O}\mathrm{ }\left(\mathrm{l}\right)\to \mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}\left(\mathrm{a}\right)+4{\mathrm{H}}^{+}\left(\mathrm{a}\right)+4{\mathrm{e}}^{-}$ 0.46
$\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\left(\mathrm{a}\right)+6{\mathrm{H}}^{+}\left(\mathrm{a}\right)+6{\mathrm{e}}^{-}$ 0.26
$\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+6{\mathrm{H}}^{+}\left(\mathrm{a}\right)+6{\mathrm{e}}^{-}$ 0.26
$\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+2{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{C}{\mathrm{O}}_{2}\left(\mathrm{g}\right)+8{\mathrm{H}}^{+}\left(\mathrm{a}\right)+8{\mathrm{e}}^{-}$ 0.17
$2\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)\to {\mathrm{C}}_{2}{\mathrm{H}}_{6}\left(\mathrm{g}\right)+2{\mathrm{H}}^{+}\left(\mathrm{a}\right)+2{\mathrm{e}}^{-}$ 0.35
$2\mathrm{C}{\mathrm{H}}_{4}\left(\mathrm{g}\right)\to {\mathrm{C}}_{2}{\mathrm{H}}_{4}\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{a}\right)+4{\mathrm{e}}^{-}$ 0.44
$2{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{O}}_{2}\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{a}\right)+4{\mathrm{e}}^{-}$ 1.23
The inherent inertness of CH4 and its low solubility in electrolytes are significant reasons for the low current density and low reaction activity[39-42]. Currently， a large number of studies have shown that developing new catalysts with high reactivity and selectivity for efficient activation of C―H bonds and promotion of C―C coupling through common methods such as doping[43-45]， interface engineering[46-48]， defect engineering[49-50]， and morphology engineering is extremely critical[51-52]. These strategies can enhance mass transfer diffusion and electron transfer during the CH4 oxidation reaction， which is beneficial for the adsorption and activation of CH4， thus improving the conversion efficiency. This article summarizes the recent research progress on the low-temperature electrooxidation of methane from aspects such as C―H bond activation， C―C coupling mechanisms， catalyst design and preparation， and provides perspectives on catalyst design.

2 The mechanism of low‑temperature electrooxidation of methane

Electrocatalytic oxidation of CH4 is a complex process involving multiple electron transfers， and exploring the reaction mechanism is crucial for designing efficient catalysts and enhancing the selectivity and activity. The CH4OR reaction process typically involves steps such as adsorption， decomposition， reaction， and product desorption on the catalyst surface. Due to advancements in in-situ characterization techniques （such as in situ Raman， infrared spectroscopy， and electron microscopy） and computational theoretical methods （like molecular dynamics and first-principles calculations）， researchers are now more frequently employing computer simulations to investigate the reaction processes. By constructing possible adsorption models of reactants， intermediates， and products on the catalyst surface， they can infer the reaction pathways and clarify the entire reaction mechanism[53-54]. Currently， most studies indicate that CH4OR can preferentially produce low-carbon products with high economic value （such as CH3OH， HCOOH， C2H4O， CH3COOH， and CH3COCH3）， and it becomes increasingly difficult as the carbon chain increases （Fig.2）[55-56]. There are two pathways for electrocatalytic CH4 activation， and typically， the rate-determining step is the cleavage of the first CH3—H bond.
图2 电化学甲烷氧化反应的反应途径[53]

Fig.2 Proposed reaction pathways of the electrochemical methane oxidation reaction[53]

2.1 Direct activation mechanism of methane dehydrogenation

Direct activation of dehydrogenation occurs via anode active sites （e.g.， Pt， Au， Pd）， following the reactions CH4（g） +* → *CH4 → *CH3 + *H or CH4（g） +* → *CH4 → *CH3 + H+ + e-. The dehydrogenation reaction of *CH3 is an energy-dependent process[4]. It occurs on the catalyst through a sequence of steps： *CH3 → *CH2 → *CH → *CHO → *CHOH → *CO， which preferentially adsorbs， dissociates， and oxidizes to produce COx （Fig.3）[57]. The *CH3 exhibits a strong interaction with the catalyst’s active sites， forming a robust *M—CH3 bond. This bond inhibits the desorption of certain intermediate products and promotes excessive oxidation to CO2[56].
图3 Pt（211）表面甲烷完全电化学氧化为二氧化碳的吸附转移机制示意图[57]

Fig.3 Schematic diagram of the adsorption transfer mechanism for the complete electrochemical oxidation of methane to carbon dioxide on Pt（211）[57]

2.2 Mechanism of methane dehydrogenation activated by reactive oxygen species

Some sources， including strong oxidants （H2O2， O3） as well as oxygen donors like CO2 and CO32-， can contribute to the generation of reactive oxygen species such as *O and *OH under electrochemical conditions[58-59]. These species attack the surface of adsorbed CH4， breaking its first C―H bond. Subsequently， electron and proton transfers occur to form *CH3 （e.g.， CH4 （g） + *OH → *CH3 + H2O）[60-61]. However， the activation of CH4 by *OH and *O radicals to form *CH3 can lead to sequential and thermodynamically spontaneous oxidation， resulting in the generation of *CH2， *CH， and ultimately *C species， which are prone to further oxidation into undesired COₓ products. Transition metal oxides （such as NiOx， CoOx， and FeOx） activate CH4 to form *CH3 via reactive oxygen species such as *OOH generated on the catalyst surface （NiⅢOOH[62]， CoⅢOOH[63] and FeⅣO[64]）. The interaction between *CH3 and the catalytic active center is relatively weak due to sp2 hybridization， which does not generate strong *M-CH3 adsorption bonds. This results in the easy desorption of the methane intermediate， facilitating the release of a complete *CH3OH molecule， which can then be further oxidized to produce CH2O， CH3OH， CH3CH2OH， CH3COCH3， HCOOH， CH3COOH， or other products[56]. It is noteworthy that the activation of the initial C—H bond by active species involves a thermochemical process， whereas the electrochemical process involves the continuous provision of active sites or active substances. However， these reactive oxygen species may further oxidize and evolve as molecular oxygen （OER） from the anode， leading to competitive reactions with CH4OR that hinder the generation of high-value-added products （Fig. 4）.
图4 在低电位范围内CH4对双电层结构的影响对比示意图[56]

Fig.4 Schematic comparing the double layer in the absence and presence of CH4 at the low potential region[56]

The regulation of reactive oxygen species during the electrochemical CH4 oxidation reaction （CH4OR） is crucial for suppressing the oxygen evolution reaction （OER） and improving the selectivity of the CH4OR reaction. Among all the oxygen intermediates （*OH， *O， and *OOH）， *O is considered the active species for CH4 activation due to the activation barrier of the *OH-assisted pathway （1.2 eV）， which is approximately twice that of the *O-assisted pathway （0.6 eV）. Jaehyun Lee and colleagues[65] investigated the influence of active species （*O） supplied from either H2O and CO32- in the electrolyte on the CH4OR using α-Fe2O3 as a cocatalyst. Since the *OH → *O step in the oxygen evolution reaction （OER） is the potential-determining step， applying a high potential （2.0 V vs. RHE） promotes this step， leading to an exothermic and spontaneous overall reaction that rapidly generates O2. This makes it difficult to maintain surface *O， implying that *O cannot be efficiently sourced from the H2O electrolysis process on the α-Fe2O3 （0001） surface because the generated *O rapidly reacts to form O2， thereby inhibiting CH4OR （Fig. 5）. Consequently， a alternative pathway was proposed utilizing CO32- as an intermediate to supply the active species： CO32- is first adsorbed with an activation energy of 1.6 eV， followed by overcoming a 0.8 eV energy barrier for the reaction *CO3 → CO2 + *O to form *O. This process effectively bypasses the thermodynamically spontaneous progression of *O generated from H2O electrolysis at high potentials （which proceeds as *O + OH- → *OOH + e- → *O2 + H+）， thus avoiding the competing OER effect. Moreover， electrochemically generated chlorine radicals （·Cl）， primarily derived from the oxidation of chloride ions （Cl-） via the chlorine evolution reaction pathway， can effectively facilitate the activation of C―H bonds in CH4 under ambient conditions[66]. The ·Cl radicals exhibit superior stability and higher reactivity compared to hydroxyl-derived *O. Similar to *O， the formation of ·Cl promotes the breaking of the first CH3―H bond to form CH3Cl. Meanwhile， CH3Cl easily desorbs at the anode， preventing its excessive oxidation to CO2， and the generated CH3Cl can be readily converted to CH3OH after subsequent hydrolysis in water.
图5 （a） Fe2O3催化剂上氧电解反应的自由能；（b） CO32-在0 V和0.8 V（vs. RHE）电位下吸附解离的自由能[65]

Fig.5 （a） Free energy diagram for the OER on the Fe2O3 catalyst， （b） Free energy diagram for adsorption dissociation of CO32- at potentials of 0 V and 0.8 V （vs. RHE）[65]

2.3 Kinetic and thermodynamic control in the CH4OR

In the low-temperature CH4OR， tailoring the electrolyte composition， pH， and ionic environment serves to thermodynamically widen the potential window between CH4OR and the OER， while kinetically suppressing the OER pathway and promoting the conversion of CH4 to high-value products. In conventional alkaline or neutral electrolytes， the OER typically follows an adsorption evolution mechanism. While the reactive oxygen species （e.g.， *OOH， *O） generated along the OER pathway can participate in activating the C―H bond of CH4， they are also highly prone to further conversion into O2， leading to diminished Faradaic efficiency for CH4OR products. Research indicates that introducing specific electrolyte additives， such as sulfate （SO42-） or phosphate （PO43-）， can alter the surface charge distribution of the catalyst and the adsorption energy of key intermediates， thereby inhibiting the formation and transformation of *OOH and delaying the progression of the OER[67]. For instance， phosphate additives can facilitate the formation of a metal phosphate passivation layer on the electrode surface. This layer moderately suppresses the OER while simultaneously providing a favorable active interface for CH4 activation， promoting the formation of target products like CH3OH. In weakly acidic electrolytes， the OER rate is often limited by the water dissociation step. In contrast， the energy barrier for the *O-mediated activation pathway of CH4 is relatively low， creating an opportunity to preferentially drive CH4OR at lower potentials. Furthermore， utilizing anions present in the electrolyte， such as carbonate （CO32-）， as a source of *O enables the establishment of an active oxygen generation channel independent of the OER pathway， effectively avoiding competitive OER. By rationally designing the electrolyte composition and constructing a favorable interfacial microenvironment through electrode surface engineering， the potential window between CH4OR and OER can be effectively expanded on both thermodynamic and kinetic levels， ultimately enhancing the selectivity and energy efficiency of electrochemical methane oxidation.

3 Methane electrooxidation catalyst

In order to avoid deep oxidation， it is necessary to promote C―H bond activation and the coupling processes of C―C and C―O. Electrocatalysts play a decisive role in the conversion， yield， selectivity， and Faradaic efficiency （FE） of the processes. Therefore， constructing catalytic active sites based on relevant reaction mechanisms particularly important. In electrochemical processes， the catalyst must possess high stability， conductivity， high activity， and favorable environmental characteristics. Currently， methane oxidation catalysts predominantly consist of noble metal catalysts， alloy catalysts， single-atom catalysts， and transition metal oxide catalysts （Fig. 6）. Additionally， we have compiled a summary of catalysts （Table 2）. Noble metal-based catalysts （e.g.， Pt， Pd， Au， Ru） are regarded as high-performance catalysts due to their unique intrinsic activity and high stability. Additionally， unique nanostructured materials， such as alloys， metal-organic frameworks （MOFs）， nanosheets， and single-atom structures， can effectively enhance the catalytic activity and metal utilization， thereby reducing material costs.
图6 低温甲烷电氧化催化剂

Fig.6 Types of low-temperature methane electro oxidation catalysts

表2 甲烷低温电氧化催化剂的催化活性

Table 2 Catalytic activity of methane low-temperature electrooxidation catalysts

Catalysts Electrolyte Potential /Current density（vs. RHE） C2+ product selectivity Yield Stability Ref
Ethanol Methyl formate Aceticacid Athylene glycol Acetate
NiO/Ni 0.1 mol/L NaOH 1.4 V 89% - - - - 25 μmol/（g·h） 25 h 67
WO3 Nanobar Arrays 0.1 mol/L Na2SO4 1.3 V - - - 23.9% - 0.47 mmol/（cm2·h） 12 h 68
WO3 nanosheets 0.1 mol/L Na2SO4 1.2 V 50.7% - - - - 125090 μmol/（g·h） 12 h 69
FeNi（OH）x 0.1 mol/L NaOH 1.46 V 87% - - - - 9.09 mmol/（g·h） 2.5 h 70
NiO/NiHF 0.1 mol/L NaOH 1.44 V 85% - - - - - 3 h 71
ZnO nanosheets NaHCO3 （pH=9） 1.3 V - 75.46% - - 347.31 mmol/（g·h） 12 h 72
Ov-V2O5 [BMIM]BF4 2.2 V ~45% - - - - 352.5 μmol/（g·h） 0.5 h 73
Ni3N/Ni interface 0.1 mol/L NaOH 1.7 V 78% - - - - - 1 h 74
Mg-MOF-74 1 mol/L KOH 1.65 V - - - - 7.5 % 126.6 μmol /（g·h） - 75
Ultrathin WO3 nanosheets 0.1 mol/L Na2SO4/H2SO4 0.9 V 63.6% - - - - 98.9 mmol/（g·h） - 76
Rh/ZnO 0.1 mol/L KOH 2.2 V 22.5% - - - - 789 μmol/（g·h） 24 h 77
Zr-doped Fe2O3 0.5 mol/L Na2CO3 1.6 V 87% - - - - 1831 μmol/（g·h） 18 h 65
6% PTEF-Ni（OH）2 0.5 mol/L Na2CO3 1.56 V ~73% - - - - - 16 h 79
NiCo alloy/CNTs 0.1 mol/L NaOH 1.6 V 75.6% - - - - - ~12 h 80
Cu@Ni-NiO 0.1 mol/L NaOH 1.9 V 59.8% - - - - 66 h 81
TiO2-CuOx [bmim][BF4] 2.0 V 74% - - - - 59.8 mmol/（g·h） - 82
Cu2O/CuO 0.5 mol/L Na2SO4 2.2 V 69.2% 441.3 μmol/（g·h） 8 h 83
NiCuCoMnAg high-entropy alloys （HEAs） 1 mol/L Na2CO3 1.4 V 95.3% - - 84
Fe-N-C single atom 0.1 mol/L KOH 1.6 V 68% - - - - 4668.3 μmol/（g·h） 100 h 85
Ni-Co PBA-VCN 0.1 mol/L NaOH 2.0 V 56.7% - 160 h 86

3.1 Noble metal catalysts

Pt-based electrocatalysts have been extensively studied， providing a substantial experimental and theoretical foundation for the oxidation of CH4 under mild conditions[87-91]. In polymer electrolyte fuel cells （PEFCs）， when Pt acts as a catalyst in saturated CH4 solutions at a potential of 0.3 V vs. RHE， *CO can be observed to accumulate rapidly on the surface within the first few minutes， with coverage gradually increasing over a longer duration. Additionally， compared to “platform sites”， the abundant “step sites” on rough polycrystalline Pt surfaces facilitate easier dissociation of CH4 molecules， leading to a faster generation of *CH3. It is also found that *CH， produced from the activation of CH4 at the “step sites” on the Pt surface， is the most abundant and relatively stable *CHx species， exhibiting a lower energy barrier for the formation of *CHO and *CO. As the electrode potential increases to 1.1 V vs. RHE， high coverages of *O and *OH active species are generated； under these conditions， the interaction between CH4 and the Pt surface can yield more oxidized *COOH species. This suggests that the rate of decomposition and oxidation of CH4 on the Pt surface strongly depends on the applied potential and the relevant coverages of *H， *OH， and *O derived from the aqueous electrolyte. The likely reaction centers for the activation of CH4 into CHx are localized regions with reduced Pt coordination， particularly along step edges. These sites are prone to interact with nearby oxygen-containing species （originating from *O or *OH）， where their composition is dependent on the electrochemical potential and pH （with *H adsorption being unfavorable for the dissociative activation of CH4）.
Electrophilic trivalent metal ion Pd（III） is also an effective intermediate for the catalytic functionalization of CH4[93-95]. This electro-generated high-valent Pd complex can rapidly activate CH4， with a relatively low activation energy barrier of （25.9±2.6） kcal/mol （1 kcal/mol=4.184 kJ/mol）[96-97]. The conversion of CH4 to the methyl sulfate precursor （CH3OSO3H） and methanesulfonic acid （CH3SO3H） is achieved through concurrent Faradaic and non-Faradaic reaction pathways[96]. This is attributed to the double-electron oxidation that occurs as Pd（II） in the electrolyte undergoes an initial electron transfer step， followed by a chemical reaction step and a subsequent second electron transfer step. The rate of the chemical reaction increases with the concentration of Pd（II）. Moreover， in a 20% SO3/H2SO4 electrolyte at 70 ℃， a current density of 0.65 mA/cm2 can be sustained for 5 h. However， due to the simultaneous Faradaic and non-Faradaic methane methylation reactions， approximately 3.4 molecules of methane are methylated per electron transferred， with 7% of the products being oxidized to carbon dioxide.
To address the issue of products on the anode being oxidized to CO2 or the competitive OER， a method for the electrochemical partial oxidation of methane （EMPO） through in situ generated partially oxygenated active species at the cathode is proposed. Using Au foil or carbon powder as selective cathode catalysts， H2O2 is generated via oxygen reduction reaction （ORR）， providing *OH and *OOH active species that can effectively activate CH4 and CH3OH. Initially， CH4 undergoes partial oxidation to form unstable CH3OOH （*CH3+*OOH↔*CH3*OOH）， which is then reduced at the cathode to CH3OH （*CH3+*OH→*+CH3OH）， and ultimately is oxidized to HCOOH under the action of *OH[98]. In the EMPO system， the selectivity and rate of the product HCOOH can reach up to 80.7% and 18.9 μmol/h， respectively. The overall flow rate of the reaction process has little effect on the reaction； as the flow ratio of CH4/O2 increases， the partial concentration of CH4 rises from 20% to 50%， resulting in an increased yield of HCOOH， indicating that CH4 is the limiting reagent affecting the reaction rate. Furthermore， as the applied potential changes from +0.2 V vs. RHE to -0.2 V vs. RHE， the increase in cathodic current enhances the production of H2O2 and the quantity of oxygen-containing liquid products.
The high-voltage electro-Fenton （HPEF） strategy accelerates reaction kinetics[99-100]， thereby addressing the low reactivity and solubility of CH4 in electrolytes （Fig.7）. First， the oxygen reduction reaction （ORR） was conducted on a silver （Ag） electrode to generate H2O2. Subsequently， H2O2 decomposition was facilitated by doping Fe2+ onto Ag， resulting in the production of hydroxyl radicals （•OH） as reactive oxygen species. At an overpotential of only 0.38 V vs. RHE， we achieved a Faradaic efficiency of 81.4% and a selectivity of 95.2% for HCOOH[59]. During the HPEF process， highly active •OH is generated from self-produced H2O2 through the action of Fe2+， enabling the continuous oxidation of CH4 to form HCOOH. The oxidized Fe3+ is subsequently reduced to Fe2+ on the silver foil cathode. Furthermore， a substantial increase in reaction pressure from 0.1 MPa to 4.0 MPa （CH4/O2/Ar = 9/0.5/0.5） significantly enhances the solubility of CH4 in the electrolyte， increases the collision frequency between CH4 and •OH， and inhibits the quenching of •OH， thereby facilitating the conversion of O2 to CH4. Additionally， the high selectivity of HCOOH can be attributed to its elevated solubility energy in water， which decreases the chemical potential of HCOOH and contributes to its stabilization. Although noble metal catalysts exhibit high catalytic activity and reaction rates， thereby lowering energy barriers and minimizing side reactions， the high costs， limited storage capacity， and susceptibility to poisoning significantly impede their large-scale industrial applications[101].
图7 （a） 电辅助甲烷选择性部分氧化系统（EMPO）；（b） EMPO液态产物生成量随时间的变化趋势；（c） 异相-均相耦合电催化过程中甲烷转化为甲酸的示意图；（d） 银箔阴极经10个反应循环的稳定性测试[59]

Fig.7 （a） Schematic illustration of the electro-assisted selective CH4 partial oxidation system （EMPO） system. （b） Time-dependent change in the amounts of liquid products of EMPO. （c） Schematic diagram of CH4 conversion to HCOOH by the hetero-homogeneous coupling electrocatalytic process. （d） Stability test of the Ag foil cathode for 10 reaction cycles[59]

3.2 Alloy catalysts

Nickel （Ni） exhibits excellent abilities for CH4 adsorption and dissociation； however， subsequent activation processes can be relatively difficult[102-104]. Alloy catalysts formed from two or more metals can alter the electronic structure and geometric state of the metals， enhancing catalytic activity and accelerating reaction rates. Molybdenum （Mo） possesses strong oxygen （*O） adsorption capacity and electro-negativity； thus， the Mo-Ni dual-site may facilitate C―H activation and *O adsorption for partial oxidation of methane[105]. Mo atoms tend to reside in the subsurface layer of MoNi （100）， and with an increase in the number of surface *O species， the segregation energy （Eseg） of Mo becomes more negative， which may lead to the migration of Mo atoms from the subsurface to occupy surface positions. The primary role of Mo is to promote O2 activation while inhibiting the dissociative adsorption of O2 on the Ni site of MoNi （with Ni serving as the CH4 activation center）. Notably， the *O adsorbed on Mo will first migrate to Ni rather than directly bond with *C to form *CO.
In addition， Pt/TiO2 catalyst with surface Pt-Ti alloy layer reveals a unique reaction mechanism for CO-assisted O2 activation， which effectively promotes the generation of oxygenated products from CH4 under mild conditions[87]. By precisely controlling the loading of Pt （0.1%） and facilitating the diffusion of Ti atoms to the surface of the Pt nanocrystals， a unique surface Pt-Ti alloy structure is formed. Under optimized reaction conditions （150 ℃， a CH4/CO/O2 gas mixture at 31 bar， in an aqueous phase）， a total yield of oxygenated products of 749.8 mmol/gPt is achieved， with methanol （CH3OH） selectivity of 62.3%. Notably， even under milder conditions （such as 90 ℃）， the catalyst maintains considerable activity （oxygenated product yield of 288.4 mmol/gPt） and high methanol selectivity （60.4%）， demonstrating its potential for practical applications. Crucially， CO facilitates the dissociation of O2， generating highly active oxygen species at the Pt-Ti alloy sites， which can effectively activate the stubborn C―H bonds in methane [106].
In the electrochemical CH4OR process， Co atoms have been found to facilitate the C―H bond dissociation followed by the C―C coupling process[107-110]， making it easier to generate C2 or C3 products. Ni-Co alloys have been considered for the preparation of C2+ products； however， conventional NiCo alloy catalysts tend to agglomerate due to the recrystallization process during calcination. Carboxylated multi-walled carbon nanotubes （MWCNTs-COOH） can be introduced as dispersants to prepare highly dispersed NiCo alloy catalysts[80]. Initially， individual Ni-Co PBA is fixed using ―COOH ends， and upon heating， pyrolysis leads to the cleavage of ―C≡N― in Ni―C≡N―Co， resulting in highly dispersed NiCo alloys. Here， the NiCo bimetal acts as a tandem catalyst， first performing the dehydrogenation of CH4 on Ni atoms to generate *CHx species （x=2， 3）， then facilitating the coupling of *CH2OH （*CH2+*OH→*CH2OH） and *CH3 to produce *CH3CH2OH （Fig. 8）. The NiCo alloy also exhibits outstanding short-term stability （maintaining a stable current density for 16 hours at 1.6 V vs. RHE） and selectivity for CH3CH2OH （with a Faradaic efficiency of 75.6%）. However， under prolonged operation， alloy catalysts remain thermodynamically susceptible to oxidation by active species in the electrolyte， potentially leading to gradual deactivation or structural changes over time[111].
图8 （a） NiCo合金/碳纳米管合成流程图；（b） NiCo合金/碳纳米管上乙醇稳定性测试及部分法拉第效率变化（16 h）；（c） 用于电解CH4制备乙醇的双室电解槽示意图及反应机理[80]

Fig.8 （a） Flowchart for the synthesis of NiCo alloy/CNTs. （b） Stability tests and partial Faraday efficiency changes for ethanol on NiCo alloy/CNTs （16 h）. （c） Schematic diagram of a two-chamber electrolytic cell for the electrolysis on CH4 to produce ethanol and the reaction mechanism[80]

3.3 Transition metal oxide catalysts

So far， in the existing electrochemical CH4OR processes， only C1 compounds （CH3OH， HCOOH） dominate as products， and their yields are relatively low[112-115]. The electron transfer at the interface between metals （M） and metal oxides （MOx） can alter the electronic structure of the metals， facilitating the coupling of C―C to generate CH3CH2OH （Fig.9）. Song et al.[67] constructed an optimized xNiO/Ni interface catalyst to study the role of the interface structure in the CH4 activation. In a 0.1 mol/L NaOH solution， by comparing the linear sweep voltammetry （LSV） curves of nickel foam （Ni foam） and xNiO/Ni under saturated CH4 and argon （Ar） atmospheres， it was found that Ni foam did not exhibit distinct oxidation peaks in CH4 and Ar， and the LSV curves were nearly overlapping， indicating that Ni foam exhibits kinetic sluggishness towards CH4. In contrast， the LSV curve of the xNiO/Ni catalyst showed a significant increase in oxidation current in CH4 compared to that in Ar， displaying clear oxidation peaks （originating from the oxidation of NiO to NiOOH）， suggesting that CH4OR readily occurs on the xNiO/Ni electrode. Furthermore， by collecting the cathode products （H2）， it was found that the yield variation trend of CH3CH2OH and CH3OH on the anode catalyst closely matched （the mole ratio aligns well with theoretical values）， implying that the conversion of CH4 adheres to stoichiometry （2CH4 + H2O→C2H5OH + 2H2， CH4 + H2O→CH3OH + H2）. This reveals that the *OH provided by the electrolysis of H2O can be significantly utilized for CH4OR instead of competing with the OER reaction to oxidize to O2. However， due to the dielectric properties of the NiO component， an increase in its content leads to an increase in charge transfer resistance[116]. Throughout the reaction process， the formation of *CH3 to CH3OH （*CH3 + *OH → *CH3OH） is an endothermic reaction with an energy level of 0.32 eV， while the hydroxylation of *CH2 （*CH2 + *OH → *CH2OH， ΔG = -0.77 eV） and the coupling process of *CH3 and *CH3-*CH2OH （*CH3 + CH2OH → CH3CH2OH， ΔG = -1.64 eV） are both exothermic reactions. This reveals the excellence of CH3CH2OH production. Therefore， the xNiO/Ni catalyst can achieve 89% Faradaic efficiency of CH3CH2OH at 1.40 V vs. RHE and a product rate of 25 μmol/（gNiO/Ni·h）.
图9 （a） ZrO2纳米管/Co3O4催化剂及整体电化学甲烷氧化反应系统的制备；（b） ZrO2纳米管/Co3O4催化剂电化学氧化甲烷3、6和12 h后的产物选择性[119]；（c） 甲烷在NiO/Ni界面电氧化为乙醇与甲醇的示意图；（d） 不同施加电位下xNiO/Ni催化剂上阳极产物乙醇与甲醇的电化学电位；（e） 采用NiO@NiHF阳极进行电催化甲烷转化的示意图；（f） xNiO@NiHF复合催化剂在1.40~1.50 V内阳极产物乙醇与甲醇的电化学电位及电流密度曲线；（e） NiO@NiHF阳极实现电催化甲烷转化的示意图；（f） 在CH4饱和的0.1 mol/L NaOH溶液中，xNiO@NiHF复合催化剂于1.40~1.50 V（相对红外氢电极）条件下的电化学势与电流密度曲线[71]

Fig.9 （a） Fabrication of the ZrO2 NT/Co3O4 catalyst and overall electrochemical methane oxidation reaction system. （b） product selectivity after 3， 6 and 12 h of electrochemical CH4 oxidation by ZrO2NT/Co3O4[119]. （c） Schematic illustration of electrooxidation of methane to ethanol and methanol on the NiO/Ni interface. （d） FEs of the anodic products ethanol and methanol over xNiO/Ni catalysts at various applied potentials. （e） Schematic illustration of electrocatalytic methane conversion using a NiO@NiHF anode. （f） FEs and current densities of xNiO@NiHF composite catalysts at 1.40~1.50 V （vs. RHE） in CH4-saturated 0.1 mol/L NaOH[71]

A NiO@Ni hollow fiber electrode with a compact three-dimensional geometric structure （providing a large area of gas-liquid-solid three-phase boundary） can also efficiently conduct the CH4 oxidation reaction （CH4OR） and generate low-carbon alcohols （CH3CH2OH， CH3OH）[71]. In this work， at a potential of 1.44 V vs. RHE， CH3OH achieves a Faradaic efficiency of 54% and a current density of 20 μA/cm2， while at a potential of 1.46 V vs. RHE， CH3CH2OH reaches a Faradaic efficiency of 85% and a current density of 40 μA/cm2. A Cu@Ni-NiO interfacial coated structure catalyst prepared using O2 plasma can activate the C―H bonds of Ni0 while producing reactive *O through the electrolysis of H2O （H2O → *OH → *OOH → *O）[81]. During the reaction， the generated *CH2 species quickly transfers from Ni0 to Ni2+， allowing *O to immediately activate the next CH4 molecule rather than transitionally oxidizing to CO2. Simultaneously， *CH2 on Ni2+ will hydroxylate with *OH to generate *CH2OH， which then couples with the newly formed *CH3 on Ni0， achieving a high selectivity for CH3CH2OH （59.8%）.
In addition to NiO， cobalt oxide （Co3O4）-based catalysts also facilitate the coupling of C―C bonds during the CH4 oxidation reaction. Due to the spinel structure of Co3O4， it possesses relatively weaker M―O bonds， making oxygen vacancies on the surface easy to migrate and providing good surface oxygen exchange capability[117-118]. Oh et al.[119] designed a nano-structure zirconia nanotubes （ZrO2 NT） with high specific surface area which has good electron acceptance ability and adsorption capacity for CO32-. By loading Co3O4 onto ZrO2 NT to prepare the mixed catalyst， they achieved the electrochemical production of C3 alcohols （1-propanol， 2-propanol） in lower potential region （1.6 V vs. RHE）， with yield as high as 2416 μmol/（gcat·h） after continuous reaction for 12 h. Because ZrO2 and Co3O4 are simply physically combined （weak interaction）， this prevents a decrease in the reducibility of Co3O4， benefiting the CH4OR process. Here， CH4 can easily enter the surface of Co3O4 through the ZrO2 nanostructure， thereby allowing charge to be easily transferred to the reactants， which increases the reaction rate and induces a lower onset potential. The study found that CH3CHO （primarily derived from the oxidation of CH3CH2OH） was identified as a key intermediate for the formation of C3 alcohols. Specifically， •CH3 radicals undergo free radical addition with CH3CHO to generate 1-propanol， while CH4 participates via nucleophilic addition to CH3CHO， yielding 2-propanol.
Particularly， the transition metal oxide-based catalysts discussed in this section are primarily designed with interface structures as the core strategy. This is because single-component transition metal oxides （e.g.， pure NiO or Co3O4）， when employed independently as electrocatalysts for the CH4 oxidation reaction （CH4OR）， typically face multiple challenges such as low intrinsic activity， poor electrical conductivity， difficulties in controlling product selectivity， and insufficient structural stability. These limitations originate from their homogeneous electronic structure， inadequate charge transport capability， and instability under anodic potentials. By constructing metal/oxide or oxide/oxide interfaces， it is possible to effectively modulate the electronic structure of active sites， create synergistic catalytic centers， and improve charge and mass transport， thereby cooperatively enhancing the activity， selectivity， and stability of the CH4OR process. However， it should be noted that MOx-based interface catalysts still face issues such as uncontrollable structures， poor stability， and low conductivity， which limit their large-scale application in industrial catalysis.

3.4 MOFs catalysts

It is well-known that metal-organic framework （MOF） materials possess a large surface area and porous characteristics， providing abundant active sites for catalytic reactions[120-123]， which makes them particularly effective in capturing CH4 and optimizing catalytic reactions （Fig.10）. Furthermore， the tunable pore sizes and chemical functionalities of MOFs enable precise control over reaction pathways and product selectivity， allowing for the efficient production of specific types of products[124]. According to previous reports， Mg-MOF-74 demonstrates significant CH4 adsorption capability due to its periodically uniform metal-oxide-metal （Mg-oxo-Mg） structure， with numerous unsaturated Mg sites readily binding to adsorbates[75]. The Mg-MOF-74 electrocatalyst exhibit a total Faradaic efficiency of 10.9% （FE for formate = 4.9%， FE for acetate = 6.0%） and a total yield of 126.6 μmol/（gcat·h）（yield for formate = 66.07 μmol/（gcat·h）， yield for acetate = 60.56 μmol/（gcat·h）） at an applied potential of 1.6 V vs. RHE， with current densities for formate and acetate being 30 and 36 μA/cm2， respectively. During the electrooxidation of CH4 （as shown in the equation （1） to （4））， it can be observed that methoxy （CH3O-） belongs to the first intermediate. Additionally， analysis of the post-electrolysis samples and long-duration electrochemical tests （10000 s） reveals that their crystalline framework structure and electrochemical performance are relatively stable[125].
$\mathrm{C}{\mathrm{H}}_{4}-2{\mathrm{e}}^{-}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{C}{\mathrm{H}}_{3}{\mathrm{O}}^{-}+{\mathrm{H}}_{2}\mathrm{O}$
$\mathrm{C}{\mathrm{H}}_{3}{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{C}{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{H}}_{2}\mathrm{O}$
$\mathrm{C}{\mathrm{H}}_{2}{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{H}}_{2}\mathrm{O}$
$\mathrm{C}\mathrm{H}{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_{2}\mathrm{O}$
图10 （a） 示意图展示简化版金属有机框架结构及其路径[130]；（b） Mg-MOF-74中Mg-氧-Mg位点吸附CH4的示意图；（c） 甲酸的法拉第效率与生成速率[75]；（d） 含CN空位的Ni-Co PBA-VCN催化剂上CH4电催化转化为甲酸甲酯的拟议机理；（e） Ni-Co PBA-VCN在2.0 V vs. RHE条件下进行的CH4OR稳定性测试（160 h）结果[86]

Fig.10 （a） Scheme representing simplistic MOF structure and pathways[130]. （b） Scheme of the CH4 adsorption on Mg-oxo-Mg of Mg-MOF-74. （c） Faradic efficiency and production rate of formate[125]. （d） Proposed mechanism of CH4 electrocatalytic to methyl formate on Ni-Co PBA-VCN catalyst with CN vacancies. （e） The CH4OR stability test （160 h） results of Ni-Co PBA-VCN at 2.0 V vs. RHE[86]

Among various metal-organic framework （MOF） materials， developing a series of catalysts with dual functional catalytic capabilities through organic coordination is an effective approach to regulate the active centers and enhance the electronic coupling between metals[126-128]. A recent study described the development of a Ni-Co PBA-VCN catalyst with significant CN vacancies， synthesized using wet chemistry and H2 cold plasma treatment techniques. The ―C≡N coordination defect caused unsaturated coordination of Ni and Co， leading to an upward shift of the metal d-band center and facilitating the adsorption and activation of CH4 molecules （CH4→*CH4，ΔG=0.47 eV）[86] The tandem effect of the Ni-Co bimetallic system enables activated *CH2 on Ni atoms to transfer directly to two low-coordinate Co atoms adjacent to the same VCN vacancy. This results in dehydrogenation and oxidation to produce *HCOOH， which subsequently couples with *CH3OH on Ni atoms （Ni*CH3OH + Co*HCOOH→HCOOCH3） to generate methyl formate. This process achieves a Faraday efficiency of 56.7% for HCOOCH3 at a potential of 2.0 V vs. RHE， while maintaining a nearly constant current density of 1.8 mA/cm2 over 120 h. Despite the promising applications of MOFs in CH4 electrocatalysis， their low conductivity remains a crucial factor that limits performance and application. This limitation arises from insufficient overlap between the organic linkers’ frontier molecular orbitals and the metal clusters/ions， as well as the mismatch in ionic bonds between metal and carboxylic acid or imidazole linkers[129].

3.5 Single atom catalysts

Single-atom catalysts （SACs） with uniform metal distribution and well-defined coordination environments exhibit superior performance in methane （CH4） conversion and product selectivity. SACs feature atomically dispersed active sites， which endow them with extremely high surface free energy and specific activity. The strong interaction between these sites and the substrate further promotes efficient C―H bond activation[131-134]. It is noteworthy， however， that the term “SACs” does not imply that isolated zero-valent metal atoms are the only active centers[135-136]. Instead， the synergy between metal atoms and their surrounding coordinating atoms has emerged as a key factor enhancing the catalytic activity and selectivity （Fig.11）. Inspired by the sophisticated structure and working mechanism of natural methane monooxygenase （MMO）， significant breakthroughs have been made in the design of CH4 oxidation reaction （CH4OR） catalysts and the regulation of reaction pathways. By precisely tailoring the local coordination environment and spatial geometry of single-atom active sites， researchers have simultaneously addressed two critical challenges in methane conversion： efficient C―H bond activation and prevention of over-oxidation of products. For instance， a dual-site synergistic mechanism involving single-atom ruthenium （Ru） and zirconium-oxo clusters in the UiO-66 support enabled near 100% selective hydroxylation of methane to oxygenates under mild conditions， achieving a high turnover frequency （TOF） of 185.4 h-1. In this system， the electron-deficient Ru $\stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{＝}$O species activates the C―H bond of methane， while the electron-rich ZrₓOₓ·OH sites facilitate the formation of target products. Moreover， the support decomposes excess H2O2， thereby suppressing over-oxidation[137]. More remarkably， inspired by the steric “gating” effect of the protein matrix in MMO， which prevents product over-oxidation （Cu1/CN）. This catalyst features a nitrogen-copper-oxygen （N2-Cu1-O） active site situated within a “V”-shaped structure at the zigzag edges of a carbon nitride （C3N4） support. This configuration not only lowers the energy barrier for homolytic C―H bond cleavage of methane to as low as 0.58 eV， achieving a remarkable TOF of 405.3 h-1 at 50 ℃， but also incorporates a “V”-shaped channel entrance with an opening of approximately 3.87 Å （1 Å=0.1 nm）. This pore allows methane molecules （kinetic diameter ~3.78 Å （1 Å=0.1 nm）） to access the active site， while effectively blocking the return of larger product molecules （e.g.， CH3OOH， kinetic diameter ~4.23 Å）， thus kinetically preventing over-oxidation and enabling nearly 100% selectivity toward oxygenates[138]. This strategy elegantly mimics the gating mechanism of enzymes， highlighting the powerful synergy between local geometric structure and electronic properties in catalyst design.
图11 （a） 表面自由能与最小化金属之间关系的示意图[131]；（b） 流动电池中甲烷与电解质流动及催化剂表面反应的示意图；（c） 对应于每个反应坐标的原子构型；（d） 不同电极电位下乙醇的生成速率；（e） 不同电位下含氧产物的选择性；（f） 乙醇产量随反应时间的变化曲线[85]

Fig.11 （a） Schematic illustration of the relationship between surface free energy and minimizing metal[131]. （b） Scheme for the flow of methane and electrolyte and the reaction at the catalyst surface in the flow cell. （c） Atomic configurations corresponding to each reaction coordinate. （d） The production rate for ethanol at various electrode potentials. （e） The selectivity of oxygenate products at various potentials. （f） The ethanol production as a function of reaction time[85]

Leveraging the unique advantages of single-atom catalysts （SACs）， Kim et al.[139] developed an Fe-N-C SAC for the electrochemical conversion of CH4， assisted by the oxygen evolution reaction （OER）， in a flow cell equipped with a gas diffusion electrode. By precisely controlling the electrode potential， they achieved high performance under conditions of 1.6 V vs. RHE and a CH4 flow rate of 50 sccm （1 sccm=1 mL/min）， with an ethanol （CH3CH2OH） production rate of 11480.6 mmol/（gcat·h） and selectivity of 85%. The system remained stable for over 100 h. The overall reaction consists of two main stages： the supply of active oxygen species and the activation of CH4. In the first stage， the OER is carefully regulated to generate *O species within a potential window of 1.14~1.79 V vs. RHE. This begins with *OH adsorption （occurring above a threshold potential of ~0.90 V vs. RHE）， followed by deprotonation to form *O （dominant above 1.14 V vs. RHE）. Subsequent reaction with *OH yields *OOH， which rapidly undergoes further deprotonation to form O2 at potentials above 1.79 V vs. RHE， marking the onset of rapid OER. In the second stage， CH4 oxidation proceeds via *O species. CH4 reacts with *O to form *CH3OH （CH4 + *O → *CH3OH， ΔG = -1.06 eV）， accompanied by C—H bond cleavage. The *CH3OH intermediate then undergoes two sequential deprotonation steps： first to CH3O （CH3OH - e- → *CH3O + H+， ΔG = +0.66 eV）， and then to CH2O （CH3O - e- → *CH2O + H+， ΔG = +0.94 eV）. Finally， due to the highly electrophilic carbon in *CH2O， it can directly react with another CH4 molecule to form ethanol. These steps become thermodynamically favorable under anodic potentials. At lower potentials， desorption of *CH3OH to methanol is favored over its deprotonation. As the potential increases， deprotonation becomes dominant， steering the pathway toward ethanol formation. Despite the promising features， SACs also face inherent limitations. When metal particles are reduced to the atomic level， the specific surface area increases dramatically， leading to a sharp rise in surface free energy. This makes SACs highly prone to agglomeration into larger clusters during both synthesis and reaction， resulting in catalyst deactivation. Therefore， significant challenges remain in terms of stability and metal loading capacity.

4 Defect engineering： material design strategy for catalytic performance optimization

Recent years have seen defect-engineered nanomaterials becoming a research hotspot in the field of catalysis due to their unique redox reaction capabilities and physicochemical and optical properties[139-140]. This is an emerging technology aimed at precisely adjusting the characteristics and catalytic performance of nanomaterials by artificially introducing or regulating defects， thus meeting different application needs. Defects such as reconstructed defects， vacancies， doping， and both metal-based and non-metallic defect-derived nanocatalysts have been widely proven to play a crucial role in promoting various electrochemical processes， such as the oxygen evolution reaction （OER）[141-143]， hydrogen evolution reaction （HER）[144-145]， nitrogen reduction reaction （NRR）[146]， and carbon dioxide reduction reaction （CO2RR）[147]. Based on the geometric structure of the defects， they can be categorized into four main types： point defects （vacancies， doping）， line defects （steps， dislocations）， surface defects （grain boundaries， stacking faults）， and bulk defects （pores， edges）. Defects can significantly alter the adsorption of reactant molecules on the catalyst surface during catalytic reactions and provide a large number of active sites， thereby triggering chemical transformations to generate target products[148-152]. This occurs because， compared to fully coordinated sites， defects create unsaturated and more reactive coordination sites （Fig.12）. Specifically， in Ov-ZnO and Znv-ZnO the presence of surface vacancies has been observed to reinforce metal-ligand coordination， favoring the formation of pivotal intermediates CH3* and COOH*[72]. Similarly， oxygen vacancies （Ov） serve as pivotal active sites that effectively lower the activation energy barrier for the initial C―H bond cleavage in methane by modulating the electronic structure of tungsten trioxide （WO3）， thereby significantly enhancing the reaction kinetics. For instance， the Ov-induced formation of W5+-O species not only facilitates the further dehydrogenation of *CH3 intermediates and the formation of *CH2OH but also optimizes the subsequent C―C coupling pathway， steering the reaction selectivity towards ethanol rather than complete oxidation products and enabling highly selective directional conversion[69].
图12 （a） 分别在p-ZnO、Ov-ZnO和Znv-ZnO表面上CH3COOH形成的MEO反应能曲线，插图：反应中间体的球棍模型[72]；（b） WO3表面与（c） Ov-WO3表面上CH4电催化氧化为乙醇的反应能及反应路径分布曲线[69]

Fig.12 （a） MEO reaction energy profiles calculated for the formation of CH3COOH on the surface of p-ZnO， Ov-ZnO and Znv-ZnO respectively. Insets： Ball-and-stick models of the reaction intermediates[72]. （b） Reaction energy and （c） reaction path profiles for CH4 electrocatalytic oxidation to ethanol on the surfaces of WO3 and Ov-WO3 respectively[69]

5 Conclusions and prospects

Electrochemically converting methane into high-value oxygenated products at low temperatures provides an attractive pathway for achieving renewable electricity storage and high-value utilization of natural gas. This technology possesses unique advantages such as intermittent and modular operation and adaptability to a wide range of working conditions， holding promise for the distributed production of oxygenated chemicals. However， this technology is still in its early development stages and faces multiple challenges， including catalytic efficiency， selectivity， and stability， making commercialization a significant hurdle.
Currently， research on various catalytic systems faces notable bottlenecks （Fig.13）. Although noble metal catalysts exhibit excellent low-temperature activation capabilities and high reaction rates， the high cost and scarcity limit large-scale applications. Alloy catalysts show potential in activity and product regulation but are prone to oxidation in complex multi-electron reactions and reactive oxygen species environments， leading to structural deactivation. Transition metal oxides facilitate C―C coupling to generate C2₊ products but suffer from poor intrinsic conductivity， resulting in suboptimal activity and selectivity. MOF materials face similar conductivity challenges. While single-atom catalysts can provide extremely high atomic utilization and intrinsic activity， they encounter severe challenges regarding stability and high loading due to the difficulty in avoiding aggregation of metal sites during preparation and reaction because of the very high surface free energy.
图13 低温甲烷电氧化催化剂设计与制备中需解决的挑战

Fig. 13 Challenges to be addressed in the design and preparation of low-temperature methane electro oxidation catalysts

Methane low-temperature electrooxidation is a multiscale systematic issue involving materials， interfaces， and reaction engineering. To overcome existing bottlenecks， efforts should focus on two aspects： （1） the continued advancement of catalyst design by constructing precise active sites within structures such as alloys， oxides， and single atoms to synergistically optimize C―H bond activation and the selectivity of oxygen-containing products. （2） It is essential to expand the research perspective from the catalyst itself to the entire reaction system. There is an urgent need for systematic investigation into how reaction conditions， such as electrolytic cell structure， electrode design， electrolyte composition， pH value， ionic effects， and temperature and pressure， influence mass transfer， interfacial electric fields， and reaction pathways. In particular， a deeper understanding of the microscopic environment of the electrode-electrolyte double layer will provide theoretical guidance for creating optimal reaction conditions. Through this multiscale， systematic collaborative innovation， methane electrooxidation technology can achieve breakthroughs in activity， selectivity， and stability， ultimately advancing toward practical applications.

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