Challenges in the selective electrochemical oxidation of methane: Too early to surrender

The selective electrochemical oxidation of methane to value-added chemicals has been pursued for decades without breakthroughs and developments beyond academic research. Main setbacks encountered in virtually every report are poor methane conversion rate and selectivity. For tangible progress, research should focus on tackling CH 4 mass transport and concentration limitations. At the same time, harmonized research protocols must be developed, e.g. to define standard control experiments and key metrics. This will facilitate data comparison and accelerate electrocatalyst discovery, which so far remained challenging due to inconsistent data-reporting practices. Fundamental research on model (well-defined) electrocatalysts should also be intensified, along with in-situ spectroscopic investigations to understand the reaction mechanism and design catalysts to prevent overoxidation.


Methane valorization by partial oxidation: Do we need it?
The controlled activation of the CeH bond in methane (CH 4 ) is considered a "holy grail" in chemistry and catalysis due to the industrial relevance of value-added chemicals that could be directly produced by selective CH 4 oxidation [1].The direct conversion of CH 4 to methanol (CH 3 OH), according to reactions 1 or 2, is of great interest because the produced CH 3 OH could be used as feedstock for chemical syntheses and as an energy source [2].Such direct CH 4 -to-CH 3 OH route is, however, hampered by the chemical inertness of CH 4 , in which CeH bonds are weakly polarized, sterically hindered, and hence very stable, making it difficult to react CH 4 partially and not fully to CO and CO 2 .
While an efficient direct route remains unviable, the chemical industry produces large amounts of CH 3 OH from synthesis gas through an indirect process via steam methane reforming (SMR) and the water-gas shift reaction, according to reactions 3e5: CO ðgÞ þ 2H 2ðgÞ /CH 3 OH ðlÞ DG 0 R;298K ¼ À29:4 kJ mol À1 ; E 0 cell ¼ þ0:08V This route is energy-intensive and causes CO 2 emissions of about 300 million tons per annum (MTPA), corresponding to ca. 10% of the total CO 2 emissions of the chemical sector [3].In the last decade, CH 3 OH production has nearly doubled to 98 MTPA and will potentially rise to 500 MTPA by 2050 [3].The reliance on SMR is incongruent with global ambitions to phase out fossil fuels and reach net-zero carbon emissions by 2050.The increasing demand for CH 3 OH underscores its relevance for the chemical industry and society, but the associated CO 2 emissions, projected to skyrocket, call for the development of alternative technologies for CH 4 valorization and CH 3 OH production.In line with the ongoing electrification of the chemical industry, electrochemical conversion processes appear to be a renewable pathway for selective CH 4 oxidation, ideally directly to CH 3 OH, in a decentralized fashion, and powered by green electricity (from wind and solar), hence potentially with zero carbon emission [4].
Interestingly, the motivation provided by academic literature on selective electrochemical oxidation of methane (SEOM) mainly revolves around conceiving alternatives to CH 4 flaring (combustion of CH 4 on offshore oil drilling platforms causing CO 2 emissions) and avoiding atmospheric CH 4 emissions.However, oil producers have available options to reduce flaring by up to 95% within 2030 [5].Likewise, CH 4 emissions by the energy sector (accounting for 35% of the global CH 4 emissionsdthe remainder being associated with microbial processes, ruminants, wetlands, etc. [6]) will likely be reduced by 30e60% in the coming years, achieving zero emissions by 2050 [7].
Research efforts on SEOM remain, in our opinion, primarily justified by the need for alternatives to SMR, hence, to reduce CO 2 emissions, even if newly developed technologies would rely on fossil CH 4 .Nevertheless, biogenic sources can be considered as a nonfossil feedstock for CH 4 valorization and toward carbon circularity.In fact, the current world production of biomethane through anaerobic digestion, landfill, and gasification technologies is at around 35 MTPA (3% of total bioenergy demand, 0.1% of natural gas demand), and the production is projected to increase to around 720 MTPA (stated policies scenario) or 2000 MTPA (sustainable development scenario) by 2040 [8,9].

Selective electrochemical oxidation of methane-we need orchestrated research efforts
The SEOM has attracted scientific interest since the 1960s.However, the research community is growing slowly compared to what is seen for H 2 O electrolysis or the electrochemical CO 2 reduction reaction (CO 2 RR).
The lack of a critical mass of researchers investigating this subject is a first hurdle to advance in the fielddthis may originate from the earlier disillusion of the heterogeneous catalysis community that invested huge efforts on direct CH 4 conversion without making a breakthrough.From 2014 to 2022, about 4 0 000 articles were published worldwide on the electrochemical CO 2 RR [10] and >20 0 000 patents were filed on H 2 O electrolysis [11].In contrast, only about 110 articles have been published on the SEOM in the last ten years, as shown in Figure 1d herein, we limit ourselves to low-temperature electrochemical processes.Interestingly, out of these publications, about 30 articles are reviews.It is surprising that so many reviews have been published in a field that stagnates since decades, and hence remains still in its infancy.While with the present opinion we are shifting the balance further in favor of review articles, we hope to provide a constructively critical analysis of the state of the art and encourage intensified and orchestrated research efforts to advance in the field.

(Electro)chemical methane activation mechanisms-in essence
Although discussed in various articles and reviews [12e15], a brief recap of the mechanisms of electrochemical CH 4 activation is provided in the following to substantiate the content of the following sections.While mechanistic details have yet to be fully uncovered, it is common to encounter in the recent literature two main pathways for the cleavage of the CeH bond [15]: In the direct pathway, adsorption of CH 4 on the electrocatalyst surface (anode) is a key step to enable polarization of the CeH bond.This lowers the activation barrier for proton abstraction or for nucleophilic attack by a surface-adsorbed oxygen species generated electrochemically in the vicinity of the adsorbed CH 4 molecule.CH 4 is found to overoxidize to CO and CO 2 on noble-metal electrocatalysts (typically, platinumgroup metals [PGMs]) [16].The community seems to not have reached yet a consensus on the main cause of methane full oxidation, i.e., whether it results from the ease of adsorption of methane and its oxidation intermediates on the electrocatalyst surface, leading to consecutive proton abstraction steps, or from formation of adsorbed oxygenated species (by partial oxidation) that rapidly oxidize further due to a low redox potential (discussed in the next session).In contrast, metal oxide anodes have been proven not only to adsorb CH 4 moderately but also to mediate its reaction with adsorbed oxygen species (by so called "water discharge"), thereby forming CH  [20], Pd II /Pd III [21], Pt II /Pt IV [22], Rh II /Rh III [23]) and surface-adsorbed chlorine intermediates (*Cl [24]), have also been successfully used as redox mediators for CH 4 activation.In homogeneous catalysis, complexes based on high-valent metal ions were found to be able to activate the methane's CeH bond [25,26].Their application in an electrochemical system, where their regeneration (reoxidation) would take place electrochemically (via a Faradaic process at the electrode surface), might be a promising approach to explore.Nevertheless, since the application of such redox mediators requires downstream separation steps, the use of water as the redox mediator, i.e., via formation of ROS, remains a most preferred approach.In addition, in general, the indirect pathway currently appears more promising than developing electrode surfaces capable of selective conversion of CH 4 (i.e., via direct pathway), as electrocatalysts that can generate ROS efficiently have meanwhile been developed In the following, we briefly discuss the mechanism of two selected electrocatalytic systems presented in recent literature on the SEOM.
Overoxidation of CH 4 to CO 2 above 1 V vs. SHE: *OCH 2 / ./ CO 2 (10) The first reaction step combines the electrochemical activation of the CeH bond of CH 4 with OH À of the CoOOH surface and the concerted rebinding of the CH 3 fragment with a surface-adsorbed OH (*OH) (6).
The formed adsorbed *CH 3 OH on the surface has two options: (i) the CH 3 OH can desorb, and the hydroxyl group on the surface can be replenished electrochemically by hydroxyl ion to give back the initial catalyst (CoOOH) (7); and (ii) the *CH 3 OH can further oxidize to *OCH 3 (8).At 1 V vs. SHE, the thermodynamic favorability of the two reactions switches and (8) becomes more favorable.*OCH 3 then oxidizes to *OCH 2 , a step again thermodynamically favorable (9).These are the first steps toward the overoxidation of CH 4 to CO 2 .
The other favorable oxidation steps after *OCH 2 were not explicitly calculated (10).
For the indirect pathway of the SEOM, ROS can be generated cathodically over a carbon-based electrode toward the production of HCOOH [19].The proposed mechanism proceeds according to the following reactions (11e18): ROSs formation: CH 4 activation: CH 3 OOH formation: CH 3 OH formation: HCOOH formation: In In general, for a catalyst to perform well in the SEOM, it must effectively lower the activation barrier for CeH bond cleavage and prevent overoxidation.The CeH bond activationdas exemplified earlierdcan occur directly through methane adsorption on the catalyst surface or indirectly via regenerable in-situ generated oxidizing agents such as peroxocarbonates or hydroxy radicals.In nonelectrochemical methane oxidation, the involvement of reactive oxygen species suggests a homolytic cleavage pathway [27].However, for transitionmetal catalysts, the pathway (homolytic or heterolytic) remains debated due to the lack of definitive spectroscopic evidence, making both routes plausible [28e31].
Preventing overoxidation is particularly challenging as the activation barriers for subsequent oxidation steps are lower once methane is activated.Therefore, it is crucial to ensure that methanol can be efficiently transported away from the reaction sites, thus avoiding its further oxidation.Future research should focus on improving insitu characterization techniques (to provide clearer mechanistic insights), and developing catalysts with precise control over the nature and surface concentration of active sites and reactor designs to avoid overoxidation.

Few achievements, many challenges-what are promising directions?
Most reports on SEOM deal with producing CH 3 OH in aqueous electrolytes, using three-electrode or H-cell configurations, at atmospheric pressure and close-toambient conditions [14].Analyzing the results, it emerges that virtually every study reports small current densities (total current densities in the 1 mA cm À2 regime or lower) and low CH 3 OH production rates (in the 0.1e1 nmol s À1 cm À2 regime).Moreover, the tested electrode materials often feature complex compositions with poor scalability perspectives.In other words, no breakthrough has yet been achieved in the field.Why is this so?We outline in the following crucial factors that, in our view, hamper progress in SEOM and provide our opinion on most promising directions to pursue.

Poor selectivitydundesired methanol oxidation
The direct pathway for SEOM is challenging from both thermodynamic and kinetic standpoints.The standard CH 4 oxidation potential to methanol (reaction 19) is þ0.63 V vs. SHE (see Figure 2).This potential is less positive than that for the water oxidation reaction (WOR, þ1.23 V vs. SHEdreaction 20).In principle, in the 0.63e1.23V potential range, CH 4 can be activated without competition from WOR.In practice, however, slow CH 4 conversion kinetics are typically observed.
Operating at larger overpotential to accelerate the CH 4 conversion is counterproductive as WOR begins to compete.
Figure 2 Thermodynamic potentials relevant for selective electrochemical oxidation of methane and competing reactions.The different equilibria are written as oxidation reactions.
Most importantly, even if the kinetics of CH 4 activation in the þ0.63 to 1.23 V potential range was favorable, the CH 3 OH -CO 2 redox potential is of (ca.) 0.05 V vs. SHE (reaction 21), i.e., it is less positive than that of CH 4 .CH 3 OH is more strongly polarized and less sterically hindered than CH 4 , leading to a higher susceptibility of the former to electrophilic and nucleophilic attacks, i.e., a more pronounced reactivity.Hence, the formed CH 3 OH can easily oxidize at the electrode surface to CO and CO 2 , leading to poor selectivity and product yields, and a low Faradaic efficiency (FE) to CH 3 OH [1].This is the real culprit of the SEOMdoften not highlighted in the literature.
In the indirect pathway, ROSs are usually generated (anodically) at more positive potentials than CH 3 OH oxidation (reactions 22e24).However, due to the lack of in-situ mechanistic studies, it is unclear whether the oxidation of CH 3 OH to CO 2 proceeds homogeneously in solution through ROS, or heterogeneously through the adsorption of CH 3 OH on the electrode surface.To minimize CH 3 OH oxidation and losses in FE, we would need an electrocatalyst that is efficient towards ROS generation but lousy in CH 3 OH oxidation.Boron-doped diamond is known to oxidize CH 3 OH indirectly through ROS and not on the electrode surface [32]dbased on a similar concept, SnO 2 might also be promising.
Based on what is mentioned earlier, accumulation of CH 3 OH in the electrolyte, by simply increasing the electrolysis time, cannot be achieved, independent of a direct or indirect pathway.Product removal from the electrolyte, on the other hand, is challenging as CH 3 OH is highly soluble in aqueous electrolytes and thus, it is difficult to remove in the case of batch electrolysis.Therefore, batch operation is not ideal if the electrocatalyst is not selective.A promising approach to remove CH 3 OH from the electrode surface is to opt for a flow cell.
While direct and indirect pathways are often mentioned in SEOM literature, we still have poor understanding of the reaction mechanism and little evidence with respect to which reaction intermediates form.Unfortunately, a limited number of studies is devoted to such aspects, more likely because the reaction complexity, in terms of variety of intermediates and time scales, along with operating conditions (e.g., gas concentration or pressure, mass transport), makes the application of in-situ and operando spectro-electrochemical techniques challenging [33].
As discussed in the following, the main roadblock remains to achieve reasonable conversion rates for the SEOM.
Most studies have dealt with developing new electrocatalysts, whereas the main obstacles toward high current densities are the low solubility and poor mass transport of CH 4 in aqueous electrolytes (regardless of the direct or indirect pathway)dhence, the community should deal with cell and process engineering aspects too.

Low CH 4 concentration and mass transport
The solubility of CH 4 in water at room temperature and atmospheric pressure is 1.4 mM only [34].This limits the kinetics of SEOM in aqueous electrolytes.Higher CH 4 concentrations can be achieved in organic solvents, e.g. up to 24 mM in tetrahydrofuran [35].Besides concerns associated to their toxicity and volatility, organic solvents might be more reactive than CH 4 itself, and hence undergo degradationdnonetheless, it remains worth exploring organic solvents at least at the lab scale [23,36,37], e.g., using rotating-(ring)-disk electrode setups (R(R)DE).
To improve substantially the solubility of CH 4 in water (by a factor 10e100), an approach can be increasing the pressure up to 10e100 bar [38].A proof of concept has been demonstrated, leading to a 220-fold increase in the yield compared to atmospheric pressure [39]*.Note, however, that the CH 4 solubility reached at 20 bar in water is comparable to that of CO 2 at atmospheric pressure (34 mM) [34].
For both direct and indirect pathways to achieve high conversions, it is necessary to enable efficient CH 4 mass transport to the electrode surface.This can be attained by flowing CH 4 through a porous electrode, the socalled gas diffusion electrode (GDE, consisting of a porous transport layer/gas diffusion layer and a catalyst layer) [40]*.In general, the use of GDE-based cell has received minor attention for SEOM, whereas we and other authors [41,42] advocate for intensifying the research efforts in this direction.Research on such applied and engineering aspects could attract the interest of the industry, hence strengthening the research capabilities by publicdprivate partnerships.
When operating with GDEs, gas-flow rate and pressure can be controlled to tune the reaction kinetics and achieve an optimal tradeoff between high conversion (low flow rates) and high selectivity (high flow rates).Temperature can be controlled too.When operating with a liquid electrolyte, the solubility of CH 4 increases with decreasing the electrolyte temperature.However, when operating with a humidified CH 4 feed [43], the increase of temperature could be beneficial to tune the CH 4 :H 2 O molar ratio (i.e., partial pressures), hence providing an additional knob that together with pressure and gas-flow rate could enable control over reaction kinetics and product distribution.
For SEOM, worth mentioning are the three GDE-based cell configurations shown in Figure 3. Cell configurations presented in Figure 3-left (the so-called microfluidic cell) and in Figure 3-middle (zero-gap cell) have been successfully applied in PEM water electrolysis and fuel cells, and in CO 2 RR.From an engineering point of view, these technologies can be promising to advance the field of SEOM, not only in electrolysis mode but also considering the perspective selective oxidation of methane when used as fuel in fuel cells [44].The hollow-fiber electrode, shown in Figure 3-right, represents an emerging concept [45].Selective oxidation of propylene has been achieved with such electrode and cell configuration [46], but beneficial effects of enhanced mass transport were not evident from this study.Being still a relatively novel approach, it needs first to be further explored for processes that are better understood and developed at a higher technology readiness level (TRL) before considering its suitability for the SEOM.Regardless of the cell or electrode configuration, the SEOM should initially be paired with well-known half-cell processes (indicated with "?" in Figure 3), e.g., the hydrogen or oxygen evolution reactions in the case of anodic and cathodic SEOM, respectively.
A final point: out of the few reports available on GDEs for SEOM, most of them use carbon-based materials.
This choice is questionable due to the potential instability of carbon electrodes, particularly when used as anodes or, in general, when exposed to ROS.Moreover, their degradation can lead to formation of CO and CO 2 and can affect and falsify the estimation of conversion rates, product yields and FEs.The use of metal-based materials as gas-diffusion layers/porous transport layers (frits, felts, meshes, foams, etc.) is recommended, as frequently applied in water electrolyzers [47].

Questionable selection and characterization protocols of electrocatalysts
Practical formation rates of CH 3 OH, or value-added products in general, are in the range of or below background levels of 0.1 nmol cm À2 s À1 (equal to 0.002 ppb cm À2 s À1 or 10 mA cm À2 partial current density), whereas ultrahigh purity CH 4 gas (99.999%) has hydrocarbon impurity levels in the 0.5-to 2-ppm range (this can already easily lead to false positives).Such low yields (ascribed to low currents and poor selectivity as discussed above) make material discovery challenging.
In dealing with the issue of low product formation rates, the community studying the electrochemical dinitrogen reduction to ammonia has set a virtuous example by agreeing that ammonia production rates can be considered significant only if above 10 nmol cm À2 s À1 [48].In addition, quantitative isotope labeling and control experiments (e.g., under inert gas atmospheres, etc.) became a must-include in publications.Such modus The use of gas-diffusion (left and middle) and hollow-fiber (right) electrodes enables increased mass transport of CH 4 to the reaction sites.
operandi is still not a standard protocol within the SEOM community.Consequently, it is difficult to judge if products identified and reported formation rates are significant or instead due, e.g., to decomposition of carbon-based current collectors, membranes, or impurities.This is worsened by the often-incomplete description of experimental conditions adopted for the electrochemical testsdwe believe the more the details the better!There is also no consensus on how to normalize and report product formation rates.Both nmol cm À2 s À1 and mmol h À1 g À1 catalyst are commonly used.We also recognized a trend in reporting the electrocatalysts' performance as mass-specific current or product formation rate (e.g.mmol g À1 cat: or mA mg À1 cat: ).These nonharmonized figures complicate data comparison across different labs and publicationsdin fact, the electrocatalyst loading on electrodes (e.g., mg cm À2 geo: ) and the electrode surface area are often not reported.Thus, the SEOM community would clearly benefit from more harmonized, thorough, and rigorous research protocols and data-reporting metrics to make tangible progress.
Finally, we noted that several studies deal with complex catalysts with poorly defined composition, morphology, and structure.In fundamental research to screen and test working electrodes (i.e., in a three-electrode cell, RDE setups, etc.), we advocate for the use of defined (model) electrode surfaces [49], such as single crystals, physicalvapor-deposited or chemical-vapor-deposited (PVD or CVD) films and highly-defined layers of nanoparticles [50].This is particularly relevant to discover active catalysts for CH 4 activation by the direct pathway (where CH 4 surface adsorption is a key step).For the indirect pathway, bifunctional electrocatalysts as depicted in Figure 4 should be investigated, e.g., electrodes consisting of well-known materials for efficient partial water oxidation (to ROS), surface-modified with metals (e.g., nanoparticles) to enable CH 4 surface adsorptiondthis approach could facilitate the SEOM kinetics by bringing in close vicinity ROS and CH 4 .In general, materials investigated as working electrodes must be selected bearing in mind scalability perspectives and should be based on carbon-free materials (to avoid degradation and false positives) and tested under controlled CH 4 mass transport conditions, that is, by using R(R)DE setups [17].Even more ideal would be closed gas-tight R(R)DE setups that could operate under high-CH 4 -pressure conditions [39].
Selected promising electrocatalysts should then be tested at relevant currents and examined in view of their stability in a full-cell configuration, e.g. using GDE-based cell designs outlined earlier.Before doing so, however, a first step should be to test the chosen cell design and process parameters to validate that CH 4 oxidation can be operated at relevant conversion rates and currents.For this, PGM electrocatalysts can be a suitable choice, even if the main product is CO 2 .

Additivesdelectrolyte composition
Tuning the electrolyte composition and the formation of solideelectrolyte interface (SEI) provide additional knobs to steer selectivity in electrochemical conversion processes.For example, in the oxidative decarboxylation of acetic aciddthe so-called Kolbe oxidation processda passivating organic layer is formed on PtO x electrodes, which prevents undesired formation of oxygen in favor of ethane formation with substantial FEs.Interestingly, when the (local) carbonate concentration is increasing, the selectivity of acetic acid (acetate) oxidation changes from ethane toward the formation of CH 3 OH, the socalled Hoefer Most reaction.Although the exact reason for the change in selectivity in high local carbonate concentrations is not yet fully understood, these observations could be exploited for the selective conversion of CH 4 .For example, addition of specific amounts of acetate or carbonate to the electrolyte, or mixtures thereof, could be used to determine how surface modification affects the conversion of CH 4 [51].Evidently, this requires isotopic labeling studies to discriminate whether ethane or CH 3 OH would form by partial oxidation of CH 4 or stem from acetate.Moreover, the formation of an SEI might increase the local concentration of the gaseous reactant (CH 4 in this case) at the electrode surfaceda concept successfully applied in the lithium-mediated electrochemical reduction of dinitrogen gas to ammonia [52].
To summarize the discussion above, we provide in Table 1 factors and experimental parameters that influence the SEOM.

Conclusions
Despite many challenges, the concept of methane-tomethanol conversion remains a promising alternative to the energy-intense and environmentally unfriendly SMR route.Currently, the SEOM is hampered by poor conversion rates and selectivity.The use of suboptimal process conditions such as low methane concentration and poor mass transport, common in a significant number of reports on the subject, makes the supply of reactant (CH 4 ) the main rate-limiting step.To ramp up conversion rates and to get insights into the mechanism of CeH activation and selective oxidation, research should be intensified in investigating a broader process-parameter space, e.g., by operating at high pressure, by tuning the electrolyte composition, or using nonaqueous media.This might enable operation at CH 4 concentrations that are orders of magnitude higher than in water at atmospheric pressure.Physical and chemical vapor deposition techniques have become a standard in electrode preparation for fundamental study on water and CO 2 electrolysis.We argue that the use of highly defined (model) electrocatalysts instead of the often-used complex and highly heterogeneous counterparts, in combination with in-situ spectroelectrochemical techniques, will advance the mechanistic understanding of selective methane electrochemical conversion and unleash the discovery of promising electrocatalyst materials.Moreover, we attribute the limited success also to the lack of control experiments, such as quantitative isotope labeling, which makes it complicated to assess if the identified products and reported yields originate from SEOM or, instead, are due to contamination or degradation of cell components.Similarly, unconventional and nonhomogenized reporting of figures of merit complicates comparing results across different publications and from different labs, whereas a more orchestrated research effort based on defined research protocols could accelerate progress in the field.Finally, we suppose that, even if not successful in achieving high currents and product yields, the publication of well-executed investigations with thorough descriptions of used methods and of experimental procedures can also be beneficial for the community.

Declaration of competing interest
The authors declare the following financial interests/ personal relationships which may be considered as potential competing interests 3158-3165, https://doi.org/10.1039/d3ee00027c.This study deals with a Fe-N-C electrocatalyst that shows ethanol formation by an oxygen evolution reaction-assisted methane oxidation.A potential region is identified that maintains stable active oxygen on the Fe-N-C electrocatalyst where the potential limiting step for the oxygen evolution reaction is OOH* formation.A reaction pathway is proposed for the spontaneous oxidation of methane by the active oxygen, leading to production of methanol, and conversion to ethanol by deprotonation (isotope labeling experiments are included).The work also shows the application of a gas diffusion flow cell to enhance the mass transfer of methane, reaching stable operation for 100 h at a total current density of 7 mA cm −2 .

Figure 4
Figure 4 such a reaction mechanism, first O 2 is reduced to H 2 O 2 on the electrode, then the ROSs are formed by electrochemical reduction of H 2 O 2 or chemical reaction between H 2 O 2 and the previously formed radicals [11e13].Second, $OH radicals activate CH 4 to produce CH [15]adicals in the electrolyte[14].Then, CH 3 OOH is generated by a reaction between CH 3 $ and $OOH radicals[15].Subsequently, CH 3 OH is generated by electrochemical reduction of CH 3 OOH or radical reaction between CH3$ and $OH [16e17].Finally, HCOOH is formed in the presence of $OH radicals[18].

Table 1
Relevant factors and experimental parameters for the selective electrochemical oxidation of methane.
Electrochemical oxidation of methane to methanol on electrodeposited transition metal oxides.J Am Chem Soc 2023, 145:6927-6943, https://doi.org/10.1021/jacs.3c00441.This work describes an electrochemical method for the deposition of a family of thin-film transition metal (oxy)hydroxides as catalysts for SEOM via a direct pathway.The work systematically investigates the dependence of activity and methanol selectivity on catalyst film thickness, overpotential, temperature, and electrochemical cell hydrodynamics using a gas tight RDE setup.Optimal conditions of intermediate overpotentials, intermediate temperatures, and fast methanol transport are identified to favor methanol selectivity.18. Ramos AS, Santos MCL, Godoi CM, Oliveira Neto A, De Souza R Fernando B: Obtaining C2 and C3 products from methane using Pd/C as anode in a solid fuel cell-type electrolyte reactor.ChemCatChem 2020, 12:4517-4521, https://doi.org/10.1002/cctc.202000297.Kim JH, Oh C, Yun H, Lee E, Oh HS, Park JH, Hwang YJ: Electro-assisted methane oxidation to formic acid via in-situ cathodically generated H2O2 under ambient conditions.Nat Commun 2023, 14, https://doi.org/10.1038/s41467-023-40415-6.This study deals with the SEOM by using cathodically generated ROS (i.e., indirect pathway) at ambient temperature and pressure, showing production of oxygenates such as CH 3 OOH, CH 3 OH, and HCOOH, validated by isotope labeling.Mechanistic analysis reveals that produced ROS such as $OH and $OOH activate CH 4 and CH 3 OH.The work also demonstrates the applicability of the approach to activate C 2 H 6 . .Song Y, Yang X, Liu H, Liang S, Cai Y, Yang W, Zhu K, Yu L, Cui X, Deng D: High-pressure electro-fenton driving CH 4 conversion by O 2 at room temperature.J Am Chem Soc 2024, 146:5834-5842, https://doi.org/10.1021/jacs.3c10825.This study reports on a high-pressure electro-Fenton strategy for SEOM at room temperature via an indirect pathway mediated by ROS cathodically produced from O 2 .The elevated pressure promotes the electrocatalytic reduction of O 2 to H 2 O 2 and increases the reaction collision probability between CH 4 and $OH radicals, the latter being formed by decomposition of H 2 O 2 by reaction with Fe 2+ species.Operating at elevated pressure increases by 220 times the product (HCOOH) formation rate compared to ambient pressure conditions.The formation of oxygenates by SEOM is validated by isotope labeling.40 * .Kim C, Min H, Kim J, Moon JH: Boosting electrochemical methane conversion by oxygen evolution reactions on Fe-N-C single atom catalysts.Energy Environ Sci 2023, 16: JC, Primc D, Kawashima K, Wygant BR, Verma S, Spanu L, Mullins CB, Bell AT, Weber AZ: A perspective on the electrochemical oxidation of methane to methanol in membrane electrode assemblies.ACS Energy Lett 2020, 5: 2954-2963, https://doi.org/10.1021/acsenergylett.0c01508.