Electrocatalytic Conversion of CH₄ to Oxygenates over Ni and Ce Doped LaCoO₃ Perovskite in Aqueous Carbonate Electrolyte

Abstract

In this study, an electrochemical system for methane conversion was developed, employing Ni- and Ce-doped LaCoO₃ perovskite as the anode catalyst in an Na₂CO₃ electrolyte. Structural characterization revealed that the La_1−yCe_yCo_1−xNi_xO₃ (x = 0–0.5, y = 0–0.12) synthesized by the sol–gel method maintains the perovskite structure, but is rich in oxygen vacancies. Electrochemical studies revealed that the performance of methane activation is related to the presence of Ni(III) in the catalyst, and reactive oxygen species (•OH and HOO⁻) are provided through water oxidation reactions (WOR) in the Na₂CO₃ electrolyte. The electrocatalytic performance of the synthesized La_0.92Ce_0.08Co_0.5Ni_0.5O₃ during methane conversion was verified in an electrolysis cell, and ethanol and acetic acid were identified as the methane conversion oxygenates. Under ambient conditions, the formation rate of ethanol reached 577.0 μmol g_cat⁻¹ h⁻¹ at 0.90 V (vs. Ag/AgCl) in 0.5 mol L⁻¹ Na₂CO₃. The catalyst was found to retain structural integrity and sustain catalytic activity over multiple reaction cycles.

1. Introduction

Methane (CH₄), the main component of natural gas and shale gas, is regarded as a greenhouse gas with a global warming potential (GWP) more than 25 times that of CO₂ [1,2,3]. Nowadays, it is a policy requirement to develop technical proposals for the capture and conversion of methane released from oilfields and petrochemical plant sites under ambient conditions [4]. However, methane possesses much greater chemical stability than methanol, and the thermodynamically favorable products of methane oxidation reactions are CO₂ and H₂O rather than the oxygenates. The mainstream process for converting methane to methanol is therefore achieved through a two-stage procedure: steam reforming of methane followed by Fischer–Tropsch synthesis [5,6,7].

Inspired by the methane monooxygenase found in methane-oxidizing bacteria, researchers have developed various mild-condition methane conversion systems in the field of enzymatic catalysis, photocatalysis, and electrocatalysis [8,9,10,11]. The electrocatalytic system has the potential for application in renewable electricity sources (such as solar energy), making it possible to collect and convert methane in remote environments like oil fields [12,13]. Studies have demonstrated that the design of electrocatalytic methane conversion systems necessitates two critical strategies: methane activation is realized through chemisorption of CH₄ molecules at defined active sites on the catalyst, which facilitates C-H bond elongation [14]; then the activated CH₄ molecules are attacked by reactive oxygen species (ROS) to transform to oxygenates, and the ROS, such as superoxide radicals (•O²⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH), are continuously generated through oxygen reduction reaction (ORR) on the cathode or water oxidation reaction (WOR) on the anode [15,16,17,18].

Recently, Sun and co-workers integrated dynamic infrared spectroscopy with theoretical calculations to demonstrate that the initial C–H bond cleavage occurs independently of applied potential, which means that methane activation is not the rate-determining step for electrocatalytic methane oxidation [19,20]. This shows that the efficiency of methane conversion might be controlled by the abundance and oxidative capability of ROS. In the past five years, the authors established different electrochemical reaction systems to generate different ROS: (1) by using ionic liquids as non-aqueous electrolytes, superoxide radicals (•O²⁻) are generated as the dominant ROS through the 1-electron pathway of ORR on the cathode; (2) by using an aqueous or water-containing ionic liquid electrolyte and selecting appropriate catalysts, H₂O₂ or HOO⁻ are continuously generated via the 2-electron pathway of ORR on the cathode; and (3) hydroxyl radicals (•OH) produce through WOR on the anode in an aqueous electrolyte. Notably, both methane oxidation (including C–H bond activation) and ROS generation occur on the anode in the third system, which might improve the solubility of methane in a pressure-tight container [21,22,23]. However, higher oxidation state products such as aldehydes or carboxylic acids were the main oxygenates when •OH was the predominant ROS, whereas lower alcohols including methanol and ethanol were the dominant oxygenates when milder ROS (•O²⁻, HOO⁻) were used [18,24,25].

Several studies have reported that noble and transition metal oxides demonstrate the performance of methane activation [26,27,28]. It is generally recognized that oxygen vacancies existing at heterogeneous interfaces of metal oxides serve as effective adsorption sites for CH₄. These are formed through the interaction between multivalent ions and high-valence oxide structures [29,30,31]. However, high-valence metals are prone to redox cycling and leaching under reaction conditions, leading to structural degradation and eventual catalyst deactivation [32,33,34]. The special lattices of perovskite-type oxides (ABO₃) have better tolerance for reversible redox transitions of A or B metals, which might provide a viable strategy to solve the above problem [35,36,37,38]. On the other hand, precisely doping A and B metals with certain components would cause oxygen vacancies to form, thereby maintaining structural integrity but yielding highly stable catalysts for methane activation [39,40,41,42].

In this study, LaCoO₃ doped with Ce and Ni elements was synthesized using the sol–gel method, which maintains the perovskite structure rich in oxygen vacancies. In electrochemical characterization, the Ni and Ce co-doped LaCoO₃ perovskite demonstrated a catalytic performance for methane activation and ROS generation in carbonate electrolyte at an anodic potential, and ethanol and other oxygenates were identified as methane conversion products after continuous electrolysis under ambient conditions.

2. Materials and Methods

2.1. Materials and Reagents

Materials for perovskite synthesis: Na₂CO₃ (99.8%), Na₂SO₄ (99.0%), La(NO₃)₃·6H₂O (99.0%) and Ni(NO₃)₃·6H₂O (99.0%) were acquired from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Co(NO₃)₃·6H₂O (99.99%), Ce(NO₃)₃·6H₂O (99.99%), Triton X-100 (98%) and citric acid (99.5%) were acquired from Macklin (Shanghai, China). Ethylene glycol (C₂H₆O₂, AR) and isopropyl alcohol (C₃H₈O, AR) were purchased from Fuyu Chemical Co., Ltd. (Tianjin, China). Carbon paper was purchased from Toray Industries, Inc. Fluorine-doped tin oxide (FTO) was obtained from Nippon Sheet Glass Co., Ltd. (Osaka, Japan).

2.2. Preparation of La_1−yCe_yCo_1−xNi_xO₃ Perovskite

LaCoO₃ perovskites were synthesized via the sol–gel method. The procedure is described as follows: La(NO₃)₃·6H₂O (1732.0 mg, 4.00 mmol) and Co(NO₃)₂·6H₂O (1164.1 mg, 4.00 mmol) were dissolved in 40 mL deionized water, followed by the addition of citric acid (2101.4 mg, 10.00 mmol) and glycol (172 μL, 1.92 mmol). The mixture was stirred under an 80 °C water bath until a sol was obtained (the sol was also used for a film-coated FTO electrode as described below). After that, the sol was evaporated in a vacuum oven at 110 °C overnight to obtain a dry gel. Then, the ground dry gel powder was subjected to programmed calcination in a muffle furnace: room temperature to 200 °C at 1 °C min⁻¹ and kept for 0.5 h, then raised to 700 °C at 5 °C min⁻¹ and kept for 6 h. After cooling to room temperature, the product was thoroughly ground to obtain LaCoO₃ powder.

The preparation procedure of Ni and/or Ce doped LaCoO₃ perovskites was almost the same as that of LaCoO₃, except for certain amounts of Co(NO₃)₂ and La(NO₃)₃ were substituted by Ni(NO₃)₂ and Ce(NO₃)₃. Taking La_0.92Ce_0.08Co_0.5Ni_0.5O₃ as an example, La(NO₃)₃·6H₂O (1593.5 mg, 3.68 mmol), Ce(NO₃)₃·6H₂O (208.4 mg, 0.36 mmol), Co(NO₃)₂·6H₂O (582.06 mg, 2.00 mmol), and Ni(NO₃)₂·6H₂O (581.6 mg, 2.00 mmol) were dissolved in 40 mL deionized water, followed by the addition of citric acid (2101.4 mg, 10.00 mmol) and glycol (172 μL, 1.92 mmol) (Figure S1). The Ni- and/or Ce-doped LaCoO₃ perovskites were designated as La_1−yCe_yCo_1−xNi_xO₃ (hereinafter as LCO-Ce_yNi_x), where x represents the molar ratio of Ni to Co, and y represents the molar ratio of Ce to La.

2.3. Structural Characterization

The morphological features and distribution of the particles were characterized by scanning electron microscopy (SEM, Zeiss SUPRA 55, Carl Zeiss, Oberkochen, Germany). The X-ray diffraction (XRD) patterns of the powders were collected using an X-ray diffractometer (XRD, Rigaku Ultima III, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å), scanned in the 2θ range of 10–90° at a scanning rate of 5° min⁻¹. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on an Al Kα X-ray source (Thermo Scientific ESCALAB Xi+ spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) with a pass energy of 30 eV, where the binding energy scale was calibrated by referencing the C 1s peak at 284.8 eV.

2.4. Electrocatalytic Performance Measurements

The catalyst-loaded carbon paper (LCO-Ce_yNi_x@CP) electrode was fabricated as follows: 15.0 mg LCO-Ce_yNi_x was dispersed in a mixed solvent consisting of 3.5 mL isopropanol, 1.4 mL deionized water and 100 μL of 5 wt% Nafion solution, and the suspension was ultrasonicated to form the ink. Then, the ink was loaded onto a piece of carbon paper at 150 °C, and the loading was carefully controlled as 3.0 mg cm⁻².

The catalyst-coated FTO (LCO-Ce_xNi_y@FTO) electrode was fabricated as follows (Figure S2). The sol of LCO-Ce_xNi_y was mixed with Triton X-100 at about 1% (v/v) to form a precursor solution for spin coating on the surface of an FTO slide; then, a dry gel film was formed after drying at 150 °C. After that, the slide was heated at 350 °C for 7 min. Then, we repeated the above procedures three times. The coated slide was then calcined at 550 °C for 6 h. The loading of the catalyst was calculated as the difference in weight before and after coating.

The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) curves of these catalyst-loaded electrodes were characterized using an electrochemical workstation (CHI 660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) in a three-electrode system with a platinum plate (10 mm × 10 mm) as the cathode, and an Ag/AgCl electrode as the reference. An undivided cell containing 18 mL of 0.5 mol L⁻¹ Na₂CO₃ electrolyte was used, after purging with high-purity methane or nitrogen gas for 30 min. All potentials mentioned in this work are referenced to the standard Ag/AgCl electrode.

The electrocatalytic performance of LCO-Ce_xNi_y for methane conversion under ambient pressure was evaluated in an undivided cell with methane gas bubbled at a flow rate of 10 mL min⁻¹. As for performance evaluation under pressurized conditions, a self-designed gastight electrolysis bath was employed, where methane gas was preloaded at 0.15 MPa.

The oxygenates in the electrolyte after electrolysis were identified and quantitatively determined by using ¹H-NMR (Bruker AVANCE III HD 400 MHz, Bruker Corporation, Billerica, MA, USA). In catalytic performance testing, the initial electrolyte (before reaction) was used as a blank sample to confirm that all detected oxygenates originated from the electrocatalytic oxidation of methane. A calibration curve established based on standard solutions showed excellent linearity, enabling quantitative analysis of the products (details in Supplementary Materials, Figure S3).

3. Results and Discussion

3.1. Structural Characterization of LCO-Ce_xNi_y

All the LCO-Ce_xNi_y powders prepared by the sol–gel method exhibit an agglomerated morphology (Figure S4), while the EDS mapping images demonstrate that the doped Ni and Ce elements are uniformly distributed (Figure 1a). Figure 1b,c show the top and cross-section views of LCO-Ce_xNi_y@FTO. It seems that a dense and smooth film was formed on the surface of FTO slide, and the layer of LCO-Ce_xNi_y was tightly adhered to the FTO film and the glass substrate.

The XRD patterns of LCO-Ni_0.4, LCO-Ni_0.5, LCO-Ni_0.6 are shown in Figure 1d, and the position and relative intensity of peaks at 2θ = 23.22°, 33.24°, 40.62°, 47.48° and 58.96° are aligned with the PDF of LaCoO₃ (JCPDS No.48-0123). That implies the perovskite structure could be maintained even when 60% of Co(NO₃)₂ was replaced by Ni(NO₃)₂ during the sol–gel preparation. Furthermore, it is found that the peak position of plane (104) shifted from 33.24° to 33.02° as the Ni doping increased from 40% to 60%, indicating a slight enlargement in lattice spacing occurred. As shown in Figure 1e, with 50% Ni doping (x = 0.5), the perovskite structure was still maintained when 12% of La(NO₃)₃ was substituted by Ce(NO₃)₃ (y < 0.12), but several extra diffraction peaks assigned to CeO₂ and NiO phases were observed when the doping percentage reached 15% (y > 0.15). The synthesized LCO-Ce_yNi_x (x = 0.5, y = 0.12) with pure perovskite structure was used for further investigation. XRD patterns confirmed the formation of the target crystalline phase in the catalyst thin film (Figure 1f).

Figure 1g shows the O 1s XPS spectra of several powders of LCO-Ce_yNi_x (x = 0.5, y = 0.12), in which the deconvolved peaks at 528.4 eV, 531.3 eV, and 533.8 eV correspond to lattice oxygen, oxygen vacancies, and adsorbed oxygen species, respectively [43]. It is found that the relative proportions of oxygen vacancies increased with more Ni and Ce doping, and the oxygen vacancy content is about 62.1% in LCO-Ce_0.08Ni_0.5 (Table S1).

The La 3d_5/2 XPS spectrum indicates that La ions in LCO-Ce_0.08Ni_0.5 have a single valence state of La³⁺ (Figure S5a). In the XPS spectra of Co 2p, two main peaks at 781.3 and 794.6 eV are ascribed to Co³⁺, whereas the Co²⁺ corresponds to the peaks at 796.5 and 779.4 eV (Figure S5b). Derived from the relative areas of the main peaks, Co³⁺/Co²⁺ is approximately 1.95. As shown in Figure S5c, the Ni XPS spectrum features two characteristic peaks: the Ni²⁺ 2p_3/2 peak centered at 853.8 eV with a corresponding satellite at 861.7 eV, and the Ni³⁺ 2p peak at 855.1 eV accompanied by a satellite at 865.1 eV. The ratio of Ni³⁺/Ni²⁺ is approximately 1.26. Additionally, an interfering La 3d_3/2 peak is observed at 852.4 eV [44]. The Ce 3d XPS spectrum confirms successful doping of Ce atoms into the perovskite structure in a mixed valence state of Ce³⁺ and Ce⁴⁺. The v and u peaks represent the Ce 3d_5/2 and Ce 3d_3/2 orbitals, respectively (Figure S5d) [45]. The oxygen vacancies are formed through the incorporation of Ni (II/III) and Ce (III/IV) into the LaCoO₃ crystal lattice, which makes the ideal perovskite structure perturbated at both A- and B-sites. This structural perturbation triggers charge compensation through oxygen vacancy formation, leading to form a perovskite structure rich in oxygen vacancies.

3.2. Electrochemical Behaviors of LCO-Ce_yNi_x Perovskite

The LSV curves of LCO-Ce_0.08Ni_0.5@CP, LCO-Ni_0.5@CP, LaCoO₃@CP and LCO-Ce_0.08Ni_0.5@FTO electrodes in N₂-saturated Na₂CO₃ electrolyte are shown in Figure 2a. Since it is measured in a N₂ atmosphere, the anodic current could only originate from WOR on these electrodes. Notably, a certain slight current occurred before obvious oxygen evolution (below 0.90 V), which might be related to the generation of ROS. Judging from the current densities in the range of 0.65 V and 0.90 V, the order for the amount of generated ROS should be LCO-Ce_0.08Ni_0.5@FTO, LCO-Ce_0.08Ni_0.5@CP, LCO-Ni_0.5@CP, and LaCoO₃@CP. In further experiments, we confirmed the existence of H₂O₂ and •OH in N₂-saturated 0.5 mol L⁻¹ Na₂CO₃ electrolyte after potentiostatic electrolysis for 0.5 h by the Ce(IV) and bromopyrogallol red qualitative test (Figures S6 and S7). However, it is worth mentioning that no H₂O₂ was identified when a N₂-saturated 0.5 mol L⁻¹ Na₂SO₄ solution was used as the electrolyte, which is consistent with the information disclosed from LSV characteristics (Figure 2b). The function of carbonate in electrolyte in the formation of H₂O₂ through WOR has been well discussed [46,47,48].

Furthermore, the LSV curves of these electrodes in CH₄ and N₂ atmospheres are compared in Figure 2c. There is almost no change for LaCoO₃@CP and Ce-doped LCO@CP (see more in Figure S8), but for LCO-Ce_0.08Ni_0.5-loaded electrodes, the anodic currents in the CH₄ atmosphere are obviously higher than those in the N₂ atmosphere. This incremental anodic current originated from the presence of CH₄, indicating that CH₄ molecules are involved in the electrode reaction occurring on the anode.

In the CV curves of the LCO-Ce_0.08Ni_0.5@CP electrode in Figure 2d, the pair of anodic and cathodic peaks at 0.65 V and 0.42 V are assigned to the reversible transformation between Ni(II) and Ni(III). In contrast, the CV curves of the LaCoO₃@CP and the LCO-Ce₀.₀₈@CP exhibit no discernible redox peaks (Figure S9). It could be reasonably inferred that Ni(III) in LCO-Ce_yNi_x serves as the active site for chemisorption of CH₄ molecules, with the recovery of desorbed Ni(II) to Ni(III) causing the anodic current to increase.

3.3. Evaluation of Electrocatalytic Performance for Methane Conversion

The electrocatalytic performance of LCO-Ce_yNi_x for methane conversion was evaluated in an undivided cell with aqueous carbonate solution as the electrolyte, and ¹H-NMR spectroscopy was applied for qualitative and quantitative analysis of oxygenates in the electrolyte after electrolysis. The effect of the components in LCO-Ce_yNi_x, the anodic potential, and the concentration of carbonate solution on the electrocatalytic performance were investigated.

A typical ¹H-NMR spectrum of the electrolyte after electrolysis is shown in Figure 3a, in which ethanol and acetic acid can be identified as methane conversion products. In most of the tests in this study, ethanol was identified as the main product. Furthermore, other oxygenates like acetic acid and acetone could be detected in the electrolyte when electrolysis was conducted for an extended period or at a higher positive potential. In the following discussion, the formation rates of ethanol and acetic acid calculated by the quantitative analysis results of ¹H-NMR were used to evaluate the electrocatalytic performance of LCO-Ce_yNi_x.

A comparison of electrolysis results by different LCO-Ce_yNi_x@CP electrodes operating under the same conditions is shown in Figure 3b. It is worth noting that no oxygenates were detected when the LaCoO₃-loaded electrode was used (not shown in Figure 3b), and only acetic acid was detected using the LCO-Ce_0.08Ni_0.3@CP electrode, while quite a large amount of ethanol and acetic acid was detected when using other LCO-Ce_yNi_x@CP (x = 0.4, 0.5) electrodes. For instance, the formation rate of ethanol on the LCO-Ce_0.08Ni_0.5@CP is about 245.6 μmol g_cat⁻¹ h⁻¹, and it is 319.1 μmol g_cat⁻¹ h⁻¹ on the LCO-Ce_0.12Ni_0.5@CP electrode. Additionally, in most of the electrolysis experiments, the detected amount of acetic acid exhibited little variation. This observation indicates an apparent formation rate of 70 μmol g_cat⁻¹ h⁻¹, suggesting that acetic acid is an intermediate product for further oxidation.

Under identical conditions, the effect of the anode potential on the performance of the LCO-Ce_0.08Ni_0.5@CP electrode was investigated, and the experimental results are presented in Figure 3c. It is shown that the formation rate of ethanol exhibited an inverted-V profile as the potential changed from 1.15 V to 1.35 V, and the maximum was obtained at 1.25 V. According to the CV curve, Ni(II) in the catalyst can be oxidized to Ni(III) at potentials more positive than 0.65 V, which serves as the active site for chemisorption of CH₄, while abundant ROS could be generated near 0.9 V. This is the reason that the ethanol formation rate increases as the potential increases from 1.10 V to 1.25 V. The oxygen evolution reaction (OER) then becomes violent at potentials higher than 1.25 V, leading to the further oxidation of ethanol and acetic acid.

Electrolysis in a pressurized cell using LCO-Ce_0.08Ni_0.5@CP as the working electrode under the same conditions was performed to investigate the effect of methane solubility on the conversion rate. As shown in Figure 3d, the ethanol formation rate showed only a slight increase compared to that in atmospheric pressure. This phenomenon observed in this work is obviously different from our previous work [22], which indicates that methane solubility is not the rate-limiting factor in this electrocatalytic system.

It was found that the ethanol formation rate increased to 474.2 μmol g_cat⁻¹ h⁻¹ when the Na₂CO₃ concentration of the electrolyte increased from 0.5 mol L⁻¹ to 2.0 mol L⁻¹ (Figure S10). Correspondingly, no products were detected when using 0.5 mol L⁻¹ Na₂SO₄ solution as an electrolyte. It is reported that the carbonate ions in the electrolyte are involved in the generation of H₂O₂ through 2-electron pathway of WOR on anode. The oxidizing capacity of H₂O₂ is milder than •OH, which is beneficial to the formation of oxygenates in methane conversion. However, the low selectivity for H₂O₂ production results in low Faradaic efficiency for electrocatalytic methane oxidation to ethanol (Figure S11).

Figure 3e,f show the catalytic performance of LCO-Ce_0.08Ni_0.5@FTO electrodes, and the highest ethanol formation rates reached 577.0 μmol g_cat⁻¹ h⁻¹ and 609.9 μmol g_cat⁻¹ h⁻¹ at 0.9 V under ambient and pressurized conditions, respectively. LSV curves indicate that these electrodes demonstrate superior WOR activity compared to LCO-Ce_0.08Ni_0.5@CP, which promotes ROS formation at more negative potentials. However, the enhanced oxygen evolution at more positive potentials competes with the generation of methane oxidation products. This observation confirms the relationship between methane oxidation and WOR-derived ROS.

The optimized electrolysis conditions for methane conversion on LCO-Ce_0.08Ni_0.5@CP loaded electrode were determined as constant-potential electrolysis at 1.25 V in 2.0 M Na₂CO₃ solution, under which an ethanol formation rate of 474.2 μmol g_cat⁻¹ h⁻¹ under atmospheric pressure and a rate of 482.6 μmol g_cat⁻¹ h⁻¹ under pressurized conditions were achieved.

3.4. Stability of the Catalyst

The stability of the catalyst was evaluated through consecutive electrolysis. The ethanol formation rate gradually stabilized with increasing cycle number during multiple 30 min electrolysis cycles of the identical electrode. As shown in Figure 4a, the ethanol formation rate gradually stabilized with increasing cycle number during multiple 30 min electrolysis cycles of the identical LCO-Ce_0.08Ni_0.5@CP electrode in 0.5 mol L⁻¹ Na₂CO₃ solution at 1.25 V. Figure 4c shows an increase in current during prolonged electrolysis, followed by stabilization. XRD results indicated a limited extent of La-ion exsolution from the LaCoO₃ matrix during the reaction, leading to surface reconstruction with a small amount of La₂O₂CO₃ (JCPDS No. 23-0320, Figure S12) [49]. Considering the relatively low diffraction peak intensity and the stable catalytic performance in repeated tests, the crystal structure of the catalyst remains essentially stable. The performance degradation is due to OER-induced destabilization of the catalyst, leading to partial detachment and reduced methane conversion efficiency.

Figure 4b shows the ethanol formation rate of the LCO-Ce_0.08Ni_0.5@FTO electrode during repeated electrolysis cycles. This electrode exhibits better stability than the LCO-Ce_0.08Ni_0.5@CP electrode. The enhanced stability can be attributed to the direct deposition of perovskite films on the FTO surface, where tight interfacial attachment effectively prevents catalyst detachment. The film morphology remains stable through repeated electrolysis, further confirming the reliability of the loading strategy. These phenomena demonstrate that the Ni and Ce co-doped LaCoO₃ catalyst integrates the structural stability of perovskites with the catalytic durability of the active phase.

3.5. Mechanisms for Electrochemical Methane Conversion

Studies have shown that the type of electrolyte significantly influences product distribution in catalytic methane oxidation by altering intermediate reaction pathways. In ionic liquid systems, activated methane rapidly reacts with reactive oxygen species (ROS), forming short-chain oxygenates such as CH₃OH [21]. In contrast, aqueous-phase environments promote the coupling of surface-bound *O and *C intermediates at reaction interfaces, leading to the formation of C2 and C3 products. The point is that aqueous solutions can reduce the solvation free energy of oxygenates (e.g., ${\Delta \mathrm{G}}_{{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}}$ = −0.22 eV at 298K), thereby preventing further oxidation and facilitating their accumulation [50,51]. Simultaneously, at the reaction interface, •CH₃ first undergoes dehydrogenation to form •CH₂, which then undergoes hydroxylation to form the key intermediate •CH₂OH. The resulting •CH₂OH then undergoes C–C coupling with •CH₃ to yield ethanol (C2 product) [14]. By contrast, in mild catalytic conditions for methane conversion, where H₂O₂ serves as the dominant ROS, methanol is found to be the primary oxidation product preferentially generated [52,53]. However, these oxygenated intermediates can undergo further dehydrogenation and participate in radical reactions, subsequently driving C–C coupling and the formation of highly oxidized products (e.g., aldehydes and carboxylic acids) in the presence of highly reactive ROS such as •OH [54,55,56].

Based on existing theories and the experimental results, we propose a reaction mechanism for methane electrocatalytic oxidation in this system. First, Ni(II) sites on the catalyst are oxidized to Ni(III) at an anodic potential, generating active sites for chemisorption of methane molecules [4]. Subsequently, the adsorbed methane molecules at these sites undergo C–H bond stretching, leading to methane activation [19]. Meanwhile, the Ce/Ni co-doping strategy induces the formation of abundant oxygen vacancies within the catalyst, which enabled the anode to simultaneously catalyze the 1e-WOR for •OH generation and the CO₃²⁻-mediated 2e-WOR for H₂O₂ [46,57,58]. Since H₂O₂ is a relatively mild ROS, it partially inhibits the deep oxidation of intermediates, thereby promoting ethanol formation. In contrast, the more reactive ·OH promotes both further oxidation and C–C coupling of ethanol, yielding oxygenates such as acetic acid and acetone.

4. Conclusions

In this study, a Ni and Ce co-doped LaCoO₃ with a perovskite structure was applied as an anodic catalyst to convert methane in a Na₂CO₃ electrolyte under ambient conditions, and ethanol was found to be the main conversion product. The doped Ni provides chemisorption sites for methane activation, and the introduction of oxygen vacancies caused by doping and the presence of CO₃²⁻ in electrolyte promote the generation of •OH and H₂O₂ through WOR on the anode. As a result, the ethanol generation rate of 577.0 μmol g_cat⁻¹ h⁻¹ was achieved on LCO-Ce_0.08Ni_0.5@FTO electrode at 0.9 V in 0.5 mol L⁻¹ Na₂CO₃ under ambient conditions, and good performance stability was also tested.