The authors fabricated a NiO/Ni heterostructure electrocatalyst on nickel foam via a thermal oxidation strategy. By optimizing electrolysis parameters such as current density, electrolysis time, and electrolyte pH gradient, they significantly enhanced EOM performance. In a pH-asymmetric 0.1 M NaOH||0.1 M HCl system, the catalyst achieved a high liquid product formation rate of 2.7 mmol gNiO – 1 h –1 at 0.4 mA and a remarkable 90.1% Faradaic efficiency for liquid products at 0.1 mA. Notably, by harnessing electrochemical neutralization energy, the EOM||HER system stably operated at an ultralow cell voltage of 1.86 V, saving up to 37% energy compared to conventional electrolyzers, while simultaneously producing high-purity hydrogen at the cathode ( Figure 2 ).
Through comprehensive in situ characterization and control experiments, the authors unraveled the reaction mechanism and identified key active species in EOM. In situ Raman spectroscopy revealed potential-dependent evolution of surface species, confirming NiOOH ( Ni III –O –Ni III –O – ) as the crucial intermediate for methane activation ( Figure 3 ). 18 O isotope labeling and kinetic isotope effect experiments demonstrated that the oxygen atoms in products originated directly from OH – in the electrolyte, with the reaction proceeding via a proton-coupled electron transfer mechanism. Combined XPS depth profiling and electrochemical active species studies revealed that dynamic formation and maintenance of highly active Ni 3+ species at the heterointerface are essential for efficient EOM, directly impacting the selectivity towards low-carbon alcohols.
Density functional theory (DFT) calculations further elucidated how the NiO/Ni interface promotes methane activation and regulates product selectivity ( Figure 4 ). Simulations indicated that applying an external field (1.45 V vs. RHE) substantially enhances CH₄ adsorption on Ni sites, changing the Gibbs free energy from endergonic (+1.16 eV) to exergonic (–0.29 eV). Reaction pathway analysis showed that *CH 4 dehydrogenation to *CH 3 is the rate-determining step of EOM. At low applied potentials, *CH 3 tends to hydroxylate directly into methanol, while at higher potentials, it favors further dehydrogenation via the *OCH 2 intermediate to form ethanol. However, the high desorption energy barrier of ethanol (0.91 eV) leads to an extended residence time on active sites, thereby facilitating its deep oxidation to acetic acid. This also accounts for the phenomenon that ethanol is prone to deep oxidation to acetic acid. Moreover, the charge redistribution at the heterointerface enhances the electron affinity of Ni 3+ -O· active centers, boosting *CH 4 polarization, which matches the experimentally observed high EOM efficiency and potential-dependent selectivity.
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