NiCo ₂ O ₄ −PMA With Subnanoscale Dual‐Domain Sites Enables Near‐Unity Acetate Selectivity in Photoelectrochemical Methane Oxidative Coupling

A promising strategy revolves around the direct partial oxidation of methane, an important carbon feedstock for chemical synthesis, into liquid oxygenates under mild reaction conditions. However, having both a high conversion quality and thus being a good selection to achieve the desired product remains a significant challenge, largely due to the intrinsic chemical stability of methane. This challenge becomes even more pronounced when targeting higher value chemicals that require C‒C coupling. Herein, we construct dual‐domain site configurations at the subnanoscale by integrating nickel cobaltite (NiCo 2 O 4 ) and phosphomolybdic acid (PMA), both of which effectively catalyze the oxidation of methane. Experimental results demonstrate that the resulting composite (NiCo 2 O 4 −PMA) exhibits a subnanosheet morphology and a periodically arranged heterostructure. Notably, the incorporated PMA not only stabilizes carbonyl (*CO) intermediates to prevent over‐oxidation but also serves as a secondary active site adjacent to NiCo 2 O 4 , facilitating C‒C coupling between methyl (*CH 3 ) and *CO species. This process leads to the formation of acetyl (*CH 3 ‒CO) intermediates, which are subsequently converted into acetate. Consequently, NiCo 2 O 4 −PMA achieves a Faradaic efficiency for acetate of 26.01%, a productivity of up to 5.8 mmol h −1 , and a selectivity of ∼99% under ambient conditions (1 bar, 25°C) in photoelectrochemical systems.