Enhanced Hydrogen Supply to Atomically Dispersed Copper Sites through Close Cooperation with Oxygen Vacancies in Black TiO₂ to Promote CH₄ Production in CO₂ Electrolysis

CO₂ electroreduction (eCO₂R) holds promise as an environmentally friendly approach to reducing greenhouse gas emissions. Cu is a representative catalyst with high eCO₂R activity. However, its selectivity for CH₄ synthesis is still insufficient due to the slow eight-electron transfer to a single carbon, the predominance of C-C coupling reactions toward C₂₊ products on Cu, as well as occurrence of the hydrogen evolution reaction. Here, for high CH₄ selectivity, we demonstrate a genuine hydrogen supply to atomically dispersed Cu sites (AD-Cu) via the cooperative function of oxygen vacancy (V_O) formed on defective black anatase TiO₂ (Cu-TiO₂-H₂), that is prepared by exposing Cu-doped TiO₂ (Cu-TiO₂) to hydrogen gas. Cu-TiO₂-H₂ exhibited a remarkable Faradaic efficiency for CH₄ production of 63% and a partial current density of −120 mA cm^-2. The catalytic mechanism for the high CH₄ selectivity was elucidated using a variety of spectroscopies, such as electron spin resonance, reversed double-beam photoacoustic spectroscopy (RDB-PAS) and in situ Raman measurements, with the support of quantum chemical calculations. In situ Raman measurements revealed that Cu-TiO₂-H₂ greatly accelerates proton consumption for the hydrogenation of *CO intermediates and that the surface pH on Cu-TiO₂-H₂ is sufficiently high to stabilize *CHO intermediates, key species for CH₄ formation. DFT calculations support the stability of the intermediates during the process of forming *CHO. All our results suggest that V_O contiguous to AD-Cu on Cu-TiO₂-H₂ promotes water dissociation and smoothly supplies hydrogen to AD-Cu on Cu-TiO₂-H₂, thus facilitating CH₄ formation in eCO₂R.