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 (VO) 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 VO 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.