S‐Vacancy‐Induced "Proton Fence Effect" Enables Selectivity Switching Between CH4 and CO in Photo‐Assisted CO2 Electroreduction

Upcycling CO2 into high‐value C1 products is impressive for achieving carbon neutrality and energy sustainability, while rational modulation of C1 product selectivity is one of the biggest challenges in electrocatalytic CO2 reduction reaction (eCO2RR) due to the competing reaction pathways and thermodynamic limitation. Here, we showcase a ‘proton fence’ strategy enabled by in situ adsorbed *OH on sulfur vacancies (SV) to ultraselectively switch the C1 product between CH4 and CO during CO2RR, with Faraday efficiency of 93.6% and 95.3%, respectively. In situ measurements uncover that the photo‐generated holes counteract Cu2+ electroreduction to retain the intact structure of CuInS2/CuS, while *OH dissociated from water can spontaneously anchor toward SV to hinder the local proton migration, completely circumventing multiproton products. Meanwhile, the preferential desorption of *CO from Cu centers adjacent to the *OH‐anchored SV renders the exclusive formation of CO. In the absence of SV, *CO can be further hydrogenated in a lower free energy/even spontaneously to afford CH4. The proposed proton confinement effect furnishes a promising reference for the selectivity control of eCO2RR, and the photo‐assisted electroreductive protocol demonstrates a paradigm of in situ stabilization of electron‐intolerant catalytic structures.