S‐Vacancy‐Induced “Proton Fence Effect” Enables Selectivity Switching between CH ₄ and CO in Photo‐Assisted CO ₂ Electroreduction

Abstract Upcycling CO 2 into high‐value C 1 products is impressive for achieving carbon neutrality and energy sustainability, while rational modulation of C 1 product selectivity is one of the biggest challenges in electrocatalytic CO 2 reduction reaction (eCO 2 RR) 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 (S V ) to ultraselectively switch the C 1 product between CH 4 and CO during CO 2 RR, with Faraday efficiency of 93.6% and 95.3%, respectively. In situ measurements uncover that the photo‐generated holes counteract Cu 2+ electroreduction to retain the intact structure of CuInS 2 /CuS, while *OH dissociated from water can spontaneously anchor toward S V to hinder the local proton migration, completely circumventing multiproton products. Meanwhile, the preferential desorption of *CO from Cu centers adjacent to the *OH‐anchored S V renders the exclusive formation of CO. In the absence of S V , *CO can be further hydrogenated in a lower free energy/even spontaneously to afford CH 4 . The proposed proton confinement effect furnishes a promising reference for the selectivity control of eCO 2 RR, and the photo‐assisted electroreductive protocol demonstrates a paradigm of in situ stabilization of electron‐intolerant catalytic structures.