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.

Keywords: C1 chemistry; CO2 electroreduction; Photo-assisted electrocatalysis; selectivity control; sulfur vacancy.

© 2025 Wiley‐VCH GmbH.

将二氧化碳（CO₂）转化为高价值的一碳产品对于实现碳中和和能源可持续性具有重要意义，而在电催化二氧化碳还原反应(eCO2RR)中合理调节一碳产品的选择性是由于竞争反应路径和热力学限制而面临的主要挑战之一。在这里，我们展示了一种由硫空位（SV）上原位吸附的OH引发的“质子屏障”策略，在CO₂电解过程中能够超高选择性地切换CH₄和CO产品，其法拉第效率分别为93.6%和95.3%。原位测量揭示了光生空穴可以对抗Cu²⁺的电还原，从而保持CuInS₂/CuS结构的完整性，而从水中解离出来的OH可以自发地锚定在SV上阻碍局部质子迁移，完全避免了多质子产物的形成。同时，优先从Cu中心附近与OH锚定的SV脱附的CO导致了CO的选择性生成。在没有SV的情况下，*CO可以在较低自由能/甚至自发氢化得到CH₄。所提出的质子限制效应为eCO2RR选择性的控制提供了一个有前景的参考，并且光辅助电还原方法展示了原位稳定电子不耐受催化结构的一个范例。

关键词：一碳化学；二氧化碳电还原；光辅助电催化；选择性控制；硫空位。

© 2025 Wiley‐VCH GmbH.