Unveiling the Role of Topological Surface States in Boosting Electrocatalytic Nitrate Reduction to Ammonia

Abstract Designing efficient catalysts for nitrate reduction reaction (NO ₃ ⁻ RR) poses a challenge in advancing the selectivity and yield of ammonia (NH ₃ ). Unlike conventional catalytic descriptors, topological surface states (TSSs) represent an orthogonal avenue for tailoring catalytic properties, while its role in NO ₃ ⁻ RR remains unknown. Here, the semimetallic character of Co ₃ Sn ₂ S ₂ , endowed with robust TSSs is leveraged and enhances charge transport characteristics, to establish this system as a prototypical platform for decoding surface state‐governed NO ₃ ⁻ RR mechanism. The catalyst exhibits exceptional NO ₃ ⁻ RR performance, achieving a maximum NH ₃ Faradaic efficiency of 91.6% at −0.5 V _RHE and a high NH ₃ yield of 22.4 mg h ⁻¹ cm ⁻² at −0.6 V _RHE , while maintaining excellent stability over 200 h in a membrane–electrode assembly electrolyzer, outperforming its semiconductor counterparts. In situ experiments and density functional theory calculations reveal that the TSSs accelerate charge transfer kinetics as well as alleviate the energy barrier for the *NOH → *N step. This work highlights the critical role of TSSs in governing electrocatalytic mechanisms and advances the rational design of high‐performance topological NO ₃ ⁻ RR catalysts.