Controllable product selectivity in electrochemical nitrate reduction over CuCo₂S₄ catalyst

Electrochemical nitrate reduction (eNO₃RR) offers a promising route for nitrate removal and conversion into value-added nitrogen products such as nitrite, hydroxylamine, ammonia, and dinitrogen. However, controlling product selectivity remains a major challenge, as the reaction pathway is highly sensitive to catalyst surface properties and operating conditions. Here, we address this challenge by demonstrating that selectivity can be systematically tuned through electronic structure of catalyst, applied potential and electrolyte-controlled reaction environments. In particular, the thiospinel CuCo₂S₄ catalyst exhibits unique active sites that regulate the adsorption and reduction of nitrate-derived intermediates, enabling selective ammonia formation. Comparative studies on respective monometallic sulfides reveal that CuS favors nitrite production, whereas CoS and CuCo₂S₄ show enhanced ammonia selectivity, highlighting the critical role of catalyst structure in directing reaction pathways. Beyond catalytic structure, applied potential and nitrate concentration further modulate selectivity. On CuS, ammonia selectivity increases from 44% to 88% as the potential shifts from 0.0 to −0.5 V vs. RHE. More importantly, nitrate concentration governs surface intermediate coverage, driving a transition in product distribution from hydroxylamine (47%) at ultra-low concentrations to ammonia (96%) at moderate concentrations (>0.05 M), and to N₂ formation (64%) at high concentrations, reflecting a shift between hydrogenation and N–N coupling pathways. Within this framework, as synthesized CuCo₂S₄ catalyst coated electrode delivers outstanding performance, achieving 96.4 ± 1.8% ammonia selectivity, 83.6 ± 5.6% Faradaic efficiency, and an ammonia yield rate of 11.6 ± 1.0 mg h⁻¹ cm⁻² at −0.4 V vs. RHE. A sulfur re-enrichment strategy substantially restores catalytic activity by surface reconstruction. Overall, this work establishes catalyst design, reaction-condition engineering and surface reconstruction as key principles for tuning eNO₃RR selectivity.