Unravelling the Ru-promoted dynamic evolution of Cobalt hydroxide during nitrate reduction towards ammonia production

Geen ammonia synthesis through electrochemical nitrate reduction (e-NO₃R) using cost-effective Co-based catalysts is promising, but the inevitable structural evolution induced by reductive potentials compromises long-term stability and hinders practical implementation. Focusing on β-Co(OH)₂, which serves as the active phase in e-NO₃R systems, this work combines experimental analysis and computational studies to reveal a dynamic surface *OH evolution process: *OH cleavage under negative potentials and *OH generation by the dissociation of NO₃^-. Notably, Ru nanoparticles anchored on β-Co(OH)₂ nanosheets promote structural evolution by facilitating *OH cleavage and generation, thereby sustaining a highly active and selective OH-terminated surface. Simultaneously, Ru provides moderate *H adsorption, accelerates the conversions from NO₃^- to NO₂^- and from NO₂^- to NH₃, and thus enhances ammonia synthesis. The optimized Co(OH)₂-Ru catalyst achieves an ammonia yield of 98 ± 0.91 mg·h^-1·cm^-2 with a Faradaic efficiency (FE) of 97.7 ± 0.90 % at -0.7 V versus reversible hydrogen electrode (vs. RHE), while maintaining NH₃ FEs above 95% across a broad potential window. This work elucidates structural evolution dynamics, offering a design principle for robust electrocatalysts.