Unraveling the Structural Evolution of Cobalt Sulfides in Electrocatalytic NO₃RR: the Inescapable Influence of Cl

Electrochemical nitrate reduction (NO₃RR) to ammonia is an attractive approach for mitigating NO₃^- pollution and producing valuable NH₃. Cobalt-sulfur compounds are widely considered to be potential electrocatalysts for NO₃RR. However, there is still a lack of research on the probable structural evolution, long-term stability, and reactive sites of cobalt-based sulfides during catalysis. Herein, we have employed three cobalt sulfides (CoS, where x = 8/9, 2, 1.097) with different sulfur contents as catalysts for electrocatalytic NO₃RR under alkaline conditions. At -0.8 V vs RHE, all these CoS show promising performances that Faradaic efficiencies of >80% and a high yield of >1780 mmol h^-1 g_cat^-1 for NH₃ production are achieved. Through a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and other characterizations, it is revealed that all these cobalt sulfides are easily converted into cobalt hydroxide during the NO₃RR. This phenomenon is seemingly contradictory to the thermodynamic prediction that, according to the Pourbaix diagram, these CoS compounds should be stable even under the catalytic condition. We suggest that this is due to the presence of Cl^- ions in the electrolyte that promote the transformation of CoS toward Co(OH)₂. Chloride ions are commonly found in both industrial settings and natural water bodies and are challenging to remove. The evolved Co(OH)₂ species is proposed to be responsible for catalyzing NO₃RR, especially during a long-term catalytic process. This study highlights the inevitable structural evolution of CoS catalysts under current alkaline electrocatalytic NO₃RR conditions, offering theoretical guidance for the judicious selection and design of future catalysts.