Accelerating the proton-coupled electron transfer on Co₉S₈ hollow polyhedrons for electrochemical reduction of nitrate to ammonia

Electrocatalytic NO₃⁻-to-NH₃ conversion via nitrate reduction reaction (NO₃⁻RR) is a complex proton-coupled electron transfer (PCET) process. The valence band of an electrocatalyst and the release and delivery of protons at electrocatalyst-electrolyte interfaces directly govern electron and proton transfer, respectively. In this study, Co₉S₈ hollow polyhedrons were designed as an effective electrocatalyst for NO₃⁻-to-NH₃ conversion in the phosphate buffer saline (PBS), a protic auxiliary electrolyte. Experimental evidence suggests that Co₉S₈ presents metallic characteristics with lower work function and outstanding electronic conductivity, facilitating electron transfer at electrocatalyst-electrolyte interfaces. In-situ spectroscopy confirms that Co₉S₈ promotes the dissociation of HPO₄²⁻ and the proton transfer at electrocatalyst-electrolyte interface, thereby accelerating the hydrogenation process of adsorbed NO₃⁻ and other intermediates. Benefiting from these advantages, Co₉S₈ demonstrates a significantly lower onset potential and higher NH₃ yield compared to the commonly used and stable electrocatalyst Co₃O₄ for NO₃⁻-to-NH₃ conversion. This study proposed a new insight about the electron and proton transfer process for NO₃⁻RR.