Accelerating the Proton-Coupled Electron Transfer on Co9S8 Hollow Polyhedrons for Electrochemical Reduction of Nitrate to Ammonia

Electrocatalytic NO 3 − -to-NH 3 conversion via nitrate reduction reaction (NO 3 − 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, Co 9 S 8 hollow polyhedrons were designed as an effective electrocatalyst for NO 3 − -to-NH 3 conversion in the phosphate buffer saline (PBS), a protic auxiliary electrolyte. Experimental evidence suggests that Co 9 S 8 presents metallic characteristics with lower work function and outstanding electronic conductivity, facilitating electron transfer at electrocatalyst-electrolyte interfaces. In-situ spectroscopy confirms that Co 9 S 8 promotes the dissociation of HPO 4 2− and the proton transfer at electrocatalyst-electrolyte interface, thereby accelerating the hydrogenation process of adsorbed NO 3 − and other intermediates. Benefiting from these advantages, Co 9 S 8 demonstrates a significantly lower onset potential and higher NH 3 yield compared to the commonly used and stable electrocatalyst Co 3 O 4 for NO 3 − -to-NH 3 conversion. This study proposed a new insight about the electron and proton transfer process for NO 3 − RR.