Boosting Electrocatalytic Reduction of Nitrate to Ammonia over Co3O4 Nanosheets with Oxygen Vacancies

Electrocatalytic nitrate reduction into ammonia is promising for its restricted activity and selectivity in wastewater treatment, however, it remains challenging. In this work, Co3O4 nanosheet electrodes with rich oxygen vacancies (OVs) (Co3O4−x/NF) are prepared and then applied as efficient catalysts for selective electrocatalytic reduction of nitrate to ammonia. The resulting Co3O4−x/NF electrodes exhibit high NO3−-N removal efficiency and NH4+-N selectivity, at 93.7% and 85.4%, respectively. X-Ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectra (EPR) results clearly reveal the formation of OVs in Co3O4−x/NF. The electrochemical characterization results confirm that OVs can effectively improve electron transfer as well as the electrochemically active area. The Co2+/Co3+ ratio of Co3O4−x/NF increases after the electrocatalytic reduction of nitrate, highlighting the crucial role played by Co2+ in mediating ammonia production via the Co2+/Co3+ cycle. These findings offer valuable guidelines for the development of more efficient and sustainable approaches for nitrate-contaminated wastewater treatment and ammonia synthesis.