Engineering the Co/CoO heterostructure to trigger the in-situ generation of abundant high-valent cobalt species for enhanced electroreduction of nitrate to ammonia

The CoO-based materials are promising candidates for electrochemical nitrate reduction reaction to ammonia (eNO3RR). Herein, Zn/Co bimetallic MOFs are adopted to construct Co/CoO Schottky heterostructures, where the Co-CoO interfaces are engineered by controlling the Zn/Co ratio. The interface-optimized Co/CoO displays an NH3 yield of 713 mu mol h-2 cm- 2 and a maximum Faradaic efficiency of 99.16 % due to the synergistic effect between CoO and Co and abundant oxygen vacancies. The more the Co-CoO interfaces, the more the in-situ generated high-valent cobalt species (CoOOH), and the higher the catalytic performance. Therefore, the highvalent cobalt species are considered the true active sites. When used as a cathode in a rechargeable Zn-NO3 battery, the Co/CoO heterostructure achieves a power density of 4.7 mW cm-2 and an NH3 yield of 131.7 mu mol h-2 cm- 2 with robust working stability.