High-valent cobalt active sites derived from electrochemical activation of metal-organic frameworks for efficient nitrate reduction to ammonia

Metal-organic frameworks (MOFs) exhibited great potential in the electrocatalytic reduction of nitrate to ammonia (eNO3RR), but the structural transformation and the identification of the active sites in the electrochemical process have rarely been reported. Herein, a series of MOFs have been fabricated for eNO3RR. The Cobased MOF (Co-TPA) is selected as a representative for exploration through in-situ and ex-situ techniques to reveal the structural evolution during eNO3RR. The structural transformation from the Co-TPA to electrochemically activated catalyst (Co-TPA-E) is verified, and it is found that the in-situ formed high-valent Co species (CoOOH) are the active sites for eNO3RR. Moreover, the formation of the high-valence CoOOH species can be further facilitated by introducing highly electronegative Cu ions into the Co-TPA-E, resulting in excellent catalytic activity with an NH3 yield rate of 1.12 mmol h-1 cm-2 at - 0.326 V vs. RHE and a maximum Faradaic efficiency of 99.62 %.