Enhanced Activity and Stability for Electrocatalytic Nitrate Reduction to Ammonia over Low-Coordinated Cobalt

It is still challenging to develop an effective strategy to simultaneously enhance the activity and stability of electrocatalysts for electrocatalytic nitrate reduction reaction (eNO₃RR). Herein, taking metallic cobalt as an example, it is demonstrated that the construction of low-coordinated cobalt nanosheets (L-Co NSs) by H₂ plasma etching of electrodeposited cobalt nanosheets (Co NSs) can greatly enhance the activity and stability of metallic cobalt for eNO₃RR. Compared with Co NSs, at −0.4 V versus RHE, the nitrate removal rate, ammonia partial current density, and ammonia yield are increased by L-Co NSs from 82.14% to 98.57%, from 476 to 683 mA cm⁻², and from 2.11 to 2.54 mmol h⁻¹ cm⁻², respectively. In addition, L-Co NSs demonstrate negligible activity decay after 30 cycles of stability test, while the Co NSs show significant activity decline. In situ electrochemical characterizations and theoretical calculations verify that the abundance of Co vacancies in L-Co NSs not only contribute to the optimized electronic structure and enhanced desorption of key intermediate to boost the activity but also facilitate the transformation of Co(OH)₂ to Co⁰ to promote the stability. Furthermore, L-Co NSs exhibit favorable performance in removing nitrate from simulated wastewater and air plasma discharge-electrocatalytic reduction cascade system to produce ammonia.