Unveiling the dynamic transformation of Cu species in high entropy oxides during the electrochemical nitrate reduction to ammonia

Ammonia production is one of the industrial processes that boosted human development and life quality but has an enormous energy cost and environmental impact. Electrochemical nitrate reduction represents a way to tackle the downsides of traditional thermal processes, including lower energy consumption and environmental remediation. In this work, we synthesized high entropy oxides (HEO) with different calcination temperatures to control the morphological, crystallographic, and electrocatalytic properties. Operando-XAS measurements were used to evaluate the speciation of the Cu atoms in the catalysts as a function of the applied potential, and electrochemical impedance spectroscopy (EIS) was used to gain insights into the reaction mechanism during the nitrate-to-ammonia reaction. Several Cu species were determined to be present at high reduction potentials, confirming the presence of mixed interfaces, and a potential gradient was observed to affect the catalyst interface. The prepared electrodes showed a good ammonia production rate above 20 mgNH3gcat bility for over 24 h to obtain concentrations over 800 mu MNH3. We ultimately highlight the importance of conducting operando and in-situ measurements to correctly evaluate the catalyst surface in electrochemical systems.