Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy

Determining the active phase of an electrocatalyst at work is key to understanding its properties. However, the operating morphology of electrocatalysts is challenging to investigate because they can restructure into different motifs under applied potential due to changes in their oxidation state. These transformations will further alter their catalytic properties. Here, we employ a multi-modal approach centered on electrochemical liquid cell transmission electron microscopy (EC-TEM) to study the evolution of cubic Cu ₂ O pre-catalysts during the electrocatalytic nitrate reduction reaction and unveil how redox kinetics determine the working catalyst morphology. We found drastic differences in catalyst restructuring during operation and a strong dependency of its composition on the applied potential and the chemical environment. Moreover, by matching the timescales of morphological changes observed in EC-TEM with time-resolved chemical state information obtained from operando transmission soft X-ray microscopy, hard X-ray absorption spectroscopy and Raman spectroscopy, we reveal that Cu ₂ O can be kinetically stabilized for extended durations under moderately reductive conditions due to the formation of surface hydroxides. Finally, we rationalize how the interaction between the electrolyte and the catalyst influences the ammonia selectivity.