Watching copper catalysts evolve: Temporally resolved in situ scanning transmission X-ray microscopy during electrochemical CO₂ reduction

Understanding the time-dependent evolution of the chemical state, structure, and interfacial properties of an electrocatalyst during continuous operation is essential for identifying performance descriptors and degradation mechanisms. However, temporally resolved in situ investigations of electrochemically active materials under realistic operating conditions remain challenging. Here, we use in situ scanning transmission X-ray microscopy (STXM) to perform real-time, spatially resolved visualization of chemical and morphological evolution of Cu catalysts under CO 2 reduction (CO 2 R) conditions with continuous electrochemical control. By synchronizing high-speed STXM image acquisition with applied electrochemistry, redox processes which are inaccessible to conventional in situ STXM under static conditions, can be studied. During linear sweep and cyclic voltammetry measurements, the interconversion among Cu, Cu₂O, and CuO is directly visualized under continuously changing electrode potentials. While bulk-averaged analysis shows overall reduction of Cu oxides to metallic Cu at CO₂R-relevant potentials, spatially resolved measurements reveal pronounced regional heterogeneity in reduction kinetics, which correlates with the local electrical conductivity of different Cu phases. Under potential cycling conditions, Cu dissolution and redeposition continuously modifies the morphology and location of catalyst particles on the electrode surface. These results provide additional insight into the dynamic behavior of Cu CO₂R catalysts under changing electrode potential conditions. They highlight the power of continuous acquisition during in situ STXM for capturing dynamic and heterogeneous processes occurring at catalyst surfaces and interfaces during electrochemical reactions.