Copper CO2 Reduction Electrocatalysts Studied by In-situ Soft X-ray Spectro-Ptychography

A micro-chip based three-electrode electrochemical reactor enabling controlled, variable electrolyte flow, rapid electrolyte change and applied electrode potentials was used for in-situ soft X-ray spectro-ptychography of Cu particle catalysts under electrochemical CO2 reduction (CO2R) conditions. In comparison to scanning transmission X-ray microscopy (STXM), the spatial resolution was improved by a factor of three through measuring patterns of diffracted photons via spectro-ptychography. We present here a detailed study of how individual cubic Cu particles change morphology and oxidation state as a function of applied potential during CO2R. Quantitative chemical mapping by in-situ spectro-ptychography demonstrated that as-deposited, primarily mixed Cu(I) and Cu(0) particles were completely reduced to pure Cu(0) at an electrode potential of -0.2 VRHE, above the potential at which CO2R commences. At increasingly negative potentials, in the regime of CO2R, these Cu(0) particles underwent morphological changes, losing the initial cubic structure and forming irregular dendritic-like structures. This initial demonstration of in-situ soft X-ray spectro-ptychography sheds insight on the morphological and chemical structural changes of Cu particles in the CO2R regime and paves the way for more detailed in-situ studies of electrochemical materials and processes.