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.

A micro-fluidic enabled electrochemical device has been developed to investigate electrochemically active materials under reaction conditions using in-situ scanning transmission soft X-ray microscopy (STXM). In-situ STXM measurements at the Cu 2p edge were conducted on electro-deposited Cu catalysts under electrochemical CO2 reduction (CO2R) conditions. The study provides detailed, quantitative results about the changes in the morphology and chemical structure (oxidation state) of the catalyst particles as a function of applied electrode potentials. The initially electrochemically deposited Cu particles contain both Cu(0) and Cu(I). As an increasingly cathodic potential is applied, the Cu(I) species gradually converted to Cu(0) over the potential range of +0.4 to 0 V versus the reversible hydrogen electrode (VRHE). During this process, Cu(I) particles of various sizes are converted to metallic Cu at different reaction rates and at slightly different electrode potentials, indicating a degree of heterogeneity in the electrochemical response of discrete particles. At CO2R relevant potentials, only metallic Cu is observed, and the morphology of the particles is fairly stable within the spatial resolution limits of STXM (~ 40 nm). We also prepared a working electrode with relatively thick Cu-based electro-deposits. The spatially resolved chemical analysis by STXM identified that Cu-oxide species can persist under CO2R conditions, but only when the catalyst particles are electronically isolated from the working electrode and therefore are catalytically irrelevant. Thus, in-situ STXM is presented as a technique to gain advanced morphological and spatially-resolved chemical structure insight into electrochemically active materials, which was used to provide improved understanding regarding Cu electrodes under CO2 reaction conditions.