In-situ Studies of Copper-based CO2 Reduction Electrocatalysts by Scanning Transmission Soft X-ray Microscopy

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.