IN SITU SOFT X-RAY SPECTRO-MICROSCOPIC CHARACTERIZATION OF CATALYSTS FOR ELECTROCHEMICAL CO2 REDUCTION

Abstract

Carbon dioxide electroreduction (CO2R) is a promising and sustainable route to generate valuable feedstocks through the electrochemical conversion from CO2 with electricity generated by renewable energy resources, to reduce greenhouse gas emissions, thereby protecting the global environment. One of the critical challenges for developing practical CO2R developments is understanding the structures and chemistry of CO2R electrocatalysts, and then generating fundamental insights to guide the design and optimization of high-performance electrocatalysts. During my Ph.D. studies, synchrotron-based X-ray spectro-microscopy techniques, scanning transmission X-ray microscopy (STXM) and X-ray spectro-ptychography, were used to study nickel-nitrogen-carbon (Ni-N-C) and electrodeposited Cu-based CO2R electrocatalysts. STXM and ptychography were upgraded to in situ characterizations to provide spectroscopic characterization and quantitative, chemically selective imaging of these catalytic materials under CO2R conditions. To achieve in situ STXM and spectro-ptychography, a micro-fluidic based, liquid-flow electrochemical in situ device was developed, fabricated, and implemented. The in situ device is optimized from previous versions developed by Vinod Prabu, past graduate student of Hitchcock group, and the initial concept was provided by Pablo Ingino and Dr. Martin Obst, collaborators at the University of Bayreuth. In situ STXM and spectro-ptychography provided a detailed chemical and morphological evaluation of catalyst materials at different applied potentials during electrochemical processes. The in situ STXM studies of Cu-based catalysts showed that electrodeposited Cu2O particles are converted to metallic Cu with different reaction rates at applied potentials less negative than that for initiation of CO2R. The in situ STXM results show a degree of heterogeneity in the electrochemical response of discrete nanoparticles and metallic Cu as the active catalyst for CO2 reduction which is structurally relatively stable at CO2R-relevant potentials within the spatial resolution of STXM. In situ spectra-ptychography was used to follow morphological changes of a single Cu-based catalytic particle in the electrochemical regime of CO2R. Our results show that the Cu particle lost the initial cubic structure and formed irregular dendritic-like structures during the CO2R process. To the best of my knowledge, this is the first time in situ STXM has been applied to CO2R electrocatalysts under flow liquid and electrochemical conditions and the first report of in situ spectro-ptychography studies. In summary, my research has successfully achieved the in situ STXM and spectro-ptychography experiments and contributed to an improved understanding of Cu nanoparticle CO2R electrocatalysts.