UC Berkeley

The electrochemical CO2 reduction reaction (CO2RR) represents a compelling strategy for closing the anthropogenic carbon cycle, enabling the conversion of CO2 into value-added fuels and chemicals using renewably sourced electricity. Realizing this potential demands a deep mechanistic understanding of the catalytic processes occurring at the electrochemical interface, an interface that is far from static. Electrocatalysts undergo complex structural and chemical transformations under reaction conditions, and the local environment surrounding the catalyst surface plays an equally decisive role in governing selectivity and activity. Advancing CO2RR therefore requires characterization tools capable of probing catalysts not before or after, but during operation. In this thesis, I present a series of operando and in situ characterization studies spanning X-ray spectroscopy and electron microscopy, which together build a more complete picture of CO2RR catalysis: from the dynamic evolution of active sites and the molecular driving forces behind it, to their degradation pathways, to the chemical environment at the catalytic solid-electrolyte interface during turnover.In Chapter 1, I introduce the electrochemical CO2 reduction reaction and its relevance to climate change remediation, followed by a comprehensive overview of the operando characterization techniques that form the methodological backbone of this thesis, including X-ray-based methods and transmission electron microscopy.In Chapter 2, I demonstrate that operando high energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) can resolve surface-specific chemistry in CO2RR with sub-eV energy resolution. Applied to 5 nm copper (Cu) nanoparticles, the high energy resolution proves critical for capturing ligand desorption from the Cu surface under electrochemical bias, a signal inaccessible to conventional fluorescence-yield XAS. Extended X-ray absorption fine structure (EXAFS) tracks the continuous evolution of oxidation state and coordination number during operation, pointing to undercoordinated metallic copper nanograins as the true active state reached after approximately one hour of electrolysis.Chapter 3 investigates the fate of these Cu nanograin active sites after electrolysis is complete. Using in situ 4-dimensional scanning transmission electron microscopy (4D-STEM) and electron tomography, I show that upon removal of applied bias, nanograins spontaneously oxidize and 2 reconstruct into hollow single-crystalline Cu2O nanocubes via a dissolution-redeposition mechanism governed by crystalline domain size, with water serving as the main structure-directing agent. This process is sensitive to pH: slightly elevated pH promotes a passivating Cu2+shell while preserving the metallic core, revealing a mechanistically distinct deactivation pathway and suggesting concrete strategies for improving catalyst durability. Chapter 4 broadens the investigation of Cu catalyst dynamics by examining the molecular driving forces behind structural reconstruction across two complementary model systems. Operando electrochemical liquid-cell (4D-)STEM captures the transformation of shape-controlled Cu nanocubes into polycrystalline metallic nanograins, while HERFD-XAS and EXAFS of heterogenized CuPc provide quantitative evidence for CO-driven ejection of single Cu atoms and their progressive aggregation into metallic clusters. Together, these systems converge on a mechanism in which CO induces the formation of mobile copper carbonyl species, supporting CO-driven Cu migration as a broadly relevant phenomenon in Cu-based CO2RR catalysis. Chapter 5 shifts focus from catalyst structure to active site chemistry, introducing valence-to-core X-ray emission spectroscopy (VtC-XES) as a ligand-sensitive probe of intermediate-active site interactions. Using heterogenized cobalt phthalocyanine as a model system and combining VtC-XES with HERFD-XAS and density functional theory (DFT), I resolve the sequential adsorption of *COO(H) and *CO intermediates and identify a mixed intermediate state at the onset potential for methanol production. This demonstrates that transient reaction intermediates can be directly detected under operando conditions by X-ray spectroscopy.Finally, in Chapter 6 I investigate the role of the electrochemical double layer in CO2RR using surface-sensitive total electron yield (TEY)-XAS, supported by DFT. I show that desolvated Cs⁺ ions, confined within a ligand-modified silver nanocatalyst interface, interact with surface-bound intermediates through partial covalent character, reducing the overpotential for CO2-to-CO conversion and demonstrating that cation effects operate through direct electronic interactions rather than purely electrostatic stabilization, with implications for catalyst and electrolyte design across a wide range of electrochemical systems.Taken together, this thesis demonstrates that operando and in situ characterization are not merely complementary to conventional catalyst studies, but are indispensable for capturing the true structure, chemistry, and dynamics of CO2RR electrocatalysts under working conditions, and offers new mechanistic insights for the rational design of more efficient, selective, and stable systems.