In Situ Spectroscopic Studies of Electrolyte and Organic Films Effects on Electrochemical CO₂ Reduction

This thesis employs in-situ spectroscopy and electrochemical techniques to elucidate how electrolyte composition and organic film modification control selectivity in electrochemical CO₂ reduction on copper electrodes. The central objective is to establish molecular-level understanding of interfacial phenomena that govern product distributions, with particular focus on disentangling electric field effects from non-electric field contributions of electrolyte cations and on clarifying the role(s) of organic surface coatings. The first chapter motivates the development of selective CO₂ reduction technologies and introduces the unique challenges of controlling reaction pathways at copper electrochemical interfaces. Theoretical foundations used in this dissertation are established in the second chapter. The third chapter demonstrates that a local alkaline pH, instead of the presence of cations, is required for CO₂ reduction to occur in acidic media. Spectroscopic measurements reveal that enhanced CO₂ reduction performance with the addition of cations arises from suppressed H⁺ transport and modified interfacial solvation, motivating the use of transport-modulating organic films. Using organic film-modified copper then yields selective CO₂ reduction over hydrogen evolution with low cation concentrations even in strongly acidic (pH 1) electrolyte. The fourth chapter investigates the interplay between electrolyte cations and organic coatings to show that organic films modify transport of reactants and intermediates, amplifying cation effects. Mixed lithium-cesium electrolytes are used, which reveal that organic films reduce the cesium concentration required for selective catalysis not by altering cation accumulation, but by modifying how accumulated cations influence reaction chemistry through changes in water structure and interfacial CO₂ and CO populations. The final chapter introduces composite film design to show that film functionality can be tuned via doping provided there is structural control. Amine functionalization is used, which concentrates CO₂ at the interface through hydrogen bonding interactions. Composite films with small amine concentrations achieve the highest ethylene selectivity across acidic, neutral, and alkaline electrolytes, demonstrating how tuning of organic coatings via precursor mixing can optimize interfacial properties.