Organic Films at the Electrode-Electrolyte Interface in CO₂ Reduction

Citation

Watkins, Nicholas B. (2024) Organic Films at the Electrode-Electrolyte Interface in CO₂ Reduction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7t8e-7j20. https://resolver.caltech.edu/CaltechTHESIS:09272023-154324412

Abstract

This thesis focuses on the use of use high-throughput experimentation and analytical electrochemistry techniques to understand how organic films on (photo)electrode surfaces alter catalyst selectivity. Specifically, the objective has been to deconvolute effects associated with the organic film from the atomic identity of the catalyst, reactant and intermediate concentration polarization effects, and temperature in the context of electrochemical CO₂ reduction. The first chapter provides the motivations behind the transformation of CO₂ into value-added materials using electricity and the challenges that the field faces. The second chapter introduces the data-driven identification of a scaling relationship between the partial current densities of methane and C₂₊ products among 14 bulk copper bimetallic alloys. This strict dependence represents an intrinsic limitation of selectivity tuning through alloying. However, it can be disrupted to favor C₂₊ products by the presence of an organic additive, highlighting the potential of hybrid organic–inorganic catalysts to tune branching ratios in the CO₂R reaction network. The third chapter highlights that with the wide band gap CuGa₃Se₅ chalcopyrite absorber, organic coatings can not only provide dramatic increases in selectivity toward CO₂R products compared to the unmodified system, but also and significantly moderate catalyst corrosion. The fourth chapter unveils a new class of molecular films on polycrystalline copper, derived from aryl diazonium and iodonium salts, that are corrosion resistant even at pH 1 and have the potential for many future electrochemical applications. In the fifth chapter, we demonstrate that increased mass transport at the electrode surface directly resulted in changes to the ethylene and methane Tafel slope values on copper electrodes. These findings emphasize that the apparent Tafel slope reported for any copper system is not necessarily representative of the catalyst’s intrinsic kinetics alone, but also contains information about the cell geometry and electrolyte convective transport. The final chapter investigates the combined effect of organic films, mass transport, and electrode heating on electrocatalysis. We find that we can use surface heating to replace bulk heating, but that the complexity of CO₂R prevents predictable behavior. However, the addition of additive films to the electrode surface enables idealized electrochemical CO₂ reduction kinetics, and therefore the calculation of important parameters such as the activation energy for C₂₊ product formation.

Thesis Files