Affiliation: College of Arts and Sciences, Department of Chemistry
The multi-decade proliferation of electrochemical hydrogen evolution catalysts has resulted in a relatively small handful of excellent catalysts. Thus, research has turned towards understanding catalytic mechanism in the hope of rationally guiding the next generation of catalysts. Specifically, recent focused effort has sought understanding of how homogeneous catalysts mediate the combination of two electrons from the electrode and two protons from solution-based sources. The individual proton and electron movements are frequently coupled such that the movement of one triggers the movement of the other, sometimes resulting in simultaneous transfer. Intentional catalyst designs that favor stronger coupling have shown impressive catalytic rates. This realization has promoted increased scrutiny of proton-coupled electron transfer (PCET) reactivity. PCET reactions have both kinetic and thermodynamic components; this dissertation focuses on thermodynamic aspects of electrochemical PCET through the development of relationships between applied potential and non-aqueous acidity. Non-aqueous potential-pKa theory is demonstrated through the construction of two experimental diagrams. A third candidate example of a potential-pKa diagram is discussed in the context of the challenges and opportunities of gaining thermodynamic information from irreversible electrochemical data. Two challenges were encountered during this research. First, direct reduction of acids by the electrode can obscure the desired PCET reactivity. Electrochemical analysis of over twenty acids in acetonitrile yielded a dataset of direct acid reduction potentials, information that was used to guide later PCET studies. This work additionally summarized unique considerations associated with acid-base behavior in non-aqueous solvents. Second, metal complexes used for PCET and hydrogen evolution studies can degrade at the electrode. Successful identification of when decomposition/transformation occurs allows accurate interpretation of electrochemical data and provides a guide for selecting metal complexes likely to be more robust. This guidance, coupled with better understanding of PCET mechanism, will help enable economical and efficient catalysts for solar fuel production.