Mechanistic Studies of Interfacial Proton-Coupled Electron Transfer to Molecularly Defined Surface Sites

Abstract

Nearly all electrocatalytic reactions and all aqueous electrochemical systems involve either the reductive formation or oxidative scission of a surface-hydrogen bond in an interfacial proton-coupled electron transfer (I PCET) reaction. Whether as an intermediate step in electrocatalysis or a stoichiometric charging step in a psuedocapacitor, regardless if involved in product formation or solution degradation, I PCET can occur in any protic electrolyte. Despite I-PCET reactions’ integral role in electrochemical energy storage and value-added chemical synthesis, molecular-scale models for I-PCET mechanisms have historically been lacking. The general heterogeneity and dynamism of electrode surfaces make it difficult to identify relevant surface active sites and therefore nearly impossible to correctly assign changes in reactivity between surface-based or electrolyte-based effects. In contrast to standard heterogeneous surfaces, graphite-conjugated carboxylate (GC COOH) electrodes display stable, isolated, unique, and atomically precise active sites. Investigating I-PCET at GC-COOH electrodes therefore introduces unprecedented clarity into the chemical nature of surface-H bonds and eliminates convolution from differences in electrode structure between electrolyte conditions. This thesis utilizes GC-COOH electrodes to explore how two fundamental electrolyte properties, pH and ionic strength, control I PCET kinetics with an understanding of both properties’ kinetic dependence leading to new mechanistic insights for I PCET reactivity.

Chapter 2 concerns how I-PCET kinetics are controlled by electrolyte pH and how the observed rate dependence informs I-PCET mechanisms. Equilibrium apparent rate constants (kapp) for I-PCET were measured to be fastest at both pH extremes but reach a minimum at pH 10. The lack of pH-independent regions and the asymmetric slopes of the “V”-shaped kapp vs pH dependence observed for I-PCET stand in stark contrast to the established rate-pH dependence and path-dependent mechanism established for outer-sphere proton-coupled electron transfer. Such differences highlight the need for an alternative mechanistic model for I-PCET. With these observations, a donor-identity-dependent model for I-PCET is developed. In this model, I-PCET occurs through one of two proton donor/proton acceptor couples, either a hydronium/water couple predominating at low pH and slowing with increased pH or a water/hydroxide couple predominating at high pH and slowing with decreasing pH. These studies constitute the first molecular-scale mechanistic understanding of elementary I-PCET reactions.

Chapter 3 investigates how high concentrations of proton-neutral supporting electrolytes effect I-PCET kinetics. We measure proton activity with the reversible hydrogen electrode and I PCET kinetics with GC-COOH from 1 mole kg⁻¹ to 17 mole kg⁻¹ NaClO₄ in unbuffered perchloric acid, acetate buffered, and unbuffered sodium hydroxide aqueous electrolytes. While the proton activity of unbuffered acidic conditions increases drastically across this concentration range, that of the buffered and basic electrolytes changes little. Additionally, a significant decrease in I-PCET rates versus the rate expected for the measured proton activity is observed for the acidic and buffered electrolytes but not the basic electrolytes. With these observations we construct a mechanistic model in which I-PCET is not a single step, but a multi-step reaction sequence in which elementary I-PCET is gated by an ion exchange reaction between proton donor/acceptor species and proton-neutral supporting electrolyte at the electrode-electrolyte interface. These findings demonstrate how supporting electrolyte can be leveraged as a design parameter to independently control electrolyte pH and the rates of I-PCET-based reactions.