First-Principles Studies of Quinone Proton-Coupled Electron Transfer Kinetics

Understanding and controlling fundamental redox kinetics in flow batteries requires theoretical and experimental techniques to probe the sensitivity of reaction rates and selectivity to electrolyte chemistry. In protic electrolytes, electron transfer can be coupled to charge-compensating proton transfer processes. Such proton-coupled electron transfer (PCET) reactions may proceed via sequential proton–electron transfer (SPET) or concerted proton–electron transfer (CPET). The thermodynamics and intrinsic kinetics of possible elementary ET, PT, and CPET steps in an overall, multi-step, reaction mechanism influence observed (apparent) electrochemical kinetics. Here, we use density functional theory calculations to compute thermodynamic (reaction free energies, redox potentials) and kinetic parameters (reorganization energies) associated with proton-coupled redox reactions of different quinone derivatives in aqueous and non-aqueous media. Theoretical predictions are used alongside experimental kinetic measurements to elucidate the influence of electrolyte composition on reaction kinetics. These theoretical concepts are extended to the analysis of interfacial PCET at metal–solution interfaces. Periodic DFT is used to derive kinetic parameters for the reduction of imidazole components of non-aqueous concentrated hydrogen-bonded electrolytes. In combination with voltammetry and impedance measurements, the interfacial simulations are used to understand the kinetics of hydrogen evolution at the reductive stability limit of non-aqueous electrolytes. These studies highlight how elucidating fundamental thermodynamic and kinetic parameters can be used to guide understanding and design of redox-active and electrolyte species for tailored electrochemical reactivity.