Wavelength-Resolving Catalytic Turnover Frequencies and Identifying Alternate Proton Donors in Solar-Fuel-Forming Reactions

In electrocatalysis, catalysts minimize the energy needed to drive redox half-reactions at electrode surfaces by lowering the overpotential requirement for achieving a desired reaction rate. Two benchmark parameters have emerged for comparing the performance of molecular electrocatalysts. These are the maximum turnover frequency─equivalent to the rate constant for chemical catalysis, kcat─and the effective overpotential─which is the potential required to activate half the population of catalysts at an electrode surface. Herein, we extend these concepts and related analysis techniques to photoelectrosynthetic reactions, where, in addition to electrons and chemical substrates, photons are required as reagents. Our results show how approaches relevant to benchmarking electrocatalysts can result in erroneous metrics for catalytic components operating in light-activated assemblies if the observed maximum reaction rates are limited by fluxes of photons rather than inherent catalytic properties. This work utilizes a model assembly for hydrogen evolution and highlights the complexity of applied electrochemical bias potentials, pH effects, and illumination intensities, including how, at relatively low overpotentials, buffering species can outcompete water as a source of protons/chemical substrates. We also introduce a general yet useful formalism relating illumination intensities to photoelectrosynthetic turnover frequencies, enabling the generation of what we term wavelength-resolved TOF plots and photoelectrosynthetic Tafel-like plots from experimental data.