(27f) Importance of Proton Sources during Proton-Coupled Electron Transfer in Cathodic Reduction Reactions

The development of electrocatalytic reactors that operate in water or protonated solvents and the discovery of the accompanying family of catalytic systems that will support these technologies call for a profound understanding of the processes occurring at multiple scales and emphasizes the importance of broadening catalyst design principles beyond the transition state theory to include fundamentals of mass and charge transport. Cathodic reduction reactions involve the transfer of protons and electrons at the electrode/electrolyte interface. However, not all molecular sources of protons are equal in the electrode double layer where the reductive reactions take place. The barrier for the extraction of a proton from a hydronium ion, for example, is lower than that for the dissociation and extraction of a proton from molecular water on an absolute potential scale. The addition of buffer ions to these systems adds one layer of complexity as buffers start to regulate proton concentration near the surface of the electrode when the local pH approaches the buffer's pKa. This talk discusses the relation between the absolute potential at which a buffer becomes the proton source and the buffer's pka. It is shown experimentally that buffers with a lower pka become the source of protons for the hydrogen evolution reaction at less negative applied potentials compared to buffers with a higher pka. In this work, theory and experiments are combined to gain fundamental understanding in various cathodic reduction reactions through the systematic brake-down of the kinetics for the multiple electrochemical and thermochemical steps involved in the cathodic reduction of dissolved species (i.e. CO₂, O₂, H₂O₂) over heterogeneous electrocatalysts. This talk also discusses our efforts in decoupling mass transfer effects from intrinsic electrode kinetics.