Co-Ion Identity Affects Double Layer Structure and Reactivity Trends in the Hydrogen Evolution Reaction

The formation of an electric double layer is ubiquitous in electrolytes undergoing electrochemical reactions. Currently, models for the double layer rely on the assumptions of dilute electrolyte theory where ions in solution do not interact. Dilute electrolyte theory predicts that solely hydrated counterions, or ions opposing the surface charge, occupy the near surface region and impact reactivity. Since this theory does not account for conditions like high applied potentials and high ion concentrations, understanding the relationship between double layer structure and electrochemical reactivity is critical for opening new domains for electrochemical modulation. Notably, our recent work shows that co-ions, or ions with the same charge as the surface, can also have a definitive influence on reaction rates, particularly under conditions of high applied potential and large bulk ion concentrations. In this talk, I will discuss how to use the hydrogen evolution reaction as a model system to explore co-ion effects in electrocatalytic reactions. In our work, we systematically vary co-ion identity and track hydrogen evolution reactivity as a function of concentration to highlight how the co-ion becomes increasingly relevant in the concentrated regime. In particular, we highlight the unique ability of the tetrafluoroborate co-ion to undergo dynamic ligand exchange between hydroxide and fluoride. This allows tetrafluoroborate to act as a hydroxide sink to stabilize the hydrogen evolution byproduct. Furthermore, I will discuss our use of Raman spectroscopy to describe how co-ions impact interfacial hydrogen bonding networks, which can have a significant impact on aqueous electrochemistry. Our results highlight how co-ions influence double layer structures and have significant implications for tuning other aqueous reactions such as carbon dioxide and nitrate electroreduction.