Choice of Electrolyte Impacts the Selectivity of Proton-Coupled Electrochemical Reactions on Hydrogen Titanate

Abstract

Proton-coupled electron transfer (PCET) reactions involving transition metal oxides are prevalent in aqueous electrochemical systems used for energy storage and conversion. Here, we elucidate the role of electrolyte on PCET mechanisms in transition metal oxides in aqueous acidic electrolytes using layered hydrogen titanate (H2Ti3O7) as an example. We identify three processes by which electrolyte protons interact with hydrogen titanate at the electrochemical interface: (1) adsorption at the surface and/or insertion into the bulk, (2) adsorption as part of the hydrogen evolution reaction (HER) at the surface, and (3) dissolution of the hydrogen titanate. We utilize a combined experimental and computational (ReaxFF) approach to probe how the competition for protons and electrons among these processes influences electrochemical properties, including the energy storage, Coulombic efficiency (CE), rate capability, and lifetime. In an acidic buffered electrolyte (1 M H3PO4), the CE increases from an average of 48% to 71% and the specific capacity increases from 83 to 90 mAh g–1 as compared to a strong acid electrolyte (1 M H2SO4). We propose that H3PO4 mitigates the HER and hydrogen titanate dissolution, thereby increasing the operating potential window for proton adsorption/insertion for charge storage in hydrogen titanate. Material characterization and computational results indicate that adsorption of phosphate species onto the surface of hydrogen titanate may decrease its dissolution upon reduction, thereby improving electrode performance. We offer a preliminary solution to improve energy storage performance via electrolyte tuning by decreasing the prevalence of the HER and electrode dissolution.