Understanding the pH Dependence of the Electrocatalytic Reversible Hydrogen Reaction
Authors:   , Rebollar Tercero Luis , Tang Maureen H. 
Published: November 2022

It is well known that the hydrogen reaction rates on Pt and other electrocatalysts are fast at low pH but 1-2 orders of magnitude slower at high pH. Although the electrochemical hydrogen evolution and oxidation reactions (HER and HOR) are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on reaction kinetics has defied simple explanation for decades. This longstanding puzzle not only limits applied catalyst design, but also exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. The goal of this work is to examine the possible sources of the strong pH dependence in hydrogen electrocatalysis. This dissertation delves in logical progression into the interfacial processes that govern this anomalous pH dependence, beginning with fundamental experimental and theoretical investigations of the chemical and physical processes occurring at the interface, and ending with the incorporation of "double-layer dopants" able to further enhance the kinetics of highly active electrocatalysts in alkaline media. In this work, we have applied electroanalytical techniques to the reversible hydrogen reaction on single crystal Pt surfaces to gain insight into the role of coadsorbed species, the potential of zero charge (Epzc), transition state barrier heights, and "double-layer dopants" in the alkaline hydrogen reaction mechanism. Combining experimental results with microkinetic modeling, we determined that adsorbed hydroxide being an active participant in the alkaline hydrogen reaction mechanism is thermodynamically unfeasible, and therefore the anomalous effect of pH on kinetics must come from changes in kinetic parameters rather than from changes in thermodynamic binding energies. Focused on interfacial water mobility as the basis for such changes in kinetic parameters, we performed kinetic isotope effect (KIE) voltammetry measurements to quantify the importance of solvent mobility on the overall hydrogen reaction kinetics. Large KIE values of up to 3.4 for HOR in KOD on Pt(111) compared to no measurable effects in DClO4 confirmed that interfacial water mobility drives the pH dependence of the reversible hydrogen reaction. With this insight, we further assessed the impact of interfacial electric field strength, as described by the electrode's Epzc, on electrochemical reaction kinetics in the context of hydrogen electrocatalysis. Previous findings from our group demonstrated the use of molecular interfacial additives such as adsorbed caffeine for enhancing the alkaline HER/HOR kinetics, possibly by disrupting the double layer structure and affecting the orientation and dynamics of water. Hence, using caffeinated Pt as a model surface, we correlated reaction kinetics to the electrode's Epzc and to the solvent mobility. The activity of composite Pt surfaces was found to correlate with the proximity of the Epzc, measured by CO displacement, to the equilibrium potential of HER/HOR, but larger KIE values measured on caffeinated Pt at high pH indicate that this is not a causal relationship and Epzc is not a mechanistic descriptor of the alkaline hydrogen reaction kinetics. The use of "double-layer dopants" such as caffeine represents a new approach for designing not just HER/HOR catalysts with increased activity but also experiments that may bring light to interfacial phenomena dictating reaction kinetics. Hence, to uncover the mechanistic origin of the observed enhancement in alkaline HER/HOR kinetics, we have performed single-crystal voltammetry measurements of Pt(111) electrodes covered with surface additives in buffered electrolytes of varying pH. The results show a decrease/increase transition in kinetics around the additive's pKa, suggesting that double-layer dopants reduce transition state barriers for HER/HOR by regulating the interfacial pH. We have also made steps towards determining the relationship between Epzc and interfacial water structure through in-situ XAS measurements on caffeinated Pt and Au electrodes. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and other molecular "double layer dopants".