The Role of Surface Coverage of Reaction Intermediates in Heterogeneous Electrocatalysis

Abstract

The population of reaction intermediates is intimately related to the kinetic landscape and the overall rate of any multi-step reaction. For efficient interconversion of chemical and electrical energy, it is critical to understand the role of catalyst surface coverage and how it relates to two important mechanistic parameters, reaction orders and Tafel slopes. On heterogeneous catalysts, lateral interactions between neighboring intermediates can lead to non-linear correlations between reaction conditions and the overall reaction rate, and complicate the mechanistic analysis. To avoid this mathematical challenge, an empty or a fully saturated surface coverage of intermediates is often assumed in the field of heterogeneous electrocatalysis, but this idealistic assumption can lead to predictions far from empirical observations. Herein, we show that fractional coverage of reaction intermediates is key to understanding kinetic behaviors of energy conversion reactions and developing design principles for optimal catalysts.

In Chapter 2, we report activation-controlled hydrogen evolution reaction (HER) data from pH 1 to 12 on Au and Pt catalysts and find that reaction orders in hydronium are between 0 and 1 across the pH range. Observed reaction orders and Tafel slopes that depart from the customary values are rationalized with a fractional metal–H coverage model. HER kinetics are coverage-dependent, and we propose that the sluggish proton-coupled electron transfer kinetics from H2O vs H3O+ is the reason for HER efficiency loss in alkaline electrolytes.

In Chapter 3, we build upon the fractional coverage model in Chapter 2 to show that anything in the electrolyte, such as buffering species and supporting electrolyte ions, can adsorb on the surface and influence interfacial HER kinetics. The reaction order in sodium phosphate buffer in pH 7 is 0.6 across more than an order of magnitude in phosphate buffer concentration. Our data suggest that a fractional coverage of adsorbed dihydrogenphosphate manifests in non-integer reaction orders.

Chapter 4 explores proton donor control as a complementary strategy to tune the reaction rate of the HER. As a model system, we study the donor-dependent selectivity of CO2 reduction catalysis on Au for which the HER is a parasitic reaction. We identify acetate buffer in dimethylacetamide as a stable nonaqueous medium where we can explicitly tune the proton donor identity and activity. By decreasing the proton activity in the electrolyte, we dramatically suppress HER to improve the selectivity of CO vs H2 production from 20 to 80%.