Understanding the effects of the microenvironment on the performance of electrocatalysts has been an area of interest for optimizing electrochemical energy conversion processes. Proton exchange membrane fuel cells (PEMFCs), which operate in acidic conditions, employ the oxygen reduction reaction (ORR) at the cathode with conventional catalyst materials being platinum (Pt)-based. Anions present in the electrolyte can influence ORR activity and selectivity via surface electric field effects or competitive adsorption with oxygen intermediates. Most research efforts targeting higher activity and 4e– selectivity on non-Pt materials for the ORR engineer the solid catalyst phase with techniques such as alloying or nanostructuring. However, the electrolyte phase can alter ORR performance considerably through changes to the surface-electrolyte microenvironment. While some anion effects in acidic media have been studied using Pt electrocatalysts, these effects on other ORR catalysts are not widely reported. In our work, we systematically assess ORR activity and selectivity on weak-binding silver (Ag) and strong-binding palladium (Pd) thin films by utilizing cyclic voltammetry on a rotating (ring) disk electrode in several pH 1 electrolytes (HClO4, H2SO4, H3PO4, HNO3, HCl, HBr). First, we observe higher 4e– ORR selectivity in nitric acid compared to in perchloric acid. Additionally, we note a delayed onset potential in the hydrogen halide acids compared to in the oxyacids to different extents on both metals. Both trends, among others, indicate performance dependence on the anions present in the electrolyte phase. Post-ORR physical characterization of the electrocatalyst with x-ray photoelectron spectroscopy, atomic force microscopy, and inductively coupled plasma optical emission spectroscopy indicates that the surface remained metallic, experienced negligible roughening, and underwent minimal dissolution, respectively. We also combine experimental results with physics-based modeling and exploratory data science techniques to evaluate anion adsorption binding energies and anion property trends with ORR performance, providing additional insight into the fundamental reasons causing relative differences in anion effects between the two metals. Our work enhances understanding of the ORR from the electrolyte perspective and elucidates some of the effects of acid electrolyte anion identity on a dynamic electrochemical surface microenvironment.

Role of acid electrolyte anions in the oxygen reduction reaction double layer microenvironment

ACS Fall 2021

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