Multimodal Operando X-ray Mechanistic Studies of a Bimetallic Oxide Electrocatalyst in Alkaline Media

TL;DR

This study employs multimodal operando X-ray techniques to investigate structural and oxidation state changes in a Co-Mn spinel oxide electrocatalyst during the oxygen reduction reaction, revealing insights into stability and degradation mechanisms.

Contribution

It introduces a multimodal operando X-ray approach to simultaneously study structure and oxidation states in electrocatalysts during operation, providing new understanding of their stability and degradation.

Findings

Reversible tensile strain increase at low potentials indicating structural stability

Irreversible phase transition from tetragonal to cubic at low potential holds

Strain and oxidation state changes linked to catalyst degradation

Abstract

Furthering the understanding of the catalytic mechanisms in the oxygen reduction reaction (ORR) is critical to advancing and enabling fuel cell technology. In this work, we use multimodal operando synchrotron X-ray diffraction (XRD) and resonant elastic X-ray scattering (REXS) to investigate the interplay between the structure and oxidation state of a Co-Mn spinel oxide electrocatalyst, which has previously shown ORR activity that rivals Pt in alkaline fuel cells. During cyclic voltammetry, the electrocatalyst exhibited a reversible and rapid increase in tensile strain at low potentials, suggesting robust structural reversibility and stability of Co-Mn oxide electrocatalysts during normal fuel cell operating conditions. At low potential holds, exploring the limit of structural stability, an irreversible tetragonal-to-cubic phase transition was observed, which may be correlated to reduction in both Co and Mn valence states. Meanwhile, joint density-functional theory (JDFT) calculations provide insight into how reactive adsorbates induce strain in spinel oxide nanoparticles. Through this work, strain and oxidation state changes that are possible sources of degradation during the ORR in Co-Mn oxide electrocatalysts are uncovered, and the unique capabilities of combining structural and chemical characterization of electrocatalysts in multimodal operando X-ray studies are demonstrated.