(308f) Uncovering the Dynamic Nature of a Ag-MnOx Oxygen Reduction Catalyst Using Operando X-Ray Absorption Near-Edge Spectroscopy

As the consequences of climate change become increasingly apparent, the global drive toward the generation and use of sustainable fuels is accelerating. For the widespread generation of sustainable electricity using technologies such as hydrogen fuel cells, cheap and abundant catalysts for the kinetically slow oxygen reduction reaction (ORR) need to be developed and optimized.¹ To uncover the catalyst surface dynamics during operation to generate a platform to design new stable catalysts, in-situ/operando experiments are needed.

Herein, using operando/in-situ X-ray absorption near-edge spectroscopy (XANES) the Mn valance changes of a promising, recently demonstrated ORR catalyst, an ultra-thin, porous MnO_x layer deposited on top of a Ag thin film (MnO_x@Ag),² are tracked depending on the electrochemical microenvironment and ORR conditions. Overall, we found that the MnO_x surface consists of a mixture of Mn₂O₃/Mn₃O₄ and MnO₂ in the voltage window between 0.8 and 1.2 V_RHE. In both O­₂- and N₂-saturated electrolyte the MnO_x reduces with decreasing potential. Counterintuitively, the Mn oxidation state is substantially more reduced under ORR conditions at 0.8 V_RHE than at the same potential in a N₂-saturated electrolyte. Furthermore, the MnO_x redox is found to be reversible in N₂-saturated electrolyte, while in a O₂-saturated environment the Mn valance depends on the catalyst pre-conditioning and catalysis rate. With ex-situ atomic force microscopy and X-ray photoelectron spectroscopy we hypothesize that the non-reversibility in the O₂-saturated electrolyte could be traced back to differences in the catalyst surface morphology and the Ag valance. By uncovering the dynamic nature of our MnO_x@Ag catalyst during electrochemical measurements we demonstrate the importance of in-situ/operando studies on catalyst properties, offering new directions for catalyst pre-conditioning pathways or in-situ stabilization.

(1) Md. M. Hossen, et al., Appl Catal B, 2023, 325, 121733. (2) J. A. Zamora Zeledón, et al., Energy Environ Sci, 2022, 15, 1611-1629.