Understanding the Active Surface of a MnO_x@Ag Fuel Cell Catalyst Using Operando X-Ray Absorption Near-Edge Spectroscopy

The global drive toward the sustainable generation of fuels and the use of sustainable fuels is accelerating as the challenges posed by climate change become increasingly apparent. Hydrogen fuel cells or metal-air batteries using renewable electricity represent uniquely promising technology areas for this purpose.¹ The optimization of the kinetically hindered oxygen reduction reaction (ORR) is thereby one of the key components for a widespread use of such renewable energy conversion and storage technologies. Current state-of-the-art catalysts for hydrogen fuel cells are composed of expensive platinum. However, to reduce the material costs and increase the sustainability of the catalyst material, non-Pt group metals and non-critical raw material are of high interest. Due to stability limitations, non-Pt group metals are currently only employed in alkaline media. Bimetallic silver (Ag) catalysts are very promising non-critical raw materials for the ORR.<br/>In our recent work Zamora Zeledón <i>et al.</i>² presented ultra-thin Mn films (maximum 1 nm) deposited on top of Ag bulk material prepared by physical vapor deposition that are substantially more active under ORR conditions at 0.8 V_RHE than single metal Ag and Mn thin films. Herein, we used operando X-ray absorption near-edge spectroscopy (XANES) to understand the surface dynamics of the highly active MnAg thin films. Applying a potential of 1.2 V_RHE, above the onset of electrochemical oxygen reduction, the pre-edge features in the Mn K-edge indicate a mixture of MnO₂ and Mn₂O₃ in oxygen and nitrogen saturated electrolyte. At 0.8 V_RHE, a potential within the ORR window, the Mn is more reduced. Notably, shifts in both the main edge and distinct pre-edge features indicate that at 0.8 V_RHE the Mn is more reduced during ORR, namely, in oxygen vs. nitrogen saturated electrolyte. In terms of reversibility, consecutively applying 1.2, 0.8, and again 1.2 V_RHE revealed a non-reversible oxidation state change in oxygen in contrast to nitrogen saturated electrolyte. This could be explained by stability changes and a possible MnAg reconstruction. By coupling operando X-ray absoprtion spectroscopy (XAS) and electrocatalysis we are able to correlate activity changes to distinct changes in oxidation state. Those insights gained from operando XAS offer new directions for in situ stabilization or enhancement of electrocatalytic materials.<br/><br/>References:<br/>(1) M. Li, X. Bi, R. Wang, Y. Li, G. Jiang, L. Li, C. Zhong, Z. Chen, J. Lu, <i>Matter</i>, <b>2020</b>, <i>2</i>, 32–49.<br/>(2) J. A. Zamora Zeledón, G. T. K. Kalhara Gunasooriya, G. Ashish Kamat, M. E. Kreider, M. Ben-Naim, M. A. Hubert, J. E. Avilés Acosta, J. K. Nørskov, M. Burke Stevens, T. F. Jaramillo, <i>Energy Environ Sci</i> <b>2022</b>, <i>15</i>, 1611–1629.