(Invited) X-Ray Spectroscopic View on Transition Metal-Based Electrocatalysts Structure

Abstract

Significant progress has been made in replacing noble metal catalysts for the oxygen evolution and reduction reactions with materials based on transition metal (TM) compounds such as oxides1, perovskites2, molecular nitrogen-carbon catalysts, etc.3 However, the design of stable and active TM-based catalysts is jeopardized by the lack of understanding of the active sites structure, the reaction mechanism as well as the degradation pathway of these materials.4,5 In this perspective, soft and hard X-ray-based spectroscopies are used as state-of-the art methods for the analysis of the catalyst structure, namely, oxidation state of its components, their electronic structure, coordination number, metal – ligand bond distance, etc. Due to the constant instrumentation development, both soft and hard X-rays have been successfully applied for in situ and operando spectroscopic studies of electrocatalysts providing new insights on the behavior of TM-based materials.6–8 This talk will be focused on the application of X-ray Photoelectron, Absorption and Emission Spectroscopies (XPS, XAS and XES, respectively) towards the investigation of TM-based electrocatalysts. Some examples will be discussed illustrating the possibilities of each of these methods both ex and in situ to analyze TM-based electrocatalysts (where TM = Mn, Co, Fe) used in fuel cells and electrolyzers. The talk will also cover possible sources of error arising during data acquisition and subsequent treatment. Additionally, there will be a discussion of the pros and cons of the various designs of spectroelectrochemical cells currently used for in situ studies of fuel cells and electrolyzers catalysts.References Fabbri, E., Habereder, A., Waltar, K., Kötz, R. & Schmidt, T. J. Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 4, 3800–3821 (2014).Grimaud, A. et al. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 4, 2439–2446 (2013).Banham, D. & Ye, S. Current Status and Future Development of Catalyst Materials and Catalyst Layers for Proton Exchange Membrane Fuel Cells: An Industrial Perspective. ACS Energy Lett. 2, 629–638 (2017).Hu, C., Zhang, L. & Gong, J. Recent progress made in the mechanism comprehension and design of electrocatalysts for alkaline water splitting. Energy Environ. Sci. 12, 2620–2645 (2019).Hong, W. T. et al. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 8, 1404–1427 (2015).Saveleva, A. V. A. et al. Potential-induced spin changes in Fe/N/C electrocatalysts assessed by in situ X-ray emission spectroscopy. Angew. Chemie Int. Ed. 133, 1–7 (2021).Abbott, D. F. et al. Operando X-ray absorption investigations into the role of Fe in the electrochemical stability and oxygen evolution activity of Ni 1−x Fe x O y nanoparticles. J. Mater. Chem. A 6, 24534–24549 (2018).Zitolo, A. et al. Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nat. Commun. 8, 957–968 (2017).