(Invited Plenary) Oxygen Electrocatalysis over Transition Metal Oxides: Combining Electrochemistry with Modeling and in Situ Spectroscopy

Abstract

Transition metal oxides (TMOs) are attracting an increasing attention as promising materials for oxygen electrode in polymer electrolyte fuel cells (PEMFCs) and electrolyzers (PEMEC). Despite significant number of publications oxygen electrocatalysis over TMOs is still insufficiently understood. Pending questions relate to the reaction mechanisms, the state of the surface during the oxygen reduction (ORR) and oxygen evolution reaction (OER), structure-activity relationships, and degradation phenomena which are particularly detrimental during the OER. This slows down development of potent and durable materials for the oxygen electrode of PEMFCs and PEMECs. This presentation will consist of two parts. The first deals with the ORR on noble-metal free single and complex Mn, Co and other TMOs in view of their application for PEMFCs based on anion-exchange membranes. In this work we seek to understand the relations between the structure and the composition of TMOs oxides on the one hand, and the kinetics and the mechanism of the electrocatalytic ORR on the other hand. An experimental rotating ring disc electrode investigation of the ORR is complemented with the rotating disc electrode study of the oxidation/reduction of the stable ORR intermediate, hydrogen peroxide, and combined with microkinetic modeling in order to arrive at a self-consistent model of the ORR on TMOs oxides. For Mn oxides we conclude that the potential of the Mn(III/IV) red-ox transition of surface cations can be used as a descriptor of the catalytic activity of Mn oxides in the ORR. We further note that the reaction mechanism and the selectivity for the 4e ORR depend on the structure and composition of Mn oxides. Finally, we propose a tentative explanation for the discovered relationship between the catalytic activity and the crystalline structure. The second part of the presentation is related to the OER on mixed IrxRu1-xO2 oxide in acid medium and on noble-metal free TMOs in alkaline media. We apply synchrotron-radiation-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) for studying OER on IrxRu1-xO2 anodes of PEM electrolyzers with the objective to shed light on the long time known but still insufficiently understood stabilization effect of Ir. We conclude that various forms of Ru coexist in the anode during the OER, and their relative contributions strongly depend on the electrode potential and on the presence of Ir. Based on the in situ spectroscopic data we propose a tentative mechanism of the OER on mixed Ir(Ru) anodes and the stabilization effect of Ir. Finally, we will discuss stability of various oxide materials in alkaline and in acid media during the OER. Acknowledgements The author is indebted to Anna S. Ryabova, Ivan S. Filimonenkov, Galina A. Tsirlina, Sergey Y. Istomin, Evgeny V. Antipov, Filipp S. Napolskiy, Artem M. Abakumov, Tiphaine Poux, Antoine Bonnefont, Gwenaelle Kerangueven, Spyridon Zafeiratos, Viktoriia A. Saveleva, Li Wang, Aldo S. Gago, K. Andreas Friedrich, M. Haevecker and A. Knop-Gericke for their contribution to the work. Financial support received in the framework of ERA.NET.RUS.PLUS (project ID # 270 – NANO-MORF) and European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 621237 (INSIDE) is gratefully appreciated.