Manganese Oxide Electrocatalysts: Degradation in Alkaline Energy Conversion Devices

Abstract

Anion exchange membrane (AEM) fuel cells (FC) as well as alkaline electrolyzers (EL) offer an environment in which non-noble metal based electrocatalysts are considered to be stable in a broad potential window.1 Taking into account scarcity of and stability issues with Pt and Ir, used in alternative proton exchange membrane (PEM) environment, AEMFC and alkaline EL can be considered as advantageous technologies. On the other hand, experimental confirmation of long-term stability of non-noble metal catalysts is still to be shown. As for now, studies with a thorough analysis of stability and degradation mechanism of non-noble electrocatalysts are still rare. In this work we scrutinize the stability of Mn-based ORR and OER catalysts. Using inductively coupled plasma mass spectrometry (ICP–MS) connected on-line to an electrochemical scanning flow cell (SFC), we investigate the Mn dissolution from manganese oxide (MnOx) polymorphs MnO2 (α, β, δ) as well as α–Mn2O3 in both ORR and OER potential ranges. In terms of FC application (ORR), composite materials added to the main ORR catalyst are considered as scavengers for reactive oxygen species (ROS), significantly increasing its selectivity towards full O2 reduction to water.2 Herein, we confirm this for the investigated MnOx by rotating ring disk electrode (RRDE) measurements. While ORR activity is only marginally affected, hydrogen peroxide reduction reaction (HPRR) is facilitated when MnOx is added to an Fe-N-C catalyst. At the same time, however, we show that ROS are not only harmful to membrane materials3 and carbon support4 but also to the MnOx catalyst itself. All MnOx catalysts exhibit increased dissolution in common accelerated stress test (AST) potential ranges, if the electrolyte is purged with oxygen as compared to the commonly accepted AST during inert gas purging. Control experiments with H2O2 intentionally added to electrolyte confirm the detrimental impact of ROS on MnOx stability. In the context of EL application, we employ a similar approach, using a recently developed metric to evaluate OER stability5, to correlate MnOx stability to their activity as well as physical properties. As an outcome of this study we show that the thermodynamic limits of stability in MnOx have to be accounted for OER and ORR potential ranges individually. Further, we need to call specific attention to minimizing the H2O2 yield of Mn based ORR catalysts since it leads to previously unquantified, increased degradation of the catalyst independent of its thermodynamic limitations. Finally, our work shows that the use of MnOx as ROC scavengers and as bifunctional catalysts needs a critical revision. References M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (1974).Anna K. Mechler, Nastaran Ranjbar Sahraie, Vanessa Armel, Andrea Zitolo, Moulay Tahar Sougrati, Jan N. Schwämmlein, Deborah J. Jones and F. Jaouen, J. Electrochem. Soc., 166, 1084 (2018).N. Ramaswamy, N. Hakim and S. Mukerjee, Electrochimica Acta, 53, 3279 (2008).P. Trogadas, T. F. Fuller and P. Strasser, Carbon, 75, 5 (2014).S. Geiger, O. Kasian, M. Ledendecker, E. Pizzutilo, A. M. Mingers, W. T. Fu, O. Diaz-Morales, Z. Li, T. Oellers, L. Fruchter, A. Ludwig, K. J. J. Mayrhofer, M. T. M. Koper and S. Cherevko, Nature Catalysis, 1, 508 (2018). Acknowledgements: The project CREATE leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 721065.