Phosphoric Acid Tolerant Oxygen Reduction Reaction Catalysts for High-Temperature Polymer Electrolyte Fuel Cells

Abstract

The limitations of low-temperature polymer electrolyte fuel cells (LT-PEFCs) can be addressed by elevating the operating temperature above 100 °C. High-temperature (HT-) PEFC systems benefit from a simplified heat and water management, a higher tolerance toward fuel impurities such as carbon monoxide and faster oxygen reduction reaction (ORR) kinetics [1,2]. Operating temperatures higher than 100 °C involve the usage of other electrolytes than conventional perfluorosulfonic acid membranes (e.g. Nafion) due to the absence of liquid water in the system and the resulting loss of proton conductivity in the membrane. For PEFC operation in the range of 150-200 °C, humidification-independent phosphoric acid doped polybenzimidazole (PBI) is typically used as membrane material [1,2]. However, the usage of phosphoric acid determines also the activity of the catalysts due to the strong adsorption of phosphate species on the platinum surface and the corresponding reduced number of available active sites. In order to overcome the significant deactivation of Pt, the employed catalyst system needs to be modified. It was shown that combining Pt with Au via electroplating results in increased ORR activity also in the presence of phosphoric acid [3]. We present a facile and straight-forward synthesis of Pt-Au nanoparticles supported on high surface area carbon with varying metal stoichiometry. The as-prepared Pt-Au/C catalysts are further benchmarked to a standard Pt/C catalyst for usage as cathode catalysts in HT-PEFC systems. The ORR activity and stability of the catalyst systems are evaluated ex situ by means of cyclic voltammetry, Levich-Analysis and accelerated stress tests using a rotating disk electrode (RDE) setup. Furthermore, the catalyst systems are characterized in situ by means of polarization curves, continuous operation, accelerated stress tests and electrochemical impedance spectroscopy measurements at single cell and stack level. The as-prepared Pt-Au/C catalysts show increased tolerance toward phosphoric acid in comparison to the standard Pt/C (see Figure 1 and Figure 2). Acknowledgment Financial support was provided by The Climate and Energy Fund of the Austrian Federal Government and The Austrian Research Promotion Agency (FFG) through the program Energieforschung (e!Mission). [1] T. Ossiander, M. Perchthaler, C. Heinzl, F. Schönberger, P. Völk, M. Welsch, A. Chromik, V. Hacker, C. Scheu, Influence of membrane type and molecular weight distribution on the degradation of PBI-based HTPEM fuel cells, J. Memb. Sci. 509 (2016) 27–35. [2] A. Schenk, C. Grimmer, M. Perchthaler, S. Weinberger, B. Pichler, C. Heinzl, C. Scheu, F.-A. Mautner, B. Bitschnau, V. Hacker, Platinum–cobalt catalysts for the oxygen reduction reaction in high temperature proton exchange membrane fuel cells – Long term behavior under ex-situ and in-situ conditions, J. Power Sources. 266 (2014) 313–322. [3] J.E. Lim, U.J. Lee, S.H. Ahn, E. Cho, H.J. Kim, J.H. Jang, H. Son, S.K. Kim, Oxygen reduction reaction on electrodeposited PtAu alloy catalysts in the presence of phosphoric acid, Appl. Catal. B Environ. 165 (2015) 495–502. Figure 1