Durability and Degradation of Direct Dimethyl Ether High Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

Dimethyl ether (DME) combines high energy density with low toxicity and good electro-oxidation kinetics due to the absence of carbon-carbon bonds. It has therefore been identified as an attractive fuel for various fuel cell types, including the high temperature polymer electrolyte membranes (HT-PEM) fuel cells utilizing phosphoric acid doped polybenzimidazole (PBI) membranes [1, 2]. The HT-PEMFC technology has been intensively developed during the last two decades, and recently lifetimes exceeding 9000 h with degradation rates as low as 0.5 μV h-1 have been achieved at 160 °C and 200 mA cm-2 with hydrogen and air [3]. This type of cell was originally developed for a direct methanol fueled system [4], but lifetimes exceeding a few hundred hours have yet not been reported in this mode. We recently demonstrated that this is, at least partly, due to severe chemical incompatibility between methanol and the phosphoric acid within the polymer membrane. It results in formation of organic phosphoric acid ester derivatives, which not only reduces the conductivity of the membrane but also promotes the acid loss [5]. This work presents durability studies of direct DME HT-PEM fuel cells, and extends to a discussion about the predominating degradation mechanisms responsible for the limited lifetimes. The cells were operated at a constant current load at 160 or 200 °C and periodically characterized by polarization curves and electrochemical impedance spectroscopy. Supported by studies on a model system of phosphoric acid and DME, a main degradation pathway is suggested, where methyl phosphates are formed within the membrane. In analogy with the direct methanol cells, this reduces the proton conductivity of the membrane and promotes the acid loss, which leads to gradually increasing internal resistance. [1] J.O. Jensen, A. Vassiliev, M.I. Olsen, Q.F. Li, C. Pan, L.N. Cleemann, T. Steenberg, H.A. Hjuler, N.J. Bjerrum, J. Power Sources, 211 (2012) 173-176. [2] J. Neutzler, G. Qian, K. Huang, B. Benicewicz, J. Power Sources, 216 (2012) 471-474. [3] T. Søndergaard, L.N. Cleemann, H. Becker, D. Aili, T. Steenberg, H.A. Hjuler, L. Seerup, Q. Li, J.O. Jensen, J. Power Sources, 342 (2017) 570-578. [4] J.S. Wainright, J.T. Wang, D. Weng, R.F. Savinell, M. Litt, J. Electrochem. Soc., 142 (1995) L121-L123. [5] D. Aili, A. Vassiliev, J.O. Jensen, T.J. Schmidt, Q. Li, J. Power Sources, 279 (2015) 517-521