Phosphoric Acid Loss Mechanism of High Temperature Proton Exchange Membranes

Abstract

High temperature-polymer electrolyte fuel cells (HT-PEMFCs) traditionally utilize phosphoric acid-doped polybenzimidazole as proton exchange membrane.1 However, the retention of phosphoric acid within polymer matrix has been pointed out as one of the most critical factors that affect HT-PEMFC durability. Previously, we reported a new HT-PEMFC system in which a quaternary ammonium-functionalized polymer imparted a stronger ionic interaction which we deemed “ion-pair coordination” with phosphoric acid compared with that of polybenzimidazole,2 giving rise to improved water tolerance. In this study, we use a combination of 31P NMR and DFT computations to compare the ionic interactions between representative small molecule organic bases, benzimidazole or tetramethylammonium hydroxide, with phosphoric acid and water. Based of these results, we propose a new phosphoric acid loss mechanism in which the strongly interacting ion pairs can increase the phosphoric acid partition composition towards accommodating more phosphoric acid for better phosphoric acid retention. References Li, J.O. Jensen, R.F. Savinell, N.J. Bjerrum, Prog. Polym. Sci., 2009, 34, 449-477.-S. Lee, J.S. Spendelow, Y.K. Choe, C. Fujimoto, Y. S. Kim, Nature Energy, 2016, 1, 16120-16124.