Electric Response and Conductivity Mechanism in H3PO4-Doped Polybenzimidazole-4N-HfO2 Nanocomposite Membranes for High Temperature Fuel Cells

Relaxation and polarization phenomena of phosphoric acid-doped [PBI4N(HfO2)(x)](H3PO4)(y) nano composite membranes for high-temperature proton-exchange membrane fuel cells are studied using Dynamic Mechanical Analysis (DMA) and Broadband Electrical Spectroscopy (BES). The membranes are obtained by casting combinations of a polybenzimidazole polymer (PBI4N) with increasing amounts of hafnium oxide nanofiller, resulting in [PBI4N(HfO2)(x)] hybrid systems with 0 <= x <= 0.32. Phosphoric acid at varying content levels (0 divided by 18 wt%) is used as a doping agent, giving rise to [PBI4N(HfO2)(x)](H3PO4)(y), membranes. DMA and BES studies lead us to determine that the electric response of the membranes is modulated by polarization phenomena and by alpha and beta dielectric relaxation events of the polymer matrix. Additionally, the experimental results suggest that in [PBI4N(HfO2)(x)](H3PO4)(y) membranes the conductivity occurs owing to three conductivity pathways: two mechanisms involving inter-domain proton migration phenomena by hopping events; and one mechanism in which proton exchange occurs between delocalization bodies. These results highlight the significant effect of the hafnium oxide nanofiller content on the conductivity of [PB14N(HfO2)(x)](H3PO4)(y) where, at x >= 0.04, demonstrates conductivity higher (9.0 x 10(-2) S/cm) than that of pristine H3PO4-doped PBI4N (4.8 x 10(-2)S/cm) at T >= 155 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.