Influence of the PBI structure on PBI/CsH5(PO4)2 membrane performance for HT-PEMFC application

Conventional high-temperature proton exchange membranes use phosphoric acid (PA) as a proton conductor. However, PA loss, poor mechanical stability, and catalyst poisoning represent major limitations. Solid acids are an alternative to PA that does not require humidification and is less prone to leaching. In this study, we prepared two composite membranes by high-temperature impregnation to investigate the effects of the structure of polybenzimidazole (PBI) on the anchoring of solid acids: a laboratory-synthesized m-PBI/CsH5(PO4)2 and the commercial OPBI/CsH5(PO4)2. Compared to PA-doped membranes, CsH5(PO4)2-doped membranes showed excellent mechanical properties, particularly m-PBI/CsH5(PO4)2. Scanning electron microscopy and energy dispersive spectroscopy images showed that CsH5(PO4)2 was uniformly dispersed on the surface and cross-section of both membranes, conducive to the formation of proton transport channels following the CsH5(PO4)2 melting in both samples. The characterization of the physical structures proved the interaction between the imidazole ring and solid acid. Furthermore, the proton conductivity without humidification was similar in m-PBI/CsH5(PO4)2 and OPBI/CsH5(PO4)2, demonstrating the suitability of m-PBI for fuel cells production. To test this, we assembled a m-PBI/CsH5(PO4)2 cell that showed competitive performances, with a peak power density of 361.28 mW cm−2 at 160 °C in H2/O2 atmosphere.