Structure and Interactions in Polybenzimidazole Composite Membranes for High-Temperature Polymer Fuel Cells: A Full Multinuclear Solid-State NMR Study

Fabrication of composite membranes including inorganic or hybrid fillers into polybenzimidazole (PBI) membranes is a promising strategy for improving the performance of a membrane-electrode assembly (MEA) for high-temperature fuel cells. To this aim, a full understanding of the structure and interactions in such a complex system, which includes polymer, filler, and phosphoric acid, is mandatory. In this paper, we used multinuclear magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy to investigate the inclusion of inorganic and hybrid mesoporous fillers into a pyridine-based polybenzimidazole (PBI_5N) matrix and the effects of the subsequent H₃PO₄ doping. Composite membranes were prepared through an innovative spray deposition method, with 30 wt % of filler loading and SBA-15 or propylsulfonic-functionalized SBA-15-type silicas as filler. ¹³C CP MAS NMR experiments showed the structural changes induced by the acid doping on the polymer backbone. ³¹P­(¹H) 2D heteronuclear correlation experiments confirmed strong interactions between H₃PO₄ molecules and imidazole protons through a hydrogen-bonding network. ²⁹Si­(¹H) CP MAS experiments revealed different interactions between acid and filler particles, depending on the filler type. Here, for the first time, a complete understanding of the interactions among polymer, filler, and phosphoric acid is provided. We demonstrate that a fully inorganic mesoporous filler seems to be more suitable with respect to the hybrid homologues for increasing the proton conductivity of PBI, because of its higher affinity with H₃PO₄. We gave evidence of a possible proton conductivity inside the mesoporous structure of SBA-15, which became hindered in the propylsulfonic-functionalized silicas due to the presence of −SO₃H moieties.