New hybrid materials based on cardo polybenzimidazole PBI-O-PhT and modified silica with covalent silanol cross-linking

Polybenzimidazoles (PBI) doped with phosphoric acid are a promising electrolyte for medium-temperature fuel cells. However, to be effective at high temperatures in the presence of acid, the mechanical and conductive properties of the material must be stable and no critical increase in gas permeability is required. This work proposes an approach to improve the properties of PBI-O-PhT-based materials by combining two previously known methods: covalent crosslinking with silane (3-bromopropyl)trimethoxysilane (SiBr) and doping with silicon oxide (SiO₂), including grafted imidazolinpropyl groups (SiO₂Im). The silanol cross-linked samples exhibited higher stability when tested with Fenton's reagent and retained their morphological integrity even after 360 h of testing. The study shows that covalent crosslinking improves the stability of dopant particles in the membrane matrix and prevents their leaching during acid treatment. Additionally, the incorporation of silicon oxides enhances the proton conductivity of samples with covalent cross-linking and reduces gas permeability compared to the original PBI membrane. Proton conductivity of the covalent cross-linked samples reaches 50 and 55 mS·cm⁻¹ at oxide contents of 5 wt% SiO₂Im and 10 wt% SiO₂, respectively.