Polybenzimidazole/silica nanocomposites: Organic-inorganic hybrid membranes for PEM fuel cell

Despite the myriad studies on polybenzimidazole based polymer electrolyte membranes for fuel cells operating above 100 °C, the development of membranes with higher proton conductivity without compromising mechanical stability has continued to be the prime challenge. In this article, organic/inorganic hybrid nanocomposites of poly (4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) were prepared with surface functionalized silica nanoparticles to address this key issue. Structural and morphological studies probed by SAXS, WAXD and TEM, respectively revealed the formation of self-assembled clusters of nanoparticles when the OPBI nanocomposites were made with amine modified silica (AMS) whereas a well dispersed structure was obtained for OPBI and the unmodified silica (UMS) composite. The OPBI/AMS nanocomposites displayed a significant enhancement in the thermal stabilities compared to the pristine OPBI and OPBI/UMS nanocomposite. The OPBI/AMS nanocomposite membranes exhibited a larger mechanical reinforcement than the pristine OPBI and OPBI/UMS nanocomposite. The formation of nanoparticle clusters in the OPBI matrix in the case of OPBI/AMS was found to be the driving force for the higher thermal and mechanical stability of OPBI/AMS than those of OPBI/UMS. The incorporation of AMS in the OPBI matrix shielded the polymer chains from the attack of oxidative radicals, resulting in a huge enhancement of oxidative stability of the nanocomposite membranes compared to the pure OPBI membrane. The OPBI/AMS nanocomposite membranes have significantly higher phosphoric acid (PA) loading compared to the pure OPBI membrane which resulted in the higher proton conductivities of the former. The self-assembled clusters of AMS in the OPBI matrix facilitated the proton transport process.