Low acid leaching PEM for fuel cell based on polybenzimidazole nanocomposites with protic ionic liquid modified silica

Abstract

Despite the remarkable advances in recent times on polybenzimidazole (PBI) based proton exchange membranes (PEM), there are two most important limitations that restrict their uses: (1) declining of the mechanical strength at high acid doping level and (2) leaching of doped acid when membrane is in contact with water. In continuation with our earlier success (J. Mater. Chem. 2011, 21, 14897; ACS Appl. Mater. Interfaces 2014, 6, 21286) on the former concern, in this article we also addressed the latter problem associated with 'Acid leaching' along with the issue of mechanical strength at high PA loading. We have made an attempt to study how the surface modifier molecule plays a role in altering the morphology and structure of poly (4, 4′-diphenylether-5, 5′-bibenzimidazole) (OPBI) nanocomposite membranes thereby enhancing the PEM properties especially acid retention capability. Silica nanoparticles of 25 nm size were prepared and successfully modified with phosphate anion containing imidazolium ionic liquid (IL) and were incorporated in OPBI by solution blending method. The chemical interactions between the polymer and ionic liquid modified silica (ILMS) were confirmed by FTIR, NMR and WAXD studies. TEM images disclosed how these interactions led to the formation of self-assembled clusters in the OPBI matrix. All the ILMS nanocomposite membranes displayed high thermal, mechanical and oxidative stabilities. The IL-decorated silica nanoparticles prevented the leaching of phosphoric acid (PA), by hydrogen bonding interactions, from the PA doped membranes. This resulted in higher PA doping levels of the nanocomposites. The ability to retain more PA was also reflected in the high proton conductivity data of the ILMS nanocomposites - 15% ILMS loaded membrane which showed almost two fold increment in proton conductivity compared to the neat OPBI. This was attributed to the self - assembled clusters of ILMS nanoparticles in the matrix.