Novel polyolefin/silicon dioxide/H3PO4 composite membranes with spatially heterogeneous structure for phosphoric acid fuel cell

Novel composite membranes based on polyolefins for intermediate and high temperature (120–160 °C) phosphoric acid fuel cells with polymer matrices have been synthesized and their properties have been studied, including testing in operating fuel cells. In contrast to polybenzimidazoles uniformly swelling with H₃PO₄, which are typically used as membrane-separators in such a type of fuel cells, the proposed materials have heterogeneous internal structure with spatially separated condensed bundles of non-swelling rigid polymer-silica composite matrix and proton-conducting channels filled with phosphoric acid. Such a heterogeneous structure may potentially provide improved balance between proton conductivity and mechanical stability of the membranes in comparison with the homogeneously swollen PBI structures. The composite porous films based on polyethylene and polypropylene have been prepared in several different ways and filled with network of silicon dioxide. The SiO₂ phase forms hydrophilic three-dimensional well-percolated channels. The affinity between the SiO₂ phase and the liquid phosphoric acid is responsible for capillary retention of the liquid electrolyte in the porous matrix (phosphoric acid wets SiO₂ surface). Besides, the framework of SiO₂ phase enhances the mechanical stability of the membranes at high temperatures. Maximum proton conductivity of 0.033 S/cm is achieved at 160 °C for fuel cell with the obtained polyethylene-based membrane. The best performance is detected for fuel cells on polypropylene-based membrane, which provides 0.5 V at 0.4 A/cm² at 140 °C being supplied with hydrogen and air. The proposed concept is aimed to mimic spatially-non-uniform Nafion-type membranes instead of using uniformly swollen polybenzimidazoles.