Novel aryloxy-polybenzimidazoles as proton conducting membranes for high temperature PEMFCs

Polybenzimidazole (PBI) is the material of choice to fabricate proton exchange membranes for high temperature PEMFCs. Among the most recent trends in the design of PBI polymers, we recall the introduction of oxygen atoms in the polymer backbone. In fact, the presence of ether groups improves the polymer solubility in polar solvents and, consequently, the membrane and MEA processability. In addition, it provides reactive points for functionalization processes and further chemical modifications. Here we reported on the synthesis and characterization of new arylether-based PBIs, and namely Poly 1,4-bis-(4-(1H,1′H-2,5′-bibenzo[d]imidazol-2′-yl)phenoxy) benzene and Poly 2′,2″-(4,4′-oxybis(4,1-phenylene))bis(1H, 1′H-2,5′-bibenzo[d]imidazole), labelled in the following as PBI-108 and PBI-109, respectively. The polymers differ for the number of the ether-based spacers, which are one in case of PBI-108, and two for PBI-109. The H 3PO4-doped membranes were characterised in terms of thermal and chemical stability, proton conductivity and fuel cell performances. In particular, the MEAs properties were investigated with respect to the acid doping level of the electrodes, temperature, pressure and gas flow rates. The monomer structure does not remarkably affect the electrochemical properties of the membranes. However, the PBI-109 membrane is chemically more stable in presence of oxy- and hydroxyl-free radicals with respect to PBI-108 and oxygen-free PBI systems. Proton conductivity of 8 mS cm-1 was measured at 120 °C and RH = 50% in the case of aryloxy-PBI with the shorter spacer. The power density increases with temperature, pressure and air stoichiometry. Values as high as to 400 mW cm-1 were measured at 150 °C, λair = 6 and a backpressure of 2 bar