Highly proton conductive porous membranes based on polybenzimidazole/ lignin blends for high temperatures proton exchange membranes: Preparation, characterization and morphology- proton conductivity relationship

A new porous blend membranes based on polybenzimidazole (PBI) and lignin as a renewable material were prepared and structurally characterized by FT-IR for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs). LiCl salt was used as a porogen to create micro-void in the membranes. The effect of presence of lignin and LiCl salt in the PBI on the structure of the membrane was investigated using scanning electron microscopy (SEM) analysis. Impedance spectroscopic (IS) technique was utilized to evaluate proton conductivity of the membranes at different temperatures. Through combining the SEM and IS results, a relationship between proton conductivity and the morphology of the membranes was established. The results showed that by increasing the content of lignin (0–20 wt%) in the membrane, phosphoric acid (PA) doping level increases from 4 to 26.8 and water uptake increases from 18 to 26.8 wt% because number and size of the pores formed in the membrane structure increase. The obtained results showed that increasing lignin content from 5 to 20 wt% results in increasing membrane proton conductivity from 14.6 to 69 mS/cm. This is the result of increasing doping level and decreasing the membrane resistance at ambient conditions (due to increasing numbers and diameters of micro-void in the membrane). The proton conductivities of PA-PBI/lignin membranes were also measured in a temperature range of 25–160 °C. Proton conductivity of membranes increased with temperature. Using the Arrhenius plot, the relationship between proton conductivity and temperature was investigated. At the evaluated range of temperatures, the proton conductivity of 5, 15, and 20 membranes increases 328%, 110%, and 120% compared to the ambient temperature, respectively. The highest proton conductivity was observed in sample PBI/lignin (20 wt%), to be 152 mS/cm at 160 °C.