Polybenzimidazole (PBI) is a typical polymer electrolyte membrane for high temperature PEMFC. PBI transfers proton conductivity with phosphoric acid as a mediator and has excellent thermal, chemical and mechanical properties at high temperature up to 200℃. However, PBI has poor processability due to low solubility in organic and organic solvents due to its rigid chemical structure. In addition, it is necessary to improve the proton conductivity by increasing phosphoric acid doping level.
In this study, different two types of monomers were introduced into the PBI structure to improve the limitations of PBI. Suberic acid was introduced to improve the solubility. In order to compensate for weakening physiochemical properties, the PBI random copolymers were synthesized by introducing 5-aminoisophthalic acid. The polymer was synthesized at a ratio of 1:9, 3:7, 1:1, 7:3, 9:1 in order to investigate the difference of the membrane performance as the ratio of each monomer was changed. In addition, to improve the proton conductivity, mesoporous SiO2 was introduced to fabricate a composite membrane and its performance was evaluated. The physical and chemical properties of the modified PBI and mesoporous SiO2 coposite membranes were compared by TGA, DSC, acid uptake, and oxidative stability, respectively. The chemical structure of the polymer and inorganic materials were analyzed by FT-IR and XRD. The TGA graph shows that the thermal stability of all the membranes is higher than that of m-PBI considering that 65% or more of the polymer membranes were retained up to 800°C, and the mesoporous SiO2 containing membranes had higher thermal stability as the ratio of mesoporous SiO2 increases. In DSC, the lower the ratio of methylene group, the higher the Tg the polymer have and the highest value was 240°C. As a result of acid uptake measured at the concentration of phosphoric acid between 2 and 14 M, it was found that the more the methylene group was, the more phosphoric acid was absorbed. The oxidative stability measured with Fenton reagent also showed the same results.
In addition, the surface of the composite membrane was confirmed by SEM-EDAX analysis, and it was confirmed that compared to the PBI membrane, the roughness of the composite membrane was incereased. Also, the proton conductivity was measured under the conditions of no humidification and high temperature (100 to 180°C). X1Y9 was the highest at 15.9 mS/cm, and 15 wt% of the membrane among composite membranes had the best electrochemical performance of 19 mS/cm.
In conclusion, in order to resolve the problems of PBI, the modified membrane was fabricated by introducing new structures. As a result, the polymer membranes showed higher solubility in acid solvents than m-PBI and their thermal and oxidative stability were as good as m-PBI. However, acid uptake and proton conductivity are relatively low compared to m-PBI, and still need to be improved. On the basis of research, if the proton conductivity is incresed for the polymer by making greater for acid uptake of the polymers, it can be applied to study the polymer electrolyte membrane with better performance than the present one.