Development and characterization of polybenzimidazole as a solid polymer electrolyte

Fuel cells are a kind of battery in which the oxidizer and reducer are stored outside of the system rather than in the electrodes. Direct methanol fuel cell technology demands a polymer electrolyte with low fuel and air cross over, good thermooxidative stability, good mechanical properties, and high conductivity operating at 150-200{dollar}spcirc{dollar}C, because catalysts for the methanol anode are not very active.; H{dollar}sb3{dollar}PO{dollar}sb4{dollar} doped polybenzimidazole (PBI) has been investigated as a potential polymer electrolyte for use in hydrogen/air and direct methanol/air fuel cells. The advantages of this electrolyte includes good mechanical properties, high proton conductivity, thermal stability and low vapor permeability.; It was found that the tensile modulus of polybenzimidazole(PBI) film cast from DMAc solution doubles after annealing at 250{dollar}spcirc{dollar}C for one hour. Density measurements show that when PBI is annealed at 250{dollar}spcirc{dollar}C, the polymer density increases by 2.5%. A Positron Annihilation Lifetime Spectroscopy (PALS) study of free volume changes in PBI as a function of temperature indicates that the free volume fraction (h) starts decreasing above 145{dollar}spcirc{dollar}C and decreases by about 0.02 when the sample is heated up to 300{dollar}spcirc{dollar}C. DMA analysis of PBI shows a sub-Tg transition ({dollar}beta{dollar} transition) at 150-350{dollar}spcirc{dollar}C. Above the {dollar}beta{dollar} transition temperature, polymer chains have enough mobility so that they can undergo volume relaxation. The relatively large change in free volume leads to a significant change in mechanical properties such as change in modulus and toughness.; The solubilities of H{dollar}sb3{dollar}PO{dollar}sb4{dollar} and H{dollar}sb2{dollar}SO{dollar}sb4{dollar} in PBI films has been interpreted in terms of the "dual-mode" sorption model. Experimental data agree very well with the model. The Langmuir sorption capacity of PBI is lower for H{dollar}sb2{dollar}SO{dollar}sb4{dollar} (divalent) than for H{dollar}sb3{dollar}PO{dollar}sb4{dollar} (monovalent). The Langmuir affinity constant depends strongly on the acidity of the permeant and increases as the permeant acidity increases.; The permeability and diffusion coefficients of PBI films to H{dollar}sb3{dollar}PO{dollar}sb4{dollar} and H{dollar}sb2{dollar}SO{dollar}sb4{dollar} solutions have been studied. Both the permeability and the time lag decrease with increasing H{dollar}sb2{dollar}SO{dollar}sb4{dollar} concentration. Analysis of the experimental results has been based on dual mode sorption considering partial immobilization of the localized permeant molecules. Results indicate that 1.4% of the molecules in the Langmuir sorption mode are mobile. The concentration dependency of the diffusion coefficients is discussed based on this model.; Thermogravimetric analysis shows that dehydration of phosphoric acid occurs above 130{dollar}spcirc{dollar}C to produce pyrophosphoric acid and, at higher temperatures, polyphosphoric acid. Differential scanning calorimetry and wide angle x-ray diffraction measurements indicate that phosphoric acid doped PBI crystallizes. Dynamic mechanical analysis and tensile tests of PBI and H{dollar}sb3{dollar}PO{dollar}sb4{dollar} doped PBI show that H{dollar}sb3{dollar}PO{dollar}sb4{dollar}, at concentrations lower than ca. 300 mole%, raises the modulus of PBI. Water, and/or higher concentrations of H{dollar}sb3{dollar}PO{dollar}sb4{dollar}, plasticize PBI. Formation of pyrophosphoric acid in PBI and/or crystallization result in a higher storage modulus.; This study indicates that acid doped PBI is a promising solid polymer electrolyte that functions as a good separator and has good conductivity. More study should be conducted about the long life stability of PBI in a fuel cell.