High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC

Abstract

High temperature operation of proton exchange membrane fuel cells under ambient pressure has been achieved by using phosphoric acid doped polybenzimidazole (PBI) membranes. To optimize the membrane and fuel cells, high performance polymers were synthesized of molecular weights from 30 to 94 kDa with good solubility in organic solvents. Membranes fabricated from the polymers were systematically characterized in terms of oxidative stability, acid doping and swelling, conductivity, mechanical strength and fuel cell performance and durability. With increased molecular weights the polymer membranes showed enhanced chemical stability towards radical attacks under the Fenton test, reduced volume swelling upon the acid doping and improved mechanical strength at acid doping levels of as high as about 11 mol H3PO4 per molar repeat polymer unit. The PBI‐78kDa/10.8PA membrane, for example, exhibited tensile strength of 30.3 MPa at room temperature or 7.3 MPa at 130 °C and a proton conductivity of 0.14 S cm–1 at 160 °C. Fuel cell tests with H2 and air at 160 °C showed high open circuit voltage, power density and a low degradation rate of 1.5 μV h–1 at a constant load of 300 mA cm–2.

Influences of the structure of imidazolium pendants on the properties of polysulfone-based high temperature proton conducting membranes

“…As seen from the figure, all membranes showed conductivities of higher than 0.01 S cm -1 and the conductivities steadily increased with the temperature up to 160 o C. However, the increase in the conductivity was almost negligible as the temperature was increased further, especially for membranes of M-1# to M-4#. As previously reported [39][40][41], the doping phosphoric acid would dehydrate at temperatures higher than 160 o C, which leads to the formation of oligomeric species of polyphosphoric acids with apparently lower conductivity than that of the orthophosphoric acid. In addition, the enhancement in the conductivity by increasing temperatures is restricted by the low ADLs of the membrane [42].…”

“…Becasue the failure of a HT-PEMFC might result from chemical and mechanical degradations of the polymer membranes, leaching of the doping acids and loss of catalyst activities [3]. As previously reported, macromolecular crosslinking [9] and increasing molecular weight [39] of the polymer could effectively improve the durability of PBI-based fuel cells. though they had the same functional group of methylimidazolium [15].…”

Epoxides cross-linked hexafluoropropylidene polybenzimidazole membranes for application as high temperature proton exchange membranes

“…In order to improve the mechanical strength without or with less sacrifice of the proton conductivity, various approaches have been developed. The resultful explorations include increasing molecular weight of the mPBI polymer [9,10], synthesis of PBI variants or copolymers [11][12][13][14][15][16][17][18][19], fabrication of composite membranes with nano inorganic compounds [10,20], ionically and covalently crosslinking [6,21].…”

Recent Progress on the Key Materials and Components for Proton Exchange Membrane Fuel Cells in Vehicle Applications

“…Thermal cycling stability tests showed that the open circuit voltage (OCV) degradation rate was 118% less than the pure PBI membrane. An extremely high molecular weight (from 30 to 94 kDa) PBI membrane was designed and studied by Yang et al [19]. With increased molecular weights, the PBI membrane properties including chemical stability, swelling, mechanical strength, and proton conductivity were all improved.…”