Polybenzimidazole and benzyl-methyl-phosphoric acid grafted polybenzimidazole blend crosslinked membrane for proton exchange membrane fuel cells

• –C6H5CH2PO3H2 (BP) graft polybenzimidazole (PBI) has higher H3PO4 doping than PBI.
• PBI membrane conductivity is improved by grafting –C6H5–CH2–PO3H2 side chain.
• Crosslink PBI/PBI-BP blend membrane shows better fuel cell performance than PBI.

We synthesize polybenzimidazole (PBI; Mw = 1.65 × 105 g mol−1) and benzyl-methyl-phosphoric acid grafted PBI (PBI-BP; 24 mol% degree of grafting). We demonstrate that blending 20 to −40 wt.% PBI-BP in the PBI membrane enhances the H3PO4 doping level, proton conductivity, and mechanical strength. However, the membrane is highly dissolved in an 85 wt.% H3PO4 aqueous solution as the PBI-BP content in the blend membrane is larger than 50 wt.%. To prevent PBI-BP from being dissolved out of the blend membrane by the H3PO4 aqueous solution, we fabricated a PBI/PBI-BP/epoxy (8/2/1.23 by wt.) crosslinked membrane. The crosslinked membrane demonstrated good fuel cell performance and excellent stability after a 23 on/off (12 h on at 160 °C with a current density of 200 mA cm−2 and 12 h off at room temperature) fuel cell cycle test with an unhumidified H2/O2.