Membrane electrode assemblies for high-temperature polymer electrolyte fuel cells based on poly(2,5-benzimidazole) membranes with phosphoric acid impregnation via the catalyst layers

A novel strategy for introducing phosphoric acid as the electrolyte into high-temperature polymer electrolyte fuel cells by using acid impregnated catalyst layers instead of pre-doped membranes is presented in this paper. This experimental approach is used for the development of membrane electrode assemblies based on poly(2,5-benzimidazole) (ABPBI) as the membrane polymer. The acid uptake of free-standing ABPBI used for this work amounts to ABPBI × 3.1 H3PO4 which has a specific conductivity of ∼80 mS cm−1 at 140 °C. Rather thick catalyst layers (20% Pt/C, 1 mg Pt cm−2, 40% PTFE as binder, d = 100–150 μm) are prepared on gas diffusion layers with a dense hydrophobic microlayer. After impregnation of the catalyst layers with phosphoric acid and assembling them with a mechanically robust undoped ABPBI membrane a fast redistribution of the electrolyte occurs during cell start-up. Power densities of about 250 mW cm−2 are achieved at 160 °C and ambient pressure with hydrogen and air as reactants. Details of membrane properties, preparation and optimization of gas diffusion electrodes and fuel cell characterization are discussed. We consider our novel approach to be especially suitable for an easy and reproducible fabrication of MEAs with large active areas.