Promising Membrane for Polymer Electrolyte Fuel Cells Shows Remarkable Proton Conduction over Wide Temperature and Humidity Ranges

A step in the direction of the real-life application of fuel cells (FCs) has been realized through the fabrication of a promising proton conductive membrane comprising a perfluorosulfonic-acid ionomer and nitrogen-rich poly[2,2′-(4,4′-bipyridine)-5,5′-bibenzimidazole] (BiPyPBI). The BiPyPBI-perfluorosulfonic acid membranes displayed remarkable oxidative and mechanical stabilities with significant proton conduction over wide ranges of temperatures (40 to 140 °C) and humidities (30 to 90% RH). A 0.5 molar BiPyPBI feed ratio increased the proton conduction of perfluorosulfonic acid by 2.6- and 1.5-fold at 40 and 80 °C, respectively, due to the enhancement in the ion-exchange capacity (1.9 mmol/g, which was twofold higher than that of bare Nafion). The protonic conductivity reached 0.171 S/cm at 140 °C. Using a BiPyPBI feed increased the stability of the Nafion membrane, corresponding to a 3.5-fold increase in the mechanical stress (9.6 MPa) and a 2.2-fold decrease in the elongation at break. In addition, the oxidation stability of the Nafion membrane increased by 26%. The measured activation energy suggested that the presence of BiPyPBI created an easier proton transport pathway (by the Grotthuss mechanism) because of a stronger hydrogen-bonding network than in bare Nafion. Compared to the power density of a perfluorosulfonic-based MEA, the power density of the BiPyPBI-perfluorosulfonic-based membrane electrode assembly (MEA) at 140 °C increased by approximately 20-fold to 175 mW cm–1 at 30% RH and by approximately 5-fold to 201 mW cm–1 at 90% RH. Impedance spectra confirmed the improvement of the FC performance of the BiPyPBI-perfluorosulfonic-based MEA, indicating enhanced charge transfer. After 10,000 cycles of relative humidity stress testing, the BiPyPBI-perfluorosulfonic-based MEA showed a power density of 146 mW cm–1 (corresponding to a 16% loss in the initial power density measured at 30% RH). The MEA lost only 26% of its initial power density upon relative humidity stress cycling.