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

Berber, M.R. and Ismail, M.S. and Pourkashanian, M. and Zakaria Hegazy, M.B. and Apfel, U.-P.

ACS APPLIED POLYMER MATERIALS
Volume: 3 Pages: 4275-4286
DOI: 10.1021/acsapm.1c00869
Published: 2021

Abstract
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. © 2021 American Chemical Society.

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