Investigation of the new polymer binders including phosphate functional groups with vinylphosphonic acid for HT-PEMFC

Abstract

The high-temperature PEMFC (HT-PEMFC) is operated between 120℃ and 180℃, so it can’t use the Nafion as the binder at electrodes because proton conductivity of the Nafion is low without water. We made new polymers (poly(phenylene oxide), PPO) with a phosphate group including methyl phosphonic acid (PPO-MPA) or 6-oxohexyl phosphonic acid (PPO-HPA), binders including phosphate functional groups, that have higher proton conductivity at dry condition. To enhanced proton conductivity in electrodes, vinylphophonic acid (VPA) was added at the electrodes that were made by PPO-MPA and PPO-HPA. The wetting angle of the PPO-MPA electrode was 46.87°, which is lower than that of the PPO-HPA electrode (104.04°). After adding the VPA, the PPO-MPA electrode became more hydrophobic (increased wetting angle), but the PPO-HPA electrode became more hydrophilic (decreased wetting angle). In the FT-IR spectra of the PPO-MPA electrode, the peak associated with the PO32- asymmetric stretch is more blue-shifted than the peak of the polyvinylidene difluoride electrode at a high temperature. It occurred to increase amounts of released protons from PPO-MPA, and thus, it indicated that the binder with a phosphate group improved the proton conductivity in the HT-PEMFC. At 0.2 A/cm2, the MEA performance (0.61 V) of the PPO-HPA electrode was higher than that of the PPO-MPA electrode (0.55 V). In the EIS spectra, PPO-HPA presents a lower Rpro value (0.060 Ω cm2) than that of PPO-MPA (0.064 Ω cm2). And the Rct of the PPO-HPA electrode (0.35 Ω cm2) is lower than that of the PPO-MPA electrode (0.63 Ω cm2). However, the adding VPA made the MEA performance change. For the MEA with the PPO-MPA electrode, the addition of VPA increased the performance, whereas the voltage of the MEA with the PPO-HPA electrode decreased. In EIS spectra, the Rct of PPO-MPA electrode decreased but that of PPO-HPA electrode increased. The addition of VPA was expected to increase the MEA performance by proton hopping via phosphate group, but therefore, it affected the MEA performance by changing the electrode hydrophobicity.