Effect of vinylphosphonic acid and polymer binders with phosphate groups on performance of high-temperature polymer electrolyte membrane fuel cell

Abstract

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are operated between 120 °C and 180 °C. As a result, they cannot employ Nafion® as an electrode ionomer because the proton conductivity of Nafion® is very low without water. In this study, new polymers (poly(phenylene oxide), PPO) with a phosphate group including methyl phosphonic acid (PPO-MPA) or 6-oxohexyl phosphonic acid (PPO-HPA), which show good proton conductivities, are applied as ionomers and binders to form an electrode. In addition, vinylphosphonic acid (VPA) is investigated as an electrode additive. The PPO-MPA electrode shows a lower wetting angle than that of the PPO-HPA electrode. After the addition of VPA, the PPO-MPA electrode becomes more hydrophobic, whereas the PPO-HPA electrode becomes more hydrophilic. 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 due to increased amounts of released protons from PPO-MPA, indicating that the binder with a phosphate group improved the proton conductivity of the ionomer in the HT-PEMFC. At 0.2 A/cm2, the performance of the membrane electrode assembly (MEA) using the PPO-MPA cathode increases from 0.55 V to 0.59 V by the introduction of VPA, whereas that using the PPO-HPA cathode decreases from 0.61 V to 0.57 V, which exhibits the same propensity as the hydrophobicity change. The value of charge transfer resistance at 0.6 V estimated by fitting of the electrochemical impedance spectra follows the same trend as the MEA performance. This is attributed to a change in catalyst coverage by phosphoric acid in the electrode by the addition of VPA and not a change in proton conductivity. The possibility of new polymer binders for the HT-PEMFC is demonstrated, and the MEA performance is shown to be influenced by the incorporation of VPA, which is accounted for the change in hydrophobicity of the electrode.