Concentrated aqueous perfluoroethylene-1,2-bis-phosphonic acid, (OH)2OPCF2CF2PO(OH)2, has been investigated as an alternative to phosphoric acid as the electrolyte in the H2/O2 acid fuel cell. Phosphoric and bis-phosphonic acid solutions were equilibrated with pure O2 (P = 1 atm) and were used in (i) chronoamperometry experiments performed with a Pt microelectrode to estimate the O2 diffusion coefficient, D, and solubility, C, and (ii) O2 electroreduction polarization studies done with a gas-fed Teflon-bonded carbon electrode with a high surface area Pt catalyst in a microfuel cell. At 22°C in 85% bis-phosphonic acid, C was found to be 3.4 ± 0.3 × 10-4 mol/liter and D was 5.0 ± 0.5 × 10-7 cm2/s as compared with C = 2.8 ± 0.3 × 10-4 mol/liter and D = 1.2 ± 0.1 × 10-6 cm2/s in 85% phosphoric acid at T = 22°C. The over-potential for O2 reduction on the microfuel cell cathode was smaller in 85% bis-phosphonic acid than in 85% phosphoric acid for current densities up to 200 mA/cm2 at 100°C. This indicates that the oxygen electroreduction kinetics on Pt increased when phosphoric acid was replaced with the perfluorinated bis-phosphonic acid. At higher current densities (≥ 200 mA/cm2) where mass transport may influence cathode performance, the performance with bis-phosphonic acid was not as good as with phosphoric acid at 100°C, which is consistent with the higher viscosity and lower D found for bis-phosphonic acid relative to phosphoric acid at 22°C. As the temperature was raised from 100 to 200°C, the cathode performance improved with the bis-phosphonic acid electrolyte. The enhanced O2 reduction kinetics with the bis-phosphonic acid persisted in the fuel cell environment at temperatures up to 200°C for 500 h which demonstrated a major improvement in stability for a high performance perfluorinated fuel cell electrolyte used at higher temperatures such as 200°C.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry