Perfluoro-ethylene-1,2-bis-phosphonic acid fuel cell electrolyte

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.