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

Concentrated aqueous perfluoroethylene-1,2-bis-phosphonic acid, (OH)(2)OPCF2CF2PO(OH)(2), has been investigated as an alternative to phosphoric acid as the electrolyte in the H-2/O-2 acid fuel cell. Phosphoric and bis-phosphonic acid solutions were equilibrated with pure O-2 (P = 1 atm) and were used in (i) chronoamperometry experiments performed with a Pt microelectrode to estimate the O-2 diffusion coefficient, D, and solubility, C, and (ii) O-2 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 degrees C in 85% bis-phosphonic acid, C was found to be 3.4 +/- 0.3 x 10(-4) mol/liter and D was 5.0 +/- 0.5 x 10(-7) cm(2)/s as compared with C = 2.8 +/- 0.3 x 10(-4) mol/liter and D = 1.2 +/- 0.1 x 10(-6) cm(2)/s in 85% phosphoric acid at T = 22 degrees C. The overpotential for O-2 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/cm(2) at 100 degrees 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 (greater than or equal to 200 mA/cm(2)) where mass transport may influence cathode performance, the performance with bis-phosphonic acid was not as good as with phosphoric acid at 100 degrees C, which is consistent with the higher viscosity and lower D found for bis-phosphonic acid relative to phosphoric acid at 22 degrees C. As the temperature was raised from 100 to 200 degrees C, the cathode performance improved with the bis-phosphonic acid electrolyte. The enhanced O-2 reduction kinetics with the bis-phosphonic acid persisted in the fuel cell environment at temperatures up to 200 degrees 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 degrees C.