Perfluoro‐ethylene‐1,2‐bis‐phosphonic Acid Fuel Cell Electrolyte

Concentrated aqueous perfluoroethylene-1,2-bis-phosphonic acid has been investigated as an alternative to phosphoric acid as the electrolyte in the H{sub 2}/O{sub 2} acid fuel cell. Phosphoric and bis-phosphonic acid solutions were equilibrated with pure O{sub 2} (P = 1 atm) and were used in (1) chronoamperometry experiments performed with a Pt microelectrode to estimate the O{sub 2} diffusion coefficient, D, and solubility, C, and (2) O{sub 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 C in 85% bis-phosphonic acid, C was found to be 3.4 {+-} 0.3 {times} 10{sup {minus}4} mol/liter and D was 5.0 {+-} 10{sup {minus}7} cm{sup 2}/s as compared with C = 2.8 {+-} 0.3 {times} 10{sup {minus}4} mol/liter and D = 1.2 {+-} 0.1 {times} 10{sup {minus}6} cm{sup 2}/s in 85% phosphoric acid at T = 22 C. The over-potential for O{sub 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/mcm{sup 2} at 100 C. This indicates that the oxygen electroreduction kinetics on Pt increased when phosphoric acid was replaced with the perfluorinated bis-phosphonic acid.more » At higher current densities 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 O{sub 2} reduction kinetics with the bis-phosphonic acid persistend 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.« less