FUEL-CELL CATHODE STUDIES IN AQUEOUS K2CO3 AND KOH

The galvanostatic steady-state performance of PTFE-bonded Pt-on-carbon gas diffusion electrodes (GDEs) has been measured with variable oxygen partial pressures in 2 to 11M KOH and 2 to 5.5M K2CO3. The catalyst in the porous electrodes was characterized with cyclic voltammetry in a special cell with low uncompensated solution resistance. Cyclic voltammograms yielded measurements of the wetted areas of carbon and Pt and the local electrolyte composition. Corrections for differences in the area of Pt wetted by the KOH electrolyte collapsed oxygen reduction (OR) performance curves for several different electrodes into a single curve of log-specific current density vs. potential. Comparisons with kinetic results suggest that OR on carbon contributes to the high current densities in 6.9M KOH at high overpotentials. In K2CO3 electrolyte, current density was found to be proportional to the square root of both wetted Pt area and O2 pressure, indicating that mass-transfer rates were not limited by O2 transport. Hysteresis measurements and cyclic voltammetry were used to identify slow transport of OH− ion away from the reaction site as the limiting phenomenon. The generally lower wettability and pH rise in the carbonate-containing electrodes resulted in lower performance when compared with KOH. The results suggest that modern GDEs, which have been optimized primarily for efficient O2 transfer, are not adequate for use in aqueous carbonate electrolytes. Modifications of the GDE structure to provide for more rapid OH− transport will be necessary to obtain good performance with aqueous carbonate electrolytes. © 1990, The Electrochemical Society, Inc. All rights reserved.