THE KINETICS OF ELECTROCHEMICAL REACTIONS ON HIGH-TEMPERATURE FUEL-CELL ELECTRODES

The rates of electrochemical reactions relevant for use in high-temper ature solid oxide fuel cells (SOFC) has been investigated as a functio n of electrode potential, temperature and composition of the gas mixtu re. From Arrhenius plots, apparent activation energies, E(a), and appa rent pre-exponential factors, A, were calculated for the oxygen-reduct ion and oxygen-evolution reactions at La0.84Sr0.16MnO3 cathodes. At lo w overpotentials (\eta\less-than-or-equal-to 0.2 V), both apparent act ivation energies and apparent pre-exponential factors are much higher in the temperature range T = 800-1000-degrees-C (E(a) almost-equal-to 160-210 kJ/mol, log A almost-equal-to 6-9) compared with those in the range T= 500-800-degrees-C (E(a) almost-equal-to 80-110 kJ/mol, log A = 2-4). For oxygen reduction, reaction orders of z(e) = 1 at p(O2) > 0 .2 bar and z(e) = 0.5 at p(O2) < 0.2 bar were obtained. These values m ay be related to either oxygen adsorbed as molecules or atoms as the r eacting species. From impedance spectroscopy, it follows that the rate of the oxygen-exchange reaction is determined not only by charge tran sfer, but also by another process, possibly the adsorption or surface diffusion of intermediates. For the nickel zirconia cermet anode fabri cated by wet powder spraying (WPS), an increase in sintering temperatu re to 1400-degrees-C results in an increase in current density. A curr ent density of 0.27 A cm-2 at an overvoltage of 0.1 V may be achieved. From Arrhenius plots, an energy of activation of 130 +/- 10 kJ mol-1 was determined.