AC AND DC ELECTROCHEMICAL INVESTIGATION OF PROTONIC CONDUCTION IN CALCIUM-DOPED BARIUM CERATE CERAMICS

Protonic conduction in barium cerates doped with a divalent metal, BaCe1-xCaxO3-alpha (x = 0.02, 0.05, 0.10, 0.15), has been investigated at high temperature in a moist atmosphere. Conduction by protons was confirmed by emf measurements using specimen ceramics as the solid-electrolyte membrane separating moist and dry nitrogen atmospheres. Bulk electrical conductivity, sigma(b), measurements were carried out using impedance techniques. Plots of log(sigma(b)T) versus 1/T showed apparent Arrhenius behaviour. Conductivity increases markedly with increasing x for 0 < x less-than-or-equal-to 0.05, but not for x greater-than-or-equal-to 0.10. The protonic conductivity for a 10% Ca-doped ceramic at 900-degrees-C is ca. 2 X 10(-3) S cm-1 with an 'activation energy' of 54 kJ mol-1. At high temperature, dc measurements for x = 0.02 gave 'conductivities' approaching sigma(b) values obtained by impedance measurements, but contributions other than bulk resistance became increasingly significant at lower temperatures. Of those contributions, the electrode-electrolyte charge-transfer resistance was of particular importance.