Experiment and Analysis of Mass Transfer in SrCe_<0.95>Yb_<0.05>O_<3-a> Fuel Cell using Complex Impedance Plot

For the application of proton-conducting ceramics to tritium pumps in nuclear fusion technology, mass-transfer process in a SrCe0.95Yb0.05O3−a fuel cell was studied experimentally and analytically using complex impedance plot based on an alternate current method. Impedance plots on a complex plane of the SrCe0.95Yb0.05O3−a fuel cell with the supply of hydrogen and oxygen under a dry or moist condition revealed the presence of a composite imaginary impedance comprised of several mass- or charge-transfer processes as follows: hydrogen or oxygen dissociation on a porous Ni or NiO electrode, hydrogen or oxygen diffusion through the Ni or NiO electrode, reaction of hydrogen and oxygen, and proton diffusion through the ceramic electrolyte. The values of parameters appearing in the mathematical model of their respective mass-transfer and charge-transfer processes were determined independently under different conditions of the hydrogen and oxygen partial pressures, temperature and dry or moist atmosphere. The temperature dependence of the parameters was made clear experimentally, and their physical meaning was discussed. The resistance of the reaction between proton and oxygen ion on the cathode electrode was found to be a key factor.