Overall Conductivity and Electromotive Force of SrZr_<0.9>Yb_<0.1>O_<3-a> Cell System Supplied with Moist CH_4

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A proton-conducting ceramic cell for recovering tritium from process streams was investigated for its application to a fusion reactor system. The ceramic cell tested here was composed of a SrZr0.9Yb0.1O3−a tube, one end of which was closed, and Ni/SiO2 and NiO/SiO2 porous electrodes. Its anode was supplied with moist CH4 or H2 and its cathode with moist O2. All of the j-V curves obtained by a direct-current method were correlated to the relation V=E0−jd⁄σ at 600–700°C regardless of the two different conditions of the CH4 + H2O and H2 + H2O supply. The rate-controlling step of charged hydrogen ion transfer was determined from the dependences of the overall conductivity σ and the electromotive force E0 on the anode H2O partial pressure and temperature. The E0 value under the condition of the CH4 + H2O supply was affected by the diffusion of reaction products of CH4 + H2O = CO + 3H2 through the porous anode. On the other hand, the σ value was limited by the oxygen reduction rate at the cathode interface between the ceramic and the Ni electrode regardless of the different conditions between CH4 + H2O and H2 + H2O. These results were consistent with our results obtained by an alternating-current method. The activation energy of the overall conductivity was 60 kJ/mol.
2007-10-25