Overall conductivity and electromotive force of SrZr_0.9Yb _0.1O_3-a cell system supplied with moist CH₄

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 SrZr_0.9Yb _0.1O_3-a tube, one end of which was closed, and Ni/SiO ₂ and NiO/SiO₂ porous electrodes. Its anode was supplied with moist CH₄ or H₂ and its cathode with moist O ₂. 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 + H ₂O 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.