Shung-Ik Lee

A Study of SOFC Anodes Based on Cu-Ni and Cu-Co Bimetallics in CeO2 ­ YSZ

A series of solid oxide fuel cells (SOFCs) with anodes containing mixtures of either Cu and Ni or Cu and Co, prepared by impregnation of the metal salts into porous yttria-stabilized zirconia (YSZ), were examined for operation with both H 2 and n-butane fuels at 973 and 1073 K. The Cu-Ni mixtures formed single-phase alloys; however, anodes prepared with Cu 0.8 Ni 0.2 were found to be unstable in n-butane at 973 K if the alloy was reduced at 973 K, but stable if reduced at 1073 K, indicating a probable change in surface composition with reduction temperature. The addition of Ni to Cu significantly increased the performance of the cells for operation in H 2 , but caused no change in performance for n-butane. Unlike the Cu-Ni system, Cu-Co bimetallics formed two phases. It appears that Cu covers the surface of most of the Co, based on reduced CO adsorption and the fact that Cu 0.5 Co 0.5 mixtures were stable for at least 3 h in n-butane at 973 K. Again, at 973 K, the addition of Co to Cu significantly increased the performance of the cells for operation in H 2 , but caused no change in performance for n-butane. At 1073 K, improved performance was observed for both H 2 and n-butane with anodes based on Cu and Co. The results suggest that bimetallics are worth considering for fuel cells that operate on hydrocarbons.

The Effect of Ca, Sr, and Ba Doping on the Ionic Conductivity and Cathode Performance of LaFeO3

bKorea Institute of Energy Research, Daejeon 305-343, Korea The influence of ionic conductivity on the performance of solid oxide fuel cell cathodes was studied for electrodes prepared by infiltration of 40 wt % La0.8Ca0.2FeO3 LCF ,L a0.8Sr0.2FeO3 LSF, and La0.8Ba0.2FeO3 LBF into 65% porous yttria-stabilized zirconia YSZ. The ionic conductivities of LCF, LSF, and LBF, measured between 923 and 1073 K using permeation rates in a membrane reactor, showed that LSF exhibited the highest ionic conductivities, followed by LBF and LCF. When electrodes were calcined to 1123 K, the performance characteristics of each composite were essentially identical, exhibiting current-independent impedances of 0.2 cm2 at 973 K. When the composites were calcined to 1373 K, the open-circuit impedances were much larger and showed a strong dependence on current density. The open-circuit impedances followed the ionic conductivities, with LSF‐ YSZ electrodes showing the lowest impedance and LCF‐YSZ electrodes the highest. Scanning electron microscopy images and Brunauer‐Emmett‐Teller surface areas indicate that calcination at 1373 K causes the perovskites to form dense layers over the YSZ pores. A model is proposed in which diffusion of ions through the perovskite film limits the performance of the composite electrodes calcined at 1373 K.

An Examination of Carbonaceous Deposits in Direct-Utilization SOFC Anodes

The deposition, stability, and function of carbonaceous films formed by exposing porous yttria-stabilized zirconia (YSZ) anodes in YSZ-based solid oxide fuel cells (SOFCs) to n-butane at elevated temperatures was studied using a combination of four-probe conductivity, impedance spectroscopy, and cell polarization measurements. The carbonaceous deposits were found to have high electronic conductivity and to be relatively stable for steam-to-carbon ratios as high as 3.75. Comparison of the performance of cells in which carbon films were used as the sole current collector in the anode with anodes containing both Cu and carbon films indicated that in the latter case, the carbon layer plays an important role in providing electronic conductivity near the three-phase boundary.

Cu-Co Bimetallic Anodes for Direct Utilization of Methane in SOFCs

Cu-Co bimetallic mixtures with ceria and YSZ have been investigated as solid oxide fuel cell (SOFC) anodes. While pure Co is unstable in dry CH 4 at 1073 K due to severe carbon formation, Cu-Co mixtures with even 90 wt % Co showed only small amounts of carbon following exposure to dry CH 4 for 2 h at 1073 K. Cells based on Cu-Co mixtures showed improved performance in both H 2 and CH 4 compared to cells prepared with only Cu. Finally, a cell with a 50:50 ratio of Cu and Co was shown to be stable for 500 h in humidified (3 mol % H 2 O) CH 4 at 1073 K.