Fuel cells directly and efficiently convert chemical energy to electrical energy. Of the various fuel cell types, solid-oxide fuel cells (SOFCs) combine the benefits of environmentally benign power generation with fuel flexibility. However, the necessity for high operating temperatures (800-1,000 degrees C) has resulted in high costs and materials compatibility challenges. As a consequence, significant effort has been devoted to the development of intermediate-temperature (500-700 degrees C) SOFCs. A key obstacle to reduced-temperature operation of SOFCs is the poor activity of traditional cathode materials for electrochemical reduction of oxygen in this temperature regime. Here we present Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta)(BSCF) as a new cathode material for reduced-temperature SOFC operation. BSCF, incorporated into a thin-film doped ceria fuel cell, exhibits high power densities (1,010 mW cm(-2) and 402 mW cm(-2) at 600 degrees C and 500 degrees C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. We further demonstrate that BSCF is ideally suited to 'single-chamber' fuel-cell operation, where anode and cathode reactions take place within the same physical chamber. The high power output of BSCF cathodes results from the high rate of oxygen diffusion through the material. By enabling operation at reduced temperatures, BSCF cathodes may result in widespread practical implementation of SOFCs.

CeO 2 (Alfa Aesar 11328) was ball-milled with 16 wt% starch (Mallinckrodt 8188-2) for 19 to 24 h in anhydrous ethanol, and, after drying, mixed with 42 wt% graphite powders (-100 mesh, Alfa Aesar 14735). The composite powder was uniaxially pressed into quarter-circular-arc pieces. Subsequent heat-treatment at 1,500° C for 5 hr induced both thermal decomposition of the starch and graphite poreformers and sintering to yield suitably mechanically robust porous monoliths. The pieces measured approximately 40 mm in arc length, 8 mm in thickness, 9 mm in height. Porosity was determined by a simple measurement of the geometric dimensions and sample mass and comparing to the theoretical density of CeO 2 , 7.2 g cm-3 ; it was found to be 80 %. X-ray powder diffraction data were collected using a Philip X'Pert PRO diffractometer (Cu kα, 45 kV, 40mA). Scanning electron microscopy was performed on a Carl-Zeiss 1550 VP.

Cell voltage versus current density (polarization curves) for several CsHSO4 fuel cells with differing Pt loadings operated under the conditions indicated. Platinum content was varied by changing the thickness of the electrode layer while maintaining the overall composition.

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