A Solid Oxide Fuel Cell with a Novel Geometry That Eliminates the Need for Preparing a Thin Electrolyte Film

Abstract

We propose a solid oxide fuel cell design based on a configuration of two electrodes on the same surface of the electrolyte in a flowing mixture of different hydrocarbons and air between 500 and 600°C. The ohmic resistance can be reduced without using a thin electrolyte film due to a significantly enhanced performance by the approach of the two electrodes to each other on the smooth electrolyte surface. The fuel cell performance, especially at reduced temperatures, is further improved by using a more reactive hydrocarbon fuel and a more catalytically active anode. The resulting power density reaches 122 mW cm Ϫ2 using 2 mm thicker electrolyte at 500°C.Conventional solid oxide fuel cell ͑SOFC͒ designs are based on a configuration of the two electrodes on opposite surfaces of the electrolyte. This essentially requires the use of a thin electrolyte film to reduce the ohmic resistance of the electrolyte, especially at reduced temperatures. 1-4 For self-supporting electrolytes, the mechanicalshock resistance of the SOFC becomes gradually lower as the thickness of the electrolyte decreases. For anode-or cathode-supported electrolytes, high fabrication cost is referred as one of their major problems. However, the requirement described above would be eliminated by the fabrication of a SOFC which arranges the two electrodes on the same surface of the electrolyte, because a reduction of the ohmic resistance could be achieved by the placement of the two electrodes close together. Accordingly, it permits the use of a strong and thick electrolyte sheet, thus making the cell more mechanically shock resistant than conventional SOFCs. It also allows the use of low cost traditional ceramics processing techniques such as tape casting and calendering. However, this SOFC design creates a serious problem that makes it very difficult to separate the supply of fuel and air.Many researchers have proposed a type of potentiometric device with only one gas chamber, where the anode and the cathode are exposed to the same mixture of reducing gas ͑hydrogen, carbon monoxide, or hydrocarbons͒ and oxidizing gas ͑oxygen, water vapor, or carbon dioxide͒. [5][6][7][8][9][10][11][12] We have recently succeeded in applying this single-chamber device to a SOFC constructed from a Sm 3ϩ -doped ceria ͑SDC͒ electrolyte with a SDC-Ni anode and a perovskite cathode, although the two electrodes were attached to opposite surfaces of the electrolyte. 13 Based on this result, it could be extrapolated that if the two electrodes were placed on the same surface of the electrolyte in a mixture of fuel and air, one would fabricate exactly the SOFC intended above. In this study, we report the performance of such a SOFC using methane, ethane, propane, and butane fuels, where details are investigated by changing the electrolyte species, their surface morphology, and the electrode form or configuration. An improvement of the SOFC performance by the addition of a metal oxide to the anode is also reported. ExperimentalFabrication of SOFC.-A Ce 0.8 Sm 0.2 O 1.9 ͑SDC͒ elect...

Single-Chamber Solid Oxide Fuel Cell Technology—From Its Origins to Today’s State of the Art

“…Similarly, reduction of the ohmic cell resistance and increase in peak power density by reducing the electrolyte surface roughness were found for an SDC electrolyte [147]. A decrease in surface roughness from 1.6 to 0.06 m led to an increase in power density from 68 to 90 mW·cm In addition to the electrolyte surface morphology, the electrode feature sizes, that is, electrode width w and inter-electrode gap d (see Figure 9), affect the ohmic cell resistance [69,107,117,147,150,180]. In a theoretical study on SC-SOFCs with coplanar electrodes, based on electrode and electrolyte resistance considerations [173], maximum performance was calculated for very small electrode widths and gap sizes on the order of only a few micrometers to minimize the ohmic resistance.…”

“…Addition of Pd, however, favored the complete oxidation of butane and a performance decrease was observed, in contrast to [147] where the addition of Pd was beneficial to the performance of a SC-SOFC with coplanar electrodes.…”

“…In the case of a BaCe 0.8 Gd 0.2 O 3-α electrolyte, a smoothly polished surface enabled a surface ionic conductivity similar to its bulk conductivity and led to a smaller ohmic resistance and better cell performance, as compared to cells composed of electrolytes with roughened surface [174]. Similarly, reduction of the ohmic cell resistance and increase in peak power density by reducing the electrolyte surface roughness were found for an SDC electrolyte [147]. A decrease in surface roughness from 1.6 to 0.06 m led to an increase in power density from 68 to 90 mW·cm In addition to the electrolyte surface morphology, the electrode feature sizes, that is, electrode width w and inter-electrode gap d (see Figure 9), affect the ohmic cell resistance [69,107,117,147,150,180].…”

“…Reducing the inter-electrode distance from 5 to 0.5 mm led to an increase in the maximum current from 8 to 24 mA at 950 °C in a methane-oxygen mixture (R mix = 2). The OCV was found to be independent of the variation of the inter-electrode gap [180,184,185], which principally affected the ohmic resistance of the electrolyte [147].…”

“…Similarly, for a cell with coplanar electrodes of Ni-SDC and SSC on an SDC electrolyte tested at 600 °C in a C 2 H 6 -air mixture, a reduction of the gap size from 3 to 0.5 mm (electrode width fixed at 0.5 mm) caused an increase in peak power density from 38 to 193 mW·cm when the electrode width was decreased from 1 to 0.5 mm (gap size fixed at 1 mm) [147]. Compared to a dual-face cell with an electrolyte thickness equal to the inter-electrode gap of the single-face SC-SOFC, the latter exhibited a higher ohmic resistance and lower cell performance.…”