The performance of a single-chamber solid oxide fuel cell was studied using a ceria-based solid electrolyte at temperatures below 773 kelvin. Electromotive forces of approximately 900 millivolts were generated from the cell in a flowing mixture of ethane or propane and air, where the solid electrolyte functioned as a purely ionic conductor. The electrode-reaction resistance was negligibly small in the total internal resistances of the cell. The resulting peak power density reached 403 and 101 milliwatts per square centimeter at 773 and 623 kelvin, respectively.
There is worldwide interest in the development and commercialization of fuel cells for vehicles and portable electric devices. It is a presently accepted notion that polymer electrolyte fuel cells (PEFCs) are the only devices capable of operating at low temperatures. We believe, however, that PEFCs are not perfect from a practical point of view because they require hydrogen as the fuel, which is impractical in terms of storage and handling. An external reformer, therefore, must be used to convert more viable alcohols and hydrocarbons into hydrogen, thereby defeating their portability. There have been recent successes with solid oxide fuel cells (SOFCs) which perform well between 500 and 700ЊC directly using alcohols and hydrocarbons as the fuels. 1-3 A further reduction in the operating temperature of internal-reforming SOFCs and an enhancement of their thermal-and mechanical-shock resistance would make this technology a promising alternative to PEFCs.A novel type of fuel cell, which is distinguished from conventional fuel cells in design and principle, has been proposed by many researchers. 4-8 This fuel cell consists of only one gas chamber, where both the anode and the cathode are exposed to the same mixture of fuel and air. We will use a single-chamber fuel cell (SCFC) as a notation for this type of fuel cell. Because there is no need to separate the supply of fuel and air, it is more thermally and mechanically shock resistant than conventional fuel cells. We have recently succeeded in applying this cell design to an SOFC constructed from an yttria-stabilized zirconia (YSZ) electrolyte with a Ni-based anode and a strontium-doped lanthanum manganite (LSM) cathode. 9 This SCFC exhibits high power density in a flowing mixture of methane and air, but it must operate at the high temperature of 950ЊC in order to achieve sufficient ionic conduction in the solid electrolyte.The operation of SOFCs at reduced temperatures causes excessive ohmic and polarization losses in the cell. Thus, it is necessary to use a highly conductive electrolyte together with a highly active anode and cathode. Lanthanum gallate-10-14 or ceria-based 15-18 oxides would be promising electrolytes because of their much higher ionic conductivities than that of YSZ. In addition, Ni-ceria cermets 19 and Co-based perovskite oxides, 20,21 which exhibit mixed ionic and electronic conduction under each of their respective operating conditions, have been generally regarded as suitable anodes and cathodes, respectively, at reduced temperatures.In this study, we demonstrate that it is possible to operate a thermally and mechanically shock-resistant SOFC at reduced temperatures by combining the advantages of using a highly conductive electrolyte with the single-chamber cell design. We also show that ethane, propane, and liquefied petroleum gas (LPG) can be successfully used as the fuels in the present SCFC, especially at operating temperatures below 550ЊC. Figure 1a shows an SCFC constructed for fuel-cell tests at reduced temperatures. ExperimentalThe perf...
Electrocatalytic oxidation of methane over anodes in single-chamber solid oxide fuel cells, 0-10 wt % Pd-30 wt % Ce 0.8 Sm 0.2 O 1.9 (samaria-doped ceria, SDC)- Ni | SDC | Sm 0.5 Sr 0.5 CoO 3 , was studied in a mixture of methane and air between 450 and 550°C. The addition of a small amount of Pd (0.145 mg cm−2) to the anode significantly promoted the partial oxidation of methane by oxygen to form hydrogen and carbon monoxide, which resulted in electromotive forces of ca. 900 mV from the cell and extremely small electrode-reaction resistances of the anode. The peak power densities, when using a 0.15 mm thick SDC electrolyte, reached 644, 467, and 269 mW cm−2 at 550, 500, and 450°C, respectively. © 2002 The Electrochemical Society. All rights reserved.
The performance of a solid oxide fuel cell ͑SOFC͒ with the configuration, H 2 , 3 wt % Pd-loaded FeO͉25 mol % Y 3ϩ -doped BaCeO 3 (BCY25)͉Ba 0.5 Pr 0.5 CoO 3 , air, was studied between 350 and 600°C. The BCY25 electrolyte showed higher ion conductivities than 8 mol % yttria-stabilized zirconia ͑YSZ͒ below 800°C and 20 mol % Sm 3ϩ -doped ceria ͑SDC͒ below 600°C, thus having the smallest ohmic resistance loss during cell discharge below 600°C among the three electrolytes. The overpotentials of the Pd-loaded FeO anode and the Ba 0.5 Pr 0.5 CoO 3 cathode at 600°C were 25 and 53 mV, respectively, at 200 mA cm Ϫ2 , which were less than one-fourth those of a Pt electrode. The resulting peak power density was higher than those obtained for two SOFCs using the YSZ and SDC electrolytes with a Ni-SDC anode and a Sm 0.5 Sr 0.5 CoO 3 cathode below 600°C.Considerable research efforts have recently been devoted to developing solid oxide fuel cells ͑SOFCs͒ capable of operating below 600°C for vehicular applications. 1-10 A key subject for such development is the use of a highly ion-conductive electrolyte with a highly active anode and cathode. Different cations-doped ceria, notably Sm 3ϩ -doped ceria ͑SDC͒, are promising electrolytes, because they show much higher ion conductivities than those of the commonly used yttria-stabilized zirconia ͑YSZ͒. However, it is difficult for these materials to exhibit high performance around 400°C. For example, even a 5 m thick SDC electrolyte is calculated to show a large ohmic resistance of 3 ⍀ cm Ϫ2 at 400°C from its ion conductivity. Accordingly, alternative electrolytes to ceria-based electrolytes are required for the commercialization of the low operating temperature SOFCs.Barium cerate (BaCeO 3 ) materials exhibit proton conduction over the wide temperature range of 300 to 1000°C by substituting trivalent cations such as Y 3ϩ , Gd 3ϩ , and Nd 3ϩ on some Ce 4ϩ sites. 11-17 Although the conductivities of presently available BaCeO 3 are near or lower than those of the doped ceria, these materials have the possibility that meet the above criterion for the following two reasons. First, proton conduction in the doped BaCeO 3 is governed by a mechanism based on the hopping of an extremely small proton between adjacent oxide ions, 12,13,15-17 which could result in lower activation energies for proton conduction than those for oxide-ion conduction in YSZ and the doped ceria. Second, the solubility of protons in the BaCeO 3 bulk increases as the temperature is reduced, 13,15,18 which could enhance the preexponential of proton conduction with decreasing temperature.Electrode kinetics are also crucial to the operation of SOFCs at low temperatures. Several studies on the kinetics for the electrode reaction over supported catalysts have showed that Pd, Pt, Rh, and Ru promote the anodic reaction of fuels. [19][20][21][22] In addition, it is generally accepted that Co-based perovskite oxides are suitable cathodes at reduced temperatures. 23-25 More recently, Pr-doped perovskite oxides have also been rep...
Performance of a single-chamber solid oxide fuel cell was evaluated using a 0.15 mm thick Sm-doped ceria ͑SDC͒ electrolyte together with a 30 wt % SDC-Ni anode and a Sm 0.5 Sr 0.5 CoO 3 cathode at heating temperatures below 500°C in a flowing mixture of butane and air. A large quantity of reaction heat, which was evolved by the partial oxidation of butane by oxygen at the anode, caused a temperature rise of more than 100°C at the anode, followed by thermal conduction to the cathode through the electrolyte. Simultaneously, the cell generated a large electromotive force of ca. 900 mV between the two electrodes. The resulting peak power density reached 245, 180, 105, and 38 mW cm Ϫ2 at heating temperatures of 450, 400, 350, and 300°C, respectively. The comparison of the butane fuel with the other hydrocarbon fuels showed that the fuel cell performance became enhanced, especially at reducing temperatures, as the carbon number of the hydrocarbon increased, and the chain structure was branched.Currently, there is great interest in the development of solid oxide fuel cells ͑SOFCs͒ capable of starting up at low temperatures for vehicular applications. 1-6 A key subject for this development is the use of a highly conductive electrolyte with a highly active anode and cathode, because the operation of SOFCs under such conditions causes excessive ohmic and polarization losses in the cell. Ceriabased oxides are promising electrolytes which exhibit much higher ionic conductivities than that of yttria-stabilized zirconia ͑YSZ͒. 7-13 In addition, metal-ceria cermets 14-16 and Co-based perovskite oxides [17][18][19] have been generally regarded as suitable anodes and cathodes, respectively, at reduced temperatures. It is, however, difficult for these materials to exhibit high performance below 400°C. For example, even a 5 m thick Sm-doped ceria ͑SDC͒ electrolyte is calculated to show a large ohmic resistance of 7 ⍀ cm Ϫ2 at 300°C from its ionic conductivity.In this study, we demonstrate a novel technique enabling the SOFC to meet rapid low temperature startup criteria for electric vehicles. The conversion of hydrocarbon fuels into hydrogen and carbon monoxide is mainly achieved by two reactions: the steam reforming of hydrocarbons 20-23 and their partial oxidation by oxygen. [24][25][26][27] Reaction 1 is endothermic, so that it requires a large amount of heat to sustain an adequate reaction rate. In contrast, Reaction 2 is exothermic, thus evolving reaction heat according to the carbon number of the hydrocarbons CH 4 ϩ 1 2 O 2 → 2H 2 ϩ CO ⌬H ϭ Ϫ25.03 kJ mol Ϫ1 at 423°C ͓3͔ C 2 H 6 ϩ O 2 → 3H 2 ϩ 2CO ⌬H ϭ Ϫ120.66 kJ mol Ϫ1 at 423°C ͓4͔ C 3 H 8 ϩ 3 2 O 2 → 4H 2 ϩ 3CO ⌬H ϭ Ϫ208.29 kJ mol Ϫ1 at 423°C ͓5͔ C 4 H 10 ϩ 2 O 2 → 5H 2 ϩ 4CO ⌬H ϭ Ϫ292.54 kJ mol Ϫ1 at 423°C ͓6͔If Reaction 2 of a higher hydrocarbon, such as butane, is used as the internal reforming reaction in the SOFC, the anode temperature can effectively increase due to the evolution of a large quantity of reaction heat. A similar increase in the temperature can also be ex...
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