Design And The Performance Study Of New Type Solid Oxide Fuel Cells

With the rapid growth of population and economy, global energy consumptionincreases strongly, which has stimulated intense research on renewable energy conversionand storage systems with high efficiency, low cost and environmental friendliness. Fuelcells which is an energy conversion device, directly convert the chemical energy ofgaseous or liquid fuels into electrical energy by a highly efficient and cleanelectrochemical oxidation process. The fuel will not need to be fired, so the fuel cell notlimited by the Carnot cycle. Fuel cells offer high chemical-to-electrical conversionefficiency. In order to guarantee future energy requirements and reduce pollutantemissions preclude, the wide spread development of fuel cells technology will becomemost important for staff scientist. Solid oxide fuel cells （SOFCs） are a forward lookingtechnology for a highly efficient，environmental friendly power generation. The traditionalSOFCs are operated at high temperature, the high operating temperature promotes reactionkinetics for gas oxidation or reduction, eliminates the need of precious metal catalysts. Atsuch a high temperature, it allows internal reforming of hydrocarbon fuels into H₂and COto suitable for fuel cells fuel gas. All the above advantages proved that SOFCs are verypromising for further research.So far， from the economics perspective, in energy production, SOFCs cannotcompeted with thermal power generation or hydraulic electro generating. As aconsequence, significant effort has been devoted to the development of intermediatetemperature SOFCs. However，it is still a challenge for SOFCs to reduce the operationtemperature to the intermediate temperature. The over all electrochemical performance ofan SOFC will decrease with the reduction in the operating temperature due to increasedpolarization resistances of the electrode reaction and decreased electrolyte conductivity. Akey obstacle to reduce temperature operation is the poor activity of traditional cathodematerials, which has become the limiting factor in determining the overall cellperformance. Therefore, the developments of new cathode with high catalytic performancefor the oxygen-reduction reaction or design new cell structure are critical for intermediate temperature SOFCs, so cathode is becoming more and more the center of attention.The development of practical application of SOFCs has been still hindered by someproblems. New material research and structure design need further study to optimize theSOFCs allocation. In this paper we put forward the concept of―Thermoelectricsolid-oxide fuel cells‖. The basic concept is that: although the operating factor of fuel gasfor SOFCs is high, during the SOFCs operating, it is unavoidable for SOFC to create greatheat by ohmic resistance and electrode polarization. Thermoelectric materials are a kind ofsemiconducting functional materials, which can be used to interconvert heat energy andelectricity energy directly. Thermoelectric materials will convert some waste heat intoelectricity under different temperature condition. This character can be used in SOFCs toconvert heat into electricity. We put forward to unite the SOFCs and thermoelectric powergeneration. The―thermoelectric SOFCs‖performs the follow sequence operations: withsuitable thermoelectric materials as replacement of parts for conventional SOFCs subunitconstruction, such as cathode, anode or connect materials. The SOFCs will work on theassumption that no more additional components was used, by redesigning or reworkingthe new electrode structure, if the thermoelectric materials can be used as SOFC electrode,connection or sealing material with a special structure or shape, it will not only play thesame effect as the original device, but also can be used to reduce the cell temperaturedifference to a certain extent. So the thermoelectric components will be useful for theSOFC thermal system. And then the SOFCs design and flow chart opens up the possibilityof getting a favor for the stack thermal system by using special cell component from thethermoelectric materials.For insight into the additional contribution from the thermoelectric voltage producedby the thermoelectric cathode owing to the temperature gradient in the SOFC, a porouscolumned cathode was fabricated for testing. Below are some simple yet specific steps wecan take to test the thermoelectric voltage: first, we fabricate a special structure SOFCswith conventional anode, buffer layer and electrolyte, a new structure cathode was used toverify the thermoelectric voltage can produced by the thermoelectric cathode owing to thetemperature gradient. The new structure cathode was a porous column with1cm indiameter and1.5cm in height. The porous column was used as cathode and attached to the SOFCs cathode side with Ag paste as a binder. In SOFCs, the electrochemical reaction forH₂or CH4oxidation is exothermic in nature (for example,2H₂（g）+O₂（g）=2H₂O （g）,H727°C=-236.46kJ mol^-1). In addition, the ohmage and overpotential will inevitablygive rise to heat loss in the cell. Therefore, the operational SOFCs produce a lot ofadditional heat energy, which provides an appreciable temperature gradient to ambienttemperature. We use the proper P-type thermoelectric semiconductor material as cathode,and the created thermoelectric voltage by waste heat is consistent with the cell voltage, sothe total voltage is the sum of cell voltage and thermoelectric voltage.In the second chapter, we proved that the P-type material NaCo₂O₄can be used asthermoelectric cathode, further, the high temperature thermal conductivity and Seebeckcoefficient was tested. The thermoelectric voltage at800°C is13.9mV for NaCo₂O₄, andthe average voltage increase from1.1639V to1.1497V. Indicating that a certain electricvoltage difference was created by the columned cathode. In our experiment, we found thatNaCo₂O₄could not connect well with La0.8Sr0.2Ga0.83Mg0.17O3-（LSGM） or Ce0.8Sm0.2O₂-（SDC） electrolyte after sintering. To solve this problem, CuCo₂O₄with the spinel cubicstructure was introduced between the electrolyte and the cathode. Nominal solidsNa1–xCuxCo2O4（0≤x≤1, NCCO） were employed as cathode materials. The single cellwith this nominal solid as cathode gets a very superior power density, the chemicalstability and cell durability is very well too. This proved that nominal solidsNa1–xCuxCo2O4（0≤x≤1, NCCO） is very suitable for SOFCs thermoelectric cathode.In the third chapter, the power density and thermoelectric performance was testedwhen Ca₂Co₂O₅was used as thermoelectric cathode. The maximum power density reached522mW cm-2at800°C and415mW cm-2at750°C. As previously reported, the highpower density demonstrates that Ca₂Co₂O₅is an excellent cathode candidate for a SOFC.The difference in average voltage between the two ends of the columned cathode is11.5mV, which proves the existence of a thermoelectric voltage generated between the twoends of the elongated cathode. The coexistence of high-spin Co4+:t3e2and Co3+:t4e2onoctahedral sites sharing common edges would give facile polaron hopping along theoctahedral-site ribbons in analogy from hopping of elections from d6to d5high-spinconfigurations across shared octahedral-site edges. Finally, the excess oxygen ion allows the interstitial oxygen of a double-well potential to move in three dimensions to give goodoxide-ion mobility at the operating temperature of the thermoelectric SOFC.In the fourth chapter, the maximum power density Pmaxfor Ca₃Co₂O₆as SOFCsthermoelectric cathode reached to1.47W/cm2at800oC and1.14W/cm2at750oC with100μm LSGM as electrolyte. Ca₃Co₂O₆cathode not only can be used for LSGMelectrolyte, but also with intermediate-temperature （IT） SDC and BaZr0.1Ce0.7Y0.2O3（BZCY） as electrolytes. The ξ is1.54for Ca₃Co₂O₆and1.21for BSCF. The bigger ξindicates that Ca₃Co₂O₆is more sensitive than BSCF for oxygen catalyzing reaction asSOFC cathode. For the thermodynamic testing, we test the TEC distribution,thermal-stress curves and the interfacial shearing stress. All the three testing proved thatCa₃Co₂O₆can match better with LSGM than BSCF. Finally, it is proved that Ca₃Co₂O₆also can be used as thermoelectric cathode to produce thermoelectric voltage owing to thetemperature gradient.In the fifth chapter, Sr₂Co_1+xMo_1-xO_6-δ（x=0.1,0.15,0.2） were used as anode, cathodeand symmetric electrodes for SOFCs, respectively. When x=0.1was used as anode whichexhibits the maximum power density （Pmax） approximately660mWcm-2among all the9cells. The conductivity for SCMO in H₂is dominated by electrons from the Mo⁶⁺/Mo⁵⁺couple. When x=0.2was used as cathode, the Pmaxis540mW cm-2. The cobalt-basedcathode materials, the main contribution of catalytic activity for oxygen decompositioncomes from Co ions, and the Co³⁺–O–Co2+couples provide small polarizationconductivity. When x=0.15was used as symmetric electrode, which exhibit both highstability and conductivity both in air and hydrogen atmosphere. Cell with x=0.15assymmetric electrode shows the power density as high as460mW cm-2at800°C.