| The solid oxide fuel cell(SOFC) is a whole-solid-state structure energy conversion device which can convert the chemical energy into electrical energy. SOFC has been widely investigated due to its high energy conversion efficiency, low pollution emission and feul flexibility. SOFC can not only use hydrogen and hydrocarbon but also use solid carbon as fuel. The DC-SOFC is defined as a SOFC directly using solid carbon as fuel without fedding gas. DC-SOFC has attracted increasing attention because of its simple construction, whole solid state construction and high volume energy density. At present, the research of DC-SOFC is still in its early stage. This thesis focuses on the anode reaction mechanism of the DC-SOFC, firstly verified the reaction mechanism of DC-SOFC, and then demonstrated that the DC-SOFC device can be taken as an electricity-gas cogeneration system, aiming to provide certain theoretical basis for the practical application of DC-SOFC.For the purpose of simple fabrication of stable SOFC for the fundamental research of DC-SOFC, dip-coating technique which is fast, low-cost and especially suitable for complex-shaped ceramic component is developed to fabricate tubular YSZ supported electrolyte membrane. The critical technological parameters of the dip-coating process are investigated in details. The sintering behavior and ionic conductivity of YSZ are improved by adding 1wt%Al2O3, consistents with the previous studies. It is found that higher solid content of the electrolyte slurry resulted in higher density of the electrolyte membrane after sintered at 1400 oC for 5h. Tubular YSZ membrane in thickness of 160μm, relative density of 96% and with high mechanical strength is successfully prepared using the slurry with solid content of 38%. All these properties meet the requirements for preparing stable and high performance electrolyte-supporting SOFCs.It has been presumed that the reaction mechanism of a DC-SOFC consists of the cycle of the electrochemical oxidation of CO(CO + O2- ò CO2) taking place on the anode and the Boudouard reaction on the carbon fuel(C + CO2ò2CO). In the work presented in this thesis, the anode reaction of a DC-SOFC is simulated by fueling a SOFC directly with CO fuel, in order to direct demonstration the proposed anode reaction mechanism of DC-SOFC. Tubular SOFC single cell with the construction of Cu-Ce O2-YSZ|YSZ|Ag is fabricated by dip-coating and infiltration techniques. The output performances, AC impedance of the SOFCs operated on solid carbon and pure CO, are compared and analyzed. The results show that both I-V curves and Ac impedances of the two cells are quiet alike, especially at higher temperatures(800, 850oC). The experimental results strongly support that the two cells have the same electrochemical reaction, which is the electrochemical oxidation of CO. Therefore, the assumption that the reaction mechanism of a DC-SOFC is the cycle of the electrochemical oxidation of CO taking place on the anode and the Boudouard reaction on the carbon fuel has been directly verified.On the basis of the above works, we proposed that the carbon in a DC-SOFC is generally partly oxidized because CO is the favored product at high temperature rather than CO2. The emitted gas is CO dominated CO-CO2 mixture. The output CO is a valuable product with high chemical energy. Therefore, the DC-SOFC device can be taken as an electricity-gas cogeneration system. Tubular DC-SOFCs with the construction of Ag-GDC|YSZ|LSM-YSZ are fabricated and operated on 5wt% Fe-loaded activated carbon. The performances of the DC-SOFCs are investigated through characterizing the electrical power output, CO production rate. The results demonstrated that electricity and CO gas can be cogenerated through a DC-SOFC. It turns out that a rapid rate of the Boudouard reaction is necessary for getting high electrical power and CO production. When the produced CO is considered as a kind of produced energy, the overall conversion efficiency is about twice of the electrical conversion efficiency. The over all conversion efficiency of 76.5% for a DC-SOFC under constant current of 1A is obtained.To improve the discharge time and capacity of a tubular DC-SOFC, a durable performance tubular DC-SOFC electricity-gas cogeneration system with external anode is fabricated, with larger quartz tube as carbon fuel container. The stability and discharge tests of the DC-SOFCs are analyzed at 800 oC under different constant currents. The DC-SOFCs filled with 8.0g 5wt% Fe-loaded carbon fuel show discharge times of 36, 21, 10 and 8 h at constant currents of 0.75, 1.5, 3.0 and 4.0A, respectively. CO producing rates of 6.5, 13.5, 22.1 and 28.8ml min-1 are obtained, respectively, under corresponding currents. The electricity conversion efficiency and overall conversion efficiency of the DC-SOFCs at different currents are analyzed in details.To promote the practical application of DC-SOFC, the performance of DC-SOFC fuelling with coke is investigated based on an YSZ-supported SOFC. The results show that the electricity-gas cogeneration of coke can be realized through a DC-SOFC. The DC-SOFC shows OCV and maxium power density of 0.92 V and 116 m W cm-2 at 800 oC. The CO producing rate reduced quickly during the discharge test. Obviously, its performance is much lower than that operated on activated carbon due to lower Boudouard reaction activity of coke. The BET and Raman spectroscopy analysis show that the specific surface area and the number of defect structure of coke are much lower than that of activated carbon, respectively. Therefore, improving the Boudouard reaction activity of coke, such as adding superior catalyst, is the key to improve the performance of the DC-SOFC fuelling with coke.Under the inspiration of the above works, we proposed and investigated the performance of solid oxide electrolysis cells(SOECs) with Ag-GDC cathode for pure CO2 reduction. Electrolysis of CO2 can be preformed in YSZ-supporting SOEC with Ag-GDC fuel electrode. The SOEC operated on pure CO2 apparently shows lower minimum electrolytic voltage than that with reducing gas feeding. At lower voltages, the SOEC is operated as an oxygen pumper, while the main electrochemical process at higher voltages is the electrochemical reduction of CO2. The SOEC achieved higher Faraday efficiency and energy conversion efficiency at higher voltages. Stability test demonstrated the feasibility of using Ag-GDC as the cathode of SOECs for pure CO2 reduction. The impedance spectra show that the ohmic resistance is the main loss at high voltages; therefore the performance of the SOEC may be further improved by reducing the thickness of the electrolyte. |
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