Direct Carbon Solid Oxide Fuel Cell

Direct carbon fuel cell (DCFC), which converts the chemical energy of carbon directly to electricity with a theoretical conversion efficiency a little bit over 100%, can use simply processed coal as fuel, provides a new way for using coal cleanly and efficiently. Direct carbon solid oxide fuel cell (DC-SOFC) is a whole-solid state device which has no problem of liquid corrosion and leakage. As carbon has a theoretical energy density of ⁹Ah/g, there is great potential to developt DC-SOFC into a high-performance battery. So DC-SOFC has a considerable development prospect. At present, the research of DC-SOFC is still in its early stage.In the work presented in this paper, YSZ electrolyte tubes have been prepared by slip casting. After sintered at 1600oC for 4h, they become dense and their mechanical strength is high. All these properties meet the requirements for preparing the SOFCs.In the present work, we have designed a YSZ electrolyte supporting tubular DC-SOFC device which has a simple and whole-solid-state structure. The output performance of the DC-SOFC using pure graphite as fuel has been researched. The results show that the designed DC-SOFC is feasible. However, the cell performance decayes seriously. The composition of the product produced by the reaction of sufficient carbon and O₂ has been calculated through the thermodynamic equilibrium theory. When the temperature is larger than 750oC, the produced gas is mainly CO, while when the temperature is smaller than 750oC, the produced gas is mainly CO₂. The reaction mechanism of DC-SOFC has been analysed through the experimental results and the theoretical calculation. It is pointed out that the electrochemical oxidation reaction of CO on the anode and the Boudouard reaction of carbon fuel are the key reactions in a DC-SOFC.In the present work, DC-SOFCs with activated carbon are prepared and their output performance, resistance and stability have been studied. The results show that the interfacial polarization resistance of the cell dominates the total loss in the cell. A 37h stable operation of a DC-SOFC with activated carbon has been realized, demonstrating that such DC-SOFCs can be self-sustaining. The cell performance is related to the nature of carbon fuel. It is suggested that the electrochemical oxidation reaction of CO on the anode and the Boudouard reaction of carbon fuel can maintain long-term stable operation as their reaction rates are coordinate.In the present work, GDC and Fe catalysts have been added to the anode and carbon fuel, respectively, and their influence on the cell output performance, impedance performance and stability have been studied. The change of the cell microstructure and carbon fuel micro-morphology before and after the cell reaction also has been characterized. The results show that the added GDC improves the anode structure and catalyses the electrochemical oxidation reaction of CO on the anode, and reduces the interfacial polarization resistance, so that the cell performance is greatly improved. Fe catalyst catalyses the Boudouard reaction of carbon fuel and makes the carbon fuel fully consumed. The stability of the cell is also improved by the added catalysts. EDX analysis shows that after the cell reaction, the fuel residue contains very little carbon that can be ignored. XRD analysis shows that the fuel residue is mainly nanoscale Fe₂O₃.In this paper, the influence of the content of Fe and Ni catalysts on the Boudouard reaction has been studied. When the content of Fe is 5-7% and Ni is of 5% or more, the catalytic effect is the best.In the present work, YSZ electrolyte supporting tubular SOFCs with Ag-SDC and Ni-SDC anode, respectively, have been prepared. The output performance, the impedance performance and stability operated on CO fuel have been studied. Performance of Ag-SDC anode is much better than that of Ni-SDC anode on dry CO fuel. After stable operation of a 95h, the performance of Ag-SDC anode declined only 8.3%, while the performance of Ni-SDC anode declined 77.6%, after stable operation of a 37h. SEM characterization shows that Ag is slightly sintered after the stability test of Ag-SDC anode, while the Ni-SDC anode structure changes considerably and Ni is sintered seriously. EDX and XRD analysis show that carbon deposition on Ag-SDC anode surface is rare that can be ignored, while carbon deposition on Ni-SDC anode surface is very serious.