Low Cost Anode-supported Electrolyte Film Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are solid-state electrochemical device that convert the chemical energy of reaction directly into electrical energy. It possesses several distinct advantages, such as environmental friendship, higher conversion efficiency and broad adaptive fuels, which is a new green energy developed in the world for 21st century and attracting more and more attentions of researchers. It is necessary to lower the operation temperature and develop the intermediate SOFCs (IT-SOFCs)for the commercialization of SOFCs. Reducing the thickness of electrolyte to prepare the anode-supported electrolyte film SOFCs is an effective approach to lower the operation temperature and reduce the cost of SOFCs while to retain the high performance and extend the life-span of SOFCs. Therefore, development of cost-effective fabrication technology of anode-supported SOFCs is a key step for the industrialization and commercialization of SOFCs.Colloidal spray coating technique was developed to fabricate Y2O3-stabilized ZrO2 (YSZ) electrolyte thin films onto NiO-YSZ porous anode substrates. The important technological parameters of colloidal spray coating, including the preparation of YSZ suspension and the skills of the technology, were investigated in detail. The results showed that the properties of the YSZ suspension exerted a significant effect on the microstructure of the YSZ films. 15μm-thick YSZ film was successfully coated on the anode substrate by colloidal spray coating. Cells with dense YSZ film provided open-circuit voltage (OCV) of over 1V above 600℃, which is very close to the OCV calculated by Nernst equation. The maximum power density of the cell based on YSZ film using La0.8Sr0.2MnO3 (LSM)-YSZ composite cathode is as high as 0.99W/cm2 at 800℃using humidified hydrogen as fuel and ambient air as oxidant.Anode-supported SOFCs with Smo.2Ce0.8O1.9 (SDC) electrolyte films were fabricated by colloidal spray coating technique as described above and tested using CHI604B electrochemical instrument. Three kinds of SDC powders used to fabricate SDC electrolyte films were synthesized by glycine-nitrate process, citrate-nitrate method and solid-state reaction, respectively. The microstructure of SDC films and the performance of the single cell were compared. The results showed that SDC powders which had been synthesized by citrate-nitrate method were suitable for preparing electrolyte films by colloidal spray coating technique for SOFCs. SDC film and the electrodes contacted well, and the surface of the sintered SDC film was crack-free while there were some pinholes in the cross section. The electrochemical test results revealed that the cell based on SDC film can provide OCV as high as 0.9V.It is an effect approach to reduce the preparation costs of SOFCs by partially replacing Ni with Fe in the anode. Ni1-xFexO (x=0.01,0.04,0.08,0.1,0.15,0.2,0.4,0.5) anode powders were synthesized by glycine-nitrate process (GNP). The single cells consisted of Ni1-xFexO-YSZ anode, YSZ electrolyte film, LSM-YSZ composite cathode were prepared and tested at the temperature from 600℃to 850℃with humidified hydrogen and methane (75ml/min), respectively, as fuel and ambient air as oxidant. The results showed that the cell performance had been improved by partial substitution of Ni with Fe. And the cell with Ni0.9Fe0.1O-YSZ anode showed the highest power density,1.238W/cm2 at 850℃, among the cells with different anode composition. It was found that Ni1-xFexO-YSZ anode might decrease coking, extend the life of the cell and maintain the stability of the anode.A simple and cost-effective slip casting technique was successfully developed to fabricate NiO-YSZ anode substrates for tubular anode-supported single SOFCs. The key technological parameters of slip casting were investigated in detail. The results showed that the optimum technological parameters of slip casting were using 10wt%～15wt% Arabic resin (balata) as dispersant; pH with 4～5; ball milling the raw materials for 2 hours; ball milling the Arabic resin (balata) for 20 minutes and using glycine-nitrate process to prepare NiO. A single cell, NiO-YSZ/YSZ(20μm)/LSM-YSZ, using the tubular anode support with YSZ coating, was assembled and tested to demonstrate the feasibility of the techniques applied. Using humidified hydrogen (75ml/min) as fuel and ambient air as oxidant, the maximum power densities of the cell were 760mW/cm2 and 907mW/cm2 at 800℃and 850℃, respectively. The results proved that slip casting technique may be promising for tubular anode substrate fabrication.The cone-shaped tubular anode substrates were fabricated by slip casting technique based on the technique as mentioned above. And the YSZ electrolyte films were deposited onto the anode tubes by dip coating method. A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC was fabricated by invaginating one by one. Its typical operating characteristics were investigated, particularly with respect to the thermal cycling test. The results showed that the single cell with anode functional layer, NiO-YSZ/YSZ (7μm)/LSM-YSZ, provided a maximum power density of 1.78W/cm2 at 800°C, using moist hydrogen (75ml/min) as fuel and ambient air as oxidant. And the ohmic resistance of the cell was 0.04Ωcm2 the polarization resistance was 0.14Ωcm2。Both the ohmic resistance and the polarization resistance of the cell were reduced by introducing the anode functional layer. The two-cell-stack presented an open circuit voltage (OCV) of about 1.7V and a maximum output power of 2.64W (1=3A) at 800℃. The two-cell-stack had experienced twelve periods of thermal cycling test and showed good thermo-mechanical properties. The developed segmented-in-series SOFC stack is highly promising for portable applications.Anode-supported cone-shaped tubular solid oxide fuel cells were successfully fabricated by a single-step cosintering process. The key facts of single-step cosintering process, which significantly influences the characterization of YSZ electrolyte and electrode, including sintering procedure and highest sintering temperature were investigated. The results showed the microstructure and dense property of YSZ was the key step in single-step cosintering process. Cell electrochemical test results and scanning electron microscope graphs showed that the optimal procedure for the single cone-shaped tubular SOFCs was that, first raising the temperature to 400℃with a heating rate of 1℃/min and dwelling at 400℃for 1 h, and then raising to 1000℃with 2℃/min and dwelling there for 2h, finally raising to 1300℃with 2℃/min and dwelling for 2h.