Preparation And Performance Of Nanostructured Electrodes For Intermediate/Low Temperature Solid Oxide Fuel Cells

Nowadays,intermediate/low temperature solid oxide fuel cells(SOFCs)have attracted many attentions in the resaerch of power sources area.Main research works concentrate on the following areas,such as preparing thin electrolyte membranes on the electrodes,optimizing the electrode microstructure and developing new anode materials and new structures for hydrocarbon with high electrochemical catalytic activity at low temperature and strong ability to suppress carbon deposition.This thesis focuses on the above mentioned issues to carry out studies on the anode-supported SOFC with Sm doped CeO₂ as electrolyte through designing and preparing nanostructured electrodes.These researches are summarized as:(1)In order to reduce the sintering temperature of Ce_0.8Sm_0.2O_1.9(SDC)ceramic, nanocrystalline SDC powders were prepared by both a combustion process and a colloidal seed-assisted spray drying and pyrolysis method.SDC powders were prepared by a novel solution combustion process using molten stearic acid as dispersive medium and reducing agent.The particle size and distribution of SDC powders were influenced by the molar ratio of NO₃^- to stearic acid(N/s).The powders prepared with the molar ratio of N/s at 1:1.5 exhibited a narrower distribution in size and the average agglomerate size was about 157 nm.The primary particle size was in the range 10～40 nm.The as-combusted powders showed a low crystallinity and exhibited an excellent sintering performance,easily achieving high dense SDC ceramics with lager grains(0.85μm).The nanocrystalline SDC electrolyte powders were prepared by colloidal seed-assisted spray drying and pyrolysis.(NH₄)₂CO₃(AC)and NH₄HCO₃(AHC)were used as precipitation reagents to prepare colloidal solutions,and SDC powders with different particle size and morphology were obtained.The SDC powders prepared with AC colloidal solution showed spherical in morphology and smaller particle size, which showed excellent sintering behavior and could be sintered up to 96%of the theoretical at a lower sintering temperature of 1250℃,and the average grain size of ceramics was 0.86μm.(2)In order to obtain nanosized anode materials with uniform distribution of NiO and SDC,nanocrystalline NiO-SDC composite powders were prepared by the colloidal spray drying and pyrolysis method.Two series of mixed colloidal solutions were prepared.One contained Ni sols and Ce-Sm solution;the other contained Ce-Sm sols and Ni solution.The results showed that the primary particle size of the NiO-SDC composite powders slightly increased with the concentration of spray mixed colloidal solutions.Furthermore,the composite powders from mixed colloidal solutions with Ni sols showed more uniform distribution of two phases and smaller particle size than those from mixed colloidal solutions with Ce-Sm sols.The primary particle size was about 30 nm amd the powders had a very narrow particle size distribution centered at 250 nm.In addition,The NiO-SDC powders were also synthesized by a glycine-nitrate method.The ball-milled powders became fine particles with 20-40 nm for primary particle size,and 170 nm for average agglomerated particle size and a narrower distribution in sizes.(3)In order to obtain the anode with high catalytic activity at intermediate/low temperature,the porous NiO-SDC ceramics were prepared with monodispersed polystyrene(PS)spheres as the pore template,which was used as the anode in the anode-supported SOFCs.Considering the difference in the particle size between PS spheres and NiO-SDC powders,the powders and PS spheres were mixed under ultrasonic in ethanol to make them mixed and distributed uniformly.The relations among sintering temperature,microstructure and mechanical strength of the ceramics were studied.The results indicated that the NiO-SDC ceramic sintered at 1200℃showed to have an interconnected pore structure,about 150 nm grains in the pore walls,and the compression strength with 20.83 MPa.The maximum power density as high as 333 mWcm^-2was obtained operated at 600℃.The high power output showed that such an anode could provide high surface areas and more catalytically active sites and counteract the negative influence of the increasing ohmic resistance in electrolyte.(4)To reduce the cathode-electrolyte interfacial polarization resistance of low temperature SOFCs,a nanostructured porous thin cathode consisting of Sm_0.5Sr_0.5CoO₃(SSC)and SDC was fabricated on an anode-supported electrolyte sheet(membrane)using spin-coating technique.A suspension with nanosized cathode powders,volatilizable solvents and a soluble pore former was prepared.In the cathode, the SDC grains distributed uniformly on and cross the SSC matrix.The cathode had a good interconnecting porous structure with the porosity about 31%and there were many 50～100 nm grains on the frameworks.The results indicated that the cell with the nanostructured porous thin cathode sintered at 950℃showed relatively high maximum power density of 212 mW cm^-2at 500℃and 114 mW cm^-2at 450℃,and low interfacial polarization resistance of 0.79Ωcm² at 500℃and 2.81Ωcm² at 450℃.The good performance is attributed to that the nanostructured porous thin cathode can provide high surface area for gas adsorption/desorption and has high triple-phase boundaries(TPBs)length,which increases the electrochemical reactions sites and decreases the interfacial polarization resistance effectively.(5)Considing the strong ability of Cu to suppress the carbon deposition when dry methane is directly oxidized in anode,a Cu/Ni/SDC anode was designed.The anode was prepared by the impregnation method,whereby a small amount of Cu was incorporated into the previously formed Ni/SDC porous matrix.Cu nanoparticles adhered to and were uniformly distributed on the pore surface of the Ni/SDC prous matrix,and maximized the role of Cu in suppressing carbon deposition.The higher electronic conductivity of Cu/Ni/SDC anode was obtained,which provided the effective passage for more electrons to be transported from the electrochemical reaction sites so that the power output of the cell can be enhanced.For the resulting Cu/Ni/SDC anode-supported cell,maximum power density of 317 mW cm^-2was achieved at 600℃.The power density showed only～2%loss after 12-h operation. The results demonstrate that the Cu/Ni/SDC anode effectively suppresses carbon deposition by decreasing the Ni surface area available and the level of carbon monoxide disproportionation.This combination of effects results in very low power density loss over the operating time.According to the catalytic mechanism of Cu to the hydrocarbon,it was found that Cu participates in the electrochemical reaction of the fuel and the catalysis of Cu was carried out through Cu₂O at the interface between Cu and SDC.