Improvement On The Coking Resistance Of Solid Oxide Fuel Cells With Nickel-based Anodes

Solid oxide fuel cell?SOFC?is a solid-state power-generation system which can directly convert chemical energy into electricity with a high efficiency at high temperature.SOFCs have very good prospects from a sustainable development and have been generally thought to be widely used in the future as energy utilization devices.The efficient and clean utilization of hydrocarbon fuels in SOFCs is an important development direction of SOFC technology at present.SOFCs based on nickel cermet anode show excellent performance when H₂ is used as the fuel.In the Western and Japan,the demo devices have already been in operation.However,traditional Ni cermet anode is suffering from serious carbon deposition when hydrocarbon fuel is used directly,which results in anode deactivation and cell performance decline.Therefore it is necessary to improve the coking resistance of the nickel-based anode.In this thesis,we fabricated a cell configuration using the 8%-Y₂O₃ stabilized ZrO₂?YSZ?as the electrolyte,the 20%Sr-doped LaMnO₃?LSM?as the cathode and the traditional Ni-YSZ anode with an independent catalyst layer which is separated from SOFC anode.The application of a separated catalyst layer avoids the mutual interaction between catalyst and anode material because of the mismatch of the thermal expansion performance,also retains the high electrocatalytic activity and electronic conductivity of Ni cermet anode.What's more,hydrocarbon fuel passes through the catalyst layer and is reformed to produce CO and H₂ before reaching the anode layer.The hydrocarbon concentration inside the anode layer is minimized through the barrier of catalyst layer,thus carbon deposition over the nickel-based anodes is reduced.In this work,two catalysts NiO/BaO/CeO₂?NiBaCe?and NiO/BaO/CeO₂/Al₂O₃?NiBaCeAl?were synthesized via glycine combustion method?GNP?.Using two substrates?YSZ and Al₂O₃?,the catalyst and substrate is co-pressed to form the dual layer catalyst slice,respectively.Thus four types of catalyst layer,NiBaCe-YSZ,NiBaCe-Al₂O₃,NiBaCeAl-YSZ,NiBaCeAl-Al₂O₃,were prepared.The electrochemical performance of cellswith and without the independent catalyst layer is tested using H₂,wetmethane and wet coal bed methane?CBM?fuels.The main results are asfollows:?1?Catalyst NiBaCeAl shows a good activity for steam reforming ofmethane.Methane conversion reaches 92.1%at 850 °C.After at least 400min stability test at 800 °C,methane conversion is stable at above 80%.Thetwo cells covering the catalyst NiBaCeAl as an independent catalyst layerachieve a similar and considerable power output with peak power density?PPD?of 0.65 W·cm-2 operating on 3%H₂O-97%CH4 fuel,which is higherthan that of the blank cell?0.60 W·cm-2?.At the same time,a good operationstability over at least 12 hours is achieved.While for the blank cell,the cellvoltage declines to zero after discharging for 20 min.The post-testcharacterization of the cells shows carbon nanotubes are formed on thesurface of two catalysts,maybe the acidic NiAl₂O₄ promotes carbondeposition and growth of carbon nanotubes.EDX results indicate that nocoking is found on the surface of the Ni cermet anode.All of these resultsreveal that the application of catalyst layer greatly improves the cokingresistance of the Ni cermet anode.Based on the formation of carbonnanotubes on the catalyst surface,it is supposed that the cells with thisstructure can be used to achieve carbon electric symbiosis.?2?Catalyst NiBaCe has a high catalytic activity and stability for thesteam reforming of methane.Methane conversion reaches 90.5%at 900 °C.During the whole 400 min operation at 800 °C,methane conversion retainsstable at above 78%.The PPD of cell covering the catalyst layer shows anobvious improvement.The fuel cell with NiBaCe-YSZ catalyst layer deliversa PPD of 0.67 W·cm-2,slightly higher than that of NiBaCe-Al₂O₃?0.65W·cm-2?at 850 °C when operating on 3%H₂O-97%CH4 fuel.Both the PPDsof the cells are higher than that of the blank cell?0.6 W·cm-2?.Discharge testsunder constant current show that the cells with catalyst have a good operationstability for at least 12 h.Whereas the cell voltage of the blank cell rapidly drops to zero after discharge for 20 min due to the severe coking.Resultsindicate that the application of NiBaCe catalyst layer greatly improves thecoking resistance of the Ni cermet anode.The cell with Al₂O₃ substrateshows a better coking resistance that that with YSZ substrate.?3?With the four catalyst layers,NiBaCe-YSZ,NiBaCe-Al₂O₃,NiBaCeAl-YSZ and NiBaCeAl-Al₂O₃,respectively,the cells are fueled with3%H₂O-97%coal bed methane?CBM??components:CH4,82.9975%;O2,2.1853%;N2,10.1839%;C2-C8,3.4731%;CO2,1.1602%?.Results show agood operation stability for at least 12 h at 800 0 C.The post-testcharacterization of the cells shows carbon nanotubes are formed on thesurface of NiBaCeAl catalysts.Based on this phenomenon,the cells with thisstructure can be used to achieve carbon electric symbiosis.Furthermore,thecell with Al₂O₃ substrate also shows a better coking resistance that with YSZsubstrate.No obvious coking on the anode surface indicates that the use ofcatalyst layer not only increases the cell power output and anti-coking ability,but also promotes the application of CBM as SOFC fuel.?4?In conclusion,the results indicate that the cell with NiBaCe-Al₂O₃shows superior performance to the other three cells when operating on 3%H₂O-97%CH4 fuel.The cell with NiBaCeAl-Al₂O₃ shows superiorperformance to the other three cells when operating on 3%H₂O-97%CBMfuel.The cells with catalyst layer improce electrochemical performance,coking resistance and operation stability.