| Microwave-induced pyrolysis of sewage sludge can produce large amount of biogas,which contains high ratio of hydrogen and carbon monoxide.However,this kind of biogas can not be directly utilized by traditional thermoelectric power system due to its relative low heat value.Solid oxide fuel cell,an electrochemical device that can directly convert chemical energy from different fuels into electricity with high energy conversion efficiency and fuel flexibility,can be employed to convert energy from this kind of biogas.However,carbon deposition and sulfur poisioning of the SOFC anode caused by the carbon-hydrogen fuels and hydrogen sulfide from the biogas,may lead to the degradation of the cell performance in the long run,of which these problems should be overcome in practical use.This research aimed at exploiting and modifying alternative anode materials to replace the traditional Ni based anode,in order to improve the electrochemical catalytic activity,output stability,carbon and sulfur resistance when the SOFC was fed with biogas.Alternative peroviskite material LCFN?La0.9Ca0.1Fe0.9Nb0.1O3-??,with high lattic stability and catalytic activity was manufactured.LCFN-SDC composite anode was also prepared by further modification of LCFN with SDC,which showed higher electrochemical performance and output stability in SOFC anode.Besides,the LCFN based materials also be suitable for symmetrical SOFC electrode,which was benifical for direct biogas fuel cell due to it feasibility to recover electrode activity by converting fuel and air atmospheres.The basic physicochemical properties of LCFN were studied.The LCFN showed high lattic stability at high temperatures,as well as good thermal and chemical compatibility with SSZ,SDC materials.LCFN also showed high conductivity for SOFC electrode,especially for anode material.The electrochemical performance and output stability of LCFN based anode in SOFCs fed with the biogas were studied,which indicated that biogas is suiatable for LCFN based SOFC to generate electricity with high power output and stability.At 700,750 and 800 ?,peak power desities of 253?411 ? 575 m W·cm-2 can be reached using LCFN|SSZ|LSM cells.Further modification of LCFN anode with SDC improved not only the peak power but the output stability.Peak power desities of 346,495 and 651 m W·cm-2 can be reached using LCFN-SDC|SSZ|LSM cells,and the degradation rate was reduced to 0.28 mV·h-1 at the relative stable period in the long run of 100 h at 750 ?.The electrochemical performance and output stability of LCFN based symmetrical SOFC were also studied when fed with biogas.Peak power desities of 235.3,337.1,405.5 and 481.3 m W·cm-2 can be reached at 700,750,800 and 850 ? using LCFN-SDC|SSZ|LCFN-SDC cells,and the degradation rate of 0.23 m V·h-1 was reached at the relative stable period in the long run of 50 h at 750 ?.The redox stability tests for ten times also demonstrated that LCFN based electrodes were suitable for symmetrical SOFC for direct biogas fuel cell with high output stability,since it was feasible to recover the anode side electrode activity by converting fuel and air atmospheres intermittently.The carbon and sulfur resistance of LCFN based SOFC were also studied when fed with biogas.Fuel selective oxidation was occurred in LCFN based anode,of which hydrogen oxidation was the easiest part,while on the contrary,methane was most difficult to oxide.The relative high fuel efficiencies of ethylene and ethane were mainly benefit from the pyrolysis and reforming reaction at high temperature.Carbon monoxide also showed high fuel efficiencies,for which the disproportionated reaction of CO was the main sources of carbon deposition in the inlet tube.No evidence of carbon and sulfur deposition was detected in SEM and EDS mapping on the surface of the LCFN based anode after long term stability tests in biogas,which contains carbon-hydrogen fuels and 100 ppm H2 S,indicating that LCFN based anode had high resistance upon carbon deposition and sulfur poisoning.In summary,LCFN based anode were suitable for direct biogas SOFC with high electrochemical catalytic activity,output stability,carbon and sulfur resistance,especially the modified LCFN-SDC anode.The symmetrical SOFC consisted of LCFN based electrode also exhibited high potential for direct use of biogas due to its robust redox stability.This research has great significance on the research and application of direct biogas SOFC and the development of SOFC electrode material. |
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