Preparation And Performance Of Nanofiber Symmetrical Electrodes For Solid Oxide Fuel Cells

Solid oxide fuel cells(SOFCs)are new kinds of devices for converting chemical energy into electrical energy.High manufacturing cost and poor lifespan are the main problems to restrict the further commercial developments of SOFC.In this thesis,gas diffusion in SOFC electrodes,conductivity,and long-term stability of single cell under different conditions were researched in details.In order to achieve the above purposes,we optimized the composition and micro-structure of electrodes firstly,and then prepared novel composite electrode materials with excellent electrochemical performance.Single symmetrical structure cells with excellent electrochemical properties were prepared through the optimization of the cell structure and the manufacturing process.To improve the long-term working stability of cells,the carbon deposition and sulfur poisoning were solved by using the special structure of symmetric electrodes.The single phase of Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-8(PSCFN)nanoparticles were synthesized by solid phase method.The Rp values of PSCFN nanoparticle electrodes sintered at 750 ? and 800 ? under air atmosphere for 2 h were 0.069 ?·cm2 and 0.048 ?·cm2,respectively.The PSCFN-GDC nanoparticle composite electrodes were prepared by impregnating GDC material to PSCFN nanofiber-structured skeletons to further improve the electrochemical performance of the nanoparticle electrodes.When the mass ratio of PSCFN and GDC was 1:0.05,the Rp values of composite electrodes measured at 750 ? and 800 ? were 0.041 ?·cm2 and 0.035 ?·cm2.In this condition,after long-term thermal cycling test and constant current test,there was an obvious increase in Rp values of the nanoparticle composite electrodes,indicating a poor long-term stability of the composite electrodes.The PSCFN nanofibers were prepared by electro-spinning technology.The Rp values of PSCFN nanofiber electrodes were measured 0.462 ?·cm2 and 0.298 ?·cm2 at 750 ? and 800 ? in air atmosphere,respectively.The polarization resistance of electrodes can be obviously reduced by dipping GDC into PSCFN nanofiber electrode.When the mass ratio of PSCFN: GDC was 1: 0.10,the Rp values of nanofiber composite electrodes decreased to 0.039 ?·cm2 and 0.018 ?·cm2 under air atmosphere at 750 ? and 800 ? respectively.In this condition,after long-term thermal cycling and constant current tests in air atmosphere,the Rp value of nanofiber composite electrodes did not show any obvious change,suggesting a good long-term stability.In humid hydrogen(97% H2 and 3% H2O)and air atmosphere,PSCFN was proved to be the redox reversible material by the oxidation-reduction-oxidation treatment.The Rp values of PSCFN nanofiber electrodes were measured 1.704 ?·cm2 and 1.573 ?·cm2 at 800 ? and 850 ? in humid hydrogen atmosphere,respectively.The performance of electrodes can obviously improved by dipping GDC into PSCFN nanofiber electrode material under the reducing atmosphere.When mass ratio of PSCFN: GDC was 1: 0.56,the Rp values of the nanofiber composite electrodes were 0.301 ?·cm2 and 0.212 ?·cm2 at 800 ? and 850 ? under humid hydrogen atmosphere,respectively.Under the humid hydrogen conditions,PSCFN-GDC nanofiber composite anodes were researched by thermal cycling stability tests,constant current tests and oxidation-reduction cycle test.After these long-term tests,the Rp values of nanofiber composite electrodes did not show any obvious change,suggesting a good long-term stability in humid hydrogen atmosphere.Under the humid methane(97% CH4 and 3% H2O)condition,PSCFN-GDC nanofiber composite anodes were researched by constant current tests.After the long-term test,there was an obvious increase in Rp values of the nanofiber composite electrodes,indicating the the application of methane fuel will have impact on the long-term stability of composite electrodes.The investigations on the nanofiber PSCFN-GDC in different gas atmosphere indicates that PSCFN-GDC symmetrical cell,which is supported by LSGM electrolyte,can be successfully prepared by firing PSCFN-GDC(1:0.56)on both sides of LSGM electrolyte.The Rp value and long-term stability tests of PSCFN-GDC symmetrical cell were carried out with cathode under air atmosphere and anode under wet hydrogen,wet methane or hydrogen-hydrogen sulfide atmosphere,respectively.The results showd that the symmetrical cell could remain good power output in both hydrogen and methane atmosphere,while the power would rapidly decrease in hydrogen-hydrogen sulfide atmosphere.By using consideration of the symmetrical structure and redox characteristic of PSCFN-GDC composite electrode,the inversion electrode methods were applied to solve the problem of carbon deposition and sulfur poisoning on SOFC anode.The results show that the carbon deposition could be significantly eliminated and sulfur poisoning could also be removed by oxidation.However,the sulfur poisoning could cause aggregation of the metal particle,and finally lead to irreversible effect.