The Application Of K₂NiF₄-type And Ni-based Materials In Intermediate Temperature Solid Oxide Fuel Cell

A solid oxide fuel cell(SOFC)converts chemical energy in fuels to electrical energy directly with high system efficiency and low emissions.The high operating temperature(800-1000?)could improve cell performance,but it also causes many serious problems.As a result,decreasing operating temperature region to 400-800?and using hydrocarbon as the fuel are the main trends of SOFCs.To this end,some attempts of this thesis are as follows:(1)La2Ni0.9Fe0.1O4+?(LNF)with a K2NiF4-type structure is synthesized and used as an anode material of a solid oxide fuel cell.It is reduced to La2O3 and Ni-Fe alloy phases in H2 atmosphere and shows low electrical conductivity.The electrical conductivity of the anode material is enhanced by using CH4 and methanol as the carbon sources.The electrical conductivity of the anode material after treated with methanol is higher than that after treated with CH4.which is due to the carbon deposited from methanol has a higher degree of graphitization than that from CH4,bringing about a higher electrical conductivity.The conductivity of the reduced LNF at 800? increases from 0.23 to 2.47 S cm-1 after the carbon deposition in methanol.The increase of the anode conductivity results in an improvement of the fuel cell performance.A maximum power density of 166 mW cm-2 is obtained at 800? from a single cell with La0.8Sr0.2Ga0.83Mg0.17O3(LSGM)as the electrolyte and Ba0.5Sr0.5Co0.8Fe0.2O3(BSCF)as the cathode.(2)Furthermore,a single layer solid oxide fuel cell composed of La2NiO4 and Ce0.8Sm0.2O1.9-(Li0.67Na0.33)2CO3(SDC-LN)is fabricated and characterized.La2NiO4 is decomposed to Ni and La2O3 in a reducing atmosphere at the operating temperature of the cell.The electrical conductivity of the composite material increases with the increase of the content of La2NiO4.At 700?,the single cell composed of 30 wt.%La2NiO4 exhibits the highest power output of 340 and 331 mW cm-2 with H2 and methanol as fuels,respectively,which is higher than the previous report using perovskite as the material for single layer solid oxide fuel cell.(3)Ni-Mo-SDC composites are prepared and investigated as anodes of solid oxide fuel cells with methanol as fuel.The addition of Mo improves the catalytic activity for methanol pyrolysis and the resistance to carbon deposition of Ni-SDC anode.When the molar ratio of Mo to Ni is 0.03 to 1,the cell shows a maximum power density of 680 mW cm-2 at 700?.The stability of the cell is enhanced with the increase of the content of Mo in the anode(4)To further improved the resistance to carbon deposition,La or Sm is added into Ni-Mo-SDC anode,which shows highest cell performance with methanol as fuel.The rare earth elements improve the catalytic oxidation of the fuel,resulting in the decrease of anodic polarization resistance.The single cell with La2O3-Ni-Mo-SDC as the anode exhibits a maximum power density of 765 mW cm-2 at 700?.The addition of La or Sm in the anodes also improves the stability of single cells due to the accelerated oxidation of the deposited carbon and the suppression of the deposition of carbon with a high graphitization degree.