Fabrication And Electrochemical Properties Of Solid Oxide Fuel Cells Supported On Ceramic Electrodes With Straight Pore Structures

Solid oxide fuel cells(SOFCs)have been considered as sone of the most efficient energy power generation technologies.The main features of SOFCs are efficient,clean,quiet,and all-solid-state.SOFCs can also be operated in the electrolysis mode to convert CO2 and H2O into CO and H2.The state-of-art SOFCs are supported on the nickel-yttria stabilized zirconia(Ni-YSZ)electrodes.The maj or limitation with the Ni-YSZ supporting electrodes is the large volume expansion and contraction occurring in the redox cycling of Ni(?)NiO,resulting in irreversible degradation or even a mechanic failure.Moreover,the Ni-YSZ electrodes are prone to oxidation in pure CO2 or coking formation under high conversions of CO2 to CO.In contrast,ceramic supporting electrodes possess better redox stability and coking-resistance,thus presenting a potential solution to overcome the limitation of the Ni-YSZ electrodes.The limitation with the ceramic supporting electrodes is the low electrical conductivity and catalytic activity.As a result,the SOFCs supported on ceramic electrodes demonstrate much worse electrochemical performance than those supported on the Ni-YSZ electrodes.This thesis is thus devoted to improving the electrochemical performance of the SOFCs by optimizing the pore structure and modifying inner-surfaces of the ceramic supporting electrodes.In Chapter 1,the history,working principle,polarization losses,key materials for SOFCs and the preparation methods are reviewed.In particular,the supporting electrodes and their preparation methods are discussed.The scope of this thesis is then described.In Chapter 2,La0.8Sr0.2Cr0.5Fe0.5O3-?(LSCrF)-Y0.16Zr0.84O1.92(YSZ)composites were adopted as the supporting electrode for SOFCs.The electrode was prepared by phase inversion tape casting/sintering method,and modified with Sr2Fe1.5Mo0.5O6-?(SFM)nanoparticles using the impregnation method.Single cells were fabricated,comprising the YSZ-LSCrF as the fuel electrode,thin YSZ film as the electrolyte and(La0.8Sr0.2)0.95MnO3-?(LSM)-YSZ as the air electrode.The cell with the electrode modified with 15 wt%SFM exhibited a maximum power density of 423 mW·cm-2 at 800?,much higher than that for the bare cell(51 mW·cm-2).When tested in the electrolysis mode with pure CO2 at 1.5 V at 800?,a current density of 1.02 A·cm-2 was observed for the former,while only 0.41 A·cm-2 for the latter.The former operated stably with a constant electrolysis current over 60h.It is concluded that SOFCs supported on SFM-modified YSZ-LSCrF fuel electrodes hold promise for application in CO2 electrolysis without the safe gas.In Chapter 3,the phase inversion-derived porous LSCrF-YSZ support was adopted as the air electrode for SOFC.The cell consisted of a thin YSZ film as the electrolyte and a thin LSCrF-YSZ layer as the fuel electrode.Both the air and fuel electrode were modified with SFM nanoparticles using the impregnation method.When tested in the fuel cell mode?a maximum power density of 592 mW·cm-2 was obtained at 800? for the SFC-modified cell,much higher than that for the bare cell(94 mW·cm-2).In the electrolysis mode with pure CO2 at 800? and 1.5 V,a current density of 1.42 A·cm-2 was attained for the former,while only 0.35 A·am-2 for the latter.The improved electrochemical performance is attributed to the formation of continuous SFM nano-networks in the electrodes,thereby improving current collection and increasing active reaction sites.The cell supported on SFM-modified YSZ-LSCrF air electrode operated stably under constant electrolysis current,promising for application in electrolysis of CO2 without the safe gas.In Chapter 4,porous La0.8Sr0.2MnO3-?(LSM)-Y0.16Zr1.84O1.92(YSZ)supports prepared by the phase inversion tape casting method were used as the air electrode for SOFC.The cell was composed of a thin dense YSZ film as the electrolytes and porous NiO-YSZ layer as the fuel electrode.The LSM-YSZ support was modified with Sm0.2Ce0.8O1.95-?(SDC)nanoparticles using the impregnation method,and the optimal SDC loading was determined to be 15 wt%.The SDC-modified cell exhibited a maximum power density of 652 mW·cm-2 at 800?,while only 211 mW·cm-2 for the bare cell.The polarization resistance for the SDC-modified cell at 800? was determined to be 0.34 ?·cm2,much smaller than 1.5 ?·cm2 for the bare cell.The cell supported on the LSM-YSZ air electrode was also found to demonstrated excellent redox cycling stability.In Chapter 5,the influences of the pore structure in the LSM-YSZ electrodes on the cells' electrochemical performance was investigated.Two types of LSM-YSZ supports were prepared,one by the phase-inversion tape-casting(Support-1),and the other by the conventional tape-casting method using graphite as the pore former(Support-2).Support-1 contained finger-like straight open pores,while Support-2 had randomly distributed pores with tortuous pathways for gas transport.The LSM-YSZ supports were modified with nano-scale SDC catalysts using the liquid impregnation method.Cells were fabricated with the LSM-YSZ supports as the air electrodes,thin dense YSZ films as electrolytes and thin porous Ni-YSZ layer as the fuel electrodes.Cells with Support-1(Cell-1)showed a maximum power density of 652 mW·cm-2 at 800?,much higher than the value of 308 mW·am-2 for cells with Support-2(Cell-2).When measured in the electrolysis mode under 67%CO2-33%CO,the current densities at 800? and 1.4 V were 0.88 and 0.43 A·cm-2 with the corresponding CO production rate of 6.1 and 3.0 ml·min-1·cm-2 for Cell-1 and Cell-2,respectively.The much better performance for Cell-1 was attributed to the presence of the straight pore structure in the LSM-YSZ electrode,which allowed for easy access of the reactant gas species to the electrolyte-electrode interface and full contact with the catalyst on the electrode,thereby improving the electrochemical reaction activity and reducing the activation loss of the electrode,and improving the electrochemical performance.In Chapter 6,the LSM-YSZ support with straight pore structure was adopted as electrodes for the electrochemical oxygen pump.The pump comprising a thin YSZ electrolyte layer sandwiched by two LSM-YSZ supports were fabricated by the phase inversion tape casting/lamination/sintering method.The LSM-YSZ electrodes were modified with SDC nanoparticles using the impregnation method.At 700? and 1V,a current of 1.08 A·cm-2,equivalent to oxygen generation rate 3.76 mL(STP)·cm-2 min-1,was obtained with the SDC-modified cell,much larger than those with the bare cell(0.37 A·cm-2 and 1.29 mL·cm-2.min-1).The electrolytic current remained stable over 250 h of testing.The electrochemical cell with symmetric geometry and straight pore structure in electrodes developed in the present study is promising for use as an oxygen pump.Chapter 7 describes the summaries and prospects of the full work.