Fabrication And Electrochemical Properties Of Solid Oxide Cells With Straight Open Pores In The Supports

Solid oxide cells(SOCs)have been extensively studied as a clean and highly efficient energy conversion device,which can work in the solid oxide Fuel Cells(SOFCs)mode to convert chemical energy contained in fuels,or in the solid oxide electrolysis cells(SOECs)mode to convert electrical energy into chemical energy.Reducing the operation temperatures of SOCs to the intermediate-to-low range will not only lead to the decrease of the cost of cell components such as interconnectors,but also the increase of the durability and lifetime of the devices.Electrode-supported planar SOCs have attracted increasing attention among various cell configurations due to the small thickness of the electrolyte layer and resultant low ohmic polarization.Currently,the porous electrode support is typically prepared using pyrolyzable pore formers such as graphite or starch.The as-obtained pores are tortuous and may thus impose a large resistance to the gas phase mass transport.This thesis was intended to investigate and improve the phase inversion tape casting method for preparation of SOCs’ electrode supports with straight open porese,and to study the effects of pore structures in the electrode supports on the electrochemical performance,contributing to the development of SOCs technology.Chapter 1 briefly introduces the development history,operating principle,factors affecting the performance of SOCs.The typical materials for SOCs’ electrode and electrolyte are subsequently reviewed.And the main configuration and preparation methods of SOCs are summarized.Finnally,the scope and content of the thesis is presented.In Chapter 2,the phase-inversion tape casting was modified to prepare a planar NiO-yttria-stabilized zirconia(YSZ)anode with straight open pores.Two slurries,one consisting of NiO and yttria-stabilized zirconia(YSZ),the other containing graphite,were co-tape cast onto a Mylar sheet,and transferred into a water bath for solidification via the phase-inversion mechanism.The as-formed green tape possessed layered straucture along the thickness direction:the top and middle layer derived from the slurry of NiO and YSZ,and the bottom layer from the graphite slurry.The graphite bottom layer was removed later by firing at elevated temperatures,leaving the finger-like porous middle layer exposed to the gas phase.A cell supported on the as-prepared anode substrate exhibited satisfactory electrochemical performances with a maximum power density of 986 mW cm-2 at 800 0C.The cell did not show a convex-up curvature in I-V plots at high current density,indicating the absence of concentration polarization which is attributed to the open pore structure of the phase-inversion derived anode.After introducing a thin anode functional layer between electrolyte and as-prepared anode,the maximum power density was increased to 1195 mW cm-2 at 800 ℃.In Chapter 3,a functional layer and a porous support that together constitute an anode for a SOFC were formed in one step by the phase-inversion tape casting method.Two slurries,one of NiO and YSZ,and the other of NiO,YSZ and graphite,were co-cast,and then solidified by immersion in a water bath via the phase inversion mechanism.The as-formed green tape consisted of sponge-like thin layer and a finger-like thick porous layer,derived from the first and the second slurry,respectively.The former acted as anode functional layer(AFL)while the latter as the anode support.The thickness of AFL was varied between 20-60 μm by adjusting the blade gap for the tape casting.With the as-structured anode support,cells with a configuration NiO-YSZ/YSZ/YSZ-La0.8Sr0.2)0.95MnO3-δ(LSM)were fabricated,and their electrochemical properties measured using air as oxidant and hydrogen as fuel.The maximum power densities obtained at 750℃ were 720,821 and 988 mWcm-2 with the AFL thickness at 60,40 and 20 μm,respectively.The satisfactory electrochemical performance is attributed to the two-layered structure of the anode,in which the sponge-like AFL layer provides plenty of triple-phase-boundaries for hydrogen oxidation and the finger-like thick porous support enables facile fuel transport.In Chapter 4,the effects of the pore structure of the electrode supports on the electrochemical performance of supported-SOC were investigated.Two planar NiO-8 mol%yttria-stabilized zirconia supports were prepared,one by the phase-inversion tape casting,and the other by conventional tape casting method using graphite as the pore former.The former contained finger-like straight open large pores,while the latter contained randomly distributed and tortuous pores.The electrochemical properties of the cells with different pore structure in the electrode supports were measured.In the SOFC mode,the cell with the straight open pores in the electrode support exhibited a maximum power density of 980 mWcm-2 at 750 ℃,much higher than the cell with tortuous pores(487 mWcm-2).In the SOEC mode,the former showed a current density of 1.42 A cm-2 and a H2 production rate of 9.89 mL(STP)crr-2min-1 at 1.3 V and 50 vol.%humidity and 750 ℃,while only 0.91 A cm-2 and a H2 production rate of 6.34 mL cm-2min-1 for the latter.It is concluded that the introduction of large straight open pores into the electrode support allows fast gas phase transport and thus minimizes the concentration polarization.The straight pores in the support could also provide better access to the reaction site(the electrode functional layer),thereby reducing the activation polarization as well.Since the YSZ-LSM composite shows low electrochemeical activity in the intermediate temperature(IT)range of 500-750℃,the Sm0.2Ce0.8O2-δ(SDC)-La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)composite has been used as the air electrode for IT-SOCs.In Chapter 5,a planar NiO-YSZ electrode support was prepared by the phase-inversion tape casting method.The as-prepared electrode support possessed a two-layered structure with a functional layer of thickness 20μm and a finger-like porous layer of thickness 820μm.For comparison,a two-layered electrode support was prepared by the conventional tape casting/lamination method using graphite as pore former.Thess electrode supports were used to fabricate SOEC with a configuration of Ni-YSZ/YSZ/SDC/SDC-LSCF.The electrochemical performance of the cells with different electrode supports was measured.In the SOFC mode,the maximum power densities of the cell with the phase-inversion formed supports was 1190 mWcm-2 at 700℃,much higher than that for the cell with the conventional support(815 mWcm-2).In the SOEC mode,the former showed a current density of 1.24 A cm-2 and H2 production rate of 8.63 mL cm-2min-1 at 1.3 V and 50 vol.%humidity,while only 0.92 A cm-2 and a H2 production rate of 6.46 mL cm-2min-1 for the latter.In Chapter 6,SOFC with flat tube anode support was prepared via phase-inversion tape casting/lamination method.The flat tube anode consists of three layers:a top layer acting as anode working layer,the middle layer as support with built-in gas passage,and the bottom layer as current collector.A thin dense YSZ electrolyte layer was deposited onto the anode working layer by dip-coating/sintering.A composite cathode of YSZ-LSM with a weight ratio of 50:50 was screen-printed on the surface of the YSZ electrolyte layer.The as-prepared flat-tube cell showed a maximum power density of 245 mW at 800 ℃ using H2-3%H2O(100 mlmin-1)as the fuel and ambient air as the oxidant.The method developed in this study is applicable to the preparation of other ceramic devices with flat tube configuration.In Chapter 7,the summary of this dissertation is presented,and future work concerning the phase-inversion tape casting method and SOCs research is discussed.