Electrochemical Modification Of PrBaCo 2 O 6-δ Cathode Of Solid Oxide Fuel Cell

As the third generation fuel cell technology,solid oxide fuel cells（SOFCs）have attracted much attention because of their high energy-conversion efficiency,low pollution emission and fuel flexibility.Conventional SOFCs are usually operated at high temperatures（800-1000℃）,which leads to some practical problems,such as chemical diffusion and instability,limited choices of cell components,sealing difficulty and high-cost manufacture etc.In order to overcome these drawbacks,tremendous efforts have been focused on constructing SOFCs operated at intermediate-and low-temperatures（350-700℃）.To develop novel cathode materials with high electrochemical performance and good chemical compatibility with electrolyte at high temperatures,in this thesis,ionic and electronic conductor PrBaCo₂O_6-δwith double perovskite structure is selected as research aim.The cathode materials of PrBaCo_2-xO_6-δ,PrBaNb_xCo_2-xO_6-δand PrBaCo_2-xO_6-δ–xCGO are prepared through Co-deficiency,Nb-doping and CGO composite strategies,and systematically evaluated as potential cathode materials for SOFCs.Firstly,PrBaCo_2-xO_6-δ（x=0,0.02,0.06 and 0.10）oxides are synthesized by a conventional solid-state reaction,and their electrochemical properties are tested.The phase and structural information are characterized by room-temperature X-ray diffraction（XRD）technique.The results indicate that PrBaCo_2-xO_6-δoxides crystalize in an orthorhombic double structure with space group Pmmm.XRD analyses confirm that as-prepared materials possess a good chemical compatibility with CGO electrolyte at high temperatures.The electrical conductivity of samples is measured by a four-probe method,and their conductivity values significantly exceed the requirement of cathode material(>100 S cm^-1).The polarization resistance（R_p）of PBC_1.94O reaches 0.059Ωcm² at 700 ^oC,which is reduced about 16%compared to that of PBCO.A maximum power density of 893 mW·cm^-2 is achieved for anode-supported single cell with configuration of Ni-YSZ|YSZ|CGO|PBC_1.94O at 700℃,while PBCO cathode-based single cell delivers a maximum power density of 764 mW·cm^-2.The rate-limiting steps of oxygen reduction reaction of the PBC_1.94O cathode are changed with temperature by impedance measurements under different oxygen partial pressures.To investigate the oxygen nonstoichiometry with increasing Co-deficiency level,iodometry titration and TG experiments are performed on PBC_2-xO samples.The results show that the oxygen content（6-δ）decreases with increasing Co-deficiency level from 0 to 0.06,accompanied with increase in concentration of oxygen vacancies.When x=0.06-0.1,the valence state of Co ions is almost constant（3.098+0.003）.Therefore,we can conclude that the concentration of oxygen vacancies is enhanced by Co-deficiency strategy,which improves the electrochemical performance of materials.To further optimize electrochemical performance of the PBC_2-xO cathodes,PrBaCo_1.94O_6-δ–xCGO（x=10,20,30 and 40 wt.%）composite cathode materials are prepared by mechanical mixing technique.The thermal expansion coefficients and polarization resistances of composite materials are remarkably reduced.When x=10%,the polarization resistance reaches the lowest value of 0.061Ωcm² at 700℃.The PBC_1.94O-10CGO cathode-based anode-supported single cell delivers a maximum power density of 958 mW·cm^-2,which is much higher than that of PBCO cathode-based single cell.The main rate-limiting step of oxygen reduction reaction of the composite cathode is determined to be the charge transfer reaction by impedance measurements under different oxygen partial pressures.PrBaNb_xCo_2-xO_6-δ（x=0.02 and 0.04）cathode materials are synthesized by a conventional solid-state reaction.Room-temperature XRD analysis suggests that the single perovskite phase is identified as x>0.04.The results show that PrBaNb_xCo_2-xO_6-δhas a good chemical compatibility with CGO electrolyte at high temperatures.The electrical conductivity of as-prepared samples is measured by a four-probe method,and is much higher than the requirement of cathode material.The Pr BaNb_0.04Co_1.96O_6-δcathode gives an extreme low polarization resistance of 0.035Ωcm².Anode-supported single cell delivers a maximum power density of 831 mW·cm^-2 at 700℃,which is higher than that of PBCO cathode-based single cell.