Performance And Reaction Processes Of Novel Cobalt-Based Perovskite Cathode Materials BaCoFeNbO For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC) has seized the attention of the world more and more widely because of its clean, efficient characteristics. It has become the indispensable power supply device in the future energy system. Traditional SOFC is operated at high temperature around 1000 °C. It is easy to cause the material to aging, interface reaction between materials and the high cost. Lowing the operating temperature to intermediate temperature range(600-800°C) can solve this problem. But the electrolyte ohmic losses, especially cathode polarization loss which serious influence the cell performance also increase significantly along with the decrease of operating temperature. The electrolyte ohmic losses problem has been solved through the development of high ionic conductivity of the electrolyte materials and electrolyte membrane technology. Therefore, the development of new cathode materials, the reduction of cathode polarization loss has become the important direction of the development of SOFC. An important approach to improve the catalytic activity is to analyse the cathodic reaction mechanism, optimize the cathodic reaction steps, and then improve the cathodic reaction rate. One of the main current research ways for IT-SOFC is to improve the electrochemical reaction rate through the development of new cathode materials.ABO3 type cobalt base perovskite oxide has good mixed ionic and electronic conductivity and oxygen catalytic reduction ability, it has become the main IT-SOFC cathode materials. In this paper, we disscuss the cobalt base mixed conductive cathode materials and composite cathode materials, studies the impact of chemical composition, microstructure, material reaction mechanism. Through regulating the material composition and structure, exploring the feasible way of material performance optimization, and discussing the influence law of electrochemical properties and physical and chemical nature, aims at analyzing the oxygen reduction reaction steps of the cathode, finding the key of lower cathodic polarization loss and improve the cathode catalytic activity, then providing certain material and technical reserves which can promote the development of the IT-SOFC. The main research content includes:1. Perovskite oxide Ba Co0.7Fe0.2Nb0.1O3-δ(BCFN) has good mixed ionic and electronic conductivity(MIEC), who shows good electrochemical performance for IT-SOFC cathode material. However, the main factors of improving the performance of this material, the main steps of oxygen reduction reaction, and the main process of controlling the reaction rate are still no clear analysis. Therefore, in order to further explore the overall reaction process of BCFN material, improve the reaction rate of oxygen reduction reaction, improve the performance of cathode, we choose in A-site with Sr to analyze its feasibility and the root cause of performance optimization. Ba1-x Srx Co0.7Fe0.2Nb0.1O3-δ(x=0.0-0.4) oxides with cubic perovskite structure are synthesized by the conventional solid state reaction method and investigate as cathodes for IT-SOFC. The study finds that Ba1-x Srx Co0.7Fe0.2Nb0.1O3-δ materials formed a single-phase cubic perovskite structure after sintering at 1000 oC for 10 h. The material thermal expansion coefficient is reduced after doping. A significantly decrease in the polarization resistance RP When the concentration of Sr increase to x=0.2. A further increase in the concentration of Sr results in a higher polarization resistances. The appropriate concentration of Sr can improve the concentration of small polaron, make the small polarons and oxygen vacancies reach the optimal concentration ratio, which results in the conductive ability of material is enhanced, the electrochemical performance is improved. The electrochemical reaction mechanism study shows that the cathode reaction including adsorption and dissociation of gaseous oxygen, reduction reaction of atom adsorbed oxygen into adsorbed oxygen ion and oxygen ion transfer into the electrolyte. For BCFN sample, in the range of 1 atm-0.01 atm, the process of atom adsorbed oxygen into adsorbed oxygen ion is the major rate limiting step; For B0.8S0.2CFN sample, the process of atom adsorbed oxygen into adsorbed oxygen ion is the major rate limiting step when oxygen partial pressure is higher than 0.1 atm. The adsorption and dissociation of gaseous oxygen process is the major rate limiting step when oxygen partial pressure is lower than 0.1 atm. The performance of the cell with Ba0.8Sr0.2Co0.7Fe0.2Nb0.1O3-δ cathode is higher than that of other cells. It suggests that Ba0.8Sr0.2Co0.7Fe0.2Nb0.1O3-δ is a very promising cathode material for IT-SOFC.2. B0.8S0.2CFN as IT-SOFC cathode material shows excellent performance. In order to enhance the ionic conductivity of materials, improve the oxygen reduction reaction rate, we synthesize the BSCFN-x SDC composite oxides. The XRD result shows no reaction product is found between BSCFN cathode and SDC electrolyte after heat-treatment at 1000 oC for 10 h. The thermal expansion coefficient is reduced after composite, the matching of cathode and electrolyte is improved. The addition of a highly ionic conductive phase to the cathode layer is effective in improving the electrocatalytic activity of cathode due to enlargement of the electrochemically active area, i.e., the triple phase boundary(TPB), at which the oxygen reduction reaction occurs. BSCFN-30 SDC cathode shows the best performance, which RP much smaller than pure BSCFN cathode at 800 oC. Comparison of the impedance spectra results demonstrate that the process of generate lattice oxygen is the major rate limiting step when oxygen partial pressure is higher than 0.05 atm. The adsorption and dissociation of gaseous oxygen process is the major rate limiting step when oxygen partial pressure is lower than 0.05 atm. The electrochemical activity of composite cathode is enhanced, the reaction rate is accelerated. Introduction of ionic conductive SDC to BSCFN-x SDC composite cathode can make the oxygen reduction reaction processes from the three processes into two processes, the adsorbed oxygen atom directly generated lattice oxygen, promote the charge transfer process. The reaction rate is accelerated and the electrochemical performance is improved. Using BSCFN-x SDC cathodes, Ni0.9Cu0.1-SDC anode, SDC electrolyte achieves excellent performance. The power densities are 620.37,656.79,717.56 and 663.00 m Wcm-2 at 800 oC. These results suggest that BSCFN-30 SDC is a potential cathode material for use in IT-SOFC.3. For MIEC material, the electronic conductivity often several orders of magnitude higher than the ionic conductivity, thus an effective way to improve the performance of cathode is to improve the ionic conductivity. Some researchers have found that A-site cation deficiencies introduce into the lattice structure of perovskite oxides, the crystal structure, oxygen vacancy concentration and thermal expansion coefficient are likely to be affected, thus changing the cathode reaction process. Therefore, we use solid state reaction method prepare Ba1-x Co0.7Fe0.2Nb0.1O3-δ(B1-x CFN,x=0,0.05,0.10,0.15) cathode materials. The influence of crystal structure, thermal expansion and electrochemical reaction through XRD, SEM, ac impedance spectroscopy and single cell test are studied. X-ray diffraction(XRD) is used to confirm the crystal structure of B1-x CFN powder. The morphologies of B1-x CFN cathode are characterized by a scanning electron microscope. The study finds that B1-x CFN fully formed cubic perovskite structure after sintering at 1000 oC for 10 h. The crystal cell volume with the increase of A-site deficiency is not a linear relation. The cell volume of B0.9CFN is the biggest. A-site deficiency in Bx CFN will render the formation of oxygen vacancies, rather than the oxidation of B-site ions as charge compensation. It is clear that the porosity decreases as A-site deficiency increase, B0.9CFN has the smallest porosity. When the A-site deficiency is 0.15(B0.85CFN), the porosity is increase due to the particle agglomeration. Interfacial polarization resistance is reduced when Ba deficiency in Bx CFN. The interfacial polarization resistance drop nearly 66.2% when x=0.10(B0.90CFN) at 800 oC. The process of generate lattice oxygen is the major rate limiting step when oxygen partial pressure is higher than 0.01 atm. The adsorption and dissociation of gaseous oxygen process is the major rate limiting step when oxygen partial pressure is 0.01 atm. Ba defects result in the formation of oxygen vacancies. The adsorbed oxygen atom directly generate lattice oxygen, which promote the reaction rate and cathodic oxygen reduction reaction activity. The interfacial resistance and the performance of single cell of B0.90 CFN are lower than other cathodes in the operation temperature of 800 oC, which means B0.90 CFN is a better cathode for IT-SOFC.4. In order to promote the ionic conductivity for material, we adopt composite method in material. The addition of SDC to the cathode layer is effective in improving the ionic conductivity and electrocatalytic activity, decrease the interfacial polarization resistances Rp of cathode. Electrochemical reaction mechanism study shows that the process of generate lattice oxygen is the major rate limiting step when oxygen partial pressure is higher than 0.05 atm. The adsorption and dissociation of gaseous oxygen process is the major rate limiting step when oxygen partial pressure is lower than 0.05 atm. The join of ion conductive phase SDC affects the charge transfer process and adsorption and dissociation process of oxygen, which accelerates the reaction rate of the two processes. In the whole temperature range of measurement, the Rp is the smallest when the content of SDC is 30 wt.%. The composite cathodes have lower thermal expansion coefficient, the thermal stability is improved. B0.9CFN-30 SDC has appropriate grain size and more adequate porosity. The internal of cathode forms a continuous B0.9CFN conductive phase also forms a continuous SDC ion diffusion channel.