Novel Cobalt-Based Peroverskite Cathode Materials And Their Application In Intermediate-Temperature Solid Oxide Fuel Cells

To enter the 21st century, environmental pollution and energy crisis are becoming increasingly serious, efficient and clean energy use patterns gradually become a future direction of energy development and utilization. As one of the most promising energy conversion devices, Solid Oxide Fuel Cells (SOFCs) with high efficiency, low pollutant and fuel flexibility have received a great attention in the past 20 years.Reducing the cost of the production and use is a critical issue for popularization and application of SOFCs technology. It has been widely recognized that an effective way to reduce costs is to lower the operating temperature of SOFCs to intermediate temperature range (500-800oC). However, the lowering of the operation temperatures results in the increase of the electrolyte ohmic and cathode polarization and this consequently leads to a rapid decrease in the performance of the fuel cells. The problem coming from electrolyte has been solved to a certain extent by using new electrolyte materials or adopting film technique. Therefore, the polarization of electrodes, particularly cathode overpotential losses is one of the major issues to develop IT-SOFCs. For the traditional electron conductive cathode material such as Pt or LSM, the oxygen reduction reaction is entirely confined to two-dimensional interface between the cathode and electrolyte. Because of its small length of three-phase boundaries (TPBs), the catalytic activity of oxygen is very low. The mixed electronic and ionic conductors (MIEC) cathode materials have not only high electronic conductivity, but also high ionic conductivity. Cathodic oxygen reduction reaction is a complex electrochemical reaction process of heterogeneous catalysis, involving a series of charge-transfer and charge transport in both aerobic species and oxygen vacancy and electron participation, therefore, MIEC cathode provides a convenient condition for oxygen reduction reaction. It is generally accepted that MIEC cathodes can extend the electrochemically active sites from just TPBs at the cathode/electrolyte interface to the entire surface of the porous cathode, consequently, there is a decrease in cathode polarization resistance.The cathode material in SOFCs has not only good compatibility and long-term stability with the adjacent material, but also high catalytic activity for oxygen reduction reaction. Therefore, the work of this paper is around developing new cathodes with high catalytical activity, long life-time, less thermal mechanical mismatch with the other components, suitable to practical applications of IT-SOFCs. Chemical composition, micro-structure, sintering temperature, and other influence factors on the properties of cathode were studies.The Pr_0.7Sr_0.3Co_1-yCu_yO_3-δ (y=0.05-0.40) were synthesized by a sol-gel method, and then the structure, the thermal character, the electrical character as well as the electrochemistry character were researched. The results demonstrated that Pr_0.7Sr_0.3Co_1-yCu_yO_3-δ have orthorhombic perovskite structure as Pr0.7Sr0.3CoO3-δ. The samples with y≤0.2 were single phase, while in the case of y≥0.3, traces of impurity phase were observed. The replacement of the smaller Co3+ (0.545?) by the larger Cu2+ (0.730?) in Pr_0.7Sr_0.3Co_1-yCu_yO_3-δ leads to a decrease in the tolerance factor t, and the consequent causes a bending of the (Co,Cu)–O–(Co,Cu) bonds. This in turn causes the localization of charge carriers, which leads to an increase in the polaron binding energy, resulting in an increase in the activation energy with increasing Cu doping.The partial substitution of Cu for Co could reduce the TECs of cobalt-containing materials to some extent by the decrease in Co content and depressing the transitions of Co3+ from the low spin state to the high spin state, thus increase thermal compatibility with electrolyte. When the B site cation becomes large keeping the A site cation to be fixed, the lattice free volume increases. A large free volume is favored to obtain higher larger oxygen ion mobility. The substitution of Co by Cu increases the lattice free volume, enhances the surface exchange of oxygen and increases migration of oxygen ion, which reduces cathodic overpotential. The sample (PSCC) with y=0.1 exhibits the lowest polarization overpotential. With a PSCC as the cathode, Sm0.2Ce0.1O1.9 (SDC) thin film as electrolyte, a fuel cell provides maximum power densities of 91, 191, 406 and 481mWcm-2 at 600, 650, 700 and 750oC, respectively.With an aim to further improve the electrocatalytic activity of PSCC, the PSCC-SDC composite cathodes were prepared by mixing PSCC and SDC. The electrochemical properties of PSCC-SDC were investigated by Galvanostatic current interruption technique and impedance spectroscopy technique in air. The results demonstrate that there are no obvious reactions between PSCC and SDC, or solid solution formation, this indicates that there is very good chemical compatibility between them. Adding SDC particles to PSCC has two functions: on the one hand it helps to reduce the thermal expansion coefficient of the cathode and thus makes it mechanically more compatible with the electrolyte; on the other hand it also results in the suppression of growth of PSCC particles, thereby maintaining the porosity and optimizing the connectivity and size distribution between particles of each solid phase to yield a larger three-phase contract area that is accessible for oxygen reduction. The composite containing 35% in weight SDC exhibited the smallest interfacial resistance (Rp) and the lowest polarization overpotential. At 700oC, the interfacial resistance Rp was about 0.282Ωcm2 for pure PSCC, meanwhile, the optimal composition, PSCC-35SDC, yielded Rp less than 0.137Ωcm2. The overpotential for the pure PSCC is 152 mV at 0.5Αcm-2. For the PSCC?35SDC cathode, the overpotential is reduced to 70mV under the same current conditions. Using a PSCC–35SDC cathode, the anode supported single cell with SDC electrolyte membrane achieved excellent performance. The maximum power densities of the single cell were 204, 388 and 615 mW cm?2 at 600, 650 and 700oC, respectively. The total resistances of the cell are about 0.349, 0.296 and 0.236Ωcm2 at the corresponding temperatures, respectively. The performance of the cell is much better than that of the cell using pure PSCC as cathode at the same work temperature.The oxide with the ordered structures types in the A sites such as the double or layered perovskites, PrBaCo₂O_5+δ(PBCO), has received great attention as IT-SOFCs cathode, due to its rapid oxygen ion diffusion and surface exchange kinetics, and high activity for activation and migration of oxygen at low temperatures(300-500oC). PrBaCo₂O_5+δ (PBCO) powder was prepared by a combined EDTA and citrate complexing method. This paper seeks to address this issue by evaluating the performance of this material working as a cathode of IT-SOFCs. The results indicate that a single phase double perovskite with an orthorhombic structure has been formed after calcination at 1100oC for 10 h. As the temperature increases, the conductivity of PBCO suddenly decreases at ¹50oC, suggesting a transition to semiconductor-metal behavior. Lattice oxygen begins loss at high temperature, which decreases the conductivity and increases the TEC. The TEC of PBCO is about 22.8×10-6 K-1 at the temperature range of 30-800oC. Sintering temperature has an important effect on the microstructure and electrochemical properties of PBCO cathode. The optimizing sintering temperature of PBCO is 950oC. The PBCO cathode sintered at 950 oC shows the best microstructure and the smallest Rp. The Rp is 0.164Ωcm2 at 700oC. A porous layer of PBCO is deposited on a 42μm thick electrolyte of SDC prepared by a dry pressing process. A fuel cell of PBCO/SDC/Ni-SDC provides the maximum power densities of 866, 583, 313 and 115mWcm-2 at 650, 600, 550 and 500oC, respectively, using hydrogen as fuel and stationary air as oxidant. The total resistances of the cell are about 0.41, 0.51, 0.57, and 0.77Ωcm2 at the corresponding temperatures, respectively. The present cell exhibits much higher power outputs than that with Sm0.5Sr0.5CoO3-δcathode, which implies that PBCO possesses a better cathode performance than that for Sm0.5Sr0.5CoO3-δ.Although PBCO shows high activity for activation and migration of oxygen, the electronic conductivity is much higher than ionic conductivity in PBCO. Theory and experiment have proved that oxygen ion diffusion and surface exchange is still a major factor to impact MIEC cathode performance. Therefore, enhancing ionic conductivity and surface oxygen exchange kinetics in cathode bulk and surface are key factors to improve cathode performance. Single cathode materials often can not simultaneously meet the needs of IT-SOFCs cathode for high electron conductivity and high ionic conductivity requirements. Electrolyte composite cathode can obtain higher ionic conductivity, and existence of electrolyte phase can greatly improve compatibility and adhesion between the cathode and electrolyte, and greatly extend the length of TPBs, so that the main factors, which impact the interface charge transfer properties of cathode, become smaller or negligible level. We present the electrochemical properties of PBCO-SDC composite cathodes. We research the sintering temperature and SDC content, which can influence the microstructure and electrochemical performance of composite cathode. The results show that the PBCO and SDC mixture after heat-treated at 1100 oC for 3 h, they still remained their structures unchanged, and there are no obvious reactions between PBCO and SDC. The electrochemical performance PBCO-SDC cathode is very sensitive to the sintering temperature. The PBCO-SDC cathode sintered at 900oC shows the smallest interfacial polarization resistance. As the SDC content increases up to 30 wt%, composite cathode has formed a continuous electronic conductive phase and ionic conductive phase where there are the greatest catalytic synergy between the SDC and PBCO in the composite cathode and largest TPBs, which leads to the smallest Rp. The Rp values are only about 0.035Ωcm2 at750oC, which is much lower than that of pure PBCO cathode (0.121Ωcm2). Using PBCO?30SDC as the cathode and an LSGM film as the electrolyte, SDC as buffer layers in order to prevent the reactions inducing high resistance products between LSGM and NiO or the Co-contained cathode, The maximum power densities of the cell reach 130, 231, 364, 521 and 741 mW cm?2 at 600, 650, 700, 750 and 800 oC, respectively, illustrating that good performance can be obtained for a single cell using PBCO?30SDC as the cathode and an LSGM film as the electrolyte.Finally, the BaCo_0.7Fe_0.3-yNb_yO_3-δ (BCFNy, y=0.00–0.12) perovskite, formerly as oxygen permeation membrane, was investigated as a novel cathode for SOFCs. It is found that BCFNy can obtain single phase perovskite structure after calcinations at 1100oC in air for 15 h. Nb-doping enhances the phase transformation of hexagonal to cubic perovskite. Nb-doping has a significant impact on the TECs of BCFNy. At 30⁸00oC, the TEC of the sample without Nb-doping is 17.17×10-6 K-1, whereas the TECs of the sample doped with Nb is in the range of 18.50-18.96×10-6 K-1. The sudden increase In TECs was attributed to the phase transformation of hexagonal to cubic perovskite. In addition, the thermal expansion curve for BCFNy displays a steeper inflection at 450oC⁵50oC, which corresponds to the lattice expansion. At this temperature range, the lattice oxygen becomes extremely active, and begins loss, resulting in a large number of oxygen vacancies formation and arousing the Co4+ and Fe4+ ion reduction, which leads to increase of the TECs. The increase of Nb doping concentration caused an evident decrease in the Rp. When the Nb doping concentration increased up to 0.10, the Rp reached a minimum, i.e. 0.9406Ωcm2 at 500 oC, 0.1300Ωcm2 at 600 oC, 0.0211Ωcm2 at 700oC, and 0.0082Ωcm2 at 800 oC. The low Rp values are also comparable to those of BSCF cathode (1.4063, 0.1724, and 0.0423Ωcm2 at 500, 600, and 700oC, respectively), PBCO and PBCO-30SDC. The high electrochemical performance of BCFN0.10 cathode can be mainly attributed to large ionic radius Ba2+ fully occupied A-site of coblt-based peroverskite with a strong stabilization effect of Nb for peroverskite structure in B-site. Using Ni0.9Cu0.1-SDC as the anode, SDC as electrolyte, and BCFNy (y = 0.10, 0.12) as the cathode, the single cell give a satisfactory output performance. The maximum power densities of the cell with BCFN0.10 reach 202,350,569,820 and 1006 mW cm?2 at 600, 650, 700, 750 and 800oC respectively, while the maximum power densities of the cell with BCFN0.12 reach 140,235,381,576 and 806 mW cm?2 at the same temperature.In this paper, three kinds of new MIEC cathode materials were investigated. And based on composite cathode materials design idea, we explored effective way of enhancing ion conductivity in cathode. The properties of these mixed conducting cathodes were systemically studied, which exhibited attractive performances for IT-SOFCs.