Characterization Of Pr_1-xSr_xCo_0.8Fe_0.2O_3-δ Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cell

A fuel cell is an energy conversion device with a high efficiency and a low pollution. Different from the traditional cells that can only reserve energy, it generates electricity from fuels such as hydrogen, natural gas and other hydrocarbons. The fuel cell is also called a cell because it is composed of electrolyte, anode and cathode, which are the same for a normal cell. The electrolyte is sandwiched by the two electrodes. The fuel cell is also different from the traditional power generation methods. Because it is not limited by the Carnot cycle, fuel cell has advantages of higher energy conversion efficiency and lower polluted gases emission over the traditional generator. Recently with the natural resource exhaustion and environment deterioration, developing efficient and environmental friendly energy techniques is necessary. Since fuel cell just matches such requirements, it attracts the interests all over the world.As the fourth generation fuel cell, SOFC (Solid Oxide Fuel Cell) has many outstanding advantages, which is better than other fuel cells. Firstly, equipped with all solid components, it eliminates the problems that liquid electrolyte fuel cell faces, such as corrosion and leakage of liquid electrolytes. Secondly operating at high temperatures, its electrode reaction is so fast that it is unnecessary to use noble metals as electrodes. Thus the cost of the cells can be minimized. The most outstanding advantage of SOFC is that it uses a large scale of fuels, from the hydrogen, carbon monoxide to the natural gas or even other combustive gases.Intermediate temperature solid oxide fuel cell (IT-SOFC) is the tendency of the solid oxide fuel cell commercializing development. The traditional cathode material La1?xSrxMnO3 (LSM) is deemed to be one of the most promising cathode materials for high temperature SOFCs because of its outstanding electrochemical performance, thermal and chemical stability and relatively good compatibility with yttria-stabilized zirconia (YSZ). However, with decreasing temperature, its electrical conductivity is greatly reduced and the cathode resistance is increased rapidly. so it is important to develop new cathode materials with high performance at intermediate temperature.In recent years, Ln1?xSrxCo1?yFeyO3?δ(Ln = La, Sm, Nd, Gd, Dy) has become the cathode material of choice for IT-SOFCs, because of its high electronic and ionic conductivity as well as its high catalytic activity for oxygen reduction. In our study, Cathode materials consisting of Pr1-xSrxCo0.8Fe0.2O3?δ(x=0.2–0.6) were prepared by the sol–gel process for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). Pr1-xSrxCo0.8Fe0.2O3?δ(x=0.2–0.6) had an orthorhombic perovskite structure after sintering at 900°C for 6h. The electrical conductivities were all higher than 279 S cm-1, which met the demand for IT-SOFC. At the same time, we compared the conductivity values for each proportion at the 450°C, 600°C, 750°C. We found that Pr0.6Sr0.4Co0.8Fe0.2O3?δhad the highest conductivity, which is 989S/cm, 901 S/cm and 730 S/cm at 450°C, 600°C, 750°C, respectively. Therefore, we choose Pr1-xSrxCo0.8Fe0.2O3-δcathode to make other tests, for the Pr1-xSrxCo0.8Fe0.2O3-δ(0.2≤x≤0.6) system. The average thermal expansion coefficients of Pr0.6Sr0.4Co0.8Fe0.2O3?δbetween 30 to 850°C are about 19.69×10-6 K-1. The result of these samples was slightly higher than that of Ce0.85Gd0.15O1.925 (GDC) (12.56×10-6 K-1, measured by our lab.) electrolyte. In order to decrease thermal expansion coefficient we fabricated composite electrodes which can match well with GDC electrolyte. The SEM images of Pr0.6Sr0.4Co0.8Fe0.2O3?δ(PSCF) cathodes sintered at 1000oC showed a smaller grain size and reasonable porosity to ensure gas diffusion. Also the cathode appeared to be in good contact with the dense electrolyte pellet.Using impedance spectra, the electrochemical performance of PSCF–GDC composite cathode in the temperature range of 650-800 oC on GDC electrolyte, and the effect of sintering temperature on performance of cathode were investigated. The area specific resistance (ASR) value of PSCF–GDC sintered at 1000oC on GDC electrolyte is 0.046Ωcm2 at 800oC, which is much lower than the 0.516Ωcm2 measured at 800oC for PSCF–GDC sintered at 1050oC. Its value is a little lower than that of PSCF–GDC sintered at 950oC, which is 0.05Ωcm2. And considering the SEM photos of PSCF–GDC sintered at 1000oC, we think 1000oC is the appropriate sintering temperature for this composite cathode. At the same time, we obtained the activation energy from the slope of lnR vt 1000 / T curve at different sintering temperature for PSCF-GDC composite cathode, which can further demonstrates that the best sintering temperature is 1000 oC. We also tested the overpotential for PSCF-GDC composite cathode, it can be predicted that the weak polarization properties of this cathode can improve performance of electrode significantly, and the PSCF-GDC composite cathode is a potential cathode material. We measured the power densities of the GDC-supported fuel cells by using PSCF–GDC as composite cathode. The maximum power density of the cell with the PSCF–GDC composite cathode was 520mWcm-2, 435 mWcm-2 and 303 mWcm-2 at 800 oC, 750 oC and 700 oC, respectively.These results above mentioned indicate that PSCF–GDC composite cathode is a promising cathode material for IT-SOFC.