Properties Of Perovskite-like Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cell

Solid oxide fuel cells (SOFCs) are a kind of electricity generator with high efficiency and low pollution. The cathode material as SOFC important component is always the priority subject of studies. Strontium doped lanthanum manganate (La_1-xSr_xMnO₃, LSM) is commonly used as ABO₃ type cathode in traditional high temperature solid oxide fuel cells. However, LSM is not suitable to operate in intermediate temperature range, due to the low conductivity and electrochemical activity. For the further improvement of cell performance, the development of high performance cathode material at intermediate temperature is critical. Studies showed that perovskite-like A₂BO₄ type oxide materials exhibited mixed ionic and electronic conducting properties, substantial oxygen permeability, high electrochemical properties, and a relatively high xygen diffusion and surface exchange coefficients.With the ABO₃ type oxides cathode materials for SOFC, A₂BO₄ type oxides generally are lower thermal expansion coefficient (TEC). The TEC of these oxides match reasonably well with the yttrium stabilized zirconia (YSZ) or Ce_0.9Gd_0.1O_1.9 (CGO) electrolyte materials. Furthermore, A₂BO₄ type oxides show higher electrical conductivity at intermediate temperature, which is extremely advantageouso to the cathode material. All these results imply that A₂BO₄ type oxides are likely to be new cathode materials for (intermediate temperature solid oxide fuel cell) IT-SOFCs. Some preliminary studies on A₂BO₄ type oxides materials in therms of electrochemical properties have been reported. But there are seldom deep researches on electrochemical properties of doping in the A site of the A₂BO₄ type oxide.Cathode materials Ln_2-xSr_xMO₄ (Ln=La, Nd, Sm; M=Ni, Cu) for IT-SOFC were prepared by glycine-nitrate process (GNP). The chemistry compatibility of the electrode and electrolyte at high temperature and effects of the sintering temperature on the microstructure and electrochemical properties were investigated, respectively. The results showed the sintering temperature has effect on the polarization resistance of the electrode. At moderate temperature, a fine microstructure with moderate porosity and well-necked particles has been formed, and the electrode gave the lowest polarization resistance. Oxygen partial pressure effect experiments have been performed to study the mechanism of the reaction occurred on the electrode. We have studied variations of the electrode polarization resistance with temperatures and oxygen partial pressures, respectively. The results showed that the possible rate limiting step for cathode reaction depends on the oxygen ion transfer process, oxygen adsorption-desorption process and oxygen ion transfer from the TPB to the CGO electrolyte process. The rate limiting step is differect at different temperature and oxygen partial pressure. Sr doped La2CuO4 electrode exhibits fine chemical stability and higher electrochemical catalytic properties. In order to study the chemical stability of electrode with electrolyte materials at high temperature, the sample was prepared by mixing thoroughly La_1.7Sr_0.3CuO₄ with CGO powders, and after heat-treating at 1000 oC for 144 h in air. XRD measyrement, this result indicates that La_1.7Sr_0.3CuO₄ has a good chemical compatibility with the CGO electrolyte. It was found that La_1.7Sr_0.3CuO₄ electrode showed the lowest polarization resistance. That is 0.16Ω.cm² at 700 oC. The polarization resistance of La_1.7Sr_0.3CuO₄ is comparable to that of La1-xSrxCoyFe1-yO₃ (LSCF) Material.La_1.6Sr_0.4NiO₄-CGO and La_1.6Sr_0.4NiO₄-Ag composite electrode were prepared in order to, optimize the electrode preparation technology. Has systematically studied the effect of the CGO or Ag doping amount to electrochemical performance of the composite cathode, and high-performance SOFC composite was prepared cathode by select optimum doping amount. The results showed the addition of oxygen ion conductor CGO notably decreased polarization resistance of the electrode. The addition of 40 wt.% CGO in LSN resulted in the lowest polarization resistance of 0.76Ω.cm² at 700 oC in air, about four times smaller than that of pure LSN cathode. The La_1.6Sr_0.4NiO₄-Ag composite cathode also showed fine electrochemical performance. The addition of 7 wt.% Ag in LSN resulted in the lowest polarization resistance of 0.26Ω.cm² at 700 oC in air, about ten times than that of LSN cathode. Furthermore, LSN-7Ag composite cathode exhibited the lowest overpotential of about 32 mV at a current density of 144 mA.cm^-2 at 700 oC in air. In conclusion, we have explore a new kind of cathode material and we got some nice results when user in on IT-SOFC.