Electrochemical Properties Of Pr ₂ CuO Cathode Materials For Solid Oxide Fuel Cells

The development of new mixed ionic–electronic conducting materials(MIECs) is important for the electrochemical applications, including solid oxide fuel cells(SOFCs). Up to now, many perovskite-type MIECs such as La1-xSrxMnO3, La1-xSrx Co1-yFeyO3 and Ba0.5Sr0.5CoxFe1-xO3 have been extensively studied as possible cathodes in SOFCs. Recently, some mixed ionic-electronic conducting materials with K2NiF4-type structure have been attracted extensive attention. These oxides are usually formulated as A2BO4, with A of a lanthanide(La, Pr, Nd) or alkaline(Sr, Ba), and B of a transition metal cation(Co, Ni, Cu). These results imply that perovskite-like type oxide Pr2CuO4 is likely to be new cathode materials for SOFC.In this paper,the Pr2CuO4 powders were synthesized using the glycinenitrate process(GNP). These compounds have the good compatibility with CGO electrolyte at high temperature. The thermal expansion coefficient was clearly to that of the CGO, and effects of the sintering temperature on the microstructure and electrochemical properties were investigated, respectively. The microstructure of the electrode sintered at 950 oC showed a structure with reasonable porosity and well-necked particles. Pr2CuO4 cathode also exhibits a low polarization resistance, 0.56 ? cm2 at 700 oC. 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 is the charge transfer process. The exchange current density increases with the temperature(i0 = 6.3 mA cm2 at 600 oC, i0 = 8.5 mA cm2 at 650 oC, i0 = 17.4 mA cm2 at 700 oC).In order to improve the electrode property further, the composite cathode of Pr2CuO4-CGO(PCO-CGO) and Pr2CuO4-Ag(PCO-Ag) electrodes were studied systematically. The results showed that the addition of CGO powders into PCO electrode was effective in improving the electrode performance and the bonding of the electrode and electrolyte. The addition of 40 wt.% CGO in PCO resulted in the lowest polarization resistance of 0.12 ? cm2 at 700 oC, about five times smaller than that of pure PCO cathode. The PCO-Ag composite cathode also showed fine electrochemical performance. The addition of 6 wt.% Ag in PCO resulted in the lowest polarization resistance of 0.37 ? cm2 at 700 oC, about two times smaller than that of pure PCO cathode. Furthermore, PCO-CGO and PCO-Ag composite cathode also exhibited the fine exchange current density.In order to futher study the effect of doping on electrochemical performance, Pr2-xSrxCuO4(0.2-1.0) cathode materials were synthesized by a conventional solid-state reaction. Doping amount of Sr is studied to see the influence of the thermal expansion coefficient, ion-electron conductivity and electrochemical performances. The results showed that the electrochemical performance of the cathode was significantly improved. Pr1.6Sr0.4CuO4 cathode has a polarization resistance of 0.22 ? cm2 at 700 oC. Oxyge reduction kinetics proves that the rate-limiting step for oxygen reduction reaction on Pr1.6Sr0.4CuO4 cathodes is the charge transfer process.