Study On Modification Of PrBaCo₂O_5+? Cathode Material For Solid Oxide Fuel Cells

Fuel cells are the most promising new generation environment friendly energy power systems. They will be widely used in portable electronics, general household power supplies, power vehicles and so on. Undoubtedly, it is helpful to ease the global energy crisis and environmental pollution problems. Fuel cells are a kind of devices that transform the chemical energy of the fuel into electrical energy with high efficiency and little pollution. As a kind of fuel cells, solid oxide fuel cell is composed of solid electrodes and electrolyte, having a wide range of sources and applications.This work is based on intermediate temperature solid oxide fuel cell, choosing mixed conductor PrBaCo₂O_5+? as the cathode material. The main purpose is to improve the comprehensive properties of the cathode material. The PrBaCo₂O_5+? cathode materials with Ni doping and Sr/Ni co-doping are studied systematically. The crystal structure, electrical conductivity, thermal expansion coefficient and fuel cells' performances are investigated systematically.The cathode materials PrBaCo_2-xNi_xO_5+??x=0, 0.1, 0.2 and 0.3? and PrBa_0.5Sr_0.5Co_2-xNi_xO_5+??x=0, 0.1, 0.2 and 0.3? were prepared by a combined EDTA-citrate complexing sol-gel method. XRD results showed that after calcination at 1000 °C in air for 5h the PrBaCo_2-xNi_xO_5+? exhibited a tetragonal structure with the space group of P4/mmm; while the PrBa_0.5Sr_0.5Co_2-xNi_xO_5+? exhibited an orthorhombic structure with the space group of Pmmm. After sintered at 1100 °C in air for 10 h, the cathode pellets pressed at 150 MPa were dense enough. The electrical conductivity was measured by a DC four-probe method. The results revealed that the materials with Ni-doping and Sr/Ni co-doping showed elevated conductivity compared to the undoped ones. Besides, the conductivity reached a maximum at x=0.1. However, it decreased with the Ni content increase further. The conductivity of PrBaCo_1.9Ni_0.1O_5+? was 1397 S cm-1 at 400 °C; the conductivity of PrBa_0.5Sr_0.5Co_1.9Ni_0.1O_5+? was 1463 S cm-1 at 400 °C. The thermal expansion test showed that the thermal expansion coefficient of the cathode material decreased with the Ni content increase, which was helpful to increase the matching between the electrode and electrolyte.The electrolyte-supported fuel cells were prepared by a coating method. The test results of the symmetrical fuel cell and the single fuel cell revealed that the electrochemical performances of PrBaCo_1.7Ni_0.3O_5+? and PrBa_0.5Sr_0.5Co_1.9Ni_0.1O_5+? were better than those of other materials of the same series. The polarization resistance of PrBaCo_1.7Ni_0.3O_5+? was 2.628, 1.183, 0.474, 0.161 and 0.042 ? cm2 at 600, 650, 700, 750 and 800 °C, respectively; the power density of the single fuel cell was 100, 185, 360, 495 and 660 m W cm-2, respectively. The polarization resistance of PrBa_0.5Sr_0.5Co_1.9Ni_0.1O_5+? was 8.556, 4.822, 1.640, 0.647 and 0.297 ? cm2 at 600, 650, 700, 750 and 800 °C, respectively; the power density of the single fuel cell was 12, 40, 57, 73 and 120 m W cm-2, respectively.