Properties Of La_1.6-xPr_xSr_0.4NiO_4+δ As Cathodes For Intermediate Temperature Solid Oxide Fuel Cells

The cathode are one of the most important component influenced theperformance of solide oxide fuel cells （SOFCs）. The polarization damage of cathodereach up to65%of the whole ohms losses in single cells, which can limit thedevelopment of SOFCs. Recently, much more attention has been focused onintermediate-temperature and low-temperature SOFCs, This need put forward higherrequirement for cathode materials of SOFCs, which is terribly essential to researchthe intermediate-temperature cathode material to improve its properties.K₂NiF₄-type structure oxide consists of the alternating AO rock salt layers andABO₃perovskite layers, exhibiting high mixed oxide ion and electron holeconductivity in combination with relatively fast surface exchange kinetics, whichhas the potential to expand the triple-phase boundary （TPB） to whole cathode anddecrease the activation energy for oxygen reduction. Compared to the tranditionalABO₃cathode, this oxide with K₂NiF₄-type structure exhibits high oxygen diffusionand relatively fast surface kinetics, possesses higher electrical conductivitiy andelectrochemical activity at intermediate temperature. Besides, its thermal expansioncoefficients （TECs） agree very well with a variety of conventional electrolyte. Theoxide with a serials of sound properties is a kind of promising cathode for SOFCoperating at500–700oC.K₂NiF₄-type structure oxide La₂NiO_4+δas the cathode of SOFC have beenreported by many groups. For the substituted lanthanum nickelates, the B-site hasbeen doped with Cu, Fe, or Co, and few studies focused on the A-site. It is generallybelieved that cations at different sites have different effects on cathode materials. Inorder to know the oxide La₂NiO_4+δclearly, the rare-earth element Pr-substitutedmaterials La_1.6-xPr_xSr_0.4NiO_4+δ（x=0–1.6） cathodes would be investigatedsystematically for the requirement of the cathode of IT-SOFCs.La_1.6-xPr_xSr_0.4NiO_4+δ（x=0,0.1,0.4,0.7,1.0,1.3,1.6） powders were synthesizedby a modified sol-gel method, and the powders were checked with single phaseK₂NiF₄-type structure. The nonstoichiometry δ of oxygen ranging from0.03to0.44shows that the mechanism of ionic conduction for LPSNx is interstitial oxygen. Itcan be clearly seen that the values of conductivity decrease slightly from x=0.1to1.0and then increase a little from x=1.0to1.6above400oC, The maximumconductivity value is around113Scm^-1for LPSN00and the minimum is about82Scm^-1for LPSN10. The conductivity of LPSNx at700oC lasting12h indicates thatLPSNx can work at high temperature stably. The calculated average TECs from 30oC to900oC are in the range of13.35–15.03×10-6K^-1for LPSNx cathodes, whichmatch well with that of GDC, SDC, YSZ, LSGM and so on. From chemicalcompatibility analysis between LPSNx and clectrolytes, we can know that Pr₂Zr₂O₇and La₂Zr₂O₇are detected between YSZ and LPSNx and no reactions betweenLPSNx and GDC when sintering at1000oC. Impedance spectra of LPSNx cathodetested at700oC in air with the symmetrical cells show that LPSN10presents thelowest polarization resistance value （⁰.40cm²） in air, which is about one-sixth ofthe un-doped LPSN00（2.3cm²）. We can easily know that the limiting step on thecathode reactions is adsorption-desorption process, involving oxygen diffusion forLPSNx in the whole range of measuring oxygen partial pressure. These results ofcathode polarization under different temperature show that temperature can facilitatethe catalytic activity of cathodes. The values of activation energy are calculatedfrom97.5±5.9to118.4±1.2kJmol^-1.The performance of composite electrode LPSN10-GDC drops seriously,because the GDC caclined at a low temperature800oC can not form the big sizegrain to improve the catalytic activity and connectivity. The properties of nanoLPSN10cathode modified with Ag changes with the increasing of AgNO3concentration, the minimum polarization resistance is0.08cm²at the5molL^-1AgNO3concentration. In order to study the effect of H₂O on LPSN10cathode, theLPSN10dealed with deionized water is studied alone, and the result shows that thedeionized water can dissolve some ions of LPSN10.Based on researches above, we choose the LPSN10to prepare different singlecells. From the result of each single cell, we can know that the single cellLPSN10/GDC/YSZ/Ni-YSZ with GDC isolation layer is the best one. Thesemaximum power densities are0.42Wcm-2,0.63Wcm-2and0.73Wcm-2at700oC,750oC and800oC respectively.