Study On The High Temperature Electrochemical Properties Of Manganese Perovskite Materials

Solid Oxide Fuel Cell?SOFC?is a kind of electricity generator which convert directly chemical energy into electrical energy through electrochemical reactions.SOFC is a new energy conversion devices with the advantages of high efficiency and low pollution.As an important component in SOFCs,the cathode material is always the priority subject of studies.In this thesis,Mn-based perovskite oxides Ln_1-xSr_xCr_0.5Mn_0.5O₃?Ln=La,Pr,Nd,Sm,Gd??x=0.1,0.3,0.5?,double-perovskite oxides La_1-xBiMn₂O₆?x=0.0-0.20?and La_2-xNiMnO₆?x=0.0-0.4?)are systematically investigated.In order to develop new cathode materials with high electrochemical performance,good thermal compatibility and promising chemical stability with electrolyte,the electrochemical properties of these materials have been evaluated by the formation of CGO-composite cathodes and the cation-deficiency doping strategy?Firstly,Ln_1-xSr_xCr_0.5Mn_0.5O₃ was synthesized by a conventional solid-state reaction.This material has good chemical stability with CGO electrolyte at high temperatures.Sr doping in Ln_1-xSr_xCr_0.5Mn_0.5O₃ leads to an increase in electrical conductivity and promots the electrochemical properties.However when the Sr²⁺doping concentration is up to x=0.7,second phase SrCrO₄ is formed.Therefore the electrochemical study is limited to the samples with x=0.5.It is found that the cathode polarization resistance?Rp?increases with the atomic number of the lanthanide elements.The La_0.5Sr_0.5Cr_0.5Mn_0.5O₃ cathode exhibits the lowest polarization resistance of 2.0?.cm² at700^oC in air.Depending on the temperatures,the reaction rate-limiting step for oxygen reduction reaction on La_0.5Sr_0.5Cr_0.5Mn_0.5O₃ cathode would be a process involving successively the diffusion of O^2-at TPB to electrolyte and the charge transfer reaction at interface.La_1-xBiMn₂O₆?x=0.0-0.2?cathode materials were synthesized by solid-state reaction.These materials have good chemical stability with CBO electrolyte at high temperatures.The Rp of LaBiMn₂O₆ cathode decreases with temperature and reaches0.71?cm² at 700 ^oC in air,the lowest overpotential of 85 mV is found under current density of 216 mA cm^-22 at 700 ^oC in air.La³⁺vacancy in La_1-xBiMn₂O₆ materials clearly promotes the cathode property.La_0.80BiMn₂O₆ cathode shows the highest electrical conductivity and promising electrochemical performance,the Rp decreases to 0.18?cm² at 700 ^oC in air.The cathode overpotential of 180 mV is obtained at the current density of 975 mA cm^-2 at 700 ^oC.The dependence of Rp with oxygen partial pressures indicates that the rate-limiting step for oxygen reduction reaction on La_0.80BiMn₂O₆cathode is the diffusion of O^2-at TPB to electrolyte.La_2-xNiMnO₆?x=0.0-0.4?)materials were synthesized by solid-state reaction.These materials have good chemical stability with CGO electrolyte at high temperatures.At 700 ^oC,La₂NiMnO₆ cathode has a polarization resistance of 0.4?cm².The La-deficiency leads to an increase in electrical conductivity and improvement of ORR performance for cathode.At 700 ^oC,La_1.8NiMnO₆ cathode has a polarization resistance of 0.22?cm².This indicates that it is a potential cathode for IT-SOFCs.Oxygen reduction kinetics proves that the rate-limiting step for oxygen reduction reaction on La_1.8NiMnO₆ cathodes is charge transfer reaction at electrolyte/electrode interface.