Perovskite Anode Materials For Solid Oxide Fuel Cell

Solid oxide fuel cells（SOFCs）convert chemical energy in fuels into electricity with high efficiency.Its main advantages include no need of noble metals as catalysts,high fuel flexibility and low pollution.Conventional nickel-based cermet anodes suffer from serious carbon deposition when using hydrocarbons as the fuels.Metal oxides with a perovskite structure（ABO3）exhibit high coking resistence but low catalytic activity.In this work,the catalytic activity of perovskite anodes is improved with cation substitution and anion doping.The relationship between composition-catalytic activity-electrochemical property of the perovskite anodes is systemically investigated.These anodes show promising performance and stability in direct hydrocarbon SOFCs.（1）Sm0.5Ba0.5Mn O3-δ（SBMO）is synthesized with the Pechini method and investigated as an anode material of solid oxide fuel cells with H2 and methanol as fuels.A tetragonal structure is formed after reduction.The reduction process of SBMO is studied with X-ray photoelectron spectroscopy and temperature programmed reduction techniques.The electrical conductivities of an SBMO sample sintered at 950 ^oC are 1.15 and 0.10 S cm^-1 at 850 ^oC in air and H2,respectively.A single cell with the SBMO anode layer and a 300-μm-thick La0.9Sr0.1Ga0.8Mg0.2O3-δelectrolyte layer exhibits a maximum power density（Pmax）of about 150 m W cm^-2 at850 ^oC with H2 as fuel.The electrical conductivity of the anode layer with methanol as fuel is improved by the moderate carbon deposition,and the Pmax increases to about415 m W cm^-2.The cell fed with methanol also shows a promising stability.（2）Ln0.5Ba0.5Mn O3-δ（Ln BM,Ln=La,Pr,Nd,Sm）anodes are synthesized with the pechini method.The powders show a cubic-hexagonal heterogeneous stucture before reduction,which change to a tetragonal phase after reduction.The oxygen non-stoichiometry is studied with iodometric titration at room temperature combined with thermal gravimetric analysis in a reducing atmosphere.The oxygen vacancy concentration of the anodes at a high temperature is in the order of LBM<NBM<PBM<SBM.The results of H2 temperature programmed reduction and CH4 temperature programmed surface reaction suggest that the oxidation process has the lowest initial temperature on the surface of LBM,and followed by NBM.LBM and NBM both show high catalytic activities.The electrooxidation rate of the fuel on the surface of the anodes is determined by the oxygen surface exchange and charge transfer steps.The Pmax of the single cells with SBM,PBM,LBM and NBM anodes fueled with wet H2 are 754,926,962 and 1087 m W cm^-2,respectively,which drop to219,362,337 and 530 m W cm^-2 respectively when CH4 is used as the fuel.（3）Co-Fe co-doped La0.5Ba0.5Mn O3-δanode with a cubic-hexagonal heterogeneous stucture is synthesized.An A-site ordered double perovskite with Co0.94Fe0.06 alloy-oxide core-shell nanoparticles on its surface is formed after reduction.The phase transition and the exsolution of the nanoparticles are investigated with X-ray diffraction,thermogravimetric analysis and high-resolution transmission electron microscope.The exsolved nanoparticles with the layered double perovskite supporter show a high catalytic activity.A single cell with that anode and a300μm-thick La0.8Sr0.2Ga0.8Mg0.2O3-δelectrolyte layer exhibits maximum power densities of 1479 and 503 m W cm^-2 at 850 ^oC with wet hydrogen and wet methane fuels,respectively.Moreover,the single cell fed with wet methane exhibits a stable power output at 850 ^oC for 200 h,demonstrating a high resistance to carbon deposition of the anode due to the strong anchor of the exsolved nanoparticles on the perovskite parent.The oxide shell also preserves the metal particles from coking.（4）A highly active and stable perovskite La0.5Ba0.5Fe O3-δanode material is prepared through the sol-gel method,in which O^2-is partially replaced by F^-.The oxygen surface exchange and charge transfer steps are the rate-determining steps of the anode process,and the former is accelerated with fluorine doping on the anion sites due to the lowering of metal-oxygen bond energy.The oxygen surface exchange coefficients of La0.5Ba0.5Fe O3-δand La0.5Ba0.5Fe O2.9-δF0.1 at 850°C are 1.4×10^-4 and2.8×10^-4 cm s^-1,respectively.A single cell with La0.5Ba0.5Fe O3-δanode shows maximum power densities of 1446 and 691 m W cm^-2 at 850°C with wet hydrogen and methane fuels,respectively,which increase to 1860 and 809 m W cm^-2respectively when La0.5Ba0.5Fe O2.9-δF0.1 is used as the anode.The cell exhibits a short-term durability of 40 h using wet methane as fuel without carbon deposition on the anode.