Investigation Of Nickle-based Anode Materials For Intermediate Temperature Solid Oxide Fuel Cell

Solid oxide fuel cells（SOFCs）can convert chemical energy into electrical energy without the limitation of Carnot cycle and so it can effectively improve the energy efficiency of fossil energy.The main products of the reaction are water and carbon dioxide,which greatly reduces the emission of pollutants.Ni-based cermets are widely used as anode materials for SOFC due to their excellent conductivity and catalytic activity for the electrochemical oxidation of fuels.However,when hydrocarbons are used as fuels,Ni is prone to carbon deposition,resulting in cell degradation.In order to alleviate carbon deposition and enhance the catalytic activity of anode simultaneously,Ni-based cermet anode materials are modified and optimized in this work.Some improvements have been achieved:（1）Ni-MnO_x-Ce_0.8Sm_0.2O_1.9（SDC）composites are synthesized through a hydrothermal process and investigated as anode materials of SOFC fed with methanol.The lowest anodic polarization resistance is obtained when the molar ratio of Mn to Ni is 0.05:0.95.The high catalytic activity is attributed to the transfer of electrons from Ni to Mn and the increase of the content of the lattice oxygen in the anode.The single cell with that anode and SDC-carbonate composite electrolyte exhibits a maximum power density(P_max)of 0.72 W cm^-2 at 700℃.Mn also increases the resistance to carbon deposition of the anode due to the high lattice oxygen content and the redox cycle of the Mn species.The stability of the single cell is enhanced with the increase of the content of Mn in the anode.（2）Ni-SDC with a three-dimensionally ordered macroporous（3DOM）structure is prepared with poly（methyl methacrylate）as the template.Lattice oxygen in 3DOM SDC shows a higher activity than that in hydrothermally synthesized SDC（HT）.3DOM SDC also exhibits a stronger interaction with Ni compared to SDC（HT）,which facilitates the release of lattice oxygen and the formation of oxygen vacancies in SDC in a reducing atmosphere.Therefore,Ni-3DOM SDC shows a much higher catalytic activity for electrochemical oxidation than Ni-SDC（HT）as the anode material of solid oxide fuel cells.With Ni-3DOM SDC as the anode,a cell supported by a 0.50 mm-thick SDC-carbonate composite electrolyte layer exhibits P_max of 1.28 and 1.63 W cm^-2 at700℃ with H₂ and methanol as fuels,respectively,more than twice the P_max of a similar cell with a Ni-SDC（HT）anode.The Ni-3DOM SDC anode also shows a high resistance to coking.（3）Sm_0.20Ni_xCe_0.80-xO_2-δ（SNDC）materials are prepared through the EDTA-citric acid sol-gel method and evaluated as anodes for SOFC with H₂ and methanol as fuel.Ni nanoparticles are exsolved from SNDC after reduction.Compared with the traditional impregnation method,the exsolution process is a simpler and more attractive method to introduce catalytic nanoparticles to improve the catalytic activity.Increasing the Ni content can effectively extend reaction sites（three-phase boundary,TPB）and decrease the polarization impedance.P_max of 1.26 and 1.48 W cm^-2 are obtained using an SDC-carbonate composite electrolyte-supported single cell with a SNDC15 anode at 700℃ using H₂ and methanol as the fuels,respectively.Due to the strong interaction between Ni nanoparticles and support,the formation and growth of carbon depositsis effectively inhibited.Therefore,the amount of carbon deposition on SNDC15 is only12.4%,much lower than that on other anode materials with the same Ni content.（4）Ni-SSn_xDC composites are prepared and studied as anode materials for SOFC with H₂ and methanol as fuels.After reduction,intermetallic compounds are formed accompanied with Ce-Sn-O solid solution.With the addition of Sn in the anode,the oxidation temperature of CH₃OH decreases remarkably because the lattice oxygen activity is improved.The anode reaction rate is mainly determined by O^2-diffusion.The cell with Ni-SSn₁₀DC anode exhibits the P_max of 1.99 and 2.11 W cm^-2 at 700℃ with H₂ and methanol as fuels,respectively.The activation energy of Ni-SSn_xDC anode is0.32-0.65 e V,which is promising for the application of low temperature solid oxide fuel cell.The high oxygen activity of the Ni-SSn_xDC anodes also results in a high carbon resistance.