Study On Applications Of F-Doped Electrode Materials In Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFCs）convert the chemical energy stored in fuel directly into electrical energy with low pollution,high efficiency and no noble metal catalysts.Compared with traditional power generation technology by combusting,SOFCs has no combustion process and working mechanism of mechanical movement,and less energy loss in the process of energy conversion.So SOFCs have attracted widespread attention.However,high operating temperature is one major obstacles to the commercialization of SOFCs.As the temperature decreases,the reduction rate of oxygen decreases,which leads to the loss of electrochemical performance and efficiency.Thus,to develop highly efficient catalysts for oxygen reduction（ORR）is a key factor in the development of low-temperature SOFCs（IT-SOFCs）with high performance below 600°C.Previous studies have proposed a series of perovskite cathodes with enhanced ORR activity,and a strategy of using F to replace O-site anion doping has been frequently used to modify cathodes.In order to solve the influence of high temperature bottleneck on SOFCs,the electrode materials with good catalytic activity at low temperature were developed by the strategy of fluorine anion（F^-）doping LNCA,and the influence of F^-doping on the catalytic activity of ORR was explored.Then,the influence of composite F^-doped electrode materials as electrolyte on SOFCs properties was discussed.First,LNCA,LNCAF_0.1 and LNCAF_0.2 cathode materials were synthesized by solid phase method and applied to the cell of Ni-LNCA/SDC/LNCAF_X-Ni structure.The electrochemical mechanism and properties of IT-SOFCs with LNCAF_X（X=1,2）oxide as cathode,LNCA as anode and SDC as electrolyte were studied.It was found that the successful doping of F-at the oxygen site of LNCA can improve the oxygen ion conductivity,promote the surface exchange and volume diffusion of oxygen in LNCAF_0.1 and LNCAF_0.2,and thus increase the catalytic activity of ORR at 450-550°C.At 550°C,the maximum power density of LNCAF_0.1 and LNCAF_0.2 cathode fuel cell is 497 and 591 m W cm^-2,respectively,which is higher than that of LNCA cathode fuel cell.In addition,the single cell of LNCAF_0.2based cathode with the best performance showed good stability for 110 h at 550°C.The results show that LNCAF_0.2 doped with F is the most promising candidate among the three cathode materials.Our study provides a feasible F anion doping strategy for the development of IT-SOFCs cathode catalysts.Then,LSCF and F-doped LSCFF_X（X=1,2）were prepared by the sol-gel method,and combined with YSZ as electrolyte.The electrochemical mechanism and properties of SOFCs with LSCFF_X/YSZ as electrolyte and LNCA as symmetrical electrode were studied.Compared with undoped composite electrolyte（LSCF/YSZ）cells,Ni-LNCA/LSCFF_X:YSZ/LNCA-Ni cells showed higher electrochemical performance.At 550°C,the maximum power densities of the cell with LSCFF_0.1/YSZ and LSCFF_0.2/YSZ composite electrolyte are583 and 661 m W cm^-2,respectively.The results show that F^-doping can promote the electrochemical performance of the cell.The rectifier curve proves that there is a Schottky junction in metal-semiconductor contact between the electrolyte layer of LSCFF_X and the anode layer of Ni-LNCA.The barrier of Ni/LSCFF_0.2 is 2.4 e V greater than the 2.13 e V of Ni/LSCF,which means that electrons need more energy to overcome the barrier,namely the electron is more difficult for passing through the Ni/LSCFF_0.2 interface.