PrFe_0.9W_0.1O₃-Based Electrode Materials For Symmetrric Solid Oxide Fuel Cells

As a solid-state energy conversion device with high energy conversion efficiency and broad fuel adaptability,the solid oxide fuel cell（SOFC）will play an important role in achieving the"double carbon"goal.The development of symmetric solid oxide fuel cell（SSOFC）provides a realistic opportunity to achieve this goal.Therefore,it is necessary to develop SSOFC electrode materials that meet the high catalytic activity under oxygen electrode conditions and withstand hydrogen reduction to maintain the stability of the phase structure under fuel electrode conditions.Compared with other SSOFC electrode materials,rare-earth ferrite perovskite electrode materials have good oxygen reduction activity and reasonable catalytic performance for fuels.However,their phase structure stability in reducing atmosphere and low catalytic activity when carbon-based fuels are used limit their application.The tungsten（W）has a stable valence state（+6）and a matching ionic radius（0.60?）to the Fe ion.Therefore,the stability of perovskite electrode materials in reducing atmospheres is taken as a starting point in this work,the study systematically examines the W-doped PrFeO₃-based SSOFC electrode materials and also investigates the effects of doping the A-and B-site elements and non-stoichiometry of the A-site on their stability and electrode performance,as well as the mechanisms of hydrogen oxidation reaction（HOR）and oxygen reduction reaction（ORR）in the electrode.First,the electrode material Pr_0.6Sr_0.4Fe_0.9W_0.1O_3-?（PSFW）was prepared by doping with W at the B-site of Pr_0.6Sr_0.4FeO_3-?（PSF）.W-doping greatly improves the phase structure stability of perovskite materials under wet H₂ conditions.The electrical conductivity of PSFW in air and 5%H₂-Ar（3 vol%H₂O）is 28.7 and 0.21 S cm^-1,respectively.The average thermal expansion coefficient（TEC）is 12.44×10^-6 K^-1,which is the same as the thermal expansion of the oxygen ion conducting electrolyte La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-?(LSGM,TEC=12.11×10^-6 K^-1).The mechanism of W-doping enhancing the structural stability of the PSFW phase was elucidated using the average metal-oxygen binding energy（ABE）and density functional theory（DFT）.The polarization impedance（R_p）of the SSOFC single cell with PSFW as electrode and LSGM as electrolyte in oxidation（air）and reduction（wet H₂,3 vol%H₂O）atmospheres at 800 ^oC is 0.086 and 0.215Ωcm²,respectively.By using the distribution of relaxation times（DRT）method to analyze the electrochemical impedance spectroscopy（EIS）data,it was found that the rate-limiting steps for HOR and ORR are the hydrogen adsorption-dissociation process on the porous hydrogen electrode surface and the oxygen adsorption-dissociation process,respectively.The symmetric PSFW cell shows suitable electrocatalytic performance and redox cycling stability in SOFC,SOEC（electrolytic H₂O）and RSOC modes.Based on the stable perovskite phase structure of W-doped at B-site,the physical and chemical properties of A-doped Pr_0.5A_0.5Fe_0.9W_0.1O_3-?（PAFW）electrode material doped with divalent alkaline earth metals（Ca,Sr and Ba）were investigated.The doping of different alkaline earth metal elements at the A-site can lead to different crystal structures of perovskite oxides,thereby affecting their electrochemical catalytic performance.The electrode materials doped with Ca and Sr both exhibit orthorhombic perovskite structure.Although their catalytic activities are different,their catalytic kinetics are consistent.The rate-limiting steps for HOR and ORR are the hydrogen adsorption-dissociation process on the porous hydrogen electrode surface and the oxygen adsorption-dissociation process,respectively.The Ba-doped electrode material exhibits a tetragonal double perovskite structure at room temperature.The rate-limiting step in the HOR mechanism is the charge transfer process rather than the adsorption-dissociation process of hydrogen.The rate-limiting step in ORR is the adsorption-dissociation process of oxygen.To promote Fe dissolution,a(Pr_0.6Sr_0.4)_0.95Fe_0.9W_0.1O_3-?（PSFW95）electrode material was developed and tested as a proton conducting SSOFC electrode material.The electrode material treated with 700°C wet H₂（3 vol%H₂O）for 5 h shows dissolution of Fe metal nanoparticles while maintaining the perovskite structure.The dissolved nanoparticles can enhance the electrocatalytic activity of the electrode material in a reducing atmosphere.PSFW95 exhibits good chemical compatibility and thermal expansion with the proton conducting electrolyte material Ba Zr_0.1Ce_0.7Y_0.2O_3-?（BZCY）.The R_p value of SSOFC with PSFW95 as electrode and BZCY as electrolyte in humid air and H₂ at 700°C is 0.259 and 0.667?cm²,respectively.DRT-EIS studies show that the rate-limiting steps of ORR and HOR are the process of extracting electrons from the oxygen lattice and the hydrogen diffusion process at the porous fuel electrode.The maximum power density of a symmetric cell at 700°C is 182.9 m W cm^-2,with an R_p of 0.307?cm²;after 60 hours of continuous current discharge at a current density of 120 m A cm^-2 at 650°C,the single cell still exhibits a stable output voltage.By doping the transition metals Cu,Ni and Co at the B-site of PSFW to further improve the electrode performance,Pr_0.6Sr_0.4Fe_0.8B_0.1W_0.1O_3-?（PSFBW,B=Cu,Ni and Co,PSFCu W,PSFNi W and PSFCo W）electrode materials were prepared by the sol-gel method.Although PSFCu W retains an orthorhombic perovskite structure under reducing atmosphere,Cu doping inhibits the electrocatalytic activity under reducing atmosphere.Ni and Co doped electrode materials show dissolution of alloy nanoparticles under reducing atmosphere,which improves the electrocatalytic performance of the electrode.Although the Ni and Co doped electrode materials show partial decomposition of phase structure under reducing conditions,they can still restore the original perovskite structure after high temperature oxidation treatment.It means that two electrode materials exhibit good stability in the oxidation-reduction cycle.DRT-EIS investigations showed that the electrode materials doped with Ni and Co have the same rate-limiting steps.The electrode material doped with Ni exhibits the lowest polarization resistance and the highest electrical power density.