Characterization And Preparation Of Proton SOFC New Layer Oxide Sr₃Fe₂O_7-δ Cathode Materials

Solid oxide fuel cells have some advantages such high energy conversion efficiency,unaffected by Carnot cycle,environmentally friendly,which have been kown as the clean,efficient new energy technologies in the 21st century.Compared to oxygen ion-conducting solid oxide fuel cells（O-SOFC）,water is generated at the cathode of a proton-conducting solid oxide cells（H-SOFC）,of which the theoretical electromotive force is high,and the high-temperature proton conductor electrolyte materials have high conductivity with low activation energy at low temperatures.However,the conventional cathodic polarization rapidly increases as the reduced operating temperature,greatly limiting the electrochemical performance.Meanwhile,they also have poor stability in the steam-containing air environment.Therefore,it is necessary to develope high-performance cathode materials operating at low temperatures.Based on Ruddlesden-Popper（R-P）type Sr₃Fe₂O_7-δlayered oxide material with high oxygen defect concentration,high catalytic activity and stability,we use some methods such as B-site doped,A-site doped and composite cathode to further improve the catalytic activity and stability in Sr₃Fe₂O_7-δfor H-SOFC.DifferentNi-dopedSr₃Fe_2-xNi_xO_7-δ（x=0.5,1）（SFN）,La-doped La_0.25Sr_2.75NiO_7-δ（LSFN）layered oxide cathode material and three different components of LSFN-BZCY composite cathode materials（9:1,8:2,7:3）were prepared and then characterized in the electrochemical performance,respectively.The results show that the interplanar spacing and the oxygen non stoichiometryδincrease,with the increase of Ni content,while the thermal expansion coefficient decreases with the increase of Ni content.The conductivity of SFN（x=1）cathode material in air is higher than that of SFN（x=0.5）cathode,exhibitings typical semiconductor-type conductive behavior.Using SFN as the cathode,the anode supportteds Ni-BZCY/Ni-BZCY/BZCY/SFN cells are assembled and then tested.At700°C,the maximum power density of SFN（x=0.5）and the SFN（x=1）cathodes reach 619.1 mW·cm^-2,766.1 mW·cm^-2,respectively,indicating that the SFN（x=1）cathode shows better electrochemical performance.The results show that the oxygen non-stoichiometriyδreduces with La doping,and there are good chemical compatibility of LSFN-BZCY mixuture below 1000°C.The conductivity of LSFN in air decreases with increasing temperature（223 S/cm at 300°C）,exhibiting typical metal-type conductive behavior.Using LSFN as the cathode material for H-SOFCs,the maximum power density and the polarization resistance 518.1 mW·cm^-2 and 0.208Ω·cm² at 700°C,respectively.Good long-term stability futher investigate LSFN is promising single-phase cathode.The results show that the maximum power density and the open circuit voltage of LSFN-BZCY（7:3）are 426.0 mW·cm^-2 and 1.045 V at650°C,resepectively,while the corresponding values of LSFN-BZCY（9:1）and LSFN-BZCY（8:2）are 290.1 mW·cm^-2 and 0.99 V,363.8 mW·cm^-2 and 1.007 V at the same condition.The results show that the LSFN-BZCY（7:3）composite cathode has the best performance and is the ideal component of H-SOFC.