Preparation And Investigation On Self-Assembled High Performance Composite Electrodes For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFC）can directly convert chemical energy in fuels to electricity with high efficiency,low emission,and fuel flex ibility.However,the high cost and unsatisfied durability of conventional high temperature SOFCs（800-1000℃）are bottle necks to impede the commercialization of the technique.Decreasing operating temperature to intermediate-range（IT,600-800℃）has been expected to expand material selection,improve the durability of SOFCs and prolong their lifetime.Nevertheless,lower temperature will significantly decrease the efficiency of ionic transportation in electrolyte and electro-catalytic performance of electrodes.Therefore,designing and exploring high performing and stable electrodes（cathodes and anodes）are great challenges in R&D of IT-SOFCs.One of the critical problems of cathode for IT-SOFC is the segregation of A s ite alkaline earth cation at the surface of perovskite at high temperature,which deteriorates the performance and stability of cathodes.Till now,there is no efficient to suppress the segregation and its side effects.For anodes,the conventional Ni-YSZ cermet anodes normally show oxidation-reduction instability and are vulnerable to carbon deposition and sulfur poisoning when us ing carbon-containing fuels.Although composite anodes prepared through in-s itu exsolution by reducing perovskite precursors have been proved to effectively improve the electrocatalytic activity and the oxidation-reduction stability of anodes,the exsolved metal nanoparticles are prone to agglomeration at high temperature and under long-time operation,which will impair the catalytic activity and durability of the anodes.In this work,we designed a two-phase composite oxide which is composed of single perovskite（SP）and a Ruddlesden–Popper（RP）phase with the formula of La_0.5Sr_0.6Fe_0.8Cu_0.15Nb_0.05O_3-δ(LSr_0.6FCN),and successfully prepared it for the first time by self-assembly in one step solid state method.There are several advantages of the composite as cathode and anode precursor,respectively,comparing with the SP control La_0.5Sr_0.5Fe_0.8Cu_0.15Nb_0.05O_3-δ(LSr_0.5FCN):1.The segregation of A site Sr was relatively suppressed while maintaining the excellent cathode performance;2.The agglomeration of the exsolved metal nanoparticles was impeded through formation of nanofibers（with a diameter of 20 nm）3.The performance of the resultant anode was significantly improved(the half-cell impedance of the anode R-LSr_0.6FCN was reduced about 48%comparing with the R-LSr_0.5FCN).The maximum power density（MPD）of the electrolyte supported single cells with LSr0.6FCN as both cathode and anode achieved 1.03W·cm^-2 and 0.85 W·cm^-2 at 850 and 800℃,respectively,which rank the highest level in similar cells previously reported in literature.Furthermore,the current density of the cells remained stable in various fuels（wet hydrogen,wet syngas 48.5%CO+48.5%H₂,50ppm H₂S+H₂,wet methane）during 100 hours test under 0.7V constant voltage,indicating that the anode has excellent stability and tolerance to sulfur poisoning and carbon deposition.Interestingly,the nanofiber metal exsolution evolved to network structure after working for100h in wet hydrogen（3wt%H₂O）,as well as in wet syngas.The results hints that the existence of RP phase in the LSr_0.6FCN play an important role in inhibiting the grain growth and improving surface oxygen exchange dynamics of the material.Subsequently,we changed the stoichiometric ratio of A-site Sr and prepared a group material of LSr_xFCN（0.4≤x≤0.65）.The calculated and experimental results suggest that the RP phase will generate when x>0.5,and the proportion of RP increases with the increase of x.A site excess samples（x>0.5）show better cathode performance and anode stability comparing to A site deficient samples（x<0.5）,which can be attributed to their better ability to suppress Sr segregation in air and regulate metal exsolution under reduction.As a result,the MPD value of the LSr_xFCN based SSOFC,fueled by wet hydrogen（3wt%H₂O）and oxidized by air,firstly rises then declines with increas ing x.The maximum MPD has been achieved at x=0.6,which indicates that the content of RP in LSr_xFCN has the optimum value.