Study Of Strontium-free Perovskite-based Composite Cathode For Proton-conducting Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)has attracted widespread attention in the research and industrial communities due to their excellent chemical-electric conversion efficiency and flexible fuel adaptability.However,the high operating temperature of SOFC based on traditional yttrium oxide stabilized zirconia(YSZ)electrolyte hinders their commercialization due to high costs and cell component compatibility challenges.Reducing the operating temperature to 600-800?(medium temperature,IT)even below600?(low temperature,LT)while maintaining sufficient power output is a key step in the commercialization of SOFC technology.Due to the low temperature dependence of proton conduction,proton-conducting solid oxide fuel cell(H-SOFC)using proton conductors as electrolytes has broad prospects for applications at intermediate-to-low temperatures.However,the lack of suitable cathode materials has hindered the practical application of H-SOFC technology.The research focuses on the development of high-performance and high-stability H-SOFC cathode.To address the problems of Sr segregation that are common in perovskite materials containing strontium(Sr),calcium(Ca)is used to replace Sr to develop two new Sr-free perovskite materials:simple perovskite La_0.5(Ba_0.75Ca_0.25)_0.5Co_0.8Fe_0.2O_3-?anddoubleperovskite Nd(Ba_0.75Ca_0.25)Co_1.5Fe_0.4Ni_0.1O_5+?(NBCCFN).The two materials were systematically characterized and tested,and the experimental results were analyzed in depth.The research results are as follows:(1)A simple perovskite LBCCF was synthesized.LBCCF exhibited a cubic structure with a lattice parameter of a=3.874(5)(?)and excellent chemical compatibility with Ba Zr_0.1Ce_0.7Y_0.1Yb_0.1O_3-?(BZCYYb).The thermal expansion coefficient was decreased by doping with Ca,but it still has a large thermal expansion mismatch with BZCYYb.Therefore,the composite cathode LBCCF-BZCYYb is used in subsequent cell tests.XPS result indicates LBCCF has a high proportion of surface oxygen vacancies,which will bring excellent catalytic activity.Meanwhile,the electrical conductivity was relatively high at the H-SOFC operating temperature due to the increase in the hole concentration caused by the substitution of Sr with Ca.The electrochemical test was performed on a single cell developed with the LBCCF-BZCYYb composite cathode.The cell achieved high peak power density(1102 m W cm^-2 at 750°C),and the performance did not significantly deteriorate after the 100 h constant current test.The low-frequency polarization resistance was higher than the high-frequency polarization resistance within the test temperature range by analyzing the electrochemical impedance spectroscopy data,demonstrating that the adsorption,dissociation,and diffusion of oxygen are the rate-limiting steps in the ORR process.(2)Layered double perovskite NBCCFN was prepared.NBCCFN has a tetragonal crystal structure with lattice parameters a=b=3.889(9)(?)and c=7.629(1)(?),and NBCCFN has excellent chemical stability with proton conductor BZCYYb and oxygen ion conductor Gd_0.1Ce_0.9O_2-?(GDC).XPS and TGA analysis show that NBCCFN has sufficient oxygen vacancies,which helps it maintain high catalytic activity.The conductivity of NBCCFN is higher than 300 S cm^-1 at the test temperature,which meets the conductivity requirements of cathode applications(?100 S cm^-1).Compared with Nd Ba Co₂O_5+?(NBC)matrix,NBCCFN successfully reduced its thermal expansion coefficient through co-doping strategy.To overcome the thermal mismatch problem,two composite cathodes(NBCCFN-BZCYYb and NBCCFN-GDC)were assembled separately.The single cell with NBCCFN-GDC cathode exhibits lower the polarization resistances and higher the peak power densities than that of the NBCCFN-BZCYYb cathode.Distribution of relaxation time analysis reveals that the the oxygen reduction and diffusion processes are the rate-limiting steps of the NBCCFN-BZCYYb cathode reaction.Under the synergistic effect of NBCCFN and GDC,the oxygen reduction(O^-?O^2-)and oxygen ion migration capabilities are enhanced,the NBCCFN-GDC cathode showed better performance.So even in H-SOFC,enhancing the oxygen ion diffusion ability is an effective way to reduce the obvious oxygen polarization.