Preparation Of Cathode Film And Its Optimization Of Electrochemical Performance For Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)is an electrochemical energy conversion device that can directly convert chemical energy stored in fuel and oxidizer into electrical energy in an efficient and environmentally friendly manner.It has received widespread attention due to its high energy conversion and wide fuel adaptability.However,a series of problems such as high cost,poor cycle performance and poor compatibility of adjacent components from high operating temperature(>850°C)hampers widespread commercialization of SOFC technology.Therefor,it is necessary to reduce the operating temperature to the medium temperature region(600-800°C).But reducing temperature substantially retards the oxygen reduction reaction(ORR)kinetics at the cathode.Because the low temperature will seriously affect the catalytic activity of the electrode material,the activation energy of the oxygen reduction reaction will increase,the electrode reaction kinetics will become slow,the oxygen ion transportation will be difficult,and the oxygen ion conductivity will drop rapidly.Therefore,it is very important to develop electrode materials with high catalytic activity at medium and low temperatures.In this paper,the perovskite-like A₂BO₄(Referred to as RP configuration)electrode material was mainly studied.The La₂CuO₄(LCO)cathode films were grown on YSZ(100)substrates to study its electrochemical performance by using pulsed laser deposition.By controlling the deposition conditions during the deposition process and the partial substitution of Ba and Sr ions on La site,so as to control the crystal structure of La₂CuO₄film,and to explore the influence of different crystalline structures on the catalytic activity of the oxygen reduction reaction(ORR).The effects of crystal structure of La₂CuO₄ on electrochemical properties were investigated by controlling the deposition conditions of thin films.A series of La₂CuO₄films were deposited on YSZ(100)single crystal substrate by pulsed laser deposition equipment,and La₂CuO₄ films with different crystal structures were prepared by adjusting the oxygen pressure during deposition.The study showns that the crystal structure of the thin film changed from T(?)phase?T*phase?T phase with changing oxygen pressure.T(?)phase is the single crystal with tetragonal symmetry which preferred growing along the c axis and showed a larger polarization resistance.The value of R_p is2.351?cm² at 850°C.T*is a mixed phase whose resistance value at 850°C is between T(?)phase and T phase.T phase is orthorhombic which shown lower polarization resistance compared with the other phase structure,and the R_p value of T phase is 0.783?cm² when the oxygen pressure was 100 m Torr and the temperature of 850°C,which is about 67%lower than the Rp value of T(?)phase.Besides,the orthorhombic phase surface has relatively higher oxygen vacancy concentration,which is conducive to the absorption and diffusion of oxygen and enhances the oxygen reduction reaction.Therefore,the La₂CuO₄with orthorhombic structure show excellent electrochemical property than other symmetries.The results also show that the polarization resistance can be reduced by changing the crystal structure of the material,and the electrochemical performance of the cathode film can be improved.Highly textured,c-axis oriented thin films of undoped,Ba-doped,and Sr-doped La₂CuO₄ are successfully prepared on YSZ(100)substrates by using pulsed laser deposition.Intriguingly,the films undergo a structural transformation from T(?)(square-planar)to T(octahedral)crystal structure with partial substitution of Ba and Sr ions on La site.Meanwhile,the addition of both Ba and Sr dopants lead to reducing in polarization resistance and alternative kinetics of oxygen reduction reaction(ORR),hereinto La_1.8Ba_0.2CuO₄ film presents superior electrochemical properties because of it can accommodate much more oxygen vacancies than the other two films.First-principles calculations reveal that much lower O-defect formation energy result fundamentally from the increase of Cu-O bond length along c-axis orientation after structural transformation from T'-phase to T-phase.These findings unveil the relationship between the structural transformation and ORR activity,and provide a novel approach to rational design film cathodes for higher electrochemical performance.