Fabrications and electrochemical properties of superlattice（Ce0.8SmO2-)/YSZ)N electrolyte films

Abstract

The growing demand for the energy conversion and storage of miniaturized system has promoted extensive researches aiming at fabricating solid-state ionic devices in thin-film form. Recent developments in the field of thin-film growth technologies have controlled the films at an atomic level of deposited layers, thus opening new perspectives in the field of engineering of multilayers and heterostructures based on complex oxides. This work focuses on the characterizations of the low-temperature properties of Ce0.8Sm0.2O2-/Y2O3:ZrO2(SDC/YSZ)N superlattice films.(SDC/YSZ)N superlattice electrolytic films with various periods(N=4, 6, 10 and 20) are fabricated on monocrystal MgO substrates by the pulsed laser sputtering method. Here, SiTrO3(STO) is used as a buffer layer, SDC and YSZ are deposited alternately in the whole process. The total thickness values of samples are all fixed at 400 nm no matter how many periods the samples have. The surface morphologies, phase structures and electric properties of the as-deposited samples are characterized by scanning electron microscopy(SEM), X-ray diffraction and alternating current(AC) impedance spectroscopy. It is indicated that the films have excellent superlattice structures after STO has been used as a buffer layer and the substrate temperature has heated to 700℃. The interface between two layers are clearly observed by SEM. Moreover, neither cracks nor snaps are found at the interface. The grains uniformly grow on the surfaces of films and are arranged into cylinder structures, leading to compact films. Through AC impedance analysis, the samples which have more periods exhibit smaller activation energies. With increasing the number of interfaces, the activation energy of film decreases whereas the ionic conductivity increases. When the number of periods reaches 20, the activation energy is measured to be approximately 0.768 eV. The conductivity enhancement of(SDC/YSZ)N superlattice electrolyte film can be attributed to the large lattice mismatch near the interface between two different layers. That is to say, the interface between the highly dissimilar structures stabilizes a disordered oxygen sublattice with an increased number of oxygen vacancies, which promotes oxygen diffusion to increase the ionic conductivity of sample. Furthermore, the ionic conductivity of the(SDC/YSZ)20 film with a thickness ratio of m SDC: YSZ of 2:1 is much higher than that of the film witha thickness ratio of 1:1. Finally, it is noted that the STO buffer layer provides the proper lattice match for CeO2, inducing the good epitxial growth of superlattice electrolyte film(SDC/YSZ)20. And the conductivity enhancement could be attributed to the increase of SDC thickness in a bilayer. Therefore,(SDC/YSZ)20 superlattice electrolyte film is more ideal low-temperature fuel cell electrolyte material due to higher ionic conductivity.