Enhanced Oxygen Ion Conductivity in Composite Film

COMMUNICATION Composite Film Electrolytes

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Enhanced Oxygen Ion Conductivity in Composite Film Electrolytes with Aligned Nanowires Mengfei Zhang, Tianjun Li, Zheng Li, Yan Xing, Xiaohui Zhao, Muhammad Shahid, Yajie Yuan, Hiroki Nishijima, and Wei Pan* Currently, with the rapid development of thin film fabrication technology, strain engineering in composite electrolytes have attracted considerable attention as a means to enhance oxygen ion conductivity in intermediate temperature range.[4] Strain can alter the conductivity through changes in the hopping sites and frequencies due to variations of the enthalpies of oxygen-vacancy migration and association.[5] To investigate the effects of strain in composite electrolytes, epitaxial thin films are typically used where the strain is induced by lattice mismatch at the interface.[3b,6] Even though some findings are debated, it is clear that strain constitutes a new avenue for tuning ion conduction in oxides.[5b,7] Recently, composite electrolytes with aligned ceramic nanowires have emerged as a very simple and promising type of nanostructure systems for enhancing the proton or lithium ion conductivity.[8] The nanowires with high aspect ratio embedded in electrolyte created position charged oxygen vacancies on the surfaces and then release more lithium ion, or reduced the junction cross significantly. In this work, nanowires were used as strain-inducing material, composited with nanometer-thick dense film. As a result, it introduced tensile strain in the film and then enhanced the electric conductivity. Herein, we fabricated composite samarium neodymium codoped ceria (Ce0.8Sm0.1Nd0.1O1.9; SNDC) thin film electrolytes with aligned YSZ nanowires to increase the oxygen conductivity by creating tensile strain (Figure 1a). A significant advantage of using nanowires as the strain-inducing material is that the stress can be varied by controlling the diameter of nanowires. To the best of our knowledge, it is the first time to improve the oxygen-ion conductivity and maintain the longterm chemical stability by tuning the strain in composite film electrolytes with nanowires. First, we prepared YSZ nanowires of different diame ter through electrospinning and followed by calcination at 800 °C. The surface morphology of YSZ nanowires with different diameter are demonstrated in Figure S1a,b, of the Supporting Information demonstrating long continuous structures and smooth surfaces of the wires. As shown in Figure S1c,d, of the Supporting Information the average diameters of YSZ nanowires are about 93 and 188 nm, abbreviated as YSZ(Ø90 nm) and YSZ(Ø180 nm), respectively. Figure S2a of the Supporting Information shows X-ray diffraction (XRD) patterns of the YSZ nanowires calcined at 800 °C. The main diffraction peaks are indexed to cubic phases (JCPDS card No.

Recently, tuning oxygen ionic transport in film through tensile stress has generated much interest. Herein, composite samarium neodymium codoped ceria (SNDC) thin film electrolytes are fabricated with aligned yttria-stabilized zirconia (YSZ) nanowires to increase the oxygen conductivity by introducing tensile strain. The morphologies and oxygen-ion conductivities are studied according to the diameter of YSZ nanowires in the composite electrolytes. The composite SNDC film electrolytes with YSZ nanowires exhibit great longterm chemical stability with excellent conductivity at intermediate temperature, which is more than twice times higher than that of SNDC film. By using Raman spectrum and sin2 ψ-methodology, it is found that this conductivity enhancement originates from the bending surface of the film on the nano wires that can produce tensile strain paralleled to the film surface. Over the years, lowering the operation temperature of solid oxide fuel cells (SOFCs) to an intermediate temperature range (500–700 °C) has been an important issue, as it helps to reduce fabrication cost, elevate stability, and increase durability, resulting in the significant challenge of using a material with high ionic conductivity as an electrolyte.[1] The state-of-the-art yttria-stabilized zirconia (YSZ) is considered to be one of the most reliable electrolyte materials in terms of structural and thermodynamic stability. However, the low ionic conductivities of YSZ at intermediate temperature restrict its application in SOFCs. In comparison with YSZ, a ceria-based solid electrolyte with higher electric conductivity has been proposed as a more promising candidate.[2] In addition, several approaches have been developed to improve the ionic conductivity of ceria-based electrolytes by the use of dopants, fabricating composite electrolytes or introducing strain.[3] M. F. Zhang, T. J. Li, Z. Li, Y. Xing, X. H. Zhao, M. Shahid, Y. J. Yuan Prof. W. Pan State Key Laboratory of New Ceramics and Fine Processing School of Materials Science and Engineering Tsinghua University Beijing 100084, P. R. China E-mail: [email protected] H. Nishijima Functional Material Department Inorganic Material Engineering Division Toyota Motor Corporation Material Development Division Toyota Motor Corporation Toyota, Aichi 471-8572, Japan The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201800098.

DOI: 10.1002/admi.201800098

Adv. Mater. Interfaces 2018, 1800098

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(Figure S2c,d, Supporting Information) and the results confirmed our expectation. Figure S4 of the Supporting Information shows the XRD patterns of SNDC film, YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/ SNDC composite electrolytes. The measured results were in good agreement with the SDC patterns (JCPDS card No. 75–0158) concerning both peak intensity and position. Moreover, the composite electrolytes showed nontextured structure and we could not observe any peaks belonging to YSZ nano wires. The reason is not only relatively low concentration of YSZ nanowires (the concentration number of YSZ nanowires is about 400 mm−1) but also the influence of SNDC film deposited on nanowire. As the nanowires are located underneath the film, it is hard to be detected by XRD. Figure 1b,c shows the surface morphologies of the composite film electrolytes with aligned nanowires (YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/ SNDC) after post-annealing at 600 °C. The SNDC film was continuous and dense throughout the whole region, even for those joint parts of nanowire and film, as shown in Figure 1b,d and Figure S5 (Supporting Information). Particularly worth mentioning is that the film on the top of the nanowires formed a continuous bending structure with nanowires embedded between the substrate and the film, which might thus generate strain between at those bending parts, resulting in the change in conductivity of film. The percentage of bending parts in composite electrolytes is about 8% (400*200/1000000 = 8%) and the tensile strain would be induced at these parts. Figure 1e illustrates the schematic diagram of composite structure consisted with film Figure 1. a) Schematic of the fabrication of composite film electrolytes with aligned nanowires. and embedded nanowires. b,c) The surface morphology images of the YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/SNDC To further verifying the strain state of composite electrolytes. The inset in (b) shows the continuous bending structure. d) Cross-sec- composite structures, laser confocal microtional structure of YSZ(Ø180 nm)/SNDC. e) The schematic diagram of composite electrolytes. Raman spectroscopy was used to analyze the stress conditions of SNDC film at different parts of composite electrolytes. Most commonly, compressive 70–4431), and there is no other phase. We calculated the strain leads to an increase in wavenumber, while the opposite average crystallite sizes of the YSZ(Ø90 nm) and YSZ(Ø180 nm) is observed for tensile strain.[10] We tested the Raman spectrosnanowires using the Scherrer equation from XRD peaks (Figure S2b, Supporting Information) and further verified copy at different positions of the composite electrolytes, one these crystallite sizes by direct transmission electron microwas at the region without nanowires and another was located scope (TEM) observation (Figure S3, Supporting Informaprecisely on the top of a nanowire, as is shown in Figure 2a. A tion). It could be seen that the crystalline size remained shift of 2.0 cm−1 on the peak position of F2g mode is observed almost constant at ≈16.08 nm in the two specimens at plat region and bending region (Figure 2b,c). The SNDC film (Table S1, Supporting Information). According to the prewith plat morphology could be considered as strain-free after vious reports,[9] the conductivity of the same nanomaterial annealed, which served as a reference for the estimation of strain effects (Figure 2a). Therefore, the negative shift in waveis mainly dependent on the crystallite size, and we believed number could be ascribed to the appearance of tensile strain. that the two specimens we obtained were with similar conHowever, it is too hard to compare the strain conditions using ducting ability. We measured the conductivity of YSZ(Ø90 nm) Raman spectroscopy only due to the spatial resolution limit, and YSZ(Ø180 nm) nanowires at different temperatures


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Figure 2. a) The different area of Raman detection. b,c) Raman spectra and normalization Raman spectra of SNDC film and YSZ(Ø180 nm)/SNDC composite electrolytes. d–f) The XRD spectra, g–i) Sin2 ψ-d graph calculated for the (220) planes, and j–l) the schematic diagram of strain state for SNDC film, YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/SNDC composite electrolytes.

which is larger than the diameter of nanowires used in present research. The shift of the XRD peaks also indicates the tensile strain in the composite electrolytes (Figure S6, Supporting Information). To further analyze the strain conditions within the SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/SNDC electrolytes, XRD with the mode of ψ-scan is displayed in Figure 2d–f. Ψ is the tilting angle of the specified diffracting plane in the crystallite from the normal to the film surface. With the change of tilting angle ψ, the diffraction peak of SNDC (2 2 0) plane changed accordingly. The peak position shifted toward the lower angle with the increase of ψ, indicating that the increase of


interplanar spacing d, which further illustrated the existence of tensile strain. This result was in accordance with the observation of Raman spectrum. When a film is sustaining a stress, the interplanar spacing d of the selected diffracting lattice plane (h k l) in the film can be expressed as follows[11] d=

1+ v v d0σ sin 2 ψ + 1 − (σ 1 + σ 2 ) d0 (1) E  E 

where v and E are, respectively, the Poisson ratio and Young’s modulus of the specimen, d0 is the interplanar spacing under a stress-free state, σ is the stress in the grain,

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and σ1 and σ2 are the principal stresses paralleled to the film surface.[12] Equation (1) indicates that the interplanar spacing d is directly proportional to sin2 ψ; therefore, based on the slope of the plot of the straight line d versus sin2 ψ, the stress σ in the film can be determined by

σ=

∂d E (2) (1 + v )d0 ∂sin 2 ψ

The dependence of the lattice spacing d of the (2 2 0) plane on sin2 ψ for the SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/SNDC are shown in Figure 2g–i. As the Poisson ratio and Young’s modulus are the same for the SNDC films in present research, we can compare the strain condition based on the change of slope with linear fitting for convenience. All the curves showed positive reliance with the change of sin2 ψ, indicating the existence of tensile strain in all specimens. Additionally, the residual stress in SNDC film after post-annealing was much smaller than that in composite film electrolytes according to the value of slope which is in good agreement with our assumption. The tensile strain of composite electrolytes increased with increase of the diameter of YSZ nanowires. Figure 2j–l shows the schematic diagrams of stress state for SNDC film, YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/SNDC composite film electrolytes. The SNDC films were deposited on

substrates covered with nanowires, forming the bending region where the nanowires were embedded, and thus induced tensile stress on the films which was determined by the radius of curvature. Larger diameter of these embedded nanowires would result in higher degree of bending effect of films and therefore an observably increased tensile strain was found in the specimen. The in-plane electrical properties of the composite film electrolytes were measured using electrochemical impedance spectroscopy, where Figure S7a of the Supporting Information shows the schematic diagrams of test condition. Typical impedance spectra of SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/ SNDC electrolytes at 480 °C are shown in Figure S7b of the Supporting Information, where all the curves consisted of a semicircular arc at higher frequencies and a tail at lower frequencies. Two R-CPE elements connected in a series were used as an equivalent circuit to fit the spectra (inset of Figure S7b, Supporting Information). Figure 3a shows the Arrhenius plots of the conductivity for SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/SNDC electrolytes. Obviously, it could be found that the conductivity of composite film electrolytes with aligned nanowires were greater than that of SNDC film. The composite film electrolytes with larger diameter of nanowires exhibited superior conductivity than the smaller one, which showed the similar trend as the variation of strain state. As was reported by Fluri, the introduction of tensile strain in electrolyte materials would enlarge the migration channel and

Figure 3. a) Arrhenius plots of conductivity, b) activation energy of SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/SNDC composite electrolytes, and c,d) the first time and the second time of electrical stability of SNDC, YSZ(Ø90 nm)/SNDC, and YSZ(Ø180 nm)/SNDC composite electrolytes measured at 600 °C in air.


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thus improve the conduction ability of material.[6a] Here we ascribe the change of conductivity to the alteration of strain state. In this work, the specimen of YSZ(Ø180 nm)/SNDC composite film electrolytes showed the highest conductivity of 0.026 S cm−1 at 600 °C, which is more than 2.5 times higher than that of single SNDC film. Figure 3b shows the change of the activation energies (Ea) of different specimens, where the composite film electrolytes were in the range of 0.86–0.90 eV. The activation energies of composite film electrolytes are lower than that of SNDC film (0.965 eV) and SNDC bulk (0.971 eV),[13] further verifying the effect of strains. To investigate the thermal stability of composite materials, long-term heat treatment at 600 °C in air was carried out for these specimens. As shown in Figure 3c, the conductivity of YSZ(Ø90 nm)/SNDC and YSZ(Ø180 nm)/SNDC composite film electrolytes declined slightly during the first 20 h and then remained indistinguishable after treating for 50 h with the value more than two times higher than that of SNDC film. The decrease at first few hours may result from the release of residual stress between the interface of film and nanowires. In order to verify the assumption, we again treated the specimens at 600 °C after cooling to the room temperature. As is shown in Figure 3d, the electrical conductivity remained unchanged for the entire time region without any decreasing behavior, indicating high stability of composite film electrolytes. In summary, we fabricated composite SNDC film electrolytes with aligned YSZ nanowires to increase the oxygen conductivity by creating tensile strain. First, well-aligned YSZ nanowires were collected on quartz substrate by electrospinning. Then, the composite film electrolytes were simply assembled by depositing SNDC film on the surface. The film on the top of the nanowires formed a continuous bending structure with nanowires embedded between the substrate and the film. The composite film with larger diameter of nanowires was found the existence of higher tensile strain according to the Raman spectrum and sin2 ψ-methodology results, resulting in the greater improvement in conductivity. High conductivity and stability of the composite material as we checked make it applicable for future usage such as intermediate temperature SOFCs and oxygen sensors.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by Toyota Motor Corporation and the National Natural Science Foundation of China (Grant No. 51323001). The authors thank X. Q. Xi for help with ion cutting experiment.

Conflict of Interest The authors declare no conflict of interest.


Keywords composite film electrolytes, nanowires, oxygen conductivity, samarium neodymium codoped ceria, tensile strains Received: January 18, 2018 Revised: March 15, 2018 Published online:

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