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Oriented PrBaCo2O5+δ thin films for solid oxide fuel cells
Accepted Manuscript Oriented PrBaCo2O5+δ Thin Films for Solid Oxide Fuel Cells Yang Gao, Dengjie Chen, Chi Chen, Zongping Shao, Francesco Ciucci PII:
S0378-7753(14)02138-7
DOI:
10.1016/j.jpowsour.2014.12.110
Reference:
POWER 20389
To appear in:
Journal of Power Sources
Received Date: 28 July 2014 Revised Date:
20 December 2014
Accepted Date: 22 December 2014
Please cite this article as: Y. Gao, D. Chen, C. Chen, Z. Shao, F. Ciucci, Oriented PrBaCo2O5+δ Thin Films for Solid Oxide Fuel Cells, Journal of Power Sources (2015), doi: 10.1016/j.jpowsour.2014.12.110. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Oriented PrBaCo2O5+δ Thin Films for Solid Oxide Fuel Cells
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Yang Gao,a Dengjie Chen,a Chi Chen,a Zongping Shao c,d and Francesco Ciuccia,b* a
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong, SAR China. Tel: +852 2358 7187; E-mail: [email protected] b
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, SAR China
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c
College of Energy, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
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Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
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d
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ABSTRACT Oriented PrBaCo2O5+δ (PBC) thin films are prepared on yttria-stabilized ZrO2 (YSZ)
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substrates with orientations (001), (110) and (111) via pulsed laser deposition (PLD). Electrochemical impedance spectroscopy (EIS) experiments at various temperatures and oxygen partial pressures reveal a good oxygen reduction reaction (ORR) performance of
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all the thin films. However, the films’ performance has considerable variance. PBC thin film deposited on (111) oriented YSZ had the best performance, followed by (110) and
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(001). Experiments including high resolution X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and computation, namely molecular dynamics (MD), are used to elucidate the substrate-determined orientations, examine the morphology and composition of the thin films, and to explain the variance of the ORR performance. Our results support the anisotropy of the oxygen
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vacancy pathway in the a-b plane and indicate that the substrate orientations can have
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great influence on the thin films properties.
KEYWORDS: solid oxide fuel cells; PrBaCo2O5+δ thin films; pulsed laser deposition;
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oriented layered structure; electrochemical impedance spectroscopy; anisotropic oxygen transport.
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INTRODUCTION With the current environmental crisis facing humanity, the demand for more efficient and
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sustainable energy resources has become urgent. It is suggested that solid oxide fuel cells (SOFCs) may be employed to help overcome this challenge[1, 2]. SOFCs utilize electrochemical processes to efficiently transform chemical energy to electricity, without
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combustion, therefore reducing the environmental impact. Furthermore this technology has a high energy density and low noise compared to heat engines [3, 4]. These
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advantages have drawn worldwide development efforts [5], and the performance of SOFCs has advanced substantially in the past few decades. Nevertheless, to achieve a competitive level, there remain several practical challenges for this technology. One crucial obstacle inhibiting the commercial feasibility of SOFCs is the high operating temperature, which results in operation difficulty and, high manufacturing and
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maintenance costs [6]. Lowering the operation temperature results in decreased performance for the current technology and therefore methods to improve it are required
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[7, 8].
The sluggish ORR kinetics at the cathode is one of the main causes for the SOFCs’ lower
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performance in the intermediate temperature range (500-800 °C) [9, 10]. Therefore, research groups worldwide have focused on developing advanced cathode materials with enhanced kinetics [11, 12]. Mixed ionic and electronic conductors (MIECs) may improve the ORR performance since the active reaction sites can be extended from the triplephase boundary to the entire surface [13-17]. As a type of MIEC, double perovskites have remarkable oxygen exchange capabilities at intermediate temperatures [18-22]. The materials in the LnBaCo2O5+δ (Ln=La, Pr, Nd, Sm, Gd, and Y) series were found to be
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characterized with fast surface oxygen exchange kinetics and high bulk diffusion rates [23, 24]. Among them, PrBaCo2O5+δ (PBC) was suggested to have the highest ORR performance, with a high concentration of oxygen vacancies and a high anisotropic
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oxygen ion mobility [23, 25, 26].
Considering the good ORR properties of PBC, several researchers have employed PBC
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as cathode material for SOFCs [23, 27-30]. Kim et al. deposited PBC/SrTiO3 (STO) thin films and measured the surface exchange rate [27]. Liu et al. made PBC symmetric half
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cells on LaAlO3 (LAO) and measured their electrochemical impedance spectroscopy (EIS) response [28]. Since STO and LAO are not SOFC electrolytes materials, these results cannot be fully extended to SOFCs, as the electrolyte oxygen vacancy cross flow is not considered.
Furthermore,
the
area
specific
resistance
(ASR)
of
a
simple
PBC/electrolyte/PBC symmetric cell for thin films has yet to be measured [27, 28].
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Moreover, although research has shown that PBC has extraordinary surface oxygen exchange kinetics and bulk diffusion rate (e.g. D* = 3.6 × 10−7 cm2 s-1 and k* = 6.9 × 10−5 cm s-1 for PrBaCo2O5+δ at T = 500 °C [26]), its EIS response is not comparable to
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traditional perovskites such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ or even Ba0.95La0.05FeO3−δ [31, 32].
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Recent studies have shown that the oxygen diffusion is anisotropic in PBC [33, 34], where the oxygen anion transport pathways in the Co-O and Pr-O layers, were visualized using neutron scattering [35]. However, it remains unclear if and how these two dimensional oxygen pathways, which can be tuned according to the PBC film orientation, affect the actual ORR behavior. It should be noted that similar work has been performed for ceria [36] and brownmillerites [37], suggesting that the substrate orientation has a
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significant influence not only on thin film growth direction, but also on the catalytic reactivity and surface exchange kinetics.
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In this work, we have successfully grown PBC oriented thin films on YSZ substrates via PLD. Three different terminations ((001), (110) and (111)) of single crystalline YSZ were used as substrates, and the ORR performance was examined after fabrication. The ORR
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performance of the thin films was found to be linked to the substrate orientation. This was further investigated using experimental methods including X-ray diffraction (XRD),
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atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), as well as simulations, i.e., molecular dynamics (MD). Different orientations and varying cobalt valance states were found, and anisotropic oxygen vacancy pathways were confirmed.
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EXPERIMENTAL
Synthesis. PBC powder was synthesized via the sol-gel method [34]. Stoichiometric amounts of Pr(NO3)3, Ba(NO3)2 and Co(NO3)3 were mixed together with ethylene
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diamine tetraacetic acid (EDTA) and citrate (CA), with the ratio of the total metal ions:
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EDTA:CA as 1:1:2. NH3 was added to the solution to maintain a weak acid pH of 6. The solution was heated to 100 °C, being continuously stirred to form a thick gel, and then heated to 250 °C in an oven to obtain a carbonized precursor. The precursor was calcined at 950 °C for 10 h to achieve the powder, and then compressed to a disc and calcined at 1100 °C for 5 h to form the dense solid pellet as the target for PLD. The Sm-doped CeO2 (SDC) target was synthesized in a similar way with the Sm and Ce nitrates, and calcined at 800 °C during the last step.
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Deposition. The oriented thin films were grown by the Neocera JP-788 PLD system and Continuum Electro-Optics SLIII-10 Nd:YAG laser system, with the laser wavelength at 266 nm, fluence of 3.5 J cm-2 and frequency of 1 Hz. The previously synthesized SDC
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and PBC targets were put inside the PLD chamber, and 10×10×0.5 mm single crystalline YSZ films of three different orientations (001), (110) and (111) (MTI Corporation) were used as the substrates. 500 shots of SDC were pre-deposited at 700 °C in 75 mTorr pure
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oxygen. Subsequently, 5000 shots of PBC were deposited on the surface of SDC in a
For
simplicity,
PBC
thin
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similar manner at 170 mTorr. films
are
indicated
as
PBC(001)//YSZ(001),
PBC(110)//YSZ(110) and PBC(110)//YSZ(111). These notations emphasize the orientations of the PBC thin films and YSZ substrates, respectively. It will be explained later that the orientation of the PBC thin films may not be identical to that of the substrate,
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while the SDC interlayer has an identical orientation as YSZ, and therefore will not be stated in the expression.
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Characterization. In order to measure the ORR behavior of the thin films, symmetric cells of PBC/SDC/YSZ/SDC/PBC were prepared by repeating the deposition on both
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sides of the YSZ substrates. Silver was employed for current collection. The EIS experiments were conducted using a VSP BioLogic potentiostat and impedance analyzer in the frequency range between 400 mHz to 100 kHz, every 50 °C from 550 °C to 700 °C, and oxygen partial pressure from 0.1 atm to 1 atm, mixed with nitrogen to reach 1 atm. High resolution XRD (PANalytical Empyrean) was used to characterize the lattice structure and orientations of the thin films with a Cu Kα anode, a graphite
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monochromator and a 3D PIXcel detector. Normal and off-normal XRD were employed to determine the orientation of the crystal lattice with respect to the substrates. Rocking curves (omega scan) and X-ray reflectivity (XRR) were utilized to check the quality and
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thickness of the thin films.
The surface composition and roughness were examined by XPS and AFM, respectively.
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A PHI 5600 (Physical Electronics) XPS machine with monochromatic Al Kα (1486.6 eV) was employed for quantitative analysis. As the identification peaks of the sample may
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overlap, carbon was adopted for correction and the lines of Pr 3d3/2, Ba 4p, and Co 3p were chosen for quantification. NanoScope IIIa/Dimension 3100 Tapping-mode AFM was used to explore the morphology as well as the roughness of the surface. Calculations. MD simulations were carried out using LAMMPS [38], where the ions
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were assumed to be fully charged and interactions between ions were treated as pairwise. Specifically, the ionic pair interaction was simulated via the Buckingham potential model, which took the following analytic form:
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(1)
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The potential parameters were taken from published data, as listed in Table 1 [39, 40]. In the LAMMPS implementation, the simulated system contained a supercell of 8×8×4 PrBaCo2O5.5 cells, containing 2432 atoms. The isobar-isothermal ensemble (NPT) was used with the Nose-Hoover thermostat and the pressure controlled at 1 atm. The integration of equations used the Verlet algorithm with a time step of 1 fs and a time span of 400 ps. The potential cutoff for the short range interaction was set to 1.2 nm and the
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long range Coulomb force was solved by the Ewald summation, for which the force accuracy was set to 10-4. Oxygen diffusion was simulated at temperatures ranging from 873 K to 1573 K with an interval of 100 K. The oxygen diffusivity data was obtained by
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the method described in the previous work [41]. After the simulation, the oxygen transport trajectory was mapped to a 2x2x1 supercell and the oxygen density was
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calculated. The cell structure and oxygen density was visualized with VESTA [42].
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RESULTS AND DISCUSSION
The three layered thin films PBC/SDC/YSZ were first examined using normal XRD (Fig. 1a). The XRD patterns of PBC, SDC, and YSZ in PBC(001)//YSZ(001) showed (00l) peaks only, demonstrating a c-axis orientation and oriented structure. Similarly, in
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PBC(110)//YSZ(110), the XRD patterns of PBC, SDC, and YSZ all revealed a (110) orientation. For the (111) orientated YSZ substrate the deposited thin films may not show a coherent orientation. For PBC thin films a PBC(110)//YSZ(111) was formed, yet SDC
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still showed a coherent (111) orientation as the YSZ substrates. The X-ray reflection (XRR) showed the thickness of the thin films to be approximately 54 nm (Fig. 1c). It is
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important to note that an identical thickness was observed for all thin films. The small full width at half maximum (FWHM) in the rocking curve, approximately 0.85o, suggested the deposited films were of good quality (Fig. 1d) [43-45]. In conclusion, XRD characterizations showed oriented thin films with layered structures were obtained for all samples.
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The EIS data was used to identify the ORR performance [46-48]. The polarization resistance (Rp) was found as the horizontal intercept from the EIS spectroscopy. This was then divided by two to account for the symmetric cell as shown in Fig. 2 and Fig. 3. Since
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the surface area of the electrodes was made to 1 cm × 1 cm, the ASR and Rp are equal, therefore these two terms will not be distinguished in the remainder of the article. As ASR is a direct indicator for the ORR activity, this was compared among the different
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thin films. At conditions of 600 °C and an oxygen pressure (pO2) of 0.21 atm, the ASR for PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) was found to
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be 0.706, 0.575 and 0.302 Ωcm2, respectively (Fig. 2). This trend was observed for other atmospheres and temperatures, indicating that PBC(110)//YSZ(111) had the best ORR performance, followed by PBC(110)//YSZ(110) and PBC(001)//YSZ(001). The Rp and pO2 was fitted using the expression 1/Rp~pO2m. Physically, m indicates the
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oxygen partial pressure dependence at a certain temperature. As represented in Fig. 3a, we obtained the dependence m as 0.22 at 700 °C for PBC(001)//YSZ(001). The activation energy of the ORR at certain atmosphere can be calculated with the equation
the
oxygen
partial
pressure
of
0.21atm,
the
activation
energy
for
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Under
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Rp~exp(Ea/RT) (Fig. 3b), where Ea is the activation energy and R is the ideal gas constant.
PBC(001)//YSZ(001) was 107.6 kJ mol-1 (~1.12 eV). It can be observed from Fig. 3a and Fig. 3b that the activation energy increased slightly as the oxygen pressure increased. In addition, m increased slightly as the temperature increased, and this may be attributed to the larger energy barrier at a lower temperature, which inhibits the charge transfer and prevents oxygen from being involved in the ratedetermining step.
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Using the same analysis (Fig. 3c-3f), we can obtain that the reaction orders m of our samples with different orientations are all around 0.25. This indicates that the charge transfer step + 2 + ∙∙ = × is most likely reaction rate-determining [49, 50]. The
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value of m was within the range of reported experiments (0.15-0.4), and the activation energies of our thin films were also in agreement with other studies (80-130kJ mol-1) [18,
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27, 51, 52].
Currently, the oxygen vacancy channels (OVCs) theory is usually used to explain the
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ASR performance of different oriented thin films. It has been shown theoretically that the OVCs are mostly distributed in the Pr-O layer, which provides a preferential channel for oxygen transfer [37]. Since there is no OVC through the (001) direction, it justifies the large ASR observed for PBC(001)//YSZ(001).
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We further performed MD simulations to elucidate the transport mechanism. MD simulations have been widely used in SOFC research, particularly in the study of perovskite materials [53-55]. The key quantities that can be extracted from the
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simulations include static properties, e.g., the lattice parameters, and dynamic properties, e.g., oxygen diffusion coefficients. For PBC, the oxygen diffusivity is of primary
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importance, since it is closely related to the ORR reactant delivery, and therefore to the overall material performance. Fig. 4a shows the self-diffusivity data for oxygen anions at temperatures ranging from 873 K to 1573 K. By comparison with the existing data for both PBC and GBC [15, 18, 24, 50], we find that reasonable diffusivities are obtained from our simulations. This supports the validity of the potential model. The oxygen transport trajectories were processed to render the oxygen density map, as shown in Fig. 4b. The oxygen density map indicates that the oxygen transport within the material is
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highly anisotropic, suggesting it is limited to the PrO0.5 and CoO2 planes. Similar results were obtained by MD simulation and neutron diffraction experiments [56, 57]. The reason for the anisotropic diffusion may be attributed to the large difference between the
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ionic radii of the two cation species at the A site. This difference results in a higher oxygen vacancies formation energy around the larger cation than the smaller cation [58]. The oxygen vacancies around Ba are therefore unstable, and the fraction of occupied
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oxygen in the Ba-O plane is close to unity.
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The difference between PBC(110)//YSZ(110) and PBC(110)//YSZ(111), on the other hand, is more complicated. The normal XRD patterns indicate that both films have a preferential (110) orientation. A further investigation of the lattice structure was carried out by the off-normal XRD. Considering PBC(001)//YSZ(001) for example, the out-ofplane rotation Х was set as 45o, and YSZ (202), SDC (202), and PBC (204) planes were
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measured. Four peaks were found in the 360o scan due to the lattice symmetry, and a difference of 45o was found between the two patterns of PBC and YSZ (Fig. 1b). This result agrees with the previous study on thin films, where it was found that perovskite
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materials deposited on (001) oriented electrolyte substrates, e.g., SDC and YSZ, result in
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a 45o in-plane rotation [43]. This could also be deduced from the lattice parameter, since a(SDC)≈ √2 a(PBC) (a(SDC) = 5.43 Å, a(PBC) = 3.91 Å). By applying this type of inplane rotation, the lattice mismatch between PBC and SDC will be reduced from 28% to 2%. For the other two orientations, the situation is slightly more complicated. In the case of PBC(110)//YSZ(110), a 15o in-plane rotation was found between PBC and YSZ, and approximately a 45o rotation for PBC(110)//YSZ(111). This type of rotation may also be assigned to the stretching effect caused by the lattice mismatch at the junction between
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SDC and PBC. The original mismatch (without rotation) can be obtained by comparing the side length. This mismatch is 34% between PBC(110) and SDC(110) and 32% between PBC(111) and SDC(111). The relatively high lattice mismatch can be a driving
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force for the abnormal orientation on (111) YSZ and for the in-plane rotation. Upon reorientation and rotation, the calculated mismatch is reduced to ~13% and 15% for PBC(110)//YSZ(110) and PBC(110)//YSZ(111) respectively. The results of the
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calculation are summarized in Table 2. Additional characterizations and mechanism
planes.
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studies are still needed due to the complex interfacial connections between higher index
With the help of AFM, the topography of the films can be revealed along with the roughness information (Fig. 5).
The 3-dimensional images of 5 µm×5 µm×20 nm
indicate that the sample is rather flat with the roughness of 0.48 nm, 3.1 nm and 4.2 nm
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for PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) respectively. We should note that the lattice parameter c is approximately 0.7 nm. It is expected that the ASR scales with the inverse of the surface roughness; since a higher roughness
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indicates more active surface area, and therefore an increased ORR performance, and
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decreased ASR. However, considering that the deposition procedure was identical, a difference in the roughness was not expected. The AFM characterization was repeated with re-deposited thin films, and it was confirmed that PBC(001)//YSZ(001) had a much lower roughness in comparison to the other two films. This difference may be attributed to the substrate used and to the growth mechanism. In fact, oriented crystals tend to exhibit the terminations with a lower plane index when growing in order to lower the surface energy. The oriented crystal surface with a larger plane index tends to have more
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disordered atoms on the top, i.e., clusters and stepped atoms and therefore exhibits higher roughness [59, 60]. In addition, thin films deposited on substrates with different terminations may also have different growth mechanisms which are directly related to the
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surface roughness. The Frank-van der Merwe growth is a layer-by-layer mechanism and guarantees low roughness, while the Volmer-Weber growth tends to yield three dimensional islands with relatively high roughness [61-65]. For PBC(001)//YSZ(001),
in
Frank-van
der
Merwe
growth,
while
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the low-index surface and small lattice mismatch achieved by in-plane rotation may result for
PBC(110)//YSZ(110)
and
index and large lattice mismatch.
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PBC(110)//YSZ(111), Volmer-Weber growth may dominate due to the high surface
The surface composition may also play a significant role since the surface oxygen exchange is the main rate-determining step in ORR [66-68]. The XPS data was analyzed
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to determine the valence states and quantify the amounts of Pr, Ba, Co in the samples, by locating their characteristic peaks (Fig. 6a). As cobalt, Co is a transition metal element, it may exhibit different valence states such as +3 and +4 in double perovskites [69-71]. We
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obtained the actual surface valence state by fitting the Co 3p peak as part of two
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characteristic peaks (Fig. 6b). These data are listed in Table 3, with the nonstoichiometric formula Kim et al. reported for reference [27]. According to Table 3, one of the most notable differences among the thin films is the ratio and valence state of cobalt. As previously explained, the oxygen transport is strongly related to the oxygen vacancy in the Pr-O plane, which is directly determined by the cobalt valance state. We can see a clear trend of cobalt valence decrease and oxygen vacancy
increase
from
PBC(001)//YSZ(001),
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PBC(110)//YSZ(110)
to
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PBC(110)//YSZ(111). This is also consistent with the ORR performance sequence. In addition, it has also been suggested that surface cobalt enrichment may decrease the ORR performance by blocking the oxygen tunnels [66]. Furthermore, it can also be observed
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from the XPS study, that PBC(110)//YSZ(110) and PBC(110)//YSZ(111) have different level of A-site element excess, which may also contribute to their different ORR performance. The different surface composition between PBC(001)//YSZ(001) and the
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other two (110) oriented PBC thin films may be due to the nature of the material (PBC) itself [72, 73]. The difference between PBC(110)//YSZ(110) and PBC(110)//YSZ(111)
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may be determined by the different growth mechanism and surface reconstruction.
Conclusions
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Oriented PBC thin films have been successfully deposited on (001), (110) and (111) oriented SDC/YSZ electrolyte substrates via PLD system. XRD data revealed abnormal orientations and in-plane rotations of those PBC thin films, which was attributed to lattice
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mismatch. The symmetric cells exhibited good performance as shown by the low ASR. performance
differences
among
the
films
were
observed.
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PBC(110)//YSZ(111) had the lowest ASR, followed by PBC(110)//YSZ(110) and then PBC(001)//YSZ(001). Our characterization and analysis suggests that the difference can be mainly attributed to the anisotropic oxygen diffusivity in the a-b plane and the valence of Co. The relatively slow oxygen diffusion in the [001] direction inhibits the activity of PBC(001)//YSZ(001), and the relatively low cobalt valence, on the other hand, provides PBC(110)//YSZ(111) with the best performance. The orientation of the thin film is
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mainly determined by the substrate, while the lattice mismatch between the deposited material and the substrate also has an effect. The latter explains the coincidently same orientation between PBC(110)//YSZ(110) and PBC(110)//YSZ(111). The different
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surface roughness may have resulted from the different surface energy on different terminations and the different growth mechanisms. The surface composition variance may result from the thin films themselves, as expected from the different orientations,
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while the interaction between the thin films and the different growth mechanisms may also have an influence. Even though open questions remain, this work provides a
orientation and surface composition.
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Acknowledgements
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reference for further development of enhanced SOFC cathode materials by tuning the
The authors gratefully acknowledge HKUST for providing start-up funds, and the Research Grants Council of Hong Kong for support through the projects, DAG12EG06
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and ECS 639713.
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[33] M. Burriel, J. Peña-Martínez, R.J. Chater, S. Fearn, A.V. Berenov, S.J. Skinner, J.A. Kilner, Chem. Mater., 24 (2012) 613-621. [34] D. Chen, R. Ran, Z. Shao, J. Power Sources, 195 (2010) 7187-7195. [35] R.A. Cox-Galhotra, A. Huq, J.P. Hodges, J.-H. Kim, C. Yu, X. Wang, A.J. Jacobson, S. McIntosh, J. Mater. Chem. A, 1 (2013) 3091. [36] M. Tsuchiya, N.A. Bojarczuk, S. Guha, S. Ramanathan, J. Chem. Phys., 130 (2009) 4711. [37] H. Jeen, Z. Bi, W.S. Choi, M.F. Chisholm, C.A. Bridges, M.P. Paranthaman, H.N. Lee, Adv. Mater., (2013). [38] S. Plimpton, P. Crozier, A. Thompson, Sandia National Laboratories, (2007). [39] J. Hermet, G. Geneste, G. Dezanneau, Appl. Phys. Lett., 97 (2010) 174102. [40] M.T. Buscaglia, V. Buscaglia, M. Viviani, P. Nanni, J. Am. Ceram. Soc., 84 (2001) 376-384. [41] C. Chen, D. Chen, Y. Gao, Z. Shao, F. Ciucci, J. Mater. Chem. A, (2014) 1157311583. [42] K. Momma, F. Izumi, J. Appl. Crystallogr., 41 (2008) 653-658. [43] J. la O' G, S.J. Ahn, E. Crumlin, Y. Orikasa, M.D. Biegalski, H.M. Christen, Y. Shao-Horn, Angew. Chem. Int. Edit., 49 (2010) 5344-5347. [44] Y.-H. Lee, J.-M. Wu, J. Cryst. Growth, 263 (2004) 436-441. [45] W.C. Chueh, Y. Hao, W. Jung, S.M. Haile, Nat. Mater., 11 (2012) 155-161. [46] F. Ciucci, T. Carraro, W.C. Chueh, W. Lai, Electrochim. Acta, 56 (2011) 5416-5434. [47] N.P. Brandon, S. Skinner, B.C.H. Steele, Annu. Rev. Mater. Res., 33 (2003) 183213. [48] C. Chen, D. Chen, W.C. Chueh, F. Ciucci, Phys. Chem. Chem. Phys., 16 (2014) 11573-11583. [49] Y. Takeda, R. Kanno, M. Noda, Y. Tomida, O. Yamamoto, J. Electrochem. Soc., 134 (1987) 2656-2661. [50] M.J. Escudero, A. Aguadero, J.A. Alonso, L. Daza, J. Electroanal. Chem., 611 (2007) 107-116. [51] C.-Y. Yoo, B.A. Boukamp, H.J.M. Bouwmeester, Solid State Ionics, (2013). [52] D. Chen, R. Ran, K. Zhang, J. Wang, Z. Shao, J. Power Sources, 188 (2009) 96-105. [53] M.-Y. Yoon, K.-J. Hwang, D.-S. Byeon, H.-J. Hwang, S.-M. Jeong, J. Power Sources, 248 (2014) 1085-1089. [54] D.-S. Byeon, S.-M. Jeong, K.-J. Hwang, M.-Y. Yoon, H.-J. Hwang, S. Kim, H.-L. Lee, J. Power Sources, 222 (2013) 282-287. [55] Y. Sun, C. Wang, Y. Chen, J. Power Sources, 233 (2013) 131-138. [56] I. Seymour, A. Tarancon, A. Chroneos, D. Parfitt, J. Kilner, R. Grimes, Solid State Ionics, 216 (2012) 41-43. [57] Y.-C. Chen, M. Yashima, J. Peña-Martínez, J.A. Kilner, Chem. Mater., 25 (2013) 2638-2641. [58] J. Hermet, B. Dupe, G. Dezanneau, Solid State Ionics, 216 (2012) 50-53. [59] D. Mori, H. Oka, Y. Suzuki, N. Sonoyama, A. Yamada, R. Kanno, Y. Sumiya, N. Imanishi, Y. Takeda, Solid State Ionics, 177 (2006) 535-540. [60] J. Ventura, I. Fina, C. Ferrater, E. Langenberg, L.E. Coy, M.C. Polo, M.V. GarcíaCuenca, L. Fàbrega, M. Varela, Thin Solid Films, 518 (2010) 4692-4695. [61] W. Lorenz, G. Staikov, Surf. Sci., 335 (1995) 32-43.
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[62] E. Bauer, J.H. van der Merwe, Physical Review B, 33 (1986) 3657. [63] J.A. Venables, Surf. Sci., 299 (1994) 798-817. [64] M. Copel, M. Reuter, E. Kaxiras, R. Tromp, Phys. Rev. Lett., 63 (1989) 632. [65] J.H. van der Merwe, E. Bauer, Physical Review B, 39 (1989) 3632. [66] H.-I. Ji, J. Hwang, K.J. Yoon, J.-W. Son, B.-K. Kim, H.-W. Lee, J.-H. Lee, Energ. Environ. Sci., 6 (2013) 116-120. [67] W. Jung, H.L. Tuller, Energ. Environ. Sci., 5 (2012) 5370-5378. [68] A.-K. Huber, M. Falk, M. Rohnke, B. Luerßen, L. Gregoratti, M. Amati, J. Janek, Phys. Chem. Chem. Phys., 14 (2012) 751-758. [69] A. Maignan, C. Martin, D. Pelloquin, N. Nguyen, B. Raveau, J. Solid State Chem., 142 (1999) 247-260. [70] A. Yan, V. Maragou, A. Arico, M. Cheng, P. Tsiakaras, Appl. Catal., B-Environ., 76 (2007) 320-327. [71] S.P. Simner, M.D. Anderson, M.H. Engelhard, J.W. Stevenson, Electrochem. SolidState Lett., 9 (2006) A478-A481. [72] L.F. Zagonel, N. Barrett, O. Renault, A. Bailly, M. Bäurer, M. Hoffmann, S.J. Shih, D. Cockayne, Surf. Interface Anal., 40 (2008) 1709-1712. [73] R. Saha, W.D. Nix, Acta Mater., 50 (2002) 23-38.
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Table 1. Potential parameters for the Buckingham potential 6
A /
/ Å
/ Å
Ref.
O2----O2-
22764.3
0.1490
43.00
[39]
Ba2+---O2-
1214.4
0.3522
0
[39]
Co3+---O2-
1329.82
0.3087
0
Pr3+---O2-
1445.2
0.3608
0
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Ion pairs
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[39]
[40]
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Table 2. Lattice mismatch between PBC and SDC in PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films before and after rotation and
PBC(110) //YSZ(110) 34% 13% 15o /
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Mismatch before rotation Mismatch after rotation In-plane rotation Orientation change
PBC(001) //YSZ(001) 28% 2% 45o /
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PBC(110) //YSZ(111) 32% 15% 45o (111) to (110)
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Table 3. Calculated surface compositions of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films by XPS, compared with the data from the literature.
valance state of cobalt rather than the XPS peak of oxygen.
PBC(110) //YSZ(111) 1.00 1.02 1.41 3.23
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PBC(110) //YSZ(110) 1.00 1.09 1.50 3.42
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Pr Ba Co Co valence
PBC(001) //YSZ(001) 1.00 1.03 1.20 3.48
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Composition
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The ratio of Pr is fixed at 1 for comparison, and the ratio of oxygen is deduced from the
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PrBaCo2O5.65 1.00 1.00 2.00 3.15
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FIGURE CAPTIONS Figure 1. Normal XRD patterns of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and
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PBC(110)//YSZ(111) (a), Off-normal XRD patterns of PBC(001)//YSZ(001) (b), X-ray reflection of PBC(001)//YSZ(001) indicating the thickness as 54 nm (c), and rocking curve of PBC(001)//YSZ(001) as 0.85o (d).
PBC(110)//YSZ(111) thin films at 600 °C, 0.21atm.
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Figure 2. Polarization resistances of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and
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Figure 3. EIS data of polarization resistance related to temperature (a) and oxygen pressure (b) of PBC(001)//YSZ(001) thin films. (c)(d) and (e)(f) represent PBC(110)//YSZ(110) and PBC(110)//YSZ(111), respectively.
Figure 4. The left image is the comparison of oxygen self-diffusivities between
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PrBaCo2O6-δ (PBC) and GdBaCo2O6-δ (GBC) (a). The diffusivities are determined from electrical conductivity relaxation (ECR), mass relaxation (MR) and molecular dynamics (MD). The right image is the structural visualization of a 2x2x1 supercell with the red-
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Figure 5. AFM topographic maps of PBC(001)//YSZ(001) (a), PBC(110)//YSZ(110) (b) and PBC(110)//YSZ(111) (c). Figure 6. Overall XPS survey scan spectroscopy (a) and cobalt individual peaks (b) of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films.
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ACCEPTED MANUSCRIPT PrBaCo2O5+δ oriented thin films are fabricated with pulsed laser deposition. Performance of thin films with different oriented substrates is evaluated. Thin film on (111) oriented substrate shows a polarization resistance of 0.302 Ωcm2 at 600 oC, 0.21atm.
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Orientation of the substrates has a strong influence on the thin films performance.
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The difference is attributed to anisotropic oxygen transport and the substrates effect.
S0378-7753(14)02138-7
DOI:
10.1016/j.jpowsour.2014.12.110
Reference:
POWER 20389
To appear in:
Journal of Power Sources
Received Date: 28 July 2014 Revised Date:
20 December 2014
Accepted Date: 22 December 2014
Please cite this article as: Y. Gao, D. Chen, C. Chen, Z. Shao, F. Ciucci, Oriented PrBaCo2O5+δ Thin Films for Solid Oxide Fuel Cells, Journal of Power Sources (2015), doi: 10.1016/j.jpowsour.2014.12.110. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Oriented PrBaCo2O5+δ Thin Films for Solid Oxide Fuel Cells
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Yang Gao,a Dengjie Chen,a Chi Chen,a Zongping Shao c,d and Francesco Ciuccia,b* a
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong, SAR China. Tel: +852 2358 7187; E-mail: [email protected] b
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong, SAR China
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c
College of Energy, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
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Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
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d
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ABSTRACT Oriented PrBaCo2O5+δ (PBC) thin films are prepared on yttria-stabilized ZrO2 (YSZ)
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substrates with orientations (001), (110) and (111) via pulsed laser deposition (PLD). Electrochemical impedance spectroscopy (EIS) experiments at various temperatures and oxygen partial pressures reveal a good oxygen reduction reaction (ORR) performance of
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all the thin films. However, the films’ performance has considerable variance. PBC thin film deposited on (111) oriented YSZ had the best performance, followed by (110) and
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(001). Experiments including high resolution X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and computation, namely molecular dynamics (MD), are used to elucidate the substrate-determined orientations, examine the morphology and composition of the thin films, and to explain the variance of the ORR performance. Our results support the anisotropy of the oxygen
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vacancy pathway in the a-b plane and indicate that the substrate orientations can have
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great influence on the thin films properties.
KEYWORDS: solid oxide fuel cells; PrBaCo2O5+δ thin films; pulsed laser deposition;
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oriented layered structure; electrochemical impedance spectroscopy; anisotropic oxygen transport.
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INTRODUCTION With the current environmental crisis facing humanity, the demand for more efficient and
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sustainable energy resources has become urgent. It is suggested that solid oxide fuel cells (SOFCs) may be employed to help overcome this challenge[1, 2]. SOFCs utilize electrochemical processes to efficiently transform chemical energy to electricity, without
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combustion, therefore reducing the environmental impact. Furthermore this technology has a high energy density and low noise compared to heat engines [3, 4]. These
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advantages have drawn worldwide development efforts [5], and the performance of SOFCs has advanced substantially in the past few decades. Nevertheless, to achieve a competitive level, there remain several practical challenges for this technology. One crucial obstacle inhibiting the commercial feasibility of SOFCs is the high operating temperature, which results in operation difficulty and, high manufacturing and
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maintenance costs [6]. Lowering the operation temperature results in decreased performance for the current technology and therefore methods to improve it are required
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[7, 8].
The sluggish ORR kinetics at the cathode is one of the main causes for the SOFCs’ lower
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performance in the intermediate temperature range (500-800 °C) [9, 10]. Therefore, research groups worldwide have focused on developing advanced cathode materials with enhanced kinetics [11, 12]. Mixed ionic and electronic conductors (MIECs) may improve the ORR performance since the active reaction sites can be extended from the triplephase boundary to the entire surface [13-17]. As a type of MIEC, double perovskites have remarkable oxygen exchange capabilities at intermediate temperatures [18-22]. The materials in the LnBaCo2O5+δ (Ln=La, Pr, Nd, Sm, Gd, and Y) series were found to be
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characterized with fast surface oxygen exchange kinetics and high bulk diffusion rates [23, 24]. Among them, PrBaCo2O5+δ (PBC) was suggested to have the highest ORR performance, with a high concentration of oxygen vacancies and a high anisotropic
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oxygen ion mobility [23, 25, 26].
Considering the good ORR properties of PBC, several researchers have employed PBC
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as cathode material for SOFCs [23, 27-30]. Kim et al. deposited PBC/SrTiO3 (STO) thin films and measured the surface exchange rate [27]. Liu et al. made PBC symmetric half
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cells on LaAlO3 (LAO) and measured their electrochemical impedance spectroscopy (EIS) response [28]. Since STO and LAO are not SOFC electrolytes materials, these results cannot be fully extended to SOFCs, as the electrolyte oxygen vacancy cross flow is not considered.
Furthermore,
the
area
specific
resistance
(ASR)
of
a
simple
PBC/electrolyte/PBC symmetric cell for thin films has yet to be measured [27, 28].
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Moreover, although research has shown that PBC has extraordinary surface oxygen exchange kinetics and bulk diffusion rate (e.g. D* = 3.6 × 10−7 cm2 s-1 and k* = 6.9 × 10−5 cm s-1 for PrBaCo2O5+δ at T = 500 °C [26]), its EIS response is not comparable to
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traditional perovskites such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ or even Ba0.95La0.05FeO3−δ [31, 32].
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Recent studies have shown that the oxygen diffusion is anisotropic in PBC [33, 34], where the oxygen anion transport pathways in the Co-O and Pr-O layers, were visualized using neutron scattering [35]. However, it remains unclear if and how these two dimensional oxygen pathways, which can be tuned according to the PBC film orientation, affect the actual ORR behavior. It should be noted that similar work has been performed for ceria [36] and brownmillerites [37], suggesting that the substrate orientation has a
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significant influence not only on thin film growth direction, but also on the catalytic reactivity and surface exchange kinetics.
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In this work, we have successfully grown PBC oriented thin films on YSZ substrates via PLD. Three different terminations ((001), (110) and (111)) of single crystalline YSZ were used as substrates, and the ORR performance was examined after fabrication. The ORR
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performance of the thin films was found to be linked to the substrate orientation. This was further investigated using experimental methods including X-ray diffraction (XRD),
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atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), as well as simulations, i.e., molecular dynamics (MD). Different orientations and varying cobalt valance states were found, and anisotropic oxygen vacancy pathways were confirmed.
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EXPERIMENTAL
Synthesis. PBC powder was synthesized via the sol-gel method [34]. Stoichiometric amounts of Pr(NO3)3, Ba(NO3)2 and Co(NO3)3 were mixed together with ethylene
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diamine tetraacetic acid (EDTA) and citrate (CA), with the ratio of the total metal ions:
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EDTA:CA as 1:1:2. NH3 was added to the solution to maintain a weak acid pH of 6. The solution was heated to 100 °C, being continuously stirred to form a thick gel, and then heated to 250 °C in an oven to obtain a carbonized precursor. The precursor was calcined at 950 °C for 10 h to achieve the powder, and then compressed to a disc and calcined at 1100 °C for 5 h to form the dense solid pellet as the target for PLD. The Sm-doped CeO2 (SDC) target was synthesized in a similar way with the Sm and Ce nitrates, and calcined at 800 °C during the last step.
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Deposition. The oriented thin films were grown by the Neocera JP-788 PLD system and Continuum Electro-Optics SLIII-10 Nd:YAG laser system, with the laser wavelength at 266 nm, fluence of 3.5 J cm-2 and frequency of 1 Hz. The previously synthesized SDC
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and PBC targets were put inside the PLD chamber, and 10×10×0.5 mm single crystalline YSZ films of three different orientations (001), (110) and (111) (MTI Corporation) were used as the substrates. 500 shots of SDC were pre-deposited at 700 °C in 75 mTorr pure
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oxygen. Subsequently, 5000 shots of PBC were deposited on the surface of SDC in a
For
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PBC
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similar manner at 170 mTorr. films
are
indicated
as
PBC(001)//YSZ(001),
PBC(110)//YSZ(110) and PBC(110)//YSZ(111). These notations emphasize the orientations of the PBC thin films and YSZ substrates, respectively. It will be explained later that the orientation of the PBC thin films may not be identical to that of the substrate,
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while the SDC interlayer has an identical orientation as YSZ, and therefore will not be stated in the expression.
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Characterization. In order to measure the ORR behavior of the thin films, symmetric cells of PBC/SDC/YSZ/SDC/PBC were prepared by repeating the deposition on both
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sides of the YSZ substrates. Silver was employed for current collection. The EIS experiments were conducted using a VSP BioLogic potentiostat and impedance analyzer in the frequency range between 400 mHz to 100 kHz, every 50 °C from 550 °C to 700 °C, and oxygen partial pressure from 0.1 atm to 1 atm, mixed with nitrogen to reach 1 atm. High resolution XRD (PANalytical Empyrean) was used to characterize the lattice structure and orientations of the thin films with a Cu Kα anode, a graphite
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monochromator and a 3D PIXcel detector. Normal and off-normal XRD were employed to determine the orientation of the crystal lattice with respect to the substrates. Rocking curves (omega scan) and X-ray reflectivity (XRR) were utilized to check the quality and
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thickness of the thin films.
The surface composition and roughness were examined by XPS and AFM, respectively.
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A PHI 5600 (Physical Electronics) XPS machine with monochromatic Al Kα (1486.6 eV) was employed for quantitative analysis. As the identification peaks of the sample may
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overlap, carbon was adopted for correction and the lines of Pr 3d3/2, Ba 4p, and Co 3p were chosen for quantification. NanoScope IIIa/Dimension 3100 Tapping-mode AFM was used to explore the morphology as well as the roughness of the surface. Calculations. MD simulations were carried out using LAMMPS [38], where the ions
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were assumed to be fully charged and interactions between ions were treated as pairwise. Specifically, the ionic pair interaction was simulated via the Buckingham potential model, which took the following analytic form:
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(1)
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The potential parameters were taken from published data, as listed in Table 1 [39, 40]. In the LAMMPS implementation, the simulated system contained a supercell of 8×8×4 PrBaCo2O5.5 cells, containing 2432 atoms. The isobar-isothermal ensemble (NPT) was used with the Nose-Hoover thermostat and the pressure controlled at 1 atm. The integration of equations used the Verlet algorithm with a time step of 1 fs and a time span of 400 ps. The potential cutoff for the short range interaction was set to 1.2 nm and the
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long range Coulomb force was solved by the Ewald summation, for which the force accuracy was set to 10-4. Oxygen diffusion was simulated at temperatures ranging from 873 K to 1573 K with an interval of 100 K. The oxygen diffusivity data was obtained by
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the method described in the previous work [41]. After the simulation, the oxygen transport trajectory was mapped to a 2x2x1 supercell and the oxygen density was
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calculated. The cell structure and oxygen density was visualized with VESTA [42].
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RESULTS AND DISCUSSION
The three layered thin films PBC/SDC/YSZ were first examined using normal XRD (Fig. 1a). The XRD patterns of PBC, SDC, and YSZ in PBC(001)//YSZ(001) showed (00l) peaks only, demonstrating a c-axis orientation and oriented structure. Similarly, in
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PBC(110)//YSZ(110), the XRD patterns of PBC, SDC, and YSZ all revealed a (110) orientation. For the (111) orientated YSZ substrate the deposited thin films may not show a coherent orientation. For PBC thin films a PBC(110)//YSZ(111) was formed, yet SDC
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still showed a coherent (111) orientation as the YSZ substrates. The X-ray reflection (XRR) showed the thickness of the thin films to be approximately 54 nm (Fig. 1c). It is
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important to note that an identical thickness was observed for all thin films. The small full width at half maximum (FWHM) in the rocking curve, approximately 0.85o, suggested the deposited films were of good quality (Fig. 1d) [43-45]. In conclusion, XRD characterizations showed oriented thin films with layered structures were obtained for all samples.
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The EIS data was used to identify the ORR performance [46-48]. The polarization resistance (Rp) was found as the horizontal intercept from the EIS spectroscopy. This was then divided by two to account for the symmetric cell as shown in Fig. 2 and Fig. 3. Since
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the surface area of the electrodes was made to 1 cm × 1 cm, the ASR and Rp are equal, therefore these two terms will not be distinguished in the remainder of the article. As ASR is a direct indicator for the ORR activity, this was compared among the different
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thin films. At conditions of 600 °C and an oxygen pressure (pO2) of 0.21 atm, the ASR for PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) was found to
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be 0.706, 0.575 and 0.302 Ωcm2, respectively (Fig. 2). This trend was observed for other atmospheres and temperatures, indicating that PBC(110)//YSZ(111) had the best ORR performance, followed by PBC(110)//YSZ(110) and PBC(001)//YSZ(001). The Rp and pO2 was fitted using the expression 1/Rp~pO2m. Physically, m indicates the
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oxygen partial pressure dependence at a certain temperature. As represented in Fig. 3a, we obtained the dependence m as 0.22 at 700 °C for PBC(001)//YSZ(001). The activation energy of the ORR at certain atmosphere can be calculated with the equation
the
oxygen
partial
pressure
of
0.21atm,
the
activation
energy
for
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Rp~exp(Ea/RT) (Fig. 3b), where Ea is the activation energy and R is the ideal gas constant.
PBC(001)//YSZ(001) was 107.6 kJ mol-1 (~1.12 eV). It can be observed from Fig. 3a and Fig. 3b that the activation energy increased slightly as the oxygen pressure increased. In addition, m increased slightly as the temperature increased, and this may be attributed to the larger energy barrier at a lower temperature, which inhibits the charge transfer and prevents oxygen from being involved in the ratedetermining step.
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Using the same analysis (Fig. 3c-3f), we can obtain that the reaction orders m of our samples with different orientations are all around 0.25. This indicates that the charge transfer step + 2 + ∙∙ = × is most likely reaction rate-determining [49, 50]. The
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Currently, the oxygen vacancy channels (OVCs) theory is usually used to explain the
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ASR performance of different oriented thin films. It has been shown theoretically that the OVCs are mostly distributed in the Pr-O layer, which provides a preferential channel for oxygen transfer [37]. Since there is no OVC through the (001) direction, it justifies the large ASR observed for PBC(001)//YSZ(001).
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We further performed MD simulations to elucidate the transport mechanism. MD simulations have been widely used in SOFC research, particularly in the study of perovskite materials [53-55]. The key quantities that can be extracted from the
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simulations include static properties, e.g., the lattice parameters, and dynamic properties, e.g., oxygen diffusion coefficients. For PBC, the oxygen diffusivity is of primary
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importance, since it is closely related to the ORR reactant delivery, and therefore to the overall material performance. Fig. 4a shows the self-diffusivity data for oxygen anions at temperatures ranging from 873 K to 1573 K. By comparison with the existing data for both PBC and GBC [15, 18, 24, 50], we find that reasonable diffusivities are obtained from our simulations. This supports the validity of the potential model. The oxygen transport trajectories were processed to render the oxygen density map, as shown in Fig. 4b. The oxygen density map indicates that the oxygen transport within the material is
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highly anisotropic, suggesting it is limited to the PrO0.5 and CoO2 planes. Similar results were obtained by MD simulation and neutron diffraction experiments [56, 57]. The reason for the anisotropic diffusion may be attributed to the large difference between the
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ionic radii of the two cation species at the A site. This difference results in a higher oxygen vacancies formation energy around the larger cation than the smaller cation [58]. The oxygen vacancies around Ba are therefore unstable, and the fraction of occupied
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oxygen in the Ba-O plane is close to unity.
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The difference between PBC(110)//YSZ(110) and PBC(110)//YSZ(111), on the other hand, is more complicated. The normal XRD patterns indicate that both films have a preferential (110) orientation. A further investigation of the lattice structure was carried out by the off-normal XRD. Considering PBC(001)//YSZ(001) for example, the out-ofplane rotation Х was set as 45o, and YSZ (202), SDC (202), and PBC (204) planes were
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measured. Four peaks were found in the 360o scan due to the lattice symmetry, and a difference of 45o was found between the two patterns of PBC and YSZ (Fig. 1b). This result agrees with the previous study on thin films, where it was found that perovskite
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materials deposited on (001) oriented electrolyte substrates, e.g., SDC and YSZ, result in
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a 45o in-plane rotation [43]. This could also be deduced from the lattice parameter, since a(SDC)≈ √2 a(PBC) (a(SDC) = 5.43 Å, a(PBC) = 3.91 Å). By applying this type of inplane rotation, the lattice mismatch between PBC and SDC will be reduced from 28% to 2%. For the other two orientations, the situation is slightly more complicated. In the case of PBC(110)//YSZ(110), a 15o in-plane rotation was found between PBC and YSZ, and approximately a 45o rotation for PBC(110)//YSZ(111). This type of rotation may also be assigned to the stretching effect caused by the lattice mismatch at the junction between
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SDC and PBC. The original mismatch (without rotation) can be obtained by comparing the side length. This mismatch is 34% between PBC(110) and SDC(110) and 32% between PBC(111) and SDC(111). The relatively high lattice mismatch can be a driving
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force for the abnormal orientation on (111) YSZ and for the in-plane rotation. Upon reorientation and rotation, the calculated mismatch is reduced to ~13% and 15% for PBC(110)//YSZ(110) and PBC(110)//YSZ(111) respectively. The results of the
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calculation are summarized in Table 2. Additional characterizations and mechanism
planes.
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studies are still needed due to the complex interfacial connections between higher index
With the help of AFM, the topography of the films can be revealed along with the roughness information (Fig. 5).
The 3-dimensional images of 5 µm×5 µm×20 nm
indicate that the sample is rather flat with the roughness of 0.48 nm, 3.1 nm and 4.2 nm
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for PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) respectively. We should note that the lattice parameter c is approximately 0.7 nm. It is expected that the ASR scales with the inverse of the surface roughness; since a higher roughness
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indicates more active surface area, and therefore an increased ORR performance, and
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decreased ASR. However, considering that the deposition procedure was identical, a difference in the roughness was not expected. The AFM characterization was repeated with re-deposited thin films, and it was confirmed that PBC(001)//YSZ(001) had a much lower roughness in comparison to the other two films. This difference may be attributed to the substrate used and to the growth mechanism. In fact, oriented crystals tend to exhibit the terminations with a lower plane index when growing in order to lower the surface energy. The oriented crystal surface with a larger plane index tends to have more
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disordered atoms on the top, i.e., clusters and stepped atoms and therefore exhibits higher roughness [59, 60]. In addition, thin films deposited on substrates with different terminations may also have different growth mechanisms which are directly related to the
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surface roughness. The Frank-van der Merwe growth is a layer-by-layer mechanism and guarantees low roughness, while the Volmer-Weber growth tends to yield three dimensional islands with relatively high roughness [61-65]. For PBC(001)//YSZ(001),
in
Frank-van
der
Merwe
growth,
while
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the low-index surface and small lattice mismatch achieved by in-plane rotation may result for
PBC(110)//YSZ(110)
and
index and large lattice mismatch.
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PBC(110)//YSZ(111), Volmer-Weber growth may dominate due to the high surface
The surface composition may also play a significant role since the surface oxygen exchange is the main rate-determining step in ORR [66-68]. The XPS data was analyzed
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to determine the valence states and quantify the amounts of Pr, Ba, Co in the samples, by locating their characteristic peaks (Fig. 6a). As cobalt, Co is a transition metal element, it may exhibit different valence states such as +3 and +4 in double perovskites [69-71]. We
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obtained the actual surface valence state by fitting the Co 3p peak as part of two
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characteristic peaks (Fig. 6b). These data are listed in Table 3, with the nonstoichiometric formula Kim et al. reported for reference [27]. According to Table 3, one of the most notable differences among the thin films is the ratio and valence state of cobalt. As previously explained, the oxygen transport is strongly related to the oxygen vacancy in the Pr-O plane, which is directly determined by the cobalt valance state. We can see a clear trend of cobalt valence decrease and oxygen vacancy
increase
from
PBC(001)//YSZ(001),
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to
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PBC(110)//YSZ(111). This is also consistent with the ORR performance sequence. In addition, it has also been suggested that surface cobalt enrichment may decrease the ORR performance by blocking the oxygen tunnels [66]. Furthermore, it can also be observed
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from the XPS study, that PBC(110)//YSZ(110) and PBC(110)//YSZ(111) have different level of A-site element excess, which may also contribute to their different ORR performance. The different surface composition between PBC(001)//YSZ(001) and the
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other two (110) oriented PBC thin films may be due to the nature of the material (PBC) itself [72, 73]. The difference between PBC(110)//YSZ(110) and PBC(110)//YSZ(111)
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may be determined by the different growth mechanism and surface reconstruction.
Conclusions
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Oriented PBC thin films have been successfully deposited on (001), (110) and (111) oriented SDC/YSZ electrolyte substrates via PLD system. XRD data revealed abnormal orientations and in-plane rotations of those PBC thin films, which was attributed to lattice
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mismatch. The symmetric cells exhibited good performance as shown by the low ASR. performance
differences
among
the
films
were
observed.
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PBC(110)//YSZ(111) had the lowest ASR, followed by PBC(110)//YSZ(110) and then PBC(001)//YSZ(001). Our characterization and analysis suggests that the difference can be mainly attributed to the anisotropic oxygen diffusivity in the a-b plane and the valence of Co. The relatively slow oxygen diffusion in the [001] direction inhibits the activity of PBC(001)//YSZ(001), and the relatively low cobalt valence, on the other hand, provides PBC(110)//YSZ(111) with the best performance. The orientation of the thin film is
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mainly determined by the substrate, while the lattice mismatch between the deposited material and the substrate also has an effect. The latter explains the coincidently same orientation between PBC(110)//YSZ(110) and PBC(110)//YSZ(111). The different
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surface roughness may have resulted from the different surface energy on different terminations and the different growth mechanisms. The surface composition variance may result from the thin films themselves, as expected from the different orientations,
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while the interaction between the thin films and the different growth mechanisms may also have an influence. Even though open questions remain, this work provides a
orientation and surface composition.
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Acknowledgements
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reference for further development of enhanced SOFC cathode materials by tuning the
The authors gratefully acknowledge HKUST for providing start-up funds, and the Research Grants Council of Hong Kong for support through the projects, DAG12EG06
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and ECS 639713.
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Table 1. Potential parameters for the Buckingham potential 6
A /
/ Å
/ Å
Ref.
O2----O2-
22764.3
0.1490
43.00
[39]
Ba2+---O2-
1214.4
0.3522
0
[39]
Co3+---O2-
1329.82
0.3087
0
Pr3+---O2-
1445.2
0.3608
0
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[40]
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Table 2. Lattice mismatch between PBC and SDC in PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films before and after rotation and
PBC(110) //YSZ(110) 34% 13% 15o /
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Mismatch before rotation Mismatch after rotation In-plane rotation Orientation change
PBC(001) //YSZ(001) 28% 2% 45o /
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PBC(110) //YSZ(111) 32% 15% 45o (111) to (110)
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Table 3. Calculated surface compositions of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films by XPS, compared with the data from the literature.
valance state of cobalt rather than the XPS peak of oxygen.
PBC(110) //YSZ(111) 1.00 1.02 1.41 3.23
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PBC(110) //YSZ(110) 1.00 1.09 1.50 3.42
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Pr Ba Co Co valence
PBC(001) //YSZ(001) 1.00 1.03 1.20 3.48
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The ratio of Pr is fixed at 1 for comparison, and the ratio of oxygen is deduced from the
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PrBaCo2O5.65 1.00 1.00 2.00 3.15
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FIGURE CAPTIONS Figure 1. Normal XRD patterns of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and
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PBC(110)//YSZ(111) (a), Off-normal XRD patterns of PBC(001)//YSZ(001) (b), X-ray reflection of PBC(001)//YSZ(001) indicating the thickness as 54 nm (c), and rocking curve of PBC(001)//YSZ(001) as 0.85o (d).
PBC(110)//YSZ(111) thin films at 600 °C, 0.21atm.
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Figure 2. Polarization resistances of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and
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Figure 3. EIS data of polarization resistance related to temperature (a) and oxygen pressure (b) of PBC(001)//YSZ(001) thin films. (c)(d) and (e)(f) represent PBC(110)//YSZ(110) and PBC(110)//YSZ(111), respectively.
Figure 4. The left image is the comparison of oxygen self-diffusivities between
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Figure 5. AFM topographic maps of PBC(001)//YSZ(001) (a), PBC(110)//YSZ(110) (b) and PBC(110)//YSZ(111) (c). Figure 6. Overall XPS survey scan spectroscopy (a) and cobalt individual peaks (b) of PBC(001)//YSZ(001), PBC(110)//YSZ(110) and PBC(110)//YSZ(111) thin films.
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ACCEPTED MANUSCRIPT PrBaCo2O5+δ oriented thin films are fabricated with pulsed laser deposition. Performance of thin films with different oriented substrates is evaluated. Thin film on (111) oriented substrate shows a polarization resistance of 0.302 Ωcm2 at 600 oC, 0.21atm.
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Orientation of the substrates has a strong influence on the thin films performance.
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The difference is attributed to anisotropic oxygen transport and the substrates effect.