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Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure Kunal Mukherjee,*,† Yoshiaki Hayamizu,†,‡ Chang Sub Kim,† Liudmila M. Kolchina,§ Galina N. Mazo,§ Sergey Ya. Istomin,§ Sean R. Bishop,† and Harry L. Tuller† †
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 908-8579, Japan § Department of Chemistry, Moscow State University, Leninskie Gory, Moscow 119991, Russia ABSTRACT: Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4 were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T′ and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr1.6Sr0.4CuO4 is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La0.6Sr0.4Co0.8Fe0.2O3−δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance. KEYWORDS: solid-oxide fuel cells, thin-film cathodes, mixed ionic electronic conductors, combinatorial deposition, orientation control
I. INTRODUCTION The sluggish kinetics of the oxygen reduction reaction (ORR) at temperatures below 800 °C is a key limitation in increasing the power density and efficiency and thus reducing the cost of solid oxide fuel cells (SOFCs). Mixed ionic and electronic conductors (MIECs, materials with simultaneously high oxide ion and electron conduction) have been demonstrated to increase oxygen exchange rates by enabling ORR over the entire electrode surface and reducing the energy barrier to ORR, as compared to composites of separate ionic and electronic conducting phases.1 One of the premier examples of fast ORR in MIECs is in the acceptor doped (Ba,La)(Co,Fe)O3 and, to a lesser extent, (Sr,La)(Co,Fe)O3 systems.2 More recently, fast kinetics have been found in the more complex perovskite-based structures, including Ruddlesden−Popper layered structured materials such as La2NiO4 and double perovskites such as GdBaCo2O5.3,4 The ability to rapidly transport oxygen as interstitials along crystallographic planes is believed to play a key role in their exceptional ORR properties at intermediate temperatures (∼600 °C). In the search for improved cathode materials based on layered perovskites, developing a link between the bulk properties and the surface exchange kinetics is essential.5,6 The layered perovskites offer interesting avenues for this study due to the ability to accommodate both interstitial and vacancy type anion defects, enhanced by donor and acceptor type © XXXX American Chemical Society
doping, respectively. The study of donor doped cathodes is especially very limited in the literature.7 The role of surface orientation on ORR in these materials is also important because unlike simple cubic materials they have a large anisotropy in oxygen diffusivity.8 Finally, a comparison of materials similar in composition but with different crystal structures (and hence different ionic and electronic mobility) would provide a convenient method to understand the relative importance of doping. To assist in elucidating such key factors impacting ORR, we take advantage of the nonequilibrium nature of pulsed laser deposition to systematically investigate the role of crystal structure, donor and acceptor doping, and film orientation on the area specific resistance (ASR) of a relatively new class of cathode material: layered praseodymium cuprate, shown schematically in Figure 1. As described in detail in this study, the thin film cuprate system offers considerable flexibility in the control of crystal structure, ionic and electronic conductivity, and orientation without significant changes in the composition. Deposition schemes, such as buffer layers for orientation control and monolayer-by-monolayer synthesis of metastable phases, enable a study of these effects, not accessible via bulk techniques. Further, dense smooth thin film electrodes allow Received: July 20, 2016 Accepted: November 17, 2016
A
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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oxygen transport, as some of the best SOFC electrolytes have the fluorite structure, but in Pr2CuO4 the fluorite layers are reported to be compressed, leading to slow diffusion.13 The thermal expansion coefficient (TEC) of Pr2CuO4 is reported to be 11.8 ppm/K in the temperature range of 100−1000 °C and is comparable to that of YSZ (10.8 ppm/K).12 This is an important parameter in determining the long-term reliability of SOFC cathodes. Lyskov et al. fabricated screen-printed Pr2CuO4 electrodes on Ce0.9Gd0.1O1.95 and obtained an ASR of 1.7 Ω·cm2 at 700 °C.14 The incorporation of the larger Sr2+ cations on the A-site leads to the T* structure, as in Pr1.6Sr0.4CuO4, comprising both T and T′ blocks. Intermediate Sr2+ doping between x = 0.1 and x = 0.3 in bulk samples leads to phase separation into T′ and T* phases.15 Pr1.6Sr0.4CuO4 is slightly oxygen deficient (0.02− 0.07 formula units at 700 °C) as prepared, with the Sr2+ doping leading primarily to the creation of holes with a fraction of both Cu and Pr reportedly oxidized to 3+ and 4+, respectively.11 High temperature transport measurements by Mazo et al. show that while the oxygen diffusivity is significantly improved to 6.7 × 10−10 cm2/s at 700 °C, the electronic conductivity drops slightly to 30 S/cm, despite the Sr acceptor doping.15 It is unclear if the improvement in diffusivity stems only from a change in crystal structure from T′ to T* or also from the increase in oxygen deficiency. While there are no data on the anisotropy in oxygen diffusivity in T*-Pr1.6Sr0.4CuO4 or T′Pr2CuO4, Opila et al. showed that the T-La2−xSrxCuO4+δ shows higher diffusivity along the rock-salt layers compared to the perpendicular direction by a factor of 600 at 500 °C, a reason to suggest that a similar level of anisotropy might exist in the other layered cuprate structures.16 Oxygen isotope exchange studies by Mazo et al. show moderately better surface exchange at 700 °C of 7.7 × 10−8 cm/s in Pr1.6Sr0.4CuO4 as compared to 1.2 × 10−8 cm/s in Pr2CuO4, despite the large difference in oxygen diffusivity. The TEC of Pr1.6Sr0.4CuO4 is reported to be 14.9 ppm/K up to 500 °C but increases to 17.3 ppm/K in the range of 500−1000 °C due to chemical expansion from oxygen loss.15 There are no reports of electrochemical characterization of Pr1.6Sr0.4CuO4 as SOFC cathodes. The solubility limit of cerium in T′-Pr2−xCexCuO4+δ is reported to be x = 0.15 and leads to electron doping.17 This is a very useful aspect of the cuprate system as compared to the nickelates owing to the possibility of Cu being stable in a reduced state below 2+. Here the Ce is assumed to exist in a 4+ state. Thermogravimetric analysis on bulk single crystals of Pr1.85Ce0.15CuO4+δ showed a small oxygen excess of δ = 0.03, which was reduced to δ = 0.01 upon annealing in an Ar atmosphere.17 A study of the crystal field of Pr3+ using infrared transmission also confirmed the presence of interstitial oxygen in Pr2−xCexCuO4+δ but not in Pr2CuO4.18 More interestingly, the study showed that during reduction of Pr2−xCexCuO4+δ (900 °C, Ar atmosphere), the interstitial oxygen atoms were stabilized with Ce4+ and the equatorial oxygen atoms on the CuO2 planes were removed instead. There are no previous reports related to high temperature electronic and ionic transport in Pr 2 − x Ce x CuO 4 + δ , but studies on T′Nd2−xCexCuO4+δ show an increase in electronic conductivity with Ce doping.19
Figure 1. Schematic of the material parameters (crystal structure, doping, and orientation) investigated in this study for their effect on oxygen reduction. The fluorite and rock-salt intergrowths of the T′ and T* layers, potentially fast oxygen diffusion pathways in the layered cuprates, are shown with dashed boxes.
for a quantitative comparison between different compositions given their well-defined and reproducible geometries. The layered rare-earth cuprates have been widely studied as model superconducting materials with many reports of the effects of hole and electron doping on the critical temperature.9,10 Unlike the nickelates and cobaltates, the cuprates exhibit three different but related crystal structures due to the ability of copper to form pyramidal and square planar coordination blocks in addition to octahedral blocks. A2CuO4 undergoes structural transformations from T (octahedral) to T* (pyramidal) to T′ (square-planar) crystal structure with decreasing size of the A-site cation.11 The alternating perovskite blocks have a rock-salt intergrowth layer in the T-structure and a fluorite intergrowth layer in the T′ structure. The T* is a hybrid between the T and T′ structures with alternating rocksalt and fluorite intergrowths (Figure 1). Pr2CuO4 has the T′ structure, and Hwang et al., using a combination of thermogravimetric analysis (TGA) and X-ray absorption spectroscopy, determined that it remains largely stoichiometric, with the oxidation states of Pr and Cu being 3+ and 2+, respectively.11 Kaluzhskikh et al. showed that while Pr2CuO4 has a high electronic conductivity of 100 S/cm, it has a very low oxygen diffusivity of 7.2 × 10−13 cm2/s at 700 °C.12 Typically a fluorite intergrowth layer is not expected to be detrimental to
II. EXPERIMENTAL METHODS Ablation targets of Pr2CuO4 (T′) and Pr1.6Sr0.6CuO4 (T*) were prepared for pulsed laser deposition via a conventional solid-state synthesis route using powders of Pr6O11, SrCO3, and CuO (SigmaB
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) Symmetric 2θ−ω X-ray diffraction scans of the films as a function of composition on YSZ single crystal substrates. The (004) and (006) peaks are visible in c-axis oriented films, and the (200) peak is seen in a-axis oriented films. The vertical dashed lines correspond to peak positions from powder diffraction data in the literature of T′-Pr2CuO4 and T*-Pr1.6Sr0.4CuO4.11 (b) AFM micrographs of as-grown T′-Pr2CuO4 and T*-Pr1.6Sr0.4CuO4 films deposited on YSZ. with an electrolyte material) thin film cathodes are not the optimum design for fuel cell efficiency but they lend themselves well to a systematic comparison between different material systems.
Aldrich, 99.99%). The powders were mechanically milled using zirconia media in ethanol for 20 h followed by calcination at 1100 °C for 24 h and repeat milling for 20 h. The pelletized targets were then sintered (1070 °C, 8 h for Pr1.6Sr0.6CuO4 and 1100 °C, 5 h for Pr2CuO4 to achieve a higher density). The synthesis of Pr2−xCexCuO4 targets (x = 0.05 and 0.15) followed a similar two-step solid-state route. Initially, prefired CeO2, Pr6O11, and CuO (99.99%) were mixed, pelletized, and annealed at 950 °C for 18 h in air followed by regrinding and pelletizing procedures. Finally, the samples were annealed at 1100 °C for 36 h in air. Single crystals of epi-polished 1 cm × 1 cm × 0.5 mm (001) SrTiO3 and YSZ (ZrO2 with 8 mol % Y2O3, MTI Corporation) were used as substrates. The thin films were deposited using pulsed laser deposition (Neocera) at a heater temperature of 700 °C (corresponding to a substrate temperature of ∼550 °C) with an oxygen background of 0.01 Torr using a 248 nm KrF excimer laser with pulse energy of 400 mJ at a repetition rate of 10 Hz. The growth rates of the films were approximately 0.07 nm/s, and the film thicknesses were between 150 and 300 nm. Following deposition, the samples were left in the growth chamber with an oxygen background of 8 Torr during the cool-down period (typically 30 min). Metastable, lightly Sr-doped films (x = 0.05 and 0.15) were prepared by a monolayer-by-monolayer growth method by alternating Pr2CuO4 and Pr1.6Sr0.6CuO4 targets with a weighted number of laser pulses to achieve the desired composition and relying on interdiffusion near the surface. We note that this composition is within the twophase region for bulk samples. The thin films were structurally characterized via high-resolution X-ray diffraction (Bruker D8 HRXRD) with Cu Kα1 radiation, atomic force microscopy (AFM, Veeco Nanoscope IIIa), wavelength dispersive spectrometry (JEOL JXA-8200 superprobe), and transmission electron microscopy (JEOL JEM 2011, 200 kV). Lithographically defined gold electrodes deposited onto the cathode thin films served as current collectors, while porous silver paste served as the counter electrode. Electrochemical impedance spectroscopy (EIS) was performed with the aid of a Solartron 1260 impedance analyzer using a 20 mV amplitude ac voltage from 0.03 Hz to 30 MHz. The ASR measurements, as determined from EIS, were performed as a function of temperature on as-grown samples with no prior heat treatments, followed by measurements as a function of pO2 on the aged samples. We note that the nonporous, single-phase (as opposed to a two-phase mixture
III. RESULTS (a) Effect of Composition on Structure and ASR. Figure 2a shows symmetric coupled 2θ−ω scans of the thin cuprate films on YSZ as a function of composition. Single-phase, highly textured, c-axis oriented T′-Pr2CuO4 was observed to grow on both SrTiO3 (not shown) and YSZ substrates despite the lattice mismatch of 1.4% and 9.4%, respectively. The in-plane orientation of the film on the YSZ substrate was rotated by 45° with respect to the YSZ cube, similar to that reported for cubic perovskite film growth on fluorite substrates. There was no strain in the film, as the peak positions corresponded well to that reported in the literature for bulk.11 A surface roughness of 0.76 nm was measured using the AFM (Figure 2b) with nearly square shaped grains ranging from 60 to 80 nm separated by low-angle grain boundaries at angles 45° to the [100] substrate edge (aligned to the figure boundary), consistent with the XRD analysis above. Yamamoto et al. have also previously observed a c-axis orientation in MBEgrown Pr2CuO4 films on SrTiO3 substrates, with improving crystallinity following an increase in growth temperature from 500 to 650 °C.20 A study to examine the effects of growth conditions on the microstructure and ASR could be undertaken in future work. Thin films of Ce- and Sr-doped Pr2CuO4 were also synthesized on (001) YSZ substrates. The Ce-doped samples were observed to remain c-axis oriented but with increasing XRD peak width, suggesting an increase in sources of inhomogeneous strain such as grain boundaries and dislocations. Bulk single crystals of Pr2−xCexCuO4 have also reported to have peak broadening with increasing Ce content.17 A slight decrease in the c-axis lattice constant with increasing cerium content was also observed in the (004) peak, consistent with the substitution of Pr3+ with the smaller Ce4+ ion. Surprisingly, C
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces a corresponding shift in the (006) peak was not observed. The substitution of Pr3+ with larger Sr2+ also resulted in increased XRD peak broadening with the emergence of (110) oriented domains and, unexpectedly, a decrease of the c-axis lattice constant. While we note that these lightly Sr-doped compositions are not stable in bulk and will phase-separate, no significant volume of the T* phase was observed. This does not rule out the presence of such a phase existing in amorphous form, but the nonequilibrium nature of PLD combinatorial growth has previously been used for the synthesis of metastable phases.21 The XRD peaks from both films are much closer to the T′ phase of Pr2CuO4 leading us to label the film as T′ phase. The electrochemical results from these two samples (Sr = 0.05 and 0.15) are directly compared with the remaining samples without further discussion of phase purity. Finally, single phase T*-Pr1.6Sr0.4CuO4 and metastable Pr1.8Sr0.2CuO4 films were also obtained on both SrTiO3 (not shown) and YSZ substrates, but the out-of-plane orientation of the films on both substrates was a-axis instead of c-axis. Nevertheless, a 45° inplane rotation with respect to the YSZ cube was seen in the films deposited on YSZ substrates, similar to that in Pr2CuO4. The out-of-plane strain was measured to be 1.3% tensile in Pr1.6Sr0.4CuO4 with respect to the bulk value. AFM measurements showed a low rms roughness of 1.19 nm with elongated grains along the [110] directions of the YSZ substrate (Figure 2b). To the best of our knowledge, there are no reports on growth of epitaxial T*-Pr1.6Sr0.4CuO4 films, and the mechanism behind the orientation change (a- vs c-axis) is presently not clear. Lattice mismatch can be ruled out because a-axis growth actually results in greater strain than c-axis growth. Lee et al. have reported a switch in orientation with increasing strontium content in thin films of La2−xSrxNiO4 on (001) YSZ, albeit from a-axis to c-axis.22 This phenomenon was explained by difference in surface energy. As will be shown in section IIIb, it is possible to use a nearly domain-matched interface to the film to switch the orientation of the film. This led us to speculate that it is a change in the substrate−film interface energy related to the different crystal structures that might be responsible for the preferred orientation (i.e., T* and T′ have similar lattice parameters; thus using a T* seed layer to switch orientation of a T′ film is not expected to be strain-related). It was also noted that the metastable Pr1.8Sr0.2CuO4 film exhibited very high crystallinity, comparable to the films grown from the two constituent targets, leading us to label it as single-phase T*. Figure 3 shows a plot of an exemplar complex impedance spectrum from an asymmetric cell with Pr2CuO4 as cathode measured at 600 °C and pO2 = 0.21 atm. The spectrum shows two arcs, a distorted high frequency arc offset from the origin and an adjacent nearly semicircular low frequency arc. This spectrum was fit using an equivalent circuit consisting of a resistor (R) in series with two RQ elements (resistor in parallel with a constant phase element [Q, CPE]), shown by the inset in Figure 3. The resistor in series, corresponding to the high frequency horizontal offset, was seen to be independent of pO2 and is therefore attributed to oxygen ion conduction through the YSZ electrolyte. Additionally, previous work on similar PrxCe1−xO2−δ and Sr(Ti1−xFex)O3−δ based asymmetric thin film cells identified the smaller high frequency arc as corresponding to the porous Ag counter electrode and the larger low frequency arc to the dense thin-film working electrode.23,24 In films thinner than the characteristic length LC ≪ k/D (where k is the oxygen surface exchange rate and D is the oxygen diffusion rate in the bulk), oxygen transport is limited by
Figure 3. Exemplar EIS spectra of the T′-Pr2CuO4 thin film measured at 600 °C and pO2 = 0.21 atm. A fit to the spectra using the equivalent circuit drawn above is included. The ASR is extracted from the low frequency semicircle.
surface exchange and thus the low frequency arc resistance corresponds to the film’s resistance to oxygen exchange, or area specific resistance (ASR) when normalized by the area of the exposed film. From diffusion and surface exchange data of praseodymium cuprates from the literature, LC is calculated to be greater than 1 cm at 700 °C for all cases. The electrochemically determined oxygen surface exchange coefficient (kq) was obtained from the ASR (Rcathode) using the following equation:25 kq =
kBT 2
4e C0R cathode
(1)
where kB, T, e, and Co are the Boltzmann constant, temperature, elementary charge of an electron, and oxygen concentration in the film. Figure 4a shows an Arrhenius plot of the ASR in the range of 500−650 °C at pO2 = 0.21 atm. Pr2CuO4 is found to exhibit a relatively high ASR with an activation energy of 1.6 eV, corresponding to a kq of 3.5 × 10−8 cm/s at 650 °C using eq 1. This value is lower than that of other common perovskite MIEC cathodes.26 Using isotope exchange measurements, Kaluzhskikh et al. report a higher activation energy of k* of 2.04 eV in bulk ceramic pellets of Pr2CuO4 (between 700 and 900 °C), and by extrapolation of their results to 650 °C, k* is about 3 × 10−9 cm/s, lower than our results.12 The addition of both Sr and Ce dopants into the T′ structure resulted in up to an order of magnitude reduction in ASR. The activation energies of the ASR lie between 1.3 and 1.6 eV in the doped samples with no particular correlation to the amount or type of dopant. The similar behavior of both donor and acceptor doped Pr2CuO4 suggests that the rate limiting step is not linked to the electron concentration in the conduction band, unlike the case of SrTixFe1−xO3−δ.27 T*-Pr1.6Sr0.4CuO4 was seen to exhibit a considerably lower ASR at 650 °C but with a higher activation energy (2.1 eV), making it comparable in resistance to the doped T′ films at 500 °C. Using eq 1, kq of 1.3 × 10−6 cm/s is calculated for T*-Pr1.6Sr0.4CuO4 at 650 °C, comparable to, if not better than, other well-studied cubic materials such as (La,Sr)(Co,Fe)O3 (LSCF) and Sr(Ti0.65Fe0.35)O3 (STF35).24,26 However, these results are not in agreement with transport measurements on bulk pellets of Pr1.6Sr0.4CuO4 performed by Mazo et al. using isotope exchange measurements, where a nearly 1 order of magnitude slower exchange is noted, despite a very small activation energy of 0.88 eV.15 Similar discrepancies D
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. ASR for doped and undoped thin films as a function of (a) inverse temperature (pO2 = 0.21 atm) and (b) oxygen partial pressure (T = 600 °C) as determined by electrochemical impedance spectroscopy. Data for LSCF and BSCF are from Baumann et al.,26 and data for STF35 are from Jung et al.24 Crystal structure (T′ and T*) and orientation (c-axis, a-axis, and 110) are indicated.
Figure 5. (a) Symmetric 2θ−ω scans of Pr2CuO4, Pr1.8Sr0.2CuO4, and Pr1.6Sr0.4CuO4 deposited directly and on Pr1.6Sr0.4CuO4, Pr2CuO4, and Pr2CuO4 30 nm seed layers respectively on single crystal (001) YSZ substrates. Expected peak positions for the bulk samples are shown with dotted lines. (b) YSZ (100) cross-sectional TEM image obtained using mass−thickness contrast of a Pr2CuO4 deposited onto a buffer layer of Pr1.6Sr0.4CuO4 on YSZ.
of 2 eV, despite having a lower amount of acceptor doping. The ASR was also lower than that of T′-Pr1.85Sr0.15CuO4. This indicates an effect of the combination of the crystal structure change from T′ to T* and the orientation change from c-axis to a-axis, both potentially important factors. Section IIIb presents an approach to discriminate between these two factors.
in the value and activation energy of the surface exchange rate determined from ASR and isotope exchange have been reported by Mauvy et al. in Nd2NiO4 films, the cause remaining unclear, but differences in the magnitude of the driving force between the two experiments are speculated.28 Finally, the ASR of metastable T*-Pr1.8Sr0.2CuO4 was measured to be comparable to that of T*-Pr1.6Sr0.4CuO4 with an activation energy E
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Figure 4b shows the dependence of ASR on pO2 in the range 0.001−1 atm at 600 °C. The ASR was seen to decrease with increasing pO2 with a power-law exponent ranging between −0.2 and −0.35 for all compositions studied. As stated by Wang et al., a similarity in the exponents suggests that oxygen incorporation occurs via a common mechanism, facilitating an examination of correlations between bulk properties with surface exchange.29 These results are also qualitatively similar to that from screen-printed Pr2CuO4 electrodes from Lyskov et al., who report a power-law exponent of −0.4 (−3.5 < log pO2 < −1.5) increasing to −0.1 (−1.5 < log pO2 < 0) at 600 °C, indicative of a transition in the rate limiting step with increasing pO2.14 (b) Effect of Orientation on ASR. Measuring the relationship between degree of anisotropy and oxygen surface exchange rate can be readily performed with bulk single crystals but is often challenging in thin-film studies due to limited orientation control. Isotope exchange/diffusion profiles have been studied by several groups for thin films grown on different substrates or substrate orientations.8 By way of example, Nd2NiO4 films have been grown a-axis oriented on (Sr,La)AlO4 substrates and c-axis oriented on SrTiO3 substrates.30 However, such experiments preclude the use of EIS to study oxygen exchange, as the substrates are highly insulating and/or mixed ionic−electronic conductors. In this section, results are presented demonstrating orientation control on YSZ substrates for the first time by use of seed layers of one cuprate film grown on another cuprate film of a different native orientation. This was possible because, fortuitously, T*-Pr1.6Sr0.4CuO4 and T′Pr2CuO4 grow with nearly 100% a-axis and c-axis orientations on YSZ, respectively. Such a strategy was previously employed in the growth of oriented YBa 2 Cu 3 O 7−x films using PrBa2Cu3O7−x seed layers in Josephson junctions.31 In this study T′-Pr2CuO4 was deposited on a 30 nm seed layer of aaxis oriented Pr1.6Sr0.4CuO4, and T*-Pr1.6Sr0.4CuO4 and T*Pr1.8Sr0.2CuO4 were grown on a 30 nm seed layer of c-axis oriented Pr2CuO4. Figure 5a shows symmetric coupled 2θ−ω scans of the films along with that from their native orientations previously discussed in Figure 2a. A nearly pure a-axis oriented T′-Pr2CuO4 was obtained using this technique. Figure 5b shows a cross-sectional TEM image of this film along the (100) face of the YSZ substrate, showing sub-100 nm columnar grains due to two perpendicular in-plane orientations of a-axis films on cubic substrates. The position of the 200 diffraction peak is in good agreement with that of the T′-bulk indicating that no change in the crystal structure and film strain was observed as a result of the seed layer. Unfortunately, orientation control was not as successful in the other films. A majority of T*Pr1.8Sr0.2CuO4 grains were converted to c-axis orientation, but (110) and a-axis oriented grains were also detected. T*Pr1.6Sr0.4CuO4 showed only a small fraction of c-axis oriented grains with the film largely remaining a-axis oriented. Figure 6 shows ASR as a function of temperature from these films alongside data already presented in Figure 4a. The EIS measurements measure surface exchange, and the seed layer is not expected to change this due to its MIEC nature. With a complete orientation change of T′-Pr2CuO4 from c-axis to aaxis, a reduction in the ASR by a factor of 4 was measured with no change in activation energy. In the T*-Pr1.8Sr0.2CuO4 film with mixed a-axis and c-axis domains (majority c-axis), we noted an increase in ASR by nearly an order of magnitude, also with no change in activation energy. The difference in ASR in
Figure 6. ASR as a function of inverse temperature (pO2 = 0.21 atm) determined by EIS of films grown with (empty symbols) and without (filled symbols) seed layers to influence the crystal face exposed to air. Films with seed layers are also marked with an −O.
the T*-Pr1.6Sr0.4CuO4 films was small as expected due to little change in the orientation as a result of the seed layer.
IV. DISCUSSION A key assumption in the selection of materials for SOFC cathodes has been that bulk properties of a material serve as a good indicator for surface oxygen reduction kinetics. Accordingly, as a rule of thumb, it is accepted that some combination of a large oxygen defect concentration, fast diffusion, and high electronic conductivity is key to fast oxygen surface exchange. However, this hypothesis is currently being re-examined. Model systems in which it is possible to change the ionic and electronic properties over several orders of magnitude have shown a weak dependence of surface exchange on these bulk parameters.24 Skinner and Kilner showed that the addition of Sr acceptor dopants in La2NiO4+δ, lowered the diffusivity, partly due to a reduction in the number of interstitials, but the surface exchange was largely unaffected.32 The present results for the praseodymium cuprate system provides a window into the complex interplay between various parameters of the layered compounds. Considering that Pr2CuO4 has a high electronic conductivity, the order of magnitude improvement in ASR as a result of both Ce and Sr doping suggests that the increase in exchange rate is related to changes in oxygen nonstoichiometry, with oxygen vacancies (formed by acceptors) and interstitials (formed by donors) both important to surface exchange. The faster oxygen reduction kinetics of T*-Pr1.6Sr0.4CuO4 as compared to T′Pr2CuO4 reported in the literature are in agreement with our results. However, despite a lower amount of doping, metastable T*-Pr1.8Sr0.2CuO4 has a surface exchange rate just as high as T*-Pr1.6Sr0.4CuO4. Thus, fast oxygen reduction kinetics in T*Pr1.6Sr0.4CuO4 is only partly due to the acceptor doping, the change in crystal structure and orientation also playing a role. F
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observations, the rate limiting step in the oxygen reduction reaction for both the doped and undoped films is likely to be charge transfer to neutral oxygen adsorbates.
From the dependence of the ASR on pO2 (Figure 4b), involvement of molecular species of oxygen in the rate limiting step can be ruled out, as others have indicated this would have resulted in an exponent less than −0.5.33 A charge transfer reaction involving an electron and a neutral dissociated oxygen atom with a fast oxygen dissociation reaction in the preceding step is often suggested to be rate-limiting in cathodes that have an exponent close to −0.25.33 Changes in the defect equilibria of the cathode with pO2 modifies this exponent slightly, increasing it if oxygen vacancies are important and decreasing it if oxygen interstitials play a role. Unfortunately, no correlation between the exponent and type of doping was seen, likely due to the generally large errors present in surface-exchange dominated ASR measurements (typically ∼100%). Note that a limiting charge transfer step need not necessarily arise out of a deficiency of charge carriers (majority or minority) but could be due to a mismatch in energy levels between the conduction or valence band and the adsorbed oxygen 2p band or a low surface coverage of neutral oxygen adsorbates. Finally, an effect of the film orientation on surface exchange is indeed observed, but this effect is at best an order of magnitude. Therefore, it is likely that the improvement in surface exchange attributed to the change in crystal structure from T′ to T* is actually due to the accompanying change in orientation from c-axis to a-axis and not due to the higher oxygen diffusivity of the T* phase. It would, however, be premature to conclude that this provides evidence for a disconnect between diffusivity and surface exchange in Pr2CuO4, as there does not yet exist any data in the literature studying anisotropy in either bulk electronic or ionic transport properties in the layered praseodymium cuprates. The use of seed layers to control orientation in the present work demonstrates the potential for the examination of surface exchange and bulk transport coupling using EIS in other layered compounds such as the nickelates where an anisotropy in bulk properties is known to exist. Some recent reports on layered perovskites have also shown that the surface exchange determined using isotope depth profiles is only moderately anisotropic or in some cases even isotropic despite several orders of magnitude difference in oxygen diffusivity.8,34,35 Other reports have indicated that cation segregation effects may dominate over the impact of orientation.35 Indeed, preliminary X-ray photoelectron spectroscopy (XPS) on these samples show Pr (and Sr) segregation after annealing the samples in air for 18 h at 650 °C. However, a systematic study of ASR has to be performed to establish the roles of the segregates.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kunal Mukherjee: 0000-0002-2796-856X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was carried out as part of the activity of the Skoltech-MIT Center for Electrochemical Energy Storage. Structural characterization of the films was conducted in the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under Award DMR-1419807.
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V. CONCLUSIONS The praseodymium cuprate-based cathodes were shown to be flexible in terms of changing dopant type, crystal structure, and orientation. Highly textured thin films based on Pr2CuO4 were synthesized on single crystal YSZ substrates for the first time to determine their electrochemical activity toward oxygen reduction using electrochemical impedance spectroscopy. The addition of both acceptor and donor type dopants improved the ASR by nearly an order of magnitude in T′-Pr2CuO4, an effect not previously seen in perovskite-based cathodes. T*Pr1.6Sr0.4CuO4 was identified as a potential SOFC cathode material with fast surface exchange at intermediate temperatures (1.3 × 10−6 cm/s at 650 °C). A strategy of seed layers to control film orientation was used to find that surface exchange could be orientation dependent, but the effect was found to be less than an order of magnitude. On the basis of our G
DOI: 10.1021/acsami.6b08977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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