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Ce0.9Gd0.1O1.95 vertically aligned nanocomposite thin film as interlayer for thin film solid oxide fuel cells
Electrochimica Acta 62 (2012) 147–152
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Microstructure and electrochemical properties of PrBaCo2 O5+ı /Ce0.9 Gd0.1 O1.95 vertically aligned nanocomposite thin film as interlayer for thin film solid oxide fuel cells Sungmee Cho a , Young Nam Kim b , JoonHwan Lee a , Arumugam Manthiram b , Haiyan Wang a,∗ a b
Department of Electrical and Computer Engineering, Texas A & M University, College Station, TX 77843, USA Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA
a r t i c l e
i n f o
Article history: Received 23 November 2011 Accepted 3 December 2011 Available online 13 December 2011 Keywords: Vertically aligned nanocomposite structure PBCO/GDC interlayer Area specific resistance Pulsed laser deposition Thin film solid oxide fuel cells
a b s t r a c t A thin layer of a vertically aligned nanocomposite (VAN) structure of PrBaCo2 O5+ı (PBCO) and Ce0.9 Gd0.1 O1.95 (GDC) between the cathode and the electrolyte was demonstrated by a pulsed laser deposition (PLD) technique for thin film solid oxide fuel cells (TFSOFCs). The VAN structured PBCO/GDC thin film shows high quality epitaxial growth on single crystal SrTiO3 substrate at high deposition temperatures. It also has a lattice matching relation between the PBCO and GDC vertical nanocolumns. The symmetric cells with the VAN interlayer are found to have a lower area specific resistance (ASR) of 2.1 cm2 compared to that of the ones without the interlayer (2.5 cm2 ) at 400 ◦ C. The maximum power density of the single cell prepared with the VAN interlayer was 151, 263, and 350 mW cm−2 , respectively, at 650, 700, and 750 ◦ C with an open circuit voltage (OCV) of 0.88 V at 750 ◦ C. The PBCO/GDC VAN interlayer could perform as a transition layer that enhances adhesion and relieves stress from different thermal expansion coefficients (TECs) and lattice parameters between the cathode and the electrolyte in TFSOFCs. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are considered to be one of the most promising energy conversion devices because of their high energy conversion efficiency. To enable their wide applications, intermediate operation temperature SOFCs (400–700 ◦ C) are needed for the considerations of cost reductions (for both materials and manufacturing process), structural integrity of the cells, extensive selection of SOFC materials, and lifetime of the cells [1]. However, the increased cathode polarization resistance at low operating temperatures becomes one of the major limiting factors in the overall performance of the intermediate temperature SOFCs. The general requirement of the SOFC cathode material is high oxygen reduction rate, which is firmly limited to triple phase boundary (TPB) sites, where the oxygen molecules combine with electrons and form ions. An extension of the TPB active zone over the entire cathode plays an important role to increase the cell performance. In this aspect, many oxide materials containing Fe, Mn, Co and Ni have been studied for the cathode materials [2–5] and several perovskite-type mixed ionic electronic conducting (MIEC) ceramic materials such as doped LaCoO3 , BaCoO3 , and SmCoO3 have been
∗ Corresponding author. Tel.: +1 979 845 5082; fax: +1 979 845 6259. E-mail address: [email protected] (H. Wang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.008
introduced as promising cathode candidates for the intermediate temperatures SOFCs [6–8]. Recently, as a new cathode material of the layered or double perovskite-type structure with ordered A-cations, PrBaCo2 O5+ı (PBCO) is found to be a potential cathode candidate for SOFCs because of its high electrical conductivity, rapid oxygen ion diffusion, and surface exchange kinetics at intermediate temperatures. It has been reported that the distribution of vacancies could greatly improve the diffusivity of oxide ions in the bulk material and possibly provide surface defect sites with enhanced reactivity towards molecular oxygen [9–14]. On the other hand, surface modification by adding new nanostructure interlayer for improved interface area [15] or by implementing a thin interlayer for catalytic effect [16,17] could also provide enhanced cell performance. In our previous work [15], a vertically aligned nanocomposite (VAN) structure of La0.5 Sr0.5 CoO3 (LSCO)/GDC, as an interlayer between the cathode and the electrolyte has been demonstrated which effectively increases the cathode/electrolyte interface as well as the TPBs and thus leads to enhanced power density of the cells. This again suggests that the microstructural variations in the electrolyte and the electrode could affect the reaction kinetics of the cell, and the main power loss occurs due to the polarization resistance at the cathode/electrolyte interface in SOFCs. This is consistent with previous reports [18–23]. Here, a new binary VAN interlayer consisting of the new cathode material PBCO and the electrolyte GDC is deposited for thin film
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Scheme 1. (a) Schematic diagram of an anode supported single cell and (b) VAN interlayer where “P” and “G” refer, respectively, to PBCO and GDC columns.
SOFCs (TFSOFCs). A schematic sketch of the binary VAN interlayer is given in Schemes 1(a) and (b). The goal of this work is to enable this new VAN structure by optimizing the deposition parameters and implementing the VAN structure in a single cell. This new cell structure could take advantage of the new cathode material as well as the increased cathode/electrolyte interfacial area and achieve a better overall cell performance. 2. Experimental Thin film depositions of the yttria-stabilized zirconia (YSZ)/GDC electrolyte, and the PBCO–GDC cathode were performed using a KrF excimer laser (Lambda Physik 210, = 248 nm, 2–10 Hz) in a pulsed laser deposition (PLD) chamber. The laser beam was focused to obtain an energy density of approximately 5 J cm−2 at a 45◦ angle of incidence. Oxygen partial pressure was optimized for the depositions. Various substrates such as SrTiO3 (STO) (0 0 1), pressed GDC, and 60 wt.% NiO + 40 wt.% GDC (Praxair Inc) anode disks were selected for the VAN growth, symmetric cell growth, and single cell deposition, respectively. The pressed targets of PBCO–GDC (50:50 wt.%), 8 mol% YSZ, and GDC were all prepared by mixing the stoichiometric amounts of raw powders. To make the PBCO powder, the raw powders including Pr6 O11 , BaCO3 , and Co3 O4 were mixed in ethanol and calcinated at 1000 ◦ C for 12 h in air. The calcinated powders were then ground, pressed into pellets, and annealed at 1200 ◦ C for 12 h in air. The anode substrates were prepared with 5 wt.% starch as a pore former to increase the anode porosity. The NiO–GDC anode powders were ball-milled for 48 h and then the resultant NiO–GDC with 5 wt.% starch cermet powder was compacted into disks under a uniaxial press using a die set of 1 inch in diameter and sintered at 1300 ◦ C for 3 h in air. YSZ (∼1 m)/GDC (∼6 m) electrolyte and PBCO–GDC composite cathode (∼0.8 m) films were deposited onto the NiO–GDC anode substrates under the stated conditions by PLD. The growth ˚ rates of these thin films were about ∼0.86 A/s at 600 ◦ C and ˚ ∼0.64 A/s at 750 ◦ C for PBCO–GDC interlayer under an oxygen ˚ ˚ partial pressure of 200 mTorr and ∼1.4 A/s and 3.2 A/s, respectively, for the YSZ and GDC layers in an oxygen partial pressure of 20 mTorr. The substrate temperature was varied from 300 to 750 ◦ C to optimize the film crystallinity and the nanostructure of the interlayer. To make a thick cathode layer, the PBCO–GDC
powder mixed with an organic binder (Heraeus V006) was applied onto the sample by the screen printing method. The cathode is about ∼20 m after annealing at 1150 ◦ C for 5 h in air. Microstructural characterizations of these films were performed by the cross-sectional transmission electron microscopy (TEM, JEOL2010) and scanning transmission electron microscopy (STEM, FEI Tecnai F20). Surface morphology study for the thin films was conducted with a field emission scanning electron microscopy (FESEM). The symmetric cells were fabricated and subsequently conducted for electrochemical evaluation with an AC impedance analyzer (Gamry Instruments) in the frequency range of 10−2 –300 kHz from 350 to 650 ◦ C in air. The cell performance with the PBCO–GDC interlayer was examined with an anode-supported single cell consisting of PBCO/PBCO–GDC/electrolyte/NiO–GDC using a potentiostat (Autolab-Eco Chemie BV). During the single cell performance test, humidified H2 (∼3% H2 O at 25 ◦ C) and air were supplied as a fuel and oxidant, respectively, at a constant flow rate of 100 mL min−1 . The effective electrode area of the cell was 1.13 cm2 . Measurement details can be found elsewhere [17]. 3. Results and discussion The microstructural characterization of PBCO and PBCO/GDC thin films was first performed by X-ray diffraction (XRD). A set of PBCO/GDC nanocomposite films were prepared on STO (001) substrates in the temperature range of 300–750 ◦ C. Fig. 1 shows the XRD patterns of the PBCO thin film and the PBCO/GDC nanocomposite films deposited at 500 ◦ C and 650 ◦ C. The thickness of the PBCO and PBCO/GDC films is ∼ 40 and ∼ 100 nm, respectively. The PBCO thin film in Fig. 1(a) is orientated predominantly along the c-axis and all the PBCO peaks are very close to the STO peaks, e.g., the PBCO (0 0 2) diffraction peak (c = 0.7636 nm) is almost overlapping with the STO (0 0 1) peak (a = 0.3904 nm). The PBCO/GDC nanocomposite film shows a similar structure as the PBCO thin film with highly textured growth along (0 0 l) orientation as shown in Fig. 1(b). Note that the XRD pattern of the PBCO thin film exhibits excellent crystallinity with the deposition temperature beyond 500 ◦ C. In addition, the strain state of both films can be concluded as below. First, the calculated out-of-plane lattice parameter of the PBCO thin film is 0.780 nm based on PBCO (0 0 2) peak position, which is in tension out-of-plane on the STO substrate compared to that of its bulk lattice parameter (c = 0.7636 nm). Second, the PBCO phase in
S. Cho et al. / Electrochimica Acta 62 (2012) 147–152
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Fig. 1. XRD patterns of (a) PBCO thin film deposited at 500 ◦ C and (b) PBCO/GDC composite film deposited at 600 ◦ C.
PBCO/GDC nanocomposite film has a higher tensile stress along the out-of-plane direction based on the calculated lattice parameter of 0.790 nm. This suggests that a further tensile stress is introduced by the GDC secondary phase. Fig. 2 shows the cross-sectional TEM images of the PBCO/GDC thin films on the STO substrates. In Fig. 2(a), the binary VAN thin film deposited at 600 ◦ C grows as a columnar structure with two phases clearly separated. The one deposited at 750 ◦ C (Fig. 2(b)) instead shows an interesting nanolayered structure within the pillars. High resolution cross-sectional TEM images of the PBCO/GDC thin films at different temperatures are shown in Figs. 3 and 4. The film deposited at 600 ◦ C shows a typical VAN structure with excellent epitaxial quality (Fig. 3(a)). The corresponding selected area diffraction pattern (SAD, inset in Fig. 3(a)), shows the distinguished diffraction dots from both the film and the substrate indicating the excellent epitaxy of the VAN film. In Fig. 3(b), clear alternating columns of PBCO and GDC are shown in the high resolution TEM image. The average column width for PBCO and GDC is ∼9 and ∼7 nm, respectively. High angle annular dark field (HAADF) images were taken under STEM mode. It is also called Zcontrast image, where the contrast is proportional to Z2 . One such image in Fig. 3(c) shows well-aligned vertical columns with the
different contrast from the PBCO (darker contrast) and GDC (brighter contrast) columns, which is consistent with the TEM observations. Fig. 4 shows the high resolution cross-sectional TEM images of the PBCO/GDC thin film deposited at 750 ◦ C. As seen in Fig. 4(a), the lateral layers are the alternating PBCO and GDC layers indicated by alternating bright and dark contrasts. The nanolayer thickness is about ∼2.3 and ∼1.0 nm for the PBCO and GDC nanolayers, respectively. The corresponding SAD pattern clearly indicates the high quality epitaxial growth of the PBCO and GDC phases. As shown in Fig. 4(c), the STEM image confirms the apparent nanolayer structure of PBCO and GDC with high epitaxial quality and smooth interface. It is interesting to note that the deposition temperature strongly affects the architecture of the PBCO/GDC nanocomposite thin film, i.e., from the vertically aligned columnar structure (Fig. 3) to the lateral layered structure (Fig. 4). It has been reported for the phase separation of the VAN structure in perovskite systems that the fast growth kinetics can be reached by spinodal decomposition in proper timescales [24,25]. However, the nanocomposite structure is strongly affected by the growth kinetics. For example, the VAN structure is formed only when the desired adatom surface mobility and the deposition rate were achieved. Fig. 5(a) shows the AC impedance spectra in the form of a Nypuist plot for a symmetric cell with a 150 nm PBCO/GDC interlayer at 350 ◦ C. This measured impedance data is fitted well to an equivalent circuit as shown in Fig. 5(b). In this model, a constant phase element (CPE) is used to study the inhomogeneous
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Fig. 5. AC impedance data of the symmetric cell with the PBCO/GDC interlayer at 350 ◦ C: (a) Nyquist plot and (b) equivalent circuit model.
Fig. 3. Cross-sectional (a) low magnification TEM image, (b) high resolution TEM image, and (c) STEM (Z-contrast) image of the PBCO/GDC VAN structure grown on the STO substrate at 600 ◦ C.
porous electrode/electrolyte systems for exact fitting results. The first small arc at the high and medium frequency intercepts of the spectrum is related to the contributions of the Pt wire and bulk electrolyte resistance, which is represented as R0 and (R1 -CPE1 ) circuits. The second semicircle is associated with the electrode polarization resistance at the low frequency range. With the fitting results, the (R2 -CPE2 ) circuit is for the oxide ion transfer through the electrode/electrolyte interface and both (R3 -CPE3 ) and (R4 -CPE4 ) are attributed to an electron charge transfer in the typical range at the interface, which is in good agreement with the previous literature [26,27].
Fig. 4. Cross-sectional (a) low magnification TEM image, (b) high resolution TEM image, and (c) STEM (Z-contrast) image of the PBCO/GDC nanolayered structure grown on the STO substrate at 750 ◦ C.
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Fig. 8. Cross-sectional SEM images of the anode supported single cell with the PBCO/GDC interlayer after the power measurement. Fig. 6. Area specific resistance (ASR) of the symmetric cells with and without the VAN PBCO/GDC structure grown by PLD.
To compare the area specific resistance (ASR) of symmetric cells without and with the 150 nm interlayer, the ASR of the cathode was [(R =R +R +R )×(Area)] calculated by the following equation:ASR = p 2 3 2 4 The calculation was based on the AC impedance data under air in the temperature range of 350–650 ◦ C. As shown in Fig. 6, the ASR of the cell with the VAN interlayer shows a lower value in comparison to the results obtained for the cell without the interlayer. For example, the cell without the interlayer has a higher ASR (2.5 cm2 ) than the one with the interlayer (2.1 cm2 ) at 400 ◦ C. Therefore, the VAN PBCO/GDC interlayer incorporation into the PBCO cathode can affect the total cathode polarization with increasing effective area of the TPBs. The interlayer also improves the catalytic reaction probability at the gas–cathode–electrolyte TPBs. Accordingly, a decrease of the ASR of the cathode could lead to a better performance in TFSOFCs. An anode-supported single cell with the PBCO/GDC interlayer was measured for the I–V characteristic at the temperatures of 650–750 ◦ C. Fig. 7 presents the cell voltage and power density as 1.2
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a function of the current density by a two-electrode set-up under humidified hydrogen at the rate of 100 mL min−1 , using Pt wires as electrical contacts on both sides of the cell. An open circuit voltages (OCV) of 0.88 V was obtained at 750 ◦ C by implementing the bilayer GDC/YSZ. Adding a thin YSZ can suppress ceria reduction at the anode side and improve the power output based on our previous work [28]. The general trend is that the cell voltage drops with respect to increasing current density at each temperature. The maximum power density of the cell with the PBCO/GDC interlayer is 151, 263, and 350 (highlighted as red arrow) mW cm−2 at 650, 700, and 750 ◦ C, respectively. A single cell without the interlayer was also prepared and measured as comparison. The power density of the cell is 268 mW cm−2 at 750 ◦ C. The improvement in the cell performance is largely because of the increased interface area by the interlayer structure. Besides, it is worth to note that concentration polarization in the I–V curves in Fig. 7 is observed under a current drain of ∼0.8 A/cm2 which is possibly related to the limited fuel supply due to the low porosity in the anode side. The true maximum power density of the cell at 750 ◦ C could be higher under the optimized porosity in the anode side. Cross-sectional SEM image in Fig. 8 shows the anode-supported single cell after the power measurements at high temperatures. The microstructures of PBCO/GDC interlayer and YSZ/GDC bilayer can be clearly observed as ∼1 and ∼ 6 m, respectively. The bilayer electrolyte shows a fully dense structure with an obvious interface between the YSZ and GDC layers in the cell. The PBCO cathode and NiO–GDC anode exhibit a porous structure with uniform pore distribution. It is evident that there is no crack or delamination issue observed at the interfaces among the cathode, interlayer, and electrolyte thin film layers. This suggests that the PBCO/GDC interlayer acts as an effective transition layer and improve the adhesion properties between the cathode and electrolyte. 4. Conclusions The VAN structures of the PBCO/GDC interlayer between the cathode and electrolyte for thin film solid oxide fuel cells (TFSOFCs) were prepared by PLD. The PBCO/GDC thin film interlayer shows either the vertically ordered columnar structure (600 ◦ C) or the nanolayered structure (750 ◦ C). In both cases, the nanocomposite films have high quality epitaxial growth. The symmetric cell with the VAN PBCO/GDC interlayer shows a lower ASR of 2.1 cm2 at 400 ◦ C compared to the cell without the interlayer (2.5 cm2 ).
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The single cell with the PBCO/GDC interlayer shows an OCV of 0.9 V at 750 ◦ C and maximum power densities of 151, 263, and 350 mW cm−2 at 650, 700, and 750 ◦ C, respectively. Overall the VAN PBCO/GDC interlayer acts as an effective transition layer between the cathode and electrolyte, enhancing the adhesion properties in between the layers, and thus leading to improved overall cell performance. Acknowledgements The work was in part funded by the National Science Foundation (NSF 0846504 and NSF 1007969), the Air Force Office of Scientific Research (Contract No.: FA9550-07-1-0108 and FA955009-1-0114), and DOE Center for Integrated Nanotechnology (CINT). The work at the University of Texas at Austin was funded by the Welch Foundation grant F-1254. References [1] N.P. Brandon, S. Skinner, B.C.H. Steele, Annu. Rev. Mater. Res. 33 (2003) 183. [2] C. Zuo, S. Zha, M. Liu, M. Hatano, M. Uchiyama, Adv. Mater. 18 (2006) 3318. [3] M. Liu, D. Dong, L. Chen, J. Gao, X. Liu, G. Meng, J. Power Sources 176 (2008) 107. [4] E. Boehm, J.-M. Bassat, M.C. Steil, P. Dordor, F. Mauvy, J.-C. Grenier, Solid State Sci. 5 (2003) 973. [5] Z. Tao, L. Bi, L. Yan, W. Sun, Z. Zhu, R. Peng, W. Liu, Electrochem. Commun. 11 (2009) 688. [6] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27. [7] Z. Shao, S.M. Haile, Nature 431 (2004) 170.
[8] C. Xia, W. Rauch, F. Chen, M. Liu, Solid State Ionics 149 (2002) 11. [9] G. Kim, S. Wang, A. Jacobson, Z. Yuan, W. Donner, C. L. Chen, L. Reimus, P. Brodersen, C. A. Mims, Appl. Phys. Lett. 88 (2006) 024103/1. [10] G. Kim, S. Wang, A.J. Jacobson, L. Reimus, P. Brodersen, C.A. Mims, J. Mater. Chem. 17 (2007) 2500. [11] M. Yadav, W. Gong, A.J. Jacobson, J. Solid State Electrochem. 15 (2011) 293. [12] A. Yu Suntsov, I.A. Leonidov, A.A. Markov, M.V. Patrakeev, Ya N. Blinovskov, V.L. Kozhevnikov, Russ. J. Phys. Chem. A 83 (2009) 832. [13] H. Ding, Y. Xie, X. Xue, J. Power Sources 196 (2011) 2602. [14] Y. Lin, R. Ran, C. Zhang, R. Cai, Z. Shao, J. Phys. Chem. A 114 (2010) 3764. [15] J. Yoon, S. Cho, J.-H. Kim, J.H. Lee, Z. Bi, A. Serquis, X. Zhang, A. Manthiram, H. Wang, J. Adv. Funct. Mater. 19 (2009) 3868. [16] Z. Fan, F.B. Prinz, Nano Lett. 11 (2011) 2202. [17] Y.B. kim, T.P. Holme, T.M. Gür, F.B. Prinz, J. Adv. Funct. Mater. (2011) 1. [18] B. Kenney, K. Karan, Solid State Ionics 178 (2007) 297. [19] M.H. Hung, M.V.M. Rao, D.S. Tsai, Mater. Chem. Phys. 101 (2007) 297. [20] J. Deseure, Y. Bultel, L. Dessemond, E. Siebert, P. Ozil, Appl. Electrochem. 37 (2007) 129. [21] M. Sase, J. Suzuki, K. Yashiro, T. Otake, A. Kaimai, T. Kawada, J. Mizusaki, H. Yugami, Solid State Ionics 177 (2006) 1961. [22] E. Koep, C.M. Jin, M. Haluska, R. Das, R. Narayan, K. Sandhage, R. Snyder, M.L. Liu, J. Power Sources 161 (2006) 250. [23] S.P. Yoon, J. Han, S.W. Nam, T.H. Lim, S.A. Hong, J. Power Sources 136 (2004) 30. [24] J.L. MacManus-Driscoll, J. Adv. Funct. Mater. 20 (2010) 2035. [25] H. Yang, H. Wang, B. Maiorov, J. Lee, D. Talbayev, M.J. Hinton, D.M. Feldmann, J.L. MacManus-Driscoll, A.J. Taylor, L. Civale, T.R. Lemberger, Q.X. Jia, J. Appl. Phys. 106 (2009) 093914. [26] D. Chen, R. Ran, K. Zhang, J. Wang, Z. Shao, J. Power Sources 188 (2009) 96–105. [27] J.-H. Kim, Y.N. Kim, S. Cho, H. Wang, A. Manthiram, Electrochim. Acta 19 (2010) 5312. [28] S. Cho, Y.N. Kim, J.-H. Kim, A. Manthiram, H. Wang, J. Electrochim. Acta 56 (2011) 5472.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Microstructure and electrochemical properties of PrBaCo2 O5+ı /Ce0.9 Gd0.1 O1.95 vertically aligned nanocomposite thin film as interlayer for thin film solid oxide fuel cells Sungmee Cho a , Young Nam Kim b , JoonHwan Lee a , Arumugam Manthiram b , Haiyan Wang a,∗ a b
Department of Electrical and Computer Engineering, Texas A & M University, College Station, TX 77843, USA Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA
a r t i c l e
i n f o
Article history: Received 23 November 2011 Accepted 3 December 2011 Available online 13 December 2011 Keywords: Vertically aligned nanocomposite structure PBCO/GDC interlayer Area specific resistance Pulsed laser deposition Thin film solid oxide fuel cells
a b s t r a c t A thin layer of a vertically aligned nanocomposite (VAN) structure of PrBaCo2 O5+ı (PBCO) and Ce0.9 Gd0.1 O1.95 (GDC) between the cathode and the electrolyte was demonstrated by a pulsed laser deposition (PLD) technique for thin film solid oxide fuel cells (TFSOFCs). The VAN structured PBCO/GDC thin film shows high quality epitaxial growth on single crystal SrTiO3 substrate at high deposition temperatures. It also has a lattice matching relation between the PBCO and GDC vertical nanocolumns. The symmetric cells with the VAN interlayer are found to have a lower area specific resistance (ASR) of 2.1 cm2 compared to that of the ones without the interlayer (2.5 cm2 ) at 400 ◦ C. The maximum power density of the single cell prepared with the VAN interlayer was 151, 263, and 350 mW cm−2 , respectively, at 650, 700, and 750 ◦ C with an open circuit voltage (OCV) of 0.88 V at 750 ◦ C. The PBCO/GDC VAN interlayer could perform as a transition layer that enhances adhesion and relieves stress from different thermal expansion coefficients (TECs) and lattice parameters between the cathode and the electrolyte in TFSOFCs. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are considered to be one of the most promising energy conversion devices because of their high energy conversion efficiency. To enable their wide applications, intermediate operation temperature SOFCs (400–700 ◦ C) are needed for the considerations of cost reductions (for both materials and manufacturing process), structural integrity of the cells, extensive selection of SOFC materials, and lifetime of the cells [1]. However, the increased cathode polarization resistance at low operating temperatures becomes one of the major limiting factors in the overall performance of the intermediate temperature SOFCs. The general requirement of the SOFC cathode material is high oxygen reduction rate, which is firmly limited to triple phase boundary (TPB) sites, where the oxygen molecules combine with electrons and form ions. An extension of the TPB active zone over the entire cathode plays an important role to increase the cell performance. In this aspect, many oxide materials containing Fe, Mn, Co and Ni have been studied for the cathode materials [2–5] and several perovskite-type mixed ionic electronic conducting (MIEC) ceramic materials such as doped LaCoO3 , BaCoO3 , and SmCoO3 have been
∗ Corresponding author. Tel.: +1 979 845 5082; fax: +1 979 845 6259. E-mail address: [email protected] (H. Wang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.008
introduced as promising cathode candidates for the intermediate temperatures SOFCs [6–8]. Recently, as a new cathode material of the layered or double perovskite-type structure with ordered A-cations, PrBaCo2 O5+ı (PBCO) is found to be a potential cathode candidate for SOFCs because of its high electrical conductivity, rapid oxygen ion diffusion, and surface exchange kinetics at intermediate temperatures. It has been reported that the distribution of vacancies could greatly improve the diffusivity of oxide ions in the bulk material and possibly provide surface defect sites with enhanced reactivity towards molecular oxygen [9–14]. On the other hand, surface modification by adding new nanostructure interlayer for improved interface area [15] or by implementing a thin interlayer for catalytic effect [16,17] could also provide enhanced cell performance. In our previous work [15], a vertically aligned nanocomposite (VAN) structure of La0.5 Sr0.5 CoO3 (LSCO)/GDC, as an interlayer between the cathode and the electrolyte has been demonstrated which effectively increases the cathode/electrolyte interface as well as the TPBs and thus leads to enhanced power density of the cells. This again suggests that the microstructural variations in the electrolyte and the electrode could affect the reaction kinetics of the cell, and the main power loss occurs due to the polarization resistance at the cathode/electrolyte interface in SOFCs. This is consistent with previous reports [18–23]. Here, a new binary VAN interlayer consisting of the new cathode material PBCO and the electrolyte GDC is deposited for thin film
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Scheme 1. (a) Schematic diagram of an anode supported single cell and (b) VAN interlayer where “P” and “G” refer, respectively, to PBCO and GDC columns.
SOFCs (TFSOFCs). A schematic sketch of the binary VAN interlayer is given in Schemes 1(a) and (b). The goal of this work is to enable this new VAN structure by optimizing the deposition parameters and implementing the VAN structure in a single cell. This new cell structure could take advantage of the new cathode material as well as the increased cathode/electrolyte interfacial area and achieve a better overall cell performance. 2. Experimental Thin film depositions of the yttria-stabilized zirconia (YSZ)/GDC electrolyte, and the PBCO–GDC cathode were performed using a KrF excimer laser (Lambda Physik 210, = 248 nm, 2–10 Hz) in a pulsed laser deposition (PLD) chamber. The laser beam was focused to obtain an energy density of approximately 5 J cm−2 at a 45◦ angle of incidence. Oxygen partial pressure was optimized for the depositions. Various substrates such as SrTiO3 (STO) (0 0 1), pressed GDC, and 60 wt.% NiO + 40 wt.% GDC (Praxair Inc) anode disks were selected for the VAN growth, symmetric cell growth, and single cell deposition, respectively. The pressed targets of PBCO–GDC (50:50 wt.%), 8 mol% YSZ, and GDC were all prepared by mixing the stoichiometric amounts of raw powders. To make the PBCO powder, the raw powders including Pr6 O11 , BaCO3 , and Co3 O4 were mixed in ethanol and calcinated at 1000 ◦ C for 12 h in air. The calcinated powders were then ground, pressed into pellets, and annealed at 1200 ◦ C for 12 h in air. The anode substrates were prepared with 5 wt.% starch as a pore former to increase the anode porosity. The NiO–GDC anode powders were ball-milled for 48 h and then the resultant NiO–GDC with 5 wt.% starch cermet powder was compacted into disks under a uniaxial press using a die set of 1 inch in diameter and sintered at 1300 ◦ C for 3 h in air. YSZ (∼1 m)/GDC (∼6 m) electrolyte and PBCO–GDC composite cathode (∼0.8 m) films were deposited onto the NiO–GDC anode substrates under the stated conditions by PLD. The growth ˚ rates of these thin films were about ∼0.86 A/s at 600 ◦ C and ˚ ∼0.64 A/s at 750 ◦ C for PBCO–GDC interlayer under an oxygen ˚ ˚ partial pressure of 200 mTorr and ∼1.4 A/s and 3.2 A/s, respectively, for the YSZ and GDC layers in an oxygen partial pressure of 20 mTorr. The substrate temperature was varied from 300 to 750 ◦ C to optimize the film crystallinity and the nanostructure of the interlayer. To make a thick cathode layer, the PBCO–GDC
powder mixed with an organic binder (Heraeus V006) was applied onto the sample by the screen printing method. The cathode is about ∼20 m after annealing at 1150 ◦ C for 5 h in air. Microstructural characterizations of these films were performed by the cross-sectional transmission electron microscopy (TEM, JEOL2010) and scanning transmission electron microscopy (STEM, FEI Tecnai F20). Surface morphology study for the thin films was conducted with a field emission scanning electron microscopy (FESEM). The symmetric cells were fabricated and subsequently conducted for electrochemical evaluation with an AC impedance analyzer (Gamry Instruments) in the frequency range of 10−2 –300 kHz from 350 to 650 ◦ C in air. The cell performance with the PBCO–GDC interlayer was examined with an anode-supported single cell consisting of PBCO/PBCO–GDC/electrolyte/NiO–GDC using a potentiostat (Autolab-Eco Chemie BV). During the single cell performance test, humidified H2 (∼3% H2 O at 25 ◦ C) and air were supplied as a fuel and oxidant, respectively, at a constant flow rate of 100 mL min−1 . The effective electrode area of the cell was 1.13 cm2 . Measurement details can be found elsewhere [17]. 3. Results and discussion The microstructural characterization of PBCO and PBCO/GDC thin films was first performed by X-ray diffraction (XRD). A set of PBCO/GDC nanocomposite films were prepared on STO (001) substrates in the temperature range of 300–750 ◦ C. Fig. 1 shows the XRD patterns of the PBCO thin film and the PBCO/GDC nanocomposite films deposited at 500 ◦ C and 650 ◦ C. The thickness of the PBCO and PBCO/GDC films is ∼ 40 and ∼ 100 nm, respectively. The PBCO thin film in Fig. 1(a) is orientated predominantly along the c-axis and all the PBCO peaks are very close to the STO peaks, e.g., the PBCO (0 0 2) diffraction peak (c = 0.7636 nm) is almost overlapping with the STO (0 0 1) peak (a = 0.3904 nm). The PBCO/GDC nanocomposite film shows a similar structure as the PBCO thin film with highly textured growth along (0 0 l) orientation as shown in Fig. 1(b). Note that the XRD pattern of the PBCO thin film exhibits excellent crystallinity with the deposition temperature beyond 500 ◦ C. In addition, the strain state of both films can be concluded as below. First, the calculated out-of-plane lattice parameter of the PBCO thin film is 0.780 nm based on PBCO (0 0 2) peak position, which is in tension out-of-plane on the STO substrate compared to that of its bulk lattice parameter (c = 0.7636 nm). Second, the PBCO phase in
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Fig. 1. XRD patterns of (a) PBCO thin film deposited at 500 ◦ C and (b) PBCO/GDC composite film deposited at 600 ◦ C.
PBCO/GDC nanocomposite film has a higher tensile stress along the out-of-plane direction based on the calculated lattice parameter of 0.790 nm. This suggests that a further tensile stress is introduced by the GDC secondary phase. Fig. 2 shows the cross-sectional TEM images of the PBCO/GDC thin films on the STO substrates. In Fig. 2(a), the binary VAN thin film deposited at 600 ◦ C grows as a columnar structure with two phases clearly separated. The one deposited at 750 ◦ C (Fig. 2(b)) instead shows an interesting nanolayered structure within the pillars. High resolution cross-sectional TEM images of the PBCO/GDC thin films at different temperatures are shown in Figs. 3 and 4. The film deposited at 600 ◦ C shows a typical VAN structure with excellent epitaxial quality (Fig. 3(a)). The corresponding selected area diffraction pattern (SAD, inset in Fig. 3(a)), shows the distinguished diffraction dots from both the film and the substrate indicating the excellent epitaxy of the VAN film. In Fig. 3(b), clear alternating columns of PBCO and GDC are shown in the high resolution TEM image. The average column width for PBCO and GDC is ∼9 and ∼7 nm, respectively. High angle annular dark field (HAADF) images were taken under STEM mode. It is also called Zcontrast image, where the contrast is proportional to Z2 . One such image in Fig. 3(c) shows well-aligned vertical columns with the
different contrast from the PBCO (darker contrast) and GDC (brighter contrast) columns, which is consistent with the TEM observations. Fig. 4 shows the high resolution cross-sectional TEM images of the PBCO/GDC thin film deposited at 750 ◦ C. As seen in Fig. 4(a), the lateral layers are the alternating PBCO and GDC layers indicated by alternating bright and dark contrasts. The nanolayer thickness is about ∼2.3 and ∼1.0 nm for the PBCO and GDC nanolayers, respectively. The corresponding SAD pattern clearly indicates the high quality epitaxial growth of the PBCO and GDC phases. As shown in Fig. 4(c), the STEM image confirms the apparent nanolayer structure of PBCO and GDC with high epitaxial quality and smooth interface. It is interesting to note that the deposition temperature strongly affects the architecture of the PBCO/GDC nanocomposite thin film, i.e., from the vertically aligned columnar structure (Fig. 3) to the lateral layered structure (Fig. 4). It has been reported for the phase separation of the VAN structure in perovskite systems that the fast growth kinetics can be reached by spinodal decomposition in proper timescales [24,25]. However, the nanocomposite structure is strongly affected by the growth kinetics. For example, the VAN structure is formed only when the desired adatom surface mobility and the deposition rate were achieved. Fig. 5(a) shows the AC impedance spectra in the form of a Nypuist plot for a symmetric cell with a 150 nm PBCO/GDC interlayer at 350 ◦ C. This measured impedance data is fitted well to an equivalent circuit as shown in Fig. 5(b). In this model, a constant phase element (CPE) is used to study the inhomogeneous
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Fig. 5. AC impedance data of the symmetric cell with the PBCO/GDC interlayer at 350 ◦ C: (a) Nyquist plot and (b) equivalent circuit model.
Fig. 3. Cross-sectional (a) low magnification TEM image, (b) high resolution TEM image, and (c) STEM (Z-contrast) image of the PBCO/GDC VAN structure grown on the STO substrate at 600 ◦ C.
porous electrode/electrolyte systems for exact fitting results. The first small arc at the high and medium frequency intercepts of the spectrum is related to the contributions of the Pt wire and bulk electrolyte resistance, which is represented as R0 and (R1 -CPE1 ) circuits. The second semicircle is associated with the electrode polarization resistance at the low frequency range. With the fitting results, the (R2 -CPE2 ) circuit is for the oxide ion transfer through the electrode/electrolyte interface and both (R3 -CPE3 ) and (R4 -CPE4 ) are attributed to an electron charge transfer in the typical range at the interface, which is in good agreement with the previous literature [26,27].
Fig. 4. Cross-sectional (a) low magnification TEM image, (b) high resolution TEM image, and (c) STEM (Z-contrast) image of the PBCO/GDC nanolayered structure grown on the STO substrate at 750 ◦ C.
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Fig. 8. Cross-sectional SEM images of the anode supported single cell with the PBCO/GDC interlayer after the power measurement. Fig. 6. Area specific resistance (ASR) of the symmetric cells with and without the VAN PBCO/GDC structure grown by PLD.
To compare the area specific resistance (ASR) of symmetric cells without and with the 150 nm interlayer, the ASR of the cathode was [(R =R +R +R )×(Area)] calculated by the following equation:ASR = p 2 3 2 4 The calculation was based on the AC impedance data under air in the temperature range of 350–650 ◦ C. As shown in Fig. 6, the ASR of the cell with the VAN interlayer shows a lower value in comparison to the results obtained for the cell without the interlayer. For example, the cell without the interlayer has a higher ASR (2.5 cm2 ) than the one with the interlayer (2.1 cm2 ) at 400 ◦ C. Therefore, the VAN PBCO/GDC interlayer incorporation into the PBCO cathode can affect the total cathode polarization with increasing effective area of the TPBs. The interlayer also improves the catalytic reaction probability at the gas–cathode–electrolyte TPBs. Accordingly, a decrease of the ASR of the cathode could lead to a better performance in TFSOFCs. An anode-supported single cell with the PBCO/GDC interlayer was measured for the I–V characteristic at the temperatures of 650–750 ◦ C. Fig. 7 presents the cell voltage and power density as 1.2
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a function of the current density by a two-electrode set-up under humidified hydrogen at the rate of 100 mL min−1 , using Pt wires as electrical contacts on both sides of the cell. An open circuit voltages (OCV) of 0.88 V was obtained at 750 ◦ C by implementing the bilayer GDC/YSZ. Adding a thin YSZ can suppress ceria reduction at the anode side and improve the power output based on our previous work [28]. The general trend is that the cell voltage drops with respect to increasing current density at each temperature. The maximum power density of the cell with the PBCO/GDC interlayer is 151, 263, and 350 (highlighted as red arrow) mW cm−2 at 650, 700, and 750 ◦ C, respectively. A single cell without the interlayer was also prepared and measured as comparison. The power density of the cell is 268 mW cm−2 at 750 ◦ C. The improvement in the cell performance is largely because of the increased interface area by the interlayer structure. Besides, it is worth to note that concentration polarization in the I–V curves in Fig. 7 is observed under a current drain of ∼0.8 A/cm2 which is possibly related to the limited fuel supply due to the low porosity in the anode side. The true maximum power density of the cell at 750 ◦ C could be higher under the optimized porosity in the anode side. Cross-sectional SEM image in Fig. 8 shows the anode-supported single cell after the power measurements at high temperatures. The microstructures of PBCO/GDC interlayer and YSZ/GDC bilayer can be clearly observed as ∼1 and ∼ 6 m, respectively. The bilayer electrolyte shows a fully dense structure with an obvious interface between the YSZ and GDC layers in the cell. The PBCO cathode and NiO–GDC anode exhibit a porous structure with uniform pore distribution. It is evident that there is no crack or delamination issue observed at the interfaces among the cathode, interlayer, and electrolyte thin film layers. This suggests that the PBCO/GDC interlayer acts as an effective transition layer and improve the adhesion properties between the cathode and electrolyte. 4. Conclusions The VAN structures of the PBCO/GDC interlayer between the cathode and electrolyte for thin film solid oxide fuel cells (TFSOFCs) were prepared by PLD. The PBCO/GDC thin film interlayer shows either the vertically ordered columnar structure (600 ◦ C) or the nanolayered structure (750 ◦ C). In both cases, the nanocomposite films have high quality epitaxial growth. The symmetric cell with the VAN PBCO/GDC interlayer shows a lower ASR of 2.1 cm2 at 400 ◦ C compared to the cell without the interlayer (2.5 cm2 ).
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The single cell with the PBCO/GDC interlayer shows an OCV of 0.9 V at 750 ◦ C and maximum power densities of 151, 263, and 350 mW cm−2 at 650, 700, and 750 ◦ C, respectively. Overall the VAN PBCO/GDC interlayer acts as an effective transition layer between the cathode and electrolyte, enhancing the adhesion properties in between the layers, and thus leading to improved overall cell performance. Acknowledgements The work was in part funded by the National Science Foundation (NSF 0846504 and NSF 1007969), the Air Force Office of Scientific Research (Contract No.: FA9550-07-1-0108 and FA955009-1-0114), and DOE Center for Integrated Nanotechnology (CINT). The work at the University of Texas at Austin was funded by the Welch Foundation grant F-1254. References [1] N.P. Brandon, S. Skinner, B.C.H. Steele, Annu. Rev. Mater. Res. 33 (2003) 183. [2] C. Zuo, S. Zha, M. Liu, M. Hatano, M. Uchiyama, Adv. Mater. 18 (2006) 3318. [3] M. Liu, D. Dong, L. Chen, J. Gao, X. Liu, G. Meng, J. Power Sources 176 (2008) 107. [4] E. Boehm, J.-M. Bassat, M.C. Steil, P. Dordor, F. Mauvy, J.-C. Grenier, Solid State Sci. 5 (2003) 973. [5] Z. Tao, L. Bi, L. Yan, W. Sun, Z. Zhu, R. Peng, W. Liu, Electrochem. Commun. 11 (2009) 688. [6] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27. [7] Z. Shao, S.M. Haile, Nature 431 (2004) 170.
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