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Cobalt-doped BaZrO3: A single phase air electrode material for reversible solid oxide cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 5 2 2 e1 2 5 2 7
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Cobalt-doped BaZrO3: A single phase air electrode material for reversible solid oxide cells Yuanyuan Rao a, Shenghong Zhong a, Fei He a, Zhenbin Wang a, Ranran Peng a,*, Yalin Lu a,b,** a
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China b Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, PR China
article info
abstract
Article history:
BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) are prepared and identified as single phase
Received 29 March 2012
air electrode materials for reversible solid oxide cells. BZC-x shows a typical perovskite
Received in revised form
structure with x 0.4, and slight BaCoO3 second phase is observed with x ¼ 0.5. Conduc-
30 April 2012
tivity measurements suggest that BZC-x is an oxygen ion and electron mixed conductor in
Accepted 4 May 2012
dry oxygen atmosphere. The electronic conductivity of BZC-x increases when increasing Co
Available online 27 June 2012
content in BZC-x specimen, while the ionic conductivity reduces. For BaZr0.6Co0.4O3d, the electronic and ionic conductivity are 5.24 S cm1 and 1.20 103 S cm1, respectively,
Keywords:
measured at 700 C in dry oxygen. With BZC-0.4 as a single phase air electrode, the dis-
Air electrodes
charging and electrolysis current densities of a single cell are 299 mA cm2 (at 0.7 V) and
Protoneelectron mixed conductor
935 mA cm2 (at 1.5 V), respectively, measured at 700 C. The polarization resistance of
Proton conducting electrolytes
cells using this new air electrode is only 0.19 U cm2, approximately 65% lower than that
Reversible solid oxide cells
using a traditional Sm0.5Sr0.5CoO3d/BaCe0.5Zr0.3Y0.2O3d composite air electrode.
Solid oxide electrolysis cells
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Solid oxide fuel cells
1.
Introduction
One unavoidable difficulty in using solar cells and windmills as power generators is that both sunlight and wind are neither continuous nor steady, which may cause fluctuations harmful to standard electrical grids [1,2]. Reversible solid oxide cells (RSOCs) could provide an effective means to minimize such fluctuations. They can work as the solid oxide electrolysis cells (SOECs) to store excess electrical energy as chemical energy (as hydrogen) and as the solid oxide fuel cells (SOFCs) to release electrical energy when necessary [3]. Comparing to traditional oxygen ion conducting electrolytes, RSOCs with
reserved.
proton conducting electrolytes (H-RSOCs) have drawn an increasing attention for their unique characteristics [4,5], including a relatively lower bulk resistance with the proton conducting electrolytes [6], a lower concentration polarization resistance in the traditional hydrogen electrodeesupported configuration [7], and the ability to generate pure hydrogen at the hydrogen electrode [3]. The polarization resistances of air electrode have been identified as the main resistance source of H-RSOC working in both SOFC mode and SOEC mode [8]. Till now, the air electrode materials of H-RSOC were almost the same as those used for conventional proton conducting SOFCs (H-SOFC), of which
* Corresponding author. Tel./fax: þ86 551 3600594. ** Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China. Tel./fax: þ86 551 3603004. E-mail addresses: [email protected] (R. Peng), [email protected] (Y. Lu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 5 2 2 e1 2 5 2 7
composites cathodes constituted of a proton conductor and an oxygen ioneelectron mixed conductor were found to present low polarization resistances [9e14]. For example, Fabbri et al. [9] used La0.6Sr0.4Fe0.8Co0.2O3d (LSCF)/ BaCe0.9Yb0.1O3d (BCYb) as a composite air electrode for HSOFC, and a polarization resistance for the cells of approximately 0.14 U cm2 at 700 C was achieved. Wu et al. [10] used BaCe0.8Sm0.2O3d (BCS)/Sm0.5Sr0.5CoO3d (SSC) as composite cathode for SOFC, and a minimum interfacial polarization resistance of 0.21 U cm2 at 700 C was achieved. Unfortunately, a larger polarization resistance was observed using composite air electrode in H-RSOC. He et al. [3] used a BaCe0.5Zr0.3Y0.2O3d (BCZY)/SSC as air electrode, and the cell polarization resistances were 0.38 and 0.41 U cm2 working in SOFC and SOEC mode, respectively, measured at 700 C with 30% H2O-70% air injected into air electrode. The large polarization resistance of composite cathode might result from the low oxygen partial pressure in air electrode of H-RSOC. Exploring new air electrode materials with an improved electrochemical reaction rate at air electrodes will be the key to further improve electroperformance of a H-RSOC using a thin film electrolyte [3]. In this work, a type of new perovskite oxides, BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5), were reported and identified as single phase air electrode materials for HRSOCs. The electron and ion conducting properties of these new materials were systematically studied as a function of the Co content. Performances of single cells using such BZC-x as the single phase air electrodes were characterized and compared with that using conventional Sm0.5Sr0.5CoO3d (SSC)/BaCe0.5Zr0.3Y0.2O3d (BCZY) composite air electrodes.
2.
Experimental section
2.1.
Powders preparation
BaZr1xCoxO3d (x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) powders were prepared by a citric acidenitrate process. Stoichiometric amounts of Ba(NO3)2, Zr(NO3)4$5H2O and Co(NO3)2$6H2O were dissolved in deionized water to form a nitrate solution. Citric acid was then added to the solution with a mole ratio of citric acid:Ba2þ of 3:1. The precursor solution was then heated on a hot plate, which resulted in grey powders after fully polymerization and ignition. The powders were fired at 1100 C for 2 h to remove the carbon residues and to form the perovskite phase. BaZr1xCoxO3d were abbreviated as BZC-x, where the number x after BZC denoted the mole ratio of Co3þ in B sites. For example, BaZr0.9Co0.1O3d is abbreviated as BZC-0.1. Structural characterizations of BZC-x powders were carried out by powder X-ray diffraction (XRD) using a Philips X’pert PROS diffractometer with Cu-Ka radiation at room temperature.
2.2.
Conductivity measurements
The total conductivity and ionic-only conductivity were measured using a four-probe direct current (DC) technique and an electron blocking method, respectively. For the total conductivity measurements, BZC-x powders were cold pressed into a rectangular bar of 38.04 mm in length, 6.38 mm
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in width and 2.16 mm in height, and then sintered at 1300 C for 5 h. Four silver wires were wound tightly around the sintered bars as current collector, in which two outer electrodes 27.22 mm apart supplied the current and two inner electrodes 17.20 mm apart measured the potential drop. The conductivity measurements were carried out in a tube furnace with temperature changing from 400 to 700 C in oxygen atmospheres. For the ionic-only conductivity measurement, BZC-x powders were cold pressed into 1.55 mm-thick pellets, and then sintered at 1300 C for 5 h Zr0.85Y0.15O3d (YSZ) was chosen as the electronic barrier layer because the electronic conductivity of doped BaZrO3 cannot be ignored for BZC-x samples [15]. The resistances of YSZ pellets, approximately 0.86 mm in thickness and 11.6 mm in diameter after sintered at 1400 C for 5 h, were measured by the DC technique. YSZ pellets were then adhered to the sintered BZC-x pellets by Pt paste to overcome the interface resistance [16]. Pt paste was also applied onto the outer surface of the adhered YSZ and BZC-x samples as current collectors. Glass powders mixed with water were painted onto the edge of the sandwich samples and heated at 750 C for 1 h to form a glass seal layer to prevent oxygen leakage at the edges of the samples. The ionic-only conductivity measurements were done with a HewlettePackard multimeter (34401A) in the range of 400e700 C in the same temperature range discussed above.
2.3.
Fabrication and testing of cells
The hydrogen electrode-supported cells were prepared by a co-pressing method, with BaCe0.5Zr0.3Y0.2O3d (BZCY) as the electrolyte and a NiO/BZCY (65:35 in weights) mixture as the hydrogen electrode, where BCZY and NiO powders were prepared by the citric method [3] and glycine nitrate process [17], respectively. The thickness of the dense BCZY electrolyte films was approximately 20 mm after sintered at 1400 C for 5 h in air. BZC-x powders mixed with ethocel and abietyl alcohol were screen-printed onto the BCZY electrolyte surface and fired at 1100 C for 2 h in air to form the air electrode and to complete the cell fabrication. The thickness and the area of the BZC-x air electrode were 30 mm and 23.76 mm2, respectively. Two silver wires were connected to each electrode using silver paste as current leads. The single cells were sealed onto a stainless steel tube and tested with a home-developed cell-testing system. Hydrogen and 30% H2O-70% air mixture were introduced into the hydrogen electrode and the air electrode, respectively, with the flow rates of H2 and dry air at 70 and 100 mL min1. The fuel cell performances were measured using an electrochemical workstation (IM6e, Zahner) at 700 C.
3.
Results and discussion
3.1.
Phase identification
Fig. 1 shows the XRD spectra of BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) powders fired at 1100 C for 2 h. With x 0.4, BZC-x shows a typical perovskite structure (PDF No.74-1299).
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Fig. 1 e XRD patterns of BZC-x (x [ 0.1, 0.2, 0.3, 0.4, 0.5) powders calcined at 1100 C for 2 h.
A secondary phase of BaCoO3 was identified in the XRD pattern for BZC-0.5. This result indicates that BaZrO3 has an almost five times higher Co ion incorporation capability than ˚ )eCo3þ BaCeO3 because of the closer radius of Zr4þ (0.72 A ˚ (0.53 A) [18]. The lattice parameters of BZC-x are simulated as ˚ for x ¼ 0.1, 0.2, 0.3 and 0.4 4.1922, 4.1846, 4.1642 and 4.1577 A using GSAS software [19].
3.2.
Conductivities of BZC-x
Fig. 2 shows the total conductivity of BZC-x samples measured in dry oxygen gas using a four-probe direct current technique. The conductivities of BZC-x increase with both testing temperature and the Co content. For BZC-0.4, the total conductivities are 2.75, 3.95 and 5.24 S cm1 at 500, 600 and 700 C, respectively. The activation energies for BZC-x are in the range of 0.28e0.34 eV. These low activation energies suggest that the electronic conductivity should be the major conductivity contributor.
Fig. 2 e Temperature dependence of the total conductivity for BZC-x samples measured in dry oxygen atmosphere.
Fig. 3 shows the ionic conductivities of the BZC-x samples measured in the dry oxygen atmosphere. The ionic conductivities increase with the increase of testing temperature, but reduce with the increase of Co content. The ion conductivities of BZC-0.4 are 8.91 105, 3.46 104 and 1.20 103 S cm1 measured at 500, 600, and 700 C, respectively, approximately 3e5 orders of magnitude lower than their own total conductivities (mainly the electronic conductivities). For BZC-0.1, the ionic conductivity is 3.10 103 S cm1 at 700 C, approximately one order of magnitude lower than its total conductivity. The simulated activation energies of these ionic conductivities are approximately 0.80e0.91 eV. These large activation energies suggest that the high ion conductivities of BZC-x might come from the diffusion of oxygen vacancies. The ionic and electron conductivities of these new materials in dry oxidant atmosphere could be explained as Equations (1)e(3) in the Kro¨gereVink notation: BaZrO3
Co2 O3 ! 2Co0Zr þ VO þ 3O O
(1)
2Co0Zr þ V O þ 1=2O2 /2CoZr þ OO
(2)
0 Co Zr /CoZr þ h
(3)
where oxygen vacancies ðVO Þ and electron holes ðh Þ could be created in order to maintain the electro-neutrality requirement in both conditions when Co3þ is introduced into the lattice and the incorporation of oxygen [20]. With the increase of Co concentration, more oxygen vacancies will be formed as shown in equation (1), while the oxygen catalytic dissociation would be faster at the same time which results in a easier incorporation of oxygen and thus less oxygen vacancies. Therefore an increased electronic conduction but reduced oxygen ion conduction was achieved with the increase of Co content.
3.3. Electrochemical performance of the cells with BZC-x as air electrodes Electro-performance of the cells using BZC-x as air electrodes is shown in Fig. 4, where the positive current density refers to operating in the SOFC mode and the negative current density
Fig. 3 e Temperature dependence of ionic conductivity for BZC-x samples measured in dry oxygen atmosphere.
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Fig. 4 e IeV curves of reversible solid oxide cells (RSOCs) measured at 700 C with different air electrodes. The measurements were conducted with 30% H2O-70% air and H2 as the reactant gas.
refers to the SOEC mode. Measured at 700 C, the open circuit voltages (OCVs) are all approximately 0.95 V, independent of the Co content in the air electrodes. In the SOEC mode, the current density initially increases with the Co content (x) in BZC-x and reaches a maximum at x ¼ 0.4. Measured at 1.5 V, the current densities were 419, 620, 935, and 823 mA cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. The area specific resistances (ASRs) of single cells simulated from IeV curves in the SOEC mode were approximately 1.17, 0.70, 0.53, and 0.60 U cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. In the SOFC mode, the maximum current density is also achieved with x ¼ 0.4. The discharging current densities are approximately 87, 162, 299, and 187 mA cm2 measured at 0.7 V with x ¼ 0.2, 0.3, 0.4, and 0.5. The simulated ASRs in the SOFC mode are 2.18, 1.43, 0.86, and 0.98 U cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively, much larger than those measured in the SOEC mode. Fig. 5(a, b, c, d) shows impedance spectra of the cells with BZC-x air electrodes measured at 700 C under an open circuit condition (OCC). Since the bi-layer of anodes and electrolytes were fabricated using identical procedures, the difference in impedance spectra should result from the different conducting behaviour of the air electrode materials. As shown in Fig. 5, both the bulk resistance (Rb) and the interfacial polarization resistance (Rp) reduce with the increase of Co content, and reach a minimum with x ¼ 0.4, the value of which are 0.59 and 0.19 U cm2, respectively. The low polarization resistance suggest a high electrochemical rate with BZC-0.4 air electrode. Two depressed arcs could be observed for each impedance spectrum, as shown in Fig. 5, implying two rate-limiting steps. An equivalent circuit comprising two RQ elements and one R in series, Rb(RHQH)(RLQL), is proposed to analyse these spectra, where RH and RL represent high- and low-frequency polarization resistances, and QH and QL represent high- and lowfrequency constant phase elements, respectively. For BZC0.4, the simulated RH and RL are 0.08 and 0.11 U cm2, respectively, at 700 C. Arrhenius plots of RH and RL for BZC-0.4 are shown in Fig. 6. The activation energies of RH and RL are 0.55 and 0.93 eV, respectively, which are close to those of proton
Fig. 5 e Impedance spectra of H-RSOCs measured in open circuit mode at 700 C with a) BZC-0.2; b) BZC-0.3; c) BZC0.4; d) BZC-0.5 as air electrode.
conduction and oxygen ion migration [14,21]. This result suggests that transports of both protons and oxygen ions are still the rate-limiting steps. This might be the right reason for the relatively low electrochemical rates with BZC-0.5. It should be noted that the BZC0.1e0.3 have higher proton conductivities while larger polarization resistances. This might result from the insufficient catalytic activity of BZC-x with low Co content which is also a key factor to evaluate an electrode.
Fig. 6 e Arrhenius plots of RH (solid) and RL (open) measured in open circuit condition with BZC-0.4 as air electrode.
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The impedance spectra in different operating modes are shown in Fig. 7 with BZC-0.4 as air electrode. The bulk resistance of cells in both SOFC and SOEC operating modes is 0.59 U cm2, which is the same as that measured under OCC. However, the interfacial polarization resistance differs significantly in the two operating modes. The polarization resistances of cells are 0.04 and 0.30 U cm2 operating in SOEC and SOFC mode, respectively. The large difference of Rp in the two operating modes should result from their different ratelimiting steps. Previous studies on the reaction mechanism at air electrodes have shown that the rate-limiting steps in the SOFC mode were the transferring of both the protons and the dissociated oxygen ions to the triple-phase boundaries (TPBs) [21], and in the SOEC mode was the transferring of the generated protons to the TPBs and into the electrolytes at last [3]. The extremely low polarization resistance in SOEC mode suggests a much improved proton conduction with the new air electrode, which might be formed as equation (4). Another reason for the improved proton production might result from a much improved large volume ratio (w60%) of the single phase air electrode. The low volume ratio (w30%) of the proton conducting component in a composite air electrode usually make the proton conductivity in such composite-structured air electrodes 2e3 orders of magnitude lower than their intrinsic proton conductivity [22]. In the SOFC mode, the improvement in the reaction rate might not be sufficient for the relatively low diffusion of oxygen ions of this new mixed conductor in wet atmosphere [3,21]. Additionally, the diffusion of oxygen gas, corresponding to an additional low-frequency arc, has also been observed in the SOFC mode, which enlarges the polarization resistance. This observation is consistent with the simulation results in ref. [8] that the air electrode concentration loss was higher for an SOFC with proton conducting electrolyte due to the existence of steam. Modification of electrode’s microstructure should be an effective way to intensively reduce the polarization resistances. V O þ H2 O þ OO /2OHO
Fig. 8 e Stability test of H-RSOCs with BZC-0.4 air electrode measured at 700 C under SOEC mode (1.3 V).
and polarization resistance of the cells with SSC/BCZY electrodes is 0.52 and 0.55 U cm2. Compared with that of the SSC/ BCZY composite electrode, the polarization resistance using the new BZC-0.4 air electrode is approximately 65% lower, suggesting a much faster electrode reaction when using the new single phase air electrode. The electro-performance of cell using SSC/BCZY as air electrodes is shown in Fig. 4. Its electrolysis current density at 1.5 V is 620 mA cm2, approximately 34% lower than that with the BZC-0.4 air electrode. Its discharging current density is 261 mA cm2 at 0.7 V, about 38 mA cm2 less. The stability of BZC-0.4 air electrode is shown in Fig. 8. Obvious drop has not been detected in the tested time range. The high electroperformance of RSOCs with BZC-0.4 air electrode greatly suggests that proton-electron mixed electrode should be a promising single phase air electrode for RSOCs.
(4)
The impedance spectra of a cell with a SSC/BCZY air electrode are also shown in Fig. 7 for comparison. Both anode and electrolyte of the cell with SSC/BCZY air electrode were fabricated with the same procedure for that with BZC-0.4 air electrode. Under the OCC mode (shown in Fig. 7(b)), the bulk
4.
Conclusions
Co-doped BaZrO3 (BZC-x) samples were proposed as a promising single phase air electrode for reversible solid oxide cells. Conductivity measurements suggest that BZC-x were oxygen ion and electron mixed conductors in dry oxygen atmosphere. The electronic and ionic conductivities of BaZr0.6Co0.4O3d were 5.24 S cm1 and 1.20 103 S cm1, respectively, measured at 700 C in dry oxygen. The polarization resistance of RSOC using the new BaZr0.6Co0.4O3d air electrode is only 0.19 U cm2 measured at 700 C, approximately 65% lower than that using a traditional Sm0.5Sr0.5CoO3d/BaCe0.5Zr0.3Y0.2O3d composite electrode.
Acknowledgements Fig. 7 e Impedance spectra of the H-RSOCs with BZC-0.4 (-) and SSC-BZCY (,) as air electrode measured at 700 C in: a) SOFC mode (0.7 V); b) Open circuit mode; and c) SOEC mode (1.3 V).
This work was financially supported by the Natural Science Foundation of China (51072193), National Basic Research Program of China (973 Program, 2012CB922001), and by the Fundamental Research Funds for the Central Universities.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 5 2 2 e1 2 5 2 7
references
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[12] Liu Bin, Chen Xiaobo, Dong Yonglai, Mao Samuel S, Cheng Mojie. A high-performance, nanostructured Ba0.5Sr0.5Co0.8Fe0.2O3d cathode for solid oxide fuel cells. Adv Energy Mater 2011;10:343e6. [13] Taillades G, Dailly J, Taillades-Jacquin M, Mauvy F, Essouhmi A, Marrony M, et al. Intermediate temperature anode-supported fuel cell based on BaCe0.9Y0.1O3d electrolyte with novel Pr2NiO4 cathode. Fuel Cells 2010;10:166e73. [14] Wu Tianzhi, Zhao Yingqi, Peng Ranran, Xia Changrong. Nano-sized Sm0.5Sr0.5CoO3d as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9. Electrochim Acta 2009;54:4888e92. [15] Babilo Peter, Hailew Sossina M. Enhanced sintering of yttrium-doped barium zirconate by addition of ZnO. J Am Ceram Soc 2005;88:2362e8. [16] Li Xue, Zhao Hailei, Gao Feng, Zhu Zhiming, Chen Ning, Shen Wei. Synthesis and electrical properties of Co-doped Y0.08Sr0.92TiO3d as a potential SOFC anode. Solid State Ionics 2008;179:1588e92. [17] Ranran Peng, Yan Wu, Lizhai Yang, Zongqiang Mao. Electrochemical properties of intermediate-temperature SOFCs based on proton conducting Sm-doped BaCeO3 electrolyte thin film. Solid State Ionics 2006;177:389e93. [18] Yang Lei, Wang Shizhong, Lou Xiaoyuan, Liu Meilin. Electrical conductivity and electrochemical performance of cobalt-doped BaZr0.1Ce0.7Y0.2O3d cathode. Int J Hydrogen Energy 2011;36:2266e70. [19] Larson AC, Von Dreele RB. General structure analysis system (GSAS). Los Alamos National Laboratory Report LAUR; 1994. 86e748. [20] Hjalmarsson Per, Søgaard Martin, Mogensen Mogens. Electrochemical performance and degradation of (La0.6Sr0.4)0.99CoO3d as porous SOFC-cathode. Solid State Ionics 2008;179:1422e6. [21] He Fei, Wu Tianzhi, Peng Ranran, Xia Changrong. Cathode reaction models and performance analysis of Sm0.5Sr0.5CoO3deBaCe0.8Sm0.2O3d composite cathode for solid oxide fuel cells with proton conducting electrolyte. J Power Sources 2009;194:263e8. [22] McLachlan David S, Blaszkiewicz Michael, Newnham Robert E. Electrical resistivity of composites. J Am Ceram Soc 1990; 73:2187e203.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Cobalt-doped BaZrO3: A single phase air electrode material for reversible solid oxide cells Yuanyuan Rao a, Shenghong Zhong a, Fei He a, Zhenbin Wang a, Ranran Peng a,*, Yalin Lu a,b,** a
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China b Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, PR China
article info
abstract
Article history:
BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) are prepared and identified as single phase
Received 29 March 2012
air electrode materials for reversible solid oxide cells. BZC-x shows a typical perovskite
Received in revised form
structure with x 0.4, and slight BaCoO3 second phase is observed with x ¼ 0.5. Conduc-
30 April 2012
tivity measurements suggest that BZC-x is an oxygen ion and electron mixed conductor in
Accepted 4 May 2012
dry oxygen atmosphere. The electronic conductivity of BZC-x increases when increasing Co
Available online 27 June 2012
content in BZC-x specimen, while the ionic conductivity reduces. For BaZr0.6Co0.4O3d, the electronic and ionic conductivity are 5.24 S cm1 and 1.20 103 S cm1, respectively,
Keywords:
measured at 700 C in dry oxygen. With BZC-0.4 as a single phase air electrode, the dis-
Air electrodes
charging and electrolysis current densities of a single cell are 299 mA cm2 (at 0.7 V) and
Protoneelectron mixed conductor
935 mA cm2 (at 1.5 V), respectively, measured at 700 C. The polarization resistance of
Proton conducting electrolytes
cells using this new air electrode is only 0.19 U cm2, approximately 65% lower than that
Reversible solid oxide cells
using a traditional Sm0.5Sr0.5CoO3d/BaCe0.5Zr0.3Y0.2O3d composite air electrode.
Solid oxide electrolysis cells
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Solid oxide fuel cells
1.
Introduction
One unavoidable difficulty in using solar cells and windmills as power generators is that both sunlight and wind are neither continuous nor steady, which may cause fluctuations harmful to standard electrical grids [1,2]. Reversible solid oxide cells (RSOCs) could provide an effective means to minimize such fluctuations. They can work as the solid oxide electrolysis cells (SOECs) to store excess electrical energy as chemical energy (as hydrogen) and as the solid oxide fuel cells (SOFCs) to release electrical energy when necessary [3]. Comparing to traditional oxygen ion conducting electrolytes, RSOCs with
reserved.
proton conducting electrolytes (H-RSOCs) have drawn an increasing attention for their unique characteristics [4,5], including a relatively lower bulk resistance with the proton conducting electrolytes [6], a lower concentration polarization resistance in the traditional hydrogen electrodeesupported configuration [7], and the ability to generate pure hydrogen at the hydrogen electrode [3]. The polarization resistances of air electrode have been identified as the main resistance source of H-RSOC working in both SOFC mode and SOEC mode [8]. Till now, the air electrode materials of H-RSOC were almost the same as those used for conventional proton conducting SOFCs (H-SOFC), of which
* Corresponding author. Tel./fax: þ86 551 3600594. ** Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China. Tel./fax: þ86 551 3603004. E-mail addresses: [email protected] (R. Peng), [email protected] (Y. Lu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.022
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composites cathodes constituted of a proton conductor and an oxygen ioneelectron mixed conductor were found to present low polarization resistances [9e14]. For example, Fabbri et al. [9] used La0.6Sr0.4Fe0.8Co0.2O3d (LSCF)/ BaCe0.9Yb0.1O3d (BCYb) as a composite air electrode for HSOFC, and a polarization resistance for the cells of approximately 0.14 U cm2 at 700 C was achieved. Wu et al. [10] used BaCe0.8Sm0.2O3d (BCS)/Sm0.5Sr0.5CoO3d (SSC) as composite cathode for SOFC, and a minimum interfacial polarization resistance of 0.21 U cm2 at 700 C was achieved. Unfortunately, a larger polarization resistance was observed using composite air electrode in H-RSOC. He et al. [3] used a BaCe0.5Zr0.3Y0.2O3d (BCZY)/SSC as air electrode, and the cell polarization resistances were 0.38 and 0.41 U cm2 working in SOFC and SOEC mode, respectively, measured at 700 C with 30% H2O-70% air injected into air electrode. The large polarization resistance of composite cathode might result from the low oxygen partial pressure in air electrode of H-RSOC. Exploring new air electrode materials with an improved electrochemical reaction rate at air electrodes will be the key to further improve electroperformance of a H-RSOC using a thin film electrolyte [3]. In this work, a type of new perovskite oxides, BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5), were reported and identified as single phase air electrode materials for HRSOCs. The electron and ion conducting properties of these new materials were systematically studied as a function of the Co content. Performances of single cells using such BZC-x as the single phase air electrodes were characterized and compared with that using conventional Sm0.5Sr0.5CoO3d (SSC)/BaCe0.5Zr0.3Y0.2O3d (BCZY) composite air electrodes.
2.
Experimental section
2.1.
Powders preparation
BaZr1xCoxO3d (x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) powders were prepared by a citric acidenitrate process. Stoichiometric amounts of Ba(NO3)2, Zr(NO3)4$5H2O and Co(NO3)2$6H2O were dissolved in deionized water to form a nitrate solution. Citric acid was then added to the solution with a mole ratio of citric acid:Ba2þ of 3:1. The precursor solution was then heated on a hot plate, which resulted in grey powders after fully polymerization and ignition. The powders were fired at 1100 C for 2 h to remove the carbon residues and to form the perovskite phase. BaZr1xCoxO3d were abbreviated as BZC-x, where the number x after BZC denoted the mole ratio of Co3þ in B sites. For example, BaZr0.9Co0.1O3d is abbreviated as BZC-0.1. Structural characterizations of BZC-x powders were carried out by powder X-ray diffraction (XRD) using a Philips X’pert PROS diffractometer with Cu-Ka radiation at room temperature.
2.2.
Conductivity measurements
The total conductivity and ionic-only conductivity were measured using a four-probe direct current (DC) technique and an electron blocking method, respectively. For the total conductivity measurements, BZC-x powders were cold pressed into a rectangular bar of 38.04 mm in length, 6.38 mm
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in width and 2.16 mm in height, and then sintered at 1300 C for 5 h. Four silver wires were wound tightly around the sintered bars as current collector, in which two outer electrodes 27.22 mm apart supplied the current and two inner electrodes 17.20 mm apart measured the potential drop. The conductivity measurements were carried out in a tube furnace with temperature changing from 400 to 700 C in oxygen atmospheres. For the ionic-only conductivity measurement, BZC-x powders were cold pressed into 1.55 mm-thick pellets, and then sintered at 1300 C for 5 h Zr0.85Y0.15O3d (YSZ) was chosen as the electronic barrier layer because the electronic conductivity of doped BaZrO3 cannot be ignored for BZC-x samples [15]. The resistances of YSZ pellets, approximately 0.86 mm in thickness and 11.6 mm in diameter after sintered at 1400 C for 5 h, were measured by the DC technique. YSZ pellets were then adhered to the sintered BZC-x pellets by Pt paste to overcome the interface resistance [16]. Pt paste was also applied onto the outer surface of the adhered YSZ and BZC-x samples as current collectors. Glass powders mixed with water were painted onto the edge of the sandwich samples and heated at 750 C for 1 h to form a glass seal layer to prevent oxygen leakage at the edges of the samples. The ionic-only conductivity measurements were done with a HewlettePackard multimeter (34401A) in the range of 400e700 C in the same temperature range discussed above.
2.3.
Fabrication and testing of cells
The hydrogen electrode-supported cells were prepared by a co-pressing method, with BaCe0.5Zr0.3Y0.2O3d (BZCY) as the electrolyte and a NiO/BZCY (65:35 in weights) mixture as the hydrogen electrode, where BCZY and NiO powders were prepared by the citric method [3] and glycine nitrate process [17], respectively. The thickness of the dense BCZY electrolyte films was approximately 20 mm after sintered at 1400 C for 5 h in air. BZC-x powders mixed with ethocel and abietyl alcohol were screen-printed onto the BCZY electrolyte surface and fired at 1100 C for 2 h in air to form the air electrode and to complete the cell fabrication. The thickness and the area of the BZC-x air electrode were 30 mm and 23.76 mm2, respectively. Two silver wires were connected to each electrode using silver paste as current leads. The single cells were sealed onto a stainless steel tube and tested with a home-developed cell-testing system. Hydrogen and 30% H2O-70% air mixture were introduced into the hydrogen electrode and the air electrode, respectively, with the flow rates of H2 and dry air at 70 and 100 mL min1. The fuel cell performances were measured using an electrochemical workstation (IM6e, Zahner) at 700 C.
3.
Results and discussion
3.1.
Phase identification
Fig. 1 shows the XRD spectra of BaZr1xCoxO3d (BZC-x, x ¼ 0.1, 0.2, 0.3, 0.4, 0.5) powders fired at 1100 C for 2 h. With x 0.4, BZC-x shows a typical perovskite structure (PDF No.74-1299).
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Fig. 1 e XRD patterns of BZC-x (x [ 0.1, 0.2, 0.3, 0.4, 0.5) powders calcined at 1100 C for 2 h.
A secondary phase of BaCoO3 was identified in the XRD pattern for BZC-0.5. This result indicates that BaZrO3 has an almost five times higher Co ion incorporation capability than ˚ )eCo3þ BaCeO3 because of the closer radius of Zr4þ (0.72 A ˚ (0.53 A) [18]. The lattice parameters of BZC-x are simulated as ˚ for x ¼ 0.1, 0.2, 0.3 and 0.4 4.1922, 4.1846, 4.1642 and 4.1577 A using GSAS software [19].
3.2.
Conductivities of BZC-x
Fig. 2 shows the total conductivity of BZC-x samples measured in dry oxygen gas using a four-probe direct current technique. The conductivities of BZC-x increase with both testing temperature and the Co content. For BZC-0.4, the total conductivities are 2.75, 3.95 and 5.24 S cm1 at 500, 600 and 700 C, respectively. The activation energies for BZC-x are in the range of 0.28e0.34 eV. These low activation energies suggest that the electronic conductivity should be the major conductivity contributor.
Fig. 2 e Temperature dependence of the total conductivity for BZC-x samples measured in dry oxygen atmosphere.
Fig. 3 shows the ionic conductivities of the BZC-x samples measured in the dry oxygen atmosphere. The ionic conductivities increase with the increase of testing temperature, but reduce with the increase of Co content. The ion conductivities of BZC-0.4 are 8.91 105, 3.46 104 and 1.20 103 S cm1 measured at 500, 600, and 700 C, respectively, approximately 3e5 orders of magnitude lower than their own total conductivities (mainly the electronic conductivities). For BZC-0.1, the ionic conductivity is 3.10 103 S cm1 at 700 C, approximately one order of magnitude lower than its total conductivity. The simulated activation energies of these ionic conductivities are approximately 0.80e0.91 eV. These large activation energies suggest that the high ion conductivities of BZC-x might come from the diffusion of oxygen vacancies. The ionic and electron conductivities of these new materials in dry oxidant atmosphere could be explained as Equations (1)e(3) in the Kro¨gereVink notation: BaZrO3
Co2 O3 ! 2Co0Zr þ VO þ 3O O
(1)
2Co0Zr þ V O þ 1=2O2 /2CoZr þ OO
(2)
0 Co Zr /CoZr þ h
(3)
where oxygen vacancies ðVO Þ and electron holes ðh Þ could be created in order to maintain the electro-neutrality requirement in both conditions when Co3þ is introduced into the lattice and the incorporation of oxygen [20]. With the increase of Co concentration, more oxygen vacancies will be formed as shown in equation (1), while the oxygen catalytic dissociation would be faster at the same time which results in a easier incorporation of oxygen and thus less oxygen vacancies. Therefore an increased electronic conduction but reduced oxygen ion conduction was achieved with the increase of Co content.
3.3. Electrochemical performance of the cells with BZC-x as air electrodes Electro-performance of the cells using BZC-x as air electrodes is shown in Fig. 4, where the positive current density refers to operating in the SOFC mode and the negative current density
Fig. 3 e Temperature dependence of ionic conductivity for BZC-x samples measured in dry oxygen atmosphere.
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Fig. 4 e IeV curves of reversible solid oxide cells (RSOCs) measured at 700 C with different air electrodes. The measurements were conducted with 30% H2O-70% air and H2 as the reactant gas.
refers to the SOEC mode. Measured at 700 C, the open circuit voltages (OCVs) are all approximately 0.95 V, independent of the Co content in the air electrodes. In the SOEC mode, the current density initially increases with the Co content (x) in BZC-x and reaches a maximum at x ¼ 0.4. Measured at 1.5 V, the current densities were 419, 620, 935, and 823 mA cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. The area specific resistances (ASRs) of single cells simulated from IeV curves in the SOEC mode were approximately 1.17, 0.70, 0.53, and 0.60 U cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. In the SOFC mode, the maximum current density is also achieved with x ¼ 0.4. The discharging current densities are approximately 87, 162, 299, and 187 mA cm2 measured at 0.7 V with x ¼ 0.2, 0.3, 0.4, and 0.5. The simulated ASRs in the SOFC mode are 2.18, 1.43, 0.86, and 0.98 U cm2 with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively, much larger than those measured in the SOEC mode. Fig. 5(a, b, c, d) shows impedance spectra of the cells with BZC-x air electrodes measured at 700 C under an open circuit condition (OCC). Since the bi-layer of anodes and electrolytes were fabricated using identical procedures, the difference in impedance spectra should result from the different conducting behaviour of the air electrode materials. As shown in Fig. 5, both the bulk resistance (Rb) and the interfacial polarization resistance (Rp) reduce with the increase of Co content, and reach a minimum with x ¼ 0.4, the value of which are 0.59 and 0.19 U cm2, respectively. The low polarization resistance suggest a high electrochemical rate with BZC-0.4 air electrode. Two depressed arcs could be observed for each impedance spectrum, as shown in Fig. 5, implying two rate-limiting steps. An equivalent circuit comprising two RQ elements and one R in series, Rb(RHQH)(RLQL), is proposed to analyse these spectra, where RH and RL represent high- and low-frequency polarization resistances, and QH and QL represent high- and lowfrequency constant phase elements, respectively. For BZC0.4, the simulated RH and RL are 0.08 and 0.11 U cm2, respectively, at 700 C. Arrhenius plots of RH and RL for BZC-0.4 are shown in Fig. 6. The activation energies of RH and RL are 0.55 and 0.93 eV, respectively, which are close to those of proton
Fig. 5 e Impedance spectra of H-RSOCs measured in open circuit mode at 700 C with a) BZC-0.2; b) BZC-0.3; c) BZC0.4; d) BZC-0.5 as air electrode.
conduction and oxygen ion migration [14,21]. This result suggests that transports of both protons and oxygen ions are still the rate-limiting steps. This might be the right reason for the relatively low electrochemical rates with BZC-0.5. It should be noted that the BZC0.1e0.3 have higher proton conductivities while larger polarization resistances. This might result from the insufficient catalytic activity of BZC-x with low Co content which is also a key factor to evaluate an electrode.
Fig. 6 e Arrhenius plots of RH (solid) and RL (open) measured in open circuit condition with BZC-0.4 as air electrode.
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The impedance spectra in different operating modes are shown in Fig. 7 with BZC-0.4 as air electrode. The bulk resistance of cells in both SOFC and SOEC operating modes is 0.59 U cm2, which is the same as that measured under OCC. However, the interfacial polarization resistance differs significantly in the two operating modes. The polarization resistances of cells are 0.04 and 0.30 U cm2 operating in SOEC and SOFC mode, respectively. The large difference of Rp in the two operating modes should result from their different ratelimiting steps. Previous studies on the reaction mechanism at air electrodes have shown that the rate-limiting steps in the SOFC mode were the transferring of both the protons and the dissociated oxygen ions to the triple-phase boundaries (TPBs) [21], and in the SOEC mode was the transferring of the generated protons to the TPBs and into the electrolytes at last [3]. The extremely low polarization resistance in SOEC mode suggests a much improved proton conduction with the new air electrode, which might be formed as equation (4). Another reason for the improved proton production might result from a much improved large volume ratio (w60%) of the single phase air electrode. The low volume ratio (w30%) of the proton conducting component in a composite air electrode usually make the proton conductivity in such composite-structured air electrodes 2e3 orders of magnitude lower than their intrinsic proton conductivity [22]. In the SOFC mode, the improvement in the reaction rate might not be sufficient for the relatively low diffusion of oxygen ions of this new mixed conductor in wet atmosphere [3,21]. Additionally, the diffusion of oxygen gas, corresponding to an additional low-frequency arc, has also been observed in the SOFC mode, which enlarges the polarization resistance. This observation is consistent with the simulation results in ref. [8] that the air electrode concentration loss was higher for an SOFC with proton conducting electrolyte due to the existence of steam. Modification of electrode’s microstructure should be an effective way to intensively reduce the polarization resistances. V O þ H2 O þ OO /2OHO
Fig. 8 e Stability test of H-RSOCs with BZC-0.4 air electrode measured at 700 C under SOEC mode (1.3 V).
and polarization resistance of the cells with SSC/BCZY electrodes is 0.52 and 0.55 U cm2. Compared with that of the SSC/ BCZY composite electrode, the polarization resistance using the new BZC-0.4 air electrode is approximately 65% lower, suggesting a much faster electrode reaction when using the new single phase air electrode. The electro-performance of cell using SSC/BCZY as air electrodes is shown in Fig. 4. Its electrolysis current density at 1.5 V is 620 mA cm2, approximately 34% lower than that with the BZC-0.4 air electrode. Its discharging current density is 261 mA cm2 at 0.7 V, about 38 mA cm2 less. The stability of BZC-0.4 air electrode is shown in Fig. 8. Obvious drop has not been detected in the tested time range. The high electroperformance of RSOCs with BZC-0.4 air electrode greatly suggests that proton-electron mixed electrode should be a promising single phase air electrode for RSOCs.
(4)
The impedance spectra of a cell with a SSC/BCZY air electrode are also shown in Fig. 7 for comparison. Both anode and electrolyte of the cell with SSC/BCZY air electrode were fabricated with the same procedure for that with BZC-0.4 air electrode. Under the OCC mode (shown in Fig. 7(b)), the bulk
4.
Conclusions
Co-doped BaZrO3 (BZC-x) samples were proposed as a promising single phase air electrode for reversible solid oxide cells. Conductivity measurements suggest that BZC-x were oxygen ion and electron mixed conductors in dry oxygen atmosphere. The electronic and ionic conductivities of BaZr0.6Co0.4O3d were 5.24 S cm1 and 1.20 103 S cm1, respectively, measured at 700 C in dry oxygen. The polarization resistance of RSOC using the new BaZr0.6Co0.4O3d air electrode is only 0.19 U cm2 measured at 700 C, approximately 65% lower than that using a traditional Sm0.5Sr0.5CoO3d/BaCe0.5Zr0.3Y0.2O3d composite electrode.
Acknowledgements Fig. 7 e Impedance spectra of the H-RSOCs with BZC-0.4 (-) and SSC-BZCY (,) as air electrode measured at 700 C in: a) SOFC mode (0.7 V); b) Open circuit mode; and c) SOEC mode (1.3 V).
This work was financially supported by the Natural Science Foundation of China (51072193), National Basic Research Program of China (973 Program, 2012CB922001), and by the Fundamental Research Funds for the Central Universities.
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