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Characterization of PrBa0.92CoCuO6−δ as a potential cathode material of intermediate-temperature solid oxide fuel cell
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Characterization of PrBa0.92CoCuO6¡d as a potential cathode material of intermediate-temperature solid oxide fuel cell Xuening Jiang a,*, Jiao Wang a, Guoqiang Jia a, Zijian Qie a, Yuchao Shi a, Asim Idrees a, Qingyu Zhang a, Lei Jiang b a
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China b Dalian Institute of Chemical and Physics, CAS, Dalian, 116023, China
article info
abstract
Article history:
PrBa0.92CoCuO6d (PB0.92CoCu), a layered perovskite oxide with Ba2þ-deficiency at A-site
Received 26 September 2016
and Cu2þ doping at B-site was synthesized and characterized as cathode material of
Received in revised form
intermediate-temperature solid oxide fuel cell (IT-SOFC) in comparison with performance
12 December 2016
of the parent oxide of PrBaCo2O6d (PBCoO). PB0.92CoCu had the same phase structure as
Accepted 15 December 2016
that of PBCoO but showed a slight lattice expansion. Results of iodometric titration and
Available online xxx
thermogravimetry (TG) measurements demonstrated that PB0.92CoCu had a higher concentration of oxygen vacancy than PBCoO did. Electrical conductivities of PB0.92CoCu
Keywords:
ranged from 255 S cm1 at 400 C to 134 S cm1 at 800 C in air, meeting the requirement for
Solid oxide fuel cell
the cathode of SOFCs. Thermal expansion coefficient (TEC) of PB0.92CoCu was about one
Cathode
quarter decreased compared with TEC of PBCoO. PB0.92CoCu showed improved electro-
Point defects
chemical performance than PBCoO characterized by low area specific resistances (ASRs)
Thermal expansion coefficient
ranging from 0.12 U cm2 at 600 C to 0.017 U cm2 at 750 C. High peak power densities,
Electrochemical performance
1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C and 930 mW cm2 at 700 C were achieved in a single cell using the PB0.92CoCu cathode. This single cell also showed an operational stability using hydrogen fuel at 650 C without any decay for 100 h. These results have demonstrated that PB0.92CoCu is a promising cathode material of IT-SOFCs. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) that work at intermediatetemperatures (IT) of 600e800 C have been intensively studied in recent years with the aim of reducing fabrication and working cost, enhancing structural durability and expanding practical application fields of SOFCs [1e5]. Lowering the
working temperature, however, is unfortunately accompanied with degradation of cell performance due to increased resistances from the component materials of cathode, electrolyte and anode. Particularly, polarization resistance of the cathode increases dramatically with the lowering temperature because oxygen reduction reaction (ORR) occurring over it has a relatively high activation energy and thus becomes the
* Corresponding author. Fax: þ86 41184708389. E-mail address: [email protected] (X. Jiang). http://dx.doi.org/10.1016/j.ijhydene.2016.12.076 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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dominant contribution to the performance degradation of the IT-SOFC [1e7]. Therefore, as a cathode material of IT-SOFCs, it must firstly have high catalytic activity for ORR (i.e. low polarization resistances). Furthermore, since the SOFCs are usually fabricated by multiple steps of high-temperature calcination and a long lifetime is required, the cathode materials must also have proper thermal expansion coefficients (TECs) that match with TECs of the commonly used electrolyte materials of Ce0.9Gd0.1O1.95 (GDC), Ce0.8Sm0.2O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3d (LSGM) in order to avoid structural cracking in the cells [1e7]. Double-layered perovskite oxides with the general formula of LnBaCo2O5þd (Ln ¼ lanthanide element) have recently been investigated as promising cathode materials for IT-SOFCs because of their high oxygen surface exchange coefficient, superior oxygen ion bulk diffusivity, and the resultant high ORR catalysis [6e14]. Among this series of perovskite oxides, PrBaCo2O5þd (PBCoO) has the highest surface exchange coefficient and bulk diffusion coefficient [6,9,10]; therefore, it has been demonstrated to have better electrochemical performance than the other double-layered perovskites as IT-SOFC cathode materials [10,13,14]. However, as a characteristic of cobalt-based perovskite oxides, PBCoO also has the problem of high TEC value that mismatches with TECs of the electrolyte materials. Besides, PBCoO has high ORR catalytic activity at temperatures above 700 C, but its polarization resistances at the lower temperatures (<700 C) are still too high to meet the requirement of resistance (<0.15 U cm2) for a cathode material [15]. Therefore, lowering the TEC value and improving lowtemperature electrochemical performance are key issues for practical application of PBCoO as cathode material of ITSOFCs. For improving overall performance of the PBCoO cathode, various strategies have been pursued such as A-site/B-site doping and fabrication of composite cathodes [16e27]. It's known that high TEC value of cobalt-based perovskite oxides is caused by reduction of B-site Conþ (n ¼ 3, 4) ions and low/ high spin transition of Co3þ [1,6,16e20]. Therefore, substitution of Conþ ions with other relatively stable transition-metal ions such as Fe, Cu and Ni can effectively lower the TEC values [17e20], which, however, has no significant effect on improving electrochemical performance and sometimes the electrochemical performance even shows a downtrend with the substitution due to the less catalytic reactivity of the valence stable transition-metal ions. Zhao et al. [17] reported that TEC of PrBaCo2xFexO5þd was ~20% decreased at Fe doping content of x ¼ 1.5 while its ASR values increased with the higher Fe doping content. TEC of PrBaCoCuO5þd with B-site Cu2þ doping significantly decreased from 24.1 106 K1 down to 15.2 106 K1, better matching with TECs of the electrolytes, but the ASR value was as high as 0.207 U cm2 at 600 C [18]. A-site cationic doping is another way of improving performance of perovskite oxides [21e23]. Park et al. [21] studied effect of A-site Sr2þ doping on performance of PBCoO cathode and the minimum ASR values were observed at the Sr2þ doping content of x ¼ 0.5 and 0.75 in PrBa1xSrxCo2O5þd (x ¼ 0, 0.25, 0.5, 0.75, and 1.0). Fu et al. [22] found that A-site Ca2þ doping led to a slightly lower TEC value for PBCoO but its polarization resistance increased with the higher Ca2þ doping
content. In recent years, introduction of A-site cationic deficiency has proved to be an effective way of enhancing ORR catalytic activities of the LnBaCo2O5þd double-layered perovskite oxides [25e32]. Systematic studies were made in our group on structures and properties of the PrBa1xCo2O6d oxides with various Ba2þ-deficiency contents (x ¼ 0.00e0.10) at A-sites [25]. Among the samples, the oxide with Ba2þ deficiency content of x ¼ 0.08 showed the best electrochemical performance characterized by 50% decrease in polarization resistance at 600 C as compared with the parent oxide [25]. In work of Dong et al. [30] on PrBa1xCo2O5þd (x ¼ 0.00, 0.05, 0.10) cathodes, the best electrochemical performance was obtained at Ba2þ deficiency content of x ¼ 0.10 and the improved performance was ascribed to enhanced oxygen surface exchange and bulky diffusion due to increase in concentration of oxygen vacancies. However, TEC values of the oxides were only slightly lowered due to the A-site cationic deficiency [25,32]. It has been proposed and experimentally demonstrated that co-substitution at A/B sites, rather than the aforementioned or other A-site or B-site sole substitution, can lead to synergistic effects with respect to electrochemical properties and TEC-related thermal stability of the cathode materials [33e35]. For example, PrBa0.5Sr0.5Co2xFexO5þd with co-doping of Sr2þ on A-site and Fe3þ on B-site showed fast oxygen ion diffusion and rapid surface oxygen exchange while maintained compatibility with the electrolytes for IT-SOFCs and the durability under operating conditions [33]. Considering the aforementioned effects of A-site Ba2þ deficiency and B-site Cu2þ doping on improving electrochemical performance and lowering TEC respectively [18,25], their synergistic effects are expected to improve overall performance of the perovskite oxides. Therefore, with the aim of further improving performance of the PBCoO cathode, a combined strategy, both A-site Ba2þ-deficiency and B-site Cu2þ-doping, was adopted for PBCoO. As an example, the PrBa0.92CoCuO6d (PB0.92CoCu) oxide with 8 mol% Ba2þ-deficiency at A-site and 50 mol% Cu2þdoping at B-site was firstly studied. It was evaluated as cathode material of IT-SOFC with respect to chemical stability, electrical conductivities, thermal expansion behavior and electrochemical performance. As expected, PB0.92CoCu showed better performance than the parent oxide of PBCoO characterized by much lower ASR values (i.e. enhanced ORR catalytic activity), smaller TEC with improved matching with electrolytes and high peak power densities of the single cell. Oxygen content and point defects of PB0.92CoCu and PBCoO were measured and compared for deep understanding of the performance improvement.
Experimental PB0.92CoCu was synthesized by a combined EDTA-citrate complexing solegel method [36]. Briefly, the reagents of Pr(NO3)3$6H2O (AR), Ba(NO3)2 (AR), Co(NO3)3$6H2O (AR) and Cu(NO3)2$6H2O (AR) at stoichiometric proportions of PB0.92CoCu were firstly dissolved into EDTA-NH3$H2O solution (pH z 6) to form an aqueous solution, and then citric acidNH3$H2O solution (pH z 6) was added at a mole ratio of 1:1:2 for EDTA: total metal ions: citric acid. The mixed solution was subsequently heated at 80 C to obtain a dark dry foam
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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structure, which was then decomposed on a hot plate to make the precursors followed by calcination in air at 600 C for 6 h and 950 C for 3 h in sequence. The PBCoO oxide was synthesized with the similar process except that the final calcination temperature was increased to 1050 C for 3 h. The PB0.92CoCu powders were mixed with GDC electrolyte in a 50/ 50 weight ratio followed by calcination at 950 C for 12 h in air for study of their chemical compatibility. A symmetrical cell with the configuration of PB0.92CoCu/GDC/PB0.92CoCu was fabricated for electrochemical impedance spectra (EIS) studies. The PB0.92CoCu ink, prepared by mixing the cathode powders with a-terpineol and ethyl cellulose, was screenprinted onto both sides of the GDC pellet, followed by calcination at 950 C for 2.5 h in air. The obtained cathodes have the active electrode areas of 0.283 cm2. An anode-supported single cell of PB0.92CoCu/GDC/YSZ/NiO-YSZ was fabricated. The ~400 mm thick NiO-YSZ (1:1 weight ratio) anode was tape cast as the support and then a ~6 mm thick dense 8YSZ layer and ~4 mm thick GDC electrolyte buffer layer were subsequently screen-printed onto the support substrate followed by sintering at 1200 C. The PB0.92CoCu paste was screen-printed onto the center of the GDC layer and calcined at 950 C for 2.5 h to function as the cathode layer. Phase structures of the as-synthesized PB0.92CoCu, PBCoO and the calcined PB0.92CoCu-GDC mixed powders were characterized by X-ray diffraction measurement (XRD, Rigaku D/ Max 2400). Oxygen contents of PB0.92CoCu and PBCoO were determined by iodometric titrations at room temperature with blank tests and five parallel analyses to reduce the measurement error. Thermogravimetric Analysis (TGA, Netzsch TG209F3) of the samples was carried out in the temperature range of 50e1000 C in air with heating rate of 5 C/min to check their thermal-driven oxygen releasing behaviors. Thermal expansion data of PB0.92CoCu and PBCoO were collected with a dilatometer (Netzsch DIL 402PC) at 30e900 C in air with a heating rate of 5 C/min. Electrical conductivities of both samples were measured at 400e800 C in air by a DC four-electrode method. Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiostat was used for EIS measurements of the symmetrical cells under open circuit voltage (OCV) condition at 600e800 C in air. IeV polarization curves were measured for assessing performance of the anode-supported single cell of PB0.92CoCu/GDC/YSZ/NiYSZ within the temperature range of 650e800 C using the Solartron 1287 potentiostat. The anode side and cathode side were supplied with H2 fuel and pressed air respectively at a flow rate of 60 mL min1.
Results and discussion Phase structure and chemical reactivity with electrolyte Fig. 1 shows XRD patterns of the as-synthesized PB0.92CoCu and PBCoO powders as well as the PB0.92CoCu-GDC mixture calcined at 950 C for 12 h in air. It is observed that PB0.92CoCu and PBCoO oxides have the diffraction peaks that match well, indicating that they have the same phase structures that were indexed with an orthorhombic Pmmm space group. The lattice parameters of PB0.92CoCu were calculated to be
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Fig. 1 e XRD patterns of the as-synthesized PB0.92CoCu and PBCoO powders as well as the PB0.92CoCu-GDC mixture calcined at 950 C for 12 h in air.
a ¼ 3.936 Å, b ¼ 3.916 Å, c ¼ 7.699 Å and cell volume of 118.67 Å3, while the results of PBCoO are a ¼ 3.906 Å, b ¼ 3.929 Å, c ¼ 7.650 Å and cell volume of 117.40 Å3. The doubled c-axis (c z 2a z 2b) for both oxides indicates that they have the same double-layered perovskite structure but PB0.92CoCu shows slight lattice expansion as compared with PBCoO. It was found in our previous work [25] that Ba2þ deficiency at A-sites of PBCoO led to a slight structural shrinkage of the oxide. In contrast, substitution of B-site cobalt ions with Cu2þ ions was reported [37] to cause a structural expansion of the oxides due to the larger ionic radius of Cu2þ (r ¼ 73 pm) than the Conþ ions (rCo4þ ¼ 53 pm, rCo3þ ¼ 54.5 pm, rCo2þ ¼ 65 pm). Therefore, effect of Cu2þ substitution seems to be dominant in structure of PB0.92CoCu. It is also observed in Fig. 1 that no additional diffraction peaks indicative of impurity phases were observed in the XRD pattern of the calcined PB0.92CoCu-GDC mixture, thus no chemical reaction occurred between PB0.92CoCu and GDC at 950 C in air. Since pure phase of PB0.92CoCu was obtained at 950 C and it is decomposed at the higher temperatures as shown by the following TG result, it's not necessary to check its chemical reactivity with GDC at temperatures above 950 C.
Oxygen contents and point defects Electrical and electrochemical properties of the perovskitetyped cathode materials of SOFC are intrinsically determined by concentration and distribution of point defects of oxygen vacancies (Vo ) and Co4þ (h , charge carriers of p-type semiconductors) [2,17,26,33]. These point defects change with the A-site/B-site cations in the oxides as well as the measurement temperature and atmosphere. To get a deep understanding of properties of the PB0.92CoCu cathode, oxygen contents (6d) and average valence (n) of the Conþ ions were measured by iodometric titration method at room tempera ture, and then content of Vo and distribution of Co4þ/Co3þ ions in PB0.92CoCu and PBCoO were calculated based on electricneutrality of oxides and the results were listed in Table 1. PB0.92CoCu has lower oxygen content (5.153) than the parent oxide of PBCoO (5.71), thus its content of Vo (0.847) is much higher than that of PBCoO (0.29). In both oxides, the average
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Table 1 e Lattice parameters, oxygen content and point defects of PB0.92CoCu and PBCoO oxides. Samples
PB0.92CoCu PBCoO
Space group
Pmmm Pmmm
Cell volume (Å3)
118.67 117.40
Lattice parameters (Å) a
b
c
3.936 3.906
3.916 3.929
7.699 7.650
6d
Vo
n
[Co4þ]
[Co3þ]
5.153 5.71
0.847 0.29
3.466 3.21
0.932 0.42
1.068 1.58
Calculated assuming Cu exists as Cu2þ as in Ref. [37].
valence of the Conþ ions is n > 3, indicating coexistence of Co4þ and Co3þ ions; however, the larger n value of PB0.92CoCu suggested more Co4þ and less Co3þ ions than in PBCoO. These results have indicated that PB0.92CoCu has different point defects from PBCoO. More negative charges were introduced by the A-site Ba2þ-deficiency in PB0.92CoCu, which needed more positive charges, Vo and h (Co4þ) for the charge compensation. Besides, the Cu2þ cations needed less O2 ions for charge compensation than the Conþ (n > 3) cations with the higher valence, which could also lead to the less oxygen content and more Vo in PB0.92CoCu. It is known that lattice oxygen in the perovskite oxides can release at high temperatures and lead to changes in the point defects. To get such knowledge, thermogravimetric analysis (TGA) was carried out for PB0.92CoCu and PBCoO oxides at temperatures of 50e1000 C in air and the results were shown in Fig. 2. Different TG curves were observed for the two oxides. At the low temperatures, below 252 C for PBCoO and below 311 C for PB0.92CoCu, a slight weight gain was observed for both oxides while the change in PB0.92CoCu was more obvious. Similar phenomena were also observed in other PBCoOrelated oxides at the similar temperatures [30,38], which was ascribed to the process of oxygen surface absorption followed by oxygen bulky diffusion. The more distinct weight gain in PB0.92CoCu can be reasonably interpreted based on this mechanism. Firstly, it was reported [39] that B-site Cu2þ doping could promote oxygen surface adsorption over the perovskite oxides. Secondly, PB0.92CoCu had more Vo than PBCoO (Table 1), and a higher concentration of Vo was advantageous for oxygen surface adsorption and bulky diffusion in the oxide [2]. These two factors have probably contributed to the more evident weight gain at the low temperature in PB0.92CoCu. With the increasing temperatures, a gradual weight loss was observed in both oxides due to thermaldriven releasing of lattice oxygen while an abrupt weight
Fig. 2 e TG curves of PB0.92CoCu and PBCoO oxides measured at 50e1000 C in air.
loss occurred at the temperature above 960 C in PB0.92CoCu, which could be caused by decomposition or melting of the sample. Therefore, the PB0.92CoCu cathode layer should be calcined at temperatures lower than 960 C during fabrication of SOFCs. Based on the iodometric titration results (Table 1) of the assynthesized PB0.92CoCu and PBCoO oxides and the TGA results shown in Fig. 2, temperature-dependence of oxygen content (6d), Vo concentration and average valence (n) of Conþ ions were calculated in the temperature range of 50e800 C and the results were shown in Fig. 3. Decrease ratio of oxygen content t 100%, where a0 is the oxygen content of was defined as a0aa 0 the sample at the room temperature and at is the value at a specific temperature. It was found that the decrease ratio of oxygen content up to 800 C in air was 10.22% for PB0.92CoCu, larger than that of PBCoO (7.60%), indicating that the lattice oxygens were easier to be released in PB0.92CoCu probably due to the weaker CueO bonds than the CoeO bonds [40]. In company with the changes in oxygen content, more Vo were generated in PB0.92CoCu than in PBCoO (Fig. 3b). As shown in Fig. 3c, the average valence (n) of Conþ ions decreased from 3 < n < 4 down to 2 < n < 3 with the increasing temperature for both oxides, suggesting sequent reduction of Co4þ to Co3þ and Co2þ ions with releasing of the lattice oxygen. The change was more complicated in PB0.92CoCu because the average valence of Conþ ions varied with both the A-site Ba2þ-deficiency and the B-site Cu2þ-doping. It was also found that the n value of PB0.92CoCu became smaller than that of PBCoO at the temperatures above ~350 C, indicating that more Co4þ ions were reduced in PB0.92CoCu due to releasing of more lattice oxygens.
Electrical conductivity It's known that cobalt-containing perovskite oxides are mixed ionic and electronic conductors (MIECs) with Vo and h as the respective charge carriers [2,10,17,20,26]. In these MIECs, the electronic conductivity is usually several orders of magnitude larger than the oxygen ionic conductivity; therefore the total electrical conductivities are dominated by the electronic conduction [2,6,10]. As shown in Fig. 4, the conductivity values of PB0.92CoCu range from 255 S cm1 at 400 C to 134 S cm1 at 800 C in air, meeting the requirement of conductivity for the cathode of SOFCs (>100 S cm1) [41]. These values are smaller than the conductivities of PBCoO but are comparable to the values of PrBaCoCuO5þd [18]. Decreased conductivities due to the B-site Cu2þ-doping have also been found in other perovskite oxides [37]. The following factors have probably contributed to the lower conductivities of PB0.92CoCu. As is known, electronic conduction in the MIECs occurs by a Zerner
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Fig. 3 e Temperature-dependence of oxygen content (6¡d) (a), content of Vo (b) and average valence (n) of Conþ ions (c) for PB0.92CoCu and PBCoO oxides at 50e800 C in air.
of Vo in the oxide can break the BeOeB bonds, inhibit the electronic hopping and decrease electronic conductivity [2,28], which could lead to the lower electrical conductivities of PB0.92CoCu as well since it had more Vo than PBCoO did (Fig. 3b).
Thermal expansion coefficients
Fig. 4 e Electrical conductivities of PB0.92CoCu and PBCoO oxides measured at various temperatures in air.
double exchange mechanism, i.e. the electronic hopping occurs along the eBnþeO2-eB(nþ1)þe pathway that is formed by multi-valenced B-site ions (like Conþ with n ¼ 4, 3 and 2) and the lattice oxygen, thus the electrical conductivity is determined by both concentration and mobility of the charger carrier h [2,42]. As discussed above (Fig. 3c), PB0.92CoCu has less h than PBCoO does in the temperature range of 400e800 C, which could lead to the decreasing conductivity. Moreover, Zeng et al. [42] proposed that, existence of the B-site ions with fixed valences would act as blocks for electronic conduction, decrease mobility of the charge carriers and thus decrease electrical conductivities in the oxides. This was probably another factor that contributed to the lower electrical conductivities of PB0.92CoCu considering the relatively stable Cu2þ ions compared to the Conþ ions. Besides, existence
Fig. 5 shows thermal expansion curves of PB0.92CoCu and PBCoO oxides measured in the temperature range of 30e900 C in air. An inflection was observed at ~300 C in each curve, which was probably caused by reduction of Co4þ to Co3þ ions due to thermal-driven releasing of the lattice oxygen, as found in other cobalt-containing perovskite oxides [10,18,25]. The average TEC value of PB0.92CoCu at 30e900 C is
Fig. 5 e Thermal expansion curves and average TEC values of PB0.92CoCu and PBCoO oxides measured at 30e900 C in air.
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18 106/K. It is 25% smaller than TEC of PBCoO (24 106/K), demonstrating that the combined strategy of A-site Ba2þdeficiency and B-site Cu2þ-doping has decreased the TEC value as expected. The TEC value of PB0.92CoCu is also smaller than TEC of PrBa0.92Co2O5þd with sole A-site Ba2þ-deficiency (TEC ¼ 21 106/K) [25] but is still bigger than that of with the pure B-site Cu2þ-doping PrBaCoCuO5þd 6 (TEC ¼ 15.2 10 /K) [18]. This suggested that the A-site Ba2þdeficiency and B-site Cu2þ-doping had exerted a synergistic effect on the thermal expansion behavior of PB0.92CoCu, which could probably be associated with the changes in oxygen content and point defects (Table 1). Significantly, the TEC of PB0.92CoCu is much smaller than TECs of other A-site or B-site doped PBCoO such as PrBaCoFeO5þd (TEC ¼ 25.3 106/K) [17], PrBaCo1.5Sc0.5O5þd (TEC ¼ 20.5 106/K) [20] and the PBCoOSDC composite cathode (TEC ¼ 20.1 106/K) [14], suggesting that PB0.92CoCu has better TEC matching with electrolyte materials of SOFCs as compared to the above related materials.
Electrochemical performance of the cathode Electrochemical performance of PB0.92CoCu cathode was investigated with AC-impedance spectra method at various temperatures in air using the GDC-based symmetrical cells and the obtained results in comparison with the results of PBCoO measured at the same conditions were shown in Fig. 6. The ohmic resistances arising from the GDC electrolyte and lead wires were normalized to zero for clarity. Area specific resistances (ASRs) at various temperatures of the PB0.92CoCu and PBCoO cathodes were calculated from the impedance spectra and were shown in Arrhenius plot in Fig. 7. The ASRs of PB0.92CoCu are 0.12 U cm2 at 600 C, 0.059 U cm2 at 650 C, 0.032 U cm2 at 700 C and 0.017 U cm2 at 750 C respectively. These values are smaller than ASRs of PBCoO that range from 0.182 U cm2 at 600 C to 0.021 U cm2 at 750 C. In particular, 34% decrease in ASR of 600 C has been obtained in PB0.92CoCu, demonstrating enhanced ORR catalytic activity of PB0.92CoCu at the low temperature, which is consistent with its smaller reaction activation energy (Ea ¼ 1.04 eV) than that of PBCoO (Ea ¼ 1.21 eV) calculated from the Arrhenius plots. The ASR values of PB0.92CoCu are also much smaller than ASRs of many PBCoO-related cathodes such as A-site/B-site doped oxides of
Fig. 7 e Arrhenius plots of ASR for PB0.92CoCu and PBCoO cathodes with respective activation energy (Ea). PrBaCoFeO5þd (0.105 U cm2 at 700 C) [17], PrBaCuCoO5þd (0.207 U cm2 at 600 C) [18], PrBa0.5Sr0.5Co2O5þd (0.23 U cm2 at 650 C) [23], and the composite cathode of PBCoO-SDC (0.073 U cm2 at 700 C) [14]. Although possibly different microstructures of the cathodes made by different groups can influence the cathode reaction, the ASR value provides a metric of the cathode performance, which can be considered as a level of performance that can be achieved at minimum with the possibility that a higher performance can be attained if the electrode microstructure is optimized [2,10,17,33,43]. Particularly for the cathodes of PB0.92CoCu and PBCoO in this work, this influence can be neglected because we have ensured that the two cathode layers have the similar microstructures (Fig. 8). Concentration and distribution of oxygen vacancies (Vo ) in the perovskite oxides is one more important intrinsic factor that dominates electrochemical performance of the cathodes because Vo can promote oxygen transport kinetics in the oxides and therefore enhance the ORR catalytic activities [2,6,9,25,44]. As shown in Fig. 3b, PB0.92CoCu has much more Vo than PBCoO does, which should be the main contribution to its improved electrochemical performance.
Performance of single-cell In order to further evaluate PB0.92CoCu as a cathode material of IT-SOFC, an anode-supported single cell of PB0.92CoCu/GDC/
Fig. 6 e Nyquist EIS plots of symmetrical cells of PB0.92CoCu/GDC/PB0.92CoCu and PBCoO/GDC/PBCoO measured at various temperatures in air. Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Fig. 8 e Cross-sectional SEM images with inserted surface SEM images of the cathode layers in the symmetrical cells of PB0.92CoCu/GDC/PB0.92CoCu and PBCoO/GDC/PBCoO respectively.
Fig. 9 e (a) Cross-sectional SEM image with inserted image of the cathode surface of the PB0.92CoCu/GDC/YSZ/Ni-YSZ single cell; (b) IeV and IeP polarization curves of the single cell. YSZ/Ni-YSZ was characterized. This single cell was fabricated based on a GDC/YSZ electrolyte double-layer (Fig. 9a), in which the dense YSZ thin film (~6 mm thick) was used as an electronic blocking layer to minimize internal loss and increase opencircuit voltage (OCV) of the cell [2,7,45] and the GDC buffer layer (~4 mm thick) was used to provide high oxygen ionic conduction at 600e800 C and also to avoid chemical reaction between YSZ and the cathode [7,46]. Porous structured cathode and anode layers were prepared for facile in/out gas diffusion. As shown in Fig. 9b, OCVs of the single cell reached up to ~1.1 eV at the measurement temperatures and high peak power densities, 1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C, 930 mW cm2 at 700 C and 632 mW cm2 at 650 C respectively were achieved. These results indicated that good performance of the single cell could be obtained using the PB0.92CoCu cathode. To determine working stability of the single cell with the PB0.92CoCu cathode, the single cell of PB0.92CoCu/GDC/YSZ/NiYSZ was first polarized under a constant current density of 350 mA cm2 at 650 C for 20 h by operating on hydrogen fuel until stable performance was obtained, and then the stability tests were conducted. Fig. 10 showed time dependence of voltage of the single cell under the current density of 350 mA cm2 at 650 C. It was found that the single cell maintained a stable voltage at around 0.7 V throughout a test period of 100 h, demonstrating performance stability of the single cell with the PB0.92CoCu cathode.
Fig. 10 e The time dependence of the voltage of the PB0.92CoCu/GDC/YSZ/Ni-YSZ single cell under a specific current density of 350 mA cm¡2 at 650 C.
Conclusions In this work, PB0.92CoCu was synthesized and characterized as cathode material of IT-SOFCs in comparison with the results of the parent oxide of PBCoO. PB0.92CoCu had the same doublelayered perovskite structure with PBCoO but showed a slight structural expansion. Lattice oxygen was easier to be released in PB0.92CoCu, leading to its lower oxygen content (more oxygen vacancies) than in PBCoO. Electrical conductivities of
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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PB0.92CoCu were smaller than the results of PBCoO but satisfied the conductivity requirement for a SOFC cathode. The average TEC value of PB0.92CoCu was 25% smaller than TEC of PBCoO with better TEC matching with electrolytes. ASRs of PB0.92CoCu ranged from 0.12 U cm2 at 600 C to 0.017 U cm2 at 750 C, which were much smaller than ASRs of PBCoO and some related cathodes, indicating highly enhanced ORR catalytic activity of PB0.92CoCu. High peak power densities, 1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C and 930 mW cm2 at 700 C were obtained with the single cell using PB0.92CoCu as the cathode. Working stability of the single cell was also demonstrated. These results have demonstrated that the combined strategy of A-site Ba2þ deficiency and B-site Cu2þ-doping has improved the overall performance of PB0.92CoCu, which is therefore a promising cathode material for IT-SOFCs.
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Acknowledgements This work was financially supported by the “Liaoning Provincial Natural Science Foundation of China (2013020106)” and “The Fundamental Research Funds for the Central Universities (DUT15LAB08)”.
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Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
Available online at www.sciencedirect.com
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Characterization of PrBa0.92CoCuO6¡d as a potential cathode material of intermediate-temperature solid oxide fuel cell Xuening Jiang a,*, Jiao Wang a, Guoqiang Jia a, Zijian Qie a, Yuchao Shi a, Asim Idrees a, Qingyu Zhang a, Lei Jiang b a
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China b Dalian Institute of Chemical and Physics, CAS, Dalian, 116023, China
article info
abstract
Article history:
PrBa0.92CoCuO6d (PB0.92CoCu), a layered perovskite oxide with Ba2þ-deficiency at A-site
Received 26 September 2016
and Cu2þ doping at B-site was synthesized and characterized as cathode material of
Received in revised form
intermediate-temperature solid oxide fuel cell (IT-SOFC) in comparison with performance
12 December 2016
of the parent oxide of PrBaCo2O6d (PBCoO). PB0.92CoCu had the same phase structure as
Accepted 15 December 2016
that of PBCoO but showed a slight lattice expansion. Results of iodometric titration and
Available online xxx
thermogravimetry (TG) measurements demonstrated that PB0.92CoCu had a higher concentration of oxygen vacancy than PBCoO did. Electrical conductivities of PB0.92CoCu
Keywords:
ranged from 255 S cm1 at 400 C to 134 S cm1 at 800 C in air, meeting the requirement for
Solid oxide fuel cell
the cathode of SOFCs. Thermal expansion coefficient (TEC) of PB0.92CoCu was about one
Cathode
quarter decreased compared with TEC of PBCoO. PB0.92CoCu showed improved electro-
Point defects
chemical performance than PBCoO characterized by low area specific resistances (ASRs)
Thermal expansion coefficient
ranging from 0.12 U cm2 at 600 C to 0.017 U cm2 at 750 C. High peak power densities,
Electrochemical performance
1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C and 930 mW cm2 at 700 C were achieved in a single cell using the PB0.92CoCu cathode. This single cell also showed an operational stability using hydrogen fuel at 650 C without any decay for 100 h. These results have demonstrated that PB0.92CoCu is a promising cathode material of IT-SOFCs. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) that work at intermediatetemperatures (IT) of 600e800 C have been intensively studied in recent years with the aim of reducing fabrication and working cost, enhancing structural durability and expanding practical application fields of SOFCs [1e5]. Lowering the
working temperature, however, is unfortunately accompanied with degradation of cell performance due to increased resistances from the component materials of cathode, electrolyte and anode. Particularly, polarization resistance of the cathode increases dramatically with the lowering temperature because oxygen reduction reaction (ORR) occurring over it has a relatively high activation energy and thus becomes the
* Corresponding author. Fax: þ86 41184708389. E-mail address: [email protected] (X. Jiang). http://dx.doi.org/10.1016/j.ijhydene.2016.12.076 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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dominant contribution to the performance degradation of the IT-SOFC [1e7]. Therefore, as a cathode material of IT-SOFCs, it must firstly have high catalytic activity for ORR (i.e. low polarization resistances). Furthermore, since the SOFCs are usually fabricated by multiple steps of high-temperature calcination and a long lifetime is required, the cathode materials must also have proper thermal expansion coefficients (TECs) that match with TECs of the commonly used electrolyte materials of Ce0.9Gd0.1O1.95 (GDC), Ce0.8Sm0.2O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3d (LSGM) in order to avoid structural cracking in the cells [1e7]. Double-layered perovskite oxides with the general formula of LnBaCo2O5þd (Ln ¼ lanthanide element) have recently been investigated as promising cathode materials for IT-SOFCs because of their high oxygen surface exchange coefficient, superior oxygen ion bulk diffusivity, and the resultant high ORR catalysis [6e14]. Among this series of perovskite oxides, PrBaCo2O5þd (PBCoO) has the highest surface exchange coefficient and bulk diffusion coefficient [6,9,10]; therefore, it has been demonstrated to have better electrochemical performance than the other double-layered perovskites as IT-SOFC cathode materials [10,13,14]. However, as a characteristic of cobalt-based perovskite oxides, PBCoO also has the problem of high TEC value that mismatches with TECs of the electrolyte materials. Besides, PBCoO has high ORR catalytic activity at temperatures above 700 C, but its polarization resistances at the lower temperatures (<700 C) are still too high to meet the requirement of resistance (<0.15 U cm2) for a cathode material [15]. Therefore, lowering the TEC value and improving lowtemperature electrochemical performance are key issues for practical application of PBCoO as cathode material of ITSOFCs. For improving overall performance of the PBCoO cathode, various strategies have been pursued such as A-site/B-site doping and fabrication of composite cathodes [16e27]. It's known that high TEC value of cobalt-based perovskite oxides is caused by reduction of B-site Conþ (n ¼ 3, 4) ions and low/ high spin transition of Co3þ [1,6,16e20]. Therefore, substitution of Conþ ions with other relatively stable transition-metal ions such as Fe, Cu and Ni can effectively lower the TEC values [17e20], which, however, has no significant effect on improving electrochemical performance and sometimes the electrochemical performance even shows a downtrend with the substitution due to the less catalytic reactivity of the valence stable transition-metal ions. Zhao et al. [17] reported that TEC of PrBaCo2xFexO5þd was ~20% decreased at Fe doping content of x ¼ 1.5 while its ASR values increased with the higher Fe doping content. TEC of PrBaCoCuO5þd with B-site Cu2þ doping significantly decreased from 24.1 106 K1 down to 15.2 106 K1, better matching with TECs of the electrolytes, but the ASR value was as high as 0.207 U cm2 at 600 C [18]. A-site cationic doping is another way of improving performance of perovskite oxides [21e23]. Park et al. [21] studied effect of A-site Sr2þ doping on performance of PBCoO cathode and the minimum ASR values were observed at the Sr2þ doping content of x ¼ 0.5 and 0.75 in PrBa1xSrxCo2O5þd (x ¼ 0, 0.25, 0.5, 0.75, and 1.0). Fu et al. [22] found that A-site Ca2þ doping led to a slightly lower TEC value for PBCoO but its polarization resistance increased with the higher Ca2þ doping
content. In recent years, introduction of A-site cationic deficiency has proved to be an effective way of enhancing ORR catalytic activities of the LnBaCo2O5þd double-layered perovskite oxides [25e32]. Systematic studies were made in our group on structures and properties of the PrBa1xCo2O6d oxides with various Ba2þ-deficiency contents (x ¼ 0.00e0.10) at A-sites [25]. Among the samples, the oxide with Ba2þ deficiency content of x ¼ 0.08 showed the best electrochemical performance characterized by 50% decrease in polarization resistance at 600 C as compared with the parent oxide [25]. In work of Dong et al. [30] on PrBa1xCo2O5þd (x ¼ 0.00, 0.05, 0.10) cathodes, the best electrochemical performance was obtained at Ba2þ deficiency content of x ¼ 0.10 and the improved performance was ascribed to enhanced oxygen surface exchange and bulky diffusion due to increase in concentration of oxygen vacancies. However, TEC values of the oxides were only slightly lowered due to the A-site cationic deficiency [25,32]. It has been proposed and experimentally demonstrated that co-substitution at A/B sites, rather than the aforementioned or other A-site or B-site sole substitution, can lead to synergistic effects with respect to electrochemical properties and TEC-related thermal stability of the cathode materials [33e35]. For example, PrBa0.5Sr0.5Co2xFexO5þd with co-doping of Sr2þ on A-site and Fe3þ on B-site showed fast oxygen ion diffusion and rapid surface oxygen exchange while maintained compatibility with the electrolytes for IT-SOFCs and the durability under operating conditions [33]. Considering the aforementioned effects of A-site Ba2þ deficiency and B-site Cu2þ doping on improving electrochemical performance and lowering TEC respectively [18,25], their synergistic effects are expected to improve overall performance of the perovskite oxides. Therefore, with the aim of further improving performance of the PBCoO cathode, a combined strategy, both A-site Ba2þ-deficiency and B-site Cu2þ-doping, was adopted for PBCoO. As an example, the PrBa0.92CoCuO6d (PB0.92CoCu) oxide with 8 mol% Ba2þ-deficiency at A-site and 50 mol% Cu2þdoping at B-site was firstly studied. It was evaluated as cathode material of IT-SOFC with respect to chemical stability, electrical conductivities, thermal expansion behavior and electrochemical performance. As expected, PB0.92CoCu showed better performance than the parent oxide of PBCoO characterized by much lower ASR values (i.e. enhanced ORR catalytic activity), smaller TEC with improved matching with electrolytes and high peak power densities of the single cell. Oxygen content and point defects of PB0.92CoCu and PBCoO were measured and compared for deep understanding of the performance improvement.
Experimental PB0.92CoCu was synthesized by a combined EDTA-citrate complexing solegel method [36]. Briefly, the reagents of Pr(NO3)3$6H2O (AR), Ba(NO3)2 (AR), Co(NO3)3$6H2O (AR) and Cu(NO3)2$6H2O (AR) at stoichiometric proportions of PB0.92CoCu were firstly dissolved into EDTA-NH3$H2O solution (pH z 6) to form an aqueous solution, and then citric acidNH3$H2O solution (pH z 6) was added at a mole ratio of 1:1:2 for EDTA: total metal ions: citric acid. The mixed solution was subsequently heated at 80 C to obtain a dark dry foam
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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structure, which was then decomposed on a hot plate to make the precursors followed by calcination in air at 600 C for 6 h and 950 C for 3 h in sequence. The PBCoO oxide was synthesized with the similar process except that the final calcination temperature was increased to 1050 C for 3 h. The PB0.92CoCu powders were mixed with GDC electrolyte in a 50/ 50 weight ratio followed by calcination at 950 C for 12 h in air for study of their chemical compatibility. A symmetrical cell with the configuration of PB0.92CoCu/GDC/PB0.92CoCu was fabricated for electrochemical impedance spectra (EIS) studies. The PB0.92CoCu ink, prepared by mixing the cathode powders with a-terpineol and ethyl cellulose, was screenprinted onto both sides of the GDC pellet, followed by calcination at 950 C for 2.5 h in air. The obtained cathodes have the active electrode areas of 0.283 cm2. An anode-supported single cell of PB0.92CoCu/GDC/YSZ/NiO-YSZ was fabricated. The ~400 mm thick NiO-YSZ (1:1 weight ratio) anode was tape cast as the support and then a ~6 mm thick dense 8YSZ layer and ~4 mm thick GDC electrolyte buffer layer were subsequently screen-printed onto the support substrate followed by sintering at 1200 C. The PB0.92CoCu paste was screen-printed onto the center of the GDC layer and calcined at 950 C for 2.5 h to function as the cathode layer. Phase structures of the as-synthesized PB0.92CoCu, PBCoO and the calcined PB0.92CoCu-GDC mixed powders were characterized by X-ray diffraction measurement (XRD, Rigaku D/ Max 2400). Oxygen contents of PB0.92CoCu and PBCoO were determined by iodometric titrations at room temperature with blank tests and five parallel analyses to reduce the measurement error. Thermogravimetric Analysis (TGA, Netzsch TG209F3) of the samples was carried out in the temperature range of 50e1000 C in air with heating rate of 5 C/min to check their thermal-driven oxygen releasing behaviors. Thermal expansion data of PB0.92CoCu and PBCoO were collected with a dilatometer (Netzsch DIL 402PC) at 30e900 C in air with a heating rate of 5 C/min. Electrical conductivities of both samples were measured at 400e800 C in air by a DC four-electrode method. Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiostat was used for EIS measurements of the symmetrical cells under open circuit voltage (OCV) condition at 600e800 C in air. IeV polarization curves were measured for assessing performance of the anode-supported single cell of PB0.92CoCu/GDC/YSZ/NiYSZ within the temperature range of 650e800 C using the Solartron 1287 potentiostat. The anode side and cathode side were supplied with H2 fuel and pressed air respectively at a flow rate of 60 mL min1.
Results and discussion Phase structure and chemical reactivity with electrolyte Fig. 1 shows XRD patterns of the as-synthesized PB0.92CoCu and PBCoO powders as well as the PB0.92CoCu-GDC mixture calcined at 950 C for 12 h in air. It is observed that PB0.92CoCu and PBCoO oxides have the diffraction peaks that match well, indicating that they have the same phase structures that were indexed with an orthorhombic Pmmm space group. The lattice parameters of PB0.92CoCu were calculated to be
3
Fig. 1 e XRD patterns of the as-synthesized PB0.92CoCu and PBCoO powders as well as the PB0.92CoCu-GDC mixture calcined at 950 C for 12 h in air.
a ¼ 3.936 Å, b ¼ 3.916 Å, c ¼ 7.699 Å and cell volume of 118.67 Å3, while the results of PBCoO are a ¼ 3.906 Å, b ¼ 3.929 Å, c ¼ 7.650 Å and cell volume of 117.40 Å3. The doubled c-axis (c z 2a z 2b) for both oxides indicates that they have the same double-layered perovskite structure but PB0.92CoCu shows slight lattice expansion as compared with PBCoO. It was found in our previous work [25] that Ba2þ deficiency at A-sites of PBCoO led to a slight structural shrinkage of the oxide. In contrast, substitution of B-site cobalt ions with Cu2þ ions was reported [37] to cause a structural expansion of the oxides due to the larger ionic radius of Cu2þ (r ¼ 73 pm) than the Conþ ions (rCo4þ ¼ 53 pm, rCo3þ ¼ 54.5 pm, rCo2þ ¼ 65 pm). Therefore, effect of Cu2þ substitution seems to be dominant in structure of PB0.92CoCu. It is also observed in Fig. 1 that no additional diffraction peaks indicative of impurity phases were observed in the XRD pattern of the calcined PB0.92CoCu-GDC mixture, thus no chemical reaction occurred between PB0.92CoCu and GDC at 950 C in air. Since pure phase of PB0.92CoCu was obtained at 950 C and it is decomposed at the higher temperatures as shown by the following TG result, it's not necessary to check its chemical reactivity with GDC at temperatures above 950 C.
Oxygen contents and point defects Electrical and electrochemical properties of the perovskitetyped cathode materials of SOFC are intrinsically determined by concentration and distribution of point defects of oxygen vacancies (Vo ) and Co4þ (h , charge carriers of p-type semiconductors) [2,17,26,33]. These point defects change with the A-site/B-site cations in the oxides as well as the measurement temperature and atmosphere. To get a deep understanding of properties of the PB0.92CoCu cathode, oxygen contents (6d) and average valence (n) of the Conþ ions were measured by iodometric titration method at room tempera ture, and then content of Vo and distribution of Co4þ/Co3þ ions in PB0.92CoCu and PBCoO were calculated based on electricneutrality of oxides and the results were listed in Table 1. PB0.92CoCu has lower oxygen content (5.153) than the parent oxide of PBCoO (5.71), thus its content of Vo (0.847) is much higher than that of PBCoO (0.29). In both oxides, the average
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Table 1 e Lattice parameters, oxygen content and point defects of PB0.92CoCu and PBCoO oxides. Samples
PB0.92CoCu PBCoO
Space group
Pmmm Pmmm
Cell volume (Å3)
118.67 117.40
Lattice parameters (Å) a
b
c
3.936 3.906
3.916 3.929
7.699 7.650
6d
Vo
n
[Co4þ]
[Co3þ]
5.153 5.71
0.847 0.29
3.466 3.21
0.932 0.42
1.068 1.58
Calculated assuming Cu exists as Cu2þ as in Ref. [37].
valence of the Conþ ions is n > 3, indicating coexistence of Co4þ and Co3þ ions; however, the larger n value of PB0.92CoCu suggested more Co4þ and less Co3þ ions than in PBCoO. These results have indicated that PB0.92CoCu has different point defects from PBCoO. More negative charges were introduced by the A-site Ba2þ-deficiency in PB0.92CoCu, which needed more positive charges, Vo and h (Co4þ) for the charge compensation. Besides, the Cu2þ cations needed less O2 ions for charge compensation than the Conþ (n > 3) cations with the higher valence, which could also lead to the less oxygen content and more Vo in PB0.92CoCu. It is known that lattice oxygen in the perovskite oxides can release at high temperatures and lead to changes in the point defects. To get such knowledge, thermogravimetric analysis (TGA) was carried out for PB0.92CoCu and PBCoO oxides at temperatures of 50e1000 C in air and the results were shown in Fig. 2. Different TG curves were observed for the two oxides. At the low temperatures, below 252 C for PBCoO and below 311 C for PB0.92CoCu, a slight weight gain was observed for both oxides while the change in PB0.92CoCu was more obvious. Similar phenomena were also observed in other PBCoOrelated oxides at the similar temperatures [30,38], which was ascribed to the process of oxygen surface absorption followed by oxygen bulky diffusion. The more distinct weight gain in PB0.92CoCu can be reasonably interpreted based on this mechanism. Firstly, it was reported [39] that B-site Cu2þ doping could promote oxygen surface adsorption over the perovskite oxides. Secondly, PB0.92CoCu had more Vo than PBCoO (Table 1), and a higher concentration of Vo was advantageous for oxygen surface adsorption and bulky diffusion in the oxide [2]. These two factors have probably contributed to the more evident weight gain at the low temperature in PB0.92CoCu. With the increasing temperatures, a gradual weight loss was observed in both oxides due to thermaldriven releasing of lattice oxygen while an abrupt weight
Fig. 2 e TG curves of PB0.92CoCu and PBCoO oxides measured at 50e1000 C in air.
loss occurred at the temperature above 960 C in PB0.92CoCu, which could be caused by decomposition or melting of the sample. Therefore, the PB0.92CoCu cathode layer should be calcined at temperatures lower than 960 C during fabrication of SOFCs. Based on the iodometric titration results (Table 1) of the assynthesized PB0.92CoCu and PBCoO oxides and the TGA results shown in Fig. 2, temperature-dependence of oxygen content (6d), Vo concentration and average valence (n) of Conþ ions were calculated in the temperature range of 50e800 C and the results were shown in Fig. 3. Decrease ratio of oxygen content t 100%, where a0 is the oxygen content of was defined as a0aa 0 the sample at the room temperature and at is the value at a specific temperature. It was found that the decrease ratio of oxygen content up to 800 C in air was 10.22% for PB0.92CoCu, larger than that of PBCoO (7.60%), indicating that the lattice oxygens were easier to be released in PB0.92CoCu probably due to the weaker CueO bonds than the CoeO bonds [40]. In company with the changes in oxygen content, more Vo were generated in PB0.92CoCu than in PBCoO (Fig. 3b). As shown in Fig. 3c, the average valence (n) of Conþ ions decreased from 3 < n < 4 down to 2 < n < 3 with the increasing temperature for both oxides, suggesting sequent reduction of Co4þ to Co3þ and Co2þ ions with releasing of the lattice oxygen. The change was more complicated in PB0.92CoCu because the average valence of Conþ ions varied with both the A-site Ba2þ-deficiency and the B-site Cu2þ-doping. It was also found that the n value of PB0.92CoCu became smaller than that of PBCoO at the temperatures above ~350 C, indicating that more Co4þ ions were reduced in PB0.92CoCu due to releasing of more lattice oxygens.
Electrical conductivity It's known that cobalt-containing perovskite oxides are mixed ionic and electronic conductors (MIECs) with Vo and h as the respective charge carriers [2,10,17,20,26]. In these MIECs, the electronic conductivity is usually several orders of magnitude larger than the oxygen ionic conductivity; therefore the total electrical conductivities are dominated by the electronic conduction [2,6,10]. As shown in Fig. 4, the conductivity values of PB0.92CoCu range from 255 S cm1 at 400 C to 134 S cm1 at 800 C in air, meeting the requirement of conductivity for the cathode of SOFCs (>100 S cm1) [41]. These values are smaller than the conductivities of PBCoO but are comparable to the values of PrBaCoCuO5þd [18]. Decreased conductivities due to the B-site Cu2þ-doping have also been found in other perovskite oxides [37]. The following factors have probably contributed to the lower conductivities of PB0.92CoCu. As is known, electronic conduction in the MIECs occurs by a Zerner
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Fig. 3 e Temperature-dependence of oxygen content (6¡d) (a), content of Vo (b) and average valence (n) of Conþ ions (c) for PB0.92CoCu and PBCoO oxides at 50e800 C in air.
of Vo in the oxide can break the BeOeB bonds, inhibit the electronic hopping and decrease electronic conductivity [2,28], which could lead to the lower electrical conductivities of PB0.92CoCu as well since it had more Vo than PBCoO did (Fig. 3b).
Thermal expansion coefficients
Fig. 4 e Electrical conductivities of PB0.92CoCu and PBCoO oxides measured at various temperatures in air.
double exchange mechanism, i.e. the electronic hopping occurs along the eBnþeO2-eB(nþ1)þe pathway that is formed by multi-valenced B-site ions (like Conþ with n ¼ 4, 3 and 2) and the lattice oxygen, thus the electrical conductivity is determined by both concentration and mobility of the charger carrier h [2,42]. As discussed above (Fig. 3c), PB0.92CoCu has less h than PBCoO does in the temperature range of 400e800 C, which could lead to the decreasing conductivity. Moreover, Zeng et al. [42] proposed that, existence of the B-site ions with fixed valences would act as blocks for electronic conduction, decrease mobility of the charge carriers and thus decrease electrical conductivities in the oxides. This was probably another factor that contributed to the lower electrical conductivities of PB0.92CoCu considering the relatively stable Cu2þ ions compared to the Conþ ions. Besides, existence
Fig. 5 shows thermal expansion curves of PB0.92CoCu and PBCoO oxides measured in the temperature range of 30e900 C in air. An inflection was observed at ~300 C in each curve, which was probably caused by reduction of Co4þ to Co3þ ions due to thermal-driven releasing of the lattice oxygen, as found in other cobalt-containing perovskite oxides [10,18,25]. The average TEC value of PB0.92CoCu at 30e900 C is
Fig. 5 e Thermal expansion curves and average TEC values of PB0.92CoCu and PBCoO oxides measured at 30e900 C in air.
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18 106/K. It is 25% smaller than TEC of PBCoO (24 106/K), demonstrating that the combined strategy of A-site Ba2þdeficiency and B-site Cu2þ-doping has decreased the TEC value as expected. The TEC value of PB0.92CoCu is also smaller than TEC of PrBa0.92Co2O5þd with sole A-site Ba2þ-deficiency (TEC ¼ 21 106/K) [25] but is still bigger than that of with the pure B-site Cu2þ-doping PrBaCoCuO5þd 6 (TEC ¼ 15.2 10 /K) [18]. This suggested that the A-site Ba2þdeficiency and B-site Cu2þ-doping had exerted a synergistic effect on the thermal expansion behavior of PB0.92CoCu, which could probably be associated with the changes in oxygen content and point defects (Table 1). Significantly, the TEC of PB0.92CoCu is much smaller than TECs of other A-site or B-site doped PBCoO such as PrBaCoFeO5þd (TEC ¼ 25.3 106/K) [17], PrBaCo1.5Sc0.5O5þd (TEC ¼ 20.5 106/K) [20] and the PBCoOSDC composite cathode (TEC ¼ 20.1 106/K) [14], suggesting that PB0.92CoCu has better TEC matching with electrolyte materials of SOFCs as compared to the above related materials.
Electrochemical performance of the cathode Electrochemical performance of PB0.92CoCu cathode was investigated with AC-impedance spectra method at various temperatures in air using the GDC-based symmetrical cells and the obtained results in comparison with the results of PBCoO measured at the same conditions were shown in Fig. 6. The ohmic resistances arising from the GDC electrolyte and lead wires were normalized to zero for clarity. Area specific resistances (ASRs) at various temperatures of the PB0.92CoCu and PBCoO cathodes were calculated from the impedance spectra and were shown in Arrhenius plot in Fig. 7. The ASRs of PB0.92CoCu are 0.12 U cm2 at 600 C, 0.059 U cm2 at 650 C, 0.032 U cm2 at 700 C and 0.017 U cm2 at 750 C respectively. These values are smaller than ASRs of PBCoO that range from 0.182 U cm2 at 600 C to 0.021 U cm2 at 750 C. In particular, 34% decrease in ASR of 600 C has been obtained in PB0.92CoCu, demonstrating enhanced ORR catalytic activity of PB0.92CoCu at the low temperature, which is consistent with its smaller reaction activation energy (Ea ¼ 1.04 eV) than that of PBCoO (Ea ¼ 1.21 eV) calculated from the Arrhenius plots. The ASR values of PB0.92CoCu are also much smaller than ASRs of many PBCoO-related cathodes such as A-site/B-site doped oxides of
Fig. 7 e Arrhenius plots of ASR for PB0.92CoCu and PBCoO cathodes with respective activation energy (Ea). PrBaCoFeO5þd (0.105 U cm2 at 700 C) [17], PrBaCuCoO5þd (0.207 U cm2 at 600 C) [18], PrBa0.5Sr0.5Co2O5þd (0.23 U cm2 at 650 C) [23], and the composite cathode of PBCoO-SDC (0.073 U cm2 at 700 C) [14]. Although possibly different microstructures of the cathodes made by different groups can influence the cathode reaction, the ASR value provides a metric of the cathode performance, which can be considered as a level of performance that can be achieved at minimum with the possibility that a higher performance can be attained if the electrode microstructure is optimized [2,10,17,33,43]. Particularly for the cathodes of PB0.92CoCu and PBCoO in this work, this influence can be neglected because we have ensured that the two cathode layers have the similar microstructures (Fig. 8). Concentration and distribution of oxygen vacancies (Vo ) in the perovskite oxides is one more important intrinsic factor that dominates electrochemical performance of the cathodes because Vo can promote oxygen transport kinetics in the oxides and therefore enhance the ORR catalytic activities [2,6,9,25,44]. As shown in Fig. 3b, PB0.92CoCu has much more Vo than PBCoO does, which should be the main contribution to its improved electrochemical performance.
Performance of single-cell In order to further evaluate PB0.92CoCu as a cathode material of IT-SOFC, an anode-supported single cell of PB0.92CoCu/GDC/
Fig. 6 e Nyquist EIS plots of symmetrical cells of PB0.92CoCu/GDC/PB0.92CoCu and PBCoO/GDC/PBCoO measured at various temperatures in air. Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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Fig. 8 e Cross-sectional SEM images with inserted surface SEM images of the cathode layers in the symmetrical cells of PB0.92CoCu/GDC/PB0.92CoCu and PBCoO/GDC/PBCoO respectively.
Fig. 9 e (a) Cross-sectional SEM image with inserted image of the cathode surface of the PB0.92CoCu/GDC/YSZ/Ni-YSZ single cell; (b) IeV and IeP polarization curves of the single cell. YSZ/Ni-YSZ was characterized. This single cell was fabricated based on a GDC/YSZ electrolyte double-layer (Fig. 9a), in which the dense YSZ thin film (~6 mm thick) was used as an electronic blocking layer to minimize internal loss and increase opencircuit voltage (OCV) of the cell [2,7,45] and the GDC buffer layer (~4 mm thick) was used to provide high oxygen ionic conduction at 600e800 C and also to avoid chemical reaction between YSZ and the cathode [7,46]. Porous structured cathode and anode layers were prepared for facile in/out gas diffusion. As shown in Fig. 9b, OCVs of the single cell reached up to ~1.1 eV at the measurement temperatures and high peak power densities, 1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C, 930 mW cm2 at 700 C and 632 mW cm2 at 650 C respectively were achieved. These results indicated that good performance of the single cell could be obtained using the PB0.92CoCu cathode. To determine working stability of the single cell with the PB0.92CoCu cathode, the single cell of PB0.92CoCu/GDC/YSZ/NiYSZ was first polarized under a constant current density of 350 mA cm2 at 650 C for 20 h by operating on hydrogen fuel until stable performance was obtained, and then the stability tests were conducted. Fig. 10 showed time dependence of voltage of the single cell under the current density of 350 mA cm2 at 650 C. It was found that the single cell maintained a stable voltage at around 0.7 V throughout a test period of 100 h, demonstrating performance stability of the single cell with the PB0.92CoCu cathode.
Fig. 10 e The time dependence of the voltage of the PB0.92CoCu/GDC/YSZ/Ni-YSZ single cell under a specific current density of 350 mA cm¡2 at 650 C.
Conclusions In this work, PB0.92CoCu was synthesized and characterized as cathode material of IT-SOFCs in comparison with the results of the parent oxide of PBCoO. PB0.92CoCu had the same doublelayered perovskite structure with PBCoO but showed a slight structural expansion. Lattice oxygen was easier to be released in PB0.92CoCu, leading to its lower oxygen content (more oxygen vacancies) than in PBCoO. Electrical conductivities of
Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076
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PB0.92CoCu were smaller than the results of PBCoO but satisfied the conductivity requirement for a SOFC cathode. The average TEC value of PB0.92CoCu was 25% smaller than TEC of PBCoO with better TEC matching with electrolytes. ASRs of PB0.92CoCu ranged from 0.12 U cm2 at 600 C to 0.017 U cm2 at 750 C, which were much smaller than ASRs of PBCoO and some related cathodes, indicating highly enhanced ORR catalytic activity of PB0.92CoCu. High peak power densities, 1541 mW cm2 at 800 C, 1228 mW cm2 at 750 C and 930 mW cm2 at 700 C were obtained with the single cell using PB0.92CoCu as the cathode. Working stability of the single cell was also demonstrated. These results have demonstrated that the combined strategy of A-site Ba2þ deficiency and B-site Cu2þ-doping has improved the overall performance of PB0.92CoCu, which is therefore a promising cathode material for IT-SOFCs.
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Acknowledgements This work was financially supported by the “Liaoning Provincial Natural Science Foundation of China (2013020106)” and “The Fundamental Research Funds for the Central Universities (DUT15LAB08)”.
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Please cite this article in press as: Jiang X, et al., Characterization of PrBa0.92CoCuO6d as a potential cathode material of intermediatetemperature solid oxide fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.076