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electrolyte interface
Journal of Power Sources 320 (2016) 86e93
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Effects of NiO on the conductivity of Ce0.85Sm0.15O1.925 and on electrochemical properties of the cathode/electrolyte interface Haopeng Wang, Xiaomei Liu*, Hailin Bi, Shenglong Yu, Fei Han, Jialing Sun, Lili Zhu, Huamin Yu, Li Pei Key Laboratory of Physics and Technology for Advanced Batteries, Ministry of Education, College of Physics, Jilin University, Changchun 130012, China
h i g h l i g h t s Apparent specific grain boundary conductivity could be significantly increased. Diffusion of Si from electrolyte to electrode/electrolyte interface is mitigated. Electrode interfacial polarization resistance can be significantly decreased. The addition of NiO to SDC significantly improves the single cell performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2016 Received in revised form 13 April 2016 Accepted 15 April 2016
Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) are investigated as electrolytes for solid oxide fuel cells (SOFCs). Impedance spectroscopy measurements reveal that the grain boundary resistance can be significantly reduced by adding 0.5 at.% NiO to SDC. Symmetric cells of the BaCo0.7Fe0.2Nb0.1O3d (BCFN) electrode on SDC and SDCN electrolytes are fabricated and the electrochemical properties of the electrode/electrolyte interface are investigated. The polarization resistance of the BCFN electrode on the SDCN electrolyte is much lower than that of the BCFN electrode on the SDC electrolyte, mainly because of the increase in the electrolyte conductivity and the decrease in the Si content at the electrode/electrolyte interface. NiO is able to restrict the diffusion of the siliceous impurity from the electrolyte to the electrode/electrolyte interface. Single cells based on SDC and SDCN electrolytes are fabricated using Ni0.9Cu0.1Ox-SDC as the anode and BCFN as the cathode. At 800 C, the maximum power density of the SDCN-based cell is 0.745 W cm2, which is much higher than that of the SDC-based cell. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Apparent specific grain boundary conductivity Impurity diffusion Electrochemical properties Cathode/electrolyte interface
1. Introduction Solid oxide fuel cells (SOFCs) convert chemical energy to electrical power with a high efficiency and low emissions [1]. Conventional SOFCs based on yttria-stabilized zirconia (YSZ) operate at high temperatures (850e1000 C), giving rise to material degradation and operational problems as well as high cost. Therefore, a reduction of the working temperature of SOFCs is in high demand to achieve broad commercialization. An intensive research effort aiming to reduce the operating temperature of such cells to 500e800 C has been conducted. Doped ceria materials are
* Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jpowsour.2016.04.074 0378-7753/© 2016 Elsevier B.V. All rights reserved.
considered to be good candidates for use in intermediatetemperature solid oxide fuel cells (IT-SOFCs) because in the intermediate-temperature range, their ionic conductivities are higher than those of YSZ [2,3]. Similar to most polycrystalline ceramics, the overall physicochemical properties of ceria-based electrolytes are determined by both grain bulk and grain boundary features. In the intermediatetemperature region, the grain boundary resistivity contribution to the overall resistivity is significant. To a large extent, the blocking effect of grain boundaries is attributed to the presence of thin siliceous films [4,5]. The SiO2 impurity is ubiquitous in ceria precursor materials. Furthermore, SiO2 contamination can be inadvertently introduced during the manufacturing process. It is difficult to eliminate the negative effect of grain boundaries on the total conductivity [6]. To improve the grain boundary conductivity
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of acceptor-doped CeO2, many attempts have been made to reduce the effect of the grain boundaries by adding various dopants, such as MnO2, Fe2O3, Co3O4, MgO and CaO [5e9]. Nickel is the most commonly used anode for SOFCs. Li et al. [10] reported that metallic Ni could diffuse from the NieCe0.8Gd0.2O1.9 (Ni-GDC) cermet anode into the Ce0.8Gd0.2O1.9 (GDC) electrolyte along grain boundaries, leading to microstructural changes at the GDC grain boundary regions. However, the effect of Ni diffusion on the electrical conductivity of GDC has not been investigated. It is reported that Ni ions do not incorporate into the cerium gadolinium oxide surface or bulk when sintered at 1300 C, but rather react with the Gd ions to form the GdNiO3 phase. Ni-impregnated Ce0.85Gd0.15O1.925 (0.5 at.%) sintered at 1500 C shows enhanced grain boundary conductivity that probably indicates that Ni incorporates into Ce0.85Gd0.15O1.925 above 1300 C [11]. The effect of the grain size on the grain boundary conductivity has not been investigated. Furthermore, the oxygen ion conductivity of the electrolyte and the average grain size of the electrolyte have a significant effect on the electrode interfacial polarization resistance of the electrolyte. According to our previous investigation of the electrochemical properties of Ce0.85Sm0.15O1.925 (SDC) BaCe0.83Y0.17O3-d (BCY) composite electrolytes, SDCBCY composite electrolytes exhibit lower electrode interfacial polarization resistance than the SDC electrolyte and BCY electrolyte. The decrease in the electrode interfacial polarization resistances of the composite electrolytes could be attributed to the increase in the oxygen ion conductivity and the decrease in the average grain size [12]. To the best of our knowledge, very little information is available regarding the effect of a small amount of NiO addition on the ionic conductivity of the SDC electrolyte, and there is no report on the effect of NiO addition to the SDC electrolyte on the electrochemical properties of the electrode/electrolyte interface. The aim of this work is to systematically investigate the electrical properties of the Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) electrolytes and to study the effect of NiO addition to the SDC electrolyte on the electrochemical properties of the cathode/electrolyte interface. In this work, SDC and SDCN electrolytes are synthesized by the glycine-nitrate process. The results of XRD phase analysis and SEM micrographs of the samples are reported herein. AC impedance spectroscopy is employed to study the electrical conductivities of the SDC and SDCN electrolytes. Symmetric cells of the BaCo0.7Fe0.2Nb0.1O3d (BCFN) electrode on the SDC and SDCN electrolytes are fabricated and the electrochemical properties of the electrode/electrolyte interface are investigated. The power densities and current densities of single cells based on the SDC and SDCN electrolytes are evaluated. 2. Experimental 2.1. Sample preparation Ce0.85Sm0.15O1.925 (SDC) powders were synthesized via the glycine-nitrate process (GNP) as reported previously [12]. The preparation of Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) powders followed the same route as that for the SDC powders. The starting materials were Ce(NO3)3$6H2O (A.R.), Ni(NO3)2$6H2O (A.R.) and Sm2O3 (A.R.). The electrolyte pellets were prepared by uniaxially pressing SDC and SDCN powders and sintering at 1450 C for 10 h. Prior to the electrical measurements Ag paste was brushed onto both sides of the sintered pellets and heat-treated at 175 C for 3 h. The projected area of the Ag electrode was 0.16 cm2. BaCo0.7Fe0.2Nb0.1O3-d (BCFN) powders were synthesized by the conventional solid state reaction process as reported previously [13]. Symmetric cells of BCFN/SDC/BCFN and BCFN/SDCN/BCFN
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were fabricated. The electrolyte pellets were polished to reduce the thickness to 350 mm. The prepared BCFN powders were mixed with ethyl cellulose and terpineol to form a slurry, and then, the electrode slurry was coated symmetrically onto both sides of the electrolyte pellets by the screen-printing technique. The coated electrode layers were sintered in air at 1000 C for 2 h. The electrolyte-supported cells with the Ni0.9Cu0.1Ox-SDC anode and BCFN cathode were also fabricated. The electrolyte pellets were polished to reduce the thickness to 240 mm. The details on fabrication of Ni0.9Cu0.1Ox-SDC and BCFN can be found in Ref. [14]. Ag paste was coated on each electrode surface to act as a current collector. A single cell was attached to one end of an alumina tube with the anode inside by sealing with Ag paste. 2.2. Characterization Crystal phase identification of the electrolyte samples was performed using a Rigaku D/Max-2550 diffractometer with Cu-Ka radiation. Intensities were collected in the 2q range between 20 and 80 , with a step size of 0.02 . The morphologies of the electrolyte pellets and symmetric cells were characterized using a JEOL JSM7500F field emission scanning electron microscope (FE-SEM) equipped with an APOLLO XL energy dispersive X-ray (EDX) spectrometer. The average grain sizes were estimated using the linear intercept method by randomly selecting more than 200 grains from the SEM micrographs. The densities of the sintered pellets were measured using the Archimedes method. Electrical conductivities of the electrolyte pellets were measured in air by AC impedance spectroscopy. The measurements were carried out using a Solartron 1260 frequency response analyzer combined with a Solartron 1287 potentiostat in the frequency range of 101 Hze106 Hz and in the temperature range of 300e800 C. The obtained impedance spectra were fitted using the Zview 2.0 software. The impedance spectra of the symmetric cells were obtained in the frequency range of 102 Hze106 Hz and in the temperature range of 500e800 C. The single cell performance was tested at temperatures from 600 to 800 C with the same electrochemical workstation. Dry H2 was fed into the alumina tube for use as the fuel with a flow rate of 160 ml min1, while oxygen in the air was used as oxidant. 3. Results and discussion 3.1. Results of XRD Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the electrolytes. It can be observed that both samples exhibit a singlephase fluorite structure. In the SDCN pattern, no extra peaks or obvious peak shifts are observed. Ni2þ could hardly be dissolved into the SDC lattice due to the much smaller radius of Ni2þ compared to that of Ce4þ. The filling of an interstice by a cation smaller than the precise interstice size is unstable [8]. 3.2. Morphology SEM micrographs of the SDC and SDCN electrolytes are shown in Fig. 2a and b. Both samples are densified. The relative densities and average grain sizes of the samples are listed in Table 1. It is clear that the addition of NiO has a remarkable influence on the grain growth of SDC. SDCN exhibits a larger average grain size than SDC. NiO is beneficial in terms of sinterability, apparently due to the emergence of viscous flow sintering [15]. Fig. 2c and d shows the micrographs of the cross-sectional views of the BCFN/SDC and BCFN/SDCN interfaces. Examination of the figures shows that the SDC and SDCN electrolytes adhered well to
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H. Wang et al. / Journal of Power Sources 320 (2016) 86e93 Table 2 Normalized compositions (wt%) of BCFN/SDC and BCFN/SDCN samples after sintered at 1000 C for 2 h. Ce BCFN/SDC Area 1 83.0 Area 2 35.4 Area 3 BCFN/SDCN Area 1 82.7 Area 2 58.7 Area 3
Fig. 1. XRD patterns of SDC and SDCN electrolytes; both samples were heat treated at 1450 C for 10 h.
the porous BCFN electrode. To investigate the effect of NiO addition
Sm
Si
16.0 6.8
0.4 0.4
16.0 11.4
0.5 0.3
Ni
0.8 0.6
Ba
Co
Fe
Nb
39.4 68.1
12.1 21.2
3.3 5.6
2.6 4.8
19.9 68.6
6.2 20.8
1.7 5.6
1.3 4.7
to the SDC electrolyte on the interface chemical composition, EDX analysis is performed on the samples. Table 2 lists the normalized local compositions observed at three representative areas: Area 1the electrolyte region; Area 2the electrode/electrolyte interface region; and Area 3the electrode region. It can be seen from Table 2 that the Si content in the SDCN electrolyte (Area 1) is higher than that in the SDC electrolyte (Area 1). However, the Si content at the BCFN/SDCN interface (Area 2) is lower than that at the BCFN/ SDC interface (Area 2). The siliceous impurity phase is extremely dynamic in nature [16]. The siliceous impurity can diffuse from the electrolyte to the electrode/electrolyte interface. The difference in
Fig. 2. SEM micrographs of the surface of (a) SDC and (b) SDCN sintered at 1450 C for 10 h and the fractured surface of (c) BCFN/SDC and (d) BCFN/SDCN bilayers after electrochemical tests.
Table 1 Relative density, average grain size and activation energies for bulk and grain boundary conductivities. Sample
Relative density (%)
Average grain size (mm)
Bulk activation energy (eV) 300e600 C
Grain-boundary activation energy (eV) 300e600 C
SDC SDCN
95.8 95.5
1.84 2.81
0.70 0.70
1.01 1.01
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the Si distribution between the two symmetric cells indicates that addition of NiO to SDC restricts siliceous impurity diffusion from the electrolyte to the electrode/electrolyte interface. The XRD results demonstrate that Ni2þ could not incorporate into the SDC lattice. Large amounts of silicon in SDCN could be removed by NiO to the multiple grain junctions and Ni- and Si-rich secondary phase may be formed. The mobility of the Ni- and Si-rich secondary phase is lower, mitigating the effect of siliceous impurity diffusion in SDCN. 3.3. Electrical properties Fig. 3 shows the impedance spectra of the SDC and SDCN electrolytes measured at 400 C and the corresponding equivalent circuit. Instead of capacitors, constant phase elements (CPEs) are applied to model the obtained experimental data. CPEs account for the non-ideal behavior of the capacitive elements due to a variety of physical phenomena such as surface heterogeneity that originate from the effects of surface roughness, impurities, dislocations, and grain boundaries [17]. Contributions, in the order of decreasing frequency, of the grain bulk, the grain boundary and the electrode polarization behavior can be clearly identified. As shown in Fig. 3, the grain boundary resistance Rgb can be significantly decreased by adding 0.5 at.% NiO to SDC. Fig. 4 shows the temperature dependences of the bulk conductivity sbulk, grain boundary conductivity sgb, apparent specific grain boundary conductivity divided by the bulk conductivity sas gb/ sbulk and total conductivity stot. The bulk conductivity is calculated based on sbulk ¼ L/SRbulk, where L is the sample thickness and S is the electrode area on the sample surface. Thus, the grain boundary conductivity sgb and the total conductivity stot can be obtained. The Arrhenius relationship can be expressed as s ¼ A/Texp(Ea/kbT), where A, Ea, and kb represent the pre-exponential factor, the activation energy, and the Boltzmann constant, respectively. Here, ln(sT) is plotted against 1000/T, and the plot should show a linear relationship between these two quantities. The calculated activation energy values are listed in Table 1. Inspection of Fig. 4a shows that the addition of NiO has a negligible effect on the bulk conductivity and on the activation energy for bulk conductivity. This could be attributed to the very low solubility of Ni2þ in the SDC
lattice, as proved by XRD. As shown in Fig. 4b, the sgb value of SDCN is greater than that of SDC. As shown in Table 1, the relative densities of both samples are above 95%, suggesting that relative density is not the key factor leading to the difference in sgb between SDC and SDCN. The increase in the sgb value can be explained as follows. The grain boundary conductivity sgb depends on the bulk conductivity, space-charge potential, siliceous impurity at the grain boundaries and average grain size [3]. The average grain size of SDCN is larger than that of SDC, as shown in Table 1. To eliminate the influence of the number of grain boundaries on sgb, the apparent specific grain boundary conductivity sas gb is calculated. sas gb is an experimental datum and can be calculated from sas gb ¼ Ldgb/SRgbdg [3,4], where dgb is the grain boundary thickness and dg is the average grain size. For simplicity, the grain boundary thickness is taken to be 5.0 nm while calculating sas gb [4,18]. Because as sas gb is associated with sbulk, it is necessary to compare the sgb/sbulk values of SDC and SDCN. Fig. 4c shows the temperature dependence of sas gb/sbulk. For materials with normal purity values, based on the space-charge layers and siliceous impurity at grain boundaries, the apparent specific grain boundary conductivity sas gb can be expressed as [3,18].
sas u2 4eD4ð0Þ 2eD4ð0Þ gb exp ¼ 2 kB T sbulk dg kB T
(1)
where u2 is the area of grain-to-grain contact and D4(0) is the space-charge potential. The u2/d2g term is called the “impurity blocking term” and represents the clean fraction of resistive siliceous films at the grain boundaries. In our previous research, we reported that the space-charge potential D4(0) is proportional to the slope of the ln(sas gbT/sbulk)e1000/T curve [18]. As shown in Fig. 4c, the slope of the curve remains almost unchanged, indicating that 0.5 at.% NiO has little influence on the space-charge potential D4(0). As also shown in Fig. 4c, addition of 0.5 at.% NiO to SDC leads as to an increased sas gb/sbulk. According to Eq. (1), the increase in sgb/ 2 2 sbulk should result from the increase in u /dg. As discussed in section 3.2, large amounts of silicon in SDCN could be removed by NiO to the multiple grain junctions and Ni- and Si-rich secondary phase may be formed. The blocking effect of the grain boundaries contributed by the siliceous impurity in the SDCN sample is thus reduced, leading to the increase in u2/d2g. This indicates that the addition of NiO can mitigate the harmful effect of siliceous impurity on grain boundary conduction. The temperature dependence of the total conductivity is shown in Fig. 4d. Over the entire temperature range, SDCN exhibits higher total conductivity values than SDC. At 800 C, the total conductivity of SDCN is 4.93 102 S cm1. In our previous work [6], Fe2O3 was identified to be an effective additive for improving the grain boundary conductivity of SDC. At 800 C, the total conductivity of Ce0.85Sm0.15O1.9250.5 at.% FeO1.5 is 5.97 102 S cm1, higher than that of SDCN in this study. Nevertheless, an investigation of SDCN is highly important because nickel is the most commonly used anode for SOFCs. Fig. 5a shows the impedance spectra of the symmetric cells measured at 500 C and the corresponding equivalent circuit. For these two symmetric cells, the impedance spectra show two successive semi-circles. The characteristic capacitance (C) and angular relaxation frequency (f) are calculated based on the following equations:
1n n
Fig. 3. Impedance spectra for SDC and SDCN measured at 400 C in air. The adopted equivalent circuit model is presented in the plot.
89
C¼R
1
Qn
(2)
90
H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
Fig. 4. Temperature dependence of (a) bulk, (b) grain boundary, (c) apparent specific grain boundary conductivity divided by the bulk conductivity and (d) total conductivity for SDC and SDCN electrolytes.
ðRQ Þn f ¼ 2p 1
(3)
where R is resistance and Q and n are the parameters that characterize the CPE used for the fitting. The capacitance values of the electrolytes are calculated using Eq. (2) by multiplying the impedance data by a volume factor, whereas those of the electrode responses are obtained by multiplying the impedance data by the electrode area. The intercepts of these spectra with real axis (Z') at HF correspond to the ohmic resistances Ro of the symmetric cells; this includes the bulk resistance of the electrolyte, ohmic resistance of the electrode, contact resistance at the electrode/electrolyte interface and contact resistance between the electrode and the current collector. The corresponding capacitances are ~109 F cm1 for the middle-frequency (MF) semi-circle (RgbCPE1) of the samples. Because the symmetric cell consists of an electrolyte and an electrode, these capacitance values can be attributed to the grain boundary response of the electrolyte and the grain boundary response of the electrode. The capacitance values extracted from the low-frequency (LF) semi-circle (RelCPE2) for the samples are ~102 F cm2, which can be attributed to the electrode process. Fig. 5b presents the temperature dependence of the electrode interfacial polarization resistance Rel. As shown in Fig. 5b, the Rel
value of the symmetric cell using the SDCN electrolyte is lower than that of the symmetric cell using the SDC electrolyte. As discussed in our previous work, Rel decreases with increasing oxygen ion conductivity of the electrolyte and increases with increasing average grain size of the electrolyte [12]. As shown in Fig. 4d, SDCN exhibits a higher oxygen ion conductivity than SDC, which is expected to result in a decrease in the Rel of the symmetric cell using the SDCN electrolyte. At the same time, as shown in Table 1, the average grain size of SDCN is larger than that of SDC, which is expected to give rise to an increase in the Rel of the symmetric cell using the SDCN electrolyte. Furthermore, as shown in Table 2, the Si content at the BCFN/SDCN interface is lower than that at the BCFN/SDC interface, which is beneficial for the oxygen ion transfer process at the electrode/electrolyte interface. The symmetric cell using the SDCN electrolyte exhibits a lower Rel value than the symmetric cell using the SDC electrolyte. This is mainly caused by the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface.
3.4. Single cell performance Fig. 6a shows the IeV (current density and cell voltage) characteristics and corresponding IeP (power density derived from IeV)
H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
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Fig. 5. (a) Impedance spectra for the symmetric cells of BCFN/SDC/BCFN and BCFN/ SDCN/BCFN measured at 500 C in air and (b) temperature dependence of electrode interfacial polarization resistance Rel.
Fig. 6. (a) Single cell performance based on SDC and SDCN electrolyte with Ni0.9Cu0.1Ox-SDC as anode and BCFN as cathode tested at 600 C and (b) impedance spectra of single cells based on SDC and SDCN measured under open circuit conditions at 600 C.
curves for fuel cells based on the SDC and SDCN electrolytes tested at 600 C. The obtained open circuit voltages (OCVs) and maximum power densities are listed in Table 3. It is clear that the SDCN-based cell shows a much better performance than the SDC-based cell. Fig. 6b shows the impedance spectra of the SDC- and SDCN-based cells measured under open circuit conditions at 600 C. For these cells, the impedance spectra show two successive semi-circles. The characteristic capacitance values for the semi-circles are also calculated using Eq. (2). The corresponding capacitance values are ~104 F cm2 for the MF semi-circle of the cells. Generally, these capacitance values could be related to the oxygen ion transfer followed by the charge-transfer process. The capacitance values for the LF semi-circle of the cells are ~102 F cm2. These capacitance values are too large to be associated with the double layer capacitance. This indicates that the LF process is most likely related to the oxygen exchange and diffusion process. As shown in Fig. 6b, the resistance values for the frequencies greater than 105 Hz are the ohmic resistances Roc of the cells. The intercepts of the spectra with a real axis at low frequencies represent the total resistances Rtc of the cells. The interfacial polarization resistances Rpc of the cells can
be calculated as the difference between Rtc and Roc (Rpc ¼ Rtc e Roc). The Roc, Rpc and Rtc values of the cells are listed in Table 3. Examination of the data in Table 3 shows that the SDCN-based cell exhibits a lower Roc value than the SDC-based cell. The main reason for this is that SDCN exhibits a higher oxygen ion conductivity than SDC, as shown in Fig. 4d. Additionally, the SDCN-based cell exhibits a lower Rpc value than the SDC-based cell, which can be attributed to the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface, as mentioned in section 3.3. Examination of the data presented in Table 3 also shows that the SDCN-based cell exhibits a lower Rtc value than the SDC-based cell. Because NiO in SDCN can be reduced to Ni under hydrogen atmosphere, the stability of the SDCN-based cell is examined in a short-term cell test. Fig. 7 shows the time dependence of the open circuit voltage (OCV) of the SDCN-based cell tested at 600 C. The cell shows a very stable performance with no significant cell voltage degradation during the 10 h testing period, indicating that extra electronic conduction cannot be introduced. This is because the NiO content in SDCN is quite small and the electronic
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Table 3 Open circuit voltages, maximum power densities, ohmic resistances, interfacial polarization resistances and total resistances of single cells tested at 600 C. Sample Open circuit voltage (V) Maximum power density (W cm2) Ohmic resistance (U cm2) Interfacial polarization resistance (U cm2) Total resistance (U cm2) SDC SDCN
0.888 0.919
0.144 0.208
0.76 0.49
0.48 0.36
1.24 0.85
larger than that of SDC. The EDX results indicate that the addition of 0.5 at.% NiO to SDC is effective for mitigating the diffusion of siliceous impurities from the electrolyte to the electrode/electrolyte interface. Analysis of the electrical properties show that the addition of 0.5 at.% NiO to SDC has little influence on the space-charge potential. The increase in the apparent specific grain boundary conductivity arises from the increase in the conduction path width determined by the siliceous impurity. The electrode interfacial polarization resistance of the symmetric cell using the SDCN electrolyte is lower than that of the symmetric cell using the SDC electrolyte, which can be attributed to the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface. The SDCN-based cell shows a much better performance than the SDC-based cell. The maximum power densities of the single cell based on the SDCN electrolyte are 0.208, 0.324, 0.471, 0.630 and 0.745 W cm2 at 600, 650, 700, 750 and 800 C. Acknowledgements Fig. 7. Time dependence of the open circuit voltage (OCV) of the cell based on SDCN tested at 600 C.
This work was supported by the National Natural Science Foundation of China (Nos. 51272087 and 50872041) and the National Foundation for Fostering Talent in Basic Science of China (No. J1103202). References
Fig. 8. Single cell performance based on SDCN electrolyte with Ni0.9Cu0.1Ox-SDC as anode and BCFN as cathode tested at different temperatures.
conduction path through Ni cannot be formed. Fig. 8 shows the IeV and power density curves of the SDCN-electrolyte-based cell at different temperatures. The obtained maximum power densities are 0.208, 0.324, 0.471, 0.630 and 0.745 W cm2 at 600, 650, 700, 750 and 800 C.
4. Conclusions Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) are examined from the standpoint of their application in SOFCs as electrolytes. The SDC and SDCN electrolytes exhibit the single-phase fluorite structure. The average grain size of SDCN is
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[17] C. Peng, J. Melnik, J.-L. Luo, A.R. Sanger, K.T. Chuang, BaZr0.8Y0.2O3d electrolyte with and without ZnO sintering aid: preparation and characterization, Solid State Ionics 181 (2010) 1372e1377. [18] D.T. Yan, X.M. Liu, Q. Liu, H.P. Wang, J.H. Xu, S. Qi, M.Z. Zheng, W.H. Su, Study on rare earth electrolyte SDC, J. Rare Earth 28 (2010) 914e916.
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Effects of NiO on the conductivity of Ce0.85Sm0.15O1.925 and on electrochemical properties of the cathode/electrolyte interface Haopeng Wang, Xiaomei Liu*, Hailin Bi, Shenglong Yu, Fei Han, Jialing Sun, Lili Zhu, Huamin Yu, Li Pei Key Laboratory of Physics and Technology for Advanced Batteries, Ministry of Education, College of Physics, Jilin University, Changchun 130012, China
h i g h l i g h t s Apparent specific grain boundary conductivity could be significantly increased. Diffusion of Si from electrolyte to electrode/electrolyte interface is mitigated. Electrode interfacial polarization resistance can be significantly decreased. The addition of NiO to SDC significantly improves the single cell performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2016 Received in revised form 13 April 2016 Accepted 15 April 2016
Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) are investigated as electrolytes for solid oxide fuel cells (SOFCs). Impedance spectroscopy measurements reveal that the grain boundary resistance can be significantly reduced by adding 0.5 at.% NiO to SDC. Symmetric cells of the BaCo0.7Fe0.2Nb0.1O3d (BCFN) electrode on SDC and SDCN electrolytes are fabricated and the electrochemical properties of the electrode/electrolyte interface are investigated. The polarization resistance of the BCFN electrode on the SDCN electrolyte is much lower than that of the BCFN electrode on the SDC electrolyte, mainly because of the increase in the electrolyte conductivity and the decrease in the Si content at the electrode/electrolyte interface. NiO is able to restrict the diffusion of the siliceous impurity from the electrolyte to the electrode/electrolyte interface. Single cells based on SDC and SDCN electrolytes are fabricated using Ni0.9Cu0.1Ox-SDC as the anode and BCFN as the cathode. At 800 C, the maximum power density of the SDCN-based cell is 0.745 W cm2, which is much higher than that of the SDC-based cell. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Apparent specific grain boundary conductivity Impurity diffusion Electrochemical properties Cathode/electrolyte interface
1. Introduction Solid oxide fuel cells (SOFCs) convert chemical energy to electrical power with a high efficiency and low emissions [1]. Conventional SOFCs based on yttria-stabilized zirconia (YSZ) operate at high temperatures (850e1000 C), giving rise to material degradation and operational problems as well as high cost. Therefore, a reduction of the working temperature of SOFCs is in high demand to achieve broad commercialization. An intensive research effort aiming to reduce the operating temperature of such cells to 500e800 C has been conducted. Doped ceria materials are
* Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jpowsour.2016.04.074 0378-7753/© 2016 Elsevier B.V. All rights reserved.
considered to be good candidates for use in intermediatetemperature solid oxide fuel cells (IT-SOFCs) because in the intermediate-temperature range, their ionic conductivities are higher than those of YSZ [2,3]. Similar to most polycrystalline ceramics, the overall physicochemical properties of ceria-based electrolytes are determined by both grain bulk and grain boundary features. In the intermediatetemperature region, the grain boundary resistivity contribution to the overall resistivity is significant. To a large extent, the blocking effect of grain boundaries is attributed to the presence of thin siliceous films [4,5]. The SiO2 impurity is ubiquitous in ceria precursor materials. Furthermore, SiO2 contamination can be inadvertently introduced during the manufacturing process. It is difficult to eliminate the negative effect of grain boundaries on the total conductivity [6]. To improve the grain boundary conductivity
H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
of acceptor-doped CeO2, many attempts have been made to reduce the effect of the grain boundaries by adding various dopants, such as MnO2, Fe2O3, Co3O4, MgO and CaO [5e9]. Nickel is the most commonly used anode for SOFCs. Li et al. [10] reported that metallic Ni could diffuse from the NieCe0.8Gd0.2O1.9 (Ni-GDC) cermet anode into the Ce0.8Gd0.2O1.9 (GDC) electrolyte along grain boundaries, leading to microstructural changes at the GDC grain boundary regions. However, the effect of Ni diffusion on the electrical conductivity of GDC has not been investigated. It is reported that Ni ions do not incorporate into the cerium gadolinium oxide surface or bulk when sintered at 1300 C, but rather react with the Gd ions to form the GdNiO3 phase. Ni-impregnated Ce0.85Gd0.15O1.925 (0.5 at.%) sintered at 1500 C shows enhanced grain boundary conductivity that probably indicates that Ni incorporates into Ce0.85Gd0.15O1.925 above 1300 C [11]. The effect of the grain size on the grain boundary conductivity has not been investigated. Furthermore, the oxygen ion conductivity of the electrolyte and the average grain size of the electrolyte have a significant effect on the electrode interfacial polarization resistance of the electrolyte. According to our previous investigation of the electrochemical properties of Ce0.85Sm0.15O1.925 (SDC) BaCe0.83Y0.17O3-d (BCY) composite electrolytes, SDCBCY composite electrolytes exhibit lower electrode interfacial polarization resistance than the SDC electrolyte and BCY electrolyte. The decrease in the electrode interfacial polarization resistances of the composite electrolytes could be attributed to the increase in the oxygen ion conductivity and the decrease in the average grain size [12]. To the best of our knowledge, very little information is available regarding the effect of a small amount of NiO addition on the ionic conductivity of the SDC electrolyte, and there is no report on the effect of NiO addition to the SDC electrolyte on the electrochemical properties of the electrode/electrolyte interface. The aim of this work is to systematically investigate the electrical properties of the Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) electrolytes and to study the effect of NiO addition to the SDC electrolyte on the electrochemical properties of the cathode/electrolyte interface. In this work, SDC and SDCN electrolytes are synthesized by the glycine-nitrate process. The results of XRD phase analysis and SEM micrographs of the samples are reported herein. AC impedance spectroscopy is employed to study the electrical conductivities of the SDC and SDCN electrolytes. Symmetric cells of the BaCo0.7Fe0.2Nb0.1O3d (BCFN) electrode on the SDC and SDCN electrolytes are fabricated and the electrochemical properties of the electrode/electrolyte interface are investigated. The power densities and current densities of single cells based on the SDC and SDCN electrolytes are evaluated. 2. Experimental 2.1. Sample preparation Ce0.85Sm0.15O1.925 (SDC) powders were synthesized via the glycine-nitrate process (GNP) as reported previously [12]. The preparation of Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) powders followed the same route as that for the SDC powders. The starting materials were Ce(NO3)3$6H2O (A.R.), Ni(NO3)2$6H2O (A.R.) and Sm2O3 (A.R.). The electrolyte pellets were prepared by uniaxially pressing SDC and SDCN powders and sintering at 1450 C for 10 h. Prior to the electrical measurements Ag paste was brushed onto both sides of the sintered pellets and heat-treated at 175 C for 3 h. The projected area of the Ag electrode was 0.16 cm2. BaCo0.7Fe0.2Nb0.1O3-d (BCFN) powders were synthesized by the conventional solid state reaction process as reported previously [13]. Symmetric cells of BCFN/SDC/BCFN and BCFN/SDCN/BCFN
87
were fabricated. The electrolyte pellets were polished to reduce the thickness to 350 mm. The prepared BCFN powders were mixed with ethyl cellulose and terpineol to form a slurry, and then, the electrode slurry was coated symmetrically onto both sides of the electrolyte pellets by the screen-printing technique. The coated electrode layers were sintered in air at 1000 C for 2 h. The electrolyte-supported cells with the Ni0.9Cu0.1Ox-SDC anode and BCFN cathode were also fabricated. The electrolyte pellets were polished to reduce the thickness to 240 mm. The details on fabrication of Ni0.9Cu0.1Ox-SDC and BCFN can be found in Ref. [14]. Ag paste was coated on each electrode surface to act as a current collector. A single cell was attached to one end of an alumina tube with the anode inside by sealing with Ag paste. 2.2. Characterization Crystal phase identification of the electrolyte samples was performed using a Rigaku D/Max-2550 diffractometer with Cu-Ka radiation. Intensities were collected in the 2q range between 20 and 80 , with a step size of 0.02 . The morphologies of the electrolyte pellets and symmetric cells were characterized using a JEOL JSM7500F field emission scanning electron microscope (FE-SEM) equipped with an APOLLO XL energy dispersive X-ray (EDX) spectrometer. The average grain sizes were estimated using the linear intercept method by randomly selecting more than 200 grains from the SEM micrographs. The densities of the sintered pellets were measured using the Archimedes method. Electrical conductivities of the electrolyte pellets were measured in air by AC impedance spectroscopy. The measurements were carried out using a Solartron 1260 frequency response analyzer combined with a Solartron 1287 potentiostat in the frequency range of 101 Hze106 Hz and in the temperature range of 300e800 C. The obtained impedance spectra were fitted using the Zview 2.0 software. The impedance spectra of the symmetric cells were obtained in the frequency range of 102 Hze106 Hz and in the temperature range of 500e800 C. The single cell performance was tested at temperatures from 600 to 800 C with the same electrochemical workstation. Dry H2 was fed into the alumina tube for use as the fuel with a flow rate of 160 ml min1, while oxygen in the air was used as oxidant. 3. Results and discussion 3.1. Results of XRD Fig. 1 shows the powder X-ray diffraction (XRD) patterns of the electrolytes. It can be observed that both samples exhibit a singlephase fluorite structure. In the SDCN pattern, no extra peaks or obvious peak shifts are observed. Ni2þ could hardly be dissolved into the SDC lattice due to the much smaller radius of Ni2þ compared to that of Ce4þ. The filling of an interstice by a cation smaller than the precise interstice size is unstable [8]. 3.2. Morphology SEM micrographs of the SDC and SDCN electrolytes are shown in Fig. 2a and b. Both samples are densified. The relative densities and average grain sizes of the samples are listed in Table 1. It is clear that the addition of NiO has a remarkable influence on the grain growth of SDC. SDCN exhibits a larger average grain size than SDC. NiO is beneficial in terms of sinterability, apparently due to the emergence of viscous flow sintering [15]. Fig. 2c and d shows the micrographs of the cross-sectional views of the BCFN/SDC and BCFN/SDCN interfaces. Examination of the figures shows that the SDC and SDCN electrolytes adhered well to
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H. Wang et al. / Journal of Power Sources 320 (2016) 86e93 Table 2 Normalized compositions (wt%) of BCFN/SDC and BCFN/SDCN samples after sintered at 1000 C for 2 h. Ce BCFN/SDC Area 1 83.0 Area 2 35.4 Area 3 BCFN/SDCN Area 1 82.7 Area 2 58.7 Area 3
Fig. 1. XRD patterns of SDC and SDCN electrolytes; both samples were heat treated at 1450 C for 10 h.
the porous BCFN electrode. To investigate the effect of NiO addition
Sm
Si
16.0 6.8
0.4 0.4
16.0 11.4
0.5 0.3
Ni
0.8 0.6
Ba
Co
Fe
Nb
39.4 68.1
12.1 21.2
3.3 5.6
2.6 4.8
19.9 68.6
6.2 20.8
1.7 5.6
1.3 4.7
to the SDC electrolyte on the interface chemical composition, EDX analysis is performed on the samples. Table 2 lists the normalized local compositions observed at three representative areas: Area 1the electrolyte region; Area 2the electrode/electrolyte interface region; and Area 3the electrode region. It can be seen from Table 2 that the Si content in the SDCN electrolyte (Area 1) is higher than that in the SDC electrolyte (Area 1). However, the Si content at the BCFN/SDCN interface (Area 2) is lower than that at the BCFN/ SDC interface (Area 2). The siliceous impurity phase is extremely dynamic in nature [16]. The siliceous impurity can diffuse from the electrolyte to the electrode/electrolyte interface. The difference in
Fig. 2. SEM micrographs of the surface of (a) SDC and (b) SDCN sintered at 1450 C for 10 h and the fractured surface of (c) BCFN/SDC and (d) BCFN/SDCN bilayers after electrochemical tests.
Table 1 Relative density, average grain size and activation energies for bulk and grain boundary conductivities. Sample
Relative density (%)
Average grain size (mm)
Bulk activation energy (eV) 300e600 C
Grain-boundary activation energy (eV) 300e600 C
SDC SDCN
95.8 95.5
1.84 2.81
0.70 0.70
1.01 1.01
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the Si distribution between the two symmetric cells indicates that addition of NiO to SDC restricts siliceous impurity diffusion from the electrolyte to the electrode/electrolyte interface. The XRD results demonstrate that Ni2þ could not incorporate into the SDC lattice. Large amounts of silicon in SDCN could be removed by NiO to the multiple grain junctions and Ni- and Si-rich secondary phase may be formed. The mobility of the Ni- and Si-rich secondary phase is lower, mitigating the effect of siliceous impurity diffusion in SDCN. 3.3. Electrical properties Fig. 3 shows the impedance spectra of the SDC and SDCN electrolytes measured at 400 C and the corresponding equivalent circuit. Instead of capacitors, constant phase elements (CPEs) are applied to model the obtained experimental data. CPEs account for the non-ideal behavior of the capacitive elements due to a variety of physical phenomena such as surface heterogeneity that originate from the effects of surface roughness, impurities, dislocations, and grain boundaries [17]. Contributions, in the order of decreasing frequency, of the grain bulk, the grain boundary and the electrode polarization behavior can be clearly identified. As shown in Fig. 3, the grain boundary resistance Rgb can be significantly decreased by adding 0.5 at.% NiO to SDC. Fig. 4 shows the temperature dependences of the bulk conductivity sbulk, grain boundary conductivity sgb, apparent specific grain boundary conductivity divided by the bulk conductivity sas gb/ sbulk and total conductivity stot. The bulk conductivity is calculated based on sbulk ¼ L/SRbulk, where L is the sample thickness and S is the electrode area on the sample surface. Thus, the grain boundary conductivity sgb and the total conductivity stot can be obtained. The Arrhenius relationship can be expressed as s ¼ A/Texp(Ea/kbT), where A, Ea, and kb represent the pre-exponential factor, the activation energy, and the Boltzmann constant, respectively. Here, ln(sT) is plotted against 1000/T, and the plot should show a linear relationship between these two quantities. The calculated activation energy values are listed in Table 1. Inspection of Fig. 4a shows that the addition of NiO has a negligible effect on the bulk conductivity and on the activation energy for bulk conductivity. This could be attributed to the very low solubility of Ni2þ in the SDC
lattice, as proved by XRD. As shown in Fig. 4b, the sgb value of SDCN is greater than that of SDC. As shown in Table 1, the relative densities of both samples are above 95%, suggesting that relative density is not the key factor leading to the difference in sgb between SDC and SDCN. The increase in the sgb value can be explained as follows. The grain boundary conductivity sgb depends on the bulk conductivity, space-charge potential, siliceous impurity at the grain boundaries and average grain size [3]. The average grain size of SDCN is larger than that of SDC, as shown in Table 1. To eliminate the influence of the number of grain boundaries on sgb, the apparent specific grain boundary conductivity sas gb is calculated. sas gb is an experimental datum and can be calculated from sas gb ¼ Ldgb/SRgbdg [3,4], where dgb is the grain boundary thickness and dg is the average grain size. For simplicity, the grain boundary thickness is taken to be 5.0 nm while calculating sas gb [4,18]. Because as sas gb is associated with sbulk, it is necessary to compare the sgb/sbulk values of SDC and SDCN. Fig. 4c shows the temperature dependence of sas gb/sbulk. For materials with normal purity values, based on the space-charge layers and siliceous impurity at grain boundaries, the apparent specific grain boundary conductivity sas gb can be expressed as [3,18].
sas u2 4eD4ð0Þ 2eD4ð0Þ gb exp ¼ 2 kB T sbulk dg kB T
(1)
where u2 is the area of grain-to-grain contact and D4(0) is the space-charge potential. The u2/d2g term is called the “impurity blocking term” and represents the clean fraction of resistive siliceous films at the grain boundaries. In our previous research, we reported that the space-charge potential D4(0) is proportional to the slope of the ln(sas gbT/sbulk)e1000/T curve [18]. As shown in Fig. 4c, the slope of the curve remains almost unchanged, indicating that 0.5 at.% NiO has little influence on the space-charge potential D4(0). As also shown in Fig. 4c, addition of 0.5 at.% NiO to SDC leads as to an increased sas gb/sbulk. According to Eq. (1), the increase in sgb/ 2 2 sbulk should result from the increase in u /dg. As discussed in section 3.2, large amounts of silicon in SDCN could be removed by NiO to the multiple grain junctions and Ni- and Si-rich secondary phase may be formed. The blocking effect of the grain boundaries contributed by the siliceous impurity in the SDCN sample is thus reduced, leading to the increase in u2/d2g. This indicates that the addition of NiO can mitigate the harmful effect of siliceous impurity on grain boundary conduction. The temperature dependence of the total conductivity is shown in Fig. 4d. Over the entire temperature range, SDCN exhibits higher total conductivity values than SDC. At 800 C, the total conductivity of SDCN is 4.93 102 S cm1. In our previous work [6], Fe2O3 was identified to be an effective additive for improving the grain boundary conductivity of SDC. At 800 C, the total conductivity of Ce0.85Sm0.15O1.9250.5 at.% FeO1.5 is 5.97 102 S cm1, higher than that of SDCN in this study. Nevertheless, an investigation of SDCN is highly important because nickel is the most commonly used anode for SOFCs. Fig. 5a shows the impedance spectra of the symmetric cells measured at 500 C and the corresponding equivalent circuit. For these two symmetric cells, the impedance spectra show two successive semi-circles. The characteristic capacitance (C) and angular relaxation frequency (f) are calculated based on the following equations:
1n n
Fig. 3. Impedance spectra for SDC and SDCN measured at 400 C in air. The adopted equivalent circuit model is presented in the plot.
89
C¼R
1
Qn
(2)
90
H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
Fig. 4. Temperature dependence of (a) bulk, (b) grain boundary, (c) apparent specific grain boundary conductivity divided by the bulk conductivity and (d) total conductivity for SDC and SDCN electrolytes.
ðRQ Þn f ¼ 2p 1
(3)
where R is resistance and Q and n are the parameters that characterize the CPE used for the fitting. The capacitance values of the electrolytes are calculated using Eq. (2) by multiplying the impedance data by a volume factor, whereas those of the electrode responses are obtained by multiplying the impedance data by the electrode area. The intercepts of these spectra with real axis (Z') at HF correspond to the ohmic resistances Ro of the symmetric cells; this includes the bulk resistance of the electrolyte, ohmic resistance of the electrode, contact resistance at the electrode/electrolyte interface and contact resistance between the electrode and the current collector. The corresponding capacitances are ~109 F cm1 for the middle-frequency (MF) semi-circle (RgbCPE1) of the samples. Because the symmetric cell consists of an electrolyte and an electrode, these capacitance values can be attributed to the grain boundary response of the electrolyte and the grain boundary response of the electrode. The capacitance values extracted from the low-frequency (LF) semi-circle (RelCPE2) for the samples are ~102 F cm2, which can be attributed to the electrode process. Fig. 5b presents the temperature dependence of the electrode interfacial polarization resistance Rel. As shown in Fig. 5b, the Rel
value of the symmetric cell using the SDCN electrolyte is lower than that of the symmetric cell using the SDC electrolyte. As discussed in our previous work, Rel decreases with increasing oxygen ion conductivity of the electrolyte and increases with increasing average grain size of the electrolyte [12]. As shown in Fig. 4d, SDCN exhibits a higher oxygen ion conductivity than SDC, which is expected to result in a decrease in the Rel of the symmetric cell using the SDCN electrolyte. At the same time, as shown in Table 1, the average grain size of SDCN is larger than that of SDC, which is expected to give rise to an increase in the Rel of the symmetric cell using the SDCN electrolyte. Furthermore, as shown in Table 2, the Si content at the BCFN/SDCN interface is lower than that at the BCFN/SDC interface, which is beneficial for the oxygen ion transfer process at the electrode/electrolyte interface. The symmetric cell using the SDCN electrolyte exhibits a lower Rel value than the symmetric cell using the SDC electrolyte. This is mainly caused by the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface.
3.4. Single cell performance Fig. 6a shows the IeV (current density and cell voltage) characteristics and corresponding IeP (power density derived from IeV)
H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
91
Fig. 5. (a) Impedance spectra for the symmetric cells of BCFN/SDC/BCFN and BCFN/ SDCN/BCFN measured at 500 C in air and (b) temperature dependence of electrode interfacial polarization resistance Rel.
Fig. 6. (a) Single cell performance based on SDC and SDCN electrolyte with Ni0.9Cu0.1Ox-SDC as anode and BCFN as cathode tested at 600 C and (b) impedance spectra of single cells based on SDC and SDCN measured under open circuit conditions at 600 C.
curves for fuel cells based on the SDC and SDCN electrolytes tested at 600 C. The obtained open circuit voltages (OCVs) and maximum power densities are listed in Table 3. It is clear that the SDCN-based cell shows a much better performance than the SDC-based cell. Fig. 6b shows the impedance spectra of the SDC- and SDCN-based cells measured under open circuit conditions at 600 C. For these cells, the impedance spectra show two successive semi-circles. The characteristic capacitance values for the semi-circles are also calculated using Eq. (2). The corresponding capacitance values are ~104 F cm2 for the MF semi-circle of the cells. Generally, these capacitance values could be related to the oxygen ion transfer followed by the charge-transfer process. The capacitance values for the LF semi-circle of the cells are ~102 F cm2. These capacitance values are too large to be associated with the double layer capacitance. This indicates that the LF process is most likely related to the oxygen exchange and diffusion process. As shown in Fig. 6b, the resistance values for the frequencies greater than 105 Hz are the ohmic resistances Roc of the cells. The intercepts of the spectra with a real axis at low frequencies represent the total resistances Rtc of the cells. The interfacial polarization resistances Rpc of the cells can
be calculated as the difference between Rtc and Roc (Rpc ¼ Rtc e Roc). The Roc, Rpc and Rtc values of the cells are listed in Table 3. Examination of the data in Table 3 shows that the SDCN-based cell exhibits a lower Roc value than the SDC-based cell. The main reason for this is that SDCN exhibits a higher oxygen ion conductivity than SDC, as shown in Fig. 4d. Additionally, the SDCN-based cell exhibits a lower Rpc value than the SDC-based cell, which can be attributed to the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface, as mentioned in section 3.3. Examination of the data presented in Table 3 also shows that the SDCN-based cell exhibits a lower Rtc value than the SDC-based cell. Because NiO in SDCN can be reduced to Ni under hydrogen atmosphere, the stability of the SDCN-based cell is examined in a short-term cell test. Fig. 7 shows the time dependence of the open circuit voltage (OCV) of the SDCN-based cell tested at 600 C. The cell shows a very stable performance with no significant cell voltage degradation during the 10 h testing period, indicating that extra electronic conduction cannot be introduced. This is because the NiO content in SDCN is quite small and the electronic
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H. Wang et al. / Journal of Power Sources 320 (2016) 86e93
Table 3 Open circuit voltages, maximum power densities, ohmic resistances, interfacial polarization resistances and total resistances of single cells tested at 600 C. Sample Open circuit voltage (V) Maximum power density (W cm2) Ohmic resistance (U cm2) Interfacial polarization resistance (U cm2) Total resistance (U cm2) SDC SDCN
0.888 0.919
0.144 0.208
0.76 0.49
0.48 0.36
1.24 0.85
larger than that of SDC. The EDX results indicate that the addition of 0.5 at.% NiO to SDC is effective for mitigating the diffusion of siliceous impurities from the electrolyte to the electrode/electrolyte interface. Analysis of the electrical properties show that the addition of 0.5 at.% NiO to SDC has little influence on the space-charge potential. The increase in the apparent specific grain boundary conductivity arises from the increase in the conduction path width determined by the siliceous impurity. The electrode interfacial polarization resistance of the symmetric cell using the SDCN electrolyte is lower than that of the symmetric cell using the SDC electrolyte, which can be attributed to the increase in the oxygen ion conductivity of the electrolyte and the decrease in the Si content at the electrode/electrolyte interface. The SDCN-based cell shows a much better performance than the SDC-based cell. The maximum power densities of the single cell based on the SDCN electrolyte are 0.208, 0.324, 0.471, 0.630 and 0.745 W cm2 at 600, 650, 700, 750 and 800 C. Acknowledgements Fig. 7. Time dependence of the open circuit voltage (OCV) of the cell based on SDCN tested at 600 C.
This work was supported by the National Natural Science Foundation of China (Nos. 51272087 and 50872041) and the National Foundation for Fostering Talent in Basic Science of China (No. J1103202). References
Fig. 8. Single cell performance based on SDCN electrolyte with Ni0.9Cu0.1Ox-SDC as anode and BCFN as cathode tested at different temperatures.
conduction path through Ni cannot be formed. Fig. 8 shows the IeV and power density curves of the SDCN-electrolyte-based cell at different temperatures. The obtained maximum power densities are 0.208, 0.324, 0.471, 0.630 and 0.745 W cm2 at 600, 650, 700, 750 and 800 C.
4. Conclusions Ce0.85Sm0.15O1.925 (SDC) and Ce0.85Sm0.15O1.9250.5 at.% NiO (SDCN) are examined from the standpoint of their application in SOFCs as electrolytes. The SDC and SDCN electrolytes exhibit the single-phase fluorite structure. The average grain size of SDCN is
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