Effect of ZnO Addition on the Sintering and Electrical Properties of Ceria‐based Electrolyte Materi

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Effect of ZnO Addition on the Sintering and Electrical Properties of Ceria‐based Electrolyte Materials Liu Ying1, Wang Xiuping1, Zhou Defeng1*, Ning Dezheng1, Zhang Guanming1, Meng Jian2 School of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, People’s Republic of China 1

State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China 2

*154074924@qq.com

Abstract The Ce0.9Gd0.1O2‐δ ceramics with 500 ppm SiO2 and with the different dopant of Zn (0.5, 1.0, 2.0, 3.0 at.%) were prepared by sol‐gel method. All the materials were single phase with a cubic fluorite structure. The relative densification reached a maximum at 1.0 at.% ZnO sintering at 1500 ℃ for 12 h. The conductivities showed sharp increase for the ceria system that contained of ZnO in the range of 0.5 to 1.0 at.% and beyond 1.0 at.%, the conductivity slightly decreased. For the solution adding 1.0 at.% ZnO had the highest total conductivity and grain boundary conductivity (  t = 3.8×10‐3 S∙cm‐1,



gb

= 3.3×10‐2 S∙cm‐1 at 550 ℃) compared to the ceria

system without ZnO (  gi = 2.4×10‐3 S∙cm‐1,  = 6.4×10‐3 gb

S∙cm‐1 at 550 ℃).

Keywords Ce0.9Gd0.1O2‐δ; ZnO‐doping; Scavenging Effect; Grain Boundary Conductivity

Introduction In recent years, due to the higher ionic conductivity and lower interfacial losses with electrode, ceria‐based electrolytes are considered as the promising candidate for developing intermediate temperatures solid oxide fuel cell (IT‐SOFC) (Anjaneya, K. C. et al., 2013).

Among the doped ceria‐based electrolytes, the GDC stands out for its excellent electrical conductivity (Anjaneya, K. C. et al., 2013; Arabacι, A. et al., 2012). As reported by Jadhav et al. (Chourashiya, M. G. et al., 2008), all GDC samples sintered at 1500 ℃ showed uniform and smoother surfaces with conductivity 0.1 S∙cm−1 at 800 ℃ and activation energies less than 0.9 eV. However, critical challenges limit seriously its applications because they are difficult to densify below 1500 ℃. Additionally, in the low‐temperature region, the grain boundary (GB) behavior usually dominates

the total conductivity for doped ceria (Zhang, T. S. et al., 2006; Kim, D. S. et al., 2006; Gil, V. et al., 2007), while the grain and grain boundary conductivity all work above 550 ℃ (Yang, M. et al., 2012). But the grain boundary conductivity of non‐pure system is mainly connected with silicate film distributing in grain boundary, they are inadvertently incorporated in the starting ceramic powders during manufacturing process and consequently form amorphou layers (Verkerk, M. J. et al., 1982; Tian, C. et al., 2000), and thus block the transportation of charging carries, which lead to higher grain boundary resistivity (Gerhardt, R. et al., 1986; Gerhardt, R. et al., 1986).

Therefore, a large number of the transition and alkaline earth metal oxides (ZnO (Zhou, H. F. et al., 2008), MoO3 (Zhao, G. C. et al., 2011), SrO (Gao, Z. et al., 2011) and MgO (Cho,Y. H. et al., 2007)) as additives doped ceria‐basedsolid solutions have been systematically investigated to improve the grain boundary performance and decrease the sintering temperature. In MgO‐doped CGOSi, the MgO react with siliceous phase and form the high conductivity of Mg2SiO4, thereby enhancing the grain boundary conductivity (Cho,Y. H. et al., 2007). The Fe2O3 is not only good sintering aids but also scavenger of SiO2 impurity in NDC/NDCSi (Dong, H. L. et al., 2009; Li, B. et al., 2010; Liu, J. W. et al., 2012). For the sintering aid, CuO has been reported effectively improving the densification but it has no significant effect on the grain boundary conductivity (Fagg, D. P. et al., 2003).

Also many researches focus on the zinc oxide, because it can promote ceramics densification and control grain growth during the sintering process. Therefore the effects of ZnO additive on the densification and electrical conductivity have been examined widely (Ge,

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L. et al., 2011). For example, the addition of 1 wt.% ZnO reduce the sintering temperatures from 1700 ℃ to 1500 ℃ in the GdSmZr2O7 ceramic (Liu, Z. G. et al., 2013). It is also reported that the ZnO‐doped BaZr0.85Y0.15O3−δ exhibited much lower grains boundary activation energy values than that of pure BaZr0.85Y0.15O3−δ due to the increased grain size and consequently decreased grain boundary space (Peng, C. et al., 2009). However, the ZnO‐doped ceria has been rarely reported (Gao, L. et al., 2010). In this paper, we prepared the powders of the Ce0.9Gd0.1O2‐δ with silica content of 500 ppm (GDCSi) with the different dopant of ZnO. The optimal amount of ZnO is confirmed at the different samples, at the same time we discuss the effects of ZnO doping on densification and conductivity (especially the grain boundary conductivity) in detail.

Experimental The Ce0.9Gd0.1O2‐δ with and without zinc doping is prepared by a modified sol–gel method. Analytical reagents Ce(NO3)3∙6H2O (≥99.5%), Gd2O3 (≥99.99%), C6H8O7∙H2O (≥99.5%) and Zn(NO3)2∙6H2O (≥99.5 %) are used as the starting materials. These starting materials were dissolved in the distilled water or nitric acid to form solutions, separately, and then mixed according to the stoichiometric ratios of the samples. Then solid citric monohydrate and 463 ppm SiO2 (about 37 ppm in Ce0.9Gd0.1O2‐δ determined by inductive coupled high frequency plasma (Zhu, J. X. et al., 2007)) that in the form of tetraethyl orthosilicate were added into the mixture with stirring at 50 ℃ . The transparent solution was vaporized overnight by water bath at 70 ℃, resulting in a dry gel. The precursors were calcined at 300 ℃ for 4 h. After pre‐calcination, the precursors were ball‐milled and pressed into pellets (diameter about 13.4 mm) under 45 MPa followed by sintering at 1300, 1400, 1500 ℃ for 12 h with Pt plate as the sample holder. Densities of the sintered pellets were determined by Archimedean method. The X‐ray diffraction (XRD) patterns were recorded on a Rigaku D/max‐IIB X‐ray diffractometer with Cu Kα1 radiation (λ = 0.15405 nm) at room temperature. Micrographs of the polished and thermal etched pellets sintered at 1200 ℃ and the synthesize powders were studied by the Field emission scanning electron microscopy (FE‐SEM, XP30, Philips) equipped with an energy dispersive X‐ray spectrometer (EDX) analyzer (XP30, Philips). Oxide ionic conductivity was analyzed by electrochemical impedance spectroscopy (Solartron 1260, UK) with a frequency response analyzer (Solatron 1255) and an electrochemical interface

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(SI1287). Silver electrodes were coated on both sides of the pellets and heated at 550 ℃ for 30 min. Impedance measurement was conducted on cooling from 800 ℃ to 300 ℃ in a frequency range from 1 MHz to 0.1 Hz with an increment of 50 ℃. The conductivity was calculated according to the thickness L, resistance R, and surface area S of the sample, using the expression

σ=

L R S

.

Results and Discussion X‐ray Diffraction Study Fig. 1 shows XRD patterns of un‐doped and ZnO‐doped GDCSi samples after sintered at 1500 ℃ for 12 h. We see clearly that all the samples formed a single solid solution with cubic structure having no secondary phase resulting from the limit of amount of it. In Fig. 2, the results show that the lattice constants decrease with the increase of doping ZnO in the cubic lattice of the GDCSi structure, which probably due to entering the GDCSi (Ce4+: 0.097 nm) lattice of Zn2+ (0.074 nm). However, under the 1.0 at.%, the lattice constants no longer change with the Zn content, implying that the solubility limit of Zn which enters GDCSi interstitially is estimated to be less than 1.0 at.%.

That indicate the solid solubility of ZnO in GDCSi is tiny.

(a) GDCSi (10GDC + 500 ppm SiO2), (b) GDCSi+0.5ZnO (GDCSi + 0.5 at.% ZnO), (c) GDCSi+1ZnO (GDCSi + 1.0 at.% ZnO), (d) GDCSi+2ZnO (GDCSi + 2.0 at.% ZnO) , (e) GDCSi+3ZnO (GDCSi+3.0 at.% ZnO) FIG. 1 THE XRD DIFFRACTION PATTERM OF ALL SAMPLES SINTERED AT 1500 ℃ FOR 12 H

FIG. 2 LATTICE PARAMETER OF GDCSi AS A FUNCTION OF THE ZnO CONCENTRATIONS. ALL THE SAMPLES SINTERED

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AT 1500 ℃ FOR 12 H. TABLE 1 FITTING EQUIVALENT CIRCUIT PARAMETERS AND CONDUCTIVITIES OF THE GDCSi AND GDCSi+1ZnO SAMPLES

Factor and samples

L0×10‐6/Hz

Rgi/Ω

Rgb/Ω

CPEgb/nF



t

/ s∙cm‐1



gb

/ s∙cm‐1

GDCSi 13.1 216.4 5647.3 71.6 3.1×10‐6 3.2×10‐5 GDCSi+1ZnO 10.5 136.2 3079.8 68.5 4.5×10‐5 4.9×10‐5 1.4×10‐4 GDCSi 6.9 168.5 867.6 79.4 1.2×10‐4 400 ℃ ‐4 2.2×10‐4 GDCSi+1ZnO 5.7 99.7 641.5 73.3 1.7×10 5.1×10‐4 GDCSi 4.2 167.4 386.8 89.5 3.7×10‐4 450 ℃ 1.1×10‐3 GDCSi+1ZnO 2.5 92.6 183.0 68.7 6.1×10‐4 ‐4 2.4×10‐3 GDCSi 2.6 72.9 50.4 84.6 9.7×10 500 ℃ 5.8×10‐3 GDCSi+1ZnO 2.3 60.1 24.0 71.8 1.7×10‐3 6.4×10‐3 GDCSi 2.2 67.8 18.6 87.3 2.4×10‐3 550 ℃ 3.3×10‐2 GDCSi+1ZnO 1.7 44.8 4.3 78.6 3.8×10‐3

GDCSi 1.8 22.5

5.3×10‐3 600 ℃

GDCSi+1ZnO 1.5 16.1

8.7×10‐3 * All the specimens contain 500 ppm of SiO2 impurities and were sintered at 1500 ℃ for 12 h L0: inductance of the experiment setup, Rgi: grain resistance, Rgb: grain boundary resistance, CPEgb: constant phase element of the grain boundary Rel: electrode resistance, CPEel: constant phase element of the electrode 350 ℃

Microstructure Development Three sintering temperatures (i.e., 1300, 1400 and 1500 ℃), were chosen in this work to evaluate the effect of ZnO doping levels on the sintered density.

The results are shown in Fig.3, it can be seen that the density of all samples increase with the increasing of the temperature and a small amount of ZnO addition.

For instance, the relative density of the GDCSi increases from 95.6% (un‐doped) up to 97.6% (1.0 at.% ZnO‐doped) respectively for the sample sintered at 1500 ℃ for 12 h, whereas the relative density has almost no increase when ZnO content was increased to 3.0 at.% after sintered from 1250 to 1500 ℃ . The change of relative density could be connected with grain size and porosity. Hence, we will continue to research the scanning electron micrographs of all samples that after thermal etching. Fig. 4 shows the FE‐SEM micrographs of the un‐doped GDC and ZnO‐doped GDCSi samples after sintered at 1500 ℃ for 12 h. It can be seen that the GDCSi sample is porous, while there are a few or even no porous in the GDCSi doped with ZnO content range of 0.5‐3.0 at.% (Figs.4b‐e). And the tendency of the grain sizes is presented in Fig. 3. The grain size has the same tendency as that of the relative density. The GDCSi+1ZnO sample shows the perfect grain size, and why the grain size has this variation trend? The reason will be analyzed from two aspects. On the one hand, the lattice constants decreasing with the doping content increasing to 1.0 at.% in Fig. 2. But with the doping content increasing to 1.0 ‐ 2.0 at.%, the grain size remain unchanged, indicating that the solid solubility limit of ZnO is 1.0 at.%. While little ZnO doping into the crystal lattice of GDC, which cause the



gi

/ s∙cm‐1

6.2×10‐4 8.2×10‐4 8.3×10‐4 1.2×10‐3 1.3×10‐3 1.3×10‐3 1.6×10‐3 2.3×10‐3 2.7×10‐3 4.3×10‐3 5.3×10‐3 8.7×10‐3

lattice distortion on account of the radius of Zn2+ ion (0.074 nm) smaller than Ce4+ ion (0.097 nm), and the lattice distortion can cause grain boundary migration that lead to the growth of grain at last (Zając, W. et al., 2009). However, when the doping content of ZnO keep increasing to 3.0 at.%, part of the ZnO existing in grain skin, which has the deleterious effect on grain boundary migration, and thus leading to suppress the growth of grain. Therefore, the addition of ZnO is higher than the 1.0 at.% can restrain the growth of the grain. Above all, small amount of ZnO doping promote the growth of grain, which thus lead to a remarkable densification of GDCSi+1ZnO electrolyte.

FIG. 3 EFFECT OF ZnO‐DOPING LEVEL ON THE SINTERED DENSITY FOR 12 H AT 1300 ℃ (A), 1400 ℃ (B), 1500 ℃ (C) AND GRAIN SIZE OF GDCSi SAMPLES SINTERED AT 1500 ℃ FOR 12 H.

Electrical Properties Fig. 5 shows impedance spectra of GDCSi+1ZnO electrolytes. Such a type of impedance spectra is well fitted to the conventional equivalent circuit consisting of a series of 3‐sub‐circuits of parallel resistors‐ constant phase elements (R‐CPE) as shown in Fig. 5(e) and (f) (Zając, W. et al., 2009). A part of fitting parameters with the fitting error less than 5% are

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GDCSi+2ZnO and GDCSi+3ZnO. The second reason is that with the increasing of ZnO‐doped concentration, " ) increased, and the concentration of dopant ion ( ZnCe as the charge compensation, the oxygen vacancy concentration increased, which leads to the promotion of conductivity. However, beyond 1.0 at.% of ZnO, the grain boundary and total conductivities have slight change and even decrease slightly.

of high frequency (HF), middle frequency (MF), and low frequency (LF), respectively. Nevertheless, in order to estimate the bulk and grain boundary resistances more exactly, we may extrapolate the grain boundary resistance (Rgb, ext) from the total resistance (Rt) to estimate the grain resistance (Rgi ≈ Rt −Rgb,ext) (Pérez‐Coll, D. et al., 2006, Pérez‐Coll, D. et al., 2006, Pérez‐Coll, D. et al., 2007). It is observed that the GDCSi with ZnO (1.0 at.%) has a half of arcs than the un‐doped GDCSi especially the grain boundary arcs, indicating a small grain boundary resistance. In table 1, the grain boundary resistance of GDCSi+1ZnO is 183.0 Ω (Table 1) at 450 ℃, which is about a half of the grain boundary resistance of the GDCSi.

t

higher than GDCSi (  = 2.4×10‐3 S∙cm‐1) at 550 ℃. The t total conductivity is common influenced by the grain

FIG. 4 FE‐SEM MICROGRAPHS OF ALL SAMPLES SINTERED AT 1500 ℃ FOR 12 H (A) GDCSi, (B) GDCSi +0.5ZnO, (C) GDCSi+1ZnO, (D) GDCSi+2ZnO (E) GDCSi+3ZnO

-2

Z''()

Fig. 6a shows Arrhenius plots of the total conductivity for the GDCSi and with ZnO‐doped samples. From it we can see the total conductivity increases with increasing ZnO content and until up to 1.0 at.%, but decreases when the concentration of ZnO increases further. GDCSi+1ZnO showed the maximum conductivity (  = 3.8×10‐3 S∙cm‐1), which is much

-40

700oC

a

GDCSi GDCSi + 1ZnO

-1

Rgi

Z''()

shown in Table 1. From Fig. 5, we can see that not all these three arcs are observed at any temperature owing to the limited frequency range of the equipment (0.1 Hz to 1 MHz) and the character of the samples, and with temperature increasing, the frequency range shifts to higher and the relaxation frequency of the grain boundary is significantly lower at the intermediate temperature due to the higher capacitance values. Their interceptions with real axis (Z) corresponded to bulk, grain boundary, and electrolyte/electrode interfacial polarization resistances ( Rgi , Rgb and Rel ) according to their frequency range

GDCSi GDCSi + 1ZnO

-20

Rgi+Rgb Rgi

t

0 6

8 Z'( )

-400

GDCSi GDCSi + 1ZnO

-10k

R

40 Z'( )

R el

60

80

o

d

450 C

Rgi R +R gi gb

20

-20k

Z''()

-200

0 0

10 o

c Z''()

and grain boundary conductivity. The bulk conductivity depends on the character of sample. Thus the grain interior behavior of GDCSi was almost unaffected by the additives (ZnO) used in this experiment. We can pay more attention to the behavior of grain boundary.

550oC

b

300 C

Rgi Rgi+Rgb

el

GDCSi GDCSi + 1ZnO

R el

0 0

200

400 Z'()

600

800

0k 0k

20k Z'( )

40k

Fig. 6b shows the grain boundary conductivity of all the samples. Note that the conductivity of grain boundary has the same tendency as that of the total, and we obtained the maximum at 1.0 at.% (  = gb

3.3×10 S∙cm at 550 ℃). The increase in the grain boundary conductivity is mainly due to three reasons.

The first reason is the GDCSi+1ZnO has the higher relative density, which is similar to that of the ‐2

64

‐1

FIG. 5 IMPEDANCE SPECTRA (A‐D) OF GDCSi SINTERED AT 1500 ℃ WITH AND WITHOUT 1.0 AT.%ZnO DOPING, THE EQUIVALENT CIRCUITS USED IN ANALYZING THE IMPEDANCE SPECTRA (E, F).

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2

a

t

0

GDCSi GDCSi + 0.5ZnO GDCSi + 1.0ZnO GDCSi + 2.0ZnO GDCSi + 3.0ZnO

-2 -4 -6 0.8 1.0 1.2 1.4 1.6 1.8 1000/T(K-1)

4 ln[T.]( K.s.cm-1)

ln[T.]( K.s.cm-1)

4

b

2 0

gb

(No. 20101549).

GDCSi GDCSi + 0.5ZnO GDCSi + 1.0ZnO GDCSi + 2.0ZnO GDCSi + 3.0ZnO

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FIG. 6 ARRHENIUS PLOTS FOR GDCSi SAMPLES WITH AND WITHOUT ZnO DOPING AT 1500 ℃ FOR 12 H: (A) TOTAL CONDUCTIVITY; (B) GRAIN BOUNDARY CONDUCTIVITY

On the basis of interrelated reports before (Zhou, D. F. et al., 2009), if the amount of dopant ions is too much, it may tap into ordered vacancy clusters (e.g. Zn ''Ce VO.. / Zn ''Ce VO.. Zn ''Ce ) at the expense of free oxygen vacancies，that might astrict the transport of oxygen vacancy on crystal lattice and at last the conductivity decreased. The last reason is probably that of the ZnO is likely to be as a scavenging aid that induced the strong aggregation of the siliceous intergranular phases at the grain boundary, because of the trace amounts of ZnO modifying the viscosity and wettability of the siliceous Phase (Ge, L. et al., 2013).

However, when the addition of ZnO is more than 1.0 at.% in GDCSi, which causes the macrosegregation of superfluous ZnO and silicon dioxide phase in the grain boundary, which leads to the increases of grain boundary and total resistances. Therefore, the optimum adding amount of ZnO is found to be 1.0 at.% in GDCSi.

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Conclusions

Dong, H. L.; Kwang, S. C.; Young, S. L.; Kyoung, S. K.; Chu, S.

We have successfully synthesized the solid solutions of GDCSi with ZnO‐doped by a sol–gel method. With the dopant of ZnO, the relative density, grain size, and microstructure have improved evidently and the sintering temperature of GDCSi decreases. The relative density of the GDCSi increases from 95.6% (un‐doped) up to 97.6% (1.0 at.% ZnO‐doped) for the group of samples sintered at 1500 ℃ for 12 h. The total conductivity especially grain boundary conductivity also increases，and the GDCSi with 1.0 at.% ZnO‐doped has highest conductivity (  = 3.8×10‐3

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This work is financially support by National Natural Science Foundation of China under the project No. 20871023 and by the Jilin Provincial Science Research Foundation of China

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