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8YSZ – Carbonate composite electrolyte-conductivity enhancement
Journal Pre-proof 8YSZ – Carbonate composite electrolyte-conductivity enhancement Santanu Basu, Md. Nazmul Alam, Swarnavo Basu, Himadri S. Maiti PII:
S0925-8388(19)33807-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152561
Reference:
JALCOM 152561
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: S. Basu, M.N. Alam, S. Basu, H.S. Maiti, 8YSZ – Carbonate composite electrolyte-conductivity enhancement, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.152561. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
8YSZ – Carbonate Composite Electrolyte-conductivity enhancement a b b b Santanu Basu# , Md. Nazmul Alam , Swarnavo Basu and Himadri S. Maiti
b
a
Department of Ceramic Technology, Govt. College of Engineering & Ceramic Technology, India
SMSE, IIEST-Shibpur, India
#Corresponding Author Email: [email protected]
1
Abstract A composite electrolyte based on 8YSZ and Li/Na carbonate eutectic is prepared and characterized according to requirement of electrolytes. The powdered eutectic is mixed with 8YSZ powder in two different proportions. Sintered samples have been characterized by XRD, SEM and impedance spectroscopy. Fractured micrographs of the compositions show distinctly different morphology of the carbonate phase due the quantity of the carbonate phase addition. Impedance spectroscopy provides separate conductivities of the carbonate phase, YSZ phase, inter-phase boundary and that of the electrolyte-electrode interface. Equivalent circuits of the transport phenomena have been proposed. The conductivity of the carbonate phase is significantly higher than that of the YSZ phase and accordingly the overall conductivity of the composite electrolyte increases with increasing quantity of the carbonate phase. At 600oC the composite with 20wt% carbonate phase shows the conductivity of 1.6x10-3S-cm-1 compared to 1.78x10-4Scm-1for pure 8YSZ.
Keywords: YSZ, Carbonate Eutectic, Composite Electrolyte ,Impedance Spectroscopy.
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1. Introduction Intermediate-temperature solid oxide fuel cells (ITSOFCs) become a potential technology in the field of non conventional energy generation. Even using thin-film technology traditional yttrium-stabilized zirconia(YSZ) electrolyte incapable to suit necessity of low-temperature operation [1]. Therefore research thrust to develop new electrolyte materials with high ion conductivities at lower temperature for advanced ITSOFCs become important [2,3]. Recently a new type of composite electrolyte was developed with doped-cerium oxide as parent material and carbonates as the second phase. Second phases of these composite electrolytes improved ionic conductivity and adjustable multi-ionic conductivity of the electrolytes [4,5]. Improvement of ionic conductivity in such composite ceria carbonate electrolytes extended the concept to other solid electrolytes [6]. While composite electrolyte based on ceria system as well as other oxide systems have been studied extensively, systems based on YSZ, the most established oxygen ion conductor has not been studied much. Thus opportunity exists to study preparation and characterization of carbonate – YSZ composite material in terms of sintering and electrical conduction measurements. Impregnation of carbonates in YSZ has been studied by Ahn et al for application of CO2 separation [7], whereas most of the ceria carbonate composites approach based on sintering of oxide in the presence of carbonate eutectic [8]. Our present study is aimed at preparation of a composite electrolyte based on sintering of 8 mol% YSZ in presence of a eutectic mixture of Li2CO3 and Na2CO3 followed by its complete micro-structural and electrical characterization.
2. Experimental Li2CO3 (99.0%, Loba, India) and Na2CO3 (99.0%, Loba, India) were mixed to obtain a eutectic composition having 43.1 wt% Li2CO3 and rest Na2CO3. The mixture was melted at 600°C in a vitreous silica crucible and held for 2 hours for effective melting and homogenization. The eutectic melt is then cooled down to room temperature and ground in an agate mortar. This fine eutectic mixture is then mixed with 8YSZ (Tosho, Japan) powder in the weight percentages of 10 and 20. The composite mixtures were then uni-axially pressed in a steel mould with diameter of 15mm and thickness of around 5mm. The pellets, thus formed, were sintered for 2 hours at 1100 °C for the samples containing 20 and 10 wt% eutectic respectively.
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During sintering at high temperature special precaution has been taken to prevent carbonate decomposition by controlling sintering atmosphere. The phase identification of the sintered samples was confirmed from the x-ray powder diffraction (XRD) data collected at a scan rate of 2° per minute on a Philips PW 3207 (Holland) diffractometer with CuKα radiation. The fracture surfaces of the sintered samples were examined by field emission scanning electron microscope (FE-SEM) using Supra 35VP (Carl Zeiss, Germany). Platinum paste (Gwent Electronics, U.K.) was applied on both surfaces of the sintered samples (~13mm dia. and 3mm thick) and cured at 800°C in air. Two probe ac impedance measurements were conducted on the samples in the frequency range 0.1 Hz to 10 MHz with an applied voltage of 0.1 V using a Solartron frequency response analyzer (FRA 1260) coupled with a Solartron 1296 dielectric interface unit in the temperature range 300–850°C. Impedance data were collected at every 50°C interval and a heating rate of 3°C/min was used for increasing the temperature.
3. Results and Discussion 3.1.
XRD and Density: Variation of bulk densities (BD) obtained from geometrical dimensions of the sintered
samples is shown in Fig. 1. As the density of the eutectic composition is much lower than that of pure 8YSZ, increasing eutectic content decreases in the bulk density of the composite material. Apparent density (A.D.) were calculated by Archimedes principle using distilled water as liquid medium. The A.P. and B.D. for composites have been found to be very close in value(as shown in Fig.1) indicating dearth of open pores. XRD of the sintered samples are shown in Fig. 2. Major reflections are indexed for 8YSZ (JCPDS file no. 30-1468). For 10wt% composite specimen no other phase except YSZ is evident, thus the presence of the eutectic phase is not detected. In contrast to this, 20wt% composite shows a few extra reflections, which are identified as those of Li - Na carbonate (JCPDS no. 852296). This finding is in contrast to that of Jaswal et al. [9], who have reported absence of any such crystalline phase even after incorporating as high as 35 wt % of eutectic mixture to ceria based electrolyte. Thus for 10wt% composite the eutectic may be in amorphous state. But for 20wt% composite the situation is different. Li - Na carbonate peaks indicates the presence of crystalline phases of the eutectic composition. Both 10wt% and 20wt% compositions do not indicate the presence of any Li2ZrO3 or other zirconate phases as reported by Wade et al. [10]. 4
3.2.
Microstructure: FE-SEM pictures of fracture surfaces of 10 and 20 wt% composite electrolytes are shown
in Fig. 3 (a) and (b) respectively. It is interesting to note that the micro-structural characteristics of the samples are distinctly different from those previously reported for similar studies with ceria based materials [11, 12] as most of them reported surface morphology. Both the pictures shown in Fig. 3 indicate microstructures with nearly zero porosity which was not evident in case of SDC – eutectic compositions reported earlier [13]. Higher temperature (10000C) of sintering compared to the eutectic temperature of around 500°C used in this work may have resulted in such an impervious microstructure. It is evident from Fig. 3 that the eutectic carbonate phase is present either in the form of thick thread-like morphology (Fig. 3a) or fine needles (Fig. 3b) within the matrix of YSZ grains. Such appearance of acicular crystalline phase has been reported by Huang et al. [14] for SDC with 45 wt% eutectic composition without any plausible explanation for such difference. It is also interesting to note that the threads present in the 10wt% sample is much thicker and resemble a glass phase
whereas the fine needles observed in 20wt% sample are fine
crystalline needles dispersed in composite matrix of glass-YSZ. The plausible reason is at high temperature of sintering for the former (10 wt %) having less volume of molten liquid phase at which the viscosity of the eutectic is expected to higher than the 20wt% composition. Quenching rate being high the relaxation ceases and the eutectic phase remains in the amorphous state for high viscous 10wt% composition. In case of 20 wt % eutectic compositions due to presence of higher amount liquid at high temperature viscosity become low enough to complete atomic relaxation ending with acicular crystals of carbonates. Another very interesting observation from these micrograph is that the volume percentages of eutectic and YSZ in the 10% eutectic composition, Fig 3(a) indicates that they are approximately 25% and 75% respectively. In contrast, for 20% eutectic composition Fig 3(b) the volume percentages are nearly same (50:50). A composite like structure of homogeneously distributed non directional network of an acicular phase embedded in a fine particulate structure is evident in this figure. Considering the density of 8YSZ (6.10g/cc), Li2CO3 (2.11 g/cc) and NaCO3 (2.54g/cc), the theoretical volume percentages of eutectic and YSZ for 10% eutectic composition are calculated to be 22.4% and 77.6%. The same density data results in theoretical volume percentages of 39.3% and 60.7% eutectic and YSZ respectively for the sample containing 20% eutectic. For the 10% eutectic composition the predicted volume percentage is almost same as that of actual finding as described above; whereas a huge mismatch is evident for 20% eutectic composition. The 50 volume % of eutectic is much higher than the theoretically calculated value 5
of 39.3%. The presence of much higher volume of eutectic mixture can only be satisfactorily explained by the assumption of dissolution of YSZ in eutectic at higher sintering temperature which in term may be the nucleating centre of heterogeneous crystallization of eutectic phase.
3.3.
Electrical Conductivity: As mentioned before, electrical impedance of the samples has been measured as functions
of frequency at temperatures ranging from 500 to 700°C. The data are presented in “Nyquist Plot” of real and imaginary parts of the impedance (Z′′ vs Z′) as parametric function of the frequency (Fig. 4). Complex plane impedance plots for the sintered samples of 8YSZ with 10 and 20 wt% eutectic composites at different temperatures in the range of 500 – 700°C are shown in Figs. 4 (a) and (b), respectively. There are certain similarities between the two sets of plots; for examples in both the figures the low temperature plot appears on the right side of x-axis (away from origin), indicating higher resistance of the sample and it slowly shifts to the left (closer to the origin) with increasing temperature due to lowering of the overall resistance. The shape of the plot, however, changes quite significantly as the temperature increases with consequent decrease in resistance. The semicircles become more distinct but with reduced size as the temperature increases. In Fig 4 both (a) and (b) the insets show magnified plots corresponding to a few temperatures for which the shape of the curves are not clearly visible in the main plot. In both the cases, the semicircular nature of the plots becomes progressively distinct at increasing temperatures with consequent reduction in overall resistance. As the temperature increases the relaxation time (τ) for conductive species decreases as the conduction of all the ionic species are thermally activated. The peak of complex plane plot is determined by the relation τ×ω=1, where ω is angular frequency, thus distinct peaks appear at higher temperature. The high frequency data at 500°C for both compositions are compared in Fig. 5. The intersection with the real axis for 20wt% is closer to the origin indicating that it has lower resistance than the 10wt% specimen. Additionally, the 20wt% specimen shows an indication of negative impedance below the real axis indicative of the predominance of an inductive part of the impedance due to relatively low resistance. The same is not evident for 10wt% sample except above 550°C (not shown in this figure). The appearance of such inductive component is normally observed for regular solid electrolytes e.g. YSZ at temperatures higher than 1000°C. The data at lower temperatures and at low measurement frequencies for both the samples indicate a nearly linear plot primarily due to very high resistance of the samples. This linearity is not due to predominance of any diffusion controlled charge transfer reaction normally observed at 6
the electrode-electrolyte interface. With increasing measurement temperature the semicircles become better visible even though they are not perfect but depressed ones due to distributed relaxation times. This is in contrast with the observation reported by Chockalingam et al. [15] and Lapa et al. [16] for samarium oxide doped ceria (SDC) and (NaLi)2CO3 composite electrolyte. They proposed that the presence of a single high frequency semi circle corresponds to composite effect due to the coexistence of numerous processes with different relaxation times. The lowest measurement temperature used in this investigation is 500°C, which is marginally higher than the eutectic temperature of the carbonate mixture (~490°C). At this temperature the carbonate phase is expected to form a soft or semi-solid phase having a significantly different relaxation time for carbonate ion migration than that of the oxygen ion in the solid YSZ phase whose resistance is expected to be much higher than that of the carbonate phase. This may give rise to the occurrence of separate semicircles; the higher frequency one being due to carbonate ion migration and the lower one arising from the oxygen ion migration through YSZ grains. In both the cases, the experimental data have been fitted to several depressed semi-circles with the help of Z-view curve fitting software. Proposed equivalent circuits are presented as insets to these figures. Complex plane semicircles are often described by a parallel circuit with one resistor and one capacitor (R||C) while depressed semicircles can be explained by a simple parallel circuit with one resistor and one constant phase element (R||CPE). In case of 10% sample Fig. 6(a) two distinct semicircles are visible. The highest frequency semicircle is not visible, may originate from CO32- conduction in the eutectic phase. The next semicircle (actually observed in this investigation) may be due the O2- conduction in YSZ along with its inter-phase boundary with eutectic carbonate. The third dispersed semicircle is attributed to the combined effect of electrode phenomena and the carbonate-oxide interface transport. For the 20% sample depicted in Fig. 6(b), there appear to be three different semicircles; once again there is no experimental data for the highest frequency one. The First high frequency data (not observed) may originated from the carbonate conduction in glassy phase, second one from carbonate conduction in crystalline phase and inter-phase boundary, third and the fourth ones possibly correspond to the two interface phenomena: i) boundary between the carbonate and the oxide phases and ii) electrolyte-electrode interface. As several ions (Li+1,Na+1, VO••, CO3= in crystalline and glassy phase) presents in these systems the overlapping of relaxation time of their transportation causes depression in the formed impedance semicircles. Resistances calculated from the impedance plots as described above are converted to conductivity using geometrical factors and plotted as Arrhenius plots as shown in Fig.7. In Fig. 7
7(a) the data for the eutectic carbonate phase, inter-phase boundary and the overall conductivity of 10wt% composition are shown; all these data appear to follow a linear relationship. In Fig. 7(b) similar plots for 20wt% composition are shown. As discussed earlier in this case several semicircles can be resolved and they are designated as those arising from the conduction in the carbonate in eutectic phase, bulk YSZ and inter-phase boundary. Total resistance of the samples has been calculated from the intercept (extended) of the last semicircle (without the low frequency electrode contribution) with Z/ axis. The resistance thus obtained is then converted to conductivity with the help of geometrical factors. For both the samples no conductivity jump has been observed near the eutectic melting point as reported by Soares et al. [13] for SDC-eutectic composite as in the present work impedance measurement temperatures are confined above 500oC, eutectic of carbonates. For 10wt% composition grain boundary conductivity appeared to be low compare to CO32- conductivity but at higher temperature it overtakes CO32- conductivity. At higher temperatures, a cooperative effect of CO32-and O2- conduction is the plausible conduction mechanism. In case of 20wt% composition oxygen ion conductivity of YSZ always remains less than that of CO32- conductivity in the eutectic phase. For both the compositions O2conductivity of YSZ and also CO32-conduction in the eutectic do not show any change of slope, a characteristics of oxide ion conduction in a ceramic phase due to vacancy clustering [17,18]. Activation energy values of conduction are evaluated from the straight line fitting of these plots and are presented in Table 1. The activation energy values are in close proximity of SDC – eutectic composite reported by Chockalingam et al. [15]. In case of SDC due to low temperature sintering usually a carbonate coated SDC occurs where as in YSZ due to high temperature of sintering YSZ dissolution, re crystallization occurs which differentiate microstructures as well as conduction paths, which depicted in the EIS. Fig. 8 compares the Arrhenius plot of total conductivity of the composite samples containing 10wt% and 20wt% eutectic. Both the compositions show conductivity improvement compared to that of pure 8YSZ. The conductivity of these two compositions along with the conductivity of pure 8YSZ at different temperatures is presented in Table 2. It is evident that irrespective of temperature, conductivity of 8YSZ has been improved with addition of the eutectic mixture. For 20wt% compositions the conductivity is highest irrespective of temperature. At higher temperature end the conductivity values increased more than one order of magnitude compared to that of pure 8YSZ. This increase of conductivity may be a cooperative effect of carbonate as well as oxide ion conduction.
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4. Conclusions: The composite electrolyte prepared from 8YSZ and (Li-Na)2CO3 eutectic compositions show significantly enhanced electrical conductivity. The 20wt% composition shows much higher conductivity compared to that of 10wt% composition. Higher mobility of the CO32- ion through the carbonate phase is the primary reason for this enhanced conductivity. The microstructures of the two samples are also distinctly different. While in the 10wt% sample, the carbonate phase is present in the form a course rod like glassy morphology, the 20wt% composition, indicates the presence of the dual carbonate phase in glassy as well as a much finer needle like morphology. The difference in the quantity of the carbonate is the plausible reason for the morphological difference. Impedance spectroscopy of the samples shows three or four depressed semicircles corresponding to CO32- ion conduction, O2- ion conduction, interfacial transport between the two different phases and electrolyte-electrode transport phenomenon, particularly observed for the 20wt% specimen. Overall conductivity of YSZ has been enriched by addition of carbonate eutectic compositions. Based on these findings these improvised composite electrolytes based on YSZ carbonates may be used in ITSOFCs.
Acknowledgement: The authors are grateful to Dr. R. N. Basu, Head of Fuel Cell and Battery Division of CSIR-Central Glass and Ceramic Research Institute, Kolkata for his help in preparation and electrical characterization of the samples.
References: 1. R. Maric, S. Seward, P. W. Faguy, M. Oljaca, Electrolyte Materials for Intermediate Temperature Fuel Cells Produced via Combustion Chemical Vapor Condensation, Electrochem. Solid-State Lett., 6 (2003) A91-A95 2. T. Hibino, A. Hashimoto, T. Inoue, J.-i. Tokuno, S.-i. Yashida, M. Sano, A LowOperating-Temperature Solid Oxide Fuel Cell in Hydrocarbon-Air Mixtures, Science, 288 (2000) 2031-33 3.
T. Fukui, S. Ohara, K. Murata, H. Yoshida, K. Miura, T. Inagaki, Performance of intermediate temperature solid oxide fuel cells with La(Sr)Ga(Mg)O3 electrolyte film, J. Power Sources, 106 (2002)142-145.
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4. I.Khan, P. K. Tiwari, S. Basu, Development of melt infiltrated gadolinium doped ceriacarbonate composite electrolytes for intermediate temperature solid oxide fuel cells, Electrochimica Acta 294 (2019) 1-10. 5. M. Anwar, M. Ali, A. Muchtar, M. R. Somalu, Synthesis and characterization of M-doped ceria-ternary carbonate composite electrolytes (M = erbium, lanthanum and strontium) for low-temperature solid oxide fuel cells, J. of Alloys and Compounds, 775 (2019) 571-580. 6. Yu-Jun Jin, Zhan-Guo Liu, Zhao-Ying Ding, Gui Cao, A. Henniche, Hai-Bin Zhang, XuYang Zhen, Jia-Hu Ouyang, Preparation and characterization of GdSmZr2O7– (Li0.52Na0.48)2CO3 composite electrolyte for intermediate temperature solid oxide fuel cells, Electrochimica Acta 283 (2018) 291-299. 7. H. Ahn, D. Kim, Vı´ctor Manuel Aceituno Melgar, J. Kim, Mohd Roslee Othman, Hoang Viet Phuc Nguyen, J. Han, Sung Pil Yoon, YSZ-carbonate dual-phase membranes for high temperature carbon dioxide separation, J. of Industrial and Eng. Chem., 20 (2014) 3703– 3708. 8. R. Razaa, X.Wang, Y.Mac, X. Liua, Bin Zhua, Improved ceria–carbonate composite electrolytes, Int. J. Hydrogen Energy, 35(2010) 2684 – 2688. 9. N.Jaiswal, S.Upadhyay, D.Kumar, O.Parkash, Enhanced ionic conductivity in La3+ and Sr2+ co-doped ceria: carbonate nanocomposite, Ionics 21 (2015) 2277-2283. 10. J. L. Wade, C. Lee, A.C. West, K.S. Lackner, Composite electrolyte membranes for high temperature CO2 separation, J. Mem Sc., 369 (2011) 20. 11. A.I.B. Ronda˜o, N. C. T. Martins, S. G. Patrício, F. M. B. Marques F.M.B. Marques, Ionic transport in (nano)composites for fuel cells, Int. J. Hydrogen Energy, 41 (2016) 7666-7675. 12. A.S.V. Ferreira, C.M.C. Soares, F.M.H.L.R. Figueiredo, F.M.B. Marques, Intrinsic and extrinsic compositional effects in ceria/carbonate composite electrolytes for fuel cells, Int. J. Hydrogen Energy, 36 (2011) 3704-3711. 13. C.M.C. Soares, S. G. Patrício, F.M.L. Figueiredo, F.M.B. Marques, Relevance of the ceramic content on dual oxide and carbonate-ion transport in composite membranes, Int. J. Hydrogen Energy 39 (2014) 5424-5432. 14. J. Huang, Z. Mao, L. Yang, R. Peng, SDC-Carbonate Composite Electrolytes for LowTemperature SOFCs, Electrochem. Solid State Lett., 8 (2005) A437-A440. 15. R. Chockalingam, S. Basu, Impedance spectroscopy studies of Gd-CeO2-(LiNa)CO3 nano composite electrolytes for low temperature SOFC applications, Int. J Hydrogen Energy 36 (2011) 14977-14983. 10
16. C.M. Lapa, F.M.L. Figueiredo, D.P.F. Desouza, L. Song, B. Zhu, F.M.B. Marques, Synthesis and characterization of composite electrolytes based on samaria-doped ceria and Na/Li carbonates, Int. J. Hydrogen Energy 35 (2010) 2953-2957. 17. A.I.B. Ronda˜o, S.G. Patrı´cio, F.M.L. Figueiredo, F.M.B. Marques, Impact of ceramic matrix functionality on composite electrolytes performance, Electrochim. Acta. 109 (2013) 701-709. 18. S. Basu, P.S. Devi, H.S. Maiti, Nb-Doped La2Mo2O9: A New Material with High Ionic Conductivity, J. Electrochem. Soc., 152 (2005) A2143-A2147.
11
B.D. and A.D. ( g/c.c.)
5.8
Apparent Density
5.6
B.D. 5.4
5.2
0
5
10
15
20
Eutectic mixture added (Wt.%) Fig. 1: Variation of Bulk Density and Apparent density of the sintered composite as function of the amount of Eutectic mixture of carbonate added.
12
(1 11 ) (2 00 )
(2 00 )
(1 11 )
20 % Eute ctic M ixtur e
10 % Eute ctic M ixtur e
10 20 30
2θ (d egree s)
40 50
Fig.2: XRD of sintered samples
13
(a)
(b)
a
b
Fig. 3: FE-SEM pictures of fracture surfaces (a) composite with 10 wt% (b) composite with 20 wt% eutectic.
14
3
4x10
(a)
10 wt% eutectic+8YSZ 3
3x10
o
4
500 C
Z (ohm)
5.0x10
Z (ohm)
3
200
0.0 0.0
//
2x10
4
5.0x10 /
Z (ohm)
100 o
700 C
//
Z (ohm)
//
300
0 300
o
200
C
100 /
o
C
Z (ohm)
55 0
o
C
60 0
o
C
70 0
0
3
0
50 0
0
3
1x10
3
1x10
3
2x10
3
3x10
4x10
/
Z (ohm) 3
1.5x10
4
1.0x10
4
5.0x10
3
(b)
//
Z (ohm)
20 wt% eutectic+8YSZ
o
3
500 C
1x10
//
Z (ohm)
2x10
0.0 3
0.0
5.0x10
4
/
4
1.0x10
1.5x10
55 0 50 0
o
C
65 0
70 0
o
C
o
C
60 0
o
C
o
C
Z (ohm)
0 3
0 /
1x10
3
2x10
Z (ohm) Fig. 4: Complex plane impedance plots for the sintered composites with (a) 10wt% eutectic and (b) 20wt% eutectic at different temperatures of measurement (500-700°C).
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3
5x10
3
4x10
o
at 500 C 3
//
Z (ohm)
3x10
3
2x10
10% eutectic 20% eutectic
3
1x10
0
3
-1x10
3
2.0x10
/
3
4.0x10
Z (ohm)
Fig. 5: Comparison of complex plane impedance plots at high frequencies for the sintered composites with (a) 10wt% eutectic and (b) 20wt% eutectic measured at 500°C.
16
3
2.0x10
o
10 wt% eutectic+8YSZ at 600 C
3
//
Z (ohm)
1.5x10
3
1.0x10
Simulated data 2
5.0x10
0.0 0.0
2
5.0x10
3
1.0x10
3
3
1.5x10
2.0x10
/
Z (ohm)
(b)
4
2.5x10
4
Simulation data
4
1.5x10
//
Z (ohm)
2.0x10
4
1.0x10
o
20 wt% eutectic+8YSZ at 600 C 3
5.0x10
0.0 0.0
3
5.0x10
4
1.0x10
4
1.5x10
4
2.0x10
4
2.5x10
/
Z (ohm) Fig. 6: Complex plane impedance plots of the samples with (a) 10 wt% eutectic and (b) 20wt% eutectic. Proposed equivalent circuits are shown in the inset. Solid lines are the simulated plots based on the respective equivalent circuits (proposed), while the circles are the experimental data points.
17
6
(a)
5
Carbonate Oxide ion and interphase Total
4
-1
Ln(σT) (Scm K)
3 2 1 0 -1 -2 -3
10 wt% eutectic+8YSZ
-4 0.9
1.0
1.1
1.2
1.3
-1
1000/T (K )
4
Carbonate Oxide ion Interphase Total
3
1
-1
Ln(σT) (Scm K)
2
(b)
0 -1 -2 -3 -4
20 wt% eutectic+8YSZ 0.9
1.0
1.1
1.2
1.3
1.4
-1
1000/T (K )
Fig. 7: Arrhenius plots for different components of conductivity for composites with (a) 10 wt% eutectic and (b) 20 wt% eutectic.
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4 3
8YSZ 8YSZ+10 wt% Eutectic 8YSZ+20 wt% Eutectic
1
-1
Ln(σT) (Scm K)
2
0 -1 -2 -3
Total conductivity
-4 0.9
1.0
1.1
1.2
1.3
-1
1000/T (K ) Fig. 8:
Comparative Arrhenius plots of Total conductivity of 8-YSZ and two composites prepared in this investigation.
19
Table 1. Total conductivity of different compositions at different temperatures
Temperature (oC) 500 550 600 650 700 750 800 850
Conductivity(S cm-1)
1.07×10-4
YSZ – 10% Eutectic 1.78×10-4
YSZ – 20% Eutectic 3.567×10-4
-1.78×10-4 2.68×10-4 8.92×10-4 0.0031 -0.0043
3.15×10-4 6.95×10-4 0.002 --0.008 0.0145
7.13×10-4 0.0016 0.003 0.006 0.011 0.0223 0.0487
Pure YSZ
Table 2: Activation energies of conductivity for different species/regions in the two different compositions.
E act (eV) YSZ – 20% Eutectic
Carbonate
YSZ – 10% Eutectic 1.02
Oxide ion
----
1.23
Inter-phase
1.70
1.53
Overall
1.27
1.40
1.10
20
• • •
Carbonate –YSZ composite electrolyte prepared which otherwise rare in literature. Composite microstructures are unique and drastically changes with composition. Total conductivity of YSZ has been increased substantially with the composite additionencouraging for its use in ITSOFC.
S0925-8388(19)33807-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152561
Reference:
JALCOM 152561
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: S. Basu, M.N. Alam, S. Basu, H.S. Maiti, 8YSZ – Carbonate composite electrolyte-conductivity enhancement, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.152561. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
8YSZ – Carbonate Composite Electrolyte-conductivity enhancement a b b b Santanu Basu# , Md. Nazmul Alam , Swarnavo Basu and Himadri S. Maiti
b
a
Department of Ceramic Technology, Govt. College of Engineering & Ceramic Technology, India
SMSE, IIEST-Shibpur, India
#Corresponding Author Email: [email protected]
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Abstract A composite electrolyte based on 8YSZ and Li/Na carbonate eutectic is prepared and characterized according to requirement of electrolytes. The powdered eutectic is mixed with 8YSZ powder in two different proportions. Sintered samples have been characterized by XRD, SEM and impedance spectroscopy. Fractured micrographs of the compositions show distinctly different morphology of the carbonate phase due the quantity of the carbonate phase addition. Impedance spectroscopy provides separate conductivities of the carbonate phase, YSZ phase, inter-phase boundary and that of the electrolyte-electrode interface. Equivalent circuits of the transport phenomena have been proposed. The conductivity of the carbonate phase is significantly higher than that of the YSZ phase and accordingly the overall conductivity of the composite electrolyte increases with increasing quantity of the carbonate phase. At 600oC the composite with 20wt% carbonate phase shows the conductivity of 1.6x10-3S-cm-1 compared to 1.78x10-4Scm-1for pure 8YSZ.
Keywords: YSZ, Carbonate Eutectic, Composite Electrolyte ,Impedance Spectroscopy.
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1. Introduction Intermediate-temperature solid oxide fuel cells (ITSOFCs) become a potential technology in the field of non conventional energy generation. Even using thin-film technology traditional yttrium-stabilized zirconia(YSZ) electrolyte incapable to suit necessity of low-temperature operation [1]. Therefore research thrust to develop new electrolyte materials with high ion conductivities at lower temperature for advanced ITSOFCs become important [2,3]. Recently a new type of composite electrolyte was developed with doped-cerium oxide as parent material and carbonates as the second phase. Second phases of these composite electrolytes improved ionic conductivity and adjustable multi-ionic conductivity of the electrolytes [4,5]. Improvement of ionic conductivity in such composite ceria carbonate electrolytes extended the concept to other solid electrolytes [6]. While composite electrolyte based on ceria system as well as other oxide systems have been studied extensively, systems based on YSZ, the most established oxygen ion conductor has not been studied much. Thus opportunity exists to study preparation and characterization of carbonate – YSZ composite material in terms of sintering and electrical conduction measurements. Impregnation of carbonates in YSZ has been studied by Ahn et al for application of CO2 separation [7], whereas most of the ceria carbonate composites approach based on sintering of oxide in the presence of carbonate eutectic [8]. Our present study is aimed at preparation of a composite electrolyte based on sintering of 8 mol% YSZ in presence of a eutectic mixture of Li2CO3 and Na2CO3 followed by its complete micro-structural and electrical characterization.
2. Experimental Li2CO3 (99.0%, Loba, India) and Na2CO3 (99.0%, Loba, India) were mixed to obtain a eutectic composition having 43.1 wt% Li2CO3 and rest Na2CO3. The mixture was melted at 600°C in a vitreous silica crucible and held for 2 hours for effective melting and homogenization. The eutectic melt is then cooled down to room temperature and ground in an agate mortar. This fine eutectic mixture is then mixed with 8YSZ (Tosho, Japan) powder in the weight percentages of 10 and 20. The composite mixtures were then uni-axially pressed in a steel mould with diameter of 15mm and thickness of around 5mm. The pellets, thus formed, were sintered for 2 hours at 1100 °C for the samples containing 20 and 10 wt% eutectic respectively.
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During sintering at high temperature special precaution has been taken to prevent carbonate decomposition by controlling sintering atmosphere. The phase identification of the sintered samples was confirmed from the x-ray powder diffraction (XRD) data collected at a scan rate of 2° per minute on a Philips PW 3207 (Holland) diffractometer with CuKα radiation. The fracture surfaces of the sintered samples were examined by field emission scanning electron microscope (FE-SEM) using Supra 35VP (Carl Zeiss, Germany). Platinum paste (Gwent Electronics, U.K.) was applied on both surfaces of the sintered samples (~13mm dia. and 3mm thick) and cured at 800°C in air. Two probe ac impedance measurements were conducted on the samples in the frequency range 0.1 Hz to 10 MHz with an applied voltage of 0.1 V using a Solartron frequency response analyzer (FRA 1260) coupled with a Solartron 1296 dielectric interface unit in the temperature range 300–850°C. Impedance data were collected at every 50°C interval and a heating rate of 3°C/min was used for increasing the temperature.
3. Results and Discussion 3.1.
XRD and Density: Variation of bulk densities (BD) obtained from geometrical dimensions of the sintered
samples is shown in Fig. 1. As the density of the eutectic composition is much lower than that of pure 8YSZ, increasing eutectic content decreases in the bulk density of the composite material. Apparent density (A.D.) were calculated by Archimedes principle using distilled water as liquid medium. The A.P. and B.D. for composites have been found to be very close in value(as shown in Fig.1) indicating dearth of open pores. XRD of the sintered samples are shown in Fig. 2. Major reflections are indexed for 8YSZ (JCPDS file no. 30-1468). For 10wt% composite specimen no other phase except YSZ is evident, thus the presence of the eutectic phase is not detected. In contrast to this, 20wt% composite shows a few extra reflections, which are identified as those of Li - Na carbonate (JCPDS no. 852296). This finding is in contrast to that of Jaswal et al. [9], who have reported absence of any such crystalline phase even after incorporating as high as 35 wt % of eutectic mixture to ceria based electrolyte. Thus for 10wt% composite the eutectic may be in amorphous state. But for 20wt% composite the situation is different. Li - Na carbonate peaks indicates the presence of crystalline phases of the eutectic composition. Both 10wt% and 20wt% compositions do not indicate the presence of any Li2ZrO3 or other zirconate phases as reported by Wade et al. [10]. 4
3.2.
Microstructure: FE-SEM pictures of fracture surfaces of 10 and 20 wt% composite electrolytes are shown
in Fig. 3 (a) and (b) respectively. It is interesting to note that the micro-structural characteristics of the samples are distinctly different from those previously reported for similar studies with ceria based materials [11, 12] as most of them reported surface morphology. Both the pictures shown in Fig. 3 indicate microstructures with nearly zero porosity which was not evident in case of SDC – eutectic compositions reported earlier [13]. Higher temperature (10000C) of sintering compared to the eutectic temperature of around 500°C used in this work may have resulted in such an impervious microstructure. It is evident from Fig. 3 that the eutectic carbonate phase is present either in the form of thick thread-like morphology (Fig. 3a) or fine needles (Fig. 3b) within the matrix of YSZ grains. Such appearance of acicular crystalline phase has been reported by Huang et al. [14] for SDC with 45 wt% eutectic composition without any plausible explanation for such difference. It is also interesting to note that the threads present in the 10wt% sample is much thicker and resemble a glass phase
whereas the fine needles observed in 20wt% sample are fine
crystalline needles dispersed in composite matrix of glass-YSZ. The plausible reason is at high temperature of sintering for the former (10 wt %) having less volume of molten liquid phase at which the viscosity of the eutectic is expected to higher than the 20wt% composition. Quenching rate being high the relaxation ceases and the eutectic phase remains in the amorphous state for high viscous 10wt% composition. In case of 20 wt % eutectic compositions due to presence of higher amount liquid at high temperature viscosity become low enough to complete atomic relaxation ending with acicular crystals of carbonates. Another very interesting observation from these micrograph is that the volume percentages of eutectic and YSZ in the 10% eutectic composition, Fig 3(a) indicates that they are approximately 25% and 75% respectively. In contrast, for 20% eutectic composition Fig 3(b) the volume percentages are nearly same (50:50). A composite like structure of homogeneously distributed non directional network of an acicular phase embedded in a fine particulate structure is evident in this figure. Considering the density of 8YSZ (6.10g/cc), Li2CO3 (2.11 g/cc) and NaCO3 (2.54g/cc), the theoretical volume percentages of eutectic and YSZ for 10% eutectic composition are calculated to be 22.4% and 77.6%. The same density data results in theoretical volume percentages of 39.3% and 60.7% eutectic and YSZ respectively for the sample containing 20% eutectic. For the 10% eutectic composition the predicted volume percentage is almost same as that of actual finding as described above; whereas a huge mismatch is evident for 20% eutectic composition. The 50 volume % of eutectic is much higher than the theoretically calculated value 5
of 39.3%. The presence of much higher volume of eutectic mixture can only be satisfactorily explained by the assumption of dissolution of YSZ in eutectic at higher sintering temperature which in term may be the nucleating centre of heterogeneous crystallization of eutectic phase.
3.3.
Electrical Conductivity: As mentioned before, electrical impedance of the samples has been measured as functions
of frequency at temperatures ranging from 500 to 700°C. The data are presented in “Nyquist Plot” of real and imaginary parts of the impedance (Z′′ vs Z′) as parametric function of the frequency (Fig. 4). Complex plane impedance plots for the sintered samples of 8YSZ with 10 and 20 wt% eutectic composites at different temperatures in the range of 500 – 700°C are shown in Figs. 4 (a) and (b), respectively. There are certain similarities between the two sets of plots; for examples in both the figures the low temperature plot appears on the right side of x-axis (away from origin), indicating higher resistance of the sample and it slowly shifts to the left (closer to the origin) with increasing temperature due to lowering of the overall resistance. The shape of the plot, however, changes quite significantly as the temperature increases with consequent decrease in resistance. The semicircles become more distinct but with reduced size as the temperature increases. In Fig 4 both (a) and (b) the insets show magnified plots corresponding to a few temperatures for which the shape of the curves are not clearly visible in the main plot. In both the cases, the semicircular nature of the plots becomes progressively distinct at increasing temperatures with consequent reduction in overall resistance. As the temperature increases the relaxation time (τ) for conductive species decreases as the conduction of all the ionic species are thermally activated. The peak of complex plane plot is determined by the relation τ×ω=1, where ω is angular frequency, thus distinct peaks appear at higher temperature. The high frequency data at 500°C for both compositions are compared in Fig. 5. The intersection with the real axis for 20wt% is closer to the origin indicating that it has lower resistance than the 10wt% specimen. Additionally, the 20wt% specimen shows an indication of negative impedance below the real axis indicative of the predominance of an inductive part of the impedance due to relatively low resistance. The same is not evident for 10wt% sample except above 550°C (not shown in this figure). The appearance of such inductive component is normally observed for regular solid electrolytes e.g. YSZ at temperatures higher than 1000°C. The data at lower temperatures and at low measurement frequencies for both the samples indicate a nearly linear plot primarily due to very high resistance of the samples. This linearity is not due to predominance of any diffusion controlled charge transfer reaction normally observed at 6
the electrode-electrolyte interface. With increasing measurement temperature the semicircles become better visible even though they are not perfect but depressed ones due to distributed relaxation times. This is in contrast with the observation reported by Chockalingam et al. [15] and Lapa et al. [16] for samarium oxide doped ceria (SDC) and (NaLi)2CO3 composite electrolyte. They proposed that the presence of a single high frequency semi circle corresponds to composite effect due to the coexistence of numerous processes with different relaxation times. The lowest measurement temperature used in this investigation is 500°C, which is marginally higher than the eutectic temperature of the carbonate mixture (~490°C). At this temperature the carbonate phase is expected to form a soft or semi-solid phase having a significantly different relaxation time for carbonate ion migration than that of the oxygen ion in the solid YSZ phase whose resistance is expected to be much higher than that of the carbonate phase. This may give rise to the occurrence of separate semicircles; the higher frequency one being due to carbonate ion migration and the lower one arising from the oxygen ion migration through YSZ grains. In both the cases, the experimental data have been fitted to several depressed semi-circles with the help of Z-view curve fitting software. Proposed equivalent circuits are presented as insets to these figures. Complex plane semicircles are often described by a parallel circuit with one resistor and one capacitor (R||C) while depressed semicircles can be explained by a simple parallel circuit with one resistor and one constant phase element (R||CPE). In case of 10% sample Fig. 6(a) two distinct semicircles are visible. The highest frequency semicircle is not visible, may originate from CO32- conduction in the eutectic phase. The next semicircle (actually observed in this investigation) may be due the O2- conduction in YSZ along with its inter-phase boundary with eutectic carbonate. The third dispersed semicircle is attributed to the combined effect of electrode phenomena and the carbonate-oxide interface transport. For the 20% sample depicted in Fig. 6(b), there appear to be three different semicircles; once again there is no experimental data for the highest frequency one. The First high frequency data (not observed) may originated from the carbonate conduction in glassy phase, second one from carbonate conduction in crystalline phase and inter-phase boundary, third and the fourth ones possibly correspond to the two interface phenomena: i) boundary between the carbonate and the oxide phases and ii) electrolyte-electrode interface. As several ions (Li+1,Na+1, VO••, CO3= in crystalline and glassy phase) presents in these systems the overlapping of relaxation time of their transportation causes depression in the formed impedance semicircles. Resistances calculated from the impedance plots as described above are converted to conductivity using geometrical factors and plotted as Arrhenius plots as shown in Fig.7. In Fig. 7
7(a) the data for the eutectic carbonate phase, inter-phase boundary and the overall conductivity of 10wt% composition are shown; all these data appear to follow a linear relationship. In Fig. 7(b) similar plots for 20wt% composition are shown. As discussed earlier in this case several semicircles can be resolved and they are designated as those arising from the conduction in the carbonate in eutectic phase, bulk YSZ and inter-phase boundary. Total resistance of the samples has been calculated from the intercept (extended) of the last semicircle (without the low frequency electrode contribution) with Z/ axis. The resistance thus obtained is then converted to conductivity with the help of geometrical factors. For both the samples no conductivity jump has been observed near the eutectic melting point as reported by Soares et al. [13] for SDC-eutectic composite as in the present work impedance measurement temperatures are confined above 500oC, eutectic of carbonates. For 10wt% composition grain boundary conductivity appeared to be low compare to CO32- conductivity but at higher temperature it overtakes CO32- conductivity. At higher temperatures, a cooperative effect of CO32-and O2- conduction is the plausible conduction mechanism. In case of 20wt% composition oxygen ion conductivity of YSZ always remains less than that of CO32- conductivity in the eutectic phase. For both the compositions O2conductivity of YSZ and also CO32-conduction in the eutectic do not show any change of slope, a characteristics of oxide ion conduction in a ceramic phase due to vacancy clustering [17,18]. Activation energy values of conduction are evaluated from the straight line fitting of these plots and are presented in Table 1. The activation energy values are in close proximity of SDC – eutectic composite reported by Chockalingam et al. [15]. In case of SDC due to low temperature sintering usually a carbonate coated SDC occurs where as in YSZ due to high temperature of sintering YSZ dissolution, re crystallization occurs which differentiate microstructures as well as conduction paths, which depicted in the EIS. Fig. 8 compares the Arrhenius plot of total conductivity of the composite samples containing 10wt% and 20wt% eutectic. Both the compositions show conductivity improvement compared to that of pure 8YSZ. The conductivity of these two compositions along with the conductivity of pure 8YSZ at different temperatures is presented in Table 2. It is evident that irrespective of temperature, conductivity of 8YSZ has been improved with addition of the eutectic mixture. For 20wt% compositions the conductivity is highest irrespective of temperature. At higher temperature end the conductivity values increased more than one order of magnitude compared to that of pure 8YSZ. This increase of conductivity may be a cooperative effect of carbonate as well as oxide ion conduction.
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4. Conclusions: The composite electrolyte prepared from 8YSZ and (Li-Na)2CO3 eutectic compositions show significantly enhanced electrical conductivity. The 20wt% composition shows much higher conductivity compared to that of 10wt% composition. Higher mobility of the CO32- ion through the carbonate phase is the primary reason for this enhanced conductivity. The microstructures of the two samples are also distinctly different. While in the 10wt% sample, the carbonate phase is present in the form a course rod like glassy morphology, the 20wt% composition, indicates the presence of the dual carbonate phase in glassy as well as a much finer needle like morphology. The difference in the quantity of the carbonate is the plausible reason for the morphological difference. Impedance spectroscopy of the samples shows three or four depressed semicircles corresponding to CO32- ion conduction, O2- ion conduction, interfacial transport between the two different phases and electrolyte-electrode transport phenomenon, particularly observed for the 20wt% specimen. Overall conductivity of YSZ has been enriched by addition of carbonate eutectic compositions. Based on these findings these improvised composite electrolytes based on YSZ carbonates may be used in ITSOFCs.
Acknowledgement: The authors are grateful to Dr. R. N. Basu, Head of Fuel Cell and Battery Division of CSIR-Central Glass and Ceramic Research Institute, Kolkata for his help in preparation and electrical characterization of the samples.
References: 1. R. Maric, S. Seward, P. W. Faguy, M. Oljaca, Electrolyte Materials for Intermediate Temperature Fuel Cells Produced via Combustion Chemical Vapor Condensation, Electrochem. Solid-State Lett., 6 (2003) A91-A95 2. T. Hibino, A. Hashimoto, T. Inoue, J.-i. Tokuno, S.-i. Yashida, M. Sano, A LowOperating-Temperature Solid Oxide Fuel Cell in Hydrocarbon-Air Mixtures, Science, 288 (2000) 2031-33 3.
T. Fukui, S. Ohara, K. Murata, H. Yoshida, K. Miura, T. Inagaki, Performance of intermediate temperature solid oxide fuel cells with La(Sr)Ga(Mg)O3 electrolyte film, J. Power Sources, 106 (2002)142-145.
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4. I.Khan, P. K. Tiwari, S. Basu, Development of melt infiltrated gadolinium doped ceriacarbonate composite electrolytes for intermediate temperature solid oxide fuel cells, Electrochimica Acta 294 (2019) 1-10. 5. M. Anwar, M. Ali, A. Muchtar, M. R. Somalu, Synthesis and characterization of M-doped ceria-ternary carbonate composite electrolytes (M = erbium, lanthanum and strontium) for low-temperature solid oxide fuel cells, J. of Alloys and Compounds, 775 (2019) 571-580. 6. Yu-Jun Jin, Zhan-Guo Liu, Zhao-Ying Ding, Gui Cao, A. Henniche, Hai-Bin Zhang, XuYang Zhen, Jia-Hu Ouyang, Preparation and characterization of GdSmZr2O7– (Li0.52Na0.48)2CO3 composite electrolyte for intermediate temperature solid oxide fuel cells, Electrochimica Acta 283 (2018) 291-299. 7. H. Ahn, D. Kim, Vı´ctor Manuel Aceituno Melgar, J. Kim, Mohd Roslee Othman, Hoang Viet Phuc Nguyen, J. Han, Sung Pil Yoon, YSZ-carbonate dual-phase membranes for high temperature carbon dioxide separation, J. of Industrial and Eng. Chem., 20 (2014) 3703– 3708. 8. R. Razaa, X.Wang, Y.Mac, X. Liua, Bin Zhua, Improved ceria–carbonate composite electrolytes, Int. J. Hydrogen Energy, 35(2010) 2684 – 2688. 9. N.Jaiswal, S.Upadhyay, D.Kumar, O.Parkash, Enhanced ionic conductivity in La3+ and Sr2+ co-doped ceria: carbonate nanocomposite, Ionics 21 (2015) 2277-2283. 10. J. L. Wade, C. Lee, A.C. West, K.S. Lackner, Composite electrolyte membranes for high temperature CO2 separation, J. Mem Sc., 369 (2011) 20. 11. A.I.B. Ronda˜o, N. C. T. Martins, S. G. Patrício, F. M. B. Marques F.M.B. Marques, Ionic transport in (nano)composites for fuel cells, Int. J. Hydrogen Energy, 41 (2016) 7666-7675. 12. A.S.V. Ferreira, C.M.C. Soares, F.M.H.L.R. Figueiredo, F.M.B. Marques, Intrinsic and extrinsic compositional effects in ceria/carbonate composite electrolytes for fuel cells, Int. J. Hydrogen Energy, 36 (2011) 3704-3711. 13. C.M.C. Soares, S. G. Patrício, F.M.L. Figueiredo, F.M.B. Marques, Relevance of the ceramic content on dual oxide and carbonate-ion transport in composite membranes, Int. J. Hydrogen Energy 39 (2014) 5424-5432. 14. J. Huang, Z. Mao, L. Yang, R. Peng, SDC-Carbonate Composite Electrolytes for LowTemperature SOFCs, Electrochem. Solid State Lett., 8 (2005) A437-A440. 15. R. Chockalingam, S. Basu, Impedance spectroscopy studies of Gd-CeO2-(LiNa)CO3 nano composite electrolytes for low temperature SOFC applications, Int. J Hydrogen Energy 36 (2011) 14977-14983. 10
16. C.M. Lapa, F.M.L. Figueiredo, D.P.F. Desouza, L. Song, B. Zhu, F.M.B. Marques, Synthesis and characterization of composite electrolytes based on samaria-doped ceria and Na/Li carbonates, Int. J. Hydrogen Energy 35 (2010) 2953-2957. 17. A.I.B. Ronda˜o, S.G. Patrı´cio, F.M.L. Figueiredo, F.M.B. Marques, Impact of ceramic matrix functionality on composite electrolytes performance, Electrochim. Acta. 109 (2013) 701-709. 18. S. Basu, P.S. Devi, H.S. Maiti, Nb-Doped La2Mo2O9: A New Material with High Ionic Conductivity, J. Electrochem. Soc., 152 (2005) A2143-A2147.
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B.D. and A.D. ( g/c.c.)
5.8
Apparent Density
5.6
B.D. 5.4
5.2
0
5
10
15
20
Eutectic mixture added (Wt.%) Fig. 1: Variation of Bulk Density and Apparent density of the sintered composite as function of the amount of Eutectic mixture of carbonate added.
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(1 11 ) (2 00 )
(2 00 )
(1 11 )
20 % Eute ctic M ixtur e
10 % Eute ctic M ixtur e
10 20 30
2θ (d egree s)
40 50
Fig.2: XRD of sintered samples
13
(a)
(b)
a
b
Fig. 3: FE-SEM pictures of fracture surfaces (a) composite with 10 wt% (b) composite with 20 wt% eutectic.
14
3
4x10
(a)
10 wt% eutectic+8YSZ 3
3x10
o
4
500 C
Z (ohm)
5.0x10
Z (ohm)
3
200
0.0 0.0
//
2x10
4
5.0x10 /
Z (ohm)
100 o
700 C
//
Z (ohm)
//
300
0 300
o
200
C
100 /
o
C
Z (ohm)
55 0
o
C
60 0
o
C
70 0
0
3
0
50 0
0
3
1x10
3
1x10
3
2x10
3
3x10
4x10
/
Z (ohm) 3
1.5x10
4
1.0x10
4
5.0x10
3
(b)
//
Z (ohm)
20 wt% eutectic+8YSZ
o
3
500 C
1x10
//
Z (ohm)
2x10
0.0 3
0.0
5.0x10
4
/
4
1.0x10
1.5x10
55 0 50 0
o
C
65 0
70 0
o
C
o
C
60 0
o
C
o
C
Z (ohm)
0 3
0 /
1x10
3
2x10
Z (ohm) Fig. 4: Complex plane impedance plots for the sintered composites with (a) 10wt% eutectic and (b) 20wt% eutectic at different temperatures of measurement (500-700°C).
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3
5x10
3
4x10
o
at 500 C 3
//
Z (ohm)
3x10
3
2x10
10% eutectic 20% eutectic
3
1x10
0
3
-1x10
3
2.0x10
/
3
4.0x10
Z (ohm)
Fig. 5: Comparison of complex plane impedance plots at high frequencies for the sintered composites with (a) 10wt% eutectic and (b) 20wt% eutectic measured at 500°C.
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3
2.0x10
o
10 wt% eutectic+8YSZ at 600 C
3
//
Z (ohm)
1.5x10
3
1.0x10
Simulated data 2
5.0x10
0.0 0.0
2
5.0x10
3
1.0x10
3
3
1.5x10
2.0x10
/
Z (ohm)
(b)
4
2.5x10
4
Simulation data
4
1.5x10
//
Z (ohm)
2.0x10
4
1.0x10
o
20 wt% eutectic+8YSZ at 600 C 3
5.0x10
0.0 0.0
3
5.0x10
4
1.0x10
4
1.5x10
4
2.0x10
4
2.5x10
/
Z (ohm) Fig. 6: Complex plane impedance plots of the samples with (a) 10 wt% eutectic and (b) 20wt% eutectic. Proposed equivalent circuits are shown in the inset. Solid lines are the simulated plots based on the respective equivalent circuits (proposed), while the circles are the experimental data points.
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6
(a)
5
Carbonate Oxide ion and interphase Total
4
-1
Ln(σT) (Scm K)
3 2 1 0 -1 -2 -3
10 wt% eutectic+8YSZ
-4 0.9
1.0
1.1
1.2
1.3
-1
1000/T (K )
4
Carbonate Oxide ion Interphase Total
3
1
-1
Ln(σT) (Scm K)
2
(b)
0 -1 -2 -3 -4
20 wt% eutectic+8YSZ 0.9
1.0
1.1
1.2
1.3
1.4
-1
1000/T (K )
Fig. 7: Arrhenius plots for different components of conductivity for composites with (a) 10 wt% eutectic and (b) 20 wt% eutectic.
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4 3
8YSZ 8YSZ+10 wt% Eutectic 8YSZ+20 wt% Eutectic
1
-1
Ln(σT) (Scm K)
2
0 -1 -2 -3
Total conductivity
-4 0.9
1.0
1.1
1.2
1.3
-1
1000/T (K ) Fig. 8:
Comparative Arrhenius plots of Total conductivity of 8-YSZ and two composites prepared in this investigation.
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Table 1. Total conductivity of different compositions at different temperatures
Temperature (oC) 500 550 600 650 700 750 800 850
Conductivity(S cm-1)
1.07×10-4
YSZ – 10% Eutectic 1.78×10-4
YSZ – 20% Eutectic 3.567×10-4
-1.78×10-4 2.68×10-4 8.92×10-4 0.0031 -0.0043
3.15×10-4 6.95×10-4 0.002 --0.008 0.0145
7.13×10-4 0.0016 0.003 0.006 0.011 0.0223 0.0487
Pure YSZ
Table 2: Activation energies of conductivity for different species/regions in the two different compositions.
E act (eV) YSZ – 20% Eutectic
Carbonate
YSZ – 10% Eutectic 1.02
Oxide ion
----
1.23
Inter-phase
1.70
1.53
Overall
1.27
1.40
1.10
20
• • •
Carbonate –YSZ composite electrolyte prepared which otherwise rare in literature. Composite microstructures are unique and drastically changes with composition. Total conductivity of YSZ has been increased substantially with the composite additionencouraging for its use in ITSOFC.