Introduction

Among the various fuel cell technologies, solid oxide fuel cell (SOFC) has proved as more efficient and best promising electrochemical energy conversion technology, in which the chemical energy in the form of fuel is converted into electricity with high efficiency. SOFCs have a great interest due to its clean environment-friendly energy conversion, in which only heat and water are produced as byproducts [1]. Solid electrolyte material plays a tremendous role in the SOFC system by facilitating the mobilization of ions from anode to cathode in order to increase the performance of the cell. Therefore, solid electrolyte should exhibit high ionic conductivity to enhance the performance of SOFC [2]. Zirconia-based solid electrolytes exhibit the highest ionic conductivity at higher temperatures (above 800 °C). However, at such high temperatures, there is a significant decrease in the output efficiency of SOFC which may lead to short circuit [3]. Extended investigations have been made on doped ceria solid electrolytes due to their improved performance and reduced operating temperature to intermediate range (i.e., 600–800 °C) over traditional zirconia-based electrolytes in order to develop intermediate temperature solid oxide fuel cells [3, 4]. However, the single-doped ceria electrolyte materials are facing some difficulties due to the reduction of ceria in reducing environment and the growth of defect clusters in the ceria structure which results in lower ionic conductivity [5,6,7]. To overcome these problems, further doping in ceria is required. In the past few decades, plenty of research has been made on co-doped ceria which exhibits the superior properties compared to single-doped ceria [5,6,7,8,9,10,11,12]. Anderson et al. [13] have investigated the relation between the lanthanide dopants and structural oxygen vacancies using density functional theory (DFT) and reported that the dopant with an effective atomic number between 61 (Pm3+) and 62 (Sm3+) is an optimized dopant in order to enhance the electrical properties of ceria. Unfortunately, Pm3+ is not suitable/useful for required applications. Therefore, this theory recommends that the dopants which are having an average atomic number close to Pm3+ and Sm3+ result in enhanced electrical properties [13]. Recently, Ramesh et al. [10], Rai et al. [14], Anirban et al. [15], Babu et al. [16], and Venkataramana et al. [17] have extensively investigated ceria-based solid electrolytes using DFT theory, such as Ce1 − x(Gd0.5Pr0.5)xO2, SmxNd0.15 − xCe0.85O2 − δ, Ce0.7PrxSmyEu0.3 − (x + y)O2 − δ, CeO2-8Gd2O3−2Nd2O3, and Ce1 − x(Pr1/3Sm1/3Gd1/3)xO2 − δ and reported improved performance of ceria. Moreover, triple-doping or multi-doping in ceria has been studied and succeeded to improve the ionic conductivity of ceria [15,16,17,18,19,20,21]. However, further investigations are still required on tri- or multi-doped ceria in order to develop new electrolytes for future SOFC applications. In view of the above and based on DFT theory, recently, we have chosen the triple dopants, such as Pr3+ (59), Sm3+ (62), and Gd3+ (64), which are having an average atomic number about 61.67 (which is lies in between 61 and 62) and reported the studies on triple-doped ceria Ce1 − x(Pr1/3Sm1/3Gd1/3)xO2 − δ system prepared via sol–gel auto-combustion route followed by conventional sintering at 1300 °C for 4 h. It has been found that the composition Ce0.76Pr0.08Sm0.08Gd0.08O2 − δ has the enhanced total ionic conductivity among the other compositions [17].

On the other hand, researchers have been focusing to reduce the sintering temperature and time with enhanced ionic conductivity. However, in the conventional sintering process, only sample surface retains the heat energy from the heating element in the furnace and by reducing the sintering temperature and time may decrease the relative density, which leads to lower ionic conductivity of a material. Therefore, researchers have been developing sintering techniques based on microwave energy, named as microwave sintering technique, which has gained much popularity compared to conventional sintering due to their less energy consumption, low sintering temperature, and time. Microwave sintering process facilitates the molecular-level rapid heating instead of surface-level conventional heating, which can enhance the density of the microstructure [22,23,24,25,26].

In the present paper, we have investigated the electrolyte material Ce0.76Pr0.08Sm0.08Gd0.08O2 − δ sintered by means of microwave sintering (1300 °C for 15, 30, and 60 min) and studied the effect of microwave sintering on structural, electrical, and thermal properties.

Experimental

The triple-doped ceria Ce0.76Pr0.08Sm0.08Gd0.08O2 − δ (PSG) as solid electrolyte material for IT-SOFC has been synthesized by adopting the sol–gel auto-combustion route. The starting materials for this synthesis have been taken in the form of nitrates, i.e., ceric ammonium nitrate (Ce(NH4)2(NO3)6), praseodymium (III) nitrate hexahydrate (Pr(NO3)36H2O), samarium (III) nitrate hexahydrate (Sm(NO3)36H2O), and gadolinium (III) nitrate hexahydrate (Gd(NO3)36H2O). The detailed synthesis process has been discussed in our previous study reported elsewhere [17]. The final synthesized powder was calcined at 800 °C for 2 h. The obtained powder was then pressed into circular pellets at a pressure of 5 tons per square inch with the help of hydraulic press. The dense specimens have been obtained by sintering with a microwave furnace (microwave energy with 2.45 GHz) at 1300 °C for 15, 30, and 60 min, respectively. The nomenclature has been assigned to above said samples as PSG-MS15, PSG-MS30, and PSG-MS60.

The characterization of structural, electrical, and thermal properties of prepared triple-doped ceria electrolyte PSG materials has been carried out by powder X-ray diffraction (PXRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, impedance spectroscopy, and thermal expansion studies.

The powder X-ray diffraction at room temperature has been done with Philips Xpert PRO ALPHA1 Panalytical diffractometer using Cu Kα1 monochromatic radiation. The microstructure and elemental composition analyses of the sintered specimens have been carried out by SEM and EDS using ZEISS EVO 18 special edition. Sample formation and structural O2-vacancy study of the samples have been done by Raman spectroscopy using a Horiba-Jobin Yvon micro-Raman spectrometer. The studies of impedance spectroscopy for all the samples in the temperature range from 200 to 600 °C have been carried out using Wayne Kerr impedance analyzer 6500B. The measurement of thermal expansion study has been done with Netzsch DIL 402 PC dilatometer from room temperature to 1000 °C.

Results and discussion

PXRD analysis

Figure 1 represents the PXRD patterns of all the microwave-sintered PSG-MS15, PSG-MS30, and PSG-MS60 samples. The PXRD patterns of all the samples reveal the formation of single phase with cubic-fluorite structure. The (hkl) parameters of all the samples indexed to (111), (200), (220), (311), (222), (400), (331), and (420) indicate the phase formation of compositions [Fm\( \overline{3} \)m space group and JCPDS PDF: 34–0394]. Rietveld analysis was carried out for structural refinements of the PXRD patterns. The structural Rietveld refinements of all the samples were shown in Fig. 2. The straight line indicates that the difference between calculated and observed patterns is small. The structural refinement parameters were in good agreement, and lattice parameter values were calculated from the refinements of PXRD patterns and were listed in Table 1. Relative densities of all the samples were calculated from the equation below,

$$ {D}_{\mathrm{rel}}\left(\%\right)=\frac{D_{\mathrm{m}}}{D_{\mathrm{th}}}\times 100 $$
(1)

where, “Drel” is relative density, “Dm” is measured density measured by Archimedes principle, and “Dth” is theoretical density. The relative densities of all the PSG-MS15, PSG-MS30, and PSG-MS60 samples were found to be above 95% and were listed in Table 1. The Debye-Scherer equation was used to calculate the crystallite size (D) from PXRD patterns of the finely grounded sintered samples [17]. Table 1 represents the crystallographic data of prepared samples, i.e., crystal structure, lattice parameter, volume, relative density, and crystallite size for sintered samples, and Rietveld refinement parameters are Rp, Rwp, Rexp, χ2, Bragg R-factor, RF factor, and goodness-of-fit (GOF).

Fig. 1
figure 1

PXRD patterns of PSG-MS15, PSG-MS30, and PSG-MS60

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Fig. 2
figure 2

Rietveld refinements of the PXRD patterns of PSG-MS15, PSG-MS30, and PSG-MS60

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Table 1 Crystallographic data of PSG-MS15, PSG-MS30, and PSG-MS60
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SEM and EDS analyses

SEM micrographs of PSG-MS15, PSG-MS30, and PSG-MS60 specimens were shown in Fig. 3. In order to characterize the SEM analysis, prepared triple-doped ceria pellets were sputtered by Au coating. From Fig. 3, it can be observed that the grains were uniformly distributed in cluster-like form agglomeration and no large pores were observed on the surface of all samples. The calculated grain sizes at 248, 229, and 245 nm values were observed for PSG-MS15, PSG-MS30, and PSG-MS60, respectively. The EDS spectra of all the PSG-MS15, PSG-MS30, and PSG-MS60 specimens were shown in Fig. 3. Three selective regions of EDS spectra were analyzed for all the samples in order to observe the elemental composition. Presence of Ce, Pr, Sm, Gd, and O elements and the average percentage of elemental composition of all the samples were confirmed and listed in Table 2.

Fig. 3
figure 3

SEM and EDS images of PSG-MS15, PSG-MS30, and PSG-MS60

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Table 2 Elemental composition analysis (EDS) of PSG-MS15, PSG-MS30, and PSG-MS60
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Raman spectroscopy

Raman spectroscopy is a powerful tool to study the complete sample formation and oxygen vacancy confirmation in ceria-based solid electrolytes. Raman spectroscopy is an evidence for the electrical properties of ceria-based materials, i.e., the total ionic conductivity can be influenced by the structural oxygen vacancies in the ceria lattice [7, 9, 11, 27,28,29,30,31]. Figure 4 illustrates the Raman spectra for all the PSG-MS15, PSG-MS30, and PSG-MS60 samples. It can be observed from Fig. 4 that the Raman spectra of all the samples show two peaks: One intense peak centered at/around 460 cm−1, which is assigned to the characteristic Raman mode of cerium ions surrounded with oxygen ions and confirms the formation of PSG-MS15, PSG-MS30, and PSG-MS60 solid electrolytes. Another peak positioned at/around 570 cm−1 is related to structural oxygen vacancies created in the solid solution due to triple doping with Pr3+, Gd3+, and Sm3+ in ceria [27,28,29,30,31]. Previously, it was reported that the concentration of structural oxygen vacancies in ceria-based electrolytes was estimated by (1) full-width half maxima of intense peak, i.e., FWHM460; (2) intensity ratio of weak peak to the intense peak, i.e. I570/I460; and (3) area ratio of weak peak to the intense peak, i.e., A570/A460 [29, 30].

Fig. 4
figure 4

Raman spectra of PSG-MS15, PSG-MS30, and PSG-MS60

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Therefore, in the present study, we have made calculations on the full-width half maxima of intense peak, intensity ratio of weak and intense peak, and also the area ratio of weak and intense peaks for the entire microwave-sintered PSG-MS15, PSG-MS30, and PSG-MS60 samples and were presented in Table 3. It is found that the PSG-MS30 shows the high value of full-width half maxima (FWHM460), intensity ratio (I570/I460), as well as area ratio (A570/A460), which indicates the high concentration of oxygen vacancies in the composition over others.

Table 3 Raman analysis of PSG-MS15, PSG-MS30, and PSG-MS60
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Due to the highly concentrated structural oxygen vacancies of microwave-sintered PSG-MS30 sample, one may expect higher total ionic conductivity for the PSG-MS30 sample.

Electrical analysis

The electrical properties of triple-doped ceria solid electrolytes were analyzed by a well-established tool, i.e., impedance spectroscopy. Generally, impedance spectra, i.e., complex impedance spectra is a plot of the real part of the impedance (Z′) and imaginary part of impedance (−Z″) and is a characteristic nature of the conducting material. The complex impedance spectra have three successive contributions in different frequency regions, such as electrode contribution that corresponds to lower frequency region, grain boundary contribution to a moderate frequency region, and grain/bulk contribution to a higher frequency region [9, 10, 32, 33]. The complex impedance plots of all the sintered samples at 200 and 400 °C were presented in Fig. 5, and the three distinguishable semi-circles correspond to grain, grain boundary, and electrode processes. Further, it is confirmed that the increase in temperature decreases the resistance of grain and grain boundary. Z-View software is used to fit the complex impedance plots with an equivalent circuit, which consists of the combination of resistances (R) and constant phase elements (CPE). Fitting results of the complex impedance plots for PSG-MS15, PSG-MS30, and PSG-MS60 samples were listed in Table 4. The grain resistance is represented by “Rg,” the grain boundary resistance is represented by “Rgb,” and the total resistance is calculated from the grain and grain boundary resistances, which is represented by “Rt” (Rt = Rg + Rgb). The total resistance, i.e., the sum of grain resistance and grain boundary resistance was taken into account to calculate the total ionic conductivity (σt) of all the specimens using the following equation:

$$ {\sigma}_{\mathrm{t}}=\frac{l}{R_{\mathrm{t}}A} $$
(2)

where, “l” is the thickness of the sample and “A” is collected electrode area. The calculated total ionic conductivity values for PSG-MS15, PSG-MS30, and PSG-MS60 samples were presented in Table 5. The highest total ionic conductivity was observed for PSG-MS30 (3.47 × 10−2 S cm−1) as compared to PSG-MS15 and PSG-MS60 samples. The high value of total ionic conductivity is because of the fast migration of ions in the prepared triple-doped ceria solid solution due to the formation of concentrated oxygen vacancies in the material. The Raman spectroscopy is an evidence for the formation of high concentrated oxygen vacancies, which were analyzed by calculating the 460FWHM, I570/I460, and A570/A460. The comparison has been made for the total ionic conductivity of present optimized microwave-sintered sample with recently studied different samples, such as single-doped, double-doped and triple/multi-doped ceria samples via different sintering techniques presented in Table 6.

Fig. 5
figure 5

Fitted complex impedance plots of PSG-MS15, PSG-MS30, and PSG-MS60

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Table 4 Fitting results of the complex impedance plots for PSG-MS15, PSG-MS30, and PSG-MS60
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Table 5 Total ionic conductivities and activation energies of PSG-MS15, PSG-MS30, and PSG-MS60
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Table 6 Comparison study of total ionic conductivity for PSG-MS30
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To calculate the activation energy for conduction, the logarithm of total ionic conductivity with the inverse of the temperature (i.e., log (σT) v/s 1000/T) was plotted using Arrhenius relation,

$$ \upsigma \mathrm{T}={\upsigma}_0\exp \left(-\frac{E_{\mathrm{a}}}{KT}\right) $$
(3)

where, “Ea” represents the activation energy for conduction, “T” denotes the absolute temperature, “K” indicates the Boltzmann constant, and “σ0” is the pre-exponential factor.

Figure 6a–c shows the grain, grain boundary, and total ionic conductivity of all the samples presented in the form of Arrhenius relation within the temperature range of 200 and 600 °C. The activation energies of all the PSG-MS15, PSG-MS30, and PSG-MS60 samples were then calculated from the linearized Arrhenius plots. From Fig. 6c at/above 450 °C, it can be observed that there is a slight slope change in Arrhenius plots incorporated as an inset in Fig. 6c. This is due to the presence of distinct activation energies in the form of association enthalpy and migration enthalpy. The vacancy associated with the cerium cation is trapped; results in activation energy in the form of association enthalpy and the free migration of oxygen vacancies in the lattice results in activation energy in the form of migration enthalpy. In the low-temperature region, the activation energy is the sum of association and migration enthalpies, whereas in the high temperature region, only migration enthalpy contributes to the activation energy [10]. Therefore, at higher temperatures (≥ 450 °C), the free migration of concentrated oxygen vacancies results in activation energy in the form of migrated enthalpy and leads to changes in the slope. It is observed from Fig. 6 that the change in the slopes results in a change in the activation energies for all the samples and the activation energy values were listed in Table 5. The presence of mobile oxygen vacancies (defects) and interaction between dopant ions and vacancies leads to the low activation energy for microwave-sintered PSG-MS30 sample (i.e., 0.69 eV) [34].

Fig. 6
figure 6

Arrhenius plots for grain (a), grain boundary (b), and total ionic conductivity (c) of PSG-MS15, PSG-MS30, and PSG-MS60

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From the above discussions, as the sintering time is increased, it can be concluded that (1) the density of the samples increased up to 30 min then slightly decreased for 60 min and (2) the grain size becomes fine up to 30 min and then increased for 60 min. A finer grain size with uniform dense microstructure will result in lower activation energy, i.e., more oxygen vacancies in the composition were observed for the PSG-MS30 composition [7, 23, 25, 26]. Lowest activation energy indicating the presence of free migrated oxygen vacancies in the composition leads to enhanced highest total ionic conductivity of the sample. In addition to this, the concentration of oxygen vacancies with the sintering time was studied and estimated using Raman analysis (Table 3), and the high concentration of oxygen vacancies of PSG-MS30 composition is responsible for the enhanced ionic conductivity [26, 29, 30].

Thermal expansion analysis

Thermal expansion analysis of a ceria-based material plays an important role in IT-SOFC applications. To sustain ceria as solid electrolyte for IT-SOFC, in addition to structural and electrical properties, it must have matched thermal expansion coefficient with the other components of a SOFC. The moderate, comparable thermal expansion coefficient of cell modules avoids the mismatch/μ-cracks between them [34, 35]. In view of the above, we have made measurement of thermal expansion for PSG-MS30 sample in the temperature range from room temperature to 1000 °C. The temperature-dependent linear thermal expansion (dL/dL0) curve for the PSG-MS30 sample was presented in Fig. 7. The thermal expansion coefficient (TEC) was then calculated from the curve of linear thermal expansion by the following equation:

$$ \mathrm{TEC}=\frac{\left(\mathrm{L}-{\mathrm{L}}_0\right)/{\mathrm{L}}_0}{\mathrm{T}-{\mathrm{T}}_0} $$
(4)

where “L0” is the initial and “L” is the final lengths of the specimen, “T0” is the start and “T” is the end temperatures of the measurement. It is observed that the TEC value of PSG-MS30 sample (13.43 × 10−6 °C−1 for 30–800 °C and 14.02 × 10−6 °C−1 for 30–1000 °C) shows good thermal compatibility with the reported electrolyte and electrode materials, like Ce0.76Sm0.2Pr0.04O2 − δ electrolyte (13.31 × 10−6 °C−1 for 30–800 °C and 13.78 × 10−6 °C−1 for 30–1000 °C), La0.6Sr1.4MnO4 anode (14.15 × 10−6 °C−1 for 30–1000 °C) and Ba0.6Sr0.4Co0.9Nb0.1O3 – δ–(20%)Gd0.1Ce0.9O1.95composite cathode (14.90 × 10−6 °C−1 for 30–1000 °C) [9, 35,36,37,38,39,40,41].

Fig. 7
figure 7

Temperature-dependent thermal expansion curve for PSG-MS30

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Conclusions

The Ce0.76Pr0.08Sm0.08Gd0.08O2 − δ (PSG) material as solid electrolyte for IT-SOFCs has been successfully synthesized by sol–gel auto-combustion route. The effect of microwave sintering on structural, electrical, and thermal properties of prepared electrolyte material was studied and compared. PXRD analysis revealed the single-phase confirmation of all the samples with cubic fluorite structure. SEM and EDS analyses revealed the dense microstructure and distribution of elemental analysis for all the samples. Microwave sintering resulted in improved electrical properties of the sample which is evidenced by the concentration of oxygen vacancies evaluated from Raman analysis. The highest total ionic conductivity, i.e., 3.47 × 10−2 S cm−1 at 600 °C with minimum activation energy of 0.69 eV, is observed for microwave-sintered PSG-MS30 sample when compared to PSG-MS15 and PSG-MS60. The moderate value of coefficient of thermal expansion, 14.02 × 10−6 °C−1, is observed for the PSG-MS30 sample. The highest total ionic conductivity with minimum activation energy and moderate thermal expansion coefficient of microwave-sintered PSG-MS30 sample makes the possibility of its use as solid electrolyte in IT-SOFC applications.