Structural, optical and electrical properties of M_0.025Sm_0.175Ce_0.8O_2-δ: an electrolyte for IT-SOFCs

Abstract

The system of compositions M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) are investigated in the present study as electrolytes for IT-SOFCs. These samples are prepared by a low-temperature sol-gel process modified by sucrose-pectin to obtain a nanocrystalline powder. The synthesized nanocrystalline powder calcined at 600°C. The average crystallite size is in the range of 11–16 nm. The co-dopants Al, Ca, Mg & Sr promote densification of the SDC 20 at relatively lower sintering temperatures (<1300°C). These samples are characterized by XRD, Williamson-Hall plots, UV-Visible, Raman, FESEM, and impedance spectroscopy. The electrical conductivity is measured in the temperature range of 300–700°C. The sample with composition Sr_0.025Sm_0.175Ce_0.80O_2-δ (SrSDC) exhibits higher electrical conductivity of 0.03342 S/cm at 700°C. Hence, SrSDC can be used as an electrolyte for IT-SOFC applications.

GRAPHICAL ABSTRACT

KEYWORDS:

• Electrolyte
• porosity
• IT-SOFC
• electrical properties
• M_ySm_x-yCe_1-xO_2-δ

1. Introduction

Solid oxide fuel cells (SOFCs) are clean, green, and environmentally friendly energy conversion devices. The efficiency of the fuel cell depends mainly on electrolyte materials [1,2]. The electrolyte performance mainly depends on oxide ion conductivity. Commercial electrolyte materials, like YSZ, requires higher operating temperature (>800°C). Higher operating temperatures always restrict the choice of materials and result in disadvantages like materials stability, degradation, chemical stability, and compatibility across the other cell components [3,4].

Cerium dioxide (CeO₂) is the alternate choice to use as an electrolyte in intermediate-temperature solid oxide fuel cells (IT-SOFC). The CeO₂ is a poor conductor even at 600°C [4]. However, when doped with proper dopant and dopant concentration, it exhibited improved oxide ion conductivity in the temperature range of 550–750°C. Due to the improved oxide ions, doped CeO₂ became the choice for many researchers in the field of fuel cells [2].

Reported results in the literature evidenced that the Sm³⁺ or Gd³⁺ doped (20%) ceria showed improved oxide ion conductivity; besides co-dopants like Sr²⁺, Ca²⁺ etc., doped ceria also enhanced the oxide ion conductivity [1–4]. As reported in the literature, Pr-Gd co-doped ceria [5,6], Sm-Gd co-doped ceria [6,7], Mg-Gd co-doped ceria [8], Sr addition (10 mol%) on SDC20 [9], Effect of Sr on Sm₂O₃-doped ceria [10], Sr-Sm co-doped ceria [11,12] showed improved oxide ion conductivity, whereas La³⁺ and Y³⁺ co-doped ceria [13], Sm³⁺ and Y³⁺ co-doped ceria [14] does not show improvement. The oxide ion conductivity depends on different parameters like the type of dopant, its concentration, ionic radii, particle size, sintering temperature, and synthesis method play an essential role in the optimization of the oxide ion conductivity of an electrolyte [1].

Recently, reported results on alkaline earth and rare earth co-doped ceria systems showed the optimization of density, and electrical properties of Sm or Gd doped ceria, for example, Ca, Sr co-dopants [2], Al co-dopant [15], Mg co-dopant [16]. Moreover, Alkaline earth oxides are cheaper than Rare earth oxides [17,18]. Furthermore, Alkaline earth ions such as Ca²⁺, Sr²⁺, Mg²⁺, Ba^2+, etc., as a co-dopant resulted in increased grain boundary conductivity, i.e. scavenging SiO₂ impurities to grain boundaries. During the sintering process, chemicals that contain SiO₂ impurities that are segregated into grain boundaries; consequently, grain boundaries become more resistive.

Kim [19] and Ciotera [20] reported that SrO reacts with SiO₂ and scavenges SiO₂ impurities into grain boundaries, resulting in decreased grain boundary resistance. It is reported [21–24] that Ca²⁺, Sr²⁺, Mg²⁺, and Ba²⁺ ions act as a scavenger for siliceous impurities at the grain boundary for GDC. It is observed that Sr²⁺ ions react with SiO₂ and clean grain boundaries, which resulted in a decrease in grain boundary resistance so that oxide ions diffuse freely through grain boundaries. Furthermore, it is observed that Alkaline earth oxides are used as an additive to improve density and grain boundary conductivity [25].

In the present study, the selection of divalent (Sr²⁺, Ca²⁺, Mg²⁺) and trivalent (Al³⁺, Sm⁺³) as co-dopants is based on average ionic radii and effective index of ionic radii. Besides, the effective index of ionic radii [26] values is calculated using Eq. 1&2 and presented in Table 1. The effective index of ionic radii values is near 1 in the case of Sr-co-doped ceria, whereas it is away from 1 in the case of Al-co-doped ceria. (1) $\begin{array}{rl}\text{Effective index}& =(\text{avg}.r_c/\text{eff}.r_o)\times (r_d/r_h)\end{array}$(1) (2) $\begin{array}{rl}\text{eff}.r_o& =1.4\times \{(2-\delta )/2\}\end{array}$(2) where symbols have their usual meaning [26].

Table 1. Effective index of ionic radii of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples.

Sample	Avg. rc/eff ro	rd/rh	Effective index = (avg. rc/eff.ro) × (rd/rh)
SDC	0.7457	1.1123	0.8295
AlSDC	0.7354	1.0422	0.7664
CaSDC	0.7502	1.1099	0.8327
SrSDC	0.7540	1.1364	0.8569
MgSDC	0.7470	1.0880	0.8127

Mori et al. [26] proposed that if the sample composition effective index is closer to 1, that composition should exhibit higher ionic conductivity.

In the present paper, the samples of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) are prepared and characterized. The compositions are named AlSDC (Ce_0.8Sm_0.175Al_0.025O_2-δ); CaSDC (Ce_0.8Sm_0.175Ca_0.025O_2-δ); MgSDC (Ce_0.8Sm_0.175Mg_0.025O_2-δ); SrSDC (Ce_0.8Sm_0.175Sr_0.025O_2-δ), and SDC 20 (Ce_0.8Sm_0.2O_2-δ). The present investigation aims to study the structure, impedance, and electrical properties of the Al, Ca, Mg & Sr co-doped ceria samples compared with samarium-doped ceria SDC 20 (Ce_0.8Sm_0.2O_2-δ).

2. Experimental

In the present study, the samples SDC20, AlSDC, CaSDC, MgSDC, and SrSDC are synthesized through a low-temperature-modified sol-gel process [27,28] using sucrose-pectin with compositional formula M_ySm_x-yCe_1-xO_2-δ. The modified sol-gel process employed in the present study differs from the sol-gel process reported in the literature [29,30]. Cerium nitrate (Ce(NO₃)₃·6H₂O), Aluminium nitrate (Al(NO₃)₃·9H₂O), calcium nitrate (Ca(NO₃)₂.4H₂O), magnesium nitrate (Mg(NO₃)₂.4H₂O), strontium nitrate (Sr(NO₃)₂.4H₂O), samarium nitrates (Sm(NO₃)₃·6H₂O), sucrose (C₁₂H₂₂O₁₁), and pectin were used in the preparation. The sucrose and pectin dissolved in distilled water in a beaker. The nitrates are dissolved in another beaker. Now the nitrate solution beaker is kept on a magnetic stirrer at 80°C, sucrose-pectin is added slowly under continuous stirring (1 h) and further kept on stirrer 24 h–36 h at 90°C. The dried material was ground (30 min.), resulting in a nanocrystalline powder that was calcined at 600°C. Then the calcined nanocrystalline powder is pressed into green pellets, sintered at different temperatures, and sintered at 1250°C for 6 h with a programmed heating rate of 5°C /min. The experimental densities (ρ_e) of sintered ceramics were measured by the Archimedes method. The sintered samples’ relative density is over 92% of the theoretical density in all specimens. The theoretical density was calculated by the formula using eq. 3: (3) ${\rho }_t=(n\ast A)/(V\ast N)$(3) where ρ is the density (g/cm³), n is the number of atoms per unit cell, A is the atomic weight (g/mol), V is the volume per unit cell (cm³/cell), and N is the Avogadro’s number The relative density is calculated using the formula (ρ_rel) = ρ_e/ρ_t.

Powder XRD patterns (PXRD) were recorded using BRUKER (D8 Advanced model). The surface morphology of the samples was recorded using SEM (scanning electron microscope) (ZEISS model). Optical absorption was carried out by Shimadzu UV-Vis spectrometer. A Lab RAM-HR instrument was used to record the Raman spectra. The electrical conductivity of samples was measured by a two-probe a.c impedance spectrometer (4294A, Agilent Technologies) on sintered pellet specimens with a 10-mm diameter and 2-mm thickness. The electrical conductivity measurements were recorded at various temperatures in the range of 300°C–700°C in air.

3. Results and discussion

3.1. Powder XRD analysis

Figure 1(a) presents the powder XRD patterns of SDC20, AlSDC, CaSDC, MgSDC, and SrSDC samples. The sample structure is cubic, and the space group is $\mathrm{Fm}\overline{3}\mathrm{m}$. The intense peaks are observed at the planes (111), (200), (220), (311), (222), (400), (331), and (420), and these peaks are matched with the JCPDS file; there are no other extra diffraction peaks of Sm₂O₃ or SrO₂, CaO₂, MgO₂, and Al₂O₃ or impurities. This indicates the formation of a solid solution, i.e. the doping of Sr²⁺, Ca²⁺, Mg²⁺, Al³⁺, and Sm⁺³ in the CeO₂ lattice. There is a shift in the peak (1 1 1) towards the lower Bragg’s angle, as shown in Figure 1(b). This shift is changed with dopant concentration. The shift is towards the lower Bragg’s angle in the case of the Sr, Ca co-dopants, whereas in the case of Al, Mg co-dopants, it is completely towards the higher Bragg’s angle. This shift depends on lattice expansion and contraction; when Ce⁴⁺ (Ionic radii = 0.97 Å) is doped with higher ionic radii [31] Sm³⁺ (Ionic radii = 1.06 Å) and Sr²⁺ (1.26 Å), lattice expands, and the peak shift is slight to the left, whereas it is to the right when the lattice contracts i.e. when Ce⁴⁺ (Ionic radii = 0.97 Å) is doped with lower ionic radii Al³⁺ (Ionic radii = 0.46 Å).

Figure 1. (a) PXRD patterns (b) shift in the plane (111), (c) Lattice parameter vs. Ionic radius, and (d) Williamson-Hall plots of SDC20, AlSDC, MgSDC, CaSDC, and SrSDC samples.

Figure 1(c) presents the lattice parameters versus ionic radii. The introduction of Sm³⁺, and Ca²⁺, Sr²⁺ into Ce⁴⁺ results in the lattice expansion, whereas lattice contraction in case of Sm³⁺, and Al³⁺, Mg²⁺, due to this the X-ray diffraction patterns shifted to lower or higher Bragg’s angle depending on the ionic radii mismatch. The lattice parameter (a) is calculated using Eq.4. (4) $a=d\sqrt{h^2+k^2+l^2}$(4) where “a” is the lattice parameter, d is the interplanar spacing, and (h, k, $l$) are the miller indices.

Figure 1(c) presents the lattice parameter changed with Sm³⁺, Al³⁺, Ca²⁺, Mg²⁺, and Sr²⁺ dopant concentrations. This is due to different ionic radii [31] of Ce⁺⁴ (0.97 Å), Sm³⁺ (1.079 Å), Al³⁺ (0.535 Å), Ca²⁺ (1.06 Å), Mg²⁺ (0.89 Å), and Sr²⁺ (1.26 Å) [6] in a solid oxide solution. Due to ionic radii mismatch, it is observed that the lattice parameter increased for Sm³⁺, Ca²⁺, and Sr²⁺, whereas it decreased for dopants Al³⁺ and Mg²⁺.

Figure 1(d) presents the average crystallite size (D_c) calculated (D_c= 0.9λ/βcosθ) from Williamson–Hall plots. The average crystallite sizes are in the range of 11 nm–16 nm. The modified sol-gel process resulted in the uniform nanocrystalline powder [27,28].

Figure 2 shows the Relative density versus sintering temperatures of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) of specimens. Table 2 presents the relative densities of SDC20, AlSDC, CaSDC, MgSDC, and SrSDC samples, and it is noticed that the co-doped ceria nanocrystalline powder actively absorbs the heat. Its relative density increased from 64% to over 92% within 200°C.

Figure 2. The Relative density versus temperature of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025).

Table 2. Relative densities of the M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples.

Temperature (°C)	SDC20	AlSDC	CaSDC	SrSDC	MgSDC
1000	52	52	50	51	53
1100	68	60	63	67	65
1150	76	71	71	78	70
1200	91	92.3	92.7	93.5	92.4
1250	92.3	94.2	95.6	96.1	94.3

3.2. UV–Vis spectra

Figure 3(a) presents the UV–Vis spectra of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) of specimens. Figure 3(b) presents the Tauc plots; a graph is plotted between (αhν)² versus hν, extrapolating the linear part of the graph results in band gap energy. The band gap energy for AlSDC, SDC20 MgSDC, CaSDC, and SrSDC are 2.78, 2.73, 2.70, 2.66, and 2.62 eV, respectively. The reported band gap energy of CeO₂ was 3.33 eV [32].

Figure 3. UV–Vis spectra (a) Absorbance versus wavelength, (b) Tauc plot of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples.

3.3. Raman spectroscopy

Figure 4 presents the Raman spectra of the M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples. The Raman spectrum is the signature of the presence of oxygen vacancies. It is noticed that there are two prominent peaks; one is strong at around 470 cm⁻¹, and the other one is a weak band at around 520–600 cm⁻¹. Compared with pure ceria [33], the extra peak formed at around 520–600 cm⁻¹ is due to the formation of additional oxygen vacancies formed due to the doping of co-dopants into the host material, i.e. Ce⁴⁺. Furthermore, it can be seen that there is no second prominent peak is observed for Al co-doping, besides the main peak intensity further decreased with different co-dopants.

Figure 4. Raman spectra of the M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples.

3.4. Microstructure analysis

Figure 5 shows the SEM images of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) of specimens sintered at 1250°C. The figures show that the surface clearly shows the grain, and few pores are seen. This result also supports the relative density calculations.

Figure 5. The SEM images of (a) SDC20, (b) AlSDC, (c) MgSDC, (d) CaSDC, and (e) SrSDC samples.

3.5. Impedance spectroscopy

Figure 6 presents the Nyquist plots of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) at 300°C and 500°C. Figures 7 and 8 presents the Nyquist plots for SrSDC and AlSDC samples, respectively.

Figure 6. Nyquist plots of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025).

Figure 7. Nyquist plots of M_ySm_x-yCe_1-xO_2-δ (M = Sr, x = 0.20; y = 0.025).

Figure 8. Nyquist plots of M_ySm_x-yCe_1-xO_2-δ (M = Al, x = 0.20; y = 0.025).

It is noticed that two arcs are clear, and one arc is not clear. The splitting of curves depends on the sample nature, temperature, and applied frequency. The left side high-frequency curve is known as the grain resistance, R_g, the middle region medium frequency curve is the grain boundary resistance, R_gb, and the right side low-frequency curve is the electrode resistance. In the present study, electrode resistance is not considered for total resistance calculation.

It can be further noticed that the depressed semi-circles are present due to the non-uniform distribution of the grains in the sample microstructure. The total resistance is calculated by fitting the Nyquist plots using EIS software [34]. The equivalent circuit models were employed to fit the Nyquist plots; here, the constant phase element is used instead of a capacitor due to the presence of depressed semi-circles in impedance plots. The equivalent circuits model (Resistance-capacitor)_g-(Resistance-capacitor)_gb was used up to 400°C, whereas above 400°C, the equivalent circuit model (Resistance)_g-(Resistance-capacitor)_gb was used to fit the date because the grain curve is not resolved properly.

Furthermore, it is noticed from Figures 6–9 that the impedance spectra resolved properly as grain, grain boundary, and electrode components up to 400°C; after that, the grain component is not resolved correctly due to exceeding the experimental frequency. The total resistance is then calculated as R_t = R_g + R_gb where R_g is the grain resistance (length of the grain curve on the X-axis), R_gb is the grain boundary resistance (length of the grain boundary curve on the X-axis). The capacitance values range from 10⁻¹¹ to 10⁻⁷ F for the grain and grain boundary resistance, respectively [35,36].

Figure 9 presents the typical Nyquist plots of SDC20, AlSDC, MgSDC, CaSDC, and SrSDC samples at 300°C. It is observed that the SrSDC, CaSDC and MgSDC samples exhibited lower total resistance than the SDC, whereas higher resistance is observed for the sample AlSDC. These results can be correlated with microstructure, i.e. grain size (Figure 5). The sample SrSDC exhibits a lower grain boundary resistance than the rest due to higher density. Similarly, other samples’ grain and grain boundary resistance changed with the grain size.

Figure 9. Nyquist plots of SDC20, AlSDC, MgSDC, CaSDC, and SrSDC samples at 300°C.

3.6. Electrical properties

The total resistance R_t, the grain resistance R_g, and the grain boundary resistance R_gb are taken from the impedance spectra at different temperatures. The grain σ_b, grain boundary σ_gb, and the total conductivity σ_t were then calculated using Eq. 5, (5) ${\sigma }_g=\frac{L}{R_{g}A},{\sigma }_{\mathrm{gb}}=\frac{L}{R_{\mathrm{gb}}A},{\sigma }_t=\frac{L}{R_{t}A},$(5) where L is the thickness, and A is the cross-sectional area of the sample.

Figure 10(a–c) presents the grain, grain boundary, and total electrical conductivity of SDC20, AlSDC, MgSDC, CaSDC, and SrSDC samples. It is noticed that the MgSDC sample showed higher grain conductivity than the rest of the samples, as presented in Figure 10(a). In contrast, the SrSDC sample showed higher grain boundary conductivity than the other samples. Furthermore, it is observed that the SrSDC sample exhibited higher total conductivity, whereas the AlSDC sample showed lower total conductivity. The main contribution of the conductivity of ceria-based compounds in the air is oxide ionic conductivity and electronic conductivity (negligible) [1,26]. In the present study, the conductivity measured in the air can be treated as oxide ionic conductivity [1,3].

Figure 10. The Arrhenius plots of (a) Grain conductivity, (b)Grain boundary conductivity, and (c) Total conductivity of SDC20, AlSDC, MgSDC, CaSDC, and SrSDC samples.

Figure 10(a–c) shows the variation of log σ with 1000/T for M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) ceramics sintered at 1250°C. The total ionic conductivity is the bulk value, the sum of the grain, and the grain boundary contributions [17,18]. The ionic conductivities are significantly enhanced in SDC20 ceramic by co-dopants Ca, Mg & Sr, which introduced oxygen vacancies in the host material. The SrSDC sample showed maximum ionic conductivity of 0.03342 S/cm at 700°C, that of SDC20 (0.01935 S/cm). The oxide ion mobility increases with temperature, so the conductivity increases at higher temperatures.

Eq.6 is used to calculate the activation energy from Figure 10(a–c): (6) $\mathrm{\sigma T}={\sigma }_0\exp \left(-\frac{E_{\text{a}}}{\mathrm{kT}}\right)$(6)

The activation energy for conduction is obtained by plotting the ionic conductivity data in the Arrhenius relation for thermally activated conduction. The calculated activation energy values are presented in Table 3, and it can be seen that the SrSDC sample exhibits lower activation energy (i.e. 0.81 eV) than the rest of the samples. It is worth noting that the activation energy values are changed with different co-dopants in the M_ySm_x-yCe_1-xO_2-δ system. Besides, with increasing temperature, the activation energy values are decreased.

Table 3. The effective index, electrical conductivity, and activation energy of M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) samples.

			Activation energy (eV)
Sample	Effective index = (avg. rc/eff.ro) × (rd/rh)	Conductivity (S/cm) at 700°C	300–500°C	550–700°C
SDC	0.8295	0.01935	0.97	0.83
AlSDC	0.7664	0.01751	0.98	0.85
CaSDC	0.8327	0.03064	0.94	0.82
SrSDC	0.8569	0.03342	0.92	0.81
MgSDC	0.8127	0.02451	0.95	0.84

The activation energy (E_a) for conduction is the sum of the association energy (H_a) and migration energy (H_m). The activation energy at lower temperatures is the sum of these two contributions, i.e. E_a = H_a+ H_m. At higher temperatures, on the other hand, the activation energy is only equal to the migration energy, i.e. E_a = H_m, [1,3].

According to Mori et al. [26], it is assumed that if the effective index approaches 1, it is possible to obtain the maximum oxide ion conductivity. Table 3 shows that the SrSDC sample’s effective index approached 1 compared to the rest of the samples. The SrSDC sample showed maximum oxide ion conductivity than the other samples in the present study.

Table 4 presents the SrSDC conductivity and activation energy. It can be seen that the SrSDC sample conductivity compared well with the reported in the literature.

Table 4. Comparison of the electrical conductivity and activation energy of the SrSDC sample with reported values in the literature.

Sample	Conductivity (S/cm)	Activation energy (eV)	Reference
Ce0.8Sm0.175Sr0.025O2-δ (SrSDC)	3.34 × 10^−2 at 700°C	0.81	Present
Ce0.78Sm0.2Sr0.02O1.88	≅ 2.1 × 10^−2 at 700°C	0.84	[10]
Ce0.8Sm0.15Sr0.05O2-δ	2.21 × 10^−4 at 750°C	0.66	[12]
Ce0.90Mg0.06Sr0.04O1.90	2.00 × 10^−2 at 700°C	0.85	[17]
Ce0.82Sm0.16Sr0.02O1.90	2.67 × 10^−2 at 600°C	0.66	[18]
Sr-GDC	2.30 × 10^−2 at 700°C	0.59	[25]
Ce0.8Gd0.1Sr0.1O1.85	2.40 × 10^−2 at 700°C	0.61	[37]
Ce0.77Sm0.17Sr0.03O2-δ	3.02 × 10^−2 at 700°C	0.97	[38]
0.94SDC–0.06Sr	3.21 × 10^−2 at 700°C	0.71	[39]
Ce0.8Sm0.15Mg0.05O2-δ	3.06 × 10^−2 at 700°C	0.86	[40]
Ce0.8Sm0.1Y0.1O1.9	3.17 × 10^−2 at 700°C	0.64	[41]
Ce0.8Y0.18Sr0.02O2-δ	2.20 × 10^−2 at 800°C	0.89	[42]
Ce0.8Sm0.15Ca0.05O1.875	1.69 × 10^−2 at 600°C	0.91	[43]

4. Conclusions

A series of co-doped ceria nanocrystalline samples of the M_ySm_x-yCe_1-xO_2-δ (M = Al, Ca, Mg & Sr, x = 0.20; y = 0.025) system are successfully synthesized by a sol-gel process modified using sucrose-pectin. XRD results confirm the single phase with a cubic structure. The average crystallite size is in the range of 11–16 nm. The co-dopants Sr, Ca, and Mg decreased the band gap energy of SDC20 from 2.85 eV to 2.47 eV. Raman spectra confirm the formation of more oxygen vacancies in the case of co-dopants like Sr, Ca, and Mg than the SDC20. The nanocrystalline powder is subjected to a sintering temperature of 1250°C to prepare dense ceramics (more than 92% TD). The modified sol-gel process resulted in a uniform nanocrystalline powder and highly dense samples (more than 92%TD) at a lower sintering temperature of 1250°C. The impedance results showed that the grain and the grain boundary resistance of SDC20 changed with Al, Ca, Mg & Sr co-dopants. The grain boundary conductivity contribution is more than the grain to the total conductivity. Higher density resulted in grain growth, which decreased the grain boundary resistance of Sr, Ca, and Mg co-doped SDC samples. Based on the effective index calculations, the SrSDC sample’s effective index is near 1, and experimentally, the SrSDC sample exhibited higher oxide ion conductivity than the rest of the samples. The co-dopant Sr introduced more oxygen vacancies in the SDC 20 than the other dopants, resulting in higher oxide ion conductivity (3.34 × 10⁻² S/cm at 700°C) and minimum in the activation energy (0.81 eV) than the rest of the samples. Co-dopant Sr significantly improved the density, grain boundary, and total conductivity of SDC20 to the rest of the samples. Therefore, the SrSDC sample is a potential candidate as an electrolyte for IT-SOFC applications.

Acknowledgement

The present work has been supported by GITAM, funding under the scheme of Research Seed Grant-RSG-(F. No 2021/0060).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by GITAM, funding under the scheme of Research Seed Grant-RSG-(F. No 2021/0060).