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Tailoring the activation energy and the ionic properties of bismuth codoped SDC powder prepared by solid-state reaction method

Research on fuel cell components has received great attention owing to the growing need for sustainable energy sources. Bismuth (Bi3+) codoped samarium-doped cerium oxide [Ce1−xSmx−yBiyO2−δ (x = 0.2 and y = 0, 0.05 and 0.1)] nanosystems were prepared by solid-state reaction method. Rietveld structure refinement of X-ray diffraction pattern confirms the cubic fluorite structure along the (111) plane with the decrease in lattice distortion. At the same sintering temperature, pellets exhibit good morphology with better mechanical strength. The conductivity measurements carried out using the Nyquist plot, as well as the modulus spectra, indicate the effect of grain and grain boundary conduction at high temperatures. With the increase in the incorporation of Bi dopant, there is a gradual decrease in ionic conductivity and activation energy. The composition of Ce0.8Sm0.1Bi0.1O2−δ exhibits less ionic conductivity compared to other samples due to the oxygen vacancies attracted by dopant cations. The effect of Bi3+ dopants on samarium-doped ceria lattice structures and the electrical properties of the systems have been discussed.


Introduction
Rapid industrialization and urbanization increased the usage of energy in day-to-day life. The main crisis that we are facing is the massive discharge of contaminants and pollutants due to the heat engines utilizing fossil fuel. All these factors boost the prospect of global warming [1]. In this scenario, renewable energy technology has been considered one of the most appealing options for the survival of humankind. The fuel cell is regarded as the best candidate for energy conversion [2].
The researchers are focusing their attention on the solid oxide fuel cell (SOFC), due to their high energy conversion and low environmental impact. Yttriastabilized zirconia (YSZ) was one of the most used electrolytes in high-temperature SOFCs. The high sintering temperature restricts its application in the SOFC applications [3]. Literature review reports that samarium-doped ceria (SDC) exhibits superior oxygen ion conductivity compared to YSZ electrolyte at intermediate temperature. The main drawback of SDC prepared by a solid-state method is its high sintering temperature to obtain desirable chemical homogeneity and density. The pellet sintered at a high temperature cannot be easily cofired with the electrode material. To overcome these inherent limitations, it is desirable to prepare a high-density pellet at reduced sintering temperature to reduce the cost of fabricating SOFCs [4][5][6].
Bismuth oxide systems possess high oxide ion conductivity to make them the best suitable candidate for good electrolyte material. There are certain drawbacks for bismuth oxide systems and doped bismuth oxide systems such as unstable under low partial pressure oxygen conditions which limits its application as an alternative electrolyte material [7]. Doped bismuth oxide systems possess a complex array of structures and their properties can be varied by changing the dopant concentration, temperature, and atmosphere. d-Bi 2 O 3 can be stabilized at room temperature by doping with smaller sized rare earth and/or donor-dopant cations. The rare earth elements such as Gd, Ho, Nd, Sm, Y, Dy, Er, and higher valency cations such as W or Nb are used as dopants for Bi 2 O 3 [8][9][10][11]. Biesuz et al. reported that samariumdoped ceria (SDC) was prepared and sintered at a temperature of * 500°C. The sintered pellet exhibits nanometric grain size with relatively high density [12]. Karaca et al. worked on the preparation of nanocrystalline samarium-doped CeO 2 by modified sol-gel process and hydrothermal content treatment and obtained ultrafine grain at the sintering temperature of 1300°C [13]. Prekasji et al. reported that the highest ionic conductivity for bulk and grain boundary to be 3.14 9 10 -3 and 9.32 9 10 -4 Scm -1 , respectively, at 700°C for Ce 0.8 Bi 0.2 O 2-d composition sintered using microwave (MS) sintering technique [14]. Accadro et al. investigated the microstructure of the gadolinium-doped ceria electrolyte, materials codoped ceria with bismuth oxide (1-5 mol%). The study reveals that high ionic conductivity was reported for the optimal 3% composition of bismuth [15]. Zhao et al. studied the impact of the incorporation of a small amount of Bi 2 O 3 to SDC prepared by Pechini-type gel routine. The analysis concluded that Bi 3? dopant plays a significant role in the anion transport of the materials [16]. Different preparation methods like solid-state reaction [17], solution combustion [18], citrate gel [19], and hydrothermal process [20] are used for synthesizing SDC nanopowder. The nanocrystalline materials exhibit better electrochemical properties like rapid densification kinetics, lower sintering temperatures, etc. Synthesizing electrolytes in nanoscale may result in improved performance of the cell. A detailed analysis of the ionic conductivity of the nanoscale material is to be explored. In this work, solid-state reaction is used to prepare bismuth-codoped SDC electrolyte, since it is a simple and inexpensive method.
In this work, attempts are made to investigate the effect of doping the trivalent bismuth ion on the SDC electrolyte. Codoping SDC with trivalent ion tends to reduce the sintering temperature without compromising the ionic conductivity. A detailed study on ionic transport helps to tailor the material for desired applications.

Experimental method
Bismuth-doped SDC electrolyte BiSDC 5(Ce 0.8 Sm 0.15-Bi 0.05 O 2-d ) and BiSDC 10(Ce .8 Sm 0.1 Bi 0.1 O 2-d ) samples of varying concentrations of the dopant were processed using the solid-state reaction method. For the solid-state synthesis method, the stoichiometric amount of cerium oxide (CeO 2 ), samarium oxide (Sm 2 O 3 ), and bismuth oxide (Bi 2 O 3 ) were mixed and grinded using acetone as a solvent for 5 days (approximately 48 h) in an agate mortar. After the grinding process, the powders were calcined at 600°C for 2 h. The powders were uniaxially pressed by applying a pressure of 2.5 MPa in a cylindrical mold. Each pressed pellet was sintered at the same temperature 800°C for 10 h. There was a shrink in the diameter of the pellets after the sintering process. Different dopant concentration was studied to determine the change in the ionic conductivity of the doped material.
The nanosized powder was characterized using an XPERT-PRO X-ray diffractometer (XRD) under Cu-K a1 radiation (1.5406 Å ) operating at 40 kV, 30 mA at room temperature. The least square method analysis was used to tabulate unit cell parameters. Rietveld analyses were performed using Fullprof Suite software. The morphology of sintered pellets was examined using Bruker FEI-NOVA NANOSEM 450 and JEOL Model JSM-6390LV. The electrical measurements of the sintered pellets were performed using an Impedance analyzer, Solatron 1260A with the aid of SMART software.

XRD studies
The Rietveld-simulated X-ray diffractograms of Bismuth-doped ceria lattice are shown in Fig. 1. After the calcination process, the XRD pattern gives a clear indication of the cubic fluorite structure (ICCD File No. 34-0394) which indicates there is no presence of segregation that occurred between Bi 2 O 3 and SDC. This structure is more suitable as an ionic conductor and is related to cerium oxide. Scherrer equation is used to calculate the average crystallite size [21] and the computed lattice parameter, volume, and density of the sample obtained from refinement are listed in Table 1. From the reported literature, it is understood that the ionic conductivity of oxides is mainly affected by the degree of lattice distortion [22]. The degree of lattice distortion depends on the discrepancy in the ionic radii of the host lattice and dopant cations. The lattice parameter of the composition was found to decrease with increasing bismuth due to the oversized Bi 3? ions (0.117 nm) generates distortion in the ceria lattice. As a result, lattice contracts due to the decrease in grain boundary mobility. Larger sized Bi 3? ions (0.117 nm) codoped SDC lattice resulted in lattice contraction compared to singly doped SDC lattice. The crystallite size reached the nanometric range with the addition of dopants. Fig. 2 shows the morphology of SDC, BiSDC 5, and BiSDC 10 pellets sintered at 800°C for 10 h. The morphology of the SDC pellets sintered at 800°C for 10 h indicates, separately, the grains and grain boundaries. The grain boundary conductivity is blocked due to the presence of a siliceous impurity layer and porous structure present in SDC pellets. It is noted that on the BiSDC 5 and BiSDC 10 sample surface, the nanopowders are agglomerated in the form of clusters. As the size of the cluster becomes bigger, the densification of the electrolyte gets retarded. This demonstrates that the incorporation of Bi in SDC lattice supports the generation of clusters on precursor material.  Figure 3 shows the disparity of loss tangent(tand) with frequency for different concentrations of the dopants. There is a decline in loss tangent(tand) value with the increasing frequency in all samples owing to the mobile ion charge carriers. At higher frequency, the loss tangent parameter becomes almost frequency-independent in all samples. At higher temperatures, an increase in tand is observed which contributes to the high ionic conductivity in the material. It is noted that at a higher temperature in all samples, there is a sudden rise in loss tangent at the low-frequency region owing to the space charge polarization [23]. There is a valley in loss tangent (tand) value in low frequency at 500°C indicates an increase in dielectric loss owing to the motion of molecule from one quasi-stable to another quasistable position. The relaxation phenomena occur on the higher-frequency side marked by the reduction in dielectric loss. Figure 4 shows the disparity of the imaginary part of impedance with the change in frequency at a selected temperature. As the temperature increases, the peaks are found to be deviating towards the higher-frequency side. The same behavior is exhibited for both the sample. The change in the peak frequency with a temperature rise obeys the Arrhenius relation [24]. Figure 5 Illustrates the Nyquist plot of SDC, BiSDC 5, and BiSDC 10 observed at a different temperature. At low temperatures for the three samples, only one large arc is visible, indicating prominent grain conduction and/or parallel grain boundary conduction through the sample. As the temperature increases, there is a relative contribution of grain interior (GI) and grain boundary (GB) seen in fitted Nyquist plot of SDC sample at 600°C. Prominent grain boundary occurs in SDC samples at the higher temperature. In BiSDC 5 and BiSDC 10 samples, there is a tendency for the formation of another arc at a higher temperature which indicates grain boundary effect. Fitted values of capacitances are in nano-and picofarad for SDC samples which mean grain interior as well as parallel grain boundary conduction mechanism occurring in the sample. The fitted capacitance value of BiSDC 5 and BiSDC 10 is in picofarads, hence prominent grain interior/bulk conduction at the higher temperature [25].

Electrical studies
The ionic conductivity of the prepared pellet sintered at the same temperature in the air with variation in temperature is shown in Fig. 6. As can be seen, the ionic conductivity of the prepared pellet at a higher temperature under study. The total conductivity is screened in the form of an Arrhenius plot. There is a change in the slope of the Arrhenius plot for different concentrations of the dopants Bi 3? at a temperature of 400°C. So, the Arrhenius plot is divided into two parts: one at low temperature and another at high-temperature regime related to different activation energies [26][27][28]. The literature report Bi 3? is highly polarizable due to the existence of 6s 2 lone pair of electrons. The presence of 6s 2 lone pair of electrons helps them to incorporate into the relatively disordered surroundings [29]. Wachsman et al. [30,31] reported that the oxygen ion transport mechanism involves the migration of oxygen ions from regular 8c sites to interstitial 32f sites. Thus, the migration of oxygen ions gives support to Bi 2 O 3 segregation at the grain boundaries and enhances the oxygen ion conductivity. Electrical studies show greater ionic conductivity for samarium-doped ceria with an increased amount of samarium due to the less deformation in the ceria lattice. This is mainly due to the ionic radius of samarium near that of host ion ceria (Ce 4? ). But the ionic conductivity of the Ce 0.8 Sm 0.1 Bi 0.1 O 2-d was found to be lower than Ce 0.8 Sm 0.15 Bi 0.05 O 2-d and Ce 0.8 Sm 0.2 O 2-d . As the amount of Bi 3? dopant increases, the oversized Bi 3? produces a strong distortion in the ceria lattice. There is a decrease in grain boundary mobility due to the  [16]. The total ionic conductivity of the samples is tabulated and given in Table 2.

Modulus spectroscopy
The modulus spectroscopy technique is one of the most powerful tools to determine the electrical inhomogeneity and carriers transport mechanism in electrolytes. Modulus formalism helps in suppressing electrode effect and dominating the bulk effects on the frequency domain. The complex modulus spectra are usually obtained from the impedance using the relation.
where x = 2Gf is the angular frequency, i = p -1, C 0 = e 0 S/d is the empty cell capacitance, e 0 is the absolute permittivity, S is the surface area, and d is the thickness of the sample [24]. Figure 7 shows the disparity of M 0 with frequency in the temperature range of 200-500°C. There is a change in the value of M 0 for both low and high frequencies. In the lowfrequency region, very low M 0 is noted in all the samples. But as the temperature varies, there is a steady rise in the dispersion as frequency changes from 1 Hz to 1 Mhz. This is a clear indication of lacking restoring force controlling the mobility of the charge carriers [32]. From the observation, it is evident that there is no electrode effect in the samples shown in Fig. 7. Figure 8 shows the disparity of M 00 with frequency in the temperature range of 200-500°C. There is a deviation in the relaxation peak towards the higher-frequency sides in all samples which reveals the existence of hopping of carriers in all the samples [33].

Conclusion
The bismuth-codoped SDC samples were processed by the solid-state reaction method. XRD images show the cubic fluorite structure with high intensity along the (111) plane. A difference in surface morphology is noted for the codoped samples at the same sintering temperature due to the varying concentration of Bi 2 O 3 . It is noted that there is a reduction in dissipation factor with the rise in frequency. The grain and grain boundary resistance decrease with a rise in temperature from the Nyquist plot in all the codoped samples. The total ionic conductivity and activation energy are obtained from the Arrhenius plot at the  temperature of 400-750°C. At higher temperatures, the activation energies are noted to be less than 1 eV in all the samples. This work provides an insight that the varying concentration of the dopants has a great