Structural and Electrochemical Performance of Sr0.9La0.1WO4 Electrolyte Materials for Solid Oxide Fuel Cells

A possible component of power generation technologies, solid oxide fuel cells (SOFCs) offer a high fuel-to-power conversion efficiency and no negative environmental impact. The solid-state sintering process was used to synthesize the scheelite-structured Sr0.9La0.1WO4 (SLW1) electrolyte material, and phase stability and ionic conductivity were evaluated for their technological applicability for SOFC applications. The resulting mixture, which was a single phase in a tetragonal crystal system, produced a scheelite structure of the space group I41/a. Its symmetry, space group, and structural characteristics are confirmed by X-ray diffraction (XRD) at room temperature and the Rietveld analysis that follows. A highly dense crystal structure was revealed by SEM examination. The ionic conductivity of the SLW1 sample is higher than SBW materials and lower than the conventional BCZY perovskite structure. At 900°C in both the wet Ar and dry Ar conditions, the SLW1 sample’s ionic conductivity was 2.41 × 10−5 S cm-1 and 1.76 × 10−5 Scm−1. This scheelite-like compound showed a thick microstructure and substantial conductivity, making it a potential mixed ion-conducting electrolyte for SOFC.


Introduction
Due to its high efficiency, versatility in terms of fuel, and low pollutant emissions, solid oxide fuel cells have emerged as a major boon in the modern development of renewable and sustainable energy technology [1][2][3][4].The components of SOFCs were developed using a variety of materials.Proton (H+ ion) conducting materials performed better among them because they are thermally activatable at lower temperatures, whereas oxygen ion-conducting materials need high activation temperatures [5].Most perovskite-type ceramic oxides have demonstrated higher protonic conductivity in H 2 and H2Ocontaining atmospheres with a higher efficiency and low activation energy at 400 to 700°C, which is called the intermediate temperature (IT) range [6][7][8].Since it can be produced more affordably utilizing economical stack connection materials, the IT-SOFC has become an eco-friendlier system than the traditional HT-SOFCs [9,10].In general, it has been discovered that oxides with oxygen 1305 (2024) 012025 IOP Publishing doi:10.1088/1757-899X/1305/1/012025 2 deficit in the form of oxygen vacancies act as high-temperature proton conductors, with protons dissolving as hydroxide defects at the expense of the vacancies.It is extremely difficult to find and create the optimum combination of extremely dense and highly chemically stable proton-conducting electrolyte materials for electrolytes at a lower sintering temperature.Oxides with oxygen vacancies and protons can be provided by only the acceptor-doped perovskites.Updated proton conductivity of roughly 0.01 Scm -1 is demonstrated by several perovskites that contain Ba and Sr, such as BaCe 0.9 Y 0.1 O 3 [11][12][13][14].Meanwhile, materials based on BaCeO3 and BaZrO3 showed strong conductivity and better chemical stability [15][16][17][18].At the intermediate temperature range, a proton-conducting electrolyte, BaCe 0.7 Zr 0.25-x Y x Zn 0.05 O 3 has been developed recently and showed a highly conductive and dense electrolyte [19,20].Based on the scheelite structure, it has recently been proposed to utilize acceptor-doped rare-earth minerals, MTO 4 as other proton-conducting materials, where M = Sr, La, Ba, Ca, Y, Cd, Nd, Tb, Gd, Tb, Pb, Er and T = Mn, W, Nb, Mo, can provide large CO 2 tolerances [21][22][23][24][25].The role that p-type electrical conduction plays, is large in oxidizing conditions over 800°C, where proton conductivity predominates under moist conditions up to about 1,000°C temperatures.LaNbO 4 materials have acceptable conductivity despite being practically pure proton conductors, and they are renowned for remaining stable in environments with water vapor and CO 2 [26,27].When LaNbO 4 has small acceptor substitutions in A site, like Ca 0.01 La 0.99 NbO 4-δ , it has the highest proton conductivity yet observed at 800°C [28].The scheelite crystal, BaWO 4 is most effective for the creation of Raman lasers [29].Similar to other Scheelites crystals, BaWO 4 has a straight band gap and less dispersive valence and conduction bands, according to structural simulations.At 800°C, Pb 0.9 Sm 0.1 WO 4+δ exhibits an ionic conductivity near 2×10 -2 Scm -1 , with compare to YSZ's conductivity of 3.6×10 -2 Scm - 1 [30,31].Additionally, the oxides of the scheelite class exhibit significant oxide ion conduction.While SrWO 4 materials were initially used as optical fibers, photoluminescence medium, laser heating elements, photocatalysts, and antibacterial materials, they can also be used in fuel cell applications and accepted the insertion of other lanthanide ions that can be employed as matrices for laser-active components that nonlinearly convert light into a new spectral range [32].
In the present study, SLW1 was composited with scheelite to provide an adequate amount of very dense electrolyte for use in SOFC.This material's strong conductivity and high-density coupling make it ideal for SOFC applications.The abbreviation SLW1 refers to the freshly mixed ion-conducting scheelite Sr 0.9 La 0.1 WO 4 .Solid state reaction (SSR) was used to manufacture the sample compound, Xray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize atomic and morphological structure, and electrochemical impedance spectroscopy (EIS) was used to characterize ionic conductivity.

Experimental
The SLW1 ceramic compound was made using the solid-state reaction technique.Using a mortar and pestle, ethanol was combined with stoichiometric proportions of SrCO 3 (98% purity, Aldrich, China), La 2 O 3 (98% purity, Aldrich, China), and WO 3 (99% purity, Aldrich, USA).The finely powdered material was first dried at 100℃, and then it was heated at a rate of 2°C min -1 for 10 hours at 700°C.Using a hydraulic press, the pellets (13 mm dia) were produced under 5 tons of pressure, sintered in air for 10 hours at 900°C, and then cooled at 5°C min -1 .In the air for 10 hours, the final sintering temperature was 1000°C.X-ray powder diffraction was used to study the phase characterization in the 2 range from 10° to 90° using a Bruker axs-D8 advance diffractometer (CuK1, = 1.5406).The data was gathered using a 0.01° step size and a 60 sec/step count time.The Rietveld approach was utilized to further refine the data that was gathered using the Full Prof (ABC Publishers) program [33].FEG-SEM was used to investigate the produced electrolyte's morphological characteristics (JSM-7610F).A chamber with an isolated atmosphere was used to gather the SEM morphological data.Using EIS, the electrochemical characteristics were investigated.In order to measure the impedance spectroscopy in the frequency range of 6 MHz to 1 mHz, a ProboStat (NorECs, Norway) system was connected to a Solartron 1260 frequency response analyzer.One V rms was the applied sine wave's amplitude.The impedance analysis was carried out using the sintered pellets of the as-prepared material (13 mm in diameter and 0.5 cm 2 platinum pasted electrodes).The conductivity cell was filled with a dry and wet Ar environment that had been dried by passing Ar gas separately through two beds of P 2 O 5 desiccant before being used to measure impedance during the cooling cycle in 50°C steps from 1000 to 150°C temperature.Before taking impedance spectra at any temperature, ample time was given to guarantee stability.The experimental impedance data were fitted with the Z-View impedance refinement program (Scribner Associates Inc.).The electrical response of the samples was modeled using a brick-layer structure.The experimental data showed that each arc was a parallel assembly comprising a resistance (R) and a constant-phase component (CPE).At low temperatures, such as T 200°C, the high impedance made it difficult to reliably extract the resistance.The results for conductivity did not get any sample porosity correction.

Phase Analysis
On the prepared samples, the XRD mechanism was applied.The patterns fall within the category of I4 1 /a space group single phase scheelite type tetragonal symmetry.Fig. 1 shows the Rietveld refinement of the SLW1 sample.No extra or intermediary phases could be found in SLW1.There are no phase changes in SLW1 due to the behavior of the inherently stable and highly reactive elements strontium and lanthanum, as well as the heaviest and most stable element, tungsten.The bulk densities, refinement factors, and unit cell parameters of the Rietveld refinement analysis are displayed in Table 1.Tungsten (W) served as the B-site component in this composition, while strontium (Sr) and lanthanum (La) made up the A-site.The chi-square, R wp, and R p values are a little bit high due to the U V W refinement and especially the negative value of U refinement.The arrangement of the A cations and vacancies inside the structures, as well as the presence of Sr vacancies, can provide novel ways to change the properties of the structures.Sr, La, and W had atomic radii of 2.19, 1.95, and 2.1 correspondingly.10% La was doped with Sr in a cation with different valences (2 and 3), but the same chemical reactivity and stability.The compound's lack of cations might be thought of as making it a good ionic conductor.Consequently, enhanced conductivity results from the presence of a modest number of dopants [34].

Morphology analysis
An investigation using a scanning electron microscope was done to see how the SLW1 electrolyte's microstructure morphology.The surface microstructure of the SLW1 electrolyte is depicted in Figure 3.The SLW1's surface was flawless and unbroken.The sample's grains were big and well-developed, entirely compressed next to one another.In the sample under investigation, there were no signs of secondary phases or liquids near the grain boundary.This suggests that the electrolyte material is nonporous and has a high density.The grain sizes range from 1 to 100 microns.Lower grain boundary resistance is provided by the high grain size, which is advantageous for ion conduction.

Ionic Conductivity
Using AC impedance spectroscopy, the ionic conduction of the SLW1 sample was examined as a component of its electrochemical characteristics.Figure 4 displays the impedance spectra of the SLW1 with the bulk response and grain boundary (GB) response, which was obtained at 700 to 600 °C under wet Ar circumstances.The estimated bulk and GB resistance of the sample compositions were calculated using a circuit model.The fitting was performed without accounting for the electrode interface response, and the total resistance was determined by adding the GB and bulk resistances.By seeing many semicircles about 600°C under moist Ar conditions, it can be concluded that the processed samples include GB resistance.From the intercept at a higher frequency with the real axis, the bulk is resolved between 600°C and 900°C.The temperature range of 500°C to 900°C resulted in an observed total frequency range of 20 Hz to 9.0E5 Hz.The frequency range, for instance, was 600 Hz to 9.0E5 Hz at 600 °C.Even though two RC (resistance in combination with parallel to CPE) comparable circuits are generally employed in series (see Figure 4 inset), it was difficult to distinguish between the bulk and grain boundary conductivities over 700°C.The Arrhenius plot of an SLW1 sample in a dry Ar and a wet Ar atmosphere is shown in Figure 5.In a dry Ar and wet Ar environment, respectively, the total conductivities of SLW1 were 9.63×10 -8 , 7.75×10 -7 , 4.56×10 -6 and 1.76×10 -5 Scm -1, and 1.7×10 -7 , 1.34×10 -6 , 6.43×10 -6 and 2.41×10 -5 Scm -1 at 600, 700, 800 and 900℃.Total conductivity activation energies (Ea) were 0.67 eV and 0.63 eV, respectively, for dry Ar and wet Ar environments.SLW1 outperformed the previously investigated SrWO4 molecule and SBW series in terms of total ionic conductivity and activation energies.[32,35].

Conclusion
The SLW1 electrolyte that followed was effectively synthesized and characterized in this research work after a single-phase SLW1 electrolyte was created via a solid-state reaction.The tetragonal scheelite structure (S.G.I4 1 /a) was revealed by the Rietveld analysis of the XRD data.High-density and non-porous materials were seen in SEM morphological images, which is significant for electrolyte application.The scheelite structure sample has a modest level of ionic conductivity.Under dry argon circumstances, SLW1 demonstrated an ionic conductivity of 2.41 × 10 −5 S cm-¹ at 900°C under wet Ar atmosphere.These materials can be used as SOFC electrolyte materials because they have a very good microstructure, considerable conductivity, and good stability, according to the results.

Acknowledgments
The researchers, AA, would like to express gratitude to Universiti Brunei Darussalam for offering a UGS scholarship to carry out this study.We are appreciative to Professor Sten Eriksson for setting up the three-month summer fellowship at Chalmers University of Technology's Department of Chemistry and Chemical Engineering to carry out this study.Additionally, we appreciate Md.Aminul Islam from the Department of Geological Science, Faculty of Science (FOS), Universiti Brunei Darussalam, for his outstanding assistance.This scientific endeavor was supported and funded by the UBD CRG project titled UBD/OVACRI/CRGWG (006)/161201.

Figure 1 .
Figure 1.Rietveld refinement profile of SLW1 in a unit cellThe tetragonal scheelite structure of the SLW1 compound is depicted schematically in Fig.2using a 3D polyhedral diagram created with the VESTA software.Fig.2(a) and 2(b), also depicts the polyhedral relationship and the bonding of the B and A cations to the oxygen atoms (b).Each of the octahedral A cations (Sr and La) and the tetrahedral B cations (W) are coordinated to eight oxygen atoms in the scheelite-type ABO 4 structure, which is a frequent binary oxide in both natural and

Figure 2 .
Figure 2. Schematic 3D atomic diagram of SLW1 in a unit cell

Figure 4 .
Figure 4. Impedance spectra of SLW1 at 600 to 700℃ under wet Ar

Figure 5 .
Figure 5. Arrhenius plot of SLW1 under dray and wet ArAdditionally, we noticed that the addition of inductance significantly improved the fit, particularly at higher temperature ranges (around 1000 °C).The equation provided by Afif et al. and Azad et al. can be used to compute the actual capacitance[36].Concerning the bulk or grain-boundary and sample or electrode responses, respectively, the capacitance measured with the high-frequency portion semicircle was in the range of 10-12 -10-8 F and that with the intermediate frequency range was 10 -8 -10 -6 F[37,38].

Table 1 .
Analysis of SLW1 X-ray diffraction data