Synthesis and ionic conductivity of calcium doped ceria relevant to Solid oxide fuel cell applications

In the ongoing situation, much impulse is being given to the improvement of sophisticated commercial electrolyte materials for intermediate temperature solid oxide fuel cells. The Ca-doped ceria samples having a...


Introduction
Solid oxide fuel cells (SOFCs) are known to convey high electrical proficiency and fuel adaptability with ensured natural advantages.In SOFCs, the hopping mechanism regulates the ions through electrolytes between the cathode and anode.The high ionic and insignificant electronic conductivity, permanence in redox conditions, mechanical strength (among other qualities) are viewed as great properties for electrolyte materials.An assortment of oxide-ion electrolytes have been examined up to this point, similar to zirconia-based oxides, cubic fluorite structure based doped ceria, stabilized d-Bi 2 O 3 and others.The issues related to their cost, electronic conduction, working and sintering temperature discourage its mass scale commercialization. 1 The doped ceria-based ceramics were found to show almost the same results at intermediate temperature (IT), 773-973 K for SOFC applications.The conduction mechanism in doped ceria was influenced by the mobility and distribution of oxygen vacancies.It is created by substituting suitable cations with lower valences than the host lattice ions.The rare earth-doped ceria-based materials have been examined because of their ability to yield high ionic conductivity at lower temperatures and are thus viewed as a prospective electrolyte for IT-SOFCs. 2,3The Sm and Gd ionic radii coordinate well with Ce ions, and hence high oxide-ion conductivity has been accounted for GDC10 and SDC20.The Gd-, Sm-, and Y-doped ceria having a high dopant concentration of 15-20 mol%, showed improved ionic conductivity at elevated temperatures. 4,5In any case, the ideal ionic conductivity at lower temperatures actually has not been accomplished, and subsequently, their expense for 10-20 mol% dopant amount is not economically reasonable. 6Thus, there exists an expansive degree in the advancement of new economically reasonable ceria-based electrolyte materials.The alkaline earth-doped ceria materials have been viewed as substantially more powerful and potential candidate to yield huge ionic conductivity at lower temperatures.For example, Ce (r i = 1.11Å) doped with Ca (r i = 1.12 Å) has been researched among the alkaline earthdoped ceria-based materials.The ionic conductivities of alkaline earth-doped ceria-based materials are viewed as critically as those of rare earth-doped materials.A high conductivity on the order of 10 À3 S cm À1 was reported by Yamashita et al. 7 at 873 K for the Ce 0.9 Ca 0.1 O 2Àd composition.Similarly, Banerjee et al. 8 reported 1.29 Â 10 À2 S cm À1 for oxide ion conductivity at 873 K for the Ce 0.8 Ca 0.2 O 2Àd composition prepared through the auto-combustion method.Calcium is abundant in nature and cheaper than other rare earth elements, 9 and its doping is found to create higher oxygen vacancies.Therefore, it has been regarded as an excellent and advanced oxygen storage material. 10Subsequently, for economical reasonability, cheap dopants such as Ca having sufficient electrochemical performance have been considered favorable.Recently, Schottky junction (SJ)-based n or p-type low-cost semiconductor -ionic conductor composite materials have shown potential to cause in-filed barriers and confine the electron's mobility or holes from the passage of the electrolyte edge to the opposite side.2][13][14][15][16][17][18][19][20][21][22] Still, there are few studies related to the Ca-doped ceria framework for IT-SOFCs.From the point of the ionic-radius, compatibility of the dopant, commercial viability, and abundant nature, calcium (Ca) has been picked as a dopant for the host cerium cross-section.In the present work, the CDC samples were prepared by the conventional solid-state method, and calcium has likewise been viewed as a sintering aid for the ceria-based electrolytes. 23Sintering ceria-based electrolytes above 1573 K prompts the reduction of some Ce 4+ to Ce 3+ ions, causing electronic leakage and affecting the performance of the cell. 24herefore, attempts have been made to achieve the high densification at a lower sintering temperature of 1473 K than the required high-temperature sintering in the range of 1873-1973 K (Yamashita et al. 1995). 8The oxide ion conductivities of CDC samples are studied as a function of dopant concentrations between the temperature range of 623-973 K in air using electrochemical impedance spectroscopy.This work accordingly plans to investigate the new low-cost Ca-doped ceria ceramics for IT-SOFCs. 25

Experimental
The Ca-doped CeO 2 ceramic oxide powders with the composition Ce 1Àx Ca x O 2Àd (x = 0.03, 0.05, 0.07 and 0.1) are prepared from conventional solid-state route by using cerium oxide [A.R grade 99.99% pure, Sd-Fine Mumbai-India] and hydrated calcium acetate [C 4 H 6 O 4 CaÁXH 2 O]; [A.R grade 99.99% pure, Sd-Fine Mumbai-India].The stoichiometric ratios of precursors were blended with isopropyl alcohol manually for 1 h, and calcined at 1273 K for 6 h in a muffle furnace.The obtained powders were again ground and pelletized (10 f Â 2 mm dimensions), and sintered at 1473 K in the air for 4 h.The final obtained products having different compositions are hereinafter referred to as 0.03 CDC, 0.05 CDC, 0.07 CDC and 0.10 CDC individually.

Characterization
The XRD analysis of CDC samples was carried out using a Rigaku MiniFlex 600 powder X-ray diffractometer (Cu Ka rad., l = 1.54 Å) instrument.The Archimedes principle was used to determine the relative densities of the pellets. 26The DILOR-JOBIN-YVON-SPEX-623 nm was used to record the Raman spectra of the CDC pellets.XPS spectra of the CDC samples were recorded from a Thermo Fisher Scientific Instrument (Al K a + hn = 1486.68eV, spot size 400 mm).The total reflectance (spectral + diffused) spectra of the CDC samples were recorded using a JASCO spectrophotometer, V-77.The Bruker instrument (Nano GmbH Berlin, Germany) was used to record the FESEM images, EDX spectra and elemental mapping of the CDC samples.The a.c.impedance analysis was carried out in air at 1 atm using a CH Instrument (Inc CHI604D, USA).The CDC pellets were silver painted and baked at 773 K for 1 h for electrical connections.The Nyquist plots were obtained between the temperature ranges of 623-973 K and frequency range of 1-10 6 Hz with an a.c signal of 10 mA.The obtained data were fitted to an equivalent electrical circuit, [R(QR)(QR)] with ZSimpWin software.The conductivity was calculated using eqn (1): with thickness (t), area of cross-section (A), and R is the resistance. 27sults and discussion View Article Online vacancies in the cerium lattice will extend the unit cell to limit the strain. 30,31Likewise, expansion of the lattice constant changes the d-spacing, causing a shift of the diffraction peaks to bring down the 2y positions.Hence, the lattice parameters of the CDC samples increase with the dopant (Ca) concentration, as shown in Table 1.Furthermore, the high-temperature sintering of the pellets (1473 K) results in sharp peaks of increased intensity with large particle sizes, indicative of the highly crystallinity due to the grain growth. 32,33he Scherrer equation was used to determine the crystallite size from the width of the preferential planes.The dislocation density was found to increase with dopant content.
The lower values of dislocation density observed in the CDC pellets are likely due to the excellent crystallinity of the CDC samples. 34The computed theoretical surface area of the CDC samples appreciably varied with the dopant content and dislocation density. 35The average crystallite size, dislocation density and specific surface area values are summarized in Table 1.The relative density of the CDC pellets was found to be around 95% and is listed in Table 1.The relative density of the CDC pellets shows an increasing trend with dopant concentrations.
Fig. 2 shows the Raman spectra of the CDC pellets.The presence of a distinct intense peak at 464-461 cm À1 has been credited to the F 2g Raman active mode of the fluorite structure.7][38] Furthermore, the Raman active mode F 2g shifts to a lower frequency amid a rise in the dopant (Ca) content.The adjustment of the bond length, cross-section dispersion, and nuclear geometry of CeO 2 , which is the result of Ce-O bond breaking, emerged because of the generation of oxygen vacancies during the replacement of Ca-ions into the cerium lattice.The produced oxygen vacancies are facilitated with Ce and Ca ions, and decrease to change the length of the Ce-O and Ca-O bonds, and determine the moving of the F 2g mode to bring down the frequencies.In Ca-doped CeO 2 , the F 2g Raman-active mode showed more extensive and asymmetric peaks in contrast to the unadulterated CeO 2 .][41][42] Furthermore, no other peaks are detected for impurities (like CaO), which in corroboration with the XRD study, established the formation of single-phase solid solutions.
The XPS analysis was employed to study the microstructure and chemical state of the CDC samples.Fig. 3(a)-(c) shows the typical XPS spectra of the 0.05 CDC, 0.07 CDC and 0.10 CDC samples sintered at 1473 K.The presence of three different sections at around 880-890 eV, 895-910 eV, and at 916 eV of the Ce 3d core-level spectra is evident in all CDC samples. 43The existence of the Ce 4+ state consigned to the 4f 0 orbital transition is designated by the peak at 916 eV. 44The charge transfer from the oxygen 2p to Ce 4f states could resulted in the three doublets having configurations of 3d 4f 0 , 3d4f 1 (L), and 3d 4f 2 (L) 2 . 45The Ce 3d spectra of these samples displayed the six characteristic peaks in the binding energy (BE) range of 870 to 930 eV.The Ce 3d XPS spectrum consists of two spinorbit splitting of multiplets of 3d 5/2 and 3d 3/2 core holes.The final state of 4f 0 -O 2p 6 was represented by u 0 0 0 and v 0 0 0 , and corresponds to high binding energy peaks.The u 00 , v 00 , u and v represent the 4f 1 -O 2p 5 and 4f 2 -O 2p 4 states and correspond to lower binding energy peaks. 46The binding energy peaks observed for the 3d 5/2 core hole multiplets (0.05 CDC: 883.3, 889, 898), (0.07 CDC: 883.2, 888.8, 898 eV) and (0.10 CDC: 882.4,887.5, 897 eV) are nearly in the same  range, and a marginal decrease in the binding energy for V 0 0 0 has been observed in all samples.The decreasing trend may be attributed to the increasing content of the dopant.Similarly, the binding energy peaks observed for the 3d 3/2 core hole multiplets (0.05 CDC: 900.5, 907.6, 916.2), (0.07 CDC: 900.2, 907.6, 917 eV) and (0.10 CDC: 899.4,906.7, 915.17 eV) are in good agreement with those that have been reported.The Ce 3d XPS spectra consist of some indistinguishable overlapped peaks corresponding to both Ce 4+ and Ce 3+ states, which make the spectra more complex.Most of the peaks in all of the spectra correspond to the Ce 4+ state, and the BE peaks at 889, 888.8 and 887.8 eV of 3d 5/2 and 900.5, 900.View Article Online spectra show two different peaks at 528.5, 528.8, and 528.9 eV, and at 531.7, 531.3, and 531 eV for the 0.05, 0.07 and 0.010 CDC, respectively.The sharp intense peaks were attributed to the lattice oxygen of the CDC samples, and the other peaks are assigned to the existence of hydroxyl groups due to exterior contaminations on the sample.The high content of Ca in 0.10 CDC may have shifted the binding energy peak, as observed in Fig. 5(c). 49he 1s oxygen spectra were deconvoluted into contributions from the Ca-O, Ce 3+ -O, and Ce 4+ -O bound species because the cerium atom in the +3 oxidation state would be more tightly bound than cerium in the +4 state. 50Upon examining the peaks' relative area, a portion of the CeO x exists in the Ce 3+ state. 51The obtained XPS spectra of the CDC samples show apparent CeO 2 features and the existence of a Ca 2p signal, indicating that the Ca particles have been doped into the CeO 2 lattice.This is corroborated by the XRD and Raman analyses of the CDC samples, and matches well with results reported in the literature. 52he diffuse reflectance (DR) UV-visible absorption spectra of the CDC samples are depicted in Fig. 6.The appearance of sharp intense peaks in the ultraviolet absorption region around 290-295 nm is ascribed to the charge transition from the O 2p to Ce 4f, and is assumed to be caused by the oxygen vacancies. 53his red-shift is attributed to the interfacial polaron effect emerging from the electron-phonon interaction. 54The Kubelka-Munk plots of the CDC samples for the optical band gap are shown in Fig. 4S (ESI †).The obtained band gap energy values are summarized in Table 1.
The FESEM images of the CDC pellets are shown in Fig. 7. Some particles were seen on the surface.This may be due to the powder, which is used as a bed during the sintering process.Generally, high-temperature sintering yields a larger particle size. 55All CDC samples have been observed to be highly dense with the absence of intra-granular pores.Hence, it may be concluded that these samples achieved good densification.The EDX and elemental mapping of the CDC samples are shown in Fig. 5S (ESI †), and it confirmed the existence of Ce, Ca, and O elements.The EDX analysis revealed that the elemental composition of all CDC samples are in good agreement with those with the proposed stoichiometric amounts.The EDX elemental mapping analysis revealed that the amount of dispersion for Ca was low compared to Ce, and confirmed the uniform distribution of elements throughout the matrix. 56ig. 8-11 show the Nyquist plots of the 0.03 CDC, 0.05 CDC, 0.07 CDC, and 0.10 CDC samples, respectively.To distinguish the overlapped grain, grain boundary (GB), and electrode contributions, an equivalent electrical circuit model [R(QR)(QR)] was used, as shown in the inset of Fig. 8.This comparable circuit comprises two RC components in series for the grain resistance (R g ) and GB (R gb ), and others for the electrode polarization (R e ).A constant phase element (CPE = Q) is applied rather than a capacitor.This Q is comparable to the dispersion of the capacitor in parallel.The component R g , R gb 8Q represents the ionic conductivity through the grain and GB, respectively. 57he total resistance is calculated by using the equation: 58 where R g and R gb are the grain and GB resistance, respectively.

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Impedance analysis revealed that the grain and GB conductivity are a function of the temperature rise for all CDC samples.The high and intermediate frequency ranges of the CDC samples for the grain and GB contributions exhibited a rapid ion migration process.The impedance data obtained from the ZSimpWin software confirmed the ionic conduction in all CDC solid solutions.The oxygen ion vacancies on the surface of the CDC are responsible for the ionic conduction.The lower grain resistance (R g ) values at higher temperatures show that all CDC samples have better ionic conductivity. 16The impedance parameters of all CDC samples extracted using the ZSimpWin software are shown in Tables 2-5.The total oxide ion conductivity is estimated by utilizing eqn (1) and is shown in Table 6.][61] The effect of temperature on the oxide ion conductivity was determined by using the Arrhenius relation.
where s, A o , T, E a , and K are the conductivity, pre-exponential constant, temperature, activation energy, and Boltzmann   Table 4 The EIS fitted data of the 0.07 CDC at different temperatures, 573-973 K   constant, respectively.The Arrhenius plots of the total, grain and GB conductivity of the CDC samples are shown in Fig. 12, and the determined activation energies are shown in Table 7.It has been observed that the activation energy decreases with a rise in the dopant content.The grain conductivity contribution is a major factor for the total oxide ion conductivity in all CDC samples. 62Thus, the amount of calcium doping affects the conductivity of the ceria.With the ascent in the dopant (Ca) content, a huge expansion in the grain conductivity was seen because of the improvement of the charge transporters emerging from the creation of oxygen vacancies as a result of doping.Hence, this prompted the expansion of the crystal structure.
The rearranged anion opening at the surface and connection with the electrode assumes a significant part in the improvement of the oxide ion conductivity.The change in the surface oxygen vacancies fundamentally adjusts the physical and chemical properties of the materials.The insertion of the Ca 2+ dopant into the cerium lattice generates the negatively charged defect, and hence increases the anion vacancies.The effect of the Ca 2+ doping in CeO 2 is depicted by using Kro ¨ger-Vink notation as shown in eqn (4).
At elevated temperature, the surplus electrons and anion vacancies are in equilibrium with the gas phase, as shown in eqn (5). 63The surplus electrons may arise due to small Ce 3+ polarons, which is in corroboration with the obtained XPS data and small conduction bandwidth.
Thus, the improved mobility for the oxygen ions could lead to the high oxide ion conductivity for the CDC samples. 64Since calcium has likewise been viewed as a sintering aid for the  The conductivity improves as a function of the dopant concentration.The 0.10 CDC shows the most noteworthy conductivity in the order of 8.01 Â 10 À3 S cm À1 at 973 K with a lower E a of 0.78 eV, among all CDC samples studied here.The obtained conductivity values for the CDC were viewed as competent when contrasted with the ionic conductivity of 7.7 Â 10 À2 S cm À1 of Ce 0.8 Sm 0.2 O 2Àd , 67 3.6 Â 10 À2 S cm À1 of Ce 0.9 Gd 0.1 O 2Àd , 68  S cm À1 at 973 K with activation energies of 0.84, 0.86 and 0.89 eV) sample, synthesized from the combustion route using citric acid, glycine, and urea as a fuel, as reported by Ong et al. 72 The high grain conductivity of the prepared CDC may be connected with the lower confinement of the dopant (Ca) at the GB.Along these lines, bringing down the comparison space charge impacts could bring about a huge augmentation of oxide ion conductivity.In the current work, the 0.05, 0.07, and 0.10 CDC synthesized from the facile solid-state route showed the ionic conductivity in the order of 10 À3 S cm À1 at 973 K.The lower E a obtained for 0.1 CDC (0.78 eV) is nearly identical to the oxide ion conductivity of Ce 0.9 Ca 0.1 O 1.9 synthesized from the auto -combustion route with citric acid, glycine and urea as fuels, as reported by Ong et al. 58 Thus, the substantially more affordable Ca-doped ceria (CDC) materials have showed significant outcomes in contrast to those more costly rare-earth doped ceria-based materials, and could assist with decreasing the expense for SOFCs.Consequently, Cadoped ceria-based ceramics could be viewed as potential electrolyte materials for IT-SOFC applications.

Conclusion
The Ca-doped ceria (CDC) samples having composition Ce (1Àx) Ca x O 2Àd [x = 0.03-0.1]were successfully prepared by using a cost-effective and conventional solid -state method.All CDC were sintered at lower temperature of 1473 K than required.The XRD, Raman, XPS, and FESEM -EDX analysis confirmed the formation of the single-phase solid solution of these CDC.The 0.10 CDC showed an essentially upgraded oxide ion conductivity in the order of 8.01 Â 10 À3 S cm À1 at 973 K with a lower activation energy, E a = 0.78 eV, in contrast to the other CDC.The critical improvement in the physical and electrochemical properties of the CDC through solid -state synthesis has been accomplished.Thus, monetarily reasonable CDC samples might be viewed as a potential electrolyte material for IT-SOFC applications.

Fig. 1
Fig.1illustrates the XRD patterns of the pellet CDC samples.The XRD patterns of the powder CDC samples are shown in Fig.1Sas ESI.† The XRD patterns exhibited the same cubicfluorite structure as per the space group Fm% 3m.The planes of the preferential peaks were indexed as per the ICDD card no.34-0394.It was observed that no peaks of CaO or other impure phases were detected in all XRD patterns of the powder and pellet CDC samples.The XRD analysis revealed that the samples consist of a characteristic lattice structure that matches the standard CeO 2 .28,29A trivial expansion in the lattice constant upon doping has been attributed to the substitution of the Ce ion by Ca, which caused strain and a slight deformation of the structure.To hold the charge balance, the initiated oxygen
2 and 899.4 eV of 3d 3/2 appeared for the 0.05 CDC, 0.07 CDC and 0.10 CDC samples, correspond to the Ce 3+ state respectively.The XPS spectra of the Ca 2p core-level with deconvolution peaks are shown in Fig.4(a)-(c) of the CDC samples, and display the 2p 1/2 and 2p 3/2 states, which are split into two final states, I and II.Due to the spin-orbit splitting, these are distinguishable at around 347.2, 347.2 and 347.1 eV, corresponding to the 2p 3/2 state, and 350.3, 350.2 and 352 eV, corresponding to the 2p 1/2 state of the 0.05 CDC, 0.07 CDC and 0.10 CDC samples respectively.47,48Fig.5(a)-(c) show the O1s XPS spectra deconvoluted into the different symmetrical signals of the CDC samples.The O 1s

Fig. 6
Fig. 6 UV-Visible diffuse reflectance spectra of the CDC samples.

Table 2
The EIS fitted data of the 0.03 CDC at different temperatures, 773-973 K

Table 3 The
EIS fitted data of the 0.05 CDC at different temperatures, 573-973 KTemp.(K) R g (O) R gb (O) R e (O)

Table 5
The EIS fitted data of the 0.10 CDC at different temperatures, 573-973 K

Table 7
65,66ctivation energy of the CDC between the temperatures, 573-973 K Open Access Article.Published on 13 October 2022.Downloaded on 9/16/2023 11:02:24 PM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.-based electrolytes, in this way it helps to achieve the high densification at a lower sintering temperature.65,66Subsequently,withtheascent in the Ca content, the augmentation in the relative densities of these CDC samples has been clearly observed, as displayed in Table1.The high densification of the CDC samples could facilitate the versatility of charge transporters at GB, and consequently it assists with improvement on the conductivity of the CDC.From the obtained results, it has been seen that the high oxide conductivities of 1.66 Â 10 À4 , 4.42 Â 10 À3 , 5.76 Â 10 À3 , and 8.01 Â 10 À3 S cm À1 were recorded ceria 8.89 Â 10 À3 S cm À1 of Ce 0.9 La 0.1 O 2Àd 69 at 973 K, 5.9 Â 10 À3 S cm À1 of Ce 0.9 La 0.1 O 2Àd 70 and 9.4 Â 10 À4 S cm À1 of Ce 0.85 La 0.15 O 2Àd at 873 K 71 like potential electrolyte materials of IT -SOFCs.The obtained conductivity for 0.10 CDC in the order of 8.01 Â 10 À3 S cm À1 at 973 K with E a of 0.78 eV can likewise be contrasted best with the conductivities of Ce 0.9 Ca 0.1 O 2Àd (8.02 Â 10 À3 , 6.31 Â 10 À3 , 1.13 Â 10 À3