on Doped Ceria: A Composite Electrolyte for Low-Temperature Solid

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The effect of alkali-carbonates (single, binary, and ternary) on doped ceria, a composite electrolyte for low temperature solid oxide fuel cells Amjad Ali, Asia Rafique, Muhammad Kaleemullah, Ghazanfar Abbas, Muhammad Ajmal Khan, Muhammad Ashfaq Ahmad, and Rizwan Raza ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17010 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

The effect of alkali-carbonates (single, binary, and ternary) on doped ceria, a composite electrolyte for low temperature solid oxide fuel cells Amjad Ali1,2, Asia Rafique1, Muhammad Kaleemullah1, Ghazanfar Abbas1, M. Ajmal Khan1, M.Ashfaq Ahmad1,and Rizwan Raza1,3* 1 Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan. 2 Department of Physics, University of Okara, 56300, Pakistan. 3 Department of Energy Technology, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden. * Email:[email protected]_______________________________________________________________________ ABSTRACT Samarium doped ceria-carbonate (SDC) has become an attractive electrolyte for fuel cells due to its remarkable ion conductivity and high performance. Different doped ceria-carbonate (single carbonate-SDC, binary carbonate-SDC, and ternary carbonate-SDC) electrolytes were synthesized by the co-precipitation/oxalate method, to optimize the electrochemical performance. The structure, morphology, and thermal, optical, and surface properties, have been studied using a variety of techniques. The X-ray diffraction results confirmed the successful incorporation of samarium into ceria as a crystalline structure and inclusion of carbonate is an amorphous in nature. To analyze the conduction mechanism, DC conductivity was measured in a H2/O2 atmosphere. Doped ceria-binary carbonate (Li/Na)CO3-SDC) showed the best conductivity of 0.31Scm-1, and power density of 617 mWcm-2, at 600oC. The enhancement in the ionic conductivity and performance of the composites is due to the contribution of hybrid ions (O2-,H+). The crystallite size of the composites was in the range 21–41 nm. For the calculation of band gaps, the optical absorption spectra of the synthesized powders were analyzed, and showed a red shift with band gap energy in the range 2.6–3.01eV, when compared with pure ceria (3.20 eV). Keywords: alkaline carbonate, electrochemical impedance spectroscopy, band gap, crystallite size, red shift. 1. Introduction Solid oxide fuel cells (SOFCs) are the most efficient technology for energy demands due to salient features such as high efficiency, fuel flexibility, and low emission1. The hindrance for the commercialization of this technology is its high operating temperature of 1000oC. Researchers are trying to decrease this operating temperature to below 600oC2. The lower temperatures of the

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SOFCs demand high ionic conductivity of electrolytes, using thinner layers and improved electrode reaction activity. The reduced temperature increases the ohmic losses of the electrolyte, which is the major drawback3. Recently, many doping electrolyte materials have been prepared to increase the ionic conductivity. Examples include yttria stabilized zirconia (YSZ), Scandia stabilized zirconia, and gadolinium or samarium doped ceria (GDC, SDC)4–5. The commonly used electrolyte material YSZ possesses high ionic conductivity, chemical stability, and compatibility with electrode materials5. It has been reported that this electrolyte operates at higher temperature (>800oC), and shows reduced ionic conductivity at lower temperatures, when compared to ceriabased GDC and SDC6. Therefore, doped ceria materials are considered the ideal electrolytes for intermediate temperature SOFCs having ionic conductivity of 0.1 Scm-1 at about 600oC7. Unfortunately, doped ceria shows mixed conductivity (ionic-electronic), which reduces the efficiency and performance of the cell due to short circuiting of the anode-cathode8. To increase the ionic conductivity of doped ceria electrolytes, the electrical properties of these materials must be improved. Some researchers have discovered two-phase ceria-based composite materials which enhanced the ionic conductivity and cell performance below 600oC. Doped ceria-carbonate electrolytes exhibit higher ionic conductivity and low electronic conductivity compared to doped ceria in the temperature range 400–600oC9. The first phase is doped ceria, and the second phase is the addition of a small amount of salts which behave in an amorphous fashion, causing the conductivity enhancement10-11. These two phase materials have ionic conductivity (>0.1 Scm−1 below 600oC), and are found to be co-ionic (O2-, H+) conductors, effectively reducing the electronic effects and exhibiting higher electrochemical performance (>1 Wcm−2 at 600oC with H2 fuel)12–15. Wang et al. prepared nanocomposite electrolyte material (N-SDC) with super-ionic conductivity of 0.1 Scm−1 above 300oC, and obtained a power density of 0.8Wcm−2at 550oC12. Gao et al. observed the electrochemical properties and conduction mechanism of a SDC/Na2CO3 nanocomposite electrolyte, and obtained enhanced conductivity in a H2 atmosphere, and measured a performance of 375mWcm−2 at 550oC16. Xia et al. studied the effect of binary carbonates on morphology, conductivity, and performance of the cell with SDC/ (Li/Na)2CO3 composite electrolytes17. Huang et al. investigated the effects of binary carbonates, (Li/ Na)2CO3, (Li/K)2CO3, and (Na/K)2CO3, on samarium doped ceria composite electrolytes for low temperature SOFCs18. In another study, Huang et al. examined the effect of binary carbonates (SDC/(Li-Na)2CO3), as a second phase, on the electrical properties of SDC/carbonate nanocomposite electrolytes. The


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obtained power density was 600mWcm-2 at 600oC19. Jing et al. studied the samarium doped ceria/ternary alkaline carbonate (Li2/Na2/K2)CO3, and obtained the highest ionic conductivity of 0.4 Scm-1 at 600oC20. Chen et al. also investigated the effect of carbonate content in samarium doped ceria, and measured fuel cell performance greater than 600 mWcm-2 at575oC21. Asghar et al. studied the LNK-SDC (35:65) and obtained the highest conductivity of 0.55 Scm-1 at 600oC22. In view of the above, the effect of single, binary, and ternary, alkali carbonates was investigated in doped ceria electrolyte. The electrolyte materials were synthesized using a coprecipitation/oxalate technique and analyzed by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and UV-visible spectroscopy. Fuel cell performance and DC conductivity of the electrolytes was measured and compared in the low temperature range. 2. Experimental Section 2.1 Synthesis of materials Electrolyte materials consisting of doped ceria-carbonates were prepared by a coprecipitation technique. To synthesize the ceria-based carbonate composites, the following chemicals were used: samarium nitrate hexahydrate Sm(NO3)36H2O, cerium nitrate hexahydrate Ce(NO3)36H2O, sodium carbonate Na2CO3, lithium carbonate Li2CO3, and potassium carbonate K2CO3 (Sigma Aldrich, 99.9% USA). Initially, samarium nitrate and cerium nitrate were mixed in 1000 ml of deionized water with the molar ratio Ce:Sm=4:1. The nitrate solution was stirred at 80oC for two hours. Single, binary, and ternary alkaline carbonates were used as the second phase in nitrate solution with a molar ratio of SDC:carbonate=1:2. The mixed solutions were further stirred for two hours at 80oC to obtain the white precipitants. The precipitants were washed with deionized water, followed by vacuum filtration, and dried in an oven for three hours at 120oC. Finally, the dried powders were fired in a furnace at 800oC for four hours to achieve a dense electrolyte. The prepared samples were Na2CO3-SDC, K2CO3-SDC, Li2CO3-SDC, (Li/Na)2CO3SDC, (Na/K)2CO3-SDC, (Li/K)2CO3-SDC, and (Li/Na/K)2CO3-SDC, named as single carbonatesSDC (N-SDC, K-SDC, L-SDC), binary carbonates-SDC (LN-SDC, NK-SDC, LK-SDC), and ternary carbonates-SDC (LNK-SDC) , respectively. A flow chart of the prepared electrolyte powder is shown in Figure1.



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Figure.1 Flow chart of electrolyte powder prepared using a co-precipitation method. 2.2 Characterization The phase structure was analyzed with an X-ray diffractometer (PANalytical X’Pert Pro MPD, Netherlands) with Cu Kα source (λ =1.5418 Å ). From the XRD peaks, lattice parameters “a” were calculated using the following equations;

d=

λ , 2 sin θ

(1) while the crystallite size,

a = d h2 + k 2 + l 2

(2)

the Scherer equation

τ=

0.9λ β cos θ

(3)


τ , was calculated using

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Here, λ is the wavelength of the source, β is the full width half maxima, and θ is the Bragg angle. Rietveld refinement of the diffraction pattern of the sample was performed by GSASII software to obtain the crystal structural parameters23. Thermal behavior was analyzed using TGA (Q600, TA Instruments, USA) with the sample heated in the range 25–1000oC at a rate of 10oC min-1 in a N2 atmosphere. The microstructure was analyzed using SEM (Ultra 55, Carl ZEISS, USA). For band gap measurements, a Perkin Elmer Lambda 750 UV Vis-NIR spectrometer was used and spectra were recorded in the range 300–800 nm (wavelength). The FTIR was observed using Perkin Elmer spectrum RX I with a spectral range of 4000–500 cm-1. 2.3 Conductivity, fuel cell performance, and impedance analysis A circular pellet of each sample was prepared at 300 MPa, with 13 mm diameter, 0.64 cm2 active area, and 1 mm thickness for conductivity measurements. The pellets were sintered at 700oC for two hours, and then silver paste was glued on both sides of the pellets to collect current. The ionic conductivity was measured using a four probe apparatus (2450 SMU, Keithley). For measurement of EIS and cell performance, a three-layer pellet (same dimensions as above) was fabricated using an SDC-carbonated electrolyte and LiNiCuZn-oxide electrode. The EIS of the prepared cell (three layers) was measured with a PARSTAT 4000 (Princeton Applied Research, USA) in H2 atmospheres, over the frequency range 0.1Hz–5MHz, with a 10 mV signal. The performance of the cell was obtained (S12, China) at 600oC. Hydrogen gas was used as a fuel with a flow rate of 100 ml min−1 at 1 atm pressure, and air as an oxidant. The hydrogen used has a purity of 99.999%.

3. Results and Discussions 3.1 Structural analysis Figure 2(a–e) shows the XRD results of doped ceria-carbonate electrolytes after heat treatment at 800oC for four hours. In XRD patterns, all detected peaks were indexed as SDC crystal planes with planes indices (111), (200), (220), (311), (222), (400), (331), and (420). For a number



of reasons, no peaks from carbonates were observed in the XRD spectra. Firstly, the crystal structure factor of the carbonate phase and SDC is different. SDC is made up of heavy cations of Ce (Z=58) and Sm (Z=62), along with oxygen, while the atomic numbers of carbonates are very low compared to Ce and Sm24. Secondly, a large portion of carbonates may be in the amorphous phase. The amorphous structure of carbonates is formed during heat treatment, creating a protective layer over the SDC phase; this is known as the second phase, and has been reported in the literature8,25-26. Doped ceria-carbonates have a fluorite structure with space group 𝐹𝐹𝐹𝐹3𝑚𝑚

(JCPDS 34-0394) and Rietveld refinement of diffraction patterns and schematic structure of CeO2 is shown in Figure 2(d–f). Refinement results are presented in Table 1. Atomic positions have been fixed by the symmetry of space group 𝐹𝐹𝐹𝐹3𝑚𝑚. Ceria and samarium cations are at the 4a site with atomic position (0 0 0) while oxygen atoms are situated at the 8c site with atomic positions (0.25 0.25 0.25). Based on the XRD results, it can be established that neither a chemical reaction nor formation of a new compound takes place in doped ceria-carbonates27. The average crystallite size was observed to be in the range 21–41 nm, and the lattice parameter ranged from 5.442–5.447 Å. Similar XRD results from doped-ceria/carbonates have already been reported25,28–30. The doping of Sm3+ into Ce4+ shifts the ceria peaks towards lower angles because of the large ionic radii of the samarium ion (0.109 nm) compared to the cerium ion (0.097 nm)31. This expansion changes the lattice-plane spacing, and so an increase in the unit cell volume and lattice parameter occurs on samarium doping32. The lattice parameter expansion has been observed in previous studies33-34. However, single, binary and ternary carbonates are amorphous in nature (no XRD peaks) so it has no effect on the lattice parameters. The goodness of fitting was calculated by the following reliable parameters; Rp: 4.09, Rwp:5.27,RB:1.02, Rexp:3.98, and χ2:1.32.


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Figure 2. XRD patterns of electrolyte a) single carbonate-SDC, b) binary carbonate-SDC,

c)

ternary carbonate-SDC d) Rietveld refinement of the composition L-SDC e) Rietveld refinement of the composition LN-SDC, and f) Schematic structure of CeO2



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Table 1. Crystallographic parameters of LN-SDC/L-SDC by Rietveld refinement Atom Wyckoff Symbol x

y

z

Uiso [Å2] Occupancy

Ce

4a

0.0

0.0

0.0

0.0143

0.80

Sm

4a

0.0

0.0

0.0

0.0143

0.20

O

8c

0.25 0.25 0.25 0.0348

1.90

SEM analysis was performed to observe the surface morphology of electrolyte materials, and the results are shown in Figure 3. The particles are uniformly dispersed on the surface, indicative of the homogeneity of the materials. The SEM descriptions confirm that the surface of the SDC particles is covered in carbonates shaped like white loops. Such microstructure of electrolyte materials might enhance the ionic conduction mechanism35.


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Figure 3. SEM microstructure of doped ceria/carbonate electrolytes.(a) N-SDC,(b)K-SDC, (c)LNSDC, (d)NK-SDC,(e) LK-SDC, and (f) LNK-SDC. 3.2 Thermal analysis TGA and DSC curves of carbonates-SDC, in the temperature range 25–800oC, are shown in Figure 4. Thermal analysis of all samples revealed two endothermic peaks at two temperatures. The first peak is attributed to the vaporization of moisture content, and the second revealed the melting transition and formation of a binary eutectic salt32-33. The eutectic point of salts varies with carbonate content in SDC. The exothermic peaks in all prepared materials may give an estimate for the activation point for crystallization33. The weight loss in single carbonate-SDC is about 5%, whereas in binary carbonate-SDC and ternary carbonate-SDC it is 2.6% and 3% respectively. The amorphous nature of carbonates on the SDC surface facilitates ionic transport, and at higher temperature the amorphous softened nature of the carbonates overcomes the electronic conduction and enhances ionic conductivity32.



Figure 4. TGA/DSC curve (a) SDC-single carbonate, (b) SDC-binary carbonates, and (c) SDCternary carbonates.


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3.3 UV-visible spectroscopy UV-visible spectra of the doped ceria/carbonates were obtained to explore the band gap as a function of wavelength in the spectral range from 300 to 800 nm, and are shown in Figure 5. The band gap was calculated using.

(

(αhυ) n = A hυ − E g

)

(4)

where α is the absorbance coefficient, h is Plank’s constant, Eg is the band gap, A is a constant, and n defines the type of band gap (direct or indirect). An indirect band gap for samarium doped ceria was determined from absorption data. The plots of ( αhυ )1/2 versus photon energy ( hυ ) are given in the insets of Figure 5(a–c). A line was drawn on the linear portion of the graph which cut at the x-axis. The point on the x-axis gives the band gap value. A strong absorption is observed in the range 300–400 nm, indicating a transition from O2- (2p) to Ce4+ (4f) orbital34. The increase of UV absorption for SDC-carbonates is the indication of replacement of Ce4+ ions with Sm3+ ions, increasing the oxygen vacancy concentration35. The calculated band gap (Eg) is shown in Table 2. Table 2. Crystallite size, lattice parameter, and band gap energy of electrolytes Composition

Crystallite size (nm)

Lattice Parameter (Å) Band gap (eV)

N-SDC

36

5.446

2.9

L-SDC

39

5.447

3.01

K-SDC

41

5.446

2.95

NK-SDC

25

5.446

2.6

LN-SDC

21

5.445

2.7

LK-SDC

29

5.445

2.8

LNK-SDC

32

5.442

2.87

The band gap of carbonates-SDC shows a red shift compared to pure ceria at 3.20eV36. Ionic conductivity in ceria is due to the generation and movement of oxygen vacancies37. Oxygen vacancies are generated in ceria to balance the charge of dopant cations. The oxygen vacancy depends on the nature and ratio of the dopants. Thus, the concentration of oxygen vacancies can be enhanced on the substitution of lower valent cations in ceria, which significantly enhanced the ionic conductivity38-39. Samarium dopant cation with a positive vacancy-dopant association energy element reduces the band gap (red shift) lower than ceria, by creating associated oxygen vacancies,



resulting in an increase in the conductivity and fuel cell performance. On the other hand, dopant cations with negative vacancy-dopant association energy elements (Er) increase the band gap (blue shift)higher than ceria, suppressing the formation of associated oxygen vacancies40, resulting in decreased conductivity and fuel cell performance41-42. The obtained results on the band gaps agreed with the reported work in the literature36,43-44.


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Figure 5. UV-Vis spectrum (inset Tauc Plot). (a) single carbonate-SDC, (b) binary carbonateSDC, and (c) ternary carbonate-SDC.



FTIR spectroscopy was used to find information on the structural and phase transformations of cerium oxide. Figure 6 shows FTIR spectra of carbonates-SDC samples taken in the range 4000– 500 cm-1. The spectrum in Figure 6(a) shows a large absorption band at 1492 cm-1, and another at 856 cm-1. These are attributed to the CO3- group in the absorption spectrum. The presence of CO3

might be responsible for co-ionic conduction that may enhance ionic conductivity. The band at

1071 cm-1 is attributed to sodium carbonate. The narrow band below 700 cm-1 is due to a metaloxygen band, which indicates the formation of SDC32,45–47. In Figure 6(b) the band at 1043 cm-1 might be due to sodium carbonate. The peak at 1484 cm-1 is due to the CO32- group. In Figure 6(c) the peak at 1215 cm-1is the stretching frequency of metal oxygen Ce-O bonds. Modes of vibration of doped ceria-carbonates are shown in Table 3.


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Figure 6. FTIR absorbance spectra of the prepared material. (a)single carbonate-SDC, (b) binary carbonate-SDC, and (c) ternary carbonate-SDC.



Table 3. Identification of vibration modes of doped ceria-carbonate31,45–48 Peaks/Band (cm-1) 3.4

Bonding

Peak at 1492 cm-1, 1484 cm-1, and 856 cm-1 Band at 1043 cm-1,1071 cm-1 Narrow band below 700 cm-1 Peak at 2926cm-1 and 1215 cm-1 Peaks at 1559 cm-1 and 2465 cm-1 Peak at 2840 cm-1 Absorption band at 3659 cm-1

CO-3 group in the absorption spectra. Due to sodium carbonate. Due to metal-oxygen band. Due to due to γ(C-H). Stretching frequency of metal oxygen Ce-O bond. Stretching vibration of COO- group. CH2stretching vibration. O-H stretching frequency of aquatic and hydroxyl groups.

Ionic conductivity The ionic conductivities of electrolytes were measured in a H2/O2 atmosphere at comparatively low temperatures (350–600oC). The measured values are shown in Table 4. For single carbonates, N-SDC has the best ionic conductivity of 0.052 Scm-1at 600oC, when compared to L-SDC and K-SDC. Binary carbonates LN-SDC showed a maximum conductivity of 0.31Scm1

, higher than NK-SDC and LK-SDC at 600 oC. Lapa et al. observed an ionic conductivity of 0.2

S cm-1 for doped ceria-carbonate (Li/Na)2CO3 at 500oC9. Finally, the ternary carbonate LNK-SDC has a conductivity of 0.094Scm-1 at 600oC. From the conductivity data, Arrhenius plots were drawn to calculate the activation energy (Ea), and the results are shown in Figure 7.


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Figure 7. Arrhenius plots of (a) single carbonate-SDC, (b) binary carbonate-SDC, (c) ternary

carbonate-SDC, and (d) comparison of LN-SDC, LNK-SDC,N-SDC


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Activation energies (Ea) were calculated from the Arrhenius plot using the equation; σ=

𝐴𝐴 𝐸𝐸𝑎𝑎 exp � � 𝑇𝑇 𝐾𝐾𝐾𝐾

(5)

where σ is the ionic conductivity, k is Boltzmann’s constant, T is temperature in degree kelvin , and A is a pre-exponential factor. Figure 7(a) shows the ionic conductivity of doped ceria-single

carbonates. The measurement reveals that the doped ceria-carbonate electrolyte has a higher conductivity than SDC (0.01 Scm-1 at 600oC)49. SDC-carbonate is dual phase; one phase is SDC in which O2- ions contribute50, and the second phase consists of carbonates which contribute the H+ ions51. Similar behavior was discussed in the core-shell nanostructure for carbonates-SDC, where the SDC surface was covered with amorphous carbonates52. However, doped ceria-salt enhanced ionic conductivity due to the hybrid ion conduction mechanism53. Rahmawati et al. also reported the contribution of H+ ions which pass through the oxide-carbonate interface via a hopping mechanism8. The Arrhenius plot in Figure 7(a), of single doped ceria-salts, shows a lower conductivity below 450oC where carbonates are in solid state form and only oxygen ions passed through the electrolyte. Above 450oC conductivity rises due to the formation of an interface between SDC and carbonates54–56. Figures 7(b) and (c) show the conductivities of various doped ceria-binary and ceria-ternary carbonates. Single carbonates have a very high melting point, but in binary carbonates the eutectic melting point decreases and solid carbonates transform into a soft phase at lower temperature. The eutectic melting point of binary carbonates LK, LN, and NK are 490oC, 499oC, and 695oC, respectively. However, the eutectic melting points varied with carbonate concentration. An abrupt jump in conductivity is observed at a temperature of 450oC for almost all composites, above which conductivity increases due to softening of carbonate phases. According to a literature survey, this behavior is mentioned as a superionic phase transition, and can be associated with the melting point of the binary carbonates57. At temperatures below 450oC the conductivity is decreased due to the block effect of solid state carbonates26. This can also be interpreted in another way: at low temperatures the cation defects in nanocomposite interface phases are not highly activated, and are less mobile because of the activation barrier, resulting in low conductivity54. In the case of ternary carbonates (LNK) the jump in conductivity is observed at 400oC, which is the eutectic melting point of the ternary carbonate where glass transitions occur. Enhancement of conductivity is due to O2- and H+ ions55–57. The Arrhenius plot for total



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conductivity is not linear over the entire temperature range. At low/intermediate temperatures the activation energy has two parts, namely association enthalpy ∆H a and migration enthalpy ∆H m . ∆H m does not depends upon the temperature and dopant concentrations58. Ea is mostly determined by ∆H a . At high temperatures, Ea depends only on ∆H m and at ∆H a → 0 ,all the defects are fully dissociated. In the Arrhenius plots there is a transition for defects from associated states to dissociated states. From Figure 7 the values of ∆H a , ∆H m can be calculated in the temperature ranges 350– 450oC and 450–600oC using equation 6. 𝐴𝐴

σ = exp(− 𝑇𝑇

∆H a + ∆H m 𝐾𝐾𝐾𝐾

)

(6)

At higher temperatures dissociation of vacancies takes place59. The activation energy at high temperature is known as migration enthalpy does not vary much with composition, whereas the association enthalpy does. 3.5 Impedance analysis Electrochemical impedance measurements of the prepared cells were taken in a H2 atmosphere at 600oC and the results are shown in Figure 8(a–d). The Nyquist plot of LN-SDC at 300oC, shown in Figure 8(a), reveals one semicircle arc corresponding to the bulk composite response at high frequency, and a small arc at low frequency is ascribed to the electrolyte-electrode interface impedance. At 300oC carbonates are in solid form so the ionic conductivity of the electrolyte is controlled by the carbonate phase, which shows high resistance at low temperature56. At 600oC the bulk response of the composite is invisible but electrode behavior is dominant. At 600oC carbonates are in molten form, resulting in a decrease in the impedance of the cell. However, in the case of doped ceria/carbonates, bulk resistance is reduced compared to single phase SDC21. It can therefore be predicted that the second phase of carbonates overcomes the barrier of the conventional single phase electrolyte material.


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Figure 8. The Nyquist plots of (a) LN-SDC (best sample @300 oC), (b) single carbonate-SDC, (c) binary carbonate-SDC, and (d) ternary carbonate-SDC. Equivalent circuits are also shown inside the figures.


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To find the total resistance of the pellet, experimental data was simulated with Z SimpWin software, and the equivalent circuit was drawn. The equivalent circuit containing L inductance, R1, R2 resistance of the bulk and electrolyte-electrode interface, respectively, and Q is a constant phase element (CPE). The two elements R1||Q and R2||Q are in series and R1||Q shows the bulk ionic conductivity57–63. Here, the electrolyte-electrode interface impedance (R2) is not considered because the total resistance of the electrolyte is given by the bulk resistance (R1). The semicircle arcs at 300oC, corresponding to bulk, indicate that grain distribution is uniform inside the samples, confirming SEM results. The doped ceria-carbonates reduce the bulk resistance above 450oC, which plays a dominant role in electrolyte conductivity9,63-66. The important difference of SDCcarbonate is the absence of a grain boundary arc due to the relevant carbonate phase at high temperature. The associated capacitance value for the semicircle was calculated using the formula;

(7 )

2π f RC = 1

The value of the capacitances is shown in the Table 4 and found to be in the order of 10-10 F, therefore it has been attributed to a conduction process through the bulk (grain). Carbonates play a vital role in raising the densification of electrolytes at lower sintering temperatures compared to SDC, resulting in less particle aggregation (because particle aggregation may reduce the triple phase boundary (TPB) for electrochemical reactions), which may affect the conductivity67. The phase of carbonate is supposed to be amorphous and continuous after melting without internal interfaces68. Table 4. Conductivity, activation energy, simulated values of R1, capacitance, and power density of carbonate-SDC Composition σ(Scm-1) @600oC

Ea(eV)

Ea(eV)350 450 oC

450 - 600

R1

C(1010) OCV PowerDensity

(Ω)

F

(V)

(mW cm-2)

o

C

N-SDC

0.052

1.04

0.53

1.6

199

0.91

488

L-SDC

0.020

0.73

0.58

2.0

167

0.89

435

K-SDC

0.006

0.35

0.61

1.9

159

0.91

463

LN-SDC

0.31

1.6

0.45

1.4

227

0.98

617

NK-SDC

0.20

1.86

0.40

1.8

138

0.97

582



Page 26 of 38

LK-SDC

0.12

1.6

0.49

2.3

172

0.96

562

LNK-SDC

0.094

1.3

0.51

1.85 176

0.92

525

3.6. Fuel cell performance Fuel cell performance, using hydrogen fuel, is shown in Figure 9. The measured values of power densities and open circuit voltages (OCVs) are shown in Table 4. The results show that the LN-SDC (one of the binary carbonate) cell exhibited the highest power density of 617mWcm-2 at 600oC. However, a comparison of (single, binary, ternary) carbonates-SDC is shown in Figure 9(d). The electrolyte with higher values of conductivity has a maximum power density. Doping Sm3+ into ceria slightly shifts the peaks towards lower angles, resulting in an increase in the lattice parameter. The increase in lattice parameter decreases the particle size, which increases the conductivity and performance. The change in conductivity can be linked to lower lattice binding energy, which increases the number of oxygen vacancies. Oxygen vacancies enhance the conductivity of the electrolytes. The enhancement in ionic conductivity may be ascribed due to the close ionic radii of both elements and small binding energy69. The high power density of the LNSDC electrolyte material may be due to fast transportation of oxygen and proton ions, called superionic conduction. The enhanced conductivity and power density may be due to the formation of ceria-carbonate interfaces which behave as superionic highways facilitating the transportation of ions61.


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Figure 9. Fuel cell performance of (a) single carbonate-SDC, (b) binary carbonate-SDC, (c) ternary carbonate-SDC, and (d) comparison N-SDC,LN-SDC, and LNK-SDC.


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4. Conclusions Doped ceria-carbonate (single, binary, and ternary) nanocomposite electrolyte powders were successfully synthesized by a co-precipitation technique. This technique is considered to be the best at controlling the structural, optical, and thermal properties of the prepared powder. The first phase is a cubic crystal phase of doped ceria, and the second phase is an alkali carbonate, which is amorphous in nature. XRD Rietveld refinement of diffraction patterns established the creation of a cubic fluorite structure 𝐹𝐹𝐹𝐹3𝑚𝑚 . Doping of samaria into ceria (SDC) led to peak broadening and decreased the intensity. This indicates a decrease in crystallinity and crystallite size. The lattice parameter is increased due to the slightly large ionic radii of samarium compared to ceria. The SDC phase is a crystalline in nature, so the XRD peaks slightly shift towards lower angles, and the lattice parameter is increased compared to CeO2. But single, binary, and ternary carbonates are amorphous in nature, so it has no effect on the lattice parameter. SEM results also confirmed that particles are uniformly distributed. Amongst all the samples, LN-SDC has the highest conductivity and performance of 0.31Scm-1 and 617mWcm-2, respectively, at 600oC. The LN-SDC crystallite size decreased (21nm), while the lattice parameter increased, on doping with samarium. The optical absorption spectra of all the electrolytes showed a red shift with band gap energy in the range 2.6–3.01eV compared to pure ceria (3.20eV). The Sm+3 dopant cation reduces the band gap of ceria, due to an increase in associated oxygen vacancies, resulting in an increase in conductivity and fuel cell performance. TGA of the samples showed that endothermic peaks near 100 oC can be ascribed to the water content of the sample, whereas other endothermic peaks show the eutectic point formation of the different salts. Doped ceria-carbonate composites are found to be co-ionic (O2-, H+) conductors, and they decrease the electronic conduction. It has been found that doped ceriacarbonates, especially LN-SDC, are suitable for LT-SOFC. These exhibit high conductivity and fuel cell performance while reducing the operating temperature. Acknowledgments We acknowledge the Higher Education Commission, Pakistan (HEC). The authors also acknowledge the help of Dr. M. Shoaib (SKKU, South Korea) for XRD reviled refinement analysis. References



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Graphical Abstract

Co-ionic (H+, O-2) conduction mechanism is proved in doped ceria-carbonate electrolytes. Carbonates-SDC shows a red shift in band gap compared to ceria due to the formation of the associated oxygen vacancies, so the mobility of ions increases, result an increase the conductivity and fuel cell performance.


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on Doped Ceria: A Composite Electrolyte for Low-Temperature Solid

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