Effect of MgO and Fe2O3 dual sintering aids on the microstructure and electrochemical performance of the solid state Gd0.2Ce0.8O2-δ electrolyte in intermediate-temperature solid oxide fuel cells

Solid state electrolytes have been intensively studied in the solid oxide fuel cells (SOFCs). The aim of this work is to investigate the effects of MgO and Fe₂O₃ dual sintering aids on the microstructure and electrochemical properties of solid state Gd_0.2Ce_0.8O_2-δ (GDC) electrolytes, which are prepared by a sol-gel method with MgO and Fe₂O₃ addition to the GDC system. It is found that the addition of MgO and Fe₂O₃ can reduce the sintering temperature, increase densification and decrease the grain boundary resistance of the electrolyte. The 2 mol% MgO and 2 mol% Fe₂O₃ co-doped GDC (GDC-MF) exhibits the highest grain boundary conductivity. At 400°C, the grain boundary conductivity and total conductivity of GDC-MF are 15.89 times and 5.56 times higher than those of GDC. The oxygen reduction reaction (ORR) rate at the electrolyte/cathode interface of GDC-MF is 47 % higher than that of GDC. Furthermore, the peak power density of a single cell supported by GDC-MF is 0.45 W cm⁻² at 700°C, 36.7% higher than that of GDC. Therefore, the GDC-MF should be a promising electrolyte material for intermediate-temperature solid oxide fuel cells (IT-SOFCs).

1 Introduction

As the critical components of solid oxide fuel cells (SOFCs), solid electrolytes have the advantages of high ionic conductivity, high density, and high chemical compatibility (Santos et al., 2018; Spiridigliozzi et al., 2019; Singh et al., 2021). Electrolytes usually require high sintering temperatures (≥1400°C) to obtain sufficiently high densities and electrical conductivities (Wang et al., 2019). However, the high sintering temperature will lead to a series of challenges, e.g., high manufacturing cost, fast aging rate of the cell, and the poor compatibility of the components. Therefore, it is of great significance to develop new solid electrolytes with high ionic conductivity in the intermediate-temperature (500–700°C) range.

CeO₂-based solid electrolytes were the widely investigated IT-SOFCs electrolytes because of the high ionic conductivity at low/medium temperature and excellent compatibility with the commonly used electrode materials (Coduri et al., 2018; Zhang et al., 2021). A small amount of low melting point additives were often added to CeO₂-based solid electrolyte as sintering aids during the process of sintering, e.g., Fe₂O₃ (Xu et al., 2011; Zheng et al., 2011; Mehranjani et al., 2017), MgO(Cho et al., 2007; Li et al., 2008; Bi et al., 2017), ZnO (Nicollet et al., 2017), and Bi₂O₃ (Accardo et al., 2018) etc. This method can significantly reduce the sintering temperature and improve the density of the electrolyte. Fe₂O₃ and MgO have been reported to be the excellent sintering aids for improving the ionic conductivity and relative density of Gd_0.2Ce_0.8O_2-δ (GDC) electrolyte (Zheng et al., 2011; Bi et al., 2017; Shajahan et al., 2020). Zheng et al. added 0.25 mol% Fe₂O₃ to Ce_0.8Sm_0.2O_1.9 electrolyte and obtained the improved densification and excellent total conductivity (σ_t) (Zheng et al., 2011). Similarly, Mehranjani et al. reported that with 2 mol% Fe₂O₃ as sintering aids, yttria-stabilized zirconia (YSZ) and GDC bilayer electrolytes could exhibit the reduced sintering temperature from 1450 to 1300°C and the improved densification (Mehranjani et al., 2017). Moreover, the liquid-phase sintering mechanism of Fe₂O₃ gave rise to the good continuity and permeability at the GDC grain boundary, which effectively promoted the migration of oxygen ions (Zhang et al., 2004a). In addition, MgO is another effective sintering aid, which can simultaneously improve the density and conductivity of the electrolyte. For instance, adding 1.0 mol% MgO to GDC electrolyte as sintering aids could reach the relative densities of 95% after sintering at 1200°C for 10 h (Bi et al., 2017). Molten MgO can promote GDC grain growth and the degree of densification of the sample. Moreover, the difference of the ionic radii between Mg²⁺ (0.89 Å) and Ce⁴⁺ (0.97 Å) will arouse the lattice distortion, which helps grain boundary movement and grain growth during the sintering process. Additionally, the solid solubility of Mg²⁺ in the CeO₂ lattice was as low as ∼1 mol% (Cho et al., 2007), when the amount of Mg²⁺ exceeds the solubility, the redundant MgO will populate at the grain boundaries. Therefore, the addition of MgO can optimize the structure of the space charge layer and remove the silicon phase at the grain boundaries, improving the overall ionic conductivity (Li et al., 2008).

In recent years, intense research has been focused on the double-sintering aids strategy on ceria-based electrolytes (Jia et al., 2015; Chen et al., 2020; Babar et al., 2022). For example, the Nd_0.2Ce_0.8O_2-δ (NDC) solid electrolyte with CoO-Bi₂O₃ (molar ratio of 1:1) dual sintering aids achieved the density of 95.3 % after sintered at 1100°C for 10 h (Chen et al., 2020). At 800°C, the σ_t was 5.765 × 10⁻² S cm⁻¹, which was higher than that of the NDC electrolytes added with single sintering aids CoO (3.802 × 10⁻² S cm⁻¹) or Bi₂O₃ (4.649 × 10⁻² S cm⁻¹). Similarly, the electrochemical property and sintering behavior of the Ce_0.8Y_0.2-xCu_xO_2-δ electrolytes were improved by adjusting the Y/Cu ratio (Jia et al., 2015). Ce_0.8Y_0.2-xCu_xO_2-δ electrolyte sintered at 1300°C for 4 h could achieve a high relative density of 95%. It can be seen that selecting a suitable sintering aid is an important approach to improve the conductivity of the ceria-based electrolyte. Compared with single sintering aids, adding dual sintering aids is also an effective strategy to improve material densification. The addition of MgO or Fe₂O₃ can form liquid phase sintering and promote electrolyte densification at low temperatures.

Usually the doping amount of the sintering aid is less than 5 mol% (Zhang et al., 2017; Han et al., 2018). With this in mind, in this paper, the addition of 4 mol% MgO-Fe₂O₃ dual sintering aids were proposed to promote the electrochemical performance of Gd_0.2Ce_0.8O_2-δ solid state electrolyte in the IT-SOFCs.

2 Experiment methodology

2.1 Chemicals

The chemicals in this paper including Ce(NO₃)₃·6H₂O (99.99%), Gd(NO₃)₃·6H₂O (99.99%), Mg(NO₃)₂·6H₂O (99%), Fe(NO₃)₃·9H₂O (99%), La(NO₃)₃·6H₂O (99.9%), Sr(NO₃)₂ (99%), Fe(NO₃)₃·9H₂O (99.99%), Co(NO₃)₂·6H₂O (99.99%), glycine (99%), NiO (99.9%), Polyethylene glycol (PEG 2000) and solid citric acid (C₆H₈O₇·H₂O, 99.5%), were purchased from Aladdin. Those chemicals were used as received without further purification.

2.2 Material preparation and cell fabrication

2.2.1 GDC-based electrolytes preparation

GDC and GDC-based electrolyte powders with MgO and Fe₂O₃ sintering aids were prepared by sol-gel method. According to the stoichiometric ratio, Ce(NO₃)₃·6H₂O, Gd(NO₃)₃·6H₂O, Mg(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O were accurately weighed and dissolved in distilled water, orderly. Polyethylene glycol and solid citric acid weighed 1.5 times of metal ions were added to the solution under continuous stirring at the room temperature to obtain the precursor solution of GDC. Then, the solution was dehydrated at 75°C to obtain a dry gel, and calcined in a muffle furnace at a heating rate of 5°C min⁻¹ to 650 °C for 6 h to obtain GDC and GDC-based electrolyte powders with different proportions of MgO and Fe₂O₃ sintering aids. Subsequently, after all powder samples were thoroughly ground in an agate mortar, the GDC samples with 4 mol% MgO, 4 mol% Fe₂O₃, 2 mol% MgO+2 mol% Fe₂O₃ added were labeled as GDC-M, GDC-F and GDC-MF, respectively. In order to obtain samples with similar thickness, ∼0.5 g of different electrolyte powders were weighed and pressed into discs with a diameter of 13 mm and a thickness of 1.2 mm under the pressure 10 MPa. Then, all pressed samples were sintered in a muffle furnace at 1200°C for 10 h at a heating rate of 5°C min⁻¹ to obtain dense electrolyte sheets. Finally, both sides of the electrolyte sheets were coated with silver paste and heated at 700°C for 30 min for electrochemical performance test after polishing and ultrasonic cleaning.

2.2.2 Electrodes preparation

La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) cathode powder was prepared by glycine-nitrate method (Bai et al., 2020). According to the stoichiometric ratio, La(NO₃)₃·6H₂O, Sr(NO₃)₂, Fe(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O were accurately weighed and dissolved in distilled water, orderly. Glycine was added to the solution at the ratio of 1.5 times higher than the $N{O}_3^{-}$ under continuous stirring at 30°C. The mixture was heated and stirred at 110°C for combustion. After the combustion was completed, the product was calcined at 750°C for 4 h at a heating rate of 5°C min⁻¹ in a muffle furnace to obtain LSCF cathode power.

The GDC and GDC-MF electrolyte powders were mixed with LSCF cathode and NiO anode (mass fraction ratio of 50:50) in ethanol, respectively, and then calcined in air at 950°C for 15 h to evaluate the SOFC performance and compatibility with the electrolyte.

2.2.3 Cell fabrication

The symmetrical cells and single-cells were prepared to investigate the electrochemical behavior of fuel cells. The electrode powders were homogeneously mixed with 3 wt% terpineol/ethylene cellulose and fully ground in an agate mortar for 30 min to produce the cathode slurry. The LSCF cathode slurry was symmetrically coated on both sides of the GDC-MF and GDC pellets using screen printing technique. The effective area and thickness of the cathode were controlled to be ∼0.25 cm² and ∼20 μm, respectively, and calcined at 950 °C for 2 h. The Ag slurry was applied to the grid structure on both sides of the cell as the current collector, and then dried at 100°C for 2 h to obtain a symmetrical cell.

Both sides of the GDC and GDC-MF electrolyte sheets were polished to a thickness of ∼300 μm and placed in ethanol for ultrasonic cleaning. The NiO-GDC (percentage ratio of 6:4) anode were prepared by ball milling, and 6 wt% terpineol/ethylene cellulose was added to form the anode slurry. The anode slurry was screen-printed on one side of the GDC-MF and GDC pellets and calcined at 1250°C for 4 h. The cathode ink was coated on the other side of the electrolyte pellets and calcined at 950°C for 2 h. The effective area and thickness of the electrode were controlled to be consistent with the symmetric cells. The anode of the fuel cell was attached and sealed to one end of the quartz tube by using silver paste.

2.3 Characterization

The X-ray diffractometer (XRD, Rigaku D/MAX-2000/PC, Japan) equipped with Cu Kα₁ radiation (λ = 0.15406 nm, 40 kV and 40 mA; scanning rate: 4° min⁻¹) was used to characterize the crystal structure and compatibility of the sintered samples. The Raman spectrometer (Jobin-Yvon Horiba Scientific) with 532 nm excitation wavelength was carried out to analyze oxygen vacancy concentration of powders from 100 cm⁻¹ to 1000 cm⁻¹. Surface chemistry analysis of the GDC-based electrolyte powders were identified by X-ray Photoelectron Spectroscopy (XPS). The microstructure of electrolyte sheets was determined by field emission scanning electron microscopy (FE-SEM, SUPPA 40, ZEISS, Germany). The thermal expansion coefficient (TEC) of the sintered samples was characterized by thermal expansion analyzer (Netzsch DIL 402C) in the range of 50–850°C (heating rate: 3°C min⁻¹; standard sample: Al₂O₃). The relative densities of all sintered samples were tested by the Archimedes method and calculated by Eq. 1:

${\mathit{\rho }}_{\mathit{rel}}\mathit{=}\frac{{\mathit{\rho }}_{\mathit{A}}}{{\mathit{\rho }}_{\mathit{T}}}\mathit{\times }\mathbf{100}\mathrm{\%}(1)$

where ρ_A is the actual density measured using Archimedes’ principle, ρ_T is the theoretical density of the sintered sample obtained from X-ray diffraction data using Jade software, and ρ_rel is the relative density of the sintered sample.

The grain size of the sintered samples was calculated according to the following Debye Scherrer formula (Eq. 2):

${\mathit{d}}_{\mathit{XRD}}\mathit{=}\frac{\mathit{k\lambda }}{\mathit{\beta cos\theta }}(2)$

where d_XRD、K、λ、θ and β are the crystalline size, Scherrer constant (0.89), X-ray wavelength (0.154,056 nm), Bragg diffraction angle and diffraction peak half-width, respectively.

2.4 Electrochemical performance

The Electrochemical Impedance Spectroscopy (EIS) was carried out on a Solartron 1260–1287 workstation under the following conditions: open-circuit voltage (OCV), AC perturbation voltage of 10 mV, frequency range of 100 KHz-0.1 Hz, temperature range of 300–800°C and temperature interval of 50°C. The EIS data was analyzed and fitted by Ziew software. The conductivity of the samples was calculated according to the following Eq. 3:

$\mathit{\sigma =}\frac{\mathit{D}}{\mathit{RS}}(3)$

in which D, S and σ correspond to the thickness, surface area and conductivity of the sample respectively.

3 Results and discussion

3.1 Microstructural analysis

3.1.1 Crystalline phase analysis

In order to determine the effect of the addition of MgO and Fe₂O₃ on the structure of GDC electrolyte, the XRD spectra of all samples are shown in Figure 1, and the unit cell parameters of the samples are listed in Table 1. As can be seen from Figure 1A, the diffraction peaks of all samples correspond to the characteristic peaks of the cubic fluorite structure (JCPDS #46–0507) of GDC, and no additional impurity peaks were observed. To further clarify the effect of MgO and Fe₂O₃ addition on GDC structure, the diffraction peak (111) of the sample was amplified (2θ = 27.0–30.0°) and shown in Figure 1B. Combined with Figure 1B and Table 1, it can be seen that the diffraction peak (111) of GDC and GDC-M samples have almost no deviation. Moreover, the unit cell parameters of GDC and GDC-M were also almost unchanged, this could be discussed from the following two aspects. On the one hand, Mg²⁺ entering the GDC lattice would lead to a decrease in unit cell parameters due to the radius of Mg²⁺ (0.89 Å) is smaller than that of Ce⁴⁺ (0.97 Å); On the other hand, it is extremely low of the solid solubility of MgO (∼1 mol%) in GDC. When the solid solubility is exceeded, the remaining MgO will remain at the grain boundaries, which increases the unit cell parameters. In addition, compared with the diffraction peak (111) of GDC, the diffraction peak of GDC-F was slightly shifted toward the high diffraction angles, indicating that the addition of Fe₂O₃ could reduce the cell volume of GDC (Table 1). This result could be attributed to the solid solubility of Fe₂O₃ in GDC more than 4 mol% (Zheng et al., 2011; Abbas et al., 2015). Therefore, Fe³⁺ (0.78 Å) with smaller radius could easily enter the GDC lattice to replace Ce⁴⁺ (0.97 Å) ions, resulting in the shrinkage of the unit cell volume. It is worth noting that the diffraction peak of GDC-MF (111) was shifted to the high diffraction angles less than that of GDC-F (Figure 1B), and the unit cell parameters was also similar to GDC-M sample. This is mainly attributed to the co-addition of 2 mol% Fe₂O₃ and 2 mol% MgO which increases the lattice parameters of the GDC samples as a result of phase equilibrium.

FIGURE 1

FIGURE 1. (A) X-ray diffraction (XRD) patterns and (B) Zoom-in XRD patterns in a 2θ range of 27.00–30.00° of GDC-based electrolytes.

TABLE 1

TABLE 1. Relevant parameters of GDC-based electrolytes sintered at 1200°C for 10 h.

3.1.2 Raman spectroscopy

The effect of MgO and Fe₂O₃ additions on the GDC structure and new defect sites were tested by the Raman spectrometer at room temperature, as shown in Figure 2A. From the Raman spectrum, all samples had a weak absorption band at 245 cm⁻¹, which could be attributed to a double degenerate transverse optical mode (Li et al., 2009). In addition, the Raman peak around 465 cm⁻¹ was associated with the F_2g vibrational mode of the cubic lattice, indicating that the CeO₂-based electrolytes still maintain the perfect cubic fluorite structure (Nakajima et al., 1994; Li et al., 2020; Song et al., 2021), which was consistent with the XRD results. Moreover, the positions of the vibrational peaks around 465 cm⁻¹ for GDC-M and GDC samples were consistent, basically. However, for GDC-F and GDC-MF samples, the vibrational peaks around 465 cm⁻¹ were slightly shifted toward higher frequencies and slightly broadened, which may be related to the formation of solid solutions (Accardo et al., 2021). The shift of the vibrational peak suggests that the addition of Fe₂O₃ to CeO₂ might form a solid solution and this difference might be related to the low solubility of MgO in GDC (∼1 mol%). In addition, the defects caused by doping could also interfere with the signal at ∼465 cm⁻¹ (Sartoretti et al., 2019), suggesting that there might be dopant ions entering the CeO₂ lattice to replace Ce⁴⁺ to generate oxygen vacancies.

FIGURE 2

FIGURE 2. (A) Raman spectra of all samples, inset is the magnified F_2g band and (B) Oxygen vacancy pattern.

The locally enlarged Raman spectra was shown in Figure 2B. Characteristic absorption peaks associated with the vibration of oxygen vacancies in the defect space were observed at ∼550 and ∼600 cm⁻¹ (Anjaneya et al., 2014; Song et al., 2021). Among them, the signal at ∼550 cm⁻¹ was attributed to the vibration of oxygen vacancies in the defect space generated by the introduction of low-valent cations into the CeO₂ lattice (Guo and Waser, 2006). With the addition of sintering aids MgO and Fe₂O₃, the signal intensities of GDC-F, GDC-M and GDC-MF were stronger than that of GDC at 550 cm⁻¹ (Figure 2B). This was because the small amount of Mg²⁺ or Fe³⁺ enters the CeO₂ lattice to replace Ce⁴⁺ to generate oxygen vacancies, resulting in the enhanced signal intensity at 550 cm⁻¹. The corresponding defect equations are shown in Eqs 4, 5:

$\mathit{F}{\mathit{e}}_{\mathit{2}}{\mathit{O}}_{\mathit{3}}\stackrel{\mathit{Ce}{\mathit{O}}_{\mathit{2}}}{\to }\mathit{2F}{\mathit{e}}_{\mathit{Ce}}^{\prime }\mathit{+}{\ddot{\mathit{V}}}_{\mathit{O}}\mathit{+2}{\mathit{O}}_{\mathit{O}}(4)$

$\mathit{MgO}\stackrel{\mathit{Ce}{\mathit{O}}_{\mathit{2}}}{\to }\mathit{M}{\mathit{g}}_{\mathit{Ce}}^{\prime \prime }\mathit{+}{\ddot{\mathit{V}}}_{\mathit{O}}\mathit{+}{\mathit{O}}_{\mathit{O}}(5)$

Since the signal at 550 cm⁻¹ was related to the oxygen vacancy, the area ratio of this peak to the peak of 464 cm⁻¹, i.e., A550/A464, could estimate the oxygen vacancy concentration (Pu et al., 2007). Compared to the GDC (0.0279) sample, the oxygen vacancy concentrations of GDC-F (0.0649), GDC-M (0.0367) and GDC-MF (0.0584) were all increased. This result confirms that the Ce⁴⁺ of GDC lattice were replaced by Mg²⁺ or Fe²⁺ to generate $\mathit{M}{\mathit{g}}_{\mathit{Ce}}^{\prime \prime }$ or $\mathit{F}{\mathit{e}}_{\mathit{Ce}}^{\prime }$ and the positively charged oxygen vacancies (${\ddot{\mathit{V}}}_{\mathit{O}}$) were generated to maintain the charge balance. The increase in oxygen vacancy concentration of the GDC-M was significantly smaller than that of GDC-F. Similarly, the oxygen vacancy concentration of the GDC-MF was larger than that of GDC-M and slightly smaller than that of GDC-F. This difference was related to the lower solid solubility of MgO ∼1 mol%, resulting in only a small amount of Mg²⁺ entering the CeO₂ lattice to replace Ce⁴⁺.

3.1.3 Elemental analysis

In order to further verify the effect of MgO and Fe₂O₃ addition on the oxygen vacancies of GDC, X-ray photoelectron spectroscopy (XPS) was carried out to characterize the chemical environment of the powder surfaces. The fitted XPS spectra of the O 1s orbitals of the GDC and GDC-MF samples were shown in Figure 3. It can be seen that the O 1s spectrum was consisted of three typical peaks located at ∼527.6, ∼530.0 and ∼531.1 eV, representing lattice oxygen (O_lattice), surface oxygen (O_surface) (e.g. OH⁻, O^2-, O⁻ and carbonate) and moist oxygen, respectively (Zhang et al., 2019). A slight increase in the O_surface/O_lattice ratio could be observed, meaning that O_lattice was decreased and O_lattice vacancy was increased (Bai et al., 2020). According to the fitted peak areas, it can be concluded that the corresponding O_surface contents of the GDC and GDC-MF were 23.9 and 25.0%, and the O_surface contents were 76.1 and 75.0%, respectively. For the GDC-MF electrolyte, the increase of O_surface content was only 1.1%, which was similar to the variation trend estimated by Raman. The reason for its slight increase may be that only small amounts of Mg²⁺ and Fe³⁺ enter the CeO₂ lattice and Fe₂O₃ may connect with some of the oxygen vacancies [$\mathit{2F}{\mathit{e}}_{\mathit{Ce}}^{\prime }\cdot {\ddot{\mathit{V}}}_{\mathit{O}}$], resulting in the consumption of oxygen vacancies (Zheng et al., 2011). With the addition of MgO and Fe₂O₃, it can be presumed that MgO/Fe₂O₃ mainly played the role of liquid-phase sintering to promote densification. At the same time, only a small portion of Mg²⁺ or Fe³⁺ replaced high-valent Ce⁴⁺ to produce oxygen vacancies.

FIGURE 3

FIGURE 3. XPS spectra of O 1s for (A) GDC and (B) GDC-MF samples.

3.1.4 Morphology characterization

All samples sintered at 1200°C for 10 h were characterized by field emission scanning electron microscopy (FE-SEM). The results of the microscopic morphology of the samples were shown in Figure 4. There were many holes and relatively poor density (86.9%) in the GDC (Figure 4A) samples. The grain sizes of the GDC-M (Figure 4B), GDC-F (Figure 4C) and GDC-MF (Figure 4D) samples were larger than those of GDC (Table 1), indicating that MgO or Fe₂O₃ could promote the growth of grains and improve the density of the samples. For the GDC-M samples, the grain size and relative density were larger than those of GDC (Table 1). It is due to the large difference between the ionic radii of Mg²⁺ (0.89 Å) and Ce⁴⁺ (0.97 Å). During the sintering process, the lattice distortion promoted the movement of grain boundaries, which accelerated the growth of grains and increased the density of the samples. Compared with the GDC-M, the GDC-F had more obvious grain growth and higher relative density (95.1%). On the one hand, this due to the mismatch of the radii of Fe³⁺ (0.78 Å) and Ce⁴⁺ (0.97Å). On the other hand, it was mainly attributed to the low melting point of Fe₂O₃ and the presence of partially melted Fe₂O₃ on the grain surface as the sintering temperature increase. Therefore, the surface tension and friction of the grains would be changed, and the rearrangement of the grains would be promoted to make the electrolyte more densified, i.e., the viscous flow mechanism was occurred (Zhang et al., 2004b). The GDC-MF sample had the largest grain size and the highest densities (96.6%), which was produced by the combined action of two sintering aids. Firstly, the densification of the electrolyte was facilitated by the viscous flow mechanism of Fe₂O₃. Secondly, due to the difference in the ionic radius of Mg²⁺/Fe³⁺ and Ce⁴⁺, the GDC lattice distortion and defects were produced, which accelerated the migration of grain boundaries, promoted grain growth, reduced the porosity between grains, and increased the relative density of the electrolyte. Therefore, the co-action of the Fe₂O₃ viscous flow mechanism and the radius effect of Mg²⁺/Fe³⁺ and Ce⁴⁺ were important factors to improve the sintering performance of the GDC-MF electrolyte.

FIGURE 4

FIGURE 4. Cross-sectional FE-SEM image of (A) GDC, (B) GDC-M, (C) GDC-F and (D) GDC-MF electrolytes sintered at 1200°C for 10 h.

3.2 EIS analysis

EIS was used to evaluate the electrocatalytic performance of GDC-based electrolytes that were sintered at 1200°C. Figure 5 exhibits the Nyquist spectra and equivalent circuit diagrams of the GDC-based electrolyte measured at 350 and 800°C, where the CPE and R_el represent the constant phasor element and polarization resistance, respectively. As shown in Figure 5A, at 350°C, the impedance spectrum was consisted of a semicircle and a small ray arc. The plots could be divided into high and low frequency arcs, and the intersection of the high frequency arc and the low frequency arc with the real axis (Z′) were the grain resistance (R_gi) and the total resistance (R_t). The difference between the R_t and the R_gi was the grain boundary resistance (R_gb). As the temperature increased to 800°C (Figure 5B), the impedance spectrum was changed to a ray arc with a tail, and the R_gb was disappeared, where R_t = R_gi. The associated resistance data were listed in Table 2. Combining Figure 5A and Table 2, it can be seen that the R_gi, R_gb and R_t of the GDC-M, GDC-F and GDC-MF were lower than that of GDC electrolyte (R_GDC-MF < R_GDC-F < R_GDC-M < R_GDC). At 350°C, the R_gb (3.14 Ω) and R_gi (70.03 Ω) of the GDC-MF were only 4.85 and 22.11% of the GDC electrolyte, indicating that the co-addition of MgO and Fe₂O₃ could effectively reduce the R_gb and R_gi. Comparing the R_gi/R_t of the GDC-M, GDC-F and GDC-MF, it can be seen that the ratio of R_gb to R_t of the GDC-MF (0.04) was significantly lower than that of GDC-M (0.15) and GDC-F (0.06). This indicated that the addition of Fe₂O₃-MgO dual sintering aids in GDC electrolyte was exhibited a more significant effect on reducing the R_gb. This was attributed to the liquid phase sintering mechanism of Fe₂O₃ (Wang et al., 2017), which increased the effective densification of the sample and reduced the R_gi and R_gb, thereby reducing the overall resistance. In addition, the radius effect of Mg²⁺ and Fe³⁺ was also an important factor to improve the sintering performance and electrical conductivity of GDC-MF.

FIGURE 5

FIGURE 5. EIS analysis of GDC-based electrolytes and Equivalent circuit diagrams at (A) 350 and (B) 800°C.

TABLE 2

TABLE 2. Resistance of GDC-based electrolytes at 350 and 800°C.

3.3 Conductivity analysis

Figures 6A–C was the Arrhenius curve of the grain boundary conductivity (σ_gb), grain conductivity (σ_gi), and total conductivity (σ_t) of GDC-base electrolytes. It can be seen from Figure 6 that in the temperature range of 300–800°C, the conductivity of the GDC electrolyte was always the lowest. Compared with the GDC electrolyte, the σ_gi and σ_gb of the GDC-M, GDC-F and GDC-MF were improved significantly, and the GDC-MF had the maximum σ_gi, σ_gb and σ_t. At 400°C, the σ_gi and σ_gb of the GDC-MF were 3.56 × 10⁻³ S cm⁻¹ and 2.40 × 10⁻¹ S cm⁻¹, which were 5.42 and 15.89 times higher than the σ_gi (6.57 × 10⁻⁴ S cm⁻¹) and σ_gb (1.51 × 10⁻² S cm⁻¹) of the GDC. Additionally, the increase of σ_gb was more significant. At 800°C, the conductivity of the GDC-MF (6.97 × 10⁻² S cm⁻¹) was much higher than the conductivity of 2.5 mol% CoO-Bi₂O₃ co-doped NDC (5.765 × 10⁻² S cm⁻¹) (Chen et al., 2020) and the Ce_0.8Gd_0.2-XSr_xO_1.9-x/2 (x = 0.01) composite electrolyte (3.6 × 10^–2 S cm⁻¹) (Kashyap et al., 2014). And the σ_t of GDC electrolyte with over 95% relative density calcined at 1400°C was 3.25 × 10⁻² S cm⁻¹, which was only 46% of the GDC-MF electrolyte (Öksüzömer et al., 2013). Supplementary Table S1 summarize the total conductivity comparison of different electrolytes. In addition, the total conductivities of GDC and GDC-MF under different partial oxygen pressures were also explored (Supplementary Figure S1), as discussed in detail in Supplementary Material. The corresponding activation energy (E_a) was calculated according to the Arrhenius formula and expressed in Figure 6C. From the picture, the GDC-MF (E_a = 0.58 eV) was showed the lowest activation energy similar to the GDC-F (E_a = 0.57 eV), but lower than the GDC (E_a = 0.72 eV) and GDC-M (E_a = 0.68 eV). The reduction in activation energy was meant that high ionic conductivity could be achieved at reduced temperatures. In summary, the improvement of GDC based-electrolytes performance could be attributed to: (I) The liquid phase sintering mechanism of MgO/Fe₂O₃ promoted the movement of grain boundaries and improved the densification of the samples, thus reducing the grain/R_gb; (II) The molten Fe₂O₃ had good continuity and permeability at the GDC grain boundary, which could effectively promote the migration of oxygen ions; (III) The MgO remaining at the grain boundary could remove the SiO₂ impurities (Bi et al., 2017), change the structure of the space charge layer (Zhang et al., 2004b), and improve the conductivity of the grain boundary; (IV) A small amount of Mg²⁺ or Fe²⁺ could enter the lattice to generate oxygen vacancies, provide oxygen ion transport channels and improve the σ_gi.

FIGURE 6

FIGURE 6. Arrhenius curves of (A) σ_gb, (B) σ_gi, and (C) σ_t of GDC-based electrolytes.

3.4 Sintering mechanism

To illustrate the excellent performance of GDC-MF electrolyte, the possible mechanism for the improvement of conductive was proposed in Figure 7. The Fe₂O₃ and MgO sintering aids were represented by red and golden yellow skeleton, respectively. During the sintering process, the Fe₂O₃/MgO particles were gradually melted at the grain boundaries to form a liquid phase, which promoted the sintering process by diffusing and surrounding the GDC particles. After sintering, the cross-section of GDC-MF was analyzed by EDX mapping (Supplementary Figure S2), and it was found that the distribution of Ce, Gd and Fe elements contained in the sample was relatively uniform, indicating that Fe³⁺ easily entered the GDC lattice. In addition, line scan analysis (Supplementary Figure S3) showed that the content of Mg element in the grain boundaries was higher than that in the grains. This is due to the low solid solubility of MgO ∼1 mol%, and only a small amount of Mg²⁺ enters the CeO₂ lattice (Cho et al., 2007). Other MgO might act as a grain boundary scavenger and interact with SiO₂ to generate Mg₂SiO₄, reducing the grain boundary resistance, thereby increasing the grain boundary conductivity of GDC (Bi et al., 2017). However, there is no peak of Mg₂SiO₄ in XRD due to the relatively small amount. Therefore, the addition of Fe₂O₃/MgO dual sintering aids in GDC electrolyte proposed in this work was an effective strategy to obtain highly dense electrolyte materials.

FIGURE 7

FIGURE 7. Schematic diagram of the full-cell model supported by GDC-MF electrolyte.

3.5 Physical and chemical properties

Previously, the GDC-MF composite electrolytes with high ionic conductivity and good densification have been demonstrated. In addition, the GDC-MF electrolytes should have good TEC, chemical compatibility, oxygen reduction reaction (ORR) activity and long-term stability (Zhang et al., 2004c; Zhu et al., 2017).

3.5.1 Thermal expansion coefficient

The thermal expansion behavior of GDC and GDC-MF electrolytes in the temperature range of 50–800°C was shown in Figure 8. The average TEC was calculated by fitting the dL/L₀ data as the function of temperature. As shown in Figure 8, the thermal expansion curve of the electrolyte was increased with the increase of temperature, and the average TEC of GDC and GDC-MF was calculated to be 10.9 × 10⁻⁶ K⁻¹ and 11.2 × 10⁻⁶ K⁻¹, respectively. The GDC and GDC-MF electrolytes were exhibited similar TECs with LSCF (15.4 × 10⁻⁶ K⁻¹) cathode (Tai et al., 1995) and NiO-based anode (16.9 × 10⁻⁶ K⁻¹) (Chen et al., 2008) materials. The results indicate that the addition of Fe₂O₃-MgO dual sintering aids had no effect on the TEC of the GDC electrolyte. The GDC-MF and GDC electrolytes have good thermal matching with the electrodes, and each component will not break in the SOFC operating temperature range.

FIGURE 8

FIGURE 8. Thermal expansion curves of sintered samples versus temperature.

3.5.2 Chemical compatibility

The chemical compatibility between the perovskite cathode material LSCF and NiO anode with the GDC and GDC-MF electrolyte was studied. The GDC and GDC-MF electrolyte powders were mixed with NiO anode powder and LSCF cathode powder at the weight ratio of 1:1, respectively, and calcined at 950°C for 15 h in air. The XRD patterns measured at room temperature were shown in Figures 9A,B. It can be seen that the main diffraction peaks could be assigned to the characteristic diffraction peaks of GDC, GDC-MF, NiO and LSCF, and no new diffraction peaks were generated. It was confirmed that the GDC-MF electrolyte exhibits good chemical compatibility with NiO and LSCF electrodes at the high temperature of 950°C.

FIGURE 9

FIGURE 9. Chemical compatibility of the GDC-MF electrolyte with (A) LSCF cathode and (B) NiO anode material.

3.5.3 Oxygen reduction activity

To evaluate the effect of the composite electrolyte on the ORR activity at the cathode/electrolyte interface, electrochemical impedance spectroscopy tests of symmetrical cells (LSCF/GDC/LSCF and LSCF/GDC-MF/LSCF) were performed in the temperature range of 650–800°C. As shown in Figure 10A, the ohmic area-specific resistance (ASR_oh) corresponds to the real axis intercept in the high frequency region, and the total area-specific resistance (ASR_t) corresponds to the real axis intercept in the low frequency region. The polarization area-specific resistance (ASR_p) can be estimated according to the formula $\mathit{AS}{\mathit{R}}_{\mathit{t}}\mathit{-AS}{\mathit{R}}_{\mathit{oh}}\mathit{=AS}{\mathit{R}}_{\mathit{P}}$.

FIGURE 10

FIGURE 10. Electrochemical performance of symmetric cells: (A) Electrochemical impedance spectra at 800 °C. (B) ASR versus temperature curves of LSCF cathodes on GDC and LSCF cathodes on GDC-MF (left Y-axis), and percentage reduction of ASR of LSCF cathodes on GDC-MF relative to GDC (right Y-axis).

The ASR_p value versus temperature is shown in Figure 10B. It can be seen that the ASR_p of the symmetric cells assembled with GDC-MF electrolytes was significantly decreased in the temperature range of 650–800 °C compared to the GDC electrolyte. Furthermore, the ASR_p value of LSCF in GDC-MF electrolytes was reduced by more than 47% compared to GDC (right y-axis). It is shown that adding Fe₂O₃-MgO dual sintering aids effectively reduced the ASR_p between the GDC-based electrolyte and the LSCF cathode and increased the ORR rate. The enhanced ORR rate could be attributed to the improved density of the GDC electrolyte with the addition of Fe₂O₃ and MgO.

3.5.4 Single cell electrochemical performance

The electrolyte-supported single cells were assembled to evaluate the effect of Fe₂O₃ and MgO on the output performance of GDC-based SOFCs. NiO and LSCF powders were used as anode and cathode materials. The single cells supported with GDC and GDC-MF electrolytes were labelled as Cell-I and Cell-II, respectively. It was compared the current density-voltage (I-V) and current density-power density (I-P) curves of single cells Cell-I and Cell-II in the temperature range of 600–700°C, using wet H₂ as the fuel and air as the oxidant (Figures 11A,B). The maximum OCV of Cell-I and Cell-II was 0.71 and 0.83 V, respectively. It can be seen that the maximum OCVs of the 2 cells were lower than the theoretical OCV of 1.04–1.1 V (Steele, 2000; Leah et al., 2005). This is mainly attributed to that part of Ce⁴⁺ in the electrolyte was reduced to Ce³⁺ at low oxygen partial pressure (under reducing atmosphere) and high temperature, resulting in a certain n-type electronic conductivity. Due to the existence of electrons, a short-circuit current is formed inside the electrolyte, The short-circuit current was formed inside the electrolytes due to the presence of electrons, so that its OCVs were less than the theoretical electromotive force (Zhou et al., 2022). In addition, the OCV of the single cells decreased slightly with increasing temperature, which was conformed to the trend predicted by the Nernst equation (Zhao et al., 2014). The peak power densities of Cell-I at 600°C, 650 and 700°C were 0.13, 0.21, and 0.33 W cm⁻², and the peak power densities of Cell-II were 0.27, 0.37 and 0.45 W cm⁻², respectively. Compared with Cell-Ⅰ, the peak power densities of cell-Ⅱ have been greatly improved. At 700°C, the peak power density of Cell-II was 36.4% higher than that of Cell-Ⅰ. The improved power density of Cell-II could be attributed to the enhanced electrolyte density, improved electrical conductivity, matching of TEC, and excellent ORR rate. As shown in Figure 11C, Cell-II was tested for long-term stability at 600°C and the current density of 0.3 A cm⁻² for 70 h. It can be seen from the figure that the voltage and power density of the single cell did not change during the test time. These results confirm that the GDC-MF electrolyte is a promising electrolyte material for IT-SOFC.

FIGURE 11

FIGURE 11. Electrochemical performance measured at 600°C, 650°C, and 700°C of (A) the GDC, (B) GDC-MF electrolyte-based SOFC, and (C) long-term stability of Cell-II measured at 600°C for 70 h.

4 Conclusion

In summary, the effects of the addition of MgO and Fe₂O₃ on the sintering behavior, phase composition, microstructure, electrochemical performance and compatibility with classical electrodes of GDC electrolytes were systematically investigated. All the GDC-based electrolytes samples were prepared by a sol-gel method and exhibited the cubic fluorite structure. MgO-Fe₂O₃ acted as a dual sintering aid could increase the effective densification of the samples and reduce the R_gi, R_gb and R_t of the GDC-MF electrolyte. At 400 °C, the σ_gb and σ_t of the GDC-MF were 2.40 × 10⁻¹ S cm⁻¹ and 3.50 × 10⁻³ S cm⁻¹, which were 15.89 and 5.56 times higher than those of GDC σ_gb (1.51 × 10⁻² S cm⁻¹) and σ_t (6.30 × 10⁻⁴ S cm⁻¹). Additionally, the GDC-MF electrolyte had matchable thermal expansion coefficient and excellent chemical compatibility with commonly used electrodes. Furthermore, the single cell supported by GDC-MF electrolyte could reach the highest power density of 0.45 W cm⁻² at 700°C, and demonstrate the stable voltage and power density in the long-term stability test within 70 h. Therefore, adding Fe₂O₃-MgO dual sintering aids to GDC electrolyte is an effective approach to improve the electrochemical performance, and the GDC-MF electrolyte should be a promising electrolyte for IT-SOFC.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

QL and PT completed the experiments, analyzed the data and wrote the manuscript. XZ and TD provided resources and funding acquisition for this manuscript. DZ, JZ, JB and WY designed the experimental direction and helped in the analysis and review of data. NW reviewed and revised the manuscript.

Funding

This study was supported by the Science Research Foundation of Jilin Province (20190201230JC and 20200403154SF), the Guangdong Basic and Applied Basic Research Foundation (2019A1515010743), the Open Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2020–31); the National Natural Science Foundation of China (21471022, 21971113, 22175094). Opening Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, China (No. 2020–31).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2022.991922/full#supplementary-material

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Glossary

ASR_p Polarization area-specific resistance

Cell-I Single cells supported with GDC electrolyte

Cell-II Single cells supported with GDC-MF electrolyte

E_a Activation energy

EIS Electrochemical impedance spectroscopy

FE-SEM Field emission scanning electron microscopy

GDC Gd_0.2Ce_0.8O_2–δ

GDC-F GDC Samples with 4 mol% Fe₂O₃ addition

GDC-M GDC Samples with 4 mol% MgO addition

GDC-MF GDC Samples with 2 mol% MgO+2 mol% Fe₂O₃ addition

IT-SOFCs Intermediate-temperature solid oxide fuel cells

LSCF La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ

OCV Open-circuit voltage

O_lattice Lattice oxygen

ORR Oxygen reduction reaction

O_surface Surface oxygen

R_gb Grain boundary resistance

R_gi Grain resistance

R_t Total resistance

SOFCs Solid oxide fuel cells

TEC Thermal expansion coefficient

XPS X-ray photoelectron spectroscopy

XRD X-ray diffractometer

σ_gb Grain boundary conductivity

σ_gi Grain conductivity

σ_t Total conductivity