Yb₂O₃ Doped Zr_0.92Y_0.08O_2-α(8YSZ) and Its Composite Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Yb³⁺ and Y³⁺ double doped ZrO₂ (8YSZ+4Yb₂O₃) samples were synthesized by a solid state reaction method. Moreover, 8YSZ+4Yb₂O₃-NaCl/KCl composites were also successfully produced at different temperatures. The 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples were characterized by x–ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that a dense composite electrolyte was formed at a low temperature of 800 °C. The maximum conductivities of 4.7 × 10⁻² S·cm⁻¹, 6.1 × 10⁻¹ S·cm⁻¹, and 3.8 × 10⁻¹ S·cm⁻¹ were achieved for the 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples at 700 °C, respectively. The logσ~log (pO₂) plot result showed that the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite electrolyte is a virtually pure ionic conductor. An excellent performance of the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite was obtained with a maximum power density of 364 mW·cm⁻² at 700 °C.

1. Introduction

Solid electrolytes for high temperature fuel cells have many advantages over liquid electrolytes such as high power density, good sealing, a broad test temperature range, etc. [1,2,3,4,5,6,7,8,9,10,11,12,13]. Electrolytes based on an oxide ion conducting divalent or trivalent cations stabilized zirconia have been widely studied [14,15,16,17]. In order to avoid deleterious phase transition, a stable tetragonal (cubic) structure of doped ZrO₂ electrolytes can also be obtained. For instance, N.M. Rendtorff et al. synthesized the tetragonal structure of 3 mol % Y₂O₃ stabilized ZrO₂ (3YSZ) through the mechanochemical activation technique [14]. However, solid oxide fuel cells (SOFCs) using cation stabilized ZrO₂ as electrolytes have usually operated at high test temperature (800–1000 °C) as the conductivity of doped ZrO₂ is significantly reduced for temperatures lower than 800 °C.

Composite electrolyte materials, which have high conductivities, are pivotal to the development of intermediate temperature fuel cells. In recent years, two-phase composite electrolytes consisting of doped BaCeO₃, SrCeO₃, CeO₂, chloride, and carbonate have exhibited enhanced ionic conductivities and intermediate temperature fuel cell performance [18,19,20,21,22,23,24]. For example, Park et al. combined a perovskite-type BaCeO₃ based electrolyte with a binary eutectic carbonate to obtain a high ionic conductivity of 0.176 S·cm⁻¹ at 550 °C [18]. Fu et al. studied a gadolinium-doped ceria chloride composite electrolyte with a good power output density of 240 mW·cm⁻² at 500 °C [24].

Many studies have shown that double cation doped ZrO₂ could reduce the test temperature and improve its conductivity when compared to the single cation stabilized zirconia [25,26,27,28,29,30]. For instance, Liu et al. found that 1 wt % Al₂O₃ doped 8 mol % Y₂O₃ stabilized ZrO₂ (8YSZ) reduced the sintering temperature of YSZ and improved the output of the fuel cell [26]. Wang et al. stabilized the structure phase when the substitution of Yb₂O₃ was over 2 mol % in the Yb₂O₃-Sc₂O₃-ZrO₂ system. Furthermore, the radius of Yb³⁺ and Y³⁺ were both close to that of Zr⁴⁺ [27]. Therefore, we tried to fabricate a new composite electrolyte by using Yb³⁺ and Y³⁺ double doped ZrO₂ together with a binary eutectic chloride.

In this paper, new Yb₂O₃ doped Zr_0.92Y_0.08O_2-α(8YSZ)-NaCl/KCl composite electrolytes were prepared at different temperatures. The morphology and structure were characterized and the ionic conductivity and fuel cell were systematically evaluated.

2. Experimental

Yb³⁺ and Y³⁺ double doped ZrO₂ was produced via a solid state reaction method. A total of 4 mol % Yb₂O₃ (99.9%) and 8YSZ (Xuancheng Jingrui New Material Co., Ltd., Xuancheng, China, sol-gel method, 50 nm) powders were fully mixed in ethanol under continuous stirring with an agate mortar and dried by an infrared lamp three times. Subsequently, the obtained powder was calcined at 1200 °C for 6 h to obtain the 8YSZ and 8YSZ+4Yb₂O₃ samples.

The binary eutectic chloride of KCl (0.202 g)–NaCl (0.158 g) (mole ratio = 1:1) was heated at 700 °C twice [31]. All of the used reagents were analytical grade (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). Then, 80 wt % of the 8YSZ+4Yb₂O₃ and 20 wt % of the KCl/NaCl powders were mixed and ground. The mixtures were sieved through 200 mesh and pressed into round disks under 200 MPa. Finally, the obtained disks were heated at 800 °C and 1000 °C for 2 h, respectively, to obtain 8YSZ+4Yb₂O₃-NaCl/KCl composite electrolytes.

The crystalline phases of the 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples were determined by X-ray diffraction (XRD, X’pert Pro MPD, Holland’s company, Amsterdam, The Netherlands). The morphology of the sintered pellets was observed by using a scanning electron microscope (SEM, S-4700, Hitachi, Tokyo, Japan).

All samples were ground and pressed into thin slices (thickness = 1.0–1.2 mm) and 80% silver-20% palladium paste (areas = 0.5 cm²) was used with silver wires as the electrodes. Electrochemical impedance spectroscopy (EIS) techniques were used to obtain the conductivity of the 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C). The ac amplitude was 20 mV in a three-electrode system over the frequency range from 1 Hz to 1 MHz. The conductivity can be calculated from: σ = $\frac{L}{R·S}$, where σ is conductivity, L is thickness, R is resistance, and S is the surface area of the electrolyte pellet [32,33]. The effects of different synthetic temperature, operating temperature, and oxygen partial pressure on the electrical conductivities were determined with an electrochemical analyzer (CHI660E, Chen Hua company, Shanghai, China) at 400–700 °C [34,35]. Oxygen concentration discharge cell: air, Pd-Ag|8YSZ+4Yb₂O₃-NaCl/KCl (800 °C)|Pd-Ag, O₂ at 700 °C and H₂/O₂ fuel cells using 8YSZ+4Yb₂O₃ and 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) as electrolytes (thickness = 1.1 mm) were constructed. The fuel cell was tested by using the linear scanning of current and voltage method within the CHI660E electrochemical analyzer [33].

3. Results and Discussion

Figure 1 presents the X-ray diffraction (XRD) patterns of the 8YSZ, 8YSZ (1200 °C), 8YSZ+4Yb₂O₃ (1200 °C), 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples together with the standard diffraction patterns of Yb₂O₃ (JCPDS 88-2161) and t-Zr_0.9Y_0.1O_1.95 (JCPDS 82-1241). The 8YSZ (Xuancheng Jingrui New Material Co., Ltd., sol-gel method, 50 nm) powder possessed coexisting tetragonal and monoclinic phases, where the tetragonal was the major phase, as shown in Figure 1a. When the synthesis temperature reached 1200 °C, it was observed that the 8YSZ and 8YSZ+4Yb₂O₃ showed an entire t-ZrO₂ phase. Yb³⁺ really formed a solid solution with YSZ and there was no trace of Yb₂O₃. Additionally, NaCl and KCl diffraction peaks also existed in the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples, that is to say that the binary eutectic chloride did not react with 8YSZ+4Yb₂O₃. This was in good agreement with the studies of ceria-carbonates or doped SrCeO₃-NaCl-KCl composite electrolytes [36,37].

The surface (a,c,e) and cross-sectional (b,d,f) morphologies of the 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) ,and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) pellets, as seen using SEM, are exhibited in Figure 2. No pores or cracks on the surface (e) and cross-sectional (f) photos of the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) were seen along the entire experiment, which meant that the Yb³⁺ and Y³⁺ double doped ZrO₂ and the binary eutectic chloride sintered uniformly. The 8YSZ+4Yb₂O₃ and NaCl/KCl are indicated by arrows in Figure 2f. However, as can be observed from Figure 2c,d, a few scattered small pores were found in the 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) pellet, which may influence the electrical properties of the sample. The sintering performance of the 8YSZ+4Yb₂O₃-NaCl/KCl pellet increased with the increase in temperature, nevertheless, the vapor pressure of the molten inorganic salts rapidly increased at the same time. This was consistent with the ceria-carbonates or doped SrCeO₃-chloride composite electrolytes prepared under similar heat treatments [36,37,38,39]. Therefore, the most suitable synthetic temperature was 800 °C.

The plots of log (σT) versus 1000 T⁻¹ of the 8YSZ, 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) pellets in air at 400–700 °C are given in Figure 3. The maximum conductivities achieved for 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) were 4.7 × 10⁻² S·cm⁻¹, 6.1 × 10⁻¹ S·cm⁻¹, and 3.8 × 10⁻¹ S·cm⁻¹ at 700 °C, respectively. Wang et al. [27] demonstrated that the conductivities of Yb³⁺ and Sc³⁺ double doped ZrO₂ could maintain 1.0 × 10⁻² S·cm⁻¹ at 700 °C, which is the threshold value for application as an electrolyte. The experimental result of Bohnke et al. [28] showed that the conductivity of the (Sc₂O₃)_0.07-(Fe₂O₃)_0.03-(ZrO₂)_0.90 was lower than 1.0 × 10⁻² S·cm⁻¹ at 700 °C. Our result was equivalent to the former. The conductivities of electrolytes have been generally found to be higher in a wet atmosphere in comparison with a dry atmosphere [33]. Furthermore, 8YSZ+4Yb₂O₃ exhibited a highest electrical conductivity of 4.7 × 10⁻² S·cm⁻¹ when compared to the 8YSZ of 2.3 × 10⁻² S·cm⁻¹ at 700 °C. The measured conductivities of the composite electrolytes were much higher than those of 8YSZ+4Yb₂O₃ and in our previous study of SrCe_0.9Sm_0.1O_3-α-NaCl-KCl (1.43 × 10⁻¹ S·cm⁻¹). This revealed that the introduction of the binary eutectic chloride allows for ionic charge carriers to move quickly and freely through it, which is beneficial for the long-range transfer ability of ions [36,40]. In Figure 3, with the increase in sintering temperature, the conductivities of the 8YSZ+4Yb₂O₃-NaCl/KCl samples decreased. The conductivity of 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) was lower than that of the sample sintered at 800 °C, especially at a low temperature range. This might be associated with the breaking of the long-range transfer of ionic charge carriers to a certain extent after heating at 1000 °C, according to the result of Figure 2.

To reveal the ionic conduction of the 8YSZ+4Yb₂O₃ and 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) samples, the variation of the conductivities with the partial pressures of oxygen in the range of pO₂ = 10⁻²⁰~1 atm were evaluated. It can be seen from Figure 4 that the conductivities were almost independent of pO₂ at 700 °C, implying that the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite electrolyte is a virtually pure ionic conductor, which agrees with previous reports [20,37,38]. The result illustrates that the conductivities of the two electrolytes in Figure 3 are purely ionic.

To explore the oxide ionic conduction of the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite under an oxygen-containing atmosphere, an oxygen concentration discharge cell was fabricated and tested at 700 °C, as illustrated in Figure 5. The calculated electromotive forces (EMF_cal) of the oxygen concentration discharge cell can be obtained from EMF_cal = $\frac{RT}{4F}$ t_O ln[pO_2(A)/pO_2(B)] when t_O = 1. It was obvious that the measured open circuit voltage exhibited a value of 33 mV, which was consistent with the calculated EMF (32.7 mV). In addition, the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite was believed to be an oxide ionic conductor under an oxygen-containing atmosphere due to the exhibited stable discharge curve [41,42].

Figure 6 is the typical impedance spectra of the 8YSZ+4Yb₂O₃ and 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) measured at 700 °C under open-circuit conditions. The spectra gave an incomplete semicircle at intermediate to high frequencies, and an arc at low frequency. The high frequency related to the ohmic resistance (R_o) and the low frequency can be attributed to the resistance between the electrode and the electrolyte [43]. The interval between the high and low frequencies corresponded to interfacial polarization resistance (R_p) where the R_p and R_o of 8YSZ+4Yb₂O₃ were 0.41 Ω·cm² and 5.45 Ω·cm², while those of 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) were 0.16 Ω·cm² and 0.87 Ω·cm² at 700 °C, correspondingly.

The single cells based on 8YSZ+4Yb₂O₃ and 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) were operated at 700 °C, hydrogen and oxygen were supplied at the anode side and cathode side, respectively, and the I-V and power density curves are displayed in Figure 7. The following two reactions occur at the cathode and anode compartments: cathode reaction: 2H⁺ + O₂ + 4e⁻ = H₂O + O²⁻ and anode reaction: 2H₂ + O²⁻ = 2H⁺ + H₂O + 4e⁻ [44]. As can be observed from Figure 7, the open circuit voltages were as high as 1.09 V, which confirmed that the samples possessed high densities. This can be attributed to the fact that NaCl–KCl eutectic melt fills the pores inside the composite electrolyte, leading to the increase in the density of the composite electrolyte at 700 °C. In this state, the 8YSZ+4Yb₂O₃ electrolyte and NaCl/KCl were between the continuous and discontinuous phases. The conductivities can be ascribed to the mobility of various species (Na⁺, K⁺, H⁺, Cl⁻ and O²⁻). And protons vacancies are the predominated defects under wet conditions, especially in a hydrogen containing atmosphere. Therefore, the fuel cell based on 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) exhibited good cell performance and gave a power output density of 364 mW·cm⁻² at 700 °C. The result was much better than the best performance of 8YSZ+4Yb₂O₃ and that ever reported for intermediate temperature SOFCs based on SrCe_0.9Eu_0.1O_3-α-NaCl-KCl of 207 mW·cm⁻² [37] at 700 °C. However, the durabilities of the electrolytes were not been tested, and will be done in future work.

4. Conclusions

In this study, Yb³⁺ and Y³⁺ double doped ZrO₂ and its composite electrolytes were successfully fabricated by a solid state reaction method. The result of the logσ~log (pO₂) plot indicated that the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite electrolyte was a virtually pure ionic conductor. Furthermore, the oxygen concentration discharge cell illustrated that the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite was an oxide ionic conductor under an oxygen-containing atmosphere. The R_p and R_o were 0.41 Ω·cm² and 5.45 Ω·cm² for 8YSZ+4Yb₂O₃, and 0.16 Ω·cm², and 0.87 Ω·cm² for 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) under open-circuit conditions at 700 °C correspondingly. Finally, an excellent performance of 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) was obtained with a maximum power density of 364 mW·cm⁻² at 700 °C.