Dramatic Enhancement of Long-Term Stability of Erbia-Stabilized

Dec 1, 2017 - Dramatic Enhancement of Long-Term Stability of Erbia-Stabilized Bismuth Oxides via Quadrivalent Hf Doping. Byung-Hyun Yun†, Chan-Woo L...
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Communication Cite This: Chem. Mater. 2017, 29, 10289−10293

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Dramatic Enhancement of Long-Term Stability of Erbia-Stabilized Bismuth Oxides via Quadrivalent Hf Doping Byung-Hyun Yun,† Chan-Woo Lee,‡ Incheol Jeong,† and Kang Taek Lee*,† †

Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea ‡ R&D Platform Center, Korea Institute of Energy Research (KIER), 152 Gajeong-Ro, Daejeon, Yuseong-Gu 34129, Korea S Supporting Information *

A

YSB, resulting in enhanced long-term stability in terms of conductivity.18 Later, Huang et al. demonstrated the stable performance of YSB with CeO2 doping for 300 h.19 Thus, these results indicate that aliovalent doping effectively suppresses the conductivity degradation kinetics of YSB at 600 °C. However, the effect of quadrivalent doping on the stability of conductivity at 600 °C of ESB, which has superior conductivity to YSB, has not yet been systematically studied. Herein, inspired by the above studies, we developed a highly stable and conductive ESB with quadrivalent Hf doping. Surprisingly, only 1 mol % of Hf doping dramatically enhanced the long-term stability of ESB for >1000 h due to suppression of its phase transition kinetics to a rhombohedral phase. Additionally, we systematically investigated the effect of the size and amount of the quadrivalent dopants on the time-dependent stability behavior of ESB. Finally, a mechanism was proposed for the quadrivalent doping and its relationship to enhancing the kinetic stability of the ESB. The empirical evidence in terms of the hypothesis was discussed. Rietveld refinement of powder X-ray diffraction (XRD) revealed that both as-synthesized pristine ESB and 1 mol % Hfdoped ESB (1HESB) had the same cubic fluorite phase of the δ-Bi2O3 type with Fm3̅m space-group symmetry (Figure 1a,b). In addition, a higher resolution transmission electron microscopy (TEM) image of 1HESB (inset of Figure 1b) indicates that the interplanar spacing computed from the fringe pattern is 2.75 Å, which belongs to the (200) plane of the cubic phase of the Er0.4Bi1.6O3 (PDF #34-0377). Based on above analysis, the calculated lattice constants of ESB and 1HESB were almost identical at 5.50150(3) Å and 1HESB 5.50093(8) Å, respectively. Figure 1c shows the Arrhenius plots of the temperature dependency in ionic conductivity as a function of inverse temperature for ESB and 1HESB, showing almost identical conductivities at temperatures from 400 to 700 °C. Thus, these results indicate that an additional 1 mol % Hf doping onto a Bi (or Er)-lattice did not affect the oxygen ion transport properties of the ESB at the initial phase formation stage. The time-dependent conductivity behavior of ESB and 1HESB was monitored in situ using electrochemical impedance spectroscopy (EIS) at 600 °C for >1000 h (Figure 2). Figure 2a shows the impedance spectra of both samples. The Nyquist

wide variety of contemporary applications are seeking to utilize solid electrolytes with fast oxygen-ion conduction, including solid oxide fuel cells (SOFCs),1 oxygen sensors,2 memristors,3 oxygen separation membranes,4 and catalysts.5 Some of oxide families, such as ZrO2-, CeO2-, or Bi2O3-based fluorites, consist of structures with face-centered cubic (FCC) packing of cations with anions in tetrahedral interstices of the cation sublattice, and are well-known as “superionic” conductors.6 Among these families, δ-Bi2O3 exhibits the highest known oxygen-ion conductivity (≈1 S/cm at 800 °C), which is 2 orders of magnitude greater than that of the most widely used yttria-stabilized zirconia (YSZ).1 The superior ionic conductivity of δ-Bi2O3 is attributed to its inherently large concentration (25%) of oxygen vacancies with high mobility due to weak bonding in the Bi−O bond and a highly polarizable Bi3+ with its lone-pair 6s2 electrons.7 Additionally, Bayliss et al. recently reported the remarkably high oxygen surface exchange coefficient (k) of δ-Bi2O3, which is comparable to state-ofthe-art SOFC cathodes, such as La0.6Sr0.4Co0.8Fe0.2O3‑δ (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3‑δ (BSCF).8 However, pure δ-Bi2O3 is stable only within a limited temperature range from 729 to 824 °C, and below 729 °C it transforms to a monoclinic α-phase, resulting in a discontinuous drop in conductivity.9 To overcome this limitation, researchers have found that high temperature δ-Bi2O3 can be stabilized down to room temperature by forming a solid solution with various rare earth oxides.10,11 Among these combinations, the singly doped Bi2O3s, Er2O3-stabilized Bi2O3 (Er0.2Bi0.8O1.5, ESB) is known to have the highest oxygen ion conductivity (e.g., 0.32 S cm−1 at 700 °C) due to having the lowest dopant (Er3+) concentration required to achieve the cubic δ-phase stabilization.12 Recently, the feasibility of highly conductive ESB as an SOFC electrolyte was repeatedly demonstrated in cells with high power densities of 2 W cm−2 at intermediate temperatures below 700 °C.1,13,14 However, ESB is known to undergo conductivity degradation due to the phase transformation from the cubic phase to the rhombohedral phase over a long period of operation at 600 °C.15 In addition, the Virkar group observed similar phase transitions in Gd2O3-stabilized Bi2O3 (GSB) and Y2O3stabilized Bi2O3 (YSB).16 Therefore, Watanabe raised the question of whether all singly doped stabilized Bi2O3s are just quenched metastable phases sustained from the high-temperature δ-Bi2O3 phase.17 Fung et al. attempted to address this issue with the additional aliovalent doping of ZrO2 or ThO2 on © 2017 American Chemical Society

Received: September 14, 2017 Revised: November 28, 2017 Published: December 1, 2017 10289

DOI: 10.1021/acs.chemmater.7b03894 Chem. Mater. 2017, 29, 10289−10293

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Chemistry of Materials

decayed during 30 to 150 h of annealing. After 250 h, it gradually converged to ∼0.006 S/cm, which is only 4% of the initial conductivity. This value is 3−4 times lower than that of Gd-doped ceria (GDC, 0.018 S/cm)1 and Sr- and Mg-doped lanthanum gallate (LSGM, 0.025 S/cm),6 and even equal to that of YSZ (0.006 S/cm)20 at the same temperature (Figure 2b). In comparison, the high ionic conductivity of 1HESB at 600 °C (∼0.141 S/cm) was retained up to 1180 h without any observable degradation. We repeatedly tested 1HESB samples from different batches, and confirmed the same results, demonstrating excellent stability with high credibility (Figure S1). To the best of our knowledge, this result with 1HESB is the highest ionic conductivity at 600 °C with high stability (>1000 h), among any reported oxygen ion conductors. XRD analysis after long-term operation test of 1000 h shows that ESB has a large phase transition to the rhombohedral phase at the cubic phase (Figure 2c). Moreover, unlike the initial SEM image of the ESB (Figure S2), the surface roughness was greatly increased, and platelet-type microstructures were observed (Figure 2d). This result is a characteristic phenomenon when the cubic phase of the stabilized bismuth oxide is transformed to rhombohedral phase as mentioned earlier. Meanwhile, even after 1180 h, 1HESB retained a pure cubic phase without any impurity phase (Figure 2c), and there was no observed change in microstructure (Figure 2d and Figure S2), which is in sharp contrast to ESB. Thus, we believe that quadrivalent Hf doping onto Bi (or Er)-lattice effectively slowed down the kinetics of the phase transition of the ESB to the rhombohedral phase. In order to better understand the effect of quadrivalent doping on phase transformation kinetics and time-dependent conductivity characteristics of ESB, different sizes of quadrivalent dopants and their contents were studied. Assuming that the loss of oxygen vacancy of ESB with a small amount of the additional quadrivalent doping is negligible, 25% of the anion sublattice of ESB are still vacant.7 In this case, the cation coordination number (CN) of ESB is 6, and according to Shannon et al., the radius of Hf4+ ion is 0.85 Å.21 Thus, we selected two other quadrivalent dopants, including Ti (0.745 Å) and Ce (1.01 Å), i.e., Ti < Hf < Ce. Each dopant was further doped with ESB by 1, 3, 5, and 7 mol %. Here, each sample was referred to as xMESB, where x is the doped content (mol %), and M represents the quadrivalent dopant (H = Hf, T = Ti, and C = Ce). Figure 3 shows the conductivity change of HESBs, TESBs, and CESBs over time with 1, 3, 5, and 7 mol % quadrivalent doping. For comparison purposes, 1HESB data from Figure 2b was overlapped in Figure 3a. For HESBs, all doping compositions up to 7 mol % showed an excellent stabilizing effect with no deterioration of conductivity at over 200 h (Figure 3 and Figure S3a). However, the stabilization effect of both Ti and Ce dopants decreased as the dopant amounts increased. TESB and CESB showed different behaviors of conductivity deterioration over time. In the case of TESB, the stabilization effect diminished sharply as Ti doping content increased (Figure 3 and Figure S3b), and 7TESB exhibited almost the same conductivity deterioration behavior as ESB (Figure 3d). Meanwhile, all the CESBs showed stable conductivity up to ∼100 h, but thereafter showed significant deterioration in a similar tendency (Figure S3c). In a previous study, Huang et al. reported that, by doping of 7 mol % Ce, the YSB showed stable conductivity for 300 h at 600 °C.19 This discrepancy with our CESB results is probably due to the fact that there is a correlation between the size of an

Figure 1. XRD patterns with Rietveld refinement of (a) ESB (Rp = 10.13%, Rwp = 13.07%) and (b) 1HESB (Rp = 10.75%, Rwp = 14.03%). (c) Arrhenius plot of total conductivity of ESB and 1HESB in air.

Figure 2. (a) Nyquist plots of ESB and 1HESB exposed to different periods of time. (b) Total conductivity of ESB and 1HESB as a function of time: LSGM,6 GDC,1 and YSZ.20 (c) XRD spectra and (d) SEM images of ESB and 1HESB before and after long-term operation at 600 °C for 1000h and 1180h, respectively.

plots of for ESB were shifted rapidly to the right as time passed, indicating a significant increase in ohmic resistance, which is estimated from the x-axis (high frequency) intercept of the Nyquist plot. This offset resistance of the doped Bi2O3 directly reflects the resistance related to oxygen ion transport in the bulk.7 From these results, and considering the geometry of the samples (Table S1), the ionic conductivity was calculated. In comparison, doping with a small amount (1 mol %) of Hfdopant dramatically amended the time-dependent behavior of ionic conductivity of ESB, resulting in a constant ohmic resistance for 1180 h (Figure 2a). There was only a minor vibration of the impedance spectra within the experimental error range (inset of Figure 2a). Considering the sample geometries, the calculated ionic conductivities of both ESB and 1HESB were plotted as a function of annealing time at 600 °C (Figure 2b). The ionic conductivity of the pure ESB sharply 10290

DOI: 10.1021/acs.chemmater.7b03894 Chem. Mater. 2017, 29, 10289−10293

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Figure 3. Time-dependent conductivity behavior of HESBs, TESBs, and CESBs with different content of the quadrivalent dopant (a) 1 mol %, (b) 3 mol %, (c) 5 mol % and (d) 7 mol %. Inset of (c) indicates change of oxidation state of Ce in 5CESB before and after annealing.

additional quadrivalent dopant, and the size of the primary stabilizer (Er or Y) in terms of stabilizing the conductivity at 600 °C. After a long-term annealing test, for CESBs in all compositions, severe phase transition to the rhombohedral phase was observed to be similar to ESB as shown in Figure S4f. In contrast, although the deterioration of TESB was worse than that of CESB, TESB showed no phase transition to a rhombohedral phase, and only a few impurity peaks were observed (Figure S4d). At this time, the aging mechanism of TESB is unclear. However, the result here implies that there is a critical ionic radius of the quadrivalent dopant (∼0.85 Å) that suppresses the rhombohedral phase transformation of ESB at 600 °C. Also, X-ray photoelectron spectroscopy (XPS) analysis before and after long-term annealing for 5CESB (Figure S5) provided an interesting point about the charge state of Ce in Bi (or Er)-lattice. The initial Ce4+ ratio was 88.5%, but after 200 h of annealing at 600 °C, it decreased to 75.7%, and thus the Ce3+ value increased by 12.9% (Inset of Figure 3c). Thus, we postulate the time-dependent change in the charge state of Ce also accelerated the phase change of CESB and contributed to deterioration. Consequently, our results suggest that minor quadrivalent doping greatly modulates the phase transition kinetics of stabilized bismuth oxide at 600 °C, and that the effect is strongly influenced by the size and amount of the quadrivalent dopant. More importantly, Hf (0.85 Å) seems to be the optimum quadrivalent dopant for effective suppression of the kinetics of the phase transformation of ESB at 600 °C. Phase transformation from the cubic phase to the rhombohedral phase of the stabilized Bi2O3 requires rearrangement of the cation sublattice, which is highly correlated to the diffusion coefficient of the cation.18 In this case, the role of quadrivalent dopant in improving the phase stability of ESB can be explained as schematically shown in Figure 4a. From the standpoint of the lattice strain, for pure ESB, there is only strain in the lattice due to the interaction between intrinsic defects (i.e., cation vacancies and interstitials), and the anions (left of Figure 4a). On the other hand, when a quadrivalent dopant (M4+) is doped as MO2 into ESB, the following defect reaction can be considered:

Figure 4. (a) Schematic illustration of the effect of quadrivalent dopant on local strain in (100) of ESB. (b) Schematic descriptions of Er and Hf arrangements in 1HESB supercell (type A). (c) Defect formation energy of cation interstitials (upper) and vacancies (lower) in ESB and 1HESB. (d) Concentration profiles of Bi in diffusion couples with and without Hf. (e) Composition-dependent cation interdiffusion coefficient in ESB diffusion couples with different quadrivalent dopant conditions.

3 3 1 ′ MO2 → M•Er,Bi + 3OOX + V ′′Er,Bi 2 2 2

(1)

Thus, when a small amount of quadrivalent dopant is doped, the concentration of vacancy, [V], is increased. Accordingly, the concentration of cation interstitial, [I], is slightly decreased because the total intrinsic defects in the cation sublattice are controlled by the Frenkel defect (KF = [V][I]). As a result, quadrivalent doping onto a Bi (or Er) lattice can induce strain in the total lattice over a wider range. Furthermore, with a smaller ionic radius of the quadrivalent dopant compared to Bi3+(1.17 Å), the induced lattice strain would be stronger due to the ionic radius mismatch (right of Figure 4a). To validate above interpretation on defect chemistry of the ESB with quadrivalent doping, density-functional theory (DFT) calculations have been performed on model systems of ESB and 1HESB. To describe defect structures of them, four different arrangements of Er in the cation lattice were considered, where each arrangement was named as A, B, C, and D, respectively (Figure 4b and Figure S6). Computational details about the DFT calculations, and concentrations of Er and Hf in the structure models of ESB and 1HESB follows experimental counterparts in the Supporting Information. Figure 4b shows formation energies of cation interstitial (EF,IBi, upper in Figure 4b), and cation vacancy (EF,VBi, lower in Figure 4b). Note that two defects are formed subsequently (VBi followed by IBi), and the large EF indicates easy formation of the corresponding 10291

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of time-dependent stability enhancement of ESB at 600 °C. Finally, the Boltzmann−Matano method was utilized to quantify interdiffusion coefficients of the cation in ESB with different secondary dopant conditions, resulting in a 53% decrease in cation interdiffusion coefficient with Hf doping on ESB compared to pure ESB. This result supports our theory that the mechanism of long-term stability enhancement with Hf doping is primarily due to the lower diffusion coefficient of the cation sublattice in ESB, thus providing suppression of phase transformation kinetics. We believe our results will provide important insights and can guide design of next-generation superionic solid electrolytes with high stability.

defect. The results show that IBi in 1HESB is relatively more difficult to form than that in ESB, vice versa for VBi. Thus, these DFT calculations predict that Hf doping in ESB reduces the concentration of cation interstitial ([IBi]) in the lattice, which is consistent with our hypothesis above. Moreover, because the diffusion of cations is caused by hopping via vacancies and interstitials, the diffusion coefficient of cations is determined by the following equation: D = [V ]D V + [I ]DI

(2)

where DV and DI are the diffusion coefficients of cation vacancy and interstitial, respectively. Assuming that DI is larger than DV,18 the diffusion coefficient of the cation can be decreased based on eq 2. Therefore, we postulate that the above cooperative interactions between quadrivalent dopants and host ions effectively reduce the diffusion coefficient of the host cation. To empirically demonstrate the above the hypothesis, we measured the interdiffusion coefficient of Bi3+ ions of the ESB diffusion couples of Er0.3Bi0.7O1.5|Er0.15Bi0.85O1.5 (30ESB| 15ESB) with different quadrivalent dopant conditions (5 mol % of Hf, Ti, and Ce) using the Boltzmann−Matano method.22 The concentration-dependent cation interdiffusion coefficient, D̃ (C*), at any specified time (t) can be estimated by the integro-differential expression as follows: 1 ⎛ dx ⎞ D̃ (C*) = − ⎜ ⎟ 2t ⎝ dC ⎠ C*

∫C

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* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03894. Material synthesis, experimental details, data analysis procedures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*K. T. Lee. E-mail: [email protected]. ORCID

C*

Kang Taek Lee: 0000-0002-3067-4589

x dC 0

ASSOCIATED CONTENT

S

(3)

Funding

This work was supported by the Global Frontier R&D on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2014M3A6A7074784). This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20174030201590).

where x is the length determined from reference position (x = 0), which is called as the “Matano interface”. The balance of mass is met at the boundary of the Matano interface where the loss due to diffusion on one side and the corresponding gain on the other side are equal (also see Supporting Information). Figure 4d shows the Bi concentration profiles of the diffusion couples in the absence of a quadrivalent dopant and in the case of 5 mol % Hf doping. The slope of the concentration profile with 5 mol % Hf doping is steeper compared to that of the no doping sample, suggesting a lower D̃ (C*) with the additional quadrivalent doping. The concentration profiles of the specimens doped with Ti and Ce are shown in Figure S8. Figure 4e shows the calculated cation interdiffusion coefficients of the ESB diffusion couples with different dopants as a function of Bi mol %. The calculated D̃ (C*) values for undoped, Ce-, Ti-, and Hf-doped samples were 13.7 × 10−10, 12.0 × 10−10, 10.0 × 10−10, and 6.5 × 10−10 cm2 s−1, respectively, at conditions of 80 mol % Bi and 20 mol % Er. This result clearly shows that the additional Hf doping effectively slowed down cation diffusion in the ESB lattice (∼53%). Moreover, this result is in good agreement with the trend of improved conductivity stability with different dopants as shown in Figure 3c. In summary, the long-term stability of a superionic conductive solid electrolyte based on stabilized Bi2O3 at 600 °C was significantly improved via secondary doping with quadrivalent Hf. Although pure ESB showed rapid degradation in conductivity due to cubic-to-rhombohedral transformation, 1 mol % of Hf doped ESB maintained its superior conductivity (∼0.14 S/cm) with no phase change for ∼1200 h at 600 °C. Unexpectedly, other quadrivalent dopants, such as Ti and Ce showed much less effect on stability enhancement. Thus, this result importantly suggests that the ionic radius of the quadrivalent dopant is strongly correlated to the mechanism

Notes

The authors declare no competing financial interest.

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REFERENCES

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