Dramatic Enhancement of Long-Term Stability of Erbia-Stabilized

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Dramatic Enhancement of Long-Term Stability of ErbiaStabilized Bismuth Oxides via Quadrivalent Hf Doping Byung-Hyun Yun, Chan-Woo Lee, Incheol Jeong, and Kang Taek Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03894 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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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 ABSTRACT: The Er2O3-stabilized Bi2O3 (ESB) exhibits a superior oxygen ion conductivity below 750 oC. However, there is significant conductivity decay due to a cubic-to-rhombohedral phase transformation over time at ~ 600oC. Thus, to enhance the kinetic stability of ESB, we develop a novel double-doped bismuth oxide through additional quadrivalent doping in ESB. Surprisingly, 1 mol.% of Hf-doped ESB shows no conductivity degradation without a phase transition for 1200 h at 600 °C, while the conductivity of pure ESB drops to less than 5% of the initial value within 200 h. Additionally, the stability improvement effect is strongly correlated to the ionic radius of the aliovalent dopant with an optimum value at ~ 0.85Å. When the Bi3+ diffusivity in the cation sublattice of ESB is measured using the Boltzmann-Matano method, Hf doping greatly reduces the diffusion coefficient by ~53% compared to pure ESB. This result suggests that the control of the cation interdiffusion coefficient is the key parameter for suppression of phase transformation kinetics of the stabilized bismuth oxide at 600oC, thus enhancing the long-term stability of its conductivity. The results of our work will pave the way for the use of Bi2O3-based solid electrolytes in electrochemical energy devices for a wide variety of applications.

A 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 sub-lattice, and are well-known as ‘superionic’ conductors.6 Among these families, !-Bi2O3 exhibits the highest known oxygen-ion conductivity (" 1 S/cm at 800¥), which is two orders of magnitude greater than that of the most widely-used yttria-stabilized zirconia (YSZ).1 The superior ionic conductivity of !-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 !-Bi2O3, which is comparable to state-of-the-art SOFC cathodes, such as La0.6Sr0.4Co0.8Fe0.2O3-! (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3-! (BSCF).8 However, pure !-Bi2O3 is stable only within a limited temperature range from 729 to 824 ¥, and below 729 ¥ it transforms to a monoclinic #phase, resulting in a discontinuous drop in conductivity.9 To overcome this limitation, researchers have found that high temperature !-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¥) due to having the lowest dopant (Er3+) concentration required to achieve the cubic !-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 o 1,13,14 C. 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¥.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 hightemperature !-Bi2O3 phase.17 Fung et al. attempted to address this issue with the additional aliovalent doping of ZrO2 or ThO2 on 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 oC. However, the effect of quadrivalent doping on the stability of conductivity at 600¥ of ESB, which has superior conductivity to YSB, has not yet been systematically studied.

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Chemistry of Materials sub-lattice 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Material synthesis, experimental details, data analysis procedures (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

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).

Notes The authors declare no competing financial interest.

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