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Published September 8, 2020 | Supplemental Material
Journal Article Open

Oxygen affinity: the missing link enabling prediction of proton conductivities in doped barium zirconates

Abstract

Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400–600 °C). In chemical terms, hydration energy, E_(hyd), and proton–dopant association energy, E_(as), are two key parameters that determine whether an oxide exhibits fast proton conduction, but to date ab initio studies have for the most part studied each parameter separately, with no clear correlation with proton conductivity identified in either case. Here, we demonstrate that the oxygen affinity, E_(O.dopant), defined as the energy released when an oxide ion enters an oxygen vacancy close to a dopant atom, is the missing link between these two parameters and correlates well with experimental proton conductivities in doped barium zirconates. Ab initio calculations of point defects and their complexes in Sc-, In-, Lu-, Er-, Y-, Gd-, and Eu-doped barium zirconates are used to determine E_(hyd), E_(as), E_(O.dopant), and the hydrogen affinity, EH.host, of each system. These four energy terms are related by E_(hyd) = E_(O.dopant) + 2E_(H.host) + 2E_(as). Complementary impedance spectroscopy measurements reveal that the stronger the calculated oxygen affinity of a system, the higher the proton conductivity at 350 °C. Although the proton trapping site is also an important factor, the results show that oxygen affinity is an excellent predictor of proton conductivity in these materials.

Additional Information

© 2020 American Chemical Society. Received: May 4, 2020; Revised: July 29, 2020; Published: July 30, 2020. This work was supported by JSPS KAKENHI (JP15H02287, JP16H00891, and JP18H01694), Iketani Foundation, Kyushu University Progress 100, and the Japan Science and Technology Agency CREST (JPMJCR18J3). A.K. acknowledges financial support from JSPS KAKENHI (JP25106008, JP16K06739 and JP16H06440), and S.M.H. acknowledges support from the US Department of Energy (under award DE-AR0000498). We thank J. Potticary for assistance with Er-doped BaZrO3 synthesis and Dr. K. Yamamoto for assistance with silver electrode sputtering. The authors declare no competing financial interest.

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