Engineering the next generation of solid state proton conductors: Synthesis and properties of Ba3-xKxHx(PO 4)2

Calum R I Chisholm, Eric S. Toberer, Mary W. Louie, Sossina M. Haile

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11 Scopus citations

Abstract

A new series of compounds with general chemical formula Ba3-x KxHx(PO4)2 has been successfully prepared. This particular stoichiometry was targeted as a candidate solid-state proton conductor because of its anticipated structural similarity to known M3H(XO4) 2 superprotonic conductors (M = Cs, Rb, NH4, K; X = Se, S)1 and to the known trigonal compound Ba3(PO4)2.2 The materials were synthesized from aqueous solution using barium acetate, dipotassium hydrogen phosphate, and potassium hydroxide as starting materials. Through variations in the initial solution stoichiometry or the synthesis temperature, the final stoichiometry could be controlled from x ̃ 0.5 to ̃ 1. X-ray powder diffraction, energy dispersive spectroscopy chemical analysis, 1H magic angle spinning (MAS) nuclear magnetic spectroscopy, and thermogravimetric analysis were all employed to establish potassium and proton incorporation. The diffraction data confirmed crystallization of a trigonal phase, and chemical analysis showed the (Ba-K):P ratio to be 3:2, consistent with the target stoichiometry. The conductivity of the Ba3-xKxH x(PO4)2 materials, as measured by A.C. impedance spectroscopy, is about 3 orders of magnitude greater than the end-member Ba3(PO4)2 material with only a slight dependence on x, however, it is substantially lower than that of typical superprotonic conductors and of the M3H(XO4)2 materials in particular. The close proximity of Ba to the hydrogen bond site is proposed to explain this behavior. At 250 °C, the conductivity is ~2.4 x 10-5 S/cm for the composition x = 0.80, which, when combined with the water insolubility and the relatively high thermal stability, may render Ba 3-xKxHx(PO4)2 an attractive alternative in selected electrochemical applications to known superprotonic conductors.

Original languageEnglish (US)
Pages (from-to)1186-1194
Number of pages9
JournalChemistry of Materials
Volume22
Issue number3
DOIs
StatePublished - Feb 9 2010

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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