Abstracts S S P - SSPC-15 - California Institute of Technology

O 28 (K)
High temperature protonic conduction in rare earth phosphates and borates
K. Amezawa 1 , H. Takahashi 1 , A. Unemoto 2 , H. Kuwabara 3 , N. Kitamura 4 and T. Kawada 1
1 Grad. Sch. of Environmental Studies, Tohoku Univ., 6-6-01 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
2 Insti. of Multidisciplinary Res. for Adv. Mat., Tohoku Univ., 2-2-1 Katahira, Aoba-ku, Sendai 980-8577, Japan/
3 Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan
4 Dep. of Pure and Appl. Chem., Faculty of Sci. and Tech., Tokyo Univ. of Sci., 2641 Yamazaki, Noda 278-8510, Japan
Rare earth phosphates such as orthophosphate LnPO4 and metaphosphate LnP3O9 are found
to exhibit protonic conduction at elevated temperatures [1, 2] and thus expected as an electrolyte of
fuel cells, hydrogen activity sensors, and hydrogen separation systems. Similar protonic conduction
has been also demonstrated in rare earth borates [3]. These oxoacid salts-based materials become
protonic conductors under moisturized atmospheres by doping lower valence (divalent) cations into
rare earth sites. High temperature protonic conductors (HTPCs) based on rare earth oxoacid salts, in
general, show relatively dominant protonic conduction and excellent chemical stability against acid
gases, although their conductivities are lower than those of proton-conducting perovskite-type
oxides. In this presentation, our recent researches on high temperature protonic conduction in rare
earth phosphates and borates are reviewed.
Mechanism of proton dissolution processes into the rare earth phosphates and borates has
been examined by means of spectroscopic measurements, e.g. MAS-NMR, FT-Raman, and FT-IR
spectroscopies in addition to conventional electrochemical measurements, e.g. measurements of
electrical conductivity and transport number [2]. Their defect structures were also investigated by
the first principle calculation [4]. Consequently, in the case of rare earth orthophosphates, it was
suggested that substitution of divalent cations for rare earth ions leads to the condensation of
phosphate ions, i.e. formation of P2O7 4- from two PO4 3- . Such condensation of phosphate ions is
regarded as formation of oxygen deficits like formation of oxygen vacancies in conventional
proton-conducting oxides. Protons are considered to be dissolved into phosphates forming hydrogen
phosphate ions through the equilibrium between the condensed phosphate ions and ambient water
vapor.
Electrical conduction in LaP3O9 glass was also investigated [5]. LaP3O9 glass had
considerably lower conductivity than LaP3O9 ceramics, and its charge carrier species were not
protons. However LaP3O9 glass became to exhibit relatively higher and protonic conductivity by
partial
crystallization of the glass. These results indicated that crystalline state is essentially needed
for
high temperature protonic conduction in phosphates.
References
1. T. Norby and N. Christiansen., Solid State Ionics, 77, 240 (1995) .
2. K. Amezawa, et al., Solid State Ionics, 145, 233 (2001); Electrochem. Solid-State Lett., 7, A511 (2004).
3. K. Amezawa, et al. Solid State Ionics, 175, 575 (2004); H. Takahashi, et al., Solid State Ionics, in press.
4. N. Kitamura, et al., 16th Int. Conf. on Solid State Ionics, P594 (2007).
5. K. Amezawa, J. Am. Ceram. Soc., 88, 321 (2005).
Acknowledgements: This work was supported by the Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
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