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SURF 2009: Synthesis and Characterization of Proton Conducting Solid Electrolytes: Cs5H3(SO4)4 and (Cs1-xRbx)5H3(SO4)4
Direct Mentor: Chatr Panithipongwut <chatr@caltech.edu>
Course Requirements:
Chemistry lab course; Physical chemistry course; Understanding of phase equilibria (introductory materials science, MS 115ab)Academic Standing:
Caltech students preferred, sophomore or more advanced students preferred.
Background
The compound Cs5H3(SO4)4 is an oxyanion solid acid. Materials in this broad class of compounds, which can be considered chemical intermediates of a normal salt (in this case Cs2SO4) and a normal acid (in this case H2SO4), are comprised of XO4 oxyanion groups linked together by hydrogen bonds. In many cases, upon heating to moderate temperatures (100-250C) the materials undergo a structural phase transition to a state of high proton conductivity. This characteristic renders them useful for applications such as fuel cells. The case of Cs5H3(SO4)4 has been a bit of a puzzle. It is unclear whether the material undergoes such a transition on heating, or decomposes via loss of H2O, or decomposes into two distinct solid phases.
Connected to these questions are the behavior of the related compound Rb3H(SO4)2. On heating this material this material decomposes into Rb2SO4 and another phase of unknown composition. We suspect that the unknown phase is Rb5H3(SO4)4, however, attempts to prepare this compound directly have been unsuccessful. Because the existence of Cs5H3(SO4)4 is well established, we anticipate that gradual replacement of the Cs in this phase by Rb will lead to the Rb5H3(SO4)4 compound.
Description
The SURF project involves unraveling the mystery of the behavior of Cs5H3(SO4)4 and determining if Rb5H3(SO4)4 is our unknown decomposition product (from the decomposition of Rb3H(SO4)2). The student will prepare materials in the (Cs1-xRbx)5H3(SO4)4 system from aqueous solutions and identify the resulting phases by X-ray powder diffraction. Time permitting, the high temperature properties will be examined by high-temperature X-ray diffraction and thermal analysis.
Evaluation of the electrical properties of these materials is considered beyond the scope of this ten week SURF project.
Additional Information
Interested students should review the following references prior to contacting either Prof. Haile or the research mentor, Chatr Panithipongwut. These papers will also be relevant for preparing the SURF research proposal.
L. A. Cowan, R. M. Morco, N. Hatada, A. Navrotsky and S. M. Haile, “High temperature properties of Rb3H(SO4)2 at ambient pressure: Absence of a superprotonic transition,” Solid State Ionics, 179, 305-313 (2008).
C. R. I. Chisholm, L. A. Cowan, S. M. Haile and W. T. Klooster, "Synthesis, Structure and Properties of Compounds in the NaHSO4-CsHSO4 system I: Crystal structures of Cs2Na(HSO4)3and CsNa2(HSO4)3," Chem. Mat. 13, 2574-2583 (2001).
Additional papers may also be posted.
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Last modified:
January 26, 2009