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Superprotonic Solid Acids: Structure, Properties, and Applications


Boysen, Dane Andrew (2004) Superprotonic Solid Acids: Structure, Properties, and Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/41BQ-3R07.


In this work, the structure and properties of superprotonic MHₙXO₄-type solid acids (where M = monovalent cation, X = S, Se, P, As, and n = 1, 2) have been investigated and, for the first time, applied in fuel cell devices. Several MHₙXO₄-type solid acids are known to undergo a "superprotonic" solid-state phase transition upon heating, in which the proton conductivity increases by several orders of magnitude and takes on values of ~ 0.01 S/cm. The presence of superprotonic conductivity in fully hydrogen bonded solid acids, such as CsH₂PO₄, has long been disputed. In these investigations, through the use of pressure, the unequivocal identification of superprotonic behavior in both RbH₂PO₄ and CsH₂PO₄ has been demonstrated, whereas for chemically analogous compounds with smaller cations, such as KH₂PO₄ and NaH₂PO₄, superprotonic conductivity was notably absent. Such observations have led to the adoption of radius ratio rules, in an attempt to identify a critical ion size effect on the presence of superprotonic conductivity in solid acids. It has been found that, while ionic size does play a prominent role in the presence of superprotonic behavior in solid acids, equally important are the effects of ionic and hydrogen bonding. Next, the properties of superprotonic phase transition have been investigated from a thermodynamic standpoint. With contributions from this work, a formulation has been developed that accounts for the entropy resulting from both the disordering of both hydrogen bonds and oxy-anion librations in the superprotonic phase of solid acids. This formulation, fundamentally derived from Linus Pauling's entropy rules for ice, accurately accounts for the change in entropy through a superprotonic phase transition. Lastly, the first proof-of-principle fuel cells based upon solid acid electrolytes have been demonstrated. Initial results based upon a sulfate electrolyte, CsHSO₄, demonstrated the viability of solid acids, but poor chemical stability under the highly reducing H₂ gas environment of the fuel cell anode. Later experiments employing a CsH₂PO₄ electrolyte proved quite successful. The results of these solid acid-based fuel cell measurements suggest solid acids could serve as an alternative to current state-of-the-art fuel cell electrolytes.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:electrolytes; fuel cells; proton conductors; solid acids; superprotonic
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Haile, Sossina M.
Thesis Committee:
  • Haile, Sossina M. (chair)
  • Fultz, Brent T.
  • Asimow, Paul David
  • Goddard, William A., III
  • Johnson, William Lewis
Defense Date:9 January 2004
Record Number:CaltechETD:etd-05282004-155105
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2195
Deposited By: Imported from ETD-db
Deposited On:28 May 2004
Last Modified:03 Feb 2021 19:16

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