Acid Sulfates, Phosphates and Selenates
acid sulfates and selenates such as CsHSO4, CsHSeO4
belong to a small class of compounds that exhibit superprotonic phase
transitions at slightly elevated temperatures. CsHSO4, for
example, transforms at 140°C into a superionic phase in which both protons
and oxygen atoms partially occupy many sites. At the transition, the
conductivity jumps by several orders of magnitude. In the high conductivity
state, the activation energy for proton transport is typically 0.32
to 0.4 eV.
high conductivity of solid acids, along with the fact that the proton
transport does not rely of the presence of water, renders them attractive
as electrolytes for fuel cell applications. Solid acid fuel cells offer
several advantages over polymer electrolyte fuel cells, the type currently
under development for portable power and transportation applications:
anhydrous proton transport eliminates the need for auxiliary humidification
equipment; complete impermeability of the solid acid to H2
and O2 results in measurably higher open circuit voltages;
complete impermeability to methanol makes possible the operation of
high efficiency direct methanol fuel cells; and "warm" temperature
operation leads to greater effectiveness of the Pt catalyst, which,
in turn, relaxes the purification requirements for the hydrogen gas
(improves the "CO tolerance" of the catalyst). What all of
this means, is that solid acids could lead to a tremendous reduction
in complexity (and therefore cost) of the overall fuel cell system.
Several challenges certainly remain, however, before solid acid based
fuel cells can be commercially viable.
the first fuel cell based on a solid acid in 2001, [Nature,
410 (2001) 910-913] using the electrolyte CsHSO4. Unfortunately,
sulfate and selenate based acid compounds eventually react with H2 in
the anode chamber, particularly in the presence of fuel cell catalysts.
The reaction by-product, H2S or H2Se, is a fantastic fuel cell poison
and such electrolytes are not suitable for long-term operation. More
recently we have shown that such fuel cells based on CsH2PO4 as the
electrolyte can perform stably for over one hundred hours of continuous
(2003)]. Furthermore, we have advanced the fuel cell fabrication
process so as to be able to obtain power densities of over 400 mW/cm2.
on the significance of solid acid fuel cells, see a
report in Nature written by Prof. Truls Norby that accompanied our
first published demonstration. For an informal description of what we've
done, view a video
put together by Science and Technology News Network.
to CsHSO4, there has been (and continues to be!) dispute
in the literature as to whether or not the solid acid CsH2PO4
undergoes a true, polymorphic superprotonic transition. There are those
who argue that the apparent transition, indicated by an increase in
conductivity upon heating, is simply an artifact due to thermal decomposition.
In such a case, the rapid evolution of H2O from the compound is responsible
to a transient increase in conductivity. We have approached this question
from two different angles. First, we have investigated mixed sulfate-phosphate
compounds in order to establish whether the chemistry of the phosphate
anion group can somehow interfere with the superprotonic transition.
Second, we have used hydrostatic pressure and water partial pressure
as tools for suppressing any decomposition/dehydration reactions.
we have discovered four intermediary phases, a-Cs3(HSO4)2(H2PO4),
Cs2(HSO4)(H2PO4) and Cs5(HSO4)3(H2PO4)2
in the CsHSO4-CsH2PO4 system. All four of these compounds undergo
superprotonic transitions and thus one can immediately conclude that
the chemistry of the phosphate group does not interfere with superprotonic
crystal-chemical features of the six known compounds in the CsHSO4-CsH2PO4
system are summarized in the table below. Rather surprisingly,
in some compounds S and P occupy crystallographically distinct sites,
whereas in others, they are randomly distributed over equivalent sites.
I. A comparison of selected structural features of xCsHSO4(1-x)CsH2PO4
et al., Acta Cryst. B47, 161 (1991).
SO4 ~ 1.5
+ disordered + vacancies
S(2)O4 - 2
-4 [P,S](2)O4 -4 S(3)O4 - 1
layer with some thickness
& Choudhary, Solid State Comm. 26, 823 (1978).
structures of all the compounds except CsH2PO4
are based on hydrogen bonded zig-zag chains of XO4 groups,
which alternate with zig-zag rows of Cs atoms:
1. The structure of b-Cs3(HSO4)2(H2-x(Sx,P1-x)O4)
projected on (010). The site occupancy at H(3) is only ~ 0.25 and leads
to a high proton conductivity, even at ambient temperatures.
transition is characterized by an increase in conductivity by several
orders of magnitude over a relatively small temperature range.
Although the mixed sulfate phosphates have unique structures that also
differ from that of CsHSO4, all exhibit clear transitions.
This implies that superprotonic behavior is independent of such things
as the dimensionality of the hydrogen bond network, the details of the
geometry of the hydrogen bonds, and the nature of the XO4
anions, which is in agreement with our proposed driving force for such
The conductivities of solid acids in the CsHSO4-CsH2PO4
of the superprotonic transition in CsH2PO4
the use of (i) hydrostatic pressure or (ii) water partial pressure it
is possible to suppress the dehydration reaction of CsH2PO4
-> CsPO3 + H2O
the superprotonic phase transition. We have performed A.C. impedance
spectroscopy under 1 GPa pressure in collaboration with Prof. Richard
Secco of the University of Western Ontario, Canada. Under these conditions,
the transition occurs at 250C and the activation energy for proton transport
in the high temperature phase is 0.35 eV.
behavior in CsH2PO4 0.3 atm water partial pressure (total pressure of
1 atm) has been demonstrated by Otomo et al. in 2003. Our preliminary
measurements of the thermodynamics of the dehydration reaction indicate
that a water partial pressure of 0.3 atm is sufficient to stabilize
CsH2PO4 up to a temperature of C, well beyond the superprotonic transition.
Figure 3 below shows these results. Consequently, it is possible to
implement CsH2PO4 in fuel cells that operate at ~ 250C so long as the
atmospheric water partial pressure is greater than ~0.12 atm. This level
of humidification is equivalent to passing the incoming gases through
water that is held about about 48C (that is, 100% relative humidity
at 48C). These results also make it clear why the literature describing
the conductivity of CsH2PO4 shows such discrepancy. The humidification
levels required to stabilize CsH2PO4 are met naturally in places like
Moscow and Boston, but not in Pasadena and other dry locations!
3. The dehydration behavior of CsH2PO4. The superprotonic transition
occurs at 230C under 1 atm of total pressure. At water partial pressure
above the blue curve, the compound is stabilized against dehydration.
Typical fuel cell operating conditions are indicated by the yellow dot.
Lisa Cowan, Mikhail Kislitsyn, Kenji Sasaki
Dane Boysen (currently Visiting Associate), Calum R.I. Chisholm (currently
Visiting Associate), Tetsuya Uda
of Naval Research; National Science Foundation; California Energy Commission,
California Institute of Technology
D.A. Boysen, T. Uda, C.R.I. Chisholm
and S.M. Haile, “High performance Solid Acid Fuel Cells through humidity
Chisholm, L.A. Cowan, S.M. Haile and W.T. Klooster, Synthesis,
Structure and Properties of Compounds in the NaHSO4-CsHSO4
System. 1. Crystal Structures of Cs2Na(HSO4)3
and CsNa2H(SO4)3, Chem. Mater.
13 (2001) 2574-2583.
Chisholm, L.A. Cowan and S.M. Haile, Synthesis, Structure and
Properties of Compounds in the NaHSO4-CsHSO4 System.
2. The Absence of Superprotonic Transitions in Cs2Na(HSO4)3
and CsNa2H(SO4)3, Chem. Mater.
13 (2001) 2909-2912.
D.A. Boysen, C.R.I. Chisholm and R.B. Merle, Solid Acid as Fuel
Cell Electrolytes, Nature
410 (2001) 910-913.
C.R.I. Chisholm, S.M. Haile and S.R. Narayanan, Polymer Solid
Acid Composite Membranes for Fuel Cell Applications, J. Electrochem.
Soc. 147 (2000) 3610-3614.
Chisholm and S.M. Haile, "Superprotonic Behavior of Cs2(HSO4)(H2PO4)
- a New Solid Acid in the CsHSO4-CsH2PO4
System," Solid State Ionics 136-137 (2000) 229-241.
Chisholm and S.M. Haile, "Structure and Thermal Behavior of the
New Superprotonic Conductor Cs2(HSO4)(H2PO4),"
Acta Cryst. B55 (1999) 937-946.
and W.T. Klooster, "Single-Crystal Neutron Diffraction Study of
(x ~ 0.5) at 15 K," Acta Cryst. B55 (1999) 285-296.
and P. Calkins, "X-ray diffraction study of Cs5(HSO4)3(H2PO4)2,
a New Solid Acid with a Unique Hydrogen-Bond Network," J. Sol.
State Chem. 140 (1998) 251-265.
P. Calkins and D. Boysen, "The structure and vibrational spectrum
(x ~ 0.5), a new superprotonic conductor, and a comparison with
J. Sol. State Chem. 139 (1998) 373-387.
P. Calkins and D. Boysen, "Superprotonic Conductivity in b-Cs3(HSO4)2(Hx(P,S)O4),"
Solid State Ionics 97 (1997) 145-151.
K.-D. Kreuer and J. Maier, "The Structure of Cs3H4(SO4)2(PO4)
-- a New Compound with a Superprotonic Transition," Acta Cryst
B51 (1995) 680-687.
G. Lentz, K.-D. Kreuer and J. Maier, "Superprotonic Conductivity
Solid State Ionics 77 (1995) 128-134.