Solid Acid Sulfates, Phosphates and Selenates
Solid 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.
The very 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.
We demonstrated 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 operation [ScienceExpress (2003)]. Furthermore, we have advanced the fuel cell fabrication process so as to be able to obtain power densities of over 400 mW/cm2.
For commentary 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.
In contrast 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.
Mixed sulfate-phosphates
To date, we have discovered four intermediary phases, a-Cs3(HSO4)2(H2PO4), b-Cs3(HSO4)2[H2-x(P1-x,Sx)O4], 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 behavior.
Relevant 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.
Table I. A comparison of selected structural features of xCsHSO4•(1-x)CsH2PO4 compounds.
Compound space group H:XO4 H bonds
per XO4dimensionality H-bond types source CsHSO4 (phase II) P21/c 1 SO4 - 2 1-d chain ordered Belushkin, et al., Acta Cryst. B47, 161 (1991). b-Cs3(HSO4)2
[H2-x(SxP1-x)O4]C2/c 1.17 (P,S)O4 - 4
SO4 ~ 1.53-d structure ordered + disordered + vacancies this work a-Cs3(HSO4)2(H2PO4) P21/n 1.333 PO4 - 4
S(1)O4 -2
S(2)O4 - 23-d structure ordered + disordered this work Cs5(HSO4)3(H2PO4)2 C2/c 1.4 [P,S](1)O4 -4 [P,S](2)O4 -4 S(3)O4 - 1 2-d layer with some thickness ordered + disordered this work e-Cs2(HSO4)(H2PO4) P21/n 1.5 (S,P)O4 - 3 2-d layer ordered + disordered this work CsH2PO4 P21/m 2 PO4 - 4 2-d layer ordered + disordered Nelmes & Choudhary, Solid State Comm. 26, 823 (1978).
The room-temperature 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:
Figure 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.
The superprotonic 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 transitions.
Figure 2. The conductivities of solid acids in the CsHSO4-CsH2PO4 system.
Observation of the superprotonic transition in CsH2PO4
Through the use of (i) hydrostatic pressure or (ii) water partial pressure it is possible to suppress the dehydration reaction of CsH2PO4
CsH2PO4 -> CsPO3 + H2O
and observe 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.
Superprotonic 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!
Figure 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.
People
Current: Lisa Cowan, Mikhail Kislitsyn, Kenji Sasaki
Former: Dane Boysen (currently Visiting Associate), Calum R.I. Chisholm (currently Visiting Associate), Tetsuya Uda
Acknowledgments ($$$)
Office of Naval Research; National Science Foundation; California Energy Commission, California Institute of Technology
Selected Publications
D.A. Boysen, T. Uda, C.R.I. Chisholm and S.M. Haile, “High performance Solid Acid Fuel Cells through humidity stabilization,” ScienceExpress (2003).
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. 1. Crystal Structures of Cs2Na(HSO4)3 and CsNa2H(SO4)3,” Chem. Mater. 13 (2001) 2574-2583.
C.R.I. 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.
S.M. Haile, D.A. Boysen, C.R.I. Chisholm and R.B. Merle, “Solid Acid as Fuel Cell Electrolytes,” Nature 410 (2001) 910-913.
D.A. Boysen, 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.
C.R.I. 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.
C.R.I. Chisholm and S.M. Haile, "Structure and Thermal Behavior of the New Superprotonic Conductor Cs2(HSO4)(H2PO4)," Acta Cryst. B55 (1999) 937-946.
S.M. Haile and W.T. Klooster, "Single-Crystal Neutron Diffraction Study of b-Cs3(HSO4)2[H2-x(Sx,P1-x)O4] (x ~ 0.5) at 15 K," Acta Cryst. B55 (1999) 285-296.
S.M. Haile 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.
S.M. Haile, P. Calkins and D. Boysen, "The structure and vibrational spectrum of b-Cs3(HSO4)2(H2-x(P1-x,Sx)O4) (x ~ 0.5), a new superprotonic conductor, and a comparison with a-Cs3(HSO4)2(H2PO4)," J. Sol. State Chem. 139 (1998) 373-387.
S.M. Haile, P. Calkins and D. Boysen, "Superprotonic Conductivity in b-Cs3(HSO4)2(Hx(P,S)O4)," Solid State Ionics 97 (1997) 145-151.
S.M. Haile, 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.
S.M. Haile, G. Lentz, K.-D. Kreuer and J. Maier, "Superprotonic Conductivity in Cs3H4(SO4)2(PO4)," Solid State Ionics 77 (1995) 128-134.
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