Electrical conductivity and thermal stability of (1-x)CsH 2PO4/xSiPyOz (x = 0.2-0.7) composites

Article abstract of DOI:10.1134/S0020168508090185

The physicochemical properties of (1 - x)CsH₂PO₄/xSiP _y O _z (x = 0.2-0.7) composites containing fine-particle silicon phosphates as heterogeneous additives have been studied at different humidities. The introduc

Full text of DOI:10.1134/S0020168508090185

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 9, pp. 1009–1014. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.G. Ponomareva, E.S. Shutova, G.V. Lavrova, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 9, pp. 1131–1136.
Electrical Conductivity and Thermal Stability
of (1 – x)CsH2PO4/xSiPyOz (x = 0.2–0.7) Composites
V. G. Ponomareva, E. S. Shutova, and G. V. Lavrova
Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
e-mail: ponomareva@solid.nsc.ru
Received October 22, 2007; in final form, December 3, 2007
Abstract—The physicochemical properties of (1 – ı)CsH2PO4/ıSiPyOz (ı= 0.2–0.7) composites containing fine-
particle silicon phosphates as heterogeneous additives have been studied at different humidities. The introduc-
tion of silicon phosphates suppresses the superionic phase transition of CsH2PO4 and increases the low-temper-
ature conductivity of the materials, which depends significantly on humidity. The CsH2PO4–SiPyOz materials
offer high conductivity (~3 × 10–3 to 10–2 S/cm at ~110–230°C) at low water vapor pressures (3 mol % ç2é).
Amorphization of the CsH2PO4 in the composites markedly changes its thermodynamic properties. The effect
of long-term isothermal holding (210°C, 3 mol % ç2é) on the conductivity of the composites has been studied.
DOI: 10.1134/S0020168508090185
INTRODUCTION
ume fraction of the high-conductivity layer is close to
unity, and it determines the physicochemical properties
of the material. This leads to the development of meta-
stable states, including partial or complete amorphiza-
tion of the salt [8, 9].
CsH2PO4 belongs to the family of proton conductors
having a dynamically disordered hydrogen-bond net-
work in a superionic phase [1–4]. In this family,
CsH2PO4 is one of the best conductors, with a conduc-
tivity (σ > 2 × 10–2 S/cm at t ≥ 230°C) comparable to
that of melts. In addition, it has a high melting point
(tm ~345°C). Below the phase transition, its conductiv-
ity is low (σ < 10–6 S/cm) [1]. In a dry atmosphere, the
superionic phase transition is accompanied by dehydra-
tion, and the conductivity of CsH2PO4 drops by several
orders of magnitude. At an increased water vapor pres-
sure, the salt is thermodynamically stable [5]. However,
at high humidity its mechanical properties are not good
enough for practical application.
In recent years, considerable research effort has
been focused on medium-temperature composites of
CsH2PO4 and various additives, such as SiO2 and SiP2O7
[7, 10–13]. Otomo et al. [7] used hydrophilic and
hydrophobic silicas with different specific surface areas
as heterogeneous additives. According to their results,
the highest conductivity is offered by systems contain-
ing hydrophilic silica in a 30 mol % H2O + argon atmo-
sphere. Their low-temperature conductivity, however,
exceeds that of the salt by an order of magnitude, and
their thermal stability is not high enough for practical
application. At the same time, recent work [10] has
shown that, in (1 – ı)CsH2PO4/ıSiO2 composites con-
taining silica particles with different morphologies and
specific surface areas, there is strong surface interaction
with the matrix even at ı = 0.4–0.5, which leads to par-
tial dehydration of the salt, the formation of a mixture
of cesium hydrogen and cesium dihydrogen pyrophos-
phates, and, as a consequence, low conductivity.
The ability to modify the properties of CsH2PO4 and
to synthesize proton-conducting electrolytes with
enhanced mechanical strength and high conductivity at
medium temperatures (up to the phase transition) is not
only of scientific interest but also of technological
importance because CsH2PO4 is a potential membrane
material for medium-temperature fuel cells [6].
It is well known that heterogeneous doping of a
number of acid salts with fine nonconducting oxide par-
ticles offers the possibility of producing, in a wide tem-
perature range, high-conductivity materials with
enhanced thermal stability and improved mechanical
performance owing to the properties of the oxide
matrix [7–9]. The salts forming on the surface of the
oxide nanoparticles are defect-rich and differ in proper-
ties from bulk materials. At a sufficiently high doping
level and large specific surface of the dopant, the vol-
Acid centers on the oxide surface were shown to
play an important role in the formation of high-conduc-
tivity composite systems [10, 11]. Indeed, modifying
the silica surface with phosphoric acid enabled the fab-
rication of composites with a proton conductivity of
~10–3 to 10–2 S/cm in the range ~130–230°ë (ı = 0.3) at
lower water vapor partial pressures in comparison with
the pure salt (~4–5 mol % H2O) [10, 11].
1009

1010
PONOMAREVA et al.
influence of humidity on their transport properties and
thermal stability are still not thoroughly understood.
*
*
The purpose of this work was to study the effect
of humidity on the physicochemical properties of
(1 – ı)CsH2PO4/ıSiPyOz (ı = 0.2–0.7) composites con-
taining fine-particle silicon phosphates as heteroge-
neous additives.
*
*
*
*
*
*
EXPERIMENTAL
4
3
CsH2PO4 was synthesized in a dilute aqueous solu-
tion containing cesium carbonate and phosphoric acid
in the ratio 1 : 2. The crystals were washed with acetone
and then calcined at 150°C for 2 h to remove the resid-
ual water. The x-ray diffraction (XRD) data for the syn-
thesized salt were in good agreement with earlier
results [3, 15].
The fine-particle component based on silicon phos-
phate was prepared by heat-treating silica (specific sur-
face S ~98 m2/g) with phosphoric acid in several steps.
A 1 : 1 mixture of SiO2 and ç3êé4 was first calcined at
120–220°C for two days and then heat-treated for 2 h at
300, 450, and 700°ë. The phase composition of the
reaction products was monitored using XRD. The spe-
cific surface area of the silicon phosphate was deter-
mined by temperature-programmed argon desorption
measurements (S = 11.3 m2/g).
2
1
10
20
30
40
50
60
2θ, deg
Composites were synthesized from appropriate
starting mixtures thoroughly ground in a ceramic mor-
tar and pressed into pellets (d = 0.6 cm, l = 0.15–0.3 cm)
at 300–500 MPa. The pellets were calcined in air or
in a closed space between 220 and 250°C at a con-
trolled content of water vapor generated by bubbling
air through a water layer. The density of the
(1 − ı)CsH2PO4/ıSiPyOz samples with ı = 0.2–0.67 was
Fig. 1. XRD patterns of the compound obtained by reacting
silica with phosphoric acid at (1) 200, (2) 300, (3) 450, and
(4) 700°C for (1) 3 and (2–4) 2 h.
Heating H3PO4-treated silica leads to the formation
of SiP2O7-type matrices. Matsui et al. [12, 13] investi-
gated (1 − x)CsH2PO4/xSiP2O7 composites containing
different amounts of the heterogeneous additive (ı =
0.33–0.83). They observed a rise in conductivity and
disappearance of the phase transition in these systems.
At temperatures from 100 to 270°C, the composites had
high conductivity (σmax = ~6.6 × 10–2 S/cm (ı = 0.3–
0.5) at 270°ë and 30 mol % H2O in argon), exceeding
that of CsH2PO4. According to Matsui et al. [12, 13], the
high conductivity of the composites is due to the forma-
2.5−2.8 g/cm3. In evaluating the mole fraction, the
molecular weight of the forming SiPyOz mixture was
taken to be 192 (SiP2O7).
The ac conductivity of samples sandwiched
between silver or platinum electrodes was measured
with an Instek LCR-821 precision LCR meter and Elins
Z-350 impedance meter at frequencies from 5 Hz to
500 kHz. The measurements were made during cooling
at a rate of 2–3°C/min at temperatures from 20 to
tion of interfacial CsH5(PO4)2 through the reaction 250°C and different humidities.
between the salt and SiP2O7 at temperatures above
220°C [13]. It is, however, known that, under normal
conditions, the conductivity of CsH5(PO4)2 crystals is
XRD patterns were collected on a DRON-3 powder
diffractometer (ëuKα1 radiation, continuous scan rate
of 2°/min) at room temperature. The materials were
also characterized by differential scanning calorimetry
(DSC) in argon between 20 and 400°C at a heating rate
of 10°C/min, using a Netzsch Jupiter STA 449C high-
resolution system.
well below the above values, no higher than ~10–3 S/cm
at 135°ë. At temperatures above tm ~ 151°ë,
CsH5(PO4)2 decomposes rather slowly [14]. It seems
likely that the formation of CsH5(PO4)2 in the compos-
ites in question may lead to slow degradation of the
electrolyte at high working temperatures (~270°C), in
particular at increased humidity. The formation mecha-
nism and state of CsH5(PO4)2 in composites and the
RESULTS AND DISCUSSION
Figure 1 shows XRD patterns of the compounds
obtained by reacting silica with phosphoric acid at dif-
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ELECTRICAL CONDUCTIVITY AND THERMAL STABILITY
1011
logσ [S/Òm]
logσ [S/Òm]
–1
–1
1
2
3
4
5
1
2
3
4
5
–2
–2
–3
–4
–5
–6
–7
–8
–3
–4
–5
–6
–7
–8
1.8
2.0 2.2
2.4 2.6
2.8
3.0
103/T, K–1
1.8
2.0
2.2
2.4
2.6
2.8
3.0
103/T, K–1
Fig. 3. Arrhenius plots of conductivity for (1
–
Fig. 2. Arrhenius plots of conductivity for (1) CsH PO and
ı)CsH PO /ıSiP O composites with ı = (1) 0, (2) 0.33,
2
4
2
4
y z
(2–5) (1 – ı)CsH PO /ıSiP O composites: ı = (2) 0.2,
(3) 0.5, and (4, 5) 0.67; (5) data from [13]; cooling rate,
2°C/min; 3 mol % water vapor in air.
2
4
y z
(3) 0.33, (4) 0.5, (5) 0.67; cooling in air at 2°C/min.
ferent temperatures. It can be seen that raising the tem- silicon phosphates (ı = 0.2) increases the proton con-
perature to 700°C changes the phase composition of the ductivity in the temperature range 20–230°C by three
mixture and improves its crystallinity. Note that, at orders of magnitude and suppresses the phase transi-
lower reaction temperatures (300°C), the XRD pattern tion. The high-temperature conductivity of the ı = 0.33
is more similar to that of SiP2O7 [16]. At higher temper- composite is close to that of the parent salt. Increasing
the additive content to ı = 0.5 reduces the high-temper-
ature conductivity by a factor of 3 and the low-temper-
ature conductivity by about a factor of 0.25 in compar-
ison with x = 0.33, but the conductivity remains higher
than that of the parent salt. Increasing x to 0.67 reduces
the conductivity of the composite by one order of mag-
nitude.
Measurements in the presence of water vapor (Fig. 3)
demonstrate that the conductivity of the composites
depends significantly on humidity. At 3 mol % water
vapor, the conductivity of the composites increases sig-
nificantly with temperature up to 230°ë and exceeds the
σ of the parent salt by three to four orders of magnitude.
The high-temperature conductivity is close to that of
cesium dihydrogen phosphate. The conductivity of the
composites richer in pyrophosphates (ı = 0.67) is more
sensitive to humidity. Systems differing in composition
atures (700°C), the phase composition of the matrix
corresponds to a mixture of silicon phosphates
(Si3(PO4)4 and Si5P6O25) and SiO2 (cristobalite) [16–18]
(the new reflections in Fig. 1 are marked by asterisks).
During storage, the matrix structure experiences partial
amorphization, probably because of the adsorption of
atmospheric moisture. The transport properties of the
composites with this matrix were studied at different
humidities.
Figure 2 shows the Arrhenius plots of conductivity
for (1 – ı)CsH2PO4/ıSiPyOz (ı = 0.2–0.67) composites
and the parent salt at a relative air humidity of 30%
(0.6 mol % ç2é). The high-temperature conductivity
of CsH2PO4 reaches σ ~3 × 10–2 S/cm, with an activation
energy Ö‡ ~ 0.4 eV. The transition to the low-tempera-
ture phase (Ea ~ 0.8 eV) is accompanied by a drop in
conductivity by more than three orders of magnitude, in become comparable in conductivity and its temperature
agreement with earlier results [1, 5, 10]. The addition of variation: the composites with ı = 0.3 and 0.5 are close
INORGANIC MATERIALS Vol. 44 No. 9 2008

1012
PONOMAREVA et al.
The XRD patterns of our composites (Fig. 4) attest
to the formation of an amorphous phase at ı = 0.33. At
higher additive contents, the XRD patterns show reflec-
tions characteristic of silicon phosphates with a higher
degree of crystallinity. The reflections from unreacted
materials indicate that no reaction occurs between the
salt and matrix under the conditions of this study;
CsH5(PO4)2 was not detected.
Figure 5 presents the DSC data for the composites,
CsH2PO4, and the silicon phosphate matrix. The curve
for CsH2PO4 shows endothermic peaks at 230, 275, and
318°ë, corresponding to the superionic phase transition
and the formation of di-, tri, or polymers upon partial
dehydration, and the melting endotherm at 345°C. The
total weight loss on heating to 400°C is 7.6%, which is
close to the theoretical one (removal of one water mol-
ecule per formula unit). In the DSC curves of the com-
posites with ı = 0.33 and 0.5, the endothermic peaks
due to the phase transition are almost indiscernible,
which is associated with the amorphization of the salt
in the composites and correlates with the above XRD
data. The curve of the ı = 0.67 composite shows a weak
endothermic peak at 131.3°C, presumably due to the
presence of a small amount of CsH5(PO4)2. The silicon
phosphate matrix shows major weight changes at tem-
peratures below ~210°ë. The present DSC and TG data
attest to significant changes in the thermodynamic
properties of CsH2PO4, amorphization of the salt in the
composites, and a slightly higher thermal stability of
the composites in comparison with the parent salt and
matrix at medium temperatures. The composites with
ı = 0.3 and 0.5 are more thermally stable than the ı =
0.67 material.
1
2
3
4
5
10
20
30
40
50
60
2θ, deg
Fig. 4. XRD patterns of (1) CsH PO , (2–4) (1 – ı)
CsH PO /ıSiP O composites, and (5) SiP O : x = (2) 0.33,
2
4
2
4
y
z
y z
It is of interest to assess how the thermal stability of the
composites and their conductivity at elevated temperatures
respond to long-term exposures under different conditions.
Heat treatment of the (1 – ı)CsH2PO4/ıSiPyOz (ı = 0.3)
composite at 210°C (3 mol % H2O in air) for 6 h was
found to have no effect on its conductivity and to pro-
duce little or no weight change, as determined by TG in
flowing argon.
(3) 0.5, (4) 0.67.
in conductivity and activation energy. The conductivity
as a function of temperature shows non-Arrhenius
behavior, and the activation energy gradually increases
with decreasing temperature: from 0.4 (0.38) eV at high
temperatures to 0.56 eV at low temperatures (ı = 0.3
and 0.5, respectively). The most likely reason for this
behavior of the system is that conduction occurs pre-
Thus, detailed studies of the CsH2PO4-based com-
posites demonstrate that their physicochemical proper-
dominantly along the interfaces between the salt and ties depend significantly on the nature of the heteroge-
neous additive, the acidic properties of its surface, and
air humidity. The high conductivity of the CsH2PO4–
SiPyOz composites is due to the presence of water
adsorbed on the surface of the silicon phosphates. Sys-
tems of this type have high conductivity only after
exposure to a humid atmosphere.
heterogeneous component, and that the proton trans-
port involves the acid centers produced by water
adsorption on the grain boundaries of the silicon phos-
phates and on the interfaces with the salt.
For comparison, Figure 3 presents the conductivity
data for CsH2PO4/SiP2O7 in argon containing 30 mol %
water vapor [13]. The conductivity of our composites at
3 mol % water vapor is seen to be slightly lower than
but comparable to that at high humidity. This result is
of practical importance because the mechanical proper-
The mechanical properties of the composites
depend significantly on their composites and air humid-
ity. For example, at 30 mol % H2O in air, composites based
on acid salts are easy to deform, whereas at 3 mol % H2O
such composites retain their strength. Note that the ı =
ties of the systems under consideration are consider- 0.67 composite has poor performance even at 3 mol %
H2O. Therefore, the compositions of composites based
ably more suitable for technological applications.
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ELECTRICAL CONDUCTIVITY AND THERMAL STABILITY
1013
∆Q
(‡)
∆m, %
(b)
1
100
3
2
98
96
94
92
5
4
2
3
4
5
1
50 100 150 200 250 300 350 400
150
200
250
300
350
400
t, °C
t, °C
Fig. 5. (a) DSC and (b) TG data for (1) CsH PO , (2–4) (1 – ı)CsH PO /ıSiP O composites, and (5) SiP O : x = (2) 0.33, (3) 0.5,
2
4
2
4
y
z
y z
(4) 0.67.
on cesium dihydrogen phosphate should be optimized
so that their high proton conductivity be combined with
good mechanical properties. It seems likely that, with
these two factors, the (1 – ı)CsH2PO4/ıSiPyOz compos-
ites with ı = 0.3–0.5 are optimal.
ACKNOWLEDGMENTS
This work was supported by the Federal Agency for
Science and Innovations (federal targeted science and
technology program Research and Development in the
Priority Areas of the Science and Technology Sector of
Russia in 2007–2012, state contract no. 02.513.11.3288).
CONCLUSIONS
REFERENCES
We synthesized (1 – ı)CsH2PO4/ıSiPyOz composites
and investigated their physicochemical properties in air
containing 0.6–3 mol % water vapor.
1. Baranov, A.I., Khiznichenko, V.P., Sandler, V.A., and
Shuvalov, L.A., Frequency Dielectric Dispersion in the
Ferroelectric and Superionic Phases of CsH2PO4, Ferro-
electrics, 1988, vol. 81, pp. 183–186.
The results demonstrate that the chemical nature of
the heterogeneous component has a significant effect
on the electrical transport in the composites. Compos-
ites based on silicon phosphates of different composi-
tions have high conductivity (~3 × 10–3 to 10–2 S/cm at
~110–230°C) and thermal stability (210°C) even at low
water vapor partial pressures, which is of importance
for improving their mechanical performance.
2. Romain, F. and Novak, A., Raman-Study of the High-
Temperature Phase-Transition in CsH2PO4, J. Mol.
Struct., 1991, vol. 263, pp. 69–74.
3. Bronowska, W., Comment on “Does the Structural Supe-
rionic Phase Transition at 231 Degrees C in CsH2PO4
Really Not Exist?” J. Chem. Phys., 2001, vol. 114, no. 1,
pp. 611–612.
4. Colomban, Ph. and Novak, A., Anhydrous Materials:
Oxonium Perchlorate, Acid Phosphates, Arsenates, Sul-
phates, and Selenates, Chemistry of Solid State Materi-
als, vol. 2: Proton Conductors, Colomban, Ph., Ed.,
Cambridge: Cambridge Univ. Press, 1992, pp. 165–182.
The thermodynamic and structural properties of the
CsH2PO4 in the composites studied differ significantly
from those of the individual salt because of the struc-
tural disordering and amorphization of the salt, but
there is no chemical interaction between the salt and
matrix.
5. Boysen, D.A. and Haile, S.M., High Temperature
Behavior of CsH2PO4 under Both Ambient and High
Pressure Conditions, Chem. Mater., 2003, vol. 15,
pp. 727–736.
INORGANIC MATERIALS Vol. 44 No. 9 2008

1014
PONOMAREVA et al.
6. Uda, T. and Haile, S.M., Thin-Membrane Solid-Acid 12. Matsui, T., Kukino, T., Kikuchi, R., and Eguchi, K., An
Fuel Cell, Electrochem. Solid-State Lett., 2005, vol. 8,
Intermediate Temperature Proton-Conducting Electro-
lyte Based on a CsH2PO4/SiP2O7 Composite, Electro-
chem. Solid-State Lett., 2005, vol. 8, no. 5, pp. A256–
A258.
no. 5, pp. A245–A246.
7. Otomo, J., Minagawa, N., Wen, C., et al., Protonic Con-
duction of CsH2PO4 and Its Composite with Silica in
Dry and Humid Atmospheres, Solid State Ionics, 2003,
vol. 156, pp. 357–369.
8. Ponomareva, V.G., Lavrova, G.V., and Simonova, L.G.,
The Influence of Heterogeneous Dopant Porous Struc-
ture on the Properties of Protonic Solid Electrolyte in the
CsHSO4–SiO2 System, Solid State Ionics, 1999,
vol. 118, pp. 317–323.
9. Ponomareva, V.G. and Lavrova, G.V., The Investigation
of Disordered Phases in Nanocomposite Proton Electro-
lytes Based on MeHSO4 (Me = Rb, Cs, K), Solid State
Ionics, 2001, vol. 145, pp. 197–204.
10. Ponomareva, V.G. and Shutova, E.S., High-Temperature
Behavior of CsH2PO4 and CsH2PO4–SiO2 Composites,
Solid State Ionics, 2007, vol. 178, pp. 729–734.
13. Matsui, T., Kukino, T., Kikuchi, R., and Eguchi, K.,
Intermediate-Temperature Fuel Cell Employing
CsH2PO4/SiP2O7-Based Composite Electrolytes, J.
Electrochem. Soc., 2006, vol. 153, pp. A339–A342.
14. Lavrova, G.V., Burgina, E.B., Matvienko, A.A., and
Ponomareva, V.G., Bulk and Surface Properties of Ionic
Salt CsH5(PO4)2, Solid State Ionics, 2006, vol. 177,
pp. 1117–1122.
15. Nirsha, B.M., Gudinitsa, E.N., Fakeev, A.A., et al., Ther-
mal Dehydration of CsH2PO4, Zh. Neorg. Khim., 1982,
vol. 27, no. 6, pp. 1366–1369.
16. Powder Diffraction File, Swarthmore: Joint Committee
on Powder Diffraction Standards, card 18-1165.
17. Powder Diffraction File, Swarthmore: Joint Committee
11. Ponomareva, V.G. and Shutova, E.S., Medium-Tempera-
ture Proton Conductors Based on CsH2PO4 and Modi-
on Powder Diffraction Standards, card 18-1167.
fied Silica, Elektrokhimiya, 2007, vol. 43, no. 5, pp. 521– 18. Powder Diffraction File, Swarthmore: Joint Committee
527.
on Powder Diffraction Standards, card 40-457.
INORGANIC MATERIALS Vol. 44 No. 9 2008