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SiO2 composites
Electrochimica Acta 53 (2008) 8186–8195
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Phase transition and proton transport characteristics in CsH2 PO4 /SiO2 composites Junichiro Otomo a,∗ , Takashi Ishigooka b , Tomoyuki Kitano b , Hiroshi Takahashi b , Hidetoshi Nagamoto b a b
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakanomachi, Hachioji-shi, Tokyo 192-0015, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2008 Received in revised form 7 June 2008 Accepted 12 June 2008 Available online 21 June 2008 Keywords: Cesium dihydrogen phosphate Proton conductor Heterogeneous composite Phase transition Thermal hysteresis
a b s t r a c t The stabilization of superprotonic phase in neat CsH2 PO4 and CsH2 PO4 /SiO2 composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH2 PO4 at around Tc = 230 ◦ C was confirmed under humidified and sealed conditions. In CsH2 PO4 /SiO2 composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH2 PO4 and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Cesium dihydrogen phosphate, CsH2 PO4 , abbreviated as CDP hereafter, is a highly proton-conducting solid electrolyte, which is classified as a hydrogen-bonded oxyacid salt crystal. CDP exhibits high proton conductivity (>10−2 S cm−1 ) in a superprotonic phase at a temperature above ca. 230 ◦ C [1–5]. The superprotonic phase transition in CDP takes place at ca. 230 ◦ C, being accompanied with a structural transformation from a monoclinic phase to a cubic phase [2–5]. Under a dry condition, however, dehydration of CDP (Eq. (1)) proceeds to form polymerization products at temperatures above 200 ◦ C [6–9]: CsH2 PO4 → Cs2 H2 P2 O7 → CsPO3
(1)
Since the reaction occurs in parallel with a transition to the superprotonic phase, the nature of CDP has been much discussed, from both the viewpoints of the formation of the superprotonic phase and dehydration, over the last decade [3–11]. The stabilization of the superprotonic phase in CDP without any decomposition was demonstrated under appropriate humid conditions in terms of crystallographic and electrochemical studies [3,4]. Recently, a stable fuel cell-operation using CDP as an electrolyte membrane has been demonstrated successfully in a humid atmosphere [12]. CDP
∗ Corresponding author. Tel.: +81 4 7136 4714; fax: +81 4 7136 4715. E-mail address: [email protected] (J. Otomo). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.018
has therefore attracted interest not only in fundamental aspects but also in its application to energy devices such as fuel cells [11,13]. The current status thus motivates us to further investigate specific properties in CDP. As for cesium hydrogen sulfate, CsHSO4 , its transition to a superprotonic phase has been extensively investigated [14–24]. The superprotonic phase transition of CsHSO4 occurs with a structural transformation from a monoclinic phase to a tetragonal phase. In the superprotonic phase, it is considered that the reorientational motion of a SO4 tetrahedron, which induces a disordering of the hydrogen bond network, facilitates proton transfer [18,19]. Similar mechanism of proton conduction is prospective in CDP. In this study, we confirmed phase transition phenomena in CDP without decomposition through the measurements of conductivity, Raman spectroscopy and thermal analysis. On the other hand, heterogeneous doping of ionic salts with highly dispersed oxides is an effective approach to improve ionic conductivity in low and intermediate temperature regions [25–28]. Heterogeneous composites that consist of proton-conducting oxyacid salts with mesoporous silica glass particles also exhibit the enhancement of conductivity [4,29–33]. In previous work [4], we have observed the enhancement of proton conductivity in a CDP/SiO2 composite prepared by a mechanical mixing method, but the observation of conductivity was limited in the measurement for a heating process. In this study, we employed an evaporationto-dryness method to prepare CDP/SiO2 composites with high dispersion of mesoporous silica grass particles [31]. Through the electrochemical measurements, we found a large asymmetric ther-
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mal hysteresis loop of conductivity in a heating–cooling cycle. New phases of ionic salts have been found in heterogeneous composite systems. In the present case of CDP/SiO2 composites, the interfacial effect on phase stability and proton conductivity, which will be mainly induced by shear elastic forces at the interface between CDP and silica particles, is discussed. A large thermal hysteresis in a phase transition has been reported in an Ag+ conducting composite system such as AgI/Al2 O3 composite, which originates from an interfacial interaction [34]. To our knowledge, however, the present study is the first case demonstrating explicitly a cooperative phenomenon for proton conductivity in an oxyacid salt/insulator heterogeneous composite with the large asymmetric thermal hysteresis of phase transformation. Main concern in this study is thus the investigation of phase transition behavior in CDP/SiO2 composites in terms of proton conductivity and structural phase transformation, based on the observation of phase transition in neat CDP. The results will provide new insight into fundamental aspects of phase transformation and the stabilization of a superprotonic phase in the oxyacid salt/insulator heterogeneous composite even below critical temperature. 2. Experimental 2.1. Sample preparations Polycrystalline CsH2 PO4 (CDP) was used as received (purity > 99%; Soekawa Chemical Co. Ltd., Japan). Powdered CDP sample was prepared by grinding in an agate mortar for 30 min. An XRD measurement showed all the detected peaks originated from CDP and no contaminants were observed. The powdered CDP sample was then pressed at 3 t cm−2 to form dense pellets (diameter: 13 mm; thickness: 1.3–1.7 mm). The resultant pellet of neat CDP was set in a glass cell, and calcined at 260 ◦ C for 1 h. The relative density of the neat CDP pellet was over 0.94 after having calcined. CDP/SiO2 composite was prepared by an evaporation-todryness method as described below [31]. Mesoporous silica glass powder provided by Mitsubishi Chemical Corporation was used as received. The properties of the silica particle are described as follows: average particle size, 5.1 m; specific surface area, 297 m2 g−1 ; average pore volume, 1.3 ml g−1 ; and average pore size, 16 nm. First, the mesoporous silica glass particles were introduced into a CDP aqueous solution to prepare an aqueous suspension. The suspension was then stirred constantly for 60 min under a reduced pressure condition in order to remove the air in the mesopores of the silica particles and to allow the solution to easily infiltrate into the mesopores. The suspension was evaporated at 50 ◦ C, and then dried at 90 ◦ C for 24 h. After additional heating at 150 ◦ C for 1 h in air to minimize the influence of adsorbed water, the resultant sample was mechanically ground for 10 min, and pressed at 3 t cm−2 to form pellets. Finally, the pellet of CDP/SiO2 composite was calcined at 260 ◦ C for 1 h in a glass cell as in the case of neat CDP. In this manner, (1 − x)CDP/xSiO2 composites were prepared with a variation in the volume fraction of SiO2 , x (=0–1).
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ply of water vapor is required to inhibit dehydration of CDP [4,11]. Conductivity measurements were performed by means of ac impedance spectroscopy using a Hewlett Packard 4192A impedance analyzer in the frequency range from 10 MHz to 10 Hz with an applied voltage of 50 mV or 500 mV. The conductivity as a function of temperature was measured in a cooling–heating cycle; after having maintained the samples at 250 ◦ C, the temperature was decreased stepwise in the cooling process from 250 ◦ C to 150 ◦ C, and then the temperature was increased stepwise in the heating process from 150 ◦ C to 250 ◦ C. The samples were kept for over 30 min at each temperature so they could stabilize before the ac impedance measurement was carried out. 2.3. Characterizations Phase transformation in neat CPD was observed by means of in situ Raman spectroscopy. The pelletized sample of neat CDP was set in a sealed glass cell. The cell was introduced into a furnace in order to observe the in situ Raman spectrum as a function of temperature that was changed stepwise from room temperature to 260 ◦ C. After the sample was maintained for 10 min at each temperature, a spectroscopic measurement was performed to collect data in equilibrium. A continuous wave Ar+ ion laser beam at 488 nm and 0.5 W was used as the exciting line, which was focused onto the sample through a glass window using a 2.0 cm focal length lens. The backscattered light was led inversely through the lens to a spectrometer (JASCO NRS-2000, Japan) and detected by a CCD detector. In this manner, we obtained Raman spectroscopic data on neat CDP without dehydration at temperatures above 200 ◦ C. Differential thermal analysis (DTA) of powdered samples of neat CDP and CDP/SiO2 composites was performed both in the
2.2. Proton conductivity measurements Silver electrodes were applied on both sides of the obtained pellets using silver paste after silver vapor deposition. The pellet samples were set in a glass tube, and then gaseous mixtures of Ar/H2 O (Ar:H2 O = 0.7:0.3; partial pressure of water vapor: 0.03 MPa) were supplied through the glass tube at atmospheric pressure. Because the dehydration of CDP (Eq. (1)) occurs at a temperature higher than 200 ◦ C under a dry condition, the sup-
Fig. 1. Typical impedance spectra of CDP/SiO2 composite (silica volume fraction: 0.28 vol.%).
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Fig. 2. Temperature dependence of conductivity in CDP/SiO2 composites with a variety of silica volume fractions (x = 0–60 vol.%). Open circles: cooling process; filled circles: heating process. (a) x = 0 vol.% (neat CDP), (b) x = 10 vol.%, (c) x = 16 vol.%, (d) x = 22 vol.%, (e) x = 28 vol.%, (f) x = 37 vol.%, (g) x = 45 vol.% and (h) x = 60 vol.%.
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heating process and cooling process between 100 ◦ C and 270 ◦ C (Rigaku TAS200, Japan). The heating and cooling rates were fixed at 2 K min−1 . Powdered samples were sealed in a steel capsule with an aluminum cover (withstand internal pressure: 5 MPa) in order to inhibit dehydration of CDP. All sealed samples were measured after having been annealed at 270 ◦ C. In this manner, we successfully measured heat flow accompanying the phase transformation of CDP, as is stated later. In addition to the above, the X-ray diffraction (XRD) patterns of powdered samples of neat CDP and CDP/SiO2 composites were recorded at room temperature (Rigaku Rint 2500VHF, Japan) using a Cu K␣ source. 3. Results and discussion 3.1. Proton conductivity Fig. 1 shows typical ac impedance spectra of a CDP/SiO2 composite. In a low-temperature region, semicircles were observed (Fig. 1a). These semicircles, however, disappeared gradually with an increase in temperature and were no longer seen in a high temperature region (>230 ◦ C), as shown in Fig. 1b, because the proton conduction process occurs too quickly to be observed with the present impedance analyzer. Instead of the semicircle, capacitancelike behavior was observed in the high temperature region, i.e. only linear impedance spectra were seen. The values of the specific resistance of proton conduction, R, were determined by extrapolating the impedance spectra to the real axis in the complex plane. Fig. 2a–h shows the temperature dependence of conductivity in neat CDP and CDP/SiO2 composites with increasing volume fraction of SiO2 . In Fig. 2a, the conductivity in neat CDP changes abruptly at ca. 230 ◦ C from low-conducting phase to high-conducting phase (superprotonic phase transition), and increases by over three orders of magnitude at the critical temperature, Tc . The high-conducting and low-conducting phases correspond to cubic and monoclinic phases, respectively [2–5]. The values of Tc in the heating and cooling processes differ by about 10 ◦ C (see Fig. 2a); the neat CDP shows a small thermal hysteresis loop in the cooling–heating cycle. A remarkable result is the appearance of a large thermal hysteresis loop for conductivity in CDP/SiO2 composites through cooling and heating processes, as described below. At 10 vol.% for the SiO2 volume fraction, Tc in the heating process shifts to a higher temperature while Tc in the cooling process shifts to a lower temperature (Fig. 2b), i.e. superheating and supercooling in the high-conducting phase appear with an addition of SiO2 . With an increase in the SiO2 volume fraction from 16 vol.% to 45 vol.% (Fig. 2c–g), significant supercooling in the high-conducting phase occurs; the conductivity in the cooling process retains relatively high values, and then gradually decreases with a decrease in temperature. At 45 vol.% (Fig. 2g), in particular, the high-conducting phase is retained in the cooling process from 250 ◦ C to 170 ◦ C. On the contrary, superheating is not significant even with an increase in the SiO2 volume fraction from 16 vol.% to 45 vol.%; the addition of SiO2 in the composites strongly influences the retention of the high-conducting phase. The conductivity in the CDP/SiO2 composites (16–45 vol.% for the SiO2 volume fractions) therefore shows large asymmetric hysteresis loops for the superprotonic phase transition in a cooling–heating cycle. With further increasing of the SiO2 volume fraction (60 vol.%), the hysteresis loop no longer appears at temperatures between 250 ◦ C and 150 ◦ C (Fig. 2h), which suggests that the high-conducting phase is retained even below 150 ◦ C. However, the conductivity at 250 ◦ C in the CDP/SiO2 composite (60 vol.% for the SiO2 volume fraction) is low (2.4 × 10−4 S cm−1 ), showing that the conductivity in the high-conducting phase is lowered by the addition of SiO2 particles. Finally, the proton-conducting network in the composite, with a SiO2 volume fraction higher than 80 vol.%,
Fig. 3. Conductivity in CDP/SiO2 composites as a function of silica volume fraction. (䊉) Conductivity at 180 ◦ C (low-conducing phase), () conductivity at 250 ◦ C (highconducing phase (superprotonic phase)). Dotted line: prediction curve at 180 ◦ C; solid line: prediction curve at 250 ◦ C.
is broken so that the conductivity becomes around 10−7 S cm−1 to 10−8 S cm−1 . Fig. 3 shows the conductivity in the CDP/SiO2 composite versus SiO2 volume fraction at 180 ◦ C and 250 ◦ C isotherms. At 180 ◦ C, which corresponds to the temperature in the low-conducting phase in neat CDP, the highest value of conductivity is obtained at around 0.4–0.5 for the SiO2 volume fraction in a cooling process. On the other hand, the conductivity at 250 ◦ C, corresponding to the highconducing phase, decreases monotonously as the SiO2 volume fraction increases. The addition of SiO2 powder is effective in the enhancement of proton conductivity in the low-conducting phase, while the SiO2 powder merely plays the role of an insulator in the high-conducing phase. The behavior of conductivity as a function of SiO2 volume fraction will be discussed in the latter part. 3.2. Structural studies: XRD and Raman spectroscopy The XRD measurements of the powdered samples of neat CDP and CDP/SiO2 composites were performed at room temperature. Fig. 4a shows the typical XRD patterns of CDP/SiO2 composites (SiO2 volume fractions of 0 vol.%, 28 vol.% and 45 vol.%). For neat CDP, a monoclinic phase (space group: P21 /m) was observed (JCPDS card no. 84-0122) [35]. As is the case with neat CDP, the monoclinic phase was observed in the CDP/SiO2 composites; all detected peaks originated from CDP and silica, and no other observable peaks were found in the composites. Miller indices for peaks observed mainly are given in Fig. 4a. In addition, the XRD main signal slightly broadens with increasing SiO2 volume fraction, suggesting that the grain size of polycrystalline CDP partly becomes smaller or the CDP crystal structure is partly disordered in the composite. The XRD patterns of a CDP/SiO2 composite (SiO2 volume fraction of 60 vol.%) before a conductivity measurement is plotted with that after the conductivity measurement in Fig. 4b, which shows that the FWHM of the XRD signals becomes narrow during the conductivity measurement; the crystallization of CDP proceeds in a composite by annealing. It has been reported that the superionic phases of oxyacid salts have plasticity [24,36]. The crystallization of CDP may proceed especially in the superionic phase (> ca. 230 ◦ C). The Raman spectra of the internal modes of pelletized polycrystalline CDP were measured between 300 cm−1 and 1300 cm−1 in a sealed glass cell at various temperatures. All detected lines originated from CDP even at high temperatures (∼260 ◦ C), and no other lines of dehydration materials such as Cs2 H2 P2 O7 were observed. Typical Raman spectra of CDP in Fig. 5a show the spectral shift
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Fig. 4. XRD patterns of CDP/SiO2 composites at room temperature (x: silica volume fraction). (a)—(A) x = 0 vol.% (neat CDP), (B) x = 28 vol.% and (C) x = 45 vol.%; (b) the change in the XRD pattern in CDP/SiO2 composite with x = 60 vol.% from a conductivity measurement; (A) before the conductivity measurement and (B) after the conductivity measurement.
and broadening of linewidths for the internal modes occur as the temperature increases. Our attention was focused on the spectral shift of a symmetric stretching mode (Ag ), (P–O), at ca. 920 cm−1 as a function of temperature (Fig. 5b) [37]. The red-shift of the (P–O) mode was observed with an increase in temperature, and besides, steep changes in the (P–O) mode occurred at around 230 ◦ C. The steep change in the spectral shift corresponds to the structural phase transformation from a monoclinic phase to a cubic phase in CDP. In addition, significant line-broadening was observed at around the critical temperature (∼230 ◦ C). The line-broadening of CDP was first reported by Romain and Novak [38]. The linebroadening suggests the orientational disorder of the H2 PO4 − anion (rapid reorientational motion of H2 PO4 − ) will be induced [38]. The reorientational motion (libration) of anions in acid salts has been investigated in terms of the linewidths of internal modes in crystals as a function of temperature. Concerning CsHSO4 , Pham-Thi et al. discussed the reorientational motion of the HSO4 − anion, based
Fig. 5. (a) In situ Raman spectra for neat CDP as a function of temperature: (A) 25 ◦ C, (B) 160 ◦ C, (C) 220 ◦ C, (D) 250 ◦ C and (E) 260 ◦ C. (b) Spectral shifts of (P–O) in neat CDP vs. temperature.
on the temperature dependence of the linewidths of the internal S–OH stretching and bending modes and the librational mode of the HSO4 − anion [39]. This dynamic motion of the HSO4 − anion probably accounts for fast proton diffusion in the superprotonic phase [18]. Similarly, the H2 PO4 − anion-dynamics will realize fast proton diffusion in a CDP cubic phase. Recent NMR and XRD studies have also suggested the reorientational motion of the H2 PO4 − anion in the CDP cubic phase [40,41]. An abrupt red-shift and a linebroadening at around 230 ◦ C in Fig. 5 may be induced by weakening of hydrogen bond (P–O· · ·HO–P) and subsequent orientational disorder of the H2 PO4 − anion in the CDP cubic phase.
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Fig. 6. DTA curve for neat CDP in heating–cooling cycles under a sealed condition. The peaks 1–4 in the figure are endothermic and exothermic heat flows that correspond to a phase transformation between a monoclinic phase (low-conducting phase) and a cubic phase (high-conducting phase).
3.3. Thermal analysis The DTA measurements for powdered samples of neat CDP and CDP/SiO2 composites were conducted in a heating–cooling cycle to investigate the influence of silica additive on the phase transformation of CDP in the composites. In order to suppress dehydration in CDP (Eq. (1)), the DTA measurements were performed with the powdered samples contained in sealed steel-capsules, as is the manner for in situ Raman spectroscopy. The DTA curve of neat CDP in heating–cooling cycles at the rate of 2 K min−1 is shown in Fig. 6. The DTA measurement in the sealed capsule demonstrates clearly a phase transformation between monoclinic phase and cubic phase in neat CDP; the endothermic heat flow associated with a phase transformation from a monoclinic phase (low-conducting phase) to a cubic phase (high-conducting phase) was observed at around 230 ◦ C in the heating process, and then the exothermic heat flow
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associated with the inverse phase transformation from the cubic phase to the monoclinic phase was observed at around 200 ◦ C in the cooling process, which repeats itself in heating–cooling cycles. A weight change during the thermal analysis was not observable because of employing the sealed steel-capsule (withstand internal pressure: 5 MPa), but no other signals in DTA resulting from dehydration of CDP were observed. Fig. 7 shows the DTA curves of the CDP/SiO2 composite at the rate of 2 K min−1 in a heating–cooling cycle as a function of SiO2 volume fraction. Each heat flow was normalized by the weight of CDP in the contents (i.e. heat flows per 1 mg of CDP in CDP/SiO2 composites). In the CDP/SiO2 composites, endothermic heat flows associated with a phase transformation from a monoclinic phase to a cubic phase were observed at around 230 ◦ C in the heating process (Fig. 7a). The onset temperatures of the endothermic heat flows shifted slightly to a higher temperature with an increase in the SiO2 volume fraction (∼3 ◦ C) but the superheating is not significant. In the cooling process, however, the addition of silica strongly influences the phase transformation in the CDP/SiO2 composites (Fig. 7b); the onset temperatures of the exothermic heat flows were significantly lower than that for neat CDP. As shown in Fig. 7b, the exothermic heat flows can be separated approximately into three regions A, B and C, i.e. the first is peak A, appearing at the same temperature with neat CDP (ca. 200 ◦ C), the second is peak B, which is a broad peak and appears in the temperature region between 190 ◦ C and 160 ◦ C, and the third is peak C, which is also a broad peak and appears in the temperature region between 160 ◦ C and 110 ◦ C. The area of peak A reduced abruptly even with the addition of only a small amount of silica (SiO2 volume fraction of 10 vol.%). Instead, peak B appeared when SiO2 powder was added (SiO2 volume fraction ≥ 10 vol.%). The area of peak B decreased gradually with an increase in SiO2 volume fraction, while peak C appeared and its area increased with an increase in SiO2 volume fraction (SiO2 volume fraction ≥ 16 vol.%). The peaks A, B and C that will correspond to a different status of CDP in a composite are designated as groups A, B and C, respectively, hereafter. Heats of transition in the composites (total areas of the heat flows in heating process and cooling process, respectively), which were normalized with that in neat CDP, were plotted against
Fig. 7. DTA curves for CDP/SiO2 composites with a variety of silica volume fractions (x = 0–60 vol.%) in a heating–cooling cycle under a sealed condition. (a) Heating process and (b) cooling process. The exothermic heat flows in figure (b) can be separated approximately into three regions, A, B and C. Note that neat CDP in figure (b) is scaled down (0.3 times).
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3.4. Thermodynamic model for phase transformation From the measurements of proton conductivity, Raman spectroscopy and thermal analysis (DTA) as described above, there is a clear relationship between superprotonic phase transition and monoclinic-to-cubic structural transformation in neat CDP. Additionally, it is found that a large asymmetric thermal hysteresis loop for conductivity in CDP/SiO2 composites is associated with the thermal hysteresis of monoclinic-to-cubic structural transformation affected by silica additive in the composites, which will be induced by shear elastic forces. The present hysteretic phenomena may be typical of a martensitic transformation, which has been extensively studied in metal and alloy systems [42]. The present thermal hysteresis in the composites, i.e. the stabilization of superprotonic phase (cubic phase) in the composites, will be induced by shear elastic forces as observed in the martensitic transformation [43,44]. In this study, we attempt to explain the present asymmetric thermal hysteresis with a simple thermodynamic model. The monoclinic-to-cubic phase transformation in neat CDP is a firstorder phase transition. The specific Gibbs free energy change, G, for the formation of a nucleus in neat CDP can be given by Fig. 8. Normalized heat of transition plotted against SiO2 weight fraction. () Heating process and (䊉) cooling process.
SiO2 weight fraction (Fig. 8), showing that the heat of transition decreased linearly with an increase in SiO2 weight fraction. At every SiO2 weight fraction, the heats of transition in a cooling process were mostly same as those in a heating process, which suggests that the observed transitions are reversible transitions between monoclinic phase and cubic phase in the heating–cooling cycle. In addition, we confirmed that the transition occurred reversibly during two heating–cooling cycles. CDP mole fractions of the groups A, B and C in the cooling process were evaluated as a function of SiO2 volume fraction, assuming that the CDP mole fractions of the groups A, B and C corresponded to the heats of transition in the groups A, B and C, respectively (Fig. 9). As stated in Fig. 7, the CDP mole fraction of the group A reduces abruptly, whereas that of the group B rises with the addition of silica (SiO2 volume fraction of 10 vol.%) instead. Then the CDP mole fraction of the group B decreases linearly and that of the group C increases linearly with an increase in SiO2 volume fraction. The DTA measurements show clearly that a large thermal hysteresis loop for conductivity in CDP/SiO2 composites is caused by the structural transformation of CDP affected by silica additive.
Fig. 9. CDP mole fractions of groups A, B and C in a cooling process vs. SiO2 volume fraction. (䊉) Group A, () group B and () group C.
G = (gm − gc + ge )n + n2/3
(2)
with the employment of a simple solid state nucleation model [45], where the specific Gibbs free energies of neat CDP in monoclinic and cubic phases are represented by gm and gc (the free energy per CDP structural unit); n is the number of CDP structural units; ge is the elastic energy accompanying the nucleus formation; is the specific interfacial energy between the monoclinic phase and cubic phase; and is the shape factor. The appearance of the thermal hysteresis (Tc ± T) in neat CDP can be explained by the elastic energy term, ge , and the interfacial energy term, n2/3 . Assuming the temperature dependence of gm and gc to be linear at around Tc , g (=gm − gc ) can be represented as g = C T, where C is a constant. When the nucleus formation becomes a critical size, i.e. ∂G/∂n = 0, T can be written as follows [44]: T = −
1 C
ge +
2 n−1/3 . 3
(3)
Thermal hysteresis in neat CDP is therefore represented by Eq. (3). Colomban et al. has suggested mechanical treatments (grinding and pressure) have an influence on structural transformation in CsHSO4 [46]. Thus, even in neat oxyacid salts such as CDP and CsHSO4 , their structural phase transformations can be influenced by stress. The influence of stress can be emphasized in a heterogeneous system as stated below. Next, the structural phase transformation of CDP in silica matrices is considered. According to the present thermal analysis, CDP in CDP/SiO2 composites can be divided into three groups (groups A, B and C). Group A is assigned to the structural transformation of the bulky part of CDP in the composite, which is independent of the silica additive (nbulk : the number of CDP structural units in group A). Group B is attributed to the structural transformation of CDP that is affected weakly by silica additive (nw : the number of CDP structural units in group B). Group C, which shows significant supercooling, is attributed to the structural transformation of CDP that is affected strongly by silica additive (ns : the number of CDP structural units in group C). Since nbulk can be negligible in the composite (see the area of peak A in Figs. 7b and 9), the total number of CDP structural units in the composites is represented as ntotal = nw + ns . Although microstructures in the regions of groups B and C are unclear, it can be estimated roughly as stated below; CDP in group C may exist in the pores of silica particles and/or at the surface on silica particles, and it adheres rigidly to silica walls, for which the structural phase transformation will be strongly prevented. CDP in group B may exist
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between silica particles, and thus there is a relatively weak inhibition to the structural phase transformation. According to XRD measurement data [3,35], the difference in CDP volume between monoclinic phase and cubic phase is estimated to be V = ca. 4%. The barrier in the metastable state is thus elastic stress arising in the phase transformation in the silica matrices. The condition of the phase transformation is written as gntotal = ew nw + es ns = E,
(4)
where ew is an elastic strain energy in group B, es is an elastic strain energy in group C (ew < es ), and E is the total elastic strain energy. Hence T is given by T =
1 C
ew
nw ns + es ntotal ntotal
=
1 (ew w + es s ), C
(5)
where w and s are molar fractions for groups B and C, respectively. Fig. 10 shows a schematic diagram for the formation of an asymmetric thermal hysteresis loop. The phase transformation from monoclinic phase to cubic phase (forward transition) occurs at the temperature TFT = Tc + TFT , while that from cubic phase to monoclinic phase (reverse transition) occurs at the temperature TRT = Tc − TRT . In the present case of CDP/SiO2 composites, an asymmetric thermal hysteresis loop (TRT > TFT ) was observed. This phenomenon can be explained according to Eq. (5) if the barriers, ew and es , in the forward transition are smaller than those in the reverse transition (ew(FT) < ew(RT) and es(FT) < es(RT) (i.e. EFT < ERT )). An investigation of elastic properties of CDP by Brillouin spectroscopy has been conducted between room temperature and 250 ◦ C [47]. The results show the elastic constants, Cel , in CDP decrease monotonously with an increase in temperature, and the non-linear variations in the elastic constants then occur at 233 ◦ C,
i.e. the sudden drops in the elastic constants have been observed through the phase transformation from monoclinic phase to cubic phase (Cel (monoclinic) > Cel (cubic)). Hence it is probable that the barriers, ew and es , change between forward transition and reverse transition in CDP. Furthermore, the asymmetric thermal hysteresis loop suggests that elastic stress relaxation such as plastic deformation (plasticity in oxyacid salts has been investigated in Refs. [24,36]) and microcracks should be involved in the forward and reverse transitions. This situation is represented by a hysteresis loop of A → B → C → D → A in Fig. 10; the forward transition (B → C) having a relatively low barrier occurs at TFT accompanied by elastic stress relaxation, while the reverse transition (D → A) having a relatively high barrier occurs at TRT accompanied by elastic stress relaxation (EFT (process: B → C) < ERT (process: D → A)). If no elastic stress relaxation occurs in the forward and reverse transitions, a hysteresis loop will track the loop of A → B → C → D → A in Fig. 10; the hysteresis loop becomes narrow and TFT shifts toward a temperature lower than Tc because of a large accumulation of elastic strain energy in the reverse transition, which cannot be supported by present results. In conclusion, elastic strain energy and its relaxation in the processes of B → C and D → A will determine the phase stability in CDP/SiO2 composites. 3.5. Proton-conducting network Finally, we mention the proton-conducting network. The SiO2 volume fraction influences not only the phase stability but also the proton-conducting network in CDP/SiO2 composites. The total conductivity of this system, total , is determined by two components, CDP (proton conductor, p) and SiO2 (insulator, i), and thus it can be represented by the sum of two terms [28]: a = Xp pa + Xi ia , total
Fig. 10. Schematic diagrams for the formation of an asymmetric thermal hysteresis loop. (a) Temperature dependence of the specific Gibbs free energy in the monoclinic phase (gm ) and the cubic phase (gc ) of CDP in CDP/SiO2 composites near the critical temperature (Tc ). TFT and TRT : see text. (b) An asymmetric thermal hysteresis loop between a monoclinic phase (low-conducting phase) and a cubic phase (high-conducting phase).
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(6)
where X and denote the volume fraction and conductivity of the given components (p and i): Xp + Xi = 1; p i . The a value in Eq. (6) ranges from −1 (consecutive connection) to 1 (parallel connection). In real systems, consecutive and parallel connections of particles, which differ in conductivity, alternate, and thus a certain intermediate variant (−1 < a < 1) is realized [28]. Eq. (6) indicates the total value decreases with an increase in Xi if p is independent of Xi . In real systems, however, p is a function of Xi below the critical temperature of CDP; in a cooling process, p increases with an increase in Xi because silica particles enhance the stability of cubic phase. These contradictory effects realize the appearance of a maximal value of total as a function of Xi , as has already been shown in Fig. 3. In the case of conductivity at 250 ◦ C in Fig. 3, p has a value in a high-conducting phase and is independent of Xi . The total value therefore decreases monotonously with an increase in Xi , according to Eq. (6). At 180 ◦ C in Fig. 3, which corresponds to the temperature below the critical temperature of CDP, p is strongly influenced by Xi ; p increases with an increase in Xi , whereas the second term in Eq. (6) has an effect on a decrease in the total value with an increase in Xi . As a result, the maximal value of conductivity appears for the SiO2 volume fraction, as shown in Fig. 3. The behavior of proton conduction in the composites is thus determined by the phase stability arising from shear elastic forces and proton-conducting network. According to the above, we evaluated the total value in Fig. 3. Considering the groups A, B and C of CDP in CDP/SiO2 composites, Eq. (6) can be rewritten as follows: a = total
3 k=1
Xk ka + Xi ia ,
(7)
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where the given components, k (k = 1, 2, 3) correspond to the groups A, B, C, respectively. Conductivity, k , is expressed as an Arrhenius equation: =
Ea A exp − T kB T
.
(8)
Activation energy, Ea , and prefactor, A, were determined from the conductivity of neat CDP in high-conducting phase (HCP) and lowconducting phase (LCP): Ea(HCP) = 0.4 eV, A(HCP) = 8.0 × 104 S K cm−1 , Ea(LCP) = 0.5 eV, A(LCP) = 1.0 × 102 S K cm−1 . In addition, the parameter values ( i = 1.0 × 10−13 S cm−1 ; a = 1/3 [28]) were employed to evaluate the total . A prediction curve, i.e. isothermal conductivity vs. silica volume fraction, for 180 ◦ C was plotted in Fig. 3, which was calculated assuming that the groups A and B were in LCP and the group C was in HCP at 180 ◦ C. Mole fractions of A, B and C in Fig. 9 were referred for the calculation of the prediction curve. Similarly, a prediction curve for 250 ◦ C was calculated (Fig. 3), in which all of the groups A, B and C were in HCP. The prediction curves can explain the tendency of conductivity toward silica volume fraction, but the prediction curves deviate from the experimental data especially in a higher region of silica volume fraction (>0.5). This means that present model (Eq. (7)) cannot demonstrate breaking of a protonconducting network exactly. Further improvement of a quantitative model should be made based on a percolation theory [27,28]. Previous studies have suggested that for CsHSO4 /SiO2 composites, the structurally disordering phase of CsHSO4 (interfacial amorphous phase) forms at the interface between CsHSO4 and SiO2 , and proton transfer will be accelerated via the interfacial conduction-pathway in the composite [29–32]. In addition, significant thermal hysteresis in conductivity was not observed in those cases. In the present composite system, conductivity can be partly affected by adsorbed water molecules and/or structurally disordering phase at the interface between CDP and SiO2 . Our present study, however, suggests that proton transfer in CDP/SiO2 composites prepared by the evaporation-to-dryness method is mainly governed by a crystallographic phase transition of CDP. The stabilization of a cubic phase in the composites will be induced by shear elastic forces in a cooling process, and orientational disorder of the H2 PO4 − ion (rapid reorientational motion of H2 PO4 − ) in the cubic phase will be maintained even below critical temperature. A structural variation among the bulky oxyacid salt, interface and insulator in heterogeneous systems thus generates complex phenomena in conductivity. In addition, from the standpoint of applications, the stabilization of the superprotonic phase by the shear elastic forces gives a hint for a strategy to use oxyacid salts as solid electrolyte in a wider temperature region. 4. Conclusion The correlation between proton conductivity and phase transformation from a monoclinic phase to a cubic phase in neat CsH2 PO4 (CDP) and CDP/SiO2 composites has been investigated. Raman spectroscopy and thermal analysis under sealed conditions demonstrate clearly a reversible phase transformation between monoclinic phase and cubic phase in CDP without dehydration, which provides reliable evidence that the phase transformation is the trigger of a transition to a superionic phase in CDP. The line-broadening in the Raman spectra suggests the orientational disorder of the H2 PO4 − ion (rapid reorientational motion of H2 PO4 − ) occurs in the cubic phase, which induces a disordering of the hydrogen bond network to facilitate proton transfer. A large asymmetric thermal hysteresis in the conductivity of CDP/SiO2 composites has been observed, i.e. the conductivity in the cooling process maintains relatively high values. DTA measurements suggest significant supercooling in the cubic phase of CDP is induced
by the silica additive. The metastable phase (cubic phase retained below critical temperature) in the composites will be induced by shear elastic forces, and orientational disorder of the H2 PO4 − ion (rapid reorientational motion of H2 PO4 − ) in the cubic phase will be maintained even below critical temperature. The present complex behavior of proton conduction in the composites is mainly determined by the phase stability arising from the shear elastic forces and proton-conducting network. Acknowledgements The authors are very grateful to Mitsubishi Chemical Corporation for providing the mesoporous SiO2 particles. This work was supported financially by a Grant-in-Aid for Young Scientists (B), no. 17760618, from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the “Research and Development of Polymer Electrolyte Fuel Cell” program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan, which are greatly appreciated. References [1] A.I. Baranov, V.P. Khiznichenko, V.A. Sandler, L.A. Shuvalov, Ferroelectrics 81 (1988) 183. [2] A.I. Baranov, V.P. Khiznichenko, L.A. Shuvalov, Ferroelectrics 100 (1989) 135. [3] W. Bronowska, J. Chem. Phys. 114 (2001) 611. [4] J. Otomo, N. Minagawa, C. Wen, K. Eguchi, H. Takahashi, Solid State Ionics 156 (2003) 357. [5] D.A. Boysen, S.M. Haile, H. Liu, R.A. Secco, Chem. Mater. 15 (2003) 727. [6] K.-S. Lee, J. Phys. Chem. Solids 57 (1996) 333. [7] E. Ortiz, R.A. Vargas, B.-E. Mellander, J. Chem. Phys. 110 (1999) 4847. [8] J.-H. Park, Phys. Rev. B 69 (2004) 054104. [9] Y. Taninouchi, T. Uda, Y. Awakura, A. Ikeda, S.M. Haile, J. Mater. Chem. 17 (2007) 3182. [10] A.I. Baranov, E.M. Kopnin, V.V. Grebenev, A. Sin, A. Zaopo, Y. Dubitsky, P. Caracino, Solid State Ionics 178 (2007) 657. [11] J. Otomo, T. Tamaki, S. Nishida, S. Wang, M. Ogura, T. Kobayashi, C. Wen, H. Nagamoto, H. Takahashi, J. Appl. Electrochem. 35 (2005) 865. [12] D.A. Boysen, T. Uda, C.R.I. Chisholm, S.M. Haile, Science 303 (2004) 68. [13] T. Uda, S.M. Haile, Electrochem. Solid-State Lett. 8 (2005) A245. [14] A.I. Baranov, B.V. Merinov, A.V. Tregubchenko, V.P. Khiznichenko, L.A. Shuvalov, N.M. Schagina, Solid State Ionics 36 (1989) 279. [15] Ph. Colomban, M. Pham-Thi, A. Novak, Solid State Ionics 20 (1986) 125. [16] Ph. Colomban, J.C. Badot, M. Pham-Thi, A. Novak, Phase Trans. 14 (1989) 55. [17] T. Norby, M. Friesel, B.E. Mellander, Solid State Ionics 77 (1995) 105. [18] W. Münch, K.D. Kreuer, U. Traub, J. Maier, Solid State Ionics 77 (1995) 10. [19] P. Zetterström, A.V. Belushkin, R.L. McGreevy, L.A. Shuvalov, Solid State Ionics 116 (1999) 321. [20] A. Damyanovich, M.M. Pintar, R. Blinc, J. Slak. Phys. Rev. B 56 (1997) 7942. [21] A.R. Lim, J.H. Chang, H.J. Kim, H.M. Park, Solid State Commun. 129 (2004) 123. [22] T. Uda, D.A. Boysen, S.M. Haile, Solid State Ionics 176 (2005) 127. [23] K. Itoh, T. Ozaki, E. Nakamura, Acta Crystallogr. B 37 (1981) 1908. [24] L.F. Kirpichnikova, A.A. Urusovskaya, V.V. Dolbinina, V.I. Mozgovoy, Ferroelectrics 172 (1995) 133. [25] C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289. [26] J. Maier, J. Phys. Chem. Solids 46 (1985) 309. [27] R.C. Agrawal, R.K. Gupta, J. Mater. Sci. 34 (1999) 1131. [28] A.B. Yaroslavtsev, Russ. J. Inorg. Chem. 45 (2000) S249. [29] V.G. Ponomareva, N.F. Uvarov, G.V. Lavrova, E.F. Hairetdinov, Solid State Ionics 90 (1996) 161. [30] V.G. Ponomareva, G.V. Lavrova, L.G. Simonova, Solid State Ionics 119 (1999) 295. [31] H. Shigeoka, J. Otomo, C. Wen, M. Ogura, H. Takahashi, J. Electrochem. Soc. 151 (2004) J76. [32] S. Wang, J. Otomo, M. Ogura, C. Wen, H. Nagamoto, H. Takahashi, Solid State Ionics 176 (2005) 755. [33] V.G. Ponomareva, E.S. Shutova, Solid State Ionics 178 (2007) 729. [34] J.-S. Lee, S. Adams, J. Maier, J. Electrochem. Soc. 147 (2000) 2407. [35] H. Matsunaga, K. Itoh, E. Nakamura, J. Phys. Soc. Jpn. 48 (1980) 2011. [36] T. Ozaki, K. Itoh, E. Nakamura, J. Phys. Soc. Jpn. 51 (1982) 213. [37] B. Marchon, A. Novak, J. Chem. Phys. 78 (1983) 2105. [38] F. Romain, A. Novak, J. Mol. Struct. 263 (1991) 69. [39] M. Pham-Thi, Ph. Colomban, A. Novak, R. Blinc, J. Raman Spectrosc. 18 (1987) 185. [40] K. Yamada, T. Sagara, Y. Yamane, H. Ohki, T. Okuda, Solid State Ionics 175 (2004) 557. [41] C.E. Botez, J.D. Hermosillo, J. Zhang, J. Qian, Y. Zhao, J. Majzlan, R.R. Chianelli, C. Pantea, J. Chem. Phys. 127 (2007) 194701.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Phase transition and proton transport characteristics in CsH2 PO4 /SiO2 composites Junichiro Otomo a,∗ , Takashi Ishigooka b , Tomoyuki Kitano b , Hiroshi Takahashi b , Hidetoshi Nagamoto b a b
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakanomachi, Hachioji-shi, Tokyo 192-0015, Japan
a r t i c l e
i n f o
Article history: Received 30 March 2008 Received in revised form 7 June 2008 Accepted 12 June 2008 Available online 21 June 2008 Keywords: Cesium dihydrogen phosphate Proton conductor Heterogeneous composite Phase transition Thermal hysteresis
a b s t r a c t The stabilization of superprotonic phase in neat CsH2 PO4 and CsH2 PO4 /SiO2 composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH2 PO4 at around Tc = 230 ◦ C was confirmed under humidified and sealed conditions. In CsH2 PO4 /SiO2 composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH2 PO4 and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Cesium dihydrogen phosphate, CsH2 PO4 , abbreviated as CDP hereafter, is a highly proton-conducting solid electrolyte, which is classified as a hydrogen-bonded oxyacid salt crystal. CDP exhibits high proton conductivity (>10−2 S cm−1 ) in a superprotonic phase at a temperature above ca. 230 ◦ C [1–5]. The superprotonic phase transition in CDP takes place at ca. 230 ◦ C, being accompanied with a structural transformation from a monoclinic phase to a cubic phase [2–5]. Under a dry condition, however, dehydration of CDP (Eq. (1)) proceeds to form polymerization products at temperatures above 200 ◦ C [6–9]: CsH2 PO4 → Cs2 H2 P2 O7 → CsPO3
(1)
Since the reaction occurs in parallel with a transition to the superprotonic phase, the nature of CDP has been much discussed, from both the viewpoints of the formation of the superprotonic phase and dehydration, over the last decade [3–11]. The stabilization of the superprotonic phase in CDP without any decomposition was demonstrated under appropriate humid conditions in terms of crystallographic and electrochemical studies [3,4]. Recently, a stable fuel cell-operation using CDP as an electrolyte membrane has been demonstrated successfully in a humid atmosphere [12]. CDP
∗ Corresponding author. Tel.: +81 4 7136 4714; fax: +81 4 7136 4715. E-mail address: [email protected] (J. Otomo). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.018
has therefore attracted interest not only in fundamental aspects but also in its application to energy devices such as fuel cells [11,13]. The current status thus motivates us to further investigate specific properties in CDP. As for cesium hydrogen sulfate, CsHSO4 , its transition to a superprotonic phase has been extensively investigated [14–24]. The superprotonic phase transition of CsHSO4 occurs with a structural transformation from a monoclinic phase to a tetragonal phase. In the superprotonic phase, it is considered that the reorientational motion of a SO4 tetrahedron, which induces a disordering of the hydrogen bond network, facilitates proton transfer [18,19]. Similar mechanism of proton conduction is prospective in CDP. In this study, we confirmed phase transition phenomena in CDP without decomposition through the measurements of conductivity, Raman spectroscopy and thermal analysis. On the other hand, heterogeneous doping of ionic salts with highly dispersed oxides is an effective approach to improve ionic conductivity in low and intermediate temperature regions [25–28]. Heterogeneous composites that consist of proton-conducting oxyacid salts with mesoporous silica glass particles also exhibit the enhancement of conductivity [4,29–33]. In previous work [4], we have observed the enhancement of proton conductivity in a CDP/SiO2 composite prepared by a mechanical mixing method, but the observation of conductivity was limited in the measurement for a heating process. In this study, we employed an evaporationto-dryness method to prepare CDP/SiO2 composites with high dispersion of mesoporous silica grass particles [31]. Through the electrochemical measurements, we found a large asymmetric ther-
J. Otomo et al. / Electrochimica Acta 53 (2008) 8186–8195
mal hysteresis loop of conductivity in a heating–cooling cycle. New phases of ionic salts have been found in heterogeneous composite systems. In the present case of CDP/SiO2 composites, the interfacial effect on phase stability and proton conductivity, which will be mainly induced by shear elastic forces at the interface between CDP and silica particles, is discussed. A large thermal hysteresis in a phase transition has been reported in an Ag+ conducting composite system such as AgI/Al2 O3 composite, which originates from an interfacial interaction [34]. To our knowledge, however, the present study is the first case demonstrating explicitly a cooperative phenomenon for proton conductivity in an oxyacid salt/insulator heterogeneous composite with the large asymmetric thermal hysteresis of phase transformation. Main concern in this study is thus the investigation of phase transition behavior in CDP/SiO2 composites in terms of proton conductivity and structural phase transformation, based on the observation of phase transition in neat CDP. The results will provide new insight into fundamental aspects of phase transformation and the stabilization of a superprotonic phase in the oxyacid salt/insulator heterogeneous composite even below critical temperature. 2. Experimental 2.1. Sample preparations Polycrystalline CsH2 PO4 (CDP) was used as received (purity > 99%; Soekawa Chemical Co. Ltd., Japan). Powdered CDP sample was prepared by grinding in an agate mortar for 30 min. An XRD measurement showed all the detected peaks originated from CDP and no contaminants were observed. The powdered CDP sample was then pressed at 3 t cm−2 to form dense pellets (diameter: 13 mm; thickness: 1.3–1.7 mm). The resultant pellet of neat CDP was set in a glass cell, and calcined at 260 ◦ C for 1 h. The relative density of the neat CDP pellet was over 0.94 after having calcined. CDP/SiO2 composite was prepared by an evaporation-todryness method as described below [31]. Mesoporous silica glass powder provided by Mitsubishi Chemical Corporation was used as received. The properties of the silica particle are described as follows: average particle size, 5.1 m; specific surface area, 297 m2 g−1 ; average pore volume, 1.3 ml g−1 ; and average pore size, 16 nm. First, the mesoporous silica glass particles were introduced into a CDP aqueous solution to prepare an aqueous suspension. The suspension was then stirred constantly for 60 min under a reduced pressure condition in order to remove the air in the mesopores of the silica particles and to allow the solution to easily infiltrate into the mesopores. The suspension was evaporated at 50 ◦ C, and then dried at 90 ◦ C for 24 h. After additional heating at 150 ◦ C for 1 h in air to minimize the influence of adsorbed water, the resultant sample was mechanically ground for 10 min, and pressed at 3 t cm−2 to form pellets. Finally, the pellet of CDP/SiO2 composite was calcined at 260 ◦ C for 1 h in a glass cell as in the case of neat CDP. In this manner, (1 − x)CDP/xSiO2 composites were prepared with a variation in the volume fraction of SiO2 , x (=0–1).
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ply of water vapor is required to inhibit dehydration of CDP [4,11]. Conductivity measurements were performed by means of ac impedance spectroscopy using a Hewlett Packard 4192A impedance analyzer in the frequency range from 10 MHz to 10 Hz with an applied voltage of 50 mV or 500 mV. The conductivity as a function of temperature was measured in a cooling–heating cycle; after having maintained the samples at 250 ◦ C, the temperature was decreased stepwise in the cooling process from 250 ◦ C to 150 ◦ C, and then the temperature was increased stepwise in the heating process from 150 ◦ C to 250 ◦ C. The samples were kept for over 30 min at each temperature so they could stabilize before the ac impedance measurement was carried out. 2.3. Characterizations Phase transformation in neat CPD was observed by means of in situ Raman spectroscopy. The pelletized sample of neat CDP was set in a sealed glass cell. The cell was introduced into a furnace in order to observe the in situ Raman spectrum as a function of temperature that was changed stepwise from room temperature to 260 ◦ C. After the sample was maintained for 10 min at each temperature, a spectroscopic measurement was performed to collect data in equilibrium. A continuous wave Ar+ ion laser beam at 488 nm and 0.5 W was used as the exciting line, which was focused onto the sample through a glass window using a 2.0 cm focal length lens. The backscattered light was led inversely through the lens to a spectrometer (JASCO NRS-2000, Japan) and detected by a CCD detector. In this manner, we obtained Raman spectroscopic data on neat CDP without dehydration at temperatures above 200 ◦ C. Differential thermal analysis (DTA) of powdered samples of neat CDP and CDP/SiO2 composites was performed both in the
2.2. Proton conductivity measurements Silver electrodes were applied on both sides of the obtained pellets using silver paste after silver vapor deposition. The pellet samples were set in a glass tube, and then gaseous mixtures of Ar/H2 O (Ar:H2 O = 0.7:0.3; partial pressure of water vapor: 0.03 MPa) were supplied through the glass tube at atmospheric pressure. Because the dehydration of CDP (Eq. (1)) occurs at a temperature higher than 200 ◦ C under a dry condition, the sup-
Fig. 1. Typical impedance spectra of CDP/SiO2 composite (silica volume fraction: 0.28 vol.%).
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Fig. 2. Temperature dependence of conductivity in CDP/SiO2 composites with a variety of silica volume fractions (x = 0–60 vol.%). Open circles: cooling process; filled circles: heating process. (a) x = 0 vol.% (neat CDP), (b) x = 10 vol.%, (c) x = 16 vol.%, (d) x = 22 vol.%, (e) x = 28 vol.%, (f) x = 37 vol.%, (g) x = 45 vol.% and (h) x = 60 vol.%.
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heating process and cooling process between 100 ◦ C and 270 ◦ C (Rigaku TAS200, Japan). The heating and cooling rates were fixed at 2 K min−1 . Powdered samples were sealed in a steel capsule with an aluminum cover (withstand internal pressure: 5 MPa) in order to inhibit dehydration of CDP. All sealed samples were measured after having been annealed at 270 ◦ C. In this manner, we successfully measured heat flow accompanying the phase transformation of CDP, as is stated later. In addition to the above, the X-ray diffraction (XRD) patterns of powdered samples of neat CDP and CDP/SiO2 composites were recorded at room temperature (Rigaku Rint 2500VHF, Japan) using a Cu K␣ source. 3. Results and discussion 3.1. Proton conductivity Fig. 1 shows typical ac impedance spectra of a CDP/SiO2 composite. In a low-temperature region, semicircles were observed (Fig. 1a). These semicircles, however, disappeared gradually with an increase in temperature and were no longer seen in a high temperature region (>230 ◦ C), as shown in Fig. 1b, because the proton conduction process occurs too quickly to be observed with the present impedance analyzer. Instead of the semicircle, capacitancelike behavior was observed in the high temperature region, i.e. only linear impedance spectra were seen. The values of the specific resistance of proton conduction, R, were determined by extrapolating the impedance spectra to the real axis in the complex plane. Fig. 2a–h shows the temperature dependence of conductivity in neat CDP and CDP/SiO2 composites with increasing volume fraction of SiO2 . In Fig. 2a, the conductivity in neat CDP changes abruptly at ca. 230 ◦ C from low-conducting phase to high-conducting phase (superprotonic phase transition), and increases by over three orders of magnitude at the critical temperature, Tc . The high-conducting and low-conducting phases correspond to cubic and monoclinic phases, respectively [2–5]. The values of Tc in the heating and cooling processes differ by about 10 ◦ C (see Fig. 2a); the neat CDP shows a small thermal hysteresis loop in the cooling–heating cycle. A remarkable result is the appearance of a large thermal hysteresis loop for conductivity in CDP/SiO2 composites through cooling and heating processes, as described below. At 10 vol.% for the SiO2 volume fraction, Tc in the heating process shifts to a higher temperature while Tc in the cooling process shifts to a lower temperature (Fig. 2b), i.e. superheating and supercooling in the high-conducting phase appear with an addition of SiO2 . With an increase in the SiO2 volume fraction from 16 vol.% to 45 vol.% (Fig. 2c–g), significant supercooling in the high-conducting phase occurs; the conductivity in the cooling process retains relatively high values, and then gradually decreases with a decrease in temperature. At 45 vol.% (Fig. 2g), in particular, the high-conducting phase is retained in the cooling process from 250 ◦ C to 170 ◦ C. On the contrary, superheating is not significant even with an increase in the SiO2 volume fraction from 16 vol.% to 45 vol.%; the addition of SiO2 in the composites strongly influences the retention of the high-conducting phase. The conductivity in the CDP/SiO2 composites (16–45 vol.% for the SiO2 volume fractions) therefore shows large asymmetric hysteresis loops for the superprotonic phase transition in a cooling–heating cycle. With further increasing of the SiO2 volume fraction (60 vol.%), the hysteresis loop no longer appears at temperatures between 250 ◦ C and 150 ◦ C (Fig. 2h), which suggests that the high-conducting phase is retained even below 150 ◦ C. However, the conductivity at 250 ◦ C in the CDP/SiO2 composite (60 vol.% for the SiO2 volume fraction) is low (2.4 × 10−4 S cm−1 ), showing that the conductivity in the high-conducting phase is lowered by the addition of SiO2 particles. Finally, the proton-conducting network in the composite, with a SiO2 volume fraction higher than 80 vol.%,
Fig. 3. Conductivity in CDP/SiO2 composites as a function of silica volume fraction. (䊉) Conductivity at 180 ◦ C (low-conducing phase), () conductivity at 250 ◦ C (highconducing phase (superprotonic phase)). Dotted line: prediction curve at 180 ◦ C; solid line: prediction curve at 250 ◦ C.
is broken so that the conductivity becomes around 10−7 S cm−1 to 10−8 S cm−1 . Fig. 3 shows the conductivity in the CDP/SiO2 composite versus SiO2 volume fraction at 180 ◦ C and 250 ◦ C isotherms. At 180 ◦ C, which corresponds to the temperature in the low-conducting phase in neat CDP, the highest value of conductivity is obtained at around 0.4–0.5 for the SiO2 volume fraction in a cooling process. On the other hand, the conductivity at 250 ◦ C, corresponding to the highconducing phase, decreases monotonously as the SiO2 volume fraction increases. The addition of SiO2 powder is effective in the enhancement of proton conductivity in the low-conducting phase, while the SiO2 powder merely plays the role of an insulator in the high-conducing phase. The behavior of conductivity as a function of SiO2 volume fraction will be discussed in the latter part. 3.2. Structural studies: XRD and Raman spectroscopy The XRD measurements of the powdered samples of neat CDP and CDP/SiO2 composites were performed at room temperature. Fig. 4a shows the typical XRD patterns of CDP/SiO2 composites (SiO2 volume fractions of 0 vol.%, 28 vol.% and 45 vol.%). For neat CDP, a monoclinic phase (space group: P21 /m) was observed (JCPDS card no. 84-0122) [35]. As is the case with neat CDP, the monoclinic phase was observed in the CDP/SiO2 composites; all detected peaks originated from CDP and silica, and no other observable peaks were found in the composites. Miller indices for peaks observed mainly are given in Fig. 4a. In addition, the XRD main signal slightly broadens with increasing SiO2 volume fraction, suggesting that the grain size of polycrystalline CDP partly becomes smaller or the CDP crystal structure is partly disordered in the composite. The XRD patterns of a CDP/SiO2 composite (SiO2 volume fraction of 60 vol.%) before a conductivity measurement is plotted with that after the conductivity measurement in Fig. 4b, which shows that the FWHM of the XRD signals becomes narrow during the conductivity measurement; the crystallization of CDP proceeds in a composite by annealing. It has been reported that the superionic phases of oxyacid salts have plasticity [24,36]. The crystallization of CDP may proceed especially in the superionic phase (> ca. 230 ◦ C). The Raman spectra of the internal modes of pelletized polycrystalline CDP were measured between 300 cm−1 and 1300 cm−1 in a sealed glass cell at various temperatures. All detected lines originated from CDP even at high temperatures (∼260 ◦ C), and no other lines of dehydration materials such as Cs2 H2 P2 O7 were observed. Typical Raman spectra of CDP in Fig. 5a show the spectral shift
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Fig. 4. XRD patterns of CDP/SiO2 composites at room temperature (x: silica volume fraction). (a)—(A) x = 0 vol.% (neat CDP), (B) x = 28 vol.% and (C) x = 45 vol.%; (b) the change in the XRD pattern in CDP/SiO2 composite with x = 60 vol.% from a conductivity measurement; (A) before the conductivity measurement and (B) after the conductivity measurement.
and broadening of linewidths for the internal modes occur as the temperature increases. Our attention was focused on the spectral shift of a symmetric stretching mode (Ag ), (P–O), at ca. 920 cm−1 as a function of temperature (Fig. 5b) [37]. The red-shift of the (P–O) mode was observed with an increase in temperature, and besides, steep changes in the (P–O) mode occurred at around 230 ◦ C. The steep change in the spectral shift corresponds to the structural phase transformation from a monoclinic phase to a cubic phase in CDP. In addition, significant line-broadening was observed at around the critical temperature (∼230 ◦ C). The line-broadening of CDP was first reported by Romain and Novak [38]. The linebroadening suggests the orientational disorder of the H2 PO4 − anion (rapid reorientational motion of H2 PO4 − ) will be induced [38]. The reorientational motion (libration) of anions in acid salts has been investigated in terms of the linewidths of internal modes in crystals as a function of temperature. Concerning CsHSO4 , Pham-Thi et al. discussed the reorientational motion of the HSO4 − anion, based
Fig. 5. (a) In situ Raman spectra for neat CDP as a function of temperature: (A) 25 ◦ C, (B) 160 ◦ C, (C) 220 ◦ C, (D) 250 ◦ C and (E) 260 ◦ C. (b) Spectral shifts of (P–O) in neat CDP vs. temperature.
on the temperature dependence of the linewidths of the internal S–OH stretching and bending modes and the librational mode of the HSO4 − anion [39]. This dynamic motion of the HSO4 − anion probably accounts for fast proton diffusion in the superprotonic phase [18]. Similarly, the H2 PO4 − anion-dynamics will realize fast proton diffusion in a CDP cubic phase. Recent NMR and XRD studies have also suggested the reorientational motion of the H2 PO4 − anion in the CDP cubic phase [40,41]. An abrupt red-shift and a linebroadening at around 230 ◦ C in Fig. 5 may be induced by weakening of hydrogen bond (P–O· · ·HO–P) and subsequent orientational disorder of the H2 PO4 − anion in the CDP cubic phase.
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Fig. 6. DTA curve for neat CDP in heating–cooling cycles under a sealed condition. The peaks 1–4 in the figure are endothermic and exothermic heat flows that correspond to a phase transformation between a monoclinic phase (low-conducting phase) and a cubic phase (high-conducting phase).
3.3. Thermal analysis The DTA measurements for powdered samples of neat CDP and CDP/SiO2 composites were conducted in a heating–cooling cycle to investigate the influence of silica additive on the phase transformation of CDP in the composites. In order to suppress dehydration in CDP (Eq. (1)), the DTA measurements were performed with the powdered samples contained in sealed steel-capsules, as is the manner for in situ Raman spectroscopy. The DTA curve of neat CDP in heating–cooling cycles at the rate of 2 K min−1 is shown in Fig. 6. The DTA measurement in the sealed capsule demonstrates clearly a phase transformation between monoclinic phase and cubic phase in neat CDP; the endothermic heat flow associated with a phase transformation from a monoclinic phase (low-conducting phase) to a cubic phase (high-conducting phase) was observed at around 230 ◦ C in the heating process, and then the exothermic heat flow
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associated with the inverse phase transformation from the cubic phase to the monoclinic phase was observed at around 200 ◦ C in the cooling process, which repeats itself in heating–cooling cycles. A weight change during the thermal analysis was not observable because of employing the sealed steel-capsule (withstand internal pressure: 5 MPa), but no other signals in DTA resulting from dehydration of CDP were observed. Fig. 7 shows the DTA curves of the CDP/SiO2 composite at the rate of 2 K min−1 in a heating–cooling cycle as a function of SiO2 volume fraction. Each heat flow was normalized by the weight of CDP in the contents (i.e. heat flows per 1 mg of CDP in CDP/SiO2 composites). In the CDP/SiO2 composites, endothermic heat flows associated with a phase transformation from a monoclinic phase to a cubic phase were observed at around 230 ◦ C in the heating process (Fig. 7a). The onset temperatures of the endothermic heat flows shifted slightly to a higher temperature with an increase in the SiO2 volume fraction (∼3 ◦ C) but the superheating is not significant. In the cooling process, however, the addition of silica strongly influences the phase transformation in the CDP/SiO2 composites (Fig. 7b); the onset temperatures of the exothermic heat flows were significantly lower than that for neat CDP. As shown in Fig. 7b, the exothermic heat flows can be separated approximately into three regions A, B and C, i.e. the first is peak A, appearing at the same temperature with neat CDP (ca. 200 ◦ C), the second is peak B, which is a broad peak and appears in the temperature region between 190 ◦ C and 160 ◦ C, and the third is peak C, which is also a broad peak and appears in the temperature region between 160 ◦ C and 110 ◦ C. The area of peak A reduced abruptly even with the addition of only a small amount of silica (SiO2 volume fraction of 10 vol.%). Instead, peak B appeared when SiO2 powder was added (SiO2 volume fraction ≥ 10 vol.%). The area of peak B decreased gradually with an increase in SiO2 volume fraction, while peak C appeared and its area increased with an increase in SiO2 volume fraction (SiO2 volume fraction ≥ 16 vol.%). The peaks A, B and C that will correspond to a different status of CDP in a composite are designated as groups A, B and C, respectively, hereafter. Heats of transition in the composites (total areas of the heat flows in heating process and cooling process, respectively), which were normalized with that in neat CDP, were plotted against
Fig. 7. DTA curves for CDP/SiO2 composites with a variety of silica volume fractions (x = 0–60 vol.%) in a heating–cooling cycle under a sealed condition. (a) Heating process and (b) cooling process. The exothermic heat flows in figure (b) can be separated approximately into three regions, A, B and C. Note that neat CDP in figure (b) is scaled down (0.3 times).
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3.4. Thermodynamic model for phase transformation From the measurements of proton conductivity, Raman spectroscopy and thermal analysis (DTA) as described above, there is a clear relationship between superprotonic phase transition and monoclinic-to-cubic structural transformation in neat CDP. Additionally, it is found that a large asymmetric thermal hysteresis loop for conductivity in CDP/SiO2 composites is associated with the thermal hysteresis of monoclinic-to-cubic structural transformation affected by silica additive in the composites, which will be induced by shear elastic forces. The present hysteretic phenomena may be typical of a martensitic transformation, which has been extensively studied in metal and alloy systems [42]. The present thermal hysteresis in the composites, i.e. the stabilization of superprotonic phase (cubic phase) in the composites, will be induced by shear elastic forces as observed in the martensitic transformation [43,44]. In this study, we attempt to explain the present asymmetric thermal hysteresis with a simple thermodynamic model. The monoclinic-to-cubic phase transformation in neat CDP is a firstorder phase transition. The specific Gibbs free energy change, G, for the formation of a nucleus in neat CDP can be given by Fig. 8. Normalized heat of transition plotted against SiO2 weight fraction. () Heating process and (䊉) cooling process.
SiO2 weight fraction (Fig. 8), showing that the heat of transition decreased linearly with an increase in SiO2 weight fraction. At every SiO2 weight fraction, the heats of transition in a cooling process were mostly same as those in a heating process, which suggests that the observed transitions are reversible transitions between monoclinic phase and cubic phase in the heating–cooling cycle. In addition, we confirmed that the transition occurred reversibly during two heating–cooling cycles. CDP mole fractions of the groups A, B and C in the cooling process were evaluated as a function of SiO2 volume fraction, assuming that the CDP mole fractions of the groups A, B and C corresponded to the heats of transition in the groups A, B and C, respectively (Fig. 9). As stated in Fig. 7, the CDP mole fraction of the group A reduces abruptly, whereas that of the group B rises with the addition of silica (SiO2 volume fraction of 10 vol.%) instead. Then the CDP mole fraction of the group B decreases linearly and that of the group C increases linearly with an increase in SiO2 volume fraction. The DTA measurements show clearly that a large thermal hysteresis loop for conductivity in CDP/SiO2 composites is caused by the structural transformation of CDP affected by silica additive.
Fig. 9. CDP mole fractions of groups A, B and C in a cooling process vs. SiO2 volume fraction. (䊉) Group A, () group B and () group C.
G = (gm − gc + ge )n + n2/3
(2)
with the employment of a simple solid state nucleation model [45], where the specific Gibbs free energies of neat CDP in monoclinic and cubic phases are represented by gm and gc (the free energy per CDP structural unit); n is the number of CDP structural units; ge is the elastic energy accompanying the nucleus formation; is the specific interfacial energy between the monoclinic phase and cubic phase; and is the shape factor. The appearance of the thermal hysteresis (Tc ± T) in neat CDP can be explained by the elastic energy term, ge , and the interfacial energy term, n2/3 . Assuming the temperature dependence of gm and gc to be linear at around Tc , g (=gm − gc ) can be represented as g = C T, where C is a constant. When the nucleus formation becomes a critical size, i.e. ∂G/∂n = 0, T can be written as follows [44]: T = −
1 C
ge +
2 n−1/3 . 3
(3)
Thermal hysteresis in neat CDP is therefore represented by Eq. (3). Colomban et al. has suggested mechanical treatments (grinding and pressure) have an influence on structural transformation in CsHSO4 [46]. Thus, even in neat oxyacid salts such as CDP and CsHSO4 , their structural phase transformations can be influenced by stress. The influence of stress can be emphasized in a heterogeneous system as stated below. Next, the structural phase transformation of CDP in silica matrices is considered. According to the present thermal analysis, CDP in CDP/SiO2 composites can be divided into three groups (groups A, B and C). Group A is assigned to the structural transformation of the bulky part of CDP in the composite, which is independent of the silica additive (nbulk : the number of CDP structural units in group A). Group B is attributed to the structural transformation of CDP that is affected weakly by silica additive (nw : the number of CDP structural units in group B). Group C, which shows significant supercooling, is attributed to the structural transformation of CDP that is affected strongly by silica additive (ns : the number of CDP structural units in group C). Since nbulk can be negligible in the composite (see the area of peak A in Figs. 7b and 9), the total number of CDP structural units in the composites is represented as ntotal = nw + ns . Although microstructures in the regions of groups B and C are unclear, it can be estimated roughly as stated below; CDP in group C may exist in the pores of silica particles and/or at the surface on silica particles, and it adheres rigidly to silica walls, for which the structural phase transformation will be strongly prevented. CDP in group B may exist
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between silica particles, and thus there is a relatively weak inhibition to the structural phase transformation. According to XRD measurement data [3,35], the difference in CDP volume between monoclinic phase and cubic phase is estimated to be V = ca. 4%. The barrier in the metastable state is thus elastic stress arising in the phase transformation in the silica matrices. The condition of the phase transformation is written as gntotal = ew nw + es ns = E,
(4)
where ew is an elastic strain energy in group B, es is an elastic strain energy in group C (ew < es ), and E is the total elastic strain energy. Hence T is given by T =
1 C
ew
nw ns + es ntotal ntotal
=
1 (ew w + es s ), C
(5)
where w and s are molar fractions for groups B and C, respectively. Fig. 10 shows a schematic diagram for the formation of an asymmetric thermal hysteresis loop. The phase transformation from monoclinic phase to cubic phase (forward transition) occurs at the temperature TFT = Tc + TFT , while that from cubic phase to monoclinic phase (reverse transition) occurs at the temperature TRT = Tc − TRT . In the present case of CDP/SiO2 composites, an asymmetric thermal hysteresis loop (TRT > TFT ) was observed. This phenomenon can be explained according to Eq. (5) if the barriers, ew and es , in the forward transition are smaller than those in the reverse transition (ew(FT) < ew(RT) and es(FT) < es(RT) (i.e. EFT < ERT )). An investigation of elastic properties of CDP by Brillouin spectroscopy has been conducted between room temperature and 250 ◦ C [47]. The results show the elastic constants, Cel , in CDP decrease monotonously with an increase in temperature, and the non-linear variations in the elastic constants then occur at 233 ◦ C,
i.e. the sudden drops in the elastic constants have been observed through the phase transformation from monoclinic phase to cubic phase (Cel (monoclinic) > Cel (cubic)). Hence it is probable that the barriers, ew and es , change between forward transition and reverse transition in CDP. Furthermore, the asymmetric thermal hysteresis loop suggests that elastic stress relaxation such as plastic deformation (plasticity in oxyacid salts has been investigated in Refs. [24,36]) and microcracks should be involved in the forward and reverse transitions. This situation is represented by a hysteresis loop of A → B → C → D → A in Fig. 10; the forward transition (B → C) having a relatively low barrier occurs at TFT accompanied by elastic stress relaxation, while the reverse transition (D → A) having a relatively high barrier occurs at TRT accompanied by elastic stress relaxation (EFT (process: B → C) < ERT (process: D → A)). If no elastic stress relaxation occurs in the forward and reverse transitions, a hysteresis loop will track the loop of A → B → C → D → A in Fig. 10; the hysteresis loop becomes narrow and TFT shifts toward a temperature lower than Tc because of a large accumulation of elastic strain energy in the reverse transition, which cannot be supported by present results. In conclusion, elastic strain energy and its relaxation in the processes of B → C and D → A will determine the phase stability in CDP/SiO2 composites. 3.5. Proton-conducting network Finally, we mention the proton-conducting network. The SiO2 volume fraction influences not only the phase stability but also the proton-conducting network in CDP/SiO2 composites. The total conductivity of this system, total , is determined by two components, CDP (proton conductor, p) and SiO2 (insulator, i), and thus it can be represented by the sum of two terms [28]: a = Xp pa + Xi ia , total
Fig. 10. Schematic diagrams for the formation of an asymmetric thermal hysteresis loop. (a) Temperature dependence of the specific Gibbs free energy in the monoclinic phase (gm ) and the cubic phase (gc ) of CDP in CDP/SiO2 composites near the critical temperature (Tc ). TFT and TRT : see text. (b) An asymmetric thermal hysteresis loop between a monoclinic phase (low-conducting phase) and a cubic phase (high-conducting phase).
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(6)
where X and denote the volume fraction and conductivity of the given components (p and i): Xp + Xi = 1; p i . The a value in Eq. (6) ranges from −1 (consecutive connection) to 1 (parallel connection). In real systems, consecutive and parallel connections of particles, which differ in conductivity, alternate, and thus a certain intermediate variant (−1 < a < 1) is realized [28]. Eq. (6) indicates the total value decreases with an increase in Xi if p is independent of Xi . In real systems, however, p is a function of Xi below the critical temperature of CDP; in a cooling process, p increases with an increase in Xi because silica particles enhance the stability of cubic phase. These contradictory effects realize the appearance of a maximal value of total as a function of Xi , as has already been shown in Fig. 3. In the case of conductivity at 250 ◦ C in Fig. 3, p has a value in a high-conducting phase and is independent of Xi . The total value therefore decreases monotonously with an increase in Xi , according to Eq. (6). At 180 ◦ C in Fig. 3, which corresponds to the temperature below the critical temperature of CDP, p is strongly influenced by Xi ; p increases with an increase in Xi , whereas the second term in Eq. (6) has an effect on a decrease in the total value with an increase in Xi . As a result, the maximal value of conductivity appears for the SiO2 volume fraction, as shown in Fig. 3. The behavior of proton conduction in the composites is thus determined by the phase stability arising from shear elastic forces and proton-conducting network. According to the above, we evaluated the total value in Fig. 3. Considering the groups A, B and C of CDP in CDP/SiO2 composites, Eq. (6) can be rewritten as follows: a = total
3 k=1
Xk ka + Xi ia ,
(7)
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where the given components, k (k = 1, 2, 3) correspond to the groups A, B, C, respectively. Conductivity, k , is expressed as an Arrhenius equation: =
Ea A exp − T kB T
.
(8)
Activation energy, Ea , and prefactor, A, were determined from the conductivity of neat CDP in high-conducting phase (HCP) and lowconducting phase (LCP): Ea(HCP) = 0.4 eV, A(HCP) = 8.0 × 104 S K cm−1 , Ea(LCP) = 0.5 eV, A(LCP) = 1.0 × 102 S K cm−1 . In addition, the parameter values ( i = 1.0 × 10−13 S cm−1 ; a = 1/3 [28]) were employed to evaluate the total . A prediction curve, i.e. isothermal conductivity vs. silica volume fraction, for 180 ◦ C was plotted in Fig. 3, which was calculated assuming that the groups A and B were in LCP and the group C was in HCP at 180 ◦ C. Mole fractions of A, B and C in Fig. 9 were referred for the calculation of the prediction curve. Similarly, a prediction curve for 250 ◦ C was calculated (Fig. 3), in which all of the groups A, B and C were in HCP. The prediction curves can explain the tendency of conductivity toward silica volume fraction, but the prediction curves deviate from the experimental data especially in a higher region of silica volume fraction (>0.5). This means that present model (Eq. (7)) cannot demonstrate breaking of a protonconducting network exactly. Further improvement of a quantitative model should be made based on a percolation theory [27,28]. Previous studies have suggested that for CsHSO4 /SiO2 composites, the structurally disordering phase of CsHSO4 (interfacial amorphous phase) forms at the interface between CsHSO4 and SiO2 , and proton transfer will be accelerated via the interfacial conduction-pathway in the composite [29–32]. In addition, significant thermal hysteresis in conductivity was not observed in those cases. In the present composite system, conductivity can be partly affected by adsorbed water molecules and/or structurally disordering phase at the interface between CDP and SiO2 . Our present study, however, suggests that proton transfer in CDP/SiO2 composites prepared by the evaporation-to-dryness method is mainly governed by a crystallographic phase transition of CDP. The stabilization of a cubic phase in the composites will be induced by shear elastic forces in a cooling process, and orientational disorder of the H2 PO4 − ion (rapid reorientational motion of H2 PO4 − ) in the cubic phase will be maintained even below critical temperature. A structural variation among the bulky oxyacid salt, interface and insulator in heterogeneous systems thus generates complex phenomena in conductivity. In addition, from the standpoint of applications, the stabilization of the superprotonic phase by the shear elastic forces gives a hint for a strategy to use oxyacid salts as solid electrolyte in a wider temperature region. 4. Conclusion The correlation between proton conductivity and phase transformation from a monoclinic phase to a cubic phase in neat CsH2 PO4 (CDP) and CDP/SiO2 composites has been investigated. Raman spectroscopy and thermal analysis under sealed conditions demonstrate clearly a reversible phase transformation between monoclinic phase and cubic phase in CDP without dehydration, which provides reliable evidence that the phase transformation is the trigger of a transition to a superionic phase in CDP. The line-broadening in the Raman spectra suggests the orientational disorder of the H2 PO4 − ion (rapid reorientational motion of H2 PO4 − ) occurs in the cubic phase, which induces a disordering of the hydrogen bond network to facilitate proton transfer. A large asymmetric thermal hysteresis in the conductivity of CDP/SiO2 composites has been observed, i.e. the conductivity in the cooling process maintains relatively high values. DTA measurements suggest significant supercooling in the cubic phase of CDP is induced
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