Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4

Solid State Ionics 177 (2006) 2421 – 2424 www.elsevier.com/locate/ssi

Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4 Atsunori Matsuda ⁎, Tomoya Kikuchi, Kiyofumi Katagiri, Hiroyuki Muto, Mototsugu Sakai Toyohashi University of Technology, Tempaku, 441-8580 Toyohashi, Japan Received 13 July 2005; received in revised form 17 March 2006; accepted 29 March 2006

Abstract Mixtures of CsHSO4 and CsH2PO4 were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 were formed during milling. The mechanochemically treated composite consisting of Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs2(HSO4)(H2PO4) was generated from the composite at around 100 °C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 °C on heating. The value becomes 2 × 10− 3 S cm− 1 at around 180 °C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs2 (HSO4)(H2PO4). © 2006 Elsevier B.V. All rights reserved. Keywords: Mechanochemical; Solid acid; Superprotonic; CsHSO4; CsH2PO4

1. Introduction Solid acid compounds in the CsHSO4–CsH2PO4 system have attracted great deal of attention as an electrolyte for the fuel cells in the medium temperature range (100–200 °C) [1]. Conductivity of these compounds increases several orders of magnitude to reach 10− 2 S cm− 1 on the superprotonic phase transition, and such high values are maintained under anhydrous conditions [2–4]. It is reported that mechanical grinding of CsHSO4 destroys the chain structure and induces a transition to a new crystalline phase [5]. On the other hand, Glipa et al. have reported that the proton conductivities of the composite of Zr (HPO4)2·H2O and Al2O3 increase by ball milling [6]. The enhancement in proton conductivity in the CsHSO4–MxOy systems has been extensively studied by Ponomareva et al. [7,8]. We have shown that phosphosilicate (P2O5–SiO2) gels containing large amounts of phosphorus keep a high conductivity of 1 × 10− 2 S cm− 1 even at 150 °C and 0.4% R.H. [9] and ⁎ Corresponding author. E-mail address: [email protected] (A. Matsuda). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.053

the addition of Al2O3 to the gels markedly improves the chemical durability of the resultant gels with a decrease in the proton conductivity [10]. Very recently, we have also shown that mechanochemical treatment on phosphorus-containing solid acids like AlH2P3O10·2H2O, α-Zr(HPO4)2·H2O, and AlH3(PO4)2·3H2O improves the proton conductivity [11]. In the mechanochemical treatment, a great impact is applied to a local area of the solid acids, resulting in the significant distortions and the rearrangement of the structures to form paths suitable for fast proton conduction. In the present study, mixtures of CsHSO4 and CsH2PO4 have been mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. 2. Experimental procedure Regent-grade CsHSO4 and CsH2PO4 purchased from Soekawa Chemical Co. Ltd. were used as starting materials. Mixtures of CsHSO4 and CsH2PO4 were mechanically milled using a planetary ball mill (Fritsch Pulverisette 7) for given periods of

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: Cs3(HSO4)2(H2PO4)

: Cs5(HSO4)3(H2PO4)2

: CsHSO4

: CsH2PO4

3. Results and discussion

Intensity (arb. unit)

MM 60min

MM 30min

MM 15min

without MM 20

22

24

26

28

30

2θ / deg. (CuKα) Fig. 1. XRD patterns of CsHSO4–CsH2PO4 (1:1 mole ratio) composite mechanically milled at 720 rpm for various times.

time. Agate was selected as a material for the pot and the ball. The rotation speed of the milling pot and the table was 720 rpm with a constant rotation ratio of 1:1. The volumetric capacity of the pot was 45 mL and 10 balls with a diameter of 10 mm were used for the mechanical milling. Sample weight was 1.0 g. X-ray diffraction (XRD) patterns of the mechanochemically treated CsHSO4–CsH2PO4 composite powders were obtained on a Rigaku RINT2000. Differential thermal analysis (DTA) and thermogravimetry (TG) of the composites were performed using a Rigaku Thermo Plus TG 8120. Differential scanning calorimetry (DSC) of the composites was also performed using a Perkin Elmer DSC 7. In situ measurements of XRD and DSC on heating and cooling processes were carried out using a Rigaku RINT-UltimaIII+DSC II. Electric conductivities of the mechanically milled CsHSO4– CsH2PO4 composite were determined by impedance data. Agilent 4284A and 4285A inductance–capacitance–resistance (LCR) meters were used to obtain the impedance data in a frequency range of 50 Hz to 30 MHz. Solartron SI1260 was also used to obtain the impedance data in a frequency range of 10 Hz to 8 MHz. Conductivity measurements were carried out for the sample pellets in a dry N2 atmosphere.

When the mixtures of CsHSO4 and CsH2PO4 were mechanically milled, the amounts of CsHSO4 and CsH2PO4 decreased during milling in a short time. It was found that Cs3 (HSO4)2(H2PO4) was formed and transformed to Cs5(HSO4)3 (H2PO4)2 during milling. XRD patterns of CsHSO4–CsH2PO4 (1:1 mole ratio) composite mechanically milled at 720 rpm are shown in Fig. 1. The intensities of the XRD peaks of CsHSO4 and CsH2PO4 decrease during mechanical milling. Several peaks assigned to Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3 (H2PO4)2 are newly seen after milling for 15 min. XRD peaks due to Cs5(HSO4)3(H2PO4)2 become significant during milling and a large amount of Cs5(HSO4)3(H2PO4)2 and a small amount of residual CsH2PO4 are observed after milling for 60 min. Cs5 (HSO4)3(H2PO4)2 is a new member of the sulfate–phosphate class of hydrogen-bonded compounds and generally prepared from aqueous solutions containing Cs2CO3, H2SO4 and H3PO4 by slow evaporation of water under ambient conditions [3]. It is noteworthy that crystalline Cs5(HSO4)3(H2PO4)2 is formed through the solid-phase reaction between CsHSO4 and CsH2PO4 by mechanical milling. DSC curves for CsHSO4–CsH2PO4 (1:1 mole ratio) composite on repeated heating and cooling runs (a) before and (b) after mechanical milling at 720 rpm for 30 min are shown in Fig. 2. Cooling and heating runs were repeated 10 times at 5 °C min− 1. In the DSC curves for the samples before milling (a), i.e., the mixture of CsHSO4 and CsH2PO4, an endothermic peak is seen at around 145 °C on the heating process, and an exothermic peak at around 130 °C on the cooling process in the temperature range between 0 and 190 °C (Fig. 2(a)). Almost no changes are observed in the DSC curves during the repeated runs. For pure CsHSO4 without milling, endothermic and exothermic peaks corresponding to the phase transition were observed at 147 and 132 °C, respectively and melting point of pure CsHSO4 was 207 °C. In addition, for pure CsH2PO4 without milling, endothermic and exothermic peaks due to the phase transition were observed at 233 and 172 °C, respectively. Therefore, these endothermic and exothermic peaks in Fig. 1(a) are ascribed to the phase transition of CsHSO4 from phase II with orthorhombic structure to superionic phase I with tetragonal structure [12]. On the other hand, it is noteworthy that an endothermic peak with shoulder is seen

(a)

(b) cooling

Heat flow Endo. Exo.

Heat flow Endo. Exo.

cooling heating

heating

1st heating 0

50

100

150

Temperature / °C

200

0

50

100

150

200

Temperature / °C

Fig. 2. DSC curves for CsHSO4–CsH2PO4 (1:1 mole ratio) composite on repeated heating and cooling runs. (a) The sample before milling and (b) the sample after mechanical milling at 720 rpm for 30 min. The heating rates were 5 °C min− 1.

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Temperature / °C 10

Conductivity / S cm-1

10

10

10

10

10

-2 180 150 120

90

60

30

3

3.3

-3

-4

-5

-6

-7

2.1

2.4

2.7

1000 K / T Fig. 3. Temperature dependence of conductivity of CsHSO4–CsH2PO4 (1:1 mole ratio) before and after milling at 720 rpm for 30 min. Triangles are for the sample before milling and circles for the sample after milling. Closed and open marks represent the upon heating and cooling processes, respectively.

from 105 to 125 °C only upon the first heating for the CsHSO4– CsH2PO4 composite mechanically milled at 720 rpm for 30 min (Fig. 2(b)). As shown in the XRD pattern (Fig. 1.), the CsHSO4–CsH2PO4 composite mechanically milled at 720 rpm for 30 min mainly consists of Cs3(HSO4)2(H2PO4) and Cs5 (HSO4)3(H2PO4)2, so that the endothermic peak in the first DSC heating curve suggests an irreversible solid phase reaction in the CsHSO4–CsH2PO4 system, which proceeds upon heating. It is reported that Cs3(HSO4)2(H2PO4) undergoes a transformation into a superprotonic phase at about 119 °C [2], Cs5(HSO4)3 (H2PO4)2 a two phase transition at 116 and 137 °C [3]. Cs3 (HSO4)2(H2PO4) is reported to decompose at 155 °C and the

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decomposition is maximized at 180 °C with the loss of water [2]. Preliminary DTA-TG measurements showed almost no weight loss in the temperature range from 100 to 190 °C. Melting was not observed for the mechanically milled composite. Thus, the irreversible reaction of the mechanically milled sample should be ascribed to the phase transition from Cs 3(HSO4)2(H2PO 4) and/or Cs5(HSO4) 3(H2PO 4)2 to the corresponding high temperature phases, or the formation of another new sulfate–phosphate class of solid acid compound. Temperature dependence of conductivities of CsHSO4– CsH2PO4 (1:1 mole ratio) before and after milling (720 rpm for 30 min) is shown in Fig. 3. Triangles are for the sample before milling and circles for the sample after milling. Closed and open marks represent the upon heating and cooling processes, respectively. The conductivity of the sample before milling is about 1 × 10− 6 S cm− 1 at 60 °C and increases to 3 × 10− 4 S cm− 1 at around 180 °C upon heating. On the cooling process, the values disagree with those of the heating process at a given temperature, which may be caused by the structural changes and/or the formation of minor crystalline phase in the CsHSO4– CsH2PO4 system as well as the well-known hysteresis behavior. The mechanochemically treated composite consisting of Cs3 (HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 shows higher conductivity than the untreated sample. Conductivity of the mechanochemically treated composite significantly increases at around 90 °C upon heating. The value becomes 2 × 10− 3 S cm− 1 at around 180 °C. On the other hand, no steep decrease is observed upon cooling. The activation energy of the mechanically milled sample with high conductivities was estimated to be about 0.3 eV for the cooling process. It is interesting that the milled samples maintain relatively high conductivity even at temperatures lower than 30 °C on the cooling process. To clarify the crystalline phase in the mechanochemically treated composite at temperatures higher that 100 °C, in situ

Fig. 4. XRD patterns with the corresponding DSC curve for CsHSO4–CsH2PO4 (1:1 mole ratio) composite milled at 720 rpm for 30 min. The heating rate was 5 °C min− 1.

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measurements of high temperature XRD and DSC were carried out. XRD patterns with the corresponding DSC curve for CsHSO4–CsH2PO4 (1:1 mole ratio) composite milled at 720 rpm for 30 min are shown in Fig. 4. In the measurements, the samples were heated and cooled in the temperature range from room temperature to 180 °C at 5 °C min− 1 under N2 atmosphere. In addition, the sample was kept at 110 °C for 10 min on the heating process for the identification of the crystalline phase. As shown in Fig. 1, the mechanochemically treated composite is composed of Cs3(HSO4)2(H2PO4) and Cs5 (HSO4)3(H2PO4)2. When the temperature increased, the main phase became Cs5(HSO4)3(H2PO4)2. This was confirmed by the decrease of the peak at 27.9° due to Cs3(HSO4)2(H2PO4) while the other peaks were overlapping to those due to Cs5(HSO4)3 (H2PO4)2. At temperatures higher than 110 °C, the diffraction patterns become relatively simple, which consisted of peaks at 17.9°, 25.5°, 31.3° and 36.4° (CuKα). These peaks can be assigned to the high temperature, superprotonic phase of Cs2 (HSO4)(H2PO4) [3,13], which remains in a wide temperature range of heating up to 180 °C and cooling down to about 40 °C. The other endothermic peaks at around 130 °C probably reflect the stepwise phase transition of Cs2(HSO4)(H2PO4) or the stepwise formation of Cs2(HSO4)(H2PO4) from Cs3(HSO4)2 (H2PO4) and Cs5(HSO4)3(H2PO4)2. No Cs2(HSO4)(H2PO4) was formed by heating from the mixtures of CsHSO4 and CsH2PO4 without milling. It is an interesting phenomenon that the high temperature phase of Cs2(HSO4)(H2PO4) formed by heating from Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2, which were prepared from the mixture of CsHSO4 and CsH2PO4 by mechanical milling. It is also noteworthy that the high temperature phase was retained at room temperature after cooling. In situ XRD and DSC measurements demonstrate that the high proton conductivity and a low activation energy of about 0.3 eV for the proton conduction are owing to the presence of the high temperature phase of Cs2(HSO4)(H2PO4). The stability of the high temperature phase of Cs2(HSO4) (H2PO4) under an ambient atmosphere is now under study. 4. Conclusions Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 were formed in the mixture of CsHSO4 and CsH2PO4 by mechanochemical treatment. The mixture of Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3 (H2PO4)2 was transformed to Cs2(HSO4)(H2PO4) at around

100 °C upon heating. Conductivity of the mechanochemically treated composites significantly increased through the transition and then reached to 2 × 10− 3 S cm− 1 at around 180 °C. The high conductivity at temperatures higher than the transition temperature and its retention upon cooling process are probably achieved by the presence of a high temperature, superprotonic conducting phase of Cs2(HSO4)(H2PO4). Mechanochemical treatment for the mixture of solid acids should be a promising way to prepare a new family of solid acid compounds through a solid phase reaction. Acknowledgements The authors are grateful to Prof. M. Tatsumisago and his colleagues of Osaka Prefecture University for DSC measurements and valuable discussion. They also thank Dr. T. Kubo and his coworkers at the Rigaku Corporation for high temperature XRD-DSC measurements. This work was partly supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant-in-Aid for Scientific Research on Priority Areas No. 439 “Nanoionics,” A02 No. 17041006). AM acknowledges Izumi Science and Technology Foundation and Hosokawa Powder Technology Foundation. References [1] S.M. Haile, D.A. Boysen, C.R.I. Chisholm, R.B. Merie, Nature 410 (2001) 910. [2] S.M. Haile, G. Lentz, K.-D. Kreuer, J. Maier, Solid State Ionics 77 (1995) 128. [3] S.M. Haile, P.M. Calkins, J. Solid State Chem. 140 (1998) 251. [4] C.R.I. Chisholm, S.M. Haile, Solid State Ionics 136–137 (2000) 229. [5] P. Colomban, M. Pham-Thi, A. Novak, Solid State Ionics 24 (1987) 193. [6] X. Glipa, J.-M. Leloup, D.J. Jones, J. Rozière, Solid State Ionics 97 (1997) 227. [7] V.G. Ponomareva, N.F. Uvarov, G.V. Lavrova, Solid State Ionics 90 (1996) 161. [8] V.G. Ponomareva, N.F. Uvarov, G.V. Lavrova, Solid State Ionics 145 (2001) 197. [9] A. Matsuda, T. Kanzaki, K. Tadanaga, M. Tatsumisago, T. Minami, Electrochim. Acta 74 (2001) 939. [10] A. Matsuda, Y. Nono, K. Tadanaga, T. Minami, M. Tatsumisago, Solid State Ionics 162–163 (2003) 253. [11] A. Matsuda, T. Tezuka, Y. Nono, K. Tadanaga, T. Minami, M. Tatsumisago, Solid State Ionics 176 (2005) 2899. [12] V. Varma, N. Rangavittal, N.R. Rao, J. Solid State Chem. 106 (1993) 164. [13] S. Hayashi, M. Mizuno, Solid State Ionics 176 (2005) 745.