Solid State Ionics 97 (1997) 247–252
Tetra-alkylammonium cation clathrate hydrates as novel proton conductors Marcin Opallo*, Agnieszka Tymosiak-Zielinska, Zofia Borkowska Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44 /52, 01 -224 Warszawa, Poland
Abstract The conductivity of tetra-alkylammonium halide hydrates has been studied by impedance spectroscopy in the temperature range 213–322 K. Conductivity drops around their melting point and in some cases again about 20 K below it. At 250 K it is in the range 10 26 –10 29 S m 21 . Linear, Arrhenius type conductivity dependence is observed for lower temperatures and corresponding activation energies of conductivity are within 0.45–0.62 eV. At a given temperature the conductivity of tetra-alkylammonium halide hydrates is a few orders of magnitude lower than that of tetramethylammonium hydroxide hydrates. The reasons for this difference are discussed. Keywords: Clathrate hydrate; Ionic conductivity; Impedance spectroscopy; Solid electrolyte; Proton conductor Materials: (C 4 H 9 ) 4 NF ? 32H 2 O; (C 4 H 9 ) 4 NCl ? 30H 2 O; (C 4 H 9 ) 4 NBr ? 32H 2 O; (C 2 H 5 ) 4 NF ? 5H 2 O; (CH 3 ) 4 NF ? 4H 2 O; (CH 3 ) 4 NOH ? 5H 2 O; (CH 3 ) 4 NOH ? 7.5H 2 O; (CH 3 ) 4 NOH ? 10H 2 O
1. Introduction Since 1940, it is recognized that tetra-alkylammonium cations form stoichiometric hydrates with different anions [1]. From the point of view of their structure they are clathrates, defined by Powell [2] as compounds in which two or more components are associated without ordinary chemical union but through complete enclosure of one set of molecules in a suitable structure formed by another one. When this structure is formed by water molecules they are called clathrate hydrates. From a chemical point of view they can be divided into two groups: tetraalkylammonium hydroxide and salt hydrates. Recent*Corresponding author.
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0167-2738 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0167-2738( 97 )00080-5
ly it has been shown that tetra-alkylammonium cation clathrate hydrates form a new class of solid electrolytes, probably protonic conductors [3–9]. Until now, mainly tetramethylammonium hydroxide hydrates were the subject of the conductivity studies [3–8]. The conductivity of tetra-alkylammonium salt hydrates received less attention [5,7,9], despite the fact that they were recently used as electrolytes for fundamental studies of electrochemical redox reactions [10–12]. Earlier the investigation of electrochemical reaction over the wide temperature range, below and above the melting point of stoichiometric electrolyte, upon freezing, was called FRozen Electrolyte ElectroChEmistry (FREECE), and the electrolytes used in these experiments were named frozen electrolytes [13,14]. From the point of view of their stoichiometry,
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tetra-alkylammonium halide hydrates can be divided into two classes: one with a number of water molecules # 10 per tetra-alkylammonium cation and the number of carbon atoms in the alkyl group equal to 1–3 and another with about 30 water molecules per tetra-alkylammonium cation and the number of carbon atoms in the alkyl group equal to 4–5. All these electrolytes have been reported to form stoichiometric clathrate hydrates [15–23] with open structure. It consists of an anionic ‘host’ network formed by hydrogen bonded water molecules and anions like fluorides with voids filled by tetraalkylammonium cations. The increasing number of water molecules is a consequence of the increasing size of tetra-alkylammonium cation and increasing number of carbon–hydrogen bonds. The structure of these compounds is different than that of tetramethylammonium hydroxide hydrates. The latter form closed structures where tetramethylammonium cations are trapped in cages formed by a water and hydroxide ions network [23]. Until now the only detailed conductivity studies of tetra-alkylammonium halide hydrates are that of (CH 3 ) 4 NF ? 4H 2 O, (TMAF4) [9] belonging to the first group. About a two orders of magnitude ionic conductivity (s ) drop is observed at temperatures around TMAF4 melting point. An Arrhenius type temperature dependence of s is observed for the temperature ranges corresponding to the high and low temperature phase of solid TMAF4. The corresponding activation energies of conductivity (Ea ) are relatively large and they are equal to 1.93 and 0.48 eV, respectively [9]. The aim of this paper was to study the temperature dependence of conductivity of a range of tetra-
alkylammonium halide hydrates. For this purpose, the following tetralkylammonium halide hydrates were selected: tetraethylammonium fluoride pentahydrate: (C 2 H 5 ) 4 NF ? 5H 2 O (TEAF5), and three tetrabutylammonium halide hydrates with approximately the same amount of stoichiometric water and different anions: (C 4 H 9 ) 4 NF ? 32H 2 O (TBAF32) (m.p. 5 300 K), (C 4 H 9 ) 4 NCl ? 30H 2 O (TBACl30) (m.p. 5 288 K) and (C 4 H 9 ) 4 NBr ? 32H 2 O (TBABr32) (m.p. 5 285 K). The data obtained in this work and literature [9] conductivity data for tetra-alkylammonium halide hydrates will be compared with that for tetramethylammonium hydroxide hydrates [8]. They will be also compared with the conductivities of other frozen electrolytes.
2. Experimental TEAF5, 98% pure from Janssen Chimica, was used as received. TBAF32, TBACl30 and TBABr32 were prepared by crystallization from 0.5–2 M TBAF, TBACl and TBABr aqueous solution [17], respectively. They were purified by recrystallization from water. Both crystallizations were performed at about 278 K. The purity of the obtained hydrate was checked by melting point measurements. The differences between measured melting points and literature data (see Table 1) [19] was smaller than 1 K. Its stoichiometry was also confirmed by comparing the magnitude of butyl and water proton signals on 2 H NMR spectra. The experimental details were the same as previously described [8,9]. The temperature range depends on the electrolyte and was within the 190–330
Table 1 The comparison of melting point temperatures, T, conductivities at 250 K, s250 K and conductivity activation energies, Ea , of tetralkyammonium halide hydrates Electrolyte (n-C 4 H 9 ) 4 NBr?32H 2 O (n-C 4 H 9 ) 4 NCl?30H 2 O (n-C 4 H 9 ) 4 NF?32H 2 O (C 2 H 5 ) 4 NF?5H 2 O a-(CH 3 ) 4 NF?4H 2 O Ice a b
(b↔a) solid–solid transition. Melting point.
T (K) 285 288 300 298 303 a , 319 b 273
s250 K (S m 21 ) 26
1.8310 1.7310 28 1.0310 29 3.1310 28 6.0310 29 1.0310 211
Ea (eV) 0.4560.03 0.5560.04 0.5260.04 0.6260.10 0.4860.03 0.55
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K range. The upper limit was defined by the electrolyte melting point and the lower by the input impedance of the experimental setup. Generally the frequency of the a.c. signal was in the range 10 21 – 10 5 Hz. At lower temperatures where bulk resistance was too large, the high frequency part was cut off because of the limited bandwidth of the current follower.
3. Results and discussion
3.1. The conductivity of tetra-alkylammonium halide hydrates The impedance of the cell Au u frozen tetralkylammonium halide hydrate u Au depends on temperature. As an example, the impedance spectrum of the cell Au u TBAF32 u Au, obtained at a temperature below the melting point of electrolyte is presented in the form of Nyquist plots in the admittance plane (Fig. 1). Their shape indicates at least one and two time constants at lower and higher temperatures, respectively. The fitting of the equivalent circuit to impedance data confirms this observation. In all cases these time constants are of distributed character. Such a behaviour is general in the case of all electrolytes studied. We will not analyze it in detail,
Fig. 1. Plot of the imaginary component of admittance, Y0, against real component of admittance, Y9, of the cell Au u frozen TBAF32 u Au at 285 K. The frequency difference between points marked by larger filled symbols is equal to one order of magnitude. The solid line corresponds to simulated spectra of the best fit equivalent circuit presented in the figure.
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noting only that such behaviour is typical for a polycrystalline electrolyte between two solid electrodes [24,25]. The values of s were estimated from the value of the resistive element in series with the rest of the best fit equivalent circuit or from the extrapolated value of Z9 at a frequency where Z0 is close to zero getting the same results. The corresponding data, in the form of log s vs. T 21 plots, are presented on Fig. 2 and Fig. 3. For all tetrabutylammonium halide hydrates, the temperature dependence of s exhibits some common features (Fig. 2). The s drops around the melting point of the electrolyte and around 270–275 K. The second drop on the s vs. T 21 plot may be an indication of a solid–solid phase transition. However, phase diagrams of the tetrabutylammonium halide–water system do not indicate that [22]. For the lowest temperatures approximately linear dependencies with similar slopes are observed. At temperatures below the melting point of the electrolyte some anion effect on the value of s is observed. The conductivity changes in the order Br 2 . Cl 2 . F 2 (see below). The second drop in conductivity occurs at a temperature close to the freezing point of water, therefore it is probable that at higher temperatures some part of the electrolyte, for example intergrain space, remains liquid. It is interesting to look on the
Fig. 2. Plot of the logarithm of the ohmic conductivity, s of tetrabutylammonium halide hydrates: TBAF32 (j), TBACl30 (m) and TBABr32 (d) against reciprocal of temperature, T. Dotted lines mark melting point.
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Fig. 3. Plot of the logarithm of the ohmic conductivity, s, of tetra-alkylammonium fluoride hydrates: TMAF4 (.) [8], TEAF5 (♦) and TBAF32 (j) against reciprocal of temperature, T. Dashed and dotted lines mark melting point and phase transition temperatures, respectively.
temperature dependence of the conductivity of TBAF32 containing a 1:1 mixture of K 3 Fe(CN) 6 and K 4 Fe(CN) 6 [11]. It has been proven that redox active ions segregate from the bulk electrolyte [10– 12], causing an increase in ionic conductivity, except that the temperature range between the two conductivities drops, this is observed in pure electrolyte [11]. It is possible that at this temperature range the intergrain space conductivity dominates. The behaviour of other halide hydrates, namely TEAF5 and TMAF4, is different. In this case we do not observe the second drop in conductivity (Fig. 3). On the other hand, for both electrolytes some slope change of s vs. T 21 dependence is observed (Fig. 4). In the case of TMAF4, the temperature where it occurs coincides with the temperature of the solid– solid phase transition [8,9].
3.2. The conductivity mechanism of tetraalkylammonium cation hydrates It is interesting to compare conductivities and conductivity activation energies of tetra-alylammonium cation hydrates. The corresponding data are summarized in Table 1. In the case of halide hydrates some effect of cation and anion on the value of s250 K is observed. The values of Ea are relatively large, and according to a
Fig. 4. Plot of the logarithm of the ohmic conductivity, s, of tetra-alkylammonium halide and hydroxide hydrates: TBAF32 (j), TMAF4 (.) [8], TMAOH5 (x) [7] and TMAOH7.5 (s) [7] against reciprocal of temperature, T.
generally accepted definition [32], all tetera-alkylammonium halide hydrates are so called ‘normal ionic conductors’. Their values of Ea are in the range 0.45–0.62 eV, similar to that of other halide hydrates not forming clathrate structures, like ammonium [26–28] and alkali metal hetropolycompounds [29] or (NH 4 ) 4 Fe(CN) 6 ? 1.5H 2 O [30]. This is not the case for tetramethylammonium hydroxide hydrates which can be called superionic conductors. Obviously there is a question of the nature of charge carriers and conductivity mechanism in frozen electrolytes. The view of acid and hydroxide hydrates, including (CH 3 ) 4 NOH ? 5H 2 O (TMAOH5), as protonic conductors was based on the similarity of activation energies of conductivity and that of reorientation of water molecules, obtained from the temperature dependence of 2 H NMR spectra [31,32]. There are no corresponding NMR data for tetra-alkylammonium halide hydrates. The conductivity within the TBA1 hydrates group is the smallest for halide hydrate of anion being expected to be most mobile, namely F 2 . However according to structural data, F 2 anions are immobile, being a part of the host network. Therefore, one may conclude that contribution of this anion to the conductivity of tetrbutylammonium fluoride hydrate is unprobable and it can be regarded as a proton conductor. The similar value of Ea of ice [33], where anions are practically not present, support this con-
M. Opallo et al. / Solid State Ionics 97 (1997) 247 – 252
clusion. The question arises, why conductivity of tetrabutylammonium cation hydrates is anion dependent. We cannot discuss this point from a structural point of view because the details of the structure of TBABr32 and TBACl30 are not known. However, the similar value of Ea of fluorides and ice may be the indirect indication of the protonic nature of their conductivity. The difference in their conductivity may arise from some participation of the anion in the conduction process: the anions can be charge carriers or they can interact with mobile protons affecting their mobility. Some light may be shed on this problem by comparison with hydroxide hydrates. According to the Nernst–Einstein equation, conductivity, s, at given temperature, T, is proportional to the product of concentration of charge carriers, C, and their mobility, expressed in terms of diffusion coefficient, D:
s 5 DCe 2 /kT.
(1)
The value of D is a function of the number of conduction paths and includes structural effect. The latter may be very important as it is observed in the case of (CH 3 ) 4 NOH hydrates [8]. At 250 K the conductivity of the hydrates (CH 3 ) 4 NOH ? 7.5H 2 O (TMAOH7.5) and (CH 3 ) 4 NOH?10H 2 O (TMAOH10) is by two orders of magnitude larger than that of TMAOH5. The difference in concentration of hydroxide defects is too small to account for such a change in conductivity and the observed effect was assumed structural [8]. If all defects are active in the conduction process, the difference between s values of TMAOH hydrates and tetraalkylammonium halide hydrates should be more than six orders of magnitude. This is indeed the case of the best and the poorest conductors among tetraalylammonium clathrate hydrates, namely TMAOH7.5 and TBAF32 at 250 K. The differences between conductivities of other tetra-alkylammonium hydroxides and halide hydrates are substantially smaller. It is also interesting to note that the pH of liquid tetra-alkylammonium halide hydrates is between 8–9, which may in part explain the smaller than seven orders of magnitude difference in conductivity. However, the number of charge carriers is not the only possible factor. The difference in their
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structure, namely the number or density of conduction paths, can be one of the reasons for the observed differences. A more sophisticated model, taking into account structural features of the protonic conductor and aimed directly at low temperature proton transport, was recently proposed and applied to stoichiometric acid clathrate hydrates [34]. Under several assumptions the authors were able to predict the correct order of magnitude of conductivity of HClO 4 ? 5.5H 2 O, which can be considered being successful.
4. Conclusions The tetralkylammonium cation clathrate hydrates represent a new class of proton conductors. The tetra-alkylammonium halide hydrates belong to the normal ionic conductor group, whereas more conductive tetra-alkylammonium hydroxide hydrates are superionic conductors [35].
Acknowledgments This research has been sponsored by State Committee for Scientific Research through grant 2 P303 052 05 and by research funds of the Institute of Physical Chemistry of Polish Academy of Sciences.
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