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Electrical and electrochemical processes in solid tetrabutylammonium hydroxide hydrate
Solid State Ionics 145 Ž2001. 407–413 www.elsevier.comrlocaterssi
Electrical and electrochemical processes in solid tetrabutylammonium hydroxide hydrate Agnieszka Prokopowicz, Marcin Opallo ) Institute of Physical Chemistry, Polish Academy of Sciences, ul.Kasprzaka 44 r 52, 01-224 Warsaw, Poland Received 4 September 2000; received in revised form 12 January 2001; accepted 15 February 2001
Abstract The electrical and electrochemical processes in solid tetrabutylammonium hydroxide hydrate ŽC 4 H 9 .4 NOHP 30H 2 O ŽTBAOH30. have been studied in the temperature range 80–305 K by impedance spectroscopy and linear sweep voltammetry. The shape of impedance spectra of an Au
1. Introduction Electrical w1–11x and electrochemical w12–16x processes in stoichiometric polyhydrates have recently received some attention. Large multiatomic ions are components of these solids. From the structure’s point of view, these compounds belong to the
) Corresponding author. Tel.: q48-22-6323221; fax: q48-39120238. E-mail address: [email protected] ŽM. Opallo..
clathrate family w17x. Anions like ClO4y, PF6y or BF4y or tetraalkylammonium cations are trapped in cages or voids formed by water and oxonium or hydroxide ion network w18–21x. At low temperatures, they exhibit relatively high electrical conductivity Ž s . w1–11x. Two tetramethylammonium hydroxide hydrates are superionic conductors w8–10x. This is probably due to high delocalization of protons over the water network. Indeed, the results of NMR studies of stoichiometric polyhydrates w14,22x show that protons are mobile species within their structure. The conductivity of clathrate tetramethylammonium hydroxide hydrates w6–10x is, by a few
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 9 3 7 - 7
408
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
orders of magnitude, larger than that of nonclathrate alkali metal hydroxide hydrates w2x. For example at 200 K, it is equal to 10y5 S my1 w9x and 10y1 2 S my1 w2x for ŽCH 3 .4 NOH P 7.5H 2 O and KOH P 4H 2 O, respectively. Also, the activation energy of conductivity is much smaller for clathrate hydroxide hydrates and for the compounds mentioned above is equal to 0.19 and 0.70 eV, respectively w2,9x. This may indicate that the clathrate structure is probably responsible for its high conductivity. Stoichiometric clathrate polyhydrates were also the subject of electrochemical studies w11–16x. They are good media for study of the hydrogen evolution reaction ŽHER. at low temperatures, because of the large concentration of delocalised protons and high electrical conductivity. Until now, these studies were performed in perchloric acid hydrate: HClO4 P 5.5H 2 O and its deuterated derivative at different electrodes w11–15x. Recently, we reported the results of HER studies in tetramethylammonium hydroxide hydrate ŽCH 3 .4 NOH P 10H 2 O ŽTMAOH10. down to 110 K w16x. In this paper, we would like to present electrical and electrochemical properties of another tetraalkylammonium clathrate hydrate: tetrabutylammonium hydroxide hydrate ŽC 4 H 9 .4 NOH P 30H 2 O ŽTBAO H30. w23–25x in a wide temperature range. We will focus on the conductivity of TBAOH30 and electrochemical hydrogen evolution from this electrolyte. Both homogeneous and heterogenous processes must be somehow related because they involve proton transfer. TBAOH30 belongs to the family of stoichiometric hydrates composed of tetrabutylammonium cation and halide ŽBry, Cly, Fy . or hydroxide anion. The number of water molecules in these solids ranges from 28 to 32. The melting point depends on the anion and it varies from 285 Žbromide. to 300 K Žhydroxide. w24x. The conductivity of tetrabutylammonium halide hydrates is rather low w10x. The detailed study of the ŽC 4 H 9 .4 NOH P H 2 O system shows that two tetrabutylammonium hydroxide hydrate form with very similar composition: congruently melting Žmp s 293.3 K. ŽC 4 H 9 .4 NOH P 28.3H 2 O and noncongruently melting ŽC 4 H 9 .4 NOH P 32H 2 O Žmp s 300.8 K.. According to Dyadin and Udachin w25x the structure of both compounds is typical of a clathrate polyhydrate.
2. Experimental 2.1. Chemicals TBAOH30 ŽFluka. and HClO4 ŽAldrich. were q99% pure and they were used without further purification. Differential scanning calorimetry experiments reveal that the former compound melts congruently at 303 K. The water used for the cell and electrodes preparation was purified by an ELIX system ŽMillipore.. For Pd electrode preparation hot water distilled from quartz was used. 2.2. Electrodes Two Au plate electrodes Ž S s 1 cm2 . separated by a Teflon w spacer were used in impedance spectroscopy experiments. For voltammetry, a three-electrode cell was used. The working Au Ž99.99% purity. wire electrode Ž S s 0.37 cm2 . was washed extensively with water. It was flame annealed three times in a reducing Bunsen burner flame and quenched in water w26x. After this procedure, the electrode was immediately immersed in liquid TBAOH30 at about 305 K together with an Au wire counter and PdŽH 2 . wire reference electrode. The latter was freshly prepared from Pd wire in aqueous HClO4 according to the literature procedure w27x. The potential of this electrode is stable for at least 20 h in concentrated aqueous hydroxide solution w28x. 2.3. Apparatus and procedures The Teflon w cylindrical cell tightly embedded in a stainless steel cylinder was filled with 1.5 cm3 of liquid TBAOH30. Before immersion of the cell into the cooling system, the electrolyte was saturated with bubbling argon for at least 30 min. After which a constant flow of argon over the surface of the electrolyte was maintained. The details of temperature control equipment were already described w16x. The temperature of the cell ranged from 80 to 305 K. Its upper limit is due to the slow decomposition of the electrolyte. The temperature was decreased at a rate smaller than 1 K miny1 . Before a given measurement, the cell was kept for at least 15 min at constant temperature to allow thermal equilibration.
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
The impedance spectra of the Au
409
Fig. 1. Plot of the magnitude of impedance Ž< Z <. ŽA. and phase angle ŽQ ., ŽB. against the frequency, f, of the cell Au
3. Results and discussion 3.1. ConductiÕity The shape of impedance spectra of Au
for our system. The other CPE 1 together with resistors R 1 and R 0 represent the conduction process within the bulk phase. The last parameter was used in the estimation of bulk conductivity. Some evolution of the equivalent circuit with temperature is observed. Except for the lowest temperatures, it is always composed of at least two CPEs. Such a spectrum is typical for a polycrystalline electrolyte w32x and it is similar to those obtained for other tetraalkylammonium electrolytes w8–10x. More detailed analysis of the impedance spectra is not possible, because of the lack of detailed information on the structure and size of electrolyte grains. Interestingly, the structure of the best-fit equivalent circuit is preserved also above the melting point, which may indicate structuring of the liquid electrolyte, although the recent neutron scattering w33x and FTIR w34x studies of aqueous solutions indicate
410
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
of charge carriers Ž c . and their mobility expressed in terms of the diffusion coefficient Ž D .,
s s Dce 2rkT .
Y Fig. 2. Plot of the imaginary component of impedance Ž Z ., X against its real component Ž Z ., of the cell Au
no change of water structure near the nonpolar group. However, in a concentrated solution of tetrabutylammonium salts, an increased population of water–hydrogen bonds around tetrabutylammonium cations was detected w34x. This distribution allows for speculation about the existence of protonic conduction paths also in a liquid electrolyte. The values of s are presented in the form of log s vs. Ty1 plot ŽFig. 3.. It is clear that at temperatures not far below the melting point, the activation energy is larger than at temperatures below 200 K. The values of the activation energy Ž EaŽ s . . are equal to 0.31 and 0.18 eV, respectively. One may conclude that at lower temperatures, TBAOH30 is a superionic conductor w35x. No deviation from Arrhenius behaviour at temperatures about 100 K is observed, contrary to HClO4 P 5.5H 2 O and its deuterated derivative w7x. It is interesting to compare the conductivity of TBAOH30 with other tetrabutylammonium clathrate hydrates, as well other clathrate polyhydrates. According to the Nernst–Einstein equation, the conductivity is proportional to the product of concentration
Ž 1.
At the arbitrarily selected temperature of 250 K, the value of s is approximately seven orders of magnitude larger than that of structurally related tetrabutylammonium fluoride hydrate: ŽC 4 H 9 .4 NF P 32H 2 O ŽTBAF32. w10x. This may be a simple reflection of the difference of the concentration of mobile charge carriers; equal to the difference in concentration of OHy. However, this may not be the only important factor, because the conductivity of other tetrabutylammonium halide hydrates like ŽC 4 H 9 .4 NCl P 30H 2 O or ŽC 4 H 9 .4 NBr P 32H 2 O is larger than that of TBAF32 w10x. The value of EaŽ s . of TBAOH30 is smaller than that of tetrabutylammonium halide hydrates Ž0.45– 0.52 eV w6x.. This indicates that the replacement of halide by OHy ions decreases the barrier for charge Žproton. transfer. Probably, the degree of proton delocalisation is larger in a network formed by water and hydroxide ions. In the tetrabutylammonium halide hydrates, halide anions are part of the clathrate network w25x. Their presence may make proton mo-
Fig. 3. Plot of the logarithm of the ohmic conductivity Ž s . of TBAOH30 against the reciprocal of temperature ŽT .. Dashed line marks the melting point.
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
411
tion more difficult than in hydroxide hydrates. Interestingly, the value of EaŽ s . of TBAOH30 is similar to that of two tetramethylammonium hydroxide hydrates ŽCH 3 .4 NOH P 7.5H 2 O and TMAOH10 w5x. Also, the relationship between EaŽ s . of these two compounds and their fluoride analogue ŽCH 3 .4 NF P 4H 2 O w8x is similar to that observed for tetrabutylammonium cation hydrates. This may indicate that the effect of the replacement of halide by hydroxide ions on the activation barrier of conductivity is more general. 3.2. Electrochemical hydrogen eÕolution Examples of current–potential curves corresponding to cathodic reaction obtained at selected temperatures are presented in Fig. 4. The shape of the voltammogram does not depend on the direction of the scan and the scan rate. In order to analyse data as a function of overpotential Žh ., its temperature dependence h ŽT . was calculated from the following equation,
h Ž T . s Em Ž T . y EPd Ž T . y IR Ž T .
Ž 2.
where EmŽT . is a measured potential and EPd ŽT . is a potential of PdŽH 2 . reference electrode. It has been assumed that the latter is equal to the standard
Fig. 5. Plot of the logarithm of the hydrogen evolution current density Ž J . against overpotential Žh . at selected temperatures.
potential of the hydrogen evolution reaction. The temperature dependence E Pd ŽT . Žin volts vs. normal hydrogen electrode. was taken into account by using the following equation w12x, EPd Ž T . s 0.196 y 0.491 = 10y3 T .
Ž 3.
For the relatively negative h , the semilogarithmic plot of current density Ž J . vs. h is approximately linear ŽFig. 5. indicating independence of the mechanism of HER from potential. Therefore, the data could be analysed using the Tafel equation,
h s a q Ž a n obs Fr2.3 RT . = log J
Fig. 4. Plot of the hydrogen evolution current Ž I . against potential Ž E . at selected temperatures marked on the figure.
Ž 4.
where a is a constant and a n obs is a charge transfer coefficient being a measure of the symmetry of the activation barrier. The value of a n obs is close to that observed in alkali metal hydroxide w36x or tetramethylammonium hydroxide concentrated solutions w16x. It is temperature dependent, namely the lower the temperature, the smaller the slope. This is similar to the Au
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A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
trolyte near the electrode surface may be different from that in the bulk.
4. Conclusions TBAOH30 is another example of highly conducting electrolyte within the tetraalkylammonium clathrate hydrate family. At temperatures below 200 K, it behaves as a superionic conductor. It is also another example of protonic conductor, which can be applied for electrochemical hydrogen evolution studies at low temperatures.
Acknowledgements Fig. 6. Plot of the logarithm of the hydrogen evolution current density Ž J . against the reciprocal of temperature ŽT . for selected overpotentials Žh . equal to 0.60, 0.65, 0.70, 0.75, 0.80 Žfrom the bottom to the top..
The help of Dr. Ewa Utzig in differential scan calorimetry experiments is gratefully acknowledged.
References drophobic tetrabutylammonium cations adsorbed at the negatively charged electrode w37x. In the case of TMAOH10, this phenomenon is improbable, because of the weak adsorption of the TMAq cations at the Au electrode w38x. The value of the activation energy of HER Ž EaŽH ER. . at the Au
Ž 5.
The larger energy of activation of the heterogeneous process may result from the strong penetration of electric field into electrolyte. It may hamper reorientation of water molecules w39x that should accompany proton transfer. Also, the structure of the elec-
w1x M. Cappadonia, U. Stimming, J. Electroanal. Chem. 300 Ž1991. 235. w2x Z. Borkowska, U. Frese, E. Kriegsman, U. Stimming, M. Gavish, K.J. Adamic, S.G. Greenbaum, Ber. Bunsen-Ges. Phys. Chem. 95 Ž1991. 1033. w3x Z. Borkowska, M. Cappadonia, U. Stimming, Electrochim. Acta 42 Ž1997. 841. w4x S. Aschrafi-Mahabadi, M. Cappadonia, U. Stimming, Solid State Ionics 70r71 Ž1994. 311. w5x M. Cappadonia, A.A. Kornyshev, A.M. Kuznetsov, S. Krause, U. Stimming, J. Chem. Phys. 101 Ž1994. 9672. w6x N. Kuriyama, T. Sakai, H. Miyamura, A. Kato, H. Ishikawa, J. Electrochem. Soc. 137 Ž1990. 355. w7x N. Kuriyama, T. Sakai, H. Miyamura, A. Kato, H. Ishikawa, Solid State Ionics 40r41 Ž1990. 906. w8x M. Opallo, A. Tymosiak, Z. Borkowska, J. Electroanal. Chem. 387 Ž1995. 47. w9x Z. Borkowska, A. Tymosiak, M. Opallo, J. Electroanal. Chem. 406 Ž1996. 109. w10x M. Opallo, A. Tymosiak-Zielinska, Z. Borkowska, Solid State Ionics 97 Ž1997. 247. w11x M. Cappadonia, S. Krause, U. Stimming, Electrochim. Acta 42 Ž1997. 841. w12x U. Frese, T. Iwasita, W. Schmickler, U. Stimming, J. Phys. Chem. 89 Ž1985. 1059. w13x U. Frese, U. Stimming, J. Electroanal. Chem. 198 Ž1986. 409. w14x T.H. Huang, E. Davis, U. Frese, U. Stimming, J. Phys. Chem. 92 Ž1988. 6874.
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413 w15x U. Frese, W. Schmickler, Ber. Bunsen-Ges. Phys. Chem. 92 Ž1988. 1412. w16x M. Opallo, A. Prokopowicz, Electrochem. Commun. 2 Ž2000. 23. w17x D.L. Fowler, W.V. Loebenstein, D.B. Call, C.A. Kraus, J. Am. Chem. Soc. 62 Ž1940. 1140. w18x G.A. Jeffrey, Acc. Chem. Res. 2 Ž1969. 344. w19x D. Mootz, M. Wiebke, Z. Kristallogr. 170 Ž1985. 194. w20x D. Mootz, E.-F. Oellers, M. Wiebke, J. Am. Chem. Soc. 109 Ž1987. 109. w21x D. Mootz, R. Seidel, J. Inclusion Phenom. 8 Ž1990. 139. w22x C.I. Ratcliffe, S.G. Garg, D.E. Davidson, J. Inclusion Phenom. 8 Ž1990. 159. w23x L.S. Aladko, Yu.A. Dyadin, T.M. Polyanskaya, I.S. Terekhova, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk Ž1977. 41. w24x Yu. Dyadin, K.A. Udachin, J. Inclusion Phenom. 2 Ž1984. 61. w25x Yu.A. Dyadin, K.A. Udachin, Zh. Strukt. Khim. 28 Ž1987. 76. w26x A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris ŽEds.., Modern Aspects of Electrochemistry, vol. 16. Plenum, New York, 1985, Chap. 1.
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w27x L.P. Cheng, MS Thesis, Columbia University, New York, 1989. w28x A. Tymosiak-Zielinska, PhD Thesis, Institute of Physical Chemistry, Warszawa, 1999. w29x W.R. Fawcett, Z. Kovacova, A.J. Motheo, C.A. Foss, J. Electroanal. Chem. 326 Ž1992. 91. w30x B. Piela, P.K. Wrona, J. Electroanal. Chem. 388 Ž1995. 69. w31x Z. Borkowska, M. Opallo, A. Tymosiak-Zielinska, P. Zoltowski, Colloids Surf., A: Physicochem. Eng. Aspects 134 Ž1998. 63. w32x N. Bonanos, B.C.H. Steele, E.P. Butler, in: J.R. Mcdonald ŽEd.., Impedance Spectroscopy. Wiley Interscience, New York, 1987, Chap. 4. w33x J.L. Finney, Faraday Discuss. 103 Ž1996. 1. w34x J. Stangret, T. Gampe, J. Phys. Chem. B 103 Ž1999. 3778. w35x P. Colomban ŽEd.., Proton Conductors. Cambridge Univ. Press, Cambridge, 1992, Preface. w36x T. Ohmori, M. Enyo, Electrochim. Acta 37 Ž1992. 2021. w37x A. Tymosiak-Zielinska, Z. Borkowska, in preparation. w38x A. Tymosiak-Zielinska, Z. Borkowska, Electrochim. Acta 45 Ž2000. 841. w39x A.A. Kornyshev, A.M. Kuznetsov, U. Stimming, J. Chem. Phys. 106 Ž1998. 9523.
Electrical and electrochemical processes in solid tetrabutylammonium hydroxide hydrate Agnieszka Prokopowicz, Marcin Opallo ) Institute of Physical Chemistry, Polish Academy of Sciences, ul.Kasprzaka 44 r 52, 01-224 Warsaw, Poland Received 4 September 2000; received in revised form 12 January 2001; accepted 15 February 2001
Abstract The electrical and electrochemical processes in solid tetrabutylammonium hydroxide hydrate ŽC 4 H 9 .4 NOHP 30H 2 O ŽTBAOH30. have been studied in the temperature range 80–305 K by impedance spectroscopy and linear sweep voltammetry. The shape of impedance spectra of an Au
1. Introduction Electrical w1–11x and electrochemical w12–16x processes in stoichiometric polyhydrates have recently received some attention. Large multiatomic ions are components of these solids. From the structure’s point of view, these compounds belong to the
) Corresponding author. Tel.: q48-22-6323221; fax: q48-39120238. E-mail address: [email protected] ŽM. Opallo..
clathrate family w17x. Anions like ClO4y, PF6y or BF4y or tetraalkylammonium cations are trapped in cages or voids formed by water and oxonium or hydroxide ion network w18–21x. At low temperatures, they exhibit relatively high electrical conductivity Ž s . w1–11x. Two tetramethylammonium hydroxide hydrates are superionic conductors w8–10x. This is probably due to high delocalization of protons over the water network. Indeed, the results of NMR studies of stoichiometric polyhydrates w14,22x show that protons are mobile species within their structure. The conductivity of clathrate tetramethylammonium hydroxide hydrates w6–10x is, by a few
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 9 3 7 - 7
408
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
orders of magnitude, larger than that of nonclathrate alkali metal hydroxide hydrates w2x. For example at 200 K, it is equal to 10y5 S my1 w9x and 10y1 2 S my1 w2x for ŽCH 3 .4 NOH P 7.5H 2 O and KOH P 4H 2 O, respectively. Also, the activation energy of conductivity is much smaller for clathrate hydroxide hydrates and for the compounds mentioned above is equal to 0.19 and 0.70 eV, respectively w2,9x. This may indicate that the clathrate structure is probably responsible for its high conductivity. Stoichiometric clathrate polyhydrates were also the subject of electrochemical studies w11–16x. They are good media for study of the hydrogen evolution reaction ŽHER. at low temperatures, because of the large concentration of delocalised protons and high electrical conductivity. Until now, these studies were performed in perchloric acid hydrate: HClO4 P 5.5H 2 O and its deuterated derivative at different electrodes w11–15x. Recently, we reported the results of HER studies in tetramethylammonium hydroxide hydrate ŽCH 3 .4 NOH P 10H 2 O ŽTMAOH10. down to 110 K w16x. In this paper, we would like to present electrical and electrochemical properties of another tetraalkylammonium clathrate hydrate: tetrabutylammonium hydroxide hydrate ŽC 4 H 9 .4 NOH P 30H 2 O ŽTBAO H30. w23–25x in a wide temperature range. We will focus on the conductivity of TBAOH30 and electrochemical hydrogen evolution from this electrolyte. Both homogeneous and heterogenous processes must be somehow related because they involve proton transfer. TBAOH30 belongs to the family of stoichiometric hydrates composed of tetrabutylammonium cation and halide ŽBry, Cly, Fy . or hydroxide anion. The number of water molecules in these solids ranges from 28 to 32. The melting point depends on the anion and it varies from 285 Žbromide. to 300 K Žhydroxide. w24x. The conductivity of tetrabutylammonium halide hydrates is rather low w10x. The detailed study of the ŽC 4 H 9 .4 NOH P H 2 O system shows that two tetrabutylammonium hydroxide hydrate form with very similar composition: congruently melting Žmp s 293.3 K. ŽC 4 H 9 .4 NOH P 28.3H 2 O and noncongruently melting ŽC 4 H 9 .4 NOH P 32H 2 O Žmp s 300.8 K.. According to Dyadin and Udachin w25x the structure of both compounds is typical of a clathrate polyhydrate.
2. Experimental 2.1. Chemicals TBAOH30 ŽFluka. and HClO4 ŽAldrich. were q99% pure and they were used without further purification. Differential scanning calorimetry experiments reveal that the former compound melts congruently at 303 K. The water used for the cell and electrodes preparation was purified by an ELIX system ŽMillipore.. For Pd electrode preparation hot water distilled from quartz was used. 2.2. Electrodes Two Au plate electrodes Ž S s 1 cm2 . separated by a Teflon w spacer were used in impedance spectroscopy experiments. For voltammetry, a three-electrode cell was used. The working Au Ž99.99% purity. wire electrode Ž S s 0.37 cm2 . was washed extensively with water. It was flame annealed three times in a reducing Bunsen burner flame and quenched in water w26x. After this procedure, the electrode was immediately immersed in liquid TBAOH30 at about 305 K together with an Au wire counter and PdŽH 2 . wire reference electrode. The latter was freshly prepared from Pd wire in aqueous HClO4 according to the literature procedure w27x. The potential of this electrode is stable for at least 20 h in concentrated aqueous hydroxide solution w28x. 2.3. Apparatus and procedures The Teflon w cylindrical cell tightly embedded in a stainless steel cylinder was filled with 1.5 cm3 of liquid TBAOH30. Before immersion of the cell into the cooling system, the electrolyte was saturated with bubbling argon for at least 30 min. After which a constant flow of argon over the surface of the electrolyte was maintained. The details of temperature control equipment were already described w16x. The temperature of the cell ranged from 80 to 305 K. Its upper limit is due to the slow decomposition of the electrolyte. The temperature was decreased at a rate smaller than 1 K miny1 . Before a given measurement, the cell was kept for at least 15 min at constant temperature to allow thermal equilibration.
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
The impedance spectra of the Au
409
Fig. 1. Plot of the magnitude of impedance Ž< Z <. ŽA. and phase angle ŽQ ., ŽB. against the frequency, f, of the cell Au
3. Results and discussion 3.1. ConductiÕity The shape of impedance spectra of Au
for our system. The other CPE 1 together with resistors R 1 and R 0 represent the conduction process within the bulk phase. The last parameter was used in the estimation of bulk conductivity. Some evolution of the equivalent circuit with temperature is observed. Except for the lowest temperatures, it is always composed of at least two CPEs. Such a spectrum is typical for a polycrystalline electrolyte w32x and it is similar to those obtained for other tetraalkylammonium electrolytes w8–10x. More detailed analysis of the impedance spectra is not possible, because of the lack of detailed information on the structure and size of electrolyte grains. Interestingly, the structure of the best-fit equivalent circuit is preserved also above the melting point, which may indicate structuring of the liquid electrolyte, although the recent neutron scattering w33x and FTIR w34x studies of aqueous solutions indicate
410
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
of charge carriers Ž c . and their mobility expressed in terms of the diffusion coefficient Ž D .,
s s Dce 2rkT .
Y Fig. 2. Plot of the imaginary component of impedance Ž Z ., X against its real component Ž Z ., of the cell Au
no change of water structure near the nonpolar group. However, in a concentrated solution of tetrabutylammonium salts, an increased population of water–hydrogen bonds around tetrabutylammonium cations was detected w34x. This distribution allows for speculation about the existence of protonic conduction paths also in a liquid electrolyte. The values of s are presented in the form of log s vs. Ty1 plot ŽFig. 3.. It is clear that at temperatures not far below the melting point, the activation energy is larger than at temperatures below 200 K. The values of the activation energy Ž EaŽ s . . are equal to 0.31 and 0.18 eV, respectively. One may conclude that at lower temperatures, TBAOH30 is a superionic conductor w35x. No deviation from Arrhenius behaviour at temperatures about 100 K is observed, contrary to HClO4 P 5.5H 2 O and its deuterated derivative w7x. It is interesting to compare the conductivity of TBAOH30 with other tetrabutylammonium clathrate hydrates, as well other clathrate polyhydrates. According to the Nernst–Einstein equation, the conductivity is proportional to the product of concentration
Ž 1.
At the arbitrarily selected temperature of 250 K, the value of s is approximately seven orders of magnitude larger than that of structurally related tetrabutylammonium fluoride hydrate: ŽC 4 H 9 .4 NF P 32H 2 O ŽTBAF32. w10x. This may be a simple reflection of the difference of the concentration of mobile charge carriers; equal to the difference in concentration of OHy. However, this may not be the only important factor, because the conductivity of other tetrabutylammonium halide hydrates like ŽC 4 H 9 .4 NCl P 30H 2 O or ŽC 4 H 9 .4 NBr P 32H 2 O is larger than that of TBAF32 w10x. The value of EaŽ s . of TBAOH30 is smaller than that of tetrabutylammonium halide hydrates Ž0.45– 0.52 eV w6x.. This indicates that the replacement of halide by OHy ions decreases the barrier for charge Žproton. transfer. Probably, the degree of proton delocalisation is larger in a network formed by water and hydroxide ions. In the tetrabutylammonium halide hydrates, halide anions are part of the clathrate network w25x. Their presence may make proton mo-
Fig. 3. Plot of the logarithm of the ohmic conductivity Ž s . of TBAOH30 against the reciprocal of temperature ŽT .. Dashed line marks the melting point.
A. Prokopowicz, M. Opallor Solid State Ionics 145 (2001) 407–413
411
tion more difficult than in hydroxide hydrates. Interestingly, the value of EaŽ s . of TBAOH30 is similar to that of two tetramethylammonium hydroxide hydrates ŽCH 3 .4 NOH P 7.5H 2 O and TMAOH10 w5x. Also, the relationship between EaŽ s . of these two compounds and their fluoride analogue ŽCH 3 .4 NF P 4H 2 O w8x is similar to that observed for tetrabutylammonium cation hydrates. This may indicate that the effect of the replacement of halide by hydroxide ions on the activation barrier of conductivity is more general. 3.2. Electrochemical hydrogen eÕolution Examples of current–potential curves corresponding to cathodic reaction obtained at selected temperatures are presented in Fig. 4. The shape of the voltammogram does not depend on the direction of the scan and the scan rate. In order to analyse data as a function of overpotential Žh ., its temperature dependence h ŽT . was calculated from the following equation,
h Ž T . s Em Ž T . y EPd Ž T . y IR Ž T .
Ž 2.
where EmŽT . is a measured potential and EPd ŽT . is a potential of PdŽH 2 . reference electrode. It has been assumed that the latter is equal to the standard
Fig. 5. Plot of the logarithm of the hydrogen evolution current density Ž J . against overpotential Žh . at selected temperatures.
potential of the hydrogen evolution reaction. The temperature dependence E Pd ŽT . Žin volts vs. normal hydrogen electrode. was taken into account by using the following equation w12x, EPd Ž T . s 0.196 y 0.491 = 10y3 T .
Ž 3.
For the relatively negative h , the semilogarithmic plot of current density Ž J . vs. h is approximately linear ŽFig. 5. indicating independence of the mechanism of HER from potential. Therefore, the data could be analysed using the Tafel equation,
h s a q Ž a n obs Fr2.3 RT . = log J
Fig. 4. Plot of the hydrogen evolution current Ž I . against potential Ž E . at selected temperatures marked on the figure.
Ž 4.
where a is a constant and a n obs is a charge transfer coefficient being a measure of the symmetry of the activation barrier. The value of a n obs is close to that observed in alkali metal hydroxide w36x or tetramethylammonium hydroxide concentrated solutions w16x. It is temperature dependent, namely the lower the temperature, the smaller the slope. This is similar to the Au
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trolyte near the electrode surface may be different from that in the bulk.
4. Conclusions TBAOH30 is another example of highly conducting electrolyte within the tetraalkylammonium clathrate hydrate family. At temperatures below 200 K, it behaves as a superionic conductor. It is also another example of protonic conductor, which can be applied for electrochemical hydrogen evolution studies at low temperatures.
Acknowledgements Fig. 6. Plot of the logarithm of the hydrogen evolution current density Ž J . against the reciprocal of temperature ŽT . for selected overpotentials Žh . equal to 0.60, 0.65, 0.70, 0.75, 0.80 Žfrom the bottom to the top..
The help of Dr. Ewa Utzig in differential scan calorimetry experiments is gratefully acknowledged.
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Ž 5.
The larger energy of activation of the heterogeneous process may result from the strong penetration of electric field into electrolyte. It may hamper reorientation of water molecules w39x that should accompany proton transfer. Also, the structure of the elec-
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