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Electrochemical hydrogen evolution in hydroxide hydrate down to 110 K
www.elsevier.nl/locate/elecom Electrochemistry Communications 2 (2000) 23–26
Electrochemical hydrogen evolution in hydroxide hydrate down to 110 K Marcin Opallo *, Agnieszka Prokopowicz Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 7 October 1999; received in revised form 15 October 1999; accepted 18 October 1999
Abstract Electrochemical hydrogen evolution was studied at an Au electrode in liquid and solid tetramethylammonium hydroxide hydrate (CH3)4NOHP10H2O (m.p. 253 K) down to almost 110 K. The current–potential relationships were obtained by slow scan voltammetry. The lowering of temperature causes substantial decrease of the slope of linear Tafel plots. This was interpreted as a decrease of charge transfer coefficient from 0.37 at room temperature to 0.01 at 113 K. The activation energy of the electrochemical hydrogen evolution at temperatures below 200 K is equal to 0.25"0.03 eV and is larger than the activation energy of the electrolyte conductivity. q2000 Elsevier Science S.A. All rights reserved. Keywords: Electrode reactions; Solid electrolytes; Electrochemical hydrogen evolution; Charge transfer coefficient; Hydroxide hydrate
1. Introduction Electrochemical studies at low temperatures have recently received some interest because they are important for a better understanding of electrode reactions. The hydrogen evolution reaction (HER) is the electrode reaction most extensively studied below 200 K. It had been a common view that the freezing of aqueous electrolyte sets a serious limit for low temperature HER studies. Addition of methanol makes it possible to carry out experiments down to 150 K in the liquid phase [1–3]. However, in this way another component was introduced to the studied system that could affect the obtained results. The use of solid stoichiometric polyhydrates, exhibiting high electric conductivity (s) below the melting point [4], seems to be a much better solution. Indeed, low temperature electrochemical studies of the HER in solid HClO4P 5.5H2O (m.p. 228 K) and its deuterated derivative were possible, because of their relatively high electric conductivity in the solid state [5–8]. The hydrated proton is a stoichiometric component of acid hydrate and its molar concentration apparently determines the relatively large HER current. The linear Tafel plots were obtained from voltammetric experiments at relatively large overpotentials [5,8]. Here we present the first example of a low temperature study of HER in basic media. Similarly to acidic media, this reaction also involves proton transfer and electron transfer. * Corresponding author. Tel.: q48-22-6323221; fax: q48-39-120238; e-mail: [email protected]
The electron is transferred from the electrode to the H2O molecule. As a result OHy anions and a hydrogen molecule are formed [9]. For the present study we propose one of the tetramethylammonium hydroxide polyhydrates: (CH3)4NOHP10H2O (TMAOH10) [10]. Its conductivity (10y4 S my1 at 150 K) is the highest among these polyhydrates [11]. Alkali metal hydroxide hydrates are less suitable for this study because of their much lower conductivity [12]. It should be emphasized that the conductivity of solid TMAOH10 is also larger than HClO4P5.5H2O previously used in HER studies [13,14].
2. Experimental 2.1. Chemicals (CH3)4NOHP5H2O (TMAOH5) was 99% pure from Acros. It was recrystallized from water at a temperature of 5–108C [15]. TMAOH10 (m.p. 253 K [10]) was prepared by addition of a stoichiometric amount of water to TMAOH5. HClO4 was 99.999% pure from Aldrich. The water was purified by the ELIX system (Milipore). Just before the experiment it was distilled twice from quartz. 2.2. Electrodes The working Au wire electrode (99.99% purity) was washed extensively with water. It was flame-annealed three
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Article: 144
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M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
times in a reducing Bunsen flame and quenched in water [16]. After this procedure the electrode was immediately immersed in liquid TMAOH10 at room temperature. The surface of the working electrode was 0.3 cm2. For all experiments a Pd(H2) reference electrode was freshly prepared in 0.7 M HClO4. Pd wire was charged by passing a cathodic current (3.3 mA) for 360 s. Then, anodic current was applied for 15 s [17]. The potential of this electrode is stable for at least 20 h in 2 M TMAOH aqueous solution [18]. 2.3. Apparatus and procedures The current–potential relationships were obtained from slow scan (0.5–2 mV sy1) linear sweep voltammetry with the Autolab electrochemical system (Eco Chemie). The data were acquired and analysed using GPES software. The impedance of the electrochemical cell was obtained by impedance spectroscopy using FRA software. The Teflonw cylindrical cell tightly embedded in a stainless-steel cylinder was filled with 1.5 cm3 of liquid TMAOH10 at room temperature. The electrolyte was saturated with bubbling argon for at least 30 min. Afterwards, a constant flow of argon over the surface of the electrolyte was maintained. The cell was placed in the cylindrical chamber of the copper block. The latter was installed in a Dewar flask filled with liquid nitrogen. The thin copper bar (‘cold finger’) was connected to the bottom of the block and immersed into liquid nitrogen. The temperature of the cell was adjusted by the amount of heat liberated from a resistive wire placed near the top of the ‘cold finger’. It was controlled by a temperature controller (type 680, Unipan-Thermal) connected to a Pt100 sensor (Heraeus) placed inside the wall of the copper block. For the lowest temperatures a thicker ‘cold finger’ without heating coil was used. The temperature of the electrolyte was measured using another Pt100 sensor placed in the electrochemical cell and connected to a digital thermometer (EMT10). The temperature of the cell ranged from 100 to 300 K. It was decreased at a rate lower than 1 K miny1. Before a given measurement, the cell was kept for at least 15 min at constant temperature to allow thermal equilibration at open circuit potential. Then, the potential was set at Es0 V until the steady state current was achieved and scanned in the negative direction.
from the best-fit equivalent circuit. The temperature dependence of R is presented in Fig. 1. At the lowest temperatures studied R is larger than 109 V. Fortunately the effect of temperature on R and current (I) is opposite and IR was not larger than 0.013 V. Examples of current–potential curves for hydrogen evolution obtained at selected low temperatures are presented in Fig. 2. At temperatures below the melting point of TMAOH10 a sharp current decrease was observed at some negative potential (not shown). This is probably the result of detachment of the electrolyte from the electrode surface caused by the formation of the gaseous product. The shape of the voltammogram did not depend on the direction of the scan and the scan rate. At temperatures higher than 110 K the shape of the dependence of the logarithm of current density (J) on potential (E) indicates electrode reaction (Fig. 3). For the relatively negative overpotentials (h) the log J versus E plot is linear. Therefore, data were analysed using the Tafel equation:
hsaq(a nobs F/2.3RT )=log J
(1)
where a is a constant and anobs is a charge transfer coefficient.
Fig. 1. Plot of the resistance of the cell (R) against temperature (T). Vertical, long dashed line denotes the melting point of the electrolyte.
3. Results and discussion The results obtained at low temperatures can be significantly affected by the ohmic drop in the cell (IR). Therefore, the estimation of the cell resistance (R) is essential for assessment of the data reliability. The values of R were obtained from the high frequency part of the impedance spectrum by extrapolation of the real part to the imaginary axis or/and
Thursday Dec 09 12:51 PM
Fig. 2. Plot of the hydrogen evolution current (I) against potential (E) at selected temperatures marked on the figure.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
Fig. 3. Plot of the logarithm of the hydrogen evolution current density (J) against overpotential (h) at 113 K.
25
Fig. 4. Plot of the charge transfer coefficient (anobs) against temperature (T). Vertical, long dashed line denotes the melting point of the electrolyte.
The temperature dependence of overpotential h(T) was calculated from the following equation:
h(T)sE m(T)yE Pd(T)yIR(T)
(2)
where Em(T) is a measured potential and EPd(T) is the potential of the Pd(H2) reference electrode. It has been assumed that the latter is equal to the standard potential of the hydrogen evolution reaction. The temperature dependence EPd(T) (in V versus the normal hydrogen electrode) was taken into account by using the following equation [4]: E Pd(T)s0.196y0.491=10y3 T
(3)
The slope of the Tafel plot depends on the temperature: the lower the temperature the smaller is the slope. One may conclude that this is due to the temperature dependence of anobs. The value of the latter at room temperature is equal to 0.37. It is similar to anobs recalculated from HER data obtained on the Au electrode in alkali metal hydroxide solution (0.38) [9]. In solid TMAOH10 it significantly decreases to 0.01 at 113 K (Fig. 4). The same trend of temperature dependence of anobs was observed for HER at the MeNHClO4P5.5H2O (MesCu, Ag and Au) interface [5,8]. However, in the particular case of the Au electrode it decreases to less than 0.2 at 170 K and remains constant at the lowest temperatures, indicating the effect of the electrode material [5]. The temperature dependence of the charge transfer coefficient is predicted by theoretical models of electrochemical proton transfer (see [19,20] and Refs. therein). It is a result of the potential dependence of the entropic part of the symmetry factor caused, for example, by a deeper penetration in the solid electrolyte by the electrode electric field at low temperatures [20]. However, one should be aware that anobs obtained from slow scan experiments represents a transfer coefficient of the overall reaction involving the electrochemical proton transfer step. The value of the activation energy of HER (EHER) at the AuNTMAOH10 interface obtained from the slope of the current density Arrhenius plot (Fig. 5) is equal to 0.25"0.03 eV. Some effect of the overpotential on the value of EHER is
Thursday Dec 09 12:51 PM
Fig. 5. Plot of the current density (J) against the reciprocal of the temperature (T) at selected overpotentials (h) equal to 1.00 V (d), y1.05 V (m), y1.10 V (j), y1.15 V (l) and y1.20 V (.).
observed, namely, the larger the h value, the larger is the activation energy. It is about two times smaller than EHER obtained in HClO4P5.5H2O [8]. Interestingly, EHER is larger by a factor close to 1.5 than the activation energy of conductivity (Es) of TMAOH10, equal to 0.17 eV in the same temperature region [11]. The same relationship between EHER and Es is true for HClO4P5.5H2O [8,13]. On the other hand, a strong effect of overpotential on the pre-exponential factor calculated from the Arrhenius equation is observed. For y1.2-h-y1.0 V it changes by almost two orders of magnitude. This may indicate that the transport of the reactant to the AuNTMAOH10 interface is not the rate-determining step at negative overpotentials or that it is strongly affected by the penetration of the electric field into the electrolyte. It is obvious that both electrochemical proton transfer at the electrodeNprotonic conductor interface and homogenous proton transfer must be closely related. Although there are no NMR data for TMAOH10, the comparison of the temperature dependence of spin-lattice relaxation time [21] and conductivity [11] of structurally similar TMAOH5 indicates that TMAOH10 is a protonic conductor. On the basis of the same relationship this conclusion can be drawn for
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
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M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
HClO4P5.5H2O [22]. The structure of both compounds in the solid state is typical for polyhedral clathrate. It consists of a network of cages formed by water molecules and hydroxide ions (TMAOH10) [15] or water molecules and hydronium ions (HClO4P5.5H2O) [23]. The majority of cages in the TMAOH10 structure are filled with TMAq cations whereas all cages of HClO4P5.5H2O are filled with ClO4y anions. The relatively high conductivity of these compounds is caused by high deficiency (TMAOH10) or excess (HClO4P5.5H2O) of protons caused by the presence of hydroxide or hydronium ions in the water network. The motion of a large number of protons must be accompanied by reorientation of water molecules that may be the ratedetermining step. One may speculate that at a close distance to the negatively charged electrode this motion is hampered by the strong electric field. Also, the structure of the electrolyte near the electrode surface may be different to that in the bulk.
4. Conclusions We have shown that in TMAOH10 it is possible to study HER down to almost 110 K. This is the first example of low temperature HER studies in basic media. The strong temperature dependence of the Tafel slope, previously observed in acidic media, is reproduced in basic electrolyte. The results obtained demonstrate the application of tetramethylammonium hydroxide hydrates to the study of HER at low temperatures in basic medium.
Acknowledgements Helpful instructions about the preparation of Au and Pd electrodes from Professor Zofia Borkowska and Ms Agnieszka Tymosiak-Zielinska are gratefully acknowledged.
Thursday Dec 09 12:51 PM
References [1] B.E. Conway, M. Salomon, J. Chem. Phys. 41 (1964) 3169. [2] B.E. Conway, D.J. McKinnon, B.V. Tilak, Trans. Faraday Soc. 66 (1970) 1203. [3] M. Nicolas, L. Dumoulin, J.P. Burger, J. Electroanal. Chem. 172 (1984) 389. [4] U. Frese, T. Iwasita, W. Schmickler, U. Stimming, J. Phys. Chem. 89 (1985) 1059. [5] U. Frese, U. Stimming, J. Electroanal. Chem. 198 (1986) 409. [6] U. Frese, W. Schmickler, Ber. Bunsenges. Phys. Chem. 92 (1988) 1412. [7] J.O’M. Bockris, J. Wass, J. Electroanal. Chem. 267 (1989) 325. [8] M. Cappadonia, S. Krause, U. Stimming, Electrochim. Acta 42 (1997) 841. [9] T. Ohmori, M. Enyo, Electrochim. Acta 37 (1992) 2021. [10] G.A. Jeffrey, W.J. McLean, J. Chem. Phys. 47 (1967) 414. [11] Z. Borkowska, A. Tymosiak, M. Opallo, J. Electroanal. Chem. 406 (1996) 109. [12] Z. Borkowska, U. Frese, E. Kriegsmann, U. Stimming, M. Gavish, K.J. Adamic, S.G. Greenbaum, Ber. Bunsenges. Phys. Chem. 95 (1991) 1033. [13] M. Cappadonia, U. Stimming, J. Electroanal. Chem. 300 (1991) 235. [14] M. Cappadonia, A.A. Kornyshev, S. Krause, A.M. Kuznetsov, U. Stimming, J. Chem. Phys. 101 (1994) 7672. [15] D. Mootz, R. Seidel, J. Incl. Phenom. 8 (1990) 139. [16] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 16, Plenum, New York, 1985, Ch. 1. [17] L.P. Cheng, M.S. Thesis, Columbia University, New York, 1989. [18] A. Tymosiak-Zielinska, Ph.D. Thesis, Institute of Physical Chemistry, Warsaw, 1999. [19] W. Schmickler, J. Electroanal. Chem. 284 (1990) 269. [20] A.A. Kornyshev, A.M. Kuznetsov, U. Stimming, J. Chem. Phys. 106 (1998) 9523. [21] C.I. Ratcliffe, S.K. Garg, D.E. Davidson, J. Incl. Phenom. 8 (1990) 159. [22] T.H. Huang, E. Davis, U. Frese, U. Stimming, J. Phys. Chem. 92 (1988) 6874. [23] D. Mootz, E.J. Oellers, M. Wiebke, J. Am. Chem. Soc. 109 (1987) 1200.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
Electrochemical hydrogen evolution in hydroxide hydrate down to 110 K Marcin Opallo *, Agnieszka Prokopowicz Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 7 October 1999; received in revised form 15 October 1999; accepted 18 October 1999
Abstract Electrochemical hydrogen evolution was studied at an Au electrode in liquid and solid tetramethylammonium hydroxide hydrate (CH3)4NOHP10H2O (m.p. 253 K) down to almost 110 K. The current–potential relationships were obtained by slow scan voltammetry. The lowering of temperature causes substantial decrease of the slope of linear Tafel plots. This was interpreted as a decrease of charge transfer coefficient from 0.37 at room temperature to 0.01 at 113 K. The activation energy of the electrochemical hydrogen evolution at temperatures below 200 K is equal to 0.25"0.03 eV and is larger than the activation energy of the electrolyte conductivity. q2000 Elsevier Science S.A. All rights reserved. Keywords: Electrode reactions; Solid electrolytes; Electrochemical hydrogen evolution; Charge transfer coefficient; Hydroxide hydrate
1. Introduction Electrochemical studies at low temperatures have recently received some interest because they are important for a better understanding of electrode reactions. The hydrogen evolution reaction (HER) is the electrode reaction most extensively studied below 200 K. It had been a common view that the freezing of aqueous electrolyte sets a serious limit for low temperature HER studies. Addition of methanol makes it possible to carry out experiments down to 150 K in the liquid phase [1–3]. However, in this way another component was introduced to the studied system that could affect the obtained results. The use of solid stoichiometric polyhydrates, exhibiting high electric conductivity (s) below the melting point [4], seems to be a much better solution. Indeed, low temperature electrochemical studies of the HER in solid HClO4P 5.5H2O (m.p. 228 K) and its deuterated derivative were possible, because of their relatively high electric conductivity in the solid state [5–8]. The hydrated proton is a stoichiometric component of acid hydrate and its molar concentration apparently determines the relatively large HER current. The linear Tafel plots were obtained from voltammetric experiments at relatively large overpotentials [5,8]. Here we present the first example of a low temperature study of HER in basic media. Similarly to acidic media, this reaction also involves proton transfer and electron transfer. * Corresponding author. Tel.: q48-22-6323221; fax: q48-39-120238; e-mail: [email protected]
The electron is transferred from the electrode to the H2O molecule. As a result OHy anions and a hydrogen molecule are formed [9]. For the present study we propose one of the tetramethylammonium hydroxide polyhydrates: (CH3)4NOHP10H2O (TMAOH10) [10]. Its conductivity (10y4 S my1 at 150 K) is the highest among these polyhydrates [11]. Alkali metal hydroxide hydrates are less suitable for this study because of their much lower conductivity [12]. It should be emphasized that the conductivity of solid TMAOH10 is also larger than HClO4P5.5H2O previously used in HER studies [13,14].
2. Experimental 2.1. Chemicals (CH3)4NOHP5H2O (TMAOH5) was 99% pure from Acros. It was recrystallized from water at a temperature of 5–108C [15]. TMAOH10 (m.p. 253 K [10]) was prepared by addition of a stoichiometric amount of water to TMAOH5. HClO4 was 99.999% pure from Aldrich. The water was purified by the ELIX system (Milipore). Just before the experiment it was distilled twice from quartz. 2.2. Electrodes The working Au wire electrode (99.99% purity) was washed extensively with water. It was flame-annealed three
1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 9 9 ) 0 0 1 3 4 - 4
Thursday Dec 09 12:51 PM
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
24
M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
times in a reducing Bunsen flame and quenched in water [16]. After this procedure the electrode was immediately immersed in liquid TMAOH10 at room temperature. The surface of the working electrode was 0.3 cm2. For all experiments a Pd(H2) reference electrode was freshly prepared in 0.7 M HClO4. Pd wire was charged by passing a cathodic current (3.3 mA) for 360 s. Then, anodic current was applied for 15 s [17]. The potential of this electrode is stable for at least 20 h in 2 M TMAOH aqueous solution [18]. 2.3. Apparatus and procedures The current–potential relationships were obtained from slow scan (0.5–2 mV sy1) linear sweep voltammetry with the Autolab electrochemical system (Eco Chemie). The data were acquired and analysed using GPES software. The impedance of the electrochemical cell was obtained by impedance spectroscopy using FRA software. The Teflonw cylindrical cell tightly embedded in a stainless-steel cylinder was filled with 1.5 cm3 of liquid TMAOH10 at room temperature. The electrolyte was saturated with bubbling argon for at least 30 min. Afterwards, a constant flow of argon over the surface of the electrolyte was maintained. The cell was placed in the cylindrical chamber of the copper block. The latter was installed in a Dewar flask filled with liquid nitrogen. The thin copper bar (‘cold finger’) was connected to the bottom of the block and immersed into liquid nitrogen. The temperature of the cell was adjusted by the amount of heat liberated from a resistive wire placed near the top of the ‘cold finger’. It was controlled by a temperature controller (type 680, Unipan-Thermal) connected to a Pt100 sensor (Heraeus) placed inside the wall of the copper block. For the lowest temperatures a thicker ‘cold finger’ without heating coil was used. The temperature of the electrolyte was measured using another Pt100 sensor placed in the electrochemical cell and connected to a digital thermometer (EMT10). The temperature of the cell ranged from 100 to 300 K. It was decreased at a rate lower than 1 K miny1. Before a given measurement, the cell was kept for at least 15 min at constant temperature to allow thermal equilibration at open circuit potential. Then, the potential was set at Es0 V until the steady state current was achieved and scanned in the negative direction.
from the best-fit equivalent circuit. The temperature dependence of R is presented in Fig. 1. At the lowest temperatures studied R is larger than 109 V. Fortunately the effect of temperature on R and current (I) is opposite and IR was not larger than 0.013 V. Examples of current–potential curves for hydrogen evolution obtained at selected low temperatures are presented in Fig. 2. At temperatures below the melting point of TMAOH10 a sharp current decrease was observed at some negative potential (not shown). This is probably the result of detachment of the electrolyte from the electrode surface caused by the formation of the gaseous product. The shape of the voltammogram did not depend on the direction of the scan and the scan rate. At temperatures higher than 110 K the shape of the dependence of the logarithm of current density (J) on potential (E) indicates electrode reaction (Fig. 3). For the relatively negative overpotentials (h) the log J versus E plot is linear. Therefore, data were analysed using the Tafel equation:
hsaq(a nobs F/2.3RT )=log J
(1)
where a is a constant and anobs is a charge transfer coefficient.
Fig. 1. Plot of the resistance of the cell (R) against temperature (T). Vertical, long dashed line denotes the melting point of the electrolyte.
3. Results and discussion The results obtained at low temperatures can be significantly affected by the ohmic drop in the cell (IR). Therefore, the estimation of the cell resistance (R) is essential for assessment of the data reliability. The values of R were obtained from the high frequency part of the impedance spectrum by extrapolation of the real part to the imaginary axis or/and
Thursday Dec 09 12:51 PM
Fig. 2. Plot of the hydrogen evolution current (I) against potential (E) at selected temperatures marked on the figure.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
Fig. 3. Plot of the logarithm of the hydrogen evolution current density (J) against overpotential (h) at 113 K.
25
Fig. 4. Plot of the charge transfer coefficient (anobs) against temperature (T). Vertical, long dashed line denotes the melting point of the electrolyte.
The temperature dependence of overpotential h(T) was calculated from the following equation:
h(T)sE m(T)yE Pd(T)yIR(T)
(2)
where Em(T) is a measured potential and EPd(T) is the potential of the Pd(H2) reference electrode. It has been assumed that the latter is equal to the standard potential of the hydrogen evolution reaction. The temperature dependence EPd(T) (in V versus the normal hydrogen electrode) was taken into account by using the following equation [4]: E Pd(T)s0.196y0.491=10y3 T
(3)
The slope of the Tafel plot depends on the temperature: the lower the temperature the smaller is the slope. One may conclude that this is due to the temperature dependence of anobs. The value of the latter at room temperature is equal to 0.37. It is similar to anobs recalculated from HER data obtained on the Au electrode in alkali metal hydroxide solution (0.38) [9]. In solid TMAOH10 it significantly decreases to 0.01 at 113 K (Fig. 4). The same trend of temperature dependence of anobs was observed for HER at the MeNHClO4P5.5H2O (MesCu, Ag and Au) interface [5,8]. However, in the particular case of the Au electrode it decreases to less than 0.2 at 170 K and remains constant at the lowest temperatures, indicating the effect of the electrode material [5]. The temperature dependence of the charge transfer coefficient is predicted by theoretical models of electrochemical proton transfer (see [19,20] and Refs. therein). It is a result of the potential dependence of the entropic part of the symmetry factor caused, for example, by a deeper penetration in the solid electrolyte by the electrode electric field at low temperatures [20]. However, one should be aware that anobs obtained from slow scan experiments represents a transfer coefficient of the overall reaction involving the electrochemical proton transfer step. The value of the activation energy of HER (EHER) at the AuNTMAOH10 interface obtained from the slope of the current density Arrhenius plot (Fig. 5) is equal to 0.25"0.03 eV. Some effect of the overpotential on the value of EHER is
Thursday Dec 09 12:51 PM
Fig. 5. Plot of the current density (J) against the reciprocal of the temperature (T) at selected overpotentials (h) equal to 1.00 V (d), y1.05 V (m), y1.10 V (j), y1.15 V (l) and y1.20 V (.).
observed, namely, the larger the h value, the larger is the activation energy. It is about two times smaller than EHER obtained in HClO4P5.5H2O [8]. Interestingly, EHER is larger by a factor close to 1.5 than the activation energy of conductivity (Es) of TMAOH10, equal to 0.17 eV in the same temperature region [11]. The same relationship between EHER and Es is true for HClO4P5.5H2O [8,13]. On the other hand, a strong effect of overpotential on the pre-exponential factor calculated from the Arrhenius equation is observed. For y1.2-h-y1.0 V it changes by almost two orders of magnitude. This may indicate that the transport of the reactant to the AuNTMAOH10 interface is not the rate-determining step at negative overpotentials or that it is strongly affected by the penetration of the electric field into the electrolyte. It is obvious that both electrochemical proton transfer at the electrodeNprotonic conductor interface and homogenous proton transfer must be closely related. Although there are no NMR data for TMAOH10, the comparison of the temperature dependence of spin-lattice relaxation time [21] and conductivity [11] of structurally similar TMAOH5 indicates that TMAOH10 is a protonic conductor. On the basis of the same relationship this conclusion can be drawn for
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144
26
M. Opallo, A. Prokopowicz / Electrochemistry Communications 2 (2000) 23–26
HClO4P5.5H2O [22]. The structure of both compounds in the solid state is typical for polyhedral clathrate. It consists of a network of cages formed by water molecules and hydroxide ions (TMAOH10) [15] or water molecules and hydronium ions (HClO4P5.5H2O) [23]. The majority of cages in the TMAOH10 structure are filled with TMAq cations whereas all cages of HClO4P5.5H2O are filled with ClO4y anions. The relatively high conductivity of these compounds is caused by high deficiency (TMAOH10) or excess (HClO4P5.5H2O) of protons caused by the presence of hydroxide or hydronium ions in the water network. The motion of a large number of protons must be accompanied by reorientation of water molecules that may be the ratedetermining step. One may speculate that at a close distance to the negatively charged electrode this motion is hampered by the strong electric field. Also, the structure of the electrolyte near the electrode surface may be different to that in the bulk.
4. Conclusions We have shown that in TMAOH10 it is possible to study HER down to almost 110 K. This is the first example of low temperature HER studies in basic media. The strong temperature dependence of the Tafel slope, previously observed in acidic media, is reproduced in basic electrolyte. The results obtained demonstrate the application of tetramethylammonium hydroxide hydrates to the study of HER at low temperatures in basic medium.
Acknowledgements Helpful instructions about the preparation of Au and Pd electrodes from Professor Zofia Borkowska and Ms Agnieszka Tymosiak-Zielinska are gratefully acknowledged.
Thursday Dec 09 12:51 PM
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StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 144