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Influence of perfluorinated and hydrogenated surfactants upon hydrogen evolution on gold electrodes
EkctrochimicaAce. Vol. 39, No. 18, DD.2743-2750, 1994 copyright 0 1994 Ltd Printedin Great Britnin.All rigbu resewed 0013-4686/M s7.00 + 0.00
Pergnmoa
Ebv*r sciena
0013-4686(!M)EOlgg-6
INFLUENCE OF PERFLUORINATED AND HYDROGENATED SURFACTANTS UPON HYDROGEN EVOLUTION ON GOLD ELECTRODES C.
CACHET,
M.
KEDDAM,
V. MARIOTTE and R. WIART
UPRlS du CNRS, Physique des Liquides et Electrochimie, Universite Pierre et Marie Curie, Tour 22, 4 place Jussieu, 75252 Paris Cedex 05, France (Receioed 24 January 1994; in revised form 31 March 1994)
Abstract-The influence of pertluorinated and hydrogenated surfactants on the mechanism of hydrogen evolution on gold electrodes in 1 M H,SO, has been investigated from steady-state polarization curves and impedance data. The ohmic drop due to bubbles and the reaction inhibition, consequent on both the electrode surface masking by bubbles and the surfactant adsorption, have been separated and compared. The activity of surfactants decreases with the hydrophilic head in the order cationic > anionic > nonionic. Pertluorinated surfactants produce a lower inhibition than the hydrogenated ones on the electrochemical reaction which occurs in a Tafel kinetic regime, independently of the presence of surfactants. Key words: hydrogen evolution, surfactant, impedance, ohmic drop, inhibition.
1. INTRODUCTION
It has been recently shown that the adsorption modalities of perlluorinated surfactants FORAFAC (ATOCHEM commercial products) on gold electrodes can be determined from capacitance measurements[l, 23. In Na,SO, electrolyte, it has been concluded that the adsorption mode of these compounds and the metal-molecule interactions strongly depend on the hydrophilic head of molecules. With a cationic compound (F1098), the aromatic rings probably generate strong attractive bondings which stabilize the molecules in a flat orientation independent of the electrode charge. The adsorption of an anionic compound (F1176) seems to be essentially determined by electrostatic forces which allow the molecules to set in the upright orientation, solely in the positive charge region of potentials. surfactants The behaviour of non-ionic (perfluorinated FlllO or hydrogenated HlllO) is intermediate and presents some similarity with the adsorption of the ionic compounds. In the positive charge region, the non-ionic compounds behave like the anionic one but the bonding between the metal and the hydrophobic chain in non-ionic molecules is stronger and able to inhibit the gold oxidation. In the negative charge region, the non-ionic molecules are adsorbed in a flat orientation, similar to the cationic compounds, but the bondings are weaker and allow easier desorption. The influence of perfluorinated surfactants on electrochemical processes are little known up to now, and their possible advantages on a given reaction have to be analyzed. However it is known that the non-ionic perfluorinated surfactant FlllO is an inhibitor for zinc corrosion and hydrogen evolution
on zinc[3-51 and also a levelling agent for zinc electrodeposition[6]. FORAFAC 1098 is used as emulsifier in fire extinguishers whereas FORAFAC 1176 can be used as a wetting agent in baths of chromium electroplating. With a view to energy saving, gas evolving electrodes have been extensively studied and various effects of bubbles on the electrode polarization have been separated[7-111. Different terms of the overpotential have been estimated: (i) the ohmic drop within the electrolyte; (ii) the area masking on the electrode surface which inhibits the electrochemical reaction; and (iii) the concentration overpotential. It is known that the presence of surfactants in the electrolyte can modify the different terms of the overpotential by affecting the characteristics such as growth rate, detachment diameter and population density of hydrogen bubbles[12-141. Hydrogen evolution in sulfuric acid solution has been chosen to investigate the influence of various perfluorinated and hydrogenated surfactants upon gas formation, and the results of this comparative analysis will be presented and discussed in the present paper. 2. EXPERIMENTAL The electrolyte was 1 M H,SO, of analytical grade purity (Merck). The solutions were prepared with water which was doubly ion-exchanged, passed through a carbon cartridge and a micropore filter, and twice distilled with potassium permanganate in a quartz apparatus. The measurements were performed at room temperature in an electrolysis cell (Fig. 1) made of Teflon. A diaphragm of Nafion@’ 423 (Du Pont de Nemours), was used to separate the anode and cathode compartments and avoid any possible influ-
C. Cacnm et al.
2744
ence of the anodic reaction on the cathodic process. All potentials were referred to a mercury/mercurous sulphate electrode in saturated potassium sulphate (sse) located in the cathode compartment. The counter electrode was made of a gold wire (Johnson, Matthey, Puratronic) whose surface area (3.9cm2) was much larger than that of the working electrode. The working electrode was made of a gold rod (Johnson Matthey, Specpure), coated with a cataphoretic paint and moulded in epoxide resin (Buehler), leaving the cross section uncovered thus forming a disc electrode of 0.3 cm diameter. The electrode surface was polished with emery paper (1200 grit) and with alumina powder (0.3~) to a mirrorlike finish. After polishing, alumina particles were removed from the surface with ultrasonic cleaning in water. Then the working electrode was placed in the cell, and finally an additional in situ treatment of the electrode surface was performed by cyclic voltammetry in the electrolyte, between - 0.9 and - 1.5 V, and using a slow sweep rate (1 mVs- ‘), to obtain reproducible polarization curves before the addition of surfactants. This treatment maintained the gold electrode surface in the reducing environment of hydrogen molecules, differently from the cleaning procedure consisting in a repetitive formation and reduction of oxide layer[15]. The upwards orientation of the active surface allowed the bubbles to grow and detach freely. surfactants following perfluorinated The FORAFAC (Atochem) were tested: the non-ionic FlllO: C6F,,-C2H.,(0C2H4),0H
n x 11.5
the anionic F1176: C,F1s-C2H,SO;K+ the cationic F1098:
and they were compared to the hydrogenated compounds: the non-ionic H1110 (Witco): C,oH2,-(OC2H,),iOH the anionic H1176 (Aldrich): CsF,,SO;Na+ the cationic H1098 (Aldrich):
To facilitate the comparison of the influence of surfactants on hydrogen evolution, the experimental procedure was standardized. All surfactants were added to the electrolyte at the potential E = -0.9 V, and at the same concentration (lo-’ M) higher than the critical micellar concentration of most of pro-
ducts, so that the surface tension was minimized and independent of surfactant concentration. Thereby it was possible to reduce the amplitude of current fluctuations and obtain significant results. Impedance measurements were performed under potentiostatic conditions using a frequency response analyzer (Schlumberger-Solartron 1174) controlled by an Apple II microcomputer. The electrolyte resistance, R,, between the working electrode surface and the reference electrode, was measured under primary potential distribution by the high-frequency limit of the electrode impedance approached at IOOkHz. The electrode impedance was measured in the frequency domain (IOOkHz-0.1 Hz). To improve the signal/noise ratio, the integration time was 10 periods at frequencies lower than lOHz, whereas it was 0.1 s at frequencies higher than 10Hz. 3. RESULTS AND DISCUSSION 3.1. Electrolyte
resistance
The electrolyte resistance R, is a significant parameter to characterize the screening effect of gas bubbles within the electrolyte close to the electrodeC7, 8, 10, 111. The growth and detachment of bubbles do induce fluctuations of current and electrolyte resistance. Because of these fluctuations, numerous measurements were repeated as a function of time, and the values of R, obtained for different currents I are presented in Figs 2-5, in which the solid lines correspond to the mean value of R, . In the absence of surfactants, small scattering of R, is observed when the current I remains lower than O.OlmA, and the mean value of R, is 4.4 f 0.252 (Fig. 2). With the irregular formation of bubbles occurring for I > 0.01 mA, the fluctuation amplitudes are higher and more scattered. The mean value of R, becomes around 5.4Q and the difference in R, is due to the screening effect of bubbles. With the presence of surfactants, the fluctuation amplitudes decrease for currents ranging between 0.1 and 1 mA and less scattered values of R, have been observed for a given flow of hydrogen molecules, in agreement with the reduced size of bubblesC12, 133. As shown in Figs 3-5 the mean value of R, is generally increased with the presence of surfactants, except for the non-ionic hydrogenated compound HlllO which deceases R, at currents lower than 5mA. The increased values of R, characterize an enhanced screening effect generated by smaller and more numerous bubbles formed at the electrode surface in the presence of surfactants. Moreover above approximately 0.1 mA, the electrolyte resistance is systematically observed to rise with increasing current, as shown on the curves in Figs 3-5. A similar increase in R, with increasing current density has already been reported for hydrogen evolution in alkaline solution[16]. At a given current (I = 20 mA, ie a current density of 280mA cme2) corresponding to a given flow of produced hydrogen molecules, the clear influence of a surfactant on the screening by bubbles can be characterized by the difference AR, between the electrolyte resistances with surfactant (deduced from the curves in Figs 3-5) and without surfactant (5.4R). Line 1 in Table 1 presents a comparison of the
2745
Influence of surfactants upon hydrogen evolution RF: FERENCE
COUNTER
ELECTRODE
TEFLON
CYLINDER
ELECTROLYTE
EPOXIDE
RESIN
HOLDER
Fig. 1. Electrolysis cell.
1 o- 2
16 3
loo
10-l
10’
lo2
CURRENT / mA Fig. 2. Electrolyte resistance R, versus current I: Electrolyte 1 M H,SO,
Table 1. Influence of surfactants on: (1) the excess in electrolyte resistance AR, at I = 20mA; (2) the overpotential at I = 20mA; (3) the Tafel slope p; (4) the R,I product; (5) the double layer capacitance C Surfactant
(1)
ARJf-2 (2) (AE I/mV (3)
p/mVdec- ’
(4) R,I/mV (5) C//lFcnl-*
None
F1098
H1098
F1176
H1176
-
7.0
5.6
1.7
3.1
FlllO
HlllO
1.5
0.9
-
122
161
52
70
25
49
137
127
123
122
121
119
121
51+9 20*5
55 f 7 -
55 * 4 -
49 f 4
53 + 3
51 *4
49 f 4
17 f 3
19 f 2
20 f 4
18 f 3
C. CAcmrc et al. -I
(1 l
No surfactnnt
l
1 mMFlll0
6-
i 0”
I
O’>
1
o’L
1 0’
’
1 0”
1 0’
1 02
CURRENT / mA I: influence of non-ionic surfactants.
Fig. 3. Electrolyte resistance R, versus current
results obtained with the different surfactants. In spite of the scattering of measurements, it appears that the screening effect by bubbles is maximum for cationic surfactants and minimum for the non-ionic ones. Thereby the screening effect of bubbles within the electrolyte seems more sensitive to the hydrophilic head (cationic, anionic or non-anionic) than to the hydrophobic tail (perfluorinated or hydrogenated), in spite of a considerable influence of this hydrophobic tail on the surface tension liquid/ gas which decreases from 33mNm-’ for the hydrogenated compounds to 18 mNm- ’ for the
14-
12 -
“““‘I
““.‘.
.
No surfactant
l
lmMF1098
3.2. Polarization curves The steady-state polarization curves were obtained by cyclic voltammetry at a slow sweep rate (1 mVs- ‘) for which the direct and reverse curves coincide. All curves Z(E) correspond to the mean current and the potential E has been corrected for ohmic drop with the mean value of R,. The influences of perfluorinated and hydrogenated surfactants on polarization curves are shown in Figs 6 and 7, respectively. In both cases, the inhibition strongly depends on the hydrophilic head and decreases with
“.““I
,.....I
......1
. ..-
.
0 lmMH1098
IO -
8-
:~~.~~~!~~~~~~ 10”
IO.”
IO”
1 0’
’
CURRENT
1
0”
I
0’
/ mA
Fig. 4. Electrolyte resistance R, versus current I: influence of cationic surfactants.
I
02
Influence of surfactants upon hydrogen evolution
2147
r-c,A .‘.““’ i O’J CURRENT
/ mA
Fig. 5. Electrolyte resistance R, versus current I: influence
the order cationic > ionic > non-ionic. The inhibition by surfactants can result from the adsorbed molecules on the electrode and/or from the enhancement of the area masking on the electrode surface by smaller and more numerous gas bubbles which perturb the potential and current distributions around the electrode. The inhibiting influence of the different surfactants have been compared by considering the overpotential AE at a given current. The value of IAE] at 20mA are gathered in Table 1, line 2. The hydrogenated surfactants appear to inhibit the hydrogen reaction more than the corresponding fluorinated compounds. As illustrated in Fig. 8, the Tafel plots of polarization curves are linear over more than three decades of current values and the Tafel slopes p = - AE/
A(logi) are given in Table 1, line 3. No significant influence of the surfactants has been observed on the Tafel slope whose mean value is p = 124mV dec- ’ in the whole potential domain. This value is in agreement with a Tafel kinetic regime for hydrogen evolution. In addition, the Volmer reaction (H+ + e- + Had) appears to be the rate determining step, in agreement with a slope of 120mVdec-’ calculated by the pseudo-equilibrium method for a charge transfer coeficient of 0.5[17,18]. 3.3. Electrode impedance Only one capacitive loop has been observed on complex plane impedance plots, as illustrated in Fig. 9 where the origin of plots has been offset by the electrolyte resistance R,. This loop corresponds to
.
2”
40
I
n
No smfactant
0
1mMFlllO
A
lmMF1176
0
lmMF1098
of anionic surfactants.
0
. 0
A
.
O
A
. 0
. c3
.
20 -
Fig. 6. Influence of peduorinated
O
A
A
surfactants on polarization
0
0
curves.
c. tkXiFiT
2748
I 40 t
I
.
lmMHlI10
A
lmMH1176
q
1mM H1098
n 0
.
1.3
1.2
1.1
Fig. 7.
1
Nosurfactant
0
1.0
et al.
1.4
1.5
-E / V @SE) Inlkrence of hydrogenated surfactants on polarization curves.
the charge transfer resistance R, in parallel with the double layer capacitance C. The inhibiting influence of surfactants yields an increased R,, at a given potential, and it is confirmed that the cationic surfactants have the strongest inhibiting effect. The R, I product being independent of current, the results agree with a rate-determining step being an irreversible electrochemical reaction. Furthermore, it appears that the surfactants do not change the mechanism of the hydrogen evolution reaction since they do not effect the R,I products, as shown in Table 1, line 4. The mean value of this product, 52mV, confirms that the rate-determining step is a
1mM
one-electron reaction[19]. Then the current density of the cathodic electrochemical reaction is governed by the following equations: 1 = i, exp[ - b(E - E,)]
where i, is the current density corresponding to an arbitrarily chosen potential E, . From the mean value of R,i, equation (2) yields p = 120mVdec-‘, in agreement with the experimen-
FlllO
-E / V @SE) Fig. 8. Tafel plot of the polarization
curve with 10m5 M FlllO.
2749
Influence of surfactants upon hydrogen evolution
tal Tafel slope of 124 mV dec- ‘, in view of the accuracy of measurements (about 10% due to the presence of current fluctuations). The capacitance C, calculated at various points on the capacitive loop, does not reveal any significant frequency dispersion. The mean values, deduced from the apex of the capacitive loop, are exhibited in Table 1, line 5, and in Fig. 10. No potential dependence has been observed without surfactant and with non-ionic or anionic surfactants. Table 1 also shows that the double layer
capacitance is not significantly affected by these compounds, thus indicating that C is much less sensitive to the presence of these surfactants than the electrolyte resistance R, and the electrode potential E. It consequently appears that the non-ionic and anionic surfactants do not directly adsorb on the electrode surface and the modifications to R, and E probably result from changes in the characteristics of bubbles and thereby in the electrode area masking. On the other hand, the addition of the cationic surfactants, which are the most efficient inhibitors,
___ E
80-
0
No surfactant
A
1mMFlllO
(1=0.81mA)
0
ImMF1098
(I=O.UmA
=
-1.195
V
( I = 1.25 mA 1
I
I LH,
1.6 kH/ 10 kH,
.
0
A
*
20
0
40
Re(Z)
60
80
100
/ Ohm
Fig. 9. Complex plane impedance plots at E = - 1.195V: influence of fluorinated surfactants.
0
0.9
No surfactant
1.0
1.1
1.2
1.3
1.4
-E / V (SSE) Fig. 10. Potential dependence of the double layer capacitance: influence of cationic surfactants.
C.
2750
also yields a marked decrease in the double layer capacitance. As exhibited in Fig. 10, a similar influence has been observed with FlO!X or H1098 and the decrease is more pronounced with increasing cathodic polarization, in agreement with the adsorption of cationic compounds on the electrode surface. This adsorption probably contributes to the relatively high value of 1AE 1 observed for the cationic surfactants.
et al.
Ackmowledgement~The authors thank ATOCHEM for financial support of this work, and particularly I. Devaux and E. Pou for helpful discussions.
REFERENCES C. Cachet, M. Keddam, V. Mariotte and R. Wiart, Electrochim. Acta 37,2317 (1992). C. Cachet. M. Keddam. V. Mariotte and R. Wiart. Electrochik Acta 38.2203 (1993).
4. CONCLUSION
The influence of perfluorinated and hydrogenated surfactants on the kinetics of hydrogen evolution in 1 M H,SO, has been investigated from the steadystate polarization curves and impedance data. In the absence of surfactants, the formation of hydrogen bubbles increases the electrolyte resistance due to the screening effect of gas bubbles. With the presence of surfactants, the electrolyte resistance generally increases. It also rises with increasing current, in agreement with an enhanced screening effect by smaller and more numerous bubbles. The screening effect of bubbles and the inhibiting influence of surfactant mainly due to the electrode area masking depend upon the molecular structure (hydrophilic head and hydrophobic chain) of surfactants. From the increases in both the electrolyte resistance and the electrode polarization, it can be concluded that the activity of surfactants depends strongly on the hydrophilic head and decreases with the order cationic > anionic > non-ionic. Comparing perfluorinated and hydrogenated surfactants reveals that the perfluorinated chain produces a lower inhibition of hydrogen evolution than the corresponding hydrogenated chain. The decrease in the double layer capacitance testifies to the adsorption of cationic surfactants on the electrode surface at negative potentials. The charge transfer resistances and the Tafel plots show that the reaction mechanism of hydrogen evolution on a gold electrode is not modified by the presence of surfactants, and the experimental data agree with the Volmer reaction H+ + e- --) H,, as ratedetermining step.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
C. Juhel, B. Beden, C. iamy; J. M. Leger and R. Vianaud. Electrochim. Acta 35.479 (1990). A.%rossa, M. Maja, N. Pen&, G. ‘Far& G. Sandona and F. Marcti, Proc. 7th Eur. Symp. on Corrosion Inhibitors, Ferrara, Italy, 9,711 (1990). M. Maja, N. Penazzi, G. Famia and G. Sandona, Electrochim.Acta 38, 1453 (1993). C. Cachet, B. Saidani and R. Wiart, J. electrochem. Sot. 13% 678 (1991). J. Dukovic and C. W. Tobias, .I. electrochem. Sot. 134, 331 (1987). J. A. Leistra and P. J. Sides, J. electrochem. Sot. 134, 2442 (1987). H. Vogt, .I. electrochem.Sot. 137, 1179 (1990). P. J. Sides and C. W. Tobias, J. electrochem. Sot. 129, 2715 (1982). C. Gabrielli, F. Huet, M. Keddam, A. Macias and A. Sahar, J. appl. Electrochem.19,617 (1989). H. H. Trieu, R. R. Chandran and R. F. Savinell, J. electrochem. Sot. US,2218 (1989). N. P. Brandon and G. H. Kelsall, J. appl. Electrochem. 15,475 (1985). N. P. Brandon, G. H. Kelsall, S. Levine and A. L. Smith, J. appl. Electrochem. 15,485 (1985). A. Hamelin, Modern Aspects of Electrochemistry, (Edited by B. E. Conway,-R. E. white and J. 0%. Bockris) Vol. 16. D. 1. Plenum Press. New York (1985). L. J. J. ‘Janssen.Ad E. Barendrechi, Electrochim A& 28,341(1983).
17. J. O’M. Bockris and A. K. N. Reddy, Modern Electrochemistry, Vol. 2, p. 1231. Plenum Press, New York (1970). 18. M. Enyo, Comprehensive Treatise of Electrochemistry, (Edited by B. E. Conway, J. O’M. Bockris, E. Yeager, S. U. M. Khan and R. E. White) Vol. 7, p. 241. Plenum Press, New York (1983). 19. A. J. Bard and L. R. Faulkner, Electrochemical Methods, Fundamentals and Applications, 116. John Wiley, New York (1980).
Pergnmoa
Ebv*r sciena
0013-4686(!M)EOlgg-6
INFLUENCE OF PERFLUORINATED AND HYDROGENATED SURFACTANTS UPON HYDROGEN EVOLUTION ON GOLD ELECTRODES C.
CACHET,
M.
KEDDAM,
V. MARIOTTE and R. WIART
UPRlS du CNRS, Physique des Liquides et Electrochimie, Universite Pierre et Marie Curie, Tour 22, 4 place Jussieu, 75252 Paris Cedex 05, France (Receioed 24 January 1994; in revised form 31 March 1994)
Abstract-The influence of pertluorinated and hydrogenated surfactants on the mechanism of hydrogen evolution on gold electrodes in 1 M H,SO, has been investigated from steady-state polarization curves and impedance data. The ohmic drop due to bubbles and the reaction inhibition, consequent on both the electrode surface masking by bubbles and the surfactant adsorption, have been separated and compared. The activity of surfactants decreases with the hydrophilic head in the order cationic > anionic > nonionic. Pertluorinated surfactants produce a lower inhibition than the hydrogenated ones on the electrochemical reaction which occurs in a Tafel kinetic regime, independently of the presence of surfactants. Key words: hydrogen evolution, surfactant, impedance, ohmic drop, inhibition.
1. INTRODUCTION
It has been recently shown that the adsorption modalities of perlluorinated surfactants FORAFAC (ATOCHEM commercial products) on gold electrodes can be determined from capacitance measurements[l, 23. In Na,SO, electrolyte, it has been concluded that the adsorption mode of these compounds and the metal-molecule interactions strongly depend on the hydrophilic head of molecules. With a cationic compound (F1098), the aromatic rings probably generate strong attractive bondings which stabilize the molecules in a flat orientation independent of the electrode charge. The adsorption of an anionic compound (F1176) seems to be essentially determined by electrostatic forces which allow the molecules to set in the upright orientation, solely in the positive charge region of potentials. surfactants The behaviour of non-ionic (perfluorinated FlllO or hydrogenated HlllO) is intermediate and presents some similarity with the adsorption of the ionic compounds. In the positive charge region, the non-ionic compounds behave like the anionic one but the bonding between the metal and the hydrophobic chain in non-ionic molecules is stronger and able to inhibit the gold oxidation. In the negative charge region, the non-ionic molecules are adsorbed in a flat orientation, similar to the cationic compounds, but the bondings are weaker and allow easier desorption. The influence of perfluorinated surfactants on electrochemical processes are little known up to now, and their possible advantages on a given reaction have to be analyzed. However it is known that the non-ionic perfluorinated surfactant FlllO is an inhibitor for zinc corrosion and hydrogen evolution
on zinc[3-51 and also a levelling agent for zinc electrodeposition[6]. FORAFAC 1098 is used as emulsifier in fire extinguishers whereas FORAFAC 1176 can be used as a wetting agent in baths of chromium electroplating. With a view to energy saving, gas evolving electrodes have been extensively studied and various effects of bubbles on the electrode polarization have been separated[7-111. Different terms of the overpotential have been estimated: (i) the ohmic drop within the electrolyte; (ii) the area masking on the electrode surface which inhibits the electrochemical reaction; and (iii) the concentration overpotential. It is known that the presence of surfactants in the electrolyte can modify the different terms of the overpotential by affecting the characteristics such as growth rate, detachment diameter and population density of hydrogen bubbles[12-141. Hydrogen evolution in sulfuric acid solution has been chosen to investigate the influence of various perfluorinated and hydrogenated surfactants upon gas formation, and the results of this comparative analysis will be presented and discussed in the present paper. 2. EXPERIMENTAL The electrolyte was 1 M H,SO, of analytical grade purity (Merck). The solutions were prepared with water which was doubly ion-exchanged, passed through a carbon cartridge and a micropore filter, and twice distilled with potassium permanganate in a quartz apparatus. The measurements were performed at room temperature in an electrolysis cell (Fig. 1) made of Teflon. A diaphragm of Nafion@’ 423 (Du Pont de Nemours), was used to separate the anode and cathode compartments and avoid any possible influ-
C. Cacnm et al.
2744
ence of the anodic reaction on the cathodic process. All potentials were referred to a mercury/mercurous sulphate electrode in saturated potassium sulphate (sse) located in the cathode compartment. The counter electrode was made of a gold wire (Johnson, Matthey, Puratronic) whose surface area (3.9cm2) was much larger than that of the working electrode. The working electrode was made of a gold rod (Johnson Matthey, Specpure), coated with a cataphoretic paint and moulded in epoxide resin (Buehler), leaving the cross section uncovered thus forming a disc electrode of 0.3 cm diameter. The electrode surface was polished with emery paper (1200 grit) and with alumina powder (0.3~) to a mirrorlike finish. After polishing, alumina particles were removed from the surface with ultrasonic cleaning in water. Then the working electrode was placed in the cell, and finally an additional in situ treatment of the electrode surface was performed by cyclic voltammetry in the electrolyte, between - 0.9 and - 1.5 V, and using a slow sweep rate (1 mVs- ‘), to obtain reproducible polarization curves before the addition of surfactants. This treatment maintained the gold electrode surface in the reducing environment of hydrogen molecules, differently from the cleaning procedure consisting in a repetitive formation and reduction of oxide layer[15]. The upwards orientation of the active surface allowed the bubbles to grow and detach freely. surfactants following perfluorinated The FORAFAC (Atochem) were tested: the non-ionic FlllO: C6F,,-C2H.,(0C2H4),0H
n x 11.5
the anionic F1176: C,F1s-C2H,SO;K+ the cationic F1098:
and they were compared to the hydrogenated compounds: the non-ionic H1110 (Witco): C,oH2,-(OC2H,),iOH the anionic H1176 (Aldrich): CsF,,SO;Na+ the cationic H1098 (Aldrich):
To facilitate the comparison of the influence of surfactants on hydrogen evolution, the experimental procedure was standardized. All surfactants were added to the electrolyte at the potential E = -0.9 V, and at the same concentration (lo-’ M) higher than the critical micellar concentration of most of pro-
ducts, so that the surface tension was minimized and independent of surfactant concentration. Thereby it was possible to reduce the amplitude of current fluctuations and obtain significant results. Impedance measurements were performed under potentiostatic conditions using a frequency response analyzer (Schlumberger-Solartron 1174) controlled by an Apple II microcomputer. The electrolyte resistance, R,, between the working electrode surface and the reference electrode, was measured under primary potential distribution by the high-frequency limit of the electrode impedance approached at IOOkHz. The electrode impedance was measured in the frequency domain (IOOkHz-0.1 Hz). To improve the signal/noise ratio, the integration time was 10 periods at frequencies lower than lOHz, whereas it was 0.1 s at frequencies higher than 10Hz. 3. RESULTS AND DISCUSSION 3.1. Electrolyte
resistance
The electrolyte resistance R, is a significant parameter to characterize the screening effect of gas bubbles within the electrolyte close to the electrodeC7, 8, 10, 111. The growth and detachment of bubbles do induce fluctuations of current and electrolyte resistance. Because of these fluctuations, numerous measurements were repeated as a function of time, and the values of R, obtained for different currents I are presented in Figs 2-5, in which the solid lines correspond to the mean value of R, . In the absence of surfactants, small scattering of R, is observed when the current I remains lower than O.OlmA, and the mean value of R, is 4.4 f 0.252 (Fig. 2). With the irregular formation of bubbles occurring for I > 0.01 mA, the fluctuation amplitudes are higher and more scattered. The mean value of R, becomes around 5.4Q and the difference in R, is due to the screening effect of bubbles. With the presence of surfactants, the fluctuation amplitudes decrease for currents ranging between 0.1 and 1 mA and less scattered values of R, have been observed for a given flow of hydrogen molecules, in agreement with the reduced size of bubblesC12, 133. As shown in Figs 3-5 the mean value of R, is generally increased with the presence of surfactants, except for the non-ionic hydrogenated compound HlllO which deceases R, at currents lower than 5mA. The increased values of R, characterize an enhanced screening effect generated by smaller and more numerous bubbles formed at the electrode surface in the presence of surfactants. Moreover above approximately 0.1 mA, the electrolyte resistance is systematically observed to rise with increasing current, as shown on the curves in Figs 3-5. A similar increase in R, with increasing current density has already been reported for hydrogen evolution in alkaline solution[16]. At a given current (I = 20 mA, ie a current density of 280mA cme2) corresponding to a given flow of produced hydrogen molecules, the clear influence of a surfactant on the screening by bubbles can be characterized by the difference AR, between the electrolyte resistances with surfactant (deduced from the curves in Figs 3-5) and without surfactant (5.4R). Line 1 in Table 1 presents a comparison of the
2745
Influence of surfactants upon hydrogen evolution RF: FERENCE
COUNTER
ELECTRODE
TEFLON
CYLINDER
ELECTROLYTE
EPOXIDE
RESIN
HOLDER
Fig. 1. Electrolysis cell.
1 o- 2
16 3
loo
10-l
10’
lo2
CURRENT / mA Fig. 2. Electrolyte resistance R, versus current I: Electrolyte 1 M H,SO,
Table 1. Influence of surfactants on: (1) the excess in electrolyte resistance AR, at I = 20mA; (2) the overpotential at I = 20mA; (3) the Tafel slope p; (4) the R,I product; (5) the double layer capacitance C Surfactant
(1)
ARJf-2 (2) (AE I/mV (3)
p/mVdec- ’
(4) R,I/mV (5) C//lFcnl-*
None
F1098
H1098
F1176
H1176
-
7.0
5.6
1.7
3.1
FlllO
HlllO
1.5
0.9
-
122
161
52
70
25
49
137
127
123
122
121
119
121
51+9 20*5
55 f 7 -
55 * 4 -
49 f 4
53 + 3
51 *4
49 f 4
17 f 3
19 f 2
20 f 4
18 f 3
C. CAcmrc et al. -I
(1 l
No surfactnnt
l
1 mMFlll0
6-
i 0”
I
O’>
1
o’L
1 0’
’
1 0”
1 0’
1 02
CURRENT / mA I: influence of non-ionic surfactants.
Fig. 3. Electrolyte resistance R, versus current
results obtained with the different surfactants. In spite of the scattering of measurements, it appears that the screening effect by bubbles is maximum for cationic surfactants and minimum for the non-ionic ones. Thereby the screening effect of bubbles within the electrolyte seems more sensitive to the hydrophilic head (cationic, anionic or non-anionic) than to the hydrophobic tail (perfluorinated or hydrogenated), in spite of a considerable influence of this hydrophobic tail on the surface tension liquid/ gas which decreases from 33mNm-’ for the hydrogenated compounds to 18 mNm- ’ for the
14-
12 -
“““‘I
““.‘.
.
No surfactant
l
lmMF1098
3.2. Polarization curves The steady-state polarization curves were obtained by cyclic voltammetry at a slow sweep rate (1 mVs- ‘) for which the direct and reverse curves coincide. All curves Z(E) correspond to the mean current and the potential E has been corrected for ohmic drop with the mean value of R,. The influences of perfluorinated and hydrogenated surfactants on polarization curves are shown in Figs 6 and 7, respectively. In both cases, the inhibition strongly depends on the hydrophilic head and decreases with
“.““I
,.....I
......1
. ..-
.
0 lmMH1098
IO -
8-
:~~.~~~!~~~~~~ 10”
IO.”
IO”
1 0’
’
CURRENT
1
0”
I
0’
/ mA
Fig. 4. Electrolyte resistance R, versus current I: influence of cationic surfactants.
I
02
Influence of surfactants upon hydrogen evolution
2147
r-c,A .‘.““’ i O’J CURRENT
/ mA
Fig. 5. Electrolyte resistance R, versus current I: influence
the order cationic > ionic > non-ionic. The inhibition by surfactants can result from the adsorbed molecules on the electrode and/or from the enhancement of the area masking on the electrode surface by smaller and more numerous gas bubbles which perturb the potential and current distributions around the electrode. The inhibiting influence of the different surfactants have been compared by considering the overpotential AE at a given current. The value of IAE] at 20mA are gathered in Table 1, line 2. The hydrogenated surfactants appear to inhibit the hydrogen reaction more than the corresponding fluorinated compounds. As illustrated in Fig. 8, the Tafel plots of polarization curves are linear over more than three decades of current values and the Tafel slopes p = - AE/
A(logi) are given in Table 1, line 3. No significant influence of the surfactants has been observed on the Tafel slope whose mean value is p = 124mV dec- ’ in the whole potential domain. This value is in agreement with a Tafel kinetic regime for hydrogen evolution. In addition, the Volmer reaction (H+ + e- + Had) appears to be the rate determining step, in agreement with a slope of 120mVdec-’ calculated by the pseudo-equilibrium method for a charge transfer coeficient of 0.5[17,18]. 3.3. Electrode impedance Only one capacitive loop has been observed on complex plane impedance plots, as illustrated in Fig. 9 where the origin of plots has been offset by the electrolyte resistance R,. This loop corresponds to
.
2”
40
I
n
No smfactant
0
1mMFlllO
A
lmMF1176
0
lmMF1098
of anionic surfactants.
0
. 0
A
.
O
A
. 0
. c3
.
20 -
Fig. 6. Influence of peduorinated
O
A
A
surfactants on polarization
0
0
curves.
c. tkXiFiT
2748
I 40 t
I
.
lmMHlI10
A
lmMH1176
q
1mM H1098
n 0
.
1.3
1.2
1.1
Fig. 7.
1
Nosurfactant
0
1.0
et al.
1.4
1.5
-E / V @SE) Inlkrence of hydrogenated surfactants on polarization curves.
the charge transfer resistance R, in parallel with the double layer capacitance C. The inhibiting influence of surfactants yields an increased R,, at a given potential, and it is confirmed that the cationic surfactants have the strongest inhibiting effect. The R, I product being independent of current, the results agree with a rate-determining step being an irreversible electrochemical reaction. Furthermore, it appears that the surfactants do not change the mechanism of the hydrogen evolution reaction since they do not effect the R,I products, as shown in Table 1, line 4. The mean value of this product, 52mV, confirms that the rate-determining step is a
1mM
one-electron reaction[19]. Then the current density of the cathodic electrochemical reaction is governed by the following equations: 1 = i, exp[ - b(E - E,)]
where i, is the current density corresponding to an arbitrarily chosen potential E, . From the mean value of R,i, equation (2) yields p = 120mVdec-‘, in agreement with the experimen-
FlllO
-E / V @SE) Fig. 8. Tafel plot of the polarization
curve with 10m5 M FlllO.
2749
Influence of surfactants upon hydrogen evolution
tal Tafel slope of 124 mV dec- ‘, in view of the accuracy of measurements (about 10% due to the presence of current fluctuations). The capacitance C, calculated at various points on the capacitive loop, does not reveal any significant frequency dispersion. The mean values, deduced from the apex of the capacitive loop, are exhibited in Table 1, line 5, and in Fig. 10. No potential dependence has been observed without surfactant and with non-ionic or anionic surfactants. Table 1 also shows that the double layer
capacitance is not significantly affected by these compounds, thus indicating that C is much less sensitive to the presence of these surfactants than the electrolyte resistance R, and the electrode potential E. It consequently appears that the non-ionic and anionic surfactants do not directly adsorb on the electrode surface and the modifications to R, and E probably result from changes in the characteristics of bubbles and thereby in the electrode area masking. On the other hand, the addition of the cationic surfactants, which are the most efficient inhibitors,
___ E
80-
0
No surfactant
A
1mMFlllO
(1=0.81mA)
0
ImMF1098
(I=O.UmA
=
-1.195
V
( I = 1.25 mA 1
I
I LH,
1.6 kH/ 10 kH,
.
0
A
*
20
0
40
Re(Z)
60
80
100
/ Ohm
Fig. 9. Complex plane impedance plots at E = - 1.195V: influence of fluorinated surfactants.
0
0.9
No surfactant
1.0
1.1
1.2
1.3
1.4
-E / V (SSE) Fig. 10. Potential dependence of the double layer capacitance: influence of cationic surfactants.
C.
2750
also yields a marked decrease in the double layer capacitance. As exhibited in Fig. 10, a similar influence has been observed with FlO!X or H1098 and the decrease is more pronounced with increasing cathodic polarization, in agreement with the adsorption of cationic compounds on the electrode surface. This adsorption probably contributes to the relatively high value of 1AE 1 observed for the cationic surfactants.
et al.
Ackmowledgement~The authors thank ATOCHEM for financial support of this work, and particularly I. Devaux and E. Pou for helpful discussions.
REFERENCES C. Cachet, M. Keddam, V. Mariotte and R. Wiart, Electrochim. Acta 37,2317 (1992). C. Cachet. M. Keddam. V. Mariotte and R. Wiart. Electrochik Acta 38.2203 (1993).
4. CONCLUSION
The influence of perfluorinated and hydrogenated surfactants on the kinetics of hydrogen evolution in 1 M H,SO, has been investigated from the steadystate polarization curves and impedance data. In the absence of surfactants, the formation of hydrogen bubbles increases the electrolyte resistance due to the screening effect of gas bubbles. With the presence of surfactants, the electrolyte resistance generally increases. It also rises with increasing current, in agreement with an enhanced screening effect by smaller and more numerous bubbles. The screening effect of bubbles and the inhibiting influence of surfactant mainly due to the electrode area masking depend upon the molecular structure (hydrophilic head and hydrophobic chain) of surfactants. From the increases in both the electrolyte resistance and the electrode polarization, it can be concluded that the activity of surfactants depends strongly on the hydrophilic head and decreases with the order cationic > anionic > non-ionic. Comparing perfluorinated and hydrogenated surfactants reveals that the perfluorinated chain produces a lower inhibition of hydrogen evolution than the corresponding hydrogenated chain. The decrease in the double layer capacitance testifies to the adsorption of cationic surfactants on the electrode surface at negative potentials. The charge transfer resistances and the Tafel plots show that the reaction mechanism of hydrogen evolution on a gold electrode is not modified by the presence of surfactants, and the experimental data agree with the Volmer reaction H+ + e- --) H,, as ratedetermining step.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
C. Juhel, B. Beden, C. iamy; J. M. Leger and R. Vianaud. Electrochim. Acta 35.479 (1990). A.%rossa, M. Maja, N. Pen&, G. ‘Far& G. Sandona and F. Marcti, Proc. 7th Eur. Symp. on Corrosion Inhibitors, Ferrara, Italy, 9,711 (1990). M. Maja, N. Penazzi, G. Famia and G. Sandona, Electrochim.Acta 38, 1453 (1993). C. Cachet, B. Saidani and R. Wiart, J. electrochem. Sot. 13% 678 (1991). J. Dukovic and C. W. Tobias, .I. electrochem. Sot. 134, 331 (1987). J. A. Leistra and P. J. Sides, J. electrochem. Sot. 134, 2442 (1987). H. Vogt, .I. electrochem.Sot. 137, 1179 (1990). P. J. Sides and C. W. Tobias, J. electrochem. Sot. 129, 2715 (1982). C. Gabrielli, F. Huet, M. Keddam, A. Macias and A. Sahar, J. appl. Electrochem.19,617 (1989). H. H. Trieu, R. R. Chandran and R. F. Savinell, J. electrochem. Sot. US,2218 (1989). N. P. Brandon and G. H. Kelsall, J. appl. Electrochem. 15,475 (1985). N. P. Brandon, G. H. Kelsall, S. Levine and A. L. Smith, J. appl. Electrochem. 15,485 (1985). A. Hamelin, Modern Aspects of Electrochemistry, (Edited by B. E. Conway,-R. E. white and J. 0%. Bockris) Vol. 16. D. 1. Plenum Press. New York (1985). L. J. J. ‘Janssen.Ad E. Barendrechi, Electrochim A& 28,341(1983).
17. J. O’M. Bockris and A. K. N. Reddy, Modern Electrochemistry, Vol. 2, p. 1231. Plenum Press, New York (1970). 18. M. Enyo, Comprehensive Treatise of Electrochemistry, (Edited by B. E. Conway, J. O’M. Bockris, E. Yeager, S. U. M. Khan and R. E. White) Vol. 7, p. 241. Plenum Press, New York (1983). 19. A. J. Bard and L. R. Faulkner, Electrochemical Methods, Fundamentals and Applications, 116. John Wiley, New York (1980).