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Redox reactions of hydrosulphide ions on the platinum electrode—II. An impedance spectroscopy study and identification of the polysulphide intermediates
;\ Pergamon
REDOX REACTIONS OF HYDROSULPHIDE IONS ON THE PLATINUM ELECTRODE-II. AN IMPEDANCE SPECTROSCOPY STUDY AND IDENTIFICATION OF THE POLYSULPHIDE INTERMEDIATES J. SZYNKARCZUK,* f. G. K~M~ROWSKI CANMET.
Western
Research
and J. C. DONIN]
Centre, One Oil Patch Drive, P.O. Bag 1280, Devon, Alberta, TOC ICO (Received
18 Januq
Canada
1994)
Abstract-Impedance
spectroscopy was used to confirm the presence of a poorly conductive sulphur layer on a platinum electrode during the electrolysis of HS-. The double layer capacitance was found to at open circuit potential. When electrolysis commenced the double layer cabe close to 4O~Fcm-’ pacttance was greatly reduced (< 5pFcm-*). The charge transfer resistance increased to few hundred ohmscm’ during electrolysis. Double potential chronocoulometric experiments were also performed. The chronocoulometric analysis showed that the average stoichiometry of the intermediate polysulphide ions m the electrolysis reaction was S:,, in the anodic step and S:,; in the cathodic step. Key
words:
impedance
spectroscopy,
chronocoulometry.
polysulphide
ions, hydrosulphide
INTRODUCTION
EXPERIMENTAL
The accumulation of a partially blocking layer of elemental sulphur (So) is suggested by the results of various electrochemical tests which have been performed to date[l]. The double layer capacitance and the charge transfer resistance of the electrode electrolyte interface can be measured directly by impedance spectroscopy in the frequency range (~20 kHz) where the electrode electrolyte interface effects are seen. The potential across the interface determines the reaction rate, therefore, the shape of the impedance spectrum will depend on the potential applied. Kinetic limitations will predominate at potentials below that required for electrolysis to proceed; at potentials suficiently high for electrolysis, the diffusion of reactants to, and possibly the diffusion of products from, the electrode would be the limiting factor. These effects have been successfully modelled using Randles’ equivalent circuit[2] where the double layer capacitance (Cd,) is in parallel with a series combination of resistance (R,,) and Warburg impedance (W). A solution resistance (R,) is in series with the rest of the circuit. In addition to a diffusion limiting effect, the development of a sulphur layer during electrolysis, would create a large impedance arising from the blocking layer. The existence of a soluble intermediate polysulphide at the electrode electrolyte interface was established in a previous work[l]. Further analysis of the double step chronocoulometry results was used to identify the ranks of the soluble intermediate in the anodic and cathodic steps. * Author to whom correspondence
should
ions.
be addressed,
impedance
spectroscop}
The impedance spectroscopy was performed using a Slumberger-Solartron I255 Frequency Response Analyzer with a Slumberger-Solartron 1286 Electrochemical Interface. The platinum working electrode was a disc electrode which could either be rotated or held stationary during the tests. Several potentials were tested ranging from the open circuit potential to 0.4 V. A saturated calomel electrode was used as a reference. The electrodes were conditioned for one hour prior to the recording of each impedance spectrum (20kHz to 1 Hz). The use of frequencies below 1Hz was found to be impractical since the length of time taken (several hours) to acquire an accurate result at low frequencies (-c I Hz) would be inordinate: if the electrolysis of the solution was to occur slowly then the impedance spectrum would fail to properly describe the behaviour of the electrode at very low frequencies. A 1 M Na,S solution, bubbled with argon, was tested with, and without, electrode rotation while another experiment was conducted with the presence of sulphur. A 1 M solution of NaOH was tested for comparison.
Chronocoulometric
experiments
Double potential chronocoulometric experiments were performed in solutions containing hydrosulphide ions (1 mM), together with a buffer and supporting electrolyte, 0.05 M Na,B,O,, as described in a previous article[ I]. 487
488
J.
SZVNKARCZUK et al.
Electrode Potential vs. SCE IV 3000 -r
Y-
2500
2wo
1500
1000
500
El&rode
Potential vs. SCE IV
Fig. 1. (A) The calculated values of double layer capacitance, (B) charge transfer resistance, and (C) Warburg factor, with respect to applied potentia1 for a platinum disc electrode without (1) and with (2) rotation. The insert presents a complex impedance plot at -0.2OV.
489
Redox reactions on platinum electrodes--II RESULTS
AND DISCUSSION
Impedance spectroscopy A typical impedance spectrum is shown in Fig. 1 with squares representing experimental points and a fitted curve. Calculated double layer capacitances (Fig. IA) were found to be of the order of at -0.6 V, which was just above the 40pFcm-’ open circuit potential of the system. A linear decrease in capacitance was observed with increasing potential until approximately - 0.2 V where a capacitance of approximately 10 PF cm- ’ was calculated and a slower decline in capacitance observed. At 0.2V a double layer capacitance of approximately 4pFcmm2 was calculated. The effect of either rotating or holding the electrode fixed appeared to be insignificant. Generally, the charge transfer resistance fell from the range 2.5-3 kRcm2 to below 5OORcm’ (Fig. IB) when the potential was increased from -0.6 to -0.4V, above -0.2 V the charge transfer resistance increased. The high values of resistance especially in higher potential range were caused by the formation of sulphur layer inhibiting the charge transfer. The strongest evidence for the existence of a blocking layer at the electrode-electrolyte interface was found in the behaviour of the double layer capacitance with respect to applied potential for the Na,S solutions (Fig. 1A) and the NaOH solution (Fig. 2). In the range of potentials where HS electrolysis occurred, the capacitance fell to a small value (approximately 2.5pFcm-*, Fig. IA at 0.2 V); but in the case of NaOH, where no reaction occurs and, therefore, no sulphur layer formed, the double layer capacitance demonstrated a minimum at -0.2 v. The values of Warburg factor (Fig. 1C) showed an increase during the electrolysis of HS-, which demonstrated the presence of a substantial diffusion limitation. The introduction of oxygen, by aerating the 1 M Na,S solution, had no significant effect on the
-0.40 Fig. 2. Double
layer capacitance
impedance spectroscopy results (Fig. 3). Therefore oxidation of the first solution was shown not to be a cause of sample variation. The nature of the sulphur layer might exhibit irregularities. The sulphur content of the Na,S solutions was suspected to be the cause of experimental deviations. Figure 4 shows the effect of sulphur additions on the Na,S solution. These preliminary results show that sulphur, which is a product of electrolysis, can have an effect on the impedance parameters at all potentials. The charge transfer resistance is particularly influenced which indicates faster sulphur layer dissolution from the surface of working electrode in the presence of polysulphide ions. The effect of electrolysis on the impedance parameters was consistent with the formation of a thin partially blocking layer. The double layer capacitance was at a small constant value during electrolysis. This would be expected if a coating of material at the electrode electrolyte interface had formed. The significant increase in electrode electrolyte resistance accompanied with an increase in the Warburg factor provided further evidence of electrode blocking. Chronocoulometric experiments Figure 5 shows the analysis of current-time data from the chronocoulometric/amperometric experiments. According to the Cottrell equation the current density (i) decays with respect to the inverse square root of time (t ‘j2), and is proportional to the number of electrons (n), the Faraday constant (F), the diffusion coefficient (D), and the concentration of electroactive species (c):
i = nFD’/Z~=- 1/2l~1!2
(1)
If the Cottrell equation is observed then the quantity 7it’j2 should be independent of t’12 and the horizontal line will be parallel to the (‘I2 axis for diffusion-controlled processes[2], whereas the shorttime data are affected by the double layer. Figure 5 shows a line which is not parallel to the t”’ axis in
-0.20 0.00 Electrode Potential vs. SCE / V with respect to applied
potential
0.20
0.40
for a I M NaOH
solution.
J.
490
-0.40
SZVNKARCZUK
et a[.
-0.20 0.00 Electrode Potential vs. SCE IV
0.20
0
B
0.20
C IO
Electrode Potential vs. SCE / V Fig. 3. (A) The double
layer capacitances,
and (B) charge transfer, aerated (2) solutions.
an anodic process, which proves that the kinetics of these reactions is complicated. Such a result was found previously in voltammetric experiments[ 11. Production of a sulphur layer on the surface of the electrode inhibits the diffusion of HS-; these ions have to diffuse through the layer of sulphur and, as a result, a declining line was observed in Fig. 5. The dashed line represents the expected behaviour if a simple diffusion-controlled reaction was to occur. The calculated value of the diffusion coeficient was relatively high (2.2 x IO-’ cm’s ‘) and was close to the value (3 x lo- 5 cm2 s- ‘) obtained by Kapusta rt uI.[3]. Data analyses were performed on the anodic and cathodic branches according to the work of Christie ef al.[4], and are presented in Fig. 6. Analysis of double potential step chronocoulometric data showed two straight lines on both; the anodic and cathodic branches. The two different slopes on either the anodic or cathodic branches
calculated
for deoxygenated
(1) and
suggest that two distinct anodic, and cathodic products were performed while the potential (anodic or cathodic) was applied. This proves the two-step mechanism of HS ion oxidation (HS-/Si and .Si-/Y; equations (5) and (6) from Cl]) and reduction (S’/Si and S,‘-/HS-; equations (3) and (4) from [l]) rather than the direct oxidation/reduction mechanism (HS-/So; equation (1) from Cl]). The analysis of these data suggested that there was no adsorption of HS- ions on the surface of electrode, possibly due to a changed morphology of the electrode surface. But it is known that sulphurcontaining compounds and ions are likely to adsorb on the platinum[5- 71 especially under acidic conditions. Double-layer capacitance of 3.3 PF cm-’ was found to be comparable to those values obtained by impedance spectroscopy (1-3 PF cm -’ at 0.40 V) but lower than values reported elsewhere[3], eg 8pFcmm2.
Redox reactions
on platinum
491
electrodes-11
Electrcde Potential vs. SCE
Electrode Potential vs. SCE
IV
/V
Fig. 4. (A) The double layer capacitances, and (B) charge transfer resistance, calculated solutions containing no sulphur (I), 0.01 M sulphur (2) 0.1 M sulphur (3).
for
I M Na,S
5.OE-04 1
1 .OE-04 1’ O.OE+OO! 0
I 0.2
I 0.4
0.6
0.8
1
I 1.2
1.4
1.6
18
t ‘I2 is’Fig. 5. Analysis of i&t data of anodic process, from chronocoulometric experiments in solution containing I x lo-‘M Na,S and O.OSOM Na,B,O, at pH 9.3. Potential was changed from -0.415 to 0.1 V. Dashed line indicates expected position of plateau for diffusion process.
492
al.
J.SzmiK~~czmet
ANODIC PROCESS
Charge / mC cm.* Fig. 6. Analysis of double potential step chronocoulometric data in solution containing 1 x lo- 3 M and O.OSOM Na,B,O, at pH 9.3. Potential was changed from -0.415 to 0.1 V, and after r = 3s was kept at 0.80 V for 3 s.
Na,S
The identification
of polysulphide intermediate
ions
The slopes of double potential chronocoulometric data from Fig. 6 are described by [4,8] :
dQ
anodic
-
=
dt”’
2n,
F
cR(D&r)1i2
cathodic
dQ
~
d@
0.97r,
= 2nc,(D,/n)“2
2C,
(2)
1
7r1’2
D;"T"'
(3) where @ = (t _ T)U2 + T’/2 - t’i2 In the case where there is no adsorption, tion (4) reduces to :
(4) then equa-
dQ -..A@?% = 2n,~c,(~,/~)'~2 d@
(5)
As described earlier, a two-step electrochemical reaction was detected during hydrosulphide oxidation which explained the two different slopes observed in Fig. 5. Knowing the diffusion coefficient of the hydrosulphide ions and that of .Si-, one could calculate the ratio of the number of electrons from those two reactions (equations (5)-(6) from [ 11):
range of diffusion coeflicients for polysulphide ions of various ranks had to be taken into account. In the case of sodium polysulphide ions, the highest number of sulphur atoms reported was 5[10-121. Assuming that the diffusion coefficient is inversely proportional to the square root of mass, the averaged calculated n value was 2.5 + 0.3 and it was lower than the value of 3.3 obtained by others[9] using a gold electrode. Therefore an equal mixture of Ss- and S:- might have given the average stoichiometry of 2.5. The analogous calculations were made for the two cathodic slopes; the average value of n was found to be 4.3, and was also independent of the size of the first potential step. During the cathodic reaction, it appeared that S:- was the dominant active species. According to Giggenbach[ IO] this form of polysulphide should dominate in aqueous solutions. It is difficult to explain why, on the cathodic potential step, different polysulphide species were more active than on the anodic one. It might be explained by a local reduction of pH; occurring at the electrode electrolyte interface. The stability of higher rank polysulphides increases at lower pH[ to]. Polysulphides which are in contact with the sulphur layer deposited on the anode are in an equilibrium state in solution[lO, 111. The following reactions describe the dissolution of the sulphur layer.
dQanodic t < 0.36 s dr”2
S + HSs + s:-
= s:--
(8)
dQsnodic t > 0.36 s dt”’
s + s:-
= s;-
(9)
s + s:-
= s:-
(10)
=(n-
1). $$ n2
=
1.7
(6)
The average value of the ratio from equation 6 was 1.7 and it did not depend on the first potential step within the range of 0.1 to 0.4V. To calculate n, the
+ OH_ = S;-
+ H,O
(7)
Polysulphide ions are also present in a dynamic equilibrium which can be described by the following equation: nSf+ ,S2- + HS-
+ OH-
= (n + l)S,S2-
+ H,O (Ill
Redox reactions on platinum
-3000 I -1 .oo
-0.50
0.00
0.50
493
electrodes--II
1.00
1.50
2 IO
Electrode Potential vs. SCE /V Fie. voltammoerams recorded - 7. Cvclic _ Na,Ba,O, at ph 9.3. Scan rate
in solution containing 1 x 10m3 M Na,S and 0.050M 1OOmVs ‘, rotation rate; (a), 1000 (b). and 3boO (c).
which shows that an increase in pH would shift the equilibrium to the right, ie towards lower rank of polysulphide ions: S:- and S:- were initially produced as the first step of sulphide ion oxidation. Also polysulphide ions were produced simultaneously by chemical reactions (equations 7-l 1) keeping the sulphur layer in equilibrium between electrochemical production of sulphur and chemical sulphur dissolution. These chemical processes together with the cathodic potential applied were able to remove the sulphur layer from the platinum electrode. Formation
oJsulphute
ions
The only thermodynamically stable[ 121 oxidation states for the sulphur water system at room temperature are: -2, 0, and + 6. Sulphide ions can be oxidized to sulphate according to the following reaction : HS
+ 9 OH-
= SOi-
+ 5 H,O
+ 8 e-
(12)
This reaction took place at l.lOV (Fig. 7) where a relatively high peak was recorded due to the formation of SO;ions. The current density at l.lOV deviated from the theoretical expectation based on the Levich equation, most probably due to the sulphur layer formed at O.OV. If the entire electrode area was covered with a sulphur layer one would not expect to record the response associated with the conversion of HS to SO:(HS-/SO:-) on the anodic but rather on the cathodic scan, when the sulphur layer is partially dissolved chemically. In addition, if the potential were shifted to high positive values to produce oxygen, then the oxygen bubbles so produced would clean part of the electrode and allow the electrochemical reaction (equation 12) to proceed. An absence of the HS-/SO:peak on the anodic scans was reported for a gold (c”s- = 1.4 x 10m4 M)[9] and platinum (C,,_ > 0.2 M)[3] electrodes in sulphide solutions. In this study, the peak due to HS-/SO:was already observed on the anodic scan, indicating only partial blockage of the electrode. EA 40-4-J
CONCLUSIONS Impedance spectroscopy results have confirmed the development of a partially blocking layer at the electrode electrolyte interface during electrolysis. The decline in the double layer capacitance during electrolysis is observed in Na,S solutions but not in NaOH solutions. The presence of dissolved oxygen had no significant effect on the impedance spectra. The intermediate species were found to be a mixture of polysulphides with an average stoichiometry of S:,; in the anodic step and S:.; in the cathodic step. The value of the pH at the electrode electrolyte interface during the respective steps would be a possible explanation for the observed differences in polysulphide rank. Acknowledgrmrnts~~~~Thiswork was supported by the Panel on Energy Research and Development (PERD) and by an internal CANMET grant. We would like also to thank the Natural Science and Engineering Research Council of Canada for funding Visiting Fellowships for J. Szynkarczuk and P. G. Komorowski.
REFERENCES I. P. G. Komorowski. J. Szynkarczuk Electrochim. Actu (submitted).
and J. C. Donrni,
Electrochemistry Group, Insrrumentc~l 2. Southampton Methoda in Elrctroc’htwistry, Chaps 2 and 6. Ellis Horwood, New York (1990). 3. S. Kapusta. A. Viembeck, S. M. Wilhelm and N. Hackerman, J. &ctrouna/. Chem. 153, 157 (1983). 4. J. H. Christie. R. A. Osteryoung and F. Anson. J. r/rc~trountrl. Chrm. 13, 236 (1967). 5. A. Q. Contractor and H. Lal. J. elcc~rocud. Chm~. 96, 175 ( 1979). 6. T. Loucka. J. dw~rouncd. Chw~. 31, 319 (1971); 44, 221 ( 1973). J. Chevalet and K. I. V. Svetlicki. J. Claviler, V. &tic, Elachi, J. clwrrounul. Chem. 344, 145 ( 1993). in Phy.sicrc[ Methods in Chetnisrry 8. R. W. Murray, (Edited by B. W. Rosslter and J. W. Hamilton), Vol. 2. p. 553. Wiley, New York (1986).
494 9. A. N. Buckley,
J. SZYNKARCZUK et al.
I. C. Hamilton and R. Woods, J. electroanal. Chem. 216,213 (1987). 10. W. Giggenbach, Inorganic Chem. 11, 1201 (1972).
11. W. F. Giggenbach, Inorganic Chem. 13, 1774 (1974). 12. H. G. Kelsall and 1. Thompson, J. appl. Electrochem. 23, 287 (1993).
REDOX REACTIONS OF HYDROSULPHIDE IONS ON THE PLATINUM ELECTRODE-II. AN IMPEDANCE SPECTROSCOPY STUDY AND IDENTIFICATION OF THE POLYSULPHIDE INTERMEDIATES J. SZYNKARCZUK,* f. G. K~M~ROWSKI CANMET.
Western
Research
and J. C. DONIN]
Centre, One Oil Patch Drive, P.O. Bag 1280, Devon, Alberta, TOC ICO (Received
18 Januq
Canada
1994)
Abstract-Impedance
spectroscopy was used to confirm the presence of a poorly conductive sulphur layer on a platinum electrode during the electrolysis of HS-. The double layer capacitance was found to at open circuit potential. When electrolysis commenced the double layer cabe close to 4O~Fcm-’ pacttance was greatly reduced (< 5pFcm-*). The charge transfer resistance increased to few hundred ohmscm’ during electrolysis. Double potential chronocoulometric experiments were also performed. The chronocoulometric analysis showed that the average stoichiometry of the intermediate polysulphide ions m the electrolysis reaction was S:,, in the anodic step and S:,; in the cathodic step. Key
words:
impedance
spectroscopy,
chronocoulometry.
polysulphide
ions, hydrosulphide
INTRODUCTION
EXPERIMENTAL
The accumulation of a partially blocking layer of elemental sulphur (So) is suggested by the results of various electrochemical tests which have been performed to date[l]. The double layer capacitance and the charge transfer resistance of the electrode electrolyte interface can be measured directly by impedance spectroscopy in the frequency range (~20 kHz) where the electrode electrolyte interface effects are seen. The potential across the interface determines the reaction rate, therefore, the shape of the impedance spectrum will depend on the potential applied. Kinetic limitations will predominate at potentials below that required for electrolysis to proceed; at potentials suficiently high for electrolysis, the diffusion of reactants to, and possibly the diffusion of products from, the electrode would be the limiting factor. These effects have been successfully modelled using Randles’ equivalent circuit[2] where the double layer capacitance (Cd,) is in parallel with a series combination of resistance (R,,) and Warburg impedance (W). A solution resistance (R,) is in series with the rest of the circuit. In addition to a diffusion limiting effect, the development of a sulphur layer during electrolysis, would create a large impedance arising from the blocking layer. The existence of a soluble intermediate polysulphide at the electrode electrolyte interface was established in a previous work[l]. Further analysis of the double step chronocoulometry results was used to identify the ranks of the soluble intermediate in the anodic and cathodic steps. * Author to whom correspondence
should
ions.
be addressed,
impedance
spectroscop}
The impedance spectroscopy was performed using a Slumberger-Solartron I255 Frequency Response Analyzer with a Slumberger-Solartron 1286 Electrochemical Interface. The platinum working electrode was a disc electrode which could either be rotated or held stationary during the tests. Several potentials were tested ranging from the open circuit potential to 0.4 V. A saturated calomel electrode was used as a reference. The electrodes were conditioned for one hour prior to the recording of each impedance spectrum (20kHz to 1 Hz). The use of frequencies below 1Hz was found to be impractical since the length of time taken (several hours) to acquire an accurate result at low frequencies (-c I Hz) would be inordinate: if the electrolysis of the solution was to occur slowly then the impedance spectrum would fail to properly describe the behaviour of the electrode at very low frequencies. A 1 M Na,S solution, bubbled with argon, was tested with, and without, electrode rotation while another experiment was conducted with the presence of sulphur. A 1 M solution of NaOH was tested for comparison.
Chronocoulometric
experiments
Double potential chronocoulometric experiments were performed in solutions containing hydrosulphide ions (1 mM), together with a buffer and supporting electrolyte, 0.05 M Na,B,O,, as described in a previous article[ I]. 487
488
J.
SZVNKARCZUK et al.
Electrode Potential vs. SCE IV 3000 -r
Y-
2500
2wo
1500
1000
500
El&rode
Potential vs. SCE IV
Fig. 1. (A) The calculated values of double layer capacitance, (B) charge transfer resistance, and (C) Warburg factor, with respect to applied potentia1 for a platinum disc electrode without (1) and with (2) rotation. The insert presents a complex impedance plot at -0.2OV.
489
Redox reactions on platinum electrodes--II RESULTS
AND DISCUSSION
Impedance spectroscopy A typical impedance spectrum is shown in Fig. 1 with squares representing experimental points and a fitted curve. Calculated double layer capacitances (Fig. IA) were found to be of the order of at -0.6 V, which was just above the 40pFcm-’ open circuit potential of the system. A linear decrease in capacitance was observed with increasing potential until approximately - 0.2 V where a capacitance of approximately 10 PF cm- ’ was calculated and a slower decline in capacitance observed. At 0.2V a double layer capacitance of approximately 4pFcmm2 was calculated. The effect of either rotating or holding the electrode fixed appeared to be insignificant. Generally, the charge transfer resistance fell from the range 2.5-3 kRcm2 to below 5OORcm’ (Fig. IB) when the potential was increased from -0.6 to -0.4V, above -0.2 V the charge transfer resistance increased. The high values of resistance especially in higher potential range were caused by the formation of sulphur layer inhibiting the charge transfer. The strongest evidence for the existence of a blocking layer at the electrode-electrolyte interface was found in the behaviour of the double layer capacitance with respect to applied potential for the Na,S solutions (Fig. 1A) and the NaOH solution (Fig. 2). In the range of potentials where HS electrolysis occurred, the capacitance fell to a small value (approximately 2.5pFcm-*, Fig. IA at 0.2 V); but in the case of NaOH, where no reaction occurs and, therefore, no sulphur layer formed, the double layer capacitance demonstrated a minimum at -0.2 v. The values of Warburg factor (Fig. 1C) showed an increase during the electrolysis of HS-, which demonstrated the presence of a substantial diffusion limitation. The introduction of oxygen, by aerating the 1 M Na,S solution, had no significant effect on the
-0.40 Fig. 2. Double
layer capacitance
impedance spectroscopy results (Fig. 3). Therefore oxidation of the first solution was shown not to be a cause of sample variation. The nature of the sulphur layer might exhibit irregularities. The sulphur content of the Na,S solutions was suspected to be the cause of experimental deviations. Figure 4 shows the effect of sulphur additions on the Na,S solution. These preliminary results show that sulphur, which is a product of electrolysis, can have an effect on the impedance parameters at all potentials. The charge transfer resistance is particularly influenced which indicates faster sulphur layer dissolution from the surface of working electrode in the presence of polysulphide ions. The effect of electrolysis on the impedance parameters was consistent with the formation of a thin partially blocking layer. The double layer capacitance was at a small constant value during electrolysis. This would be expected if a coating of material at the electrode electrolyte interface had formed. The significant increase in electrode electrolyte resistance accompanied with an increase in the Warburg factor provided further evidence of electrode blocking. Chronocoulometric experiments Figure 5 shows the analysis of current-time data from the chronocoulometric/amperometric experiments. According to the Cottrell equation the current density (i) decays with respect to the inverse square root of time (t ‘j2), and is proportional to the number of electrons (n), the Faraday constant (F), the diffusion coefficient (D), and the concentration of electroactive species (c):
i = nFD’/Z~=- 1/2l~1!2
(1)
If the Cottrell equation is observed then the quantity 7it’j2 should be independent of t’12 and the horizontal line will be parallel to the (‘I2 axis for diffusion-controlled processes[2], whereas the shorttime data are affected by the double layer. Figure 5 shows a line which is not parallel to the t”’ axis in
-0.20 0.00 Electrode Potential vs. SCE / V with respect to applied
potential
0.20
0.40
for a I M NaOH
solution.
J.
490
-0.40
SZVNKARCZUK
et a[.
-0.20 0.00 Electrode Potential vs. SCE IV
0.20
0
B
0.20
C IO
Electrode Potential vs. SCE / V Fig. 3. (A) The double
layer capacitances,
and (B) charge transfer, aerated (2) solutions.
an anodic process, which proves that the kinetics of these reactions is complicated. Such a result was found previously in voltammetric experiments[ 11. Production of a sulphur layer on the surface of the electrode inhibits the diffusion of HS-; these ions have to diffuse through the layer of sulphur and, as a result, a declining line was observed in Fig. 5. The dashed line represents the expected behaviour if a simple diffusion-controlled reaction was to occur. The calculated value of the diffusion coeficient was relatively high (2.2 x IO-’ cm’s ‘) and was close to the value (3 x lo- 5 cm2 s- ‘) obtained by Kapusta rt uI.[3]. Data analyses were performed on the anodic and cathodic branches according to the work of Christie ef al.[4], and are presented in Fig. 6. Analysis of double potential step chronocoulometric data showed two straight lines on both; the anodic and cathodic branches. The two different slopes on either the anodic or cathodic branches
calculated
for deoxygenated
(1) and
suggest that two distinct anodic, and cathodic products were performed while the potential (anodic or cathodic) was applied. This proves the two-step mechanism of HS ion oxidation (HS-/Si and .Si-/Y; equations (5) and (6) from Cl]) and reduction (S’/Si and S,‘-/HS-; equations (3) and (4) from [l]) rather than the direct oxidation/reduction mechanism (HS-/So; equation (1) from Cl]). The analysis of these data suggested that there was no adsorption of HS- ions on the surface of electrode, possibly due to a changed morphology of the electrode surface. But it is known that sulphurcontaining compounds and ions are likely to adsorb on the platinum[5- 71 especially under acidic conditions. Double-layer capacitance of 3.3 PF cm-’ was found to be comparable to those values obtained by impedance spectroscopy (1-3 PF cm -’ at 0.40 V) but lower than values reported elsewhere[3], eg 8pFcmm2.
Redox reactions
on platinum
491
electrodes-11
Electrcde Potential vs. SCE
Electrode Potential vs. SCE
IV
/V
Fig. 4. (A) The double layer capacitances, and (B) charge transfer resistance, calculated solutions containing no sulphur (I), 0.01 M sulphur (2) 0.1 M sulphur (3).
for
I M Na,S
5.OE-04 1
1 .OE-04 1’ O.OE+OO! 0
I 0.2
I 0.4
0.6
0.8
1
I 1.2
1.4
1.6
18
t ‘I2 is’Fig. 5. Analysis of i&t data of anodic process, from chronocoulometric experiments in solution containing I x lo-‘M Na,S and O.OSOM Na,B,O, at pH 9.3. Potential was changed from -0.415 to 0.1 V. Dashed line indicates expected position of plateau for diffusion process.
492
al.
J.SzmiK~~czmet
ANODIC PROCESS
Charge / mC cm.* Fig. 6. Analysis of double potential step chronocoulometric data in solution containing 1 x lo- 3 M and O.OSOM Na,B,O, at pH 9.3. Potential was changed from -0.415 to 0.1 V, and after r = 3s was kept at 0.80 V for 3 s.
Na,S
The identification
of polysulphide intermediate
ions
The slopes of double potential chronocoulometric data from Fig. 6 are described by [4,8] :
dQ
anodic
-
=
dt”’
2n,
F
cR(D&r)1i2
cathodic
dQ
~
d@
0.97r,
= 2nc,(D,/n)“2
2C,
(2)
1
7r1’2
D;"T"'
(3) where @ = (t _ T)U2 + T’/2 - t’i2 In the case where there is no adsorption, tion (4) reduces to :
(4) then equa-
dQ -..A@?% = 2n,~c,(~,/~)'~2 d@
(5)
As described earlier, a two-step electrochemical reaction was detected during hydrosulphide oxidation which explained the two different slopes observed in Fig. 5. Knowing the diffusion coefficient of the hydrosulphide ions and that of .Si-, one could calculate the ratio of the number of electrons from those two reactions (equations (5)-(6) from [ 11):
range of diffusion coeflicients for polysulphide ions of various ranks had to be taken into account. In the case of sodium polysulphide ions, the highest number of sulphur atoms reported was 5[10-121. Assuming that the diffusion coefficient is inversely proportional to the square root of mass, the averaged calculated n value was 2.5 + 0.3 and it was lower than the value of 3.3 obtained by others[9] using a gold electrode. Therefore an equal mixture of Ss- and S:- might have given the average stoichiometry of 2.5. The analogous calculations were made for the two cathodic slopes; the average value of n was found to be 4.3, and was also independent of the size of the first potential step. During the cathodic reaction, it appeared that S:- was the dominant active species. According to Giggenbach[ IO] this form of polysulphide should dominate in aqueous solutions. It is difficult to explain why, on the cathodic potential step, different polysulphide species were more active than on the anodic one. It might be explained by a local reduction of pH; occurring at the electrode electrolyte interface. The stability of higher rank polysulphides increases at lower pH[ to]. Polysulphides which are in contact with the sulphur layer deposited on the anode are in an equilibrium state in solution[lO, 111. The following reactions describe the dissolution of the sulphur layer.
dQanodic t < 0.36 s dr”2
S + HSs + s:-
= s:--
(8)
dQsnodic t > 0.36 s dt”’
s + s:-
= s;-
(9)
s + s:-
= s:-
(10)
=(n-
1). $$ n2
=
1.7
(6)
The average value of the ratio from equation 6 was 1.7 and it did not depend on the first potential step within the range of 0.1 to 0.4V. To calculate n, the
+ OH_ = S;-
+ H,O
(7)
Polysulphide ions are also present in a dynamic equilibrium which can be described by the following equation: nSf+ ,S2- + HS-
+ OH-
= (n + l)S,S2-
+ H,O (Ill
Redox reactions on platinum
-3000 I -1 .oo
-0.50
0.00
0.50
493
electrodes--II
1.00
1.50
2 IO
Electrode Potential vs. SCE /V Fie. voltammoerams recorded - 7. Cvclic _ Na,Ba,O, at ph 9.3. Scan rate
in solution containing 1 x 10m3 M Na,S and 0.050M 1OOmVs ‘, rotation rate; (a), 1000 (b). and 3boO (c).
which shows that an increase in pH would shift the equilibrium to the right, ie towards lower rank of polysulphide ions: S:- and S:- were initially produced as the first step of sulphide ion oxidation. Also polysulphide ions were produced simultaneously by chemical reactions (equations 7-l 1) keeping the sulphur layer in equilibrium between electrochemical production of sulphur and chemical sulphur dissolution. These chemical processes together with the cathodic potential applied were able to remove the sulphur layer from the platinum electrode. Formation
oJsulphute
ions
The only thermodynamically stable[ 121 oxidation states for the sulphur water system at room temperature are: -2, 0, and + 6. Sulphide ions can be oxidized to sulphate according to the following reaction : HS
+ 9 OH-
= SOi-
+ 5 H,O
+ 8 e-
(12)
This reaction took place at l.lOV (Fig. 7) where a relatively high peak was recorded due to the formation of SO;ions. The current density at l.lOV deviated from the theoretical expectation based on the Levich equation, most probably due to the sulphur layer formed at O.OV. If the entire electrode area was covered with a sulphur layer one would not expect to record the response associated with the conversion of HS to SO:(HS-/SO:-) on the anodic but rather on the cathodic scan, when the sulphur layer is partially dissolved chemically. In addition, if the potential were shifted to high positive values to produce oxygen, then the oxygen bubbles so produced would clean part of the electrode and allow the electrochemical reaction (equation 12) to proceed. An absence of the HS-/SO:peak on the anodic scans was reported for a gold (c”s- = 1.4 x 10m4 M)[9] and platinum (C,,_ > 0.2 M)[3] electrodes in sulphide solutions. In this study, the peak due to HS-/SO:was already observed on the anodic scan, indicating only partial blockage of the electrode. EA 40-4-J
CONCLUSIONS Impedance spectroscopy results have confirmed the development of a partially blocking layer at the electrode electrolyte interface during electrolysis. The decline in the double layer capacitance during electrolysis is observed in Na,S solutions but not in NaOH solutions. The presence of dissolved oxygen had no significant effect on the impedance spectra. The intermediate species were found to be a mixture of polysulphides with an average stoichiometry of S:,; in the anodic step and S:.; in the cathodic step. The value of the pH at the electrode electrolyte interface during the respective steps would be a possible explanation for the observed differences in polysulphide rank. Acknowledgrmrnts~~~~Thiswork was supported by the Panel on Energy Research and Development (PERD) and by an internal CANMET grant. We would like also to thank the Natural Science and Engineering Research Council of Canada for funding Visiting Fellowships for J. Szynkarczuk and P. G. Komorowski.
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