Electrochimica Acta 98 (2013) 116–122
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The effect of chloride on spatiotemporal dynamics in the
electro-oxidation of sulfide on platinum
Jiaping Yang a , Yanan Song a , Hamilton Varela b,c,∗ , Irving R. Epstein d , Wenyan Bi a ,
Huiyao Yu a , Yuemin Zhao a , Qingyu Gao a,∗∗
a
College of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, China
Institute of Chemistry of Sao Carlos of the University of Sao Paulo (USP), CP: 780, CEP: 13566-590, Sao Carlos, SP, Brazil
c
Ertl Center for Electrochemistry and Catalysis, Cheomdan-gwagiro 261, Buk-gu, Gwangju 500-712, South Korea
d
Department of Chemistry and Volen Center for Complex Systems, MS 015, Brandeis University, Waltham, MA 02454-9110, United States
b
a r t i c l e
i n f o
Article history:
Received 22 December 2012
Received in revised form 7 March 2013
Accepted 8 March 2013
Available online 15 March 2013
Keywords:
Sulfide electro-oxidation
Chloride effect
Oscillations
Spatiotemporal patterns
a b s t r a c t
The adsorption of poisonous species on the catalyst surface impacts electrocatalytic reactions to different
extents and by distinct means. The impact of adsorbing anionic species on electrocatalytic reactions has
generally been investigated in the non-oscillatory regime or, at most, with temporal oscillations. Here,
we report on spatiotemporal pattern formation in the electro-oxidation of sulfide on a platinum disk
perturbed with dissolved chloride. The spatiotemporal dynamics was followed simultaneously with an
electrochemical workstation and a charge-coupled device (CCD) camera. Perturbing the system with
chloride results in (a) enhancement of dynamic instabilities throughout the current–potential curve,
including regions of both positive and negative slope; (b) emergence of several spatial patterns; and
(c) decreasing the width of pulses with increasing [Cl− ]. The results are interpreted in terms of surface
poisoning by chloride adsorption.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Generally, the primary result of anion adsorption on eletrocatalytic reactions is to block surface sites, resulting in inhibition and a
consequent decrease in reaction rates. The consequences of halide
adsorption on platinum have been extensively studied in a number of electrocatalytic reactions [1–6]. The competition between
dissolved species for surface sites may cause quite complex behavior under oscillatory conditions [7]. Examples include the effect
exerted by dissolved anions on metal corrosion/dissolution [8,9], on
the electro-oxidation of small organic molecules [10,11] or hydrogen [12,13] and on the electro-reduction of hydrogen peroxide
[14,15] on platinum. Regarding the study of spatiotemporal patterns in electrochemical systems [16–19], the adsorption of halide
(especially chloride) in certain electrochemical systems [19–22] is
essential in producing spatiotemporal patterns.
Following previous investigations in the temporal domain
[23–25], we recently reported the occurrence of self-organized spatiotemporal patterns in the electrocatalytic oxidation of sulfide on a
platinum disk [26]. We explored the effect of sweep rate and of the
∗ Corresponding author.
∗∗ Corresponding author. Tel.: +86 516 83591088.
E-mail addresses: [email protected] (H. Varela), [email protected] (Q. Gao).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.03.042
series resistance connected between the working electrode and the
potentiostat, and were able to record patterns comprising fronts,
pulses, twinkling eyes, labyrinths and homogeneous oscillations.
From the experimental point of view, the rich dynamics and the
fact that macroscopic sulfur deposition and dissolution are easily
observed with a CCD camera are important advantages. Moreover,
in contrast with the situation for metal dissolution and/or deposition systems, the two-dimensional sulfur deposition observed in
the electro-oxidation of sulfide on platinum are expected to be
confined to the topmost layer of the surface, as observed on Cu
(1 1 0) in acidic chloride solution using a high-speed scanning tunneling microscope with atomic resolution [27,28]. Here, we use
the sulfide electro-oxidation on a platinum disk system to investigate the effect of chloride perturbation on spatiotemporal pattern
formation.
2. Experimental
All experiments were performed in a conventional threeelectrode electrochemical cell with a volume of 40 mL. A
polycrystalline platinum disk embedded in an insulator with a
diameter of 2 mm was used as the working electrode (WE). The
counter electrode (CE) was a platinum ring arranged below the
WE at a distance of 1.5 cm. The ohmic resistance of our setup
was about 11.6 ohms. The reference electrode (RE) was a saturated
J. Yang et al. / Electrochimica Acta 98 (2013) 116–122
calomel electrode (SCE) inserted into a J-shaped glass capillary,
whose tip was located between the WE and CE and close to the
surface of the WE. All the potential values in this work were
measured and are quoted with respect to the SCE. All electrochemical experiments were performed on a CHI-660 electrochemical
workstation (CH Instruments Inc., USA). A charge-coupled device
(CCD) camera was utilized to investigate pattern formation on
the WE.
Before each experiment, the platinum disk was first polished to
a mirror-like shine with fine alumina powder (0.05 ␮m) and then
immersed in a 0.5 mol/L HNO3 solution for 30 min. After that, it
was cleaned with ultrapure water (Millipore system, 18.2 M·cm)
in an ultrasonic bath for 5 min and then rinsed with ultrapure water.
Next, the potential was cycled from −0.25 to 1.30 V vs. SCE at a scan
rate of 50 mV/s in 0.5 mol/L H2 SO4 to further clean the electrode
surface. Finally, the electrode was rinsed repeatedly with ultrapure
water.
Electrolyte solutions were prepared by dissolving appropriate
amounts of analytical grade sodium sulfide and sodium chloride
(Sinopharm Chemical Reagent Co. Ltd., China) in ultrapure water.
The Na2 S concentration was 1.00 mol/L in all experiments, whereas
different concentrations of chloride were used. Before each electrochemical experiment, the electrolyte was deaerated with purified
nitrogen for 20–25 min, and a nitrogen atmosphere was maintained during all experiments. The temperature was held at
20.0 ± 0.1 ◦ C with a circulating water bath (Polyscience Instrument,
USA).
117
For 1.00 M NaOH solution without sulfide, Emin was determined to
be about −0.32 V at a scan rate of 1 mV/s (dashed lines in the lower
inset of Fig. 1a). With 1.00 M sulfide solution, whose pH is close to
that of the 1.00 M NaOH solution, and the same scan rate, the calculated Emin is about 0.60 V vs. SCE [29], which is consistent with the
experimental result in Fig. 1a. Thus, the oxidation of the Pt surface:
PtS(ads) + OH − (aq) → PtOH(ads) + S(s) + e−
−
Pt(s, u) + OH (aq) → PtOH(ads) + e
−
PtOH(ads) + OH − (aq) → PtO(ads) + H2 O(l) + e−
(R2)
(R3)
(R4)
3. Results and discussion
gives rise to an N-shaped negative differential resistance
(N-NDR), inducing positive feedback, i.e., the autocatalytic character of the double layer potential [16,19]. The OH adsorption
and oxide formation in the range of the potential scan also
can be inferred from an observed anodic current peak during
the negative potential scan [24,26,29], which was explained in
terms of oxide reduction (reverse reaction of (R4) and (R3))
and a consequent current increase due to the oxidation of
sulfide [29].
Our experimental observations (Fig. 1a) suggest that for the positive slope region of the j-E curve between 0.60 V and 1.00 V, pale
elemental sulfur (S) is produced by (R2) and also mainly by (R5),
resulting in a much larger area of the j-E curve for OH adsorption and oxide formation through reaction cycling of (R3) and (R5)
than for S-layer formation (R1). The elemental sulfur then gradually changes to soluble yellow polysulfide Sn2− in the region of
rising current density through the dissolution reactions (R6) and
(R7) [29,33,34].
3.1. Temporal dynamics
PtOH(ads) + HS − (aq) → Pt(s, u) + S(s) + H2 O(l) + e−
3.1.1. Oscillations and mechanistic analysis in the absence of
chloride
Fig. 1 shows a slow linear potential sweep (from −0.20 V to
1.80 V at dE/dt = 1 mV/s) for the electro-oxidation of 1.00 mol/L sulfide in the presence of different amounts of dissolved chloride, as
well as a cyclic voltammogram (Fig. 1a, dashed curves) in 1.00 mol/L
sodium hydroxide solution. We observe a linear current increase
associated with oxygen evolution of sodium hydroxide solution at
potentials above about 0.70 V as well as oxide formation and reduction between about −0.5 V and 0.3 V seen in the dashed curves in
the lower inset of Fig. 1a. In the absence of chloride (solid curve,
Fig. 1a), when the potential is increased from −0.20 V to ∼0.60 V
vs. SCE the current measured remains close to zero and barely fluctuates. Within this region the electrode surface is passivated by a
sulfur layer associated with a small peak [24,29] centered at −0.08 V
vs. SCE and seen in the solid line in the upper inset in Fig. 1a. This
process can be described as
−
−
Pt(s, u) + HS (aq) + OH (aq) → PtS(ads) + H2 O(l) + 2e
−
(R1)
Here s, u and ads represent solid electrode, unoccupied surface sites and adsorbed surface species, respectively. However, the
applied potential of sulfur passivation is more positive in acidic
solution (e.g. 0.70 V vs. a Ag/AgCl reference in 0.1 mol/L H2 SO4 [30]).
At pH about 14, as in the present case, HS− rather than S2− is
the predominant sulfur species. As the potential increases above
0.60 V, the current density increases slightly to about 10 mA/cm2 ,
and then it jumps abruptly to ∼120 mA/cm2 . This sudden increase
of the current density is due to surface oxide growth and removal
of electrodeposited sulfur from the electrode surface, as confirmed
by in situ surface-enhanced Raman scattering (SERS) spectroscopic
investigation [31] and triangular potential scan experiments [29].
The minimum potential (Emin ) of oxide formation [29,32] increases
with acidity, the concentration of Na2 S and the potential scan rate.
−
−
S(s) + HS (aq) + OH (aq) →
S22− (aq) + H2 O(l)
S(s) + S22− (aq) → S32− (aq)
(R5)
(R6)
(R7)
Reaction (R5) is accelerated by increasing the applied potential. The availability of more Pt sites causes a current increase,
which thus hides the NDR of OH adsorption and oxide formation,
thereby introducing a negative feedback and possibly producing
a hidden N-NDR (HN-NDR) region [16,19]. If the concentration
of Na2 S is increased further, from 1.00 mol/L to 2.00 mol/L, the
rates of (R5), (R6) and (R7) increase, enhancing the negative feedback and inducing HN-NDR oscillations, as found by Miller and
Chen [24].
Along the negative slope of the j-E curve, the limited diffusion of
HS− in the electric double layer for oxide (passive film) reduction
(R8) and the dissolution reactions ((R6) and (R7)), which cause a
current increase, constitutes a slow negative
PtO(s) + HS − (aq) → Pt(s, u) + S(s) + OH − (l)
(R8)
feedback, resulting in N-NDR oscillations [16,19] as shown in
Fig. 1a. Both the HN-NDR and N-NDR oscillations correspond to the
alternate deposition and dissolution of sulfur on the WE surface.
3.1.2. Effect of chloride on temporal dynamics
The addition of chloride causes pronounced changes in the
j-E curves, as can be seen in Fig. 1b–f. First, as [Cl− ] is increased,
the potential at which current starts to increase is shifted to more
positive values. In Fig. 1a the onset potential is ∼0.60 V vs. SCE. This
value gradually shifts, and reaches as high as ∼0.80 V vs. SCE when
the chloride concentration is 500 mmol/L (Fig. 1f). As anticipated
above, dissolved chloride ions can compete for platinum sites in
the whole potential region via R9,
Pt(s, u) + Cl− (aq) → Pt − Cl(ads) + e−
(R9)
118
J. Yang et al. / Electrochimica Acta 98 (2013) 116–122
250
140
b
120
200
2
j (mA/cm )
2
j (mA/cm )
100
80
60
150
100
40
50
20
0
0
300
250
0.0
0.3
0.6
0.9
1.2
1.5
0.0
1.8
c
250
150
100
0.9
1.2
1.5
1.8
0.3
0.6
0.9
1.2
1.5
1.8
d
150
100
50
50
0
0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0.0
e
250
viii
200
2
2
j(mA/cm )
150
j(mA/cm )
0.6
200
j(m A/cm2)
j(mA/cm2)
200
200
0.3
300
100
50
v
150
vi
100
50
0
0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
E vs. SCE (V)
vii
0.0
0.3
0.6
0.9
1.2
1.5
1.8
E vs. SCE (V)
Fig. 1. Current response during a linear sweep for the electro-oxidation of 1.00 mol/L of Na2 S, at 1 mV/s in the presence of dissolved chloride [Cl− ] = (a) 0 mmol/L, (b)
30 mmol/L, (c) 50 mmol/L, (d) 70 mmol/L, (e) 90 mmol/L, and (f) 500 mmol/L. Dashed line in Fig. 1a denotes the cyclic voltammetry curve in 1.00 mol/L NaOH solution. j-E
curves of upper and lower sets in Fig. 1a illustrate S-layer formation and oxide formation in the presence and absence of sulfide, respectively.
giving rise to another NDR, which was also observed during the
electro-oxidation of hydrogen on Pt in the presence of Cu2+ , Cl−
and H2 SO4 [12,13], where the addition of chloride adds a positive
feedback. Coupling of the NDR associated with Cl− adsorption (R9)
and the negative feedback from releasing Pt sites (R5) can also produce HN-NDR oscillations. At a chloride concentration of 70 mmol/L
(Fig. 1d), we observe this HN-NDR-type of current oscillation along
the positive slope branch of the j-E curve. This region becomes more
distinct in Fig. 1f, where the chloride concentration is 500 mmol/L.
Fig. 2 illustrates the effect of chloride on the time evolution of
the current under potentiostatic (0.90 V) conditions. In the absence
of chloride, only fluctuations are observed (Fig. 2a), whereas
well-developed oscillations are observed for [Cl− ] = 500 mmol/L,
(Fig. 2c).
3.2. Spatiotemporal patterns
3.2.1. Synchronization of sulfur deposition and dissolution
We have previously reported the occurrence of synchronized
sulfur deposition and dissolution in the NDR oscillation regions
[26]. When the slope of the j-E curve is positive, there is no synchronization, but fronts are observed in the absence of chloride. Fig. 3i
and ii show the evolution of fronts corresponding to Fig. 1a. We see
from Fig. 1a that the current density in the positive slope region of
the j-E curve increases monotonically with potential, which indicates that (R4) occurs on the electrode surface and sulfur deposits
from the rim of electrode. As a result, the waves observed in this
potential region are mainly fronts. However, when 500 mmol/L of
chloride is added, HN-NDR oscillations occur in the positive slope
J. Yang et al. / Electrochimica Acta 98 (2013) 116–122
140
150
a
2
j (mA/cm )
140
119
200
c
b
130
150
130
120
120
110
110
100
100
50
100
90
90
80
0
200
400
600
800
0
200
t (s)
400
t (s)
600
800
0
0
200
400
600
800
t (s)
Fig. 2. Current time series obtained at 0.90 V for the electro-oxidation of sulfide perturbed with different amounts of dissolved chloride: (a) 0 mol/L, (b) 70 mmol/L, and (c)
500 mmol/L.
region. The patterns observed evidence the transition from fronts
to synchronization of sulfur deposition and dissolution, shown in
Fig. 3iii and iv. In the N-NDR region, synchronized patterns of sulfur
deposition and dissolution, like those in Fig. 3v–viii, are generally
observed. An increase in surface-deposited sulfur typically accompanies the current increase.
Fig. 3. Fronts (i and ii) and synchronization (iii–viii) corresponding to Fig. 1a and f,
respectively.
3.2.2. Effect of chloride concentration on pattern formation
Fig. 4 shows time series of the current density with snapshots
of the corresponding yellow pulses on the electrode surface under
potentiostatic control at 1.30 V during the electro-oxidation of sulfide in the presence of various amounts of chloride. The sulfur
deposition patterns observed at this potential are mainly pulses,
including spirals and targets. With increasing chloride concentration the morphology of the pulses changes significantly. In the
absence of dissolved chloride, regular, smooth wave fronts are
Fig. 4. Potentiostatic time series of current density at 1.30 V and the corresponding evolution of pulses for the electro-oxidation of sulfide in the presence of (a) 0 mmol/L,
(b) 50 mmol/L, (c) 70 mmol/L, and (d) 90 mmol/L of dissolved chloride. Red circles show snapshots of pulses at different moments. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
J. Yang et al. / Electrochimica Acta 98 (2013) 116–122
observed. At [Cl− ] = 50 mmol/L, both the wave front and tail become
irregular. Only some fragments of pulses can be seen in Fig. 4b. More
regular but rough pulses are found in the presence of 70 mmol/L
chloride, and, as the current density increases, the wave fronts display a smooth, serrated structure (Fig. 4c). When the concentration
of chloride rises to 90 mmol/L, regular, smooth pulses are again
observed in Fig. 4d. Now, however, the current fluctuates with time.
These pulses are mostly triggered from the edge of the electrode
and propagate toward the center. This behavior results from the
fact that the current density of a disk electrode embedded in an
insulator is not uniform, but is higher at the rim of the electrode
than at the center [35,36]. These pulses disappear through collision
either with each other or with the edge of the disk electrode.
Perhaps the most remarkable effect of added chloride anions
is the resulting change of the pulse width. Fig. 5 shows the width
of typical pulses registered at 1.30 V, cf. Fig. 4, as a function of the
160
Pulsewidth (μ m)
120
150
140
130
0
20
40
60
80
100
cCl- (mmol/L)
Fig. 5. The width of typical pulses displayed in Fig. 4 as a function of chloride
concentration.
Fig. 6. Spatiotemporal patterns at different potentials for the electro-oxidation of sulfide in the presence of 70 mmol/L of chloride. (a) Current time series at different
potentials. (b) Respective snapshots of the electrode surface at selected times illustrated in (a).
J. Yang et al. / Electrochimica Acta 98 (2013) 116–122
chloride concentration. Increasing chloride concentration clearly
results in a decrease of the pulse width. In terms of mechanism,
activity pulses reflect local rates of (R5)–(R7). Chloride adsorption
via (R9), which competes with OH adsorption on Pt, decreases the
rate of (R5), and therefore the pulse width as well. Sufficiently high
chloride concentrations can even suppress the formation of activity
structures completely.
3.2.3. Effect of the applied potential
As discussed above, no waves (fronts and/or pulses) are
observed in the presence of 500 mmol/L of chloride. If [Cl− ] is too
low, oscillations cannot occur on the positive slope branch of the
j-E curve. We therefore investigate the effect of electrode potential in a system containing an intermediate level of 70 mmol/L of
chloride. Fig. 6 shows a time series of current density and typical
patterns at several potentials in the presence of 70 mmol/L chloride.
Below the onset potential, 0.75 V, the current density is close to
zero, which indicates that (R2)–(R4) do not occur on the electrode,
and therefore no patterns are observed on the electrode surface.
When the potential is controlled at 0.80 V, the current density goes
through an abrupt increase, during which fronts are observed, (see
Movie 1 in Supplemental Data) and is finally maintained as high
as ∼100 mA/cm2 . Thus electrochemical reactions take place, giving
rise to the pattern presented in Fig. 6b-i. If the potential is held at
0.90 V, small current density oscillations appear (see also Fig. 2b),
and the pattern is similar to the pattern in Fig. 6b-i. Large amplitude oscillations in current density are found under potentiostatic
control at 1.00 V. These oscillations have a period as long as 57 s
with synchronization of sulfur deposition and dissolution (Fig. 6bii and Movie 2 in the Supplemental Data). When the potential is
held at 1.05 V, mixed-mode current oscillations are obtained. Compared to the oscillations shown at 1.00 V, the maximum current
density of oscillations is much lower, just over 30 mA/cm2 , and the
period is only about half as long, but it is sufficient to generate
pattern propagation. Synchronization of sulfur deposition and dissolution is found again, but only local deposition patterns emerge
on the electrode during the experiment, suggesting that adsorption
of chloride acts as another positive feedback (NDR) on the surface
of the electrode, in addition to the positive feedback (NDR) from OH
adsorption and oxide formation, producing the mixed-mode oscillations. As the potential changes from 1.10 V to 1.30 V, we observe
a series of fluctuations and deposition patterns, shown in Fig. 6biv–b-viii, changing from combined evolution of synchronization
and pulses (Movie 3, Supplemental Data) to pulses (Movie 4, Supplemental Data). In this potential range, more regular pulses are
observed at the higher potentials, and the sulfur deposition rate is
smaller than at 0.80–1.05 V. When the potential is increased as high
as 1.50 V no significant deposition patterns are present (Fig. 6Bix). The current density is nearly steady at ∼40 mA/cm2 . Further
increasing the potential will cause the current to rise again, and
new feedback loops resulting from other processes, such as further surface oxidation to higher platinum oxides, may result in new
complex oscillations and spatiotemporal patterns [24–26].
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2013.03.042.
4. Conclusions
We report in this work the impact of adsorbed chloride on the
spatiotemporal electro-oxidation of sulfide on a polycrystalline Pt
electrode. The spatiotemporal dynamics was followed by means of
a CCD camera, and different chloride concentrations were used to
perturb the base system. Chloride induces another positive feedback in addition to oxide formation, and it was found to strongly
121
affect the system’s dynamics. In summary, the presence of dissolved chloride enhances potential instabilities throughout the
current–potential curve, including regions with positive and negative slopes, and also shifts the entire curve toward more positive
potentials. In terms of spatiotemporal structures, adding chloride results in the emergence of several spatial patterns and in a
decreased width of pulses with increasing chloride concentration.
The structure of the pulses is also affected by chloride. We attribute
these effects of enhancement in instability to NDR, i.e., positive
feedback, from chloride adsorption.
We suggested in an earlier study of sulfide electro-oxidation
on platinum [26] that “. . .further study and understanding of the
detailed mechanism for oscillations and spatiotemporal patterns
in this deceptively simple system may promote its application to
such challenges as making depositional materials, controlling sulfur deposition and poisoning in fuel cells, and controlling ordered
micro- and nano-structures. . .”. We believe the present contribution represents an important step in this direction. In particular,
our observation of the effect of chloride on the spatiotemporal patterns points to the importance of chemical perturbation,
in addition to electrical and transport properties. Moreover, the
addition of strongly adsorbing species such as chloride adds a
feedback loop and causes a shift in the position of the negative
differential resistance region, thus changing the parameter region
in which the positive feedback loop occurs. Experimental studies of spatiotemporal patterns on a single chemical system with
multi-feedback loops [37] are still rare [38,39], and results in
this direction pose interesting fundamental questions for future
investigations.
Besides deepening our understanding of the rich spatiotemporal dynamics of sulfur deposition and dissolution on platinum,
the findings presented here reveal the impact exerted by surface
adsorbed species on surface activity waves. This is an important
consideration for engineering spatial patterns on surfaces.
Acknowledgments
This work was supported in part by Grants (Nos. 21073232 and
50921002) from the National Natural Science Foundation of China
and Grant (No. BK2011006) from JSNSF. H.V. acknowledges Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP, grant no.
09/07629-6), the Conselho Nacional de Desenvolvimento Cientifico
e Tecnologico (CNPq, grant no. 306151/2010-3) and the PRPG/USP
for financial support. I.R.E. acknowledges Grant No. CHE-1012428
from the U.S. National Science Foundation.
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