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Electrochemical behavior and electrocatalytic activity of anthraquinonedisulphonate in solution phase and as doping species at polypyrrole modified glassy carbon electrode
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 602 (2007) 163–171 www.elsevier.com/locate/jelechem
Electrochemical behavior and electrocatalytic activity of anthraquinonedisulphonate in solution phase and as doping species at polypyrrole modified glassy carbon electrode Guoquan Zhang, Weishen Yang, Fenglin Yang
*
School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China Received 16 October 2006; received in revised form 6 December 2006; accepted 12 December 2006 Available online 15 December 2006
Abstract The electrochemical behaviors of anthraquinonedisulphonate (AQDS) dissolved in solution and incorporated into polypyrrole (PPy) matrix as doping species at PPy modified glassy carbon (GC) electrode were investigated in detail, also reported was their applications in the electrocatalytic reduction of oxygen. It was found that the redox process of AQDS in solution and doping phase both exhibit quasireversible behavior and pH dependence. The value of ionization constant pKa for the H2AQ/HAQ couple increases obviously from 7.6 in solution phase to 9.5 when AQDS is incorporated into PPy matrix. Under optimum condition (pH 6.0), the reduction of oxygen at the GC/PPy/AQDS electrode (AQDS in doping phase) occurs irreversibly at a potential about 621 mV less negative than at a bare GC electrode. The same peak potential shifts 78 mV to more positive potential when compared with the response for AQDS in solution phase. The observed electrocatalysis for the reduction of oxygen is mediated by the electrochemical reduction of AQDS. The number of electrons involved n and the kinetic parameters such as electron transfer coefficient a, catalytic reaction rate constant k, kf and the diffusive coefficient of oxygen DO2 , were determined by using various electrochemical approaches. The stability of the catalytic activity for the GC/ PPy/AQDS electrode was also examined. 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemical; Electrocatalytic; Anthraquinonedisulphonate; Polypyrrole; Modified electrode
1. Introduction Quinones and their derivatives have received great attention in the last decades, since these quinonoid compounds are good catalysts for many electrode reaction of interest [1–5]. In most cases, the catalyst was just adsorbed at the electrode surface giving good activity, but a very low stability. In addition, the modification of electrode surfaces by quinones can also be obtained by electrochemical deposition and provides surfaces that are much more stable than those modified with spontaneously adsorbed monolayer. The investigations of the electrochemical behavior of these *
Corresponding author. Tel.: +86 411 84706328; fax: +86 411 84708084. E-mail address: yangfl@dlut.edu.cn (F. Yang). 0022-0728/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.12.007
quinonoid compounds modified electrodes have shown a dramatic enhancement of the rate and a radical improvement of catalytic activity towards the reduction of oxygen [6–23]. An important feature of these quinones modified electrodes is that the reduction of oxygen stops at the peroxide stage, which enables their use as electrocatalysts for the electrosynthesis of hydrogen peroxide. However, during the electrocatalytic processes large part of catalysts tend to desorb from the surface of electrode towards the solution bulk, leading to a loss of activity with time [6]. A very convenient way to disperse electrocatalysts at the molecular level is using a conducting polymer, as a welldefined matrix to stabilize the catalytic sites [24–26]. The high stability, high electronic conductivity and useful mechanical properties show that polypyrrole (PPy) film has the ability to promote electron transfer when used in
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electrochemical reaction. The oxidative electropolymerization of pyrrole (Py) in the presence of doping anions is deemed to be a fast and simple method for anchoring counter-anions at molecular level in the matrix of electroactive polymer [25,26]. Therefore, aimed at improving the stability and the electrocatalytic activity of electrocatalyst, and taking advantage of both PPy film and quinonoid compound, a water soluble catalyst, anthraquinonedisulphonate (AQDS) was incorporated into a conducting polymer matrix during the electropolymerization in aqueous medium of Py in this article. This technique of electrode preparation allowed us to obtain active and stable electrodes, in which the catalyst was dispersed into the porous electron conducting polymer matrix. The bulky AQDS strongly interact with the PPy matrix and cannot be expelled from the polymer matrix even when PPy is reduced electrochemically [1]. In this work, the pH dependent electrochemical behaviors of AQDS, both dissolved in solution and incorporated into PPy matrix at PPy modified glassy carbon (GC) electrode was studied. Also investigated was the electrocatalytic activity of AQDS in the two different phases towards the electroreduction of oxygen. The kinetic parameters of the catalytic reduction of oxygen were determined using various electroanalytical techniques. The results showed that the GC/PPy/AQDS electrode enables a more efficient electrocatalytic performance and the number of electron transfer for oxygen reduction is about two. 2. Experimental 2.1. Reagents and chemicals Py (99%) was obtained from Aldrich, purified by vacuum distillation before use. Anthraquinone-2,7-disulphonic acid, disodium salt was bought from Shymax Chemicals Co., Ltd. and they were used without any further purification. Other reagents were A.R. and used as received. Doubly distilled water was used throughout the experiments. 0.1 M phosphate buffer or 0.1 M acetate buffer was used which was titrated directly in the electrochemical cell with H2SO4 and NaOH to obtain the required pH values across a wide range. All electrolyte and monomer solution were purged with N2 before and during the polymerization experiments. O2 with purity of 99.99% was used to saturate the solutions during oxygen reduction experiments. 2.2. Working electrode The polymerization of Py was performed on a GC rotating disk electrode (RDE, 0.126 cm2 area) polished with 0.5 lm alumina, sonicated for 5 min in doubly distilled water and rinsed with acetone and doubly distilled water before polymerization. The rotation of the disk electrode and the use of relatively low concentrations of monomer
prevented the polymeric film to protrude out of the electrode surface. In the case of the GC/PPy electrode, the polymerization was performed in an aqueous solution containing 0.1 M pyrrole and 0.5 M H2SO4. The GC/PPy/AQDS electrode was electrodeposited from an aqueous solution containing 0.1 M Py, 0.5 M H2SO4 and 5 mM anthraquinone-2,7disulphonic acid, disodium salt. The polymerization was performed during 15 voltammetric cycles at scan rate of 10 mV s1 between 0.7 and 0.7 V and with a rotation rate of 600 rpm. 2.3. Apparatus All the electrochemical investigations, including electropolymerization, cyclic voltammetry (CV), chronoamperometry/chronocoulometric and rotating disk electrode (RDE) voltammetry were carried out in a conventional one compartment electrolytic cell with three electrodes using an EG&G Princeton Applied Research Model 263A potentiostat–galvanostat (Princeton, NJ, USA). The experiments were controlled with General Purpose Electrochemical System (GPES) software. A Pt wire and a KCL-saturated calomel electrode (SCE) were used as the auxiliary electrode and the reference electrode, respectively. All the potentials mentioned in the text are given against the SCE. The electrochemical behavior of AQDS and the reduction of oxygen were studied with RDE covered with the electrodeposited PPy/AQDS and PPy film in aqueous buffer solution in the absence and presence of AQDS, corresponding to AQDS in doping and solution phase, respectively. Most of the experiments were conducted at room temperature. 3. Results and discussion 3.1. Electrochemical behavior of AQDS in solution and doping phase Fig. 1 shows the CVs of AQDS in solution and doping phase in pH 2.0 N2-saturated buffer solution. In acidic medium, quinones can be reduced and produce corresponding hydroquinone and hydroquinone anion through the protonation reaction [6,7,27–29]. As can be seen in this figure, the CVs exhibit a single redox couple with nearly symmetric anodic and cathodic peaks. The comparison with the CV at a bare GC electrode, shown in Fig. 1c, indicates clearly that the values of cathodic and anodic peak potentials obtained from Fig. 1a and b shift towards more positive and negative potential directions, respectively. The values of peak separation DEp (i.e. Epc Epa) evaluated from Fig. 1a–c were about 75 mV, 152 mV and 306 mV, with nearly the same formal potential E1/2 (the average of the anodic and cathodic peak potentials, (Epa + Epc)/2). The redox process of AQDS at the surface of a bare GC electrode shows low reversibility, but it exhibits a quasi-reversible behavior
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Fig. 1. CVs of (a) the GC/PPy/AQDS electrode (AQDS in doping phase) in pH 2.0 N2-saturated blank buffer solution, (b) and (c) the GC/PPy electrode and a bare GC electrode in pH 2.0 N2-saturated buffer solution containing 5 mM AQDS (AQDS in solution phase). Scan rate: 10 mV s1.
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of electrode at low scan rates. Similar electrochemical behavior was also reported for 1,4-naphthoquinone and its derivatives at the surface of carbon paste electrodes [8]. Moreover, the formal potential E1/2 is almost independent of the potential scan rate for sweep rates below 400 mV s1, indicating facile charge transfer between AQDS and electrode over this range of scan rate [29]. However, in the case of AQDS in doping form, the cathodic peak currents increase linearly with scan rate v up to 600 mV s1 (not shown), suggesting that in polymer matrix, the redox behavior of AQDS obeys a finite diffusion law and it is not expelled from the polymer matrix towards the solution bulk during the electrochemical reduction of PPy film. The electrochemical response of anthraquinone in aqueous solutions is pH dependent. Fig. 3A shows the CVs for AQDS as doping species in various pH buffer solutions. Up to a pH of approximately 8.0, the peak potentials are
when AQDS is incorporated into PPy matrix or adsorbed on PPy film surface. The DEp values obtained from Fig. 2 are greater than the 59/n mV expected for a reversible system and increase with the increasing of scan rate, indicating a limitation arising from charge transfer kinetics and the occurrence of quasi-reversible reductions [29,30]. The peak current ratio Ipc/Ipa was found to be greater than unity. This is most probably indicative of some kinetic and other complications in the electrode process [21,28]. In addition, at high scan rate (>20 mV s1), the cathodic peak currents of the CVs are linearly dependent on v1/2 (inset in Fig. 2) for AQDS in solution phase, suggesting that the reduction of AQDS is diffusion-controlled adsorption process which obeys a semi-infinite diffusion law. While at low scan rates, the cathodic peak currents vary linearly with v (not shown), which is the characteristic of non-diffusion-controlled process [13] and implies the adsorption of AQDS at the surface
Fig. 2. CVs of the GC/PPy electrode in pH 2.0 N2-saturated buffer solution containing 5 mM AQDS at various scan rates: (a) 20, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200, (g) 300 and (h) 400 mV s1. The inset shows the plot of Ipc vs. m1/2.
Fig. 3. (A) CVs of the GC/PPy/AQDS electrode in various pH buffered solutions at scan rate of 20 mV s1: (a) 2.0, (b) 4.5, (c) 6.2, (d) 7.8. (B) and (C) Dependence of the formal peak potential E1/2 on pH for AQDS in doping and solution phase, respectively.
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strongly pH dependent and the redox reaction involves the transfer of two electrons. The redox waves shift positively as the solution pH decreases from 7.8. The plots of the formal peak potential, E1/2, calculated as the average of the cathodic and anodic peak potentials of the corresponding CVs ((Epa + Epc)/2) against pH for the immobilized and diffusive AQDS are shown in Fig. 3B and C. As can be seen, E1/2 shifts in a negative potential direction with the increase of pH and each of the two plots results in two line regions with slope values of 0.067 V pH1 for pH < 9.5, 0.025 V pH1 for pH > 9.5 (Fig. 3B) and 0.072 V pH1 for pH < 7.6, 0.029 V pH1 for pH > 7.6 (Fig. 3C), respectively. The slope values of 0.067 and 0.072 V pH1 are close to a theoretical value of 0.0595 V pH1 expected for an electrochemical reaction involving two-electron two-proton transfer [6,7]. Over this pH range, AQDS is reduced to dihydroanthraquinone (H2AQ). While the slope values of 0.025 and 0.029 V pH1 compare favorably with the slope of 0.0295 V pH1 expected for the two-electron one-proton transfer reaction [6,7]. Over this pH range, AQDS is reduced to hydroanthraquinone anion (HAQ). The pKa values of the H2AQ/HAQ couple, which were calculated by intersection point of two neighbor linear segments, are 9.5 and 7.6 for the immobilized and diffusive AQDS, respectively. By contrast with AQDS in solution phase, the pKa has higher value when AQDS is incorporated into PPy matrix, which indicates that the H2AQ/HAQ couple exhibits the character of weaker acid in doping phase. These results suggest that the redox transformation does not involve the radical anion AQ and dianion AQ2 for pH < 11. 3.2. Electrocatalytic activity of AQDS for oxygen reduction When quinones derivatives are used as a mediator via homogeneous or heterogeneous media, an important factor affecting its catalytic activity is the pH value of the solution [7,8]. This is the key reason why greater attention must be paid to the optimization of the pH of the catalytic reaction media. Therefore, the properties of oxygen reduction catalyzed by AQDS were investigated in buffer solutions with various pH values. Although the reduction potentials of both oxygen and AQDS are pH dependent, their shift may be unequal due to their different kinetic behavior [21]. As the pH of buffer solution increase, up to pH 6.0, there is a gradual increase in the cathodic peak current for the immobilized AQDS. Whereas, in the case of AQDS in solution phase, the maximum value of cathodic peak current occurs at a pH about 7.0. The electroreduction of AQDS shows quasi-reversible peaks in the absence of oxygen, but in the presence of oxygen, it does not show any anodic reaction under the conditions mentioned above. Thus, pH 6.0 was chosen as optimum pH to study the electrocatalytic reduction of oxygen. Shamsipur and Manisankar also investigated this reaction using GC electrode modified by anthra-9,10-quinone derivatives at pH 6.0– 7.0 [6,14].
Fig. 4. CVs at the surface of a bare GC electrode (a) and the GC/PPy electrode (c) in pH 6.0 O2-saturated buffer solution containing 5 mM AQDS, (b) the GC/PPy/AQDS electrode in pH 6.0 O2-saturated blank buffer solution. Scan rate: 20 mV s1.
In pH 6.0 O2-saturated buffer solution, the electrocatalytic reduction of oxygen at the bare GC electrode occurs irreversibly with a peak potential of about 1089 mV (curve a, Fig. 4), while its reduction peaks on the GC/ PPy/AQDS (AQDS in doping phase) and the GC/PPy (AQDS in solution phase) electrode appear at 468 mV (curve b) and 546 mV (curve c), respectively. The peak potential of oxygen reduction shifts 621 and 78 mV to more positive potential for AQDS in doping phase when compared with a bare GC electrode and the response for AQDS in solution phase, suggesting that AQDS in doping phase is more active for oxygen reduction than AQDS in solution phase. The electrocatalytic reduction of oxygen has been studied at the surface of other quinonoid compounds modified electrodes as well. Our results are comparable with those reported by other research groups (see Table 1). The CVs of the GC/PPy electrode in pH 6.0 O2-saturated buffer solution containing 5 mM AQDS were recorded at various scan rates and the results are shown in Fig. 5A. As the scan rate increases, the catalytic reduction peak current increases and reduction peak potential shifts slightly in the negative direction. The cathodic peak potentials of oxygen reduction have a linear relationship with the logarithm of scan rates (Fig. 5B) and change according to Eq. (1) [29,32,33]: Epc ¼ ðb=2Þ log m þ constant
ð1Þ
Here, b is Tafel slope (b = 2.303RT/anaF, where a is the cathodic transfer coefficient, na is the number of electrons involved in the rate-determining step (rds) and F = 96 500 C mol1 is Faraday constant). The slope of the plot in Fig. 5B is 0.055 V (log m)1, thus, the Tafel slope b = 2 · 0.055 V, which indicates that an electron transfer process is the rds by assumption of a transfer coefficient a = 0.55 (na = 1). The information involved in the rds can also be obtained by using Tafel plot (inset in
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Table 1 Comparison of the efficiency of some quinonoid modified electrodes used in the electrocatalytic reduction of oxygen Electrode
Modifiera
Solvent
Supporting electrolyte
cO2
DE (mV)b
Ref.
Carbon paste
1,4-NQ
H2O
Phosphate buffer (pH 8.0)
O2-sat
350–550
[8]
380–470 321–665 518–755 592–729 280–560
[7] [14] [10] [11] [13]
621 543
Exp. Exp.
GC
1-HAQ derivatives PPy + AQ derivatives Riboflavin + 1,4–NQ Riboflavin + AQ AQ derivatives and dyes
H2O
PPy/AQDS(AQDS in doping phase) PPy + AQDS(AQDS in solution phase) a b
Acetate buffer (pH 4.5) Phosphate buffer (pH 7.0) Phosphate buffer (pH 7.0) Phosphate buffer(pH 7.0) (pH 5.0 – 8.0) Phosphate buffer (pH 6.0) Phosphate buffer (pH 6.0)
O2-sat
1,4-NQ = 1,4-naphthoquinone; 1-H AQ = 1-hydroxy-anthra-9,10-quinone; AQ = anthra-9,10-quinone. The shift of oxygen reduction potential which is calculated between oxygen reduction potential at modified and unmodified electrode.
Fig. 5B). It was drawn using the data derived from the falling part of current–potential curve at scan rate 20 mV s1 (curve d, Fig. 5A). As can be seen, a slope of 106 mV dec1 is obtained between 0.35 and 0.55 V, indicating the one electron transfer as the rds, assuming a transfer coefficient of a = 2.303RT/bnaF = 0.57(na = 1). The results obtained from these two different methods have a good agreement. The cathodic peak current Ipc versus the square root of the scan rate m1/2 shows a linear dependence (d, Fig. 5C) and behavior featuring the diffusion-controlled process. However, at higher scan rates, some deviation from linearity was observed. The negative shift of Epc and the negative deviation of Ipc indicate that a kinetics limitation exists in the course of reaction between the redox sites of electrode surface and oxygen, at higher scan rates [29,32]. The number of electrons in the overall reaction can be obtained from the slope of Ipc vs. v1/2 expressed as Eq. (2) [33,35]: I pc ¼ 2:99 105 nðana Þ
1=2
1=2
AcDO2 m1=2
ð2Þ
where A is the electrode area, c is the bulk concentration of dissolved oxygen (0.3 mM), DO2 is the diffusion coefficient of oxygen (1.64 · 105 cm2 s1) [7], n is the total number of electron transferred and ana is a parameter reflecting the irreversibility of reaction. Considering ana = 0.55, so, n = 2.07, which indicates that the mechanism for oxygen reduction catalyzed by the diffusive AQDS involves a two electrons transfer process. Also, a plot of the scan rate-normalized current (Ipcv1/2) versus scan rate (s, Fig. 5C), exhibits the characteristic shape that is typical of an electrochemical–chemical (EC) catalytic process [8,32,34]. Therefore, the probable overall mechanism for oxygen reduction catalyzed by AQDS can be summarized by Eqs. (3) and (4). It shows that the electrochemical reduction of oxygen might be controlled by the electron cross-exchange between oxygen and the redox sites and by the diffusion of oxygen. AQ þ 2Hþ þ 2e $ H2 AQ
ð3Þ
H2 AQ þ O2 ! H2 O2 þ AQ
ð4Þ
3.3. Chronoamperometric and chronocoulometric studies Chronoamperometric measurements of oxygen at different modified electrode surfaces were carried out using the double potential-step technique with an initial and a final potential of 450 and 800 mV, respectively. The net electrolysis current was obtained by subtraction of the background current using point by point subtraction method. In the absence of AQDS (curve a, Fig. 6A), the oxygen reduction current approaches zero on the PPy/GC electrode; while in the presence of AQDS (curves c and d), oxygen reduction reaction occurs with great electrocatalytic currents. The result reflects that the electrocatalytic reduction of oxygen was mainly mediated by AQ/H2AQ couple and the role of PPy layers was only a charge transporter. As the electrolysis potential was stepped from 800 to 450 mV, no significant anodic current was observed in the presence of oxygen. This indicates the irreversible nature of the catalytic reduction of oxygen [7,8]. A plot of net current, Inet against t1/2 for the GC/PPy/AQDS electrode in an O2-saturated buffer solution shows a straight line within short time, while it deviates from linearity at longer time periods (curve 1, inset in Fig. 6A). This result reveals that within shorter time, the mass transfer of oxygen is in agreement with the semi-infinite diffusion rule, but within longer time it presents the characteristic of the finite diffusion process in thin films. This result demonstrates the diffusion of oxygen from the solution bulk to the electrode surface and the slow permeation of oxygen through the polymer film. However, in the absence of oxygen, the plot of Inet vs. t1/2 shows a straight line which extrapolates close to the origin (curve 2). Since AQDS exists as doping species and the solution bulk contains no AQDS, this type of near contrellian behavior is not due to a linear semi-infinite diffusion process, but due to the occurrence of finite diffusion in a thin film [8,29]. Double potential-step chronocoulometric study was also carried out using the GC/PPy/AQDS electrode in the presence of oxygen (inset in Fig. 6B). The initial and final potentials are the same as in the chronoamperometric studies. In oxygenated buffer, a large enhancement of charge and the appearance of a nearly flat line on reversal of the
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Fig. 6. (A) Chronoamperograms response in pH 6.0 buffer solution obtained by the double potential-step technique at an initial potential of 450 mV and a final potential of 800 mV. The GC/PPy electrode in the presence of oxygen: (a) the blank buffer solution, (d) the buffer solution containing 5 mM AQDS. The GC/PPy/AQDS electrode in the absence (b) and presence (c) of oxygen. The inset shows the plot of net current Inet vs. t1/2 for the GC/PPy/AQDS electrode in the presence (1) and absence (2) of oxygen. (B) Plot of Q vs. t1/2 for the GC/PPy/AQDS electrode in the presence of oxygen at 450 mV. The inset shows corresponding chronocoulometric curve. (C) Dependence of Icat/IL on t1/2 driven from the data of choronoamperograms of (b) and (c) in (A). 1=2
Q ¼ 2nFAcDO2 p1=2 t1=2
ð5Þ 1/2
1/2
is 32.642 lC s , hence, n was The slope of Q vs. t determined as 1.96, which is almost similar to the value determined from CV. Also, the apparent rate constant (k) of oxygen reduction reaction catalyzed by the GC/PPy/AQDS electrode can be evaluated by chronoamperometry according to Eq. (6) [31,32,34]: Fig. 5. (A) Current–potential plots of the GC/PPy electrode in pH 6.0 O2saturated buffer solution containing 5 mM AQDS at various scan rates: (a) 5, (b) 10, (c) 15, (d) 20, (e) 50, (f) 100, (g) 150, and (h) 250 mV s1. (B) The plot of oxygen reduction peak potential Epc vs. log v for AQDS in solution phase. The inset shows the Tafel plot derived from the data at scan rate of 20 mV s1 in (A). (C) Variation of the electrocatalytic peak current with the square root of scan rate (d) and variation of the scan rate-normalized current (Ipcv1/2) with scan rate (s).
potential also prove the irreversible electrocatalytic reduction of oxygen [7,8]. The number of electrons involved in oxygen reduction can be calculated from the slope of Q vs. t1/2 plot (Fig. 6B) using Cottrell equation [35]:
I cat =I L ¼ c1=2 ½p1=2 erfðc1=2 Þ þ expðcÞ=c1=2
ð6Þ
where Icat is the catalytic current of the GC/PPy/AQDS electrode in the presence of oxygen (curve c, Fig. 6A), IL is the limited current in the absence of oxygen (curve b) and c = kct is the argument of the error function. In the cases of where c > 2, the error function is almost equal to 1 and the above equation can be reduced to: I cat =I L ¼ p1=2 c1=2 ¼ p1=2 ðkctÞ
1=2
ð7Þ
Based on the slope of the Icat/IL vs. t1/2 plot, the apparent rate constant of the electrocatalytic process, k, can be obtained
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with a value of 1.13 · 104 M1 s1. On the other hand, the surface coverage (CAQ) of the immobilized AQDS was evaluated by Q/nFA, where Q is the charge obtained by integrating the cathodic peak under the background correction at low scan rate. The calculated value of CAQ was 3.3 · 1010 mol cm2 at pH 6.0. Using this value, the heterogeneous rate constant of catalytic reaction was calculated, kf = kCAQ = 3.7 · 103 cm s1. 3.4. RDE investigations To distinguish the contribution of the diffusion of oxygen from that of the surface process involved in the oxygen reduction reaction catalyzed by the GC/PPy/AQDS electrode in pH 6.0 buffered solutions, the RDE technique was used and its data were analyzed. As can be seen from Fig. 7A, a large catalytic current begins to appear at ca. 0.5 V, which is close to the formal potential (E1/2) of the immobilized AQDS in pH 6.0 N2-saturated buffer solution (Fig. 3B). This result supports the fact that the catalytic reduction of oxygen proceeds through the mediation of the electrochemical reduction of AQDS. The Levich and Koutecky´–Levich plots derived from the current– potential curves at various rotation rates (x) are shown in Fig. 7B and C, respectively. The Levich plot derived from the limiting current measured at a potential of 0.7 V, is very close to the theoretically calculated line for a two-electron process (n = 2) and shows a slight negative deviation at higher x values. This non-linearity may be the result of a catalyzed reduction in which a current limiting chemical step precedes the electron transfer [7]. The relationship between the limiting current and rotating rate obeys the Levich equation: 2=3
I Lev ¼ 0:62nFADO2 c1=6 x1=2 c
ð8Þ
where x is the rotation rate, c is hydrodynamic viscosity (0.01 cm2 s1) [18] and other symbols have their usual meanings. Based on Eq. (8), the plot of the limiting current IL as a function of x1/2 should be straight line intersecting the origin. The slope and intercept of Fig. 7B are found to be 5.98 lA rad1/2 s1/2 and 2.32 lA, respectively. The fact that the Levich plot does not pass through the origin indicates the occurrence of kinetic limitation in the catalytic process [29] and the slow diffusion of oxygen in PPy matrix. In RDE approach, the observed current is related to the electrode rotation rate x and bimolecular rate constant k or kf by the Koutecky´–Levich equation [29,35]: 1 I 1 ¼ I 1 k þ I Lev I k ¼ nFAkcCAQ ðor I k ¼ nFAk f CÞ
ð9Þ ð10Þ
The slopes of the corresponding Koutecky´–Levich plots at different reduction potentials are almost parallel and close to that of the calculated line for the two-electron reduction of oxygen. This is good evidence for the accomplishment of
Fig. 7. (A) Current–potential curves for the reduction of oxygen at the GC/PPy/AQDS rotating disk electrode in pH 6.0 buffer solution at different rotation rates: (a) 200, (b) 400, (c) 600, (d) 800, (e) 1000, (f) 1200, and (g) 1400 rpm. Scan rate: 10 mV s1. (B) Levich plots of limiting currents at 0.7 V and the theoretical Levich plots for two (n = 2) and four (n = 4) electron reduction of oxygen. (C) Koutecky´–Levich plots for oxygen reduction at various reduction potentials.
the reduction of oxygen to hydrogen peroxide by the immobilized AQDS. The diffusive coefficient of oxygen, DO2 , in pH 6.0 buffered solution was determined to be 1.56 · 105 cm2 s1 using the Levich equation (8) at various
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Fig. 8. Change in the limiting currents density of oxygen reduction with time at the GC/PPy/AQDS electrode in pH 6.0 O2-saturated buffer solution. Rotation rate: 600 rpm.
rotation rates. The apparent rate constants k for the reduction of oxygen can be calculated from the intercepts of Koutecky´–Levich plots. The value of k measured at reduction potential of 0.7 V was 1.37 · 104 M1 s1 (kf = kCAQ = 4.5 · 103 cm s1). The values of k and kf are similar to those determined from chronoamperometric measurement and DO2 is in good agreement with those reported by others [7,13,14,29,32]. In addition, the Ik values can be obtained from the intercepts of the Koutecky´–Levich plots. The increases of Ik with the negative shift of the reduction potential indicate the potential-dependent kinetic behavior of the reduction reaction. The Tafel plot of E vs. log [ILI/(IL I)] for oxygen reduction catalyzed by the GC/PPy/AQDS electrode at different rotation rates were all quite linear with the slopes between 78 and 86 mV dec1. This is indicative of a redox catalytic process [7,19]. 3.5. Stability of catalytic activity for GC/PPy/AQDS electrode The change in oxygen reduction current at the GC/PPy/ AQDS electrode with time was also examined. During the study oxygen was bubbled continuously and the potential was fixed in the diffusion limited ranges. As shown in Fig. 8, the current at the modified electrode decreases rapidly, but subsequent decrease is slight. The degradation of the electrocatalytic activity of the modified electrode may be attributed to the degradation of PPy film and the loss of part of its electronic conductivity in the presence of hydrogen peroxide produced by the two-electron transfer reaction. 4. Conclusions The redox reactions of AQDS both in PPy matrix and at the surface of GC/PPy show a quasi-reversible behavior and exhibit pH dependence. The ionization constant, pKa,
of H2AQ/HAQ redox couple for AQDS in doping phase shifts towards higher value when compared with that for AQDS in solution phase. Both the immobilized and diffusive AQDS can catalyze the electroreduction of oxygen via the two-electron reduction pathway at an optimized pH value, but the former is more active. The observed electrocatalysis for oxygen reduction into hydrogen peroxide is attributed to dihydroanthraquinone (H2AQ). The chronoamperometry and RDE voltammetry show that in PPy matrix, the diffusion of oxygen is slow. The total number of electron transferred and the kinetic parameters were determined using various electrochemical approaches. The GC/PPy/AQDS electrode is sensitive to H2O2 and the degradation of PPy film results in a decrease in the electrocatalytic activity of the modified electrode. References [1] M. Hepel, Electrochim. Acta 41 (1996) 63. [2] T. Komura, Y. Mori, T. Yamaguti, K. Takahasi, Electrochim. Acta 42 (1997) 985. [3] T. Komura, Y. Yokono, T. Yamaguchi, K. Takahachi, J. Electroanal. Chem. 478 (1999) 9. [4] P. Tissot, A. Huissoud, Electrochim. Acta 41 (1996) 2451. [5] T. Ossowski, P. Pipka, A. Liwo, D. Jeziorek, Electrochim. Acta 45 (2000) 3581. [6] A. Salimi, M.F. Mousavi, H. Sharghi, M. Shamsipur, Bull. Chem. Soc. Jpn. 72 (1999) 2121. [7] A. Salimi, H. Eshghi, H. Sharghi, S.M. Golabi, M. Shamsipur, Electroanalysis 11 (1999) 114. [8] S.M. Golabi, J.B. Raoof, J. Electroanal. Chem. 416 (1996) 75. [9] A. Salimi, C.E. Banks, R.G. Compton, Phys. Chem. Chem. Phys. 5 (2003) 3988. [10] P. Manisankar, A.M. Pushpalasha, S. Vasanthkumar, J. Electroanal. Chem. 571 (2004) 43. [11] P. Manisankar, A. Gomathi, J. Power Sources 150 (2005) 240. [12] M.C. Pham, J.E. Dubois, J. Electroanal. Chem. 199 (1986) 153. [13] P. Manisankar, A. Gomathi, D. Velayutham, J. Solid State Electrochem. 10 (2005) 1007. [14] P. Manisankar, A. Gomathi, J. Mol. Catal. 232 (2005) 45. [15] A. Sarapuu, K. Vaik, D.J. Schiffrin, K. Tammeveski, J. Electroanal. Chem. 515 (2001) 101. [16] A. Sarapuu, K. Vaik, D.J. Schiffrin, K. Tammeveski, J. Electroanal. Chem. 541 (2003) 23. [17] K. Vaik, A. Sarapuu, K. Tammeveski, F. Mirkhalaf, J. Electroanal. Chem. 564 (2004) 159. [18] K. Vaik, D.J. Schiffrin, K. Tammeveski, Electrochem. Commun. 6 (2004) 1. [19] M.S. Hossain, D. Tryk, E. Yeager, Electrochim. Acta 34 (1989) 1733. [20] A. Salimi, H. Eshgi, H. Sharghi, S.M. Golabi, M. Shamsipur, Electroanalysis 11 (1999) 2. [21] J.B. Raoof, S.M. Golabi, Bull. Chem. Soc. Jpn. 68 (1995) 2253. [22] A. Salimi, M. Ghadermazi, Anal. Sci. 17 (2001) 1165. [23] K. Vaik, U. Ma¨eorg, F.C. Maschion, D.J. Schiffrin, K. Tammeveski, Electrochim. Acta 50 (2005) 5126. [24] O. EI Mouahid, C. Coutanceau, E.M. Belgsir, P. Crouigneau, C. Lamy, J. Electroanal. Chem. 426 (1997) 117. [25] C. Coutanceau, A. EI Hourch, J.M. Leger, C. Lamy, Electrochim. Acta 40 (1995) 2739. [26] C. Coutanceau, P. Crouigneau, L.M. Le´ger, C. Lamy, J. Electroanal. Chem. 379 (1994) 389. [27] R.J. Forster, J.P. O’Kelly, J. Electroanal. Chem. 498 (2001) 127. [28] M. Shamsipur, A. Salimi, S.M. Golabi, H. Sharghi, M.F. Mousavi, J. Solid State Electrocehm. 5 (2001) 68.
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[33] P. Ardiles, E. Trollund, M. Isaacs, M.J. Aguirre, J. Mol. Catal. 165 (2001) 169. [34] L. Zhang, S. Dong, J. Electroanal. Chem. 568 (2004) 189. [35] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 602 (2007) 163–171 www.elsevier.com/locate/jelechem
Electrochemical behavior and electrocatalytic activity of anthraquinonedisulphonate in solution phase and as doping species at polypyrrole modified glassy carbon electrode Guoquan Zhang, Weishen Yang, Fenglin Yang
*
School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China Received 16 October 2006; received in revised form 6 December 2006; accepted 12 December 2006 Available online 15 December 2006
Abstract The electrochemical behaviors of anthraquinonedisulphonate (AQDS) dissolved in solution and incorporated into polypyrrole (PPy) matrix as doping species at PPy modified glassy carbon (GC) electrode were investigated in detail, also reported was their applications in the electrocatalytic reduction of oxygen. It was found that the redox process of AQDS in solution and doping phase both exhibit quasireversible behavior and pH dependence. The value of ionization constant pKa for the H2AQ/HAQ couple increases obviously from 7.6 in solution phase to 9.5 when AQDS is incorporated into PPy matrix. Under optimum condition (pH 6.0), the reduction of oxygen at the GC/PPy/AQDS electrode (AQDS in doping phase) occurs irreversibly at a potential about 621 mV less negative than at a bare GC electrode. The same peak potential shifts 78 mV to more positive potential when compared with the response for AQDS in solution phase. The observed electrocatalysis for the reduction of oxygen is mediated by the electrochemical reduction of AQDS. The number of electrons involved n and the kinetic parameters such as electron transfer coefficient a, catalytic reaction rate constant k, kf and the diffusive coefficient of oxygen DO2 , were determined by using various electrochemical approaches. The stability of the catalytic activity for the GC/ PPy/AQDS electrode was also examined. 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemical; Electrocatalytic; Anthraquinonedisulphonate; Polypyrrole; Modified electrode
1. Introduction Quinones and their derivatives have received great attention in the last decades, since these quinonoid compounds are good catalysts for many electrode reaction of interest [1–5]. In most cases, the catalyst was just adsorbed at the electrode surface giving good activity, but a very low stability. In addition, the modification of electrode surfaces by quinones can also be obtained by electrochemical deposition and provides surfaces that are much more stable than those modified with spontaneously adsorbed monolayer. The investigations of the electrochemical behavior of these *
Corresponding author. Tel.: +86 411 84706328; fax: +86 411 84708084. E-mail address: yangfl@dlut.edu.cn (F. Yang). 0022-0728/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.12.007
quinonoid compounds modified electrodes have shown a dramatic enhancement of the rate and a radical improvement of catalytic activity towards the reduction of oxygen [6–23]. An important feature of these quinones modified electrodes is that the reduction of oxygen stops at the peroxide stage, which enables their use as electrocatalysts for the electrosynthesis of hydrogen peroxide. However, during the electrocatalytic processes large part of catalysts tend to desorb from the surface of electrode towards the solution bulk, leading to a loss of activity with time [6]. A very convenient way to disperse electrocatalysts at the molecular level is using a conducting polymer, as a welldefined matrix to stabilize the catalytic sites [24–26]. The high stability, high electronic conductivity and useful mechanical properties show that polypyrrole (PPy) film has the ability to promote electron transfer when used in
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electrochemical reaction. The oxidative electropolymerization of pyrrole (Py) in the presence of doping anions is deemed to be a fast and simple method for anchoring counter-anions at molecular level in the matrix of electroactive polymer [25,26]. Therefore, aimed at improving the stability and the electrocatalytic activity of electrocatalyst, and taking advantage of both PPy film and quinonoid compound, a water soluble catalyst, anthraquinonedisulphonate (AQDS) was incorporated into a conducting polymer matrix during the electropolymerization in aqueous medium of Py in this article. This technique of electrode preparation allowed us to obtain active and stable electrodes, in which the catalyst was dispersed into the porous electron conducting polymer matrix. The bulky AQDS strongly interact with the PPy matrix and cannot be expelled from the polymer matrix even when PPy is reduced electrochemically [1]. In this work, the pH dependent electrochemical behaviors of AQDS, both dissolved in solution and incorporated into PPy matrix at PPy modified glassy carbon (GC) electrode was studied. Also investigated was the electrocatalytic activity of AQDS in the two different phases towards the electroreduction of oxygen. The kinetic parameters of the catalytic reduction of oxygen were determined using various electroanalytical techniques. The results showed that the GC/PPy/AQDS electrode enables a more efficient electrocatalytic performance and the number of electron transfer for oxygen reduction is about two. 2. Experimental 2.1. Reagents and chemicals Py (99%) was obtained from Aldrich, purified by vacuum distillation before use. Anthraquinone-2,7-disulphonic acid, disodium salt was bought from Shymax Chemicals Co., Ltd. and they were used without any further purification. Other reagents were A.R. and used as received. Doubly distilled water was used throughout the experiments. 0.1 M phosphate buffer or 0.1 M acetate buffer was used which was titrated directly in the electrochemical cell with H2SO4 and NaOH to obtain the required pH values across a wide range. All electrolyte and monomer solution were purged with N2 before and during the polymerization experiments. O2 with purity of 99.99% was used to saturate the solutions during oxygen reduction experiments. 2.2. Working electrode The polymerization of Py was performed on a GC rotating disk electrode (RDE, 0.126 cm2 area) polished with 0.5 lm alumina, sonicated for 5 min in doubly distilled water and rinsed with acetone and doubly distilled water before polymerization. The rotation of the disk electrode and the use of relatively low concentrations of monomer
prevented the polymeric film to protrude out of the electrode surface. In the case of the GC/PPy electrode, the polymerization was performed in an aqueous solution containing 0.1 M pyrrole and 0.5 M H2SO4. The GC/PPy/AQDS electrode was electrodeposited from an aqueous solution containing 0.1 M Py, 0.5 M H2SO4 and 5 mM anthraquinone-2,7disulphonic acid, disodium salt. The polymerization was performed during 15 voltammetric cycles at scan rate of 10 mV s1 between 0.7 and 0.7 V and with a rotation rate of 600 rpm. 2.3. Apparatus All the electrochemical investigations, including electropolymerization, cyclic voltammetry (CV), chronoamperometry/chronocoulometric and rotating disk electrode (RDE) voltammetry were carried out in a conventional one compartment electrolytic cell with three electrodes using an EG&G Princeton Applied Research Model 263A potentiostat–galvanostat (Princeton, NJ, USA). The experiments were controlled with General Purpose Electrochemical System (GPES) software. A Pt wire and a KCL-saturated calomel electrode (SCE) were used as the auxiliary electrode and the reference electrode, respectively. All the potentials mentioned in the text are given against the SCE. The electrochemical behavior of AQDS and the reduction of oxygen were studied with RDE covered with the electrodeposited PPy/AQDS and PPy film in aqueous buffer solution in the absence and presence of AQDS, corresponding to AQDS in doping and solution phase, respectively. Most of the experiments were conducted at room temperature. 3. Results and discussion 3.1. Electrochemical behavior of AQDS in solution and doping phase Fig. 1 shows the CVs of AQDS in solution and doping phase in pH 2.0 N2-saturated buffer solution. In acidic medium, quinones can be reduced and produce corresponding hydroquinone and hydroquinone anion through the protonation reaction [6,7,27–29]. As can be seen in this figure, the CVs exhibit a single redox couple with nearly symmetric anodic and cathodic peaks. The comparison with the CV at a bare GC electrode, shown in Fig. 1c, indicates clearly that the values of cathodic and anodic peak potentials obtained from Fig. 1a and b shift towards more positive and negative potential directions, respectively. The values of peak separation DEp (i.e. Epc Epa) evaluated from Fig. 1a–c were about 75 mV, 152 mV and 306 mV, with nearly the same formal potential E1/2 (the average of the anodic and cathodic peak potentials, (Epa + Epc)/2). The redox process of AQDS at the surface of a bare GC electrode shows low reversibility, but it exhibits a quasi-reversible behavior
G. Zhang et al. / Journal of Electroanalytical Chemistry 602 (2007) 163–171
Fig. 1. CVs of (a) the GC/PPy/AQDS electrode (AQDS in doping phase) in pH 2.0 N2-saturated blank buffer solution, (b) and (c) the GC/PPy electrode and a bare GC electrode in pH 2.0 N2-saturated buffer solution containing 5 mM AQDS (AQDS in solution phase). Scan rate: 10 mV s1.
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of electrode at low scan rates. Similar electrochemical behavior was also reported for 1,4-naphthoquinone and its derivatives at the surface of carbon paste electrodes [8]. Moreover, the formal potential E1/2 is almost independent of the potential scan rate for sweep rates below 400 mV s1, indicating facile charge transfer between AQDS and electrode over this range of scan rate [29]. However, in the case of AQDS in doping form, the cathodic peak currents increase linearly with scan rate v up to 600 mV s1 (not shown), suggesting that in polymer matrix, the redox behavior of AQDS obeys a finite diffusion law and it is not expelled from the polymer matrix towards the solution bulk during the electrochemical reduction of PPy film. The electrochemical response of anthraquinone in aqueous solutions is pH dependent. Fig. 3A shows the CVs for AQDS as doping species in various pH buffer solutions. Up to a pH of approximately 8.0, the peak potentials are
when AQDS is incorporated into PPy matrix or adsorbed on PPy film surface. The DEp values obtained from Fig. 2 are greater than the 59/n mV expected for a reversible system and increase with the increasing of scan rate, indicating a limitation arising from charge transfer kinetics and the occurrence of quasi-reversible reductions [29,30]. The peak current ratio Ipc/Ipa was found to be greater than unity. This is most probably indicative of some kinetic and other complications in the electrode process [21,28]. In addition, at high scan rate (>20 mV s1), the cathodic peak currents of the CVs are linearly dependent on v1/2 (inset in Fig. 2) for AQDS in solution phase, suggesting that the reduction of AQDS is diffusion-controlled adsorption process which obeys a semi-infinite diffusion law. While at low scan rates, the cathodic peak currents vary linearly with v (not shown), which is the characteristic of non-diffusion-controlled process [13] and implies the adsorption of AQDS at the surface
Fig. 2. CVs of the GC/PPy electrode in pH 2.0 N2-saturated buffer solution containing 5 mM AQDS at various scan rates: (a) 20, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200, (g) 300 and (h) 400 mV s1. The inset shows the plot of Ipc vs. m1/2.
Fig. 3. (A) CVs of the GC/PPy/AQDS electrode in various pH buffered solutions at scan rate of 20 mV s1: (a) 2.0, (b) 4.5, (c) 6.2, (d) 7.8. (B) and (C) Dependence of the formal peak potential E1/2 on pH for AQDS in doping and solution phase, respectively.
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strongly pH dependent and the redox reaction involves the transfer of two electrons. The redox waves shift positively as the solution pH decreases from 7.8. The plots of the formal peak potential, E1/2, calculated as the average of the cathodic and anodic peak potentials of the corresponding CVs ((Epa + Epc)/2) against pH for the immobilized and diffusive AQDS are shown in Fig. 3B and C. As can be seen, E1/2 shifts in a negative potential direction with the increase of pH and each of the two plots results in two line regions with slope values of 0.067 V pH1 for pH < 9.5, 0.025 V pH1 for pH > 9.5 (Fig. 3B) and 0.072 V pH1 for pH < 7.6, 0.029 V pH1 for pH > 7.6 (Fig. 3C), respectively. The slope values of 0.067 and 0.072 V pH1 are close to a theoretical value of 0.0595 V pH1 expected for an electrochemical reaction involving two-electron two-proton transfer [6,7]. Over this pH range, AQDS is reduced to dihydroanthraquinone (H2AQ). While the slope values of 0.025 and 0.029 V pH1 compare favorably with the slope of 0.0295 V pH1 expected for the two-electron one-proton transfer reaction [6,7]. Over this pH range, AQDS is reduced to hydroanthraquinone anion (HAQ). The pKa values of the H2AQ/HAQ couple, which were calculated by intersection point of two neighbor linear segments, are 9.5 and 7.6 for the immobilized and diffusive AQDS, respectively. By contrast with AQDS in solution phase, the pKa has higher value when AQDS is incorporated into PPy matrix, which indicates that the H2AQ/HAQ couple exhibits the character of weaker acid in doping phase. These results suggest that the redox transformation does not involve the radical anion AQ and dianion AQ2 for pH < 11. 3.2. Electrocatalytic activity of AQDS for oxygen reduction When quinones derivatives are used as a mediator via homogeneous or heterogeneous media, an important factor affecting its catalytic activity is the pH value of the solution [7,8]. This is the key reason why greater attention must be paid to the optimization of the pH of the catalytic reaction media. Therefore, the properties of oxygen reduction catalyzed by AQDS were investigated in buffer solutions with various pH values. Although the reduction potentials of both oxygen and AQDS are pH dependent, their shift may be unequal due to their different kinetic behavior [21]. As the pH of buffer solution increase, up to pH 6.0, there is a gradual increase in the cathodic peak current for the immobilized AQDS. Whereas, in the case of AQDS in solution phase, the maximum value of cathodic peak current occurs at a pH about 7.0. The electroreduction of AQDS shows quasi-reversible peaks in the absence of oxygen, but in the presence of oxygen, it does not show any anodic reaction under the conditions mentioned above. Thus, pH 6.0 was chosen as optimum pH to study the electrocatalytic reduction of oxygen. Shamsipur and Manisankar also investigated this reaction using GC electrode modified by anthra-9,10-quinone derivatives at pH 6.0– 7.0 [6,14].
Fig. 4. CVs at the surface of a bare GC electrode (a) and the GC/PPy electrode (c) in pH 6.0 O2-saturated buffer solution containing 5 mM AQDS, (b) the GC/PPy/AQDS electrode in pH 6.0 O2-saturated blank buffer solution. Scan rate: 20 mV s1.
In pH 6.0 O2-saturated buffer solution, the electrocatalytic reduction of oxygen at the bare GC electrode occurs irreversibly with a peak potential of about 1089 mV (curve a, Fig. 4), while its reduction peaks on the GC/ PPy/AQDS (AQDS in doping phase) and the GC/PPy (AQDS in solution phase) electrode appear at 468 mV (curve b) and 546 mV (curve c), respectively. The peak potential of oxygen reduction shifts 621 and 78 mV to more positive potential for AQDS in doping phase when compared with a bare GC electrode and the response for AQDS in solution phase, suggesting that AQDS in doping phase is more active for oxygen reduction than AQDS in solution phase. The electrocatalytic reduction of oxygen has been studied at the surface of other quinonoid compounds modified electrodes as well. Our results are comparable with those reported by other research groups (see Table 1). The CVs of the GC/PPy electrode in pH 6.0 O2-saturated buffer solution containing 5 mM AQDS were recorded at various scan rates and the results are shown in Fig. 5A. As the scan rate increases, the catalytic reduction peak current increases and reduction peak potential shifts slightly in the negative direction. The cathodic peak potentials of oxygen reduction have a linear relationship with the logarithm of scan rates (Fig. 5B) and change according to Eq. (1) [29,32,33]: Epc ¼ ðb=2Þ log m þ constant
ð1Þ
Here, b is Tafel slope (b = 2.303RT/anaF, where a is the cathodic transfer coefficient, na is the number of electrons involved in the rate-determining step (rds) and F = 96 500 C mol1 is Faraday constant). The slope of the plot in Fig. 5B is 0.055 V (log m)1, thus, the Tafel slope b = 2 · 0.055 V, which indicates that an electron transfer process is the rds by assumption of a transfer coefficient a = 0.55 (na = 1). The information involved in the rds can also be obtained by using Tafel plot (inset in
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Table 1 Comparison of the efficiency of some quinonoid modified electrodes used in the electrocatalytic reduction of oxygen Electrode
Modifiera
Solvent
Supporting electrolyte
cO2
DE (mV)b
Ref.
Carbon paste
1,4-NQ
H2O
Phosphate buffer (pH 8.0)
O2-sat
350–550
[8]
380–470 321–665 518–755 592–729 280–560
[7] [14] [10] [11] [13]
621 543
Exp. Exp.
GC
1-HAQ derivatives PPy + AQ derivatives Riboflavin + 1,4–NQ Riboflavin + AQ AQ derivatives and dyes
H2O
PPy/AQDS(AQDS in doping phase) PPy + AQDS(AQDS in solution phase) a b
Acetate buffer (pH 4.5) Phosphate buffer (pH 7.0) Phosphate buffer (pH 7.0) Phosphate buffer(pH 7.0) (pH 5.0 – 8.0) Phosphate buffer (pH 6.0) Phosphate buffer (pH 6.0)
O2-sat
1,4-NQ = 1,4-naphthoquinone; 1-H AQ = 1-hydroxy-anthra-9,10-quinone; AQ = anthra-9,10-quinone. The shift of oxygen reduction potential which is calculated between oxygen reduction potential at modified and unmodified electrode.
Fig. 5B). It was drawn using the data derived from the falling part of current–potential curve at scan rate 20 mV s1 (curve d, Fig. 5A). As can be seen, a slope of 106 mV dec1 is obtained between 0.35 and 0.55 V, indicating the one electron transfer as the rds, assuming a transfer coefficient of a = 2.303RT/bnaF = 0.57(na = 1). The results obtained from these two different methods have a good agreement. The cathodic peak current Ipc versus the square root of the scan rate m1/2 shows a linear dependence (d, Fig. 5C) and behavior featuring the diffusion-controlled process. However, at higher scan rates, some deviation from linearity was observed. The negative shift of Epc and the negative deviation of Ipc indicate that a kinetics limitation exists in the course of reaction between the redox sites of electrode surface and oxygen, at higher scan rates [29,32]. The number of electrons in the overall reaction can be obtained from the slope of Ipc vs. v1/2 expressed as Eq. (2) [33,35]: I pc ¼ 2:99 105 nðana Þ
1=2
1=2
AcDO2 m1=2
ð2Þ
where A is the electrode area, c is the bulk concentration of dissolved oxygen (0.3 mM), DO2 is the diffusion coefficient of oxygen (1.64 · 105 cm2 s1) [7], n is the total number of electron transferred and ana is a parameter reflecting the irreversibility of reaction. Considering ana = 0.55, so, n = 2.07, which indicates that the mechanism for oxygen reduction catalyzed by the diffusive AQDS involves a two electrons transfer process. Also, a plot of the scan rate-normalized current (Ipcv1/2) versus scan rate (s, Fig. 5C), exhibits the characteristic shape that is typical of an electrochemical–chemical (EC) catalytic process [8,32,34]. Therefore, the probable overall mechanism for oxygen reduction catalyzed by AQDS can be summarized by Eqs. (3) and (4). It shows that the electrochemical reduction of oxygen might be controlled by the electron cross-exchange between oxygen and the redox sites and by the diffusion of oxygen. AQ þ 2Hþ þ 2e $ H2 AQ
ð3Þ
H2 AQ þ O2 ! H2 O2 þ AQ
ð4Þ
3.3. Chronoamperometric and chronocoulometric studies Chronoamperometric measurements of oxygen at different modified electrode surfaces were carried out using the double potential-step technique with an initial and a final potential of 450 and 800 mV, respectively. The net electrolysis current was obtained by subtraction of the background current using point by point subtraction method. In the absence of AQDS (curve a, Fig. 6A), the oxygen reduction current approaches zero on the PPy/GC electrode; while in the presence of AQDS (curves c and d), oxygen reduction reaction occurs with great electrocatalytic currents. The result reflects that the electrocatalytic reduction of oxygen was mainly mediated by AQ/H2AQ couple and the role of PPy layers was only a charge transporter. As the electrolysis potential was stepped from 800 to 450 mV, no significant anodic current was observed in the presence of oxygen. This indicates the irreversible nature of the catalytic reduction of oxygen [7,8]. A plot of net current, Inet against t1/2 for the GC/PPy/AQDS electrode in an O2-saturated buffer solution shows a straight line within short time, while it deviates from linearity at longer time periods (curve 1, inset in Fig. 6A). This result reveals that within shorter time, the mass transfer of oxygen is in agreement with the semi-infinite diffusion rule, but within longer time it presents the characteristic of the finite diffusion process in thin films. This result demonstrates the diffusion of oxygen from the solution bulk to the electrode surface and the slow permeation of oxygen through the polymer film. However, in the absence of oxygen, the plot of Inet vs. t1/2 shows a straight line which extrapolates close to the origin (curve 2). Since AQDS exists as doping species and the solution bulk contains no AQDS, this type of near contrellian behavior is not due to a linear semi-infinite diffusion process, but due to the occurrence of finite diffusion in a thin film [8,29]. Double potential-step chronocoulometric study was also carried out using the GC/PPy/AQDS electrode in the presence of oxygen (inset in Fig. 6B). The initial and final potentials are the same as in the chronoamperometric studies. In oxygenated buffer, a large enhancement of charge and the appearance of a nearly flat line on reversal of the
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Fig. 6. (A) Chronoamperograms response in pH 6.0 buffer solution obtained by the double potential-step technique at an initial potential of 450 mV and a final potential of 800 mV. The GC/PPy electrode in the presence of oxygen: (a) the blank buffer solution, (d) the buffer solution containing 5 mM AQDS. The GC/PPy/AQDS electrode in the absence (b) and presence (c) of oxygen. The inset shows the plot of net current Inet vs. t1/2 for the GC/PPy/AQDS electrode in the presence (1) and absence (2) of oxygen. (B) Plot of Q vs. t1/2 for the GC/PPy/AQDS electrode in the presence of oxygen at 450 mV. The inset shows corresponding chronocoulometric curve. (C) Dependence of Icat/IL on t1/2 driven from the data of choronoamperograms of (b) and (c) in (A). 1=2
Q ¼ 2nFAcDO2 p1=2 t1=2
ð5Þ 1/2
1/2
is 32.642 lC s , hence, n was The slope of Q vs. t determined as 1.96, which is almost similar to the value determined from CV. Also, the apparent rate constant (k) of oxygen reduction reaction catalyzed by the GC/PPy/AQDS electrode can be evaluated by chronoamperometry according to Eq. (6) [31,32,34]: Fig. 5. (A) Current–potential plots of the GC/PPy electrode in pH 6.0 O2saturated buffer solution containing 5 mM AQDS at various scan rates: (a) 5, (b) 10, (c) 15, (d) 20, (e) 50, (f) 100, (g) 150, and (h) 250 mV s1. (B) The plot of oxygen reduction peak potential Epc vs. log v for AQDS in solution phase. The inset shows the Tafel plot derived from the data at scan rate of 20 mV s1 in (A). (C) Variation of the electrocatalytic peak current with the square root of scan rate (d) and variation of the scan rate-normalized current (Ipcv1/2) with scan rate (s).
potential also prove the irreversible electrocatalytic reduction of oxygen [7,8]. The number of electrons involved in oxygen reduction can be calculated from the slope of Q vs. t1/2 plot (Fig. 6B) using Cottrell equation [35]:
I cat =I L ¼ c1=2 ½p1=2 erfðc1=2 Þ þ expðcÞ=c1=2
ð6Þ
where Icat is the catalytic current of the GC/PPy/AQDS electrode in the presence of oxygen (curve c, Fig. 6A), IL is the limited current in the absence of oxygen (curve b) and c = kct is the argument of the error function. In the cases of where c > 2, the error function is almost equal to 1 and the above equation can be reduced to: I cat =I L ¼ p1=2 c1=2 ¼ p1=2 ðkctÞ
1=2
ð7Þ
Based on the slope of the Icat/IL vs. t1/2 plot, the apparent rate constant of the electrocatalytic process, k, can be obtained
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with a value of 1.13 · 104 M1 s1. On the other hand, the surface coverage (CAQ) of the immobilized AQDS was evaluated by Q/nFA, where Q is the charge obtained by integrating the cathodic peak under the background correction at low scan rate. The calculated value of CAQ was 3.3 · 1010 mol cm2 at pH 6.0. Using this value, the heterogeneous rate constant of catalytic reaction was calculated, kf = kCAQ = 3.7 · 103 cm s1. 3.4. RDE investigations To distinguish the contribution of the diffusion of oxygen from that of the surface process involved in the oxygen reduction reaction catalyzed by the GC/PPy/AQDS electrode in pH 6.0 buffered solutions, the RDE technique was used and its data were analyzed. As can be seen from Fig. 7A, a large catalytic current begins to appear at ca. 0.5 V, which is close to the formal potential (E1/2) of the immobilized AQDS in pH 6.0 N2-saturated buffer solution (Fig. 3B). This result supports the fact that the catalytic reduction of oxygen proceeds through the mediation of the electrochemical reduction of AQDS. The Levich and Koutecky´–Levich plots derived from the current– potential curves at various rotation rates (x) are shown in Fig. 7B and C, respectively. The Levich plot derived from the limiting current measured at a potential of 0.7 V, is very close to the theoretically calculated line for a two-electron process (n = 2) and shows a slight negative deviation at higher x values. This non-linearity may be the result of a catalyzed reduction in which a current limiting chemical step precedes the electron transfer [7]. The relationship between the limiting current and rotating rate obeys the Levich equation: 2=3
I Lev ¼ 0:62nFADO2 c1=6 x1=2 c
ð8Þ
where x is the rotation rate, c is hydrodynamic viscosity (0.01 cm2 s1) [18] and other symbols have their usual meanings. Based on Eq. (8), the plot of the limiting current IL as a function of x1/2 should be straight line intersecting the origin. The slope and intercept of Fig. 7B are found to be 5.98 lA rad1/2 s1/2 and 2.32 lA, respectively. The fact that the Levich plot does not pass through the origin indicates the occurrence of kinetic limitation in the catalytic process [29] and the slow diffusion of oxygen in PPy matrix. In RDE approach, the observed current is related to the electrode rotation rate x and bimolecular rate constant k or kf by the Koutecky´–Levich equation [29,35]: 1 I 1 ¼ I 1 k þ I Lev I k ¼ nFAkcCAQ ðor I k ¼ nFAk f CÞ
ð9Þ ð10Þ
The slopes of the corresponding Koutecky´–Levich plots at different reduction potentials are almost parallel and close to that of the calculated line for the two-electron reduction of oxygen. This is good evidence for the accomplishment of
Fig. 7. (A) Current–potential curves for the reduction of oxygen at the GC/PPy/AQDS rotating disk electrode in pH 6.0 buffer solution at different rotation rates: (a) 200, (b) 400, (c) 600, (d) 800, (e) 1000, (f) 1200, and (g) 1400 rpm. Scan rate: 10 mV s1. (B) Levich plots of limiting currents at 0.7 V and the theoretical Levich plots for two (n = 2) and four (n = 4) electron reduction of oxygen. (C) Koutecky´–Levich plots for oxygen reduction at various reduction potentials.
the reduction of oxygen to hydrogen peroxide by the immobilized AQDS. The diffusive coefficient of oxygen, DO2 , in pH 6.0 buffered solution was determined to be 1.56 · 105 cm2 s1 using the Levich equation (8) at various
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Fig. 8. Change in the limiting currents density of oxygen reduction with time at the GC/PPy/AQDS electrode in pH 6.0 O2-saturated buffer solution. Rotation rate: 600 rpm.
rotation rates. The apparent rate constants k for the reduction of oxygen can be calculated from the intercepts of Koutecky´–Levich plots. The value of k measured at reduction potential of 0.7 V was 1.37 · 104 M1 s1 (kf = kCAQ = 4.5 · 103 cm s1). The values of k and kf are similar to those determined from chronoamperometric measurement and DO2 is in good agreement with those reported by others [7,13,14,29,32]. In addition, the Ik values can be obtained from the intercepts of the Koutecky´–Levich plots. The increases of Ik with the negative shift of the reduction potential indicate the potential-dependent kinetic behavior of the reduction reaction. The Tafel plot of E vs. log [ILI/(IL I)] for oxygen reduction catalyzed by the GC/PPy/AQDS electrode at different rotation rates were all quite linear with the slopes between 78 and 86 mV dec1. This is indicative of a redox catalytic process [7,19]. 3.5. Stability of catalytic activity for GC/PPy/AQDS electrode The change in oxygen reduction current at the GC/PPy/ AQDS electrode with time was also examined. During the study oxygen was bubbled continuously and the potential was fixed in the diffusion limited ranges. As shown in Fig. 8, the current at the modified electrode decreases rapidly, but subsequent decrease is slight. The degradation of the electrocatalytic activity of the modified electrode may be attributed to the degradation of PPy film and the loss of part of its electronic conductivity in the presence of hydrogen peroxide produced by the two-electron transfer reaction. 4. Conclusions The redox reactions of AQDS both in PPy matrix and at the surface of GC/PPy show a quasi-reversible behavior and exhibit pH dependence. The ionization constant, pKa,
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