Electrochemical Characteristics and Stability of Poly(1,5

Oct 25, 2007 - Minqiang Sun , Han Li , Jian Wang , Gengchao Wang ... Minqiang Sun , Gengchao Wang , Chongyang Yang , Hao Jiang , Chunzhong Li...
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17268

J. Phys. Chem. C 2007, 111, 17268-17274

Electrochemical Characteristics and Stability of Poly(1,5-diaminoanthraquinone) in Acidic Aqueous Solution Mingming Gao, Fenglin Yang,* Xinhua Wang, Guoquan Zhang, and Lifen Liu School of EnVironmental and Biological Science and Technology, Dalian UniVersity of Technology, Dalian 116024, China ReceiVed: June 7, 2007; In Final Form: August 20, 2007

The electrochemical characteristics and stability of poly(1,5-diaminoanthraquinone) (P15DAAQ) has been investigated in different acidic aqueous solution by electrochemical methods and Fourier transform infrared spectroscopy (FTIR). Cyclic voltammetry (CV) analysis suggests that electropolymerization consists of two phasessthe deposition of P15DAAQ from the oxidation of 15DAAQ monomers and the polymer growth process. The CV and chronoamperometry (CA) data provide the evidence that ion transfers are different between oxidation and reduction processes and are profoundly influenced by the sizes of ion radius during the redox process of P15DAAQ in acid solution. The H+ transfer is diffusionless, while the insertion/expulsion of large aqua anions is controlled by diffusion. A model is proposed to describe these ion transfer processes. In further CA research, the calculated diffusion coefficients of aqua anions in P15DAAQ are in the order of 3Cl- > SO24 > NO3 > PO4 . As a result of stability research, it can be seen that degradation of the polymer exists not only in overoxidation but also in the stable redox reaction. The degradation rates of P15DAAQ in the reversible potential region from -0.2 to 0.75 V obey the apparent first-order kinetic, and the degradation rate constants are 2.04, 4.93, 2.59, and 3.03 × 10-5 s-1 in HCl, H2SO4, HNO3, and H3PO4, respectively. According to FTIR and CV, the quinone-like structure is destroyed, accompanied with conjugation length of the polymer’s π-bond being decreased and chains in the polymer being broken and recombined with anions combined to the polymer, when P15DAAQ is potentiodynamically overoxidized at 1.5 V.

1. Introduction The electrodes coated by aminoanthraquinone polymer film can potentially serve as a new type of powerful polymer for many redox reactions. Electropolymerization of poly(1,5diaminoanthraquinone) (P15DAAQ) was first reported in 1999,1 and P15DAAQ exhibited high specific capacity.1-3 Similar to many other aminoanthraquinones, 1,2-DAAQ, 1,4-DAAQ,4 and 1-AAQ5,6 formed polymers that were subjected to electropolymerization by anodic oxidization. As compared with the other oxidative polymerization, one of the greatest outcomes of electrooxidative polymerization is that ultrathin, homogeneous, and dense polymer film can be easily prepared from aromatic diamine monomers. The 15DAAQ oligomer was in the category of “supramolecule” with π-π stacked structures that were rarely observed for the conventional conducting polymers.7 The electrochemical reduction of oxygen on surface-confined quinones has been extensively investigated.8-13 In principle, all of the aromatic diamines can be oxidized into polymers. However, there are limited references concerning the properties of electrochemically oxidative polymers from the aromatic diamines, despite relatively more work having already been done in electropolymerization of the aromatic diamines. To understand the functional properties of conducting polymers, it is necessary to analyze transport processes inside the polymer, charge transfers at the different phase boundaries (substratepolymer and polymer-electrolyte), and reloading processes of the polymer.14 In fact, the application of conducting polymers requires knowledge about redox properties and stability in * Corresponding author. Tel.: +86 411 84706172. Fax: +86 411 84708084. E-mail: [email protected].

reversible potential range. Moreover, any attempts to utilize the new materials for supercapacitor or as electrocatalyst material must take irreversible reactions into consideration.14-20 Electronic charge transfer at moderate potentials is due to the formation of radical cations, which further oxidize or recombine to form dications as the potential increases.20 Even though there is a stable potential window available for the conducting polymer, the anodic overoxidation process can reach deep into the zone of reversible doping/undoping. The aim of this work is to study the redox behavior and stability of a P15DAAQ film in different acidic aqueous solution by cyclic voltammetry (CV), chronoamperometry (CA), and ex situ Fourier transform infrared spectroscopy (FTIR). In more detail, this article is oriented toward a study of ion transfers during redox reaction and degradation processes in both reversible and irreversible potential ranges. In addition, the mechanism for electropolymerization has been investigated and discussed. 2. Experimental Methods A potentiostat/galvanostat model 263A from Princeton Applied Research under the control of a PC class computer was used for electropolymerization, cyclic voltammetry, and chronoamperometry studies. Cycle voltammetry experiments were performed in a conventional three-electrode one-compartment electrochemical cell without stirring. Two Pt electrodes of area 0.8 and 3.0 cm2 and a saturated calomel electrode (SCE) were used as working electrode, counter electrode, and reference electrode, respectively. All potential values are reported against SCE.

10.1021/jp074415j CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

P15DAAQ in Acidic Aqueous Solution

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17269 literature with a slight modification.1 The surface concentration of 15DAAQ monomer (ΓT) was calculated from21

ΓT )

Figure 1. CVs of P15DAAQ deposited on Pt between -0.3 and +1.8 V in an electrolyte of PC with 10 mM 15DAAQ + 0.1 M TBAClO4 + 0.45 M CF3COOH at 10 ( 0.1 ×bcC. Scan rate: 20 mV s-1.

Electrochemical deposition of P15DAAQ on a Pt electrodewith an area of 0.8 cm2 was carried out by potential cycling between -0.3 and +1.8 V in an electrolyte of propylene carbonate (PC) with 10 mM 1,5-diamino-1,9-anthraquinone (15DAAQ) (Aldrich) + 0.1 M tetrabutylammonium perchlorate (TBAClO4) (Fluka) + 0.45 M CF3COOH at a sweep rate of 20 mV s-1 at 10 ( 0.2 °C. 15DAAQ and TBAClO4 were used as received. All other chemicals were analytical grade. The electrode was cycled repeatedly. Other measuring experiments were carried out at 20 ( 2 °C. The P15DAAQ-coated electrodes were washed with deionized water, ethanol, and deionized water to remove organic chemicals before being used. They were kept at -0.2 V for 2 min before moving the electrodes from one kind of acid aqueous solution to another, in order to expel the doping anions from the polymer. The electrodes used for the stability study were all newly modified. Ex situ FTIR spectra of the samples were recorded on an IR Prestige-21 spectrophotometer (Japan, SHIMADZU) with the KBr-wafer method employed. 3. Results and Discussion 3.1. Electropolymerization of P15DAAQ. P15DAAQ was synthesized by CV according to a procedure described in the SCHEME 1: Electropolymerization Process of 15DAAQ

Q nAF

(1)

where Q is the charge from the area under the cathodic peak of the last cycle corrected for the baseline, A is the electrode area, and F is the Faraday constant; n, the number of electrons exchanged per reactant molecule, is two. According to these values, ΓT is calculated to be around (9-20) × 10-8 mol cm-2. The cyclic voltammograms (CVs) during the electropolymerization of 15DAAQ on Pt electrodes is shown in Figure 1. An evident irreversible anodic peak corresponding to oxidation of the 15DAAQ monomer is observed at 1.0-1.8 V in the first four cycles. No cathodic peak corresponding to the irreversible anodic peaks indicates a fast consumption of the electrogenerated monocation radicals and dications by followup chemical reactions to form electroactive P15DAAQ films on the electrode. From the second cycle, new semireversible anodic and cathodic peaks related to redox behavior of P15DAAQ on the Pt electrode appeared at 0.5-1.2 V, shifted in a more positive and negative direction, respectively, and increased steadily in height as the scan continued. These semireversible redox peaks confirmed the formation and increase of P15DAAQ on the electrode. The distinction between cycles 1-4 and cycles 5-16 in Figure 1 shows that the electropolymerization of 15DAAQ consists of two phasessthe deposition of P15DAAQ from the oxidation of 15DAAQ monomers and the polymer growth process, which is similar to that of electropolymerization of polyaniline22 and 1,5-diaminonaphthalene.23 Meanwhile, the oxidation peak at 1.0-1.8 V weakened from cycle 5 gradually and finally disappears, indicating that the polymer growth process grows predominant. The electropolymerization mechanisms generally involve many steps and are very complicated in nature. The polymer is generated by a succession of coupling reactions involving radical cations following an ECE-type mechanism.7,23,24 It has been accepted that 15DAAQ could form ladder polymers by electrooxidative polymerization with both amino groups oxidized, and the bonding manner of the P15DAAQ was found to be mainly in the 1,4 and 5,8 position.1,2,7 However, despite these previous studies together with CV in this study providing much information during growth of P15DAAQ, the initial deposition

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Figure 2. CVs of P15DAAQ-modified Pt electrode with different scan rates in oxygen-free 0.5 M H2SO4 solution (a), the plot of the anodic peak currents as a function of scan rates (b), and cathodic peak currents as a function of the square root of scan rates (c). The scan rates are 10, 20, 50, 100, 150, and 200 mV s-1 from inside to outside, respectively.

step and the process of propagation of electropolyerization are not clearly understood. We proposed that the electrooxidation of 15DAAQ leading polymerization follows a four-electrontransfer process which is simply shown in Scheme 1. 3.2. Redox Process in Acidic Aqueous Solution. Suematsu et al. have analyzed redox peaks of P15DAAQ in 4 M H2SO4 at potentials between 0.45 and 0.75 V by an in situ quartz crystal microbalance (QCM).2 They found that in the potential range from 0.45 to 0.65 V, the DAAQ oligomer exchanged two electrons by quinine/hydroquinone (Q/HQ) redox reactions, while two protons with hydrated water were exchanged. In the potential range from 0.65 to 0.75 V, the redox of the π-conjugated system (emelardine/quinone diimine) appeared to exchange one electron and one anion hydrated with two molecules of water. However, our electrochemical research by CV and CA about the redox process of P15DAAQ on Pt electrodes in different acid solutions found that ion transfers are different between oxidation and reduction processes and are profoundly influenced by the sizes of ion radii. CVs of P15DAAQ-modified Pt electrodes at scan rates 10, 20, 50, 100, 150, and 200 mV s-1 in O2-free 0.5 M H2SO4 are shown in Figure 2. At 0.450.65 V, anodic peaks currents iPA are linearly proportional to the scan rates V (Figure 2b), implying the anodic peak corresponding to the HQ/Q reaction is diffusionless. The following step for insertion of anions due to the formation of radical cations at 0.65-0.75 V has no evident anodic peaks. The reduction process in a wider potential range from 0.3 to 0.75 V is relatively complex. Figure 2c shows that cathodic peak currents iPC are fit well to a quadratic polynomial of the square root of scan rate V1/2. The quadratic polynomial is iPC ) 0.739 - 0.312 V1/2 - 0.012 V. The effect of scan rate V on iPC is composed of two parts, linearly proportional to the scan rate V and the square root of scan rate V1/2, suggesting that this cathodic peak can be attributed to the processes of the Q/HQ reaction and the expulsion of SO24 . The Q/HQ reaction during the reduction process is also diffusionless, while the expulsion of anions is in agreement with the semi-infinite diffusion law. A model of the redox process of P15DAAQ on Pt electrodes in acid solution is proposed in Figure 3 based on CV. It is assumed that small cations, that is, H+, can insert freely into the polymer, which makes the redox process corresponding to the Q/HQ reaction diffusionless. But the large ion size of aqua anions results in their transfer in the polymer more difficult. Thus, the insertion and expulsion of anions in the redox

Figure 3. Model of the redox process of P15DAAQ on a Pt electrode in acidic aqueous solution. Circle with “+”: H+; circle with “-”: aqua anion.

process are controlled by diffusion, which is consistent with electrochemical behavior of polypyrrole in aqueous solution.25 This model can be used to interpret the redox behaviors of P15DAAQ in acidic solution. CVs of P15DAAQ-modified Pt electrodes in different acid solutions are compared in Figures4 and 5. As the concentrations of H2SO4 change from 0.5 to 4 M, the potentials of anodic peaks shift in a more positive potential direction (from 0.55, 0.56, and 0.58 V to 0.63 V) with current enhancing (Figure 4). This means the expulsion of cations (H+) during the oxidation process is more difficult in dense solution (similar to results reported by Tian et al. earlier)17 and also confirms that the anodic peaks at 0.45-0.65 V are ascribed to the expulsion of H+. However, the influence of concentration of sulfuric acid on cathodic peaks is not evident, especially in 0.5, 1, and 2 M H2SO4. This may be due to the co-effect of anion expulsion and cation insertion during the reduction process. It can be concluded that the influence of anion species on the cathodic peak potential is dominant, although H+ concentrations vary with the kinds of acid. Therefore, when P15DAAQ-modified Pt electrodes are scanned in HCl, H2SO4, HNO3, and H3PO4 with the same concentration of 0.5 M (Figure 5), the variety of cathodic peak

P15DAAQ in Acidic Aqueous Solution

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17271

Figure 4. CVs of P15DAAQ-modified Pt electrode in oxygen-free H2SO4 solution of 0.5, 1, 2, and 4 M from inside to outside, respectively. Scan rate: 50 mV s-1.

Figure 6. Current i vs t-1/2 obtained by CA at the P15DAAQ-modified Pt electrode in different kinds of acidic solution (0.5 M, oxygen-free): a: HCl; b: H2SO4; c: HNO3; d: H3PO4. Potential-step at 0-0.7 V.

The chronoamperometric behavior of P15DAAQ-modified Pt electrodes was studied in different acidic solutions of the same concentration (0.5 M) with potential-step at 0-0.7 V. In 0.5 M HCl, H2SO4, HNO3, and H3PO4, the Cottrell plot of current i versus t-1/2 shows a linearity within a shorter time, while it deviates from linearity at a longer time (Figure 6). This result reveals that within a shorter time, the mass transfer of anions is in agreement with the semi-infinite diffusion law, but within a longer period of time it presents the characteristic of the finite diffusion process in thin films. Based on Cottrell equation of semi-infinite diffusion:

i) Figure 5. CVs of P15DAAQ-modified Pt electrode in 0.5 M different kinds of acid solution (oxygen-free): a: HCl; b: H2SO4; c: HNO3; d: H3PO4. Scan rate: 50 mV s-1.

TABLE 1: Cathodic Peak Potentials, Diffusion Rate, and First-Order Electrochemical Degradation Rate Constant of P15DAAQ Film on Pt Electrodes in Different Acidic Aqueous Solution

n2FAD1/2c

Epca (V)

Db (10-8 cm2 s-1)

kc (10-5 s-1)

HCl H2SO4 HNO3 H3PO4

0.69 067 0.63 0.60

3.4 2.4 1.9 0.14

2.46 4.93 2.46 2.85

a Obtained from CVs in Figure 5. b Calculated values of D from the slope values of Figure 6. c Calculated values of k from the slope values of Figure 8.

potentials in the CVs is mainly due to differences in the anion species. Cathodic peak potentials Epc obtained from Figure 5 are listed in Table 1. Results showed that the potentials of cathodic peaks move toward more positive potential values with the decreasing of radii of aqua anions, implying that the expulsion of anions becomes harder with the increasing of anion size. As a matter of fact, anions present in sulfuric acid solution 32are in the forms of SO24 and HSO4 , while PO4 , HPO4 , and H2PO4 simultaneously exist in phosphoric acid solution. Radii 3of SO24 and PO4 anions could only reflect the mean values. Their effects on the redox process depend on the mean radius values.

(2)

where D is the diffusion coefficient of anions, n2 is the number of electrons exchanged per reactant molecule and has a value of one,2 and c is the concentration of anions in solution (5 × 10-4 mol cm-3).26,27 The slopes (K) of the linear part of the plots of i vs t-1/2 reveal the diffusion rate of anions:

D)

acidic aqueous solution (0.5 M)

t1/2π1/2

( ) π1/2K n2FAc

2

(3)

The calculated values of D are summarized in Table 1. The same as the result obtained by CV in Figure 5, the diffusion coefficients of aqua anions are in the order of Cl- > SO24 > NO3- > PO34 . These phenomena act in accord with the model in Figure 3 that insertion and expulsion of the anions during the redox process are profoundly influenced by the size of anions. 3.3. Redox Stability and Overoxidation. As usual, the process of electrochemical reaction is accompanied by the movement of charge-compensating anions and solvent molecules within the conducting polymer film, and possible conformational changes of polymer structure as well.28 In this study, the anodic and cathodic peaks decreased slightly when the electrode coated with P15DAAQ was under a continuous scan at the reversible doping/undoping redox potential from -0.2 to 0.75 V (data not shown), which showed that P15DAAQ on Pt electrodes was comparatively stable. Similar results has been reported by Naoi et al. earlier.3 The charge of oxidation process (QO) at the potential range from 0.3 to 0.75 V, which is recorded during the CV scan, is adopted to indicate the polymer structure available for redox reaction. Figure 7 shows the effect of scan

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Figure 7. Charge QO/C of anodic peaks vs cycle (N) of potentiodynamic scan between -0.2 and +0.75 V at the P15DAAQ-modified Pt electrode in 0.5 M HCl, H2SO4, HNO3, and H3PO4 (oxygen-free). Scan rate: 50 mV s-1.

Figure 9. CVs of P15DAAQ-modified Pt electrode in H2SO4 0.5 M (oxygen-free) at a scan rate 50 mV s-1. Potential ranges are (A) -0.2 to 0.75 V, (B) -0.2 to 1.5 V for 2 cycles, and then (C) -0.2 to 0.75 V. Peak a is the oxidation peak corresponding to Q/HQ, and peak b is an irreversible overoxidation peak.

Figure 8. ln(QO) vs t of P15DAAQ-modified Pt electrode in 0.5 M HCl, H2SO4, HNO3, and H3PO4 (oxygen-free). Scan rate: 50 mV s-1.

Figure 10. CVs of P15DAAQ-modified Pt electrode in HCl 0.5 M (oxygen-free) at a scan rate 50 mV s-1 for 3 cycles, potential range is -0.2 to 1.5 V.

cycles on QO during continuous scan of 90 cycles in 0.5 M HCl, HNO3, H3PO4, and H2SO4 (O2-free). The values of QO varied markedly to reach a balance of ion transfer in the first ten cycles and decreased smoothly in following cycles due to the electrochemical degradation of P15DAAQ. Figure 8 shows the value of ln(QO) as a function of scan time (t) in different acid solutions. The degradation of P15DAAQ obeys the apparent first-order kinetic:

ln(QO) ) ln(a) - kt

(4)

where t is the reaction time and the beginning of the first cycle is recorded as 0 s, a is an empirical coefficient, and k is the first-order degradation rate constant. QO values used for calculation are from the tenth cycle, and the calculated values of k in different acid aqueous solutions are given in Table 1. In fact, the rate of polymer degradation depends on the potential applied, being greater for higher potential values.29 The value of k is only a mean rate constant within the reversible doping/undoping redox potential.30 There is no evident relation between degradation rate constant and anion radius. Also, by holding the modified electrode for some time at a controlled potential in a supporting electrolyte, similar changes in CVs are observed. P15DAAQ was potentiodynamically overoxidized at a potential positive to the reversible doping/undoping redox potential

of 0.75-1.5 V as shown in Figure 9. Two large irreversible oxidation peaks in Figure 9B, a and b, were observed at 0.40.65 V and 0.65-1.1 V at the first cycle, which, respectively, corresponded to oxidation and overoxidation of P15DAAQmodified electrodes in 0.5 M H2SO4. Anodic peaks at 0.40.65 V vanished after anodic overoxidation, but the peaks at 0.65-0.9 V still existed, although the current of anodic and cathodic peaks decreased. This result points to an incomplete degradation of the polymer. Redox activity of the quinone-like structure is destroyed, but the ability of doping/undoping of aqua anions in contact with the compact surface and amido unit partly persists after overoxidation. Similar phenomena were also mentioned in previous studies.15,16 The cathodic peak shifting to negative potential (from 0.68 to 0.15 V) as compare in Figure 9A with B, indicates that it becomes more difficult to reduce the film into its conductive state. This may be an effect of a decrease in the conjugation length of the polymer’s π-bond occurring as a result of overoxidation.13 Similar results also existed in phosphoric acid and nitric acid. But in hydrochloric acid, Cl- can be oxidized to Cl2.30 As shown in Figure 10, a big oxidation peak emerged at 1.2-1.5 V, which means the oxidation of Cl-. Meanwhile, generated Cl2 induced the quick loss of activity of P15DAAQ because of its strong oxidation. The polymer turned from black to dark red with the electroactivity completely vanishing after overoxidation in 0.5 M HCl

P15DAAQ in Acidic Aqueous Solution

Figure 11. FTIR spectra of 15DAAQ monomer (a), P15DAAQ (b), and P15DAAQ after overoxidation in 0.5 M H2SO4 (c).

solution. The following discussion about overoxidation degradation does not include the situation in HCl. It is assumed that the structure of P15DAAQ is overoxidized by nucleophilic attack of nucleophiles such as aqua anion and H2O at a more positive potential. Actually, there is no sharp borderline between reversible and overoxidation regions, and overoxidation reaches deep into the zone of reversible doping/ undoping.20 Overoxidation seems like an acceleration of degradation occurring in the reversible potential range. The attack of nucleophiles is continuous even in an open circuit, because the same CVs as in Figure 9C can be obtained after immersing P15DAAQ-modified electrodes in aqueous solution for 24 h. FTIR spectra of 15DAAQ, P15DAAQ, and P15DAAQ overoxidized in 0.5 M H2SO4 are compared in Figure 11. Almost all absorption bands of P15DAAQ are broader than those of 15DAAQ, simply due to the occurrence of the polymerization. This is consistent with the situation that happened in the polymerization of 1,8-diaminonaphthalene.31 Two bands at 3430 and 3310 cm-1 due to asymmetric and symmetric -NH2 SCHEME 2: Potentiodynamic Overoxidation of P15DAAQ

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17273 stretching vibrations, respectively, of monomeric 15DAAQ merge into one broad band centered at 3410 cm-1 of P15DAAQ. A small peak at 3060 cm-1 of 15DAAQ shifts to a lower wave number at 2930 cm-1 due to the C-H stretching vibration on the anthraquinone unit in the P15DAAQ. Particularly, P15DAAQ after overoxidation exhibits quite a different band shape in 1630-400 cm-1. Bands at 1620 and 1550 cm-1 corresponding to CdO stretching vibrations of monomeric 15DAAQ still remain in P15DAAQ, but they are weak and inexplicit when P15DAAQ is overoxidized. This means the CdO structure in P15DAAQ is destroyed after overoxidation. Two additional bands at 1170 and 1060 cm-1 can be easily attributed to the symmetric and asymmetric stretching vibrations of the SO24 group, respectively. This suggests that anions have combined to the polymer. Bands emerge at 1010-400 cm-1 in Figure 11c may contribute to the aromatic structure because of the breakage of chains in the polymer by overoxidation, such as a band at about 880 cm-1 ascribed to the CdC- in the aromatic structure. On the basis of the analysis by electrochemical methods together with FTIR and previous research,2,20 a three-step process is proposed for potentiodynamic overoxidation of P15DAAQ (Scheme 2). P15DAAQ undergoes two oxidation processes at the first two steps, then is overoxidized by nucleophilic attack of nucleophiles Nu such as the aqua anion and H2O at a more positive potential at the last step.19,20 From the charge recorded during the CVs in Figure 9B, charges for the QH/Q reaction (QQ/QH) at 0.4-0.65 V, for insertion of anions (QI) at 0.65-0.75 V, and for overoxidation (QOO) at 0.75-1.1 V, are 0.008, 0.005, and 0.023 C, respectively. The proportion of electrons lost during these three steps is supposed to be equal to QQ/QH/QI/QOO, which is close to 2:1:5. X in Scheme 2 is calculated to be about 2 from the proportion between the charge obtained during electropolymerization Q and QOO. It means that nearly 10 electrons are lost per 4 15DAAQ monomers during the overoxidation process. Structure of overoxidized 15DAAQ monomers M is not clearly understood, since the degradation process is a sum of several parallel reactions and depends on numerous variables, some of which may be elusive and immeasurable.12 Whereas, in the third step, it can be concluded

17274 J. Phys. Chem. C, Vol. 111, No. 46, 2007 that quinone-like structure is destroyed, accompanied with conjugation length of the polymer’s π-bond being decreased and chains in polymer being broken and recombined with anions combined to the polymer.

Gao et al. in the November 22, 2007 issue (Vol. 111, No. 46, pp 1726817274). An Addition and Correction appears in the March 13, 2008 issue (Vol. 112, No. 10). References and Notes

4. Conclusions CVs recorded during electropolymerization of 15DAAQ have reflected that this process consists of two phasessthe deposition of P15DAAQ from the oxidation of 15DAAQ monomers and the polymer growth process. We have shown the four-electrontransfer process of electropolymerization. CV and CA are used to research the redox process of P15DAAQ on Pt electrodes in acidic solution; meanwhile a model has been proposed about it. Ion transfer in the polymer is different between oxidation and reduction processes. Moreover, the redox process of Q/HQ corresponding to small cations H+ is diffusionless, while the insertion/expulsion of large aqua anions is controlled by diffusion. So the diffusion rate of anions is profoundly influenced by the size of anions. On the basis of the Cottrell equation of semi-infinite diffusion, the diffusion coefficients of aqua anions in P15DAAQ are in the order of Cl- > SO24 > . NO3- > PO34 Electrochemical stability has been studied by CV and FTIR. As the result shows, degradation of the polymer existed not only in overoxidation, but also in the stable redox reaction. The degradation of P15DAAQ during the stable redox reaction obeys the apparent first-order kinetic, and the degradation constants obtained by the linear fitting of ln(Q) vs t are (2.46, 4.93, 2.46, and 2.85) × 10-5 s-1 in HCl, H2SO4, HNO3, and H3PO4, respectively. It can be concluded that the stabilization of P15DAAQ has to be improved for application in aqueous solution. FTIR and CVs have been used to analyze the overoxidation process of P15DAAQ. Results of analysis point to an incomplete degradation of the polymer except in HCl. Also, it has been found that the quinone-like structure was destroyed, accompanied with conjugation length of the polymer’s π-bond being decreased and chains in the polymer being broken and recombined with anions combined to the polymer, when P15DAAQ was potentiodynamically overoxidized in 0.5 M H2SO4. Note Added after Print Publication. During production the corresponding author was incorrectly changed. The correct corresponding author is Fenglin Yang. This manuscript originally published on the Web on October 25, 2007 and in print

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