Electrochemical oxidation of hydrogen peroxide at a bromine adatom-modified gold electrode in alkaline media

Electrochimica Acta 54 (2009) 1570–1577

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Electrochemical oxidation of hydrogen peroxide at a bromine adatom-modified gold electrode in alkaline media Md. Rezwan Miah 1 , Takeo Ohsaka ∗ Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagastuta, Midori-ku, Yokohama 226-8502, Japan

a r t i c l e

i n f o

Article history: Received 6 June 2008 Received in revised form 11 September 2008 Accepted 18 September 2008 Available online 30 September 2008 Keywords: Br(ads) -submonolayer Br(ads) -submonolayer-coated Au (poly) electrode Alkaline media H2 O2 oxidation reaction Electrostatic attraction

a b s t r a c t Bromine (Br)-adatom (Br(ads) ) was in situ fabricated onto polycrystalline gold (Au (poly)) electrode in Br− containing alkaline media. The surface coverage of Br(ads) ( Br ) varied only in the submonolayer coverage within the investigated potential window under potentiodynamic condition because of the coadsorption of hydroxyl ion (OH− ) in alkaline media. The in situ fabricated Br(ads) -submonolayer-coated Au (poly) electrode was successfully used for the electrochemical oxidation of hydrogen peroxide (H2 O2 ). About five times higher oxidation current was achieved at the modified electrode as compared with the bare electrode. The enhancement of the electrode activity towards the electrochemical oxidation of H2 O2 was explained based on the enhanced electrostatic attraction between the anionic HO2 − molecules and Br(ads) adlayer-induced positively polarized Au (poly) electrode surface. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Detection of hydrogen peroxide (H2 O2 ) takes a great part in the reported amperometric biosensors as H2 O2 is not only an important analyte in many fields, including industry, clinical medicine and the environment, but also important in fabricating biosensors for various substances by combining it with H2 O2 -producing oxidases such as glucose oxidase, l-lactate oxidase, xanthine oxidase, horseradish peroxidase and carbonic anhydrase, etc. [1–5]. However, the direct electrochemistry of H2 O2 requires a higher over-potential. Chemical modification of the electrode surface is a well-established strategy for achieving wider applicability of the electroanalytical methodology. Accordingly, numerous efforts have been dedicated to fabricate suitable electrode materials for the sensitive detection of H2 O2 . Electrochemical sensing of H2 O2 has been done based on both of its reduction [6–13] and oxidation [14–22] reactions. However, the electrooxidation of H2 O2 often requires very high over-potential, particularly at the unmodified

∗ Corresponding author. Tel.: +81 45 9245404; fax: +81 45 9245489. E-mail addresses: [email protected] (Md.R. Miah), [email protected] (T. Ohsaka). 1 On leave from the Department of Chemistry, School of Physical Sciences, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.09.041

traditional electrode materials. The high working potentials may cause undesired results: (i) the coexisting electroactive substances can substantially interfere with the H2 O2 oxidation [22], (ii) the large base-current can result in an uncertainty in the estimation of H2 O2 oxidation current [22], and (iii) the electrode materials can undergo a rapid oxidative degradation as H2 O2 is a strong oxidizing agent [6,7]. Modification of the electrode surface is, therefore, essentially required to reduce the over-potential of H2 O2 oxidation. Several strategies have been used to reduce the over-potential of H2 O2 oxidation reaction. One strategy is to use the redox mediators that help to shuttle electrons between the electrode surface and H2 O2 molecules [23,24]. Another approach is to make use of the electrocatalytic effect where a difficult/kinetically slow redox process is facilitated by electrode modification. In recent years, there has been a surge of interest in the study of electrocatalytic H2 O2 oxidation by inorganic/organic-film coated electrodes [25–30]. For instance, Khoo et al. [25] have reported the catalytic ability of a series of oxymetallic films of several elements such as Co(II), Fe(II), Ni(II), Pb(II), Ce(III), Cr(III), Tl(I) and Mn(III) on glassy carbon electrodes for the electrocatalytic oxidation of H2 O2 . Wang and Angnes have described the electrocatalytic behavior of carbon–fiber electrodes, modified by electrochemical codeposition of rhodium and glucose oxidase, towards the electrochemical oxidation of H2 O2 and sensing of glucose [26]. A micro-biosensor was constructed by Li et al. [27] by incorporating the organic con-

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ducting salt (tetrathiafulvalene–tetracyanoquinodimethane) into a platinized platinum wire and glucose was sensitively determined at a low working potential based on the oxidation of H2 O2 . Halides are the hitherto most extensively studied class of specifically adsorbed anions because of the structural simplicity of these monatomic, monovalent anions as well as their abundance in the natural and technological environments. I− adsorption, compared with other halides, has so far been studied most extensively because of its highest adsorption affinity [31]. Smooth single crystalline metallic electrodes and background electrolytes of acidic pH have received much scientific interest in the arena of halide adsorption as compared with the polycrystalline rough metallic electrodes and electrolytes of higher pH [31–41]. This is probably due to the fact that the voltammetric features of adsorption/desorption of halides appear as well-defined voltammetric peaks at the single crystalline electrodes and the acidic electrolytes do not compete with the adsorption/desorption of the halides, especially with I− and Br− . In our laboratory, we have been increasingly interested to evaluate the effect of chemisorbed halogen (X(ads) )-adlayers, especially that of iodine (I(ads) ) and bromine (Br(ads) ) at the various metallic electrodes, on the different electrochemical reactions in alkaline media. For instance, we recently have reported that I(ads) -modified polycrystalline gold (Au (poly)) electrode shows a remarkably high activity towards the electrochemical reduction [6,7] and oxidation [42] of H2 O2 and oxidation of uric acid [43]. In the present report, we address a new approach to the electrocatalytic oxidation of H2 O2 at the Au (poly) electrode in bromide (Br− )-containing alkaline media based on the voltammetric and amperometric techniques. A tentative mechanism of the enhanced electrooxidation of H2 O2 in the presence of Br− is also addressed. 2. Experimental 2.1. Chemicals Sodium hydroxide (NaOH, 97%) (Kanto Chemical Co. Inc., Japan) and potassium bromide (KBr, 99.5%), hydrogen peroxide (H2 O2 , 35.5%) and sulfuric acid (H2 SO4 ) (Wako Pure Chemical Industries, Japan) were purchased and used as received. Milli-Q water was used as solvent. The solutions of NaOH and H2 O2 were prepared by dissolving the required amount of the reagents. The solution was purged with N2 gas (Nippon Sanso Co. Inc., Japan) before measurements. 2.2. Apparatus and procedures For cyclic voltammetric measurements, Au (poly) electrodes ( = 1.6 mm sealed in a Teflon jacket) with an exposed surface area of 2.01 × 10−2 cm2 were used as working electrodes. A spiral Pt wire and a Ag|AgCl|NaCl (sat.) were the counter and reference electrodes, respectively. A conventional two-compartment Pyrex glass cell was used. Prior to measurements, N2 gas was bubbled into the cell for 30 min to obtain a N2 -saturated NaOH solution. The necessary amount of H2 O2 was added into the solution. All the measurements were performed at 25 ± 1 ◦ C. The Au (poly) electrodes were polished with aqueous slurries of successively finer alumina powder (down to 0.06 ␮m), sonicated for 10 min in Milli-Q water and then electrochemically pretreated in 0.05 M H2 SO4 solution by repeating the potential scan in the range of −0.2 to 1.5 V vs. Ag|AgCl|NaCl (sat.) at 0.1 V s−1 for 10 min or until the cyclic voltammetric characteristic for a clean Au (poly) electrode was obtained. The real surface area was obtained from the charge consumed during the formation of the surface oxide monolayer, considering 482 ␮C charge consumed for the oxide monolayer formation on

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1 cm2 surface of the Au (poly) electrode [44]. The roughness factor (rf), which is defined as the ratio of the real surface area to the geometric surface area of the electrode, was calculated as 1.2. The current density was obtained using the geometric surface area of the electrode. Electrochemical measurements were performed using an ALS CHI-832A electrochemical analyzer. Chemisorption of Br− at the electrode was ensured by adding 5 mM KBr into an aqueous solution of 0.1 M NaOH. Experiments were always performed in the presence of added KBr unless otherwise noted. 3. Results and discussion 3.1. Adsorption/desorption of Br− at the Au (poly) electrode Before proceeding with Br− adsorption, the possibility of the chemical/electrochemical reaction of Br− in alkaline solution in the presence of H2 O2 was first clarified. Br− (or I− ) undergoes chemical oxidation by H2 O2 to produce Br2 (or I2 ) according to reaction (1) [45,46]: 2Br− (I− ) + 2H+ + H2 O2 = Br2 (I2 ) + 2H2 O

(1)

The reaction rate for Br− is slower than that for I− . The reaction requires strong acidic media. The rate of the reaction decreases drastically with increasing pH of the solution. Therefore, in highly alkaline solution (like 0.1 M NaOH which has been used in the present study) there is actually no possibility of the reaction. In our previous reports [6,7], it has been demonstrated that a highly reproducible voltammetric response of H2 O2 to its reduction can be obtained in I− -containing 0.1 M KOH solution when the solution is permitted to stand for a while (e.g. 2 h), suggesting that the reaction (1) is not feasible in alkaline media. The electrochemical oxidation of Br− to Br2 requires higher potential than that of I− to I2 [47,48] and does not take place within the potential range used in this study. Fig. 1 shows the cyclic voltammograms (CVs) obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing (a) 0 and (b) 5.0 mM KBr. In the case of the bare electrode, the upper potential was chosen at 0.2 V to avoid the formation of the surface oxide. The CV obtained in 0.1 M NaOH solution in the absence of Br− (CV a) shows three distinct features, namely the (i) so-called double layer charging, associated to the constant current zone at E < −0.55 V, (ii) potential-induced reversible adsorption/desorption of OH− , related to the couple of anodic and cathodic peaks at ca. −0.22 and −0.19 V, respectively [49] and (iii) irreversible partial oxidation of specifically adsorbed OH− to generate Au(OH)ads species, associated to the uptake of the anodic current at potential more positive than ca. 0.1 V [49]. Modifications of the voltammetric features of the Au (poly) electrode by the addition of 5.0 mM KBr in the solution are shown in Fig. 1A (b). Clearly, the presence of Br− generates quite significant changes in the voltammetric curve obtained in 0.1 M NaOH: (i) the double layer zone is shifted to more negative potential, that is, below −0.66 V, (ii) two pairs of well-defined reversible couples of anodic and cathodic peaks appear at potentials of ca. 0.08 V (designated as PI and PI ) and −0.29 V (designated as PII and PII ) and (iii) the uptake of the anodic current at E > 0.1 V is decreased. The changes in the voltammetric features in the presence of Br− are obviously associated to the adsorption/desorption of Br− . The voltammetric features in the presence of Br− are considerably similar to those obtained by Markovic et al. in their study of the adsorption/desorption of Br− at the Au (1 0 0) electrode in 0.1 M NaOH solution, except that the voltammetric peaks are sharper in their case [41]. Both of the cathodic peaks (PI and PII ) are assigned to the desorption of the Br(ads) , while both of the anodic peaks (PI and PII) are assigned to the adsorption of Br− . In this case, the

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Fig. 1. (A) CVs obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing (a) 0 and (b) 5.0 mM KBr. The symbol (*) represents the initial potential in both cases (a and b) and the upper potentials are (a) 0.2 and (b) 0.3 V. Potential scan rate: 0.1 V s−1 . (B) CVs obtained at various scan rates: (a) 0.05, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4 and (f) 0.5 V s−1 (from inner to outer) keeping other conditions as the same as those of Fig. 1A (b). Inset shows the change of Eocp of the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution upon the injection of KBr (the final concentration of KBr is 5.0 mM) into the solution.

two cathodic (and anodic) peaks result from the different binding strength of the Br(ads) to the different facets, namely, Au (1 1 1), Au (1 0 0) and Au (1 1 0) of the Au (poly) electrode. Previous report by our group on the desorption of self-assembled monolayer of mercaptoacetic acid (MAA) using the single crystalline Au (1 1 1), Au (1 0 0) and Au (1 1 0) electrodes in 0.5 M KOH solution showed that the binding strength of MAA follows the sequence Au (1 1 1) < Au (1 0 0) < Au (1 1 0) [50]. Similar trend may also be considered for Br− . Therefore, the couple (PI and PI ) may be assigned to the Au (1 1 1) facet of the Au (poly) electrode, while the couple (PII and PII ) to the Au (1 0 0) and Au (1 1 0) facets. The voltammograms in the presence and absence of Br− are almost the same at potential more negative than −0.66 V, which implies that Br(ads) -adlayer undergoes a complete desorption at this potential. Blizanac et al. [41] also reported that  Br of the Au (1 0 0) electrode in 0.1 M HClO4 solution became zero at a potential of about −0.4 V vs. SCE. The two reversible couples in the presence of Br− have slightly greater intensity as compared with those obtained in the absence of Br− suggesting the Br− adsorption/desorption along with the OH− adsorption/desorption in the same potential zone. Decrease of the uptake of anodic current above 0.1 V also implies the formation of the Br(ads) -adlayer at the Au (poly) electrode surface. However, the current above 0.1 V is still prominent, suggesting that the Br(ads) adlayer is not compact enough to inhibit fully the chemisorption of OH− . Our previous study showed that the adsorption of I− onto the Au (poly) electrode surface in 0.1 M KOH solution significantly decreased the current above 0 V due to the formation of a compacted monolayer of I(ads) which efficiently blocked the adsorption of OH− [6,7]. On the contrary, Br-adatom cannot inhibit the coadsorption of OH− because of its less adsorption affinity. It has also been reported by Blizanac et al. that in alkaline media of relatively higher pH, the adsorption of OH− is not fully inhibited by Br(ads) -adlayer [41]. We will recall the topic later. The peak cur-

rents during the anodic potential sweep are associated with the progress of adsorption of Br− on the electrode surface.  Br increases as the electrode potential becomes more and more positive. This potential-dependent increase of  Br is known as “electrocompression” [34,35]. The peaks are also attributed to the adlayer structure transition from one form to other. The adsorption of Br− at potential of −0.13 > E > −0.66 V (indicated by the anodic peak PII) results in a disordered Br(ads) -adlayer [41]. According to the results reported for the adsorption/desorption of Br− at different single crystalline electrodes in acidic media, it is apparently though that the second anodic peak PI is associated with the formation of an ordered √ √ c( 2 × 2 2)R45◦ -Br(ads) -adlayer structure [34,40,41]. However, a careful search was made by Blizanac et al. to find the diffraction √ √ peaks due to the c( 2 × 2 2)R45◦ -Br(ads) -adlayer structure in their study of adsorption/desorption of Br− at the Au (1 0 0) electrode in 0.1 M NaOH solution [41]. They could not observe such a response √ √ suggesting that a c( 2 × 2 2)R45◦ -Br(ads) -adlayer structure with long range order is not formed on the Au (1 0 0) electrode in the alkaline media. It is apparent that the anodic peak PI is indeed a fin√ √ gerprint of the ordering of the Br(ads) -adlayer to c( 2 × 2 2)R45◦ structure. But due to the competitive adsorption of Br− and OH− √ √ in alkaline solution the domain size of c( 2 × 2 2)R45◦ -Br(ads) adlayer structure is probably not large enough to be detected. Salaita et al. [51] also observed that in Br− -containing alkaline solutions of pH > 10 the adsorption/desorption of OH− gives prominent reversible couple of anodic and cathodic peaks at the Pt (1 1 1) electrode while Br− was not adsorbed strongly. In the present study, we also consider that a compact Br(ads) -adlayer on the Au (poly) electrode surface cannot be formed because of the competitive adsorption with OH− . Later we will give more evidence in this regard. From Fig. 1A (b), we estimated the total charge associated with the (i) anodic peaks PI and PII for the oxidative adsorption of Br− and (ii) cathodic peaks PI and PII for the reductive desorption of Br(ads) -adlayer. Values of 168 and 151 ␮C cm−2 were obtained for the oxidation and reduction processes, respectively which can be taken as identical within the experimental error suggesting that the adsorption/desorption of Br− is a reversible process. The value of  Br was calculated as 1.74 × 10−9 mol cm−2 using Eq. (2): Br =

Q neA

(2)

where n is the number of electrons involved in the Br− adsorption/desorption and in this case n was considered to be equal to unity (discussed later), e is the charge of an electron (1.602177 × 10−19 C) and A is the geometric surface area of Au (poly) electrode. The determined value of  Br is small as compared to the theoretical value for the monolayer of Br(ads) ( Br = 2.3 × 10−9 mol cm−2 ) or our reported value (2.8 × 10−9 mol cm−2 ) on the adsorption of I− on the Au (poly) electrode in 0.1 M KOH solution, in which we considered that a compact I(ads) -adlayer can be fabricated even in the alkaline media [6,7] because of the strong adsorption affinity of I− . In addition, the value of  Br should be further small as the estimated charge also includes the charge associated with the coadsorption of OH− . Therefore, in the presence case, it might be concluded that a compacted and ordered Br(ads) -adlayer at the Au (poly) electrode in alkaline media cannot be formed in the potential zone under investigation because of the coadsorption of OH− . Especially, in the potential zone of the second couple of the anodic and cathodic peaks (PII and PII ), the value of  Br is significantly small, and a √ √ compacted c( 2 × 2 2)R45◦ -Br(ads) -adlayer structure cannot exist. In other words, the Au (poly) electrode surface is partially covered with Br(ads) -adlayer. Thus a Br-submonolayer-coated Au (poly) electrode was successfully fabricated in situ in view of investigating the

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Fig. 3. Typical CVs obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing (a) 5 and (b) 20 mM KBr. Potential scan rate: 0.1 V s−1 . Inset shows the linear plots of the cathodic peak (PI and PII ) potential vs. log[Br− ]. Fig. 2. Linear plots of the cathodic and anodic peak currents vs. scan rate and the square root of scan rate. Data were taken from Fig. 1(B).

electrochemical oxidation of H2 O2 in alkaline media (discussed in the next section). The Br− adsorption/desorption process was also recorded at various potential scan rates ranging from 0.05 to 0.5 V s−1 and the corresponding results are presented in Fig. 1(B). The first and second peak currents (both anodic and cathodic) were plotted as function of scan rate () (for PI and PII peak currents) and the square root of scan rate (for PI and PII peak currents) and the obtained linear plots are given in Fig. 2. Both of the PI and PII peak currents vary linearly with  and pass through the origin suggesting a surface-confined process, that is, the cathodic process is associated with the desorption of Br(ads) -adlayer from the surface of the electrode. On the other hand, both of the PI and PII peak currents vary linearly with 1/2 and pass through the origin suggesting a diffusion-controlled process, that is, the anodic process is associated with the diffusion-controlled adsorption of Br− at the Au (poly) electrode surface. Adsorption of Br− was further investigated by monitoring the open-circuit potential (Eocp ) of the bare Au (poly) electrode upon the injection of KBr into N2 -saturated 0.1 M NaOH solution in which the final concentration of KBr was equal to 5.0 mM. Inset of Fig. 1 clearly shows that the introduction of KBr into the solution results in a sharp negative shift of Eocp . The negative shift of Eocp is obviously related to the spontaneous adsorption of Br− onto the electrode surface. The negative shift of Eocp implies that the adsorption proceeded through the donation of electron from anionic Br− species to the electrode surface. The observed change of Eocp , therefore, further confirms the adsorption of Br− onto the Au (poly) electrode surface. Details in this regard have been reported elsewhere by our group [42,43]. The positions of the anodic and cathodic peaks depend on the concentration of Br− . The dependence of the peak potential (at a constant scan rate) on the logarithm of the Br− concentration follows the relation [52]: Ep = Ero −

2.303RT log CBr− + nF

 RT   f  F

2

(3)

where Ep is the peak potential for equilibrium condition, Ero is the standard potential for electrosorption of the adsorbed species, n is

the number of electrons involved in the electrosorption reaction, f is the Frumkin interaction parameter and the other parameters have their usual meanings. The CVs were obtained in N2 -saturated 0.1 M NaOH solution containing 5, 10, 15 and 20 mM Br− . For clarity, the CVs obtained only in cases of 5 and 20 mM Br− are presented in Fig. 3. The cathodic peak (PI and PII ) potentials were derived from the obtained CVs and were plotted as a function of log[Br− ] (inset of Fig. 3). The linear plots of PI and PII peak potentials have the slopes of about −64 and −60 mV, respectively. Only the cathodic peak potentials were considered, as the anodic peak potentials could not be obtained precisely because of the broad shape of the anodic peaks. These values of the slopes point out a complete discharge of Br− to Br(ads) since they are very close to −59 mV for an electrosorption valency of −1 [52]. Similar values of electrosorption valency of Br− were obtained by other researchers for the adsorption of Br− at the Au (1 0 0) electrode surface [41,53]. Based on the obtained value of the electrosorption valency, the adsorption/desorption process Br− ion at the Au (poly) electrode surface can be presented by Eq. (4): Au + Br(ads) − ⇔ AuBr(ads) + e−

(4)

It was also observed that the peak currents (PI, PII, PI and PII ) remain almost unchanged with increasing the concentration of Br− in the range of 5–20 mM (see Fig. 3), suggesting that no faradic reaction of Br− takes place within the potential window investigated in this study. In other words, the peak currents are obviously attributed to the adsorption/desorption of Br− . 3.2. Voltammetric electrooxidation of H2 O2 in Br− -containing 0.1 M NaOH solution Fig. 4 shows the CVs obtained at the Au (poly) electrode in initially N2 -saturated 0.1 M NaOH solution containing (a and b) 5.0 and (c) 0 mM Br− and (a and c) 2.0 and (b) 0 mM H2 O2 . A remarkable change in the cyclic voltammetric features of H2 O2 was obtained in the presence of Br− in the solution (compare curves (a) and (c)). Electrochemical behaviors of H2 O2 at the clean Au (poly) electrode in alkaline solution have been described in detail in our recent reports [6,7,42]. The cathodic peak at −0.145 V at the bare Au (poly) electrode is assigned to the electroreduction of O2 to HO2 − . O2 was

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generated by the (i) electrochemical oxidation of H2 O2 at potential above 0 V and (ii) chemical decomposition of H2 O2 catalyzed by the bare Au (poly) electrode surface. This peak current was found to depend strongly on the initial upper potential limit (Eupper ). Inset of Fig. 4 clearly shows that the peak current diminished gradually as Eupper shifted to negative direction of potential. As Eupper became more negative, the electrochemical oxidation reaction of H2 O2 to generate O2 became less feasible resulting in less con-

centration of O2 and consequently the cathodic peak current at −0.145 V diminished. A small cathodic current was observed even when Eupper was −0.135 V, where the oxidation of H2 O2 to generate O2 is not feasible at all. The origin of this small peak current is the generation of O2 during the previous measurements of the CVs. Stirring the solution, after recording the previous CVs, almost completely diminished this peak current. However, small cathodic current still appeared because of the heterogeneous decomposition of H2 O2 to O2 at the bare Au (poly) electrode surface. H2 O2 -free O2 -saturated 0.1 M NaOH solution also gave the cathodic peak at the same potential for the O2 reduction to HO2 − . Therefore, the peak current at −0.145 V in the absence of Br− ion is undoubtedly assigned to the O2 reduction reaction although N2 -saturated solution was initially used. A remarkably high peak current for the oxidation of HO2 − to O2 was achieved in the presence of Br− . Unlike the bare electrode, the oxidation peak current (jpa ) in the presence of Br− appeared at −30 mV (Epa ). The oxidation peak current is ca. five times higher than the potential-independent plateau current at the bare Au (poly) electrode above 0 V for the same concentration of H2 O2 . The enhancement of the oxidation current in the presence of Br− is correlated to the increased electrostatic attraction between the anionic HO2 − molecules and the Br(ads) -adlayer-induced positively polarized Au (poly) electrode surface (discussed later). In the reverse scan, a cathodic peak current (jpc ) assigned to the reduction of the produced O2 to HO2 − appeared at −0.21 V (Epc ). The ratio of jpa to jpc is close to 1 indicating a reversible response of the HO2 − /O2 redox couple in the presence of Br− in 0.1 M NaOH solution. The formal potential (E◦ ), estimated as the average of Epa and Epc (i.e., (Epa + Epc )/2) is −120 mV vs. Ag|AgCl|NaCl (sat.) in 0.1 M NaOH solution. The peak-to-peak separation is 180 mV which is larger than theoretical value for the 2-electron reversible process. The larger peak-to-peak separation is associated with the negative shifting of the cathodic peak current by the Br(ads) -adlayer that creates an additional activation barrier for the O2 reduction reaction (compare curves a and c). Therefore, the most notable feature is that the potential-independent HO2 − oxidation at the bare Au (poly) electrode is transformed into a reversible redox reaction for

Fig. 5. Linear plots of the (a) anodic and (b) cathodic peak currents vs. the square root of scan rate. Inset shows the linear plots of the (a ) anodic and (b ) cathodic peak potentials vs. log . Both of the peak currents and peak potentials were derived from CVs obtained in initially N2 -saturated 0.1 M NaOH solution containing 5.0 mM Br− and 2.0 mM H2 O2 over scan rate ranging from 0.002 to 1.0 V (not shown).

Fig. 6. LSVs obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing 5.0 mM KBr and (a) 0, (b) 2.0, (c) 4.0 and (d) 6.0 mM H2 O2 . Potential scan rate: 0.1 V s−1 . Inset shows the linear plot of the peak current vs. concentration of H2 O2 . Data were taken from Fig. 6.

Fig. 4. CVs obtained at the Au (poly) electrode in initially N2 -saturated 0.1 M NaOH solution containing (a and b) 5.0 and (c) 0 mM Br− and (a and c) 2.0 and (b) 0 mM H2 O2 . The symbol (*) represents the initial potential in all cases. Potential scan rate: 0.1 V s−1 . Inset shows the Eupper dependence of the cathodic peak current at −0.145 V in the voltammogram (c).

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the HO2 − /O2 couple at the Br(ads) -submonolayer-coated Au (poly) electrode in Br− -containing 0.1 M NaOH solution. Both of the anodic and cathodic peak currents were recorded at scan rates () ranging from 0.002 to 1.0 V s−1 and the peak currents were plotted as a function of 1/2 . The results are shown in Fig. 5. We can see that both of the peak currents nicely (r2 = 0.999) fall on their individual straight lines passing through the origin suggesting that the reactions corresponding to the anodic and cathodic peaks are solely controlled by diffusion of the analytes, i.e., HO2 − and O2 , respectively, to the electrode surface. The peak potentials were also plotted as a function of log  (inset of Fig. 5). Both of the anodic and cathodic peak potentials fall on their individual straight lines having slopes equal to +0.0457 and −0.0636 V decade−1 , respectively. From these slopes the anodic and cathodic electron transfer coefficients were calculated as 0.64 and 0.46, respectively [54]. Linear sweep voltammograms (LSVs) were obtained at the Au (poly) electrode in 0.1 M NaOH solution containing 5.0 mM KBr and various amounts of H2 O2 in the range of 2.0–6.0 mM. The results are shown in Fig. 6. The anodic peak current increases with increasing the concentration of H2 O2 . The peak current was plotted as a function of the concentration of H2 O2 . Inset of Fig. 6 shows that the points nicely fall on a straight line passing through the origin. This observation further suggests that the anodic peak current is associated with the oxidation of H2 O2 . 3.3. Amperometric electrooxidation of H2 O2 in Br− -containing 0.1 M NaOH solution Fig. 7 shows the chronoamperograms obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing (a–d) 2.0 and (e) 0 mM H2 O2 and (a–c and e) 5.0 and (d) 0 mM Br− at various specified potentials (Esp ): (a) −0.05, (b) 0.05 and (c–e) 0.2 V. In all cases, the potential was stepped from an initial value of −0.2 V. The results obtained in the presence of Br− clearly show several features: (i) the current at all the examined Esp is relatively higher as

Fig. 8. (A) Schematic illustration of the induction of positive polarization of Au (poly) electrode surface by the Br(ads) -adlayer. (B) Enhanced electrostatic attraction between the anion HO2 − molecules and Br-adlayer-induced positively polarized Au (poly) electrode surface for enhancing the oxidation of H2 O2 .

compared with the bare electrode only at the shorter time (t < 10 s), (ii) the highest current was obtained at the lowest applied Esp and vise versa, (iii) the current decreased rapidly with time and (iv) the decrease of current became more pronounced with increasing Esp . The observed current–Esp relationship in Br− -containing solution could be correlated to the value of  Br at the applied Esp . The value of  Br increases with the increase of the applied electrode potential [34,35]. For clarification of the increase of  Br with Esp , the chronoamperograms were also obtained at (a ) −0.05, (b ) 0.05 and (c ) 0.2 V in Br− -containing solution in the absence of H2 O2 and the results are presented in the inset of Fig. 7. Curve (d ) was obtained in 0.1 M NaOH solution in the absence of both of Br− and H2 O2 . The higher observed anodic currents (curves a –c ) as compared to the background response (curve d ) are obviously attributed to the potential-induced adsorption of Br− at the Au (poly) electrode according to the Eq. (4) as presented in the earlier section. The sequence of the current density clearly implies that  Br increases with increasing Esp . We assume that the oxidation reaction of H2 O2 takes place at the free space (bare portion) of the Au (poly) electrode surface (discussed later). At lower Esp , higher oxidation current, therefore, was obtained because of lower  Br . With increasing Esp , the oxidation current decreased owing to the increase of  Br although H2 O2 oxidation is expected to become more feasible at higher electrode potential. 3.4. Mechanism of enhanced electrooxidation of H2 O2 in Br− -containing solution

Fig. 7. Chronoamperograms obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing (a–d) 2.0 and (e) 0 mM H2 O2 and (a–c and e) 5.0 and (d) 0 mM Br− by stepping the potential from an initial value of −0.2 V to various specified potentials (Esp ): (a) −0.05, (b) 0.05 and (c–e) 0.2 V. Inset shows the chronoamperograms obtained at (a ) −0.05, (b ) 0.05 and (c and d ) 0.2 V keeping other conditions the same as Fig. 7 except for the absence of H2 O2 .

Monovalent Br− undergoes an oxidative adsorption by transferring one electron to the electrode surface, resulting in an adlayer of neutral Br(ads) -adatoms [41,53]. Br atoms, being more electronegative than Au atoms, induce a positive polarization of the

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reaction of HO2 − . Enhanced electrode activity due to the electrostatic force of attraction between the reactant and the oppositely charged electrode surface has also been considered for other systems [42,56–58]. If a non-adsorbing electrolyte is added in excess into the electrolytic solution, then the degree of the electrostatic force of attraction between HO2 − molecules and the electrode surface should be decreased as the added anions (being excess) will mostly be attracted to the positively charged electrode surface and consequently the oxidation current of HO2 − should be decreased. Fig. 9 clearly shows that the oxidation current drastically decreased with increasing the concentration of KF, supporting that the electrostatic force of attraction plays a vital role in the enhanced oxidation of HO2 − . Similar consideration was also made in our previous report on the reduction of HO2 − at Pb-modified Au (poly) electrode in iodide-containing alkaline solution [59]. H2 O2 oxidation current was found to decrease rapidly with decreasing pH of the solution as the degree of dissociation of H2 O2 decreases with decreasing pH. In acidic pH the electrode activity was diminished because in acidic solution H2 O2 no longer exists as HO2 − (see Fig. 10). 4. Conclusions Fig. 9. CVs obtained at the Au (poly) electrode in N2 -saturated 0.1 M NaOH solution containing 5.0 mM KBr, 2.0 mM H2 O2 and (a) 0, (b) 100 and (c) 200 mM KF. Potential scan rate: 0.1 V s−1 .

Au (poly) electrode surface by withdrawing electron density from the electrode surface. Br(ads) -adlayer-induced positive polarization of the metallic electrode surfaces has also been considered elsewhere [42,55]. Shifting of the potential of zero charge to negative potential by about 300 mV in Br− -containing solution as compared to the Br− -free solution also indicates such a positive polarization of the electrode surface [32]. In 0.1 M NaOH solution, H2 O2 essentially exists as anionic HO2 − species as the pH of the solution is greater than the pKa value (11.6) of H2 O2 . The anionic HO2 − species feel an electrostatic attraction to the positively polarized Au (poly) electrode surface (as schematically shown in Fig. 8) resulting in a tremendous enhancement of the activity of the Au (poly) electrode towards the electrochemical oxidation

Br− undergoes an oxidative chemisorption onto the Au (poly) electrode surface leading to the formation of adlayer of neutral Bradatoms. The value of  Br (1.74 × 10−9 mol cm−2 ) is lower than the full monolayer coverage because of the coadsorption of OH− . The in situ prepared Br-submonolayer-coated Au (poly) electrode showed a substantial enhancement of the activity towards the electrochemical oxidation of H2 O2 in alkaline media. As compared with the bare Au (poly) electrode, the oxidation current at the modified electrode was about five times higher and the reaction was solely controlled by diffusion. Br, being more electronegative than Au, induces a positive polarization of the Au (poly) electrode surface by withdrawing electron density from the electrode surface. Increased electrostatic force of attraction between the anionic HO2 − molecules and the positively polarized electrode surface was considered as a probable origin of the enhancement of the electrode activity. Increase of the ionic strength and decrease of pH of the electrolytic solution supported the proposed mechanism. A further study of evaluating the kinetic parameters of the oxidation of H2 O2 at the Br(ads) submonolayer-coated Au (poly) electrode is in progress. Acknowledgments The present work was supported by the Grant-in-Aid for Scientific Research (A) (No. 19206079) to T. Ohsaka, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and also from the New Energy and Industrial Technology Development Organization (NEDO), Japan. Md. Rezwan Miah thanks the Government of Japan for awarding of a MEXT scholarship. References

Fig. 10. CVs obtained at the Au (poly) electrode in N2 -saturated (a) 0.1, (b) 0.04 and (c) 0.02 M NaOH and (d) 0.1 M H2 SO4 solution containing 5.0 mM KBr and 2.0 mM H2 O2 . Potential scan rate: 0.1 V s−1 .

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