# Electrochemical Reduction of Oxygen on Anthraquinone/Carbon Nanotubes Nanohybrid Modified Glassy Carbon Electrode in Neutral Medium

Authors: Zheng Gong; Guoquan Zhang; Song Wang
Journal: Journal of Chemistry
 (2013)

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## Abstract

The electrochemical behaviors of monohydroxy-anthraquinone/multiwall carbon nanotubes (MHAQ/MWCNTs) nanohybrid modified glassy carbon (MHAQ/MWCNTs/GC) electrodes in neutral medium were investigated; also reported was their application in the electrocatalysis of oxygen reduction reaction (ORR). The resulting MHAQ/MWCNTs nanohybrid was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the ORR at the MHAQ/MWCNTs/GC electrode occurs irreversibly at a potential about 214 mV less negative than at a bare GC electrode in pH 7.0 buffer solution. Cyclic voltammetric and rotating disk electrode (RDE) techniques indicated that the MHAQ/MWCNTs nanohybrid has high electrocatalytic activity for the two-electron reduction of oxygen in the studied potential range. The kinetic parameters of ORR at the MHAQ/MWCNTs nanohybrid modified GC electrode were also determined by RDE and EIS techniques.


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## Body

## 1. Introduction

The oxygen reduction reaction (ORR) has attracted much attention in many processes such as fuel cells, metal-air batteries, metal corrosion, sensors, electrocatalysis, and many other industrial processes. Depending on the electrode material and its surface properties as well as the solution pH, the ORR proceeds by a two-electron pathway or via direct four-electron pathway or via two two-electron pathway. A variety of electrode materials have been investigated for the ORR over the years in order to enhance the performance of the electrodes towards ORR. To date, carbon materials are the most frequently used supports for cathode catalysts. The electrochemical reduction of oxygen on these carbon-based or modified carbon electrodes has received a long-standing interest and has been extensively studied. Many mediators have been employed as electrocatalysts for the reduction of oxygen to produce H2O or H2O2. Among them, quinone (anthraquinone, naphthoquinone, and phenanthrenequinone) and its derivatives [1–19] have been regarded as the ideal metal-free electrocatalyst for ORR. Although the ORR mechanism is not entirely clear, it is generally considered that oxygen reduction on carbon-based cathodes involves the formation of hydrogen peroxide in alkaline and acidic media [20].Carbon nanotubes (CNTs) have attracted much attention and molded a new area of nanotechnology and material science because of their fascinating and extraordinary structure, electronic and mechanical properties. CNTs have two distinct structure types: the single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), while the double-walled carbon nanotubes (DWCNTs) were recently considered as a separate type of CNTs [20]. Recent various electrochemical studies including electrocatalysis and electroanalysis have demonstrated that CNTs modified carbon-based cathodes possess the ability to promote electron transfer kinetics and apparent electrocatalytic effect for ORR [6, 10, 20–27].In the present study, the combined electrocatalytic effect of monohydroxy anthraquinone (MHAQ) and MWCNTs towards the ORR was investigated. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to characterize the prepared MHAQ/MWCNTs nanohybrid. The electrochemical behavior and electrocatalytic activity of the MHAQ/MWCNT nanohybrid modified GC (MHAQ/MWCNTs/GC) electrodes towards the ORR were examined in neutral medium by cyclic voltammetry, rotating disk voltammetry (RDE), and electrochemical impedance spectroscope (EIS) along with the determination of the electrode kinetic parameters.


## 2. Experimental

### 2.1. Chemicals

Multiwalled carbon nanotubes (MWCNTs, purity >97%, outer diameter ≤10 nm, length 5–15μm) purchased from Nanotech Port Co. Ltd. (Shen Zhen, China) were used in this work. MHAQ was purchased from Sigma-Aldrich. Triply distilled water was used throughout the experiments. A 0.5% Nafion solution used in this work was prepared by diluting the 5% Nafion solution (Sigma) into ethanol. All other chemicals were analytical grade and used as received. The aqueous solution used was 0.1 mol L−1 NaH2PO4 + 0.01 mol L−1 NaOH (pH 7.0). N2 and O2 gases with purity of ca. 99.99% were used during the experiments.


### 2.2. Preparation of MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC Electrodes

#### 2.2.1. Procedure

Glassy carbon electrode (GC, CH Instruments, 0.126 cm2 of surface area) was polished mechanically with alumina powder (0.5 μm) and then ultrasonicated to remove any powder material adhered to the electrode surface. Finally, GC electrode was thoroughly washed with deionized water and acetone. Preanodization of the GC electrodes was carried out by continuous potential cycling from −0.5 to 1.8 V at a sweep rate of 50 mV s−1 in 0.5 mol L−1 H2SO4 solution until a stable voltammogram was obtained.For the preparation of MHAQ/MWCNTs/GC electrodes, 1.0 mg MWCNTs was dispersed with the aid of ultrasonic agitation in 1.0 mL of sodium dodecyl benzene sulphonates (SDBS) solution to give a 1.0 mg mL−1 black suspension. The MWCNTs film was prepared by dropping 20 μL suspensions of MWCNTs in SDBS onto the surface of GC and allowing the solvent to evaporate in an oven at 50°C. This electrode was further covered with a Nafion film by placing 4 μL of a 0.5% Nafion solution in ethanol onto electrode surface and allowing the solvent to evaporate in air. The resulting electrode is referred to as MWCNTs/GC electrode. Then, the MWCNTs/GC electrode was placed in the deposition solution of 3.0 mmol L−1 MHAQ at open circuits for 4 h. The resulting MHAQ/MWCNTs/GC electrode was rinsed thoroughly with deionized water and dried. The MHAQ/GC electrode was prepared by immersing the preanodized GC into the deposition solution of 3.0 mmol L−1 MHAQ for 4 h and allowing the solvent to evaporate in an oven at 50°C. Then, the electrode was also covered with a Nafion film by placing 4 μL of a 0.5% Nafion-ethanol solution onto electrode surface and dried in air. It is well known that in the presence of Nafion, MWCNTs can be solubilized and uniformly deposited onto electrode surfaces [20].In order to determine the electroactive surface area of MWCNTs/GC electrode, we performed cyclic voltammetric experiments relating to the electrochemical properties of bare GC and MWCNTs/GC electrodes in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl. Cyclic voltammetric experiments were carried out on the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs nanohybrid modified GC electrodes in 15 mL various pH aqueous solution which was deaerated by purging N2 gas for at least 40 min. Consequently, the same experiments were repeated after purging O2 gas for 40 min. Hydrodynamic voltammetric experiments were done on the MHAQ/MWCNTs/GC rotating disk electrode in 20 mL pH 7.0 aqueous solution deaerated by purging N2 gas for 40 min and in the presence of oxygen by purging O2 gas for 40 min. EIS measurements were also recorded in pH 7.0 aqueous solution as supporting electrolyte. All electrochemical experiments were carried out at room temperature.


#### 2.2.2. Apparatus

The morphology of the resulting MHAQ/MWCNTs nanohybrids was investigated by SEM (FEI Quanta 200 FEG, The Netherlands) and TEM (Tecnai G2 Spirit) which operated at 120 kV accelerating voltage. The samples were prepared by placing a droplet of MHAQ/MWCNT aqueous suspension on formvar/carbon-coated copper grids and dried in air.Cyclic voltammetry was performed on EG&G PAR potentiostat/galvanostat (Model 273, USA) with conventional three-electrode cell containing a saturated calomel electrode (SCE) reference electrode, a platinum wire counter electrode, and an MHAQ/MWCNTs hybrid modified GC working electrode. An EG&G PARC electrode rotator system (Model 636, USA) was employed for hydrodynamic voltammetric studies on the ORR. An EG&G PARC Frequency Response Detector (FRD 100) was used for EIS measurements. EIS was recorded in the frequency range from 100 kHz to 10 mHz. The potential amplitude of ac signal was 5 mV with the applied potential at −0.4 V. The equilibration time was set to 600 s at dc potential of −0.4 V before the EIS measurements. Nyquist representations of the impedance data were analyzed using ZSimpWin V3.10 software.


## 2.1. Chemicals

Multiwalled carbon nanotubes (MWCNTs, purity >97%, outer diameter ≤10 nm, length 5–15μm) purchased from Nanotech Port Co. Ltd. (Shen Zhen, China) were used in this work. MHAQ was purchased from Sigma-Aldrich. Triply distilled water was used throughout the experiments. A 0.5% Nafion solution used in this work was prepared by diluting the 5% Nafion solution (Sigma) into ethanol. All other chemicals were analytical grade and used as received. The aqueous solution used was 0.1 mol L−1 NaH2PO4 + 0.01 mol L−1 NaOH (pH 7.0). N2 and O2 gases with purity of ca. 99.99% were used during the experiments.


## 2.2. Preparation of MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC Electrodes

### 2.2.1. Procedure

Glassy carbon electrode (GC, CH Instruments, 0.126 cm2 of surface area) was polished mechanically with alumina powder (0.5 μm) and then ultrasonicated to remove any powder material adhered to the electrode surface. Finally, GC electrode was thoroughly washed with deionized water and acetone. Preanodization of the GC electrodes was carried out by continuous potential cycling from −0.5 to 1.8 V at a sweep rate of 50 mV s−1 in 0.5 mol L−1 H2SO4 solution until a stable voltammogram was obtained.For the preparation of MHAQ/MWCNTs/GC electrodes, 1.0 mg MWCNTs was dispersed with the aid of ultrasonic agitation in 1.0 mL of sodium dodecyl benzene sulphonates (SDBS) solution to give a 1.0 mg mL−1 black suspension. The MWCNTs film was prepared by dropping 20 μL suspensions of MWCNTs in SDBS onto the surface of GC and allowing the solvent to evaporate in an oven at 50°C. This electrode was further covered with a Nafion film by placing 4 μL of a 0.5% Nafion solution in ethanol onto electrode surface and allowing the solvent to evaporate in air. The resulting electrode is referred to as MWCNTs/GC electrode. Then, the MWCNTs/GC electrode was placed in the deposition solution of 3.0 mmol L−1 MHAQ at open circuits for 4 h. The resulting MHAQ/MWCNTs/GC electrode was rinsed thoroughly with deionized water and dried. The MHAQ/GC electrode was prepared by immersing the preanodized GC into the deposition solution of 3.0 mmol L−1 MHAQ for 4 h and allowing the solvent to evaporate in an oven at 50°C. Then, the electrode was also covered with a Nafion film by placing 4 μL of a 0.5% Nafion-ethanol solution onto electrode surface and dried in air. It is well known that in the presence of Nafion, MWCNTs can be solubilized and uniformly deposited onto electrode surfaces [20].In order to determine the electroactive surface area of MWCNTs/GC electrode, we performed cyclic voltammetric experiments relating to the electrochemical properties of bare GC and MWCNTs/GC electrodes in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl. Cyclic voltammetric experiments were carried out on the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs nanohybrid modified GC electrodes in 15 mL various pH aqueous solution which was deaerated by purging N2 gas for at least 40 min. Consequently, the same experiments were repeated after purging O2 gas for 40 min. Hydrodynamic voltammetric experiments were done on the MHAQ/MWCNTs/GC rotating disk electrode in 20 mL pH 7.0 aqueous solution deaerated by purging N2 gas for 40 min and in the presence of oxygen by purging O2 gas for 40 min. EIS measurements were also recorded in pH 7.0 aqueous solution as supporting electrolyte. All electrochemical experiments were carried out at room temperature.


### 2.2.2. Apparatus

The morphology of the resulting MHAQ/MWCNTs nanohybrids was investigated by SEM (FEI Quanta 200 FEG, The Netherlands) and TEM (Tecnai G2 Spirit) which operated at 120 kV accelerating voltage. The samples were prepared by placing a droplet of MHAQ/MWCNT aqueous suspension on formvar/carbon-coated copper grids and dried in air.Cyclic voltammetry was performed on EG&G PAR potentiostat/galvanostat (Model 273, USA) with conventional three-electrode cell containing a saturated calomel electrode (SCE) reference electrode, a platinum wire counter electrode, and an MHAQ/MWCNTs hybrid modified GC working electrode. An EG&G PARC electrode rotator system (Model 636, USA) was employed for hydrodynamic voltammetric studies on the ORR. An EG&G PARC Frequency Response Detector (FRD 100) was used for EIS measurements. EIS was recorded in the frequency range from 100 kHz to 10 mHz. The potential amplitude of ac signal was 5 mV with the applied potential at −0.4 V. The equilibration time was set to 600 s at dc potential of −0.4 V before the EIS measurements. Nyquist representations of the impedance data were analyzed using ZSimpWin V3.10 software.


## 2.2.1. Procedure

Glassy carbon electrode (GC, CH Instruments, 0.126 cm2 of surface area) was polished mechanically with alumina powder (0.5 μm) and then ultrasonicated to remove any powder material adhered to the electrode surface. Finally, GC electrode was thoroughly washed with deionized water and acetone. Preanodization of the GC electrodes was carried out by continuous potential cycling from −0.5 to 1.8 V at a sweep rate of 50 mV s−1 in 0.5 mol L−1 H2SO4 solution until a stable voltammogram was obtained.For the preparation of MHAQ/MWCNTs/GC electrodes, 1.0 mg MWCNTs was dispersed with the aid of ultrasonic agitation in 1.0 mL of sodium dodecyl benzene sulphonates (SDBS) solution to give a 1.0 mg mL−1 black suspension. The MWCNTs film was prepared by dropping 20 μL suspensions of MWCNTs in SDBS onto the surface of GC and allowing the solvent to evaporate in an oven at 50°C. This electrode was further covered with a Nafion film by placing 4 μL of a 0.5% Nafion solution in ethanol onto electrode surface and allowing the solvent to evaporate in air. The resulting electrode is referred to as MWCNTs/GC electrode. Then, the MWCNTs/GC electrode was placed in the deposition solution of 3.0 mmol L−1 MHAQ at open circuits for 4 h. The resulting MHAQ/MWCNTs/GC electrode was rinsed thoroughly with deionized water and dried. The MHAQ/GC electrode was prepared by immersing the preanodized GC into the deposition solution of 3.0 mmol L−1 MHAQ for 4 h and allowing the solvent to evaporate in an oven at 50°C. Then, the electrode was also covered with a Nafion film by placing 4 μL of a 0.5% Nafion-ethanol solution onto electrode surface and dried in air. It is well known that in the presence of Nafion, MWCNTs can be solubilized and uniformly deposited onto electrode surfaces [20].In order to determine the electroactive surface area of MWCNTs/GC electrode, we performed cyclic voltammetric experiments relating to the electrochemical properties of bare GC and MWCNTs/GC electrodes in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl. Cyclic voltammetric experiments were carried out on the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs nanohybrid modified GC electrodes in 15 mL various pH aqueous solution which was deaerated by purging N2 gas for at least 40 min. Consequently, the same experiments were repeated after purging O2 gas for 40 min. Hydrodynamic voltammetric experiments were done on the MHAQ/MWCNTs/GC rotating disk electrode in 20 mL pH 7.0 aqueous solution deaerated by purging N2 gas for 40 min and in the presence of oxygen by purging O2 gas for 40 min. EIS measurements were also recorded in pH 7.0 aqueous solution as supporting electrolyte. All electrochemical experiments were carried out at room temperature.


## 2.2.2. Apparatus

The morphology of the resulting MHAQ/MWCNTs nanohybrids was investigated by SEM (FEI Quanta 200 FEG, The Netherlands) and TEM (Tecnai G2 Spirit) which operated at 120 kV accelerating voltage. The samples were prepared by placing a droplet of MHAQ/MWCNT aqueous suspension on formvar/carbon-coated copper grids and dried in air.Cyclic voltammetry was performed on EG&G PAR potentiostat/galvanostat (Model 273, USA) with conventional three-electrode cell containing a saturated calomel electrode (SCE) reference electrode, a platinum wire counter electrode, and an MHAQ/MWCNTs hybrid modified GC working electrode. An EG&G PARC electrode rotator system (Model 636, USA) was employed for hydrodynamic voltammetric studies on the ORR. An EG&G PARC Frequency Response Detector (FRD 100) was used for EIS measurements. EIS was recorded in the frequency range from 100 kHz to 10 mHz. The potential amplitude of ac signal was 5 mV with the applied potential at −0.4 V. The equilibration time was set to 600 s at dc potential of −0.4 V before the EIS measurements. Nyquist representations of the impedance data were analyzed using ZSimpWin V3.10 software.


## 3. Results and Discussion

### 3.1. Characterization of the MHAQ/MWCNT Nanohybrid

It is well known that the catalyst activity is strongly dependent on its composition, structure, morphology, and the particle size of the catalyst. At the same time, the catalyst supports also play an important role through providing a large surface area in enhancing the performance of catalysts. Here, SEM and TEM were used to reveal the surface morphology of MWCNTs and MHAQ/MWCNTs nanohybrid on GC electrode. It was clearly observed from Figures1 and 2 that MWCNTs are ca. 10 nm in diameter and were well dispersed in MHAQ/MWCNTs nanohybrid and exhibit flaky structural morphology. From the SEM and TEM images, it is obvious that the MHAQ was adsorbed on the surface of MWCNTs.SEM micrographs of (a) MWCNTs and (b) MHAQ/MWCNTs nanohybrid.
(a)
(b)TEM micrographs of (a) MWCNTs and (b) MHAQ/MWCNTs nanohybrid.
(a)
(b)To determine the electroactive surface area of MWCNTs/GC electrode, cyclic voltammetric experiments relating to the electrochemical properties of bare GC and MWCNTs/GC electrodes were recorded in 5 mM K3[Fe(CN)6] containing 0.1 M KCl solution. As seen in Figure 3(a), MWCNTs/GC electrode obviously improved the heterogeneous electron-transfer rate between [Fe(CN)6]3− molecules and the GC electrode surface, and the peak potential separation is lower than that of bare GC electrode. In addition, a larger peak current of [Fe(CN)6]3−/4− on MWCNTs/GC electrode appears as compared to the bare GC electrode. This electrocatalytic activity toward [Fe(CN)6]3−/4− could be due to the unique electronic properties of MWCNTs which accelerates electron transfer rate via improved conductivity (see EIS data in Figure 8(a)) and the good affinity of MWCNTs to [Fe(CN)6]3−/4−. The electroactive surface areas of the MWCNTs/GC electrode can be determined using the Randles-Sevcik equation:
(1)Ip=2.99×105AD1/2n3/2v1/2c.(a) CVs of the bare GC and MWCNTs/GC electrodes in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl as supporting electrolyte. (b) CVs of the MWCNTs/GC electrodes at different scan rates in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl as supporting electrolyte. The inset shows the dependance of Ipc on v1/2.
(a)
(b)In this equation,Ip is peak current. A is the electroactive surface area (cm2), and D is the diffusion coefficient of [Fe(CN)6]3− in dilute aqueous solution, which is 6.50±0.02 × 10−6cm2 s−1 [26]. The n is 1, which is the number of transferred electron for [Fe(CN)6]3−/4− redox couple, v is the scan rate in V s−1, and c is the concentration of K3[Fe(CN)6] which is 5 × 10−6 mol cm−3. Based on the slope of Ipc versus v1/2 in the inset of Figure 3(b), the estimated electrochemical active area for MWCNTs/GC is ca. 0.351 cm2.


### 3.2. Electrochemical Voltammetric Behaviour of MHAQ/MWCNTs Nanohybrid

The voltammetric responses of the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes in N2-saturated pH 7.0 buffer solution are shown in Figure 4. As seen, the cyclic voltammograms (CVs) of the MHAQ/GC and MHAQ/MWCNTs/GC nanohybrid electrodes possess one redox couple, corresponding to the quinone/hydroquinone couple. The surface coverage ΓAQ of MHAQ/MWCNTs/GC, and MHAQ/GC can be evaluated from the equation  ΓAQ=Q/nFA, where Q is the charge (5.53 and 1.5 μC) obtained by integrating the corresponding cathodic peak under the background correction and A is the geometric surface area of bare GC electrode. Then, the ΓAQ values of 2.27 × 10−10 and 6.21 × 10−11 mol cm−2 were obtained for MHAQ/MWCNTs/GC and MHAQ/GC, respectively.Figure 4
CVs for the bare GC, MWCNTs/GC, MHAQ/GC, and MHAQ/MWCNTs/GC electrodes recorded in pH 7.0 N2-saturated solutions at scan rate of 20 mV s−1.In order to examine the stability and reproducibility of the MHAQ/MWCNTs nanohybrid, the electrochemical behaviour of MHAQ/MWCNTs/GC electrodes was examined by performing CVs of 100 repetitive potential cycles in N2-saturated 0.1 M KOH at scan rate 20 mV s−1 (Figure 5). As seen from the corresponding CVs, the MHAQ/MWCNTs nanohybrid exhibits good stability and there was a slight decrease in the peak height, and a slight negative shift in the peak potential. This result agrees well with the reports that spontaneous adsorbed quinones are easily desorbed during long-term operation, especially in alkaline solutions [9, 14].Figure 5
CVs for the MHAQ/MWCNTs/GC electrode in N2-saturated 0.1 M KOH at scan rate of 20 mV s−1. The CV curves correspond to the 1st, 10th, and 100th potential scans.


### 3.3. Electrocatalytic ORR on MHAQ/MWCNTs/GC Electrodes

In order to estimate the electrocatalytic contribution of MHAQ and MWCNTs to the overall ORR, the electrocatalytic reduction of oxygen on the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes was examined in pH 7.0 buffer solution.As shown in Figure6, a series of irreversible CVs for O2 reduction on various GC electrodes were obtained. The O2 reduction peak on the bare GC and MWCNTs/GC electrodes is observed around −0.7 and −0.55 V, respectively. It has been proposed that the strong electrocatalytic effect of MWCNTs towards ORR could be caused by the oxygen-containing groups (especially quinone-type functionalities groups) on the surface of MWCNTs, as demonstrated in the literature [10, 20–24]. The O2 reduction peaks on MHAQ/GC and MHAQ/MWCNTs/GC electrodes both located at ca. −0.5 V, while the O2 reduction current on MHAQ/MWCNTs/GC electrode is higher than that on MHAQ/GC electrode. The O2 reduction potential shift (which is the peak-to-peak separation of O2 reduction potential at the MHAQ/MWCNTs/GC electrode and that at a bare GC electrode) reaches ca. 214 mV.Figure 6
CVs for ORR on the bare GC, MWCNTs/GC, MHAQ/GC, and MHAQ/MWCNTs nanohybrid modified GC electrodes in pH 7.0 O2-saturated solutions. Scan rate: 20 mV s−1.


#### 3.3.1. Hydrodynamic Voltammetric Studies

The electrocatalytic ORR was also carried out at rotating MHAQ/GC and MHAQ/MWCNTs/GC disk electrodes in pH 7.0 solutions between 0 and −0.9 V with scan rate of 10 mV s−1. A set of current-potential curves recorded at various angular velocities are shown in Figures 7(a) and 7(c). It is obvious that the disk current of the MHAQ/GC and MHAQ/MWCNTs/GC electrodes in pH 7.0 N2-saturated solution was negligible. In both cases, the O2 reduction-limiting currents increase with increasing rotation rate and show well-defined plateaus. The ORR onset potential for the MHAQ/MWCNTs/GC electrode is −0.2 V versus SCE, ~100 mV higher than that of the MHAQ/GC electrode. In addition, the diffusion-limiting currents for the MHAQ/MWCNTs/GC electrode are also slightly higher. It is worth noting that as observed before [8, 14], the adsorbed MHAQ groups block native GC surface sites, therefore almost completely suppressing the O2 reduction prewave resulted from the native quinonoidal groups of GC. Thus, no well-defined prewave is observed in the present work at low overpotential which is in agreement with the results described in Figure 6. At potentials more negative than ca −0.4 V, the O2 reduction current increases sharply close to its diffusion-limited value.RDE voltammetry curves of oxygen reduction on (a) MHAQ/GC and (c) MHAQ/MWCNTs/GC electrodes in pH 7.0 O2-saturated solutions at various rotation rates. Scan rate: 10 mV s−1. The Koutecky-Levich plots of oxygen reduction on (b) MHAQ/GC and (d) MHAQ/MWCNTs/GC electrodes at various cathodic potentials. The insets in (a) and (c) show the potential dependence of the number of electrons transferred per O2 molecule.
(a)
(b)
(c)
(d)(a) Nyquist plots obtained from EIS measurements in pH 7.0 N2- and O2-saturated solutions on the bare GC, MWCNTs/GC, MHAQ/GC, and the MHAQ/MWCNTs nanohybrid modified GC electrodes at a cathodic polarization potential of −0.4 V. (b) the applied equivalent circuit model.
(a)
(b)The Koutecky-Levich (K-L) analyses of the RDE data derived from the limiting current measured at various potential are shown in Figures7(b) and 7(d). The number of electron transferred per O2 molecule (n) was calculated from these plots using the K-L equation [28]:
(2)1I=1Id+1Ik=-10.62nFADO22/3γ-1/6cO2ω1/2-1nFAkcO2,
where I is the measured current, Ik and Id are the kinetic and diffusion-limited currents, respectively, n is the number of electron transferred per O2 molecule, k is the rate constant for O2 reduction, F is the Faraday constant (96,485 C mol−1), A is the GC disk electrode surface area, ω is the rotation rate, cO2 is the concentration of oxygen in the bulk (cO2=1.2×10-6 mol cm−3 at pHs 5–13) [10], DO2 is the diffusion coefficient of oxygen (DO2=1.9×10-5 cm2 s−1 at pHs 5–13) [10], and γ is the kinematic viscosity of the solution (0.01 cm2 s−1).The extrapolated K-L lines showed nonzero intercepts, indicating that the O2 reduction process on the MWCNTs/GC and MHAQ/MWCNTs/GC electrodes is under the mixed kinetic-diffusion control in a large range of potentials. The inset in Figures 7(a) and 7(c) compares the n values calculated from K-L equation at various potentials and shows that the O2 reduction proceeds predominantly by the two-electron pathway on MHAQ/GC and MHAQ/MWCNTs/GC electrodes, indicating that in this potential range O2 reduction on the MHAQ/MWCNTs nanohybrid modified electrode yields hydrogen peroxide as the final product. From the intercepts of the K-L plots, the average apparent electron transfer rate constant (k0) of ORR on MHAQ/MWCNTs/GC electrode in the potential range from −0.5 to −0.9 V can be determined and was calculated as ca. 0.344 cm s−1; thus a value for the second-order rate constant K=1.52×106 M−1 s−1 was obtained using k0=KΓAQ  [14].


#### 3.3.2. Electrochemical Impedance Studies

EIS is a powerful tool for studying the interface properties of chemically modified electrodes. From the complex plane plots of an impedance spectrum, the electron-transfer kinetics and the diffusion characteristics can be determined. The EIS of the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes recorded in pH 7.0 N2- and O2-saturated buffer solutions at −0.4 V are presented in Figure 8(a).  As can be seen, the Nyquist plots present the suppressing semicircle arcs in the high-frequency region and almost straight lines in the low-frequency region for all four cases. The respective semicircle parameters correspond to the electron-transfer resistance (Rct) and the double-layer capacity (Cdl) nature of the studied electrodes. The high-frequency intercept of semicircle arc on the real impedance axis yields the solution ohmic resistance (Rs) between the electrode surface and the reference electrode, while the diameter of semicircle arc provides Rct of charge transport across the electrode/solution interface.A significant difference in the semicircle region which is associated with the electron-transfer resistance behavior of electrochemical reactions was observed for the four different operational conditions. The change of semicircle arc diameter in N2-saturated to O2-saturated solution indicates the existence of electron transfer between dissolved oxygen and the studied electrodes. The distinct decrease in the arc diameter can be regarded as a sufficientevidence to explain the enhanced electrocatalytic performance of MHAQ/MWCNTs/GC towards ORR. Furthermore, the suppressing semicircle arcs observed in O2-saturated condition clearly indicate the lower electron-transfer resistance behavior compared to that in N2-saturated case. This phenomenon is in agreement with the early results obtained by Yeh and Wang [29] that the characteristic charge-transfer-controlled semicircles featured in the high-frequency region decrease in radius with an increase in the concentration of dissolved oxygen. On the other hand, a linear part with a slope of ca. π/4 appears in the middle-frequency region for O2-saturated solution, implying the finite diffusion of oxygen across the electrode/solution interface during ORR, while in the relatively lower frequency region the slope of the linear part is greater than π/4, which indicates that the mass-transport rate of oxygen to the electrode/solution interface is also an influence factor for ORR at −0.4 V.The impedance parameters of ORR are evaluated using an equivalent circuit shown in Figure8(b). To obtain the best fit, two constant phase elements (CPE1 and CPE2) were used to replace the double-layer capacitance of electrode/electrolyte interface. The impedance of CPE is defined as [30]
(3)ZCPE=1Y0(jω)φ,
where Y0 is the CPE constant, ω is the angular frequency (in rad s−1), j2=-1 is the imaginary number, and φ is the CPE exponent. Depending on φ(0≤φ≤1), the CPE is considered to be a pure capacitor (ZCPE=C, φ=1) and a pure resistance (ZCPE=R, φ=0). When φ=0.5, the CPE is related to the well-known Warburg impedance, which represents the diffusion-controlled processes. Moreover, the theory on the kinetics of electrocatalytic reactions at three dimensional modified electrodes has been well developed in the works byAlbery et al.[31, 32] andSaveant et al.[33–36]. According to their theory, the other fitting parameters in equivalent circuit should involve (i) the electrolyte resistance (Rs), (ii) the electron-transfer resistance (Rct) corresponding to charge transfer at the electrode/solution interface, (iii) the total resistance of electrode/solution interface (Rf), and (iv) the diffusion impedance (ZD) which is associated with the diffusion of dissolved reactant (O2) and products (H2O2) across the electrode/solution interface. The apparent electron-transfer rate constant (k0) of ORR can be obtained from Rct using [37]
(4)k0=RTn2F2ACO2bRct.The apparent electron transfer rate constantk0 of ORR on the MHAQ/MWCNTs/GC electrode was determined as 1.14 × 10−2 cm s−1, which agrees well with the rate constant value (1.69 × 10−2 cm s−1) calculated by the K-L equation at the same polarization potential (−0.4 V). We hypothesize the following reason for the enhanced electrocatalytic activity for ORR on the MHAQ/MWCNTs/GC electrode. The presence of MWCNTs provides the low charge transfer resistance and high surface area, and the adsorbed MHAQ on MWCNTs/GC surface keeps excess electroactive sites resulting in the more favorable electron-transfer kinetics for ORR at the MHAQ/MWCNTs nanohybrid.


## 3.1. Characterization of the MHAQ/MWCNT Nanohybrid

It is well known that the catalyst activity is strongly dependent on its composition, structure, morphology, and the particle size of the catalyst. At the same time, the catalyst supports also play an important role through providing a large surface area in enhancing the performance of catalysts. Here, SEM and TEM were used to reveal the surface morphology of MWCNTs and MHAQ/MWCNTs nanohybrid on GC electrode. It was clearly observed from Figures1 and 2 that MWCNTs are ca. 10 nm in diameter and were well dispersed in MHAQ/MWCNTs nanohybrid and exhibit flaky structural morphology. From the SEM and TEM images, it is obvious that the MHAQ was adsorbed on the surface of MWCNTs.SEM micrographs of (a) MWCNTs and (b) MHAQ/MWCNTs nanohybrid.
(a)
(b)TEM micrographs of (a) MWCNTs and (b) MHAQ/MWCNTs nanohybrid.
(a)
(b)To determine the electroactive surface area of MWCNTs/GC electrode, cyclic voltammetric experiments relating to the electrochemical properties of bare GC and MWCNTs/GC electrodes were recorded in 5 mM K3[Fe(CN)6] containing 0.1 M KCl solution. As seen in Figure 3(a), MWCNTs/GC electrode obviously improved the heterogeneous electron-transfer rate between [Fe(CN)6]3− molecules and the GC electrode surface, and the peak potential separation is lower than that of bare GC electrode. In addition, a larger peak current of [Fe(CN)6]3−/4− on MWCNTs/GC electrode appears as compared to the bare GC electrode. This electrocatalytic activity toward [Fe(CN)6]3−/4− could be due to the unique electronic properties of MWCNTs which accelerates electron transfer rate via improved conductivity (see EIS data in Figure 8(a)) and the good affinity of MWCNTs to [Fe(CN)6]3−/4−. The electroactive surface areas of the MWCNTs/GC electrode can be determined using the Randles-Sevcik equation:
(1)Ip=2.99×105AD1/2n3/2v1/2c.(a) CVs of the bare GC and MWCNTs/GC electrodes in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl as supporting electrolyte. (b) CVs of the MWCNTs/GC electrodes at different scan rates in 5 mM K3[Fe(CN)6] solution containing 0.1 M KCl as supporting electrolyte. The inset shows the dependance of Ipc on v1/2.
(a)
(b)In this equation,Ip is peak current. A is the electroactive surface area (cm2), and D is the diffusion coefficient of [Fe(CN)6]3− in dilute aqueous solution, which is 6.50±0.02 × 10−6cm2 s−1 [26]. The n is 1, which is the number of transferred electron for [Fe(CN)6]3−/4− redox couple, v is the scan rate in V s−1, and c is the concentration of K3[Fe(CN)6] which is 5 × 10−6 mol cm−3. Based on the slope of Ipc versus v1/2 in the inset of Figure 3(b), the estimated electrochemical active area for MWCNTs/GC is ca. 0.351 cm2.


## 3.2. Electrochemical Voltammetric Behaviour of MHAQ/MWCNTs Nanohybrid

The voltammetric responses of the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes in N2-saturated pH 7.0 buffer solution are shown in Figure 4. As seen, the cyclic voltammograms (CVs) of the MHAQ/GC and MHAQ/MWCNTs/GC nanohybrid electrodes possess one redox couple, corresponding to the quinone/hydroquinone couple. The surface coverage ΓAQ of MHAQ/MWCNTs/GC, and MHAQ/GC can be evaluated from the equation  ΓAQ=Q/nFA, where Q is the charge (5.53 and 1.5 μC) obtained by integrating the corresponding cathodic peak under the background correction and A is the geometric surface area of bare GC electrode. Then, the ΓAQ values of 2.27 × 10−10 and 6.21 × 10−11 mol cm−2 were obtained for MHAQ/MWCNTs/GC and MHAQ/GC, respectively.Figure 4
CVs for the bare GC, MWCNTs/GC, MHAQ/GC, and MHAQ/MWCNTs/GC electrodes recorded in pH 7.0 N2-saturated solutions at scan rate of 20 mV s−1.In order to examine the stability and reproducibility of the MHAQ/MWCNTs nanohybrid, the electrochemical behaviour of MHAQ/MWCNTs/GC electrodes was examined by performing CVs of 100 repetitive potential cycles in N2-saturated 0.1 M KOH at scan rate 20 mV s−1 (Figure 5). As seen from the corresponding CVs, the MHAQ/MWCNTs nanohybrid exhibits good stability and there was a slight decrease in the peak height, and a slight negative shift in the peak potential. This result agrees well with the reports that spontaneous adsorbed quinones are easily desorbed during long-term operation, especially in alkaline solutions [9, 14].Figure 5
CVs for the MHAQ/MWCNTs/GC electrode in N2-saturated 0.1 M KOH at scan rate of 20 mV s−1. The CV curves correspond to the 1st, 10th, and 100th potential scans.


## 3.3. Electrocatalytic ORR on MHAQ/MWCNTs/GC Electrodes

In order to estimate the electrocatalytic contribution of MHAQ and MWCNTs to the overall ORR, the electrocatalytic reduction of oxygen on the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes was examined in pH 7.0 buffer solution.As shown in Figure6, a series of irreversible CVs for O2 reduction on various GC electrodes were obtained. The O2 reduction peak on the bare GC and MWCNTs/GC electrodes is observed around −0.7 and −0.55 V, respectively. It has been proposed that the strong electrocatalytic effect of MWCNTs towards ORR could be caused by the oxygen-containing groups (especially quinone-type functionalities groups) on the surface of MWCNTs, as demonstrated in the literature [10, 20–24]. The O2 reduction peaks on MHAQ/GC and MHAQ/MWCNTs/GC electrodes both located at ca. −0.5 V, while the O2 reduction current on MHAQ/MWCNTs/GC electrode is higher than that on MHAQ/GC electrode. The O2 reduction potential shift (which is the peak-to-peak separation of O2 reduction potential at the MHAQ/MWCNTs/GC electrode and that at a bare GC electrode) reaches ca. 214 mV.Figure 6
CVs for ORR on the bare GC, MWCNTs/GC, MHAQ/GC, and MHAQ/MWCNTs nanohybrid modified GC electrodes in pH 7.0 O2-saturated solutions. Scan rate: 20 mV s−1.


### 3.3.1. Hydrodynamic Voltammetric Studies

The electrocatalytic ORR was also carried out at rotating MHAQ/GC and MHAQ/MWCNTs/GC disk electrodes in pH 7.0 solutions between 0 and −0.9 V with scan rate of 10 mV s−1. A set of current-potential curves recorded at various angular velocities are shown in Figures 7(a) and 7(c). It is obvious that the disk current of the MHAQ/GC and MHAQ/MWCNTs/GC electrodes in pH 7.0 N2-saturated solution was negligible. In both cases, the O2 reduction-limiting currents increase with increasing rotation rate and show well-defined plateaus. The ORR onset potential for the MHAQ/MWCNTs/GC electrode is −0.2 V versus SCE, ~100 mV higher than that of the MHAQ/GC electrode. In addition, the diffusion-limiting currents for the MHAQ/MWCNTs/GC electrode are also slightly higher. It is worth noting that as observed before [8, 14], the adsorbed MHAQ groups block native GC surface sites, therefore almost completely suppressing the O2 reduction prewave resulted from the native quinonoidal groups of GC. Thus, no well-defined prewave is observed in the present work at low overpotential which is in agreement with the results described in Figure 6. At potentials more negative than ca −0.4 V, the O2 reduction current increases sharply close to its diffusion-limited value.RDE voltammetry curves of oxygen reduction on (a) MHAQ/GC and (c) MHAQ/MWCNTs/GC electrodes in pH 7.0 O2-saturated solutions at various rotation rates. Scan rate: 10 mV s−1. The Koutecky-Levich plots of oxygen reduction on (b) MHAQ/GC and (d) MHAQ/MWCNTs/GC electrodes at various cathodic potentials. The insets in (a) and (c) show the potential dependence of the number of electrons transferred per O2 molecule.
(a)
(b)
(c)
(d)(a) Nyquist plots obtained from EIS measurements in pH 7.0 N2- and O2-saturated solutions on the bare GC, MWCNTs/GC, MHAQ/GC, and the MHAQ/MWCNTs nanohybrid modified GC electrodes at a cathodic polarization potential of −0.4 V. (b) the applied equivalent circuit model.
(a)
(b)The Koutecky-Levich (K-L) analyses of the RDE data derived from the limiting current measured at various potential are shown in Figures7(b) and 7(d). The number of electron transferred per O2 molecule (n) was calculated from these plots using the K-L equation [28]:
(2)1I=1Id+1Ik=-10.62nFADO22/3γ-1/6cO2ω1/2-1nFAkcO2,
where I is the measured current, Ik and Id are the kinetic and diffusion-limited currents, respectively, n is the number of electron transferred per O2 molecule, k is the rate constant for O2 reduction, F is the Faraday constant (96,485 C mol−1), A is the GC disk electrode surface area, ω is the rotation rate, cO2 is the concentration of oxygen in the bulk (cO2=1.2×10-6 mol cm−3 at pHs 5–13) [10], DO2 is the diffusion coefficient of oxygen (DO2=1.9×10-5 cm2 s−1 at pHs 5–13) [10], and γ is the kinematic viscosity of the solution (0.01 cm2 s−1).The extrapolated K-L lines showed nonzero intercepts, indicating that the O2 reduction process on the MWCNTs/GC and MHAQ/MWCNTs/GC electrodes is under the mixed kinetic-diffusion control in a large range of potentials. The inset in Figures 7(a) and 7(c) compares the n values calculated from K-L equation at various potentials and shows that the O2 reduction proceeds predominantly by the two-electron pathway on MHAQ/GC and MHAQ/MWCNTs/GC electrodes, indicating that in this potential range O2 reduction on the MHAQ/MWCNTs nanohybrid modified electrode yields hydrogen peroxide as the final product. From the intercepts of the K-L plots, the average apparent electron transfer rate constant (k0) of ORR on MHAQ/MWCNTs/GC electrode in the potential range from −0.5 to −0.9 V can be determined and was calculated as ca. 0.344 cm s−1; thus a value for the second-order rate constant K=1.52×106 M−1 s−1 was obtained using k0=KΓAQ  [14].


### 3.3.2. Electrochemical Impedance Studies

EIS is a powerful tool for studying the interface properties of chemically modified electrodes. From the complex plane plots of an impedance spectrum, the electron-transfer kinetics and the diffusion characteristics can be determined. The EIS of the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes recorded in pH 7.0 N2- and O2-saturated buffer solutions at −0.4 V are presented in Figure 8(a).  As can be seen, the Nyquist plots present the suppressing semicircle arcs in the high-frequency region and almost straight lines in the low-frequency region for all four cases. The respective semicircle parameters correspond to the electron-transfer resistance (Rct) and the double-layer capacity (Cdl) nature of the studied electrodes. The high-frequency intercept of semicircle arc on the real impedance axis yields the solution ohmic resistance (Rs) between the electrode surface and the reference electrode, while the diameter of semicircle arc provides Rct of charge transport across the electrode/solution interface.A significant difference in the semicircle region which is associated with the electron-transfer resistance behavior of electrochemical reactions was observed for the four different operational conditions. The change of semicircle arc diameter in N2-saturated to O2-saturated solution indicates the existence of electron transfer between dissolved oxygen and the studied electrodes. The distinct decrease in the arc diameter can be regarded as a sufficientevidence to explain the enhanced electrocatalytic performance of MHAQ/MWCNTs/GC towards ORR. Furthermore, the suppressing semicircle arcs observed in O2-saturated condition clearly indicate the lower electron-transfer resistance behavior compared to that in N2-saturated case. This phenomenon is in agreement with the early results obtained by Yeh and Wang [29] that the characteristic charge-transfer-controlled semicircles featured in the high-frequency region decrease in radius with an increase in the concentration of dissolved oxygen. On the other hand, a linear part with a slope of ca. π/4 appears in the middle-frequency region for O2-saturated solution, implying the finite diffusion of oxygen across the electrode/solution interface during ORR, while in the relatively lower frequency region the slope of the linear part is greater than π/4, which indicates that the mass-transport rate of oxygen to the electrode/solution interface is also an influence factor for ORR at −0.4 V.The impedance parameters of ORR are evaluated using an equivalent circuit shown in Figure8(b). To obtain the best fit, two constant phase elements (CPE1 and CPE2) were used to replace the double-layer capacitance of electrode/electrolyte interface. The impedance of CPE is defined as [30]
(3)ZCPE=1Y0(jω)φ,
where Y0 is the CPE constant, ω is the angular frequency (in rad s−1), j2=-1 is the imaginary number, and φ is the CPE exponent. Depending on φ(0≤φ≤1), the CPE is considered to be a pure capacitor (ZCPE=C, φ=1) and a pure resistance (ZCPE=R, φ=0). When φ=0.5, the CPE is related to the well-known Warburg impedance, which represents the diffusion-controlled processes. Moreover, the theory on the kinetics of electrocatalytic reactions at three dimensional modified electrodes has been well developed in the works byAlbery et al.[31, 32] andSaveant et al.[33–36]. According to their theory, the other fitting parameters in equivalent circuit should involve (i) the electrolyte resistance (Rs), (ii) the electron-transfer resistance (Rct) corresponding to charge transfer at the electrode/solution interface, (iii) the total resistance of electrode/solution interface (Rf), and (iv) the diffusion impedance (ZD) which is associated with the diffusion of dissolved reactant (O2) and products (H2O2) across the electrode/solution interface. The apparent electron-transfer rate constant (k0) of ORR can be obtained from Rct using [37]
(4)k0=RTn2F2ACO2bRct.The apparent electron transfer rate constantk0 of ORR on the MHAQ/MWCNTs/GC electrode was determined as 1.14 × 10−2 cm s−1, which agrees well with the rate constant value (1.69 × 10−2 cm s−1) calculated by the K-L equation at the same polarization potential (−0.4 V). We hypothesize the following reason for the enhanced electrocatalytic activity for ORR on the MHAQ/MWCNTs/GC electrode. The presence of MWCNTs provides the low charge transfer resistance and high surface area, and the adsorbed MHAQ on MWCNTs/GC surface keeps excess electroactive sites resulting in the more favorable electron-transfer kinetics for ORR at the MHAQ/MWCNTs nanohybrid.


## 3.3.1. Hydrodynamic Voltammetric Studies

The electrocatalytic ORR was also carried out at rotating MHAQ/GC and MHAQ/MWCNTs/GC disk electrodes in pH 7.0 solutions between 0 and −0.9 V with scan rate of 10 mV s−1. A set of current-potential curves recorded at various angular velocities are shown in Figures 7(a) and 7(c). It is obvious that the disk current of the MHAQ/GC and MHAQ/MWCNTs/GC electrodes in pH 7.0 N2-saturated solution was negligible. In both cases, the O2 reduction-limiting currents increase with increasing rotation rate and show well-defined plateaus. The ORR onset potential for the MHAQ/MWCNTs/GC electrode is −0.2 V versus SCE, ~100 mV higher than that of the MHAQ/GC electrode. In addition, the diffusion-limiting currents for the MHAQ/MWCNTs/GC electrode are also slightly higher. It is worth noting that as observed before [8, 14], the adsorbed MHAQ groups block native GC surface sites, therefore almost completely suppressing the O2 reduction prewave resulted from the native quinonoidal groups of GC. Thus, no well-defined prewave is observed in the present work at low overpotential which is in agreement with the results described in Figure 6. At potentials more negative than ca −0.4 V, the O2 reduction current increases sharply close to its diffusion-limited value.RDE voltammetry curves of oxygen reduction on (a) MHAQ/GC and (c) MHAQ/MWCNTs/GC electrodes in pH 7.0 O2-saturated solutions at various rotation rates. Scan rate: 10 mV s−1. The Koutecky-Levich plots of oxygen reduction on (b) MHAQ/GC and (d) MHAQ/MWCNTs/GC electrodes at various cathodic potentials. The insets in (a) and (c) show the potential dependence of the number of electrons transferred per O2 molecule.
(a)
(b)
(c)
(d)(a) Nyquist plots obtained from EIS measurements in pH 7.0 N2- and O2-saturated solutions on the bare GC, MWCNTs/GC, MHAQ/GC, and the MHAQ/MWCNTs nanohybrid modified GC electrodes at a cathodic polarization potential of −0.4 V. (b) the applied equivalent circuit model.
(a)
(b)The Koutecky-Levich (K-L) analyses of the RDE data derived from the limiting current measured at various potential are shown in Figures7(b) and 7(d). The number of electron transferred per O2 molecule (n) was calculated from these plots using the K-L equation [28]:
(2)1I=1Id+1Ik=-10.62nFADO22/3γ-1/6cO2ω1/2-1nFAkcO2,
where I is the measured current, Ik and Id are the kinetic and diffusion-limited currents, respectively, n is the number of electron transferred per O2 molecule, k is the rate constant for O2 reduction, F is the Faraday constant (96,485 C mol−1), A is the GC disk electrode surface area, ω is the rotation rate, cO2 is the concentration of oxygen in the bulk (cO2=1.2×10-6 mol cm−3 at pHs 5–13) [10], DO2 is the diffusion coefficient of oxygen (DO2=1.9×10-5 cm2 s−1 at pHs 5–13) [10], and γ is the kinematic viscosity of the solution (0.01 cm2 s−1).The extrapolated K-L lines showed nonzero intercepts, indicating that the O2 reduction process on the MWCNTs/GC and MHAQ/MWCNTs/GC electrodes is under the mixed kinetic-diffusion control in a large range of potentials. The inset in Figures 7(a) and 7(c) compares the n values calculated from K-L equation at various potentials and shows that the O2 reduction proceeds predominantly by the two-electron pathway on MHAQ/GC and MHAQ/MWCNTs/GC electrodes, indicating that in this potential range O2 reduction on the MHAQ/MWCNTs nanohybrid modified electrode yields hydrogen peroxide as the final product. From the intercepts of the K-L plots, the average apparent electron transfer rate constant (k0) of ORR on MHAQ/MWCNTs/GC electrode in the potential range from −0.5 to −0.9 V can be determined and was calculated as ca. 0.344 cm s−1; thus a value for the second-order rate constant K=1.52×106 M−1 s−1 was obtained using k0=KΓAQ  [14].


## 3.3.2. Electrochemical Impedance Studies

EIS is a powerful tool for studying the interface properties of chemically modified electrodes. From the complex plane plots of an impedance spectrum, the electron-transfer kinetics and the diffusion characteristics can be determined. The EIS of the bare GC, MHAQ/GC, MWCNTs/GC, and MHAQ/MWCNTs/GC electrodes recorded in pH 7.0 N2- and O2-saturated buffer solutions at −0.4 V are presented in Figure 8(a).  As can be seen, the Nyquist plots present the suppressing semicircle arcs in the high-frequency region and almost straight lines in the low-frequency region for all four cases. The respective semicircle parameters correspond to the electron-transfer resistance (Rct) and the double-layer capacity (Cdl) nature of the studied electrodes. The high-frequency intercept of semicircle arc on the real impedance axis yields the solution ohmic resistance (Rs) between the electrode surface and the reference electrode, while the diameter of semicircle arc provides Rct of charge transport across the electrode/solution interface.A significant difference in the semicircle region which is associated with the electron-transfer resistance behavior of electrochemical reactions was observed for the four different operational conditions. The change of semicircle arc diameter in N2-saturated to O2-saturated solution indicates the existence of electron transfer between dissolved oxygen and the studied electrodes. The distinct decrease in the arc diameter can be regarded as a sufficientevidence to explain the enhanced electrocatalytic performance of MHAQ/MWCNTs/GC towards ORR. Furthermore, the suppressing semicircle arcs observed in O2-saturated condition clearly indicate the lower electron-transfer resistance behavior compared to that in N2-saturated case. This phenomenon is in agreement with the early results obtained by Yeh and Wang [29] that the characteristic charge-transfer-controlled semicircles featured in the high-frequency region decrease in radius with an increase in the concentration of dissolved oxygen. On the other hand, a linear part with a slope of ca. π/4 appears in the middle-frequency region for O2-saturated solution, implying the finite diffusion of oxygen across the electrode/solution interface during ORR, while in the relatively lower frequency region the slope of the linear part is greater than π/4, which indicates that the mass-transport rate of oxygen to the electrode/solution interface is also an influence factor for ORR at −0.4 V.The impedance parameters of ORR are evaluated using an equivalent circuit shown in Figure8(b). To obtain the best fit, two constant phase elements (CPE1 and CPE2) were used to replace the double-layer capacitance of electrode/electrolyte interface. The impedance of CPE is defined as [30]
(3)ZCPE=1Y0(jω)φ,
where Y0 is the CPE constant, ω is the angular frequency (in rad s−1), j2=-1 is the imaginary number, and φ is the CPE exponent. Depending on φ(0≤φ≤1), the CPE is considered to be a pure capacitor (ZCPE=C, φ=1) and a pure resistance (ZCPE=R, φ=0). When φ=0.5, the CPE is related to the well-known Warburg impedance, which represents the diffusion-controlled processes. Moreover, the theory on the kinetics of electrocatalytic reactions at three dimensional modified electrodes has been well developed in the works byAlbery et al.[31, 32] andSaveant et al.[33–36]. According to their theory, the other fitting parameters in equivalent circuit should involve (i) the electrolyte resistance (Rs), (ii) the electron-transfer resistance (Rct) corresponding to charge transfer at the electrode/solution interface, (iii) the total resistance of electrode/solution interface (Rf), and (iv) the diffusion impedance (ZD) which is associated with the diffusion of dissolved reactant (O2) and products (H2O2) across the electrode/solution interface. The apparent electron-transfer rate constant (k0) of ORR can be obtained from Rct using [37]
(4)k0=RTn2F2ACO2bRct.The apparent electron transfer rate constantk0 of ORR on the MHAQ/MWCNTs/GC electrode was determined as 1.14 × 10−2 cm s−1, which agrees well with the rate constant value (1.69 × 10−2 cm s−1) calculated by the K-L equation at the same polarization potential (−0.4 V). We hypothesize the following reason for the enhanced electrocatalytic activity for ORR on the MHAQ/MWCNTs/GC electrode. The presence of MWCNTs provides the low charge transfer resistance and high surface area, and the adsorbed MHAQ on MWCNTs/GC surface keeps excess electroactive sites resulting in the more favorable electron-transfer kinetics for ORR at the MHAQ/MWCNTs nanohybrid.


## 4. Conclusion

The MHAQ/MWCNTs nanohybrid modified GC electrode was successfully fabricated and characterized with SEM and TEM techniques. The electrochemical behavior, the electrocatalytic activity, and kinetic parameters of the MHAQ/MWCNTs/GC electrodes towards ORR were investigated in pH 7.0 buffer solution by CV, RDE, and EIS techniques. The results demonstrated that MHAQ adsorbed onto MWCNT/GC surface exhibited potent electrocatalytic activities towards two-electron reduction of oxygen at pH 7.0 with overpotential of about 214 mV lower than that of the bare GC electrode. Such MHAQ/MWCNTs nanohybrid can be utilized as low-cost electrode material for ORR in virtue of the advantages such as ease of fabrication, excellent electrocatalytic activity, and good stability.



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*Source: 756307-2012-10-15.xml*