cb pasta de C e polioxometalato

cb pasta de C e polioxometalato

(Parte 1 de 2)

A novel bacterial cellulose-based carbon paste electrode and its polyoxometalate-modified properties

Yan Liang a, Ping He a,*, Yongjun Ma a, Yong Zhou b, Chonghua Pei a,*, Xiaobing Li cSchool of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR ChinaSchool of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR ChinaInstitute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621900, PR China article info

Article history: Received 18 February 2009 Received in revised form 2 March 2009 Accepted 3 March 2009 Available online 10 March 2009

Keywords: Bacterial cellulose Carbon paste electrode Polyoxometalate Modified electrode Electrocatalysis abstract

A novel bacterial cellulose nanofiber-based carbon paste electrode (BCPE) was fabricated. It was characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Compared with traditional carbon paste electrode, BCPE exhibited better electrochemical reversibility with the enhancement of the redox currents and decrease of peak potential separation as well as lower charge transfer resistance in Fe(CN)6 3 /4 redox system. Keggin-type sodium phospho- polyoxomolybdate, PMo12, was successfully assembled on BCPE via cyclic voltametric scan, and the obtained PMo12/BCPE possessed not only a good electrochemical behavior but also an excellent electrocatalytic activity toward the reduction of nitrite. Because of its nano-dimension, lower cost and promi- nent electrochemical properties, bacterial cellulose-based carbonaceous materials would be a candidate of graphite for the preparation of novel carbon paste electrode. 2009 Elsevier B.V. All rights reserved.

1. Introduction

Bacterial cellulose (BC) produced by bacteria is an unbranched macromolecular compound composed of b-1,4-linked glucopyranose unit [1,2]. With so many unique properties such as high chemical purity, ultrafine network structure, good mechanical strength, biocompatibility and the controllability in the preparation process, BC and its derivatives could be widely applicable in paper, medical materials, permeable membrane and thermotropic liquid crystalline, etc. [3–9]. In our recent experiments, BC was prepared according to our former work [2]. Furthermore, it was found that, upon carbonizing at temperature up to 900 C under nitrogen atmosphere, BC was converted into a kind of carbon nanofiber, and the corresponding SEM image was shown in Fig. 1.

Carbon paste electrode (CPE) consisting of carbon powder and water-immiscible liquid binder is one of the most commonly used electrodes in electrochemical investigations. It is characteristic of many properties such as low background current, easy renewal of its surface, facile fabrication and modification with desired properties via incorporating different substances during the paste preparation [10–16]. However, its application was limited because of the low sensitivity and selectivity. To our knowledge, there have been some researches on the modification of CPEs with carbon nanotube, mesoporous materials carbon nanofiber and other car- bonaceous materials for the purpose of enhancing the sensitivity and selectivity [17–2]. Nevertheless, the modification of CPE with carbonized nanofiber BC has never been studied.

In this paper, a novel nano-dimension carbonized BC based

CPE (BCPE) was developed and its electrochemical behavior was studied by cyclic voltammetry, electrochemical impedance spectroscopy, etc. Furthermore, Keggin-type sodium phosphopolyoxo- molybdate (Mo12Na3O40P, PMo12) was assembled on BCPE via cyclic voltametric scan method [23], and the PMo12/BCPE modified electrode possesses excellent electrocatalytic activity to the reduction of nitrite.

2. Experimental

BCPE was prepared by hand-mixing of 40.0 mg carbonized BC [2] with 70 l Silicone oil in an agate mortar and ground to uniform paste. The paste was firmly packed into a cavity (U 3 m) at the end of a Teflon tube. The electrical contact was achieved by a copper wire connected to the paste in the inner hole of the tube. The fabrication procedure of a traditional carbon paste electrode was similar to BCPE just replacing bacterial cellulose with 200 mg graphite powder. All the surfaces of BCPE and CPE were smoothed on a piece of weighing paper just prior to use. PMo12 was assembled on the obtained electrode surfaces via cyclic voltametric scan for 100 segments in 0.1 M H2SO4 + 50 mM PMo12 solution in the potential range of 0.5 to 0.25 V for BCPE and 0.45 to 0.20 V for CPE, respectively.

1388-2481/$ - see front matter 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.03.001

E-mail addresses: heping@swust.edu.cn heping1971@yahoo.com.cn (P. He), peichonghua@swust.edu.cn (C.H. Pei).

Electrochemistry Communications 1 (2009) 1018–1021 Contents lists available at ScienceDirect

Electrochemistry Communications journal homepage: w.else vier.com/locate/elecom

Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed on PARSTAT 2273 Electrochemical System (Princeton Applied Research, USA). Three-electrode system was utilized including a home-made BCPE or traditional CPE as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum wire as auxiliary electrode. EIS was per- formed in 0.1 M KCl + 1 mM K3Fe(CN)6 solution with the perturbation amplitude 5 mV (versus open circuit potential), and the frequencies swept from 105 to 5 10 3 Hz. The data were analyzed and fitted with the aid of Zview software (Scribner Associates Inc.). Scanning electron microscopy (Hitachi S4800, Japan) was utilized to study the morphology of bacterial cellulose.

3. Results and discussion

Shown in Fig. 1 was SEM image of carbonized BC treated at 900 C. It was obvious that carbonized BC is a kind of nanofiber with three-dimensional network-like structure, which could significantly increase the effective electrode surface and facilitate the diffusion of analytes into the film.

Potassium ferricyanide was chosen as a probe to evaluate the performance of the proposed electrodes. Fig. 2 exhibited typical electrochemical responses of CPE and BCPE in 0.1 M KCl + 1 mM

K3Fe(CN)6 solution, respectively. Good linear relationships be- tween the redox currents and the square root of scan rates were observed for the two electrodes over 0.01–0.25 V s 1 range, revealing a solution control mechanism and a faster charge transfer kinetics. On the other hand, the peak-to-peak potential separations

(DEp) of 0.131 and 0.078 V were observed at CPE (curve a) and BCPE (curve b), respectively, and a pair of more well-defined redox peaks appeared on BCPE with larger peak current. The decrease in peak separation and enhanced currents observed on BCPE reflected a change in diffusional regime [24]. Furthermore, the results indicated that better reversibility was obtained on BCPE than that on CPE in terms of improved reversibility and enhanced sensitivity. All these could be attributed to the use of nano-dimension carbonized BC with higher active surface area as carbonaceous materials as well as its cross-linked structure, which was propitious to a faster electron transfer. As for the slightly bigger capacitive current of BCPE compared with traditional CPE, it was possibly related to the nano-dimension net structure of carbonized BC.

Illustrated in Fig. 3 are Nyquist plots of the two well-fabricated electrodes. The depressed arcs in mid-high frequency section presented typical constant phase element (CPE) characteristics containing information of kinetics of the faradic process, and they could be fitted using (RctCPE) circuit. The approximately linear curves in low-frequency section were related to capacitance and diffusion resistance, and thus could be fitted using (CW) circuit, in which Warburg impedance (W) responded to diffusion and capacitance(C) was contained in CPE mentioned above. Thus, an equivalent circuit as shown in the inset (a) of Fig. 3 was designed, by which the fitted plots were quite well in agreement with the measured plots. Shown in Table 1 was the data of kinetics parameters of the two electrodes. The value of charge transfer resistance

(Rct) of BCPE was much smaller than that of CPE, indicating a faster electron transfer process on BCPE than that on CPE [25], which could be attributed to that the carbonized BC has a higher real surface area.

Shown in Fig. 4a were the CVs of the PMo12 film growth on BCPE by potential cycling in 0.1 M H2SO4 solution containing PMo12 at the scan rate of 100 mV s 1. Three redox couples corresponding to PMo12 redox reaction appeared in the potential range from 0.5 to 0.25 V, and all the reduction and oxidation peak currents

Current / uA

Potential / V (vs. SCE)

Fig. 2. CVs of CPE (a) and BCPE (b) in 0.1 M KCl + 1 mM K Fe(CN) at scan rate of 10 mV s .

Rs CPE

Rct W -Z''/ ohm

Z' / ohm

BCPE (measured) BCPE (fitted )a

-Z'' / ohm

Z' / ohm

CPE (measured) BCPE (measured) BCPE (fitted)

Fig. 3. Nyquist plots of BCPE (1) and CPE (2) in 0.1 M KCl + 1 mM K Fe(CN) . The applied perturbation amplitude was 5 mV (versus open circuit potential) and the frequencies were swept from 10 to 5 10 Hz. Equivalent circuit (b). Rs: solution resistance; Rct: charge transfer resistance; CPE: constant phase element, which is a complex of various elements; W: Warburg resistance, which reflects diffuse barrier at low-frequency part.

Fig. 1. SEM image of carbonized BC. Y. Liang et al./Electrochemistry Communications 1 (2009) 1018–1021 1019 increased gradually with increasing cycling time until they reached a stabled state. CVs of PMo12 growing on CPE were also obtained in

0.1 M H2SO4 solution in the presence of PMo12 (not shown), and the redox peak currents were lower than those on BCPE. It could be attributed to the higher real active surface area and more surface defects of carbonized BC due to its nano-structure [20], resulting in larger number of activated sites than that on CPE. As shown in Fig. 4b, there are three pairs of well-defined redox peaks corresponding to 2-, 4-, 6-electron transfer of PMo12 on PMo12/BCPE in the potential range from 0.5 to 0.25 V and on PMo12/CPE from 0.45 to 0.2 V. Its electrochemical behavior was similar to that of PMo12 in aqueous solution, showing that the surface of electrodes were modified successfully, and the structure

and character of PMo12 on the two electrodes surface were not varied through the assembled process. There was a linear relationship between the redox peak currents and scan rates for PMo12/CPE (not shown), showing that the redox process was controlled by surface reaction on PMo12/CPE. While the redox peak currents were proportional to the square root of scan rates up to 800 mV s 1 for

PMo12/BCPE, indicating that a solution controlled reaction process happened on PMo12/BCPE with faster electron transfer kinetics in wide scan rate ranges. Moreover, the peak currents of PMo12/BCPE

were much higher than those of PMo12/CPE, which was possibly relevant to the nano-structure and more surface defects of carbon- ized BC and therefore more PMo12O40 3 anions were absorbed on the surface of BCPE.

It was well known that polyoxomalates (POMs) have been employed extensively in electrocatalytic reduction of nitrite since reduced POMs can serve as powerful electron reservoirs and are capable of delivering electrons to other species [26–30]. Therefore,

the electrocatalytic activity to the reduction of nitrite on PMo12/ BCPE was investigated in this work. As shown in Fig. 5, addition of nitrite ions to the cell produced a dramatic change in the cyclic voltammogram with an increase of the cathodic currents and a concomitant decrease of the anodic currents, indicating that an electrocatalytic process occurred on the electrode surface. Moreover, all the three redox couples presented the same feature, revealing that all the three cathodic waves of PMo12O40 3 anions indeed possessed a good electrocatalytic activity toward the reduction of nitrite, which was in agreement with the reported literature [26]. As illustrated in the inset of Fig. 5, the calibration curve was obtained according to the relationship between current response of the second cathodic peak and nitrite concentrations, which fol-

reproducibility and stability of PMo12/BCPE were assessed by successive cyclic scanning from 0.5 to 0.25 V over 100 times, and it was observed that the redox currents of PMo12 on PMo12/BCPE had no apparent change. Therefore, the PMo12-modified BCPE exhibited a good stability for the electrocatalytic application.

BCPE was fabricated with nano-dimension carbonized BC fiber and studied by SEM, cyclic voltammetry and electrochemical impedance spectroscopy. Compared with traditional CPE, a faster

Table 1 Values for the parameters R , CPE-T, CPE-P, W-R, W-T, W-P and the associated error% computed by fitting of the experimental EIS data (Fig. 3).

Value Error Value Error Value Error Value Error Value Error Value Error

500 mV s 10 mV s

Current / uA

Potential / V (vs. SCE)

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