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poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation
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Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation Sayed Reza Hosseini a,*, Jahan-Bakhsh Raoof b, Shahram Ghasemi a, Zahra Gholami a a b
Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran
article info
abstract
Article history:
In this work, for the first time, a novel and facile method for preparation of highly porous
Received 27 August 2014
poly (o-Anisidine) (POA) by two-step process, through electropolymerization after elec-
Received in revised form
trochemical pretreatment of carbon paste electrode (CPE) is reported. Electrochemical
7 October 2014
response of the prepared POA at the surface of electrochemically pretreated CPE (pCPE)
Accepted 22 October 2014
is, at least, 12 times higher than that obtained at the CPE. Copper (Cu) nanoparticles are
Available online xxx
uniformly electrodeposited onto the POA/pCPE composite. Then, platinum (Pt) nanoparticles are prepared through spontaneous and irreversible reaction via galvanic
Keywords:
replacement between [PtCl6]2 ions and the Cu nanoparticles. The prepared bimetallic
Carbon paste electrode
Pt/Cu nanoparticles are characterized by scanning electron microscopy (SEM), energy
Electrochemical pretreatment
dispersive spectroscopy and electrochemical methods. Electrochemical impedance
Poly (o-Anisidine)
spectroscopy shows that on the pCPE, charge transfer resistance is much smaller than
Bimetallic Pt/Cu nanoparticles
that the CPE. The SEM images reveal that the electrochemical pretreatment of the CPE
Redox replacement
strongly influences morphology of the polymer film. The methanol oxidation and sta-
Methanol oxidation
bility of the PteCu/POA/pCPE are investigated by various electrochemical techniques. The results indicate that the modified electrode exhibits excellent electrocatalytic activity towards methanol oxidation. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction An attractive power source with potential applications in a variety of systems, ranging from portable electronic device to automobile is direct methanol fuel cell (DMFC) [1,2]. DMFC has several advantages such as high energy conversion efficiency, low pollution emission, and safe fuel handling. It is well-
known that Pt with high activity is an ideal catalyst for methanol oxidation in an acidic medium. However, it is an expensive material, and cost of the DMFC containing Pt is prohibitively high and it becomes poisoned by COads which is formed during the methanol oxidation [3]. Development of less expensive anode materials with high electrocatalytic activity, reduced susceptibility to COads poisoning, and proton
* Corresponding author. Tel.: þ98 1135302333; fax: þ98 1135302350. E-mail address: [email protected] (S.R. Hosseini). http://dx.doi.org/10.1016/j.ijhydene.2014.10.104 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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conductivity are some effort towards overcoming the mentioned problems. Several studies have been focused on the application of bimetallic Pt-based electrocatalyst, such as PteRu [4], PtePd [5], PteSn [6] and PteWO3 [7] which significantly reduce the over-potential of methanol oxidation and offer considerable improvement in the catalytic properties relative to the separate metals. The Pt-based catalysts reduce the expensive Pt cost and improve its electrocatalytic activity. The enhanced catalytic activity of Pt-based catalyst can be attributed to the electronic and structural effects by the addition of certain metal in Pt particles [8,9]. However, these solutions are not simple and cost-effective. Conducting polymers (CPs) as convenient and cheap supporting materials with high surface area, environmental stability and porous structures improve the properties of electrode-electrolyte interface and allow a facile flow of electronic charges during the methanol oxidation on the Pt particles [10]. The activity of the Pt-based catalysts due to their high surface area, superior resistance against poisoning effect and/ or synergistic effects between the CPs and relevant Pt is greatly enhanced for the methanol oxidation [11,12]. The Pt particles dispersed into the CP support, not only provide access to large number of catalytic sites and reduce the catalyst loading under the condition of keeping high catalytic activity but also offer the possibility of spent catalyst recovery [13,14]. A number of the CPs have been investigated as conducting catalyst supports for methanol oxidation [15e18]. Among them, polyaniline has shown great promise as a cocatalyst [19e23]. As one of the polyaniline derivatives, poly (o-Anisidine) (POA) films have been prepared at the surface of copper [24] and Pt disk [25,26] by using potentiodynamic method. On the other hand, the ease and fast preparation, low background current, wide potential window, porous and reproducible surface, and low cost of carbon paste electrode (CPE) have been motivated the researchers to use the electrode substrate in diverse applications [27]. The o-Anisidine (OA) can be electrochemically polymerized on the CPE to form POA by using conventional cyclic voltammetry. However, its polymerization rate is very sluggish. To the best of our knowledge, up to now, some strategies were reported about POA formation with high porosity and conductivity. Ojani et al. [28] prepared POA on the CPE surface by using cyclic voltammetry technique in monomer solution containing sodium dodesyl sulfate (SDS) as an anionic surfactant. Adding SDS to the solution leads to increasing the polymer growth rate. Kumar [29] prepared nanofibrillar porous POA films by stepwise electrooxidation of monomer on a glassy carbon electrode by single potential time base galvanostatic mode. Raoof et al. [30] prepared POA/carbon nanotube supporting material for dispersion of nickel species for electrocatalytic oxidation of methanol in alkaline medium. Recently, Dong et al. [31] prepared POA/multi-walled carbon nanotube films onto Pt electrode by using cyclic voltammetry method. Surprisingly, the literature survey indicates that there is no report as yet on electropolymerization of OA at the popular CPE without any additive and template. In this work, as a continuation of the previous study concerning the development of electrocatalyst for methanol oxidation [32], we have described the preparation of PteCu/POA composite onto electrochemically pretreated CPE (pCPE). In order to increase the electron transfer rates, electrochemical pretreatment route for the CPE has been
developed which has been shown to produce improved electrode response compared to that of the native electrode material. In addition, this particular modification strategy has been shown to produce a marked enhancement in the polymer growth rate which exhibits very sluggish rate at untreated CPE. The experimental results indicate that the OA electropolymerized on the pCPE has a more rapid charge transfer rate compared to the normal POA and bimetallic Pt/Cu nanoparticles exhibit pronounced increase in the electrocatalytic activity and a significantly less susceptibility to COads poisoning. In fact, the main objective of the present work is to prepare controlled sizes of Pt nanoparticles in the porous POA matrix, and maintain the homogenous distribution of these particles by employing a new and facile approach.
Experimental Materials and instrumentation OA (>98%, Fluka) was used as a monomer at electrochemical polymerization process. Sulfuric acid (98%, Merck) and sodium hydroxide (>99%, Merck) were used for preparation of the supporting electrolytes. K3Fe(CN)6, K4Fe(CN)6 (>99%, Merck) and KCl (>99%, Merck) were used for the electrochemical impedance studies. K2PtCl6 (Merck), CuSO4.5H2O (99%, Merck) and methanol (>99.9%, Merck) were used as received. High viscosity paraffin oil (density: 0.86 g cm3, Merck) was used as pasting liquid in preparation of the CPE. Graphite powder (particle diameter: 0.1 mm, Merck) was used as the working electrode substrate. The solvent for preparation the reagents solutions was double distilled water. The electrochemical experiments were carried out by using potentiostat/galvanostat Autolab (Nova software model PGSTAT 302N, Metrohm, Netherlands) coupled with a personal computer. Electrochemical impedance spectroscopy (EIS) was performed by potentiostat/galvanostat Palmsense (PSTrace software version 4.2.2, Netherlands). The threeelectrode system consists of the CPE (3.4 mm in diameter) as working electrode, AgjAgCljKCl (3 M) as reference electrode and Pt wire as auxiliary electrode. The surface morphology and elemental analysis were performed by scanning electron microscopy (SEM, model KYKY-EM3200, China) and energy dispersive spectrometer (EDS, VEGA-Tescan, Razi Metallurgical Research Center, Tehran, Iran), respectively.
Preparation of the PteCu/POA/pCPE Fabrication of the CPE (graphite powder and paraffin oil in a ratio of 67:33% (w/w)) was performed according to our previous works. Then, cyclic voltammetry was performed between 0.0 and 1.0 V at 50 mV s1 in 0.5 M H2SO4 solution, until stable voltammograms were obtained. Before electropolymerization, the CPE was electrochemically pretreated at a fixed potential 1.8 V vs. AgjAgCljKCl (3 M) in 0.5 M H2SO4 solution for 2 min without any stirring. The electrode was denoted as pCPE. Later modifications were performed in three steps as shown in Fig. 1: (a) Electrochemical polymerization of 5.0 mM OA monomer in 0.5 M H2SO4 solution by using consecutive
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 1 e Schematic procedure for preparation of the PteCu/POA/pCPE composite.
potential cycling (10 cycles at y ¼ 50 mV s1) between 0.0 and 1.0 V. (b) Electrodeposition of Cu nanoparticles onto the POA/ pCPE at a fixed potential 0.17 V for 50 s in 0.1 M H2SO4 solution containing 0.02 M CuSO4. (c) Partial galvanic replacement of the Cu for deposition of Pt nanoparticles by immersing the electrode in 2.0 mM K2PtCl6 þ 0.1 M H2SO4 stirred solution under open circuit condition for 15 min. Table 1 summarizes several surface parameters such as geometric surface area, polymer mass, film thickness, Cu loading, Pt real surface area and roughness factor for the PteCu/POA/pCPE. The geometric surface area of the electrode was used to calculate the current density, except the comparison of methanol oxidation. All experiments were conducted at ambient temperature.
Results and discussion Electrochemical properties of the pCPE Fig. 2A shows the cyclic voltammograms (CVs) of the CPE (a) and pCPE (b) in 0.1 M KCl solution. As can be seen in Fig. 2A, no currents can be observed with the electrodes and background current of the pCPE is much larger than the CPE. The enhancement at the background current is ascribed to double layer charging
Table 1 e Surface parameters of the PteCu/POA/pCPE. Surface parameter
Value 2
2
Geometric area, Ag/cm (pr ) POA mass, WPOA/mg (h Qdep M/F n) POA thickness, dn/mm (h Qdep M/r Ag F n) Cu loading, WCu/mg cm2 (Qnet M/F Z) Pt loading, WPt/mg cm2 Pt real surface area, Ar/cm2 (QH/210 mC) Roughness factor, RF (Ar/Ag)
0.09 3.10 0.23 0.030a, 0.032b 0.046c, 0.049d 0.60 6.67
with the increased surface roughness after the electrochemical pretreatment [33]. The plane of carbon atoms in graphite oxide (having similar layered structure to graphite) is heavily decorated by oxygen-containing functional groups. These groups expand the interlayer distance and make the atomic-thick layers hydrophilic [34]. Formation of oxygen-containing surface states promote the removal of the inhibitory paraffin from the graphite particles and thereby cause the surface to resemble dry graphite 4 as a well-known redox couple was used to [27]. Fe(CN)3 6 / characterize the properties of electrode surface. Electrochemical response of 1.0 mM K4[Fe(CN)6] in 0.1 M KCl solution was shown on the CPE (c) and pCPE (d). The peak to peak potential separation, (DEp ¼ EpaeEpc) at the pCPE was obtained about 70 mV, while on the CPE, it was about 130 mV. The DEp is greater than that to 2.3RT/nF (or 59/n mV at 25 C) expected for a reversible system. At the same time, the redox peak current density at the pCPE surface was 2.5-fold greater than the CPE, which suggested that electrochemical pretreatment could greatly promotes the electron transfer rate [32]. The EIS can provide useful information on impedance changes of the electrode surface. The electron transfer resistance (Rct) is equal to diameter of the semicircle and can be used to describe the interface properties. The linear part in EIS represents Warburg impedance. The Nyquist diagrams of Fe(CN)3/4 6 couple at the CPE (a) and pCPE (b) were shown in Fig. 2B. It is noticeable that at the pCPE surface, semicircle part is almost eliminated and diagram comprised of only a linear part. It means that the Rct becomes unimportant in relation to Warburg impedance [35]. The CPE surface becomes hydrophilic after electrochemical activation and repulsive interaction (electrostatic and steric) does not exist between the redox marker ions and the electrode surface [36,37]. This implies that the pretreatment of the CPE with potentiostatic technique facilitates the electron transfer of the electrochemical probe and has significant favorable effect on the electrode response. The negatively charged species may be attracted to the hydrophilic pCPE surface. The slope of (b) is higher than (a) which indicates that the functional groups at the pCPE surface helps to enhance the mass diffusion.
a
As determined from chronoamperometry experiments. The calculated loading obtained from stripping of Cu in 0.5 M H2SO4 solution. c and d as maximum possible loaded Pt with assuming that all of the Cu atoms are galvanically replaced by Pt from a and b, respectively. b
Electrochemical polymerization In this work, the electropolymerization of the OA was investigated at the surface of both CPE and pCPE. The consecutive
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 2 e (A) Electrochemical responses of the CPE in 0.1 M KCl solution to: (a) 0.0 M, (c) 1.0 mM K4[Fe(CN)6] and the pCPE to: (b) 0.0 M, (d) 1.0 mM K4[Fe(CN)6] at y ¼ 50 mV s¡1. (B) Electrochemical impedance spectra for the CPE (a) and pCPE (b) þ 0.1 M KCl solution upon biasing the working electrode at 0.21 V. in 1.0 mM Fe(CN)3¡/4¡ 6
CVs recorded during the electropolymerization on the CPE were presented in Fig. 3A. On the 1st cycle, a peak related to the OA oxidation at 0.84 V appears. Deposition a part of the oxidation products at the electrode surface leads to create a POA film. As well it is obvious, by continuous potential cycling, this peak shifts towards more positive directions and its height decreases. This can reflect the loss of the surface activity when is covered by a polymer film with relatively low conductivity. The polymer peaks don't increase considerably with potential cycling. This can be attributed to this fact that the soluble products adsorb and/or react on the electrode surface and do not allow OA to reach the electrode surface to produce more cations-radical. Fig. 3B shows the electrochemical polymerization pattern of the OA at pCPE surface. As can be seen in Fig. 3B, two oxidation peaks appear at about 0.66 and 0.80 V on the 1st cycle. The first peak is attributed to the OA oxidation, and then the oxidized species are further oxidized to form POA with a
shift of scanning potential towards more positive directions. Similar type of the rare behavior at the first scan in electropolymerization has been reported for poly (pyrogallol) at surface of graphene modified glassy carbon electrode [38]. The first peak potential of the OA oxidation shifts from 0.84 V at the CPE to 0.66 V at the pCPE. Also, it should be noted that the lower onset oxidation potential (Eop) for monomer was observed at the pCPE surface indicating the facile oxidation. As the cyclic voltammetry continues, the peak current densities related to the oxidation and reduction of polymer are gradually increased. This result may be due to specific interactions between radical-cations which are formed during the 1st step of electropolymerization and hydrophilic nature of the surface. This is due to that the functional groups at the graphite oxide surface facilitate the electron exchange between the electrode and OA, so it can increase the polymer growth rate. After polymerization, the polymer modified electrodes were washed with double distilled water and the
Fig. 3 e Electrochemical polymerization pattern of 5.0 mM OA in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1 at the CPE (A) and pCPE (B). The arrows indicate the trends of currents density during the voltammetry experiments. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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potentials were cycled between 0.0 and 1.0 V (10 cycles at y ¼ 50 mV s1) in H2SO4 solution. The purpose is to remove the monomers and/or oligomers in the polymer film.
Electrochemical behavior of the POA/pCPE Fig. 4 presents the redox behavior (typical oxidation and reduction peaks) of the prepared polymer films (i.e., POA/CPE and POA/pCPE) in 0.5 M H2SO4 solution at potential range of 0.0e1.0 V at y ¼ 50 mV s1. The first (I) and second (II) redox peaks can be attributed to the POA oxidation in leucoemeraldine to emeraldine oxidation states and subsequent oxidation is corresponds to the oxidation of head-to-tail dimer, respectively [39]. The POA/pCPE (a) films showed noticeably higher peak current density than that the POA/CPE (b). The increase of surface area improves the dopingeundoping rate which is benefit to ion diffusion and migration. The DEp (¼29 mV) for the first redox peak is close to 59/n mV, which identifies that the number of involved electrons is 2 (n z 2.03). The ratio of anodic to cathodic peak current density (jpa/jpc) is almost equal to unity. The CVs of the POA/pCPE at various potential sweep rates (y ¼ 5e1000 mV s1) in 0.5 M H2SO4 solution were presented in Fig. 5. The ja and jc are linearly proportional to y. This result is attributed to the electrochemical activity of an immobilized redox couple at the electrode surface. From the slopes of these lines and by using (Ip ¼ n2F2yAG/4RT), where Ip, A and G are peak current, electrode surface area (0.09 cm2) and surface coverage, respectively, the G was derived about 1.0 106 mol cm2, which further confirmed the immobilized state of the POA film.
Electrochemical properties of the nano-Cu/POA/pCPE After Pt deposition, CVs of the POA/pCPE (a), Cu/POA/pCPE (b) and PteCu/POA/pCPE (c) were recorded in a potential window of 0.0e1.0 V in 0.5 M H2SO4 solution (Fig. 6A). As can be seen in trace (c), two apparent peaks at 0.42 and 0.60 V are assignable to the electrochemical response of the POA film and the broad peak in positive potential range corresponds to the formation of Pt oxides. It should be noted that the reduction peak of the POA is overlaid on reduction peak of Pt oxide. For comparison, CVs of the different PteCu modified electrodes in 0.5 M H2SO4 solution at potential range of 0.25e1.2 V were shown in
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Fig. 6B. The characteristic peaks (PteH redox peaks) in the negative region are connected to atomic hydrogen adsorption on Pt surface, and charge under the peaks can reveal the real surface area of the Pt particles. The larger peaks in the hydrogen adsorption/desorption regions are observed on the PteCu/POA/pCPE, which reflect the higher surface areas. The Ar for the PteCu/CPE, PteCu/pCPE, PteCu/POA/CPE and PteCu/POA/pCPE catalysts are about 0.30, 0.47, 0.36 and 0.60 cm2, respectively. Consequently, such Pt particles at the POA/pCPE enhance the active sites towards the electrocatalytic oxidation of methanol.
Surface morphology and elemental analysis In order to surface characterization, the micrographs of the CPE (a), pCPE (b), POA/CPE (c), POA/pCPE (d), Cu/POA/pCPE (e, f) and PteCu/POA/pCPE (g, h) have been investigated by SEM (Fig. 7). As can be seen on the surface of CPE (trace a), the layer of irregular flakes of graphite powder and some holes or cavities on the electrode surface was presented because carbon paste is a porous material. The surface structure of the pCPE is very complex due to many feasible factors (trace b). In this case, it seems that a change in surface morphology results from the electrochemical pretreatment. The action can be quite effective in stripping the organic inhibitory layer (nonconducting pasting liquid) since oxygen-containing surface states (much more hydrophilic surface) are formed. The SEM image moves towards that was exhibited by dry graphite. The POA film at CPE surface is completely isolated from each other and its morphology is compact (trace c). Electropolymerization at the pCPE leads to formation a homogenous thick POA film and relatively uniform coverage. In this case, more polymers successfully are distributed at the surface, indicating a larger and rougher area compared to the normal POA film (trace d). This result indicates that the surface morphology of POA film is significantly influenced by the electrochemical pretreatment. This structure enhances the electrolyte constituent access to interior of the polymer film. Also, the structure provides a larger available surface area and hence serves as a beneficial support for dispersion and/or distribution of Cu nanoparticles. Traces (e and f) show morphologies of the composite consisting of Cu nanoparticles with average sizes about 85 nm. The polymeric film prevents the Cu agglomerating and provides high degree of distribution during the electrodeposition. A bimetallic Pt/Cu catalyst can be generated by partial galvanic replacement (an irreversible and spontaneous redox process) of the Cu nanoparticles by Pt. Traces g and h show the structure of PteCu/POA/pCPE, which is decorated with Cu and Pt nanoparticles with average sizes at about 60 nm. The EDS experiment for elemental analysis was performed for the PteCu/POA/pCP composite electrode (Fig. 8). The EDS gives evidences for the presence of Pt and Cu in the nanocomposite. The obtained results also confirm that the galvanic replacement method is a surface limited process.
Electrocatalytic oxidation of methanol Fig. 4 e Electrochemical responses of the POA/pCPE (a) and POA/CPE (b) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.
The interest in the Pt nanoparticles is primarily related to the catalytic application. The electrocatalytic activity of the
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 5 e (A) CVs of the POA/pCPE in 0.5 M H2SO4 solution at: (a) 0.005, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.05, (f) 0.08, (g) 0.10, (h) 0.20, (i) 0.40, (j) 0.60, (k) 0.80 and (l) 1.0 V s¡1. (B) The dependency of anodic and cathodic peak current densities on the potential scan rate.
prepared electrodes for methanol oxidation was investigated in 0.5 M H2SO4 solution (Fig. 9). Methanol oxidation at the Pt surface showed the characteristic double voltammetry peaks that appear in the forward and reverse scans. The results show that, the peak current density (peak current normalized per Pt real surface area) for the PteCu/CPE, PteCu/POA/CPE, PteCu/pCPE, and PteCu/POA/pCPE are about 0.33, 0.45, 0.56 and 1.1 mA cm2, respectively. It is obvious that the PteCu/ CPE (a) is less active than the others towards methanol oxidation. In the case of PteCu/POA/pCPE, the catalytic ability is improved as the current density is enhanced and Eop shifted towards more negative direction (curve d). It is suggested that the PteCu/POA/pCPE leads to the less poisoning of Pt by formed poisoning intermediates during the methanol activation. Also, jp is proportional to y1/2 (jp/mA cm2 ¼ 9.0895 y1/2/
(V s1)1/2 þ 0.7475, r2 ¼ 0.9989), indicating that the methanol oxidation is a diffusion-controlled process (Figure not shown). Table 2 summarizes the main results pertaining to methanol oxidation at the different modified electrodes. The Eop and Ep in the CV are used to estimate the performance of the electrocatalyst for methanol oxidation reaction. Comparison of data (0.28 and 0.73 for Eop and Ep, respectively) indicates that the proposed electrode possesses comparable and acceptable electrocatalytic performance compared to other research works.
Parameters affecting the electrode modification In order to evaluate the effects of various parameters such as electrochemical pretreatment potential (Ep), activation time
Fig. 6 e (A) CVs of the POA/pCPE (a), Cu/POA/pCPE (b) and PteCu/POA/pCPE (c) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1. (B) Electrochemical response of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in 0.5 M H2SO4 solution in potential range ¡0.25e1.2 V at y ¼ 50 mV s¡1. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 7 e SEM images of the CPE (a), pCPE (b), POA/CPE (c), POA/pCPE (d), Cu/POA/pCPE (e, f) and PteCu/POA/pCPE (g, h). (f) and (h) are images with higher magnifications of (e) and (g), respectively.
(ta), POA thickness, electrodeposition time (td), OA concentration (COA), and replacement time (tr) on methanol electrooxidation, the anodic peak current density was monitored as an index for finding an optimum conditions and obtained results were summarized in Table 3. The data indicate that the peak current density increases extensively for Ep up to 1.8 V, ta up to 2 min, cycle numbers up to 10, COA up to 5.0 mM, tr up to 15 min and td up to 50 s and drop afterward.
Fig. 8 e Energy dispersive spectrum of the PteCu/POA/ pCPE.
Electrocatalytic oxidation of methanol in alkaline medium In order to elucidate the electrocatalytic activity of the asprepared PteCu/POA/pCPE in methanol oxidation in alkaline medium, at first voltammetry behavior of the Cu/POA/pCPE in 0.2 M NaOH solution was investigated (Fig. 10). Comparison of the CVs for the Cu/POA/pCPE in the absence and presence of methanol shows that the electrode can catalyze the methanol oxidation at 0.92 V with current density based on geometric surface area of the electrode at about 32.65 mA cm2 (trace A). It is well-known that the cyclic voltammetry in alkaline media
Fig. 9 e CVs of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in the presence of 2.78 M methanol in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Table 2 e Comparison of the methanol oxidation data in H2SO4 solution at the PteCu/POA/pCPE composite with some modified electrodes. Electrocatalyst a
Pt nanorod/Ti Pt/SWCNTa PtRu/MWCNTb Pt/CNTa PtRu/AP/MWCNTb Pt/Au/CPb Pt/PPy (PSS)a Pt/SBA-15a Pt/CCEb PteCu/POA/pCPEb
CH2 SO4 =M
CMethanol/M
Eop/V
Ep/V
Ref.
0.5 1.0 1.0 0.5 0.5 0.5 0.1 0.1 0.2 0.5
2.0 2.0 2.0 2.0 1.0 1.0 1.0 2.0 0.15 2.78
0.36 0.22 0.26 0.45 0.37 0.40 0.40 0.26 0.15 0.28
0.61 0.92 0.75 0.64 0.65 0.68 0.88 0.75 0.89 0.73
[40] [41] [42] [43] [44] [45] [46] [47] [48] This work
The potentials were referred to (a) SCE and (b) AgjAgCljKCl.
Table 3 e Effects of various parameters on the oxidation peak current density at the PteCu/POA/pCPE for 1.15 M methanol þ 0.5 M H2SO4 solution at y ¼ 50 mV s¡1. Ep/V jP/mA cm2 ta/s jP/mA cm2 Cycle number jP/mA cm2 COA/mM jP/mA cm2 td/s jP/mA cm2 tr/s jP/mA cm2
1.0 0.23 20 0.60 2 1.19 1 0.50 10 0.33 5 1.53
1.4 0.80 70 1.32 6 1.23 2 0.73 30 1.57 10 1.57
1.8 2.72 120 2.72 10 2.72 5 2.72 50 2.72 15 2.72
2.2 0.10 170 0.15 15 0.87 10 0.90 60 1.60 20 2.26
2.6 Damaged 220 0.10 20 0.61 15 0.40 70 1.33 25 0.85
produces CuIII species which is investigated an effective component that catalyzes the methanol oxidation [49,50]. The methanol oxidation in 0.2 M NaOH on the PteCu/POA/pCPE was shown in trace (B). The PteCu/POA/pCPE can catalyze the methanol oxidation in alkaline medium at about 0.01 V with current density based on Pt real surface area. It can be seen that Eop and Ep for methanol oxidation are (0.28, 0.73 V in 0.5 M
H2SO4) and (0.50, 0.01 V in 0.2 M NaOH) which is due to that adsorption strength of COads in NaOH is weaker than H2SO4 solution due to pH. However, in NaOH, produced CO2 would form CO2 3 and HCO3 resulting in the decrease at OH concentration and then diminishes the DMFC performance [51,52]. Therefore, the alkaline DMFC is not stable.
Comparison of the electrocatalyst stability The stability of the PteCu/POA/pCPE towards methanol oxidation were investigated by using electrochemical methods. At first, stability of the electrocatalyst was check by using successive potential cycling in 0.5 M H2SO4 þ 2.78 M CH3OH (Fig. 11A). The peak current density decreases gradually by continuous potential cycling and its value at the 50th scan is about 73% than the 1st scan. Additionally, performance of the modified electrode after 50 cycles was investigated again in a fresh aqueous solution of 2.78 M methanol þ 0.5 M H2SO4 (inset of Fig. 11A). The curve is similar to that shown in Fig. 9d and jp still remains at 0.95 mA cm2, which is 86% of its value at the 1st cycle (i.e., 1.1 mA cm2). This stability is attributed to less poisoning effect of the COads at the electrode surface.
Fig. 10 e Current-potential curves of the Cu/POA/pCPE (A) and PteCu/POA/pCPE (B) in the absence (a) and presence of 1.15 M methanol (b) in 0.2 M NaOH solution at y ¼ 50 mV s¡1. The current normalized pre geometric and Pt real surface area for the (A) and (B), respectively. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 11 e (A) CVs of the PteCu/POA/pCPE in the presence of 2.78 M methanol in 0.5 M H2SO4 solution: 1st cycle (a); 50th cycle (b) at y ¼ 50 mV s¡1. (B) Chronoamperograms of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in 2.78 M CH3OH þ 0.5 M H2SO4 solution. (C) Chronopotentiometric curve of the PteCu/POA/pCPE at 0.2 mA. Inset: Chronopotentiograms of the PteCu/CPE (a), PteCu/POA/CPE (b) and PteCu/pCPE (c). (D) CVs of the PteCu/POA/pCPE in 0.5 M H2SO4 solution in the presence of 2.78 M methanol at y ¼ 50 mV s¡1: 1st day (a); 5th day (b) and 10th day (c).
For further evaluation of the electrocatalytic activity and stability of the catalysts, chronoamperograms were conducted for the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) at peak potential value in continuous operation (Fig. 11B). As can be seen from Fig. 11B, a decrease in current density with time is found on the each electrodes in sequence d > c > b > a, which is in good agreement with the cyclic voltammetry data. These results indicate that the PteCu/POA/pCPE has better poisoning tolerance ability for methanol oxidation. Another technique for evaluation of the stability is chronopotentiometry (Fig. 11C). The potential increases with the polarization time and finally shifts to a higher value for oxygen evolution, implying the poisoning of electrocatalyst. From Fig. 11C, it can be observed on all electrodes that the electrode potential increases gradually for several seconds and then jumps to a higher potentials. It is obvious that the PteCu/POA/ pCPE can operates at longer times than the others at same current due to the better electrocatalytic stability and antipoisoning ability. Also, stability of the catalyst was verified by measuring of its response to methanol oxidation after ten days of storage in the laboratory atmosphere conditions (Fig. 11D). The peak current density at the 10th day is about 73% than the 1st day.
Conclusion A highly porous POA film with good stability has been achieved on the CPE by a novel and simple approach (without any template and additive), through the electropolymerization after electrochemical pretreatment. The barrier properties of the CPE and pCPE surface were investigated by using EIS and the result clearly showed that the Rct was decreased, when the carbon paste was electrochemically pretreated. The pretreatment caused the facile oxidative electropolymerization in H2SO4 solution to form POA film as a new electrode material that had rapid charge transfer ability. Higher Ar for the Pt nanoparticles on the POA/pCPE indicated that the present preparation method can effectively increase active sites which led to sensitive response to methanol oxidation with better electrocatalytic activity. The Eop on the PteCu/POA/pCPE was shifted negatively compared with other electrodes. Furthermore, stability of the modified electrodes was studied and it was found that the PteCu/POA/ pCPE showed satisfactory results. These observations confirmed positive effects of the electrochemical treatment on the methanol oxidation and consequently lower the Pt requirement. The method can be used as a simple, easy and
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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low cost preparation for effective electrodes based on the cheap carbon paste substrate. Our work paves a promising avenue for wide applications of carbon materials in the future DMFC devices.
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Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation Sayed Reza Hosseini a,*, Jahan-Bakhsh Raoof b, Shahram Ghasemi a, Zahra Gholami a a b
Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran
article info
abstract
Article history:
In this work, for the first time, a novel and facile method for preparation of highly porous
Received 27 August 2014
poly (o-Anisidine) (POA) by two-step process, through electropolymerization after elec-
Received in revised form
trochemical pretreatment of carbon paste electrode (CPE) is reported. Electrochemical
7 October 2014
response of the prepared POA at the surface of electrochemically pretreated CPE (pCPE)
Accepted 22 October 2014
is, at least, 12 times higher than that obtained at the CPE. Copper (Cu) nanoparticles are
Available online xxx
uniformly electrodeposited onto the POA/pCPE composite. Then, platinum (Pt) nanoparticles are prepared through spontaneous and irreversible reaction via galvanic
Keywords:
replacement between [PtCl6]2 ions and the Cu nanoparticles. The prepared bimetallic
Carbon paste electrode
Pt/Cu nanoparticles are characterized by scanning electron microscopy (SEM), energy
Electrochemical pretreatment
dispersive spectroscopy and electrochemical methods. Electrochemical impedance
Poly (o-Anisidine)
spectroscopy shows that on the pCPE, charge transfer resistance is much smaller than
Bimetallic Pt/Cu nanoparticles
that the CPE. The SEM images reveal that the electrochemical pretreatment of the CPE
Redox replacement
strongly influences morphology of the polymer film. The methanol oxidation and sta-
Methanol oxidation
bility of the PteCu/POA/pCPE are investigated by various electrochemical techniques. The results indicate that the modified electrode exhibits excellent electrocatalytic activity towards methanol oxidation. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction An attractive power source with potential applications in a variety of systems, ranging from portable electronic device to automobile is direct methanol fuel cell (DMFC) [1,2]. DMFC has several advantages such as high energy conversion efficiency, low pollution emission, and safe fuel handling. It is well-
known that Pt with high activity is an ideal catalyst for methanol oxidation in an acidic medium. However, it is an expensive material, and cost of the DMFC containing Pt is prohibitively high and it becomes poisoned by COads which is formed during the methanol oxidation [3]. Development of less expensive anode materials with high electrocatalytic activity, reduced susceptibility to COads poisoning, and proton
* Corresponding author. Tel.: þ98 1135302333; fax: þ98 1135302350. E-mail address: [email protected] (S.R. Hosseini). http://dx.doi.org/10.1016/j.ijhydene.2014.10.104 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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conductivity are some effort towards overcoming the mentioned problems. Several studies have been focused on the application of bimetallic Pt-based electrocatalyst, such as PteRu [4], PtePd [5], PteSn [6] and PteWO3 [7] which significantly reduce the over-potential of methanol oxidation and offer considerable improvement in the catalytic properties relative to the separate metals. The Pt-based catalysts reduce the expensive Pt cost and improve its electrocatalytic activity. The enhanced catalytic activity of Pt-based catalyst can be attributed to the electronic and structural effects by the addition of certain metal in Pt particles [8,9]. However, these solutions are not simple and cost-effective. Conducting polymers (CPs) as convenient and cheap supporting materials with high surface area, environmental stability and porous structures improve the properties of electrode-electrolyte interface and allow a facile flow of electronic charges during the methanol oxidation on the Pt particles [10]. The activity of the Pt-based catalysts due to their high surface area, superior resistance against poisoning effect and/ or synergistic effects between the CPs and relevant Pt is greatly enhanced for the methanol oxidation [11,12]. The Pt particles dispersed into the CP support, not only provide access to large number of catalytic sites and reduce the catalyst loading under the condition of keeping high catalytic activity but also offer the possibility of spent catalyst recovery [13,14]. A number of the CPs have been investigated as conducting catalyst supports for methanol oxidation [15e18]. Among them, polyaniline has shown great promise as a cocatalyst [19e23]. As one of the polyaniline derivatives, poly (o-Anisidine) (POA) films have been prepared at the surface of copper [24] and Pt disk [25,26] by using potentiodynamic method. On the other hand, the ease and fast preparation, low background current, wide potential window, porous and reproducible surface, and low cost of carbon paste electrode (CPE) have been motivated the researchers to use the electrode substrate in diverse applications [27]. The o-Anisidine (OA) can be electrochemically polymerized on the CPE to form POA by using conventional cyclic voltammetry. However, its polymerization rate is very sluggish. To the best of our knowledge, up to now, some strategies were reported about POA formation with high porosity and conductivity. Ojani et al. [28] prepared POA on the CPE surface by using cyclic voltammetry technique in monomer solution containing sodium dodesyl sulfate (SDS) as an anionic surfactant. Adding SDS to the solution leads to increasing the polymer growth rate. Kumar [29] prepared nanofibrillar porous POA films by stepwise electrooxidation of monomer on a glassy carbon electrode by single potential time base galvanostatic mode. Raoof et al. [30] prepared POA/carbon nanotube supporting material for dispersion of nickel species for electrocatalytic oxidation of methanol in alkaline medium. Recently, Dong et al. [31] prepared POA/multi-walled carbon nanotube films onto Pt electrode by using cyclic voltammetry method. Surprisingly, the literature survey indicates that there is no report as yet on electropolymerization of OA at the popular CPE without any additive and template. In this work, as a continuation of the previous study concerning the development of electrocatalyst for methanol oxidation [32], we have described the preparation of PteCu/POA composite onto electrochemically pretreated CPE (pCPE). In order to increase the electron transfer rates, electrochemical pretreatment route for the CPE has been
developed which has been shown to produce improved electrode response compared to that of the native electrode material. In addition, this particular modification strategy has been shown to produce a marked enhancement in the polymer growth rate which exhibits very sluggish rate at untreated CPE. The experimental results indicate that the OA electropolymerized on the pCPE has a more rapid charge transfer rate compared to the normal POA and bimetallic Pt/Cu nanoparticles exhibit pronounced increase in the electrocatalytic activity and a significantly less susceptibility to COads poisoning. In fact, the main objective of the present work is to prepare controlled sizes of Pt nanoparticles in the porous POA matrix, and maintain the homogenous distribution of these particles by employing a new and facile approach.
Experimental Materials and instrumentation OA (>98%, Fluka) was used as a monomer at electrochemical polymerization process. Sulfuric acid (98%, Merck) and sodium hydroxide (>99%, Merck) were used for preparation of the supporting electrolytes. K3Fe(CN)6, K4Fe(CN)6 (>99%, Merck) and KCl (>99%, Merck) were used for the electrochemical impedance studies. K2PtCl6 (Merck), CuSO4.5H2O (99%, Merck) and methanol (>99.9%, Merck) were used as received. High viscosity paraffin oil (density: 0.86 g cm3, Merck) was used as pasting liquid in preparation of the CPE. Graphite powder (particle diameter: 0.1 mm, Merck) was used as the working electrode substrate. The solvent for preparation the reagents solutions was double distilled water. The electrochemical experiments were carried out by using potentiostat/galvanostat Autolab (Nova software model PGSTAT 302N, Metrohm, Netherlands) coupled with a personal computer. Electrochemical impedance spectroscopy (EIS) was performed by potentiostat/galvanostat Palmsense (PSTrace software version 4.2.2, Netherlands). The threeelectrode system consists of the CPE (3.4 mm in diameter) as working electrode, AgjAgCljKCl (3 M) as reference electrode and Pt wire as auxiliary electrode. The surface morphology and elemental analysis were performed by scanning electron microscopy (SEM, model KYKY-EM3200, China) and energy dispersive spectrometer (EDS, VEGA-Tescan, Razi Metallurgical Research Center, Tehran, Iran), respectively.
Preparation of the PteCu/POA/pCPE Fabrication of the CPE (graphite powder and paraffin oil in a ratio of 67:33% (w/w)) was performed according to our previous works. Then, cyclic voltammetry was performed between 0.0 and 1.0 V at 50 mV s1 in 0.5 M H2SO4 solution, until stable voltammograms were obtained. Before electropolymerization, the CPE was electrochemically pretreated at a fixed potential 1.8 V vs. AgjAgCljKCl (3 M) in 0.5 M H2SO4 solution for 2 min without any stirring. The electrode was denoted as pCPE. Later modifications were performed in three steps as shown in Fig. 1: (a) Electrochemical polymerization of 5.0 mM OA monomer in 0.5 M H2SO4 solution by using consecutive
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 1 e Schematic procedure for preparation of the PteCu/POA/pCPE composite.
potential cycling (10 cycles at y ¼ 50 mV s1) between 0.0 and 1.0 V. (b) Electrodeposition of Cu nanoparticles onto the POA/ pCPE at a fixed potential 0.17 V for 50 s in 0.1 M H2SO4 solution containing 0.02 M CuSO4. (c) Partial galvanic replacement of the Cu for deposition of Pt nanoparticles by immersing the electrode in 2.0 mM K2PtCl6 þ 0.1 M H2SO4 stirred solution under open circuit condition for 15 min. Table 1 summarizes several surface parameters such as geometric surface area, polymer mass, film thickness, Cu loading, Pt real surface area and roughness factor for the PteCu/POA/pCPE. The geometric surface area of the electrode was used to calculate the current density, except the comparison of methanol oxidation. All experiments were conducted at ambient temperature.
Results and discussion Electrochemical properties of the pCPE Fig. 2A shows the cyclic voltammograms (CVs) of the CPE (a) and pCPE (b) in 0.1 M KCl solution. As can be seen in Fig. 2A, no currents can be observed with the electrodes and background current of the pCPE is much larger than the CPE. The enhancement at the background current is ascribed to double layer charging
Table 1 e Surface parameters of the PteCu/POA/pCPE. Surface parameter
Value 2
2
Geometric area, Ag/cm (pr ) POA mass, WPOA/mg (h Qdep M/F n) POA thickness, dn/mm (h Qdep M/r Ag F n) Cu loading, WCu/mg cm2 (Qnet M/F Z) Pt loading, WPt/mg cm2 Pt real surface area, Ar/cm2 (QH/210 mC) Roughness factor, RF (Ar/Ag)
0.09 3.10 0.23 0.030a, 0.032b 0.046c, 0.049d 0.60 6.67
with the increased surface roughness after the electrochemical pretreatment [33]. The plane of carbon atoms in graphite oxide (having similar layered structure to graphite) is heavily decorated by oxygen-containing functional groups. These groups expand the interlayer distance and make the atomic-thick layers hydrophilic [34]. Formation of oxygen-containing surface states promote the removal of the inhibitory paraffin from the graphite particles and thereby cause the surface to resemble dry graphite 4 as a well-known redox couple was used to [27]. Fe(CN)3 6 / characterize the properties of electrode surface. Electrochemical response of 1.0 mM K4[Fe(CN)6] in 0.1 M KCl solution was shown on the CPE (c) and pCPE (d). The peak to peak potential separation, (DEp ¼ EpaeEpc) at the pCPE was obtained about 70 mV, while on the CPE, it was about 130 mV. The DEp is greater than that to 2.3RT/nF (or 59/n mV at 25 C) expected for a reversible system. At the same time, the redox peak current density at the pCPE surface was 2.5-fold greater than the CPE, which suggested that electrochemical pretreatment could greatly promotes the electron transfer rate [32]. The EIS can provide useful information on impedance changes of the electrode surface. The electron transfer resistance (Rct) is equal to diameter of the semicircle and can be used to describe the interface properties. The linear part in EIS represents Warburg impedance. The Nyquist diagrams of Fe(CN)3/4 6 couple at the CPE (a) and pCPE (b) were shown in Fig. 2B. It is noticeable that at the pCPE surface, semicircle part is almost eliminated and diagram comprised of only a linear part. It means that the Rct becomes unimportant in relation to Warburg impedance [35]. The CPE surface becomes hydrophilic after electrochemical activation and repulsive interaction (electrostatic and steric) does not exist between the redox marker ions and the electrode surface [36,37]. This implies that the pretreatment of the CPE with potentiostatic technique facilitates the electron transfer of the electrochemical probe and has significant favorable effect on the electrode response. The negatively charged species may be attracted to the hydrophilic pCPE surface. The slope of (b) is higher than (a) which indicates that the functional groups at the pCPE surface helps to enhance the mass diffusion.
a
As determined from chronoamperometry experiments. The calculated loading obtained from stripping of Cu in 0.5 M H2SO4 solution. c and d as maximum possible loaded Pt with assuming that all of the Cu atoms are galvanically replaced by Pt from a and b, respectively. b
Electrochemical polymerization In this work, the electropolymerization of the OA was investigated at the surface of both CPE and pCPE. The consecutive
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 2 e (A) Electrochemical responses of the CPE in 0.1 M KCl solution to: (a) 0.0 M, (c) 1.0 mM K4[Fe(CN)6] and the pCPE to: (b) 0.0 M, (d) 1.0 mM K4[Fe(CN)6] at y ¼ 50 mV s¡1. (B) Electrochemical impedance spectra for the CPE (a) and pCPE (b) þ 0.1 M KCl solution upon biasing the working electrode at 0.21 V. in 1.0 mM Fe(CN)3¡/4¡ 6
CVs recorded during the electropolymerization on the CPE were presented in Fig. 3A. On the 1st cycle, a peak related to the OA oxidation at 0.84 V appears. Deposition a part of the oxidation products at the electrode surface leads to create a POA film. As well it is obvious, by continuous potential cycling, this peak shifts towards more positive directions and its height decreases. This can reflect the loss of the surface activity when is covered by a polymer film with relatively low conductivity. The polymer peaks don't increase considerably with potential cycling. This can be attributed to this fact that the soluble products adsorb and/or react on the electrode surface and do not allow OA to reach the electrode surface to produce more cations-radical. Fig. 3B shows the electrochemical polymerization pattern of the OA at pCPE surface. As can be seen in Fig. 3B, two oxidation peaks appear at about 0.66 and 0.80 V on the 1st cycle. The first peak is attributed to the OA oxidation, and then the oxidized species are further oxidized to form POA with a
shift of scanning potential towards more positive directions. Similar type of the rare behavior at the first scan in electropolymerization has been reported for poly (pyrogallol) at surface of graphene modified glassy carbon electrode [38]. The first peak potential of the OA oxidation shifts from 0.84 V at the CPE to 0.66 V at the pCPE. Also, it should be noted that the lower onset oxidation potential (Eop) for monomer was observed at the pCPE surface indicating the facile oxidation. As the cyclic voltammetry continues, the peak current densities related to the oxidation and reduction of polymer are gradually increased. This result may be due to specific interactions between radical-cations which are formed during the 1st step of electropolymerization and hydrophilic nature of the surface. This is due to that the functional groups at the graphite oxide surface facilitate the electron exchange between the electrode and OA, so it can increase the polymer growth rate. After polymerization, the polymer modified electrodes were washed with double distilled water and the
Fig. 3 e Electrochemical polymerization pattern of 5.0 mM OA in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1 at the CPE (A) and pCPE (B). The arrows indicate the trends of currents density during the voltammetry experiments. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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potentials were cycled between 0.0 and 1.0 V (10 cycles at y ¼ 50 mV s1) in H2SO4 solution. The purpose is to remove the monomers and/or oligomers in the polymer film.
Electrochemical behavior of the POA/pCPE Fig. 4 presents the redox behavior (typical oxidation and reduction peaks) of the prepared polymer films (i.e., POA/CPE and POA/pCPE) in 0.5 M H2SO4 solution at potential range of 0.0e1.0 V at y ¼ 50 mV s1. The first (I) and second (II) redox peaks can be attributed to the POA oxidation in leucoemeraldine to emeraldine oxidation states and subsequent oxidation is corresponds to the oxidation of head-to-tail dimer, respectively [39]. The POA/pCPE (a) films showed noticeably higher peak current density than that the POA/CPE (b). The increase of surface area improves the dopingeundoping rate which is benefit to ion diffusion and migration. The DEp (¼29 mV) for the first redox peak is close to 59/n mV, which identifies that the number of involved electrons is 2 (n z 2.03). The ratio of anodic to cathodic peak current density (jpa/jpc) is almost equal to unity. The CVs of the POA/pCPE at various potential sweep rates (y ¼ 5e1000 mV s1) in 0.5 M H2SO4 solution were presented in Fig. 5. The ja and jc are linearly proportional to y. This result is attributed to the electrochemical activity of an immobilized redox couple at the electrode surface. From the slopes of these lines and by using (Ip ¼ n2F2yAG/4RT), where Ip, A and G are peak current, electrode surface area (0.09 cm2) and surface coverage, respectively, the G was derived about 1.0 106 mol cm2, which further confirmed the immobilized state of the POA film.
Electrochemical properties of the nano-Cu/POA/pCPE After Pt deposition, CVs of the POA/pCPE (a), Cu/POA/pCPE (b) and PteCu/POA/pCPE (c) were recorded in a potential window of 0.0e1.0 V in 0.5 M H2SO4 solution (Fig. 6A). As can be seen in trace (c), two apparent peaks at 0.42 and 0.60 V are assignable to the electrochemical response of the POA film and the broad peak in positive potential range corresponds to the formation of Pt oxides. It should be noted that the reduction peak of the POA is overlaid on reduction peak of Pt oxide. For comparison, CVs of the different PteCu modified electrodes in 0.5 M H2SO4 solution at potential range of 0.25e1.2 V were shown in
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Fig. 6B. The characteristic peaks (PteH redox peaks) in the negative region are connected to atomic hydrogen adsorption on Pt surface, and charge under the peaks can reveal the real surface area of the Pt particles. The larger peaks in the hydrogen adsorption/desorption regions are observed on the PteCu/POA/pCPE, which reflect the higher surface areas. The Ar for the PteCu/CPE, PteCu/pCPE, PteCu/POA/CPE and PteCu/POA/pCPE catalysts are about 0.30, 0.47, 0.36 and 0.60 cm2, respectively. Consequently, such Pt particles at the POA/pCPE enhance the active sites towards the electrocatalytic oxidation of methanol.
Surface morphology and elemental analysis In order to surface characterization, the micrographs of the CPE (a), pCPE (b), POA/CPE (c), POA/pCPE (d), Cu/POA/pCPE (e, f) and PteCu/POA/pCPE (g, h) have been investigated by SEM (Fig. 7). As can be seen on the surface of CPE (trace a), the layer of irregular flakes of graphite powder and some holes or cavities on the electrode surface was presented because carbon paste is a porous material. The surface structure of the pCPE is very complex due to many feasible factors (trace b). In this case, it seems that a change in surface morphology results from the electrochemical pretreatment. The action can be quite effective in stripping the organic inhibitory layer (nonconducting pasting liquid) since oxygen-containing surface states (much more hydrophilic surface) are formed. The SEM image moves towards that was exhibited by dry graphite. The POA film at CPE surface is completely isolated from each other and its morphology is compact (trace c). Electropolymerization at the pCPE leads to formation a homogenous thick POA film and relatively uniform coverage. In this case, more polymers successfully are distributed at the surface, indicating a larger and rougher area compared to the normal POA film (trace d). This result indicates that the surface morphology of POA film is significantly influenced by the electrochemical pretreatment. This structure enhances the electrolyte constituent access to interior of the polymer film. Also, the structure provides a larger available surface area and hence serves as a beneficial support for dispersion and/or distribution of Cu nanoparticles. Traces (e and f) show morphologies of the composite consisting of Cu nanoparticles with average sizes about 85 nm. The polymeric film prevents the Cu agglomerating and provides high degree of distribution during the electrodeposition. A bimetallic Pt/Cu catalyst can be generated by partial galvanic replacement (an irreversible and spontaneous redox process) of the Cu nanoparticles by Pt. Traces g and h show the structure of PteCu/POA/pCPE, which is decorated with Cu and Pt nanoparticles with average sizes at about 60 nm. The EDS experiment for elemental analysis was performed for the PteCu/POA/pCP composite electrode (Fig. 8). The EDS gives evidences for the presence of Pt and Cu in the nanocomposite. The obtained results also confirm that the galvanic replacement method is a surface limited process.
Electrocatalytic oxidation of methanol Fig. 4 e Electrochemical responses of the POA/pCPE (a) and POA/CPE (b) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.
The interest in the Pt nanoparticles is primarily related to the catalytic application. The electrocatalytic activity of the
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 5 e (A) CVs of the POA/pCPE in 0.5 M H2SO4 solution at: (a) 0.005, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.05, (f) 0.08, (g) 0.10, (h) 0.20, (i) 0.40, (j) 0.60, (k) 0.80 and (l) 1.0 V s¡1. (B) The dependency of anodic and cathodic peak current densities on the potential scan rate.
prepared electrodes for methanol oxidation was investigated in 0.5 M H2SO4 solution (Fig. 9). Methanol oxidation at the Pt surface showed the characteristic double voltammetry peaks that appear in the forward and reverse scans. The results show that, the peak current density (peak current normalized per Pt real surface area) for the PteCu/CPE, PteCu/POA/CPE, PteCu/pCPE, and PteCu/POA/pCPE are about 0.33, 0.45, 0.56 and 1.1 mA cm2, respectively. It is obvious that the PteCu/ CPE (a) is less active than the others towards methanol oxidation. In the case of PteCu/POA/pCPE, the catalytic ability is improved as the current density is enhanced and Eop shifted towards more negative direction (curve d). It is suggested that the PteCu/POA/pCPE leads to the less poisoning of Pt by formed poisoning intermediates during the methanol activation. Also, jp is proportional to y1/2 (jp/mA cm2 ¼ 9.0895 y1/2/
(V s1)1/2 þ 0.7475, r2 ¼ 0.9989), indicating that the methanol oxidation is a diffusion-controlled process (Figure not shown). Table 2 summarizes the main results pertaining to methanol oxidation at the different modified electrodes. The Eop and Ep in the CV are used to estimate the performance of the electrocatalyst for methanol oxidation reaction. Comparison of data (0.28 and 0.73 for Eop and Ep, respectively) indicates that the proposed electrode possesses comparable and acceptable electrocatalytic performance compared to other research works.
Parameters affecting the electrode modification In order to evaluate the effects of various parameters such as electrochemical pretreatment potential (Ep), activation time
Fig. 6 e (A) CVs of the POA/pCPE (a), Cu/POA/pCPE (b) and PteCu/POA/pCPE (c) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1. (B) Electrochemical response of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in 0.5 M H2SO4 solution in potential range ¡0.25e1.2 V at y ¼ 50 mV s¡1. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 7 e SEM images of the CPE (a), pCPE (b), POA/CPE (c), POA/pCPE (d), Cu/POA/pCPE (e, f) and PteCu/POA/pCPE (g, h). (f) and (h) are images with higher magnifications of (e) and (g), respectively.
(ta), POA thickness, electrodeposition time (td), OA concentration (COA), and replacement time (tr) on methanol electrooxidation, the anodic peak current density was monitored as an index for finding an optimum conditions and obtained results were summarized in Table 3. The data indicate that the peak current density increases extensively for Ep up to 1.8 V, ta up to 2 min, cycle numbers up to 10, COA up to 5.0 mM, tr up to 15 min and td up to 50 s and drop afterward.
Fig. 8 e Energy dispersive spectrum of the PteCu/POA/ pCPE.
Electrocatalytic oxidation of methanol in alkaline medium In order to elucidate the electrocatalytic activity of the asprepared PteCu/POA/pCPE in methanol oxidation in alkaline medium, at first voltammetry behavior of the Cu/POA/pCPE in 0.2 M NaOH solution was investigated (Fig. 10). Comparison of the CVs for the Cu/POA/pCPE in the absence and presence of methanol shows that the electrode can catalyze the methanol oxidation at 0.92 V with current density based on geometric surface area of the electrode at about 32.65 mA cm2 (trace A). It is well-known that the cyclic voltammetry in alkaline media
Fig. 9 e CVs of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in the presence of 2.78 M methanol in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.
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Table 2 e Comparison of the methanol oxidation data in H2SO4 solution at the PteCu/POA/pCPE composite with some modified electrodes. Electrocatalyst a
Pt nanorod/Ti Pt/SWCNTa PtRu/MWCNTb Pt/CNTa PtRu/AP/MWCNTb Pt/Au/CPb Pt/PPy (PSS)a Pt/SBA-15a Pt/CCEb PteCu/POA/pCPEb
CH2 SO4 =M
CMethanol/M
Eop/V
Ep/V
Ref.
0.5 1.0 1.0 0.5 0.5 0.5 0.1 0.1 0.2 0.5
2.0 2.0 2.0 2.0 1.0 1.0 1.0 2.0 0.15 2.78
0.36 0.22 0.26 0.45 0.37 0.40 0.40 0.26 0.15 0.28
0.61 0.92 0.75 0.64 0.65 0.68 0.88 0.75 0.89 0.73
[40] [41] [42] [43] [44] [45] [46] [47] [48] This work
The potentials were referred to (a) SCE and (b) AgjAgCljKCl.
Table 3 e Effects of various parameters on the oxidation peak current density at the PteCu/POA/pCPE for 1.15 M methanol þ 0.5 M H2SO4 solution at y ¼ 50 mV s¡1. Ep/V jP/mA cm2 ta/s jP/mA cm2 Cycle number jP/mA cm2 COA/mM jP/mA cm2 td/s jP/mA cm2 tr/s jP/mA cm2
1.0 0.23 20 0.60 2 1.19 1 0.50 10 0.33 5 1.53
1.4 0.80 70 1.32 6 1.23 2 0.73 30 1.57 10 1.57
1.8 2.72 120 2.72 10 2.72 5 2.72 50 2.72 15 2.72
2.2 0.10 170 0.15 15 0.87 10 0.90 60 1.60 20 2.26
2.6 Damaged 220 0.10 20 0.61 15 0.40 70 1.33 25 0.85
produces CuIII species which is investigated an effective component that catalyzes the methanol oxidation [49,50]. The methanol oxidation in 0.2 M NaOH on the PteCu/POA/pCPE was shown in trace (B). The PteCu/POA/pCPE can catalyze the methanol oxidation in alkaline medium at about 0.01 V with current density based on Pt real surface area. It can be seen that Eop and Ep for methanol oxidation are (0.28, 0.73 V in 0.5 M
H2SO4) and (0.50, 0.01 V in 0.2 M NaOH) which is due to that adsorption strength of COads in NaOH is weaker than H2SO4 solution due to pH. However, in NaOH, produced CO2 would form CO2 3 and HCO3 resulting in the decrease at OH concentration and then diminishes the DMFC performance [51,52]. Therefore, the alkaline DMFC is not stable.
Comparison of the electrocatalyst stability The stability of the PteCu/POA/pCPE towards methanol oxidation were investigated by using electrochemical methods. At first, stability of the electrocatalyst was check by using successive potential cycling in 0.5 M H2SO4 þ 2.78 M CH3OH (Fig. 11A). The peak current density decreases gradually by continuous potential cycling and its value at the 50th scan is about 73% than the 1st scan. Additionally, performance of the modified electrode after 50 cycles was investigated again in a fresh aqueous solution of 2.78 M methanol þ 0.5 M H2SO4 (inset of Fig. 11A). The curve is similar to that shown in Fig. 9d and jp still remains at 0.95 mA cm2, which is 86% of its value at the 1st cycle (i.e., 1.1 mA cm2). This stability is attributed to less poisoning effect of the COads at the electrode surface.
Fig. 10 e Current-potential curves of the Cu/POA/pCPE (A) and PteCu/POA/pCPE (B) in the absence (a) and presence of 1.15 M methanol (b) in 0.2 M NaOH solution at y ¼ 50 mV s¡1. The current normalized pre geometric and Pt real surface area for the (A) and (B), respectively. Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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Fig. 11 e (A) CVs of the PteCu/POA/pCPE in the presence of 2.78 M methanol in 0.5 M H2SO4 solution: 1st cycle (a); 50th cycle (b) at y ¼ 50 mV s¡1. (B) Chronoamperograms of the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) in 2.78 M CH3OH þ 0.5 M H2SO4 solution. (C) Chronopotentiometric curve of the PteCu/POA/pCPE at 0.2 mA. Inset: Chronopotentiograms of the PteCu/CPE (a), PteCu/POA/CPE (b) and PteCu/pCPE (c). (D) CVs of the PteCu/POA/pCPE in 0.5 M H2SO4 solution in the presence of 2.78 M methanol at y ¼ 50 mV s¡1: 1st day (a); 5th day (b) and 10th day (c).
For further evaluation of the electrocatalytic activity and stability of the catalysts, chronoamperograms were conducted for the PteCu/CPE (a), PteCu/POA/CPE (b), PteCu/pCPE (c) and PteCu/POA/pCPE (d) at peak potential value in continuous operation (Fig. 11B). As can be seen from Fig. 11B, a decrease in current density with time is found on the each electrodes in sequence d > c > b > a, which is in good agreement with the cyclic voltammetry data. These results indicate that the PteCu/POA/pCPE has better poisoning tolerance ability for methanol oxidation. Another technique for evaluation of the stability is chronopotentiometry (Fig. 11C). The potential increases with the polarization time and finally shifts to a higher value for oxygen evolution, implying the poisoning of electrocatalyst. From Fig. 11C, it can be observed on all electrodes that the electrode potential increases gradually for several seconds and then jumps to a higher potentials. It is obvious that the PteCu/POA/ pCPE can operates at longer times than the others at same current due to the better electrocatalytic stability and antipoisoning ability. Also, stability of the catalyst was verified by measuring of its response to methanol oxidation after ten days of storage in the laboratory atmosphere conditions (Fig. 11D). The peak current density at the 10th day is about 73% than the 1st day.
Conclusion A highly porous POA film with good stability has been achieved on the CPE by a novel and simple approach (without any template and additive), through the electropolymerization after electrochemical pretreatment. The barrier properties of the CPE and pCPE surface were investigated by using EIS and the result clearly showed that the Rct was decreased, when the carbon paste was electrochemically pretreated. The pretreatment caused the facile oxidative electropolymerization in H2SO4 solution to form POA film as a new electrode material that had rapid charge transfer ability. Higher Ar for the Pt nanoparticles on the POA/pCPE indicated that the present preparation method can effectively increase active sites which led to sensitive response to methanol oxidation with better electrocatalytic activity. The Eop on the PteCu/POA/pCPE was shifted negatively compared with other electrodes. Furthermore, stability of the modified electrodes was studied and it was found that the PteCu/POA/ pCPE showed satisfactory results. These observations confirmed positive effects of the electrochemical treatment on the methanol oxidation and consequently lower the Pt requirement. The method can be used as a simple, easy and
Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104
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low cost preparation for effective electrodes based on the cheap carbon paste substrate. Our work paves a promising avenue for wide applications of carbon materials in the future DMFC devices.
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Please cite this article in press as: Hosseini SR, et al., Synthesis of PteCu/poly (o-Anisidine) nanocomposite onto carbon paste electrode and its application for methanol oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.104