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Reduced graphene oxide supported bimetallic Ni–Co nanoparticles composite as an electrocatalyst for oxidation of methanol
Solid State Sciences 98 (2019) 106022
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Reduced graphene oxide supported bimetallic Ni–Co nanoparticles composite as an electrocatalyst for oxidation of methanol Mohammad Ali Sheikh-Mohseni a, *, Vahdat Hassanzadeh b, Biuck Habibi b a b
Shahid Bakeri High Education Center of Miandoab, Urmia University, Urmia, Iran Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences, Azarbaijan Shahid Madani University, Tabriz, 53714-161, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal nanoparticles Reduced graphene oxide Nanostructures Fuel cells
A nanocomposite for catalysis of methanol oxidation reaction was fabricated by carbon paste electrode modified with reduced graphene oxide supported bimetallic Ni–Co nanoparticles (Ni–Co/RGO/CPE). It was characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and cyclic voltammetry. The voltammetric results of the modified electrode showed a well redox activity for Ni nanoparticles which was improved by the presence of Co nanoparticles and reduced graphene oxide nanosheets. The electrochemical oxidation of methanol was investigated in alkaline media at Ni–Co/RGO/CPE. The obtained results showed that the presence of Ni–Co/RGO in the structure of electrocatalyst greatly enhance the electrocatalytic oxidation of methanol. The anodic peak potential of methanol oxidation was decreased to 0.65 V vs saturated calomel electrode and its current was greatly enhanced by Ni–Co/RGO/CPE. Also, the chronoamperometry technique was employed for obtaining the catalytic rate constant for the electrooxidation of methanol according to method of Galus as 1.05 M 1s 1.
1. Introduction The fuel cells and water electrolyzers are two new technologies which can resolve currently energy and environmental pollution prob lems [1,2]. Because, they are working with low emission of pollutants and have good performance [3,4]. Therefore, they are highly interested subjects for the researchers in different fields from basic sciences to applied and engineering sciences [5]. The fuel cells which convert the chemical energy into electrical energy have different types. Among of them, direct methanol fuel cells (DMFC) bring some benefits such as low operational temperature and simple fuel feeding [6]. The DMFCs are suitable for use in portable electronics such as cell phones, laptop computers, and video camcorders [7,8]. In the DMFC technology, the electro-oxidation of methanol at the electrode is a significant factor and therefore the electrode should have high electrocatalytic activity. Some different electrodes constructed by Pt and Pt-binary materials are generally used as electrocatalyst in DMFC, due to its rather high catalytic activity [9–14]. But, using of Pt based catalysts cannot be extended because of their high cost and also because of their surface deactivation by intermediates or reaction products during working. Applying alloys of low cost metals such as Fe, Co, Ni,
and Cu can solve these problems if they have appropriate electro catalytic activity [15–18]. In recent years, the application of nanomaterials and nanoparticles has been widely developed in the areas of physical, chemical and applied sciences [19]. These materials with high specific surface area and high reactivity can be used in the structure of catalysts and electro-catalysts to enhance their catalytic activity [20–22]. Also, this materials decrease the catalyst loading [23–27]. The catalytic role of nickel has been confirmed due to its special oxidation properties. This element is used widely in organic synthesis reactions and also electrolysis of water as an electrocatalyst. Also, many electrodes modified with nickel can be used as a catalyst in fuel cells [28, 29]. Many studies were showed the electro-oxidation of various types of alcohols by nickel [30,31]. On the other hand, catalysts made of biphasic nanoparticles show better activity compared to pure metals [13–15]. Therefore, the catalysts which have been made with Ni and other metal nanoparticles in their structures take improved performance [16,17]. The nanostructured carbon materials are generally used as supported electrodes for obtaining higher surface area at the electrocatalyst sur faces [32]. Graphene has attracted great interest in different scientific fields due to its unique properties such as excellent flexibility, high
* Corresponding author. E-mail address: [email protected] (M.A. Sheikh-Mohseni). https://doi.org/10.1016/j.solidstatesciences.2019.106022 Received 29 July 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 Available online 1 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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surface area, extraordinary electronic quality, and its superior thermal and mechanical properties. These carbon nanosheets are used in the structure of modified electrodes [33,34]. Therefore, the application of Ni and other non-costly metal nanoparticles and graphene nanosheets in the structure of an electrocatalyst, bring several advantages simultaneously. In this study, we successfully fabricate the Ni–Co nanoparticles on reduced graphene oxide carbon paste electrode, electrochemically. After characterization of the obtained catalyst (Ni–Co/RGO/CPE) by some characterization techniques its electrochemical behavior was studied by cyclic voltammetry. Then the electrocatalytic behavior of bimetallic Ni–Co nanoparticles at RGO/CPE was investigated towards methanol oxidation. This is the first report which a composite including RGO and Ni–Co nanoparticles at a renewable CPE has been used for electro catalysis of methanol. Preparations of metal nanoparticles at the elec trode surfaces are in continue of our works in fuel cell applications [35–38].
3. Results and discussion 3.1. Morphology and elemental composition of Ni–Co/RGO/CPE nanocomposite The morphology of Ni–Co/RGO/CPE composites was investigated by scanning electron microscopy (SEM). The deposited Ni and Co nano particles were observable on RGO sheets support (Fig. 1 A and B). As can be seen, at Ni/RGO/CPE composite, Ni nanoparticles on carbon support showed an approximately homogeneous dispersion with average parti cle size of about 100 nm (Fig. 1B). On the other hand, at Ni–Co/RGO/ CPE, the Ni nanoparticles were grown on Co nanoparticles with regular distribution (Fig. 1A). Also, the nanoparticles were largely spherical in shape, indicates that the nanoparticles are not agglomerations. The morphology of the electrodes surfaces was also investigated by EDX mapping. The obtained images for the surface of the electrodes are shown in Fig. 1C and D. The results showed the formation of Ni nano particles at Ni/RGO/CPE (Fig. 1D) and formation of both Ni and Co nanoparticles at Ni–Co/RGO/CPE (Fig. 1C) with relatively regular dis tances between particles. The elemental analysis of the Ni–Co/RGO/CPE and Ni/RGO/CPE were performed by energy dispersive X-ray spectroscopy (EDX). Parts A and B of Fig. 2 show the EDX analysis results of the composites. The peaks of C, O and Ni were observed for both electrodes which were in the composition of the catalyst. The EDX of Ni–Co/RGO/CPE showed the peaks of Co, which indicate the presence of it (Fig. 2B). Fig. 2C shows the X-ray diffraction (XRD) patterns of the Ni/RGO/ CPE and Ni–Co/RGO/CPE. In both XRD patterns, the peak at 2θ 55� corresponds to carbon (006), in carbon paste as support material. The main peaks in the XRD pattern of the Ni/RGO/CPE at 2θ values of 42.6� , 43.6� , 44.76� , 46.41� , 52� , 60.15� and 77.71� corresponding to (010), (002), (111), (011), (200), (012) and (220) crystal planes for Ni, respectively. In XRD patterns of the Ni–Co/RGO/CPE, the main peaks at 2θ values of 42.6� , 43.6� , 44.76� , 46.41� , 51� , 60.15� and 77.71� cor responding to (010), (002), (111), (011), (200), (012) and (220) for pure Ni, pure Co or Ni–Co composite, respectively [16]. As can be seen, the
2. Materials and methods 2.1. Instruments and reagents All electrochemical measurements were performed in a three elec trode cell assembly by a potentiostat/galvanostat (electro analyzer system, SAMA 500, I.R. Iran). A saturated calomel electrode (SCE) was used as reference electrode and a platinum wire as counter electrode. A Milwaukee pH meter was used for adjusting the pH of the solutions. Deionized water was used for preparation of all the solutions. All re agents were from Sigma-Aldrich, excluding the graphite fine powder and paraffin oil (DC 350, 0.88 gcm 3) which both from Merck (Germany). 2.2. Reduced graphene oxide preparation The graphene oxide was synthesized from graphite powder using an improved method [39]. At first, the mixture of concentrated sulfuric acid and phosphoric acid (22.5:2.5 mL) was mixed by 1 g graphite. After adding 3 g of potassium permanganate, the mixture was stirred at 50 � C for 40 min. After that, the mixture was cooled to the room temperature and hydrogen peroxide (30%) was added to the oxidizing process in the ice bath. The solvent and solute were then separated and the solute was washed using 35% hydrochloric acid, ethanol, and distilled water. The sonicated suspension was then added to a solution containing 50 mL hydrazine solution (98%) and 200 mL ammonia solution (30%). It was refluxed at 90 � C for 12 h and then cooled to the room temperature. After centrifuging, the RGO was washed with deionized water and dried in vacuum for 24 h [39]. 2.3. Fabrication of the Ni–Co/RGO/CPE For fabrication of Ni–Co/RGO/CPE, the RGO/CPE was prepared firstly. The RGO was mixed with graphite powder by five weight percent. The paraffin oil (0.7 mL) was added to this mixture and a uni form paste was obtained by a mortar and pestle. The paste was filled into the electrode body which was a plastic tube (i.d. 3.5 mm) with a copper wire as the electrical contact. The RGO/CPE was then placed in a 10 mM solution of cobalt chloride and a 0.95 V potential was applied for 450 s by chronoamperometry, which Co/RGO/CPE was obtained. This elec trode was then inserted in a 10 mM nickel chloride solution and a po tential of 1.25 V was applied for 200 s to obtain Ni–Co/RGO/CPE. The Ni–Co/CPE was prepared in a same way but at bare CPE. Also, the Ni/ RGO/CPE was obtained in a same way but without deposition of Co.
Fig. 1. A) and B) SEM image of Ni–Co/RGO/CPE and Ni/RGO/CPE, respec tively; C) and D) EDX mapping of the surface of Ni–Co/RGO/CPE and Ni/RGO/ CPE, respectively. 2
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obtained as 0.77 m2/g metal. 3.3. Electrochemical characteristics of Ni–Co/RGO/CPE The electrochemical behavior of different electrodes was investi gated by the technique of cyclic voltammetry. The cyclic voltammo grams of the electrodes were recorded at the potentials between 0.0 and 1.0 V in NaOH solution (pH ¼ 13) at scan rate of 100 mVs 1. The Ni modified electrode (Ni/RGO/CPE) shows a redox activity with anodic and corresponding cathodic peak at 0.45 V and 0.34 V, respectively (curve a in Fig. 3a). While, the Co modified electrode (Co/RGO/CPE) have not any redox behavior and is not electroactive in the same con ditions (curve b in Fig. 3a). The redox behavior of Ni/RGO/CPE is related to oxidation and reduction of Ni(OH)2/NiOOH redox couple (equation (1)) [42]. Obvi ously, Ni(OH)2 is produced from oxidation of Ni at lower potentials when the potential is scanned anodically. Actually, different types of Ni hydroxide may be formed, however, these forms could transform into the less hydrated and more stable phase, Ni(OH)2 [43]. NiðOHÞ2 þOH ⇄NiOOH þ H2 O þ e
(1)
The cyclic voltammogram of Ni–Co/RGO/CPE is shown at curve c in Fig. 3a. As can be seen, when the Ni nanoparticles is deposited on Co nanoparticles the electrochemical current of redox activity of Ni is increased.
Fig. 2. A) and B) EDX spectrum of Ni/RGO/CPE and Ni–Co/RGO/CPE, respectively; C) XRD of Ni–Co/RGO/CPE, Ni/RGO/CPE and the reference NiCo patterns.
XRD pattern of Ni/RGO/CPE and Ni–Co/RGO/CPE are similar, because these elements have sequential atomic number, close atomic weight and same crystal structure in the normal conditions. 3.2. Electrochemically active surface area and metal loading of the electrodes The activity of an electrocatalyst may be controlled by the real sur face area. Therefore, the electrochemically active surface area of different electrodes were obtained using Randles-Sevcik equation in K4Fe(CN)6/K3Fe(CN)6 redox couple with concentration of 1 mM in KCl supporting electrolyte [40]. For each kind of electrodes, three samples were used and the average area was calculated. This area has not sig nificant difference between Ni–Co/RGO/CPE, Co/RGO/CPE and Ni/R GO/CPE and the average area was obtained as 0.208 cm2 for them. Also, this area was calculated as 0.191 cm2 for Ni–Co/CPE, Co/CPE and Ni/CPE and 0.1 cm2 for CPE. As can be seen, the presence of synthesized metal nanoparticles and RGO in the structure of the electrodes increased the real surface are of the electrodes. The electrochemically active sur face area of the modified electrodes increased more than two times, when RGO and metal nanoparticles were both on the electrodes. Also the metal loading on the electrocatalysts is an important factor. Thus, the Ni and Co loading on the prepared electrocatalyst were measured by coulombic charge consumed during the electro-deposition of metals on the RGO/CPE [41]. The calculated mass for Ni nano particles was obtained as 13.1 μg on Ni/RGO/CPE and was calculated as 13.4 μg for Co nanoparticles in Co/RGO/CPE. The total metal nano particles loading on the Ni–Co/RGO/CPE was obtained as 27 μg. Also the electrochemically active surface area of the Ni–Co/RGO/CPE can be
Fig. 3. a) Cyclic voltammograms of Ni/RGO/CPE (curve a), Co/RGO/CPE (curve b) and Ni–Co/RGO/CPE (curve c); b) Cyclic voltammograms of Ni–Co/ CPE (curve a) and Ni–Co/RGO/CPE (curve b); Conditions in all cases: pH ¼ 13 and ν ¼ 100 mVs 1. 3
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rates between 800 and 4000 mVs 1. This behavior was reported else where previously [44,45]. Fig. 4e shows the plot of the anodic and cathodic peak potentials (Ep) versus the logarithm of the potential scan rate (Log ν). There is no relation between Ep and Log ν at potential scan rates lower than 0.2 V s-1. But, at higher scan rates the peak potential is changed by variation of Log ν. This behavior was explained by Laviron for the surface confined electroactive species and the following equation (equation (2)) was derived between apparent charge transfer rate constant (ks) and transfer coefficient (α), if Epa-Epc ¼ ΔEp>200/n mV [46]: � � � Log ks ¼ αlog 1 α þ 1 α logα log RT=nFv α 1 � � α ðnα FΔEp =2:3RT (2)
The effect of RGO on the electrochemical response of the electrodes can be seen on Fig. 3b. The Ni–Co/RGO/CPE has higher electrochemical current rather than Ni–Co/CPE, because more amount of nanoparticles can be deposited at nanostructured RGO modified electrode and also the nanoparticles are more available at RGO modified electrode. This is related to the high surface area and high electron transfer ability of RGO nanosheets. The electrochemical behavior of Ni–Co/RGO/CPE was studied at different pHs by CV. The results showed the electrochemical current of the modified electrode is reduced with decreasing the pH of solution. According to equation (1) and related explanations, the presence of hydroxide is necessary for suitable redox activity of the Ni nanoparticles. Therefore, the electrochemical activity of the Ni modified electrode disappears at pHs lower than 10. Consequently, the optimum pH was selected as 13. The effects of the potential scan rate in the ranges of 50–500 and 800–4000 mV s 1 on the CV response of Ni–Co/RGO/CPE are shown in Fig. 4a and Fig. 4c, respectively. The anodic and cathodic peak currents (Ip) were plotted against potential scan rate (ν). As Fig. 4b shows the peak currents up to 500 mVs 1 is proportional to the scan rate, indi cating usual behavior for a surface confined redox material. This behavior is changed for scan rates higher than 500 mV s 1. Fig. 4d shows a linear relationship between Ip and square root of scan rate (ν1/2) at high scan rate. Therefore, there is a diffusion controlled process at scan
The transfer coefficient was calculated using the slopes of the plot of Epa and Epc versus Log ν at Fig. 4e as 0.68 and 0.47 for anodic and cathodic processes, respectively. Also, using equation (2) and the values of ΔEp corresponding to different scan rates, the average value of ks was derived as 0.59 � 0.03 s 1. 3.4. Electrocatalytic oxidation of methanol at Ni–Co/RGO/CPE The electrocatalytic activity of the proposed electrocatalyst was investigated for methanol oxidation by CV. The cyclic voltammetric responses of the Ni–Co/RGO/CPE and bare CPE were recorded in the absence and presence of methanol at scan rate of 100 mV s 1 in a so lution with pH ¼ 13.0. Fig. 5a shows the obtained results. As can be seen, the methanol is not oxidized at bare CPE even to potential 1.1 V (curve a), while its anodic peak at Ni–Co/RGO/CPE is about 0.7 V (curve b). Curve c of Fig. 5a shows the cyclic voltammogram of Ni–Co/RGO/CPE in the absence of methanol. As can be seen, in the cyclic voltammogram of the Ni–Co/RGO/CPE, the peak potential of Ni(OH)2/NiOOH con version upshifts about 50 mV in the presence of methanol, suggesting the contribution of NiOOH in electrocatalytic oxidation of methanol [42]. On the other hand, the anodic peak current of Ni(OH)2/NiOOH redox couple is greatly enhanced in the presence of methanol. In fact, the methanol reacts with the NiOOH formed on the electrode surface. This observation indicates a catalytic mechanism for oxidation of methanol at modified electrode. For comparing the catalytic activity of different modified electrodes, their cyclic voltammetric responses were obtained in the presence of methanol. The results are showed at Fig. 5b for Ni–Co/RGO/CPE, Ni–Co/CPE, Ni/RGO/CPE and Co/RGO/CPE in solution with pH 13.0 containing methanol. The Co nanoparticles at Co/RGO/CPE has not any electrocatalytic activity toward methanol oxidation (Fig. 5b, curve a). But, the presence of Co nanoparticles in Ni–Co/RGO/CPE shows a
Fig. 4. a and c) Cyclic voltammograms of Ni–Co/RGO/CPE for different scan rates (from inner to outer: 50, 60, 80, 100, 200, 300, 400 and 500 mVs 1 for a and 800, 1000, 1400, 1800, 2200, 2600, 3000, 3400 and 4000 mVs 1 for c); b and d) the plot of anodic and cathodic peak currents (versus ν in the range of 50–500 mVs 1 for b and versus ν1/2 in the range of 800–4000 mVs 1 for d); e) the plot of peak potentials versus the logarithm of the potential scan rate in the range of 50–4000 mVs 1.
Fig. 5. a) Cyclic voltammograms of 0.1 M methanol at CPE (curve a), Ni–Co/ RGO/CPE (curve b), curve c is cyclic voltammogram of Ni–Co/RGO/CPE in the absence of methanol; b) Cyclic voltammograms of 0.1 M methanol at Co/RGO/ CPE (curve a), Ni/RGO/CPE (curve b), Ni–Co/RGO/CPE (curve c)) and Ni–Co// CPE (curve d); Electrolyte: 0.1 M NaOH solution (pH 13.0), scan rate: 100 mV s 1. 4
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synergetic effect on methanol oxidation. It is can be concluded by comparing curves b and c for CV responses of Ni/RGO/CPE and Ni–Co/ RGO/CPE, respectively. As described in the section of electrochemically active surface area of the electrodes, the real surface area of the Ni–Co/ RGO/CPE and Ni/RGO/CPE are the same. Therefore, the difference observed between responses of methanol oxidation on these electrodes is not related to difference of the electrodes surface area but is related to the synergistic catalytic role of Ni and Co nanoparticles. Also, the role of RGO in electrocatalytic oxidation of methanol can be seen by comparing the electrochemical responses of Ni–Co/CPE (curve d) with Ni–Co/RGO/CPE (curve c) in Fig. 5b. The higher anodic current for methanol oxidation was obtained at Ni–Co/RGO/CPE, which con taining Ni, Co and RGO in its structure. 3.5. The effect of pH on the catalytic oxidation of methanol
Fig. 7. Cyclic voltammograms of 0.1 M methanol at Ni–Co/RGO/CPE in different scan rates (from inner to outer: 10,20,50,100 mVs 1); inset shows the plot of anodic peak current of methanol versus square root of potential scan rate.
The electrochemical behavior of methanol at Ni–Co/RGO/CPE investigated in different pHs from 10 to 13 (Fig. 6). The electrocatalytic current of methanol is higher in pH 13 and with decreasing the pH the methanol current is decreased. This behavior is affected from the behavior of Ni(OH)2/NiOOH redox couple in different pHs, which pre viously described. The electrochemical current of the Ni nanoparticles at Ni–Co/RGO/CPE is reduced with decreasing the pH of solution and therefore its catalytic activity is reduced.
and NiOOH cannot be occurred. Moreover, the anodic peak current of Ni (OH)2/NiOOH is decreased by decreasing the scan rate. This peak is disappearing completely at low scan rate (i.e. 10 mVs 1), therefore, the slowing reaction rate between NiOOH and methanol can be concluded [45].
3.6. The effect of potential scan rate on the catalytic oxidation of methanol
3.7. Chronoamperometric studies for the catalytic oxidation of methanol The chronoamperometry technique was used for obtaining the diffusion coefficient of methanol. Fig. 8 shows the chronoamperograms of various methanol concentrations at Ni–Co/RGO/CPE with an applied potential step of 650 mV. The extracted plots of current versus the in verse of the square roots of time (I-t 1/2) were constructed and showed a linear dependency (Fig. 8a). The slopes of the obtained lines were plotted versus methanol concentration (Fig. 8b). From the resulting slope and the Cottrell equation [47] the value of diffusion coefficient for methanol was found to be 8.75 � 10 8 cm2s 1. The catalytic rate constant of methanol at Ni–Co/RGO/CPE was also evaluated by chronoamperometry according to the method of Galus [48]. In this method, simplicity, there is an equation between the ratio of catalytic and limited current (IC/IL) and the catalytic rate constant (k) as
The effect of the potential scan rate on the cyclic voltammograms of Ni–Co/RGO/CPE in 0.1 M methanol and pH 13.0 are shown in Fig. 7. The inset of Fig. 7 displays the scan rate dependence of methanol oxidation peak current. This plot shows the methanol oxidation peak current at Ni–Co/RGO/CPE have linear proportional to the square root of scan rate in the range of 10–100 mVs 1. Therefore, the electro catalytic process of methanol oxidation is controlled by diffusion at Ni–Co/RGO/CPE. Another observation is the increasing the cathodic peak current of Ni(OH)2/NiOOH with increasing the scan rate. This is can be explained that at high scan rate, the reaction between methanol
Fig. 8. Chronoamperograms obtained at Ni–Co/RGO/CPE in pH 13.0 for different concentrations of methanol: from down to up correspond to 0.0, 0.1, 0.2, 0.3, and 0.4 M, Insets: a) plots of I versus t 1/2 obtained from chro noamperograms 0.1–0.4 M, b) plot of the slope of the straight lines against methanol concentration, c) plot of IC/IL versus t1/2.
Fig. 6. Cyclic voltammograms of 0.1 M methanol at Ni–Co/RGO/CPE in different pHs (10–13). 5
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equation (3): �1=2 IC =IL ¼ π1=2 kCb t
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(3)
Where, IC is the catalytic current (the current of Ni–Co/RGO/CPE in the presence of methanol), IL is the limited current (the current of Ni–Co/ RGO/CPE in the absence of methanol), Cb is the bulk concentration of methanol and t is the time. The plots of IC/IL versus t1/2 were constructed (Fig. 8c) and from the slopes of them the average value of k was calculated as 1.0 M 1s 1 or 1000 cm3 mol 1s 1. 4. Conclusion A novel Ni–Co bimetallic structure on reduced graphene oxide (RGO) is constructed. This composite was characterized by SEM, EDX and also electrochemical techniques. The nano dimensional structure of the composite as well as its good electrochemical behavior makes the composite as a suitable catalyst for methanol oxidation. The prepared electrocatalyst was used for electrocatalytic oxidation of methanol. The existence of Ni–Co nanocomposite and the presence of RGO was enhanced the electrocatalytic activity of the CPE based catalyst towards the electrooxidation of methanol. The voltammetric results, obtained in different pHs and in different scan rates, showed the role of the Ni(OH)2/ NiOOH redox couple in catalytic oxidation of methanol at the electro catalyst. Also the activity of Co nanoparticles and RGO nanosheets showed in the electrocatalytic process. Acknowledgement The authors are grateful to the Urmia University and Azarbaijan Shahid Madani University for the financial support of the work. References [1] S.G. Chalk, J.F. Miller, Key challenges and recent progress in batteries, fuel cells, and hydrogen storage for clean energy systems, J. Power Sources 159 (2006) 73–80. [2] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Appl. Energy 137 (2015) 511–536. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [4] T. Wilberforce, A. Alaswad, A. Palumbo, M. Dassisti, A.G. Olabi, Advances in stationary and portable fuel cell applications, Int. J. Hydrogen Energy 41 (2016) 16509–16522. [5] N.A. Tapan, M.E. Günay, R. Yildirim, Constructing global models from past publications to improve design and operating conditions for direct alcohol fuel cells, Chem. Eng. Res. Des. 105 (2016) 162–170. [6] S.S. Munjewar, S.B. Thombre, R.K. Mallick, Approaches to overcome the barrier issues of passive direct methanol fuel cell–Review, Renew. Sustain. Energy Rev. 67 (2017) 1087–1104. [7] A. Mehmood, M.A. Scibioh, J. Prabhuram, M.G. An, H.Y. Ha, A review on durability issues and restoration techniques in long-term operations of direct methanol fuel cells, J. Power Sources 297 (2015) 224–241. [8] Y. Zheng, H. Zhan, Y. Fang, J. Zeng, H. Liu, J. Yang, S. Liao, Uniformly dispersed carbon-supported bimetallic ruthenium–platinum electrocatalysts for the methanol oxidation reaction, J. Mater. Sci. 52 (2017) 3457–3466. [9] B. Habibi, M.H. Pournaghi-Azar, Methanol oxidation on the polymer coated and polymer-stabilized Pt nano-particles: a comparative study of permeability and catalyst particle distribution ability of the PANI and its derivatives, Int. J. Hydrogen Energy 35 (2010) 9318–9328. [10] L. Zhang, X.F. Zhang, X.L. Chen, A.J. Wang, D.M. Han, Z.G. Wang, J.J. Feng, Facile solvothermal synthesis of Pt71Co29 lamellar nanoflowers as an efficient catalyst for oxygen reduction and methanol oxidation reactions, J. Colloid Interface Sci. 536 (2019) 556–562. [11] S.M. Alia, G. Zhang, D. Kisailus, D. Li, S. Gu, K. Jensen, Y. Yan, Porous platinum nanotubes for oxygen reduction and methanol oxidation reactions, Adv. Funct. Mater. 20 (2010) 3742–3746. [12] Z. Han, A.J. Wang, L. Zhang, Z.G. Wang, K.M. Fang, Z.Z. Yin, J.J. Feng, 3D highly branched PtCoRh nanoassemblies: glycine-assisted solvothermal synthesis and superior catalytic activity for alcohol oxidation, J. Colloid Interface Sci. 554 (2019) 512–519. [13] B. Habibi, M.H. Pournaghi-Azar, H. Abdolmohammad-Zadeh, H. Razmi, Electrocatalytic oxidation of methanol on mono and bimetallic composite films: Pt and Pt–M (M¼ Ru, Ir and Sn) nano-particles in poly (o-aminophenol), Int. J. Hydrogen Energy 34 (2009) 2880–2892.
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[40] B. Rezaei, S. Damiri, Voltammetric behavior of multi-walled carbon nanotubes modified electrode-hexacyanoferrate(II) electrocatalyst system as a sensor for determination of captopril, Sens. Actuators, B 134 (2008) 324–331. [41] Y.D. Gamburg, G. Zangari, Theory and Practice of Metal Electrodeposition, Springer Science & Business Media, 2011. [42] L.R. Zhang, J. Zhao, M. Li, H.T. Ni, J.L. Zhang, X.M. Feng, Y.W. Ma, Q.L. Fan, X. Z. Wang, Z. Hu, W. Huang, Preparation of graphene supported nickel nanoparticles and their application to methanol electrooxidation in alkaline medium, New J. Chem. 36 (2012) 1108–1113. [43] R.M.A. Tehrani, S.A. Ghani, The nanocrystalline nickel with catalytic properties on methanol oxidation in alkaline medium, Fuel Cells 9 (2009) 579–587.
[44] A.A. El-Shafei, Electrocatalytic oxidation of methanol at a nickel hydroxide/glassy carbon modified electrode in alkaline medium, J. Electroanal. Chem. 471 (1999) 89–95. [45] I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal, M.G. Mahjani, Electrocatalytic oxidation of methanol on Ni and NiCu alloy modified glassy carbon electrode, Int. J. Hydrogen Energy 33 (2008) 4367–4376. [46] E. Laviron, General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems, J. Electroanal. Chem. Interfacial Electrochem. 101 (1979) 19–28. [47] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, second ed., Wiley, New York, 2001. [48] Z. Galus, Fundamentals of Electrochemical Analysis, Ellis Horwood, New York, 1976.
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Reduced graphene oxide supported bimetallic Ni–Co nanoparticles composite as an electrocatalyst for oxidation of methanol Mohammad Ali Sheikh-Mohseni a, *, Vahdat Hassanzadeh b, Biuck Habibi b a b
Shahid Bakeri High Education Center of Miandoab, Urmia University, Urmia, Iran Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences, Azarbaijan Shahid Madani University, Tabriz, 53714-161, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal nanoparticles Reduced graphene oxide Nanostructures Fuel cells
A nanocomposite for catalysis of methanol oxidation reaction was fabricated by carbon paste electrode modified with reduced graphene oxide supported bimetallic Ni–Co nanoparticles (Ni–Co/RGO/CPE). It was characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and cyclic voltammetry. The voltammetric results of the modified electrode showed a well redox activity for Ni nanoparticles which was improved by the presence of Co nanoparticles and reduced graphene oxide nanosheets. The electrochemical oxidation of methanol was investigated in alkaline media at Ni–Co/RGO/CPE. The obtained results showed that the presence of Ni–Co/RGO in the structure of electrocatalyst greatly enhance the electrocatalytic oxidation of methanol. The anodic peak potential of methanol oxidation was decreased to 0.65 V vs saturated calomel electrode and its current was greatly enhanced by Ni–Co/RGO/CPE. Also, the chronoamperometry technique was employed for obtaining the catalytic rate constant for the electrooxidation of methanol according to method of Galus as 1.05 M 1s 1.
1. Introduction The fuel cells and water electrolyzers are two new technologies which can resolve currently energy and environmental pollution prob lems [1,2]. Because, they are working with low emission of pollutants and have good performance [3,4]. Therefore, they are highly interested subjects for the researchers in different fields from basic sciences to applied and engineering sciences [5]. The fuel cells which convert the chemical energy into electrical energy have different types. Among of them, direct methanol fuel cells (DMFC) bring some benefits such as low operational temperature and simple fuel feeding [6]. The DMFCs are suitable for use in portable electronics such as cell phones, laptop computers, and video camcorders [7,8]. In the DMFC technology, the electro-oxidation of methanol at the electrode is a significant factor and therefore the electrode should have high electrocatalytic activity. Some different electrodes constructed by Pt and Pt-binary materials are generally used as electrocatalyst in DMFC, due to its rather high catalytic activity [9–14]. But, using of Pt based catalysts cannot be extended because of their high cost and also because of their surface deactivation by intermediates or reaction products during working. Applying alloys of low cost metals such as Fe, Co, Ni,
and Cu can solve these problems if they have appropriate electro catalytic activity [15–18]. In recent years, the application of nanomaterials and nanoparticles has been widely developed in the areas of physical, chemical and applied sciences [19]. These materials with high specific surface area and high reactivity can be used in the structure of catalysts and electro-catalysts to enhance their catalytic activity [20–22]. Also, this materials decrease the catalyst loading [23–27]. The catalytic role of nickel has been confirmed due to its special oxidation properties. This element is used widely in organic synthesis reactions and also electrolysis of water as an electrocatalyst. Also, many electrodes modified with nickel can be used as a catalyst in fuel cells [28, 29]. Many studies were showed the electro-oxidation of various types of alcohols by nickel [30,31]. On the other hand, catalysts made of biphasic nanoparticles show better activity compared to pure metals [13–15]. Therefore, the catalysts which have been made with Ni and other metal nanoparticles in their structures take improved performance [16,17]. The nanostructured carbon materials are generally used as supported electrodes for obtaining higher surface area at the electrocatalyst sur faces [32]. Graphene has attracted great interest in different scientific fields due to its unique properties such as excellent flexibility, high
* Corresponding author. E-mail address: [email protected] (M.A. Sheikh-Mohseni). https://doi.org/10.1016/j.solidstatesciences.2019.106022 Received 29 July 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 Available online 1 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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surface area, extraordinary electronic quality, and its superior thermal and mechanical properties. These carbon nanosheets are used in the structure of modified electrodes [33,34]. Therefore, the application of Ni and other non-costly metal nanoparticles and graphene nanosheets in the structure of an electrocatalyst, bring several advantages simultaneously. In this study, we successfully fabricate the Ni–Co nanoparticles on reduced graphene oxide carbon paste electrode, electrochemically. After characterization of the obtained catalyst (Ni–Co/RGO/CPE) by some characterization techniques its electrochemical behavior was studied by cyclic voltammetry. Then the electrocatalytic behavior of bimetallic Ni–Co nanoparticles at RGO/CPE was investigated towards methanol oxidation. This is the first report which a composite including RGO and Ni–Co nanoparticles at a renewable CPE has been used for electro catalysis of methanol. Preparations of metal nanoparticles at the elec trode surfaces are in continue of our works in fuel cell applications [35–38].
3. Results and discussion 3.1. Morphology and elemental composition of Ni–Co/RGO/CPE nanocomposite The morphology of Ni–Co/RGO/CPE composites was investigated by scanning electron microscopy (SEM). The deposited Ni and Co nano particles were observable on RGO sheets support (Fig. 1 A and B). As can be seen, at Ni/RGO/CPE composite, Ni nanoparticles on carbon support showed an approximately homogeneous dispersion with average parti cle size of about 100 nm (Fig. 1B). On the other hand, at Ni–Co/RGO/ CPE, the Ni nanoparticles were grown on Co nanoparticles with regular distribution (Fig. 1A). Also, the nanoparticles were largely spherical in shape, indicates that the nanoparticles are not agglomerations. The morphology of the electrodes surfaces was also investigated by EDX mapping. The obtained images for the surface of the electrodes are shown in Fig. 1C and D. The results showed the formation of Ni nano particles at Ni/RGO/CPE (Fig. 1D) and formation of both Ni and Co nanoparticles at Ni–Co/RGO/CPE (Fig. 1C) with relatively regular dis tances between particles. The elemental analysis of the Ni–Co/RGO/CPE and Ni/RGO/CPE were performed by energy dispersive X-ray spectroscopy (EDX). Parts A and B of Fig. 2 show the EDX analysis results of the composites. The peaks of C, O and Ni were observed for both electrodes which were in the composition of the catalyst. The EDX of Ni–Co/RGO/CPE showed the peaks of Co, which indicate the presence of it (Fig. 2B). Fig. 2C shows the X-ray diffraction (XRD) patterns of the Ni/RGO/ CPE and Ni–Co/RGO/CPE. In both XRD patterns, the peak at 2θ 55� corresponds to carbon (006), in carbon paste as support material. The main peaks in the XRD pattern of the Ni/RGO/CPE at 2θ values of 42.6� , 43.6� , 44.76� , 46.41� , 52� , 60.15� and 77.71� corresponding to (010), (002), (111), (011), (200), (012) and (220) crystal planes for Ni, respectively. In XRD patterns of the Ni–Co/RGO/CPE, the main peaks at 2θ values of 42.6� , 43.6� , 44.76� , 46.41� , 51� , 60.15� and 77.71� cor responding to (010), (002), (111), (011), (200), (012) and (220) for pure Ni, pure Co or Ni–Co composite, respectively [16]. As can be seen, the
2. Materials and methods 2.1. Instruments and reagents All electrochemical measurements were performed in a three elec trode cell assembly by a potentiostat/galvanostat (electro analyzer system, SAMA 500, I.R. Iran). A saturated calomel electrode (SCE) was used as reference electrode and a platinum wire as counter electrode. A Milwaukee pH meter was used for adjusting the pH of the solutions. Deionized water was used for preparation of all the solutions. All re agents were from Sigma-Aldrich, excluding the graphite fine powder and paraffin oil (DC 350, 0.88 gcm 3) which both from Merck (Germany). 2.2. Reduced graphene oxide preparation The graphene oxide was synthesized from graphite powder using an improved method [39]. At first, the mixture of concentrated sulfuric acid and phosphoric acid (22.5:2.5 mL) was mixed by 1 g graphite. After adding 3 g of potassium permanganate, the mixture was stirred at 50 � C for 40 min. After that, the mixture was cooled to the room temperature and hydrogen peroxide (30%) was added to the oxidizing process in the ice bath. The solvent and solute were then separated and the solute was washed using 35% hydrochloric acid, ethanol, and distilled water. The sonicated suspension was then added to a solution containing 50 mL hydrazine solution (98%) and 200 mL ammonia solution (30%). It was refluxed at 90 � C for 12 h and then cooled to the room temperature. After centrifuging, the RGO was washed with deionized water and dried in vacuum for 24 h [39]. 2.3. Fabrication of the Ni–Co/RGO/CPE For fabrication of Ni–Co/RGO/CPE, the RGO/CPE was prepared firstly. The RGO was mixed with graphite powder by five weight percent. The paraffin oil (0.7 mL) was added to this mixture and a uni form paste was obtained by a mortar and pestle. The paste was filled into the electrode body which was a plastic tube (i.d. 3.5 mm) with a copper wire as the electrical contact. The RGO/CPE was then placed in a 10 mM solution of cobalt chloride and a 0.95 V potential was applied for 450 s by chronoamperometry, which Co/RGO/CPE was obtained. This elec trode was then inserted in a 10 mM nickel chloride solution and a po tential of 1.25 V was applied for 200 s to obtain Ni–Co/RGO/CPE. The Ni–Co/CPE was prepared in a same way but at bare CPE. Also, the Ni/ RGO/CPE was obtained in a same way but without deposition of Co.
Fig. 1. A) and B) SEM image of Ni–Co/RGO/CPE and Ni/RGO/CPE, respec tively; C) and D) EDX mapping of the surface of Ni–Co/RGO/CPE and Ni/RGO/ CPE, respectively. 2
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obtained as 0.77 m2/g metal. 3.3. Electrochemical characteristics of Ni–Co/RGO/CPE The electrochemical behavior of different electrodes was investi gated by the technique of cyclic voltammetry. The cyclic voltammo grams of the electrodes were recorded at the potentials between 0.0 and 1.0 V in NaOH solution (pH ¼ 13) at scan rate of 100 mVs 1. The Ni modified electrode (Ni/RGO/CPE) shows a redox activity with anodic and corresponding cathodic peak at 0.45 V and 0.34 V, respectively (curve a in Fig. 3a). While, the Co modified electrode (Co/RGO/CPE) have not any redox behavior and is not electroactive in the same con ditions (curve b in Fig. 3a). The redox behavior of Ni/RGO/CPE is related to oxidation and reduction of Ni(OH)2/NiOOH redox couple (equation (1)) [42]. Obvi ously, Ni(OH)2 is produced from oxidation of Ni at lower potentials when the potential is scanned anodically. Actually, different types of Ni hydroxide may be formed, however, these forms could transform into the less hydrated and more stable phase, Ni(OH)2 [43]. NiðOHÞ2 þOH ⇄NiOOH þ H2 O þ e
(1)
The cyclic voltammogram of Ni–Co/RGO/CPE is shown at curve c in Fig. 3a. As can be seen, when the Ni nanoparticles is deposited on Co nanoparticles the electrochemical current of redox activity of Ni is increased.
Fig. 2. A) and B) EDX spectrum of Ni/RGO/CPE and Ni–Co/RGO/CPE, respectively; C) XRD of Ni–Co/RGO/CPE, Ni/RGO/CPE and the reference NiCo patterns.
XRD pattern of Ni/RGO/CPE and Ni–Co/RGO/CPE are similar, because these elements have sequential atomic number, close atomic weight and same crystal structure in the normal conditions. 3.2. Electrochemically active surface area and metal loading of the electrodes The activity of an electrocatalyst may be controlled by the real sur face area. Therefore, the electrochemically active surface area of different electrodes were obtained using Randles-Sevcik equation in K4Fe(CN)6/K3Fe(CN)6 redox couple with concentration of 1 mM in KCl supporting electrolyte [40]. For each kind of electrodes, three samples were used and the average area was calculated. This area has not sig nificant difference between Ni–Co/RGO/CPE, Co/RGO/CPE and Ni/R GO/CPE and the average area was obtained as 0.208 cm2 for them. Also, this area was calculated as 0.191 cm2 for Ni–Co/CPE, Co/CPE and Ni/CPE and 0.1 cm2 for CPE. As can be seen, the presence of synthesized metal nanoparticles and RGO in the structure of the electrodes increased the real surface are of the electrodes. The electrochemically active sur face area of the modified electrodes increased more than two times, when RGO and metal nanoparticles were both on the electrodes. Also the metal loading on the electrocatalysts is an important factor. Thus, the Ni and Co loading on the prepared electrocatalyst were measured by coulombic charge consumed during the electro-deposition of metals on the RGO/CPE [41]. The calculated mass for Ni nano particles was obtained as 13.1 μg on Ni/RGO/CPE and was calculated as 13.4 μg for Co nanoparticles in Co/RGO/CPE. The total metal nano particles loading on the Ni–Co/RGO/CPE was obtained as 27 μg. Also the electrochemically active surface area of the Ni–Co/RGO/CPE can be
Fig. 3. a) Cyclic voltammograms of Ni/RGO/CPE (curve a), Co/RGO/CPE (curve b) and Ni–Co/RGO/CPE (curve c); b) Cyclic voltammograms of Ni–Co/ CPE (curve a) and Ni–Co/RGO/CPE (curve b); Conditions in all cases: pH ¼ 13 and ν ¼ 100 mVs 1. 3
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rates between 800 and 4000 mVs 1. This behavior was reported else where previously [44,45]. Fig. 4e shows the plot of the anodic and cathodic peak potentials (Ep) versus the logarithm of the potential scan rate (Log ν). There is no relation between Ep and Log ν at potential scan rates lower than 0.2 V s-1. But, at higher scan rates the peak potential is changed by variation of Log ν. This behavior was explained by Laviron for the surface confined electroactive species and the following equation (equation (2)) was derived between apparent charge transfer rate constant (ks) and transfer coefficient (α), if Epa-Epc ¼ ΔEp>200/n mV [46]: � � � Log ks ¼ αlog 1 α þ 1 α logα log RT=nFv α 1 � � α ðnα FΔEp =2:3RT (2)
The effect of RGO on the electrochemical response of the electrodes can be seen on Fig. 3b. The Ni–Co/RGO/CPE has higher electrochemical current rather than Ni–Co/CPE, because more amount of nanoparticles can be deposited at nanostructured RGO modified electrode and also the nanoparticles are more available at RGO modified electrode. This is related to the high surface area and high electron transfer ability of RGO nanosheets. The electrochemical behavior of Ni–Co/RGO/CPE was studied at different pHs by CV. The results showed the electrochemical current of the modified electrode is reduced with decreasing the pH of solution. According to equation (1) and related explanations, the presence of hydroxide is necessary for suitable redox activity of the Ni nanoparticles. Therefore, the electrochemical activity of the Ni modified electrode disappears at pHs lower than 10. Consequently, the optimum pH was selected as 13. The effects of the potential scan rate in the ranges of 50–500 and 800–4000 mV s 1 on the CV response of Ni–Co/RGO/CPE are shown in Fig. 4a and Fig. 4c, respectively. The anodic and cathodic peak currents (Ip) were plotted against potential scan rate (ν). As Fig. 4b shows the peak currents up to 500 mVs 1 is proportional to the scan rate, indi cating usual behavior for a surface confined redox material. This behavior is changed for scan rates higher than 500 mV s 1. Fig. 4d shows a linear relationship between Ip and square root of scan rate (ν1/2) at high scan rate. Therefore, there is a diffusion controlled process at scan
The transfer coefficient was calculated using the slopes of the plot of Epa and Epc versus Log ν at Fig. 4e as 0.68 and 0.47 for anodic and cathodic processes, respectively. Also, using equation (2) and the values of ΔEp corresponding to different scan rates, the average value of ks was derived as 0.59 � 0.03 s 1. 3.4. Electrocatalytic oxidation of methanol at Ni–Co/RGO/CPE The electrocatalytic activity of the proposed electrocatalyst was investigated for methanol oxidation by CV. The cyclic voltammetric responses of the Ni–Co/RGO/CPE and bare CPE were recorded in the absence and presence of methanol at scan rate of 100 mV s 1 in a so lution with pH ¼ 13.0. Fig. 5a shows the obtained results. As can be seen, the methanol is not oxidized at bare CPE even to potential 1.1 V (curve a), while its anodic peak at Ni–Co/RGO/CPE is about 0.7 V (curve b). Curve c of Fig. 5a shows the cyclic voltammogram of Ni–Co/RGO/CPE in the absence of methanol. As can be seen, in the cyclic voltammogram of the Ni–Co/RGO/CPE, the peak potential of Ni(OH)2/NiOOH con version upshifts about 50 mV in the presence of methanol, suggesting the contribution of NiOOH in electrocatalytic oxidation of methanol [42]. On the other hand, the anodic peak current of Ni(OH)2/NiOOH redox couple is greatly enhanced in the presence of methanol. In fact, the methanol reacts with the NiOOH formed on the electrode surface. This observation indicates a catalytic mechanism for oxidation of methanol at modified electrode. For comparing the catalytic activity of different modified electrodes, their cyclic voltammetric responses were obtained in the presence of methanol. The results are showed at Fig. 5b for Ni–Co/RGO/CPE, Ni–Co/CPE, Ni/RGO/CPE and Co/RGO/CPE in solution with pH 13.0 containing methanol. The Co nanoparticles at Co/RGO/CPE has not any electrocatalytic activity toward methanol oxidation (Fig. 5b, curve a). But, the presence of Co nanoparticles in Ni–Co/RGO/CPE shows a
Fig. 4. a and c) Cyclic voltammograms of Ni–Co/RGO/CPE for different scan rates (from inner to outer: 50, 60, 80, 100, 200, 300, 400 and 500 mVs 1 for a and 800, 1000, 1400, 1800, 2200, 2600, 3000, 3400 and 4000 mVs 1 for c); b and d) the plot of anodic and cathodic peak currents (versus ν in the range of 50–500 mVs 1 for b and versus ν1/2 in the range of 800–4000 mVs 1 for d); e) the plot of peak potentials versus the logarithm of the potential scan rate in the range of 50–4000 mVs 1.
Fig. 5. a) Cyclic voltammograms of 0.1 M methanol at CPE (curve a), Ni–Co/ RGO/CPE (curve b), curve c is cyclic voltammogram of Ni–Co/RGO/CPE in the absence of methanol; b) Cyclic voltammograms of 0.1 M methanol at Co/RGO/ CPE (curve a), Ni/RGO/CPE (curve b), Ni–Co/RGO/CPE (curve c)) and Ni–Co// CPE (curve d); Electrolyte: 0.1 M NaOH solution (pH 13.0), scan rate: 100 mV s 1. 4
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synergetic effect on methanol oxidation. It is can be concluded by comparing curves b and c for CV responses of Ni/RGO/CPE and Ni–Co/ RGO/CPE, respectively. As described in the section of electrochemically active surface area of the electrodes, the real surface area of the Ni–Co/ RGO/CPE and Ni/RGO/CPE are the same. Therefore, the difference observed between responses of methanol oxidation on these electrodes is not related to difference of the electrodes surface area but is related to the synergistic catalytic role of Ni and Co nanoparticles. Also, the role of RGO in electrocatalytic oxidation of methanol can be seen by comparing the electrochemical responses of Ni–Co/CPE (curve d) with Ni–Co/RGO/CPE (curve c) in Fig. 5b. The higher anodic current for methanol oxidation was obtained at Ni–Co/RGO/CPE, which con taining Ni, Co and RGO in its structure. 3.5. The effect of pH on the catalytic oxidation of methanol
Fig. 7. Cyclic voltammograms of 0.1 M methanol at Ni–Co/RGO/CPE in different scan rates (from inner to outer: 10,20,50,100 mVs 1); inset shows the plot of anodic peak current of methanol versus square root of potential scan rate.
The electrochemical behavior of methanol at Ni–Co/RGO/CPE investigated in different pHs from 10 to 13 (Fig. 6). The electrocatalytic current of methanol is higher in pH 13 and with decreasing the pH the methanol current is decreased. This behavior is affected from the behavior of Ni(OH)2/NiOOH redox couple in different pHs, which pre viously described. The electrochemical current of the Ni nanoparticles at Ni–Co/RGO/CPE is reduced with decreasing the pH of solution and therefore its catalytic activity is reduced.
and NiOOH cannot be occurred. Moreover, the anodic peak current of Ni (OH)2/NiOOH is decreased by decreasing the scan rate. This peak is disappearing completely at low scan rate (i.e. 10 mVs 1), therefore, the slowing reaction rate between NiOOH and methanol can be concluded [45].
3.6. The effect of potential scan rate on the catalytic oxidation of methanol
3.7. Chronoamperometric studies for the catalytic oxidation of methanol The chronoamperometry technique was used for obtaining the diffusion coefficient of methanol. Fig. 8 shows the chronoamperograms of various methanol concentrations at Ni–Co/RGO/CPE with an applied potential step of 650 mV. The extracted plots of current versus the in verse of the square roots of time (I-t 1/2) were constructed and showed a linear dependency (Fig. 8a). The slopes of the obtained lines were plotted versus methanol concentration (Fig. 8b). From the resulting slope and the Cottrell equation [47] the value of diffusion coefficient for methanol was found to be 8.75 � 10 8 cm2s 1. The catalytic rate constant of methanol at Ni–Co/RGO/CPE was also evaluated by chronoamperometry according to the method of Galus [48]. In this method, simplicity, there is an equation between the ratio of catalytic and limited current (IC/IL) and the catalytic rate constant (k) as
The effect of the potential scan rate on the cyclic voltammograms of Ni–Co/RGO/CPE in 0.1 M methanol and pH 13.0 are shown in Fig. 7. The inset of Fig. 7 displays the scan rate dependence of methanol oxidation peak current. This plot shows the methanol oxidation peak current at Ni–Co/RGO/CPE have linear proportional to the square root of scan rate in the range of 10–100 mVs 1. Therefore, the electro catalytic process of methanol oxidation is controlled by diffusion at Ni–Co/RGO/CPE. Another observation is the increasing the cathodic peak current of Ni(OH)2/NiOOH with increasing the scan rate. This is can be explained that at high scan rate, the reaction between methanol
Fig. 8. Chronoamperograms obtained at Ni–Co/RGO/CPE in pH 13.0 for different concentrations of methanol: from down to up correspond to 0.0, 0.1, 0.2, 0.3, and 0.4 M, Insets: a) plots of I versus t 1/2 obtained from chro noamperograms 0.1–0.4 M, b) plot of the slope of the straight lines against methanol concentration, c) plot of IC/IL versus t1/2.
Fig. 6. Cyclic voltammograms of 0.1 M methanol at Ni–Co/RGO/CPE in different pHs (10–13). 5
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equation (3): �1=2 IC =IL ¼ π1=2 kCb t
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(3)
Where, IC is the catalytic current (the current of Ni–Co/RGO/CPE in the presence of methanol), IL is the limited current (the current of Ni–Co/ RGO/CPE in the absence of methanol), Cb is the bulk concentration of methanol and t is the time. The plots of IC/IL versus t1/2 were constructed (Fig. 8c) and from the slopes of them the average value of k was calculated as 1.0 M 1s 1 or 1000 cm3 mol 1s 1. 4. Conclusion A novel Ni–Co bimetallic structure on reduced graphene oxide (RGO) is constructed. This composite was characterized by SEM, EDX and also electrochemical techniques. The nano dimensional structure of the composite as well as its good electrochemical behavior makes the composite as a suitable catalyst for methanol oxidation. The prepared electrocatalyst was used for electrocatalytic oxidation of methanol. The existence of Ni–Co nanocomposite and the presence of RGO was enhanced the electrocatalytic activity of the CPE based catalyst towards the electrooxidation of methanol. The voltammetric results, obtained in different pHs and in different scan rates, showed the role of the Ni(OH)2/ NiOOH redox couple in catalytic oxidation of methanol at the electro catalyst. Also the activity of Co nanoparticles and RGO nanosheets showed in the electrocatalytic process. Acknowledgement The authors are grateful to the Urmia University and Azarbaijan Shahid Madani University for the financial support of the work. References [1] S.G. Chalk, J.F. Miller, Key challenges and recent progress in batteries, fuel cells, and hydrogen storage for clean energy systems, J. Power Sources 159 (2006) 73–80. [2] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Appl. Energy 137 (2015) 511–536. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [4] T. Wilberforce, A. Alaswad, A. Palumbo, M. Dassisti, A.G. Olabi, Advances in stationary and portable fuel cell applications, Int. J. Hydrogen Energy 41 (2016) 16509–16522. [5] N.A. Tapan, M.E. Günay, R. Yildirim, Constructing global models from past publications to improve design and operating conditions for direct alcohol fuel cells, Chem. Eng. Res. Des. 105 (2016) 162–170. [6] S.S. Munjewar, S.B. Thombre, R.K. Mallick, Approaches to overcome the barrier issues of passive direct methanol fuel cell–Review, Renew. Sustain. Energy Rev. 67 (2017) 1087–1104. [7] A. Mehmood, M.A. Scibioh, J. Prabhuram, M.G. An, H.Y. Ha, A review on durability issues and restoration techniques in long-term operations of direct methanol fuel cells, J. Power Sources 297 (2015) 224–241. [8] Y. Zheng, H. Zhan, Y. Fang, J. Zeng, H. Liu, J. Yang, S. Liao, Uniformly dispersed carbon-supported bimetallic ruthenium–platinum electrocatalysts for the methanol oxidation reaction, J. Mater. Sci. 52 (2017) 3457–3466. [9] B. Habibi, M.H. Pournaghi-Azar, Methanol oxidation on the polymer coated and polymer-stabilized Pt nano-particles: a comparative study of permeability and catalyst particle distribution ability of the PANI and its derivatives, Int. J. Hydrogen Energy 35 (2010) 9318–9328. [10] L. Zhang, X.F. Zhang, X.L. Chen, A.J. Wang, D.M. Han, Z.G. Wang, J.J. Feng, Facile solvothermal synthesis of Pt71Co29 lamellar nanoflowers as an efficient catalyst for oxygen reduction and methanol oxidation reactions, J. Colloid Interface Sci. 536 (2019) 556–562. [11] S.M. Alia, G. Zhang, D. Kisailus, D. Li, S. Gu, K. Jensen, Y. Yan, Porous platinum nanotubes for oxygen reduction and methanol oxidation reactions, Adv. Funct. Mater. 20 (2010) 3742–3746. [12] Z. Han, A.J. Wang, L. Zhang, Z.G. Wang, K.M. Fang, Z.Z. Yin, J.J. Feng, 3D highly branched PtCoRh nanoassemblies: glycine-assisted solvothermal synthesis and superior catalytic activity for alcohol oxidation, J. Colloid Interface Sci. 554 (2019) 512–519. [13] B. Habibi, M.H. Pournaghi-Azar, H. Abdolmohammad-Zadeh, H. Razmi, Electrocatalytic oxidation of methanol on mono and bimetallic composite films: Pt and Pt–M (M¼ Ru, Ir and Sn) nano-particles in poly (o-aminophenol), Int. J. Hydrogen Energy 34 (2009) 2880–2892.
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