Synthesis of Cu/Cu2O nananoparticles by bipolar electrolysis method for electrocatalyst in ethanol oxidation reaction

82 JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2017-0034 Mathematical and Physical Sci. 2017, Vol. 62, Iss. 8, pp. 82-88 This paper is available online at SYNTHESIS OF Cu/Cu2O NANANOPARTICLES BY BIPOLAR ELECTROLYSIS METHOD FOR ELECTROCATALYST IN ETHANOL OXIDATION REACTION Pham Van Vinh and Tran Xuan Bao Faculty of Physics, Hanoi National University of Education Abstract. Cu/Cu2O nanoparticles were successfully synthesized using the electrolysis method and a bipolar pulse power supply. The effect of pulse amplitude on the properties of samples was investigated. The phase analysis by XRD showed the presence of both Cu and Cu2O crystals corresponding to cubic structure. SEM images also showed the cubic shape of Cu/Cu2O with crystal size that increases with an increase in pulse amplitude. The presence of plasmon peak presents around the wavelength of 520nm and an absorption edge with a wavelength less than 450nm on the absorption spectrum reconfirmed the presence of Cu and Cu2O nanoparticles. The cyclic voltammetry measurement was used to study the ethanol oxidation activities of samples. It was found that Cu/Cu2O nanoparticle could catalyze ethanol oxidation in alkali medium. Keywords: Cu/Cu2O, ethanol oxidation, direct ethanol fuel cell, bipolar electrolysis. 1. Introduction The fuel cell is a device that has high energy conversion efficiency because it can convert fuel and oxidant directly into electricity without a Carnot cycle. Fuel cells have been expected to replace the traditional energies resources that are being depleted rapidly [1]. Indeed, fuels using for fuel cells are nontoxic and renewable. Moreover, fuel cells operate with high efficiency and they are quiet and environmentally friendly [2, 3]. Therefore, it has attracted a great deal of attention from many research groups. The conventional fuel cell operates using pure hydrogen gas and oxygen available in the air. This is not polluting but the preparation, storage and transport of hydrogen is very complicated and dangerous. Differing from this, direct ethanol fuel cell (DEFC) uses ethanol instead of hydrogen [4]. Ethanol exists as a solution so it is very convenient for storage as well as transportation. Ethanol is a hydrogen-rich liquid and has higher specific energy in the fuel cell (providing 12e in oxidation reaction in alkaline environments) [5, 6]. It is one type of green fuel that can be obtained in great quantity using a fermentation process from renewable resources [7, 8]. However, the oxidation rate of methanol on some electrode materials is still low. This is one of the major challenges for researchers in bringing DEFC into civil applications. To increase the catalytic activities for ethanol oxidation, some research directions focus on rare metals such as Au, Pt, Pd and their alloys [9-11]. Received August 10, 2017. Accepted September 7, 2017. Contact Pham Van Vinh, e-mail: vinhpv@hnue.edu.vn Synthesis of cu/cu2o nananoparticles by bipolar electrolysis method for electrocatalyst 83 However, these are expensive and cause the high cost of the devices. Other research directions focus on the catalytic properties of transition metals such as Cu, Ni, Ag in alkaline environments. This is expected to reduce the cost of DEFC. Therefore, the catalytic activity of Cu/Cu2O nanoparticles for the ethanol oxidation reaction in alkaline solution was chosen for this research. Cu and Cu2O particles have been synthesized using many different methods such as the reduction method, the polyol method and the electrolysis method [12-14]. The reduction method and the polyol method allow the synthesizing of nanoparticles with a grain size ranging from a few nanometers to dozens of nanometers in size. However, these methods are quite complicated, they have low stability and the cost is high. The electrolysis method is simple, low cost and has high stability. It has been used widely in the industry to obtain micro-Cu particles. However, for catalytic purpose, Cu particles should be nanoscale. One reason for the large particle size of Cu is because DC current is used in electrolysis process. The downside of this method is that the Cu ++ ions continuously transfer to the electrode causing the large Cu particles to grow. In this study, a bipolar pulse current is used instead of the DC current. With this method, in every cycle the current is reversed and therefore Cu ++ does not to have an opportunity to grow and become a large particle. 2. Content 2.1. Experimental Fig.1. Schematic diagram of experimental apparatus Preparing samples: Cu/Cu2O nanoparticles were synthesized using an electrolyzer (Fig. 1). The electrolyzer system consists of a computer, the copper electrodes, a bipolar pulse generator and a tank. Two electrodes made of pure copper (99.99%) were placed in a CuSO4 solution. The ends of the electrodes were connected to the bipolar pulse generator. The bipolar pulse generator was used as a power supply that can be controlled by a computer. The pulse period of 20 seconds and the electrolyte solution (containing 0.04 g CuSO4 and 0.2g PVP (99.999%; Sigma Aldrich) diluted in 80 ml DI water) were kept constant for all experiments. The pulse amplitudes were varied to determine their effects on the properties of the samples. The experiments were carried Pham Van Vinh 84 out under the assistance of ultrasonic. The Cu/Cu2O nanoparticles dispersing in the electrolyte were collected using a centrifuge. Physical properties analysis: An X-ray diffractometer (Bruker, D8 Advance) was used to determine the crystalline phases, a scanning electron microscope (SEM, HitachiS-4800) was used to analyze the morphology and a UV-vis spectrometer was used to study the optical properties of the samples. Preparing the electrode for cyclic voltammetry measurement: the electrodes were prepared as shown in [15]. Cyclic voltammetry was performed at room temperature using a three- electrode instrument (Auto lab) in 1M KOH and 1M KOH + 2M ethanol solution. The measurements were carried out at the potential sweep rate of 50 mVs -1 and the range was from - 1.00V to 1.00V. 2.2. Results and discussions The Cu/Cu2O particles were synthesized with pulse amplitudes of 11V, 13V, 15V and 17V. In the first half of the cycle, the formation of copper nanoparticles is described by the following equations: At the cathode: Cu 2+ (aq) + 2e – → Cu (1) At the anode: Cu 0 – 2e– → Cu2+(aq) Cu 2+ + SO4 2- → CuSO4 (2) With appreciable voltage and ultrasonic assistance, the copper that does not adhere well to the cathode is dispersed in the solution. Somehow, oxygen remaining in the solution reacts with Cu to form Cu2O. The formation of Cu2O is described in the equation: 4Cu + O2 = Cu2O (3) 20 30 40 50 60 70 80 0 100 200 300 400 500 600 - Cu (d) (c) (b) In te n s it y ( a .u ) 2o (a) - Cu2O Fig. 2-XRD diffractogram of Cu/Cu2O particles synthesized using pulse amplitudes of a) 11V; b) 13V; c) 15V; d) 17V Synthesis of cu/cu2o nananoparticles by bipolar electrolysis method for electrocatalyst 85 The formation of Cu and Cu2O was evidenced by the XRD analysis. Indeed, the XRD patterns in Fig. 2 show the presence of both Cu and Cu2O crystalline phases corresponding to the face-centered cubic structure. The intensity of the peaks increases with an increase of the pulse amplitude. The increase of peaks is attributed the growth of better crystals at higher current density. Fig. 3. FE-SEM micrograph of Cu/Cu2O particles: a) synthesized at 11V; b) synthesized at 13V c) synthesized at 15V; d) synthesized at 17V Fig. 3 is FE-SEM images of Cu/Cu2O particles synthesized using different pulse amplitudes. It is easy to see that the particles grown were cube shaped. This result agrees well with the results of XRD. The crystalline size increases with the increase in pulse amplitude. Therefore, the crystalline size can be controlled by changing the pulse amplitude. With the pulse amplitude of 11V, the crystalline size reached some hundreds of nanometers. This size is smaller than that of Cu or Cu2O synthesized using the conventional electrolysis [16-19]. 300 400 500 600 700 800 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 (d) (c) (b) A b s o rb a n c e ( a .u ) Wavelength (nm) (a) 520nm Fig. 4. UV–vis absorption spectra of Cu/Cu2O: a) synthesized at 11V; b) synthesized at 13V c) synthesized at 15V; d) synthesized at 17V Pham Van Vinh 86 The absorption spectra of Cu/Cu2O characterized in the range of 250nm to 800nm is shown in Fig. 4. There is an absorption edge in the wavelength lesser than 450nm and a peak at wavelength 520nm. The absorption edge is attributed to the absorption of Cu2O. Indeed, Cu2O is a direct band gap semiconductor. Therefore, it should satisfy the equation: (ahυ)2 = A(hυ - Eg). Here, a, A, hυ and Eg are the absorption coefficient, constant, the photon energy and the band gap respectively. By estimating the band gap from this equation and absorption spectrum, it is easy to see that the band gap of the samples is about 1.75ev. This is in good agreement with the band gap of Cu2O. The absorption peaks at the wavelength of 520 is original from the surface plasmon resonance (SPR) of Cu [20]. This result reconfirms the formation of Cu and Cu2O particles. -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 -0.001 0.000 0.001 0.002 0.003 0.004 0.005 (d) (c) (b)C u rr e n t( A ) Applied voltage (V) (a) -0.15V (A) -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 -0.0005 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 0.0040 (d) (c) (b) -1.15V C u re n t( A ) Applied voltage (V) 0.6V (a) (B) Fig. 5 The cyclic voltammogram of the Cu/Cu2O nanoparticles (A) in KOH 1 M ; (B) in 1 M KOH + 2M ethanol a) synthesized at 17V; b) synthesized at 15V c) synthesized at 13V; d) synthesized at 11V Cyclic voltammetry (CV) measurements were used to evaluate the ethanol oxidation ability of the Cu/Cu2O nanoparticles. The cyclic voltammetry measurements were first carried out in 1M KOH and then done in 1M KOH + 2M Ethanol. Fig. 5 (A) is the cyclic voltammogram of the Cu/Cu2O nanoparticles in 1M KOH. There is only one oxidation peak at voltage -1.5V. The presence of the peak is related to the oxidation of Cu (0) to Cu (I) due to equation : Cu + OH - → CuOH + e- (4) 2Cu + 2OH - → Cu2O + H2O + 2e- (5) Fig. 5 (B) is the cyclic voltammogram of the Cu/Cu2O nanoparticles in 1M KOH + 2 M ethanol. There are two oxidation peaks at -0.15 V and +0.60V. The peak at +0.6V presented when introducing ethanol in the electrolytic solution. This peak is attributed to ethanol oxidation due to the equations: Anode (oxidation reaction): C2H5OH + 3H2O → 12H + + 12e _ + 2CO2 (6) Cathode (reduction reaction): 3O2 + 12H + + 12e - → 6 H2O (7) Overall reaction (redox reaction): C2H5OH + 3O2 → 3H2O + 2CO2 (8) Synthesis of cu/cu2o nananoparticles by bipolar electrolysis method for electrocatalyst 87 Both the Cu and ethanol oxidation peaks increased with the decrease in pulse amplitude. The catalyst activities occur mainly on the surface of particles. The SEM image in Fig. 2 shows that the lower pulse amplitude is, the smaller the nanoparticles obtained. This means that the nanoparticles synthesized using a low pulse amplitude have a large surface area, resulting in increased ethanol oxidation peaks 3. Conclusions Cu/Cu2O nanoparticles were successfully synthesized using the bipolar pulse electrolysis method. The XRD pattern, absorption spectrum and the cubic shape in SEM demonstrated that Cu/Cu2O had a crystalline structure with a particle sizes that decreases with the decrease in pulse amplitude. The particle size of Cu/Cu2O synthesized using bipolar pulse is smaller than that synthesized using conventional electrolysis method. The cyclic voltammetry measurement indicates that Cu/Cu2O nanoparticles exhibit good electrocatalytic activities for ethanol oxidation reactions in an alkali medium. Acknowledgement. This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2013.50. REFERENCES [1] Höök, M. and X. Tang, 2013. Depletion of fossil fuels and anthropogenic climate change-A review. Energy Policy, 52: pp. 797-809. [2] Dyer, C.K., 2002. Fuel cells for portable applications. Fuel Cells Bulletin, 2002(3): pp. 8-9. [3] Winter, M. and R.J. Brodd, 2004. 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