Nickel nanoparticles were successfully synthesized by a bipolar electrolyser. The obtained Ni nanoparticles were significantly reduced and distributed homogeneous comparing to the particles synthesized by a conventional DC electrolyser.
Ha Noi Metroplolitan University 126 SYNTHESIS OF NICKEL NANOPARTICLES BY A BIPOLAR ELECTROLYSER APPLIED FOR FUEL CELL Pham Van Vinh1, Nguyen Thuy Duong1, Nguyen Thi Hao1, Nguyen Bich Ngan2 Faculty of Physics, Hanoi National University of Education Faculty of chemistry, Hanoi National University of Education Abstract: Nickel nanoparticles were successfully synthesized by a bipolar electrolyser The obtained Ni nanoparticles were significantly reduced and distributed homogeneous comparing to the particles synthesized by a conventional DC electrolyser The effect of pulse intensity on the properties of Ni nanoparticles was studied XRD patterns revealed that the sample was pure Ni crystals of FCC structure The pulse intensity has not affected much on the formation of crystal phase while it had a strong impact on the morphology SEM images illustrated that the particles were in the shape of leaf and increased in size with the increase of the pulse intensity Hysteresis loop showed the soft magnetic behaviours of the material The increase of saturation magnetism with the increase of pulse intensity was attributed to the effects of grain size The cyclic voltammogram exhibited that the Ni nanoparticles were a great catalysis for ethanol oxidation reaction This exposed that the Ni nanoparticles have a potential application for directed ethanol fuel cell Keywords: bipolar electrolyser, nickel nanoparticles, electro catalysis, ethanol oxidation, fuel cell Email: vinhpv@hnue.edu.vn Received 26 August 2018 Accepted for publication December 2018 INTRODUCTION Nowadays, non-renewable energies have been running out Therefore, new energy sources which generate green energy such as solar, wind, tide or chemical fuel related energies have been intereted Fuel cell is an outstanding candidate to solve this problem It has been studied for many years but still not popularly applied because of their high cost The catalysis made of platinum is the main reason for increasing the cost of fuel cell To find the solutions for this issue, recent studies have focused on alloying Pt with other metals such as Ni, Cu, Fe, Co,… to support for Pt [1-3] or searching a new technique to create the special Pt structures [4, 5] Besides, non-Pt catalysis have been attracting the attention of researchers [6, 7] Nickel nanoparticles have played an important role in Scientific Journal − No27/2018 127 commercial because of their various applications in electronics, magnetism, energy technology, and biomedicine,… Among these, catalysis is such an enormous utilization because it can reduce the cost of generating green energy Nickel nanoparticles are cheaper and can be synthesized with a large amount compared with noble metals Furthermore, Ni nanoparticles shows a great stability in alkaline medium [8] and high electrocatalytic activity of ethanol oxidation reaction Therefore, Ni nanoparticles promise potential applications as a catalyst for directed ethanol fuel There are many methods to synthesize Ni nanoparticles such as solution reduction [9], electrolysis method, polyol method [10], or sol-gel method [11] The electrolysis method has been preferred to use because it’s simple and cost effective The DC electrolysis method is common to be used However, using DC current in electrolyzing process results in creating the large size of particles, usually in microscale [12] To solve the grain size problem, a bipolar electrolyser is expected to synthesize Ni particles in nanoscale Indeed, with bipolar electric current, each electrode will become positive or negative polar alternatively and it can interrupt the ions agglomerating process and decrease the grain size In this study, we focused on fabricating Ni nanoparticles by a bipolar electrolyser and investigated the effect of pulse intensity between two electrodes on the properties of Ni nanoparticles EXPERIMENTAL 0.052g of NiCl2 was dissolved into 80ml of dehydrated water The solution was placed inside ultrasonic cleaner tank during the electrolyzing process with one is Ni electrode and the other one is Pt electrode The synthesized parameters were controlled by a computer The electrolyzing process took up 2h under the condition that the pulse period was 20s and the distance between two electrodes was kept constant of 1cm while the pulse intensities were varied for each sample The sample then was collected from nickel electrodes and was annealed in H2 medium in 1h at 250oC to obtain pure Ni nanoparticles The analysis method XRD, SEM, VSM and CV were used to investigate the crystal structure, morphology, magnetic and catalytic properties of Ni nanoparticles respectively For CV measurement, the sample was mixed with Carbon nanopowder and Nafion before having stuffed in an electrode RESULT AND DISCUSSION 3.1 Effect of pulse intensity on Ni crystal structure The XRD patterns of nickel nanoparticles at different pulse intensities ranging from 12V to 18V are showed in Fig.3.1 The figure demonstrated that nickel nanoparticles had Ha Noi Metroplolitan University 128 face-centered cubic lattice structure (fcc) Two peaks with the 2θ degree were 44.56o and 51.91o respectively which suggest two planes (111) and (200) of pure nickel No peaks of nickel compounds are found confirmed the purity of nickel nanoparticles It also indicates that the pulse intensity have not affected much on the process to form the phases of nickel crystal The intensity of two peaks of 12V-sample are higher compared with others exposing that it is crystalized better (111) Relative Intensity (a.u) 1000 18V 16V 12V 800 (200) 600 400 200 10 20 30 40 50 60 70 2-theta (degree) Figure 3.1 XRD patterns of Ni nanoparticles synthesized at different pulse intensities 12V, 16V and 18V The crystalline size could be determined from the XRD patterns by Scherrer equation: D= 0.9λ β cosθ where D: the average particle size λ: the wavelength of X-ray ( λ = 1.54056 A ) β: FWHM (rad) θ: the angle of peak position o Let’s calculate with 2θ = 44.56o Table 3.1 The average particle size at different pulse intensities β(rad) 0.0043 0.0041 0.0039 Pulse Intensity (V) 12 16 18 D (nm) 35 37 38 Scientific Journal − No27/2018 129 The results showed that the crystalline size depended on the pulse intensity during electrolyzing process When the pulse intensity increased from 12V to 18V the full width at half maximum went down and the crystalline size increased slightly from 35nm to 38nm It could be understood that more and more Ni2+ moved to Ni electrode to create clusters resulting in the increase of crystalline size 3.2 Effect of pulse intensity on Ni particle’s morphology Figure 3.2 shows the SEM images of synthesized nickel particles at different pulse intensities These images suggested Ni nanoparticles had a shape of leaf with a porous structure This characteristic was very important because it is evidenced that these nickel nanoparticles had high potential to be used for catalytic purposes Among three images, the 12V-sample was the most uniform compared with others It was reasoned by the slower speed of creating particles on Ni electrode’s surface As the pulse intensity increased, the particle size also increased and it was in accordance with the results drawn from XRD patterns 12V 16V 18V Figure 3.2 SEM images of Ni nanoparticles synthesized at different pulse intensities Ha Noi Metroplolitan University 130 3.3 Effect of pulse intensity on magnetic properties of Ni nanoparticles 18V 16V 20 12V M (emu/g) 10 -10 -20 -6000 -4000 -2000 2000 4000 6000 H (Oe) Figure 3.3 Hysteresis loops of Ni nanoparticles synthesized at different pulse intensities The hysteresis loops in Fig 3.3 whow that all samples synthesized at different pulse intensities behaved as soft magnetic materials because they all had small coercive force Soft magnetic property reconfirms the formation of the crystal of nickel particles The magnetic parameters are presented in the table 3.2 Table 3.2 Magnetic parameters of nickel nanoparticles synthesized at different pulse intensities Pulse intensities (V) Saturation Magnetism Ms (emu/g) Residual Magnetism Mr (emu/g) Coercive Force 12 12.7 1.0 62.5 16 18.5 2.5 92.5 18 20.1 2.2 85.2 Hc (G) The table illustrated that the magnetic parameters strongly depended on the pulse intensity The higher pulse intensity was, the larger saturation magnetism was The increase of saturation magnetism with the increase in the pulse intensity was the results of the particle size effect, which were showed in the XRD and SEM analysis 3.4 Effect of pulse intensity on catalytic ability of Ni nanoparticles In the Figure 3.4, the dash lines represented for the sample in KOH medium only whereas the solid lines stand for the sample in KOH and ethanol medium In the case of the Scientific Journal − No27/2018 131 samples that measured in KOH medium, there exist an oxidation peak at 500mV The reaction could be describe by following equations: Ni +2OH- → α-Ni(OH)2 + 2e- (1) α-Ni(OH)2 → β-Ni(OH)2 (2) β-Ni(OH)2 → NiOOH + H2O + e- (3) 400 12V 16V 18V 12V Et 16V Et 18V Et Current density (mA/cm2) 300 200 100 -100 -200 -1000 -500 500 1000 Potential applied (mV) Figure 3.4 The cyclic voltammogram of nickel nanoparticles synthesized at different pulse intensities in KOH medium only and KOH + C2H5OH medium However, the oxidation of Ni (0) to Ni (II) occurred at more negative potential which couldn’t show in the figure and at the considered peak, there was a change from α-Ni(OH)2 to β-Ni(OH)2 and a formation of NiOOH In case of adding ethanol, a peak near 500mV could not be observed while the peaks in the range of 600mV to 700mV appeared The present of new peaks overlaped the peaks near 500mV The reason lied in the fact that nickel oxyhydroxide (NiOOH) was capable of oxidizing some functional groups and that would oxidize ethanol to acetaldehyde before transforming to acetic acid as described in equation [13] 4β-NiOOH + CH3CH2OH + OH- → 4βNi(OH)2 + CH3COO- (4) The peak of 12 V-samples had the highest current density It implied that Ni nanoparticles which were synthesized at lower pulse intensity would perform as higher electrocatalytic activities toward ethanol oxidation Catalytic activity is primarily a surface phenomenon The obtained particles synthesized at lower pulse intensities were small hence increasing contact area resulting in improving the catalytic activity The ethanol oxydation reaction showed a great potential of Ni nanoparticles to be applied for directed ethanol fuel cell Ha Noi Metroplolitan University 132 CONCLUSION The nano crystal of Nickel nanoparticles in FCC structure were successfully synthesized by bipolar electrolyzing method The pulse intensity effected significantly on morphology and grain size of nanoparticles The particle size of Ni was reduced and uniformed with the decrease of the pulse intensity The optimum pulse intensity for synthesizing Ni nanoparticles was 12V The saturation magnetism of the Ni nanoparticles increases in the increase of the pulse intensity Ni nanoparticles exhibited as a good electrocatalyst toward ethanol oxidation Acknowledgments: This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number of 103.022013.50 REFERENCES Dominic Alexander Notter, K.B.K., Theodoros Karachalios, Nara Tudela Haberland, -Life cycle assessment of PEM FC applications: Electric mobility and μ-CHP - Energy & Environmental Science 2015 8(7) Lucci, F.R., Selective hydrogenation of 1,3-butadiene on platinum–copper alloys at the singleatom limit, - Nature Communications 2015 Van Vinh Pham, V.-T.T., Cho Sunglae, Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell - International Journal of Hydrogen Energy, 2017 42(18): p 13192-13197 Long NV, O.M., Nogami M, Hien TD, Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol 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Journal − No27/2018 133 11 Falong Jia, L.Z., Xiaoying Shang, and Yan Yang, Non-Aqueous Sol–Gel Approach towards the Controllable Synthesis of Nickel Nanospheres, Nanowires, and Nanoflowers Advanced Materials, 2008 20(5): p 1050 - 1054 12 GÖKHAN ORHAN, G.G.G., Effect of electrolysis parameters on the morphologies of copper powders obtained at high current densities Journal of the Serbian Chemical Society, 2012 77(5): p 651-665 13 Riyanto, M.R.O., Jumat Salimon, Analysis of ethanol using copper and nickel sheet electrodes by cyclic voltammetry - The Malaysian Journal of Analytical Sciences, 2007 11(2): p 379387 CHẾ TẠO HẠT NANO NICKEL ỨNG DỤNG CHO PIN NHIÊN LIỆU BẰNG PHƯƠNG PHÁP ĐIỆN PHÂN SỬ DỤNG DỊNG LƯỠNG CỰC Tóm tắ tắt: Hạt nano Nickel chế tạo thành công phương pháp điện phân sử dụng dòng lưỡng cực Kích thước hạt phân bố kích thước hạt cải thiện đáng kể so với hạt Nickel chế tạo phương pháp điện phân thơng thường sử dụng dòng DC Ảnh hưởng cường độ xung lên tính chất hạt nghiên cứu cách kỹ lưỡng Giản đồ nhiễu xạ tia X chứng tỏ mẫu chế tạo đơn pha tinh thể Nickel với cấu trúc lập phương tâm mặt Cường độ xung không ảnh hưởng nhiều lên hình thành pha tinh thể lại ảnh hưởng mạnh mẽ lên hình thái kích thước hạt Ảnh SEM cho thấy hạt có dạng hình kích thước hạt tăng theo chiều tăng cường độ xung Đường cong từ trễ cho thất vật liệu chế tạo vật liệu sắt từ mềm Sự tăng từ độ bão hòa theo cường độ xung cho hiệu ứng liên quan đến kích thước hạt Phép đo qt vòng tuần hoàn chứng tỏ hạt nano nickel chất xúc tác mạnh phản ứng oxy hóa ethanol Điều cho thấy tiềm ứng dụng hạt nano nickel pin nhiên liệu sử dụng ethanol trực tiếp Từ khóa: điện phân sử dụng dòng lưỡng cực, hạt nano nickel, xúc tác điện hóa, oxy hóa ethanol, pin nhiên liệu ... stability in alkaline medium [8] and high electrocatalytic activity of ethanol oxidation reaction Therefore, Ni nanoparticles promise potential applications as a catalyst for directed ethanol fuel. .. coercive force Soft magnetic property reconfirms the formation of the crystal of nickel particles The magnetic parameters are presented in the table 3.2 Table 3.2 Magnetic parameters of nickel nanoparticles. .. synthesizing Ni nanoparticles was 12V The saturation magnetism of the Ni nanoparticles increases in the increase of the pulse intensity Ni nanoparticles exhibited as a good electrocatalyst toward ethanol