Synthesis of Ag2O/CNTs Nanocomposite to be used as a cathode material for Zinc - Silver batteries - Nguyen Van Tu

Vietnam Journal of Science and Technology 56 (2A) (2018 ) 149-155 SYNTHESIS OF Ag2O/CNTs NANOCOMPOSITE TO BE USED AS A CATHODE MATERIAL FOR ZINC - SILVER BATTERIES Nguyen Van Tu 1, 2 * , Abdul Hakim Shah 3 , Mai Van Phuoc 1 1 Institute of Chemistry and Material, 17 Hoang Sam Street, Nghia Do Ward, Cau Giay District, Ha Noi, Viet Nam 2 School of Material Science and Engineering, Wuhan University of Technology 122 Luoshi Road, Wuhan, P. R. China 3 Department of Physics, Khushal Khan Khattak University, Karak, Pakistan * Email: nguyenvantu882008@yahoo.com Received: 08 April 2018; Accepted for publication: 14 May 2018 ABSTRACT In this article, Ag2O/carbon nanotubes (CNTs) nanocomposite has been prepared by chemical reduction method and used as a cathode material for zinc-silver batteries. The transmission electron microscopy (TEM) tests reveal the CNTs and Ag2O nanotubes form an interpenetrating network structure. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis confirmed that the Ag2O shows Cubic (Pn-3) crystal structure and mixture element states in the nanocomposite. The charging/discharging property of the Ag2O/CNTs nanocomposite was studied by galvanostatic charge-discharge measurement as a cathode material. The results indicated that Ag2O/CNTs nanocomposite has high specific capacity and good cycling stability. For the current density of 0.53 mA/cm 2 (2.5C), the initial specific capacity of the nanocomposite is 190 mAh/g and remains 172 mAh/g after 20 cycles. Keywords: silver oxide, silver nanocomposite, zinc-silver battery, cathode material. 1. INTRODUCTION Zinc-silver oxide batteries are under investigation of researchers since long. However, enhancement in the battery properties is a challenging and continuous run of the research community. Zinc-silver batteries are used in military and aerospace applications due to their unique properties such as stability, large currents and being of highly safety [1-2]. Envisage of nanotechnology has turned many applications of everyday life and likewise, the electrodes for the batteries are also being made of nanomaterials effectively because of their high electrical and thermal conductivities, and high specific surface area on the nanoscale, which improves their ability of exchange of electrons, ions diffusion, easier electrochemical process and the occurrence of increased charging/ discharged at high current density. Graphene and Nguyen Van Tu, Shah Abdul Hakim, Mai Van Phuoc 150 carbon nanotubes (CNTs) are the nowadays known best electrical conductors [3 - 5]. Exchange, ions diffusion and hence favors the ability to conduct electricity accordingly [6, 7]. The aim of this work is the synthesis of metals or metal oxides based nanocomposite with carbon nanotubes and investigation of their electrode properties for the battery applications. Enhanced electrochemical properties have been found for this nanocomposite such as the stable cycle, increased current density and directional application in zinc-silver batteries [8-10]. 2. MATERIALS AND METHODS 2.1. Material synthesis Raw CNTs (Purity ≥ 97 %, tube length 5 - 15 μm, CNTs diameter 10 - 20 nm; surface area 120 m 2 /g) were purchased from Shenzhen Nanotech Port Co., Ltd (China). In order to functionalize these CNTs for the removal of metallic impurities/ amorphous carbon, 1 g CNTs were treated in the aqueous solution of 45 mL H2SO4 (98 %) and 15 ml of HNO3 (65 %) at 40 o C for 5 hours. The product after treatment was washed with the deionized water and dried at 40 o C. Ag/CNTs materials used as cathodes were prepared from silver nitrate (AgNO3) and sodium bohydride (NaBH4) by a reduced chemical method reported somewhere else [3, 6]. Typically, an initial content of 10 % CNTs were dispersed in the deionized water by ultrasonication for 2 - 5 hours, until a homogeneous solution of CNTs was obtained. AgNO3 was then added slowly at a predetermined rate into the CNTs solution. In the presence of NaBH4 reducing agent, Ag + in solution (alkaline medium) is reduced to Ag precipitated on CNTs by reaction: 8Ag + + BH4 - + 8OH - = 8Ag + 6H2O (1) Finally, the Ag/CNTs nanocomposite precipitate was centrifuged, washed several times with distilled water, dried at 100 °C for 10 hours, and preserved. Cathode (10 mm diameter (area of 0.785 cm 2 ), with an average weight of 1.0 mg ± 0.01) comprised of 80 % active material (Ag/CNTs), 5 % CMC additive, 10 % conductive carbon powder, 0.1 mm thick nickel mesh, 1 mm mesh, was prepared. The electrode was heat-treated by firing in inert gas Ar or N2, at 400 - 500 °C for 5 hours. Anode polarization was carried out by current density of 0.2 mA/cm 2 in a 2M KOH electrolyte for 40 hours, washed, dried and preserved. The electrode reaction occurs as follows [5]: Ag - 2e + 2OH - = Ag2O + H2O or (2) Ag2O -2e + 2OH - = 2AgO + H2O (3) It was fabricated as CR2032 standard battery, cellophane separation (02 layers), with electrolyte: 400 g/l KOH + 100 g/l ZnO + 20 g/l additive, investigated the discharge ability of material electrode. Method of calculating the theoretical capacity of the battery as follows: Ccell = mAg × CAg (4) whereas, mAg is the actual mass of the silver active agent in the positive electrode; CAg is theoretical capacity of Ag/Ag2O, CAg = 231 mAh/g. Based on the formula (4) and the initial content of the CNTs (10 %), the capacity of the using Ag2O/CNTs based battery turned out to be of 0.166 mAh, thus the discharge current at rate 2.5C corresponded to a current density of 0.53 mA/cm 2 . Synthesis of Ag2O/CNTS nanocomposite to be used as cathode materials for zinc-silver 151 Anode made of pure zinc plate (containing 99.975 % zinc content) with a thickness of 0.12 mm ± 0.01, a diameter of 12 mm (1.13 cm 2 ), was treated in solution electrolysis before use. 2.2. Characterization The crystalline structure of the sample was characterized by a powder X-ray diffraction spectrometer (XRD, PertPro PANalytical, Netherlands) equipped with Cu K radiation (1.5418 Å). The morphology of the sample was observed by the field emission scanning electron microscope (FESEM, JSM-6700F, JEOL, Japan) and emission scanning electron microscope (SEM, S4800, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were acquired using a VG Multilab 2000, with Al K the as the radiation source. All XPS spectra were corrected by the C1s line at 284.8 eV. Raman spectroscope equipped with a 633 nm laser (Raman; model RenishawInvia, Britain) was employed to get the structural information. In addition, determination of organic content was carried out by thermal analysis method (TG, NETZSCH STA 409P/PG), under air atmosphere, temperature from 0 to 800 o C, at a heat rate of 5 K/minute. Prior to use, the calorimeter was calibrated with metal standards, an empty aluminum pan being used as a reference. Sample of around 5 mg weight was placed in the sealed aluminum pans. The galvanostatic charge-discharge test was carried out on a battery test system (Land BT2000, Wuhan, China), at current rates of 2.5C (0.53 mA/cm 2 ), in the potential of 1.15-1.80 V (vs SHE). 3. RESULTS AND DISCUSSION 3.1. Structure and morphology 3.1.1. XRD analysis The XRD patterns of Ag2O/CNTs nanocomposite electrodes before and after discharge are shown in Figure 1. Figure 1 (a) shows the characteristic lines with the interplanar spacing of 2.748 (1,1,1); 2.800 (2,0,0); 1.683 (2,2,0); 1.424 (3,1,1) and 1.374 Å (2,2,2) of Ag2O diffraction, corresponding to cubic (Cubic (Pn-3)) structure. It indicates that the silver oxide electrode mainly contains silver (I) oxide (Ag2O) composition. Figure 1 (b) is the diffraction pattern of silver oxide electrodes after discharge at rate 2.5C, after 20 cycles of discharge/charge. The peaks show that only the metallic Ag features are retained with the interplanar spacing of 2.359 (1,1,1); 2.043 (2,0,0); 1.445 (2,2,0) and 1.232Å (3,1,1), without diffraction peaks by AgO, Ag2O, hence it predicts a high discharge efficiency. 3.1.2. XPS and Raman analysis Raman and X-ray fluorescence analysis were applied to determine the carbon and silver presence of carbon in the samples. The Raman spectra as shown in Figure 2 (a) indicates two low intensity peaks around 1345 and 1587.8 cm -1 corresponding to the C-C, and C-C = O bond of the CNTs and the relatively high intensity peaks around 992 cm -1 related to the bonding C-O or Ag-O. The XPS spectra in Figure 2 (b) show that C, O and Ag elements coexist. In Figure 2 (c), only peak of the C-C bond groups (384.7 eV) indicated that at the heat treatment conditions (500 o C, 5 hours, under argon gas) -OH, -COOH groups are either completely decomposed or retained very low in the content. In Figure 2 (d), the peaks at the energy level of Nguyen Van Tu, Shah Abdul Hakim, Mai Van Phuoc 152 367.82 eV (Ag3d5/2), 375.7 eV (Ag3d3/2) correspond to the pure silver metal. This again demonstrates that at high-efficiency discharge, silver oxide forms almost completely transformed into metal silver. Figure 1. Results XRD patterns of Ag2O/CNTs electrodes nanocomposite before and after discharge (a) - Electrode before discharge; (b) - Electrode after 20 cycles. Figure 2. Raman and XPS spectra of the electrode material (after discharge with an initial content CNTs of 10 %). (a) Raman spectra of the Ag2O/CNTs sample; (b) XPS spectra of Ag2O/CNTs sample; (c) XPS spectrum of C1s in the Ag2O/CNTs sample; (d) XPS spectrum of Ag3d in the Ag2O/CNTs sample. In addition, the carbon content of the sample (after heat treatment) was determined by thermal analysis. The results of thermal analysis in air proves mass loss 9.87 %, which is corresponding to carbon content (CNTs) (Fig. 3). This means that at 500 °C, the CNTs were thermally decomposed small amount (compared to the original content of 10 %). Synthesis of Ag2O/CNTS nanocomposite to be used as cathode materials for zinc-silver 153 0 200 400 600 800 90 92 94 96 98 100 W ei g h t( % ) Temperature( o C) 9.87% Figure 3. Themal analysis of Ag/CNTs with initial CNTs content of 10 %, after 5 hours treatment at 500 °C, under argon gas. 3.1.3 FESEM and TEM image analysis The SEM and TEM images are depicted in Figure 4. Figure 4 (a) shows that the particle size of Ag/CNTs nanocomposite were about 50 - 100 nm and the silver nanoparticles were dispersed evenly over carbon nanotubes. In addition, SEM images of the electrode were remained stable even with compression and even after 10 and 20 cycles (Fig. 4 (b, c, d)). 3.2 Electrochemical properties The discharge capacity of the Ag2O/CNTs nanocomposite sample is shown in Figure 5. Figure 5 (a) reveals that the electrodes are capable of being discharge/charge at high current density (2.5C) and exhibit a stable structure. The first discharge capacity was 190 mAh/g with an efficiency up to 82.25 % (compared to the theoretically 231 mAh/g of Ag2O). After 20 cycles the capacity retention is 172 mAh/g with decreasing in the capacity by 9.47 % at a test voltage range of 1.8 V to 1.2 V (Fig. 5 (b). In the second cycle there is a sudden decrease in capacitance, possibly due to the negative electrode, which is related to the stabilization of the electrolyte-zinc electrode in the alkaline electrolyte solution [2]. By the third cycle backwards, the capacity decreases and the discharge process is stable. Compared to conventional Ag2O powders, the discharge efficiency of Ag2O/CNTs increases, the ability to discharge and stabilize at high current density, this is explained in terms of the nanostructures of the material (Figure 5(a)). For ordinary Ag2O powder of electrode, at a high current discharge rate of 2.5C, very fast decrease in capacity (90 mAh/g) is observed. The CNTs in Ag2O/CNTs gave high electrical conductivity of composite, improved the electron transfer processes on the surface of the electrode which leads to high performance. In addition to silver nanoparticles, silver oxide is evenly distributed on CNTs, with a super-large surface area (BET, 90-120 m 2 /g) allows for improved discharge at high current density (at 0.53 mA/cm 2 ). Nguyen Van Tu, Shah Abdul Hakim, Mai Van Phuoc 154 Figure 4. TEM and FESEM images of Ag/CNTs samples and electrode after pressing. (a) Sample of Ag/CNTs material; (b) electrodes after pressing; (c) Electrodes after 10 cycles; (d) Electrodes after 20 cycles. Figure 5.The charge/discharge profile of the Zn/KOH/Ag2O/CNTs composite and Zn /KOH/Ag2O batteries (powders) (at 2.5 C discharge/charge); (b). Dependence of capacity with cycle numbers at 2.5 C discharge. 4. CONCLUSIONS Ag/CNTs nanoparticles supported on carbon nanotubes (CNTs) have been synthesized successfully. Thermal degradation up to 9.87 % of the CNTs is observed. During anode polarization, silver nanoparticles preferentially convert to silver (I) oxide (Ag2O). The results of XRD analysis showed that as-synthesized Ag2O has crystalline structure cubic (Pn-3) and the positive electrode analysis before and after the discharge indicated that the electrode before discharge contained mainly Ag2O, and after discharge is the metal silver. 0 50 100 150 200 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 (1) (2) (3) (4)- The charge/discharge profile of the Zn/KOH/Ag 2 O powder (2,5 C) (1) (2)- The charge/discharge profile of the Zn/KOH/Ag 2 O/CNTs composite (2,5 C) E (V ) C (mAh/g) (4) (3) (a) 5 10 15 20 0 20 40 60 80 100 120 140 160 180 200 C ( m A h /g ) Cyclic numbers (b) Synthesis of Ag2O/CNTS nanocomposite to be used as cathode materials for zinc-silver 155 Ag2O/CNTs nanocomposite is used as a positive electrode in a silver-zinc battery, giving a 82.25 % of theoretical capacity, and capable of high discharge currents, at 2.5 C after 20 cycles, the capacity is remained 172 mAh/g (average decrease of 0.5 %/cycle). So it has the potential to be a positive electrode in zinc-silver batteries. Acknowledgments. This work is jointly supported by the National Foundation for Science and Technology Development of Vietnam (No.104.06-2017.62) and SRGP project no. 731 of Higher Education Commission of Pakistan. REFERENCES 1. Tran Quoc Tuy, Nguyen Van Tu, Do Binh Minh, Duong Chi Quan - Study on some factors affecting the discharge of silver oxide electrode in KOH electrolyte, Journal of Military Science and Technology 6 (2009) 74-79 (in Vietnamese). 2. Fleischer A, Lander J. - Zinc-silver oxide Batteries, John Wiley & Sons, INC New York, 1971, pp. 99-153. 3. Nguyen Van Tu, Mai Van Phuoc - Synthesis of Ag2O/graphene nanocomposite and application in zinc-silver batteries, Journal of Chemistry 52 (6B) (2014) 55-58 (in Vietnamese). 4. 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