- Email: [email protected]
carbon nanotubes composite as a high performance anode material for Ni–Zn secondary batteries
Electrochimica Acta 111 (2013) 581–587
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Layered double hydroxide/carbon nanotubes composite as a high performance anode material for Ni–Zn secondary batteries Bin Yang a , Zhanhong Yang a,b,∗ , Ruijuan Wang a , Tingting Wang a a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Key Laboratory of Resource Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 4 August 2013 Accepted 9 August 2013 Available online xxx Keywords: LDH/CNT composite Cycle stability Anode electrode Ni–Zn batteries
a b s t r a c t Nanostructured Zn–Al layered double hydroxide (LDH) and carbon nanotubes (CNTs) have been successfully assembled to form LDH/CNT composite by electrostatic force. The morphology and microstructure of LDH/CNT composites were investigated by transmission electron microscopy and X-ray diffractometer. The assembly mechanism of LDH with CNTs was also discussed. Furthermore, the unique three-dimensional composite thus prepared was used as a new anode material for Ni–Zn secondary batteries to enhance the cell performance for the first time. The electrochemical performances of LDH/CNT composite as anode active material for Ni–Zn cells were investigated by galvanostatic charge/discharge cycling and cyclic voltammogram. The obtained results clearly demonstrated that the LDH/CNT composite had superior cycle stability compared with the conventional ZnO and Zn–Al–LDH, and the discharge capacity could maintain 390 mAh g−1 after 200 cycling tests. At the same time, the LDH/CNT composite also exhibited lower charge plateau voltage and higher discharge plateau voltage, and the average utilization ratio of the anode could reach 95.6%. These results indicated that this kind of composite is a promising anode material for Ni/Zn cells. It exhibits a high capacity (∼400 mAh g−1 ) and high cycling stability. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The Ni–Zn rechargeable battery is an attractive power source for various electric appliances and large-scale energy storage systems due to its high specific energy, excellent specific power and high open-circuit voltage [1–5]. It has also attracted a lot of interest because the active materials and electrolytes in Ni–Zn batteries have non-toxicity and are inexpensive and very abundant in nature [1,4]. Nevertheless, the secondary zinc batteries are usually limited in widespread commercialization by poor cycling characteristics, which mainly resulted from the high solubility of discharge products of zinc electrode in alkaline electrolyte [1,4–7]. In addition, the dissolution of Zn in a strong alkaline electrolyte leads to self-discharge and discharge capacity fading [1,4,6]. Various attempts, such as the use of additives in either the anode [2,8] or the electrolyte [9,10], the modification of Zn electrode [2,11,12] and calcium zincate used as the anodic active material [13], have been made to overcome the shape change and dissolution problems of the Zn electrode. Nevertheless, these approaches could
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China. Tel.: +86 0731 88879616, fax: +86 0731 88879616. E-mail address: [email protected] (Z. Yang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.052
not effectively overcome these obstacles. Therefore, we need to find a new anode material for Ni–Zn secondary batteries which can remarkably increase cell lifetime and improve electrochemical performance. Layered double hydroxides (LDH), a family of layered inorganic compounds consisting of stacks of positively charged metal hydroxide layers with anions in the interlayer (LDH structure shown in Fig. 1), have been widely studied in fields of catalysts [14,15], drug delivery materials [16], nanofillers [17], electrode materials [18] and chemically tailored functional materials [19]. In particular, LDH is capable of undergoing an inner redox reaction within a limited potential range in alkaline medium, which have been investigated as anode active material for Ni–Zn cells in our previous study [20]. Compared with two-dimensional (2D) LDH, carbon nanotubes (CNTs) are one kind of one-dimensional (1D) nanomaterials which are commonly used as nanoscale fillers and additives, due to their outstanding and unique structural, mechanical and electrical properties [21–23]. Their sp2 -hybridized graphitic structure has high carrier mobility and good electron-accepting property, which affords an excellent electrical conductivity for storing and shuttling electrons. Yang et al. reported that electron conductivity of CNTs thin film was around (1–4) × 102 S/cm along the nanotube axis and (5–25) S/cm perpendicular to the axis [24]. Recently, the assembly of CNT-based composites or hybrids with the desired nanoscale guests, as an emerging and quickly
582
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
Fig. 1. LDH, Zn8 Al2 (OH)20 [CO3 2− ], is a clay with cationic layers and charge-balancing anions.
developing field, has been investigated for enhancing the properties of resulting versatile materials and achieving a broad range of practical applications [25–28]. Therefore, the hybridization of 1D nanotubes and 2D lamellar flakes leads to 3D composite
materials, which are very promising and crucial, as they can enable versatile and tailormade properties with excellent performances far beyond those of the individual materials. So, it is very necessary to prepare 3D LDH/CNTs composite which is anticipated to
Fig. 2. Diagram of the chemical route to the LDH/CNT composite.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
have promising application in electrochemical energy production and storage. The formation process of the LDH/CNT composite is represented in Fig. 2. Recently, our team investigated the application of Zn–Al–LDH as a novel anode material for Ni–Zn secondary cells for the first time [20]. However, the results show that the relatively poor conductivity of Zn–Al–LDH is unfavorable to electron transfer and consequently impacts the electrochemical performance of anode for Ni–Zn secondary batteries. To overcome these problems, it is very desirable to develop an electrochemical active and good conductivity material for Ni–Zn secondary cells. Aiming at a synergetic combination of outstanding physicochemical properties of two types of functional CNTs and LDH materials and thus enhanced their electrochemical performance to meet new advanced applications as novel anode material for Ni–Zn secondary batteries, we established an extremely simple approach to synthesize nanostructured LDH/CNT composites. The electrochemical performance of as-synthesized LDH/CNT composites was also investigated in detail by various analytical techniques. To the best of our knowledge, the detailed studies of LDH/CNT composites as the anode active material for Ni–Zn secondary cells have not been reported before. 2. Experimental 2.1. Materials and reagents Multi-walled CNTs with a diameter of 20–30 nm were purchased from Shenzhen Nanotech Port Co. Ltd. (elemental analysis: 97.00 wt.% C, 0.23 wt.% H and 2.77 wt.% O). Introduction of carboxyl and hydroxyl groups onto the surface of CNTs to form negatively charged CNTs was carried out by oxidation of pristine CNTs with a mixture of nitric acid and sulfuric acid (1:3 by volume) under sonication at 80 ◦ C for 8 h. Then the modified CNTs were filtered and washed with deionized water until pH = 7. Finally, the collected powder was dried under vacuum at 60 ◦ C for 24 h. As a result, the negatively charged carboxyl groups were introduced onto the surface of CNTs. The other reagents (analytical grade) were used as received without further purification. 2.2. The synthesis of nanostructured LDH/CNT composites LDH/CNT composites were prepared by a co-precipitation method. In a typical procedure, the modified CNTs (0.25 g) were added to 50 ml of an alkali solution of NaOH (1 M) and Na2 CO3 (0.2 M) and placed in an ultrasonication bath for 30 min. Subsequently, the solution was titrated with 100 ml of a salt solution of Zn(NO3 )2 (0.2 M) and Al(NO3 )3 (0.05 M) with Zn2+ /Al3+ molar ratio of 4:1 under vigorous stirring at 65 ◦ C. The pH value of the solution was adjusted to 10 by further titration with 1 M NaOH solution. Thereafter, the slurry was transferred into a 100 ml Teflonlined autoclave, and the hydrothermal reaction was allowed to continue for 24 h at 100 ◦ C. The obtained product was filtered and washed with deionized water and ethanol several times followed by vacuum-drying at 80 ◦ C for 12 h. Meanwhile, the pristine Zn–Al–LDH was also prepared without the introduction of CNTs to give a comparative standard. Considering only small losses of LDH and CNTs in the whole experiment, the chemical composition of the as-prepared LDH/CNT composites can be considered to be the same as the original mass ratio of CNTs to LDH, which is 10:100 in mass. 2.3. Structural characterization The synthesized samples were characterized by X-ray diffraction (XRD) using a RigakuD/max2550VL/PC system operated at
583
˚ Transmis35 kV and 200 mA with Cu K␣ radiation, = 1.5406 A. sion electron microscope (TEM) measurements were conducted on a JEOL 2010 FEG microscope at 200 kV. The TEM samples were prepared by dispensing a small amount of dry powder in ethanol with ultrasonication for 30 min. Then, one drop of the suspension was dropped on 300 mesh copper TEM grids covered with thin amorphous carbon films. Fourier transform-infrared measurements (FT-IR) were recorded on KBr pellets with a PE Paragon 1000 spectrophotometer. 2.4. Preparation of LDH/CNT electrodes and test cells The anode (Zn electrode) was prepared by pasting a mixture consisting of as-prepared LDH/CNTs active material and polytetrafluoroethylene (10 wt.%, in diluted emulsion) on a copper mesh substrate served as the current collector. The obtained LDH/CNT composite electrodes were dried under vacuum at 60 ◦ C for 12 h and then roll-pressed to a thickness of 0.3 mm to enhance the contact between the active materials and the current collector. For comparison, the pure Zn–Al–LDH electrodes and ZnO electrodes were also prepared on the basis of our previous report [18]. The cathode was the commercial sintered Ni(OH)2 electrode (Tianjin City Fine Chemical Research Institute). The ratio of designed capacity for the -Ni(OH)2 cathode and Zn–Al–LDH/CNTs anode was about 3:1, so the capacity obtained in this present work just reflected the performance of LDH/CNT electrodes. A solution of 6 M KOH, saturated with ZnO, was used as the electrolyte, and a polyolefin microporous membrane as separator. The LDH/CNT composite anode and -Ni(OH)2 cathode were assembled into a cell. 2.5. Electrochemical tests The galvanostatic charge–discharge tests were conducted on a Neware battery test system (NEWARE BTS-610, Neware Technology Co. Ltd., China) at room temperature. The cells were charged at 1 C for 1 h, and then discharged at 1 C down to 1.2 V cut-off. Cyclic voltammogram (CV) measurements were performed on a CHI 640B type electrochemical system at room temperature with a scanning rate of 1 mV s−1 , shifting from −0.85 to −1.65 V. A threeeletrode cell assembly was used with a -Ni(OH)2 electrode as the counter electrode, a LDH/CNT composite electrode as the working electrode and a Hg/HgO electrode as the reference electrode. The electrolyte was ZnO-saturated 6 M KOH solution. 3. Results and discussion 3.1. Structural characterization of nanostructured LDH/CNT composite The FT-IR spectra of the pristine CNTs and the acid-treated CNTs (CNT COOH) are shown in Fig. 3. It can be seen that the FT-IR spectrum of the CNT COOH (Fig. 3b) shows the absorption peak located at 1726 cm−1 , corresponding to COOH stretching [28]. In contrast, this peak does not appear in the spectrum of pristine CNTs (Fig. 3a), indicating that a large amount of carboxylic groups has been generated on the surface of the CNTs by nitric acid and sulfuric acid treatment. The XRD patterns for pristine Zn–Al–LDH and LDH/CNT composites are shown in Fig. 4. The XRD patterns of both samples display the characteristic reflections of hydrotalcite-like materials at 2 angles = 12◦ , 23◦ and 62◦ , which are indexed to the (0 0 3), (0 0 6) and (1 1 0) of 2D hydrotalcite-like materials [26], indicative of R3m symmetry and a hexagonal lattice. The peak at 2 = 26◦ was attributed to diffraction of carbon nanotubes in the LDH/CNT composites, which is indexed to the (0 0 2) plane of graphitic carbon with an
584
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
Fig. 3. FT-IR spectra of (a) pristine CNTs and (b) CNT COOH.
consists of thin hexagonal platelets. This Zn–Al–LDH possessed a high crystallinity and a hexagonal plate-like structure, which was the typical morphology for layers of LDH nanosheet. Meanwhile, the average size of LDH nanosheet was about 100–200 nm (Fig. 5b), which was affected by the reaction time and temperature [26]. The TEM image of LDH/CNT composite (Fig. 5c) clearly indicated that the CNTs was tightly absorbed and randomly distributed on the surface of LDH nanosheet. As for LDH/CNT sample, the surface combination with CNTs did not change the morphology and structure of LDH nanosheet. These results further indicated the strong interfacial electrostatic interaction between negatively charged functional CNTs and positively charged layers of LDH. The reason is that carboxylic acid groups (COOH) on the surface of the CNTs exist as carboxylate anions (COO ) in the alkaline solution, thus yielding negatively charged CNTs, and the surface of LDH nanosheet are positively charged [25,29]. Therefore, the negatively charged 1D CNTs is easily attached to the positively charged surface of 2D LDH nanosheet, which can form a stable 3D nanostructure under the dominance of charge attractions. Here, we propose a model to describe the assembling process of positively charged LDH nanosheet with negatively charged CNTs by electrostatic force in Fig. 6. The LDH/CNT composite thus obtained was used to prepare high performance anode active material for Ni–Zn secondary batteries. 3.2. The electrochemical performance of LDH/CNT composite
Fig. 4. XRD patterns of pristine Zn–Al–LDH and LDH/CNT composite.
interplanar distance of 0.337 nm (JCPDS No. 41-1487), confirming the existence of CNTs [29]. No other crystalline phases are detected in Fig. 4. The morphology and microstructure of modified CNTs, Zn–Al–LDH and as-prepared LDH/CNTs composite samples were characterized by TEM. It can be clearly observed from Fig. 5a that the modified CNTs by nitric acid and sulfuric acid are multi-walled nanotubes with inner diameter of about 15 nm and outer diameter of about 20–30 nm. As can be seen from Fig. 5b, the sample typically
The electrochemical cycle behaviors of Zn electrodes with conventional ZnO, pristine Zn–Al–LDH and LDH/CNT composite as active materials are illustrated in Fig. 7. For the conventional ZnO, the discharge capacity declines rapidly after 60 cycles. At the 80th cycle, the discharge capacity decreases to 300 mAh g−1 with retention of 45.4%. The poor cycling stability is attributed to the high solubility of ZnO in the concentrated KOH electrolyte [1,3]. Compared to conventional ZnO electrode, the electrochemical cycle stability of pristine Zn–Al–LDH electrode and LDH/CNTs composite electrode is significantly more stable. As to Zn–Al–LDH, the great electrochemical cycle stability mainly comes from its alkaline characteristic, which is the most fundamental reason for its application in the secondary alkaline cells. The nature of this alkaline characteristic is the great stability of this material in alkaline electrolyte. Between the Zn–Al–LDH electrode and LDH/CNT composite electrode, the electrochemical stability of the composite electrode is superior to that of the LDH electrode. During most cycles, LDH/CNT composite has the largest average discharge capacity (about 390 mAh g−1 ) and the lowest fading rate among all the batteries tested. In the recent reports, Tu et al. synthesized the Sn6 O4 (OH)4 -coated ZnO [2] and the ZnO nanoplates [1] as active materials for Ni–Zn battery respectively, and the discharge capacity of these materials declined rapidly and obviously throughout 80 cycles. Compared to these Zn compounds as active material, the
Fig. 5. Representative TEM images of (a) modified CNTs, (b) pristine Zn–Al–LDH, and (c) LDH/CNT composite structures.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
585
Fig. 6. Schematic description of assembling LDH/CNT composite: (a) CNTs suspension, (b) LDH suspension, and (c) LDH/CNT composite.
can form a conductive network on the surface of LDH nanosheet (can be seen in Fig. 5c). This conductive network efficiently promotes the charge transport between the active Zn centers and the electrode by its excellent electric conductivity, which is the key factor for enhancing the electrochemical performance. Therefore, it can be concluded that the CNTs immobilized onto the surface of LDH nanosheet by electrostatic force is an effective way to improve the electrochemical stability and utilization ratio of anode active materials. The typical charge/discharge curves of Ni/Zn cells with Zn–Al–LDH and LDH/CNTs composite as active materials at the 25th cycle are displayed in Fig. 8. The overall electrode reaction can be described as Eqs. (1) and (2):
Fig. 7. Electrochemical cycle behaviors of Ni–Zn cells with ZnO, pristine Zn–Al–LDH and LDH/CNT composite as active materials.
anode with LDH/CNT composite show much better cycling life and much stable throughout 200 cycles. For the LDH/CNT composite electrode, the CNTs immobilized onto the surface of LDH nanosheet by the electrostatic force may partly prohibit the direct contact of the active core material with the alkaline electrolyte, and further suppress the dissolution of Zn–Al–LDH in the electrolyte. Since the Zn–Al–LDH active material is stably retained at the electrode and partly protected by the CNTs onto the surface of LDH nanosheet, the electrochemical stability is accordingly improved. Table 1 shows the average utilization ratios of anode active materials with conventional ZnO, Zn–Al–LDH and LDH/CNT composite (average utilization ratio = average discharge capacity/theoretic capacity, theoretic capacity of ZnO is 650 mAh g−1 and Zn–Al–LDH is 400 mAh g−1 ). The average utilization ratio of conventional ZnO is only 46.7% and Zn–Al–LDH has an average utilization ratio of 86.5%. However, the LDH/CNT composite has the average utilization ratio of 95.6%, which shows the highest active material utilization of all the test cells. The improvement of electrochemical performance should be ascribed to the fact that the CNTs Table 1 The average utilization ratios of anode active materials with conventional ZnO, Zn–Al–LDH and LDH/CNT composite. Active material
Average discharge capacity (mAh g−1 )
Average utilization ratio (%)
ZnO Zn–Al–LDH LDH/CNTs
303.8 346.3 382.7
46.7 86.5 95.6
Zn + 4OH− = Zn(OH)4 2− + 2e
discharge process
(1)
Zn(OH)4 2− + 2e = Zn + 4OH−
charge process
(2)
From Fig. 8, the Ni–Zn cells using LDH/CNTs as anode materials show lower charge plateau voltage and higher discharge plateau voltage than those of the ZnO and pristine LDH. The decrease in charge plateau voltage is advantaged to the suppression of H2 formation and the improvement of charge efficiency. Furthermore, higher discharge plateau voltage associates with higher discharge potential and better performance in discharge process. It is the reason that the CNTs can form a conductive network onto the surface of LDH nanosheets, and their excellent electric conductivity is advantaged to the charge transport on the electrode in discharge process and charge process. Therefore, the LDH/CNT composite as anode active material shows the best cell performance compared with pristine LDH and conventional ZnO, in terms of charge plateau voltage, discharge plateau voltage and discharge capacity. To identify electrochemical reactions during the charge/ discharge cycles, CV measurements were performed. The typical CV curves of the pristine LDH, ZnO and LDH/CNTs as anode active material are presented in Fig. 9. It can be seen that the current response appeared between −0.85 and −1.65 V. The anodic process is corresponding to the charge process of the Zn electrode, the area of anode peak of LDH/CNTs is larger than those of the ZnO and pristine LDH, which results from the higher electrochemical activity of LDH/CNT composite. The cathodic peak is corresponding to discharging process of secondary battery. From Fig. 9, the cathodic peak of LDH/CNTs is steeper than those of the ZnO and pure LDH, which indicates that the LDH/CNT composite has rapid kinetics of reduce reaction, and the discharging process is more effective and rapid [1]. In general, the LDH/CNT composite displays better cell performance than conventional ZnO and pristine Zn–Al–LDH. The composition and structure of all tested cells are uniform, and the charge/discharge procedure is identical, thus the difference of electrochemical performance among the three kinds of cells
586
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
can decrease the resistance of the anode. From Eqs. (3) and (4): V = Ecathode − Eanode − I(R˝anode + R˝cathode + R˝electrolyte + R˝other ) discharging mode
(3)
V = Ecathode − Eanode + I(R˝anode + R˝cathode + R˝electrolyte + R˝other ) charging mode
(4)
where Ecathode and Eanode are polarization potentials [1], we know that the active material LDH/CNT composite can decrease the resistance of the anodes, which results in a decrease in charge voltage and an increase in discharge voltage. This is good consistent with the results in our work, which can be seen in Fig. 8. All of the above results indicate that the LDH/CNTs composite is much suitable for anode active material for secondary alkaline batteries. 4. Conclusions
Fig. 8. Typical charge/discharge curves of Ni–Zn cells tested at the 25th cycle: (a) conventional ZnO, (b) pristine Zn–Al–LDH, (c) LDH/CNT composite.
is attributed to anode active material. In comparison with the conventional ZnO and pristine Zn–Al–LDH, the LDH/CNTs composite has larger specific surface area than ZnO, and more excellent electric conductivity than pristine Zn–Al–LDH. These advantages
Nanostructured LDH/CNT composite was successfully synthesized by a simple precipitation technique through the electrostatic interaction between positively charged layer of LDH and negatively charged functional groups on modified CNTs. The LDH/CNT composite with unique 3D nanostructure is a novel nanomaterial which combines 2D LDH nanosheet and 1D carbon nanotubes together. Furthermore, LDH/CNT composite as anode material for Ni/Zn cell shows higher discharge capacity, better electrochemical cycle stability, lower charge plateau voltage and higher discharge plateau voltage, and the average utilization ratio of the anode is also increased, compared to the conventional ZnO and pristine Zn–Al–LDH. The improvement in the electrochemical properties results from the great stability of this material in alkaline electrolyte, and the CNTs on the surface of LDH nanosheet can form a conductive network. This conductive network efficiently promotes the charge transport between the active Zn centers and the electrode, which is a key factor for enhanced the electrochemical performance. Therefore, these results indicate that nanostructured LDH/CNT composite is much suitable for anode active material for Ni–Zn cells. Acknowledgements This work was financially supported by the Natural Science Foundation of China (No.21371180 and 91023031) and Science and technology project of Changsha city (No.k1303015-11 and k1203014-11). References
Fig. 9. The typical cyclic voltammogram curves of the pure LDH, ZnO and LDH/CNTs.
[1] M. Ma, J.P. Tu, Y.F. Yuan, X.L. Wang, K.F. Li, F. Mao, Z.Y. Zeng, Electrochemical performance of ZnO nanoplates as anode materials for Ni/Zn secondary batteries, Journal of Power Sources 179 (2008) 395–400. [2] Y.F. Yuan, J.P. Tu, H.M. Wu, C.Q. Zhang, S.F. Wang, X.B. Zhao, Influence of surface modification with Sn6 O4 (OH)4 on electrochemical performance of ZnO in Zn/Ni secondary cells, Journal of Power Sources 165 (2007) 905–910. [3] Y.F. Yuan, J.P. Tu, H.M. Wu, Y.Z. Yang, D.Q. Shi, X.B. Zhao, Electrochemical performance and morphology evolution of nanosized ZnO as anode material of Ni–Zn batteries, Electrochimica Acta 51 (2006) 3632–3636. [4] S.H. Lee, C.W. Yi, K. Kim, Characteristics and electrochemical performance of the TiO2 -coated ZnO anode for Ni–Zn secondary batteries, Journal of Physical Chemistry C 115 (2011) 2572–2577. [5] J. Jindra, Progress in sealed Ni–Zn cells, 1991–1995, Journal of Power Sources 66 (1997) 15–25. [6] M. Geng, D.O. Northwood, Development of advanced rechargeable Ni/MH and Ni/Zn batteries, International Journal of Hydrogen Energy 28 (2003) 633–636. [7] Y.D. Cho, G.T.K. Fey, Surface treatment of zinc anodes to improve discharge capacity and suppress hydrogen gas evolution, Journal of Power Sources 184 (2008) 610–616.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587 [8] R. Shivkumar, G.P. Kalaignan, T. Vasudevan, Studies with porous zinc electrodes with additives for secondary alkaline batteries, Journal of Power Sources 75 (1998) 90–100. [9] J. Zhu, Y. Zhou, C. Gao, Influence of surfactants on electrochemical behavior of zinc electrodes in alkaline solution, Journal of Power Sources 72 (1998) 231–235. [10] R. Renuka, S. Ramamurthy, K. Muralidharan, Effect of citrate, tartrate and gluconate ions on the behaviour of zinc in 3 M NaOH, Journal of Power Sources 76 (1998) 197–209. [11] M. Minakshi, M. Ionescu, Anodic behavior of zinc in Zn-MnO2 battery using ERDA technique, International Journal of Hydrogen Energy 35 (2010) 7618–7622. [12] M. Minakshi, D. Appadoo, D. Martin, The anodic behavior of planar and porous zinc electrodes in alkaline electrolyte, Electrochemical and Solid State Letters 13 (7) (2010) A77–A80. [13] J. Yu, H. Yang, X. Ai, X. Zhu, A study of calcium zincate as negative electrode materials for secondary batteries, Journal of Power Sources 103 (2001) 93–97. [14] R.D. Hetterley, R. Mackey, J.T.A. Jones, Y.Z. Khimyak, A.M. Fogg, I.V. Kozhevnikov, One-step conversion of acetone to methyl isobutyl ketone over Pd-mixed oxide catalysts prepared from novel layered double hydroxides, Journal of Catalysis 258 (2008) 250–255. [15] S.R.J. Oliver, Cationic inorganic materials for anionic pollutant trapping and catalysis, Chemical Society Reviews 38 (2009) 1868–1881. [16] P. Gunawan, R. Xu, Direct assembly of anisotropic layered double hydroxide (LDH) nanocrystals on spherical template for fabrication of drug–LDH hollow nanospheres, Chemistry of Materials 21 (2009) 781–783. [17] L. Du, B. Qu, Structural characterization and thermal oxidation properties of LLDPE/MgAl-LDH nanocomposites, Journal of Materials Chemistry 16 (2006) 1549–1554. [18] Z. Hu, Y. Xie, Y. Wang, H. Wu, Y. Yang, Z. Zhang, Synthesis and electrochemical characterization of mesoporous Cox Ni1−x layered double hydroxides as electrode materials for supercapacitors, Electrochimica Acta 54 (2009) 2737–2741.
587
[19] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum, Angewandte Chemie-international Edition 47 (2008) 2466–2469. [20] X. Fan, Z. Yang, R. Wen, B. Yang, W. Long, The application of Zn–Al-hydrotalcite as a novel anodic material for Ni–Zn secondary cells, Journal of Power Sources 224 (2013) 80–85. [21] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Carbon nanotubes—the route toward applications, Science 297 (2002) 787–792. [22] N.I. Kovtyukhova, T.E. Mallouk, L. Pan, E.C. Dickey, Individual singlewalled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes, Journal of the American Chemical Society 125 (2003) 9761–9769. [23] H. Dai, Carbon nanotubes: synthesis, integration, and properties, Accounts of Chemical Research 35 (2002) 1035–1044. [24] D.J. Yang, S.G. Wang, Q. Zhang, P.J. Sellin, G. Chen, Thermal and electrical transport in multi-walled carbon nanotubes, Physics Letters A 329 (2004) 207–213. [25] C.W. Chiu, J.J. Lin, Self-assembly behavior of polymer-assisted clays, Progress in Polymer Science 37 (2012) 406–444. [26] B. Du, Z. Fang, The preparation of layered double hydroxide wrapped carbon nanotubes and their application as a flame retardant for polypropylene, Nanotechnology 21 (2010) 315603–315608. [27] Y.F. Lan, J.J. Lin, Observation of carbon nanotube and clay micellelike microstructures with dual dispersion property, Journal of Physical Chemistry A 113 (2009) 8654–8659. [28] S. Huang, H. Peng, W.W. Tjiu, Z. Yang, H. Zhu, T. Tang, T. Liu, Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites, Journal of Physical Chemistry B 114 (2010) 16766–16772. [29] H. Wang, X. Xiang, F. Li, Facile synthesis and novel electrocatalytic performance of nanostructured Ni–Al layered double hydroxide/carbon nanotube composites, Journal of Materials Chemistry 20 (2010) 3944–3952.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Layered double hydroxide/carbon nanotubes composite as a high performance anode material for Ni–Zn secondary batteries Bin Yang a , Zhanhong Yang a,b,∗ , Ruijuan Wang a , Tingting Wang a a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Key Laboratory of Resource Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 30 April 2013 Received in revised form 4 August 2013 Accepted 9 August 2013 Available online xxx Keywords: LDH/CNT composite Cycle stability Anode electrode Ni–Zn batteries
a b s t r a c t Nanostructured Zn–Al layered double hydroxide (LDH) and carbon nanotubes (CNTs) have been successfully assembled to form LDH/CNT composite by electrostatic force. The morphology and microstructure of LDH/CNT composites were investigated by transmission electron microscopy and X-ray diffractometer. The assembly mechanism of LDH with CNTs was also discussed. Furthermore, the unique three-dimensional composite thus prepared was used as a new anode material for Ni–Zn secondary batteries to enhance the cell performance for the first time. The electrochemical performances of LDH/CNT composite as anode active material for Ni–Zn cells were investigated by galvanostatic charge/discharge cycling and cyclic voltammogram. The obtained results clearly demonstrated that the LDH/CNT composite had superior cycle stability compared with the conventional ZnO and Zn–Al–LDH, and the discharge capacity could maintain 390 mAh g−1 after 200 cycling tests. At the same time, the LDH/CNT composite also exhibited lower charge plateau voltage and higher discharge plateau voltage, and the average utilization ratio of the anode could reach 95.6%. These results indicated that this kind of composite is a promising anode material for Ni/Zn cells. It exhibits a high capacity (∼400 mAh g−1 ) and high cycling stability. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The Ni–Zn rechargeable battery is an attractive power source for various electric appliances and large-scale energy storage systems due to its high specific energy, excellent specific power and high open-circuit voltage [1–5]. It has also attracted a lot of interest because the active materials and electrolytes in Ni–Zn batteries have non-toxicity and are inexpensive and very abundant in nature [1,4]. Nevertheless, the secondary zinc batteries are usually limited in widespread commercialization by poor cycling characteristics, which mainly resulted from the high solubility of discharge products of zinc electrode in alkaline electrolyte [1,4–7]. In addition, the dissolution of Zn in a strong alkaline electrolyte leads to self-discharge and discharge capacity fading [1,4,6]. Various attempts, such as the use of additives in either the anode [2,8] or the electrolyte [9,10], the modification of Zn electrode [2,11,12] and calcium zincate used as the anodic active material [13], have been made to overcome the shape change and dissolution problems of the Zn electrode. Nevertheless, these approaches could
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China. Tel.: +86 0731 88879616, fax: +86 0731 88879616. E-mail address: [email protected] (Z. Yang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.052
not effectively overcome these obstacles. Therefore, we need to find a new anode material for Ni–Zn secondary batteries which can remarkably increase cell lifetime and improve electrochemical performance. Layered double hydroxides (LDH), a family of layered inorganic compounds consisting of stacks of positively charged metal hydroxide layers with anions in the interlayer (LDH structure shown in Fig. 1), have been widely studied in fields of catalysts [14,15], drug delivery materials [16], nanofillers [17], electrode materials [18] and chemically tailored functional materials [19]. In particular, LDH is capable of undergoing an inner redox reaction within a limited potential range in alkaline medium, which have been investigated as anode active material for Ni–Zn cells in our previous study [20]. Compared with two-dimensional (2D) LDH, carbon nanotubes (CNTs) are one kind of one-dimensional (1D) nanomaterials which are commonly used as nanoscale fillers and additives, due to their outstanding and unique structural, mechanical and electrical properties [21–23]. Their sp2 -hybridized graphitic structure has high carrier mobility and good electron-accepting property, which affords an excellent electrical conductivity for storing and shuttling electrons. Yang et al. reported that electron conductivity of CNTs thin film was around (1–4) × 102 S/cm along the nanotube axis and (5–25) S/cm perpendicular to the axis [24]. Recently, the assembly of CNT-based composites or hybrids with the desired nanoscale guests, as an emerging and quickly
582
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
Fig. 1. LDH, Zn8 Al2 (OH)20 [CO3 2− ], is a clay with cationic layers and charge-balancing anions.
developing field, has been investigated for enhancing the properties of resulting versatile materials and achieving a broad range of practical applications [25–28]. Therefore, the hybridization of 1D nanotubes and 2D lamellar flakes leads to 3D composite
materials, which are very promising and crucial, as they can enable versatile and tailormade properties with excellent performances far beyond those of the individual materials. So, it is very necessary to prepare 3D LDH/CNTs composite which is anticipated to
Fig. 2. Diagram of the chemical route to the LDH/CNT composite.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
have promising application in electrochemical energy production and storage. The formation process of the LDH/CNT composite is represented in Fig. 2. Recently, our team investigated the application of Zn–Al–LDH as a novel anode material for Ni–Zn secondary cells for the first time [20]. However, the results show that the relatively poor conductivity of Zn–Al–LDH is unfavorable to electron transfer and consequently impacts the electrochemical performance of anode for Ni–Zn secondary batteries. To overcome these problems, it is very desirable to develop an electrochemical active and good conductivity material for Ni–Zn secondary cells. Aiming at a synergetic combination of outstanding physicochemical properties of two types of functional CNTs and LDH materials and thus enhanced their electrochemical performance to meet new advanced applications as novel anode material for Ni–Zn secondary batteries, we established an extremely simple approach to synthesize nanostructured LDH/CNT composites. The electrochemical performance of as-synthesized LDH/CNT composites was also investigated in detail by various analytical techniques. To the best of our knowledge, the detailed studies of LDH/CNT composites as the anode active material for Ni–Zn secondary cells have not been reported before. 2. Experimental 2.1. Materials and reagents Multi-walled CNTs with a diameter of 20–30 nm were purchased from Shenzhen Nanotech Port Co. Ltd. (elemental analysis: 97.00 wt.% C, 0.23 wt.% H and 2.77 wt.% O). Introduction of carboxyl and hydroxyl groups onto the surface of CNTs to form negatively charged CNTs was carried out by oxidation of pristine CNTs with a mixture of nitric acid and sulfuric acid (1:3 by volume) under sonication at 80 ◦ C for 8 h. Then the modified CNTs were filtered and washed with deionized water until pH = 7. Finally, the collected powder was dried under vacuum at 60 ◦ C for 24 h. As a result, the negatively charged carboxyl groups were introduced onto the surface of CNTs. The other reagents (analytical grade) were used as received without further purification. 2.2. The synthesis of nanostructured LDH/CNT composites LDH/CNT composites were prepared by a co-precipitation method. In a typical procedure, the modified CNTs (0.25 g) were added to 50 ml of an alkali solution of NaOH (1 M) and Na2 CO3 (0.2 M) and placed in an ultrasonication bath for 30 min. Subsequently, the solution was titrated with 100 ml of a salt solution of Zn(NO3 )2 (0.2 M) and Al(NO3 )3 (0.05 M) with Zn2+ /Al3+ molar ratio of 4:1 under vigorous stirring at 65 ◦ C. The pH value of the solution was adjusted to 10 by further titration with 1 M NaOH solution. Thereafter, the slurry was transferred into a 100 ml Teflonlined autoclave, and the hydrothermal reaction was allowed to continue for 24 h at 100 ◦ C. The obtained product was filtered and washed with deionized water and ethanol several times followed by vacuum-drying at 80 ◦ C for 12 h. Meanwhile, the pristine Zn–Al–LDH was also prepared without the introduction of CNTs to give a comparative standard. Considering only small losses of LDH and CNTs in the whole experiment, the chemical composition of the as-prepared LDH/CNT composites can be considered to be the same as the original mass ratio of CNTs to LDH, which is 10:100 in mass. 2.3. Structural characterization The synthesized samples were characterized by X-ray diffraction (XRD) using a RigakuD/max2550VL/PC system operated at
583
˚ Transmis35 kV and 200 mA with Cu K␣ radiation, = 1.5406 A. sion electron microscope (TEM) measurements were conducted on a JEOL 2010 FEG microscope at 200 kV. The TEM samples were prepared by dispensing a small amount of dry powder in ethanol with ultrasonication for 30 min. Then, one drop of the suspension was dropped on 300 mesh copper TEM grids covered with thin amorphous carbon films. Fourier transform-infrared measurements (FT-IR) were recorded on KBr pellets with a PE Paragon 1000 spectrophotometer. 2.4. Preparation of LDH/CNT electrodes and test cells The anode (Zn electrode) was prepared by pasting a mixture consisting of as-prepared LDH/CNTs active material and polytetrafluoroethylene (10 wt.%, in diluted emulsion) on a copper mesh substrate served as the current collector. The obtained LDH/CNT composite electrodes were dried under vacuum at 60 ◦ C for 12 h and then roll-pressed to a thickness of 0.3 mm to enhance the contact between the active materials and the current collector. For comparison, the pure Zn–Al–LDH electrodes and ZnO electrodes were also prepared on the basis of our previous report [18]. The cathode was the commercial sintered Ni(OH)2 electrode (Tianjin City Fine Chemical Research Institute). The ratio of designed capacity for the -Ni(OH)2 cathode and Zn–Al–LDH/CNTs anode was about 3:1, so the capacity obtained in this present work just reflected the performance of LDH/CNT electrodes. A solution of 6 M KOH, saturated with ZnO, was used as the electrolyte, and a polyolefin microporous membrane as separator. The LDH/CNT composite anode and -Ni(OH)2 cathode were assembled into a cell. 2.5. Electrochemical tests The galvanostatic charge–discharge tests were conducted on a Neware battery test system (NEWARE BTS-610, Neware Technology Co. Ltd., China) at room temperature. The cells were charged at 1 C for 1 h, and then discharged at 1 C down to 1.2 V cut-off. Cyclic voltammogram (CV) measurements were performed on a CHI 640B type electrochemical system at room temperature with a scanning rate of 1 mV s−1 , shifting from −0.85 to −1.65 V. A threeeletrode cell assembly was used with a -Ni(OH)2 electrode as the counter electrode, a LDH/CNT composite electrode as the working electrode and a Hg/HgO electrode as the reference electrode. The electrolyte was ZnO-saturated 6 M KOH solution. 3. Results and discussion 3.1. Structural characterization of nanostructured LDH/CNT composite The FT-IR spectra of the pristine CNTs and the acid-treated CNTs (CNT COOH) are shown in Fig. 3. It can be seen that the FT-IR spectrum of the CNT COOH (Fig. 3b) shows the absorption peak located at 1726 cm−1 , corresponding to COOH stretching [28]. In contrast, this peak does not appear in the spectrum of pristine CNTs (Fig. 3a), indicating that a large amount of carboxylic groups has been generated on the surface of the CNTs by nitric acid and sulfuric acid treatment. The XRD patterns for pristine Zn–Al–LDH and LDH/CNT composites are shown in Fig. 4. The XRD patterns of both samples display the characteristic reflections of hydrotalcite-like materials at 2 angles = 12◦ , 23◦ and 62◦ , which are indexed to the (0 0 3), (0 0 6) and (1 1 0) of 2D hydrotalcite-like materials [26], indicative of R3m symmetry and a hexagonal lattice. The peak at 2 = 26◦ was attributed to diffraction of carbon nanotubes in the LDH/CNT composites, which is indexed to the (0 0 2) plane of graphitic carbon with an
584
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
Fig. 3. FT-IR spectra of (a) pristine CNTs and (b) CNT COOH.
consists of thin hexagonal platelets. This Zn–Al–LDH possessed a high crystallinity and a hexagonal plate-like structure, which was the typical morphology for layers of LDH nanosheet. Meanwhile, the average size of LDH nanosheet was about 100–200 nm (Fig. 5b), which was affected by the reaction time and temperature [26]. The TEM image of LDH/CNT composite (Fig. 5c) clearly indicated that the CNTs was tightly absorbed and randomly distributed on the surface of LDH nanosheet. As for LDH/CNT sample, the surface combination with CNTs did not change the morphology and structure of LDH nanosheet. These results further indicated the strong interfacial electrostatic interaction between negatively charged functional CNTs and positively charged layers of LDH. The reason is that carboxylic acid groups (COOH) on the surface of the CNTs exist as carboxylate anions (COO ) in the alkaline solution, thus yielding negatively charged CNTs, and the surface of LDH nanosheet are positively charged [25,29]. Therefore, the negatively charged 1D CNTs is easily attached to the positively charged surface of 2D LDH nanosheet, which can form a stable 3D nanostructure under the dominance of charge attractions. Here, we propose a model to describe the assembling process of positively charged LDH nanosheet with negatively charged CNTs by electrostatic force in Fig. 6. The LDH/CNT composite thus obtained was used to prepare high performance anode active material for Ni–Zn secondary batteries. 3.2. The electrochemical performance of LDH/CNT composite
Fig. 4. XRD patterns of pristine Zn–Al–LDH and LDH/CNT composite.
interplanar distance of 0.337 nm (JCPDS No. 41-1487), confirming the existence of CNTs [29]. No other crystalline phases are detected in Fig. 4. The morphology and microstructure of modified CNTs, Zn–Al–LDH and as-prepared LDH/CNTs composite samples were characterized by TEM. It can be clearly observed from Fig. 5a that the modified CNTs by nitric acid and sulfuric acid are multi-walled nanotubes with inner diameter of about 15 nm and outer diameter of about 20–30 nm. As can be seen from Fig. 5b, the sample typically
The electrochemical cycle behaviors of Zn electrodes with conventional ZnO, pristine Zn–Al–LDH and LDH/CNT composite as active materials are illustrated in Fig. 7. For the conventional ZnO, the discharge capacity declines rapidly after 60 cycles. At the 80th cycle, the discharge capacity decreases to 300 mAh g−1 with retention of 45.4%. The poor cycling stability is attributed to the high solubility of ZnO in the concentrated KOH electrolyte [1,3]. Compared to conventional ZnO electrode, the electrochemical cycle stability of pristine Zn–Al–LDH electrode and LDH/CNTs composite electrode is significantly more stable. As to Zn–Al–LDH, the great electrochemical cycle stability mainly comes from its alkaline characteristic, which is the most fundamental reason for its application in the secondary alkaline cells. The nature of this alkaline characteristic is the great stability of this material in alkaline electrolyte. Between the Zn–Al–LDH electrode and LDH/CNT composite electrode, the electrochemical stability of the composite electrode is superior to that of the LDH electrode. During most cycles, LDH/CNT composite has the largest average discharge capacity (about 390 mAh g−1 ) and the lowest fading rate among all the batteries tested. In the recent reports, Tu et al. synthesized the Sn6 O4 (OH)4 -coated ZnO [2] and the ZnO nanoplates [1] as active materials for Ni–Zn battery respectively, and the discharge capacity of these materials declined rapidly and obviously throughout 80 cycles. Compared to these Zn compounds as active material, the
Fig. 5. Representative TEM images of (a) modified CNTs, (b) pristine Zn–Al–LDH, and (c) LDH/CNT composite structures.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
585
Fig. 6. Schematic description of assembling LDH/CNT composite: (a) CNTs suspension, (b) LDH suspension, and (c) LDH/CNT composite.
can form a conductive network on the surface of LDH nanosheet (can be seen in Fig. 5c). This conductive network efficiently promotes the charge transport between the active Zn centers and the electrode by its excellent electric conductivity, which is the key factor for enhancing the electrochemical performance. Therefore, it can be concluded that the CNTs immobilized onto the surface of LDH nanosheet by electrostatic force is an effective way to improve the electrochemical stability and utilization ratio of anode active materials. The typical charge/discharge curves of Ni/Zn cells with Zn–Al–LDH and LDH/CNTs composite as active materials at the 25th cycle are displayed in Fig. 8. The overall electrode reaction can be described as Eqs. (1) and (2):
Fig. 7. Electrochemical cycle behaviors of Ni–Zn cells with ZnO, pristine Zn–Al–LDH and LDH/CNT composite as active materials.
anode with LDH/CNT composite show much better cycling life and much stable throughout 200 cycles. For the LDH/CNT composite electrode, the CNTs immobilized onto the surface of LDH nanosheet by the electrostatic force may partly prohibit the direct contact of the active core material with the alkaline electrolyte, and further suppress the dissolution of Zn–Al–LDH in the electrolyte. Since the Zn–Al–LDH active material is stably retained at the electrode and partly protected by the CNTs onto the surface of LDH nanosheet, the electrochemical stability is accordingly improved. Table 1 shows the average utilization ratios of anode active materials with conventional ZnO, Zn–Al–LDH and LDH/CNT composite (average utilization ratio = average discharge capacity/theoretic capacity, theoretic capacity of ZnO is 650 mAh g−1 and Zn–Al–LDH is 400 mAh g−1 ). The average utilization ratio of conventional ZnO is only 46.7% and Zn–Al–LDH has an average utilization ratio of 86.5%. However, the LDH/CNT composite has the average utilization ratio of 95.6%, which shows the highest active material utilization of all the test cells. The improvement of electrochemical performance should be ascribed to the fact that the CNTs Table 1 The average utilization ratios of anode active materials with conventional ZnO, Zn–Al–LDH and LDH/CNT composite. Active material
Average discharge capacity (mAh g−1 )
Average utilization ratio (%)
ZnO Zn–Al–LDH LDH/CNTs
303.8 346.3 382.7
46.7 86.5 95.6
Zn + 4OH− = Zn(OH)4 2− + 2e
discharge process
(1)
Zn(OH)4 2− + 2e = Zn + 4OH−
charge process
(2)
From Fig. 8, the Ni–Zn cells using LDH/CNTs as anode materials show lower charge plateau voltage and higher discharge plateau voltage than those of the ZnO and pristine LDH. The decrease in charge plateau voltage is advantaged to the suppression of H2 formation and the improvement of charge efficiency. Furthermore, higher discharge plateau voltage associates with higher discharge potential and better performance in discharge process. It is the reason that the CNTs can form a conductive network onto the surface of LDH nanosheets, and their excellent electric conductivity is advantaged to the charge transport on the electrode in discharge process and charge process. Therefore, the LDH/CNT composite as anode active material shows the best cell performance compared with pristine LDH and conventional ZnO, in terms of charge plateau voltage, discharge plateau voltage and discharge capacity. To identify electrochemical reactions during the charge/ discharge cycles, CV measurements were performed. The typical CV curves of the pristine LDH, ZnO and LDH/CNTs as anode active material are presented in Fig. 9. It can be seen that the current response appeared between −0.85 and −1.65 V. The anodic process is corresponding to the charge process of the Zn electrode, the area of anode peak of LDH/CNTs is larger than those of the ZnO and pristine LDH, which results from the higher electrochemical activity of LDH/CNT composite. The cathodic peak is corresponding to discharging process of secondary battery. From Fig. 9, the cathodic peak of LDH/CNTs is steeper than those of the ZnO and pure LDH, which indicates that the LDH/CNT composite has rapid kinetics of reduce reaction, and the discharging process is more effective and rapid [1]. In general, the LDH/CNT composite displays better cell performance than conventional ZnO and pristine Zn–Al–LDH. The composition and structure of all tested cells are uniform, and the charge/discharge procedure is identical, thus the difference of electrochemical performance among the three kinds of cells
586
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587
can decrease the resistance of the anode. From Eqs. (3) and (4): V = Ecathode − Eanode − I(R˝anode + R˝cathode + R˝electrolyte + R˝other ) discharging mode
(3)
V = Ecathode − Eanode + I(R˝anode + R˝cathode + R˝electrolyte + R˝other ) charging mode
(4)
where Ecathode and Eanode are polarization potentials [1], we know that the active material LDH/CNT composite can decrease the resistance of the anodes, which results in a decrease in charge voltage and an increase in discharge voltage. This is good consistent with the results in our work, which can be seen in Fig. 8. All of the above results indicate that the LDH/CNTs composite is much suitable for anode active material for secondary alkaline batteries. 4. Conclusions
Fig. 8. Typical charge/discharge curves of Ni–Zn cells tested at the 25th cycle: (a) conventional ZnO, (b) pristine Zn–Al–LDH, (c) LDH/CNT composite.
is attributed to anode active material. In comparison with the conventional ZnO and pristine Zn–Al–LDH, the LDH/CNTs composite has larger specific surface area than ZnO, and more excellent electric conductivity than pristine Zn–Al–LDH. These advantages
Nanostructured LDH/CNT composite was successfully synthesized by a simple precipitation technique through the electrostatic interaction between positively charged layer of LDH and negatively charged functional groups on modified CNTs. The LDH/CNT composite with unique 3D nanostructure is a novel nanomaterial which combines 2D LDH nanosheet and 1D carbon nanotubes together. Furthermore, LDH/CNT composite as anode material for Ni/Zn cell shows higher discharge capacity, better electrochemical cycle stability, lower charge plateau voltage and higher discharge plateau voltage, and the average utilization ratio of the anode is also increased, compared to the conventional ZnO and pristine Zn–Al–LDH. The improvement in the electrochemical properties results from the great stability of this material in alkaline electrolyte, and the CNTs on the surface of LDH nanosheet can form a conductive network. This conductive network efficiently promotes the charge transport between the active Zn centers and the electrode, which is a key factor for enhanced the electrochemical performance. Therefore, these results indicate that nanostructured LDH/CNT composite is much suitable for anode active material for Ni–Zn cells. Acknowledgements This work was financially supported by the Natural Science Foundation of China (No.21371180 and 91023031) and Science and technology project of Changsha city (No.k1303015-11 and k1203014-11). References
Fig. 9. The typical cyclic voltammogram curves of the pure LDH, ZnO and LDH/CNTs.
[1] M. Ma, J.P. Tu, Y.F. Yuan, X.L. Wang, K.F. Li, F. Mao, Z.Y. Zeng, Electrochemical performance of ZnO nanoplates as anode materials for Ni/Zn secondary batteries, Journal of Power Sources 179 (2008) 395–400. [2] Y.F. Yuan, J.P. Tu, H.M. Wu, C.Q. Zhang, S.F. Wang, X.B. Zhao, Influence of surface modification with Sn6 O4 (OH)4 on electrochemical performance of ZnO in Zn/Ni secondary cells, Journal of Power Sources 165 (2007) 905–910. [3] Y.F. Yuan, J.P. Tu, H.M. Wu, Y.Z. Yang, D.Q. Shi, X.B. Zhao, Electrochemical performance and morphology evolution of nanosized ZnO as anode material of Ni–Zn batteries, Electrochimica Acta 51 (2006) 3632–3636. [4] S.H. Lee, C.W. Yi, K. Kim, Characteristics and electrochemical performance of the TiO2 -coated ZnO anode for Ni–Zn secondary batteries, Journal of Physical Chemistry C 115 (2011) 2572–2577. [5] J. Jindra, Progress in sealed Ni–Zn cells, 1991–1995, Journal of Power Sources 66 (1997) 15–25. [6] M. Geng, D.O. Northwood, Development of advanced rechargeable Ni/MH and Ni/Zn batteries, International Journal of Hydrogen Energy 28 (2003) 633–636. [7] Y.D. Cho, G.T.K. Fey, Surface treatment of zinc anodes to improve discharge capacity and suppress hydrogen gas evolution, Journal of Power Sources 184 (2008) 610–616.
B. Yang et al. / Electrochimica Acta 111 (2013) 581–587 [8] R. Shivkumar, G.P. Kalaignan, T. Vasudevan, Studies with porous zinc electrodes with additives for secondary alkaline batteries, Journal of Power Sources 75 (1998) 90–100. [9] J. Zhu, Y. Zhou, C. Gao, Influence of surfactants on electrochemical behavior of zinc electrodes in alkaline solution, Journal of Power Sources 72 (1998) 231–235. [10] R. Renuka, S. Ramamurthy, K. Muralidharan, Effect of citrate, tartrate and gluconate ions on the behaviour of zinc in 3 M NaOH, Journal of Power Sources 76 (1998) 197–209. [11] M. Minakshi, M. Ionescu, Anodic behavior of zinc in Zn-MnO2 battery using ERDA technique, International Journal of Hydrogen Energy 35 (2010) 7618–7622. [12] M. Minakshi, D. Appadoo, D. Martin, The anodic behavior of planar and porous zinc electrodes in alkaline electrolyte, Electrochemical and Solid State Letters 13 (7) (2010) A77–A80. [13] J. Yu, H. Yang, X. Ai, X. Zhu, A study of calcium zincate as negative electrode materials for secondary batteries, Journal of Power Sources 103 (2001) 93–97. [14] R.D. Hetterley, R. Mackey, J.T.A. Jones, Y.Z. Khimyak, A.M. Fogg, I.V. Kozhevnikov, One-step conversion of acetone to methyl isobutyl ketone over Pd-mixed oxide catalysts prepared from novel layered double hydroxides, Journal of Catalysis 258 (2008) 250–255. [15] S.R.J. Oliver, Cationic inorganic materials for anionic pollutant trapping and catalysis, Chemical Society Reviews 38 (2009) 1868–1881. [16] P. Gunawan, R. Xu, Direct assembly of anisotropic layered double hydroxide (LDH) nanocrystals on spherical template for fabrication of drug–LDH hollow nanospheres, Chemistry of Materials 21 (2009) 781–783. [17] L. Du, B. Qu, Structural characterization and thermal oxidation properties of LLDPE/MgAl-LDH nanocomposites, Journal of Materials Chemistry 16 (2006) 1549–1554. [18] Z. Hu, Y. Xie, Y. Wang, H. Wu, Y. Yang, Z. Zhang, Synthesis and electrochemical characterization of mesoporous Cox Ni1−x layered double hydroxides as electrode materials for supercapacitors, Electrochimica Acta 54 (2009) 2737–2741.
587
[19] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum, Angewandte Chemie-international Edition 47 (2008) 2466–2469. [20] X. Fan, Z. Yang, R. Wen, B. Yang, W. Long, The application of Zn–Al-hydrotalcite as a novel anodic material for Ni–Zn secondary cells, Journal of Power Sources 224 (2013) 80–85. [21] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Carbon nanotubes—the route toward applications, Science 297 (2002) 787–792. [22] N.I. Kovtyukhova, T.E. Mallouk, L. Pan, E.C. Dickey, Individual singlewalled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes, Journal of the American Chemical Society 125 (2003) 9761–9769. [23] H. Dai, Carbon nanotubes: synthesis, integration, and properties, Accounts of Chemical Research 35 (2002) 1035–1044. [24] D.J. Yang, S.G. Wang, Q. Zhang, P.J. Sellin, G. Chen, Thermal and electrical transport in multi-walled carbon nanotubes, Physics Letters A 329 (2004) 207–213. [25] C.W. Chiu, J.J. Lin, Self-assembly behavior of polymer-assisted clays, Progress in Polymer Science 37 (2012) 406–444. [26] B. Du, Z. Fang, The preparation of layered double hydroxide wrapped carbon nanotubes and their application as a flame retardant for polypropylene, Nanotechnology 21 (2010) 315603–315608. [27] Y.F. Lan, J.J. Lin, Observation of carbon nanotube and clay micellelike microstructures with dual dispersion property, Journal of Physical Chemistry A 113 (2009) 8654–8659. [28] S. Huang, H. Peng, W.W. Tjiu, Z. Yang, H. Zhu, T. Tang, T. Liu, Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites, Journal of Physical Chemistry B 114 (2010) 16766–16772. [29] H. Wang, X. Xiang, F. Li, Facile synthesis and novel electrocatalytic performance of nanostructured Ni–Al layered double hydroxide/carbon nanotube composites, Journal of Materials Chemistry 20 (2010) 3944–3952.