Al Layered Double Hydroxide and Its Electrochemical

Nov 9, 2010 - Jun Wang,*,†,‡,§ Yanchao Song,† Zhanshuang Li,† Qi Liu,† Jideng Zhou,† .... (10) Tang, X. H.; Liu, Z. H.; Zhang, C. X.; Yan...
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Energy Fuels 2010, 24, 6463–6467 Published on Web 11/09/2010

: DOI:10.1021/ef101150b

In Situ Ni/Al Layered Double Hydroxide and Its Electrochemical Capacitance Performance Jun Wang,*,†,‡,§ Yanchao Song,† Zhanshuang Li,† Qi Liu,† Jideng Zhou,† Xiaoyan Jing,† Milin Zhang,‡ and Zhaohua Jiang§ †

College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China, ‡Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin 150001, People’s Republic of China, and §College of Chemical Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received June 18, 2010 . Revised Manuscript Received October 22, 2010

Electrodes of Ni/Al layered double hydroxide (Ni/Al LDH) coated on the surface of nickel foam are successfully prepared by an in situ method using a mixed aqueous solution of nickel nitrate and aluminum. Their structure and surface morphology are studied by X-ray diffraction and scanning electron microscopy analysis. Their supercapacitance performances are investigated by cyclic voltammetry and constant current charge/discharge measurements. Results show that Ni/Al LDH nanoplatelets densely cover the nickel foam substrate. The electrode shows excellent electrochemical capacitive character and displays a specific capacitance of 701 F g-1 at a current density of 10 mA cm-2. The capacitance loss is less than 6% after 400 charge-discharge cycles. The larger contact area between the nickel foam supporter and active materials greatly enhances the use of Ni/Al LDH.

oxides and hydroxides [MnO2,10 Fe3O4,11 NiO,12 Co(OH)2,13 and RuO214], and conducting polymers (polyaniline15,16). Metallic oxides are promising electrode materials for supercapacitors because of their high capacitance and fast redox kinetics as compared to carbon or conducting polymers. Among the transition-metal oxides, RuO2 is very promising because of its excellent capacitance (around 600-700 F g-1 in acid electrolytes), but its high price and toxicity limit its application. Recently, layered double hydroxides (LDHs) containing transition metals have been investigated as the active materials of supercapacitors.17-20 Liu et al. reported that Ni/Al LDH exhibits a specific capacitance of 140 F g-1 at a current density of 400 mA g-1.19 Wang et al. studied the electrochemical properties of double oxides obtained from the annealing of Co/Al LDH, which has a specific capacitance of 684 F g-1 at a current density of 60 mA g-1.20 Su et al. found that the restacked Co/Al LDH has a specific capacitance up to 145 F g-1 at a current of 2 A g-1 and long life cycle.21 LDH/C composite electrodes have been reported to retain the valve of specific capacitance after long cycling at a high current

1. Introduction The increasing demands for energy and growing concerns about air pollution and global warming have stimulated intense research on new energy storage and conversion devices. Electrochemical capacitors (supercapacitors) have attracted great interest as a potential power source for electric vehicles because of their fast energy delivery, high power density, and long life cycle.1-3 Dependent upon the different charge storage mechanisms, supercapacitors can be divided into two different categories: (1) electrical double-layer capacitors (EDLCs), where the electrical charge is stored at the interface between the electrode and the electrolyte, and (2) redox electrochemical capacitors, where capacitance arises from reversible Faradaic reactions taking place at the electrode/ electrolyte interface. The most used materials for supercapacitors include carbon (carbon aerogels,4,5 ordered mesoporous carbons,6,7 and carbon nanotubes8,9), transition-metal *To whom correspondence should be addressed. Telephone: þ86-4518253-3026. Fax: þ86-451-8253-3026. E-mail: [email protected]. (1) Yu, G. G.; Jin, S. H.; Li, J. W. Adv. Mater. 2008, 20, 2878–2887. (2) Lam, L. T.; Louey, R. J. Power Sources 2006, 158, 1140–1148. (3) Miller, J. R. Electrochim. Acta 2006, 52, 1703–1708. (4) Li, W. C.; Probstle, H.; Fricke, J. J. Non-Cryst. Solids 2003, 325, 1–5. (5) Probstle, H.; Schmitt, C.; Fricke, J. J. Power Sources 2002, 105, 189–194. (6) Wang, D. W.; Li, F.; Liu, M.; Cheng, H. M. New Carbon Mater. 2007, 22, 307–314. (7) Xing, W.; Huang, C. C.; Zhuo, S. P.; Yuan, X.; Wang, G. Q.; Hulicova-Jurcakova, D.; Yan, Z. F.; Lu, G. Q. Carbon 2009, 47, 1715– 1722. (8) Frackowiak, E.; Beguin, F. Carbon 2002, 40, 1775–1787. (9) Liu, Z. F.; Jiao, L. Y.; Yao, Y. G.; Xian, X. J.; Zhang, J. Adv. Mater. 2010, 22, 1–26. (10) Tang, X. H.; Liu, Z. H.; Zhang, C. X.; Yang, Z. P.; Wang, Z. I. J. Power Sources 2009, 193, 939–943. (11) Wang, S. Q.; Zhang, J. Y.; Chen, C. H. J. Power Sources 2010, 195, 5379–5381. (12) Wang, D. W.; Li, F.; Cheng, H. M. J. Power Sources 2008, 185, 1563–1568. r 2010 American Chemical Society

(13) Hu, Z. A.; Xie, Y. L.; Wang, Y. X.; Xie, L. J.; Fu, G. R.; Jin, X. Q.; Zhang, Z. Y.; Yang, Y. Y.; Wu, H. Y. J. Phys. Chem. C 2009, 113, 12502–12508. (14) Yan, S. C.; Wang, H. T.; Qu, P.; Zhang, Y.; Xiao, Z. D. Synth. Met. 2009, 159, 158–161. (15) Zhang, L. L.; Li, S.; Zhang, J. T.; Guo, P. Z.; Zheng, J. T.; Zhao, X. S. Chem. Mater. 2010, 22, 1195–1202. (16) Montilla, F.; Cotarelo, M. A.; Morallon, E. J. Mater. Chem. 2009, 19, 305–310. (17) Wang, J.; You, J.; Li, Z. S.; Yang, P. P.; Jing, X. Y.; Cao, D. X.; Zhang, M. L. Solid State Sci. 2008, 10, 1093–1098. (18) Wang, Y. G.; Cheng, L.; Xia, Y. Y. J. Power Sources 2006, 153, 191–196. (19) Liu, X. M.; Zhang, Y. H.; Zhang, X. G.; Fu, S. Y. Electrochim. Acta 2004, 49, 3137–3141. (20) Wang, Y.; Yang, W. S.; Zhang, S. C.; Evans, D. G.; Duan, X. J. Electrochem. Soc. 2005, 152, A2130–A2137. (21) Su, L. H.; Zhang, X. G. J. Power Sources 2007, 172, 999–1006.

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In general, their supporter to synthesize Mg/Al LDH. methods include the formation of LDHs in suspension and deposition on the surface of different substrates. Inspired by the research work of their group, in this paper, we present a facile in situ way for the fabrication of Ni/Al LDH electrodes using a three-dimensional substrate of nickel foam. Scheme 1 illustrates schematically the possible formation process of in situ Ni/Al LDH. Nickel foam with high porosity, high specific surface area, and excellent physical strength was used as the current collector and the source of Ni2þ for the synthesis of Ni/Al LDH. It will provide more channels to facilitate fast penetration of the electrolyte. In other words, these channels ensure that enough electrolyte ions contract the surface of the active materials in a short time. Besides, this in situ growth method improves the contract area between active materials and the substrate, thus ensuring the high use of the active materials.

Scheme 1. Schematic Illustration of the Possible Formation Process of in Situ Ni/Al LDH on Nickel Foam

2. Experimental Section Analytical-grade chemicals [Ni(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and KOH] and nickel foam were used for Ni/Al LDH preparation. A piece of nickel foam (10  10  1.0 mm) was degreased with acetone, etched with 6.0 mol L-1 HCl for 15 min, rinsed with water, and then dried in an oven. A total of 0.009 mol of Ni(NO3)2 3 6H2O and 0.003 mol of Al(NO3)3 3 9H2O were dissolved in a solution consisting of 80 mL of H2O and 0.04 mol of urea. The solution was magnetically stirred for 10 min in air at room temperature and transferred to an autoclave pressure vessel. The nickel foam was then immersed in the solution and heated at 140 °C for 10 h. After the completion of Ni/Al LDH growth, the nickel foam covered with LDH was taken out of the autoclave, washed with H2O, and dried at 60 °C for 8 h to obtain the final electrode (denoted as Ni/Al LDH/Ni foam). The loading of Ni/Al LDH is about 20 mg. Considering that the nickel foam gives a strong diffraction when analyzing X-ray diffraction (XRD) patterns, we scraped the Ni/Al LDH powder from nickel foam. For comparison, Ni/Al LDH powder was prepared under the same conditions in the absence of nickel foam in the growth solution. The Ni/Al LDH powder electrode was prepared by mixing 80 wt % Ni/Al LDH powder, 15 wt % conducting carbon, and 5 wt % polytetrafluoroethylene (PTFE) to make a paste, which was then pressed onto nickel foam (10101.0 mm). The obtained electrode was dried at 60 °C for 8 h (see the Supporting Information for details). Powder XRD patterns were recorded on a Rigaku D/max-IIIB diffractometer using Cu KR radiation (λ=0.154 06 nm). Scanning electron microscopy (SEM) was performed on a Philips XL30 instrument at an acceleration voltage of 20-30 kV and a working distance of 17 mm. All electrochemical tests were carried out in a conventional three-electrode electrochemical cell. The nickel-foam-supported Ni/Al LDH acted as the working electrode. A Pt foil of 1 cm2 and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The electrolyte was 6.0 mol L-1 KOH solution. The cutoff voltage was between -0.2 and 0.6 V versus SCE for cyclic voltammetry and 0-0.4 V versus SCE for chronopotentiometric tests. All of the above tests were carried out on a CHI760 electrochemical workstation.

density.22 Nickel or cobalt LDHs have the advantages of a low loss and being relatively environmentally friendly. Studies show that LDHs have great potential as electrode materials of supercapacitors. The conventional method for the preparation of electrodes is to mix the active material with a conductive agent and a binder to make a paste and then apply it to current collectors. This method, however, suffers some drawbacks, as follows: (1) it is difficult to make a homogeneous paste via sonication, and (2) the troublesome preparation technology, such as the temperature of paste, pressure, and drying time of the electrode, has a great influence on the electrochemical performance. To overcome these problems, new methods have been investigated for the preparation of electrodes, such as dropping films on different substrates, including stainless steel,23,24 glassy carbon electrodes,25,26 and Si wafers.27,28 Vinay et al. investigated potentiostatical deposition of nanostructured CoxNi1-x LDHs on stainless steel. The obtained Co0.72Ni0.28 LDH electrode shows a specific capacitance as high as 2104 F g-1 in 1.0 mol L-1 KOH electrolyte.23 However, stainless-steel substrates usually need to be polished and cleaned before coating. Another method for making thin-film LDH electrodes is pulsed laser deposition (PLD), which requires high energy, high material consumption, and complex equipment. This limited the application of LDHs in electrochemical fields. Duan and his group made significant contributions to the field of in situ growth LDHs on different substrates, such as the use anodic aluminum oxide (AAO) or aluminum as a (22) Malak-Polaczyk, A.; Vix-Guterl, C.; Frackowiak, E. Energy Fuels 2010, 24, 3346–3351. (23) Gupta, V.; Gupta, S.; Miura, N. J. Power Sources 2008, 175, 680–685. (24) Gupta, V.; Kawaguchi, T.; Miura, N. Mater. Res. Bull. 2009, 44, 202–206. (25) Wang, Y.; Yang, W. S.; Chen, C.; Evans, D. G. J. Power Sources 2008, 184, 682–690. (26) Gao, Y. F.; Nagai, M.; Masuda, Y.; Sato, F.; Seo, W. S.; Koumoto, K. Langmuir 2006, 22, 3521–3527. (27) Lee, J. H.; Rhee, S. W.; Jung, D. Y. Chem. Commun. 2003, 2740– 2741. (28) Lee, J. H.; Rhee, S. W.; Jung, D. Y. Chem. Mater. 2004, 16, 3774– 3779. (29) Yan, D. P.; Lu, J.; Wei, M.; Li, H.; Ma, J.; Li, F.; Evans, D. G.; Duan, X. J. Phys. Chem. A 2008, 112, 7671–7681. (30) Feng, J. T.; Lin, Y. J.; Evans, D. G.; Duan, X.; Li, D. Q. J. Catal. 2009, 266, 351–358.

3. Results and Discussion To elucidate the possible formation process of in situ Ni/ Al LDH, we carried out comparative experiments (see the Supporting Information). As illustrated in Scheme 1, the (31) Lei, X. D.; Yang, L.; Zhang, F. Z.; Evans, D. G.; Duan, X. Chem. Lett. 2005, 34, 1610–1611. (32) Lu, Z.; Zhang, F. Z.; Lei, X. D.; Yang, L.; Xu, S. L.; Duan, X. Chem. Eng. Sci. 2008, 63, 4055–4062.

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formation of in situ Ni/Al LDH should involve the transportation of metal ions to the nickel foam surface, adsorption and enrichment of the ions on the substrate, and nucleation and growth of Ni/Al LDH crystals. In the presence of a Ni2þ and Al3þ salt mixture, the controlled supply of CO32- and OH- by the decomposition of urea successfully leads to the formation of primary Ni/Al LDH nanoparticles. Ni2þ came from Ni(NO3)2 3 6H2O and dissolution of nickel foam. As the reaction continues, these LDH nanosheets directly grown on the substrate. Many forces cause this in situ process, including electrostatic, crystal-face interaction, chemical bond, and van der Waals forces. We propose that this method may be applied to the synthesis of other electrode materials. Figure 1 shows the XRD patterns of Ni/Al LDH/Ni foam and Ni/Al LDH powder. In Figure 1c, considering that nickel foam gives two strong diffraction peaks at 44.9° and 52.4° [Joint Committee on Powder Diffraction Standards (JCPDS) 04-0850], we scraped the Ni/Al LDH powder from nickel foam; thus, there is no Ni/Al LDH powder for nickel foam in the diffraction. All of the XRD patterns exhibit the characteristic reflections of Ni/Al LDH materials. Reflections (003), (006), (012), (015), (018), (110), and (113) can be indexed to a typical hydrotalcite-like structure (JCPDS 150087). The strong diffraction peak appearing at 11.32° ascribed to the (003) plane reveals that Ni/Al LDH is highly crystallized. Two small reflection peaks appearing at 2θ values of 60.65° and 61.66° correspond to the characteristic peaks of nickel hydroxides, indicating that the prepared materials are a layered structure belonging to a hexagonal

system. No other crystalline phases were detected in the XRD patterns. Figure 2 displays the morphology of the Ni/Al LDH/Ni foam electrode. As can be seen, the LDH powder on nickel foam is platelet-like particles with an average size of 500 nm and some small particles are about 20-50 nm and uniformly cover the nickel foam substrate (in Figure 2a, the scale is 1 μm). This morphology results from natural growth of LDH particles with a well-crystalline structure. Because nickel foam has a well-developed three-dimensional porous network structure and surface area, Ni/Al LDH particles could fill in the inner pores of the substrate, which facilitates the diffusion of OH- ions and increases the use of Ni. This in situ growth method provided a larger contact area between the nickel foam supporter and Ni/Al LDH than directly pressing Ni/Al LDH powder on it. When Ni/Al LDH powder is pressed on nickel foam, its porous structure is limited by preparation technology and fewer electroactive Ni sites could become exposed, which affected its electrochemical performance. From Figure S2 of the Supporting Information, the sample Ni/Al LDH powder also retains the platelet-like morphology with an average size of 1 μm. Figure 3 shows the cyclic voltammogram (CV) of the Ni/Al LDH/Ni foam electrode in 6 mol L-1 KOH electrolyte at different scan rates between 5 and 50 mV s-1. A pair of redox peaks were observed, which correspond to the conversion between different oxidation states of Ni according to the following equation:33 NiðOHÞ2 þ OH- TNiOOH þ H2 O þ e-

Figure 1. XRD patterns of (a) Ni/Al LDH/Ni foam (scraped powder from Ni foam), (b) Ni/Al LDH powder, and (c) Ni/Al LDH/Ni foam.

ð1Þ

Figure 3. CVs of the Ni/Al LDH/Ni foam electrode in 6 mol L-1 KOH solution at different scan rates between 5 and 50 mV s-1.

Figure 2. SEM and transmission electron microscopy (TEM) images of Ni/Al LDH/Ni foam.

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Figure 5. Discharge curves of the Ni/Al LDH/Ni foam electrode measured at different discharge current densities in 6.0 mol L-1 KOH solution.

Figure 4. First and second charge-discharge curves of the (a) Ni/ Al LDH/Ni foam electrode and (b) Ni/Al LDH powder electrode in 6.0 mol L-1 KOH solution at a galvanostatic current density of 10 mA cm-2.

The CVs indicate that the pseudo-capacitance mainly comes from the Faradaic redox reaction of Ni/Al LDH. With the increase of the scan rate, the redox current increased, the anodic peak shifted toward positive potential and overlapped with the oxygen evolution peak, and the cathodic peak shifted toward negative potential. This result indicates the quasireversible feature of the redox couples. The large potential difference (ΔEa,c) between the anodic and cathodic peaks is likely related to the low electronic conductivity of Ni/Al LDH. At different scan rates, ΔEa,c is higher than the theoretical value of 59 mV, which means that the electrode is not reversible. The result is also consistent with the Ni/Al LDH powder (see Figure S3 of the Supporting Information). Figure 4 shows the first charge-discharge curves of the Ni/Al LDH/Ni foam and Ni/Al LDH powder electrodes in 6.0 mol L-1 KOH solution at a galvanostatic current density of 10 mA cm-2 in the potential range of 0-0.39 V. The discharge curve consists of two sections, a sudden potential drop followed by a slow potential decay. The first potential drop results from the internal resistance, and a subsequent potential decay represents the capacitive feature of the electrode. The obvious deviation of the discharge curves from a straight line demonstrates that the capacitance is mainly coming from the faradic redox reaction of Ni species in Ni/Al LDH. The discharge time of Ni/Al LDH/Ni foam is longer than Ni/Al LDH powder at the same current density and mass of active materials. At a current of 10 mA cm-2, the specific capacitance of Ni/Al LDH/Ni foam is about 701 F g-1, while the specific capacitance of Ni/Al LDH powder is only 293 F g-1, a factor of 2.4 lower than the Ni/Al LDH/Ni foam electrode. In Figure S4 of the Supporting Information, the capacitance of the Ni/Al LDH powder electrode at the current densities of 20 and 40 mA cm-2 is 276 and 236 F g-1, respectively. Figure 5 shows the discharge curves of the Ni/Al LDH/Ni foam electrode measured at different discharge current densities in 6.0 mol L-1 KOH solution. The cutoff voltage of discharging is 0-0.35 V. The specific capacitance can be calculated according to the following equation: Cm ¼

IΔt ΔVm

Figure 6. Dependences of the discharge specific capacitance and the columbic efficiency of the Ni/Al LDH/Ni foam electrode on the charge-discharge cycle numbers. The charge-discharge tests were performed at 50 mA cm-2 in 6.0 mol L-1 KOH solution.

where Cm is the specific capacitance (F g-1), I is the charge or discharge current (mA), Δt is the charge or discharge time (s), ΔV is the potential (V), and m is the mass of the active material in the electrode (mg). The specific capacitance values of the Ni/Al LDH/Ni foam electrode obtained from the discharge curves are 482, 413, 401, 280, and 164 F g-1 at the current densities of 20, 30, 40, 60, and 100 mA cm-2, respectively. These values are larger than directly pressed Ni/Al LDH powder on nickel foam (see Figure S4 of the Supporting Information). The decreased capacitance with the increase of discharge current densities is likely caused by the resistance of Ni/ Al LDH film and the insufficient faradic redox reaction of the active material under higher discharge current densities. Therefore, the specific capacitance at the lowest current density is closer to full use of the active material. Figure 6 displays the cycling stability of the Ni/Al LDH/Ni foam electrode. It can be seen that the discharge specific capacitance decreased to 94% after 400 cycles and the columbic efficiency ranged from 92 to 98%, indicating that the Ni/Al LDH/Ni foam electrode has long-term electrochemical stability. An initial increase in capacitance was observed. This may be because of the fact that, at the initial stage, active materials have not been fully used. After repetitive charge/discharge cycling, the electrochemical active Ni sites inside the nickel foam electrode have become fully exposed; therefore, a maximum

ð2Þ

(33) Xu, M. W.; Bao, S. J.; Li, H. L. J. Solid State Electrochem. 2007, 11, 372–377.

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capacitance of 428 F g is displayed (in Figure S5 of the Supporting Information, the maximum capacitance of Ni/Al LDH powder by direct pressing is only 250 F g-1). From the CV curves, we know that the low electronic conductivity of Ni/Al LDH results in a decreased capacitance of active materials. The Ni/Al LDH/Ni foam electrode exhibited large specific capacitance at a high discharge current, making it a promising candidate for an electrode of a high-capacity and high-rate supercapacitor. In situ Ni/Al LDH on nickel foam has several advantages. First, in the synthesis with urea, the hydrolysis of ammonium to ammonia and carbonate to hydrogen carbonate gives a pH of about 9, which is suitable for precipitating metal hydroxides.22 Second, the controlled supply of carbonate and hydroxide by the decomposition of urea in the presence of a Ni2þ and Al3þ salt mixture successfully leads to the formation of well-dispersed Ni/Al LDH particles on the surface of nickel foam. Therefore, the Ni/Al LDH/Ni foam electrode prepared by the in situ method has a larger contact area between Ni/Al LDH and nickel foam than the conventional Ni/Al LDH powder electrode. Furthermore, from the strong chemical interaction between the substrate and active materials, the electron transport becomes easier than the conventional method. In view of the above reasons, this in situ growth method facilitates the electrolyte through the electrode quickly and reduces the electron transport distance within the electrode materials, which leads to the Ni/Al LDH/Ni foam

electrode having high specific capacitance and a high-rate charge-discharge ability. 4. Conclusions In this paper, Ni/Al LDH nanoplatelets were successfully grown on the surface of nickel foam by a simple in situ method. The obtained electrode has a high use of the active material and is more efficient for the diffusion and transport of electrolyte ions and the electron between the electrode and electrolyte. Thus, it has higher capacity and better high-rate performance than the Ni/Al LDH powder electrode prepared by the conventional method. A maximum specific capacitance of 701 F g-1 at the current density of 10 mA cm-2 was obtained, and the capacitance loss is less than 6% after 400 cycles. Acknowledgment. We gratefully acknowledge the support of this research by the Key Technology R&D Program of Heilongjiang Province (TB06A05), the Fundamental Research Funds of the Central University, and the Science Fund for Young Scholar of Harbin City (2004AFQXJ038). Supporting Information Available: Details of the synthesis and electrochemical characterizations of Ni/Al LDH powder. This material is available free of charge via the Internet at http:// pubs.acs.org.

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