J. Phys. Chem. C 2010, 114, 5203–5210
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Tuning of Capacitance Behavior of NiO Using Anionic, Cationic, and Nonionic Surfactants by Hydrothermal Synthesis P. Justin, Sumanta Kumar Meher, and G. Ranga Rao* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed: October 10, 2009; ReVised Manuscript ReceiVed: January 19, 2010
In this work, NiO powders with a spherical morphology were synthesized by a simple hydrothermal technique using organic surfactants as templates and urea as the hydrolysis controlling agent. The effect of cationic (cetyl trimethyl ammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (Triton X-100) surfactants for tuning the surface area, pore size, pore volume, and electrochemical properties of NiO powders was investigated. The NiO powders were characterized by X-ray diffraction, scanning electron microscopy, the Brunauer-Emmett-Teller method, cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. We observed that the charge-storage mechanism in our NiO-based electrodes is significantly Faradic in nature rather than capacitive type. The ionic nature of the surfactant used in the preparation of NiO powders shows a considerable effect on their capacitance behavior. The specific capacitance values were found to increase in the order of NiO-T (144 F g-1) < NiO-C (239 F g-1) < NiO-S (411 F g-1) at a current density of 200 mA g-1 in 2 M KOH aqueous electrolyte solution. The NiO-S sample exhibits the highest surface redox reactivity and shows the specific capacitance of 235 F g-1 over 100 cycles at a current density of 500 mA g-1 in a life cycle test. 1. Introduction Capacitive energy storage systems have generated significant interest due to a 10-20 times higher power density, a fast charging/discharging mechanism, and long life cycles compared with batteries.1 The devices involving capacitive energy storage mechanism are called electrochemical capacitors (ECs), which include double-layer capacitors (also termed supercapacitors or ultracapacitors) and pseudocapacitors.2 These devices occupy the region between batteries and dielectric capacitors in the Ragone plot (specific power vs specific energy).1,3 ECs can be used as peak-power sources in electric vehicles, memory backup devices, or back-up supplies to protect against power disruption.4 The electrical double-layer capacitors are electrochemical capacitors that store the charge electrostatically using reversible adsorption of ions of the electrolyte onto the active materials that are electrochemically stable. Carbon materials with a high specific surface area (1000-2000 m2 g-1) are widely used for electrical double-layer capacitors.5 Carbon materials are the most promising candidates because of their stable physicochemical properties, good conductivity, low cost, availability in different forms, and the flexibility of tuning their porous network. However, as a consequence of the electrostatic surface charging mechanism, these carbon materials suffer from limited energy density. In contrast, the pseudocapacitors show higher energy density than the electrical double-layer capacitors. In pseudocapacitors, the charges arise due to fast, reversible electrosorption or redox processes occurring at or near the solid electrode surface.2 For example, RuO2 oxide possesses pseudocapacitance properties and exhibits a much higher specific capacitance (>720 F g-1 in aqueous acid electrolyte) than that of conventional carbon materials.6 However, the high cost of RuO2 materials limits them for wider electrochemical energy storage applications. Further, acid electrolyte is required for * To whom correspondence should be addressed. Tel: +91 44 2257 4226. Fax: +91 44 2257 0545. E-mail:
[email protected].
RuO2 to show excellent capacitance for faster charge/discharge cycles and higher power density. Under acidic conditions, however, the metal oxide slowly dissolves, affecting the lifetime of the capacitor. This has initiated a search for inexpensive alternate electrode materials having good capacitance and longer lifetime. Several other metal oxides have been investigated for their pseudocapacitance behavior, which include MnO2,7-10 NiO,11-13 Co3O4,14-16 Fe3O4,17 V2O5,18 SnO2,19 TiO2,20 CuO,21 and Bi2O3.22 In addition to these oxides, hydroxides,23-25 sulfides,26,27and electronically conducting polymers28,29 have also been tested as pseudocapacitor electrode materials. Among the transition-metal oxides, NiO has been extensively studied as a pseudocapacitor electrode material due to its large surface area, high pseudocapacitance behavior, and low cost with a good possibility of enhancing its performance by improving the preparative methods.11-13 In the literature, this aspect has been investigated on NiO films and powders of varying porosities and morphologies.30-38 Earlier attempts on fabricated NiO films showed enhancement of specific capacitance from 138 to 590 F g-1, depending upon the electrodeposition parameters and the substrate used.11,30-34 However, for practical application of high power density devices, the powder forms yield better volumetric energy density than films. NiO powders of various morphologies have been synthesized by different methods, and their utility as electrode materials for pseudocapacitors has been reported.12,13,35-38 For example, Xing et al. prepared ball-shaped mesoporous NiO with a very high BET surface area of 478 m2 g-1 by using sodium dodecyl sulfate as a soft template.35 This mesoporous NiO showed a specific capacitance of 124 F g-1 at a current density of 100 mA g-1. Using SBA-15 as the hard template method, Wang and Xia prepared mesoporous NiO nanowires with a BET surface area of 47 m2 g-1 and obtained a specific capacitance of 120 F g-1 at a current density of 1 mA cm-2.36 Zheng and Zhang reported a specific capacitance of 72 F g-1 at a current density of 1.5 mA cm-2 for hexagon plate-shaped NiO powder samples
10.1021/jp9097155 2010 American Chemical Society Published on Web 02/25/2010
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prepared by molten salt synthesis.12 Lang et al. synthesized NiO nanoflakes by heat treatment of porous R-Ni(OH)2, which was obtained by an ammonia precipitation method at low temperature.13 This NiO nanoflake exhibited the highest specific capacitance of 942 F g-1 at a current density of 5 mA cm-2. Recently, Yu et al. synthesized nanoporous polyhedron-shaped NiO with a high BET surface area of 179 m2 g-1 and narrow pore distribution (6.0 nm) by controlled thermal decomposition of the oxalate precursors and obtained a specific capacitance of 165 F g-1 at a scan rate of 5 mV s-1.37 Using urea hydrolysis under hydrothermal conditions, Zheng et al. synthesized nanoflake-shaped NiO with a BET surface area of 107 m2 g-1 and measured a specific capacitance value of 137 F g-1 at a constant current density of 200 mA g-1.38 From these studies, it is clear that the specific capacitance of NiO strongly depends on the surface area, porosity, morphology, and preparation methods of the material. Because pseudocapacitance is an interfacial phenomenon, an effective way to improve the charge-storage capability is to increase the specific surface areas of the electrode materials with a suitable pore-size distribution. One of the ways of achieving this is to extend the surfactant-mediated synthesis method, originally reported for various types of mesoporous silicas,39 to prepare nonsilica mesoporous oxides with higher specific surface areas and optimum poresize distributions.40-44 Such mesoporous oxide systems with large specific surface areas are expected to favor the ion transport inside the pore system, thereby increasing the electroactive materialelectrolyte interface area, which benefits the electrochemical performance. In the present work, we report the role of anionic (SDS), cationic (CTAB) and nonionic (Triton X-100) surfactant molecules in modifying the specific surface area and pore-size distribution of NiO, by virtue of the surfactant’s ability to act as a template. We demonstrate that the hydrothermal synthesis route by urea hydrolysis enables the formation of NiO particles with a high surface area and optimum pore-size distribution, which are better suitable for electrochemical capacitor applications. The NiO prepared by using sodium dodecyl sulfate surfactant shows the highest specific capacitance of 411 F g-1 at a current density of 200 mA g-1. It exhibits an excellent cycle profile with a Coulombic efficiency of 80% and shows the specific capacitance of 235 F g-1 over 100 cycles at a current density of 500 mA g-1 in a life cycle test. 2. Experimental Section 2.1. Preparation of Porous NiO. Porous NiO samples were prepared by using analytical grade 20 mmol of Ni(NO3)2 · 4H2O (CDH, purity ) 99%), 40 mmol of urea (Thomas-Baker, purity ) 99.5%), and 10 mmol of surfactants (Sd Fine Chemicals, purity ) 99.5%). All three chemicals at specified quantities were dissolved in 200 mL of tripledistilled water and stirred for 2 h at room temperature to obtain a transparent solution. The solution was then transferred to a 250 mL capacity Teflon-lined stainless steel autoclave and kept at 120 °C for 24 h. The autoclave was cooled to room temperature, and the greenish precipitate was separated by centrifugation and repeated washing using distilled water, followed by ethanol. The greenish precipitate was then oven-dried for 24 h at 60 °C. The dried sample was calcined at 300 °C for 3 h in air to obtain the final black color product of NiO. The NiO samples prepared by using sodium dodecyl sulfate, cetyl trimethyl ammonium bromide, and Triton X-100 surfactants will be referred to in the text as NiO-S, NiOC, and NiO-T, respectively. 2.2. Physical Characterization. The room-temperature powder X-ray diffraction (XRD) patterns were recorded using a
Justin et al. Bruker AXS D8 diffractometer with Cu KR radiation (λ ) 0.15418 nm) generated at 40 kV and 30 mA. The average crystallite sizes of the NiO powder were estimated using the Scherrer equation, D ) 0.9λ/(β cos θ), where D is the crystallite size, λ is the wavelength of the incident X-ray, θ is the diffraction angle for the (111), (200), and (220) peaks, and β is the full width at half-maximum (fwhm) of peaks measured graphically. Thermogravimetric analysis (TGA) of the samples was performed on a PerkinElmer TGA-7 apparatus in N2 (20 mL/min) with a linear heating rate (20 °C/min) from room temperature to 800 °C. Nitrogen adsorption and desorption experiments were carried out by employing a Micromeritics ASAP 2020 analyzer. The sample was outgassed at 100 °C for 2 h, followed by 150 °C for 10 h in dynamic vacuum before physisorption measurements. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the porosity distribution (0.2-8 nm) was generated from the desorption branch of the isotherm by the Barrett-JoynerHalenda (BJH) analysis. Scanning electron microscopy pictures were taken using an FEI Quanta 200 microscope. The sample powders were deposited on a carbon tape before mounting on the sample holder. High-resolution TEM (HRTEM) characterization was performed with a JEOL JEM-3010 transmission electron microscope (TEM) operated at 200 kV. Samples for HRTEM analysis were prepared by drying the nanocrystal dispersion in acetone on amorphous carbon-coated copper grids. 2.3. Electrochemical Measurements. The working electrodes for evaluating the electrochemical properties of NiO were fabricated by mixing 75 wt % NiO with 15 wt % acetylene black in an agate mortar. To this mixture was added 10 wt % polyvinylidene difluoride (PVdF) binder with a few drops of 1-methyl-2-pyrrolidinone (NMP) to form a slurry. The slurry was coated (area of coating ) 1 cm2) on pretreated battery-grade Ni foil (0.2 mm thick) and dried at 80 °C for 8 h in air. Cyclic voltammetry (CV), chronopotentiometry, and electrochemical impedance spectroscopy (EIS) studies were performed on a CHI 7081C electrochemical workstation using a three-electrode configuration with Ni foil coated with NiO as working electrode, Pt foil (1 × 2 cm2) as counter electrode, and Hg/HgO (1.0 M KOH) as reference electrode, all dipped in 2.0 M KOH deoxygenated aqueous electrolyte. The scan rates employed in the CV study were 2, 5, 10, and 20 mV s-1. The electrochemical impedance spectra were measured by imposing a sinusoidal alternating voltage frequency of 10-2 to 105 Hz with an amplitude of 5 mV at a constant dc bias potential of 0.5 V. 3. Results and Discussion 3.1. Microstructure Studies. Figure 1 shows powder XRD patterns of NiO samples calcined at 300 °C for 3 h in air. The main peaks are indexed at 2θ ) 37.2° (111), 43.4° (200), and 62.8° (220) reflections, which agree well with standard powder diffraction patterns of NiO rock salt structure (JCPDS card no. 040835). No reflections could be detected corresponding to Ni(OH)2, NiOOH, Ni2O3, or NiO2, which indicates complete transformation of Ni(OH)2 f at 300 °C NiO + H2O. The loss of water can be traced in the thermogravimetric curve shown in the inset of Figure 1. The influence of surfactant on the crystallite size of NiO is significant, and the size varies as NiO-S < NiO-C < NiO-T (Table 1). The anionic surfactant sodium dodecyl sulfate is effective in producing smaller crystallites of NiO. Moreover, we have been able to obtain black NiO samples that are nonstoichiometric. These samples are conductive and may become nonconductive p-type semiconductors if heated at higher temperatures45 and lose their ability to store charge.35
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Figure 1. XRD patterns of NiO samples calcined at 300 °C. The inset shows the TG profile of the dried Ni(OH)2 sample and the respective DTG profile.
The surface textural characteristics of NiO powders calcined at 300 °C were evaluated by BET analysis using nitrogen gas adsorption-desorption shown in Figure 2. There is a steep uptake of N2 at the beginning of the measurement (p/p0 < 0.02), followed by an increased uptake of gas and hysteresis. All samples have sorption isotherms with a hysteresis loop associated with the filling and emptying of the micropores of
