Effects of Deposition Potential and Anneal Temperature on the

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Chem. Mater. 2007, 19, 3882-3891

Effects of Deposition Potential and Anneal Temperature on the Hexagonal Nanoporous Nickel Hydroxide Films Dandan Zhao, Wenjia Zhou, and Hulin Li* College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China ReceiVed NoVember 15, 2006. ReVised Manuscript ReceiVed February 16, 2007

In this study, improvement in the electrochemical performance of the nickel hydroxide film electrode has been pursued by (i) using nonionic surfactant Brij 56 as the structure-directing agent, (ii) varying the deposition potentials, and (iii) varying the anneal temperatures. Nanoporous nickel hydroxide films are successfully electrodeposited onto titanium substrates from the hexagonal lyotropic liquid crystal template of Brij 56. The films are physically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to determine the effects of deposition potentials and anneal temperatures on the surface morphology. Electrochemical techniques such as cyclic voltammetry (CV) and chronopotentiometry are used to systematically study the effects of deposition potentials and anneal temperatures on the capacitance of the films. The specific capacitance as high as 578 F g-1 is achieved for the HI-e Ni(OH)2 film deposited at -0.70 V versus SCE and heat-treated at 100 °C for 1.5 h, indicating its potential application as a low-cost, high-performance electrode material in electrochemical capacitors.

1. Introduction In recent years, the electrochemical capacitors (ECs) have received much attention because of their higher power density and longer cycle life relative to secondary batteries and their higher energy density compared to conventional electrical double-layer capacitors.1-3 They have many practical applications such as auxiliary power sources in combination with fuel cells or batteries for hybrid electric vehicles, back up and pulse power sources for mobile electric devices, and so forth.4 In particular, ECs based on hydrous ruthenium oxides can exhibit much higher specific capacitance than conventional carbon materials and better electrochemical stability than electronically conducting polymer materials. The remarkably high specific capacitance values varying from 658 to 760 F g-1 (from a single electrode) have been reported earlier.5-7 However, the high cost of this noble metal material limits it from commercialization. Hence, much effort has been aimed at searching for alternative inexpensive electrode materials with good capacitive characteristics, such as NiO,8,9 CoOx,10 MnO2,11 Ni(OH)2,12 Co(OH)2,13 and so forth. * To whom correspondence should be addressed. Telephone number: +86 931 891 2517. Fax number: +86 931 891 2582. E-mail address: [email protected].

(1) Conway, B. E. Electrochemical Supercapacitors; Kluwer Academic/ Plenum Publishers: New York, 1999. (2) Trasatti, S.; Kurzweil, P. Platinum Met. ReV. 1994, 38, 46-56. (3) Sarangapani, S.; Tilak, B. V.; Chen, C. P. J. Electrochem. Soc. 1996, 143, 3791-3799. (4) Huggins, R. A. Solid State Ionics 2000, 134, 179-195. (5) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092-4096. (6) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2005, 109, 7330-7338. (7) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699-2703. (8) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124130.

Energy storage mechanisms of ECs are explained in two ways, (i) double-layer capacitance arising from the charge separation at the electrode/electrolyte interface and (ii) faradic pseudo-capacitance arising from fast, reversible electrosorption or redox processes occurring at or near the solid electrode surface.1-3 Because the capability of electrode material is significantly influenced by its surface area and morphology, the electrode material with high surface area and a uniform, ordered pore network of nanometer dimension would be expected to exhibit superior performance in supercapacitor system. Among the existing synthetic approaches to ordered nanoporous materials,14 electrochemical techniques show unique principles and flexibility in the control of the structure and morphology of nanoporous film materials.15-25 The main advantage of the electrodeposition (9) Kalu, E. E.; Nwoga, T. T.; Srinivasan, V.; Weidner, J. W. J. Power Sources 2001, 92, 163-167. (10) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097-4103. (11) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444-450. (12) Cao, L.; Kong, L. B.; Liang, Y. Y.; Li, H. L. Chem. Commun. 2004, 14, 1646-1647. (13) Cao, L.; Xu, F.; Liang, Y. Y.; Li, H. L. AdV. Mater. 2004, 16, 18531857. (14) Sun, D.; Riley, A. E.; Cadby, A. J.; Richman, E. K.; Korlann, S. D.; Tolbert, S. H. Nature 2006, 441, 1126-1130 and references therein. (15) Tan, Y. W.; Srinivasan, S.; Choi, K. S. J. Am. Chem. Soc. 2005, 127, 3596-3604. (16) . Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838-840. (17) Nelson, P. A.; Elliott, J. M.; Attard, G. S.; Owen, J. R. Chem. Mater. 2002, 14, 524-529. (18) Nelson, P. A.; Owen, J. R. J. Electrochem. Soc. 2003, 150, A1313A1317. (19) Ganesh, V.; Lakshminarayanan, V. Electrochim. Acta 2004, 49, 35613572. (20) Bartlett, P. N.; Birkin, P. N.; Ghanem, M. A.; Groot, P.; Sawicki, M. J. Electrochem. Soc. 2001, 148, C119-C123.

10.1021/cm062720w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/06/2007

Hexagonal Nanoporous Nickel Hydroxide Films

technique is its relatively easy and accurate control of the surface microstructure of deposited films by changing deposition variables, such as electrolyte, deposition potential, bathing temperature, and so forth. For example, Yamauchi and Kuroda24 proposed a novel pathway of “solventevaporation-mediated direct physical casting (SEDPC)” for the formation of LLC templating mixtures from dilute surfactant solutions by a casting method. By combining the SEDPC method with colloidal crystal templating, they electrodeposited a well-ordered Pt thin film with a bimodal pore system. Bender et al.25 also electrodeposited mesoporous poly(1,2-diaminobenzene) films from hexagonal liquid crystal templates using two different surfactants, C16EO8 and Brij 56. It was demonstrated in their work that a low-cost surfactant (Brij 56) is sufficient to template the ion selective poly(1,2-diaminobenzene) films. This is an important prerequisite for any commercial use of this templating process. Recently, Nelson et al.17,18 electrodeposited mesoporous metallic nickel films by lyotropic liquid crystal templating and studied the Ni(OH)2/NiOOH electrochemistry behaviors for supercapacitor application. However, low capacitance obstructs its practical application attributed to the formation of just a few monolayers of Ni(OH)2 on nickel surface in 3 M KOH by the cyclic voltammetry technique. Lately, we have reported a novel method to directly electrodeposit a nanoporous Ni(OH)2 film from nickel nitrate dissolved in the aqueous domains of the hexagonal liquid crystal template of Brij 56 (designated HI-e Ni(OH)2).26 In this paper, the effect of deposition conditions such as deposition potential on the surface morphology of HI-e Ni(OH)2 films is examined and found to have a significant influence on the electrochemical capacitance of the deposited films. Meanwhile, the anneal temperature also plays a crucial role in improving the electrochemical capacitance of the deposited films. A maximum specific capacitance value of 578 F g-1 is obtained for the HI-e Ni(OH)2 film deposited at -0.70 V versus SCE and heat-treated at 100 °C for 1.5 h, which is a good candidate as an electrode material for ECs. 2. Experimental Section Materials. All chemical reagents were AnalaR (AR) grade and used as received. Nonionic surfactant polyoxyethylene (10) cetyl ether (Brij 56, C16EO10) was purchased from Aldrich. Saturated calomel electrode (SCE) and Ag/AgCl electrode were manufactured by Leici (Shanghai). All aqueous solutions were freshly prepared using high purity water (18 MΩ cm resistance) from an Ampeon 1810-B system (Jiangsu). All glassware was cleaned in a mixture of distilled water and nonionic detergent, followed by rinsing thoroughly at least three times with high purity water before drying. Preparation of Liquid Crystalline Phase. The phase diagrams were mapped by mixing various amounts of Brij 56 between 10 (21) Bartlett, P. N.; Gollas, B.; Guerin, S.; Marwan, J. Phys. Chem. Chem. Phys. 2002, 4, 3835-3842. (22) Luo, H. M.; Zhang, J. F.; Yan, Y. S. Chem. Mater. 2003, 15, 37693773. (23) Luo, H. M.; Sun, L.; Lu, Y. F.; Yan, Y. S. Langmuir 2004, 20, 1021810222. (24) Yamauchi, Y.; Kuroda, K. Electrochem. Commun. 2006, 8, 16771682. (25) Bender, F.; Chilcott, T. C.; Coster, H. G. L.; Hibbert, D. B.; Gooding, J. J. Electrochim. Acta 2007, 52, 2640-2648. (26) Zhao, D. D.; Bao, S. J.; Zhou, W. J.; Li, H. L. Electrochem. Commun. 2007, 9, 869-874.

Chem. Mater., Vol. 19, No. 16, 2007 3883 and 90 wt % with the high purity water or an aqueous solution containing 1.8 M Ni(NO3)2 with 0.075 M NaNO3 as the supporting electrolyte. To prepare the liquid crystal, the surfactant was first heated to 65 °C to be melted in a glass vial, and then the high purity water or the aqueous solution was added dropwise while mixing manually using a glass rod. After vigorously stirring for 5 min, the mixture was heated to 65 °C and kept at that temperature for 1 h in the sealed vial in a thermostat to achieve homogeneity. Then, the mixture was vigorously stirred for another 5 min by the glass rod immediately. The procedure of heating and stirring was repeated at least three times until a homogeneous mixture was obtained. A small amount of water was added to compensate for the loss of water during the whole mixing process. The resulting mixtures were stored hermetically in the thermostat for at least 3 h to allow phase equilibration before liquid crystalline phases were identified. Electrodeposition. The electrodeposition of nanostructured Ni(OH)2 film was conducted using a Chenhua CHI760B model Electrochemical Workstation (Shanghai), with a three-electrode cell consisting of a titanium working electrode (1 cm2 in area), a spiral platinum rod counter electrode (1.0 mm in diameter), and a saturated calomel reference electrode (SCE). All electrodepositions of HI-e Ni(OH)2 films were performed in the hexagonal liquid crystal template consisted of 50 wt % Brij 56 and 50 wt % aqueous solution of 1.8 M Ni(NO3)2 and 0.075 M NaNO3 under thermostatic, potentiostatic, and coulometric control. To deposit an HI-e Ni(OH)2 film of about 5 mg, the corresponding deposition charge quantity is approximately 8.326 C which was estimated by Faraday’s law. Cathodic deposition of nanostructured Ni(OH)2 films was carried out respectively at -0.70, -0.80, -0.90, -1.00, and -1.50 V versus SCE with a water bath of 40 °C to study the effect of deposition potential. After deposition, the nickel hydroxide films on titanium substrates were washed consecutively with methanol, 2-propanol, and high purity water to remove the adhering template mixture and dried under ambient conditions before further investigation. To study the effect of anneal temperature, the as-deposited films on titanium substrates were heated in air from room temperature to 100, 150, and 200 °C, respectively, held at that temperature for 1.5 h, and then cooled to room temperature. Additionally, to demonstrate fully the benefit of using a nonionic surfactant as a structure directing agent, a “usual” nickel hydroxide film was deposited on titanium substrate from a 100 wt % aqueous solution of 1.8 M Ni(NO3)2 and 0.075 M NaNO3 with a water bath of 40 °C at -0.70 V versus SCE. Characterization. The types of lyotropic liquid crystalline phases of the mixtures were identified by means of a polarized optical microscopy (POM) technique using a COEIC XSZ-HS7 optical polarizing microscope (Chongqing) equipped with a digitally controlled heating stage. The surface morphology of the Ni(OH)2 films was examined by scanning electron microscopy (SEM) using a JEOL JSM-6380LV (Japan), and the measurement parameters have been shown in the respective images. For transmission electron microscopy (TEM) studies a JEOL JEM-1230 (Japan) equipped with a GATAN BioScan Camera 792 (U.S.A.) was used. X-ray diffraction (XRD) data were collected using a Rigaku D/MAX 2400 diffractometer (Japan) with Cu KR radiation (λ ) 1.5418 Å) operating at 40.0 kV and 60.0 mA. Low-angle X-ray diffractograms recorded over the 2θ range of 1.0-10° were used to provide primary evidence for the formation of the hexagonal liquid crystal template and the nanostructured Ni(OH)2 film. Electrochemical characterization was carried out in a conventional three-electrode electrochemical cell containing 3 wt % KOH aqueous solution as electrolyte. The freshly prepared Ni(OH)2 film on titanium substrate was used as the working electrode, a platinum

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foil (area 1 cm2) was used as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode. The working electrode is 1 cm2 containing approximately 5 mg of Ni(OH)2. A Mettler Toledo AX26 Micro Balance (Switzerland) was utilized to measure the exact mass of as-prepared Ni(OH)2 films. The cyclic voltammetry and galvanostatic charge/discharge measurements were performed using a Chenhua CHI760B model Electrochemical Workstation (Shanghai) at room temperature.

3. Results and Discussion It is demonstrated that the electrodeposition of the Ni(OH)2 film from the Ni(NO3)2 precursor may involve the following processes: When electric current passes the electrolyte containing Ni(NO3)2, nitrate ions can be reduced on the cathodic surface to produce hydroxide ions. The generation of OH- at the cathode raises the local pH, resulting in the precipitation of Ni(OH)2 at the electrode surface according to the following reactions:27 NO3- + 7H2O + 8e- w NH4+ + 10OH-

(1)

Ni2+ + 2OH- w Ni(OH)2V

(2)

References in Baes and Mesmer28 have shown that the predominant species of soluble complexes in concentrated Ni(NO3)2 aqueous solutions (ca. 1.8 M in our work) is the polymeric Ni4(OH)44+. The polymeric species Ni4(OH)44+ then combines with more OH- to form Ni(OH)2 deposit as given in eq 4:29 4Ni2+ + 4OH- S Ni4(OH)44+

(3)

Ni4(OH)44+ + 4OH- w 4Ni(OH)2V

(4)

These reactions are likely to occur simultaneously during the electrodeposition process. While numerous researchers used Faraday’s law to predict the mass of the deposited films as the necessary first step in determining the specific capacitance, relatively few have provided the experimental verification of the exact film mass. For example, MacArthur30 used Faraday’s law to calculate film thickness but gave no experimental evidence other than electrochemical capacity. A few investigators have reported on the utilization of the quartz crystal microbalance to study the mass changes in nickel hydroxide films.31-34 Of particular interest, Streinz et al.34 observed that at low concentrations (e.g., 0.2 or 0.1 M) the utilization efficiency of electrochemically generated OH(27) Corrigan, D. A.; Bendert, R. M. J. Electrochem. Soc. 1989, 136, 723728. (28) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976. (29) Streinz, C. C.; Motupally, S.; Weidner, J. W. J. Electrochem. Soc. 1995, 142, 4051-4056. (30) MacArthur, D. M. J. Electrochem. Soc. 1970, 117, 729-733. (31) Bernard, P.; Gabrielli, C.; Keddam, M.; Takenouti, H.; Leonardi, J.; Blanchard, P. Electrochim. Acta 1991, 36, 743-746. (32) Cordoba-Torresi, S. I.; Gabrielli, C.; Goff, A. H.; Torresi, R. J. Electrochem. Soc. 1991, 138, 1548-1553. (33) Faria, I. C.; Torresi, R.; Gorenstein, A. Electrochim. Acta 1993, 38, 2765-2771. (34) Streinz, C. C.; Hartman, A. P.; Motupally, S.; Weidner, J. W. J. Electrochem. Soc. 1995, 142, 1084-1089.

was nearly 100%, whereas at high concentrations (e.g., 1.0 or 2.0 M) the utilization efficiency of OH- was significantly less than 100%. The inefficient utilization of OH- in concentrated Ni(NO3)2 was attributed to the formation of Ni4(OH)44+ (see eq 3), which diffuses away from the reaction interface before deposition occurs. The study showed that MacArthur’s assumption was good only at low Ni(NO3)2 concentrations; however, at high concentrations (such as 1.8 M in our work), film mass was overestimated using Faraday’s law. In addition, it is obvious that the mass of deposited film could hardly be calculated accurately based on Faraday’s law considering the loss of water, either physically absorbed or chemically absorbed water, after annealing at higher temperatures (e.g., 100, 150, or 200 °C). On the basis of the above discussion, the following two-step process was performed to determine the exact deposited film mass. First, Q, the quantity of electrical charge during electrodeposition, could be estimated by Faraday’s law Q ) mFn/M, where m and M are the mass and molecular weight of the deposited film, respectively, and n is the number of electrons in the electrodeposition reaction per atom of Ni (see eqs 1 and 2). For a deposited HI-e Ni(OH)2 film of about 5 mg, the corresponding deposition charge quantity is approximately 8.326 C. All electrodepositions of HI-e Ni(OH)2 films were conducted under such a coulometric control. Second, a microbalance was utilized to measure the exact mass of the HI-e Ni(OH)2 films prepared, usually slightly less than 5 mg. In this work, oligo(ethylene oxide) alkyl ether surfactant Brij 56 [main component, C16H33(OCH2CH2)10OH] has been employed, which contains hydrophobic blocks and connected ethylene oxide moieties as hydrophilic headgroups. When mixed with the aqueous solution of the metal salt, the hydrophobic blocks cluster together and the hydrophilic blocks dissolve in the water. Because the hydrophobic and hydrophilic blocks are covalently linked, microphase separation does not occur, but the result is rather nanostructures with characteristic dimensions on the order of 2-10 nm.35 In the hexagonal phase the surfactant molecules assemble into long cylindrical micelles, and these micellar rods then pack into a hexagonal array in which the separation between the micelles is comparable to their diameter,36 with the nickel nitrate and supporting electrolyte dissolving into the aqueous region of the mixture around the cylindrical micelles. When the hexagonal phase is used to template the electrodeposition of Ni(OH)2 films, inorganic precipitation occurs in the aqueous region around the cylindrical micelles, leading to the formation of the hexagonal nanoporous Ni(OH)2 film. On the basis of the above discussions, a mechanism as shown in Scheme 1 is proposed to describe the growth process of HI-e Ni(OH)2 film from the two-dimensional (2D) hexagonal liquid crystal template on the surface of titanium substrate. 3.1. Phase Characterization. Considering the nanostructured Ni(OH)2 film is a direct cast of the structure of the lyotropic liquid crystalline phase used to template the deposition, it is important to begin by characterizing the (35) Laughlin, R. G. The Aqueous Phase BehaViour of Surfactants; Academic Press Inc.: San Diego, 1994. (36) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1. 1983, 79, 975-1000.

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Figure 1. (a) Phase diagram for Brij 56 mixed with the high purity water. (b) Pseudo-binary phase diagram for Brij 56 mixed with an aqueous solution containing 1.8 M Ni(NO3)2 and 0.075 M NaNO3. The different phases were identified by their characteristic textures in polarizing light microscopy: L1, isotropic solution (water-rich); L2, isotropic solution (Brij 56-rich); LR, lamellar; VI, bicontinuous cubic; HI, hexagonal; II, cubic spherical micellar; and S, solidified. The solid lines denote phase boundaries, and the dotted lines show the compositions investigated.

Scheme 1. Schematic Mechanism of the Growth Process of HI-e Ni(OH)2 Film Electrodeposited from the 2D Hexagonal Liquid Crystal Template of Brij 56 on the Surface of the Titanium Substrate

phase behavior of the mixtures of nonionic surfactant Brij 56 and Ni(NO3)2 aqueous solution. Then the desired deposition system can be selected by choosing the appropriate composition and temperature. The phase diagram of Brij 56 with nickel nitrate aqueous solution is shown in Figure 1b, which indicates the relatively complex lyotropic phase behavior as determined by the characteristic appearances and textures of the phases36 under the polarizing microscope. Representative POM images of lyotropic liquid crystalline phases are shown in Figure 2. At low compositions of Brij 56, the system is the L1 (isotropic water-rich solution) phase. On increasing the amount of Brij 56, the system becomes biphasic comprising a mixture of phases HI (hexagonal) and II (cubic spherical micellar). The hexagonal (HI) phase domain exists from 30 wt % up to 80 wt % Brij 56 and is stable up to 80 °C for a composition around 40 wt % Brij 56, enabling fabrication of HI structural films over a large range of temperatures and compositions using this templating system. The hexagonal (HI) liquid crystal is anisotropic and displays focal conic fan textures under POM (Figure 2a). On decreasing the temperature, the hexagonal (HI) phase turns from transparent to gray-turbid in appearance and displays optical birefringence with homocentric circles on focal conic fan textures (Figure 2b). This domain is marked as the coagulated hexagonal phase [HI (S)]. There is a smaller region of the bicontinuous cubic (VI) phase existing between 50 wt % and 65% Brij 56 in the temperature range of 36.5-77.5 °C. At higher temperatures and higher concentrations, the lamellar (LR) phase

predominates, which is anisotropic and always sparse and indistinct in appearance (Figure 2c). As the Brij 56 concentration increases, at low temperatures, the system is anisotropic, possibly comprising a coagulated mixture of HI (hexagonal) and L2 (isotropic Brij 56-rich solution), denoted as [(HI + L2) (S)]. Increasing either the temperature or the amount of Brij 56, it gives way to the isotropic Brij 56-rich solution (L2) and its solidified phase [(L2) (S)]. In comparison with the Brij 56/water system (Figure 1a), the HI (hexagonal) and LR (lamellar) phase regions of the Brij 56 templating mixture are slightly enlarged at the expense of the L1 (isotropic water-rich solution) and L2 (isotropic Brij 56-rich solution) phase regions, respectively. The introduction of the deposition solution ions to the Brij 56/water system also decreases the stabilization of the small VI (bicontinuous cubic) domain at higher temperatures than that observed from the Brij 56/water system. The likely origin for this change is the coordination ability of the nitrate ion to the transition metal center. This lowers the charge of the complex ion and the ion density of the liquid crystal media, so it increases the stabilization of the lyotropic liquid crystalline mesophases. In all the salt-involved lyotropic liquid crystal systems, the counter anions play important roles in the stability of the liquid crystalline mesophase in water/ surfactant systems. The lyotropic anions, such as Cl- and SO42- ions, reduce the solubility between the surfactant and the water molecules (salting out effect); however, the hydrotropic anions such as NO3- and ClO4- ions increase the mutual solubility between the surfactant and the water

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Figure 3. Low-angle XRD patterns of (a) the 2D hexagonal phase of the template mixture consisting of 50 wt % Brij 56 and 50 wt % aqueous solution of 1.8 M Ni(NO3)2 and 0.075 M NaNO3 and (b) the HI-e Ni(OH)2 films deposited at various potentials and the “usual” Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V vs SCE.

Figure 2. Polarized-light optical microscopy (POM) images of (a) a hexagonal (HI) liquid crystalline phase, (b) a coagulated hexagonal liquid crystalline phase [HI (S)], and (c) a lamellar (LR) liquid crystalline phase formed by the pseudo-binary system of Brij 56/1.8 M Ni(NO3)2 and 0.075 M NaNO3.

molecules (salting in effect).37 The first and second row transition metal aqua complex salts of the nitrate ions, such as Ni(NO3)2‚6H2O, have a trend to form stable hexagonal mesophases in CnEOm-type nonionic surfactants, while perchlorate salts of the same metal ion prefer to form cubic mesophases.38 In the studies described below, a deposition mixture containing 50 wt % Brij 56 at 40 °C is chosen to prepare the nickel hydroxide films. As can be seen from Figure 1b, (37) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 19371944. (38) Dag, O ¨ .; Alayogˇlu, S.; Tura, C.; C¸ elik, O ¨ . Chem. Mater. 2003, 15, 2711-2717.

this is comfortably within the large area of the region where the hexagonal (HI) phase is stable. Figure 3a shows the lowangle XRD spectra for the hexagonal phase of the Brij 56Ni(NO3)2 electrolyte. The low-angle XRD patterns of the template mixture display four well-resolved peaks with d spacing of 58.1, 31.3, 21.0, and 16.0 Å at around 2θ ) 1.52°, 2.82°, 4.15°, and 5.53°, respectively, which can be indexed as the (100), (110), (210), and (220) planes of the P6mm space group. The pore-to-pore distance for this hexagonal array, given by d100/cos 30, is 67.1 Å. 3.2. Effect of Deposition Potential on the HI-e Ni(OH)2 Films. The cyclic voltammogram (CV) is recorded between 0 V and -1.00 V versus SCE, at 40 °C, on a titanium electrode of area 1 cm2, in the hexagonal (HI) phase of the Brij 56-Ni(NO3)2 electrolyte at a scan rate of 5 mV/s (Figure 4a). During the negative-going scan reduction commences at about -0.50 V, and a reduction peak is seen at about -0.75 V versus SCE. The electrodeposition is under mixed diffusion-kinetic control at -0.70 V, whereas the deposition is completely diffusion-controlled at potentials more negative

Hexagonal Nanoporous Nickel Hydroxide Films

Figure 4. (a) CV for a titanium electrode (1 cm2 area) in a hexagonal liquid crystal template consisting of 50 wt % Brij 56 and 50 wt % aqueous solution of 1.8 M Ni(NO3)2 and 0.075 M NaNO3 at a scan rate of 5 mV s-1 between 0 V and -1.00 V vs SCE. (b) Current time transient for the electrodeposition of HI-e Ni(OH)2 film on titanium electrode at different potentials versus SCE from the hexagonal liquid crystal template. Deposition temperature was 40 °C.

of the reduction peak. The physicochemical properties of the deposit are strongly dependent upon deposition conditions. At low deposition overpotentials, the formation of a smooth and compact mesoporous film would be expected, owing to the existence of the templating surfactant which tends to act as structure-directing agent. It would be expected that under mass transport limitations preferential formation of a rough film composed of mesoporous spherical particles would occur.39 On the basis of the CV curves, the HI-e Ni(OH)2 films were electrodeposited potentiostatically at potentials from -0.70 to -1.00 V versus SCE. Figure 4b shows typical current time transients for electrochemical deposition of HI-e Ni(OH)2 films at different potentials from the hexagonal phase. The deposition transients show an initial current decay with time, which can be attributed to double-layer charging followed by an increase in the current to a broad maximum and then a slow decay. As the deposition potential decreases, the deposition current density increases greatly. (39) Pletcher, D.; Walsh, F. Industrial Electrochemistry, 2nd ed.; Chapman & Hall: London, 1990.

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To investigate the effect of the deposition conditions on the surface morphology of deposited films, a series of nickel hydroxide films are deposited at different potentials of -0.70, -0.80, -0.90, -1.00, and -1.50 V versus SCE, respectively. The surface morphologies of nanostructured Ni(OH)2 films are found to vary significantly with the deposition potential as shown in the SEM images of Figure 5. Figure 5a shows an SEM image of an HI-e Ni(OH)2 film electrodeposited at -0.70 V versus SCE with no direct evidence of the ordered hexagonal nanostructure visible on the SEM scale. The film is relatively compact, flat, and smooth, distributing some large sized gaps similar to those of the Ni(OH)2 film deposited from the aqueous solution without the hexagonal liquid crystal template at -0.70 V versus SCE (Figure 5f). However, the film surface morphology becomes very different as the the potential is decreased to -0.80 V versus SCE (Figure 5b). More or less individual spherical nanoparticles of 150-250 nm in diameter cluster together and exist on the outer edge of the deposits or inside of the pore walls. The SEM image of the film deposited at -0.90 V versus SCE (Figure 5c) shows the dense distribution of such a large number of spherical particles agglomerated on the top of the deposits or within the pores. The surface of the nickel hydroxide films shifted to highly porous morphology at the deposition potential of -1.00 V versus SCE (Figure 5d). These pores can be seen to be distributed evenly with a typical diameter of 0.5-1.0 µm and appear to extend to the full thickness of the film. A closer examination of the images suggests that these clusters are actually made up of smaller spherical nanoparticles. With a decrease in potential to -1.50 V, the surface particles self-organize into aggregated microspheres with a narrow particle size distribution ranging over 250-400 nm (Figure 5e). It is important to note that these microspheres are not solid but have a mesoporous structure demonstrated by the corresponding TEM image (see Figure 6b). SEM analysis of Ni(OH)2 films showed that the surface morphology altered significantly as the deposition potential is lowered. Numerous nanoscale spherical particles agglomerate, densely distributing on the top of the deposits as well as inside the pores formed. Under stronger reduction conditions, the growth of the nanostructured Ni(OH)2 film is quickly triggered, leading to the formation of a large percentage of Ni(OH)2 microspheres with sponge-like morphology. The formation of spherical particles is a masstransport-controlled phenomenon39 and is enhanced by the effects of high current densities (i.e., high overpotentials), negligible convection, low diffusion coefficients, and high viscosities that the liquid crystalline phase would favor.40 Thus, it is unsurprising that mesoporous microspheres are formed under conditions of high overpotentials, whereas more compact mesoporous film are formed under mixed diffusion-kinetic control at low overpotentials. Support for this idea is also found by a great increase in the deposition current density (see Figure 4b) and the disorder degree in the nanostructure (see Figures 3b and 6) with decreasing the deposition potential. (40) Jiang, J. H.; Kucernak, A. Chem. Mater. 2004, 16, 1362-1367.

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Figure 5. SEM images of the HI-e Ni(OH)2 films deposited at (a) -0.70, (b) -0.80, (c) -0.90, (d) -1.00, and (e) -1.50 V vs SCE and (f) of the Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V vs SCE. The other parameters are shown in the respective diagrams.

Figure 6. TEM images of HI-e Ni(OH)2 films deposited at (a) -0.70 and (b) -1.50 V vs SCE and (c) of the Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V vs SCE, from the end-on view of pores.

Because there is no evidence for nanostructure visible on the scale of the SEM, it is necessary to use TEM to investigate the morphology of the electrodeposited nickel hydroxide films. The qualities of the nanostructured films are found to vary significantly with the deposition potential. Figure 6a shows a typical TEM image obtained for the HI-e

Ni(OH)2 film deposited at -0.70 V versus SCE. The film has a well-ordered nanoporous structure consisting of cylindrical holes of about 2.5 nm in diameter arranged on a hexagonal lattice with the center-to-center pore distance of about 7.0 nm and the pore wall thickness of about 4.5 nm. These values are consistent with previous observations of

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Figure 8. Wide-angle XRD spectra of the as-deposited HI-e Ni(OH)2 film obtained at -0.7 V vs SCE and the films heat-treated at different temperatures for 1.5 h.

Figure 7. Discharging curves of (a) the “usual” Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V vs SCE and the HI-e Ni(OH)2 films deposited at various potentials on titanium substrates measured in the potential range from 0 to 0.50 V vs Ag/AgCl and (b) the as-deposited HI-e Ni(OH)2 film obtained at -0.7 V vs SCE and the HI-e Ni(OH)2 films heat-treated at different temperatures for 1.5 h measured in the potential rage from 0 to 0.55 V in 3% KOH solution. The geometric surface area of the working electrode is 1 cm2, and the mass of the Ni(OH)2 film is normalized to 5 mg.

other mesoporous materials electrodeposited from the HI phase of the same surfactant Brij 56.17,18,20-23 On the basis of the pore size and wall thickness, a high specific surface area inside these pores would be expected. TEM analysis is also carried out on the Ni(OH)2 film deposited at lower potential (ca. at -1.50 V vs SCE) and confirms that the film exhibits a disordered nanostructure (Figure 6b). It is possible that the hexagonal liquid crystalline structure originally formed has been influenced by the greatly increased deposition current density on the decrease of the deposition potential. Obviously, there is no well-ordered or disordered nanoporous structure in the Ni(OH)2 film deposited from the aqueous solution without template (Figure 6c). In addition, low-angle X-ray diffractograms for HI-e Ni(OH)2 films deposited at various potentials and a “usual” Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V versus SCE are collected and shown in Figure 3b. The HI-e Ni(OH)2 film deposited at -0.70 V versus SCE exhibits a very strong diffraction peak with a d

spacing of 80.2 Å for (100) reflections, corresponding to a pore-to-pore distance of 92.6 Å. However, the HI-e Ni(OH)2 film obtained at -0.80 V versus SCE exhibits only a weak broad diffraction peak, and those deposited at or below -0.9 V versus SCE exhibit no peak at all just like the “usual” Ni(OH)2 film. In a trend similar to that of TEM, low-angle XRD results show that only the film deposited from the liquid crystal template at a potential of -0.70 V versus SCE exhibit a well-defined low-angle peak, indicating the expected hexagonal nanoporous structure. However, reducing the deposition potential from this optimum leads to a decrease in the XRD intensity, confirming a disordered nanostructure. Chronopotentiometric measurement has been used to compare the specific capacitance of the Ni(OH)2 films. Figure 7a shows the discharging curves of a “usual” Ni(OH)2 film deposited from the aqueous solution without template at -0.70 V versus SCE and the HI-e Ni(OH)2 films obtained from various potentials measured in the potential range of 0-0.50 V versus Ag/AgCl in 3% KOH at a discharging current of 5 mA. The working electrode each has a geometric surface area of 1 cm2, and the Ni(OH)2 film mass is normalized to 5 mg. The specific capacitance is calculated by I × ∆t/(∆V × m), where I is the constant discharging current, ∆t is the discharging time, ∆V is the potential drop during discharge, and m is the mass of the Ni(OH)2.10 The specific capacitance values calculated from the discharging curves are 314, 233, 144, 107, and 101 F g-1 for HI-e Ni(OH)2 films deposited at -0.70, -0.80, -0.90, and -1.00 V versus SCE and the “usual” Ni(OH)2 film deposited at -0.70 V, respectively. Obviously, the HI-e Ni(OH)2 film obtained from -0.70 V exhibits the highest specific capacitance; however, reducing the deposition potential from this optimum or depositing without template leads to a decrease in the specific capacitance. The difference of specific capacitance would be due to the surface morphology of deposited films, as the surface morphology of the electrode materials significantly affects the capacitance of an EC.8 For the HI-e Ni(OH)2 film deposited from -0.70 V, it has a more ordered nanoporous network structure, creating the faster

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Figure 9. Electrochemical properties of HI-e Ni(OH)2 film heat-treated at 100 °C for 1.5 h measured in 3% KOH solution: (a) CV curves at different scan rates within a potential window of 0 to 0.65 V vs Ag/AgCl; (b) discharging curves in the potential rage from 0 to 0.55 V at different discharging currents; and (c) cycle life at 2.5 mA. The geometric surface area of the working electrode is 1 cm2, and the mass of the HI-e Ni(OH)2 film is normalized to 5 mg.

electrochemical accessibility of the electrolyte and OH- ions to the bulk of the Ni(OH)2 phase, and thus the higher specific capacitance is achieved compared to the films deposited at lower potentials or without template. On the basis of the above discussions, we propose that the lyotropic liquid crystal template and the deposition potential both play crucial roles in optimizing electrochemical capacitive performance of the HI-e Ni(OH)2 film. Furthermore, we select an optimum electrodeposition potential of -0.70 V versus SCE in this work. 3.3. Effect of Anneal Temperature on the HI-e Ni(OH)2 Films. The effects of the heating temperature on the crystalline structure of Ni(OH)2 are studied by examining the XRD spectra. Figure 8 shows the wide-angle XRD patterns of the as-deposited HI-e Ni(OH)2 film on titanium substrate and the films heat-treated at different temperatures for 1.5 h. The XRD pattern of the as-deposited HI-e Ni(OH)2 film consists of broad peaks at 12.46° (001), 23.72° (002), 33.64° (110), and 59.92° (300), corresponding to the well-known layered nickel hydroxide hydrate [R-3Ni(OH)2‚ H2O, JCPDS no. 22-0444].30 The major diffraction peak (001) shows that the inter-sheet distance (c spacing) of R-3Ni(OH)2‚H2O is 7.1 Å. The material heat-treated at 200 °C still sustains R-Ni(OH)2 structure; however, the peaks shift slightly to higher angles, resulting in a decrease of the “c” spacing of the unit cell, which is due to water removal from planes perpendicular to the c-axis of Ni(OH)2 crystals.41 It is ascribed to the two water loss processes: (i) removal of physically absorbed water at ∼100 °C and (ii) removal of chemically absorbed water below 250 °C as depicted in the literature.9 Typical discharging curves of the HI-e Ni(OH)2 film electrodes heat-treated at different temperatures for 1.5 h measured in the potential range of 0-0.55 V in 3% KOH at a discharging current of 5 mA are shown in Figure 7b. The specific capacitance values calculated from the discharging curves are 314, 569, 251, and 198 F g-1 for the as-deposited HI-e Ni(OH)2 film and the films heat-treated at 100, 150, and 200 °C, respectively. Obviously, the HI-e Ni(OH)2 film obtained after being heat-treated at 100 °C for 1.5 h exhibits the highest specific capacitance, which is due to the fact that the physically absorbed water, other organic solvents, and (41) Mani, B.; Neufville, J. P. J. Electrochem. Soc. 1988, 135, 800-803.

surfactant residues are removed from the pores within the film; as a result, the electroactive surface area of the film in contact with the KOH solution increases. However, increasing the anneal temperature from this optimum leads to a decrease in the specific capacitance. This may be attributed to the loss of water molecules that are trapped in the interlamellar planes of the nickel hydroxide after heat treatment.29 It has been proposed that the hydrated interlayer allows facile permeation of electrolyte and OH- ions into the bulk of the material for efficient charge storage.6 These results indicate that improvement in the electrochemical capacitive performance of the HI-e Ni(OH)2 film could be pursued by varying the anneal temperature. In 3% KOH solution, CVs and chronopotentiometric measurements have been used to evaluate the electrochemical properties and quantify the specific capacitance of the HI-e Ni(OH)2 film electrode heat-treated at 100 °C for 1.5 h. For Ni(OH)2 electrode material, it is well accepted that the surface faradic reactions will proceed according to the following reaction.42 charge

Ni(OH)2 + OH- {\ } NiOOH + H2O + e discharge

(5)

As can be seen in Figure 9a, the two strong redox reaction peaks are responsible for the pseudo-capacitive capacitance. The anodic peak is due to the oxidation of Ni(OH)2 to NiOOH, and the cathodic peak is for the reverse process. The shape of the CV curves reveals that the capacitive characteristic is very distinguished from that of electric double layer capacitance in which case it is normally close to an ideal rectangular shape. Because solution and electrode resistance can distort current response at the switching potential and this distortion is dependent upon the scan rate,43 the shape of the CV changed when the scan rate increased. These results indicate that the measured capacitance is mainly based on the redox mechanism. Figure 9b shows the discharging curves of the HI-e Ni(OH)2 film electrode obtained after annealing at 100 °C for 1.5 h measured in the potential range of 0-0.55 V. The specific capacitance values are found to be 578, 569, and (42) Kamath, P. V.; Dixit, M.; Indira, L.; Shukla, A. K.; Kumar, V. G.; Munichandraiah, N. J. Electrochem. Soc. 1994, 141, 2956-2959. (43) Jiang, J. H.; Kucernak, A. Electrochim. Acta 2002, 47, 2381-2386.

Hexagonal Nanoporous Nickel Hydroxide Films

438 F g-1 corresponding to the discharging currents of 2.5, 5, and 10 mA, respectively. As discharge current increases, the voltage (IR) drop is produced, and finally the capacitance decreases. The curves obviously display two variation ranges; a linear variation of the time dependence of the potential (below about 0.3 V) indicates the double-layer capacitance behavior, which is caused by the charge separation taking place between the electrode and the electrolyte interface, and a slope variation of the time dependence of the potential (from 0.55 to 0.3 V) indicates a typical pseudo-capacitance behavior, which resulted from the electrochemical adsorption/ desorption or redox reaction at an interface between electrode and electrolyte.5 The shape of the discharge curves does not show the characteristics of a pure double layer capacitor but mainly pseudo-capacitance, which is in agreement with the result of the CV curves. The long-term electrochemical stability of the HI-e Ni(OH)2 film electrode obtained after annealing at 100 °C for 1.5 h has been examined by chronopotentiometry. As shown in Figure 9c, approximately 4.5% loss of specific capacitance after 400 cycles reveals that the repetitive charge-discharges do not induce noticeable degradation of the microstructure, indicating that this type of the HI-e Ni(OH)2 film is a good candidate as an electrode material for EC. 4. Conclusion In summary, direct templating with the lyotropic liquid crystalline phase of nonionic surfactant has been extended to fabricate a nickel hydroxide film electrode with high surface area and controlled regular nanoporous structure. Using this method, HI-e Ni(OH)2 films with various surface morphologies have been directly electrodeposited from the

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hexagonal liquid crystalline phase of an inexpensive nonionic surfactant Brij 56. The surface morphology of the HI-e Ni(OH)2 film is found to change greatly with a decrease in the deposition potential and with an increase in the deposition current density and the deposition rate. TEM and low-angle XRD studies show that the HI-e Ni(OH)2 film deposited at -0.70 V versus SCE has a regular nanostructure consisting of pores in ordered hexagonal arrays with a uniform pore diameter of about 2.5 nm and a pore center-to-center distance of about 7.0 nm. This designed nanostructure creates the fast electrochemical accessibility of the electrolyte and OH- ions to the bulk of the Ni(OH)2 phase, providing an important morphological basis for a high specific capacitance. The heat treatment of the deposited films at low temperatures seemed to cause little change in the surface morphology of the HI-e Ni(OH)2 films. The maximum specific capacitance of 578 F g-1 is observed for a highly ordered nanoporous HI-e Ni(OH)2 film electrode deposited at -0.70 V versus SCE and heat-treated at 100 °C for 1.5 h, which could be identified as a promising electrode material for ECs. According to the above discussions, it is apparent that the deposition potential and anneal temperature have played important roles in improving the electrochemical capacitive performance of the HI-e Ni(OH)2 film. This promising method reported above should be viable to extend to other low-cost and environmentally friendly transition metal hydroxide or oxide systems. Work in this direction is ongoing in our lab. Acknowledgment. This work was financially supported by theNationalNaturalScienceFoundationofChina(NNSFC60471014). CM062720W