NiO loaded on hydrothermally treated mesocarbon microbeads (h-MCMB) and their supercapacitive behaviors

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Solid State Ionics 178 (2008) 1859 – 1866 www.elsevier.com/locate/ssi

NiO loaded on hydrothermally treated mesocarbon microbeads (h-MCMB) and their supercapacitive behaviors Changzhou Yuan, Bo Gao, Linghao Su, Xiaogang Zhang ⁎ College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 13 July 2007; received in revised form 11 December 2007; accepted 11 December 2007

Abstract NiO was successfully loaded on hydrothermally treated mesocarbon microbeads (h-MCMB) to form NiO/h-MCMB composites by thermal decomposition of the precursor via a co-precipitation process in ethanol–water system. XRD, IR, SEM and HRTEM tests were applied to investigate the structure, morphology and the formation mechanism of the NiO/h-MCMB composites. Electrochemical characteristics of the composites were examined by cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) techniques in a three-electrode configuration with 1 M KOH solution. Electrochemical tests demonstrated that unsupported NiO with 5–10 nm grain sizes and higher specific area enlarged the contact area, made the best electroactive material and enhanced electrochemical reaction rate. Moreover, h-MCMB as support further increased the conductivity of the material, promoted the dispersion and utilization of NiO considerably, improved the electrochemical stability and power property of composite electrode. Specific capacitance of ca. 637 F g− 1 (CNiO) could be delivered at 6 mA cm− 2 for the NiO/h-MCMB composite with 23 wt.% NiO loading. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermally treated mesocarbon microbeads; NiO; Composite electrodes; Electrochemical capacitor; Electrochemical impedance spectroscopy

1. Introduction With the increasing prominence gained by hybrid electric vehicles (HEV) in recent years, electrochemical capacitors (ECs) for high power energy storage have received much attention [1,2] and have been studied extensively for using in acceleration and charge storage during regenerative braking. The ECs are in turn subdivided into the electrical double-layer capacitors (EDLC), which use the EDLC rechargeable on a highly-developed interfacial surface of electrodes; pseudocapacitors, wherein the charge is stored in a faradic pseudocapacitance of sufficiently reversible redox reactions and the electrical double-layer capacitance; and hybrid capacitors, which employ a variety of electrodes [3]. Commonly, the low utilization of carbon materials used in the EDLC [4] has driven attempts to identify more suitable materials

⁎ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail address: [email protected] (X. Zhang). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.12.018

for ECs application. Although the hydrous ruthenium oxide (RuO2 x H2O) was reported as the most promising material yielding a specific capacitance (SC) as high as 863 F g− 1 for a single electrode [5], high cost, low abundance and toxic nature preclude its practical application and commercial attraction as an alternative to carbon materials. This is the reason why in the last few years other materials with similar properties have been sought for as candidate materials for ECs. Nickel oxide with pseudo-capacitive behavior is easily available and even cheaper, which has drawn much attention now [6,7]. However, its lower electrochemical capacitance performance, compared with that of RuO2, makes it very difficult to be a good replacement for RuO2. So the approaches to improve the capacitive performance of NiO have been the next potential and logical step. It has been suggested [8] that transition metal oxides with poor conductivity are capable of storing high charge when such electroactive materials are highly dispersed and nanosized. In particular, it has been reported, in our research team, the efficient SC of NiO highly dispersed onto TiO2 nanotubes was enhanced greatly [9]. On the other hand, since Honda and Yamada first separated spheres from the mesophase pitches naming mesocarbon

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microbeads (MCMB) [10], this type material has been used as the precursor for high-performance carbon materials, such as high density carbon material [11,12], filler for high-performance liquid chromatography [13], active carbon with super-high surface [14], anodes of lithium ion battery [15–19] and alkaline zinc-air cells [20]. To the best of our knowledge, ECs by using hydrothermally treated MCMB (donated as h-MCMB) as a support for electroactive material NiO has not been reported as yet. In this work, the NiO/h-MCMB composites were synthesized by thermal decomposition of the precursor via a co-precipitation process in ethanol– water system (EWS), and first reported as an electrode material for ECs. Electrochemical capacitive performance of the composite electrodes in a three-electrode system with 1 M KOH electrolyte was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. 2. Experimental 2.1. Synthesis and characterization of NiO/h-MCMB composites NiO/h-MCMB composites were prepared by thermal decomposition of the precursor via co-precipitation method in EWS. In our experiment, the MCMB (specific area is 3.5 m2 g− 1, graphitized temperature is ca. 2500 °C and particle size distribution is ranged from 1 to 2 µm) is provided by Shanshan Science and Technology Corp (Shanghai). The purchased MCMB was under hydrothermal treatment in 6 M KOH at 150 °C for 1 h, and then washed with distilled water until pH reached 8. The hydrothermally treated MCMB was called h-MCMB. In a typical synthesis, a stoichiometric (with respect to h-MCMB) amount of Ni(NO3)2•6H2O was dissolved in absolute ethanol to form a green homogeneous solution (0.018 M). After 3 h, certain amount of hMCMB was added into the above solution. After stirring for 4 h, the pH of the mixture was slowly adjusted to 9 by dropwise addition of 0.025 M NaOH at room temperature (RT). The resulting suspension was stirred at the same temperature for additional 18 h. Thus, the mixture was aged for 6 h and then filtered, washed with copious amount of absolute ethanol, dried at 40 °C in air for 24 h and then calcinated at 300 °C under air for 1.5 h. The compositions of the NiO/h-MCMB composites were controlled by changing the relative ratio of Ni(NO3)2•6H2O and hMCMB support in the starting mixture. For comparison, the NiO/ MCMB composite with 23 wt.% NiO loading was also synthesized as described above just with exception of MCMB as support. The morphologies of the samples were examined by SEM (LEO 1430VP, Germany), TEM (Hitachi-600, Japan). The X-ray diffraction patterns of the samples were observed by XRD (Max 18 XCE, Japan) using a Cu Kα source. Infrared spectra were recorded with a model 360 Nicolet AVATAR FT–IR spectrophotometer. The specific area of the sample was obtained with a Micromeritics ASAP 2010 analyzer.

Fig. 1. XRD patterns of the Ni(OH)2/h-MCMB composite (1), NiO/h-MCMB composite with 23 wt.% NiO loading (2), h-MCMB (3) and NiO (4) as indicated (■ h-MCMB; ● NiO; ○ Ni(OH)2).

calomel electrode (SCE) reference electrode. Working electrodes, containing 10 mg NiO/h-MCMB composites with different NiO loadings, 1.5 mg acetylene black (AB) and 0.5 mg polytetrafluoroethylene (PTFE), were prepared by mixing the active material (NiO/h-MCMB composites) with AB and PTFE. A small amount of 1 M KOH solution was then added to this composite to form a more homogeneous mixture, which was pressed (1.2 × 107 Pa) on nickel grid (area for 1 cm2) pretreated with distilled HCl. All the measurements were carried out in 1 M KOH electrolyte at room temperature. CV and EIS measurements (the frequency limits were typically set between 100 kHz and 0.01 Hz and AC oscillation was 5 mV) were performed using CHI660 electrochemical workstation. The galvanostatic charge/discharge performance of the electrodes was evaluated with an Arbin BT2042 battery workstation system in the certain range of potentials. 3. Results and discussion 3.1. XRD analysis

2.2. Electrochemical tests A beaker-type electrochemical cell was used for the electrochemical measurement and equipped with a working electrode, a platinum plate counter electrode and a saturated

The crystalline structures of Ni(OH)2/h-MCMB and NiO/hMCMB composites with different NiO loadings are characterized using an X-ray diffractometer. From Fig. 1 (a), it is clear that the peak intensities of all the diffraction peaks of h-MCMB

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Fig. 2. SEM images of NiO/h-MCMB (23 wt.% NiO loading) composite (a, b) with different magnification and HRTEM of NiO (c).

are obviously much stronger than those of h-MCMB both in NiO/h-MCMB composite with 23 wt.% NiO loading (curve 2) and its precursor of Ni(OH)2/h-MCMB composite (curve 1), which indicates that h-MCMB in NiO/h-MCMB and its precursor Ni(OH)2/h-MCMB composites have been successfully coated with the nanocrystalline NiO and Ni(OH)2, respectively, and thus the peak intensities were weakened. In Fig. 1 (b)-1, except the typical diffraction peaks for h-MCMB in Ni(OH)2/h-MCMB composite, there are some other broadened peaks with very low intensity, which could be identified from the contribution of Ni(OH)2 according to the peak positions with the standard spectrum (JCPDS, card no 38-0715). In addition, as shown in Fig. 1 (b)-2, 4, all these diffraction peaks of NiO, not only the peak position but also their relative intensities, can be indexed into the rhombohedral crystalline structure, which is in line with the standard spectrum (JCPDS, card no 44–1159). The crystallite size of the crystalline NiO is determined from the major diffraction peak (200) (2θ = 43.26°) shown in Fig. 1(b)-4 by using well-known Scherrer's formula: d¼

0:89k Bcosh

ð1Þ

where λ is the X-ray wavelength (0.154056 nm), B the full width at half maximum of the peak (FWHM for 1.74000), and θ the Bragg's angle of the XRD peak. The above formula gives d ca. 5 nm for nanocrystalline NiO. Moreover, according to XRD data based on Fig. 1 (b)-2 and 4, the size of NiO nanoparticle is quite the same as that of NiO in the NiO/h-MCMB composite with 23 wt.% NiO loading. The formation mechanism of NiO of such little grain size in EWS is proposed as followings: Ni2 þ þ6HOC2 H5 →NiðHOC2 H5 Þ2þ 6

ð2Þ

− NiðHOC2 H5 Þ2þ 6 þ 2OH →NiðOHÞ2 þ 6C2 H5 OH

ð3Þ

300B C

2NiðOHÞ2 Y 2NiO þ H2 O:

and a surface-active agent, which is beneficial to the suppression of particle growth and further agglomeration [22]. 3.2. Morphology analysis The SEM images of NiO/h-MCMB composite (23 wt.% NiO loading) with different magnification are shown in Fig. 2 (a) and (b). It is clear that the NiO/h-MCMB composite exhibits ca. 1– 2 µm spherical grains in Fig. 2 (a) with lower magnification. From Fig. 2 (b) with higher magnification, the nanosized NiO coating onto h-MCMB is evident. Its size is ca. 5–10 nm. The HRTEM of nanocrystalline NiO without the h-MCMB support is also shown in Fig. 2 (c). Clearly, the nanocrystalline NiO disperses very well. Although some agglomeration has been found, the morphology of NiO still can be obviously observed. It can be seen that NiO displays relatively uniform grains and their sizes range from ca. 5 to 10 nm, which may be formed by the further soft agglomeration of grains and which is basically consistent with the size estimated from the XRD data. And moreover, the size of NiO without the h-MCMB support (see Fig. 2 (c)) is in good agreement with that of NiO coating on hMCMB in the NiO/h-MCMB composite with 23 wt.% NiO loading (see Fig. 2 (b)). Based on the above discussion, we propose a mechanism, as shown in Fig. 3, to describe the synthesis of NiO/h-MCMB composites. It is clear that, after the hydrothermal treatment,

ð4Þ

In addition to the contribution of lower starting concentrations of Ni2+ and the precipitating agent NaOH, absolute ethanol also plays an important role in the formation of nanocrystalline NiO. Firstly, absolute ethanol is a complexing agent [21] which restrains the speed of the formation of crystal nuclei. And secondly, it acts as a unique reaction medium, an organic wash

Fig. 3. A schematic for the synthesis of NiO/h-MCMB composites.

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there exists substantial amounts of OH-on the surface of hMCMB, which can be proved by FT–IR spectra shown in Fig. 4. Clearly, in Fig. 4 (b), there is typical hydroxyl band at 3664 cm− 1 in FT–IR spectrum of h-MCMB (curve b), while there is, in Fig. 4 (a), no band attributed hydroxyl in FT–IR spectrum of MCMB. And the existence of OH-improves the surface hydrophilic property of MCMB. Then during stirring the suspension of h-MCMB and absolute ethanol solution containing Ni2+ for 4 h, some Ni(OH)2 precipitation would form onto the surface of the h-MCMB. Thus, with the increasing pH by 0.025 M NaOH titration, initial precipitations provide nucleation centers that result in more and more Ni(OH)2 coating onto the surface of MCMB. Therefore, the OH− existing on the surface of h-MCMB plays a significant role in the formation of NiO/h-MCMB composite with good dispersion of NiO. Subsequently, the h-MCMB is successfully coated with Ni (OH)2, which can be verified by the diffraction peaks in Fig. 2 (b)-1. Then, calcinated at 300 °C under air for 1.5 h, the hMCMB coated with NiO is obtained. 3.3. Electrochemical profiles of NiO/h-MCMB composites 3.3.1. Cyclic voltammograms Fig. 5 shows the CV curves of the electrodes fabricated with h-MCMB (a), NiO/h-MCMB composite (b) and NiO/MCMB composite (d) both with 23 wt.% NiO loading and NiO (c) as indicated. Obviously, in Fig. 5 (a), the extremely small area under the current–potential curve suggests that the h-MCMB support has a very small SC which should be related to its low specific area of ca. 10 m2/g. In contrast, for the NiO/h-MCMB composite with 23 wt.% NiO loading, as seen from CV curve of Fig. 5 (b), the electrochemical response current turns out to be much larger and there exist obvious redox peaks within potential range from 0.0 to 0.5 V (vs. SCE), which is very distinguished from that of h-MCMB. Out of question, the NiO phase of the composites should be responsible for such difference. Also, the CV shape of the composite with 23 wt.% NiO is the same as that of crystalline NiO shown in Fig. 5 (c), which further confirms the capacitance of the composite with 23 wt.% NiO is mainly contributed by NiO phase in the composite. The redox peaks in the CV curves of NiO and the

Fig. 4. FT–IR spectra of MCMB (a) and h-MCMB (b).

Fig. 5. Cyclic voltammograms for h-MCMB (a), NiO/h-MCMB composite (b) and NiO/MCMB composite (d) both with 23 wt.% NiO loading, and NiO (c) in 1 M KOH electrolyte (5 mV s− 1).

NiO/h-MCMB composites reveal that the capacitance characteristic of the NiO phase is distinguished from that of the electrical double-layer capacitance in which the shape of CV curve is usually close to an ideal rectangular shape. The capacity of the composites mainly results from the pseudocapacitive character based on the surface faradaic redox mechanism of Ni2+ to Ni3+ occurring at the surface of NiO according to the following Eq.: charge

 NiO þ OH ² discharge NiOOH þ e :

ð5Þ

For comparison, the electrochemical performance of the NiO/MCMB composite with 23% NiO loading is also demonstrated in Fig. 5 (d). Obviously, the shape of the curves (c) and (d) is the same, suggesting the same electrochemical process as described by Eq. (5)) for both the electrode materials (NiO/h-MCMB (c) and NiO/MCMB (d) composites with the same NiO loading), while the electrochemical current response of the NiO/MCMB is less than that of the NiO/h-MCMB, which means the better support for NiO is h-MCMB rather than MCMB for ECs application. The reason for this is that the hMCMB is much better suited than MCMB for the dispersion of NiO due to its unique surface characteristics as discussed above. Moreover, the CV curves of NiO/h-MCMB composites with 44, 32 20 wt.% NiO loadings are also given in Fig. 6 (a), (b) and (c), respectively. Obviously, the shape of these composites is the same as that of NiO and NiO/h-MCMB composite with 23 wt.% NiO loading, which indicates the same electrochemical process for all the composites. In addition, with the increase of NiO loading from 20 to 44 wt.%, the electrochemical response current increases greatly, which also further confirms that the real electroactive material in the composites is the NiO phase. 3.3.2. Charge–discharge analysis Constant current galvanostatic charge/discharge measurements at different current densities are used to determine the electrochemical properties and the quantify the SCs of NiO/h-

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Fig. 6. Cyclic voltammograms for NiO/h-MCMB composite electrodes with different NiO loadings: (a) 44 wt%; (b) 32 wt.%; (c) 20 wt.% (10 mV s− 1).

MCMB composite electrodes in 1 M KOH electrolyte. The SCs of the NiO within the NiO/h-MCMB composites (CNiO) can be calculated as follows: CNiO ¼

C I t ¼ m DV  m

1863

addition, as shown in Fig. 7 (a), further increase or decrease of NiO loading both result in a sharp decrease in corrected SC. This result implies that the excess NiO in composition when NiO loading is N23 wt.%, such as 32 and 44 wt.% NiO, does not favor the formation of effectively dispersed NiO on MCMB, and therefore, results in the low utilization of NiO and the low corrected SC consequently. It also means that the less NiO in the composites when NiO loading is b 23 wt.%, such as 18 and 20 wt.% NiO, would cause the decrease in electroactive sites, which results in the decrease of SC of the composite. Because of the much decreased SC, and the slightly decreased NiO loading, the decrease of the corrected SC occurs; however, such corrected SC values are still much higher than that of the single NiO phase. The phenomenon has been also found in reference [23]. All the results demonstrate the real SC contribution is from NiO rather than h-MCMB. Herein, the optimum NiO loading for the electrochemical capacitance performance of NiO/h-MCMB composites has been obtained. Therefore, the electrochemical performance of NiO (just for comparison) and the NiO/h-MCMB composite with 23 wt.% NiO loading (the optimum NiO loading) are studied in detail in the next section.

ð6Þ

where “I” is the discharge current, “t” the total discharge time, “△V” the potential drop during discharging and “m” is the mount of NiO within the composite electrodes. The relationship between the SCs of NiO/h-MCMB composites and the weight percent of NiO is plotted in Fig. 7. Form the curve (b) (not corrected for the weight percentage of the effective component of NiO in the composites), evidently, SCs of the composites increase almost linearly with the increase of NiO loading, and reach a maximum of ca. 174 F/g in the case of 44 wt.% NiO loading. It is noticeable from curve (a) (after correction for the weight percentage of NiO in the composites), that after correction for the weight percentage of the redox-active phase of NiO, a maximal corrected SC of ca. 637 F/g can be derived for the composite NiO/h-MCMB with 23 wt.% NiO loading. In

Fig. 7. SCs of NiO/h-MCMB composite electrodes as a function of NiO loading at 6 mA cm− 2.

Fig. 8. Galvanostatic charging/discharging curves at different current densities based on NiO (a) and NiO/h-MCMB composite (b) with NiO 23 wt.% loading.

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Typical chronopotentiograms of NiO (a) and NiO/h-MCMB composite with 23 wt.% NiO loading (b) for various current densities in 1 M KOH within a potential window of 0 to 0.5 V (vs. SCE) are shown in Fig. 8 (a) and (b), respectively. The E–t responses behave as triangular waves during the charge– discharge process, characteristic of a reversible redox reaction occurring in both the electrodes. It is noted that the discharge profiles all contain two parts: a resistive component arising from the sudden voltage drops (linear portion parallel to y-axis) representing the voltage change due to the internal resistance, and a capacitive component (curve portion) related to the voltage change due to change in energy within the capacitance on all curves in Fig. 8. In addition, the sudden potential drops become less with decreasing the charge–discharge current density. It is worthy to be mentioned that the sudden voltage drops of NiO/h-MCMB (23 wt.% NiO loading) composite decrease in sharp contrast to those of NiO, which indicates the improvement in the conductivity due to the introduction the support h-MCMB. The SCs of both the electrode materials are calculated and typical results are shown in Table 1. To further determine the power capability of the different electrodes, high-rate dischargeability (HRD) of the electrodes was investigated in a current density range from 6 to 25 mA cm− 2. The HRD is defined as the ratio of discharge capacitance at a certain current density to that at 6 mA cm− 2 and calculated according to the following formula: HRDðkÞ ¼

Cd  100 C6

ð7Þ

where Cd and C6 are the discharge capacity of the electrodes at a certain current density and 6 mA cm− 2, respectively. (Fig. 9). It is worthy of noting that the SCs of NiO and the NiO/ MCMB composite with 23 wt.% NiO loading at 25 mA cm− 2 is 68.7% and 80.0% of those at 6 mA cm− 2, respectively. It demonstrates that the specific power of the composite electrode material after addition of the support h-MCMB is greatly enhanced. Here we prepared NiO electrode by a co-precipitation method in EWS and it has been found to own a SC of ca. 310 F g− 1 at 6 mA cm− 2. Such high SC of NiO depends on its microstructure. Usually, nano-structure electrode materials exhibit more attractive properties compared with conventional electrode materials, such as very small particle size, large exposed surface area, and high surface energy. These properties can enlarge the contact area, make the best of the electroactive materials and enhance the electrochemical reaction rate [6]. However, the decrease in particle size should increase the

Table 1 The SCs of NiO and NiO/h-MCMB composite with 23 wt.% NiO loading evaluated from charge–discharge curves Electroactive materials

SC (F g− 1) of NiO (6 mA cm− 2)

SC (F g− 1)of NiO (25 mA cm− 2)

NiO NiO/h-MCMB with 23 wt.% NiO

310 637

211 509

Fig. 9. High-rate dischargeability (HRD) for the different electrodes as indicated.

inter-particle electron-hopping resistance during the redox transition [24–26] because the number of interface between particles increases with the decreasing the particle size, which would be remitted to some extent by the introduction of the support h-MCMB. The maximal SC (CNiO) for NiO/h-MCMB (23 wt.% NiO) composite reaches ca. 637 F g− 1 at 6 mA cm− 2, which is realized by the introduction of h-MCMB as a support increasing the conductivity of material, promoting the dispersion and the utilization of electroactive phase NiO considerably. 3.3.3. Electrochemical impedance spectroscopy analysis In order to investigate the electrochemical behavior at the electrode/electrolyte interface in detail, electrochemical impedance measurement are carried out at alternating voltages from 0.0 to 0.4 V. Complex plane plots of the impedance of NiO and NiO/h-MCMB composite with 23 wt.% NiO loading electrodes in 1 M KOH electrolyte are presented in Fig. 10 (a) and (b) at various voltages as indicated, respectively. Before each impedance measurement, the potential of the working electrode is held constant with a potentiostat for 30 min to reach an equilibrium state. As seen from Fig. 10 (a) and (b), it is clear that, for both electrodes, in the high frequency region, the shape of a depressed semicircular arc related to the charge-transferring process is demonstrated; in the lower frequency range, a straight line of 45° slope can be seen, characteristic for a diffusion controlled process; at the very low frequencies the curves approach a vertical asymptote, due to the onset of finite volume effects, which indicates the characteristics of capacitive behavior of the electrode materials. From a comparison of Fig. 10 (a) and (b), several features have to be mentioned. First, from the point intersecting with the real axis in the range of high frequency, the internal resistance (Ri) of these electrodes is estimated to be in the range of ca. 1.8 to 2.6 ohm cm− 2. As shown in Table 2, it is obvious that after the introduction of h-MCMB support with a low resistance, the internal resistance (Ri) of the composite electrode decreases accordingly at all the applied voltages. Secondly, the magnitude of the Ri is found to decrease with the increase of OH− content to form NiOOH within the electrode when the electrode potential shifts in the anodic direction. This means that, as the

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Fig. 11. Life cycles of NiO (a) and NiO/h-MCMB composite (b) with 23 wt.% NiO loading at a current density of 10 mA cm− 2.

Fig. 10. Nyquist plots of NiO (a) and NiO/h-MCMB composite (b) with 23 wt.% NiO loading at various voltages between 0.0 V and 0.4 V (–★– 0.0 V; –○– 0.2 V; –●– 0.3 V; –☆– 0.4 V).

NiO is being oxidized its conductivity increases [27]. Thirdly, evidently, the charge transfer resistance is found to change with the potential. The charge transfer resistance (Rct) related to the faradic process decreases when the electrode potential shifts in the anodic direction for both the electrode materials. This demonstrates that the charge transfer takes place at the surface of the electro-active material NiO [28]. Finally, the pseudocapacitance estimated according to the curves of Fig. 8 (a) and (b), which expresses the ability of the electrode material to store charge, is found to turn out to be much better with the applied voltage from 0.0 to 0.4 V. Since long cycle life is of significance for ECs, the cycle charge–discharge test (for continuous 1500 cycles) at 10 mA cm− 2 is employed to examine the service life of the NiO and NiO/h-MCMB composite with 23 wt.% NiO loading electrodes. The SC as a function of cycle number is shown in Fig. 11. For Table 2 The values of Ri for NiO and NiO/h-MCMB composite with 23 wt.% NiO loading Potential/(V) −2

Ri for NiO/(Ω cm ) Ri for NiO/h-MCMB (23 wt.% NiO)/(Ω cm− 2)

0.0

0.2

0.3

0.4

2.32 1.91

2.23 1.88

2.20 1.83

2.15 1.78

the NiO electrode (see curve (a)), after ca. 267 charge/discharge cycles, SC of NiO first reaches ca. 287 from ca. 239 F g− 1, and then decreases to ca. 252 F g− 1. And for the NiO/h-MCMB composite electrode (see curve (b) ) with 23 wt.% NiO loading, after ca. 173 cycles, its SC reaches ca. 600 from ca. 573 F g− 1 and then decrease to ca. 553 F g− 1. Clearly, there is an initial large increase in SC in the first less than ca. 300 charge/ discharge cycles for each electrode, which must result from the activation process of electroactive material NiO, and then the SC decreases slowly, the reason for which may be that the voltage drops during the change of current directions and the degradation of the microstructure of active material in the process of OH-insertion (extraction) during oxidation (reduction) [29]. Moreover, as shown in Fig. 11, SC fades of only about 12 and 8% are observed for NiO and the NiO/h-MCMB composite with 23 wt.% NiO loading electrodes, respectively, from the reached maximal SC to that of the 1500th cycle, which indicates that with the introduction of h-MCMB, the degradation of capacitance of the electroactive NiO turns out to be a little less and demonstrates that the h-MCMB support improves the electrochemical stability of the NiO/h-MCMB composite electrode to a certain extent. So this kind of composite material can be considered as a promising material in the application of supercapacitors. 4. Conclusions NiO/h-MCMB composite materials for ECs were successfully synthesized by thermal decomposition of the precursor via a co-precipitation method in ethanol–water system. The XRD, SEM and HRTEM studies reveal that h-MCMB has been coated with 5–10 nm NiO crystalline. Electrochemical tests demonstrate that the NiO/h-MCMB composites have good electrochemical capacitive performance. Nanosized NiO possesses higher specific area, which enlarges the contact area, makes the best of the electroactive material NiO and enhances the electrochemical reaction rate. And it is much more important that the further introduction of h-MCMB as support increases the conductivity of material, and promotes the dispersion and the utilization of NiO considerably. The maximum SC (CNiO) of

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ca. 637 F g− 1 is obtained for the NiO/h-MCMB composite with 23 wt.% NiO loading at a charge/discharge current density of 6 mA cm− 2 for the first charge/discharge cycle. The NiO/hMCMB composite with 23 wt.% NiO loading owns even better electrochemical stability and power capability compared to the unsupported NiO due to the introduction of the support hMCMB. In view of the low-cost and environmentally benign nature of the composite material, the NiO/h-MCMB (23 wt.% NiO loading) composite electrode is believed to be more promising for large-scale ECs application. Acknowledgements This work was supported by National Basic Research Program of China (973 Program) (No.2007CB209703), National Natural Science Foundation of China (No.20403014, No.20633040) and the Natural Science Foundation of Jiangsu Province (BK2006196). References [1] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. [2] S. Sarangapani, B.V. Tilak, C.P. Chen, J. Electrochem. Soc. 143 (1996) 3791. [3] Y.G. Wang, Y.Y. Xia, Electrochem. Commun. 7 (2005) 1138. [4] S.T. Mayer, R.W. Pekela, J.L. Kaschmitter, J. Electrochem. Soc. 14 (1993) 446. [5] H.S. Kim, N. Branko, B.N. Popov, J. Power Sources 104 (2002) 52. [6] F.B. Zhang, Y.K. Zhou, H.L. Li, Mater. Chem. Phys. 83 (2004) 260. [7] Y.G. Wang, Y.Y. Xia, Electrochim. Acta 51 (2006) 3223.

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