NPC composite nanosheets

Accepted Manuscript Preparation, Characterization and Electrochemical Properties of Porous NiO/ NPC Composite Nanosheets Yanli Tan, Qiuming Gao, Weiqian Tian, Yunlu Zhang, Jiandong Xu PII: DOI: Reference:

S1387-1811(14)00474-0 http://dx.doi.org/10.1016/j.micromeso.2014.08.040 MICMAT 6724

To appear in:

Microporous and Mesoporous Materials

Received Date: Revised Date: Accepted Date:

30 April 2014 18 August 2014 19 August 2014

Please cite this article as: Y. Tan, Q. Gao, W. Tian, Y. Zhang, J. Xu, Preparation, Characterization and Electrochemical Properties of Porous NiO/NPC Composite Nanosheets, Microporous and Mesoporous Materials (2014), doi: http://dx.doi.org/10.1016/j.micromeso.2014.08.040

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Preparation, Characterization and Electrochemical Properties of Porous NiO/NPC Composite Nanosheets

Yanli Tan, Qiuming Gao *, Weiqian Tian, Yunlu Zhang, and Jiandong Xu

Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China.

________________________ *

Corresponding author, Tel: +86 10 82338212; Fax: +86 10 82338212; E-mail:

[email protected]. (Q. M. Gao)

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ABSTRACT: We present a facile combined hydrothermal and calcination method to prepare porous nickel oxide/nanoporous carbon (NiO/NPC) composite nanosheets. The material is characterized by powder X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), X-ray photoelectron spectrometer (XPS), as well as N2 sorption analyses. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests are performed and indicate that the NiO/NPC possesses superior pseudocapacitive performance with the specific capacitance value of 1337 F g-1 at the current density of 2 A g-1 using as the electrode material. The specific capacitance value of 457 A g-1 can be obtained at the high current density of 10 A g-1 showing its high rate performance. The material exhibits good cycling stability by retaining 95% of the max capacitance after 500 cycles of continuous charge-discharge processes. Adding NPC in the synthetic process not only changes the elemental composition, but also leads to more Ni3+ ions and abundant pores in the texture of the NiO/NPC material, which provides high electron conductivity and fast ion transport, enhances the rate performance of the NiO/NPC electrode, and stabilizes the active material. Keywords: porous composite nanosheets; nickel oxide; nanoporous carbon matrix; pseudocapacitor active material.

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1. Introduction The prospects for clean renewables such as solar and wind energies are promising because of the hope for slowing down and finally getting rid of the dependence on the fossil fuels. Those traditional resources not only emit global warming gases, e.g., carbon dioxide causing the climate change but also are faced with the danger of exhaustion due to the non-renewable nature. As important reversible energy conversion and storage devices, supercapacitors have the advantages of high power density, suitable energy density and very long cycle life filling the gap between the traditional rechargeable batteries and conventional solid state/electrolytic capacitors [14]. The supercapacitors can be used as the main or supplementary power sources of the portable electronic devices, hybrid electric vehicles, etc., and even if as the solar and wind energy conversion and storage systems in the future. Based on the electric charge storage mechanism, the supercapacitors can be generally categorized into two types [5]: one is electric double-layer capacitors (EDLCs), commonly using carbon-based materials with high surface area as the electrodes; and the other is pseudocapacitors, utilizing transition metal oxides or conducting polymers as the electrodes [6-10], since they can provide a variety of oxidation states for reversible Faradic reactions. As to the pseudocapacitor electrodes, transition metal oxides have attracted significant research attention in recent years. Among them, nickel oxide (NiO) is dramatically explored due to its environmentally benign character, low cost and highly electroactive properties [11-19]. NiO has a high

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theoretical specific capacitance of 2584 F g-1 [20]. However, the low transfer speeds of electrons and ions in the usual NiO particles lead to the problem of the inefficient active material utilization for achieving the high power density, large energy density and long cycling stability [21]. Preparation of nanosized metal oxide particles is an effective route to cut down the electrical and ionic transfer time because of the short transfer

distance

in the nanostructure.

However,

aggregation as well as

expansion/shrinkage of the crystal lattices of the nanosized metal oxide particles in the charge/discharge processes will reduce the utilization effects as the active materials of the supercapacitors. One of the efficient designs is to fabricate carbon-containing or carbon-coated metal oxide nanocomposites [22, 23]. Recently, Dai et al. reported a specific capacitance of 1335 F g-1 for Ni(OH)2/GS nanoplates at a current density of 2.8 A g-1 [24]. Chen et al. described the hydrothermal synthesis of three-dimensional graphene/Co 3O4 with a specific capacitance of 1100 F g-1 at a current density of 10 A g-1 [25]. These designs have apparent advantages as follows: (i) the nanostructure of the carbon-based composites provides a large surface area, which would improve the utilization of the active materials; (ii) the composite has an enhanced ion conductivity due to the short transfer distances resulted from the small sizes of the nanoparticles favorably accessing to the electrolyte by avoiding the binder, which improves the rate capability; and (iii) the composite shows high capacitive performance owing to the contribution of carbon-based materials compared to pure transition metal oxide. Among various options for carbon-based substrates, nanoporous carbons (NPCs) have

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been taken much more attention due to their tunable microtextures and surface functionalities, high electrical and thermal conductivities, and different forms (powders, fibers, foams and so on) [26]. Herein, we report a facile combined hydrothermal and calcination method to prepare porous NiO/NPC composite nanosheets. The NiO/NPC displays high performance as pseudocapacitor active material. Addition of NPC in the composite preparation process not only changes the pore texture, but also alters the content ratio of Ni3+ over Ni2+ for the NiO/NPC material. 2. Experimental section 2.1 Preparation of the NPC matrix All the chemicals used in the experiments are analytical grade and were used without further purification. Firstly, 1.0 mmol of Al(NO3)·9H2O and 0.5 mmol of 1,4naphthalenedicarboxylic acid were dissolved in 10 mL of deionized water. Then, the mixture was treated by ultrasonic dispersion for 5-10 min under ambient condition. The obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180℃ for 24 h in an electron oven. After that, the autoclave was cooled down naturally to room temperature. The product was collected and washed by centrifugation, followed by vacuum-drying at 60 ℃. The obtained pale yellow powders were put into a ceramic boat (1.5 cm × 3.0 cm × 6.0 cm). The furnace was then heated from room temperature to 800℃ with a heating rate of 2℃ min-1 and maintained for 5 h under a nitrogen gas flow. After that, the

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samples were cooled down naturally to room temperature. In order to remove the aluminum species, the obtained black powders were immersed in 20 vol.% HF under magnetic stirring for 72 h. The black precipitates were collected by centrifugation and washed several times in distilled water. Finally, the black precipitate NPCs were dried under vacuum conditions for 24 h at 60℃. 2.2 Preparation of the NiO sample The NiO sample is synthesized in the system of NiSO4-urea-1,2-propylene glycolH2O under hydrothermal condition and following with thermal post-treatment. In a typical synthesis, 2.0 mmol of NiSO4·6H2O was dissolved in 40 mL of a mixture containing 3.0 mL of 1,2-propylene glycol and 37 mL of deionized water. After stirring for 15 min, a certain amount of urea (0.1-0.2 g) was added into the above solution. The mixture was stirred for another 30 min. The obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 200℃ for 24 h in an electron oven. After that, the autoclave was cooled down naturally to room temperature. The product was collected and washed with deionized water and ethanol for several times by centrifugation, followed by vacuum-drying at 60℃. After calcining the collected precursor at 450℃ in air for 1 h, the NiO sample has been obtained. 2.3 Preparation of the porous NiO/NPC nanocomposite materials The NiO/NPC nanocomposite sample is prepared by calcination of the mixed nickel oxide precursor and NPC matrix with the Ni/C molar ratio of 1.8. The nickel oxide

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precursor was obtained in the system of NiSO4-urea-1,2-propylene glycol-H2O under hydrothermal condition, which is similar to that of the NiO sample without the thermal post-treatment. Typically, 1.8 mmol of the as-prepared nickel oxide precursor and 1.0 mmol of the NPC matrix were grinded homogeneously in an agate mortar. After that, the mixture was calcined at 450℃ in Ar for 1 h to give rise to the growth of the porous NiO/NPC nanocomposite sample. 2.4 Physicochemical characterization Powder X-ray diffraction (XRD) patterns were determined on the instrument of Xray 6000 diffractometor with the 2θ angle region from 10° to 80° at a scan rate of 3° min-1. Scanning electronic microscopy (SEM) measurements were obtained from a JSM-7500F (5 kV) instrument. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) were carried out on a JEM-2100F (JEOL, 200 kV) instrument. N2 adsorption-desorption isotherms were obtained using a NOVA 2200e system. X-ray photoelectron spectrometer (XPS) analyses were performed on a ESCALAB 250 instrument. 2.5 Electrochemical measurements The electrochemical tests were performed in a conventional three-electrode system. The working electrode was consisted of active material, carbon black and polymer binder (polyvinylidene fluoride; PVDF) in a weight ratio of 8:1:1. This mixture was then pressed onto the nickel foam and dried at 80℃ in a vacuum oven. 6 M KOH aqueous solution was chosen as the electrolyte. Platinum foil and Ag/AgCl electrode

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was used as the counter and reference electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests were performed using a CHI660D electrochemical workstation. 3. Results and discussion 3.1 XRD analyses for the NiO/NPC sample XRD patterns of the as-prepared NPC, NiO and NiO/NPC samples are shown in Figure 1. Two apparent XRD peaks with 2θ angles equaling to about 24° and 45° could be found for the NPC matrixes. The broad peak at about 24° can be indexed to the (002) reflection of the graphitic-type lattice and the weak XRD peak at about 45° corresponded to the (10) reflection i.e., a superposition of the (100) and (101) reflections of the graphitic-type lattice represents a certain extent interlayer condensation. The peak with 2θ angle equaling to 37.0, 43.4, 62.8, 75.5 and 79.8° can be clearly observed for the NiO sample, which corresponds to the (111), (200), (220), (311) and (222) plane of the face-centered cubic phase NiO (JCPDS card No. 04-0835), respectively. The (002) reflection peak of the NPC sample could be observed for the NiO/NPC sample, indicating the existence of NPC in the NiO/NPC composite structure. The XRD peaks of the NiO can be observed for the NiO/NPC sample, where the peak positions hardly change and the intensities are lower in a certain extent than that of the NiO. The low XRD intensities of the NiO/NPC sample are possibly due to the more defects existed in the composite structure resulting in the low crystallinity of the NiO. Besides, the NPC in the composite structure may also lead to a certain degree

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reduction of the XRD intensities for the NiO/NPC sample since the amounts of NiO per unit weight or volume in the composite structure become low compared to that of pure NiO. 3.2 SEM and HRTEM analyses for the NiO/NPC sample The morphologies and microstructures of the NPC, NiO and NiO/NPC samples were examined by SEM (Fig. 2). The obtained NPC sample has a stick-like morphology with the length of about 0.5-2 µm and the section width of about 100-300 nm (Fig. 2a). As shown in Figure 2b, the as-prepared NiO presents a typical sheet-like morphology with the length of several micrometers, width of about 0.2-2 µm and thickness of about 10-15 nm. In the case of the NiO/NPC composite, the skeletons of the carbon matrix are uniformly covered by the NiO nanosheets (Fig. 2c). The morphologies of the samples before and after introducing carbon substrate do not change very much. The NiO/NPC maintains the nanosheet array structure, but the surface of the nanosheets becomes a little pyknotic. Further observations for the NPC, NiO and NiO/NPC samples were carried out on TEM and HRTEM with the images shown in Figure 2 d-i. The TEM image (Fig. 2d) indicates that the NPC is porous and the further HRTEM image (Fig. 2g) reveals its mainly amorphous structure characteristic. The rough surfaces with many small pores could be observed for the NiO and NiO/NPC samples (Fig. 2 e and f), where the surface of NiO/NPC is a little rougher than that of the NiO due to the more defects formed in the composite structure. Enlarged HRTEM images of the NiO and NiO/NPC

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are provided in Figure 2 h and i. The pores with the diameters ranging from 6 to 9 nm could be observed for the NiO (Fig. 2h) and much more pores with a little larger diameters of 8-10 nm may be found from the HRTEM image of the NiO/NPC sample (Fig. 2i). Thus, the addition of NPC in the composite may be helpful for the formation of more pores in the NiO of the composite. Additionally, the crystal lattice fringes could be observed clearly for both NiO and NiO/NPC samples. The measured interplanar distance is 0.15 and 0.24 nm for NiO (Fig. 2h), which matches well with the (220) and (111) plane of spinel NiO, respectively. A distinct set of visible lattice fringes with an inter-planar spacing of 0.24 nm may be found for the NiO/NPC, which is in accordance with the (111) plane of spinel NiO. No lattice plane of carbon is observed for the NiO/NPC in consistent with the mainly amorphous nature of the NPC matrix. 3.3 N2 sorption analyses for the NiO/NPC sample The porosities of the NPC, NiO and NiO/NPC were confirmed by the N2 sorption measurements (Fig. 3) with the related textural parameters including BrunauerEmmett-Teller (BET) specific surface areas, total pore volumes and the Barret-JoynerHalenda (BJH) mesopore diameters summarized in Table 1. As shown in Figure 3a, distinct hysteresis loops can be observed in the range of about 0.45-1.0 P/P0, which demonstrate the presence of typical mesoporous structures for the NPC, NiO and NiO/NPC. The BET specific surface area of NiO is calculated to be 126.8 m2 g-1. The carbon matrix NPC has a smaller BET specific surface area of 117.8 m2 g-1 than that of

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the NiO. Thus, in theory, the BET specific surface area of a simple NiO and NPC mixture would be smaller than that of the NiO. Actually, the NiO/NPC composite nanosheets have a higher BET specific surface area of 159.2 m2 g-1 than that of the NiO. The BET surface area values obtained for the NiO/NPC nanosheets in the present study are apparently higher than the typical reported values (Table 2) [15, 27-29]. The high surface areas may be favorable for the high performance of the NiO/NPC using as the electrode material of supercapacitor. Besides, the NiO/NPC composite nanosheets possess higher pore volume and larger average mesopore diameter than those of the NiO. The total pore volume and the BJH average pore diameter of NiO and NiO/NPC are respectively 0.14/0.16 cm3 g-1 and 7.3/8.8 nm based on the N2 desorption curve analyses. The phenomena that the NiO/NPC nanocomposite materials possessing of higher pore volume, higher surface area and larger pore diameter than those of NiO are consistent to the results that there are more pores in the textures of NiO/NPC composite nanosheets from the XRD and HRTEM analyses. The detail pore size distribution curves (Fig. 3b) show that the NiO/NPC sample possesses abundant hierarchical pores with the small pore diameters of about 3 nm and large pore diameters mainly between 10-30 nm, which are a little larger than those of NiO and NPC. The hierarchical nanopores are not only helpful for improving the surface areas of the samples, but also beneficial for the electrolyte ions storage and transfer because of the efficient channels and short distances for the ions transfer, thus may lead to

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enhanced energy and power densities of the NiO/NPC using as the electrode material of supercapacitor. 3.4 XPS analyses for the NiO/NPC sample Further surface information on the elemental compositions, valences and environments of the as-prepared NiO and NiO/NPC samples was obtained by the XPS measurements (Fig. 4). The O 1s spectrum for the NiO sample shows three kinds of oxygen contributions (Fig. 4a): the first peak (O1) at 529.5eV is due to nickel-oxygen bonds, the second one (O2) at 531.4 eV is corresponding to hydroxyls, and the last peak (O3) at 532.9 eV is usually considered to be physi- and chemisorbed water [3033]. The ratio of three corresponding oxygen contributions is 1:0.94:0.04 via the Gaussian fitting method. In the case of Ni 2p XPS spectrum (Fig. 4b), the peaks centered at 850-865 and 870-885 eV with a main peak and a satellite peak are attributed to the Ni 2p3/2 and Ni 2p 1/2 spin orbit levels of NiO [33]. The two spin-orbit doublets are well fitted with Ni2+ (853.9 eV) and Ni3+ (856.0 eV), and two shakeup satellites (identified as “Sat.”). These results show that the chemical composition of the NiO sample contains both Ni2+ and Ni3+ with the molar ratio is 1:0.69 by using the Gaussian fitting method [34]. As to the NiO/NPC composite sample, the XPS sharp peak located at 284.9, 531.3 and 854.7 eV is assigned to the characteristic peak of C 1s, O 1s and Ni 2p, respectively (shown in Fig. 4c), indicating the existence of carbon, oxygen and nickel element on the sample surfaces of the NiO/NPC. In the C 1s spectrum (Fig. 4d) of the NiO/NPC composite sample shows three different kinds of

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peak at 284.6, 285.7 and 289.4 eV, corresponding to nonoxygenated carbon atoms (CC/C=C), carbon atoms in hydroxyl groups (C-OH/C-ONi) and carbon in carboxyl groups (HO-C=O), respectively [35]. The high-resolution spectrum for the O 1s region (Fig. 4e) shows three kinds of oxygen contributions. Specifically, the peak (O1) at 529.5 eV is related to typical nickel-oxygen bonds [30]. The peak (O2) sitting at 531.4 eV is usually associated with defects and a number of surface species including hydroxyls, chemisorbed oxygen, under-coordinated lattice oxygen, or species intrinsic to the surface of the spinel [31, 32]. The peak (O3) at 532.9 eV can be attributed to physi- and chemisorbed water [30]. Likewise, the molar ratio of the three kinds of corresponding oxygen contributions is 1:1.86:0.09 by using the Gaussian fitting method. Apparently, the peaks associated with defects (O2 and O3) of the NiO/NPC are much stronger than that of the NiO, which demonstrate more defects exist in the NiO/NPC composite sample than that of the NiO. The Gaussian fit to the Ni 2p XPS spectrum for the NiO/NPC (Fig. 4f) shows that the chemical composition of the NiO/NPC composite sample also contains Ni2+ and Ni3+ [34] with the ratio of 1:0.87. The result shows that the NiO/NPC composite sample contains more Ni3+ than that of the NiO, which further demonstrates there are more defects in the structure of NiO/NPC nanosheets. The XPS results are well in agreement with the above described structure characterization of the NiO and NiO/NPC samples. The total atomic of Ni and C elements is 1.85:1 (Table 3) based on the XPS measurements, corresponding to the ratio of the original devoted nickel oxide and carbon matrix. So, addition of the

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NPC in the composite not only change the elemental composition, but also lead to more Ni3+ ions and abundant pores in the texture of the NiO/NPC composite nanosheets. 3.5 CV analyses of the NiO/NPC material The electrochemical properties of the as-prepared NPC, NiO and NiO/NPC samples were studied as the electrode materials for supercapacitors. The CV curves were conducted at various scan rates of 1-200 mV s-1 in the voltage range from 0 to 0.45 V in 6 M KOH aqueous electrolyte. The CV curves with nearly symmetric rectangular shapes (Fig. 5a) were observed for the NPC electrode due to the electric double-layer formation. Figure 5 b and c shows the typical CV curve of the NiO and NiO/NPC electrode, respectively. A distinct pair of current peaks can be clearly identified from the CV curves during the cathodic and anodic sweeps [36]. The peak intensities improve leading to an enlarged area of the CV curve and a small distortion of the CV shape with increasing the scan rates. The shape of the CV curves possessing of the redox peaks clearly reveals the pseudocapacitive characteristic of the NiO and NiO/NPC electrodes. The processes are mainly associated with the reversible Faradic redox reaction: NiO + OH- ←→ NiOOH + e. Variation of the specific capacitance values as function of scan rates for the supercapacitors is shown in Figure 5d. The max specific capacitance value for the NPC, NiO and NiO/NPC sample is 22.8, 831 and 1220 F g-1, respectively, at the scan rate of 1 mV s-1. The capacitance of the NiO/NPC composite material is evidently larger than that of the pure NPC and NiO.

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Additionally, the specific capacitance of the NiO/NPC sample is as high as 155 F g-1 even at the scan rate of 200 mV s-1, which is higher than that of NiO (98.7 F g-1) and NPC (5.2 F g-1). These results are possibly due to the more defects, abundant pore textures, stable microstructures, as well as the improved conductivity of the composite structure confirmed by the following electrochemical impedance spectroscopy analyses, indicating the apparent advantage of the composite structure [24, 25]. It should be mentioned that the NiO/NPC composite nanosheets show twice to three times of many other reported NiO materials at the high scan rates measured by the similar CV method in a three-electrode system [14, 15, 19, 37-41]. With increasing the scan rates the specific capacitance of the electrodes decreases. This phenomenon is due to the fact that higher scan rate prevents the accessibility of ions to the pores of the electrodes, as well as the movement of ions is limited due to their slow diffusion and only the outer surface may be useful for charge storage at the higher scan rate [42]. 3.6 Charge-discharge performance analyses for the NiO/NPC material The charge-discharge performances of the NPC, NiO and NiO/NPC were measured at different current densities (2-10 A g-1) with the voltage window of 0-0.4 V. The obvious linear charge-discharge curves in Figure 6a for the NPC electrode are consistent with the typical EDLC characteristic. As to the NiO and NiO/NPC electrodes, it can be observed that there are voltage plateaus at around 0.2 V (Fig. 6 b and c) due to the Faradic reaction. This phenomenon demonstrates that the NiO and NiO/NPC samples reveal the pseudocapacitance performance, which is also similar to

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a battery-like material used for the nickel-based batteries [43]. The specific capacitance can be directly calculated based on the following equation [27]: C = I·∆t / (m·∆V), where C (F g-1) is the specific capacitance, I (A) is the discharge current, ∆t (s) is the discharge time, ∆V (V) is the voltage range and m (g) is the mass of the active material (the total mass of the NiO and carbon substrate). Thus, the specific capacitance of NiO can be calculated to be 847, 779, 526, 434 and 417 F g-1 at the different current density of 2, 4, 5, 8 and 10 A g-1, respectively. As for the NiO/NPC material, the specific capacitance is 1337, 1265, 778, 662 and 457 F g-1, respectively, at the corresponding current density. The capacitance of the NiO/NPC composite material (1337 F g-1) is parallel to that of the Ni(OH)2/GS (1335 F g-1) [24] at the low current density and much higher than that of the reported NiO-based materials at the high current density (Table 2) [14, 15, 28, 29, 37-41, 44, 45]. One thing to point out is that our facile hydrothermal and calcination two-step method as well as the low cost nickel raw material for the NiO/NPC sample may be more competitive than that of the graphene/Co 3O4 based on the chemical vapor deposition (CVD) technique using nickel foam as the substrate and the expensive cobalt resources [25], even though the specific capacitance of the graphene/Co3O4 composite electrode is higher (~768 A g-1) than that of the NiO/NPC (457 A g-1) at the high current density of 10 A g-1. Furthermore, the Coulombic efficiency of the NiO and NiO/NPC can reach as high as 95% and 84%, respectively. This result may arise from the high reversible redox reactions of the NiO and NiO/NPC electrodes in the charge-discharge processes. From the CV and charge-

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discharge curves, it can be apparently seen that the electrochemical performance of the NiO/NPC is superior to that of the NiO. In consideration of the similar topography and structure of the two samples, this may result from the larger surface area and higher conductivity of the NiO/NPC electrode. This also suggests a good rate capacity of the NiO/NPC electrode. The Ragone plots of the NPC, NiO and NiO/NPC samples are displayed in Figure 6d. At a low power density of 380 W kg-1, the energy density obtained for the NPC, NiO and NiO/NPC electrode is 1.06, 16.99 and 26.81 Wh kg-1, respectively. At a much higher power density of 1100 W kg-1, the energy density may arrive respectively at 1.03, 10.55 and 14.79 Wh kg-1 for the NPC, NiO and NiO/NPC electrode. Furthermore, the energy densities are higher than those reported carbon or nickel oxide based supercapacitors [46, 47]. These results indicate that the NiO/NPC is a very promising electrode material for the high performance supercapacitors. 3.7 Electrochemical impedance spectroscopy analyses of the NiO/NPC material Electrochemical impedance spectroscopy (EIS) was applied to investigate the electrical conductivity and ion transfer of the supercapacitors. The Nyquist plots of the NPC, NiO and NiO/NPC electrodes determined at 0.2 V (vs. Ag/AgCl) with frequency range from 100 kHz to 0.01 Hz are shown in Figure 7. All the impedance spectra are composed of a semicircle at high frequency and an oblique line at low frequency, which indicates the typical capacitor behavior [48]. It is generally believed that the frequency plots are composed of three distinct regions [49]: the internal resistance (Rs),

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the charge transfer resistance (Rct) and the diffusive resistance (Warburg impendence, W). The impedance data are stimulated by an equivalent circuit and shown in the inset of Figure 7. The magnitudes of Rs obtained for the NPC, NiO and NiO/NPC are around 0.5 Ω. Nevertheless, the electrode Rs of the NiO/NPC is lower than that of the NiO due to the addition of NPC possessing of the lowest Rs among them. The lower values indicate consistent interfacial contact between the nickel oxide nanosheet and carbon matrix. The NiO/NPC has a smaller Rct value (~0.3 Ω) than that of the NiO (~0.5 Ω) because of the addition of NPC with the lowest Rct (~0.2 Ω), which demonstrates that the NiO/NPC nanosheets provide a better pathway for ion and electron transport to the electrode-electrolyte interface due to the abundant pore structure and more Ni3+ ions in the composite. Furthermore, the slopping line of the NiO/NPC is more perpendicular to the real axis than that of the NiO caused by the addition of NPC whose slopping line is the most perpendicular to the real axis among them, which indicates that the Warburg resistance of the NiO/NPC is lower than that of the NiO. This result demonstrates that the NiO/NPC is a potential active electrode with enhanced electron conductivity due to the addition of the NPC matrix. 3.8 Rate performance and cycle life analyses of the NiO/NPC material To better understand the electrochemical behavior of the material, we further test their rate performance of the NPC, NiO and NiO/NPC samples with the electrodes cycled at various scan rates of 2-10 A g-1 as shown in Figure 8. The NiO/NPC shows an excellent rate capability with an average specific capacity of 1331, 1247, 751, 626,

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and 421 F g-1, when the current density increases stepwise to 2, 4, 5, 8, and 10 A g-1, respectively. When the current density comes back to 2 A g-1, an average specific capacity of 1251 F g-1 (about 94% of the initial) could be recovered for the NiO/NPC. The average specific capacities of NiO are lower than that of the NiO/NPC at the current density increased from 2 A g-1 stepwise to 4, 5, 8, and 10 A g-1. The average rate capacity of 724 F g-1 (86% of the initial) was obtained for the NiO electrode, when the current density turns back to 2 A g-1. The rate performance of NPC is high with the 99% of the initial specific capacitance preserved after even turns back to 2 A g-1, but its average specific capacities are much lower than that of NiO and NiO/NPC. So, addition of the NPC may remarkably enhance the electrochemical rate performance without apparent improvement of the specific capacity for the NiO/NPC composite material. The cycle life is an important factor for evaluating the supercapacitors, so we tested the long term charge-discharge properties of the NPC, NiO and NiO/NPC samples at a current density of 4 A g-1. The NiO/NPC composite nanosheets exhibit an excellent cycling stability by retaining 95% of its max capacitance after 500 cycles of continuous charge-discharge processes, which is better than the NiO electrode persisting of 90% of its max capacitance after 500 cycles. The cyclic stability of NPC is high with the 99% of the initial specific capacitance preserved after 500 cycles due to the mainly physical electric charge accumulate mechanism of EDLC. These results demonstrate that the NiO/NPC composite electrode gives excellent cycling stability

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due to the addition of NPC, promising for the development of high-performance supercapactiors. 4. Conclusions In summary, we have adopted a facile combined hydrothermal and calcination process to successfully prepare the NiO/NPC nanocomposite material using the nanoporous carbon NPC as the supports. More defects may be existed in the composite structure, where the NiO is well crystallized with more Ni3+ than that of the pure NiO. There are hierarchical pores in the NiO/NPC composite nanosheets with high BET specific surface area of 159.2 m2 g-1, total pore volume of 0.16 cm3 g-1 and BJH average pore diameter of 8.8 nm. Using NiO/NPC as the electrode material of supercapacitor, the hierarchical pores in the nanosheets may be beneficial for the electrolyte ions storage and transfer based on the efficient channels and short distances for the ions transfer. The NiO/NPC has a smaller Rct value (~0.3 Ω) than that of the NiO (~0.5 Ω), which demonstrates that the NiO/NPC nanosheets provide a better pathway for ion and electron transport to the electrode-electrolyte interface when utilizing as the electrode material of supercapacitor. The NiO/NPC electrode affords a high specific capacitance of 1337 F g-1 at the current density of 2 A g-1 and an excellent rate performance of 457 A g-1 at 10 A g-1. The carbon matrix NPC may stabilize the electrochemical active component NiO in the composite material. The NiO/NPC shows good cycling stability and about 95% of the initial capacitance can be retained after 500 cycles of continuous charge-discharge processes. The above results

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are helpful for deeply understanding the effects of NPC for preparation of porous transition metal oxide (TMO) and NPC nanocomposite materials. Further designing and constructing porous TMO/NPC nanocomposite materials with optimized element compositions and valences, high pore surface areas, large pore volumes as well as suitable pore diameters will be promising for the high-performance supercapacitors. Acknowledgments This work is supported by National Basic Research Programs of China (973 Programs, No. 2011CB935700 and 2014CB931800), Chinese Aeronautic Project (No. 2013ZF51069) and Chinese National Science Foundation (No. U0734002).

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26

Figure Captions Figure 1. XRD patterns of the NPC, NiO and NiO/NPC samples. Figure 2. SEM (a, b and c); TEM (d, e, and f) and HRTEM (g, h, and i) images of the NPC, NiO and NiO/NPC samples, respectively. The representative crystal lattice fringes in the HRTEM images (h and j) were highlighted by the black lines with the interplanar distance indicated by the arrows. The typical pores were marked by the yellow dot circles for clarity. Figure 3. N2 adsorption-desorption isotherms (a) and pore-size distribution curves (b) of the NPC, NiO and NiO/NPC samples. Figure 4. XPS spectra of (a) O 1s and (b) Ni 2p for the NiO sample; as well as (c) survey spectrum, (d) C 1s, (e) O 1s and (f) Ni 2p for the NiO/NPC sample. Figure 5. CV curves of (a) the NPC, (b) the NiO and (c) NiO/NPC samples at the scan rates of 1, 2, 5, 10, 20, 50, 100 and 200 mV s-1, respectively; and (c) the average specific capacitance at the various scan rates. Figure 6. Galvanostatic charge-discharge curves of (a) the NPC, (b) the NiO and (c) NiO/NPC samples at the current densities of 2, 4, 5, 8 and 10 A g-1; as well as (d) the Ragone plot (energy density vs power density) of the NPC, NiO and NiO/NPC samples. Figure 7. The Nyquist plots for the supercapacitors based on the NPC, NiO and NiO/NPC samples with the inserts of the enlarged Nyquist plots and an equivalent circuit.

27

Figure 8. Rate capability tests (a) at various current densities (2-10 A g-1) and comparative cycling performances (b) of the NPC, NiO and NiO/NPC samples at the same current density of 4 A g-1.

28

(200)

0

1200 600 0 1800

(311) (222)

NiO (220)

(111)

1800

(002)

NPC

1200 600 0

(10)

Intensity (cps)

300

(311) (222)

(200)

NiO/NPC (220)

600

(002)

900

(111)

1200

10 20 30 40 50 60 70 80 o

2θ ( )

Figure 1

29

Figure 2

30

b

a

0.21

NiO/NPC

NiO/NPC

120

0.14

-1

0 120

dV(logd) (cm g )

0.07

60

0

NPC

120

0.15

NiO

0.10 0.05 0.00 NPC

0.6 0.4

60 0 0.0

0.00

3

NiO

3

-1

Volume (cm g )

60

0.2 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

0

10

20

30

40

50

60

Pore Diameter (nm)

Figure 3

31

Intensity (a. u.)

a

O 1s

O1 529.5 531.4 O2

532.9 O3

515

520

525

530

535

540

545

550

Biding Energy (eV)

Intensity (a. u.)

b 2+

Ni

Ni 2p

Ni 2p3/2

853.9

Sat.

Ni 2p1/2 Sat.

872.6 3+

Ni

840

856.0

850

860

870

880

890

Binding Energy (eV)

32

c Intensity (a.u.)

Ni 2p

O 1s C 1s

0

200

400

600

800

1000

Binding Energy (eV)

Intensity (a. u.)

d

C 1s

C-C 284.6 285.7 C-OH/C-ONi 289.4 HO-C=O

275

280

285

290

295

300

Binding Energy (eV)

33

e Intensity (a. u.)

O 1s

531.4 O2 O1 529.5

533.0 O3

520

525

530

535

540

545

Binding Energy (eV)

Intensity (a. u.)

f

Ni 2p3/2

Ni 2p

Sat. 2+

Ni

Ni 2p1/2 Sat.

853.9 872.6

3+

Ni

840

856.0

850

860

870

880

Binding Energy (eV)

Figure 4

34

1.5

a

-1

200 mV s

-1

Current (A g )

1.0 0.5 0.0

-1

1 mV s -0.5 -1.0 -1.5 0.0

0.1

0.2

0.3

0.4

0.5

Potential (V vs Ag/AgCl)

30

b

-1

200 mV s

-1

Current (A g )

20 10 0

-1

1 mV s

-10 -20 0.0

0.1

0.2

0.3

0.4

0.5

Potential (V vs Ag/AgCl)

35

45

c

-1

200 mV s

-1

Current (A g )

30 15 0

-1

1 mV s

-15 -30 0.0

0.1

0.2

0.3

0.4

0.5

Potential (V vs Ag/AgCl)

-1

Specific Capacitance (F g )

1500

d

NiO/NPC NiO NPC

1200 900 600 300 0 0

50

100

150

200

-1

Scan Rate (mV s ) Figure 5

36

0.4

a

Voltage (V)

0.3

0.2 -1

10 A g 0.1

-1

8Ag

-1

5Ag

-1

-1

4Ag

0.0 0

1

2

3

4

2Ag 5

6

7

8

Time (s)

0.4

b

Voltage (V)

0.3

0.2 -1

10 A g 0.1

-1

8Ag

-1

5Ag

0.0 0

50

100

-1

-1

4Ag 150

200

2Ag 250

300

350

400

Time (s)

37

c

0.4

Voltage (V)

0.3

0.2 -1

10 A g 0.1

-1

8Ag

-1

5Ag

0.0 0

100

-1

-1

4Ag

200

300

2Ag 400

500

600

Time (s)

2

d

-1

Energy Density (Wh kg )

10

NiO/NPC NiO NPC

1

10

0

10

3

10

4

-1

10

Power Density (W kg )

Figure 6

38

Figure 7

39

-1

Specific Capacitance (F g )

2000

a

NiO/NPC NiO NPC

1500 1000 500 40 30

-1

2Ag

20

-1

4Ag

-1

5Ag

-1

-1 8 A g 10 A g-1 2 A g

10 0 0

50

100

150

200

250

300

-1

Specific Capacitance (F g )

Cycle Number

2000

b

1500 NiO/NPC 1000 500 NiO 40 30

NPC

20 10 0 0

100

200

300

400

500

Cycle Number

Figure 8

40

Table 1. Textural parameters of the obtained NPC, NiO and NiO/NPC samples. SBET

Vt

DBJH

[m2 g-1] a)

[cm3 g-1] b)

[nm] c)

NPC

117.8

0.21

6.9

NiO

126.8

0.14

7.3

NiO/NPC

159.2

0.16

8.8

Samples

a)

SBET represented the BET surface area; b) Vt represented the total pore volume; and

c)

DBJH standed for the BJH desorption average pore diameter.

41

Table 2. Electrochemical characteristics and surface areas of the nickel oxides and the related nanocomposites. Material

Surface area

Scan rate

Current density

Capacitance

Electrolyte

Reference

[m2 g-1]

[mV s-1]

[A g-1]

[F g-1]

NiO

72.6

50

1-10

340-121

2 M KOH

15

NiO

95.5

1-50

2-10

735-183

2 M KOH

27

NiO

-

5-50

0.5-5

480-252

2 M KOH

14

NiO

-

5-50

0.5-10

401-100

2 M KOH

19

NiO

-

2.5-10

0.005-0.03

942-613

2 M KOH

43

NiO

47

5

1-5

128-117

2 M KOH

28

NiO

95

10

1-10

286-193

6 M KOH

29

NiO

141

2.5-10

5

400

2 M KOH

35

NiO

216

2-10

1-4

710-500

6 M KOH

42

NiO/C

-

10-50

-

74

0.1 M NaSO4

36

NiO/MWNT

228

50

0.1-4

206

2 M KOH

37

NiO/carbon fabric -

1-10

3

230

3 M KOH

38

NiO/CNT

186

5-30

0.3-2

280-89

1 M KOH

39

NiO

126.8

1-200

2-10

847-417

6 M KOH

Our work

NiO/NPC

159.2

1-200

2-10

1337-457

6M KOH

Our work

Table 3. The molar ratios of the three kinds of oxygen contributions and Ni2+/Ni3+ of the NiO and NiO/NPC by the Gaussian fitting method, as well as the Ni/C of NiO/NPC composite material based on the XPS analyses. Sample

O1/O2/O3a)

Ni2+/Ni3+

Ni/C

NiO

1:0.94:0.04

1:0.69

-

NiO/NPC

1:1.86:0.09

1:0.87

a)

1.85:1

O1: the peak at 529.5 eV; O2: the peak at 531.4 eV; and O3: the peak at 532.9 eV.

44

Graphical abstract

45

HIGHTLIGHTS 1) Crystallized porous NiO/NPC nanocomposite materials were successfully synthesized. 2) The NiO/NPC has high specific surface, large pore volumes and suitable pore sizes. 3) Stable high capacitance and rate performance were found for the NiO/NPC materials. 4) The effect of nanoporous carbon was studied deeply for preparation of the NiO/NPC. 5) The structure of NiO/NPC may be adjusted by adding the NPC in the synthetic process.

46