Effects of highly crumpled graphene nanosheets on the electrochemical performances of pseudocapacitor electrode materials

Electrochimica Acta 133 (2014) 180–187

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Effects of highly crumpled graphene nanosheets on the electrochemical performances of pseudocapacitor electrode materials Cuihua An, Yijing Wang ∗ , Li Li, Fangyuan Qiu, Yanan Xu, Changchang Xu, Yanan Huang, Lifang Jiao Huatang Yuan Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Lab on Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, PR China

a r t i c l e

i n f o

Article history: Received 11 March 2014 Received in revised form 7 April 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Graphene sheets Nickel Phosphide Composite Electrochemical performances Effects of graphene sheets

a b s t r a c t Highly crumpled graphene nanosheets (GS) with a BET surface area as high as 722.1 m2 g−1 is prepared by a thermal exfoliation method. A systematical investigation is performed on the electrochemical performances of porous Ni2 P/GS nanocomposite synthesized by the low temperature solid state reaction method. It is shown that Ni2 P nanoparticles anchor on graphene sheets producing a porous structure and GS have beneficial effects on the electrochemical properties of Ni2 P nanoparticles when investigated as pseudocapacitor materials. The novel Ni2 P/GS composite electrode exhibits a superior specific capacitance of 1912 F g−1 and 888 F g−1 at current densities of 5 and 50 mA cm−2 , respectively. The specific capacitance decreases to 1473 F g−1 after 2500 cycles, suggesting its excellent cycling stability. It is confirmed that the GS with good electrical conductivity serve as a conducting network for fast electron transfer between the active materials and charge collector, as well as buffered spaces to accommodate the volume expansion/contraction during cycling, thus, leading to enhancement of the electrochemical properties of the Ni2 P electrode. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction High-performance, lightweight and environmentally friendly energy storage devices, such as supercapacitors, are urgently needed for sustainable and renewable power sources in modern electronic industry [1–3]. With the features of high power density than batteries, and higher energy density than conventional dielectric capacitors, supercapacitors have gained rapidly increasing attention for next-generation power devices [4–6]. Supercapacitors as energy storage devices can be used as a backup power source in portable electronic devices, pacemakers, hybrid electrical vehicles and so on. Supercapacitor commonly store energy based on either ion adsorption (electrochemical double layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors) [7–9]. This kind of carbon material which is regarded as typical EDLC supercapacitors possesses a long cycle life and good mechanical properties. However, the low specific capacitance of EDLCs can’t meet the evergrowing need for the electric vehicles [10–15]. Therefore, growing interest in using pseudocapacitive materials for supercapacitors has been triggered because the energy density associated with

∗ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.electacta.2014.04.056 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

Faradaic reactions is substantially larger by at least one order of magnitude than that of EDLCs [16–20]. It is noted that transition metal phosphide have metalloid properties, superior electrical conductivity and high theoretical capacity value (Ni2 P: ca. 1886 F g−1 ) [21–23]. Unfortunately, the transition metal phosphide is limited by poor cyclic performance due to the structural degradation of the electrode through the redox process. It is well known that carbon material have beneficial effects on the electrochemical performances of the electrode. Graphene, mother of all graphitic forms, has received tremendous attentions due to its notable physical and chemical properties [24–28]. Owing to its large surface area and superior electronic, thermal, mechanical properties, graphene has provided many promising applications in energy storage, catalysis and so on [25,27,29–31]. Recently, the pseudocapacitor active materials, which combined with the GS have been reported to display high specific capacitance and excellent cyclic stability compared to naked particles [32–35]. The GS in the above structure acted as not only a buffer zone of volume change of the active materials but also a good electron transfer medium. Therefore, with the help of GS, the activity and cycling performance of the pseudocapacitor material Ni2 P may be enhanced. An advanced approach is to hybridize the electrode materials by adding electrochemical active materials Ni2 P to conductive GS matrix. The Ni2 P/GS composites not only increase the utilization of

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the active material, but also improve the electrical conductivity and mechanical strength of the composite materials. Hence, we report a simple route to prepare a novel Ni2 P/GS composite electrode with high specific capacitance. A specific capacitance as high as 1912 F g−1 is obtained for the novel Ni2 P/GS composite which could be identified as a promising electrode material for electrochemical capacitors. 2. Experimental Section 2.1. Synthesis The nickel foam was purchased from Shanghai Zhongwei New Material Co., Ltd. The pore size of the nickel foam ranges from 0.1 mm to 10 mm and its volume density is 0.1 to 0.8 g cm−3 . All chemicals were of analytical grade and used without further purification. GS was made via a modified Hummers method followed by a thermal exfoliation (at 800 ◦ C for 1 min under a H2 /Ar atmosphere with 10 ◦ C/min heating rate). In a typical synthesis process, 5 mg GS and 0.3 g NiCl2 ·6H2 O were dispersed in 20 ml distilled water and subjected to ultrasonic vibration to form a homogeneous suspension, respectively. The two former suspensions were homogeneously mixed with each other and subjected to ultrasonic vibration for a while, and then freeze drying (named as M1). The 1.03 g NaH2 PO2 ·H2 O was grinded in a mortar and mixed with M1. Then, the mixture was calcined at 500 ◦ C for 1 h with 2 ◦ C/min heating rate and cooled to room temperature under a flow of Ar (99.999%). The solid obtained was washed thoroughly with distilled water and absolute ethyl alcohol to remove the by-products. After that, the wet products were dried at 80 ◦ C for 12 h in a vacuum oven. Pure Ni2 P nanoparticles were synthesized by the same method as described above to make Ni2 P/GS composite, except that there was no graphene sheets involved. 2.2. Characterization The crystal structure and surface configuration of samples was determined by powder X-ray diffraction (XRD,Rigaku D/Max-2500, Cu K␣ radiation), transmission electron microscope (TEM), highresolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) on a JEOL JEM-2100 TEM. The carbon contents of the Ni2 P/GS composite were conducted by Element Analysis (vario EL CUBE). The electronic states of the as-prepared products were investigated by X-ray photoelectron spectroscopy (XPS, PHI5000VersaProbe). The specific surface areas and porous nature of the GS and Ni2 P/GS composites were further investigated by nitrogen adsorption/desorption measurements on a NOVA 2200e analyzer. 2.3. Electrochemical measurements For electrochemical measurements, the working electrodes contained ≈3 mg active materials, which were constructed by mixing the active material, acetylene black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10. This mixture was then pressed onto the nickel foam electrode (1.0 cm × 1.0 cm), and dried under vacuum at 80 ◦ C for 10 h. Then, electrochemical measurements were conducted in a three compartment cell using a LAND battery test instrument (CT2001A). A nickel foam and Hg/HgO reference electrode were used as a counter electrode and reference electrode, respectively. The electrolyte was a 2 M KOH aqueous solution. Cyclic voltammetry (CV) was conducted by a Zahner IM6e electrochemical workstation with voltage scan rates of 1 mV s−1 , 5 mV s−1 , 10 mV s−1 , 20 mV s−1 and 50 mV s−1 . The galvanostatic charge-discharge tests were conducted at the current

Scheme 1. Illustration of the process for the synthesis of Ni2 P/GS composite.

of 5 mA cm−2 , 10 mA cm−2 , 20 mA cm−2 and 50 mA cm−2 . Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an AC voltage with 5 mV amplitude in a frequency range from 0.1 Hz to 100 Hz at open circuit potential. 3. Results and Discussion 3.1. Characterization of Ni2 P/GS composite The novel Ni2 P/GS composite was obtained by a heat treatment of a mixed salt precursor (nickel chloride and sodium hypophosphite) and GS in Ar atmosphere. The formation mechanism of Ni2 P/GS composite is illustrated in Scheme 1. A suspension of GS in distilled water was ultrasonicated for 2 h followed by adding another 20 mL distilled water solution of NiCl2 ·6H2 O. The mixture was ultrasonicated for another 0.5 h to complete ion exchange. In the ultrasonic process, it is possible that the GS was exfoliated in distilled water solution to form well-dispersed GS, and Ni2+ cations were attracted to the GS via electrostatic attraction. Ni2+ attracted onto GS in distilled water facilitates the reduction of nickel ions on GS by sodium hypophosphite in the subsequent heat treatment step. The reaction of Ni2+ attracted onto GS reduced by sodium hypophosphite can be expressed as follows equ. [1]: 6Ni2+ + 8H2 PO2− + 3GS → 3Ni2 P/GS + P + 4H3 PO4 + 4H + [1] Fig. 1a shows the XRD patterns of pure Ni2 P and the Ni2 P/GS composite. All the diffraction peaks of the sample without GS can be ascribed to Ni2 P (JCPDS 65-3544). Compared to the characteristic peaks of the pure Ni2 P, an additional small and low broad diffraction peak appears at around 25 o (inset in Fig. 1a), corresponding to the (002) diffraction peak of the disorderedly stacked GS. These results suggest that the hybride material is composed of GS and Ni2 P. And the calculated lattice parameters of Ni2 P in the Ni2 P/GS composite ˚ c = 3.368 A, ˚ in good agreement with the literature are a = b = 5.857 A, values (JCPDS 65-3544). The grain sizes of pure Ni2 P and Ni2 P/GS were 45.2 nm and 35.8 nm estimated by Scherrer equation according to the diffraction peaks. It is well accepted that the decreased grain size should benefit the diffusion of ions and electrons, thus, leading to an enhanced electrochemical performance. The GS content in the as-prepared Ni2 P/GS composite was characterized by Element Analysis (EA) and the amount of GS is 10.4%. The morphology and structure of the GS and Ni2 P/GS composite were examined by SEM, TEM and HRTEM, as shown in Fig. 1. The SEM and TEM images (Fig. 1b, d) of GS show many crumples exist while maintaining the 2D sheet-like structure. For the Ni2 P/GS composite, the Ni2 P nanoparticles densely grew onto the graphene sheets (see Fig. 1c). It is believed that between the Ni2 P and GS, both chemisorptions and van der Waals interactions exist at many defective sites and pristine regions of the GS. In addition, the defectively hinder diffusion, recrystallization and the growth of Ni2 P grains, resulting in smaller sizes of Ni2 P nanoparticles compared with the bare ones. And the TEM image shows that most of Ni2 P particles have a diameter of about 40 nm (Fig. 1e). The presence of even smaller Ni2 P particles with size of about 10 nm can also be observed from the TEM images

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Fig. 1. (a) XRD patterns of the pure Ni2 P, Ni2 P/GS composite and the standard card JCPDS 65-3544, (b, c) SEM images of GS and Ni2 P/GS composite, (d, e) TEM images of GS and Ni2 P/GS composite, the HRTEM image (f) and the SAED pattern (inset in f) of the Ni2 P/GS composite.

(the area circled in Fig. 1e). It seems that the GS is quite clear, indicating that there is no uncontrolled resembling on the GS. The Ni2 P nanoparticles on the surface of GS acting as spacer ensure to efficiently prevent the closely restacking of sheets, avoiding the loss of high active surface. The HRTEM image (Fig. 1f) displays that the Ni2 P nanoparticles exhibit high crystallinity with a lattice spacing of 0.52 nm corresponding to the interspacing of the (111) planes. The SAED pattern in the inset reveals well-crystallinity of the

composite, and all the diffraction points can be ascribed to the Ni2 P phase. To obtain more detailed information about element makeup and valence of the as-prepared Ni2 P/GS composite, XPS measurements were performed and the corresponding results are presents in Fig. 2a. The Ni 2p spectrum was best fitted (Fig. 2b). It can be assigned to Ni (852.74 eV and 855.86 eV) [36–38]. Furthermore, the fitted result of the P 2p XPS spectrum is shown in Fig. 2c. The P 2p

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Fig. 2. The XPS spectra (a), XPS spectra of Ni2P (b), P2P (c) of Ni2 P/GS composite, C1s (d) of GS and Ni2 P/GS composite.

peaks (133.04 eV and 129.19 eV) of the Ni2 P/GS composite correspond to phosphate and P simple substrate, respectively [37,39]. And the binding energy of 133.04 eV can be associated with phosphorous in phosphates. The C 1s XPS spectrum of the as-prepared Ni2 P/GS composite is illustrated in Fig. 2d. The spectrum contained one peak corresponding to the sp2 -hybridized C-C. The oxygenated functional groups (C-O and C = O) have been not found in the fitted spectrum which indicates the thorough reduction of the obtained material [40,41]. Obviously, the XPS data also confirms the successful synthesis of Ni2 P/GS composite. The specific surface areas and porous nature of the as-prepared GS and Ni2 P/GS composite were further investigated by nitrogen adsorption/desorption measurements, as indicated in Fig. 3. Brunauer-Emmett-Teller (BET) specific surface areas of GS are 722.1 m2 g−1 and the nitrogen adsorption and desorption isotherm curves exhibit a typical IV type isotherm with a distinct hysteresis at a relative P/P0 ranging from 0.5 to 1, and the shape of the isotherm indicates that the material contains mesopores. The GS exhibits an average pore volume of 3.0 cm3 g−1 . It is worth noting that the surface area of the thermally reduced GS in our work is larger than that of the GS prepared by the chemically reduced method (466 m2 g−1 ) [42]. It is attributed to the fact that the decomposition of the oxygenated functional groups will release a large amount of gas, thus forming many mesopores and crumple voids. The - stacking of graphene sheets during thermal exfoliation have been alleviated which also make a contribution to the high surface area of GS. Surface area measurement of the as-prepared Ni2 P/GS composite via nitrogen absorption-desorption gave a BET value of 61.3 m2 g−1 (Fig. 3b). These features are of huge benefits for the transport and diffusion of electrolyte ions during the charge-discharge process in supercapacitors, resulting in greatly enhanced capacitance.

3.2. Electrochemical performances The electrochemical properties of the Ni2 P/GS composite were evaluated as electrode for supercapacitors. Fig. 4a shows the typical cyclic voltammetry (CV) curves of the Ni2 P/GS composites with various sweep rates ranging from 1 to 50 mV s−1 . The shape of

the CV curves clearly reveals the pseudocapacitive characteristics. Specifically, a pair of redox peaks can be observed in all sweep rates. According to our previous report, the electrochemical reaction mechanism is the transformation between Ni(OH)2 and NiOOH [23] which is listed below. The anodic peak of CV curve is characteristic of the oxidation of Ni(OH)2 to NiOOH, and the cathodic peak corresponds to the reverse process. Ni2 + 2OH − → Ni(OH)2 Ni(OH)2 + OH −

Charge



NiOOH + H2 O + e−

Discharge

The average specific capacitance of the Ni2 P/GS composite electrode can be calculated from the CV curves in Fig. 4a by integrating the area under the current-potential curve according to the equ. [2]:

C=

1

Vc I(V )dV

mv(Va − Vc )

[2]

Va

In which I (A) stands for the current, m (g) is the mass of the active material in the electrode, v (V s−1 ) is the scan rate, Va and Vc are the maximum and minimum voltage in the CV curve, respectively. The average specific capacitance of the Ni2 P/GS composite was calculated to be 2240.8, 1248.4, 700.4, 562.9 and 390.9 F g−1 at the scan rates of 1, 5, 10, 20 and 50 mV s−1 , respectively. However, the specific capacitance of the composite electrode decreased with the increase of the scan rates. Since the redox reactions are usually dependent on the insertion-deinsertion of protons from the electrolyte [43,44]. At lower scan rates, the diffusion of ions from the electrolyte can enter into almost all the effective holes of the electrode. While with the increase of scan rate, the effective interaction between the ions and the electrode is greatly reduced, which led to a lower specific capacitance. The CV curves of Ni2 P/GS composite and Ni foam were also conducted, shown in the inset in Fig. 4a. The background signal due to the Ni foam was virtually negligible, which is consistent with our previous work [23,45,46]. As shown

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Fig. 3. Nitrogen adsorption and desorption isotherms of the GS (a) and Ni2 P/GS composite (b), respectively.

in Fig. 4b, a linear relationship is observed between anodic peak current and scan rate based on the results of CV, which suggests a diffusion-limited reaction for the redox reaction of the Ni2 P/GS electrode. Fig. 4c shows the discharge curves of the Ni2 P/GS composite in a potential range of 0-0.55 V at current densities between 5 and 50 mA cm−2 . A distinct plateau region can be observed during the process, which is consistent with the CV results. The nonlinear charge/discharge profiles further verified the pseudocapacitive behavior. In the discharge curves from 0.55 to 0.3 V vs. Hg/HgO, there is a variation in the slope of the time dependence of the potential, which originates from electrochemical adsorption-desorption, or a surface redox reaction on the electrode-electrolyte interface (pseudocapacitive behavior). The theoretical specific capacitance of the Ni2 P/GS composite is calculated to be 1689 F g−1 based on Faraday’s law. The specific capacitance is calculated by the equ. [3]: C=

It mV

[3]

In which I (A) is the discharge current,  t (s) is the discharge time,  V (V) is the voltage ranged and m (g) is the mass of the active

material in the electrode. Notably, the specific capacitance is as high as 1912, 1745, 1483 and 888 F g−1 at the discharge current densities of 5, 10, 20 and 50 mA cm−2 , respectively. This suggests that about 46.4% of the capacitance is still retained when the charge-discharge rate is increased from 5 mA cm−2 to 50 mA cm−2 . In addition, the cycling stability of the Ni2 P/GS electrode is also evaluated by the repeated charging-discharging measurement at a constant current density of 5 mA cm−2 , as shown in Fig. 4d. The specific capacitance increased during the first 200 cycles, which resulted from the activation process for the Ni2 P/GS electrode. Similar phenomena have also been reported [47,48]. After this process, the specific capacitance decreased from 1912 F g−1 to 1473 F g−1 after the subsequent 2300 cycles. Importantly, the Columbic efficiency was nearly 100% for each cycle of charge and discharge (inset in Fig. 4d). These results revealed the high specific capacitance and good rate capability of the Ni2 P/GS composite material for high-performance electrochemical pseudocapacitor. We carried out similar electrochemical measurements for the pure Ni2 P electrode. The discharged curves of pure Ni2 P electrode are similar to that of the Ni2 P/GS composite electrode (Fig. 5). The pure Ni2 P exhibits lower specific capacitance than Ni2 P/GS

Fig. 4. Electrochemical characterizations of Ni2 P/GS composite. (a) CV curves of Ni2 P/GS electrode at various scan rates, (inset in a) CV curves of the Ni2 P/GS composite and Ni foam at the scan rate of 50 mV s−1 , (b) Dependence of the current on scan rate1/2 plot for the Ni2 P/GS electrode, (c) Galvanostatic discharge curves of Ni2 P/GS electrode at various discharge current densities, (d) Dependences of the discharge specific capacitance and the coulombic efficiency on the charge-discharge cycle numbers, galvanostatic charge and discharge curves of the Ni2 P/GS electrode at a current density of 5 mA cm−2 (inset).

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Fig. 5. Galvanostatic discharge curves at various discharge current densities of the pure Ni2 P electrode. The charge-discharge tests were performed at 5 mA cm−2 in 2 M KOH solution.

composite electrode (Fig. 5). The specific capacitances of the pure Ni2 P electrode are 1568 F g−1 , 1395 F g−1 , 996 F g−1 and 543 F g−1 at the discharge current densities of 5 mA cm−2 , 10 mA cm−2 , 20 mA cm−2 and 50 mA cm−2 , respectively. 3.3. The effects of GS on the electrochemical performances The CV curves of Ni2 P/GS composite, pure Ni2 P electrodes at a scan rate of 50 mV s−1 were exhibited in Fig. 6a. It is clearly shown that the redox peaks are located in similar voltages, indicating the same redox reaction during the charge-discharge process. The area under the current-potential curve of the Ni2 P/GS composite electrode is bigger than that of the pure Ni2 P electrode. And the average specific capacitances of the Ni2 P/GS composite electrode and the pure Ni2 P electrode are 390.9 F g−1 and 325.3 F g−1 , respectively, at the same scan rate of 50 mV s−1 . It is shown that the introduction of GS as the matrix is beneficial to enhance the average specific capacitances and keep the electrochemical reaction mechanism unchanged. Fig. 6b compares the cyclic performances of Ni2 P and Ni2 P/GS electrodes. It can be seen that the specific capacitance of Ni2 P electrode firstly increases from 656 to 1568 F g−1 in the first 400 cycles, and then decreases from 1568 to 1079 F g−1 after 2000 cycles at a current density of 5 mA cm−2 . The capacitance retention of the Ni2 P electrode is 68.8%. In contrast, the specific capacitance of Ni2 P/GS increases in the first 200 cycles and then keeps a stable value (1912 F g−1 ) and reaches 1473 F g−1 after 2300 cycles, corresponding to 77.1% capacitance retention, which has been improved for Ni2 P by incorporating GS. The GS matrix with high electrical conductivity can beneficially disperse and/or host the nanosized Ni2 P. The tailed hybrid composite enable nanosized Ni2 P well exposed to the electrolyte, and easy transport of electrons and ions. Another advantage of the Ni2 P/GS electrode is its rate performance. The calculated specific capacitance values are plotted

185

Fig. 7. The specific capacitance of the Ni2 P/GS composite and pure Ni2 P electrodes plotted as a function of charge-discharge current densities.

as a function of current density for the Ni2 P/GS composite electrode and the pure Ni2 P electrode (Fig. 7). As we can see that the capacitance decreased with increasing current densities due to the limited diffusion on the electrode surface. The capacitance of the pure Ni2 P electrode was improved by intercalating GS to form a Ni2 P/GS composite. Depending on the current densities used, the specific capacitance gained moderate to significant improvements. The improvement was in the range of about 344 to 487 F g−1 compared to the pure Ni2 P electrode depending on the current densities. As expected, the enhancement of the electronic conductivity is essential to improve the rate performance of electrode. The enhanced ion diffusion and effective electron transfer in the Ni2 P/GS composite electrode are further confirmed by the electrochemical impedance spectrum (EIS) measurements. The Nyquist plots obtained for the Ni2 P/GS composite and the pure Ni2 P electrodes were collected for full comparison. As shown in Fig. 8a, the Nyquist plots of all the electrodes consist of a low-frequency line and a high-frequency semicircle. The inclined line in the lowfrequency region represents the Warburg impedance (Zw ). The semicircle in the middle frequency range indicates the chargetransfer resistance (Rct ), relating to charge transfer through the electrode-electrolyte interface. In Nyquist plots, the Ni2 P/GS composite electrode shows smaller real axis intersection (Fig. 8b) and negligible semicircle. This indicates a low interfacial resistance between current collector and active material, active material and electrolyte, as well as low charge transfer resistance. The Warburg impedance of the Ni2 P/GS composite electrode shows lower value than that of the pure Ni2 P electrode, which corresponds to the most vertical line leaning to imaginary axis at a low-frequency region. This suggests the more facile electrolyte diffusion to the composite surface and a more ideal capacitor behavior compared with the pure Ni2 P electrode. The results above demonstrate that the combination of fast ion diffusion as well as low electron-transfer resistance is also responsible for the high utilization of the active

Fig. 6. CV curves of Ni2 P/GS composite and pure Ni2 P electrodes at a scan rate of 50 mV s−1 (a), cyclic performance of Ni2 P/GS composite and pure Ni2 P electrodes at a current density of 5 mA cm−2 (b).

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Fig. 8. Impedance Nyquist plots of the Ni2 P/GS composite and pure Ni2 P electrodes (a), and the enlarge curve of the high frequency region of the two electrodes (b).

materials, so as to enhance electrochemical performance of the Ni2 P/GS composite. The excellent effects of GS on the electrochemical performances of the Ni2 P electrode can be considered from the following aspects. First, the combination of Ni2 P nanoparticles with highly conducting GS provides ions and electrons superhighways, allowing for rapid and efficient charge-discharge transport and leading to an increase in the whole electronic conductivity. Second, as GS have a high specific area, the electrolyte can diffuse into the electrode materials, so all the active materials can participate in the electrochemical charge storage process, leading to a higher specific capacitance. Third, grafting these Ni2 P nanoparticles onto the mechanically stable GS can effectively prevent the agglomeration of electro-active material, appropriately decrease restacking of GS ensembles, and make the structure stable. Such a stable structure helps to alleviate the structure damage caused by volume expansion during cycling process, resulting in an enhanced stability. 4. Conclusion In summary, a facile scalable synthesis of crumpled graphene nanosheets with high surface area was achieved by thermal exfoliation in H2 /Ar flow. We have prepared Ni2 P/GS nanocomposite via a low-temperature solid state reaction. It was found that the as-prepared graphene nanosheets with a delaminated flexible structure and a large amount of voids exhibited excellent effects on the electrochemical performances of the Ni2 P electrode. The novel Ni2 P/GS composition electrode showed high specific capacitance of 1912 and 888 F g−1 at current densities of 5 and 50 mA cm−2 , respectively, and desirable cycling stability and excellent rate capability. The effects of GS on the electrochemical properties of the Ni2 P electrode were systematically investigated. Based on the microstructure and electrochemical investigations including XRD, CV and EIS, it was demonstrated that the smaller GS dispersed in an irregular and disorderly manner in the composite, not the simple physical mix, providing more electroactive sites and ions diffusion channels, preventing the grain of Ni2 P sintering and agglomeration, thus leading to enhanced electrochemical performances. Acknowledgements This work was financially supported by 973 (2011CB935900, 2010CB631303), NSFC (21231005, 51071087), MOE (IRT-13R30), 111 Project (B12015), Research Fund for the Doctroral Program of Higher Education of China (20120031110001), Tianjin Sci & Tech Project (10SYSYJC27600) and Nature Science Foundation of Tianjin (11JCYBJC07700). References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359–367. [2] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854.

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