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Shape-Controlled Synthesis of CoP Nanostructures and Their Application in Supercapacitors Xiaojuan Chen, Ming Cheng, Di Chen, and Rongming Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10785 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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ACS Applied Materials & Interfaces

Shape-Controlled Synthesis of Co2P Nanostructures and Their Application in Supercapacitors Xiaojuan Chen, 1 Ming Cheng, 1 Di Chen 2 and Rongming Wang 2, * 1 Department of Physics, Beihang University, Beijing 100191, P. R. China. 2 School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, P. R. China. E-mail: [email protected] KEYWORDS: Co2P nanostructures, nanoflowers, nanorods, phosphating process, asymmetric supercapacitor ABSTRACT: Co2P nanostructures with rod-like and flower-like morphologies have been synthesized by controlling the decomposition process of Co(acac)3 in oleylamine system with triphenylphosphine as phosphorus source. Investigations indicate that the final morphologies of the products are determined by their peculiar phosphating processes. Electrochemical measurements manifest that the Co2P nanostructures exhibit excellent morphology-dependent supercapacitor properties. Compared with that of 284 F g-1 at a current density of 1 A g-1 for Co2P nanorods, the capacitance for Co2P nanoflowers reaches 416 F g-1 at the same current density. Furthermore, an optimized asymmetric supercapacitor by using Co2P nanoflowers as anode and graphene as cathode is fabricated. It can deliver a high energy density of 8.8 Wh kg-1 (at a high power density of 6 kW kg-1) and good cycling stability with over 97% specific capacitance remained after 6000 cycles, which makes the Co2P nanostructures potential

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applications in energy storage/conversion systems. This study paves the way to explore a new class of cobalt phosphide based materials for supercapacitor applications. INTRODUCTION

Energy storage devices are increasingly popular to meet modern life demand for hybrid/plug-in electric vehicles, portable electronic devices and power back-up.1-4 Supercapacitors are emerging as rechargeable energy storage devices with some irreplaceable properties such as high power density and long cycle life.5-8 However, the energy density of supercapacitors currently is relatively lower compared to that of secondary lithium ion batteries (LIBs). To boost the energy density of supercapacitors, it is an effectively alternative approach to assemble asymmetric supercapacitors (ASC) by incorporating a electrochemical double layer capacitor (EDLC) electrode and a pseudocapacitor electrode.9-10 The different potential window regions of two kinds of electrodes combined together result in an enlarged potential window and enhanced energy density of ASC. One of the attractive merits about asymmetric supercapacitors is that the output voltage can be improved in the environmentally friendly aqueous electrolyte. However, the performance of supercapacitors highly depends on the electrode materials. Various materials have been explored as the anodic materials, such as RuO2,11-13 MnO2, 14-15 Ni(OH)2,16 NiCo2O4,17 etc. Cobalt-based materials, as an important class in transition metal complexes, have been extensively researched. Among cobalt compounds, the interest of cobalt phosphides has been highlighted in the recent years due to their potential applications in magnetic, electronic, photonic and catalytic fields. The extremely fascinating properties of cobalt phosphides are the excellent catalytic performance and stability towards the hydrogen evolution reaction in all pH


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values,18-20 and good performance as anode materials for LIBs.21-22 Other cobalt compounds, such as cobalt oxides, sulphides and selenides, which have potential applications in catalysts for water splitting and anodic materials for LIBs,23-28 also show excellent pseudocapacitance characteristic in supercapacitors.29-31 Meanwhile, nickel phosphides, possessing similar properties to cobalt phosphides, have been reported to be used as electrode materials for supercapacitors.32 So, it is expected that cobalt phosphides may function as supercapacitor materials. In addition, cobalt phosphides are low cost and easily available in various morphologies, providing them more possibility for supercapacitors study.33-35 However, the study on cobalt phosphide materials for supercapacitors is rare. It is highly desirable to establish a feasible structure to address common concerns about cobalt phosphides for supercapacitors. Moreover, one dimensional and flower-like morphologies are becoming hot research directions in supercapacitors as they have remarkably higher specific capacitance relative to other morphologies on relevant reports.36 Herein, we report an easy and facile strategy to synthesize one dimensional (1D) Co2P nanorods and 3D Co2P nanoflowers just through controlling the phosphating processes. The possible formation mechanism of two different Co2P nanostructures has been also proposed. When both samples were applied as electrode materials for supercapacitors, Co2P nanorods and nanoflowers give rise to a specific capacitance of 284 F g-1 and 461 F g-1 at a current density of 1 A g-1 in a large voltage window -0.2–0.5 V versus Hg/HgO electrode, respectively. Furthermore, an asymmetric supercapacitor by using Co2P nanoflowers as anode and graphene as cathode was successfully fabricated with advanced electrochemical performance. The optimal ASC shows a high energy density of 24 Wh kg-1 at a power density of 0.3 kW kg-1. Even the power density increases by 20 times (6 kW kg-1), the energy density still remains 36.5% (8.8 Wh kg-1).


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Additionally, the ASC exhibits good long-term electrochemical stability with the specific capacitance remaining 97% after 6000 cycles. These impressive results imply that Co 2P nanostructures are promising practical energy-storage materials. EXPERIMENT SECTION Cobalt(III) acetylacetonate (Co(acac)3, chemical

purity), oleylamine (OAm, 98.0%),

triphenylphosphine (TPP, 99.0%), acetone (99.5%) and toluene (99.5%) were purchased from Chinese reagent companies. Graphene was purchased from Nanjing XFNANO. Materials Tech Co., Ltd. All reagents were chemical grade and used without further purification. Co2P nanorods and nanoflowers were synthesized via a simple thermal decomposition method according to the previous literature.33 In a typical process, 0.5 g Co(acac)3 was first dissolved in 10 ml OAm in a three neck flask before being heated to 100 C for 1 h under magnetic stirring and nitrogen bubbling. Subsequently, 1.0 g TPP was added into the homogeneous dark green solution for 1 h. The mixed solution was heated up to 300 C rapidly (20 min for nanorods and 25 min for nanoflowers) and maintained for 3 h. The color of the solution changed from dark green to black. Then the black solution was naturally cooled to room temperature. Finally, the products were collected by centrifugation and washed with acetone for several times. The crystal structures, chemical compositions and chemical states of the obtained samples were characterized by X-ray diffraction (XRD, X ’Pert Pro MPD system, Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS, ESCALAB MK II, Al Kα photon source, C1s 284.8eV). The morphologies and microstructures of the samples were characterized by field-emission scanning electron microscopy (FESEM, HITACHI, S4800) equipped with X-ray energy-dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM, JEOL,


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JEM-2100F, 200 kV). The specific surface areas of the products were determined by N2 adsorption–desorption isotherm measurements (Micrometrics, ASAP 2020). Electrochemical Measurements For preparation of the working electrodes, the as-synthesized active materials were mixed with carbon black and polyvinylidenefluoride (PVDF) with a mass ratio of 7:2:1 in N-methyl-2pyrrolidinone. The mixture was pasted on the cleaned nickel foam current collector (area ~1 cm2) and dried in an oven at 60 C for 12 h. The dried electrode was then pressed using a hydraulic press at a pressure of 8 MPa. The electrochemical tests were carried out in both three-electrode and two-electrode configurations with 6 M KOH aqueous solution as electrolyte. In the threeelectrode electrochemical cell, nanorods or nanoflowers pressed on the nickel foam, a Pt wire and a Hg/HgO electrode were used as the working, counter and reference electrodes, respectively. Here, the loading mass of the Co2P nanoflowers and Co2P nanorods is about 2.31 mg and 2.38 mg, respectively. In the two-electrode electrochemical system, an asymmetric supercapacitor was conducted by taking the Co2P nanoflowers as anode and graphene as cathode. Electrochemical measurements were performed in an electrochemical workstation (CHI660D) by the techniques of cyclic voltammetry (CV), chronopotentionmetry (CP), electrochemical impedance spectroscopy (EIS). EIS measurements were carried out using this apparatus over a frequency range of 100 kHz to 0.01 Hz at 0 V with an AC amplitude of 5 mV. RESULTS AND DISCUSSION Morphological and structural characterization Co2P nanorods and nanoflowers were successfully synthesized by decomposing Co(acac)3 in OAm with TPP providing phosphorus source at an appropriate heating rate, respectively. The


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samples obtained at the rapid heating are uniform nanorods with ~18 nm diameter and ~110 nm length, as shown in Figure 1(a, b). Figure 1(c) shows the HRTEM image of an individual Co2P nanorod, illustrating the (020), (-112) and (11-2) crystal planes with 0.174 nm, 0.212 nm and 0.212 nm lattice spacing, respectively. The inset in Figure 1c shows the corresponding selected area electron diffraction (SAED) obtained from the [201] axis of orthorhombic Co2P, including the (020), (-112) and (11-2) diffraction spots consistent with the HRTEM results, unveiling its single crystalline structure. Interestingly, when the heating time was extended to 25 min, flowerlike products were prepared as shown in Figure 1d. And Figure 1e shows a free-standing nanoflower, composing by a center core and several protrusive branches. The branch has a needle-like shape with a decreasing diameter from the central core about 20 nm and terminal tip of less than 10 nm. Figure 1f shows the typical HRTEM image of a random branch with distinct parallel lattice fringes. The corresponding fast-Fourier-transform (FFT) pattern (inset in Figure 1f) is also well indexed to the [201] zone axis of orthorhombic Co2P including the (020), (-112) and (11-2) diffraction spots. These results sufficiently confirm that the crystalline structure of the needle-like branch is consistent to the nanorod, and their preferred growth directions are both expected in the [010] crystallographic direction according to the crystal growth dynamics that the (010) plane has higher surface energy compared to other planes for orthorhombic crystal structure. In addition, the Scanning TEM (STEM) images and the corresponding EDX elemental mapping images of P and Co for a single nanoflower and nanorod are shown in Figure 1g and 1h, indicating that both P and Co elements are uniformly distributed in the whole structure ， respectively. The two prepared samples were characterized as Co2P by X-ray diffraction (Figure 2a). Five obviously peaks were observed in both samples consistent well with the orthorhombic Co2P


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phase (JCPD Card. No. 89-3030). The atomic ratios of Co/P for nanoflowers and nanorods are measured to be 1.72 and 1.80 by the energy dispersive X-ray spectroscope (Figure S1a and S1b), which are close to the ratio for Co2P. Additionally, X-ray photoelectron spectroscopy (XPS) analysis was also performed to further explore the surface chemical compositions of the Co2P nanorods and nanoflowers. High-resolution XPS spectra of Co 2p and P 2p are shown in Figure 2b and 2c. Focusing on the XPS spectrum of the Co2P nanorods, the peaks at 797.8 and 781.8 eV are assigned to the Co 2p1/2 and Co 2p3/2 of the oxidized Co2+ species respectively, along with two apparent satellite peaks at 803.3 and 786.4 eV, which are ascribed to the shakeup excitation of the high-spin Co2+ ions. Besides, the peaks at 793.7 and 778.7 eV are assigned to the Co 2p1/2 and Co 2p3/2 of Co species in Co2P, which have partial positive from that of Co metal.20, 37 In the close-up P 2p spectrum of Co2P nanorods, the peaks at 130.5 and 129.6 eV are close to binding energy of P 2p1/2 and P 2p3/2 of P in Co2P, which show negatively shift from that of the elemental P. And the broad peak at 133.38 eV is assigned to oxidized phosphorus species due to air contact. Thus, a transfer of electron density from Co to P occurs.38-39 In the case of the Co2P nanoflowers, the positions of occurring peaks in the Co 2p regions are basically similar to that of Co 2P nanorods. However, the peaks in the P 2p region all show negatively shift (about 0.2 eV) from that of Co2P nanorods. Meanwhile, the relative intensities of the peaks are distinctly different between two samples, implying that the content of each component is different, which may have effect on the electrochemical properties of Co2P nanorods and Co2P nanoflowers. Moreover, Figure S2 displays the N2 adsorption–desorption isotherm curves of two samples and the calculated Brunauer–Emmett–Teller (BET) surface areas were 25 m2 g-1 for Co2P nanorods and 29 m2 g-1 for Co2P nanoflowers, respectively. Growth mechanism


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In this experiment, Co2P nanorods were prepared with the heating time of 20 min from 100 C to 300 C, while the heating time was extended to 25 min, flower-like products were obtained. To explore the possible growth mechanism of both samples, the time-dependent experiments were detailedly investigated. Clearly, Figure 3a shows that abundant ~10 nm Co-complex nanoparticles formed at the beginning of 30 min. When reaction time arrived to 50 min, a few nanorods were observed (Figure 3b). With the reaction proceeding, more and more nanorods appeared and the average length and diameter of these nanorods increased (Figure 3c and 3d). In the end, nanoparticles almost completely disappeared and only uniform nanorods were obtained (Figure 3e and 3f). Figure S3a shows the atomic ratios of P/Co in Co2P nanorods at different time intervals. For nanorods, the phosphating process is simple that the atomic ratio of P/Co just increases linearly to time. Similarly, the morphology evolution of Co2P nanoflowers is shown in Figure 4. With the reaction going on, small nanoparticles (Figure 4a) gradually aggregated to bigger clusters (Figure 4b and 4c). Up to 100 min, small nanoparticles almost completely assembled to clusters and some distinct tiny nanoneedles formed on the surface of the clusters (Figure 4d). Then the nanoneedles become bigger and longer out from the core of the cluster. At last, the clusters successfully transformed to nanoflower structure (Figure 4e and 4f). For nanoflowers, the phosphating process is complex (Figure S3b). At the early stage, the sluggish phosphating process induces nanoparticles to aggregate. At the subsequent stage, the sudden increasing of P element results in the formation and growth of the nanoneedles on the surface of the clusters. These above results indicate that the final morphologies of products depend on the different phosphating process. And the heating speed is a crucial factor to the TPP phosphating process. Therefore, rapid heating is favor to the formation of Co2P nanorods, and the relatively


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slow heating progress benefits for the aggregation of nanoparticles, which is an essential step for the formation of nanoflowers. The electrochemical performance of the as-prepared samples was investigated as electrode materials for supercapacitors. Figure 5a shows the CV curves of the Co2P nanorods and Co2P nanoflowers, respectively. Several anodic and cathodic peaks are clearly observed in the CV curves, indicating the pseudocapacitance properties of both electrodes. And obviously, flowerlike Co2P electrode exhibits much higher current densities than Co2P nanorods electrode in the scan voltage range. So far, the investigation about cobalt phosphides electrodes for supercapacitors is rare and related reaction mechanism is not mentioned. Here, based on the relevant literatures,22,

40-44

a possible reaction mechanism is proposed and described by the

following equations: Co2P + 2OH− →Co(OH)2 + Co + P + 2e−

(1)

Co + 2OH− → Co(OH)2 + 2e –

(2)

3Co(OH)2 + 2OH−⇄ Co3O4 + 2e − + 4H2O

(3)

Co(OH)2 + OH− ⇄ CoOOH + H2O + e −

(4)

Co3O4 + OH− + H2O ⇄3CoOOH + e –

(5)

Co(OH)2 + 2OH− ⇄ CoO2 + 2H2O +2e –

(6)

CoOOH + OH−⇄ CoO2 + H2O + e –

(7)

In the basic solution, Co2P first converts to Co(OH)2, Co and P (eq 1).22 Then the formed Co metal is oxidized into Co(OH)2 under a relatively high overpotential (eq 2). Thus, in the CV curve of Co2P nanoflowers, the anodic peak at P1 (-0.07 V) can be assigned to the redox reaction (eq 3), whose corresponding cathodic peak disappears due to the poor reduction current. The pair of redox reaction peaks with anodic peak at P2 (0.09 V) and cathodic peak at P5 (0.05 V) can be attributed to the redox reaction (eq 4).The anodic peak at P3 (0.33 V) corresponds two cathodic


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peaks at P7 (0.28 V) and P6 (0.22 V), attributing to the nearby electrode potential of two redox reactions (eq 5 and eq 6). The last anodic peak at P4 (0.42 V) and cathodic peak at P8 (0.40 V) belong to the redox reaction (eq 7). For Co2P nanorods, the CV curve is different from that of Co2P flowers and some peaks are elusive, due to that the CV pattern strongly depends on the morphologies and surface properties of the electrodes. Subsequently, a series CV curves of Co2P nanoflowers electrode were collected at various scan rates with the potential window of -0.2-0.5V as shown in Figure 5b. With the increasing of the scan rate, the peak current enhances continuously, suggesting the rates of electronic and ionic transport are rapid enough in the applied scan rates. There is no significant change in the shape of CV, indicating the good reversibility of the Co2P electrode in the fast charge-discharge response. The CV curves of the Co2P nanorods electrode are also presented in Figure S4a. Figure 5c presents the typical galvanostatic charge–discharge voltage vs. time profiles of the Co2P nanoflowers electrode at various current densities. The asymmetrical curves with well-defined plateaus are observed, indicating its good pseudocapacitance behaviors. The specific capacitances of the Co2P nanoflowers can be calculated to 416, 385, 366, 354, 341, 304, 285 and 265 F g-1 at a current density of 1, 2, 3, 4, 5, 10,15 and 20 A g-1, respectively (Figure 5d). About 64% specific capacitance retention with current density increasing 20 folds shows its stable electrochemical performance and high power output property. The specific capacitances of Co 2P nanorods electrode calculated from the charge-discharge curves (Figure S4b) are also displayed in Figure 3d. Remarkably, Co2P nanoflowers possess better pseudocapacitive performance than Co2P nanorods, which can be attributed to the unique 3D structure providing a larger ion accessible area and effectively avoiding agglomeration among active particles, as shown in Figure S5. Typical Nyquist plots of the Co2P nanoflowers and nanorods electrodes are presented


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in Figure S6. The straight lines of the two electrodes are nearly parallel to the imaginary axis in the low frequency, indicating ideal capacitive type behavior. Besides, the equivalent series resistances (ESR) of Co2P nanoflowers and Co2P nanorods electrodes were evaluated to be approximately 0.76 Ω and 0.67 Ω, respectively, manifesting the good conductivity of the electrolyte and very low internal resistance of the electrode. Thus, these above-mentioned experimental results convincingly demonstrate that the Co2P nanostructures are promising electrode materials for supercapacitors. In order to further explore the application of the as-synthesized Co2P products in energy storage field, a prototype asymmetric supercapacitor device was fabricated with Co2P nanoflowers as anode and graphene as cathode. Typically, the electrochemical properties of graphene were evaluated in a three-electrode system and the results imply its excellent performance as electric double-layer capacitance (Figure S7). The specific capacitance of graphene electrode was calculated to 187 F g-1 from its galvanostatic charge-discharge curve at 1 A g-1, which is comparable to the value of the previously reported graphene electrode.45 Then, the comparative CV curves of the anode and cathode were measured at a scan rate of 5 mV s-1 (Figure 6a). The Co2P nanoflowers electrode can be operated within a potential window of -0.2 V to 0.5 V, and the graphene electrode can be operated within a potential window of -1.0 V to 0 V. To obtain the maximum capacitance of the device and meet the requirement of the charge balance between both electrodes, the optimal mass ratio m(Co2P nanoflowers)/m(graphene) was calculated to be 0.64 based on their specific capacitances and potential windows. Therefore, the asymmetric supercapacitor device can work at a voltage of 1.5 V, which can be further demonstrated by the CV curves of ASC with different voltage windows (Figure S8). As expected, the stable electrochemical window of the ASC can be extended to 1.5 V. Figure 6b shows the CV curves of


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ASC at different scan rates. Unlike the Co2P nanoflowers electrode curves with sharp redox peaks and the rectangular shape of graphene electrode curves, the asymmetric supercapacitor CV curve shows a rectangle-like shape combining the characteristic of faradic pseudocapacitance and EDLC. The galvanostatic charge-discharge measurements at various current densities with the potential window of 1.5 V were conducted (Figure 6c). The ASC exhibits a high specific capacitance of 76.8 F g-1 at the current density of 0.4 A g-1. Even at a very high current density of 4 A g-1, the specific capacitance still reach up to 46.5 F g-1, about 60.4% of the capacitance retained. Significantly, the nearly isosceles triangular shape of galvanostatic charge/discharge curves with very small voltage drops at different current densities demonstrates its excellent electron conductivity and good Coulombic efficiency. The long-time cycling performance of ASC, which is a critical parameter for supercapacitor practical applications, is shown in Figure 6d. In the first 200 cycles, the specific capacitance gradually increases mainly due to the activation of the electrode material. Then the specific capacitance was perfectly retained (>97%) after 6000 cycles. The Coulombic efficiency stayed at approximately 99%, implying that the Faradaic redox reaction is nearly reversible. The SEM images of the Co2P nanoflowers electrode and graphene electrode after the cycling test are shown in Figure S9. It shows that the Co2P nanoflowers and graphene on the electrodes aggregated and their surface structures were covered by some materials. Figure 7a shows the Nyquist plot of the ASC by EIS. From the extended spectrum in the inset, the value of ESR is about 0.87 Ω, which is desirable for high specific power density. It manifests favorable conductivity and very low internal resistance of ASC. And the semicircle region in the plot curve corresponds to charge transfer resistance, it can be determined as 8.0 Ω, which could be improved by growing Co2P nanoflowers directly on nickel foams with additive-free and


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binder-free.46 Figure 7b shows the Ragone plots of the ASC based on the galvanostatic chargedischarge curves. The assembled ASC possesses an impressively energy density of 24 Wh kg-1 at a power density of 300 W kg-1, which can still retain 8.8 Wh kg-1 at a higher power density of 6 kW kg-1. This superior energy density property of the ASC can be attributed to both its high specific capacitance and its much elevated cell voltage of 1.5 V in the aqueous electrolyte. The performance of Co2P nanoflowers//graphene asymmetric supercapacitor is comparable to other cobalt-based ACS reported previously, as shown in Table S1. From these investigations, it is recognized that Co2P could be an alternative electrode material in supercapacitor applications. CONCLUSION In summary, we have developed a simple strategy to synthesize Co2P nanorods and nanoflowers in OAm system with different formation processes. When used as electrodes for supercapacitors, Co2P nanoflowers delivered higher capacitance of 416 F g-1 than nanorods with 284 F g-1 at a current density of 1 A g-1, which might be due to the distinctive 3D microstructure and the larger BET surface area of flower-like products. Furthermore, an optimized asymmetric supercapacitor was fabricated by using the Co2P nanoflowers as anode and graphene as cathode. It achieved good performance with a specific capacitance of 76.8 F g-1 at 0.4 A g-1 current density, and the corresponding energy density was as high as 24 Wh kg-1 at a power density of 0.3 kW kg-1. Additionally, the ASC exhibits excellent cycling stability with the specific capacitance remaining 97% after 6000 cycles. These suggest that Co2P nanostructures have great potential in various energy storage technologies.


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Figure 1. (a, d) SEM images of the Co2P nanorods and Co2P nanoflowers; (b, e) TEM images of the Co2P nanorods and Co2P nanoflowers; (c) HRTEM image of a single nanorod and the inset is the corresponding SAED; (f) HRTEM image of a random branch of the nanoflower and the inset is its corresponding FFT image; (g) STEM image of a single Co2P nanorod and the corresponding EDS elemental mapping images; (h) STEM image of a single Co2P nanoflower and the corresponding EDS elemental mapping images.


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Figure 2. (a) XRD patterns of the Co2P nanorods and Co2P nanoflowers, and the black stick pattern corresponds to Co2P (JCPDS 89-3030); (b) XPS spectra in the Co 2p region of Co2P nanorods and nanoflowers; (c) XPS spectra in the P 2p region of Co2P nanorods and nanoflowers.

Figure 3. SEM images of the intermediates obtained at different time for Co2P nanorods: (a) 30 min, (b) 50 min, (c) 70 min, (d) 90 min, (e) 110 min and (f) 180 min. Scale bar: 100 nm.


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Figure 4. SEM images of the intermediates obtained at different time for Co2P nanoflowers: (a) 40 min, (b) 60 min, (c) 80 min, (d) 100 min, (e) 120 min and (f) 180 min. Scale bar: 200 nm.


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Figure 5. (a) CV curves of Co2P nanoflowers and Co2P nanorods; (b) CV curves of Co2P nanoflowers at different scan rates; (c) Galvanostatic charge-discharge curves of Co2P nanoflowers at various current densities; (d) Specific capacitance of Co2P nanoflowers and Co2P nanorods as a function of the current densities.


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Figure 6. (a) CV curves of the Co2P nanoflowers electrode and graphene electrode at 5 mV/s measured in three-electrode system; (b) CV curves of the ASC device collected at different scan rates; (c) Galvanostatic charge-discharge curves collected at different current densities for ASC devices in the voltage window of 1.5 V; (d) Cycling performance at 0.8 A g-1 current density of ASC.


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Figure 7. (a) Nyquist plot of asymmetric supercapacitor devices, and the inset is the expanded high frequency regions; (b) Ragone plot of the asymmetric supercapacitor device.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 51371015, 51331002), the Beijing Municipal Research Project for Outstanding Doctoral Thesis Supervisors (No. 20121000603), the Beijing Natural Science Foundation (No. 2142018) and the Fundamental Research Funds for the Central Universities (FRF-BR-15-009B). ACCSOCIATED CONTENT Supporting Information The calculation details of specific capacitance; EDS, Nitrogen adsorption–desorption isotherms of Co2P nanostructures; the electrochemical performances of graphene and Co2P nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *E-mail:

[email protected]

Notes The authors declare no competing financial interest.

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