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Controllable Fabrication of Amorphous Co-Ni Pyrophosphates for Tuning Electrochemical Performance in Supercapacitors Chen Chen, Ning Zhang, Yulu He, Bo Liang, Renzhi Ma, and Xiaohe Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07640 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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ACS Applied Materials & Interfaces

Controllable

Fabrication

of

Amorphous

Co−Ni

Pyrophosphates for Tuning Electrochemical Performance in Supercapacitors Chen Chen,†, ‡ Ning Zhang,*, † Yulu He,† Bo Liang,† Renzhi Ma,†, § Xiaohe Liu *, † †

School of Materials Science and Engineering, Central South University, Changsha, Hunan

410083, China ‡

School of Minerals Processing and Bioengineering, Central South University, Changsha,

Hunan 410083, China § International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

KEYWORDS Asymmetric supercapacitor, Nickel pyrophosphate, Amorphous phase, Electrochemistry, Electrode

ABSTRACT Incorporation of two transition metals offer an effective method to enhance the electrochemical performance in supercapacitors for transition metal compounds based electrodes. However, such a configuration is seldom concerned in pyrophosphates. Here, amorphous phase of Co−Ni pyrophosphates are fabricated as electrodes in supercapacitors. Through controllable adjusting the ratios of Co and Ni as well as calcination temperature, the electrochemical performance can be tuned. An optimized amorphous Ni−Co pyrophosphate

ACS Paragon Plus Environment


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exhibits much higher specific capacitance than monometallic Ni and Co pyrophosphates and shows excellent cycling ability. When employing Ni−Co pyrophosphates as positive electrode and activated carbon as a negative electrode, the fabricated asymmetric supercapacitor cell exhibits favorable capacitance and cycling ability. This study here provides facile methods to improve the transition metal pyrophosphate electrodes for efficient electrodes in electrochemical energy storage devices.

INTRODUCTION Currently, substantial research efforts are devoted to designing advanced energy storage techniques with high power density to satisfy the rapidly growing requirements for high−power applications such as electric vehicles and devices.1–4 Supercapacitors (SCs) have been realized as one of the most promising energy storage devices for the fast and highly reversible storage and release of electric energy.5,6 The redox type SCs are associated closely with a Faradaic−type charge transfer process that occurs on the electrode−electrolyte interface and result high pseudocapacitance, which have attracted considerable attention in the SC fields.7,8 More interestingly, when redox SCs are combined with electrochemical double−layer capacitor (EDLC) to form asymmetric supercapacitor cells (ASCs), the redox−capacitive materials can further boost the power and energy density of SCs.8– 13

Regarding the materials for redox supercapacitors, the metal oxides (e. g. RuO2, MnO2,

Co3O4),14–16 hydroxides (e. g., Ni(OH)2, Co(OH)2),17,18 sulfides (e. g., MoS2, Co9S8),13−14 and phosphides (e. g., Ni2P)21 are studied and exhibited promising electrochemical performances. Thus far, the exploration of electrode materials is toward low−cost, high specific energy and


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power density, and favorable cycling−ability to meet the both economical and high−energy demands. As inexpensive and electrochemical active materials, transition metal pyrophosphate such as Co2P2O7, Mn2P2O7 and Ni2P2O7, captivated the attention of researchers and exhibit broad and potential applications in energy storage.22–25 For instance, hexagonal tablets of Na+ doped Ni2P2O7, amorphous Ni2P2O7, porous Co2P2O7 microflower−like electrodes, Mn2P2O7 nanostructures, and Co2P2O7 nanorods were fabricated and applied as electrodes in energy storage fields.22–24,26,27 Although these materials were effective and exhibited favorable electrochemical performance, finding easily and efficient method to enhance the specific capacitance and cycling stability is still highly desirable for pyrophosphates. It was reported that inorganic metal compounds with doping or incorporation proper metal ions has the possibility to modify the physical and chemical properties of the materials.28–30 In faradic redox reactions, introducing transition metals ions would probably not only reduce the resistance of the electrode and raise the oxygen over potentials, but also participate in the electrochemical redox reaction.16,24,25 The co−incorporation of two transition metals offer an effective method to tailor the electrochemical performance for transition metal pyrophosphates, like the other transition metal compounds such as oxides, hydroxides, and sulfides.24,25−28 However, for pyrophosphates electrode materials, co−incorporation of two transition metals was still not achieved yet. On the other hand, materials with amorphous phases or poor crystallinity may exhibit unique physical

and

chemical

properties

due

to

their

unique

disordered

structures,

mechanical/electric isotropy and defect−rich characteristics.34–37 In the electrochemical



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reactions of SCs, amorphous phase of materials would allow deeper diffusion of the electrolyte ions to access the active materials and produce higher capacitance than corresponding crystallized materials.24,35,38,39 Furthermore, the strain and stress within amorphous materials are isotropic during charging/discharging, beneficial to the long−term electrochemical stability.10,35 Fabrication electrode materials with amorphous is now realized an effective method to increase the electrochemical performance. In present work, amorphous phase of Co−Ni pyrophosphates was fabricated. Through adjusting the ratios of Co and Ni, the optimized Co−Ni pyrophosphates exhibited superior specific capacitance than monometallic Co and Ni pyrophosphate as well as excellent cycling ability. Furthermore, the asymmetric supercapacitor cells (ASCs) were assembled by employing optimized Co−Ni pyrophosphate as a positive electrode and activated carbon (AC) as a negative electrode, which possessed favorable capacitance and cycling ability. This study here provides facile methods to construct transition metal pyrophosphate based electrode materials as efficient electrodes in supercapacitors.

EXPERIMENTAL SECTION Materials synthesis The materials were fabricated by a co-precipitation process. In a typical process, nickel sulfate hexahydrate (NiSO4·6H2O, AR grade, Sinopharm Chemical Reagent Co., Ltd), cobalt sulfate heptahydrate (CoSO4·7H2O, AR grade, Sinopharm Chemical Reagent Co., Ltd), and ammonium phosphate ((NH4)3PO4·3H2O, 0.02 mol, AR grade, Shanghai Zhanyun Chemical Co., Ltd) were mixed together and dissolved in deionized water. The precipitation was


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formed immediately. After vigorously magnetic stirring at room temperature for approximate 30 min, the solution was transferred into a teflon−lined stainless steel autoclave (40ml), which was subsequently heated to 140 °C in electronic furnace and maintained for 12 h for crystallization. After natural cooling to room temperature, the powder was washed with deionized water and ethanol for several times to remove the dissolved redundant ions and then was dried at 60 °C for 10 hours. These products were named as CoxNi1−x precursor (x is from 0 to 1, e. g. the Co−, Co0.7Ni0.3−, Co0.5Ni0.5, −Co0.2Ni0.8−, and Ni precursor stands for the products prepared at Co2+/Ni2+ ratio of 1:0, 2.3:1, 1:1, 1:4, and 0:1, respectively). Then, these precursors were calcinated at temperatures of 300, 400, 500, and 600 °C for 2 h. The calcinated products were named CoxNi1−x pyrophosphates (e. g. Co−, Co0.2Ni0.8−, Co0.5Ni0.5−, Co0.7Ni0.3−, and Ni pyrophosphates for the products prepared at Co2+/Ni2+ ratio of 1:0, 2.3:1, 1:1, 1:4, and 0:1, respectively). The detailed reactions occurred in each steps are summarized in supporting information. Materials Characterization X−ray diffraction were performed by a RIGAKU Rint−2000 X−ray diffractometer with equipped graphite monochromatized Cu−Kα radiation (λ=1.54184 Å). Thermogravimetric analysis was tested with SD T Q600 V8.0 Build 95. Scanning electron microscopy was tested with a FEI Helios Nanolab 600i field emission scanning electron microscopy. Transmission electron microscopy, energy dispersive spectrometer mapping images, and high−resolution were performed with a FEI Tecnai G2 F20 field emission transmission electron microscopy operated at 200 kV. The X−ray photoelectron spectroscopy spectra were recorded from a Thermo Fisher ESCALAB 250Xi spectrophotometer. The content elements was tested by a



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Thermo Fisher IRIS1000ICP inductively coupled plasma-atomic emission spectroscopy and an Elementar Vario Micro cube N/O analyzer. Three−electrode electrochemical measurement For fabrication of electrode, the active material mixed with acetylene black and polyvinylidene fluoride (PVDF) in 1−methyl−2−pyrrolidone (NMP) in a mass ratio of 8:1:1 to form a mushy, which was then pasted onto cleaned nickel foam and was dried at 60 °C for 10 hour. Electrochemical study was carried out on a Gamry Interface1000 electrochemistry workstation using a three electrode cell with 3 M KOH as the electrolyte, Hg/HgO electrode as reference electrode, platinum plat electrode as the electrode, and the as−prepared electrode as the working electrode. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests were recorded on the electrochemical workstation. According to the galvanostatic discharge curves, the corresponding specific capacitance was evaluated by such an equation:22 Csc = (I Δt) / (m ΔV), where the Csc was the specific capacitance, I (A) was the discharge current, Δt (s) was the discharge time, m (g) was the mass of active materials, and ΔV (V) was the potential window. Fabrication asymmetric supercapacitor cells (ASCs) The asymmetric supercapacitor was assembled by using CoxNi1−x pyrophosphates as positive electrode, active carbon (AC) working electrode as negative electrode, 3 M KOH as the electrolyte, and a diaphragm was used to separate the positive and negative electrodes. To get a best performance of ASCs, the charge balance the relationship between the two electrodes should follow q+ = q− and the mass of active materials between the two electrodes should be shown in the following the equations:10 q = C × ΔV × m, m+/m− = ( C− × ΔV−) / ( C+ × ΔV+).


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Where C (F g−1) was specific capacitance of electrode, m (g) was the active material mass on the electrode, and the ΔV (V) was the potential window. Power density and energy density were was calculated from the following equation:8 E = (C × ΔV2) / 7.2 and P = E × 3600 / (Δt), where E (Wh kg−1) was the average energy density, P (W kg−1) was average power density, C (F g−1) was specific capacitance, ΔV (V) is the potential window, and t (s) was the discharge time.

RESULTS AND DISCUSSIONS The thermogravimetric (TG) and differential thermal analysis (DTA) properties of Co−Ni precursor were given in Figure S1 of supporting information, showing that the Co0.4Ni1.6P2O7 could probably be obtained at high temperature treatment in air by such a process: 2Co0.2Ni0.8NH4PO4·H2O → Co0.4Ni1.6P2O7 + 2NH3 + 3H2O.22,23 Then, the structures of the Co0.2Ni0.8 precursor and calcinated products at different temperature were studied by XRD. As shown in Figure 1a, the precursor are readily consistent with the standard profile of NiNH4PO4·H2O (JCPDS card No. 88−0584) except the slightly shift of diffraction peaks to lower angels because of inducing Co2+. After calcinating at 300 °C for 2 h as shown in Figure 1b, the sharp diffraction peak disappears and exhibits an amorphous phase. And the amorphous phase is still kept as the calcination temperature increase to 400 °C as shown in Figure 1c. Further improving the calcination temperature to 500 °C, some diffraction peaks can be observed as illustrated in Figure 1d. These peaks are identical to the profiles of Ni2P2O7 (JCPDS card No. 74−1604), indicating that partly crystalized Co0.4Ni1.6P2O7 can be obtained. When the calcination temperature increased to 600 °C, the well crystalized Co−Ni



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pyrophosphate can be obtained, which are consistent with the TG and DTA results.

Figure 1 XRD profiles of the (a) Co0.2Ni0.8NH4PO4·H2O precursor and Co0.2Ni0.8 pyrophosphates prepared at calcination temperature of (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

The morphology of the product was characterized by SEM. The Co0.2Ni0.8NH4PO4·H2O precursor were composed of plate−like structure as shown in Figure 2a. These microplates have nonuniform sizes from 15 to 1µm. After calcinations at 300 and 400 °C, the microplate−like structure kept as illustrated in SEM images in Figure 2b−c. SEM image in Figure 2d shows that the product prepared at 400 °C has a smooth surface with thickness about 500 nm. Such morphology was maintained when further increasing the temperature to 500 and 600 °C as shown in Figure 2e and f, respectively.


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Figure 2 SEM images of the (a) Co0.2Ni0.8NH4PO4·H2O and Co0.2Ni0.8 pyrophosphates prepared at calcination temperature of (b) 300 °C, (c−d) 400 °C, (e) 500 °C and (f) 600 °C.

Figure 3a shows TEM image of a Co0.2Ni0.8 pyrophosphate typical microplate prepared at 400 °C. An uniform contrast is exhibited in this microplate, suggesting a homogeneous thickness for this microplate. Figure 3b displayed a typical SAED pattern for this product. There is no diffraction pot and rings observed, indicating amorphous phase of the product. Figure 3c shows a typical HRTEM image with anisotropy distribution, directly confirming amorphous phase of the product. The EDS mapping images taken under the high angle annular dark field (HAADF) image in Figure 3d shows that the elemental maps of Co, Ni, P, O, and N are uniformly distributed. The ratio of Co and Ni in this product is detected as 0.26 (analyzed by inductively coupled plasma-atomic emission spectroscopy, as shown in



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Table S1 of supporting information), which is closed to the ratio of 0.25 in synthetic process. It is worth noting that distinct amount of elements of N was detected and the ratio of N to P is 0.67, which is lower than the 1.0 of Co0.2Ni0.8NH4PO4·H2O but higher than the 0 in Co0.4Ni1.6P2O7 (analyzed by oxygen nitrogen analyzer, as shown in Table S1 of supporting information). The retained high content of N maybe caused by undecomposed Co0.2Ni0.8NH4PO4 and the adsorbed NH3 that produced during the transformation from Co0.2Ni0.8NH4PO4·H2O to Co0.4Ni1.6P2O7.22 For other samples, the amorphous phase properties for the Co0.2Ni0.8 pyrophosphate prepared at 300 °C, and crystalized phase for the Co0.2Ni0.8 pyrophosphate prepared at 500 and 600 °C are also confirmed by the TEM and HRTEM images as shown in Figure S2a−f of supporting information, which are consistent with the XRD results.

Figure 3 (a) TEM image, (b) SAED pattern, (c) HRTEM image, and (d) HAADF image with corresponding elemental mapping images for the Co0.2Ni0.8 pyrophosphate synthesized at the calcination temperature of 400 °C.


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The XPS technical was used to further characterize the compositions of above Co0.2Ni0.8 pyrophosphate at the calcination temperature of 400 °C. The survey spectrum of the sample is shown in Figure 4a. The signals of Ni 2p, Ni 3p, Co 2p, O 1s, N 1s, P 2s and P 2p originating from the product can be detected. The referenced C 1s from the adventitious carbon is located at binding energy of approximate 285.1 eV. The high resolution XPS of Co 2p in Figure 4b can be fitted into two major binding energy at 797.7 eV for Co 2p1/2 and 781.8 eV for Co 2p3/2 as well as two satellite peaks at 802.6 and 785.5 eV, which are identified as the binding energies of Co2+.40,41 As Ni 2p spectrum in Figure 4c illustrated, the two major binding energy at 874.4 eV for Ni 2p1/2 and 856.7 eV for Ni 2p3/2 and two satellite peaks at 879.8 and 861.7 eV are identified as the binding energies of Ni2+.42 Figure 4d shows P 2p spectrum with a symmetric peak at 134.1 eV, suggesting +5 state for all P atoms in the product.21 The O 2p spectrum in Figure 4e can be fitted as two peaks at 531.4 and 532.9 eV, corresponding to lattice oxygen and surface chemisorbed oxygen in hydroxyl, repectivly. 40,43 The deconvolution results of N 1s spectrum at binding energy of 399.8 and 401.9 eV is also shown in Figure 4f, which can be assigned to N3+ for the NH3 and NH4+ groups, repectivly.44,45



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Figure 4 XPS spectra of the Co0.2Ni0.8 pyrophosphate at the calcination temperature of 400 °C: (a) survey spectrum, (b) Co 2p spectrum, (c) Ni 2p spectrum, (d) P 2p spectrum, (e) O 1s spectrum, and (f) N 1s spectrum.

The electrochemical properties of the Co0.2Ni0.8 precursor and Co0.2Ni0.8 pyrophosphates prepared at different temperature were studied firstly. A shown from the representative cyclic voltammetry (CV) curves at scan rate of 5 mV s−1 in Figure 5a, the redox peaks appear in all the CV curves, exhibiting quasi−reversible electron−transfer for Faradaic redox reactions. The redox reactions are arisen from the active elements of Ni2+/Ni3+ and Co2+/Co3+ in Co0.2Ni0.8 precursor or / and Co0.2Ni0.8 pyrophosphates at the interface between electrolyte and electrode: CoxNi1−xNH4(OH)PO4·H2O + OH− ⇄ CoxNi1−xNH4Ni(OH)2PO4·H2O + e− or /


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and Co2xNi2−2xP2O7 + OH− ⇄ Co2xNi2−2xP2O7(OH) + e−.23,24,46 Furthermore, the Co0.2Ni0.8 pyrophosphates prepared at 400 °C shows the largest CV areas, corresponding to a highest electrochemical capacitance among these products. Then, the capacitive performances were directly evaluated by the galvanostatic charge−discharge (GCD) measurements at a potential window from 0 to 0.55 V at current density of 1.5 A g−1. As shown in Figure 5b, the charge−discharge curves display a relatively symmetric shape, implying favorable supercapacitive behaviors for all samples. The peak-shaped CV and non-linear GCD curves imply that the as prepared materials are more like a battery-type electrodes.47 On the basis of GCD curves, the specific capacitance are calculated and shown in Figure 5c, which exhibits 1004, 899, 1259, 1174, and 579 F g−1 for precursor, Co0.2Ni0.8 pyrophosphates prepared at 300, 400, 500, and 600 °C, respectively. The Co0.4Ni1.6P2O7 product prepared at 400 °C showed largest specific capacitance than the precursor and Co0.2Ni0.8 pyrophosphate at 300, 500, and 600 °C, which can be realized as a relatively optimal calcination temperature. To give possible reasons, the BET surface areas among these samples are measured firstly, which are 18.0, 5.2, 2.9, and 26.3 m2g−1 for Co0.2Ni0.8 pyrophosphate prepared at 300 °C, 400 °C, 500 °C, and 600 °C, respectively. The highest BET surface areas for Co 0.2Ni0.8 pyrophosphate prepared at 600 °C may be caused by the formed porous structure as shown TEM image in Figure S2e of supporting information. The Co0.2Ni0.8 pyrophosphate prepared at 400 °C showed a relatively low BET surface area because the growth of particle size during heat treatment. The low surface area don not benefit the redox reactions. Then, the impedance electrochemical impedance spectroscopy was compared, as shown in Figure S3 of supporting information. The solution resistance (Rs) can be calculated to be 0.76, 0.68, 0.74,



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and 0.80 Ω for Co0.2Ni0.8 pyrophosphates synthesized at 300, 400, 500, and 600 °C, respectively. Obviously, the 400 °C shows the relatively lower Rs than other products, indicating a more favorable pathway for ion and electron transport and subsequently induces a higher redox capacitance.48 In addition, the more reactive activity and deeper diffusion of the electrolyte ions to access the active materials for amorphous phase is also important for the improvement of specific capacitance over Co0.2Ni0.8 pyrophosphate prepared at 400 °C.38,49 These results indicate the feasibility of rationally constructing amorphous phase for tuning the electrochemical performance. Furthermore, the amorphous phase of microplates with adjusting Co-Ni compositions (e. g. Co, Co0.7Ni0.3, Co0.5Ni0.5, and Ni pyrophosphates) were also successfully fabricated as analyzed by XRD, SEM, and TEM characterizations (see Figure S4 and S5 in supporting information). The electrochemical performance pyrophosphates prepared at 400 °C with different ratio of Co/Ni were studied. Figure 5d present the CV curves of CoxNi1−x pyrophosphates at scan rate of 5 mV s−1. The anodic and cathodic sweepings reflect that the oxidation and reduction peaks at 0.16 V and 0.13 V shift to 0.52 V and 0.33 V respectively with the Co/Ni content continually adjusted from amorphous monometallic Co pyrophosphate to monometallic Ni pyrophosphate. It is worth noting that the CV curves expanded with increasing of Ni content and largest CV areas is appeared at amorphous bimetallic Co0.2Ni0.8 pyrophosphate, indicating that electrochemical performance can be tailored through rationally controlling the Co/Ni ratio over pyrophosphates. Figure 5e shows the GCD curves at a potential window from 0 to 0.55 V at current density of 1.5 A g−1 for CoxNi1−x pyrophosphates, exhibiting a symmetric shape for all samples. The specific


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capacitance were calculated to be 358, 849, 1056, 1259, and 945 F g−1 at 1.5 A g−1 for Co, Co0.7Ni0.3, Co0.5Ni0.5, Co0.2Ni0.8, and Ni polyphosphates, respectively. Obviously, the highest specific capacitance locates at Co0.2Ni0.8 polyphosphate, which may be the result of the electro−active sites participated in the redox reaction from valence interchange or charge hopping between Co and Ni cations.30,31 Thus, rationally adjusting the Co and Ni ratios for polyphosphate provides an efficient method to tuning the super capacitance.

Figure 5 (a) CV curves, (b) GCD curves, and (c) calculated specific capacitance from the GCD curves for the Co0.2Ni0.8NH4PO4·H2O and Co0.2Ni0.8 pyrophosphates at the calcination temperature from 300 to 600 °C; (d) CV curves, (e) GCD curves, and (f) calculated specific capacitance from the GCD curves for the products CoxNi1−x pyrophosphates at the calcination temperature of 400 °C. All measurements are carried out in three electrode system.



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Figure 6a shows the CV curves of amorphous Co0.2Ni0.8 polyphosphate prepared at 400 °C at scan rates ranging from 1 to 20 mV s−1. With increasing scan rates, the CV curves expand but clear redox peaks are kept, which displays a favorable stability in high scan rates. It can be observed that the potentials of oxidation peak in anodic reaction and reduction peak in cathodic reaction slightly shift to more positive/negative potentials respectively from 5 to 50 mV s−1, which may be caused by the increase of internal diffusion resistance.33 Figure 2b illustrates the GCD measurements within a potential window of 0 to 0.55 V at a set of current densities ranging from 1.5 to 30 A g−1. All CD curves exhibited symmetric shape, suggesting favorable redox capacitive behaviors. The specific capacitance are calculated as shown in Figure 6c, which exhibits that the specific capacitance is 1259 F g−1 at 1.5 A g−1 and decreased to 656 F g−1 at 30 A g−1. Such a capacitance (1259 F g−1 at 1.5 A g−1) is higher than the previous reported value for pyrophosphates such as Ni2P2O7 microstructures (approximate 790 F g-1 at 1.5 A g-1),24 porous Co2P2O7 microflowers (approximate 510 F g-1 at 1.5 A g-1),27 Co2P2O7 nanorods (483 F g-1 at 1.5 A g-1),26 and Na doped Ni2P2O7 (557 F g-1 at 1.5 A g-1).22 But, this specific capacitance is still lower than the nanocrystals of Ni2P2O7 (1893 F g-1 at 2 A g-1),23 which may be caused by the different crystal sizes or synthetic methods. Anyway, the incorporation of bimetallic transition metal as well as constructing amorphous phase reported here provides a promising method to enhance the electrochemical specific capacitance for pyrophosphates. Finally, the charge and discharge for approximate 1,000 cycles at a current density of 1.5 A g−1 are presented in Figure 6d. The capacitance value remained 88.9 % for amorphous Co0.2Ni0.8 polyphosphate prepared at 400 °C, exhibiting excellent cycling ability. However, there is only 34.09 % retained for the Co0.2Ni0.8NH4PO4·H2O precursor. These


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results indicate that the isotropic amorphous phase of Co0.2Ni0.8 polyphosphate is strongly beneficial to the long−term electrochemical stability.

Figure 6 (a) CV curves at different scan rates and (b) GCD curves of at different current densities and the (c) calculated specific capacitance from the GCD curves for Co0.2Ni0.8 pyrophosphate at the calcination temperature of 400 °C; (d) the CD cycling at the current density of 1.5 A g−1 for the Co0.2Ni0.8 pyrophosphate and Co0.2Ni0.8NH4PO4·H2O precursor. All measurements are carried out in three electrode system.

The ASCs was assembled by employing the amorphous Co0.2Ni0.8 pyrophosphate prepared at 400 °C as active material in positive electrode, AC as active material in negative electrode (electrochemical properties can be seen in Figure 6a-b of supporting information), KOH solution as electrolyte, and a diaphragm to separate the positive and negative electrodes. The 1.6 V is used for the measurements of ASCs as determined from the CV curves of AC (from −1.05 V to 0 V) and amorphous Co0.2Ni0.8 pyrophosphate (from 0 to 0.55 V) at scan rates of 5



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mV s−1 as shown in Figure S6c of supporting information. The CV curves of the ASC recorded at voltage windows from 0 to 1.6 V at scan rates from 5 to 50 mV s−1 are shown in Figure 7a. The shape of the CV curves exhibited a faradaic redox-capacitance and is maintained well with the can rates increasing from 5 to 50 mV s−1, indicating a favorable reversibility and high rate capability over the as fabricated ASC. The specific capacitance is measured by GCD at densities from 1 to 7 A g−1 are illustrated in Figure 7b. The charge−discharge curves show favorable electrochemical capacitance but the shape is not very symmetric, which means that there are some irreversible electrochemical reactions in charge and discharge process. According to these the GCD curves, the specific capacitance were calculated, which are 119 F g−1 at 1 A g−1 and decrease to 46 F g−1 at 7 A g−1, as illustrated in Figure 7c. The cycling performance the ASC evaluated at a current density of 1.5 A g−1 as shown in Figure 7d. There is 80 % of specific capacitance retained and the columbic efficiency of is kept approximate 96 % when the charge−discharge cycled for approximately 2,000 times, exhibiting favorable cycling stability over the fabricated ASC. To demonstrate the potential application as power devices over the ASCs, two serried ASCs that were charged to 3.0 V in the expectation to light a 2.0 V white light−emitting diode (LED). As shown in inset of Figure 7d, the LED can easily be lighted. Therefore, based on the electrochemical performance studied above, the fabricated ASCs can act as favorable stable and low cost energy storage devices.


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Figure 7 (a) CV curves at different scan rates, (b) GCD curves at different current densities, (c) calculated specific capacitance at different current densities, and (d) cycling performance at 1.5 A g−1 under the measurements of ASC, the inset is a photograph of an LED lighted by two serried cells.

Finally, in order to evaluate the performance of the fabricated ASCs as energy storage devices, the power density (P) and energy density (E) at various current densities were calculated, which are plotted on a Ragone diagram as illustrated in Figure 8. A maximum energy density of 42.4 Whkg−1 is achieved at the power density of 800 W kg−1. Furthermore, a maximum power density of 5.6 kW kg−1 can be obtained in optimized ASC. For comparison purpose, the energy and power fields for lithium ion batteries (LIBs) and EDLC are also exhibited in Figure 8. The energy densities for as fabricated ASCs are closed to the lower end of LIBs and much superior to EDLC. As shown in Figure 8 and the table S2 in supporting information, such performance exhibits superior performance to other transition metal phosphate based asymmetric supercapacitors such as Ni2P nanosheets/Ni foam//AC (26 Whkg−1 at 1.29



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kWkg−1)50 and Na−doped Ni2P2O7 // Graphene (23.4 Whkg−1 at 800 Wkg−1).22 But is lower than the high power device of porous graphitic carbon // Ni2P2O7 nanoparticles (65 Whkg−1 at 800 Wkg−1).23 The energy density in our work also shows higher or closed to other transition metal compounds based ASCs in recently reported work, such as 35.25 Wh kg−1 at 800.1 W kg−1 for Co2AlO4@MnO2 nanosheets // Fe3O4 nanoflake51 and 48 Wh kg−1 at 1.4 KW kg−1 for the MnCo2O4@Ni(OH)2 // AC devices.52

Figure 8 Ragone plot of as assembled ASC and a comparison with the fields of EDLC, Li−ion batteries, and references.

CONCLUSION In conclusion, pyrophosphates with different Co−Ni compositions and crystallinity was fabricated. Through controllable adjusting the ratios of Co and Ni as well as the calcination temperature, the electrochemical performance can be tuned. An optimized condition locates at amorphous phase of Co0.2Ni0.8 pyrophosphate prepared at calcination temperature of


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400 °C. The material shows specific capacitance of 1259 F g−1 at 1.5 A g−1 and retained 88.9 % after 1,000 cycles of charge and discharge in three electrodes system. Furthermore, the ASCs were assembled by employing the optimized Co0.2Ni0.8 pyrophosphate as a positive electrode and AC as a negative electrode, which possessed capacitance as 119 F g−1 at 1 A g−1. The specific capacitance was retained 80 % when the ASC charged−discharged for approximately 2,000 times. Furthermore, a maximum power density of 5.6 kW kg−1 can be obtained in optimized ASC. This study provides a facile method to construct inexpensive bimetallic transition metal pyrophosphate as efficient electrodes in asymmetric electrochemical capacitors.

ASSOCIATED CONTENT Supporting Information Detailed reactions in materials synthesis, TG and DTA analysis of precursor, SEM images, TEM images, EIS plots, XRD profiles, and elements analysis of Co-Ni pyrophosphates, electrochemical performances of AC, detailed information for referenced supercapacitors. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author * E−mail: [email protected], [email protected]



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Funding This work is supported by National Natural Science Foundation of China (No. 51402364), General Financial Grant from the China Postdoctoral Science Foundation (No. 2016M592443), and Shenghua Lieying project of Central South University. Notes The authors declare no competing financial interest.

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