Polypyrrole-Modified NH4NiPO4·H2O Nanoplate Arrays on Ni Foam

Research Article pubs.acs.org/journal/ascecg

Polypyrrole-Modified NH4NiPO4·H2O Nanoplate Arrays on Ni Foam for Efficient Electrode in Electrochemical Capacitors Chen Chen,† Ning Zhang,*,†,‡ Xiaohe Liu,*,† Yulu He,† Hao Wan,† Bo Liang,† Renzhi Ma,† Anqiang Pan,† and Vellaisamy A. L. Roy*,‡ †

School of Minerals Processing Bioengineering, and School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China S Supporting Information *

ABSTRACT: The growth of active materials on Ni foam was proved to be an effective strategy to enhance redox reactions for electrochemical capacitors. But for NH4NiPO4·H2O materials, a uniform nanostructure on Ni foam has not been achieved. Here, the NH4NiPO4·H2O nanoplate arrays on Ni foam are fabricated by a facile solvothermal method. To further promote the transitions of current carries and benefit the electrochemical reactions, the surfaces of NH4NiPO4·H2O nanoplate arrays were modified by a thin layer of polypyrrole through an in situ chemical polymerization method. The polypyrrole-modified NH4NiPO4·H2O nanoplate arrays on Ni foam showed much enhanced specific capacitance in comparison with the bare NH4NiPO4·H2O nanoplate arrays. When employing polypyrrole-modified NH4NiPO4·H2O/Ni foam as a positive electrode and activated carbon as a negative electrode to assemble the asymmetric supercapacitor cells, favorable capacitance and cycling ability were achieved. Such a fabrication provides a feasible method to construct efficient electrodes for sustainable electrochemical energy storage. KEYWORDS: Electrochemical capacitor, NH4NiPO4·H2O, Polypyrrole, Nanoplate, Ni foam

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INTRODUCTION Electrochemical capacitors (ECs), also called supercapacitors (SCs), have been realized as one of the most promising sustainable chemical methods to store energy because of their advantages of fast and highly reversible storage and release of electric energy.1−6 The redox capacitance-type SCs store energy by fast redox reactions on the surface of electrodes to result in high pseudocapacitance, which has attracted considerable attention in the fields of SCs.7−9 Fabrication of electrode materials that benefits the kinetics of ion and electron transport for the Faradaic redox reaction is crucial for improving electrochemical performance. Transition metal phosphates are inexpensive electrochemically active materials that exhibit broad and potential applications in catalysis, magnetism, and energy storage fields.10−13 Among them, hydrated ammonium metal(II) phosphate (NH4MIIPO4·H2O, MII = Mn, Fe, Co, and Ni) exhibits a layered structure, which has possibly improved the diffusion of ions and electrons for Faradaic redox reactions in SCs.14−16 Recently, various micro- and nanoscale NH4MIIPO4· H2O with different morphologies and compositions, such as NH4CoPO4·H2O nanoplates, NH4NiPO4·H2O microflowers, mesoporous NH4NiPO4·H2O, and NH4CoPO4-supported Au and Ag, were fabricated as electrodes and exhibited satisfactory electrochemical performance.17−22 However, the previously © XXXX American Chemical Society

reported capacitance value is still far from its theoretical value (e.g., approximately 2000 F g−1 for NH4NiPO4 at 1.5 A g−1). To efficiently improve the specific capacitance, electrode materials growing on Ni foam provide a favorable electrical conductivity, low diffusion resistance to ionic species, easy electrolyte penetration, and large electroactive area.23−25 Thus, far, although various electrode materials including transition metal oxides, hydroxides, and polymers have been successfully composited on active substrates, 26−30 the growth of NH4MIIPO4·H2O on conductive substrates has not been reported for electrodes. As most methods for fabricating NH4MIIPO4·H2O have generally involved rapid coprecipitation,14−16 which will easily generates NH4MIIPO4·H2O materials with scattered and large-sized particles on the substrate and difficult to satisfy the demands of compact and large surface areas for high-performance electrode materials. It is important to develop proper synthetic method to seeding active crystals on the surface of Ni foam. On the other hand, surface-modified conductive polymers such as polypyrrole (PPy) can improve the conductivity and transmission of charge carriers for electrochemically active Received: June 15, 2016 Revised: August 12, 2016

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DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Process for NH4NiPO4·H2O@PPy/Ni Foam Electrode

materials.31−37 If PPy layers were used to modify the surface NH4MIIPO4·H2O nanoplates on Ni foam material to form a three layers of PPy−NH4MIIPO4·H2O-conductive substrates, the interface resistance between electrolytes and NH4MIIPO4· H2O will be further decreased, which will strongly promote redox reactions. In this study, the uniformly grown NH4NiPO4·H2O compact nanoplate arrays on a Ni foam substrate (NH4NiPO4·H2O/Ni foam) were first achieved in a water−ethanol solvothermal environment. The surface of active materials was modified with a thin layer of PPy (NH4NiPO4·H2O@PPy/Ni foam) to promote the transition of electrons during electrochemical reactions. The NH4NiPO4·H2O@PPy/Ni foam exhibited excellent electrochemical performance in the three-electrode system. The asymmetric supercapacitor cells (ASCs) were assembled by employing NH4NiPO4·H2O@PPy/Ni foam as a positive electrode and activated carbon (AC) as a negative electrode, which exhibited favorable supercapacitance, cycling ability, and energy densities.

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seeding electrode crystals uniformly on the surface of Ni foam.8,9 The designed fabrication of NH4NiPO4·H2O@PPy/Ni foam materials is briefly summarized in Scheme 1. First, NH4NiPO4·H2O grew on the surface of cleaned Ni foam by a facial hydrothermal method in an ethanol/water solvent to decrease the reaction rates of precipitation and crystal growth of NH4NiPO4·H2O. Then, the pyrrole (Py) monomer was oxidized on the surface of NH4NiPO4·H2O to form a PPy layer in acetonitrile solution assisted by Fe3+. Finally, a three-layered electrode of NH4NiPO4@PPy·H2O/Ni foam was formed. Powder X-ray diffraction (XRD) was performed to characterize the as-synthesized products. Figure 1a shows the typical

EXPERIMENTAL SECTION

Materials Synthesis. All reagents are AR grade without further purifying. A hydrothermal method was used to synthesize the NH4NiPO4·H2O/Ni foam materials. NiSO4·6H2O (0.00038 mol) and (NH4)3PO4·3H2O (0.00394 mol) were mixed and dissolved into a solvent with the ratio of deionized water:ethanol = 1:2. After room temperature stirring, the solution was transferred into a stainless steel autoclave (40 mL Teflon lined). The cleaned nickel foam was put into the autoclave, which was heated to 140 °C and maintained for 12 h in an electronic furnace. After cooling to room temperature, the nickel foam was ultrasonicated and washed with deionized water and ethanol and then was dried in an electronic furnace at 60 °C for about 10 h. Pyrrole (Py) (C4H5N, 50 μL) was dissolved into acetonitrile to form a solution of 20 mL. After vigorous magnetic stirring at room temperature for 30 min, the as-prepared NH4NiPO4·H2O/Ni foam was added into the Py solution and kept for 30 min. Then, the FeCl3 solution, which prepared by dissolving FeCl3 (0.000617 mol) into 20 mL acetonitrile, was added into the above mixture of Py and NH4NiPO4·H2O/Ni foam for polymerization. The polymerization time was about 30 min. After that, the nickel foam was ultrasonicated and washed several times with deionized water and ethanol and then dried at 60 °C for 10 h. The fabrication of supercapacitors and measurements of electrochemical performance are detailed in the Supporting Information. Materials Characterization. X-ray diffraction was conducted on a RIGAKU Rint-2000 X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54184 Å). Scanning electron microscopy and energy dispersive spectrometer mapping images were recorded using FEI Helios Nanolab 600i field emission scanning electron microscopy. Transmission electron microscopy was performed using FEI Tecnai G2 F20 field emission transmission electron microscopy operated at 200 kV. Fourier transform infrared (FT−IR) spectroscopy was used on a Nicolet Nexus 670 instrument. Raman spectra were recorded using Renishaw Invia Raman spectrometry.

Figure 1. XRD profiles of (a) as-synthesized NH4NiPO4·H2O/Ni foam and (b) NH4NiPO4·H2O@PPy/Ni foam. Standard XRD profiles of Ni and NH4NiPO4·H2O are at the bottom.

XRD profile of the NH4NiPO4·H2O/Ni foam materials that were synthesized at 120 °C for 12 h by using a water−ethanol solvent. All the diffraction peaks were well indexed to metal Ni (as illustrated in the standard XRD profile of JCPDS No. 040850) and orthorhombic NH4NiPO4·H2O (as shown in the standard profile of JCPDS No. 50-0425). No other impurity was observed, indicating that the pure phase of NH4NiPO4· H2O on Ni foam was obtained. After chemical oxidization of Py on the surface, a similar XRD profile was observed as shown in Figure 1b, indicating that the compositions were maintained effectively after surface modifications. Morphologies of the as-prepared products were characterized through scanning electron microscopy (SEM). Figure 2a shows the SEM image of the Ni foam after a solvothermal process, displaying that the 3D structure of the nickel foam was perfectly maintained. The coarse surface of the Ni foam indicates that the product was compactly grown on the surface. The SEM image in Figure 2b illustrates that the precipitated products exhibited uniformly plate-like arrays with each plate having a width of approximately 5−10 μm. These plate arrays connected with each other and aligned vertically when growing on the nickel foam, resulting in a branch-like shape viewed from the top. The higher magnified SEM image in Figure 2c displays the side face of supported nanoplates in detail; a smooth face with a thickness of 100−150 nm is clearly observed. The energy

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RESULTS AND DISCUSSION Synthetic conditions for slowing reactions, proper solvents, surfactants, and reaction temperature were proved effective for B

DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. (a, b) SEM and (c, d) TEM images of NH4NiPO4·H2O@ PPy/Ni foam.

Furthermore, the thickness of the nanoplates was slightly increased to approximately 200 nm compared with the uncoated NH4NiPO4·H2O nanoplates. The core−shell structure of the PPy-coated NH4NiPO4·H2O can be verified further by the representative TEM image in Figure 3c, which exhibits nanoplates with sizes of approximately 2.5 μm. Consistent with the SEM images, there were nanoparticles on the surface of the nanoplate. The higher magnified TEM image in Figure 3d shows a notable contrast between the NH4NiPO4·H2O nanoplate core and polymer shells. The thickness of the outer layer is approximately 20 nm and completely encapsulating the nanoplate, exhibiting clearly that a layer of polymer is formed on the surface. Such a structure was quite different to the bare NH4NiPO4·H2O nanoplates as shown in Figure S2 of the Supporting Information. Furthermore, Fourier transform infrared spectroscopy and Raman spectroscopy give additional evidence of PPy on the surface of NH4NiPO4·H2O as presented and discussed in Figures S3 and S4 of the Supporting Information. The cyclic voltammetry (CV) curves of NH4NiPO4·H2O/Ni foam and NH4NiPO4·H2O@PPy/Ni foam at a scan rate of 5 mV s−1 measured in a three-electrode system are presented in Figure 4a. The redox peaks within 0 to 0.55 V are apparent in all CV curves, illustrating two quasireversible electron-transfer processes of pseudocapacitive Faradaic redox reactions: NH4NiPO4·H2O + OH− ⇄ NH4Ni(OH)PO4·H2O + e− and NH4Ni(OH)PO4·H2O + OH− ⇄ NH4Ni(OH)2PO4·H2O + e−.17,21 The CV curves of the NH4NiPO4·H2O@PPy/Ni foam have drastically expanded with a shift of redox potentials, corresponding to a markedly larger capacitance than that of the bare material of the NH4NiPO4·H2O/Ni foam. Moreover, two pairs of anodic/cathodic peaks can be observed for the NH4NiPO4·H2O/Ni foam at 0.42/0.27 and 0.48/0.33 V because of the redox reactions of Ni2+/Ni3+ and Ni3+/Ni4+. For the NH4NiPO4·H2O@PPy/Ni foam, a broad anodic reaction peak appeared at around 0.51 V, and cathodic reaction peaks shift to 0.33 and 0.39 V. The redox peaks shifts can be realized as synergistic effects between NH4NiPO4·H2O and PPy in that PPy cannot only enhance the electrical conductivity of the electrode but can also contribute to the redox reactions and subsequently improve the pseudocapacitance.31,34,37 The galvanostatic charge−discharge (GCD) measurements within a potential window from 0 to 0.55 V at current densities

Figure 2. (a−c) SEM and (d) elemental mapping images of assynthesized NH4NiPO4·H2O/Ni foam by using water/ethanol as solvent. SEM images of the products by using (e) water and (f) ethanol as solvent.

dispersive spectrometer elemental mapping images in Figure 2d show identical elemental maps of Ni, O, P, and N, indicating that these elements were uniformly distributed. According to these characterizations, the Ni foam-supported uniform NH4NiPO4·H2O nanoplate arrays were successfully synthesized. During solvothermal synthesis, solvents play a crucial role in the formation of NH4NiPO4·H2O crystals on Ni foam. In this study, when the solvent was changed to water, only irregular NH4NiPO4·H2O microplates of large size grew sparsely on the surface of the Ni foam, as reflected by the SEM image in Figure 2e. Under this condition, the precipitation reaction and crystal growth in the water solvent seemed too fast to generate uniform and compact NH4NiPO4·H2O nanoplates on Ni foam. However, when pure ethanol was used as the solvent, obtaining the grown product was difficult, as shown in the SEM image of Figure 2e. This may have been resulted from the poor solubility of (NH4)3PO4·3H2O and NiSO4·6H2O in ethanol and the product NH4NiPO4·H2O is difficult generated (the evidence is illustrated in Figure S1 of Supporting Information). In our synthesis, the deionized water/ethanol =0.5 was found to be an optimal condition for generating uniform and compact NH4NiPO4·H2O nanoplate arrays on Ni foam. After the growth of NH4NiPO4·H2O nanoplate arrays on Ni foam, the product was immersed in Py solution and then oxidized by Fe3+ for surface modification. Figure 3a shows SEM images of the product after the modification process. The NH4NiPO4·H2O vertically grown plate arrays were perfectly maintained except for some micro- and nanoscale particles on the surface. The modified structure is more apparent in the magnified SEM image of Figure 3b. The surface becomes blurry and rough with nanoparticles observed on the surface, which may have been formed by the surface polymerization of Py. C

DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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semicircle, revealing a low charge transfer resistance (Rct). The Rct values are merely 0.19 Ω for the NH4NiPO4·H2O@PPy/Ni foam and 0.26 Ω for the NH4NiPO4·H2O/Ni foam, according to the semicircular arc diameters. Such a comparison indicates a more tightly coupled interfacial contact existing between PPy@ NH4NiPO4·H2O and the Ni foam, and the conductive polymer of PPy could provide a more favorable pathway for ion and electron transport than the NH4NiPO4·H2O/Ni foam and subsequently a higher redox capacitance.9 Finally, charge and discharge measurements were studied for 500 cycles at a current density of 5 A g−1. As shown in Figure S5 of the Supporting Information, the capacitance value retained approximately 75% for the NH4NiPO4·H2O@PPy/Ni foam. But for the NH4NiPO4·H2O/Ni foam, only 58% capacitance remains. Obviously, the NH4NiPO4·H2O@PPy/Ni materials also demonstrated superior stability to those of NH4NiPO4· H2O/Ni. To study the possible application of the as-synthesized NH4NiPO4·H2O@PPy/Ni foam material, ASC was assembled. As shown in Figure 5a, the NH4NiPO4·H2O@PPy/Ni foam

Figure 4. (a) CV curves of NH4NiPO4·H2O/Ni foam and NH4NiPO4· H2O@PPy/Ni foam at scan rates of 5 mV s−1. (b) GCD curves of NH4NiPO4·H2O@PPy/Ni foam at various current densities. (c) Specific capacitance of NH 4 NiPO 4 ·H 2 O@PPy/Ni foam and NH4NiPO4·H2O/Ni foam at various current densities. All measurements were carried out in a three-electrode cell. (d) EIS plots of the two materials (inset: enlarged curves at high frequency region).

ranging from 5 to 20 A g−1 are shown in Figure 4b. The charge−discharge curves display a near symmetric shape, implying favorable supercapacitive behaviors. According to GCD curves in Figure 4b, a big voltage drop is located from 0.28 to 0 V for the NH4NiPO4·H2O@PPy/Ni foam material in the discharge curve. In considering that the Faradaic redox reactions have not occurred and the electrodes have very low capacitance at such a low voltage (e.g., from 0.28 to 0 V) according to the CV curves in Figure 4a, the discharge behaviors are very fast, and a big voltage drop is formed. The specific capacitance could be calculated as 1513 F g−1 at 5 A g−1 and decreased to 861 F g−1 at 20 A g−1 as shown in Figure 4c. In comparison with the NH4NiPO4·H2O/Ni foam, the NH4NiPO4·H2O@PPy/Ni foam showed enhanced electrochemical performance, improving not only the capacitance (1513 vs 964 F g−1 at 5 A g−1) but also the stability at high current densities (retained 56.9% vs 32.7% from 5 to 20 F g−1). In addition, such a high pseudocapacitive behavior (e.g., 1513 F g−1 at 5 A g−1) is superior to all those previously reported on an NH4MPO4·H2O-type material, such as 369.4 F g−1 at 0.625 A g−1 for NH4CoPO4·H2O nanoplates,21 approximately 900 F g−1 at 5 A g−1 for NH4NiPO4·H2O microrods,17 approximately 930 F g−1 at 1.5 A g−1 for mesoporous NH4NiPO4·H2O,18 and approximately 620 F g−1 at 3 A g−1 for NH4CoPO4·H2O microbundles.22 Electrochemical impedance spectroscopy was compared, as shown in Figure 4d. The EIS data shows a solution resistance, Rs (the resistance of the KOH aqueous solution, the intrinsic resistance of the electroactive materials, the contact resistance of material with substrate), and a charge−transfer resistance, Rct. A vertical line leaning to an imaginary axis represents the value of Rs. As shown in the inset, Rs can be found to be 0.7 Ω for the NH4NiPO4·H2O@PPy/Ni foam and 0.9 Ω for the NH4NiPO4·H2O/Ni foam, indicating that a more tightly coupled interfacial contact exists between NH4NiPO4·H2O@PPy and the nickel foam current collector. In the low frequency region, the slope of the impedance plot almost tends to a vertical asymptote, suggesting an ideal capacitor. At a high frequency region, it displays a negligible

Figure 5. (a) Structure, (b) CV curves at different scan rates, (c) GCD curves at different current densities, (d) calculated specific capacitance, and (e) cycling performance at 3 A g−1 under the measurements of ASC. (f) Image of a LED driven by two serried ASCs.

was employed as a positive electrode. AC pasted on Ni foam was used as the negative electrode, and a 3 M KOH solution served as the electrolyte. A diaphragm is used to separate the positive and negative electrodes. The electrochemical performance of AC is shown in Figure S6a−d of the Supporting Information, and a stable voltage window of 1.5 V is used for ASC based on the scan windows of AC and NH4NiPO4·H2O@ PPy/Ni foam in a three-electrode system. Figure 5b shows the CV curves of the ASC device collected at voltage windows from 0 to 1.5 V in a 3 M KOH solution at different scan rates from 10 to 50 mV s−1. The figure shows a typical Faradaic pseudocapacitive shape, which results from the fast redox reactions on the NH4NiPO4·H2O@PPy/Ni foam electrodes. D

DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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approaching the lower end of LIBs and is much superior to that of EDLC at the same power level. This performance shows better performance than other transition metal phosphatebased ASCs (for detailed information, see Table S1 of the Supporting Information) such as Ni2P nanosheets/Ni foam− AC (26 Wh kg−1 at 1.29 kW kg−1)38 and Na-doped Ni2P2O7− graphene (23.4 Wh kg−1 at 800 W kg−1).39 But the performance is lower than the high power device of porous graphitic carbon−Ni2P2O7 (65 W hkg−1 at 800 W kg−1).40 Furthermore, in comparison with some other transition metalbased asymmetric supercapacitors (for detailed information, see Table S1 of the Supporting Information), such as MnO2 PEDOT devices (9.8 Wh kg−1 at 850 W kg−1),41 Ni−Co sulfides−AC (25 Whkg−1 at 3.57 kW kg−1),42 graphene−Ni cobaltite−AC (7.6 Whkg −1 at 5.6 kW kg −1 ), 43 and Co0.5Ni0.5(OH)2/graphene/CNT−AC/CNT (29 Wh kg−1 at 210 W kg−1),44 the as-fabricated ASCs also showed superior energy density.

The shape of the CV curves are maintained effectively, even at a fast scan rate of 50 mV s−1, exhibiting a high rate capability and favorable reversibility of ASC. A GCD measurement was used to calculate the specific capacitance of the as-fabricated ASC. The charge/discharge curves at different current densities are shown in Figure 5c, reflecting a favorable charge/discharge capacitive behavior for the as-assembled ASC. According to theses charge/discharge curves, the gravimetric specific capacitance values were calculated at 133 F g−1 at 0.5 A g−1 and decreased to 38 F g−1 at 7 A g−1 as shown in Figure 5d, which retained only 28.6%. However, the capacitance retention for ASC is still higher than the three-electrode system: the capacitance retention is approximate 69.9% in ASC if the current densities are increased 4 times (from 0.5 A g−1 to 2 A g−1, 93 F g−1 at 2 A g−1), which is higher than the capacitance retention of 56.9% in the three-electrode system (a 4 times increase in current densities from 5 to 20 A g−1). The better retention for ASC is a result of the synergistic effect over the negative electrode of AC and the positive electrode of NH4NiPO4·H2O@PPy. Figure 5e shows the cycling performance of the NH4NiPO4·H2O@PPy/Ni foam−AC asymmetric supercapacitor, which was tested at a current density of 3 A g−1. The specific capacitance was kept perfectly in the first 500 cycles (at approximate 81 F g−1) but gradually decreased after cycling 500 times. The capacitance still has approximately 58 F g−1 and retained 71.6% after 5000 cycles, which exhibits favorable long-term cycling ability. The Coulombic efficiencies are also displayed during the charge and discharge cycles, which show above 98% during the 5000 cycling. In addition, the nanoplate arrays keep effectively during the charge and discharge process, indicating excellent structural stability of the electrode (as shown in SEM image of Figure S7 of the Supporting Information). Furthermore, the two serried NH4NiPO4·H2O@PPy/Ni foam−AC ASCs that were charged to 3.0 V showed potential to light a 2.0 V white light-emitting diode (LED), as shown in Figure 5f. As shown in Figure 6, the power density (P) and energy density (E) at different current densities are plotted on a

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CONCLUSIONS In summary, the growth of NH4NiPO4·H2O nanoplate arrays on an Ni foam substrate was achieved through a hydrothermal method. The water−ethanol solvent played an essential role in generating uniform and compact NH4NiPO4·H2O nanoplate arrays on Ni foam. The modification of PPy layer NH4NiPO4· H2O nanoplates/Ni foam further promoted the transition of electrons, which induced a high specific capacitance as 1513 F g−1 at 5 A g−1. The as-assembled ASC fabricated by employing NH4NiPO4·H2O@PPy/Ni foam as a positive electrode and AC as a negative electrode produced a specific capacitance of 133 F g−1 at 0.5 A g−1 and favorable long-term cycling ability. A maximum energy density of 41.6 Wh kg−1 can be achieved for the fabricated ASC. The NH4NiPO4·H2O@PPy/Ni structure reported here provides an efficient method for fabricating electrodes with favorable electrochemical performance for relatively unstable materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01347.

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Detailed experimental process for electrochemical measurements and additional figures. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N. Zhang). *E-mail: [email protected] (X. Liu). *E-mail: [email protected] (V. A. L. Roy).

Figure 6. Ragone plot of as-assembled NH4NiPO4·H2O@PPy/Ni foam−AC ACS against previous reports.

Funding

This work is supported by the Hong Kong Scholars Program, National Natural Science Foundation of China (51402364), General Financial Grant from the China Postdoctoral Science Foundation (2016M592443), and project of innovation-driven planning in Central South University.

Ragone diagram to exhibit the energy storage performance of a fabricated NH4NiPO4·H2O@PPy/Ni foam−AC device. The fields of electrochemical double layer capacitor (EDLC) and lithium-ion batteries (LIBs) are also displayed for comparison. A maximum energy density of 41.6 Wh kg−1 can be obtained at a power density of 375 W kg−1, and an optimized ASC can obtain a maximum power density of 8.5 kW kg−1, which is

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.6b01347 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX