honeycomb-like interconnected network of nickel...

Honeycomb-Like Interconnected Network of Nickel PhosphideHeteronanoparticles with Superior Electrochemical Performance forSupercapacitorsShude Liu,† Kalimuthu Vijaya Sankar,† Aniruddha Kundu,† Ming Ma,‡ Jang-Yeon Kwon,§

and Seong Chan Jun*,†

†School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea‡Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea§School of Integrated Technology and Yonsei Institute of Convergence Technology, Yonsei University, Yeonsu-gu, Incheon 406-840,South Korea

*S Supporting Information

ABSTRACT: Transition-metal-based heteronanoparticles areattracting extensive attention in electrode material design forsupercapacitors owing to their large surface-to-volume ratiosand inherent synergies of individual components; however,they still suffer from limited interior capacity and cyclingstability due to simple geometric configurations, low electro-chemical activity of the surface, and poor structural integrity.Developing an elaborate architecture that endows a largersurface area, high conductivity, and mechanically robuststructure is a pressing need to tackle the existing challengesof electrode materials. This work presents a supercapacitorelectrode consisting of honeycomb-like biphasic Ni5P4−Ni2P(NixPy) nanosheets, which are interleaved by large quantitiesof nanoparticles. The optimized NixPy delivers an ultrahigh specific capacity of 1272 C g−1 at a current density of 2 A g−1, highrate capability, and stability. An asymmetric supercapacitor employing as-synthesized NixPy as the positive electrode and activatedcarbon as the negative electrode exhibits significantly high power and energy densities (67.2 W h kg−1 at 0.75 kW kg−1; 20.4 W hkg−1 at 15 kW kg−1). These results demonstrate that the novel nanostructured NixPy can be potentially applied in high-performance supercapacitors.

KEYWORDS: nickel phosphide, nanosheets, heteronanoparticles, electrochemical performance, supercapacitors

1. INTRODUCTION

Supercapacitors have drawn considerable attention due to theirintriguing merits in terms of high power density, fast charge/discharge rate, and long life span.1−5 Battery-type materials oftransition metals are promising for supercapacitors owing totheir high theoretical specific capacities and energy densities incomparison to those of carbonaceous materials but fail todeliver the rapid kinetics of charge transport, which impedestheir potential technological applications for electrochemicalenergy storage.6,7 One way to address the issues regarding thesimultaneous improvement of the energy density and cyclicstability is to rationally optimize the intrinsic properties ofelectrode materials and delicately design ion-diffusion-favoredstructures.Transition-metal phosphides (TMPs) have emerged as new

electrode materials for energy conversion and storage owing totheir metalloid features and high electrical conductivity, whichare kinetically favorable for rapid electron transport that allowsfor a high rate capability.7 Also, they endow good thermal

stability and resistance to the ambient environment, thusenabling a good cyclic stability.7,8 In particular, multi-component transition-metal composites can synergisticallyimprove the electrochemical performances in terms ofelectrochemical activity, reversible capacity, and electricalconductivity.9,10 Nonetheless, mixed-metal phosphide materialsstill suffer from relatively low specific capacity and rapidcapacity fade during redox reactions. An effective strategy forsolving these issues is to design and construct a functionalelectrode featuring abundant active sites and a multistageporous structure, as well as a tailored reaction interface of activematerials/current collectors.11,12 Two-dimensional (2D) nano-sheets are promising in the field of electrochemical energystorage because they can provide a better durability andbuffering capacity to impede the volumetric variation during

Received: April 17, 2017Accepted: June 8, 2017Published: June 8, 2017

Research Article

www.acsami.org

© 2017 American Chemical Society 21829 DOI: 10.1021/acsami.7b05384ACS Appl. Mater. Interfaces 2017, 9, 21829−21838

www.acsami.org

http://dx.doi.org/10.1021/acsami.7b05384

cycling,13,14 while the ion-diffusion barrier is low, whichsupports the simultaneous achievement of a high energystorage capacity with good cycling stability, even at a highrate.15−17 From the viewpoint of electrode design, activematerials self-assembled on metallic substrates not only ensurehigh ionic/electronic conductivity but also favor chemicallystable interfaces.18,19 Despite all of this, the rational engineeringof zero-dimensional (0D) and 2D building blocks on metallicsubstrates possessing synergistic structure characteristics is stillan ongoing technical challenge to further improve theirelectrochemical performances.20

Herein, we propose a facile strategy to construct asophisticated architecture supported on nickel foam consistingof honeycomb-like porous NixPy nanosheets with abundantinterconnected heteronanoparticles. Benefiting from thesynergistic effect of the multicomponent systems and 0D/2Dbuilding blocks, the synthesized NixPy delivers an ultrahighspecific capacity of up to 1272 C g−1 at 2 A g−1 and a goodcycling stability with 90.9% capacity retention after 5000 cycles.The constructed asymmetric supercapacitor with optimizedNixPy and activated carbon (AC) as electrode materials displaysa high energy density of 67.2 W h kg−1 at 0.75 kW kg−1. Theseresults indicate that the rational assembly and heterogrowth ofactive materials are promising for high energy densitysupercapacitors while maintaining a high power density.

2. EXPERIMENTAL SECTION2.1. Materials Preparation of Honeycomb-Like Intercon-

nected Network of NixPy Heteronanoparticles. Prior to synthesis,a piece of nickel foam (2 × 3 cm2) was cleaned with a 2 M HClsolution in an ultrasonic bath for 20 min in order to remove the

surface oxide layer, and then was washed using absolute ethanol anddeionized water. The pretreated nickel foam was immersed in ahomogeneous solution containing 0.025 M Ni(NO3)2·6H2O, 0.15 MNH4F, and 0.3 M CO(NH2)2. After that, the solution was transferredto a Teflon-lined stainless-steel autoclave and then heated at 120 °Cfor 12 h with a heating rate of 5 °C min−1. The obtained productsanchored on nickel foam were rinsed with distilled water and ethanolseveral times and dried at 70 °C for 12 h. The dried samples andvarious amounts of NaH2PO2·H2O (10, 20, and 30 mmol) were placedin the two boundaries of a ceramic crucible, with the NaH2PO2·H2O atthe upstream side of the furnace and heated at 400 °C for 3 h under anAr atmosphere with a heating rate of 2 °C/min. The correspondingNixPy samples were labeled as NixPy-1, NixPy-2, and NixPy-3. The meanmass load and mass error of the as-synthesized samples weresummarized in Figure S1.

2.2. Materials Characterization. The morphologies andstructures were analyzed by X-ray diffraction (XRD, Philips X’pertdiffractometer) with highly intensive Cu Kα radiation (λ = 0.154 nm),field-emission scanning electron microscopy (SEM, Hitachi, SU-8010),and transmission electron microscopy (HRTEM, JEOL, JEM-2100)equipped with an energy dispersive X-ray spectrometer. Thecomposition and chemical valence states were identified by X-rayphotoelectron spectroscopy (XPS, PHI-5702). Nitrogen adsorption/desorption measurements were conducted with a Micromeritics ASAP2460 analyzer at 77 K. The specific surface area was derived from themultipoint Brunauer−Emmett−Teller (BET) model, and the pore sizedistribution was estimated on the basis of the desorption branch of thenitrogen adsorption isotherm utilizing the Barrett−Joyner−Halenda(BJH) method.

2.3. Electrochemical Performance Measurements. All of theelectrochemical measurements were carried out using the electro-chemical workstation (Iviumstat, IVIUM Technologies) using a three-electrode configuration in a 3 M KOH aqueous electrolyte. The as-prepared samples supported on nickel foam directly served as the

Figure 1. (a) Scheme for the formation of NixPy nanosheets through two steps, including hydrothermal precipitation and a solid/gas-phasephosphorization treatment. (b) XRD patterns of NixPy-1, NixPy-2, and NixPy-3. XPS deconvoluted spectra of (c) Ni 2p3/2 and (d) P 2p for NixPy-1,NixPy-2, and NixPy-3.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b05384ACS Appl. Mater. Interfaces 2017, 9, 21829−21838

21830

http://pubs.acs.org/doi/suppl/10.1021/acsami.7b05384/suppl_file/am7b05384_si_001.pdf


working electrode, while a saturated calomel electrode (SCE) and aplatinum plate were used as the reference and the counter electrodes,respectively. Cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) measurements were carried out to investigate theelectrochemical properties of the electrodes. Electrochemical impe-dance spectroscopy (EIS) was analyzed by applying an alternating-current voltage with a 5 mV amplitude in the frequency range of 100kHz to 0.01 Hz. The specific capacities (Cs, C g−1) of the NixPysamples were calculated from the GCD curves based on the followingequation:21

= × ΔC

I tms (1)

where I (mA) represents the discharge current, Δt (s) refers to thedischarge time, and m (mg) corresponds to the mass of the activematerial.The corresponding specific capacitance can be calculated according

to the following equation:22,23

∫=Δ

CI

m VV t t

2( ) d

t

t

21

2

(2)

where ΔV (V) is the potential window, V(t) is the operating ofpotential, and t1 and t2 refer to the initial and terminational dischargetime of GCD curves, respectively.2.4. Fabrication of the NixPy//AC Asymmetric Supercapaci-

tors. The asymmetric supercapacitor (ASC) device was constructedby using NixPy as the positive electrode, AC as the negative electrode,and a cellulosic paper as the separator. For the preparation of thenegative electrode, a homogeneous slurry was first obtained by stirringthe AC, polyvinylidene fluoride, and acetylene black with a mass ratioof 80:10:10 in an ethanol solvent. The mixed slurry was spread on anickel-foam current collector with a mass loading of about 3 mg cm−2

and then dried in a vacuum oven at 70 °C for 20 h. The assembledASC device was operated in a two-electrode cell in a 3 M KOHelectrolyte. To achieve the optimal electrochemical performance of theASC device, the mass ratio of the negative electrode to the positiveelectrode was evaluated by the charge balance theory (Q+ = Q−). Themass balancing can be expressed according to following equation:21

= × Δ+

−

− −

+

mm

C VC (3)

where m+ and C+ are the mass (mg) and specific capacity (C g−1) ofthe positive electrode, respectively, and m−, ΔV−, and C− are the mass(mg), potential window (V), and specific capacitance (F g−1) of thenegative electrode, respectively.The specific capacitance (Cdevice, F g−1), energy density (E, W h

kg−1), and power density (P, W kg−1) of the ASC device werecalculated by the total mass of the two electrodes using the followingequations:24,25

= × ΔΔ ×

CI tV Mdevice (4)

= × ΔE C V1

7.2 device2

(5)

= ×Δ

PE

t3600

(6)

where I is the discharge current (mA), Δt is the discharge time (s), ΔVis the potential range (V) of the device, and M is the total weight (mg)of the two electrodes.

3. RESULTS AND DISCUSSIONThe fabrication process for NixPy nanosheets is schematicallydepicted in Figure 1a. First, Ni(OH)2 nanosheets wereprepared on the nickel foam skeleton through a hydrothermalroute. In subsequent phosphorization process, the well-definedNixPy nanosheets were obtained, while each nanosheet consists

of numerous interconnected nanoparticles along with plenty ofpores. The phase and structure of the as-prepared samples onnickel foam were verified by X-ray diffraction (XRD). All of thediffraction peaks in the XRD pattern of the nickel precursor(Figure S2) can be indexed to hexagonal β-Ni(OH)2 phase(Powder Diffraction File (PDF) No. 14−0117, JointCommittee on Powder Diffraction Standards (JCPDS),1972).26 As shown in Figure 1b, the three most intensepeaks centered at about 44.7, 52.0, and 76.6° (2θ values)belong to the nickel substrate. A tunable sodium phosphidedosage, e.g., 10, 20, and 30 mmol, was used for syntheses of theNixPy nanosheets denoted as NixPy-1, NixPy-2, and NixPy-3,respectively. After the phosphorization treatment, all identifiedpeaks can be indexed as a mixture of hexagonal-phase Ni5P4(PDF No. 18−883, JCPDS, 1965)27 and Ni2P (PDF No. 65−3544, JCPDS, 1938).28 These results indicate that thesynthesized NixPy composites are biphasic systems. As observedin the XRD result, the intensities of the diffraction peaks at 2θ =14.9, 30.5, and 36.2° in the pattern of Ni5P4 gradually decreaseas the intensities of the Ni2P diffraction peaks located at 40.8,75.0, and 80.2° slightly increase with an increase in phosphorussource dosage, which can be ascribed to partial phasetransformation from Ni5P4 to Ni2P, which is in accordancewith the previous report.29 The peak at the dominant (111) ofNi2P shifts toward lower angle direction, which is associatedwith incremental phosphorus source, resulting in an intensivecharge imbalance.30 This phenomenon is similar for otherpeaks in Ni2P. X-ray photoelectron spectroscopy (XPS) wasemployed to explore the surface chemical composition andvalence states of the as-prepared sample (Figures 1c,d and S3).Spectral parameters obtained in the NixPy composites by XPSanalysis are shown in Tables S1 and S2. For NixPy-1, peakfitting of the Ni 2p3/2 spectrum shows two peaks at 853.4 and856.8 eV, which are assigned to a very small positive charge(Niδ+) in NixPy and oxidized Ni species, respectively,31,32 whilethe peak centered at 861.6 eV corresponds to the satellite of theNi 2p3/2 peak.

32,33 The binding energy of Niδ+ species down-shifts from 853.4 eV of NixPy-1 to 853.1 eV of NixPy-3, whilethe binding energy of Ni 2p3/2 peak shifts from 856.8 to 856.5eV, indicating the newly formed Ni−P bonds.34 For the P 2pregion of NixPy-1, the peak centered at 129.2 eV is assigned to P2p3/2, with a peak ascribed to P 2p1/2 at 130.1 eV.35 The peaklocated at around 133.6 eV is allocated to the P−O bond, whichcan be indexed to the oxidized phosphorus formed on thesurface.36,37 Clearly, the binding energies of P 2p3/2, P 2p1/2,and PO4

3− peaks shown in the spectrum of the NixPy-1 have aslightly down-shift compared to those of NixPy-2 and NixPy-3.Notably, the binding energy of P 2p in NixPy is lower than thatof P0 (130.2 eV), suggesting the existence of negative charge(Pδ−) and the formation of Ni−P,33,34 which is supportive tothe aforementioned XRD results.Scanning electron microscopy (SEM) observation indicates

that the Ni(OH)2 was densely packed and uniformly coveredthe entire surface of the nickel substrate (Figure S4a). Themagnified SEM images show that the Ni(OH)2 structureconsists of a large amount of homogeneous nanosheets whichare interconnected with each other to assemble a honeycomb-like architecture (Figure S4b−d). Energy dispersive X-rayspectroscopy (EDS) reveals the uniform distribution of Ni andO elements throughout the Ni(OH)2 structure (Figure S5).After the phosphorization treatment, the overall 3D network-like morphology is well-preserved, while the smooth surface ofthe nanosheets becomes rough (Figure S6a). The magnified



21831









SEM images (Figure S6b) show that the nanosheets have alength of about 2 μm and that the thickness of the nanosheetsranges from 20 to 30 nm. A closer view reveals that thenanosheets are constructed from the interconnected hetero-NixPy-1 nanoparticles of about 20 nm in size. (Figures 2a andS6c). The substructure of the NixPy-1 nanosheets was furtherexplored by transmission electron microscopy (TEM) analysis.The nanosheets consist of a large variety of interspacesthroughout the interior nanoparticles, with diameters rangingfrom 10 to 30 nm (Figure 2d). The high-resolution TEM image(Figure 2g) shows the lattice fringes with interplanar spacingsof 0.17 and 0.25 nm, which correspond to the (300) plane ofNi2P and the (104) plane of Ni5P4, respectively. The scanningTEM (STEM) image and its corresponding energy dispersiveX-ray (EDX) spectrum verify the uniform distribution of Niand P throughout the nanosheets (Figure S7). As thephosphorization process proceeded, the NixPy-2 nanosheetswere also assembled by the interconnected hetero-NixPy

nanoparticles (Figures 2b and S6d−f). However, the surfacesof the nanosheets with mesoporous features became rough, andthe interspacing between nanoparticles was slightly increased.

This characteristic feature is further revealed by TEM images(Figure 2e,h). For NixPy-3, the interconnected nanoparticlesbecame irregular and the interspace between the nanoparticleswas locally jammed (Figures 2c,f,i and S6g−i), which restrictsthe electrolyte ion accessibility, leading to the deterioratedelectrochemical performance. The textural properties of NixPywere investigated with N2 adsorption−desorption measure-ments (Figure S8). The N2 adsorption−desorption isothermsexhibit type-H3 hysteresis loops, which are characteristic ofmesoporous structures.38 The Barrett−Joyner−Halenda (BJH)pore-size distribution curves of the Ni(OH)2 and various NixPysamples show that the average pore size ranges from 10.8 to16.2 nm (inset of Figure S8), which further reveals theirmesoporous feature. The Brunauer−Emmett−Teller (BET)surface areas of the Ni(OH)2, NixPy-1, NixPy-2, and NixPy-3nanosheets calculated from N2 isotherms were determined tobe 20.9, 37.2, 44.1, and 27.8 m2 g−1, respectively. Thesecharacteristics clearly indicate that the NixPy samples aresupportive for storing more charge for an improved specificcapacity because of the enlarged active sites and goodaccessibility of electrolyte ions.

Figure 2. SEM images of the (a) NixPy-1, (b) NixPy-2, and (c) NixPy-3 samples. TEM and HRTEM images of the (d, g) NixPy-1, (e, h) NixPy-2, and(f, (i) NixPy-3 samples. (j) Schematic illustration of the proposed phosphorization and morphology evolution process at the surface of NixPynanosheets.



21832









On the basis of the aforementioned experimental analysis,the reaction mechanism of phosphorization treatment andaccompanied morphology evolution process can be illustratedin Figure 2j. Generally, NaH2PO2·H2O serving as a precursorcan generate PH3 reactant by a solid thermal decompositionreaction.39 With the absorption of PH3 reactant at the surface ofNi(OH)2 nanosheets, the ion exchange reaction betweendiffused PH3 and internal OH− occurred on the surface.According to the above XRD and XPS results (Figure 1), wecan predict that the Ni5P4 nanosheets were initially producedusing the established phosphorization treatment. As the ionexchange process progressed, the NixPy nanosheets werechemically converted from the precursor, while nanosheetswere interwoven with nanoparticles together with disorderedmesopores, which might contribute to a high chemicalreactivity and sufficient active reaction sites for fully efficientfaradaic reactions. Because of the unequal diffusion abilitybetween phosphorus anion and OH− species, the fasteroutward hydroxyl diffusion leads to the formation of porousstructure at the surface, while the slower inward penetration ofPH3 leads to the production of nanoparticles throughout thenanosheets, which is similar to the nanoscale Kirkendall effectoperating in previous reports.40−42 For higher P/Ni ratios, theparticles show a sustained increase in size, which may derivefrom the unequal diffusion process and lattice expansion toform NixPy while conserving the number of Ni atoms in eachnanoparticle during the transformation.

Figure 3a presents the typical CV curves of the pristineNi(OH)2, NixPy-1, NixPy-2, and NixPy-3 electrodes at a scanrate of 5 mV s−1 in 3 M KOH electrolyte. The CV curvespresent a pair of distinct redox peaks, corresponding to thereversible redox transition of Ni2+/Ni3+, which can be illustratedin the following equations.43,44 The phenomenon indicates atypical Faradaic capacitive behavior of battery-type materi-als.15,45

+ ↔ +− −Ni P 2OH Ni P(OH) 2e2 2 2 (7)

+ ↔ +− −Ni P 5OH Ni P (OH) 5e5 4 5 4 5 (8)

Notably, the enclosed CV curve area of the NixPy electrodeincreases along with the phosphorization treatment and reachesa maximum value for the NixPy-2 sample. However, the integralarea of the CV curve decreases with an increased phosphatedosage, which illustrates that an excess phosphorizationtreatment leads to a depressed electrochemical activity. Clearly,the pure nickel foam has only a negligible contribution to thecurrent density of the prepared electrodes (Figure S9). EachCV curve maintains well-defined redox peaks with a slightpolarization at different scan rates ranging from 2 to 20 mV s−1,indicating good electrode kinetics (Figure S10). From therelationship between the redox peak current and the squareroot of the scan rate for the cathodic peak, it can be obviouslyobserved that the cathodic peak current increases almostlinearly with the square root of the scan rate (Figure S11),

Figure 3. Electrochemical performances of Ni(OH)2, NixPy-1, NixPy-2, and NixPy-3 electrodes. (a) Comparison of CV curves at a scan rate of 5 mVs−1. (b) Comparison of GCD curves at a current density of 2 A g−1. (c) Specific capacities versus discharge current densities. (d) EIS curves. Theinset shows an equivalent circuit. (e) Cyclic stability for 5000 cycles at a current density of 8 A g−1.



21833





indicating that the redox reaction is dominated by a diffusion-controlled battery-type Faradaic process.46−48

Figure 3b presents a comparison of the GCD curves at acurrent density of 2 A g−1 within a potential window from −0.1to 0.5 V vs SCE, in which the nonlinear trend demonstrates theFaradaic character of the electrode materials.49 Note that thepotential range of the GCD curves presents a deviationcompared with that of the CV curves, which is derived from thepolarization effect of electrode materials.50 As expected, theNixPy-2 electrode exhibits a longer discharging time than theother electrodes. Figure 3c shows the calculated specificcapacities derived from GCD curves at various current densitiesfor the prepared electrodes (Figure S12). All of the NixPyelectrodes yield a much higher specific capacity than that of theNi(OH)2 electrode at the same current density. The specificcapacity for the NixPy-2 electrode (1272 C g−1) is about 1.26times that of the NixPy-1 electrode (1009 C g−1) and 1.12 timesthat of the NixPy-3 electrode (1140 C g−1) at a current densityof 2 A g−1, which is comparable to those of reported nickel-based composites (Table S3).9,47,51−56 It can be found that thespecific capacity gradually decays as the discharge currentdensity increases because of the insufficient active materialinvolved in the redox reaction at higher current densities.Impressively, even at a high current density of 10 A g−1, thecapacity retention of about 64.0% still remained for NixPy-2,which is higher than those for the NixPy-1 electrode (63.2%)and the NixPy-3 electrode (47.5%) and is also superior to thoseof previously reported transition metal materials.56−59 To fulfill

an efficient comparison with previous reports, the correspond-ing specific capacitances of as-synthesized electrode materialsare also presented in Figure S13. The specific capacitance of2638 F g−1 for NixPy-2 is delivered when the current density is1 A g−1, which is obviously higher than those of Ni(OH)2,NixPy-1, and NixPy-3. NixPy-2 especially still maintains a specificcapacitance of 1359 F g−1 at a high current density of 15 A g−1,indicating its good rate capability.To further investigate the reason that the prepared NixPy

performs with such good capacitive behavior, electrochemicalimpedance spectroscopy (EIS) measurements were carried outin the frequency range of 0.01−100 kHz at an open-circuitcondition. The measured Nyquist plots of the EIS spectra(Figure 3d) are simulated based on an equivalent circuit (insetof Figure 3d) using the complex nonlinear least-squares fittingmethod. The obtained values of each component calculatedfrom the experimental impedance spectra are shown in TableS4. As can be seen, the NixPy electrodes possess a equivalentseries resistance (Rs) smaller than that of the Ni(OH)2electrode, revealing a vital role of the phosphorization inincreasing the conductivity of integrated electrode. Thesemicircle is obviously depressed in the high-frequency region,implying a small interfacial charge-transfer resistance (Rct) atthe electrode/electrolyte interface.7 Meanwhile, the Rct ofNixPy-2 is estimated to be 4.6 Ω, which is lower than those ofthe Ni(OH)2 (12.4 Ω), NixPy-1 (6.0 Ω), and NixPy-3 (6.9 Ω),thus making for a good rate capability.

Figure 4. (a) Schematic illustration of assembled structure of the NixPy//AC ASC configuration. (b) CV curves at different scan rates. (c) GCDcurves at various current densities. (d) Current density dependence of specific capacitance. (e) Cycling performance performed at a current densityof 5 A g−1. (f) Ragone plots and previously reported nickel-based and TMP-based ASC devices.



21834







The cycling capabilities of the prepared electrodes werecarried out using a repeated charging/discharging test at acurrent density of 8 A g−1. After 5000 cycles, the pristineNi(OH)2 shows an inferior cycling stability with about 75.9% ofinitial capacity retention, which is close to the previousNi(OH)2-based electrode.60 Clearly, after the establishedphosphorization process, the NixPy-2 electrode providesmaximum capacity retention of 90.9%. The degradation fromthe NixPy-2 electrode to the NixPy-3 electrode may be derivedfrom the slight blockage among the NixPy-3 nanoparticles. Thecapacity retention values of the NixPy electrodes are worthhighlighting and are superior to recent reports (Table S5).61−65

To gain further insight into the long-term cyclic stability, SEMimages of cycled NixPy electrodes were recorded (Figure S14),showing that it still retains the initial structural configurationapart from a slight aggregation, which thus results in thedegradation of its cyclic stability to some extent.The superior capacitive performance of the synthesized NixPy

electrodes could be attributed to the following aspects: (1) Thevoids between individual nanosheets not only provide sufficientactive sites but also accommodate the drastic volumetricvariation upon cycling, which is conducive to improvingmaterial utilization and reducing capacity fading. (2) Downsiz-ing 2D nanosheets into highly defined 0D nanoparticleswithout agglomeration can endow large surface-to-volumeratios, increased active sites, and prominent edges to favorreversible conversion reactions. (3) The interspaces betweenthe interconnected nanoparticles enable an accessible iondiffusion and short diffusion path and thus facilitate anenhanced efficient utilization of active materials. (4) The activematerials chemically self-assembled on the current collectorpossess a “one-body” geometry, which makes for a highelectronic/ionic conductivity and hinders mechanical deforma-tions during long-term cycling.To further demonstrate the potential applications in energy

storage devices, an ASC device (Figure 4a) was assembledbased on the electrochemical results of the optimized NixPyelectrode as the positive electrode and AC as the negativeelectrode, respectively. The optimal mass ratio of NixPy and ACis determined to be about 0.13. As shown in Figure S15a, theCV curves of the AC electrode are collected at different scanrates with the potential window from −1 to 0 V vs SCE in 3 MKOH solution and present nearly rectangular shapes, clearlydemonstrating the manifestation of the typical double-layerbehavior of the AC electrode. Figure S15b shows that the GCDcurves of the AC electrode measured at various currentdensities are approximately symmetric, suggesting that the ACelectrode is nearly reversible.63 Calculated from the GCDcurves, the specific capacitance of the AC electrode (FigureS15c) can reach as high as 187 F g−1 at a current density of 1 Ag−1, and it still remains at 75 F g−1 at a current density of 8 Ag−1. In Figure S15d, an excellent cycling stability of the ACelectrode with about a 96.3% capacitance retention over 4000cycles at 5 A g−1 can be observed. These excellentelectrochemical properties of the AC electrode indicate that itis a promising negative electrode for this work.Figure 4b exhibits that the CV curves of the NixPy//AC ASC

device performed at scan rates ranging from 10 to 50 mV s−1 ata constant working window of 0−1.5 V. Apparently, the CVcurves of the ASC device show the characteristics including theNixPy electrode with obvious redox peaks and the approx-imately rectangular shape of the AC electrode curves, indicatingthe effective combination of fast charge−discharge properties

and ideal capacitive behavior. Moreover, the GCD curves(Figure 4c) of the ASC device present symmetric shapes,indicative of excellent reversibility. The corresponding specificcapacitance (Figure 4d) is calculated to be 215 F g−1 at acurrent density of 1 A g−1 and is maintained at 65 F g−1 evenwhen the current density reaches 20 A g−1. Figure 4e shows thecyclic stability at a current density of 5 A g−1. The retention ofthe intrinsic specific capacitance still remains at about 84.6%even after 5000 cycles, demonstrating the outstanding stabilityof the device. The corresponding Coulombic efficiency duringcycling test is also exhibited in Figure S16, which is calculatedfrom the ratio of discharge and charge time. The Coulombicefficiency decreases rapidly at the initial stage, whereas it is well-maintained at 86.4% after long-term cycle test, which perhapsdue to the fast charge transfer dynamics inside the nano-structured electrode materials. Calculated from the GCDcurves, the Ragone plot (Figure 4f) reveals that the ASCdevice delivers the maximum energy density of 67.2 W h kg−1

at a power density of 0.75 kW kg−1 and still remains at 20.4 Wh kg−1 even at a high power density of 15 kW kg−1, both ofwhich are comparable to those of previously reported nickel-based composites and TMP-based ASC devices.50,61,66−70 Theattractive electrochemical performances demonstrate the greatpotential of NixPy for the use in supercapacitors and otherelectrochemical energy storage applications.

4. CONCLUSIONSWe propose and demonstrate a promising structural design of3D self-supported biphasic NixPy nanosheets, which are knittedby numerous interwoven nanoparticles. The as-fabricated NixPycomposite can be directly employed as an electrode forsupercapacitors, exhibiting an ultrahigh specific capacity of 1272C g−1 at a current density of 2 A g−1 and considerable cyclingstability with a capacity retention of 90.9% after 5000 cycles.The constructed NixPy//AC asymmetric device displays a highenergy density of 67.2 W h kg−1 at a power density of 0.75 kWkg−1 and still retains an energy density as high as 20.4 W h kg−1

while at a high power density of 15 kW kg−1. It is anticipatedthat the present synthetic strategy will provide great potentialfor the exploration of advanced electrochemical energy-storageelectrode materials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b05384.

Mass loading, XRD pattern of Ni(OH)2, XPS surveyspectra, SEM images of Ni(OH)2 and NixPy, TEM imageof NixPy-1, nitrogen adsorption−desorption isothermsand pore size distribution curves, CV curves, therelationship of current density versus square root of thescan rate, GCD curves, specific capacitances, SEM imagesof NixPy after cycling, electrochemical measurements ofAC electrode, Coulombic efficiency of ASC device, tablesfor XPS results, EIS parameters, and electrochemicalperformance comparison with previous reports (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Kundu: 0000-0002-6508-6728



21835









http://pubs.acs.org

http://pubs.acs.org/doi/abs/10.1021/acsami.7b05384


mailto:[email protected]

http://orcid.org/0000-0002-6508-6728


Seong Chan Jun: 0000-0001-6986-8308NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was fully supported by the Korean Government(MSIP) (No. 2015R1A5A1037668) through the NationalResearch Foundation of Korea (NRF) funded by the Ministryof Education.

■ REFERENCES(1) Huang, Y.; Peng, L.; Liu, Y.; Zhao, G.; Chen, J. Y.; Yu, G.Biobased Nano Porous Active Carbon Fibers for High-PerformanceSupercapacitors. ACS Appl. Mater. Interfaces 2016, 8 (24), 15205−15215.(2) Shi, J.-L.; Du, W.-C.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J.Hydrothermal Reduction of Three-Dimensional Graphene Oxide forBinder-free Flexible Supercapacitors. J. Mater. Chem. A 2014, 2,10830−10834.(3) Wang, K.; Zhang, X.; Li, C.; Sun, X.; Meng, Q.; Ma, Y.; Wei, Z.Chemically Crosslinked Hydrogel Film Leads to Integrated FlexibleSupercapacitors with Superior Performance. Adv. Mater. 2015, 27,7451−7457.(4) Li, C.; Zhang, X.; Wang, K.; Sun, X.; Liu, G.; Li, J.; Tian, H.; Li,J.; Ma, Y. Scalable Self-Propagating High-Temperature Synthesis ofGraphene for Supercapacitors with Superior Power Density and CyclicStability. Adv. Mater. 2017, 29, 1604690.(5) Zhang, X.; Zhang, H.; Li, C.; Wang, K.; Sun, X.; Ma, Y. RecentAdvances in Porous Graphene Materials for SupercapacitorApplications. RSC Adv. 2014, 4, 45862−45884.(6) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous Metal/Oxide Hybrid Electrodes for Electrochemical Supercapacitors. Nat.Nanotechnol. 2011, 6, 232−236.(7) Zhou, K.; Zhou, W.; Yang, L.; Lu, J.; Cheng, S.; Mai, W.; Tang,Z.; Li, L.; Chen, S. Ultrahigh-Performance Pseudocapacitor ElectrodesBased on Transition Metal Phosphide Nanosheets Array viaPhosphorization: A General and Effective Approach. Adv. Funct.Mater. 2015, 25, 7530−7538.(8) Duan, S.; Wang, R. Au/Ni12P5 Core/Shell Nanocrystals fromBimetallic Heterostructures: In Situ Synthesis, Evolution and Super-capacitor Properties. NPG Asia Mater. 2014, 6, e122.(9) Zhao, Y.; Hu, L.; Zhao, S.; Wu, L. Preparation of MnCo2O4@Ni(OH)2 Core−Shell Flowers for Asymmetric SupercapacitorMaterials with Ultrahigh Specific Capacitance. Adv. Funct. Mater.2016, 26, 4085−4093.(10) Wang, J.; Zhang, X.; Wei, Q.; Lv, H.; Tian, Y.; Tong, Z.; Liu, X.;Hao, J.; Qu, H.; Zhao, J.; et al. 3D Self-Supported Nanopine Forest-Like Co3O4@CoMoO4 Core−Shell Architectures for High-EnergySolid State Supercapacitors. Nano Energy 2016, 19, 222−233.(11) Huang, L.; Chen, D.; Ding, Y.; Feng, S.; Wang, Z. L.; Liu, M.Nickel−Cobalt Hydroxide Nanosheets Coated on NiCo2O4 Nano-wires Grown on Carbon Fiber Paper for High-PerformancePseudocapacitors. Nano Lett. 2013, 13, 3135−3139.(12) Pan, Z.; Qiu, Y.; Yang, J.; Ye, F.; Xu, Y.; Zhang, X.; Liu, M.;Zhang, Y. Ultra-Endurance Flexible All-solid-state Asymmetric Super-capacitors Based on Three-Dimensionally Coated MnOx Nanosheetson Nanoporous Current Collectors. Nano Energy 2016, 26, 610−619.(13) Liu, Z.; Yu, X. Y.; Paik, U. Etching-in-a-Box: A Novel Strategy toSynthesize Unique Yolk-Shelled Fe3O4@Carbon with an UltralongCycling Life for Lithium Storage. Adv. Energy Mater. 2016, 6, 1502318.(14) Sahoo, R.; Pal, A.; Pal, T. 2D Materials for Renewable EnergyStorage Devices: Outlook and Challenges. Chem. Commun. 2016, 52,13528−13542.(15) Lai, H.; Wu, Q.; Zhao, J.; Shang, L.; Li, H.; Che, R.; Lyu, Z.;Xiong, J.; Yang, L.; Wang, X.; et al. Mesostructured NiO/NiComposites for High-Performance Electrochemical Energy Storage.Energy Environ. Sci. 2016, 9, 2053−2060.

(16) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L.X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen-Doped PorousCarbon Nanofiber Webs as Anodes for Lithium Ion Batteries with aSuperhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047−2050.(17) Liu, S.; Hui, K.; Hui, K. Flower-like Copper CobaltiteNanosheets on Graphite Paper as High-Performance SupercapacitorElectrodes and Enzymeless Glucose Sensors. ACS Appl. Mater.Interfaces 2016, 8, 3258−3267.(18) Choudhary, N.; Li, C.; Chung, H.-S.; Moore, J.; Thomas, J.;Jung, Y. High-Performance One-Body Core/Shell Nanowire Super-capacitor Enabled by Conformal Growth of Capacitive 2D WS2Layers. ACS Nano 2016, 10, 10726−10735.(19) Liu, S.; Kim, K. H.; Yun, J. M.; Kundu, A.; Sankar, K. V.; Patil,U. M.; Ray, C.; Jun, S. C. 3D Yolk−Shell NiGa2S4 MicrospheresConfined with Nanosheets for High Performance Supercapacitors. J.Mater. Chem. A 2017, 5, 6292−6298.(20) Luo, Z. Z.; Zhang, Y.; Zhang, C.; Tan, H. T.; Li, Z.; Abutaha, A.;Wu, X. L.; Xiong, Q.; Khor, K. A.; Hippalgaonkar, K.; et al.Multifunctional 0D−2D Ni2P Nanocrystals−Black PhosphorusHeterostructure. Adv. Energy Mater. 2017, 7, 1601285.(21) Jiao, Y.; Pei, J.; Yan, C.; Chen, D.; Hu, Y.; Chen, G. LayeredNickel Metal−Organic Framework for High Performance AlkalineBattery-Supercapacitor Hybrid Devices. J. Mater. Chem. A 2016, 4,13344−13351.(22) Hou, X.; Chen, L.; Xu, H.; Zhang, Q.; Zhao, C.; Xuan, L.; Jiang,Y.; Yuan, Y. Engineering of Two-Dimensional Cobalt-GlycineComplex Thin Sheets of Vertically Aligned Nanosheet Basic BuildingBlocks for High Performance Supercapacitor Electrode Materials.Electrochim. Acta 2016, 210, 462−473.(23) Laheaar, A.; Przygocki, P.; Abbas, Q.; Beguin, F. AppropriateMethods for Evaluating the Efficiency and Capacitive Behavior ofDifferent Types of Supercapacitors. Electrochem. Commun. 2015, 60,21−25.(24) Chen, S.; Yang, G.; Jia, Y.; Zheng, H. Three-DimensionalNiCo2O4@NiWO4 Core−Shell Nanowire Arrays for High Perform-ance Supercapacitors. J. Mater. Chem. A 2017, 5, 1028−1034.(25) Liu, S.; Hui, K.; Hui, K.; Jadhav, V. V.; Xia, Q. X.; Yun, J. M.;Cho, Y.; Mane, R. S.; Kim, K. H. Facile Synthesis of MicrosphereCopper Cobalt Carbonate Hydroxides Electrode for AsymmetricSupercapacitor. Electrochim. Acta 2016, 188, 898−908.(26) Xiong, X.; Ding, D.; Chen, D.; Waller, G.; Bu, Y.; Wang, Z.; Liu,M. Three-Dimensional Ultrathin Ni(OH)2 Nanosheets Grown onNickel Foam for High-performance Supercapacitors. Nano Energy2015, 11, 154−161.(27) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W. D.; Paik, U. CarbonCoated Porous Nickel Phosphides Nanoplates for Highly EfficientOxygen Evolution Reaction. Energy Environ. Sci. 2016, 9 (4), 1246−1250.(28) Chung, Y.-H.; Gupta, K.; Jang, J.-H.; Park, H. S.; Jang, I.; Jang, J.H.; Lee, Y.-K.; Lee, S.-C.; Yoo, S. J. Rationalization of Electrocatalysisof Nickel Phosphide Nanowires for Efficient Hydrogen Production.Nano Energy 2016, 26, 496−503.(29) Lu, Y.; Tu, J. P.; Xiong, Q. Q.; Xiang, J. Y.; Mai, Y. J.; Zhang, J.;Qiao, Y. Q.; Wang, X. L.; Gu, C. D.; Mao, S. X. Controllable Synthesisof a Monophase Nickel Phosphide/Carbon (Ni5P4/C) CompositeElectrode via Wet-Chemistry and a Solid-State Reaction for the Anodein Lithium Secondary Batteries. Adv. Funct. Mater. 2012, 22, 3927−3935.(30) Wrenn, T. L.; McBride, J. R.; Mares, J. W.; Rosenthal, S. J.Ferroelectric Particles Generated through a Simple, Room-Temper-ature Treatment of CdSe Quantum Dots. Chem. Mater. 2015, 27,3817−3820.(31) Zhao, Y.; Zhao, Y.; Feng, H.; Shen, J. Synthesis of NickelPhosphide Nano-Particles in a Eutectic Mixture for HydrotreatingReactions. J. Mater. Chem. 2011, 21, 8137−8145.(32) Pan, Y.; Liu, Y.; Liu, C. Nanostructured Nickel PhosphideSupported on Carbon Nanospheres: Synthesis and Application as an



21836

http://orcid.org/0000-0001-6986-8308


Efficient Electrocatalyst for Hydrogen Evolution. J. Power Sources2015, 285, 169−177.(33) Liang, H.; Gandi, A. N.; Anjum, D. H.; Wang, X.;Schwingenschlogl, U.; Alshareef, H. N. Plasma-Assisted Synthesis ofNiCoP for Efficient Overall Water Splitting. Nano Lett. 2016, 16,7718−7725.(34) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and CationModulation in Metal Compounds for Bifunctional Overall WaterSplitting. ACS Nano 2016, 10, 8738−8745.(35) Yang, J.; Ma, M.; Sun, C.; Zhang, Y.; Huang, W.; Dong, X.Hybrid NiCo2S4@MnO2 Heterostructures for High-performanceSupercapacitor Electrodes. J. Mater. Chem. A 2015, 3, 1258−1264.(36) Feng, Y.; Yu, X.-Y.; Paik, U. Nickel Cobalt Phosphides Quasi-Hollow Nanocubes as an Efficient Electrocatalyst for HydrogenEvolution in Alkaline Solution. Chem. Commun. 2016, 52, 1633−1636.(37) Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.;Liu, D.; Liu, Y.; Liu, C. Monodispersed Nickel Phosphide Nanocrystalswith Different Phases: Synthesis, Characterization and ElectrocatalyticProperties for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 1656−1665.(38) Yu, J. C.; Zhang, L.; Zheng, Z.; Zhao, J. Synthesis andCharacterization of Phosphated Mesoporous Titanium Dioxide withHigh Photocatalytic Activity. Chem. Mater. 2003, 15, 2280−2286.(39) Zhai, T.; Wan, L.; Sun, S.; Chen, Q.; Sun, J.; Xia, Q.; Xia, H.Phosphate Ion Functionalized Co3O4 Ultrathin Nanosheets withGreatly Improved Surface Reactivity for High Performance Pseudo-capacitors. Adv. Mater. 2017, 29, 1604167.(40) Yu, L.; Zhang, L.; Wu, H. B.; Lou, X. W. D. Formation ofNixCo3−xS4 Hollow Nanoprisms with Enhanced PseudocapacitiveProperties. Angew. Chem. 2014, 126, 3785−3788.(41) Mu, L.; Wang, F.; Sadtler, B.; Loomis, R. A.; Buhro, W. E.Influence of the Nanoscale Kirkendall Effect on the Morphology ofCopper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange.ACS Nano 2015, 9, 7419−7428.(42) Yu, Y.; Yin, X.; Kvit, A.; Wang, X. Evolution of Hollow TiO2

Nanostructures via the Kirkendall Effect Driven by Cation Exchangewith Enhanced Photoelectrochemical Performance. Nano Lett. 2014,14, 2528−2535.(43) Krishnamoorthy, K.; Veerasubramani, G. K.; Radhakrishnan, S.;Kim, S. J. One Pot Hydrothermal Growth of Hierarchical Nano-structured Ni3S2 on Ni Foam for Supercapacitor Application. Chem.Eng. J. 2014, 251, 116−122.(44) Dai, C.-S.; Chien, P.-Y.; Lin, J.-Y.; Chou, S.-W.; Wu, W.-K.; Li,P.-H.; Wu, K.-Y.; Lin, T.-W. Hierarchically Structured Ni3S2/CarbonNanotube Composites as High Performance Cathode Materials forAsymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5,12168−12174.(45) Brousse, T.; Belanger, D.; Long, J. W. To be Or Not to bePseudocapacitive? J. Electrochem. Soc. 2015, 162, A5185−A5189.(46) Yuan, C.; Li, J.; Hou, L.; Zhang, X.; Shen, L.; Lou, X. W. D.Ultrathin Mesoporous NiCo2O4 Nanosheets Supported on Ni foam asAdvanced Electrodes for Supercapacitors. Adv. Funct. Mater. 2012, 22,4592−4597.(47) Liu, S.; Hui, K. S.; Hui, K. N. 1 D Hierarchical MnCo2O4

Nanowire@MnO2 Sheet Core−Shell Arrays on Graphite Paper asSuperior Electrodes for Asymmetric Supercapacitors. ChemNanoMat2015, 1, 593−602.(48) Liu, S.; San Hui, K.; Hui, K. N.; Yun, J. M.; Kim, K. H. VerticallyStacked Bilayer CuCo2O4/MnCo2O4 Heterostructures on Function-alized Graphite Paper for High-performance Electrochemical Capaci-tors. J. Mater. Chem. A 2016, 4, 8061−8071.(49) Shao, M.; Li, Z.; Zhang, R.; Ning, F.; Wei, M.; Evans, D. G.;Duan, X. Hierarchical Conducting Polymer@Clay Core−Shell Arraysfor Flexible All-Solid-State Supercapacitor Devices. Small 2015, 11,3530−3538.(50) Liu, S.; Lee, S. C.; Patil, U.; Shackery, I.; Kang, S.; Zhang, K.;Park, J. H.; Chung, K. Y.; Jun, S. C. Hierarchical MnCo-layeredDouble Hydroxides@Ni(OH)2 Core−Shell Heterostructures as

Advanced Electrodes for Supercapacitors. J. Mater. Chem. A 2017, 5,1043−1049.(51) Hu, H.; Guan, B.; Xia, B.; Lou, X. W. Designed Formation ofCo3O4/NiCo2O4 Double-Shelled Nanocages with Enhanced Pseudo-capacitive and Electrocatalytic Properties. J. Am. Chem. Soc. 2015, 137,5590−5595.(52) Ke, Q.; Guan, C.; Zhang, X.; Zheng, M.; Zhang, Y. W.; Cai, Y.;Zhang, H.; Wang, J. Surface-Charge-Mediated Formation of H-TiO2@Ni(OH)2 Heterostructures for High-Performance Supercapacitors.Adv. Mater. 2017, 29, 1604164.(53) Zhang, J.; Xiao, K.; Zhang, T.; Qian, G.; Wang, Y.; Feng, Y.Porous Nickel-Cobalt Layered Double Hydroxide Nanoflake ArrayDerived from ZIF-L-Co Nanoflake Array for Battery-type Electrodeswith Enhanced Energy Storage Performance. Electrochim. Acta 2017,226, 113−120.(54) Guan, C.; Liu, X.; Ren, W.; Li, X.; Cheng, C.; Wang, J. RationalDesign of Metal-Organic Framework Derived Hollow NiCo2O4 Arraysfor Flexible Supercapacitor and Electrocatalysis. Adv. Energy Mater.2017, 1602391.(55) Shen, L.; Wang, J.; Xu, G.; Li, H.; Dou, H.; Zhang, X. NiCo2S4Nanosheets Grown on Nitrogen-Doped Carbon Foams as anAdvanced Electrode for Supercapacitors. Adv. Energy Mater. 2015, 5,1400977.(56) Dai, S.; Zhao, B.; Qu, C.; Chen, D.; Dang, D.; Song, B.; deGlee,B. M.; Fu, J.; Hu, C.; Wong, C.-P.; et al. Controlled Synthesis ofThree-Phase NixSy/rGO Nanoflake Electrodes for Hybrid Super-capacitors with High Energy and Power Density. Nano Energy 2017,33, 522−531.(57) Jiang, Y.; Chen, L.; Zhang, H.; Zhang, Q.; Chen, W.; Zhu, J.;Song, D. Two-Dimensional Co3O4 Thin Sheets Assembled by 3DInterconnected Nanoflake Array Framework Structures with EnhancedSupercapacitor Performance Derived from Coordination Complexes.Chem. Eng. J. 2016, 292, 1−12.(58) Xuan, L.; Chen, L.; Yang, Q.; Chen, W.; Hou, X.; Jiang, Y.;Zhang, Q.; Yuan, Y. Engineering 2D Multi-layer Graphene-Like Co3O4

Thin Sheets with Vertically Aligned Nanosheets as Basic BuildingUnits for Advanced Pseudocapacitor Materials. J. Mater. Chem. A2015, 3, 17525−17533.(59) Salunkhe, R. R.; Lin, J.; Malgras, V.; Dou, S. X.; Kim, J. H.;Yamauchi, Y. Large-Scale Synthesis of Coaxial Ccarbon Nanotube/Ni(OH)2 Composites for Asymmetric Supercapacitor Application.Nano Energy 2015, 11, 211−218.(60) Min, S.; Zhao, C.; Zhang, Z.; Chen, G.; Qian, X.; Guo, Z.Synthesis of Ni(OH)2/RGO Pseudocomposite on Nickel foam forSupercapacitors with Superior Performance. J. Mater. Chem. A 2015, 3,3641−3650.(61) Tang, C.-h.; Yin, X.; Gong, H. Superior PerformanceAsymmetric Supercapacitors Based on a Directly Grown CommercialMass 3D Co3O4@Ni(OH)2 Core−Shell Electrode. ACS Appl. Mater.Interfaces 2013, 5, 10574−10582.(62) Liu, S.; Jun, S. C. Hierarchical Manganese Cobalt Sulfide Core−Shell Nanostructures for High-Performance Asymmetric Super-capacitors. J. Power Sources 2017, 342, 629−637.(63) Liu, S.; Lee, S. C.; Patil, U. M.; Ray, C.; Sankar, K. V.; Zhang, K.;Kundu, A.; Kang, S.; Park, J. H.; Jun, S. C. Controllable SulfurationEngineered NiO Nanosheets with Enhanced Capacitance for HighRate Supercapacitors. J. Mater. Chem. A 2017, 5, 4543−4549.(64) Li, Y.; Cao, L.; Qiao, L.; Zhou, M.; Yang, Y.; Xiao, P.; Zhang, Y.Ni−Co Sulfide Nanowires on Nickel Foam with UltrahighCapacitance for Asymmetric Supercapacitors. J. Mater. Chem. A2014, 2, 6540−6548.(65) Shao, Y.; Zhao, Y.-Q.; Li, H.; Xu, C.-L. Three-DimensionalHierarchical NixCo1‑xO/NiyCo2‑yP@C Hybrids on Nickel Foam forExcellent Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 35368−35376.(66) Lu, X.-F.; Wu, D.-J.; Li, R.-Z.; Li, Q.; Ye, S.-H.; Tong, Y.-X.; Li,G.-R. Hierarchical NiCo2O4 Nanosheets@Hollow Microrod Arrays forHigh-performance Asymmetric Supercapacitors. J. Mater. Chem. A2014, 2, 4706−4713.



21837


(67) Hu, W.; Chen, R.; Xie, W.; Zou, L.; Qin, N.; Bao, D. CoNi2S4Nanosheet Arrays Supported on Nickel Foams with UltrahighCapacitance for Aqueous Asymmetric Supercapacitor Applications.ACS Appl. Mater. Interfaces 2014, 6, 19318−19326.(68) Ye, L.; Zhao, L.; Zhang, H.; Zan, P.; Gen, S.; Shi, W.; Han, B.;Sun, H.; Yang, X.; Xu, T. Serpent-cactus-like Co-doped Ni(OH)2/Ni3S2 Hierarchical Structure Composed of Ultrathin Nanosheets forUse in Efficient Asymmetric Supercapacitors. J. Mater. Chem. A 2017,5, 1603−1613.(69) Zhao, C.; Shao, X.; Zhang, Y.; Qian, X. Fe2O3/ReducedGraphene Oxide/Fe3O4 Composite in Situ Grown on Fe Foil forHigh-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2016,8, 30133−30142.(70) Chen, X.; Cheng, M.; Chen, D.; Wang, R. Shape-ControlledSynthesis of Co2P Nanostructures and Their Application in Super-capacitors. ACS Appl. Mater. Interfaces 2016, 8, 3892−3900.



21838


honeycomb-like interconnected network of nickel...

Documents