Honeycomb-Like Interconnected Network of Nickel Phosphide

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Honeycomb-like Interconnected Network of Nickel Phosphide Heteronanoparticles with Superior Electrochemical Performance for Supercapacitors Shude Liu, Kalimuthu Vijaya Sankar, Aniruddha Kundu, Ming Ma, Jang-Yeon Kwon, and Seong Chan Jun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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ACS Applied Materials & Interfaces

Honeycomb-like Interconnected Network of Nickel Phosphide Hetero-nanoparticles with Superior Electrochemical Performance for Supercapacitors

Shude Liua, Kalimuthu Vijaya Sankara, Aniruddha Kundua, Ming Ma b, Jang-Yeon Kwonc, Seong Chan Juna,* a

School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea

b

Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South

Korea c

School of Integrated Technology and Yonsei Institute of Convergence Technology, Yonsei

University, Yeonsu-gu, Incheon 406-840, South Korea E-mail: [email protected] (Seong Chan Jun)

Keywords:

Nickel

phosphide;

Nanosheets;

Hetero-nanoparticles;

Electrochemical

performance; Supercapacitors

Abstract Transition metal-based hetero-nanoparticles are attracting extensive attention in electrode material design for supercapacitors owing to their large surface-to-volume ratios and inherent synergies of individual components; however, they still suffer from limited interior capacity and cycling stability due to simple geometric configurations, low electrochemical activity of the surface, and poor structural integrity. Developing an elaborate architecture that endows a larger surface area, high conductivity, and mechanically robust structure is a pressing need to tackle the existing challenges of electrode materials. This work presents a supercapacitor electrode consisting of honeycomb-like biphasic Ni5P4–Ni2P (NixPy) nanosheets, which are interleaved by large quantities of nanoparticles. The optimized NixPy delivers an ultrahigh 1 ACS Paragon Plus Environment


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specific capacity of 1272 C g−1 at a current density of 2 A g−1, high rate capability, and stability. An asymmetric supercapacitor employing as-synthesized NixPy as the positive electrode and activated carbon 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 h kg−1 at 15 kW kg−1). These results demonstrate that the novel nanostructured NixPy can be potentially applied in highperformance supercapacitors.

1. Introduction Supercapacitors have drawn considerable attention due to their intriguing merits in terms of high power density, fast charge/discharge rate, and long life span.1-5 Battery-type materials of transition metals are promising for supercapacitors owing to their high theoretical specific capacities and energy densities in comparison to carbonaceous materials, but fail to deliver the rapid kinetics of charge transport, which impedes their potential technological applications for electrochemical energy storage.6-7 One way to address the issues regarding the simultaneous improvement of the energy density and cyclic stability is to rationally optimize the intrinsic properties of electrode materials and delicately design ion diffusion-favored structures.

Transition-metal phosphides (TMPs) have emerged as new electrode materials for energy conversion and storage owing to their metalloid features and high electrical conductivity, which are kinetically favorable for rapid electron transport that allows for a high rate capability.7 Also, they endow good thermal stability and resistance to the ambient environment, thus enabling a good cyclic stability.7-8

In particular, multicomponent

transition-metal composites can synergistically improve the electrochemical performances in terms of electrochemical activity, reversible capacity, and electrical conductivity.9-10 2 ACS Paragon Plus Environment

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Nonetheless, mixed-metal phosphide materials still suffer from relatively low specificcapacity and rapid-capacity fade during redox reactions. An effective strategy for solving these issues is to design and construct a functional electrode featuring abundant active sites and a multistage porous structure, as well as a tailored reaction interface of active materials/current collectors.11-12 Two-dimensional (2D) nanosheets are promising in the field of electrochemical energy storage because they can provide a better durability and buffering capacity to impede the volumetric variation during cycling,13-14 while the ion-diffusion barrier is low, which supports the simultaneous achievement of a high energy storage capacity with good cycling stability, even at a high rate.15-17 From the viewpoint of electrode design, active materials self-assembled on metallic substrates not only ensure high ionic/electronic conductivity, but also favor chemically stable interfaces.18-19 Despite all of this, the rational engineering of zero-dimensional (0D) and 2D building blocks on metallic substrates possessing synergistic structure characteristics is still an ongoing technical challenge to further improve their electrochemical performances.20

Herein, we propose a facile strategy to construct a sophisticated architecture supported on nickel foam consisting of honeycomb-like porous NixPy nanosheets with abundant interconnected hetero-nanoparticles. Benefiting from the synergistic effect of the multicomponent systems and 0D/2D building blocks, the synthesized NixPy delivers an ultrahigh specific capacity of up to 1272 C g−1 at 2 A g−1, and a good cycling stability with 90.9% capacity retention after 5000 cycles. The constructed asymmetric supercapacitor with optimized NixPy and activated carbon (AC) as electrode materials displays a high energy density of 67.2 W h kg−1 at 0.75 kW kg−1. These results indicate that the rational assembly and heterogrowth of active materials are promising for high energy density supercapacitors while maintaining a high power density. 3 ACS Paragon Plus Environment


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2. Experimental Section 2.1 Materials preparation of honeycomb-like interconnected network of NixPy heteronanoparticles Prior to synthesis, a piece of nickel foam (2 × 3 cm2) was cleaned with a 2-M HCl solution in an ultrasonic bath for 20 min in order to remove the surface oxide layer, and then was washed using absolute ethanol and deionized water. The pretreated nickel foam was immersed in a homogeneous solution containing 0.025-M Ni(NO3)2·6H2O, 0.15-M NH4F, and 0.3-M CO(NH2)2. After that, the solution was transferred to a Teflon-lined stainless-steel autoclave and then heated at 120°C for 12 h with a heating rate of 5°C min−1. The obtained products anchored on nickel foam were rinsed with distilled water and ethanol several times and dried at 70°C for 12 h. The dried samples and various amounts of NaH2PO2·H2O (10, 20, and 30 mmol) were placed in the positions of the two boundaries of a ceramic crucible, with the NaH2PO2·H2O at the upstream side of the furnace and heated at 400°C for 3 h under an Ar atmosphere with a heating rate of 2°C/min. The corresponding NixPy samples were labeled as NixPy-1, NixPy-2, and NixPy-3. The mean mass load and mass error of the as-synthesized samples were summarized in Figure S1.

2.2 Materials characterization The morphologies and structures were analyzed by X-ray diffraction (XRD, Philips X’pert diffractometer) 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. The composition and chemical valence states were identified by X-ray photoelectron spectroscopy (XPS, PHI-5702). Nitrogen adsorption/desorption measurements were conducted with a Micromeritics ASAP 2460 analyzer at 77 K. The specific surface area was derived from the multipoint Brunauer−Emmett−Teller (BET) model, and the pore-size 4 ACS Paragon Plus Environment

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distribution was estimated on the basis of the desorption branch of the nitrogen adsorption isotherm utilizing the Barrett−Joyner−Halenda (BJH) method. 2.3 Electrochemical performance measurements All of the electrochemical measurements were carried out using the electrochemical 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 working electrode, while a saturated calomel electrode (SCE) and a platinum plate were used as the reference and the counter electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were carried out to investigate the electrochemical properties of the electrodes. Electrochemical impedance spectroscopy (EIS) was analyzed by applying an alternating-current voltage with a 5-mV amplitude in the frequency range of 100 kHz to 0.01 Hz. The specific capacities (Cs, C g−1) of the NixPy samples were calculated from the GCD curves based on the following equation:21 C =

× ∆

(1)

where I (mA) represents the discharge current, ∆t (s) refers to the discharge time, and m (mg) corresponds to the mass of the active material. The corresponding specific capacitance can be calculated according to the following equation:22-23

C = ∆ ()

(2)

where ∆V (V) is the potential window, V (t) is the operating of potential, t1 and t2 refer to the initial and terminational discharge time of GCD curves, respectively. 2.4 Fabrication of the NixPy//AC asymmetric supercapacitors The asymmetric supercapacitor (ASC) device was constructed by using NixPy as the positive electrode, AC as the negative electrode, and a cellulosic paper as the separator. For the 5 ACS Paragon Plus Environment


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preparation of the negative electrode, a homogeneous slurry was first obtained by stirring the AC, polyvinylidene fluoride, and acetylene black with a mass ratio of 80:10:10 in an ethanol solvent. The mixed slurry was spread on a nickel-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 assembled ASC device was operated in a two-electrode cell in a 3-M KOH electrolyte. To achieve the optimal electrochemical performance of the ASC device, the mass ratio of the negative electrode to the positive electrode was evaluated by the charge balance theory (Q+ = Q−). The mass balancing can be expressed according to following equation:21

=

× ∆

(3)

where m+ and C+ are the mass (mg) and specific capacity (C g−1) of the positive electrode, respectively, and m−, ∆V−, and C− are the mass (mg), potential window (V), and specific capacitance (F g−1) of the negative 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 were calculated by the total mass of the two electrodes using the following equations:24-25 × ∆

C = ∆ ×

(4)

"

E = #. C × ∆V

P=

'())×* ∆

(5)

(6)

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

3. Results and discussion The fabrication process for NixPy nanosheets is schematically depicted in Figure 1a. Firstly, Ni(OH)2 nanosheets were prepared on the nickel-foam skeleton through a hydrothermal route. In subsequent phosphorization process, the well-defined NixPy nanosheets were obtained 6 ACS Paragon Plus Environment

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while each nanosheet consists of numerous interconnected nanoparticles along with plenty of pores. The phase and structure of the as-prepared samples on nickel foam were verified by Xray diffraction (XRD). All of the diffraction peaks in the XRD pattern of the nickel precursor (Figure S2) can be well indexed to hexagonal β-Ni(OH)2 phase (JCPDS No. 14-0117).26 As shown in Figure 1b, the three most intense peaks centered at about 44.7°, 52.0° and 76.6° (2θ values) belong to the nickel substrate. A tunable sodium phosphide dosage, e.g., 10, 20, and 30 mmol, was used for syntheses of the NixPy nanosheets denoted as NixPy-1, NixPy-2, and NixPy-3, respectively. After the phosphorization treatment, all identiﬁed peaks can be well indexed as a mixture of hexagonal-phase Ni5P4 (JCPDS No. 18-883)27 and Ni2P (JCPDS No. 65-3544).28 These results indicate that the synthesized NixPy composites are biphasic systems. As observed in the XRD result, the intensity of the diffraction peaks at 2θ = 14.9°, 30.5°, and 36.2° in the pattern of Ni5P4 gradually decreases together with the intensity value of the Ni2P diffraction peaks located at 40.8°, 75.0° and 80.2° slightly increases with an increase dosage in phosphorus source, which can be ascribed to partial phase transformation from Ni5P4 to Ni2P, which is in accordance with the previous report.29 The peak at the dominant (111) of Ni2P shifts toward lower angle direction, which is associated with incremental phosphorus source, resulting in an intensive charge imbalance.30 This phenomenon is similar for other peaks in Ni2P. X-ray photoelectron spectroscopy (XPS) was employed to explore the surface chemical composition and valence states of the as-prepared sample (Figure 1c-d and Figure S3). Spectral parameters obtained in the NixPy composites by XPS analysis are shown in Table S1 and Table S2. For NixPy-1, peak fitting of the Ni 2p3/2 spectrum shows two peaks at 853.4 eV and 856.8 eV, which are assigned to a very small positive charge (Niδ+) in NixPy and oxidized Ni species, respectively,31-32 while the peak centered at 861.6 eV corresponds to the satellite of the Ni 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, while the binding energy of Ni 2p3/2 peak shifts from 856.8 eV to 856.5 eV, indicating the newly formed Ni−P bonds.34 For the P 2p region of 7 ACS Paragon Plus Environment


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NixPy-1, the peak centered at 129.2 eV is assigned to P 2p3/2, with a peak ascribed to P 2p1/2 at 130.1 eV.35 The peak located at around 133.6 eV is allocated to the P–O bond, which can be indexed to the oxidized phosphorous formed on the surface.36-37 Clearly, the binding energies of P 2p3/2, P 2p1/2 and PO43− peaks shown in the spectrum of the NixPy-1 have a slightly downshifting compared to those of NixPy-2 and NixPy-3. Notably, the binding energy of P 2p in NixPy is lower than that of P0 (130.2 eV), suggesting the existence of negative charge (Pδ-) and the formation of Ni−P,33-34 which is supportive to the aforementioned XRD results.

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

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Scanning electron microscopy (SEM) observation indicates that the Ni(OH)2 was densely packed and uniformly covered the entire surface of the nickel substrate (Figure S4a). The magnified SEM images show that the Ni(OH)2 structure consists of a large amount of homogenous nanosheets which are interconnected with each other to assemble a honeycomblike architecture (Figure S4b–d). Energy dispersive X-ray spectroscopy (EDS) reveals the uniform distribution of Ni and O 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 of the nanosheets becomes rough (Figure S6a). The magnified SEM images (Figure S6b) show that the nanosheets have a length of about 2 µm, and the thickness of the nanosheets ranges from 20 to 30 nm. A closer view reveals that the nanosheets are constructed from the interconnected hetero-NixPy-1 nanoparticles of about 20 nm in size. (Figure 2a and Figure S6c). The substructure of the NixPy-1 nanosheets was further explored by transmission electron microscopy (TEM) analysis. The nanosheets consist of a large variety of interspaces throughout the interior nanoparticles, with diameters ranging from 10 nm to 30 nm (Figure 2d). The high-resolution TEM image (Figure 2g) shows the lattice fringes with interplanar spacings of 0.17 and 0.25 nm, which correspond to the (300) plane of Ni2P and the (104) plane of Ni5P4, respectively. The scanning TEM (STEM) image and its corresponding energy dispersive X-ray (EDX) spectrum verify the uniform distribution of Ni and P throughout the nanosheets (Figure S7). As the phosphorization process proceeded, the NixPy-2 nanosheets were also assembled by the interconnected hetero-NixPy nanoparticles (Figure 2b and Figure S6d-f). However, the surfaces of the nanosheets with mesoporous features became rough and the interspacing between nanoparticles was slightly increased. This characteristic feature is further revealed by TEM images (Figure 2e and 2h). For NixPy-3, the interconnected nanoparticles became irregular and the interspace between the nanoparticles was locally jammed (Figure 2c, f, i and Figure S6g–i), which restricts the electrolyte ion accessibility, leading to the deteriorated electrochemical performance. The 9 ACS Paragon Plus Environment


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textural properties of NixPy were investigated with N2 adsorption-desorption measurements (Figure S8). The N2 adsorption-desorption isotherms exhibit type-H3 hysteresis loops, which are characteristic of mesoporous structures.38 The Barrett–Joyner–Halenda (BJH) pore-size distribution curves of the Ni(OH)2 and various NixPy samples show the average pore size ranging from 10.8 to 16.2 nm (Inset of Figure S8), which further reveals their mesoporous feature. The Brunauer–Emmett–Teller (BET) surface areas of the Ni(OH)2, NixPy-1, NixPy-2, and NixPy-3 nanosheets calculated from N2 isotherms were determined to be 20.9, 37.2, 44.1, and 27.8 m2 g−1, respectively. These characteristic clearly indicates that the NixPy samples are supportive for storing more charge for an improved specific capacity because of the enlarged active sites and good accessibility of electrolyte ions.

Based on the aforementioned experimental analysis, the reaction mechanism of phosphorization treatment and accompanied morphology evolution process can be illustrated in Figure 2j. Generally, NaH2PO2·H2O serving as a precursor can generate PH3 reactant by a solid thermal decomposition reaction.39 With the absorption of PH3 reactant at the surface of Ni(OH)2 nanosheets, the ion exchange reaction between diffused PH3 and internal OH− occurred on the surface. According to the above XRD and XPS results (Figure 1), we can predict that the Ni5P4 nanosheets were initially produced using the established phosphorization treatment. As the ion exchange process progressed, the NixPy nanosheets were chemically converted from the precursor, while nanosheets were interweaved with nanoparticles together with disordered mesopores, which might contribute to a high chemical reactivity and sufficient active reaction sites for fully efficient faradaic reactions. Because of the unequal diffusion ability between phosphorus anion and OH− species, the faster outward hydroxyl diffusion makes for the formation of porous structure at the surface, while the slower inward penetration of PH3 leads to produce nanoparticles throughout the nanosheets, 10 ACS Paragon Plus Environment

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which is similar with the nanoscale Kirkendall Effect operating in previous reports.40-42 For higher P/Ni ratios, the particles show a sustained increase in size, which maybe derives from the unequal diffusion process and lattice expansion to form NixPy while conserving the number of Ni atoms in each nanoparticle during the transformation.

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



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illustration of the proposed phosphorization and morphology evolution process at the surface of NixPy nanosheets.

Figure 3a presents the typical cyclic voltammetry (CV) curves of the pristine Ni(OH)2, NixPy1, NixPy-2, and NixPy-3 electrodes at a scan rate of 5 mV s−1 in 3-M KOH electrolyte. The CV curves present a pair of distinct redox peaks, corresponding to the reversible redox transition of Ni2+/Ni3+, which can be illustrated in the following equations.43-44 The phenomenon indicates a typical Faradic capacitive behavior of battery-type materials.15, 45

+, - + 2 012 ↔ +, -(01) + 2 4 2

(7)

+,5 -6 + 5 012 ↔ +,5 -6 (01)5 + 5 4 2

(8)

Notably, the enclosed CV curve area of the NixPy electrode increases along with the phosphorization treatment and reaches a maximum value for the NixPy-2 sample. However, the integral area of the CV curve decreases with an increased phosphate dosage, which illustrates that an excess phosphorization treatment leads to a depressed electrochemical activity. Clearly, the pure nickel foam has only a negligible contribution to the current density of the prepared electrodes (Figure S9). Each CV curve maintains well-defined redox peaks with a slight polarization at different scan rates ranging from 2 to 20 mV s−1, indicating good electrode kinetics (Figure S10). From the relationship between the redox peak current and the square root of the scan rate for the cathodic peak, it can be obviously observed that the cathodic peak current increases almost linearly with the square root of the scan rate (Figure S11), indicating that the redox reaction is dominated by a diffusion-controlled battery-type Faradic process.46-48

Figure 3b presents a comparison of the galvanostatic charge/discharge (GCD) curves at a current density of 2 A g−1 within a potential window from −0.1 to 0.5 V vs. SCE, in which the 12 ACS Paragon Plus Environment

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nonlinear trend demonstrates the Faradic character of the electrode materials.49 Note that the potential range of the GCD curves presents a deviation compared with that of the CV curves, which is derived from the polarization effect of electrode materials.50 As expected, the NixPy2 electrode exhibits a longer discharging time than the other electrodes. Figure 3c shows the calculated specific capacities derived from GCD curves at various current densities for the prepared electrodes (Figure S12). All of the NixPy electrodes yield a much higher specific capacity than that of the Ni(OH)2 electrode at the same current density. The specific capacity for the NixPy-2 electrode (1272 C g−1) is about 1.26 times that of the NixPy-1 electrode (1009 C g−1) and 1.12 times of the NixPy-3 electrode (1140 C g−1) at a current density of 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 the specific capacity gradually decays as the discharge current density increases because of the insufficient active material involved in the redox reaction at higher current densities. Impressively, even at a high current density of 10 A g−1, the capacity 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 those of previously reported transition metal materials.56-59 To fulfill an efficient comparison with previous reports, the corresponding specific capacitances of as-synthesized electrode materials are also presented in Figure S13. The specific capacitance of 2638 F g−1 for NixPy-2 is delivered when the current density is 1 A g−1, which is obviously higher than those of Ni(OH)2, NixPy-1, and NixPy-3. Especially NixPy-2 still maintains a specific capacitance 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, electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.01–100 kHz at an open-circuit condition. The measured Nyquist plots of the EIS spectra (Figure 3d) are simulated based on an equivalent circuit (inset of 13 ACS Paragon Plus Environment


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Figure 3d) using the complex nonlinear least-squares fitting method. The obtained values of each component calculated from the experimental impedance spectra are shown in Table S4. As can be seen, the NixPy electrodes possess a smaller equivalent series resistance (Rs) than that of the Ni(OH)2 electrode, revealing a vital role of the phosphorization in increasing the conductivity of integrated electrode. The semicircle is obviously depressed in the highfrequency region, implying a small interfacial charge-transfer resistance (Rct) at the electrode/electrolyte interface.7 Meanwhile, the Rct of NixPy-2 is estimated to be 4.6 Ω, which is lower than that of the Ni(OH)2 (12.4 Ω), NixPy-1 (6.0 Ω), and NixPy-3 (6.9 Ω), thus making for a good rate capability.

The cycling capabilities of the prepared electrodes were carried out using a repeated charging/discharging test at a current density of 8 A g−1. After 5000 cycles, the pristine Ni(OH)2 shows an inferior cycling stability with about 75.9% of initial capacity retention, which is close to the previous Ni(OH)2-based electrode.60 Clearly, after the established phosphorization process, the NixPy-2 electrode provides maximum capacity retention of 90.9%. The degradation from the NixPy-2 electrode to the NixPy-3 electrode may be derived from the slight blockage among the NixPy-3 nanoparticles. The capacity retention values of the NixPy electrodes are worth highlighting and are superior to recent reports (Table S5).61-65 To further insight the long term cyclic stability, SEM images of cycled NixPy electrodes were performed (Figure S14), which still remains initial structural configuration apart from a slight aggregation, which thus results in the degradation of its cyclic stability to some extent.

The superior capacitive performance of the synthesized NixPy electrodes could be attributed to the following aspects: (1) the voids between individual nanosheets not only provide sufficient active sites, but also accommodate the drastic volumetric variation upon cycling, which is conducive to improving material utilization and reducing capacity fading; (2) downsizing 2D 14 ACS Paragon Plus Environment

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nanosheets into highly defined 0D nanoparticles without agglomeration can endow large surface-to-volume ratios, increased active sites, and prominent edges to favor reversible conversion reactions; (3) the interspaces between the interconnected nanoparticles enable an accessible ion diffusion and short diffusion path, and thus facilitate an enhanced efficient utilization of active materials; (4) the active materials chemically self-assembled on the current collector possess a “one-body” geometry, which makes for a high electronic/ionic conductivity and hinders mechanical deformations during long-term cycling.

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 mV s−1. (b) Comparison of GCD curves at a current density of 2 A g−1. (c) Speciﬁc capacities versus discharge current densities. (d) EIS



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curves. The inset shows an equivalent circuit. (e) Cyclic stability for 5000 cycles at a current density of 8 A g−1.

To further demonstrate the potential applications in energy storage devices, an ASC device (Figure 4a) was assembled based on the electrochemical results of the optimized NixPy electrode as the positive electrode and AC as the negative electrode, respectively. The optimal mass ratio of NixPy and AC is determined to be about 0.13. As shown in Figure S15a, the CV curves of the AC electrode are collected at different scan rates with the potential window from −1 to 0 V vs. SCE in 3-M KOH solution, which present nearly rectangular shapes, clearly demonstrating the manifestation of the typical double-layer behavior of the AC electrode. Figure S15b shows that the GCD curves of the AC electrode measured at various current densities are approximately symmetric, suggesting that the AC electrode is nearly reversible.63 Calculated from the GCD curves, the specific capacitance of the AC electrode (Figure S15c) can reach as high as 187 F g−1 at a current density of 1 A g−1, and it still remains at 75 F g−1 at a current density of 8 A g−1. In Figure S15d, an excellent cycling stability of the AC electrode with about a 96.3% capacitance retention over 4000 cycles at 5 A g−1 can be observed. These excellent electrochemical properties of the AC electrode indicate that it is a promising negative electrode for this work.

Figure 4b exhibits the CV curves of the NixPy//AC ASC device performed at scan rates ranging from 10 to 50 mV s−1 at a constant working window of 0−1.5 V. Apparently, the CV curves of the ASC device show the characteristics including the NixPy electrode with obvious redox peaks and the approximately rectangular shape of the AC electrode curves, indicating the 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 specific capacitance (Figure 4d) is 16 ACS Paragon Plus Environment

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calculated to be 215 F g−1 at a current density of 1 A g−1 and maintains 65 F g−1 even when the current density reaches 20 A g−1. Figure 4e shows the cyclic stability at a current density of 5 A g−1. The retention of the intrinsic specific capacitance still maintains about 84.6% even after 5000 cycles, demonstrating the outstanding stability of the device. The corresponding coulombic efficiency during cycling test is also exhibited in Figure S16, which is calculated from the ratio of discharge and charge time. The Coulombic efficiency decreases rapidly at the initial stage, whereas it is well maintained at 86.4% after long-term cycle test, which perhaps due to the fast charge transfer dynamics inside the nanostructured electrode materials. Calculated from the GCD curves, the Ragone plot (Figure 4f) reveals that the ASC device 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 W h kg−1 even at a high power density of 15 kW kg−1, which are comparable to those of previously reported nickel based composites and TMP-based ASC devices.50, 61, 66-70 The attractive electrochemical performances demonstrate the great potential of NixPy for the use in supercapacitors and other electrochemical energy storage applications.



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

4. Conclusions In summary, we propose and demonstrate a promising structural design of 3D self-supported biphasic NixPy nanosheets, which are knitted by numerous interweaved nanoparticles. The asfabricated NixPy composite can be directly employed as an electrode for supercapacitors, exhibiting an ultrahigh specific capacity of 1272 C g−1 at a current density of 2 A g−1 and considerable cycling stability with a capacity retention of 90.9% after 5000 cycles. The 18 ACS Paragon Plus Environment

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constructed NixPy//AC asymmetric device displays a high energy density of 67.2 W h kg−1 at a power density of 0.75 kW kg−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 can be anticipated that the present synthetic strategy will provide great potential for the exploration of advanced electrochemical energystorage electrode materials.

Acknowledgements This

work

was

fully

supported

by

the

Korean

Government

(MSIP)

(No.

2015R1A5A1037668) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (please add manuscript number). Additional details, including mass loading, XRD pattern of Ni(OH)2, XPS survey spectra, SEM images of Ni(OH)2, SEM images of NixPy, TEM image of NixPy-1, nitrogen adsorption– desorption isotherms and pore size distribution curves, CV curves, the relationship of current density vs. square root of the scan rate, GCD curves, specific capacitances, SEM images of NixPy after cycling, electrochemical measurements of AC electrode, Coulombic efficiency of ASC device, tables for XPS results, EIS parameters, electrochemical performance comparison with previous reports.



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Graphical Abstract


Honeycomb-Like Interconnected Network of Nickel Phosphide

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