NiCoP [email protected] double hydroxides nanosheet heterostructure for flexible asymmetric supercapacitors

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NiCoP nanowire@NiCo-layered double hydroxides nanosheet heterostructure for flexible asymmetric supercapacitors ⁎

Xiangyang Gao, Yafei Zhao, Kaiqing Dai, Jingtao Wang, Bing Zhang , Xiangjian Shen

⁎

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, PR China

H I GH L IG H T S

nanowire@NiCo-LDH grows • NiCoP directly on carbon cloth for the first

G R A P H I C A L A B S T R A C T

A novel core/shell heterostructure of NiCoP@NiCo-LDH with high conductivity and outstanding stability is available for flexible solid-state asymmetric supercapacitors with high energy density and long cycle life.

time.

electrode exhibits a much higher • The capacitance compared to previous materials.

CC/NiCoP@NiCo-LDH//AC • The achieves a high energy density and cycling stability.

provide theoretical assistance for • DFT the improved electrochemical performance.

A R T I C LE I N FO

A B S T R A C T

Keywords: NiCoP@NiCo-LDH Core/shell heterostructures Supercapacitors Energy storage devices Electrochemical performance

Reasonable design of multi-component composites with heterostructures is an effective strategy to improve the performances of energy storage materials. Herein, we fabricate a supercapacitor electrode consisting of a 3D hierarchical NiCoP@NiCo-LDH core/shell heterostructure on conductive carbon cloth (CC). Such special structure integrates the advantages of the highly interconnected nanosheets of NiCo-LDH, the high conductivity of NiCoP, excellent mechanical strength of CC, and the 3D hierarchical open structure. Density functional theory (DFT) calculations reveal the superior electrical conductivity of NiCoP core and the powerful affinity for OH− of NiCo-LDH shell, which provide additional theoretical assistance for the improved electrochemical performance. Benefiting from the well-defined core/shell heterostructure and efficient synergetic effects among multi-components, the CC/NiCoP@NiCo-LDH electrode exhibits a high capacitance of 4.683 F cm−2 (1951 F g−1) at the current density of 1 mA cm−2 and an excellent rate capability of 89.3% at 20 mA cm−2. In addition, a flexible asymmetric supercapacitor (FASC) composed of CC/NiCoP@NiCo-LDH as the positive electrode and activated carbon (AC) as the negative electrode achieves a high energy density of 57 Wh kg−1 at the power density of 850 W kg−1 and an outstanding cycling stability of 97% capacitance retention after 10,000 cycles. The above results suggest that CC/NiCoP@NiCo-LDH has potential to be selected as a candidate of positive electrode for constructing FASC.

⁎

Corresponding authors. E-mail addresses: [email protected] (B. Zhang), [email protected] (X. Shen).

https://doi.org/10.1016/j.cej.2019.123373 Received 10 July 2019; Received in revised form 4 October 2019; Accepted 2 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Xiangyang Gao, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123373

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heterostructured NiCoP@NiCo-LDH for flexible SCs through decorating arrayed NiCoP nanowires (NWs) on CC followed by hydrothermal treatment to grow NiCo-LDH nanosheets (NSs). Such an efficient design shows distinct merits. On one hand, the CC/NiCoP@NiCo-LDH heterostructure electrode material not only integrates the advantages of the high electronic conductivity of NiCoP NWs and the well-interconnected NiCo-LDH NSs, but also owns an open hierarchical 3D structure, which provides rapid and efficient pathways for electrons or ions, thus resulting in high utilization of electroactive species. On the other hand, NiCoP NWs provide the desirable electrical conductivity, and also act as Faraday redox materials to further increase the redox reaction kinetics. As a result, the CC/NiCoP@NiCo-LDH exhibits a high capacitance of 4.683 F cm−2 at 1 mA cm−2 and an excellent rate performance. The fabricated CC/NiCoP@NiCo-LDH//AC flexible asymmetric supercapacitor (FASC) delivers a high energy density of 57 Wh kg−1 at the power density of 850 W kg−1 and a superior electrochemical cycling stability. Our work illuminates the effective heterointerface engineering of electrodes to improve electrochemical performance of energy storage devices.

1. Introduction Supercapacitors (SCs) or ultracapacitors, as one of the most promising energy conversion and storage devices, have attracted intensive attention due to their fast charge/discharge rate, high-power density and excellent durability [1–3]. SCs can be divided into two categories: pseudocapacitors and electron double-layer capacitors (EDLCs) [4,5]. In particular, pseudocapacitors are dominated by the mechanism of fast reversible multi-electron surface Faraday redox reaction, which can provide high specific capacitance and has potential to fabricate highenergy storage devices [4,6]. Among them, transition metal hydroxides such as Ni(OH)2, Co(OH)2 and NiCo- layered double hydroxides (NiCo-LDH) with high capability of storing energy by surface Faraday redox reaction, have been extensively employed to design and fabricate electrode materials with various micro-nano structures [6–10]. Especially, the NiCo-LDH consisting of brucite-like layers with cation and interlayers with anions, have been widely explored as promising electrode materials [11–13]. Compared to the individual cobalt or nickel hydroxide, NiCo-LDH materials exhibit significantly improved energy density and overall performance, attributing to the existence of more redox reactions and surface active sites as well as the synergistic effects between bimetallic ions [14,15]. Nevertheless, the achieved capacities of NiCo-LDH is still far below the theoretical value due to the relatively low electronic conductivity and ion transfer rate, thus greatly hindering the further improvement of their performance [16,17]. Metal phosphides (MPs) of NiCoP exhibit the metalloid characteristics or rapid electron transport ability induced by strong electron delocalization in the sublattices of NiCoP, making them attractive potential electrode candidates for high performance hybrid SCs [18–21]. As previously reported, various morphologies of MPs, such as hexagonal thin-plate of NixCo3-xP [22] and hollow NiCo-P [19] nanocages, demonstrated superior electrochemical performance. However, it is generally believed that NiCoP are limited by relatively poor electrochemical stability and undesirable specific capacitance [23,24]. Previous reports indicated that designing of complex electrode materials to combine NiCoP with other stable electroactive materials is a feasible way to boost the electrochemical utilization and specific capacitance [23,25,26]. For instance, Shao and co-workers designed a 3D hierarchical NixCo1−xO/NiyCo2−yP@C hybrid electrode by introducing NixCo1−xO with excellent stability and carbon layers with mechanical stability, achieving superior electron transport and cycle stability [25]. Li and co-workers developed a facile strategy to improve the stability of the hybrid electrode by combining Ni2-xCoxP with graphene [26]. Core/shell heterostructures, such as CoNiO2/Ni(OH)2 [27], NiCo2O4@NiMoO4 [28], ZnCo2O4/NiMoO4 [29], and MnCo-LDH@Ni (OH)2 [9], consisting of a core material with high electrical conductivity and a shell material with pseudocapacitivity, are an intriguing class of hybrid materials with a promising potential for supercapacitor applications [27,30–32]. Such heterostructure with a well-defined heterointerface may generate internal electric fields and charge discontinuity at its interface, which not only has a significant improvement in electronic/ionic conductivity and redox reaction kinetics, but also exerts synergistic behavior [29,30]. To be specific, the core material with large surface area can act as both the backbone and electron storage/delivery pathway, and the thin layer of shell on core serves as a short diffusion path for the charge carriers as well as increases the contact area with the electrolyte, facilitating fast redox reaction and improving the durability of the core. Besides, both the core and shell are accessible to the electrolyte on account of porous structure of the core and thin layer of the shell respectively, contributing to the improvement of electrochemical charge storage. It is also believed that decorating active material on a conductive substrate (eg. carbon cloth (CC), foam Ni) is able to accelerate the charge transport and improve the electric conductivity. Based on the above considerations, we developed a core/shell

2. Experimental section 2.1. Materilas Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), urea (CO(NH2)2), ethanol, hexamethylenetramine (HMTA), sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl alcohol (PVA), sodium hypophosphite monohydrate (NaH2PO2·H2O), potassium hydroxide (KOH) and polytetrafluoroethylene (PTFE) were obtained from Aladdin Chemical Reagent Co., Ltd. Carbon cloth (WOS1009) was purchased from Taiwan CeTech Co., Ltd. Active carbon was purchased in Kuraray Co., Japan. 2.2. Synthesis of CC/NiCoP NWs arrays A piece of CC with size of 2 × 4 cm2 was cleaned by being sonicated in acetone, ethanol and distilled water successively for 30 min, respectively. 1.5 mmol Ni(NO3)2·6H2O, 3.5 mmol Co(NO3)2·6H2O, 0.87 mmol sodium dodecyl sulfate and 50 mmol urea were dissolved in 50 mL of 1:1 water/EtOH (v/v) co-solvent and stirred for 30 min at room temperature to form a homogenous solution. Then, CC and the above solution were transferred into a Teflon-lined stainless autoclave and maintained at 110 °C for 10 h. After being cooled to room temperature, the precursor was washed with deionized water and dried at 60 °C under vacuum. Finally, the precursor was phosphatized with NaH2PO2 at 300 °C for 2 h in Ar, and the obtained product was named as CC/NiCoP. The mass loading of NiCoP on bare CC was 1.1 mg cm−2. 2.3. Synthesis of CC/NiCoP@NiCo-LDH core/shell heterostructures 2 mmol of Ni(NO3)2·6H2O, 1 mmol of Co(NO3)2·6H2O and 5 mmol of hexamethylenetramine were dissolved in 36 mL of deionized water by being stirred for 30 min. The CC/NiCoP was immersed into the above solution and maintained at 90 °C for 24 h. The product was washed with deionized water and dried at 60 °C under vacuum to get CC/NiCoP@ NiCo-LDH. The mass loading of NiCoP@NiCo-LDH on CC was 2.4 mg cm−2. For comparison, CC/NiCo-LDH with NiCo-LDH mass loading of 1.4 mg cm−2 was also prepared using the same method. 2.4. Fabrication of flexible solid-state asymmetric supercapacitor The flexible solid-state asymmetric supercapacitor (FASC) was assembled with CC/NiCoP@NiCo-LDH, AC and PVA/KOH gel as positive electrode, negative electrode and electrolyte, respectively. The negative electrode was prepared by coating the mixture of active materials, 2

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Fig. 1. Schematic for the fabrication of CC/NiCoP@NiCo-LDH core/shell heterostructure.

(F g−1), areal specific capacitance (F cm−2), discharge current (A), discharge time (s), voltage window (V), area of the testing electrode (cm−2), energy density (Wh kg−1) and power density (W kg−1), respectively.

acetylene black and PTFE with the mass ratio of 8:1:1 on the CC (1 × 2 cm2). The mass ratio of CC/NiCoP@NiCo-LDH to AC was optimized and determined to be 0.34 based on the capacitance calculated from the galvanostatic charge/discharge (GCD) curves and charge balance principle. The PVA/KOH gel electrolyte was prepared by blending 2.13 g of KOH and 1.52 g of PVA in 20 mL of deionized water under magnetic stirring at 85 °C until the solution became clear. The two electrodes were coated with a layer of PVA/KOH gel electrolyte and assembled face to face through gentle squeezing and then wrapped with a preservative film. Finally, the CC/NiCoP@NiCo-LDH//AC FASC was dried at room temperature for 12 h.

2.7. Density function theory calculations For simulating the NiCoP, NiCo-LDH and NiCoP@ NiCo-LDH all ab initio total energy calculations were carried out based on density functional theory (DFT) within the framework of VASP (Vienna ab initio Simulation Packages) code which uses a plane wave basis set for the electronic orbitals. The electronic exchange and correlation was described within the generalized gradient approximation using the Perdew-Burke-Ernzerhof (PBE) functional. The interaction of the valence electrons with the ionic cores was treated within the projector augmented-wave (PAW) method (refer to ESI.† for details).

2.5. Material characterization The morphology of the as-synthesized electrode materials were characterized by Scanning electron microscopy (SEM, Ultra 55, Zeiss) and Transmission electron microscope (TEM, FEI Talos F200S). X-ray Diffraction (XRD) pattern was recorded by X’Pert PRO X-ray diffractometer. X-ray photoelectron spectrum (XPS) was measured by ESCALAB 250Xi X-ray photoelectron spectrometer. The surface wettability was analysed on OAC 25 optical contact angles. The N2 adsorption–desorption isotherms were measured on the Quantachrone Autosorb iQ-MP-C. The pore-size distribution was evaluated using the Barrett–Joyner–Halenda (BJH) method.

3. Result and discussion 3.1. Morphology and structure characterization The formation process of CC/NiCoP@NiCo-LDH core/shell heterostructure is showed in Fig. 1. Firstly, Ni2+ and Co2+ are absorbed on the surface of CC by strongly electrostatic and coupled interaction in alkaline environments [11]. The metal ions react with OH– produced by CO(NH2)2 hydrolysis to form the precursor particles, which act as seed crystals [33]. Then, the adjoining nanoparticles grow into 1D NWs along a particular crystal orientation to minimize the surface energy [9]. The precursor NWs are converted to NiCoP NWs by phosphorization treatment with NaH2PO2 [22]. As a skeleton, the NiCoP NWs provide abundant nucleation sites for the subsequent growth of NiCoLDH. Through the second hydrothermal treatment, NiCo-LDH nanocrystals are initially formed on the surface of NiCoP NWs. Afterwards, these NiCo-LDH nanocrystals further grow into NSs due to the oriented attachment effect, resulting in the formation of a 3D hierarchical core/ shell heterostructure [9,33]. The microstructures of the as-synthesized electrode materials are investigated by SEM (Fig. 2 and Fig. S1). As shown in Fig. 2a and 2b, CC is woven from staggered conductive fibers with a diameter of 8–10 μm. The CC with special interlaced network can be used as the growth substrate for electroactive species. Fig. 2c reveals that aligned NiCoP NWs are vertically and uniformly packed over the surface of CC. Comparing the statistical diameter value of CC with that of CC/NiCoP fibers, the growth height of the aligned NiCoP nanowire is about 0.6–1.0 μm. Fig. 2d shows the NWs present a needle-like structure with an average diameter of 50 nm. The interlaced CC fibers can be viewed as highly conductive collectors paralleled with numerous aligned

2.6. Electrochemical measurements Electrochemical measurements were performed in 6 M KOH aqueous by CHI 660E electrochemistry workstation with a three-electrode system. The as-prepared electrode, Pt foil and Hg/HgO electrode were used as working electrode, counter electrode and reference electrode, respectively. The electrochemical tests include cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The mass/areal specific capacitance, energy density and power density were calculated by using the following equations, respectively:

Cm =

I × Δt m × ΔV

(1)

CA =

I × Δt S × ΔV

(2)

E=

Cm × ΔV 2 7.2

(3)

P=

3600E Δt

(4)

where Cm, CA, I, Δt, ΔV, E and P are the mass specific capacitance 3

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Fig. 2. SEM images of (a, b) bare CC, (c, d) CC/NiCoP and (e, f) CC/NiCoP@NiCo-LDH.

LDH, respectively [11,35]. All the TEM and HRTEM results are well consistent with SEM images and further confirm the successfully fabrication of NiCoP@NiCo-LDH material. The HAADF-STEM image of NiCoP@NiCo-LDH and mappings of Ni, O, Co and P are exhibited in Fig. 3e and 3f, respectively. Obviously, the core/shell heterostructure is mainly composed of Ni, Co, P and O elements and they are uniformly distributed. Such rationalized core/shell heterostructure ensures abundant charge carrier concentration and efficient utilization of overall electrode material for rapid redox kinetics, thus potentially enhancing the efficiency of electrochemical energy-storage [8,23]. The N2 adsorption and desorption measurements were performed to further examine the NiCoP@NiCo-LDH heterostructure. As shown in Fig. S2, the NiCoP and NiCoP@NiCo-LDH show type-IV adsorption and desorption isotherms, proving its mesoporous characteristics. Compared to NiCoP (52.3 m2 g−1), the NiCoP@NiCo-LDH presents a larger BET surface area of 88.4 m2 g−1 and the pore volume is 0.568 cm3 g−1, demonstrating a reasonable design of multi-component composites with heterostructures, thus leading to an enhanced surface area. Moreover, the pore size distribution of NiCoP@NiCo-LDH is centered at about 6.8 nm (inset of Fig. S2). The NiCoP@NiCo-LDH with large surface area and suitable pore size distribution can markedly improve the contact area of the solid–liquid interface and shorten the diffusion lengths, further promoting the Faraday redox reaction kinetics [17]. The characterization of LDH precursor before phosphorization is provided in Fig. S4 and S5. After phosphorization, the peaks at 32°, 41°, 47.6° and 55.3° are indexed to the (1 0 1), (1 1 1), (2 1 0) and (2 1 1)

nanowire conductors, which can provide a rapid and convenient transmission path for electron or ions transport [8,18]. After growing NiCo-LDH by hydrothermal method, aligned NWs still exist on the surface of CC fibers (Fig. 2e), and careful observation shows needle-like NWs are completely covered by a thin layer of NiCo-LDH NSs as shown in Fig. 2f, indicating that core/shell heterostructure of NiCoP@NiCoLDH formed on CC fibers. For comparison, NiCo-LDH with NSs structure grows directly on bare CC (Fig. S1), and it shows that NSs has larger thickness and assemble to flower-like structure. The CC/NiCoP@ NiCo-LDH core/shell heterostructure possesses 3D open frameworks and thin layer of shell, which provides a large number of electroactive sites and hierarchical channels for easy transport of ions and electrons, thus facilitating electrochemical energy-storage [8,9,14]. The NiCoP and NiCoP@NiCo-LDH are stripped from the carbon cloth for TEM and HRTEM observation to get the detailed morphologies and heterointerface information. Fig. 3a shows NiCoP NWs with diameter of about 50 nm are obtained. The HRTEM image of NiCoP in Fig. 3b indicates the lattice fringes with interplanar distance of 0.22 nm, which matches well with the (1 1 1) crystal plane of NiCoP NWs [34]. Fig. 3c clearly displays the core/shell structure with ultrathin layer of NiCo-LDH NSs homogeneously depositing on the surface of NiCoP NWs. Such morphology offers open structure, which is beneficial to the permeation of electrolyte into the inner core. Distinct heterosinterface between NiCoP and NiCo-LDH can be easily found in Fig. 3d. The evaluated interplanar distance of 0.22, 0.26 and 0.29 nm correspond to the (1 1 1) plane of NiCoP, and the (0 1 2) and (1 1 1) planes of NiCo4

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Fig. 3. TEM and HRTEM images of (a, b) NiCoP and (c, d) NiCoP@NiCo-LDH, insets are the corresponding SAED patterns. (e) HAADF-STEM image of NiCoP@NiCoLDH. (f) Mappings of Ni, O, Co and P in NiCoP@NiCo-LDH.

NiCo-LDH of Ni 2p3/2 and Co 2p3/2 peaks shift about 0.4 and 0.6 eV to lower binding energies, respectively, demonstrating the existence of strong electronic interaction and a heterointerface between NiCoP NWs and NiCo-LDH NSs [8,37]. By deconvolution, Ni 2p3/2 and Co 2p3/2 spectra show peaks located at 856.1, 857 eV and 781, 782.6 eV, which indicates the presence of Ni2+, Ni3+ and Co3+, Co2+ cations, respectively [38–40]. For the P 2p spectrum (Fig. 4c), the peak located at 130.3 eV is assigned to metal phosphides (M-P), and the peak at 134.8 eV is representative of oxidized phosphorous species, which may due to the oxidation in air [25]. Remarkably, the P peaks of CC/ NiCoP@NiCo-LDH also shift 0.3 eV towards lower binding energy, further verifying the charge transfer and existence of a heterointerface between NiCoP and NiCo-LDH [37]. The strong interaction in the heterointerface can generate internal electric fields and charge

planes of the NiCoP (Fig. 4a) (JCPDS card no. 71–2336), respectively [34]. Furthermore, the peaks at 10.1°, 34.9° and 60° are indexed to (0 0 3), (0 1 2) and (1 1 0) planes of the NiCo-LDH (JCPDS card no. 40–0216), respectively [36,37]. The XRD pattern of the CC/NiCoP@ NiCo-LDH consists of the main peaks of NiCoP and NiCo-LDH, indicating the successful formation of NiCo-LDH material onto NiCoP. XPS were carried out to detect the surface composition and the corresponding oxidation states of the elements, and further elucidate the synergistic effects between NiCoP and NiCo-LDH (Fig. 4b-d and Fig. S3). From the high-resolution XPS spectra of Ni 2p (Fig. 4b), the Ni 2p3/ 2 and Ni 2p1/2 peaks are centered at 856.5 and 873 eV, respectively [38]. Similarly, the Co 2p spectrum (Fig. 4c) exhibits doublet strong peaks at 781.3 and 796.5 eV, which can be assigned to Co 2p3/2 and Co 2p1/2, respectively [39]. Compared with CC/NiCoP, the CC/NiCoP@ 5

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Fig. 4. (a) XRD patterns of the different samples. (b-d) XPS spectra of Ni2p, Co2p and P2p in CC/NiCoP and CC/NiCoP@NiCo-LDH, respectively.

The GCD curves of the CC/NiCoP@NiCo-LDH electrode at different current densities under voltage window of 0–0.5 V are displayed in Fig. 5d. Each curve has an obvious evident plateau, indicating that the highly reversibility of the redox reactions during charge/discharge process, which is in agreement with CV results [42]. Fig. 5e exhibits the GCD curves of the as-prepared electrode materials at the current density of 1 mA cm−2. As expected, the CC/NiCoP@NiCo-LDH shows longer discharge times compared with those of CC/NiCoP and CC/NiCo-LDH, suggesting that the hybrid electrodes have a larger capacitance due to the well-defined heterosinterface and efficient synergetic effects in the core/shell heterostructure [38]. The capacitance of the as-made samples can be calculated by GCD curves. The capacitance values are 1.240, 1.938 and 4.683F cm−2 (1127, 1384 and 1951F g−1) for the CC/NiCoP, CC/NiCo-LDH and CC/NiCoP@NiCo-LDH electrodes at 1 mA cm−2, respectively (Fig. 5e). These results outperform many previously reported Ni, Co-based hybrid electrode materials (Table S1). With increasing current density from 1 to 20 mA cm−2, the capacitance of these three electrodes exhibits a gradual attenuation with different rate property. Specifically, the capacity of CC/NiCoP decreases to 1.070 F cm−2 (972.7 F g−1) with 86.3% retention and the capacity of CC/NiCo-LDH decreases to 1.520 F cm−2 (1085.7 F g−1) with 78.4% retention (Fig. 5f). In comparison, the CC/NiCoP@NiCo-LDH electrode shows higher capacitance retention rate of 89.3%. The enhanced performance of CC/NiCoP@NiCo-LDH electrode is assumed to be related to the relatively low resistance, which can be verified by the following EIS analysis. To explain the better performance of CC/NiCoP@NiCo-LDH and research the ion diffusion kinetics of the as-prepared electrodes materials, EIS are surveyed in the frequency range from 0.01 to 105 Hz (Fig. 5g). Moreover, the left inset in Fig. 5g shows the equivalent circuit for understanding the relationship between each resistor intuitively. The EIS curves include a semicircle at high frequency region and a sloped line at the low frequency region. The semicircle represents charge transfer resistance (RCT), which mainly depends on the

discontinuity, which is beneficial to improvement in electronic and ionic conductivity, redox reaction kinetics and synergistic behavior of active species [37,38]. Further, deconvolution of O 1s peak (Fig. S3) shows metal-oxygen bonding and OH− locates at 529.5 and 530.7 eV, respectively [41].

3.2. Electrochemical properties The comprehensive electrochemical properties of the as-synthesized electrode materials are evaluated using a typical three-electrode cell system in 6 M KOH aqueous electrolyte solution. Fig. 5a presents the CV curves of CC/NiCoP@NiCo-LDH electrode at different scan rates. Clearly, all the CV curves show similar shapes and a pair of distinct redox peaks, demonstrating that the capacitance is mainly attributed to the fast redox reactions [18,19,38]. With increasing scanning rates, anodic and cathodic peaks are slightly shifted because of the internal diffusion resistance and incremental polarization [42]. From the CV curves of the CC/NiCoP, CC/NiCo-LDH and CC/NiCoP@NiCo-LDH at a scan rate of 7 mV s−1 (Fig. 5b), it is clearly observed that the CC/ NiCoP@NiCo-LDH exhibits more intense redox peaks intensities and larger areas compared with CC/NiCoP and CC/NiCo-LDH, revealing that the CC/NiCoP@NiCo-LDH hybrid electrodes possess a higher electrochemical activity and excellent energy storage capability. The higher redox behavior of CC/NiCoP@NiCo-LDH electrode may be attributed to the strong interaction in the heterointerface, which accelerates electron transfer and improves synergistic behavior of the overall electrode material [37,38]. In addition, the linear correlation between the square root of scanning rates and the corresponding peaks current indicates that the as-prepared hybrid electrode possesses excellent reversibility (Fig. 5c) [42,43]. The linear correlation also proves dominance of OH– diffusion controlled reactions, in which the powerful affinity for OH– greatly facilitates the fast redox reaction kinetics, confirming that the CC/NiCoP@NiCo-LDH electrode mainly stores energy by surface Faraday redox reaction [42,43]. 6

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Fig. 5. (a) CV curves of CC/NiCoP@NiCo-LDH electrode at different scanning rates. (b) CV curves of the different electrodes at a scanning rate of 7 mV s−1. (c) Plots of anodic and corresponding cathodic peak shown in (a) versus the square root of scanning rate. (d) GCD curves of CC/NiCoP@NiCo-LDH electrode at different current densities. (e) GCD curves of the different samples at 1 mA cm−2. (f) Areal capacitance of the different electrodes at various current densities. (g) Nyquist plots of the different samples; (h) 5000 cycles GCD measurement of the different samples at 20 mA cm−2.

initial capacitance after 5000 charge/discharge cycles. By contrast, cycling stability of CC/NiCoP@NiCo-LDH electrode has been significantly improved, and the specific capacitance could retain more than 80% of the initial value after 5000 charge/discharge cycles. The outstanding cycle stability of CC/NiCoP@NiCo-LDH can be attributed to two aspects: 1) NiCoP NWs with vertical structure can effectively alleviate the self-agglomeration of NiCo-LDH; 2) 3D open structures provide hierachical channels for the diffusion of ions and electrons, thus adapting to volume expansion and nano-structure changes during longterm cycling [14,28,36]. Furthermore, SEM image of the CC/NiCoP@ NiCo-LDH electrode after 5000 cycles shows that the core/shell heterostructure can be well preserved, further demonstrating the good stability of the CC/NiCoP@NiCo-LDH electrode (Fig. S6b). To better understand the role of NiCoP and NiCo-LDH in the hybrid architectures and further insight on the outstanding electrochemical performance of NiCoP@NiCo-LDH, density functional theory (DFT) calculations are performed. The optimized atomic structure models of NiCoP and NiCo-LDH are shown in Fig. 6a and Fig. 6b, respectively. First, the density of state (DOS) is used to analyze the electric properties of these species. The Ni(OH)2 exhibits a typical DOS of semionductor with a small band gap and weak DOS near the Fermi level in Fig. 6c. When the Co substitutes part of the Ni atoms in Ni(OH)2, the DOS of NiCo-LDH slightly increases at the Fermi level, which indicates that the conductivity of NiCo-LDH can be improved to some extent. Impressively, after P heteroatom doped into NiCo-LDH, the architecture of

hydrophilicity, conductivity and micro-structure of the electrode material [42,44–47]. The semicircle for the CC/NiCoP@NiCo-LDH is smaller than those of CC/NiCoP and CC/NiCo-LDH in the right inset of Fig. 5g, revealing a more facile charge transfer process for CC/NiCoP@ NiCo-LDH [46]. The slope of the straight line at the low frequency corresponds to a diffusion resistance (RW). The CC/NiCoP@NiCo-LDH electrode exhibits a more vertical line, indicating its lower RW and faster ion diffusion and electron transport compared with other samples. The CC/NiCoP@NiCo-LDH electrode exhibits an obviously lower equivalent series resistance (RS), which can be attributed to the outstanding electrical conductivity core of the NiCoP and the synergistic behavior between core and shell [45,46]. In addition, the optical contact angles of the as-prepared electrodes materials are measured to study surface wettability. As shown in Fig. S6, the bare CC exhibits hydrophobic surface, while the CC/NiCoP and CC/NiCoP@NiCo-LDH show superhydrophilic nature. The superior wettability can facilitate the diffusion of electrolyte ions, thereby increasing the contact area between the electrolyte and electrode and further improving the effective utilization of the electroactive material [17,42]. To assess the cycling stability of different material electrodes, the relationship between the specific capacitance and number of cycles was measured at a high current density of 10 mA cm−2 (Fig. 5f). The CC/ NiCo-LDH electrode exhibits a low retention rate of 50% after 5000 cycles, attributing to the serious aggregation of NiCo-LDH NSs on CC fibers (Fig. S6a). Meanwhile, CC/NiCoP can only maintain 70% of 7

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Fig. 6. Optimized atomic structure models of (a) NiCo-LDH, (b) NiCoP and (c) NiCoP@NiCo-LDH. Density of states (DOS) of (d) NiCo-LDH, (e) NiCoP and (f) NiCoP@ NiCo-LDH.

fast diffusion of ions and electrons, thus boosting the utilization of the overall electrode material and enhancing redox reaction kinetics. Thirdly, all the components of the hybrid electrode can be used as a Faraday redox reaction material, thus increasing the total specific capacitance. To evaluate the practical application potentiality of the CC/NiCoP@ NiCo-LDH core/shell heterostructure electrode, FASC is assembled with the CC/NiCoP@NiCo-LDH as the positive electrode, active carbon (AC) as the negative electrode and PVA-KOH gel as the solid electrolytecumseparator (Fig. 7a). The commercial activated carbon (AC) is extensively used as negative electrode material for FASC [36,45–47], which has a typical micro-sheet structure in Fig. S11. The CV curves of AC at different scanning rates exhibit nearly rectangular shapes (Fig. S12a), proving that AC possesses an outstanding electric double-layer characteristic. The specific capacitance of the AC is calculated to be 279F g−1 at 1 A g−1 and 182 F g−1 at 20 A g−1 respectively according to the GCD curves in Fig. S12b. In order to optimize the optimal operating voltage window, a series of CV with various voltage operation windows are demonstrated at 30 mV s−1 (Fig. 7b), and it is severely polarized due to the oxygen evolution reaction as the voltage reached 1.8 V [48]. Furthermore, the GCD curves of FASC at various operation voltage windows are shown in Fig. S14 and the voltage window can be extended up to 1.7 V. Thus the optimum operating voltage window range of the FASC is selected within 0 to 1.7 V. Fig. 7c exhibits the CV curves of FASC at various scan rates with an operating voltage window of 0–1.7 V. With the scan rate increasing, the shape of CV profiles shows a slight change, indicating superior capacitive behavior and good rate capability [4,11]. The GCD curves at various current densities exhibit no obvious internal resistance (IR) drop (Fig. 7d), which is benefit to a high discharge power delivery and superior cycling performance [23,41,49]. According to the GCD curves, the calculated capacitance values are 142, 125, 119, 108, 99 and

NiCoP extends in all directions with individual atom interacting more closely. From Fig. 6d, it is clearly seen that the NiCoP shows a high, continuous and gapless DOS at the Fermi level, demonstrating a superior electrical conductivity. The increased DOS at Fermi level of NiCoP is mainly due to the fact that P heteroatom metallizes the relaxed structure of NiCo-LDH. Besides, the NiCoP@NiCo-LDH exhibits an enhanced DOS near the Fermi level in Fig. 6c and 6f. Synergetic behavior at the interface between NiCoP and NiCo-LDH leads to an improved transfer capacity of electron, thus achieving excellent conductivity. Moreover, the hybrid nanoarchitectures simultaneously ensure abundant electron active sites and numerous charge carrier concentrations. The adsorption abilities of OH– on these two optimized atomic structure models are also investigated. The NiCo-LDH strongly absorbs OH– from the electrolyte to form a water molecule (Fig. S9), and it exhibits a high OH– adsorption energy of 3.010 eV (absolute value), indicating that the NiCo-LDH possesses powerful affinity for OH–. The strong OH– adsorption capacity facilitates the trap of electrolyte ions, thus resulting in fast redox reaction kinetics [4]. Interestingly, the edge P site of NiCoP model shows a higher OH– adsorption energy of 3.320 eV, which provides a strong driving force for NiCo-LDH growing and forming a heterointerface with intimate contact on the NiCoP surface (Fig. S10). These theoretical results further prove that NiCoP and NiCo-LDH play crucial roles in improving the reversible redox kinetics with excellent electrochemical performance of the multifunctional hybrid electrodes. The excellent electrochemical properties of core/shell heterostructure CC/NiCoP@NiCo-LDH electrodes are summarized in the following aspects. First of all, high electronic conductivity of NiCoP and superior stability of NiCo-LDH are effectively combined in a core/shell heterostructure, which simultaneously ensures good charge transport and outstanding cycle stability. Secondly, 3D open core/shell heterostructure of CC/NiCoP@NiCo-LDH hybrid electrode material possesses numerous electroactive sites and hierarchical channels, which allow the 8

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Fig. 7. (a) Schematic illustration of the flexible solid-state asymmetric supercapacitor; (b) CV curves of the FASC device collected in various scan potential windows; (c) CV curves of the FASC device at different scanning rates; (d) GCD curves of the FASC device at different current densities; (e) Cyclic stability of the FASC device at a current density of 5 A g−1 for 10,000 cycles; (f) Ragone plots of the FASC device in this work and previously reported references.

94 F g−1 at current densities of 1, 2, 3, 5, 7 and 10 A g−1, respectively (Fig. S13b). Moreover, the durability of the FASC device is conducted at current density of 5 A g−1 (Fig. 7e). Remarkably, the FASC device can retain a capacitance of 97% after 10,000 successive charge/discharge cycles, exhibiting an electrifying cycling lifespan. Energy and power density are two important parameters for achieving FASC commercial application. The Ragone plots of FASC and some other Ni, Co-based asymmetric supercapacitor are shown in Fig. 7f. The FASC device can achieve energy density as high as 57 Wh kg−1 at the power density of 850 W kg−1 and still maintain 37.7 Wh kg−1 at the extremely high power density of 8493 W kg−1, which is higher than a number of Ni, Co-based asymmetric supercapacitor, such as NiCoP/NiCo-OH//PC (34 Wh kg−1 at 775 W kg−1) [23], s-NiCoP//AC (43.54 Wh kg−1 at 150 W kg−1) [50], NiCoP@Co9S8// AC (38 Wh kg−1 at 800 W kg−1) [51], NiCo-LDH/ NiCoP@NiMn-LDH//AC (42.2 Wh kg−1 at 750 W kg−1) [48], NiCoP// AC (32.9 Wh kg−1 at 1301 W kg−1) [18], p-NiCoP//AC (45.5 Wh kg−1

124.2 W kg−1) [52] and NiCoP-CoP//PNGF (39 Wh kg−1 at 1784 W kg−1) [22]. These results demonstrate the huge potential of CC/NiCoP@NiCo-LDH core/shell heterostructure electrode material in high energy and power asymmetric supercapacitor. Flexibility and deformability are indispensable aspects of modern portable and lightweight electronics. The photographs of FASC at the bending angles of 0°, 40°, 80° and 160° are shown in Fig. 8a. It is clearly observed that FASC folded and twisted without damaging the structural integrity, demonstrating that the as-assembled device possesses excellent flexibility and stability. In addition, CV curves of the FASC device at different bending angles states exhibit no obvious changes compared with the flattened state (Fig. 8b), which further proves the outstanding deformability of the FASC device. Remarkably, the FASC device is able to light up LEDs consisting of ‘ZZU’ shape and power a hygrothermograph (Fig. 8c and 8d). Based on these excellent performances, the as-assembled FASC device shows great potential in the next-generation energy storage electronic device application. 9

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Fig. 8. (a) The photographs of the FASC under folded and twisted conditions; (b) CV curves of FASC collected at a scan rate of 30 mV s−1 under flat and bend conditions; (c) Picture show that two devices in series can light up LEDs consisting of ‘ZZU’ shape; (d) The photograph show that two devices in series can power a hygrothermograph.

4. Conclusions [2]

In summary, we have demonstrated a new strategy to combine the micromorphologies and nanostructures of core/shell heterostructures NiCoP@NiCo-LDH by judiciously heterointerface engineering with intentionally introduced highly conductive core of NiCoP. The electrochemically favorable 2D NiCo-LDH with ultrathin NSs and interconnected structures are uniformly anchored on the highly conductive core material of 1D NiCoP to form a 3D open core/shell heterostructure. In heterostructures, heterointerfaces effectively improve internal electronic properties and synergistic behavior, which facilitates rapid redox reaction kinetics. In addition, the 3D open heterostructure allows more electroactive sites to be exposed, which increases the utilization of the overall electrode material. Benefiting from these advantages, the core/ shell heterostructure electrodes of CC/NiCoP@NiCo-LDH simultaneously ensure superior charge transport and structure stability. Furthermore, the FASC consisted of CC/NiCoP@NiCo-LDH cathode and AC anode delivers a high energy density and excellent durability.

[3]

[4]

[5]

[6]

[7]

[8]

[9]

Conflicts of interest There are no conflicts to declare.

[10]

Acknowledgements

[11]

This work was supported by the National Natural Science Foundation of China (Grants 21576247, U1804140, 21706242 and 21873086).

[12]

Appendix A. Supplementary data [13]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123373. [14]

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