Ni3Se2 nanosheets in-situ grown on 3D NiSe nanowire arrays with enhanced electrochemical performances for supercapacitor and efficient oxygen evolution

Journal Pre-proof Ni3Se2 nanosheets in-situ grown on 3D NiSe nanowire arrays with enhanced electrochemical performances for supercapacitor and efficient oxygen evolution

Jian Zhao, Lina Yang, Huanyu Li, Tianqi Huang, He Cheng, Alan Meng, Yusheng Lin, Peng Wu, Xiangcheng Yuan, Zhenjiang Li PII:

S1044-5803(20)32290-7

DOI:

https://doi.org/10.1016/j.matchar.2020.110819

Reference:

MTL 110819

To appear in:

Materials Characterization

Received date:

3 September 2020

Revised date:

7 December 2020

Accepted date:

9 December 2020

Please cite this article as: J. Zhao, L. Yang, H. Li, et al., Ni3Se2 nanosheets in-situ grown on 3D NiSe nanowire arrays with enhanced electrochemical performances for supercapacitor and efficient oxygen evolution, Materials Characterization (2020), https://doi.org/10.1016/j.matchar.2020.110819

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© 2020 Published by Elsevier.

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Ni3Se2 nanosheets in-situ grown on 3D NiSe nanowire arrays with enhanced electrochemical performances for supercapacitor and efficient oxygen evolution Jian Zhaoa＃, Lina Yangb＃, Huanyu Lia, Tianqi Huangb, He Chengc, Alan Mengb,

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Yusheng Lina, Peng Wuc, Xiangcheng Yuanb, Zhenjiang Lia*

a

ro

College of Materials Science and Engineering, Qingdao University of Science and

State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and

re

b

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Technology, Qingdao 266061, Shandong, P. R. China.

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Molecular Engineering, Qingdao University of Science and Technology, Qingdao

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266042, Shandong, P. R. China. c

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Key Laboratory of Polymer Material Advanced Manufacturing Technology of

ShandongProvincial,College of Electromechanical Engineering, College of Sino-Ger man Science and Technology, Qingdao University of Science and Technology, Qingdao 266061, Shandong, P. R. China. ＃

These authors contributed equally to this work and should be considered co-first

authors. *

Corresponding Author

E-mail: [email protected]

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ABSTRACT

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A typical core-branch NiSe@Ni3Se2/NF nanostructure directly grown on Ni foam as

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an asymmetric supercapacitor (ASC) electrode and electrocatalyst is prepared

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employing a facile two-step in-situ growth procedures. The as-synthesized

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nanoarchitecture is composed of relatively thin Ni3Se2 nanosheets shell and NiSe

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nanowire arrays core (NiSe NWAs). Thanks to the favorable electric conductivity, high theoretical capacitance and the distinct micro-morphologies of the Ni-based

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selenide, it can present excellent capacitive performances. More importantly, an ASC

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constructed utilizing the as-fabricated NiSe@Ni3Se2/NF hybrids as positive electrode and active carbon (AC) as negative electrode can exhibit a large energy density of 45.5 Wh kg-1 at 1.600 kW kg-1. Moreover, it can also show outstanding ultra-long durability with a capacitance retention of ~96.1% after 12000 cycles. In addition, the as-obtained

NiSe@Ni3Se2

catalyst

can

present

favorable electrocatalytic performances for oxygen evolution reaction (OER) with a small overpotential of 281 mV at 10 mA cm-2. Thus, this strategy not only provides an efficient channel to design high-performance electrode materials and electrocatalyst, but also promotes the practical applications of the newly emerged metal selenides 2

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nanoarchitectures in energy storage and conversion systems.

Key words: Ni3Se2 nanosheets, NiSe nanowire arrays, electrochemical performances,

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asymmetric supercapacitors, oxygen evolution

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1. INTRODCTION

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With the enhancing of fossil fuel consumption demands and the environmental

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pollution issues, developing high-performance and clean energy storage and

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conversion configurations is an assignment of the utmost urgency [1,2]. Among the energy storage systems, supercapacitors are the desirable candidates, mainly owing to

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their enhanced power density, rapid charging/discharging rate, light weight and low

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cost features [3,4]. However, their fatal flaw is much lower energy density, generally, the energy density of commercial supercapacitors is only ~5 Wh kg-1 [5,6], which significantly limits their large-scale applications. As for the energy conversion configuration, the electrochemical water splitting is generally selected as a clean and efficient energy conversion approach, which is always refined by the depressed kinetics of oxygen evolution reaction (OER) [7,8]. At present, the efficient OER electrocatalysts still assign to some noble metal oxides including RuO2 and IrO2, however, these samples often encounter severely scarcity and high cost [9,10]. In this case, the current priority focuses on rational design and development of advanced multifunctional nanostructures with superior electrochemical performances for their 3

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practical SCs and OER applications.

As for SC electrode materials and OER electrocatalysts, transition metal compounds, especially Ni-based nanostructures including Ni3S2 [11], Ni(OH)2 [12] NiS [13] and NiO [14] et al have received significant interests. It is mainly benefit from their large theoretical capacitance,

good redox

features,

prominent

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electrochemical activity as well as environmentally friendly. Unfortunately, there are

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the existence of the obvious gap between their actual and theoretical capacitance, and

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an faultiness in rate property, inferior OER activity or cycling stability [15-17]. It may

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be attributed to intrinsically inferior electrical conductivity, or relatively few available electroactive sites. For the sake of solving these issues, researchers usually use

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conductive carbon-based nanomaterials as frameworks to uniformly grow these active

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materials on the surface of the supports, which can really exhibit relatively superior

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electrochemical properties than the single component [18-20]. Despite achieving some encouraging achievements, the above composites are still unable to meet the needs of high-performance supercapacitor and OER electrocatalysts. Since the strategy also fails to change the conductivity of active materials themselves. Moreover, the specific capacitance of these aforementioned carbon-based frameworks is very small, thus, the contribution of capacitance as for the hybrid electrode materials is mainly stemmed from the active materials. Based on this, synthesizing a new family of active materials and skeletons, and intelligently constructing the unique composite nanoarchitectures are extremely necessary at present.

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In addition, compared with transition metal oxides or sulfides, selenides have begun to enter researchers' visual field, and they could potentially be better candidates because the element Se owns similar electrochemical performances of O or S. Importantly, it possesses more metallicity and displays a higher electronic conductivity (1 × 10-3 S m-1) than that of S (5 × 10-28 S m-1) [21]. Among various

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Ni-based selenides, NiSe possesses excellent electrical conductivity for charge transfer, electrochemical activities and the variety of oxidation states [22-25]. Thus,

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when the NiSe on Ni foam would be considered as skeletons for hybrid electrode

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materials, apart from making the active materials uniform distribution, the supports

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also can react with electrolyte ions, then contributing capacitance to the composites.

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Besides, as for the another type of the Ni3Se2, it not only has favorable electrical

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conductivity but also exhibits stable phase, robust physical-chemical features closely relied on its composition, phase structure and morphology in accordance with the

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phase diagram [26]. Moreover, there are three Ni atoms of Ni3Se2 taking part in a three-electron reaction, leading to larger charge storage capability, which can be expected to a promising alternative electroactive materials [27]. Meanwhile, the kind of the selenides with excellent conductivity can also display remarkable catalytic activities for OER. It is therefore of interest to synthesize NiSe framework for growing Ni3Se2 to form ideal integrated nanostructures for SCs and OER catalytic properties.

In this study, a particular nanostructure consisting of the Ni3Se2 NSs grown on the surface of NiSe NWAs were successfully synthesized directly on nickel foam via 5

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one-step in-situ hydrothermal approach coupling with electrodeposition route. It can be acted as a self-supported hybrid electrode for ASCs and OER electrocatalysts. Owing to their synergistic effects, the NiSe NWAs@Ni3Se2NSs/NF can deliver good capacitive properties. Moreover, utilizing the as-prepared hybrids and AC as positive and negative electrode, an ASC device was assembled, which possesses high energy

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density and ultra-long cycling performance. Additionally, the achieved composite also exhibits low overpotential for OER. This work vastly broadens the applications of

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different transition metal selenides in high-performance energy storage devices and

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conversion systems.

Chemicals and materials

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2. EXPERIMENTAL SECTION

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Nickel chloride hexahydrate (NiCl2·6H2O), selenium dioxide (SeO2), lithium chloride

Reagent

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(LiCl) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Co.,

Ltd.

Selenium

(Se)

powder

was

obtained

from

Shanghai Macklin Biochemical Co., Ltd. Commercial activated carbon (AC) and Nickel (Ni) foam were procured from Fuzhou Yihuan carbon Co., Ltd. and Heze Tianyu Technology Development Co. Ltd., respectively. All the chemical reagents were AR grade and employed without further purification treatment. Preparation of the NiSe NWAs on Ni foam (NF) A piece of Ni foam (1×1 cm2) was washed with 3M HCl, ethanol and deionized water several times to guarantee its surface was well cleaned before use. Firstly, 0.079g Se

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powder and 0.108g KBH4 were added into 5ml deionized water, after continuous stirring for 5min, a clear KHSe solution was achieved. Secondly, the obtained KHSe solution was added into ethanol (30 mL) under N2 flow. Then, the mixed solution was transferred into 50 mL Teflon-lined stainless steel autoclave with a piece of pretreated Ni foam maintained at 140℃ for 14 h in an electric oven. The obtained product is

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NiSe NWAs/NF, and its active mass loading is ~2.0 mg.

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Synthesization of the NiSe NWAs @Ni3Se2 NSs/NF

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The NiSe NWAs @Ni3Se2 NSs/NF were gained by one-step eletrodeposition route.

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Specifically, the above obtained NiSe NWAs/NF as working electrode, a Pt wire and a

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saturated calomel as counter electrode and reference electrode respectively in a mixed electrolyte solution including 0.2377g NiCl2·6H2O, 0.111g SeO2 and 0.212g LiCl at

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-0.8 V for 400s. In contrast, the Ni3Se2 NS/NF was fabricated employing Ni foam as

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working electrode under the same conditions. The active mass loading of the NiSe NWAs @Ni3Se2 NSs/NF and Ni3Se2 NSs/NF were about 2.4 mg and 0.4mg. Assembly of asymmetric supercapacitor Active carbon (AC) was selected as negative electrode materials, which combines with the obtained NiSe NWAs @Ni3Se2 NSs/NF hybrid positive electrode to construct an ASC device. 3 M KOH and filter paper are acted as the electrolyte and separator respectively. The typical mass loading of the AC electrode materials is 3.6 mg cm-2, which was verified based on the well-known charge balance theory [28].

Materials characterization 7

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The morphology were characterized through utilizing a FESEM (Hitachi, SU8010). The specific microstructural information were analysed by employing TEM (Hitachi, H-8100). XRD (D8 X-ray diffractometer) was used to record the phase compositions. Furthermore, XPS characterization was performed to further determine the surface composition on a Thermo ESCALAB 250Xi device

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with an Al-Kα (hν=1486.6 eV) excitation source.

performances

test:

Cyclic

voltammetry

(CV),

Galvanostatic

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Capacitive

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Electrochemical measurements

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charge-discharge (GCD) as well as electrochemical impedance spectroscopy (EIS)

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were tested via CHI660e (Chenhua Instrument Co., Shanghai, China). The performances of the electrodes were recorded in a standard three-electrode systems

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with Pt wire and a saturated calomel electrode (SCE) as the counter and reference

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electrode separately. In terms of the capacitive performance of the assembled ASC, it was conducted in a two-electrode configuration.

Electrocatalytic test: the catalytic performance was measured employing Pt wire as the counter and a SCE as reference electrode in 1 M KOH (PH=14) solution. The polarization plots were tested at a scan rate of 5 mV s-1 with iR corrected.

3.RESULTS AND DISCUSSION 3.1 Characterizations and electrochemical performances of the obtained electrodes

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The core-branch structured NiSe NAs@ Ni3Se2 NSs/NF are prepared by a two-step process as depicted in Fig. 1 First of all, 3D NiSe nanowire arrays are directly grown on the Ni foam surface by the hydrothermal technique. Specifically, the added Se powder in KBH4 aqueous solution can be reduced to Se2-, then it reacts with the metal Ni foam to form the 3D NiSe NAs [29,30]. They makes the Ni foam

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surface with metallic luster change to black. Subsequently, Ni3Se2 NSs are also directly fabricated on NiSe NAs through a facile electrodeposition procedure with

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NiCl2·6H2O, SeO2 and LiCl as raw materials. The detailed synthetic route and

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reaction mechanism are indicated by the following equations [31]:

Se+2e- → Se2-

（1） （2） （3） （4）

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3Ni2++2Se2- → Ni3Se2

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H2SeO3+4H++4e- → Se+3H2O

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SeO2+H2O → H2SeO3

To achieve the morphology information of the as-synthesized products in detail, SEM technique is carried out displayed in Fig. 2(a-e). From the low-magnification SEM image (Fig. 2(a)), the NiSe nanowire arrays are evenly grown on the Ni foam. The highly magnificated SEM images presented in Fig. 2(b,c) indicates that the NiSe nanowires at the whole surface of the substrate can form a typical clusters arrays, which are intertwined with each other, constituting a 3D network structure. Moreover, the average diameter of these nanowires with smooth surface is ~50 nm, and they are tens of microns in length. Fig. 2(d,e) show SEM 9

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images of the NiSe NWAs@Ni3Se2 NSs. Clearly, the whole surface of the NiSe NWAs are homogeneously wrapped by the class of sheet-like Ni3Se2, which allows their surface to become very rough. And the composite products still preserve the wire-like architectures, which is similar to that of the NiSe nanowires. The corresponding EDS analysis of the hybrid architecture shown in inset of Fig. 2e

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reveals that it consists of Ni and Se elements. It can be seen from Fig. S1 in Supporting Information that the Ni and Se elements are evenly distributed. For

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comparison, only Ni3Se2 NSs is synthesized on Ni foam under the same fabrication

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conditions as displayed in Fig. S2 in Supporting Information. The Ni3Se2 NSs are

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arbitrarily and unevenly dispersed on Ni foam, and a part of the products are

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accumulated on the surface of the substrate. The specific structures of the as-prepared

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NiSe NWAs@Ni3Se2 NSs are also investigated through TEM and HRTEM analysis depicted in Fig. 2(f-i). The corresponding TEM images (Fig. 2(f,g)) obviously

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illustrate that the Ni3Se2 NSs are uniformly grown on the well-defined NiSe NWAs, creating a special core-branch nanostructures. The thickness of Ni3Se2 nanosheet shell is relatively thin of ~20-40 nm, which is consistent with the above SEM characterization results. Additionally, the HRTEM analysis of the hybrid product is further displayed in Fig. 2h,i, from which a set of clear lattice fringes with interplanar spacing of 0.30 and 0.27 nm can be observed. It is well indexed to the (110) and (101) planes of Ni3Se2 and NiSe, respectively. Based on the aforementioned analysis results, the Ni3Se2 nanosheets grown on NiSe nanowire arrays has been prepared successfully. Typical XRD analysis was carried out to study the crystallographic structures 10

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and phases of the prepared products, and the achieved patterns are shown in Fig. 3(a). It can be seen from Fig. 3(a), three strong characteristic peaks with green circle correspond to the Ni foam substrate (JCPDS 87-0712). Moreover, the five characteristic peaks with blue mark centered at 32.8°, 44.3°, 49.7°, 59.6° and 61.0° are ascribed to (101), (102), (110), (103) and (201) planes of NiSe (JCPDS 75-0610).

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Besides, the other diffraction peaks featured at 20.9°, 29.5°, 29.8°, 37.2°, 42.6°, 47.6°, 48.2°, 52.7° and 53.4° can be indexed to (101), (110), (012), (003), (202), (211), (113),

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(122) and (104) planes of Ni3Se2 (JCPDS 85-0754). Furthermore, the surface

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composition and chemical state of the synthesized products were discussed using XPS

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analysis (Fig. 3b-d), and Fig. 3(b) illustrates the corresponding XPS survey spectrum.

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It indicates that the coexistence of the elements Ni and Se. Fig. 3(c) presents

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high-resolution Ni 2p spectrum. Clearly, two major peaks at the binding energies of 855.8 and 873.5 eV corresponding to Ni 2p3/2 and Ni 2p1/2 respectively are

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spin-orbit characteristics of Ni2+ and Ni3+ state stemmed from the obtained products [25, 32-34]. The Se 3d features shown in Fig. 3(d) depicts two spin-orbit doublets at 54.8 and 55.7 eV respectively, reflecting the Se 3d5/2 and Se 3d3/2 signals, which suggests the presence of Se2-, and the peak at 58.9 eV can be attributed to oxidized Se [35,36]. Consequently, the above XRD and XPS analysis results further substantiate that the NiSe NWAs@ Ni3Se2 NSs composites have been synthesized successfully.

The electrochemical features of the Ni foam (NF), NiSe NWAs/NF, Ni3Se2 NSs/NF and NiSe NWAs@ Ni3Se2 NSs/NF electrodes are estimated in a 11

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three-electrode apparatus. The corresponding GCD curves (Fig. 4(a)) comparison of the obtained electrodes at 2 A g-1 obviously exhibit that there are two charging/discharging plateaus in each plot pointing to the redox behavior. The NiSe NWAs@ Ni3Se2 NSs/NF also has much longer discharge times than the individual NiSe NWAs/NF and Ni3Se2 NSs/NF, which is indicative of its larger specific capacitance, agreeing well with the above CV test results. Fig. 4(b) exhibits the CV

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curves of the NiSe NWAs@ Ni3Se2 NSs/NF at 5~100 mV s-1. A couple of redox peaks

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are clearly observed in each curve at different scan rates, mainly resulting from the

(6)

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NiSeOH + e-

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NiSe + OH- ⇌

(5)

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Ni3Se2 + 3OH- ⇌ Ni3Se2OH3 + 3e-

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corresponding redox reactions as follows [27,37]:

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Fig. 4(c) elaborates the linear relationship between the square root of the scan rate and the oxidation/reduction peak current densities. It affirms the reaction is controlled by ion diffusion, in which the energy storage mechanism of the prepared electrode materials belongs to faradic redox reactions rather than physical adsorption of the electrolyte ions [38]. The GCD profiles of the NiSe NWAs@ Ni3Se2 NSs/NF, NiSe NWAs/NF and Ni3Se2 NSs/NF at different current densities from 2 to 10 A g-1 are presented in Fig. 4(d) and Fig. S3 in the supporting information, respectively. According to the GCD curves, their specific capacitance is calculated from the equations (1) listed in the supporting information, the corresponding results are 12

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NSs/NF electrodes only exhibit 357 F g-1 and 960 F g-1 with rate capability of ~47% and 76.1% respectively. Importantly, when the current density is increased to 20 A

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g-1, the composite can still retain 1160 F g-1 with rate capability of ~73.3%.

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Meanwhile, the obtained specific capacitance of the NiSe NWAs@ Ni3Se2 NSs/NF is

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clearly superior to other transition metal compounds electrode materials reported in

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prior literature (Tab. S1, see supporting information). Fig. 4(f) shows the Nyquist

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plots of the NiSe NWAs@ Ni3Se2 NSs/NF, NiSe NWAs/NF and Ni3Se2 NSs/NF electrodes. Based on an equivalent circuit and fit results (inset in Fig. 4(f)), it is easier

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to observe that the individual NiSe NWAs/NF electrode has relatively larger equivalent series resistance (Rs ~1.02 Ω) and charge transfer resistanc (Rct ~1.45 Ω) at the high-frequency region, and the single Ni3Se2 NSs electrode possesses higher diffusion resistance (Rw) in the low-frequency field possibly owing to their partial aggregation on the substrate. However, in terms of the NiSe NWAs@ Ni3Se2 NSs/NF hybrid electrode, it shows smaller Rs of ~0.76 Ω and Rct of ~1.34 Ω. Meanwhile, it also illustrates a sharp (almost vertical) rise of the imaginary component of impedance, confirming reduced Rw for efficient ions diffusion. Thus, the uniformly dispersed Ni3Se2 NSs grown on the NiSe NWAs/NF hybrid electrode really owns favorable 13

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electric conductivity. In addition, cycling durability is another standard to determine the electrochemical property of an electrode material. As depicted in Fig. 4(g), in terms of the NiSe NWAs@ Ni3Se2 NSs/NF hybrid electrode, there is only 7.5% capacitance loss after 4000 cycles at 10A g-1, proving excellent capacitance retention. While the NiSe NWAs/NF and Ni3Se2 NSs/NF electrode only maintains ~76.2% and

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66.6% of the initial capacitance, respectively. Moreover, the cycling performance of the NiSe NWAs@ Ni3Se2 NSs/NF hybrid electrode is also much better than those of

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other transition metal compound electrode materials in previous reports listed in Tab.

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S2 in supporting information.

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3.2 Electrochemical performances of the fabricated ASCs

In order to further study the capacitive behaviors of the synthesized NiSe

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NWAs@ Ni3Se2 NSs/NF under practical working conditions of an ASC, the

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asymmetric device was constructed, where the NiSe NWAs@ Ni3Se2 NSs/NF was employed as positive electrode and AC as negative electrode materials depicted in Fig. 5(a). The AC products can exhibit good electrochemical properties (Fig. S4 in supporting information). Since the AC and NiSe NWAs@ Ni3Se2 NSs/NF correctly fills up the complementary voltage window (Fig. 5(b)), hence expanding the working potential window of the device. To gain the accurate value, the CV curves with different various voltage window at 20 mV s-1 were recorded presented in Fig. S5 (see supporting information). No apparent polarization was examined in the range of 0~1.6 V, verifying that 1.6 V is an appropriate potential window. The CV plots with the

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Journal Pre-proof 0~1.6 V working window at 10~50 mV s-1 are illustrated in Fig. 5(c), the shape of the CV curves can be well preserved as the scan rate enlarges to 50 mV s-1, suggesting the favorable rate property. In accordance with the tested GCD curves (Fig. 5d) and the equations (2) in supporting information, the specific capacitance of the device are calculated to be 128, 115.5, 103.5, 86, and 80 F g-1 at 2, 4, 8, 16 and 20 A g-1,

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respectively (shown in Fig. 5(e)). Moreover, the ASC device also possesses the low voltage drop, revealing its excellent conductivity, which is further confirmed by the

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corresponding EIS pattern (Fig. S6, supporting information). Furthermore, the energy

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and power densities of the ASC are described in the Ragone plots (Fig. 5(f)). The

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constructed device delivers a high energy density of 45.5 Wh kg-1 at 1.6 kW kg-1 and

be

referred

to

the

equations

(3)

in

supporting

information).

It

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can

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28.5 Wh kg-1 at 16 kW kg-1, respectively (the calculated method of its energy density

significantly outperforms those of previous reported ASCs with various electrode

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materials such as NiS//RGO@Fe3O4 (43.7Wh kg-1 at 0.664 kW kg-1) [39], NiAl-LDH-160//ACNF (20 Wh kg-1 at 0.750 kW kg-1) [40], Ni3Se2 NSs@CF//AC (32.8Wh kg-1 at 0.677 kW kg-1) [41], NiSe@MoSe2//AC (24.5 Wh kg-1 at 0.400 kW kg-1) [24], (Ni,Co)Se2/NiCo-LDH//PC (39 Wh kg-1 at 1.650 kW kg-1) [42], ZIF-67-LDH-CNP-110//AC (33.29 Wh kg-1 at 0.15 kW kg-1) [43], NiS-HS//AC (38.3 W h kg-1 at 0.160 kW kg-1) [44] and CoSe2//N-CNW (32.2 Wh kg-1 at 1.914 kW kg-1) [36]. Additionally, the cycling performance of the ASC is conducted at 8 A g-1 for 12000 cycles, with almost no attenuation of the capacitance (holds ~96.1% of the initial value), thereby elaborating the splendid cycling stability for long time usage 15

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(Fig. 5(g), black curve). Such excellent durability exceeds those of the ASC device constructed from different electrode material counterparts published recently summarized in Tab. S3 in the supporting information. Moreover, the coulomb efficiency of the assembled device can remain almost 100% at all times presented in Fig. 5(g) (blue curve), which is further demonstrated through the first and last 20

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cycles (Fig. S7, supporting information). To explore their practical applicability, we utilized the two fabricated devices connected in series to easily power a red LED and

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electronic watch (see inset in Fig. 5(g)), fully verifying the high performance of the

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constructed ASC device.

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3.3 OER performances of the these prepared electrodes

The as-obtained NiSe NWAs@ Ni3Se2 NSs/NF electrode also shows excellent

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electrocatalytic properties for OER in 1 M KOH (Fig. 6). Fig. 6a depicts the linear

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sweep voltammetry (LSV) curves (with iR-correction) of the NiSe NWAs/NF, Ni3Se2 NSs/NF and NiSe NWAs@ Ni3Se2 NSs/NF electrodes at 5 mV s-1. Obviously, the hybrid NiSe NWAs@ Ni3Se2 NSs electrode presents overpotential of 281 mV at a current density of 10 mA cm-2, which is lower than that of bare NiSe NWAs/NF (324 mV) or Ni3Se2 NSs (622 mV) electrode. To further investigate the catalytic performance, Tafel plots are carried out displayed in Fig. 6(b). From Fig. 6(b), the lowest Tafel slope of the composite electrode is 69.1 mV dec-1, smaller than the pristine NiSe NWAs/NF (146.3 mV dec -1) or Ni3Se2 NSs/NF (146.7 mV dec-1). It demonstrates the NiSe NWAs@ Ni3Se2 NSs electrode owns higher catalytic activity.

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Moreover, the catalytic features of the hybrid electrode exceeds other catalysts, which is listed in Tab. S4 (supporting information). To calculate the electrochemically active surface area of these electrodes, the corresponding CV curves are measured at potential window of -0.1~ 0 V (vs SCE) at 5~200 mV s-1 (Fig. 6c and Fig. S8 in Supporting information). The calculated results (Fig. 6(d)) show that the electrochemical double layer capacitance of the NiSe NWAs@Ni3Se2 NSs/NF is 3.25

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mF cm -2, which is higher than NiSe NWAs/NF (2.82 mF cm-2) and Ni3Se2 NSs/NF

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(2.67 mF cm-2). It reveals the composite electrode possesses more active reaction sites.

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In addition, the NiSe@Ni3Se2 /NF can still present remain ~94% original value

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after 35 h, verifying outstanding cycle stability (Fig. S9, see Supporting

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Information). It is superior to those of the previous reported transition metal

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compound catalysts (see Tab. S4 in the supporting information). It can be seen from Fig. S10 in Supporting Information that NiSe@Ni3Se2/NF after OER still

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presents core-shell morphology, which is almost consistent with the pristine sample (Fig. 2d-f). It further substantiates their excellent cycling performance. In addition, from Fig. S11 in Supporting Information, the HER test of the NiSe@Ni3Se2/NF composite shows overpotential of 94.5 mV at 10 mA cm-2, and its corresponding Tafel Tafel slope is 109.5 mV dec-1. Moreover, it can also deliver satisfactory cycling stability. Thus, the above results substantiate that the as-synthesized hybrid electrode possesses outstanding electrocatalytic properties for OER and HER.

The favorable electrochemical performances of the as-synthesized the 17

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core-branch NiSe NWAs@ Ni3Se2 NSs/NF is primarily ascribed to the unique design and the cooperative contribution of the support and active materials. Specifically speaking, 1) The element Se owns more metallicity, and exhibits a high electroconductibility, thus the NiSe/NF skeletons themselves possess remarkable electronic conductivity. Besides, they can also react with electrolyte ions, so they are

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capable of contributing their capacitance to the hybrid electrode materials, which is completely distinguished from carbonaceous nanomaterials. Moreover, the NiSe

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nanowires can form typical clusters arrays, and they entwine each other to create a

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typical 3D network structure. It can not only act as highways for rapid electron

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transfer, but also allow the active materials to disperse uniformly for maximizing their

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availability. 2) As for the Ni3Se2 nanosheets active materials, apart from good

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electronic conductivity, they possess typical three-electron reactions during their charging/discharging process. These merits result in higher charge storage capability.

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And the intersected Ni3Se2 nanosheets active materials can also bring about numerous volume for contacting with electrolyte ions, which is crucial for occurring more redox reactions. 3) When the active materials were anchored on the skeletons, it is able to create a typical two-phase interface, in which there are a large amount of defects, leading to abundant active sites and rapid electronic transmission. 4) The NiSe nanowire arrays are directly grown on the Ni foam, and the Ni3Se2 nanosheets are also synthesized on the surface of the skeleton. The method avoids utilization of polymeric binder, which ensures compact contact and electronic conductivity between the supports and the active materials, in favor of electron transmission during redox 18

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reaction processes.

4.CONCLUSION In summary, we have triumphantly used a facile two-step strategy to Ni3Se2 nanosheets on the surface of the 3D NiSe nanowires arrays directly deposited on Ni foam as attracting electrode materials for high-energy ASC device. The advanced

of

electrode materials with unique hierarchical architectures can show large sepcific

ro

capacitance of 1580 F g-1, good rate property and long-term cycling stability. More

-p

significantly, coupling the fabricated NiSe NWAs@ Ni3Se2 NSs/NF positive electrode

re

with AC negative electrode builds an ASC system. The device manifests an eximious

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energy density of 45.5 Wh Kg-1 at 1.600 kW kg-1, with a capacitance preservation of 96.1% over 12000 cycles. Additionally, the NiSe@Ni3Se2 composite also present a

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low overpotential of 281 mV at 10mA cm -2. This inspiring work both uncovers the

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superiority of transition metal selenides, and provides an available pathway for their practical applications in high-performance energy storage and conversion systems.

Acknowledgments

The work reported here was supported by the National Natural Science Foundation of China under Grant No. 51672144, 51572137, 51702181, Shandong Provincial Key Research and Development Program (SPKR&DP) under Grant No. 2019GGX102055, the Natural Science Foundation of Shandong Province under Grant No. ZR2017BB013, ZR2019BEM042, the Innovation and Technology Program of Shandong Province under Grant No. 2020KJA004, the Higher Educational Science and Technology 19

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Program of Shandong Province under Grant No. J17KA014, J18KA001, J18KA033, the Taishan Scholars Program of Shandong Province under No. ts201511034 and the Overseas Taishan Scholars Program. We express our grateful thanks to them for their financial support.

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Figure Caption

Figure. 1. Schematic illustration for the synthesis of NiSe NAs@ Ni3Se2 NSs composites.

Figure. 2. SEM images with various magnification of the obtained NiSe NWAs (a-c) and NiSe NWAs@Ni3Se2 NSs (d,e), the inset in (e) shows the corresponding EDS spectrum of the NiSe NWAs@Ni3Se2 NSs, and TEM images (f,g) with different magnification and HRTEM image (h,i) of the NiSe NAs@Ni3Se2 NSs.

Figure. 3. XRD patterns (a), and survey XPS spectrum (b) and high-resolution XPS spectra of Ni 2p (c) and Se 3d (d) regions for NiSe NWAs@ Ni3Se2 NSs composites.

Figure. 4. (a) CV curves (b) GCD plots of Ni foam substrate, NiSe NWAs, Ni3Se2 NSs and NiSe 29

Journal Pre-proof NAs@ Ni3Se2 NSs at 20 mV s-1 and 2 A g-1 respectively, (c) CV curves of the NiSe NWAs@ Ni3Se2 NSs at various scan rates, (d) corresponding curves of positive and negative peak densities versus the square root of scanning rate, (e) GCD profiles of the NiSe NWAs@ Ni3Se2 NSs at different current densities, (f) specific capacitance of the NiSe NWAs, Ni3Se2 NSs and NiSe NWAs@ Ni3Se2 NSs at various current densities, (g) ESI patterns and (h) cycling stability of the

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NiSe NWAs, Ni3Se2 NSs and NiSe NWAs@ Ni3Se2 NSs.

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Figure. 5. (a) Schematic illustration of the assembled ASC device structure based on NiSe

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NWAs@ Ni3Se2 NSs as positive electrode material and AC as negative electrode material, (b) CV

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curves of the NiSe NWAs@Ni3Se2 NSs and AC at 20 mV s-1, (c) CV curves at various scan rates and (d) GCD plots at different current densities of the as-assembled ASC, (e) specific capacitance

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and voltage drop of the device at different current densities, (f) Ragone plots of the fabricated ASC

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device and recently reported values for comparison, (g) the long-term cycling stability of the

Figure. 6. LSV (a) after iR correction and Tafel curves (b) of the NiSe@Ni3Se2, bare NiSe and Ni3Se2 electrodes, (c) CV curves of the hybrid NiSe@Ni3Se2, and (d) Electrochemical double-layer capacitance measurements of the hybrid NiSe@Ni3Se2, bare NiSe and Ni3Se2 electrodes.

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Ni JCPDS: 87-0712

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We hereby state that the work described has not been published previously (except in the form of an abstract or as part of a published lecture or academic thesis), that it is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out. Submission also implies that, if accepted, it will not be published

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elsewhere in the same form, in English or in any other language, without the written

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The as-prepared NiSe/Ni3Se2 nanostructures are acted as electrode materials for

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high-performance asymmetric supercapacitor and OER .

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Highlights NiSe@Ni3Se2 electrode material are prepared via a two-step in-situ growth technology. The integrated hybrid electrode delivers extraordinary capacitance and OER characteristics.

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An asymmetric supercapacitor based on NiSe@Ni3Se2 was successfully assembled.

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The device exhibits high power and energy density, and long-term cycling stability.

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