Advanced Cu0.5Co0.5Se2 nanosheets and MXene electrodes for high-performance asymmetric supercapacitors

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Advanced Cu0.5Co0.5Se2 nanosheets and MXene electrodes for highperformance asymmetric supercapacitors ⁎

Yara Abu Dakkaa, Jayaraman Balamurugana, Ravichandran Balajia, Nam Hoon Kima,b, , ⁎ Joong Hee Leea,b,c, a

Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global) & Department of BIN Convergence Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea b Regional Leading Research Center for Nanocarbon-based Energy Materials and Application Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea c Carbon Composite Research Centre, Department of Polymer – Nano Science and Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea

H I GH L IG H T S

• Novel Cu Co

strategy for the design of nanostructures has been established. The optimal (Cu0.5Co0.5)Se2 is proposed as SC electrode for the first time. The charge storage mechanism of CuxCo1−xSe2 electrodes are discussed in detail. Merits of binder-free/freestanding electrodes for SC applications is demonstrated. The assembled ASC exhibits an excellent energy density and ultralong cycle life. x

• • • •

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

1−xSe2

A R T I C LE I N FO

A B S T R A C T

Keywords: Asymmetric supercapacitors Energy density Freestanding Hierarchical Copper-cobalt selenide

Transition-metal chalcogenides (TMCs) have attracted numerous interests in the field of energy storage owing to their exceptional electrical conductivity, ultrahigh specific capacity, etc. Herein, with inspiration from the attractive nanostructures of hierarchical frameworks with interconnected networks, we endeavored to design ternary copper cobalt selenide (CuxCo1–xSe2) nanostructures through a facile and cost-effective hydrothermal and followed by selenization process. The effects of Cu2+ is investigated and shows significant enhancement in the electrochemical performances. The optimal Cu0.5Co0.5Se2 nanosheets (NSs) possess hierarchical architectures, large specific surface area, unique porous networks, and excellent intrinsic conductivity that result in superior electrochemical properties by their excellent synergistic effects. Taking advantage of the merits of the rational nanostructures, the Cu0.5Co0.5Se2 NSs significantly boost the capacitive performances as ultrahigh specific capacitance of ~1695 F g−1 at a current density of 1 A g−1, and long-term cycling stability (~94.9%). An asymmetric supercapacitor (ASC) device is fabricated using the Cu0.5Co0.5Se2 NSs as a positive electrode, and multilayered MXene (Ti3C2) as a negative electrode. Remarkably, the ASC operates at a working potential of 1.6 V and delivers a high energy density (~84.17 Wh kg−1 at 0.604 kW kg−1), high power density (~14.95 kW kg−1 at 57.73 Wh kg−1), and exceptional cycling stability (~91.1% after 10,000 charge–discharge

⁎ Corresponding authors at: Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global) & Department of BIN Convergence Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. E-mail addresses: [email protected] (N.H. Kim), [email protected] (J.H. Lee).

https://doi.org/10.1016/j.cej.2019.123455 Received 26 August 2019; Received in revised form 25 October 2019; Accepted 9 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Yara Abu Dakka, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123455

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cycles). The energy-storage properties are superior to recently reported TMCs-based ASC, proposing that the Cu0.5Co0.5Se2//MXene ASC has massive potential for next-generation energy-storage systems.

1. Introduction

asymmetric supercapacitors to boost the energy density as well as cycling stability [36]. Inspired by these studies, we envisaged that the energy storage properties of the cobalt selenide could be greatly improved by means of the Cu substitution. To the best of our knowledge, there has been no studies on ternary copper cobalt selenide electrode for high performance SC applications. In order to improve the energy storage properties of the electrode materials, it is necessary to rationally design the electrodes with numerous electroactive sites and excellent ion/electrons transport kinetics that instantaneously involve the Faradaic reaction kinetics [37,38]. Most exclusively, the former demands large electroactive surface areas, which will encourage the EDLC and accommodate a huge number of superficial electroactive species that contribute to the Faradaic redox kinetics, whereas the latter demands fast electrolyte ions diffusion as well as fast electron conduction to the active sites. This can be achieved by fabricating a large specific surface area with exclusive nanoporous networks into the active materials, fast ion transport kinetics, and excellent electrical conductivity. Nevertheless, TMCs based electrodes are usually fabricated with a polymer binder by the conventional slurry coating method for electrochemical investigations [39], which increase the “dead mass” of the active TMCs surface and obstructed from electrolyte contact to contribute in the Faradaic redox reactions for the capacitive performances. Furthermore, the binder will significantly reduce the conductivity of the electrode materials, which hinders their energy storage applications in industrial sectors. Therefore, to achieve better charge storage properties, it is essential to design and fabricate TMCs over 3D conductive substrate with desirable porous networks. By this strategy, the frequently used conventional electrode fabrication process can be circumvented and, more notably, electroactive TMCs with a large electroactive surface and excellent electrical conductivity can be in direct contact with 3D substrate and electrolyte to improve the charge storage properties at high rates. Herein, for the first time, we have demonstrated a novel strategy for the design and fabrication of CuxCo1−xSe2 (0 < x < 1) series by a simple hydrothermal and subsequent exclusive selenization process to enhance the electrochemical performances. The molar ratio of Cu and Co components in CuxCo1−xSe2 nanostructures can accurately tuned in the initial precursors to achieve hierarchical superstructures. The morphological changes and energy storage properties of the as-prepared electrodes have been studied in detail. The doping effect of Cu species is also demonstrated by the prominently improved energy storage ability in the presence of Cu species with perfect stoichiometric ratio of Cu/Co in the CuxCo1−xSe2 nanostructures. Impressively, the optimal Cu0.5Co0.5Se2 electrode shows the best energy storage behaviors with an ultrahigh specific capacitance of ~1695 F g−1 at a current density of 1 A g−1 and holds ~94.9% of its initial capacitance after 10,000 successive charge-discharge process owing to their advantages of ultrathin nanosheets with numerous electroactive sites. The assembled ASC delivers an ultrahigh energy density of ~84.17 Wh kg−1 at 0.60kW kg−1, extraordinary power density (~14.95 kW kg−1 at 57.73 Wh kg−1), and outstanding cycling performance (~91.1% of initial capacitance retention after 10,000 cycles at a high current density of 10 A g−1). These energy storage properties outperform most of the CuCo-based electrode materials, demonstrating that Cu0.5Co0.5Se2 NSs are promising freestanding and binder-free electrodes for energy storage and conversion systems.

The progress of charge storage systems with exclusive features have attracted much interest in the scientific community because of the basic requirements for renewable and sustainable energy sources [1,2]. Supercapacitors (SCs) have recently received much interest in modern electronics such as electric vehicles, smart textiles, and portable electronics because they deliver exceptional power density, ultralong cycle life, and superfast charge-discharge rates [3–5]. Generally, SCs are classified into two types owing to their energy storage properties: electric double layer capacitors (EDLCs) and pseudocapacitors. The EDLCs store the electrical energy by means of ion adsorption, which is a purely electrostatic process. Additionally, pseudocapacitors store electrical energy by means of fast and highly reversible redox kinetics on the electrode surface, which is a purely Faradaic process. Carbon allotropes such as carbon nanotubes, mesoporous carbon, and graphene sheets are used as EDLC type electrode materials and they deliver a higher power density of ~10 kW kg−1 and ultralong cycling stability (50,000 cycles), but they suffer from lower specific capacitance and energy density [6]. In contrast, pseudocapacitors based battery type electrode materials can offer greater specific capacitance and ultrahigh energy density (~100 Wh kg−1) due to their richer redox reaction kinetics, higher electrical conductivity, and tremendous structural integrity [7–11] Based on the merits of pseudocapacitors as energy storage mechanisms, extensive effort has been devoted to explore superior electrode materials with excellent conductivity, richer redox properties, high catalytic activity, and high specific capacitance for SCs to satisfy the modern electronics demands [12–15]. Transition metal chalcogenides (TMCs) have recently been discovered as promising electrode materials for high performance energy storage systems owing to their excellent electrochemical properties, delivering maximum functionality, and outstanding flexibility [16–21]. For example, Shen et al. reported FeNi2S4 based electrode material for SCs, resulting in high electrochemical performances such as ultrahigh specific capacitance and ultralong cycling life [22]. Choudhary et al. reported on a WS2 nanowires based electrode material for SCs that resulted in exceptional rate performances [23]. Liu et al. reported on an (Fe0.5Ni0.5)S2 nanocomposite based electrode, resulting in excellent energy storage capacity [24] and demonstrated that TMCs are an excellent candidate for energy storage applications [25]. When compared to metal sulfides based electrode materials, metal selenide based electrode materials show excellent electrochemical properties due to their unique properties such as high surface area, unique porous networks, and excellent electrical conductivity [26,27]. Therefore, investigation of the electrochemical energy storage properties of the Se-based TMCs is still limited but is essential for next-generation energy storage applications. We perceived that there are only a few reports of a copper selenide-based electrodes that show excellent specific capacity, but it suffers from poor rate performance and short cycling life [28–30]. On the other hand, cobalt selenide-based materials can provide exceptional cycling stability, but the specific capacitance is comparatively low [31,32]. Remarkably, bimetallic CuCo based TMCs display excellent electrical conductivity, richer redox reaction kinetics, and exceptional electrochemical stability, and thus have better electrochemical energy storage properties than their corresponding counterparts, as previously recognized for CuCo oxides and chalcogenides [33,34]. For instance, Liu et al., reported CuCoS4 nanostructures based electrode for flexible quasi-solid-state supercapacitors to achieve ultra-high energy and power densities [35]. Furthermore, we have recently reported metalorganic framework derived CuCo2S4 NS arrays based electrode for

2

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2. Experimental

electrodes were examined using an FE–SEM (SUPRA 40 VP; Carl Zeiss, Germany) in the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University, South Korea. The surface roughness and thickness of Cu0.5Co0.5Se2 NS arrays were examined by AFM (XE–100, Park System Co., Jeonju, Korea). The intrinsic morphological characteristics were examined by TEM, HR-TEM, and STEM (H–7650; Hitachi Ltd., Japan). The elemental compositions of the as-obtained electrode materials were investigated by energy dispersive X-ray analysis (EDAX, SUPRA 40 VP; Carl Zeiss, Germany) and inductively coupled plasma optical emission spectrometry (ICP-OES; J-A1100; JarrellAsh Company, Japan). The crystalline nature of the as-obtained electrodes was studied by XRD (Rigaku Corporation, Tokyo, Japan). The oxidation states and binding nature of the Cu0.5Co0.5Se2 NS arrays were analyzed using XPS (Theta Probe; Thermo Fisher Scientific, UK). The textual properties of the Cu0.5Co0.5Se2 NS arrays were examined by N2 adsorption-desorption isotherms (Micromeritics ASAP 2020).

2.1. Synthesis of CuxCo1−xSe2 nanostructures All chemicals were purchased from Sigma–Aldrich and used them without further purification. Ti3AlC2 was purchased from the Carbon–Ukraine company and used it directly. At first, the Ni foam (NF) substrates were treated with 3 M HCL, DI water, ethanol, and acetone with ultrasonication for 15 min each, then dried them at 60 °C overnight to remove impurities as well as surface residues. Typically, 0.5 mmol of Cu(NO3)2·3H2O, 0.5 mmol of Co(NO3)2·6H2O, and 2.0 mmol of hexamethylenetetramine were mixed in a vessel with 50 mL methanol and stirred for 20 min to obtain a clear solution. The mixture was transferred into a Teflon-lined stainless steel autoclave associated with a piece of NF (5 cm × 2 cm). The hydrothermal reaction was carried out at ~180 °C for 12 h and then was naturally cooled down slowly to reach ambient conditions [40]. The as-grown Cu0.5Co0.5 DHs/ NF precursor was washed with DI water and ethanol repeatedly to remove impurities and then dried at 60 °C in a vacuum oven. The asobtained Cu0.5Co0.5 DHs/NF was transferred to their corresponding selenide by the detailed procedure: 0.108 g of NaBH4 (used as a reducing agent) and 0.098 g of Se powder were dissolved in 2.5 mL DI water stirred for 20 min and then 47.5 mL of ethanol was added slowly added to the above mixture and vigorously stirred for 15 min to obtain a clear solution. The mixture was transferred into a 70 mL Teflon-lined stainless-steel autoclave, keeping the Cu0.5Co0.5 DHs/NF in a perpendicular direction, and then the reaction was carried out at 180 °C for 8 h. The following chemical reaction occurred during the selenization process: 2Se + 4NaBH4 + 7H2O → 2NaHSe + Na2B4O7 + 14H2; NaHSe + MX2 → MSe + NaX + HX (i.e., MX2 = CuxCo1−x DHs). Finally, the as-obtained Cu0.5Co0.5Se2 NS arrays/NF was washed with DI water and ethanol repeatedly and then dried it at 60 °C in a vacuum oven. For comparison, the CuxCo1−xSe2 nanostructures with different x values such as 0, 0.25, 0.33, 0.67, 0.75, and 1 were synthesized by a similar procedure, resulting materials denoted as CoSe2, Cu0.25Co0.75Se2, Cu0.33Co0.67Se2, Cu0.67Co0.33Se2, Cu0.75Co0.25Se2, and CuSe2, respectively. To calculate the accurate mass of the active CuxCo1−xSe2 electroactive materials, the samples were weighed before and after the fabrication process. To avoid experimental errors, we have fabricated five samples for each electrode, and the average value of mass loading for the samples was estimated. The mass loadings of the CuxCo1−xSe2 (x = 0, 0.25, 0.33, 0.5 0.67, 0.75, and 1) samples were calculated to be ~ 2.9, 2.5, 3.1, 2.7, 3.3, 3.2, and 2.8 mg cm−2, respectively.

2.4. Electrochemical measurements The electrochemical performance of the as-obtained electrodes was exemplified using an electrochemical workstation (CHI 660E, USA). In three-electrode configurations, the electrochemical performance of NF supported CuxCo1−xSe2 nanostructures or MXene electrodes was directly used as the working electrode, Ag/AgCl and Pt foil as the reference and auxiliary electrodes, respectively. All three-electrode performances were carried out at ~25 °C in an aqueous 2 M KOH electrolyte. EIS analysis was carried out at the frequency from 100 kHz to 0.01 Hz with an amplitude of ~5 mV. The specific capacitance of asobtained electrodes was calculated from the discharge curves as follows

Cs =

2I ∫ Vdt f

mV 2∣V i V

(1)

where Cs is the specific capacitance (F g−1), I is the current during the discharge process (A), ∫ Vdt is the area under the discharge curve, m is the mass of the electroactive materials (g), and V is the voltage with respect to the initial and final values of Vi and Vf, respectively [42]. 2.5. Device fabrication and electrochemical measurements The ASC was assembled using the optimal NF supported Cu0.5Co0.5Se2 NS arrays as the positive electrode and MXene-coated NF as the negative electrode, NKK TF40 as the separator, and PVA-KOH as the gel electrolyte. Before assembling the ASC, both Cu0.5Co0.5Se2 and MXene electrodes were soaked in the PVA-KOH gel electrolyte for 15 min, and the ASC device was sealed with polytetrafluoroethylene tape. Finally, copper wire was directly connected to both positive and negative terminals. To attain excellent electrochemical performance for the ASC device, the mass ratio was fixed to ~ 0.53 (based on the mass balance of the ASC device). The mass ratio of both positive and negative electrode was examined as follows

2.2. Synthesis of MXene MXene was prepared through a reported work with slight modifications [41]. 10 mmol of Ti3AlC2 was dissolved in 50 mL hydrofluoric acid (48%) and vigorously stirred it at ~25 °C for 30 min. The mixture was ultra-sonicated for 1 h to obtain a stable suspension, then was washed with water several times until the pH reaches around 7.0. Further, the exfoliated suspension was filtered and dried at 60 °C to obtain MXene. In order to fabricate the MXene over NF, 80 wt% of MXene, 15 wt% of carbon black, and 5 wt% of polyvinylidene fluoride (PVDF) were ground well and dispersed in 1.5 mL N-methylpyrolidiene (NMP), then finally sonicated for 30 min to obtain the ink solution. The as-prepared ink solution was coated on NF substrate (1 cm × 1 cm) slowly and then dried it at 60 °C in a vacuum oven. The SEM image in Fig. S1 shows the MXene coated NF, which clearly demonstrates the ultrathin MXene layers homogeneously coated on porous 3D NF support, which may boost the electrochemical performances of MXene.

m+ C ΔV = − − m− C+ΔV+

(2)

where m is the mass (g) of the electroactive material, C is the specific capacitance (F g−1), and ΔV is the operating potential window (V) for the positive (+) and negative (–) electrodes. Moreover, the Coloumbic efficiency (η) was calculated as follows

η=

qd qC

× 100%

(3)

where qd and qC are the total amounts of discharge and charge of the ASC device, respectively (calculated from the GCD curves). The energy density (E) and power density (P) of the ASC device were calculated as follows

2.3. Material characterizations The morphological and structural characteristics of the as-obtained 3

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E=

Ccell ΔV 2 2 × 3.6

(4)

P=

E × 3600 tdischarge

(5)

substrate. The high-resolution SEM image clearly indicates that Cu0.5Co0.5Se2 NS arrays are perpendicular to the NF substrate, resulting in hierarchical 3D superstructures. Most interestingly, the Cu0.5Co0.5Se2 NS arrays have retained their original structure after the exclusive selenization process, demonstrating that ion exchange is a powerful technique for improving the catalytic activity as well as the electrical conductivity without the structural disorder. Fig. 2b clearly shows that the Cu0.5Co0.5Se2 NS arrays are a highly porous network by interconnecting with each other. The elemental composition of the as-prepared Cu0.5Co0.5Se2 NS was studied by energy dispersive X-ray (EDX) analysis (Fig. 2c). The EDX spectroscopy analysis revealed the existence of copper, cobalt, and selenide elements in the Cu0.5Co0.5Se2 NS arrays (Fig. 2c). The stoichiometric ratio of the Cu and Co is closely to be 1:1, which is fully consistent with theoretical values. This was examined by ICP-OES and are illustrated in Table S1. Note that, the Cu0.5Co0.5Se2 NS arrays became much more porous than their corresponding LDH precursors (Fig. S3). Such unique porous networks of Cu0.5Co0.5Se2 NS arrays may shorten the ion-transport pathway to boost the ion/electron transport kinetics, resulting in extraordinary rate capability and ultralong cycling stability. Further increase of the molar ratio of the Cu:Co into 2:1 (Cu0.67Co0.33Se2) or 3:1 (Cu0.75Co0.25Se2), the SEM images clearly show the disorder nanosheets on the Ni foam substrate (Fig. S2d, e). The pure CuSe2 shows different sizes of the nanosheets loosely attached on the surface of the Ni backbone with cracks between the adjacent NS arrays (Fig. S2f). The SEM analysis clearly demonstrates the effect of the Cu doping into the CuxCo1−xSe2 nanostructures. This study clearly reveals that the CuxCo1−xSe2 nanostructures with an x value of 0.5 show the exclusive morphology with unique porous networks. Thus, we conclude that the Cu0.5Co0.5Se2 NS arrays with hierarchical nanostructures may reduce the ion pathway to improve the ion/electron transport kinetics and energy storage properties [43]. The surface roughness and the thickness of the Cu0.5Co0.5Se2 NS arrays were investigated by AFM, as shown in Fig. 2d and S4. As depicted in Fig. 2d, the high-resolution AFM of the Cu0.5Co0.5Se2 NS arrays clearly shows the surface roughness is around 1.2 nm. Further, the thickness of the

where Ccell is the specific capacitance of the ASC device, ΔV is the operating voltage window during the charge-discharge process, and tdischarge is the discharge time. The practical applicability of the assembled ASC device was tested by glowing a red light emitting diode (LED). 3. Results and discussion 3.1. Morphological and structural investigation of the CuxCo1−xSe2 nanostructures Fig. 1 is a schematic representation for the design and fabrication of hierarchical Cu0.5Co0.5Se2 NS arrays, multi-layered MXene NS and their electrochemical performance towards solid-state ASCs. At first, the CuxCo1−x LDH precursors were directly grown on NF substrate by a simple hydrothermal process. After that, the CuxCo1−x LDH precursors were effectively transformed into their corresponding selenide using an exclusive ion-exchange technique. The morphology of the as-obtained CuxCo1−xSe2 nanostructures with different x values that were investigated by FE-SEM analysis and are presented in Fig. 2 and S2. As depicted in Fig. S2a, the SEM image clearly shows that the CoSe2 nanoparticles of different sizes (~19.1 to 34.8 nm) are loosely attached to the NF substrates. When Cu ion incorporation into the initial precursors of Cu:Co with the molar ratio of 1:3 (i.e., Cu0.25Co0.75Se2), the SEM image in Fig. S2b shows the mixture of nanoparticles as well as sheetlike morphology. Whereas further increase of the molar ratio of the Cu:Co to 1:2 (i.e., Cu0.33Co0.67Se2) in Fig. S2c, the SEM image clearly depicts that the thicker nanosheets on NF substrate. When the molar ratio of Cu:Co is changed to a 1:1 in the initial precursor (Cu0.5Co0.5Se2), the SEM image clearly reveals that the ultra-thin Cu0.5Co0.5Se2 NS arrays were homogeneously grown on the 3D NF

Fig. 1. Schematic for the synthesis of CuxCo1−xSe2 nanostructures and their application in solid-state ASC. 4

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Fig. 2. (a) Low and (b) high-resolution SEM images, (c) EDX analysis, and (d) 3D high-resolution AFM image of the hierarchical Cu0.5Co0.5Se2 NS arrays.

nanosheet surface were observed, which provides more electroactive sites to improve the ion/electron transport kinetics. The HR-TEM image in Fig. 3c displays the lattice fingers of ~2.39 nm, corresponding to the (211) plane of the Cu0.5Co0.5Se2 NS arrays, which further proves the highly crystalline nature of the Cu0.5Co0.5Se2 NS arrays. Further, we confirmed the homogeneous distribution of the copper, cobalt, and selenide in the Cu0.5Co0.5Se2 NS arrays by (HAADF-STEM) analysis (Fig. 3d) This study clearly reveals that there was no any elemental segregation in the single NS arrays. HAADF-STEM image further

Cu0.5Co0.5Se2 NS arrays is around 1.5 nm, which is in good agreement with SEM observation. The copper, cobalt, and selenium with homogeneous distributions in the Cu0.5Co0.5Se2 NS arrays were further validated by SEM-EDS mapping analysis (Fig. S5a–e). The intrinsic morphological analysis of Cu0.5Co0.5Se2 NS arrays was further exemplified by TEM, HR-TEM, and HAADF-STEM analyses and the results are presented in Fig. 3a–d. The TEM images clearly reveal that Cu0.5Co0.5Se2 NS arrays possess larger size, wrinkled, and ultrathin nature (Fig. 3a and b). Meanwhile, numerous nanopores on the

Fig. 3. (a) TEM, (b) high-resolution TEM images of the Cu0.5Co0.5Se2 NS arrays, and (c) HAADF-STEM image of Cu0.5Co0.5Se2 NS arrays and their corresponding STEM-EDS color mapping of Cu-K, Co-K, and Se-K. 5

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the very close ionic radii of Cu2+ (0.57 Å) and Co2+ (0.58 Å), Cu dopant might substitute for the atomic Cu into the CoSe2 central position in the CuxCo1−xSe2 crystal, which is clear from the slight deviation in the value of lattice parameters [46]. X-ray Photoelectron Spectroscopy (XPS) is one of the best techniques to investigate detailed surface information about electronic states as well as the chemical composition of the Cu0.5Co0.5Se2 NS arrays (Fig. 4b–d). The survey XPS spectrum clearly depicts the existence of copper (~933.3 eV), cobalt (~779.6 eV), and selenium (~58.78 eV) in the Cu0.5Co0.5Se2 NS arrays (Fig. S6), whereas a negligible amount of carbon (~284.78 eV) and oxygen (~531.18 eV) perceived from the surface oxidation, indicating that the Cu0.5Co0.5Se2 was ultrapure in nature. The high-resolution XPS of Cu 2p deconvoluted into two main peaks at the binding energies of ~ 933.4 and 953.3 eV, which corresponds to Cu 2p3/2 and Cu 2p1/2, respectively (Fig. 4b). Besides, the satellite peak at the binding energy of ~944.1 eV represents the existence of a paramagnetic state of Cu2+ [47]. The high-resolution insight spectrum of Co 2p was deconvoluted into two main peaks at the binding energies of ~779.9 and 802.1 eV, corresponding to Co 2p3/2 and Cu 2p1/2, respectively (Fig. 4c). We observed the additional satellite peak at the binding energy of ~789.2 eV, which is related to the Co2+ state. Fig. 4d displays the Se 3d XPS spectrum. The high-resolution Se 3d was deconvoluted into two main peaks at the binding energies of ~54.0 and 54.8 eV, which corresponds to the Se 3d5/2 and Se 3d3/2, respectively. The binding energies of Se 3d5/2 and Se 3d3/2 spectrum are relatively higher than those of pristine CuSe2 or CoSe2, which further proves the highly synergistic effect of the Cu and Co ions in the Cu0.5Co0.5Se2 NS arrays. The XPS analysis clearly indicates the existence of the Cu2+ Cu3+, Co2+, Co3+ states in the Cu0.5Co0.5Se2 NS , arrays. The atomic weight percentage of the Cu, Co, and Se was found to be around ~13.44%, 14.76%, and 59.12%, respectively. Therefore, the atomic weight ratio of Cu:Co:Se is about 0.5:0.5:2, which is consistent with the stoichiometric ratio of Cu:Co is 1:1 in the Cu0.5Co0.5Se2

confirms the replacement of oxygen by selenium by an effective selenization process. The doping of the selenium is expected to improve the rate performance and cycling stability of the Cu0.5Co0.5Se2 NS electrode during prolonged charge-discharge cycles at higher current densities. The phase purity, as well as the crystalline nature of CuxCo1−xSe2 nanostructures, were exemplified by the XRD patterns and are presented in Fig. 4a. At x = 0, the diffraction pattern good agreement with the standard pattern of cubic CoSe2 (JCPDS Card No. 00-009-0234) and display diffraction peaks at ~30.49, 34.19, 37.62, 51.75, 56.48, 58.85, and 73.99°, corresponding scattering from (2 0 0), (2 1 0), (2 1 1), (3 1 1), (2 3 0), (3 2 1), and (4 2 1) planes, respectively. In case of the CuxCo1−xSe2 (x = 0.25), the XRD pattern almost similar with the standard diffraction pattern of CuxCo1−xSe2 (x = 0). Additionally, two diffraction peaks of ~29.78 and 46.01° were observed, corresponding to the (1 0 1) and (2 1 1) planes of the orthorhombic CuSe2 phase (JCPDS Card No. 01-071-0046), respectively. Most importantly, the appearance of Cu peak indicates the successful formation of the CuxCo1−xSe2 (x = 0.25) phase, demonstrating that the Cu effectively substituted and formed the ternary phase in CuxCo1−xSe2 nanostructures, which is well-matched with the previous report [11]. As illustrated in Fig. 4a, the diffraction planes such as (2 0 0), (2 1 0), (2 1 1), (3 1 1), (2 3 0), (3 2 1), and (4 2 1) of the Cu0.25Co0.75Se2 (x = 0.25, 0.33, 0.67, and 0.75). Whereas at x = 1, the diffraction pattern is fully fitted with the CuSe2 phase (JCPDS Card No. 01-071-0046) with an orthorhombic crystal structure. Further, the diffraction peaks of (2 1 0) plane of the nanostructures gradually shift towards smaller angles along with the Cu content increment from 0.25 to 0.75. Besides, the diffraction peak of (2 1 0) plane holds a well‐distinct shape with a highly symmetric nature. The lattice parameter of the CuxCo1−xSe2 nanostructures is calculated using XRD and the parameter value increases from 5.86 to 5.94 Å, which is a good agreement with the sequential shift of the (2 1 0) peak. This linear tendency completely follows the Vegard's law [44], specifying important proof that Cu2+ and Co2+ are effectively incorporated into the CuxCo1−xSe2 nanostructures [45]. Considering

Fig. 4. The structural and chemical composition of CuxCo1−xSe2 nanostructures: (a) XRD patterns of the CuxCo1−xSe2 nanostructures with different x values and their corresponding structure formation. High-resolution XPS spectra of (b) Cu 2p, (c) Co 2p, and (d) Se 3d in the Cu0.5Co0.5Se2 NS arrays. 6

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Cu0.5Co0.5Se2 electrodes is mentioned as follows [51].

NS arrays, which is also in good agreement with EDAX analysis. Further, the textural properties of the Cu0.5Co0.5Se2 NS arrays were investigated by N2 adsorption-desorption isotherms, as shown in Fig. S7. The N2 desorption isotherm study clearly indicates that the Cu0.5Co0.5Se2 NS arrays had a type IV isotherm with a hysteresis loop at a relative pressure of 0.45 to 1.0. The Cu0.5Co0.5Se2 NS arrays had a larger BET surface area of about 204.2 m2 g−1, which is relatively higher than the reported TMCs such as CuCo2S4 [48] and Ni-Co sulfide nanowires [49]. The pore-size distribution of the Cu0.5Co0.5Se2 arrays was examined by the Barrett–Joyner–Halenda (BJH) method. As depicted in the inset of Fig. S7, the BJH analysis showed that the Cu0.5Co0.5Se2 NS arrays had a pore diameter of ~17.9 nm, demonstrating the mesoporous nature of Cu0.5Co0.5Se2 NS arrays. This result indicates that the Cu0.5Co0.5Se2 NS arrays had a larger specific surface area with a unique mesoporous nature to boost rate capability and cycling stability of SCs by reducing the ion/electron transport pathway [50].

Cu 0.5 Co0.5 Se2 + OH− − +H2 O↔ Cu 0.5 SeOH + Co0.5 SeOH + 2e−−

(6)

−1

When increasing the sweep rates from 10 to 50 mV s , the oxidation, as well as reduction peaks, are slightly shifted towards the positive as well as negative potential windows owing to the internal resistance of the Cu0.5Co0.5Se2 electrode. It is well known that both the anode and the cathode potential difference (Epp) is the imperative factor to examine the catalytic activity of the electrodes. The Cu0.5Co0.5Se2 electrode displayed an Epp value of about 370 mV (at a scan rate of 10 mV s−1), which is higher than the theoretical value of a reversible single electron (~59 mV), demonstrating that Cu0.5Co0.5Se2 has quasi-reversible behavior. The GCD curves CuxCo1−xSe2 electrodes at various current densities from 1 to 30 A g−1 with the fixed working potential of −0.2 to 0.45 V are presented in Fig. 5b and S9. The symmetric GCD curve exhibits superior redox reaction kinetics, indicating its exceptional reversibility. Further, the Cu0.5Co0.5Se2 electrode shows a negligible IR drop even at a higher current density of 30 A g−1, demonstrating its ultra-low internal resistance. The GCD curves are in good agreement with the CV curves. In order to validate the electrochemical properties of the Cu0.5Co0.5Se2, the comparative electrodes were examined, such as CuSe2, CoSe2, Cu0.25Co0.75Se2, Cu0.33Co0.67Se2, Cu0.67Co0.33Se2, and Cu0.75Co0.25Se2 electrodes, by CV and GCD measurements, which are discussed in detail. The CV curves of CuxCo1−xSe2 electrodes were recorded at a scan rate of 10 mV s−1 and are shown in Fig. 5c. Impressively, the CV curve area and current density of the Cu0.5Co0.5Se2 electrode is much higher than for the other electrodes (CoSe2, CuSe2, Cu0.25Co0.75Se2, Cu0.33Co0.67Se2, Cu0.67Co0.33Se2, and Cu0.75Co0.25Se2), indicating that ultrahigh capacity behaviour of the Cu0.5Co0.5Se2 electrode. Further, GCD curves of CuxCo1−xSe2 electrodes were recorded at a fixed current density 1 A g−1, as presented in Fig. S10. Most interestingly, the Cu0.5Co0.5Se2 electrode shows an ultra-long discharge time of ~1586 s, which is much higher than those of the other comparable electrodes, such as the CuSe2 (~545 s), CoSe2 (~841 s), Cu0.25Co0.75Se2 (~962 s), Cu0.33Co0.67Se2 (~1303 s), Cu0.67Co0.33Se2 (~1057 s), and

3.2. Electrochemical properties of the CuxCo1−xSe2 nanostructures Reflecting on the hierarchical 3D nanostructures with unique porous networks, we envisaged the as-grown CuxCo1−xSe2 nanostructures to have exceptional electrochemical performance towards SC applications. The detailed electrochemical properties of the CuxCo1−xSe2 nanostructures with various x values were examined using a three-electrode system in 2 M KOH as the aqueous electrolyte and Ag/AgCl as the reference electrode at room temperature. In order to calculate the mass of active electrode material through the in-situ method, it is necessary to estimate it by deducting the weight before as well as after growing. We carried out this weight calculation more than three times to avoid experimental errors. The CV curves of CuxCo1−xSe2 electrodes at different scan rates from 10 to 50 mV s−1 with an operating potential window of −0.2 to 0.75 V are presented in Fig. 5a and S8. As depicted in Fig. 5a, all CV curves displayed a pair of strong redox peaks because of the exceptional redox behavior of the Cu2+/Cu3+ and Co2+/Co3+ couple. The highly reversible redox reaction kinetics of

Fig. 5. Electrochemical performance of the CuxCo1−xSe2 electrodes: (a) CVs of the Cu0.5Co0.5Se2 electrode at different scan rates from 10 to 50 mV s−1, (b) GCDs of the Cu0.5Co0.5Se2 electrode at various current densities from 1 to 30 A g−1, (c) CVs of CuxCo1−xSe2 electrodes at a fixed scan rate of 10 mV s−1, (d) specific/areal capacitance of Cu0.5Co0.5Se2 electrode at various current densities from 1 to 30 A g−1, (e) EIS of CuSe2, CoSe2, and Cu0.5Co0.5Se2 electrodes, and (f) Cycling performance of the Cu0.5Co0.5Se2 electrode at a current density of 15 A g−1 (inset shows the corresponding electrode at initial and last 10 charge-discharge cycles). 7

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10,000 charge-discharge cycles in an aqueous 2 M KOH electrolyte, further displaying the outstanding durability of the Cu0.5Co0.5Se2 electrode. Such exceptional electrochemical performance and cycling stability of the Cu0.5Co0.5Se2 electrode can be ascribed to the following reasons: (i) Cu0.5Co0.5Se2 NS arrays with excellent redox properties due to their effective incorporation of Cu into the CoSe2 framework, (ii) the Cu0.5Co0.5Se2 NS arrays with numerous mesoporous channels provide more electroactive surface that can reduce the ion/electron transport pathway to enhance the energy storage properties. This effectively avoids “dead mass” in the active electrode to boost the capacitive performances. (iii) The ultra-thin Cu0.5Co0.5Se2 NS arrays directly grown over 3D NF could enhance electrical conductivity and charge transport kinetics by avoiding the polymer binder and conductive additives. (iv) The high synergistic interaction between Cu0.5Co0.5Se2 NS arrays and NF backbone could improve the mechanical stability during prolonged charge-discharge cycles. Such ultrahigh specific capacitance, excellent rate capability, and remarkable cycling stability of the Cu0.5Co0.5Se2 electrode have effectively led us to use it as a positive electrode to fabricate ASC for future energy-storage technology.

Cu0.75Co0.25Se2 (~876 s), which further confirms that the Cu0.5Co0.5Se2 electrode has superior energy-storage properties.[52] The specific/areal capacitance of as-obtained CuxCo1−xSe2 electrodes was calculated at different current densities from the discharge curve of GCD measurements, as illustrated in Fig. 5d and S11. The Cu0.5Co0.5Se2 electrode exhibits excellent energy-storage performance with high specific capacitances of ~1695, 1600, 1510, 1428, 1364, 1309, 1272, 1253 F g−1 with the corresponding areal capacitances of ~4.20, 3.97, 3.73, 3.53, 3.4, 3.3, 3.23, and 3.16 F cm−2 at the current densities of 1, 3, 5, 10, 15, 20, 25 and 30 A g−1, respectively. Impressively, the Cu0.5Co0.5Se2 electrode possess an ultrahigh specific capacitance of 1695 F g−1 at the current density of 1 A g−1, which is significantly higher than for the other electrodes of CuSe2 (~663 F g−1) CoSe2 (~1005 F g−1), Cu0.25Co0.75Se2 (~1198 F g−1), Cu0.33Co0.67Se2 (~1326 F g−1), Cu0.67Co0.33Se2 (~1281 F g−1), and Cu0.75Co0.25Se2 (~1026 F g−1), and superior to copper and cobalt electrodes in recent reports (Table S2). The Cu0.5Co0.5Se2 electrode possesses excellent retention of about 73.95% at a higher current density of 30 A g−1, which is much higher than those of CuSe2 (31.67%), CoSe2 (49.77%), Cu0.25Co0.75Se2 (56.76%), and Cu0.33Co0.67Se2 (60.18%), Cu0.67Co0.33Se2 (54.78%), and Cu0.75Co0.25Se2 (51.17%),. Such ultrahigh specific capacitance and excellent rate capability of Cu0.5Co0.5Se2 electrode result from their hierarchical nanostructure, extraordinary surface area, and exclusive porous architecture. The exceptional rate capability can be ascribed to the fast ion-transport kinetics and improved electronic conductivity, which is further confirmed by electrochemical impedance spectroscopy (EIS) analysis [53]. As depicted in Fig. 5e and S12, the Nyquist plots clearly show that the Cu0.5Co0.5Se2based electrode displays a quite vertical slope in the low-frequency region, whereas moderately semicircle with a smaller diameter in the high-frequency region, which is suggestive of ultralow charge-transfer resistance (Rct) as well as excellent conductivity when compared to the other electrodes. The optimal Cu0.5Co0.5Se2 shows a lower internal resistance (Rs) and Rct of 0.45 Ω and 0.59 Ω, respectively, which is much lower than that of the other electrode of CuSe2 (Rs = 0.65 Ω; Rct = 0.67 Ω), CoSe2 (Rs = 0.54 Ω; Rct = 0.47 Ω), Cu0.25Co0.75Se2 (Rs = 0.45 Ω; Rct = 0.73 Ω), and Cu0.33Co0.67Se2 (Rs = 0.67 Ω; Rct = 0.39 Ω), Cu0.67Co0.33Se2 (Rs = 0.51 Ω; Rct = 0.55 Ω), and Cu0.75Co0.25Se2 (Rs = 0.49 Ω; Rct = 0.62 Ω), and superior to reported copper/cobalt-based transition-metal chalcogenides investigated for similar electrochemical performances [31,54]. Such ultra-low internal resistance of Cu0.5Co0.5Se2 electrode results from their unique properties, such as large surface area, unique porous structure, and controlled selenization process. The cyclic performance of the electrode is an imperative factor for the energy-storage system in commercial applications. We conducted the ultralong cycling stability test for the as-obtained CuxCo1−xSe2 electrodes at a current density of 15 A g−1, as illustrated in Fig. 5f and S13. The Cu0.5Co0.5Se2 electrode exhibits a more stable and highly reversible charge − discharge capacitance even after 10,000 cycles. The Cu0.5Co0.5Se2 electrode holds about 94.9% of the initial capacitance after 10,000 charge-discharge cycles, which is much higher than for the other electrodes of CuSe2 (~73.1%), CoSe2 (~75.2%), Cu0.25Co0.75Se2 (~81.3%), Cu0.33Co0.67Se2 (~90.2%), Cu0.67Co0.33Se2 (~85.6%), and Cu0.75Co0.25Se2 (~78.1%) and is also superior to reported copper/cobalt-based chalcogenides in the literature (Table S2). This result demonstrates that the Cu0.5Co0.5Se2 electrode owns outstanding electrochemical stability. Furthermore, the initial and last ten chargedischarge cycles are shown in the inset of Fig. 5f. When compared to the initial ten cycles, the last ten charge-discharge cycles retained the shape and discharge time well, further confirming its exceptional electrochemical stability. In order to further prove the exceptional electrochemical stability, a morphological analysis was done of the Cu0.5Co0.5Se2 electrode after 10,000 charge-discharge cycles; the results are presented in Fig. S14. The FE-SEM image clearly shows that the Cu0.5Co0.5Se2 NS arrays retained their original structure even after

3.3. Electrochemical performance of the Cu0.5Co0.5Se2 NS//MXene ASC In order to attain ultrahigh energy density, highly exfoliated MXene was chosen with unique porous networks as the negative terminal electrode to assemble an ASC device. The MXene was effectively synthesized by a selective etching method (see more details on the experimental section). Before assembling the ASC device, the newly developed MXene negative electrode was characterized by XRD, FE–SEM and TEM; the results are shown in Fig. S15a–e. The XRD of the Ti3AlC2 clearly displays the diffraction peaks from 33° to 43°, related to the aluminum (Fig. S15a). Note that the diffraction peaks of the aluminum were successfully eliminated after the selective etching process, which further confirms that the MXene was highly exfoliated. It also shows a small shift towards lower angles, and that the peaks in the MXene samples are wider than those in the Ti3AlC2 sample. This proves that the prepared MXene is of high purity and excellent grade. The FE–SEM clearly shows the successfully exfoliated individual sheets (Fig. S15b), which are similar to exfoliated graphite [55]. Further, the TEM image depicts the ultra-thin nanosheets, which further prove the defect-free nature of the MXene (Fig. S15c, d). The SAED pattern in the inset of Fig. S15d reveals the highly crystalline nature of MXene. Besides, the HRTEM image of the MXene clearly shows the lattice fringes with high crystalline nature (Fig. S15e). The individual electrochemical properties of the MXene in a three-electrode configuration were examined; the results are shown in Fig. S16a-d. Reflecting on the 3D structure of the multi-layered MXene with a very large surface area, the as-prepared multi-layered MXene electrode is expected to have outstanding electrochemical performance. The electrochemical properties of the multilayered MXene were examined using conditions similar to those of the positive electrode. Although MXene has been usually reported in acidic or neutral media to avoid the oxidation of the MXene, it has also been reported in KOH media with excellent performance but poor cycling stability [56–58]. The active material weight was measured prior to the manual depositing of the multi-layered MXene on the surface of NF. The CV curves of the multi-layered MXene electrode at different scan rates from 5 to 30 mV s−1 with a potential window of −1 to 0 V are presented in Fig. S16a, in which all CV curves display a pair of redox peaks. As the scan rate increases from 5 to 30 mV s−1, the redox peaks slightly shift towards the lower and higher potential windows because of the internal resistance of the multi-layered MXene electrode. GCD curves of the multi-layered MXene at different current densities from 1 to 30 A g−1 with a constant potential window from −1 to 0 V, presented in Fig. S16b, have even shapes and excellent redox reaction kinetics. In addition, the multi-layered MXene electrode exhibits negligible IR drop even at high current densities of 30 A g−1, which further confirms the ultra-low internal resistance. The GCD curves are in good agreement 8

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(Fig. 6a). To attain the high energy and power densities, the mass ratios of both positive electrode and negative electrodes was optimized and fixed them at ~0.53 in the assembled ASC device (see more details on the experimental section). The CV curves of the Cu0.5Co0.5Se2 positive electrode with the working potential window of −0.2 to 0.75 V, with the MXene negative electrode with the working potential window of −1.0 to 0 V at a constant scan rate of 10 mV s−1 are shown in Fig. S19. The CV curves of the Cu0.5Co0.5Se2 clearly show a richer redox reaction, which indicates its pseudocapacitive behavior. Moreover, the CV curves of the MXene have nearly rectangular shapes, representing the EDLC behavior. In order to further optimize the working potential window of the assembled Cu0.5Co0.5Se2//MXene ASC, CV and GCD with different voltage windows were investigated detail in the following section. The CV measurement of the Cu0.5Co0.5Se2//MXene ASC with different operating potentials of 0.6 to 1.6 V at a constant scan rate of 10 mV s−1 was conducted, as depicted in Fig. S20. For the working potential of 0.6 to 1.0 V, the ASC displayed nearly rectangular shapes, representing that the ASC holds an insufficient Faradaic redox reaction. Further increasing the working potential up to 1.2 V, the ASC showed redox reaction kinetics with imperfect shapes. When the working potential was increased up to 1.4, the ASC exhibited incomplete redox reaction kinetics with a partial Faradaic redox process. Impressively, once the working potential reached to 1.6 V, the ASC displayed complete oxidation as well as redox peaks, indicating the excellent reversibility and higher electrochemical performance of the assembled ASC device. However, once the working potential ran beyond 1.8 V, an obvious oxygen evolution reaction occurred. In order to further optimize the working potential, the GCD measurements for Cu0.5Co0.5Se2// MXene ASC were carried out with different operating potential windows from 0.6 to 1.6 V at a constant current density of 1 A g−1 (Fig. S21). As the potential window was increased from 0.6 to 1.6 V, the specific capacitance was dramatically improved from 33.5 to 236.72 F g−1. Therefore, we conclude that the assembled ASC device with an optimized potential window of ~1.6 V can obtain high energy and power densities.

with CV curves. Interestingly, the GCD curve of the multi-layered MXene showed a very long discharge time of ~398.4 s, which is much higher than that of previously reported electrodes [59]. The multilayered MXene electrode exhibits excellent energy-storage performance with high specific capacitances of ~321, 311, 303, 294, 284, 275, 268, and 263 F g−1 at current densities of 1, 3, 5, 10, 15, 20, 25, and 30 A g−1, respectively (Fig. S16c). The cyclic stability of the electrode is a dominant factor in energystorage systems for commercial applications. The ultralong stability test was conducted for the as-prepared multi-layered MXene electrode at a current density of 10 A g−1, as shown in Fig. S16d. The multi-layered MXene electrode held about 81.6% of its initial capacitance after 10,000 charge-discharge cycles. This further confirms that the multilayered MXene electrode has ultra-high electrochemical stability. The first and last ten charge-discharge cycles are shown in the inset of Fig. S16d. The last ten charge-discharge cycles retained the shape and discharge time when compared to the first ten charge-discharge cycles, which further validates the outstanding electrochemical performance of the multi-layered MXene electrode, which has an outstanding capacitance retention of about 82.1% because of its unique multi-layered nanosheet structure, which provides ultra-high surface area and exclusive porosity, Fig. S13c. The excellent electrical conductivity and electrochemical stability of the MXene electrode were further studied by EIS analysis and are illustrated in Fig. S17. The MXene electrode holds the Rct of about 1.52 Ω after the prolonged charge-discharge cycles, which is slightly higher than the initial (Rct = 1.23 Ω), indicating that the outstanding cycling stability of MXene. As illustrated in Fig. S18, the SEM image reveals that morphology and microstructure of MXene are well retained, further indicating its excellent cycling performances. Furthermore, there are few big particles observed on the surface of the multi-layered MXene which could be attributed to the MXene oxidation due to the use of KOH electrolyte [41]. The solid-state ASC device assembled using Cu0.5Co0.5Se2 as the positive electrode, the highly exfoliated MXene as the negative electrode, PVA-KOH as the gel electrolyte, and NKK TF40 as a separator

Fig. 6. Electrochemical performance of Cu0.5Co0.5Se2//MXene ASC device: (a) Schematic diagram for the assembled ASC, (b) CV curves of the ASC device at different sweep rates from 10 to 100 mV s−1, (c) GCD curves of the ASC obtained at various current densities from 1 to 20 A g−1, (d) Specific capacitance vs. current density of the ASC, (e) Cycling performance of the ASC at a current density of 10 A g−1 (the insets show the corresponding initial and last ten charge-discharge cycles), and (f) The energy density of the ASC compared with a reported ternary metal-based ASC devices (inset shows a glowing red LED using a series-connected two ASC devices). 9

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storage properties can be significantly improved by Cu species incorporated into the CoSe2, which reveals the electrochemical performances activated by Cu species. Of the CuxCo1–xSe2 series of the asobtained electrodes, the Cu0.5Co0.5Se2 NS arrays with a stoichiometric ratio of Cu:Co with 1:1 provide superior electrochemical properties for SC applications. The optimal Cu0.5Co0.5Se2 electrode achieved a specific capacitance of ~1695 F g−1 at a current density of 1 A g−1 with extraordinary rate capability (~73.95% at a high current density of 30 A g−1), and outstanding cycling stability of (~94.9% of the initial capacitance retained after 10,000 charge-discharge cycles). The assembled Cu0.5Co0.5Se2//MXene ASC delivers a wider working potential window of ~1.6 V, resulting in an ultrahigh energy density of 84.17 Wh kg−1 at 0.604 kW kg−1, extraordinary power density (14.95 kW kg−1 at 57.73 Wh kg−1), and prolonged cycle life with capacitance retention of 91.1% after 10,000 charge-discharge cycles. The electrochemical performance of our device is superior to recently reported copper and cobalt selenide-based ASCs. Our work thus develops a new strategy for freestanding ternary metal selenide electrodes as an advanced material for future energy-storage and conversion technology.

The CV curves of the Cu0.5Co0.5Se2//MXene ASC with a potential window from 0 to 1.6 V at different scan rates from 10 to 100 mV s−1 are presented in Fig. 6b. All the CV curves clearly show a pair of richer redox peaks which are attributed to both the positive and the negative electrodes. When increasing the scan rate up to 100 mV s−1, the shape of the CV curves was maintained, which indicates the exceptional Faradaic behavior and superior performance of the assembled ASC. The GCD curves of the Cu0.5Co0.5Se2//MXene ASC at different current densities ranging from 1 to 20 A g−1 were investigated with a potential window of 1.6 V (Fig. 6c). The GCD curves have clearly demonstrated battery-type behavior. The curves were also a perfect match with the CV curves for the Cu0.5Co0.5Se2//MXene ASC. All the GCD curves were almost symmetric, which demonstrates high Coulombic efficiency, which could be attributed to the low and negligible IR drops even at high current densities. The specific capacitances of the Cu0.5Co0.5Se2// MXene ASC were as high as ~236.72, 227.08, 216.11, 191.77, 175.46, and 162.36 F g−1 at current densities of 1, 3, 5, 10, 15, and 20 A g−1, respectively. Fig. 6d demonstrates the retention capability of the ASC, which retains up to 68.58% of its initial capacitance at the current density of 20 A g−1. The cyclic stability and rate capability tests for the Cu0.5Co0.5Se2// MXene ASC were conducted, as shown in Fig. 6e. The Cu0.5Co0.5Se2// MXene ASC retained 91.1% at the current density of 10 A g−1 after 10,000 cycles, which indicates that the Cu0.5Co0.5Se2//MXene ASC exhibits outstanding stability and rate performance. The first and last ten cycles of the cyclic stability are depicted as an inset in Fig. 6e. Most remarkably, a negligible shape change was observed for the last ten cycles, which further validates the exceptional cycling performance of the ASC device. The cyclic performance of the Cu0.5Co0.5Se2//MXene ASC is compared with the reported literature Table S3. Such outstanding cycling performance of our ASC results from the hierarchical structures of both electrodes, large specific surface area, and exclusive porous network. The Rct of the Cu0.5Co0.5Se2//MXene ASC was calculated to be ~0.49 Ω. It increased slightly from ~ 0.49 to 0.70 Ω after 10,000 cycles (Fig. S22). Therefore, the Cu0.5Co0.5Se2//MXene ASC possesses excellent electrical conductivity offering a very low internal resistance and hence ultrahigh performance and cyclic stability. The Ragone plots of the Cu0.5Co0.5Se2//MXene ASC are shown in Fig. S23. Most interestingly, our ASC device delivers a high energy density of ~ 84.17 W h kg−1 and power density of 0.604 W kg−1 , which is superior to reported ASCs such as Ni0.85Se@MoSe2//GNS (25.5 W h kg−1 at 420 W kg−1) [52], NiSe//rGO (38.8 W h kg−1 at 629 W kg−1) [60], Ni-Co-Fe-S@NCAs-NP//rGO-NF (35.9 W h kg−1 at 375 W kg−1) [61], CCO-NS//HCP-CNF (21.1 W h kg−1 at 400 W kg−1) [62], and Ni0.6Co0.4S4//rGO (44.1 Wh kg−1 at of 691.3 W kg−1) [63]. All these data are the plot in a Ragone graph shown in Fig. 6f and Table S3. Even at the higher power density of ~14.95 kW kg−1, the ASC keeps an ultrahigh energy density of 57.73 W h kg−1. The superior electrochemical energy-storage properties of the Cu0.5Co0.5Se2//MXene device can be ascribed to the following aspects: unique and uniform morphology with an outstanding surface area that allows the electrolyte to have excellent coactive interactions with the active parts of the Cu0.5Co0.5Se2 and MXene electrodes. This allows the ions to easily exchange by means of the electrolyte; this high conductivity helps the device maintain its exceptional stability. The practical application of the Cu0.5Co0.5Se2//MXene ASC was presented when two samples of ASC devices connected in series were enough to light a red LED, as displayed in the inset of Fig. 6f, demonstrating that our ASC can have industrial applications. Thus, the assembled Cu0.5Co0.5Se2//MXene ASC with excellent performance promises to be an energy storage device for future electronics.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The work was supported by the Basic Research Program (2019R1A2C1004983) and the Regional Leading Research Center Program (2019R1A5A8080326) through the Ministry of Science and ICT of Republic of Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123455. References [1] J.R. Miller, P. Simon, Materials science – electrochemical capacitors for energy management, Science 321 (2008) 651–652. [2] X.-C. Xie, K.-J. Huang, X. Wu, Metal–organic framework derived hollow materials for electrochemical energy storage, J. Mater. Chem. A 6 (2018) 6754–6771. [3] M. Kuang, X.Y. Liu, F. Dong, Y.X. Zhang, Tunable design of layered CuCo 2 O 4 nanosheets@ MnO 2 nanoflakes core–shell arrays on Ni foam for high-performance supercapacitors, J. Mater. Chem. A 3 (2015) 21528–21536. [4] X. Yang, Z. Lin, J. Zheng, Y. Huang, B. Chen, Y. Mai, X. Feng, Facile template-free synthesis of vertically aligned polypyrrole nanosheets on nickel foams for flexible all-solid-state asymmetric supercapacitors, Nanoscale 8 (2016) 8650–8657. [5] J. Balamurugan, T.T. Nguyen, V. Aravindan, N.H. Kim, J.H. Lee, Flexible solid-state asymmetric supercapacitors based on nitrogen-doped graphene encapsulated ternary metal-nitrides with ultralong cycle life, Adv. Funct. Mater 28 (2018) 1804663. [6] M. Vangari, T. Pryor, L. Jiang, Supercapacitors: review of materials and fabrication methods, J. Energy Eng. 139 (2012) 72–79. [7] A. Lamberti, M. Fontana, S. Bianco, E. Tresso, Flexible solid-state CuxO-based pseudo-supercapacitor by thermal oxidation of copper foils, Int. J. Hydrogen Energy 41 (2016) 11700–11708. [8] Z.-B. Zhai, K.-J. Huang, X. Wu, H. Hu, Y. Xu, R.-M. Chai, Metal–organic framework derived small sized metal sulfide nanoparticles anchored on N-doped carbon plates for high-capacity energy storage, Dalton Trans. 48 (2019) 4712–4718. [9] P. Xiong, X. Zhang, H. Wan, S. Wang, Y. Zhao, J. Zhang, D. Zhou, W. Gao, R. Ma, T. Sasaki, Interface modulation of two-dimensional superlattices for efficient overall water splitting, Nano Lett. 4518–4526 (2019). [10] Y. Jiang, Y. Song, Y. Li, W. Tian, Z. Pan, P. Yang, Y. Li, Q. Gu, L. Hu, Charge transfer in ultrafine LDH nanosheets/graphene interface with superior capacitive energy storage performance, ACS Appl. Mater. Interfaces 9 (2017) 37645–37654. [11] P. Yang, Z. Wu, Y. Jiang, Z. Pan, W. Tian, L. Jiang, L. Hu, Fractal (NixCo1−x)9Se8 Nanodendrite Arrays with Highly Exposed Surface for Wearable, All-Solid-State Supercapacitor, Adv. Energy Mater. 8 (2018) 1801392. [12] R. Ma, T. Sasaki, Two-dimensional oxide and hydroxide nanosheets: controllable

4. Conclusion In summary, we have developed a novel strategy to fabricate active CuxCo1–xSe2 nanostructures for high-performance ASCs. The energy10

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