Nanostructure selenium compounds as pseudocapacitive electrodes for high-performance asymmetric supercapacitor

The electrochemical performance of an energy conversion and storage device like the supercapacitor mainly depends on the microstructure and morphology of the electrodes. In this paper, to improve the capacitance performance of the supercapacitor, the all-pseudocapacitive electrodes of lamella-like Bi18SeO29/BiSe as the negative electrode and flower-like Co0.85Se nanosheets as the positive electrode are synthesized by using a facile low-temperature one-step hydrothermal method. The microstructures and morphology of the electrode materials are carefully characterized, and the capacitance performances are also tested. The Bi18SeO29/BiSe and Co0.85Se have high specific capacitance (471.3 F g–1 and 255 F g–1 at 0.5 A g–1), high conductivity, outstanding cycling stability, as well as good rate capability. The assembled asymmetric supercapacitor completely based on the pseudocapacitive electrodes exhibits outstanding cycling stability (about 93% capacitance retention after 5000 cycles). Moreover, the devices exhibit high energy density of 24.2 Wh kg–1 at a power density of 871.2 W kg–1 in the voltage window of 0–1.6 V with 2 M KOH solution.


Introduction
power output devices for digital communications and electric vehicles owing to their facile manufacture, fast charging/discharge, long cycle life and higher power density than batteries [1][2][3][4]. Intrinsically, supercapacitors based on the principle of charge storage are divided into two types: pseudocapacitors and electric double-layer capacitors (EDLCs) [5]. In EDLCs, the charges are stored because of the surface adsorption of the ions from the electrolyte as a result of electrostatic attraction, thus forming two charged layers (double layer). Pseudocapacitors store charges by fast and reversible oxidation/reduction (Faradaic) reactions occurring at the electrode/electrolyte interfaces, as well as in the bulk near the surface of the electrode. Pseudocapacitors show higher capacitance when compared with EDLC-type devices due to the additional charge transferred within the defined potential [6][7][8].
Up to now, electrode materials with long cycle stability and high capacitance have obtained breakthrough progress, for instance transition metal sulfides (CuS [9], FeS [10], Al 2 S 3 [11], etc.), transition metal oxides/hydroxides (NiO [12], ZnO [13], TiO 2 [14], Co(OH) 2 [15], CoNi 2 S 4 [16], etc.) as well as conducting polymer materials (polyaniline [17], polypyrrole [18], etc.). The above electrode materials have been investigated for use in asymmetric supercapacitors extensively, due to their fast reversible redox reactions, cost-effectiveness, easy processability and relatively good cyclic stability [19]. However, compared with the transition metal dichalcogenide, the two-dimensional (2D) layered transition metal selenide has been paid less attention. The weak van der Waals force of 2D layered metal selenide is beneficial for insertion of guest species. Owing to their superior electronic structure and physical properties, the 2D nanosheet structural materials have attracted abundant attention compared to the corresponding bulk materials [20]. Recently, transition metal selenide has been shown to display outstanding electrochemical performance. Balasingam et al. have synthesized a few-layered MoSe 2 nanosheet by a simple hydrothermal method and further studied its electrochemical charge storage properties. It is shown that the MoSe 2 nanosheet electrode provides a symmetric device that exhibits 49.7 F g -1 with a scan rate of 2 mV s -1 and a largest specific capacitance of 198.9 F g -1 [21]. Wang et al. made a flexible all-solid-state supercapacitor based on three-dimensional (3D) hierarchical GeSe 2 nanomaterials, which have a high specific capacitance of 300 F g −1 at a current density of 1 A g -1 [22]. Choi et al. synthesized SnSe 2 nanoplate-graphene composites and used them as a novel anode material for lithium ion batteries [23]. Tang et al. synthesized a new type of binder-free electrode material of NiSe/NF (NiSe nanowire film on nickel foam) by using the one-step hydrothermal method; it has a high specific capacitance of 1790 F g -1 at a current density of 5 A g -1 [24]. Yang et al. developed interconnected Co 0.85 Se nanomaterials on nickel foam directly through a facile single-step hydrothermal method, which exhibits a high specific capacitance of 1528 F g -1 at 1 A g -1 and excellent cycling stability [25]. Therefore, the transition metal selenide possesses outstanding electrochemical energy storage properties and deserves to be further investigated as an advanced electrode material for supercapacitor application.
In recent years, in order to improve energy density and power density to attain a win-win situation, researchers have assembled asymmetric supercapacitors (ASCs) with different electrode materials in aqueous electrolytes [26][27][28][29][30]. According to the literature, E = 1/2CV 2 (energy density formula); in order to improve energy density (E), two approaches can be used: maximizing the specific capacitance (C) and/or enlarging the operating potential window (V). Employing ionic liquids or organic electrolytes can increase the operating voltage effectively. However, their inherent deficiencies such as poor ionic conductivity and sometimes toxicity have hindered their practical application [26,27]. In comparison, an aqueous electrolyte is the best choice. The ASC is made up of two different electrodes, hence it is an essential prerequisite to select appropriate positive and negative electrode materials to assemble a highperformance ASC. Recently, the literature has depicted that the transition metal selenide has excellent electrochemical performance for a supercapacitor. But the synthesis of different selenium compounds as the positive and negative electrodes of a supercapacitor in the same system has not been well explored so far. In addition, the benzyl alcohol route in particular has been proved to be versatile for the synthesis of various metal oxide nanoparticles with good control over particle phase, size and shape [28,29]. The fact that the benzyl alcohol route is typically applied without the use of surfactants makes this approach ideal for mechanistic studies. Additives such as surfactants complicate the interpretation of the results due to their possible influence on nucleation and growth by complexation of cations [30,31]. Therefore, in a two-component system just consisting of a precursor and solvent, the complexity is decreased to a minimum, although benzyl alcohol itself may play multiple roles as reaction medium, oxygen source and/or capping agent. Another unique feature of non-aqueous systems is the possibility to monitor the organic reactions occurring in parallel to nanoparticle formation by standard analytical techniques. Thus, the use of benzyl alcohol as a solvent (without any template and surfactant) to synthesize transition metal selenium-based compounds is a very desirable route. In this work, we have used a facile low-temperature one-step hydrothermal method without any template and surfactant (benzyl alcohol as the solvent) to synthesize two different transition metal selenides: Bi 18 SeO 29 /BiSe and Co 0.85 Se. The lamella-like Bi 18 SeO 29 /BiSe and petal-like Co 0.85 Se, which are used as negative and positive electrode materials, have high specific capacitance in aqueous electrolyte. The assembled ASCs possess excellent energy density and outstanding power density with a wide voltage window as well as high cycling stability in aqueous electrolyte.

Synthesis of Bi 18 SeO 29 /BiSe nanocomposite
The Bi 18 SeO 29 /BiSe nanocomposites were synthesized using the hydrothermal method in benzyl alcohol. In the typical process, Bi(NO 3 ) 3 •5H 2 O (0.582 g) and SeO 2 (0.111 g) were dispersed in benzyl alcohol solution (35 ml), stirring uniformly for 1 h with the assistance of ultrasonic vibration. After stirring vigorously at room temperature for about 10 min, the white solution was transferred to a 50 ml Teflon-lined stainless steel autoclave and heated at 180°C for 15 h. Finally, the resulting greyish precipitate was collected by centrifugation and on cooling to room temperature naturally. Subsequently, the greyish precipitate was washed with distilled water and ethanol several times to remove any possible ions and dried in a vacuum at 65°C overnight.

Synthesis of Co 0.85 Se nanomaterials
The cobaltous selenide (Co 0.85 Se) nanosheets may also be synthesized by the hydrothermal method in benzyl alcohol solution; using 0.349 g Co(NO 3 ) 2 •6H 2 O and 0.111 g SeO 2 as raw materials, the process was similar to the synthesis of Bi 18 SeO 29 /BiSe nanocomposites.

Materials characterization
The morphologies of Bi 18 SeO 29 /BiSe and Co 0.85 Se were analysed using field emission scanning electron microscopy (SEM, Ultra Plus, Carl Zeiss) with a voltage of 5.0 kV. The microstructure of Bi 18 SeO 29 /BiSe and Co 0.85 Se were further characterized by transmission electron microscopy (TEM, JEM-2010 Japan). The crystallographic structure of the materials was degassed at 200°C before nitrogen adsorption measurement. X-ray diffraction (XRD) was conducted using a Rigaku D/Max-2400 diffractometer, with Cu Ka radiation (λ = 0.15418 nm) at 40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS) was performed using an Escalab 210 system (Germany). In the SEM, the elemental mapping of relative energy-dispersive X-ray spectrometry (EDS) was performed by a probe focused to 0.2 nm with a camera length of 20 cm.

Three-electrode system
The electrochemical performance of Bi 18 SeO 29 /BiSe and Co 0.85 Se was studied in 2 M KOH solution using a three-electrode system. High-purity carbon rods and oxidation of saturated mercury electrode (Hg/HgO) were used as the counter electrode and the reference electrode, respectively. The working electrodes were manufactured as follows: in general, 80 wt% electrode materials (16.0 mg), 10 wt% commercial carbon black (acetylene black, 2.0 mg) as well as 10 wt% polymer binder (polyvinylidene fluoride, 2 mg) were mixed with some N-methyl-2-pyrrolidone to form a homogeneous slurry. Further, the obtained slurry was coated on nickel foam with an area of 1.0 cm 2 and dried at 65°C overnight, and

Two-electrode system of asymmetric supercapacitors
The electrochemical measurements were further taken using a two-electrode system consisting of Bi 18 SeO 29 /BiSe and Co 0.85 Se electrodes in 2 M KOH electrolyte. The working electrodes of the two electrode system were fabricated similarly to the three electrodes. The as-prepared slurry was spread on the rounded nickel foam mesh current collector with an area of 1 cm 2 uniformly, and the coating was left in an oven at 65°C overnight. Afterwards, this was weighed and pressed into sheets under 13 MPa to ensure adherence between the active materials and the current collector. The total mass of each electrode was limited to 3.0-5.0 mg. Two different electrodes of the same or very close weight were selected for measurement.
Finally, the ASCs were assembled using lamella-like Bi 18 SeO 29 /BiSe as the negative electrode and flower-like Co 0.85 Se as the positive electrode with a separator (filter paper) and electrolyte solution; they were assembled into a sandwich cell construction (electrode/separator/electrode). To uniformly diffuse the KOH electrolyte solution into the pseudocapacitive material electrode, the separated and pseudocapacitive material electrodes were immersed in 2 M KOH electrolyte for 6 h and then assembled into the supercapacitor configuration.

Electrochemical testing
A CHI660D electrochemical workstation (Shanghai Chenghua Instrument Co. Ltd, China) was used in the three-electrode and two-electrode systems by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrical impedance spectroscopy (EIS). The EIS measurement was measured at a frequency ranging from 10 mHz to 100 kHz with an impedance amplitude of ±5 mV in open circuit potentials. In addition, a LAND CT2001A cell tester (Wuhan Landian electronics Co. Ltd, China) with a computer controlled system was used for the cycle-life stability test.
In the three-electrode system, the gravimetric capacitances of the Bi 18 SeO 29 /BiSe or Co 0.85 Se sample were calculated from the charge-discharge curves based on the following equation: where C s corresponds to the specific capacitance (F g -1 ), I represents the discharge current (A), t represents the discharge time (s), m refers to the total mass (g) and V refers to the voltage change (V) of the Bi 18 SeO 29 /BiSe or Co 0.85 Se. For the two-electrode cells of ASCs, it was calculated by the equation To obtain Q + = Q − , as well as to make use of the largest voltage window, the mass ratio of the positive and negative electrodes are acquired on the basis of the following equations [33]: and where C + m and C − m are the specific capacitances (F g -1 ) , and V + and V − represent the voltage ranges (V) of the Co 0.85 Se and Bi 18 SeO 29 /BiSe electrodes, respectively. C cell denotes the specific capacitance (F g -1 ) of the ASC device, I the discharge current (A), t the discharge time, m the total weight (g) of the two electrodes, V the voltage window (V), E (Wh kg -1 ) the energy density and P (W kg -1 ) the power density [34].

Negative electrode
The purity and crystallinity of negative electrode Bi 18 SeO 29 /BiSe were investigated using powder XRD, as displayed in figure 1a; the peaks can be indexed to the hexagonal Bi 18 402) and (440) planes of the hexagonal Bi 18 SeO 29 /BiSe, which exhibits a crystalline characteristic. The clear lattice fringes and SAED are in agreement with the aforementioned XRD results. Although with some reservation, an approximate electrochemically active surface area was calculated from the Brunauer-Emmett-Teller method (BET) [35]. The BET surface area of the Bi 18 SeO 29 /BiSe is 19.35 m 2 g -1 , as shown in figure 2a, which is similar to that of pure Co 3 O 4 material (18.5 m 2 g -1 ) [36] and to that of emeraldine-di-hydrogen sulfate (22.1 m 2 g -1 ) [37]. The electrochemically active surface area can facilitate intercalation and de-intercalation of electrolyte ions.
To further confirm the chemical element compositions and the surface valence state information, Bi 18 SeO 29 /BiSe was evaluated by the XPS technique and EDS, as presented in figure 2. In the XPS survey spectrum of Bi 18 SeO 29 /BiSe, the elements of Bi, Se and O as well as C can be clearly identified (figure 2b). Figure 2c shows the high-resolution spectrum of the Bi 4f region, which exhibits two asymmetrical main signals corresponding to the deconvolution of Bi 4f7/2 (159.28 ± 0.3 eV) and Bi 4f5/2 (164.45 ± 0.3 eV) with a spin-orbit splitting of about 5.4 eV [38]. The binding energies of 159.28 eV and 164.45 eV can be attributed to the Bi 3+ ion [39]. From the significant dispersion of the binding energy, it can be demonstrated that bismuth forms a mixture rather than pure metal oxide [40]. Further, the core-level spectrum of the Se 3d region is shown in figure 2d; the binding energy of 58.7 eV can be assigned to oxidized Se (SeO X ), which is in good agreement with previously reported values [30]. In addition, to verify the elemental distribution, Bi 18 SeO 29 /BiSe was further characterized by EDS (figure 2e); the corresponding element mapping images can unambiguously confirm the homogeneous distribution of bismuth, selenium and oxygen in the nanomaterial.
The electrochemical behaviour of Bi 18 SeO 29 /BiSe nanosheets was first studied by CV and GCD techniques with a three-electrode system in 2 mol l -1 KOH aqueous electrolyte. Figure 3a displays the CV curves of the Bi 18 SeO 29 /BiSe nanosheets as the negative electrode at different scan rates ranging from 5 to 30 mV s -1 in the potential window of −1 to 0 V. A pair of redox peaks can be clearly observed in the CV curves of the Bi 18 SeO 29 /BiSe composite at different scan rates, showing it has typical Faradaic pseudocapacitance behaviour. In detail, the oxidation peaks were seen in forward CV scans, while the reduction peaks were seen in the reverse CV scans. In highly alkaline solution, hydroxide ions are generated naturally, which are likely to gravitate towards the cathode at high potentials Thus, the reduction peak at 0.7 V corresponds to Bi 3+ transformed into Bi( OH) 2 + in the reduction process. The oxidation peaks at about −0.48 V and −0.35 V correspond to Bi 3+ /Bi 0 transformation. Those peaks are also seen in some other studies [27,41,42]. It may be catalysed by the induction or oxidation of some unconverted Bi 0 during the reduction process, and the process has the following reaction [27,43]: Here, Bi 0 are active atoms and Bi met is the metal bismuth.
There are characteristic redox peaks of the electrode observed from −1 to 0 V which correspond to the reversible intercalation/deintercalation of OHions occurring in the Bi 18 SeO 29 /BiSe bulk and at the interface of Bi 18 SeO 29 /BiSe and electrolyte to increase the discharge time.
From the CV curve, we can see that the shape remains similar even at high scan rates, which indicates that the Bi 18 SeO 29 /BiSe electrode material has excellent capacitance behaviour. And at higher sweep rates, the higher/lower potentials the reduction and oxidation peak potentials move, because the ions may only be immersed in the surface of the material; however at a lower sweep rates, ions can be effectively diffused into the inner active sites. As the lower scan rates are provided for a longer period of time, the active site interacts with the ions better. So it is obvious that as the scan rate increases, the specific capacitance will be reduced. The various current density GCD curves of the Bi 18 SeO 29 /BiSe electrode are presented in figure 3b; the charge/discharge curve reveals the pseudocapacitance behaviour. In the discharge process, there is a kink in the curve, which is perhaps some of the untransformed Bi 0 oxidation platforms during the reduction process. These results are consistent with that consequence of the CV curves. As exhibited in figure 3c, the specific capacitances of the Bi 18 SeO 29 /BiSe electrode at various current densities of 0.5, 1, 2, 3, 5, 8 and 10 A g -1 are 471.3, 373, 331.2, 320.4, 307, 291.2 and 285 F g -1 , respectively. This indicates outstanding rate capability [44]. This phenomenon is also present in transition metal sulfides and oxides/hydroxides, which can be attributed to the diffusion effect [45,46]. To evaluate the cycling stability, the GCD cycling of Bi 18 SeO 29 /BiSe was observed at a current density of 2 A g -1 (figure 3d), which indicates that Bi 18 SeO 29 /BiSe has good cycling stability with about 68% capacitance retention after 5000 cycles in 2 M KOH electrolyte. This result makes Bi 18 SeO 29 /BiSe a promising candidate as an advanced electrode material for supercapacitor application.

Positive electrode
The Co 0.85 Se was characterized in detail. From the XRD pattern displayed in figure 4a,   suggesting that the nanosheet is of polycrystalline nature [31], which shows good agreement with the XRD pattern and can be indexed to the structure. The specific surface area and pore size distribution of Co 0.85 Se were examined by N 2 adsorptiondesorption measurements. As displayed in figure 5a and inset, at a relative pressure of 0.9-0.99, the apparent N 2 adsorption and the hysteresis loop indicate the coexistence of a minor fraction of mesopores/macropores, which is mainly due to the 3D voids between interconnected particles [47]. The specific surface area of Co 0.85 Se calculated by the multiple points BET method is 73.3 m 2 g -1 , which is larger than that of the previously reported Co 0.85 Se electrode materials; Gong et al.   surface area of 26.44 m 2 g -1 [48] and Yang et al. described 59 m 2 g -1 [31]. A mass of the 3D porous structure will favour the penetration of electrolyte and the rapid transmission of electrons, which will improve the electrochemical properties of the Co 0.85 Se electrode material. The XPS and EDS techniques were used to evaluate the surface valence state information and the chemical element compositions of Co 0.85 Se ( figure 5b-d), mainly exhibiting the Se 3d and Co 2p peaks. Figure 5b shows the Co 2p 3/2 , 2p 1/2 and two satellite peaks (marked as 'sat.'). The Co 3+ 2p 3/2 , Co 2+ 2p 3/2 and Co 2+ 2p 1/2 are corresponding to binding energies of 779.0, 780.8 and 797.2 eV, respectively.  Figure 5. (a) Nitrogen adsorption-desorption isotherms and (inset) pore size distribution of Co 0.85 Se; (b) high-resolution XPS spectra of Co 2p1/2 and Co 2p3/2 peak; (c) high-resolution XPS spectra of Se 3d peak; and (d) the corresponding element mapping images (selected from the square region) for the Co 0.85 Se nanosheets.
The two spin-orbit doublets characteristic of Co 2+ and Co 3+ are considered and consistent with the previously reported values [37]. Furthermore, figure 5c clearly displays the core region of the binding energy of 55.4 eV corresponding to the Se 3d spectrum, and this approached the reported value [49]. The results show that the synthesized Co 0.85 Se electrode material consisted of Co 2+ , Co 3+ and Se 2 , which is consistent with the formula Co 0.85 Se. Furthermore, the Co 0.85 Se electrode material is synthesized by element mapping images of cobalt and selenium in figure 5d, in which it is obvious that the elemental distributions are very uniform.
Electrochemical behaviours of the Co 0.85 Se electrode material were investigated by CV and GCD measurements with 2 M KOH at a voltage window between −0.1 and 0.6 V (versus Hg/HgO) in a standard three-electrode system. From figure 6a, it is clear that the area surrounded by the CV curve of Co 0.85 Se exhibits two pairs redox peaks, which might be attributed to standard Faradaic pseudocapacitance behaviour. The two pairs ascribed to the reversible redox reaction can occur according to [50] Co 0.85 Se + OH − → Co 0.85 SeOH + e − Co 0.85 SeOH + OH − → Co 0.85 SeO + H 2 O + e − Figure 6a describes the typical CV curves of the Co 0.85 Se nanosheets at various scan rates from 5 to 30 mV s -1 , from which, even at a high scan rate of 30 mV s -1 , two pairs of reversible redox peaks can be obviously observed. In addition, due to the presence of polarization, with the increase of the scan rate the position of the redox peak gradually changes [50]. The GCD test results are presented in figure 6b to analyse the charge storage capacity of the electrode material. Because the redox reaction occurs at the electrode interface by the desorption and adsorption of the hydroxyl ion in the alkaline electrolyte [51], it is obvious that each charge-discharge curve indicates pseudocapacitive performance at current densities from 0.5 to 8 A g -1 , and deviates from the EDLC linear curve. All of the charge-discharge curves are almost symmetrical, which reveals that Co 0.85 Se has excellent capacitive behaviour. As displayed in figure 6c, the specific capacitance of Co 0.85 Se is calculated to be 255, 246, 237, 229, 220, 204 and 196 F g -1 at various current densities from 0.5 to 10 A g -1 , exhibiting an outstanding rate capability. The stability of the Co 0.85 Se electrode material was tested using the charging and discharge technique at a current density of 2 A g -1 . Finally, it is seen to retain about 78% of its initial capacitance after 5000 cycles, indicating that the Co 0.85 Se positive electrode material possesses outstanding cycling stability.

Asymmetric supercapacitor
In practical applications, to further evaluate the electrochemical performance of the electrode materials, we assembled an ASC, in which Bi 18  that reaches 24.2 Wh kg -1 at an outstanding power density of 871.2 W kg -1 at a current density of 1 A g -1 .
Even when the current density is as high as 15 A g -1 , the energy density still remains at 11.9 Wh kg -1 with an excellent power density of 13 387.5 W kg -1 . The energy density of the Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC device is shown in figure 7d. To verify the feasibility of energy supply for the Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC, the three-tandem cell group can light up a red light-emitting diode (figure 7d, inset). Therefore, the excellent high energy density indicates that it has great potential as a supercapacitor.
The facilitated ion or electron transport kinetics of the Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC device was investigated by EIS. The Nyquist plot of the ASC device and the corresponding equivalent circuit are shown in figure 7e and in the inset of figure 7e, respectively. It consists of three different parts at a distinct frequency range: high frequencies, an unfinished semicircular part; the middle frequencies, an inclined portion in the curve (about 45°); and the low frequencies, the linear part. The high frequency of the real axis intercept indicates the internal resistance (R s ), which is the sum of the large amount of electrolyte resistance, the contact resistance of the electrode/electrolyte interface and the intrinsic resistance of the electrode active material [52]. The charge transfer resistance (R ct ) is the diameter of the semicircle which is related to the charge transfer at the electrode/electrolyte interface and thus produced a Faraday reaction [46]; in the middle frequencies, the 45°slope of the line is called Warburg impedance (Z w ), which is caused by the diffusion process of the electrolyte [53]. Moreover, C L and C dl represent the limit capacitance and the double-layer capacitance [54]. As can be seen from the fitted results, a Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC device not only has a low R s (0.88 Ω cm 2 ), but also possesses a small R ct (1.87 Ω cm 2 ), as well as a low Warburg resistance (0.017 Ω cm 2 ). The values demonstrate that the electrolyte permeation and ion diffusion into the pore structure are very easy, and these values may belong to electrode materials with special structure. On the one hand, Bi 18 SeO 29 /BiSe is an ultrathin nanosheet, and it can provide an efficient pathway for charge transportation; on the other hand, Co 0.85 Se material has a 3D high surface area, for adsorbing ions to provide abundant electrical active sites. These results can facilitate transport of the electrolyte. The cyclic stability of the Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC device is evaluated by the GCD process with a current density of 2 A g -1 after 5000 cycles and an operating voltage of 0-1.6 V (figure 7f ). Owing to the full activation of the Bi 18 SeO 29 /BiSe and Co 0.85 Se materials, the specific capacitance increases at the beginning, and even after 1500 cycles, the degradation of the active material was not observed with maintenance of 100% initial capacity. Thus, excellent cycling performance is shown. Finally, the ASC device shows only a slight decrease in specific capacitance (about 93% of the initial specific capacitance retention after 5000 cycles), indicating excellent cycle stability. The above results mean that the high-performance ASC with excellent stability may be a candidate for energy storage devices in future electronic application.

Conclusion
In summary, the negative Bi 18 SeO 29 /BiSe and positive Co 0.85 Se electrode materials are prepared through a simple one-step hydrothermal method without any template and surfactant. Bi 18 SeO 29 /BiSe and Co 0.85 Se have high specific capacitance (471.3 F g -1 and 255 F g -1 at 0.5 A g -1 ), high conductivity, outstanding cycling stability, as well as good rate capability. The all-pseudocapacitive electrodes fabricated Bi 18 SeO 29 /BiSe//Co 0.85 Se ASC device has excellent electrochemical performance with a good cycling stability (93% capacitance retention after 5000 cycles at a current density of 2 A g -1 ) and high energy density (about 24.2 Wh kg -1 ) as well as high power density (about 871.2 W kg -1 ) in aqueous electrolyte at a wide voltage window of 0-1.6 V. The results demonstrate that the electrode material preparation approach is easy and also the resulting ASC is promising as an energy-storage device.
Data accessibility. This article has no additional data.