Constructive Electroactive 2D/2D MoS2-N-rGO and 1D/2D Bi2S3-N-rGO Heterostructure for Excellent Mo-Bi Supercapattery Applications

Metal sulfides including MoS2 and Bi2S3 materials, have been considered as a strong candidate for supercapacitor applications. However, the short-term stability and low surface area have limited the establishment of such eco-friendly materials in energy storage. In this work, an effective strategy is designed to in-situ combine transition metal sulfides with nitrogen doped reduced graphene oxide hydrogels and improve the overall supercapattery properties. Precisely, MoS2-N-rGO and Bi2S3-N-rGO hydrogels have been developed via hydrothermal route. The morphological analysis manifests two-dimensional 2D/2D heterostructure for the MoS2-N-rGO and 1D/2D heterostructure for the Bi2S3-N-rGO. The cyclic voltammetry studies showed a battery-like electrochemical behavior for the synthesized hydrogels. The calculated capacitance for MoS2-N-rGO and Bi2S3-N-rGO are about 438 F/g and 342 F/g @ 1 A/g with 50% and 41% of their capacitance initial values @ 20 A/g, respectively. The cycling performance showed that MoS2-N-rGO and Bi2S3-N-rGO can maintain 90% and 98% of their original specific capacitance after 1000 cycles life. Furthermore, the supercapattery device was fabricated using MoS2-N-rGO as cathode and Bi2S3-N-rGO as anode. The hybrid device is capable of offering 33.4 Wh/kg energy density, at 0.85 kW/kg power density, with 44.7% retention at 20 A/g. Notably, the overall electrochemical behavior of Mo-Bi supercapattery device is remarkable among the pointed behaviors for other hybrid devices.


Introduction
New energy storage technologies are needed to be commercially available soon due to the foreseeable exhaustion of fossil-based fuel and subsequent industrial and economic issues. These technologies involve devices to store electrical energy harvested from solar, wind, and other natural sustainable resources, in addition to energies recovered in the industrial sector [1,2]. Supercapacitors (SCs) are electrochemical energy storage devices that are considered promising replacement over traditional dielectric capacitors and batteries. They significantly possess great energy density, high power rates, fast power delivery needed for energy recovery technologies, and longer lifetime [3][4][5]. SCs present promising solution to bridge the critical gap between battery and conventional electrolytic capacitor in terms of energy and power densities [6][7][8].
Two types of supercapacitor electrode materials can be categorized based on the mechanism by which charge is stored. (1) EDLCs or electric double layer capacitorssuch as graphene and carbon materials, in which the charge can be stored nonfaradically, i.e., the charge is electrostatically adsorbed at the electrode/electrolyte interface. (2) pseudocapacitors conductive polymers and metal oxides [9,10]. The charge in this type can be stored through fast and reversible faradic reactions at the surface or sub-surface 1 3 of the active electrode material or through doping or dedoping, according to the material in use. In both EDLC and pseudocapacitive charge storage mechanism, the charge is directly proportional to potential difference and the shape of cycle voltammetry curves, either having a rectangular or semi rectangular shape. Additionally, since the charge storage in both types is a surface process, the increased specific surface area of the electrode leads to an increased storage capacity [11]. (3) Hybrid electrodes combine the characteristics of both EDLCs and pseudocapacitors giving rise to intermediate yet more stable performance [12][13][14]. A novel technology that combines the benefits of high energy density in batteries and high-power density in supercapacitors is named supercapattery. Supercapattery device consists of high-power density EDLC electrode in one side, and high energy density pseudocapacitive (battery-type) electrode in the other side [15,16].
A crucial component of a supercapattery device is the electrode material since it defines the mechanism, efficiency, and characteristics of the electrochemical charge storage. For example, the electrical properties of the device, such as the internal resistance, is highly affected by the electrode material and architecture [11].
Currently, transition metal sulfides (TMS) are catching more attention for supercapacitor practices because of their promising physicochemical and electrical characteristics [17,18]. They possess superior electrochemical performance, relatively high specific capacity, high redox activity, and low electronegativity [19].
Molybdenum disulfide MoS 2 is a commonly investigated TMS for energy storage applications as an electrode material [20]. MoS 2 exhibits layered structure in which one layer of molybdenum atom is inserted amid two coats of sulfur atom and all stack together by means of weak van der Waals forces [21][22][23]. This multi-layer architecture facilitates ion movement into the electrode material, together with the high surface area of the 2D nanolayers, will give rise to pseudocapacitive electrochemical behavior [24][25][26]. However, one important obstacle is the low electrical conductivity of MoS 2 that lowers the electrode interaction kinetics [27]. Overcoming this issue is by combining MoS 2 together with a graphene-like molecule, such as reduced graphene oxide (rGO), resulting in a hybrid electrode with enhanced characteristics [28][29][30].
Another important TMS is Bi 2 S 3 that possesses very good physicochemical properties and can be synthesized in various nanostructures. Bi 2 S 3 was synthesized in 1D and 3D structures and was reported to possess good supercapacitor behavior [31,32]. Same as MoS 2 , Bi 2 S 3 possesses relatively poor intrinsic conductivity, therefore it is necessary to combine this material with carbon matrix to overcome this drawback. Graphene is a 2D carbon monolayer that exhibits large surface area and excellent EDLC electrochemical behavior [33][34][35]. However, there are not much available studies that investigate either MoS 2 or Bi 2 S 3 combined with a carbonbased material for supercapacitor applications. Mixing metal sulfide is being considered the next generation composites for electrode materials [36]. This combination of TMS materials is expected to show promising results in a full cell architecture as it integrates physicochemical characteristics of both EDLC and battery-like capacitance.
Herein, we have synthesized MoS 2 -N-rGO and Bi 2 S 3 -N-rGO hydrogels via hydrothermal route. Structure, morphology, and porosity of the synthesized hydrogels were checked and their electrochemical behavior for energy storage applications was assessed, for each electrode separately. Afterwards, each hydrogel composite material was incorporated to form a full cell and we studied its electrochemical properties as well as its cyclability and overall capacitance.

Experimental Section
Graphene oxide (GO) was purchased and then processed in the lab to obtain reduced graphene oxide (rGO) and Nitrogen doped rGO. Simple hydrothermal process, shown schematically in Fig. S1, was employed for hydrogel formation, as follow:

Synthesis of MoS 2 -N-rGO
Typically, an appropriate amount of concentrated (2.5 mg/ ml) GO solution was sonicated with 3.8 mM of (NH 4 ) 2 MoS 4 for 30 min, where (NH 4 ) 2 MoS 4 was used as a source for MoS 2 and Nitrogen. The combined blend was poured in a 50 mL volume Teflon-lined stainless-steel autoclave, then the hydrothermal experiment was conducted at 200 °C for 12 h. After natural cooling down to room temperature, the obtained hydrogel was extracted, washed several times with deionized water, and freeze dried for two days.

Synthesis of Bi 2 S 3 -N-rGO
Typically, an appropriate amount of concentrated (2.5 mg/ ml) GO solution was sonicated was sonicated with 3.8 mM of Bi(NO 3 ) 3 .6H 2 O and 7 mM of thiourea for 30 min, followed by adding 3 ml of ammonia solution with concentration of 28%, where ammonia served as a source of Nitrogen. The combined blend was poured in a 50 mL volume Teflon-lined stainless-steel autoclave, then the hydrothermal experiment was conducted at 200 °C for 12 h. After natural cooling down to room temperature, the obtained hydrogel was extracted, washed several times with deionized water, and freeze dried for two days.

Synthesis of MoS 2 and Bi 2 S 3
The pristine MoS 2 and Bi 2 S 3 were synthesized following same routes but without adding GO. The drying process was operated at 65 °C overnight under vacuum.

Characterization
Structural analysis and phase formation of all electrode materials was checked by X-ray diffraction technique (XRD) D8 Advance (Cu Kα, λ = 0.154 nm) from Bruker. Analysis of the oxidation states and surface properties was assessed by X-ray Photoelectron Spectrometer (XPS) (Kratos' Axis Ultra DLD (delay line detector) photoelectron spectrometer and the spectra were calibrated using carbon spectrum as a reference). The morphological properties were assessed by using SEM from SUPRA 40 ZEISS, and TEM from JEOL 2010F, at 200 kV. Sample's porosity was investigated according to the BET method [37] by ASAP 2020 (Physisorption Analyzer).

Electrochemical Performance
Solartron System 1470E and 1400A were used to operate all measurements. The working electrode was composed of a piece of each MoS 2 -N-rGO or Bi 2 S 3 -N-rGO hydrogels sandwiched between two Ni foam pieces, while the reference electrode was Ag/AgCl, and the counter electrode was a Pt wire. The 2 M KOH was utilized as an electrolyte. The loading mass for the single electrode test was about 2.2 mg for MoS 2 -N-RGO and Bi 2 S 3 -N-GO. Three-electrode cell configuration had been employed to examine the supercapatterive performance of different samples in terms of cyclic voltammetry and galvanostatic charge-discharge in addition to cyclic stability and electrochemical impedance spectroscopy. The frequency for the impedance test varied from 10 6 to 0.1 Hz, with a potential 10 mV. The MoS 2 -N-rGO//Bi 2 S 3 -N-rGO hydrogel hybrid supercapattery full cell was then fabricated after matching the mass ratio between the two electrodes. For the full cell configuration, the loading mass was about 2 mg and we used coin cell 2032 for testing and glassy carbon as the separator.

Morphological and Structural Characterization
The low magnification SEM images of MoS 2 -N-rGO and Bi 2 S 3 -N-rGO are displayed in Fig. 1a, c respectively. SEM images reveal that MoS 2 nanosheets and Bi 2 S 3 nanowires are uniformly distributed among N doped rGO sheets. Additionally, the low magnification SEM images shown in Fig. 1b, d manifest that the 2D MoS 2 consists of few layers and the size of Bi 2 S 3 nanowire is found to be around 15-70 nm on average, respectively.
The TEM and high-resolution TEM (HRTEM) analysis were performed to further investigate the morphological and structural properties of MoS 2 -N-rGO and Bi 2 S 3 -N-rGO samples. As demonstrated in Fig. 2a, b, 2D/2D MoS 2 -N-rGO and 1D/2D Bi 2 S 3 -N-rGO heterostructures have been constructed and the metal sulfide are anchored in the N-doped rGO. The detected d-spacing of about 0.30 and 0.29 nm which come to lattice planes (100) and (211) of 2H-MoS 2 and Bi 2 S 3 , respectively.
XRD measurements were performed to check the internal structure and state of crystallinity for the studied materials. XRD pattern for MoS 2 -N-rGO depicted in Fig. 3a, shows no observed sharp peaks, except of some weak and/or broad humps for the MoS 2 , confirming the poor crystalline nature of the material. Moreover, the weakened broad peaks positioned at about 2θ = 17˚, 34˚, 57˚ was found to be corresponding to the (002), (100), and (110) planes which affirm the formation of the MoS 2 [38]. The standard pattern JCPDS. Card No. 77-0341 for MoS 2 is plotted together with the data. The peak centered at about 2θ = 25°-which is slightly broad-corresponds to the layered rGO nanosheets [39,40]. Additionally, the peak centered at about 2θ = 44˚which is intensely broad-corresponds to the N-doped rGO [41]. These results confirm the phase homogeneity and the seemingly short-range order structure of this 2D material.
On the other hand, Fig. 3b depicts the XRD patterns for Bi 2 S 3 -N-rGO sample which exhibit the very sharp diffraction peaks of Bi 2 S 3 -N-rGO nanowires. Diffraction peak analysis indicates that the peaks at 2θ: 15  These peaks could all be indexed to Bi 2 S 3 orthorhombic phase with Pbnm space group [32,43]. The standard pattern JCPDS card No. 17-0320 for Bi 2 S 3 is plotted together with the data. This could imply that the incorporation of N-rGO did not hinder the crystallinity of the synthesized 1D Bi 2 S 3 nanowires.
The XPS test was operated to establish the surface contents and define chemical state configuration of the synthesized materials. Figure 4 shows XPS spectra for the synthesized MoS 2 -N-rGO hydrogel, confirming the existence of Mo, S, C, O, and N. As observed in Fig. 4a there exist three dominant peaks at 232.8, 228.9, and 229.8 eV that belong to Mo 4+ 3d 3/2 , Mo 4+ 3d 5/2 , and 1 T phase respectively [44,45], in addition to the peak at 226.7 eV which correspond to S 2 s [46]. The observed peak at 236.1 eV which is belonged to Mo 6+ 3d may associated to the passive layers stamped on MoS 2 surface when it is exposed to atmosphere [47].
The chemical state of S in MoS 2 -N-rGO is clarified in Fig. 4b, the peaks observed at 161.8 eV and 163.8 eV belong to S 2p 3/2 and S 2p 1/2 valence states, respectively, agreeing with S 2ligands present in MoS 2 [47]. Whereas the peak at around 169.6 eV is ascribed to S-O groups because of the construction of passive materials on MoS 2 surface. While there is an additional peak recognized at 165.4 eV, it corresponds to the edge S [48]. Furthermore, the spectrum shown in Fig. 4c, for C 1 s, exhibits two main peaks at 284.8 eV and 286.3 eV, which coincides to C-C/C = C and C = O, respectively. The peak observed at around 285 eV is corresponding to C-N which indicates that the N-doping process is successful. The N 1 s XPS spectrum, shown in Fig. 4d, demonstrations three key points at 395.3 eV, 398.8 eV, and 402 eV which are belongs to nitride-like nitrogen, pyridinic nitrogen, and graphitic nitrogen, respectively [49].
XPS spectra for the synthesized Bi 2 S 3 -N-rGO hydrogel are shown in Fig. 5, confirming the existence of Bi, S, N, O and C elements. In Fig. 5a, peaks at 158.7, 159.7, 164 and 165 eV are assigned to Bi-S 4f 7/2 , Bi-O 4f 7/2 , Bi-S 4f 5/2 and Bi-O 4f 5/2 respectively [32,50]. Additionally, the peak at 161.3 eV is assigned to S 2p transitions. Figure 5b, c manifest the XPS spectra of N 1 s and C 1 s, respectively. XPS spectra for C 1 s display three peaks at 284.7, and 285.3 eV which belong to carbon bonded to carbon and nitrogen respectively, and 286.1 eV corresponds to the oxygen containing groups. XPS results for all synthesized hydrogels confirm that all samples have high purity phase of either MoS 2 or Bi 2 S 3 .
To check the influence of N-doped rGO on the porosity and surface area of MoS 2 and Bi 2 S 3 , the nitrogen (N 2 ) adsorption/desorption measurements are conducted. Figure 6a, b suggest that isotherm curves belong to type IV which suggest that the tested materials show mesoporous structure [51]. The estimated distribution of pore size is manifested in Fig. 6c, d. The distribution of pore size is centered at 2 nm and 30 nm, confirming that the formation of mesoporous structure of which is favorable for ion diffusion. The surface areas calculated for the different samples were found to have values around 92, 47, 63, 1.3 m 2 /g for MoS 2 -N-rGO, MoS 2 , Bi 2 S 3 -N-rGO, and Bi 2 S 3 , respectively. This improvement in the specific surface areas of different samples can be ascribed to the conjugation of metal sulfides with N-doped rGO which has high surface area in nature.

Electrochemical Characterization
The supercapatterive properties of the studied samples was characterized by the three-electrode mode, with 2 M KOH electrolyte.
Cyclic voltammetry (CV) measurements are shown in Fig. 7a, c, for the two studied electrodes, with different scanning rates from 1 to 100 mV/s and 0 to 0.6 V voltage window, for MoS 2 -N-rGO and from 0 to − 1.2 V for Bi 2 S 3 -N-rGO electrodes. The battery type behavior of the studied electrodes is confirmed by the presence of redox peaks [52]. Moreover, there is a systematic shift to the higher voltage due to the rapid charge and discharge processes [49]. Figure S2a presents the CV curve at scan rate 10 mV/s, where there is one oxidation peak around 0.39 V and two redox peaks around 0.21 and 0.27 V which have been reported as electrochemical characteristics for sulfide materials. Even more, such observed redox can be explained in terms of the interaction between the electrolyte ions and MoS 2 -N-rGO according to the following equation [13,22].
To go further the CV curve of Bi 2 S 3 -N-rGO @ 100 mV/s is displayed in Fig. S2b. The observed redox peaks at around − 0.44, − 0.6, and − 0.79 V manifests its typical electrochemical behavior due to the faradic reactions and this behavior can be described by the following equation [31,53]: (1) MoS 2 + 2KOH ↔ MoS 2 OH + 2K + + 2e − (2) Bi 2 S 3 + 2KOH ↔ Bi 2 S 3 OH + 2K + + 2e − Figure 7b, d provide galvanostatic charge/discharge measurement results of the two studied electrodes. The defined voltage plateaus suggest the electrochemical properties of the electrode to belong to a battery type class of materials. Such battery behavior can be ascribed to transformation of active materials from one phase to another.
For the two studied electrodes, the specific capacitance ( C s ) is shown in Fig. 8a. The calculation of C s was performed, using the charge/discharge data, by means of current i , discharge time Δt , mass m and voltage V , by the equation: The C s for MoS 2 -N-rGO and Bi 2 S 3 -N-rGO were calculated to be 438 F/g and 342 F/g @1 A/g with rate capability of about 50% and 41% @ 20 A/g, respectively. Furthermore, there are an enhancement in the conductivities (electronic and ionic) of the active materials which can be indicated from the regularity in the curves of charge/discharge with small IR values in the discharge process. Electrochemical Impedance Spectroscopy (EIS) measurements were performed to investigate the supercapatterive nature of electrodes/electrolyte interfaces. The obtained Nyquist plots for the studied two electrodes are displayed in Fig. 8b. For further explanation of the Nyquist plot, the equivalent resistance (R eq ) which represent the summation of the interface resistance, electrolyte ionic resistance, and electrode materials electronic resistance can be calculated. The R eq of MoS 2 -N-rGO and Bi 2 S 3 -N-rGO are about 0.60 and 1.00 Ω, respectively, suggesting better conductivities. The charge transfer R ct is presented by the diameter of semicircle which is the resistance at the electrode\electrolyte interface. The R ct is evaluated to be 1.15 Ω for MoS 2 -N-rGO while that of Bi 2 S 3 -N-rGO is 1.50 Ω suggesting better internal conductivities and enhance the capacitive behaviors. The ion diffusion resistance is small as indicated from the straight line at low frequency range suggesting rapid OH ˗ ion diffusion rate.
The EIS data was fitted, and the equivalent circuit model was obtained, as shown in Fig. 8c, where R S is equivalent resistance, C DL is double-layer capacitance, W o is the Warburg diffusion element, R CT is charge-transfer resistance, and CPE represents non-faradaic impedance arising from the interface capacitance.
The stabilities of the MoS 2 -N-rGO and Bi 2 S 3 -N-rGO were inspected by galvanostatic charge and discharge measurement @ 10.00 A/g, as shown in Fig. 8d, e. MoS 2 -N-rGO and Bi 2 S 3 -N-rGO can maintain 80% and 98% of their initial C s after 2000 cycles of charge/discharge, whereas coulombic efficiency was found to be around 100% and 92%, respectively. One can see that these values of MoS 2 -N-rGO and Bi 2 S 3 -N-rGO are one of the highest values among the published similar electrode materials reported hybrid supercapacitor application. For example: 386.70 F/g @ 1.00 A/g have been stated for 2D/2D heterostructure of 1 T-MoS 2 / Ti 3 C 2 MXene [54]. MoS 2 /reclaimed carbon fiber electrode provides 225 F/g specific capacitance @ 0.50 A/g with 78% retention after 2000 cycle [55]. 310 F/g @ 2.00 mV/s has been reported for free standing ultrathin heterostructures of 1 T-2H MoS 2 on carbon nanofibers, and, after 20,000 cycles, it can retain 87% of its specific capacitance [56]. Nanosheets of MoS 2 encased with MOF derived microporous carbon reported 189 F/g C s @ 1.00 A/g with 98% retention after 3000 cycles [57]. Flower-like N-co-doped MoS 2 /rGO gives C s around 340 F/g @ 0.50 A/g with rate retention 59% @ 5.00 A/g [58]. While Bi 2 S 3 showed specific capacitance of about 233 F/g at current density 1 A/g with rate capability around 42% of its original capacitance at 20 A/g [59]. The development in the MoS 2 -N-rGO and Bi 2 S 3 -N-rGO electrochemical performance is ascribed to the improvement in the surface areas, mesoporous structure and the conductivity of the two electrodes because of the synergistic effect between the active materials and N-rGO, which enhances their conductivity and enhances the charge transfer/ion diffusion.
On the other hand, for further showing the effect of incorporating nitrogen-doped rGO on the electrochemical performance of each electrode material, each pristine electrode (MoS 2 and Bi 2 S 3 ) was studied, and results are shown in Fig.  S4. It is noted that from Fig. S4e, MoS 2 can retain only 8% of its original capacitance thus the synergistic effect between N-doped reduced graphene oxide and MoS 2 enhance the specific capacitance retention. Furthermore, Bi 2 S 3 exhibits a  isotherms (a, b) and pore-diameter distribution curve (c, d) for MoS 2 based materials and Bi 2 S 3 based materials, respectively specific capacitance of 214.5 F/g @ 2 A/g with capacitance retention of about 39% at current density 20 A/g as shown in Fig. S4f.
Additionally, XPS data was used to calculate the percentage sulfides and graphene in the synthesized hydrogel material. For MoS 2 -N-rGO hydrogel, the ratios for Mo, S, N, C, and O were found to be 6.869%, 3.005%, 5.139%, 68.798%, and 16.189% respectively. For Bi 2 S 3 -N-rGO hydrogel, since peaks for Bi and S elements occur very close to each other so we calculated the percentage of both Bi and S to be about 27.589% of the total composition. Ratios for N, C, and O were found to be 4.043%, 54.489%, and 13.879% respectively.
The contribution of each electrode components to capacitance, at different current densities, is shown in Table 1.
Results are shown for current densities of 5, 10, and 20 A/g. At lower current density of 2 A/g there was no significant change in the capacitance values for MoS 2 , the effect only was shown for Bi 2 S 3 .
For further investigation of the performance of synthesized electrodes, a full cell device is assembled using MoS 2 -N-rGO as cathode and Bi 2 S 3 -N-rGO as anode, and 2 M KOH as electrolyte. To balance the mass charge, the mass ratio between the two electrodes was estimated to be around 0.94 according to the cycling voltammetry of the discrete electrode carried out in the three-electrode configuration at 20 mV/s as depicted in Fig. S3a. Figure S3b depicts the cyclic voltammetry measurements of MoS 2 -N-rGO// Bi 2 S 3 -N-rGO full cell device within various potential windows. As manifested in Fig. S3b, the CV curves area increases with increasing the operating voltage indicating the increment in the capacitance and then the specific energy density. Yet the oxygen evolution started to appear at operating voltage of 1.8 V, thus 1.7 V is chosen as the augmented functioning voltage to examine the total supercapatterive behavior of the cell.  Figure 9a presents the CV curves of the cell at various scanning rates, in which the CV displaying definite oxidation/reduction peaks coming from the electrolyte synergy with MoS 2 and Bi 2 S 3 . The symmetry in charge/ discharge curves which is investigated in accordance with current densities indicating the desired behavior as presented in Fig. 9b. The capacitance of the MoS 2 -N-rGO//Bi 2 S 3 -N-rGO cell determined based on the total mass of the two electrodes and summarized in Fig. 9c. The capacitance value of the hybrid cell is around 83.2 F/g @ 1 A/g. Additionally, it can retain 44.7% of its capacitance @ 20 A/g suggesting high-rate capability. The hybrid device showed good conductivity as indicated from Fig. 9d. The corresponding Ragone plot of the supercapattery device is displayed in Fig. 9e. It reveals that the device can supply energy density as high as 33.4 Wh/kg at power density of about 0.85 kW/ kg within charging/discharging time 313.2 s. Just after the charging span is set at 3.4 s, the energy thickness is preserved at 15 Wh/kg at 18 kW/kg power density.

Conclusion
In conclusion, we designed novel design to reinforce the overall electrochemical performance of metal sulfide by in-situ to prepare 2D/2D MoS 2 -N-rGO and 1D/2D Bi 2 S 3 -N-rGO heterostructures, with MoS 2 nanosheets consisting of few layers and Bi 2 S 3 nanorods having the size of 70 nm on average. The cyclic voltammetry shows that the candidate electrodes 2D/2D MoS 2 -N-rGO and 1D/2D Bi 2 S 3 -N-rGO have battery type behaviors. The specific capacitances are found to be around 438 and 342 F/g at 1 A/g current density, for MoS 2 -N-rGO and Bi 2 S 3 -N-rGO, respectively, with excellent rate capability. After 1000 charge/discharge cycles at current density 10 A/g, the 2D/2D MoS 2 -N-rGO electrode retains about 90%, whereas the 1D/2D Bi 2 S 3 -N-rGO maintains about 98% of their original specific capacitance. Moreover, a hybrid supercapattery cell MoS 2 -N-rGO//Bi 2 S 3 -N-rGO can supply a high specific capacitance of 83.2 F/g at current density 1 A/g and it shows high-rate capability with potential window 1.7 V. The hybrid cell delivers energy density as high as 33.4 Wh/kg at power density of about 0.85 kW/kg within charging/discharging time 313.2 s. When the charging time is set at 3.4 s, the energy density is maintained at 15 Wh/kg at power density 18 kW/kg. The total steep work proposes that the unique technique of in-situ formation of 2D/2D MoS 2 -N-rGO and 1D/2D Bi 2 S 3 -N-rGO heterostructures hydrogels bestows further possibilities in relation to advancing high-performance energy storage devices.

Declarations
Conflict of interest Authors confirm that no conflict of interest to declare.
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