A binder free synthesis of 1D PANI and 2D MoS₂ nanostructured hybrid composite electrodes by the electrophoretic deposition (EPD) method for supercapacitor application

Abstract

A facile, binder-free approach is applied, along with the electrophoretic deposition (EPD) method, to fabricate large-scale, hybrid 2D MoS₂ nanosheets and 1D polyaniline (PANI) nanowires based electrodes on a conducting substrate for supercapacitor electrode material. The entire substrate surface is uniformly decorated by electrophoretically assembled MoS₂ 2D nanosheets and 1D nanowires of PANI, revealed by structural and morphological analysis. The electrochemical capacitive measurements of the MoS₂/PANI hybrid electrode exhibit a specific capacitance of ∼485 F g⁻¹ at a low charging–discharging current density (1 mA cm⁻²). The MoS₂ : PANI composition ratio was varied as 1 : 1, 1 : 2 and 1 : 3 to achieve high supercapacitive performance. The maximum supercapacitive performance (∼812 F g⁻¹) was obtained for a 1 : 2 ratio of MoS₂ and PANI, with high energy density (112 W h kg⁻¹) and power density (0.6 kW kg⁻¹). Synergistic interactions between conductive 1D PANI and 2D MoS₂ nanosheets with high surface area lead to a high supercapacitive performance. A binder approach to the direct synthesis of hybrid electrode by the EPD method eradicates the drawbacks offered by conventional electrodes prepared by the general slurry coating technique with resistive binders.

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1. Introduction

The investigation of clean and renewable energy materials is necessary because of the present confrontation by the energy crisis and environmental problems. Apart from the existing energy storage devices, supercapacitors are being studied extensively as energy-storage devices, due to their high power density, with moderate energy and long cycling life.^1,2 Electrochemical supercapacitors can deliver energy at higher rates than batteries and maintain their specific power for extended periods.^3,4 However, the low-energy density of supercapacitors, compared to batteries, limits their applications in devices that require high-energy density with high-power.^5,6 Accordingly, to offer supercapacitors with enhanced energy density, the innovation of new materials is essential. Mainly two kinds of supercapacitor electrode materials are used: electrochemical double layer capacitors (EDLCs) and pseudocapacitors (redox), based on their charge storage mechanism.^7,8 However, EDLC based materials, which mainly contain carbon materials, such as graphene, activated carbon, carbon nanotubes, etc., suffer from low energy density.⁹ On the other hand, pseudocapacitive materials possess a redox reaction that could allow fast and reversible surface faradaic reactions, and store more charges than double-layered capacitors, thus gaining much attention for energy dense supercapacitors.^9,10 To date, various transition metal hydroxides/oxides and conductive polymers with multiple oxidation states are used as pseudocapacitive materials for supercapacitors.¹¹

Instead of contemporary pseudocapacitive materials, in recent times 2D nanomaterials of transition metal di-chalcogenide (TMD) nanosheets have been gaining attention as excellent candidates for high performance supercapacitors, because of their intrinsic characteristics, including remarkable storage capacity/reversibility, and conductivity, compared to metal oxides/hydroxides.^12,13 A molybdenum disulfide (MoS₂) two-dimensional (2D) nanosheet, consisting of hexagonal sheets of molybdenum (Mo) sandwiched between two hexagonal sheets of sulfur (S), has recently attracted significant attention, due to its unique atomic structure analogues with graphene, and intrinsic electronic properties among all TMDs.^12–15 Higher intrinsic fast ionic conductivity and theoretical capacity than transition metal oxides and graphite, respectively, along with a range of oxidation states from +2 to +6, render MoS₂ a potential supercapacitor material. However, obstacles like poor electrical conductivity of MoS₂ 2D nanosheets restrict their application as high performance supercapacitor electrodes.^16,17 Efforts have been made to improve the capacitive performance of 2D MoS₂ based electrodes in supercapacitors by studying composites with other various conducting pseudocapacitive materials.^17–19

Pseudocapacitive polymer networks (comprising good conductivity) composed of MoS₂ nanosheets are an effective approach to addressing the conductivity issue. Conducting polymers (CPs) have been recognized as potential electrode materials and have attracted significant attention, due to their multiple redox states and other fascinating characteristics, like high accessible surface area, low resistance, high stability, etc.^20,21 Among the various CPs, polyaniline (PANI) has attracted considerable attention as a pseudocapacitive material, due to its high conductivity, redox reversibility, low cost and environmental compatibility.²¹ To date, numerous synthetic methodologies have been adopted to prepare hybrid 2D MoS₂/PANI or MoS₂/conducting polymer (PPy, PEDOT) composite material, using hydrothermal, chemical assembly techniques, etc., for energy storage applications.^22–28 The randomly mixed, sandwiched, self-assembled, and layer-by-layer assembled architectures of MoS₂/PANI composites have been utilized as advanced electrode materials for energy storage application.^22–28 Nevertheless, these chemically synthesized hybrid MoS₂/CPs based composites have been fabricated as supercapacitor electrodes by using the conventional slurry coating technique, which suffers from major shortcomings. In most of the works, those chemically assembled composite structures usually suffer from serious aggregation, and such aggregation of composites hinders the rapid diffusion of electrolyte and reduces the surface area, consequently decreasing the performance of the electrodes.²⁹ The weight and conductivity of electrode material are very significant features; usually, the conventional binder-enriched slurry-coating technique involves hefty resistive polymer binders that cause contact resistance between nanostructures or MoS₂/PANI and restrict easy charge transportation towards the current collector, in addition to the increased weight of the electrode, which do not contribute to the capacitance and translate to an effectively lower energy and power density.³⁰

Therefore, the rational combination of 2D MoS₂ nanosheets with 1D nanostructured conductive polymers (PANI) is expected to be an ideal way to fabricate supercapacitors with high performance. Based on the above considerations, the direct encasing (binder free) of 2D MoS₂/PANI on a conducting substrate (stainless steel) by using a facile electrophoretic deposition (EPD) through a colloidal solution of exfoliated 2D MoS₂ and 1D PANI in acetonitrile (ACN) is described and discussed in the present manuscript. Furthermore, the supercapacitive performances of MoS₂/PANI hybrid electrodes with different compositions of MoS₂ and PANI are investigated in 1 M Na₂SO₄, in terms of specific capacitance, energy and power density.

2. Experimental

2.1 Preparation of 2D MoS₂, PANI and MoS₂/PANI composite electrodes

The nanocomposites of the 2D MoS₂ nanosheets and 1D PANI nanowires were prepared by an electrophoretic deposition method on a stainless steel substrate (SS). Initially, a few layered MoS₂ nanosheets were prepared by the Li intercalation and ultra-sonication assisted exfoliation method reported by Coleman et al.³¹ Polyaniline was synthesized by the chemical oxidative polymerization of aniline monomer using ammonium peroxydisulfate (APS) at a molar ratio of 1 : 1. The polymerization was carried out by dissolving aniline (4.6 ml) in 1 M HCl solution (50 ml) with constant stirring at room temperature; separate 50 ml aqueous solutions of APS (11.4 g) were prepared. Prior to mixing, both the reactants were pre-cooled in an ice-bath. Aqueous APS solution was slowly added (drop wise) to the prepared aniline solution with constant stirring in the cooling state for 4 h. The obtained PANI was filtered and washed with distilled water and dried at room temperature in a vacuum oven.

A stock solution of the synthesized 1D PANI nanowires and 2D MoS₂ nanosheets was prepared in ACN solution as 1 mg ml⁻¹, via washing and dispersing by ultra-sonication for 1 h. The colloidal solution of the hybrid MoS₂/PANI precursor was prepared by mixing the solutions and sonicating for 1 h. The large-scale fabrication of hybrid MoS₂, PANI and MoS₂/PANI electrodes was carried out by applying constant DC voltage (∼32 V) to the conducting substrate (stainless steel substrate, SS) against a graphite electrode in a colloidal suspension of 2D MoS₂, PANI and mixed MoS₂/PANI composites in ACN solution. Different compositions of MoS₂/PANI were prepared by varying the volume ratio of solutions by 1 : 1, 1 : 2, 1 : 3 (MoS₂ : PANI); the produced nanocomposites were named MoS₂/PANI-n (n = 1, 2, 3). The EPD deposition of MoS₂/PANI onto SS (2 cm × 4 cm) was carried out for 20 minutes and as-deposited hybrid electrodes were dried at room temperature (∼25 °C) for further supercapacitive performance testing.

2.2 Materials characterization

The electrode materials were structurally characterized by XRD, XPS, Raman and FESEM measurements. The X-ray diffraction (XRD) was carried out on a Rigaku Ultima diffractometer, using Cu-Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a thermo scientific ESCALAB 250 instrument (Thermo Fisher Scientific, UK). Raman spectra were recorded at ambient temperature on a WITeck CRM200 confocal microscopy Raman system, with 488 nm wavelength laser. The morphology of the composite was examined by field-emission scanning electron microscopy (FESEM, JSM-7001F, JEOL). The supercapacitive performance was determined by forming a three electrode system, with platinum (Pt) as the counter electrode, MoS₂/PANI on SS as the working electrode and Ag/AgCl as the reference electrode in 1 M Na₂SO₄ electrolyte. Cyclic voltammetry (CV), galvanostatic discharge tests and electrochemical impedance spectroscopy (EIS) were performed using ZIVE SP2 LAB analytical equipment (South Korea).

3. Results and discussion

3.1 Electrophoretic deposition of MoS₂/PANI on the conducting SS substrate

The EPD method is a highly efficient and facile electrochemical method for electrostatically encasing nanostructures on a conducting substrate.^32,33 Mixed colloidal suspensions of 2D MoS₂ and 1D PANI in ACN solution were used as precursors for hybrid 1D PANI and layered 2D MoS₂ nanosheet films by an electrophoretic deposition (EPD) method. A simple addition of exfoliated colloids of 1D PANI into exfoliated MoS₂ nanosheets in ACN was used to obtain homogeneously mixed colloidal suspensions of PANI and MoS₂ nanosheets. The actual photographs, shown in Fig. 1, reveal good exfoliation of 2D MoS₂ nanosheets and 1D PANI nanowires, dispersed in ACN. The colloidal suspension of 2D MoS₂ nanosheets and 1D PANI in ACN solutions was confirmed by distorted laser through the MoS₂ and PANI dispersed solution. Furthermore, the surface charges of colloidal MoS₂ and PANI suspensions were examined with zeta (ζ) potential measurements. The zeta (ζ) potentials of ∼ −11.6 mV and ∼3.1 mV were obtained for 2D MoS₂ and 1D PANI, respectively (shown in Fig. S1, see ESI†). The oppositely charged 1D PANI (positive) and MoS₂ (negative) nanosheets can form a coherent interaction. Interestingly, appositely charged nanostructures form colloidal suspension of mixed nanostructures and do not lead to composite aggregation. The actual photograph of the distorted laser beam through MoS₂/PANI (shown in Fig. 1), signifies the formation of a colloidal suspension. Surprisingly, the colloidal suspension of the MoS₂/PANI mixed solution reveals a zeta (ζ) potential of about −8.6 mV (Fig. S1, see ESI†), which is less than for bare MoS₂ sheets. This result suggests that PANI nanowires with less positive charge encase the negatively charged, large MoS₂ nanosheets and result in the formation of a suspension of less negatively charged composite material. The negative charge of the composite provides strong evidence for the homogeneous mixing of these species in mixed colloidal suspension. Further, a prepared, mixed colloidal suspension of MoS₂ and PANI was used as the precursor for the fabrication of the MoS₂/PANI hybrid material on the SS electrode by EPD deposition (schematic diagram is shown in Fig. 1). The dark brown and green colored films of hybrid MoS₂/PANI, PANI, and MoS₂ materials were obtained with a uniform surface coating over the SS substrate by applying ∼32 V DC voltage for 20 min. The weights of the deposited MoS₂/PANI on the SS substrate were measured by the weight difference method, using a sensitive microbalance; the measured weight of active materials on the SS substrate were found to be in the range of ∼0.28 to ∼0.17 mg cm⁻² (the graph of active material weights with different compositions of MoS₂/PANI is shown in Fig. S2 (see ESI†)). The actual photographs of PANI, MoS₂ and MoS₂/PANI hybrid electrodes are shown in Fig. 1 and signify that the EPD method is uniquely facile and advantageous, in terms of large area deposition over different kinds of conducting substrates with complex sizes and shapes.

Fig. 1 Schematic diagram of the electrophoretic deposition (EPD) process of the hybrid MoS₂/PANI electrode.

3.2 Morphological and elemental analysis

The FE-SEM micrographs of MoS₂-SS, PANI-SS and MoS₂/PANI-SS electrodes are shown in Fig. 2(a)–(i) at different magnifications. The fibrous network of PANI coated on SS substrate can be clearly seen in the SEM image shown in Fig. 2(a) and (c). The SEM images shown at high magnification confirm that the fibrous network of PANI consists of interconnected 1D nanowires with size of about 80 nm in diameter (shown in Fig. 2(c)). Fig. 2(d)–(f) show that the 2D nanosheet-like structure of MoS₂ is well decorated over the entire substrate surface. At high magnification, the SEM image of the MoS₂ electrode reveals the formation of a layered structure of MoS₂ nanosheets (shown in Fig. 2(f)). The MoS₂ nanosheets are laid down on the SS substrate surface with size of around a few micrometers. The SEM images of the MoS₂/PANI composite electrode are shown in Fig. 2(g)–(i). Low magnification SEM images (Fig. 2(g) reveal the uniform coating of the 1D PANI nanowires with the MoS₂ 2D nanosheets over the stainless steel substrate. At higher magnification SEM, from the image shown in Fig. 2(h), one can see that the 1D PANI nanowires are well decorated, anchored and composed of the 2D MoS₂ nanosheets. A porous structure with small voids is generated, due to the encasement of the MoS₂ nanosheets by the PANI nanowires (shown in Fig. 2(i)); this porous structure can lead to the enhancement of supercapacitive performance. Furthermore, SEM-EDS mapping analysis, shown in Fig. 3, was carried out to find particulate deposited elements and the interconnection of MoS₂ and PANI on the SS surface (EDS mapping of bare MoS₂ and PANI electrodes provided in Fig. S3†). At low magnification EDS analysis, the images reveal the complete decoration with uniform Mo, S, C, O and N elements, confirming the attachment of PANI and MoS₂ on the SS substrate. The EDS mapping analysis confirms the excellent interconnection between the 2D MoS₂ nanosheets and PANI 1D nanowires. The layered structure of the 2D nanosheets of MoS₂ and 1D nanowires of PANI synergistically generated the porous structure, which is beneficial in energy storage devices, in terms of the easy intercalation of electrolytic ions and charge transport.³⁴

Fig. 2 FE-SEM images of (a–c) MoS₂, (d–f) PANI and (g–i) hybrid MoS₂/PANI electrodes at different magnifications.

Fig. 3 The EDS mapping analysis of the hybrid MoS₂/PANI surface on the SS substrate.

3.3 Structural studies (XRD, XPS and Raman)

The crystallinity of EPD deposited thin films was investigated using X-ray diffraction. The XRD patterns of MoS₂, PANI and MoS₂/PANI films on the SS substrate are shown in Fig. 4. The peaks revealed at ∼43°, 50°, 74° and 90° originated from the SS substrate (marked as ‘*’). The bare MoS₂-SS electrode shows two significant diffraction peaks (2θ) at ∼14.5° and ∼16.8°, which are attributed to the (002) and (003) reflections from hexagonal MoS₂, (marked as ‘O’), and correspond to the c-axis growth of MoS₂ single crystals.³⁵ The reflection in the vicinities of 2θ at 14–17° (∼5–7 Å) is very broad and strong, which signifies a layered MoS₂ structure, probably due to electrochemical restacking.³⁶ The XRD pattern of the PANI-SS electrode exhibits only peaks originating from the SS substrate, without any significant diffraction peak from PANI, confirming the formation of amorphous PANI.³⁷ Moreover, the XRD pattern of the hybrid MoS₂/PANI-SS electrode reveals only two characteristic peaks, which correspond to the diffraction planes of MoS₂ with lowered intensity and confirms the encasing of amorphous PANI over the layered MoS₂ composite on the SS substrate. The XRD results confirm the formation of the electrophoretically assembled layered structure of 2D MoS₂ nanosheets and the amorphous PANI composite.

Fig. 4 The XRD patterns of MoS₂, PANI and hybrid MoS₂/PANI coated SS electrodes.

The Raman spectra of PANI, MoS₂ and the MoS₂/PANI composite, within the range of 100–2000 cm⁻¹, were studied for composite confirmation and are shown in Fig. 5. The observed peaks in the Raman spectra, indicated as “o” and “x” in the graphs, belong to MoS₂ and PANI, respectively. The MoS₂ nanosheets reveal sharp peaks at ∼382 and ∼405 cm⁻¹, and the obtained doublet confirms the layered structure.³⁸ On the other hand, the PANI sample reveals mainly four broad characteristic peaks at ∼1167, 1338, 1491 and ∼1600 cm⁻¹.^39–41 Bands near 1600 cm⁻¹ are attributed to the benzenoid C–C ring stretching vibration and the quinoid C–C stretching mode of the polymer chain. The bands near 1491 cm⁻¹ are strengthened and new bands appear on the high/low wavenumber side for each of these bands in the spectra of the polyaniline salts. The semibenzenoid polaronic (C–N⁺) broad band near 1338 cm⁻¹, and the band of the C–H in plane bending vibration can be observed below 1167 cm⁻¹. Several other less intense bands appear in the Raman spectra of PANI; due to the difference in the extent of doping, the wavenumber of the C–C stretching vibration varies. Thus, two and four distinct peaks originating from MoS₂ and PANI, respectively, are attributed to the formation of the composite, suggesting that the layered structure of MoS₂ is maintained in the composite.

Fig. 5 Raman spectra of MoS₂, PANI, and hybrid MoS₂/PANI over the SS electrode.

X-ray photoelectron spectra of MoS₂, PANI and MoS₂/PANI samples were obtained. The atomic spectra of carbon (C 1s), nitrogen (N 1s), molybdenum (Mo 3d) and sulfur (S 2p) obtained from the MoS₂/PANI sample are shown in Fig. 6. The C (1s) spectrum shown in Fig. 6(a) has four components: C–C/C–H (283.5), C–N/C=N (284.75 eV), C–O (285.82 eV) and C=O (288 eV).⁴² The N (1s) spectrum can be deconvoluted into two peaks, as shown in Fig. 6(b), attributed to the major benzenoid-amine component (–NH–) peak at 394.63 eV, along with a small shoulder peak corresponding to the quinoid-imine (–NH⁺) at 397.92 eV.²¹ The XPS spectra for Mo (3d), shown in Fig. 6(c), reveals the 1T, 2H phase and partial oxidation of MoS₂. As for the 2H-phase, Mo (3d_5/2) and Mo (3d_3/2) peaks are observed at 229.38 and 232.72 eV, respectively, and are ascribed to the presence of the Mo⁴⁺ chemical state of MoS₂ formation. Furthermore, the peak at binding energy of 226.22 eV, corresponding Mo⁶⁺ (3d_5/2), can be ascribed to the partial oxidation during lithium intercalation during the synthesis process. Also, the additional two peaks of the 1T phase at 228.63 eV and 231.94 eV suggest that MoS₂ is exfoliated to monolayers, with a small S (2s) peak located at 226 eV.⁴³ The peaks at 161.8 and 163.1 eV, corresponding to the S (2p_3/2) and S (2p_1/2) of divalent sulfide ions (S²⁻), are shown in Fig. 6(d).^43,44 These binding energies are all consistent with the reported values for MoS₂ and PANI material, and the XPS spectra indicate the formation of the MoS₂/PANI composite. XRD, Raman, and XPS studies therefore confirm the formation of the layered MoS₂ nanosheets and the amorphous PANI composite over the SS substrate.

Fig. 6 The core level XPS spectra of (a) C (1s), (b) N (1s), (c) Mo (3d) and (d) S (2p) for the EPD deposited, hybrid MoS₂/PANI coated SS electrodes.

3.4 Supercapacitive performance

The individual electrochemical performances of MoS₂-SS, PANI-SS and hybrid MoS₂/PANI-SS electrodes were investigated by forming a three electrode electrochemical half-test cell. To illustrate the capacitive behavior and quantify the specific capacitance of the prepared electrode material, cyclic voltammetry (CV) analysis is a proficient tool. The comparative CV curves of MoS₂, PANI and MoS₂/PANI hybrid electrodes measured at 20 mV s⁻¹ scan rate, are shown in Fig. 7(a). The CV curves of MoS₂ and hybrid MoS₂/PANI electrodes exhibit two intense redox peaks that arise from the reversible faradaic reaction in 1 M Na₂SO₄ electrolyte. However, the PANI electrode shows much less current under the curve than the other electrodes, and the shapes of the all CV curves are not close to the ideal rectangular shape from EDLC, which clearly indicates the pseudocapacitive behavior of the electrodes, based on a redox mechanism.⁴⁵ The pseudocapacitance, based on the redox reaction of ions involved in the aqueous electrolyte, can be stated as the following reaction:

MoS₂ + xNa⁺ + ye⁻ ↔ (MoS_2−x)^y− (1)

Fig. 7 (a) Comparative CV curves of MoS₂/PANI, PANI and MoS₂ electrodes within the optimized potential window of 0.0 to 1.0 V, in aqueous 1 M Na₂SO₄ at a 20 mV s⁻¹ scan rate. Electrochemical studies of the hybrid MoS₂/PANI electrodes with different composite ratios (1 : 1, 1 : 2, 1 : 3). (b) CV curves, (c) galvanostatic discharge curves (GDC) of MoS₂/PANI-1, MoS₂/PANI-2 and MoS₂/PANI-3 electrodes.

The higher current under the curve and maximum area with prominent redox peaks for hybrid MoS₂/PANI-SS electrode can be seen from Fig. 7(a), compared to bare MoS₂-SS and PANI-SS electrodes. The resulting maximum pseudocapacitive charge storage is due to the more porous, conducting, and higher specific area access offered by the intercalated hybrid PANI nanowire in the MoS₂ layered nanosheets framework. To obtain the optimized composition, electrodes were made with different compositions of MoS₂ and PANI, in volume ratios of 1 : 1, 1 : 2 and 1 : 3 (noted as MoS₂/PANI-1, MoS₂/PANI-2 and MoS₂/PANI-3, respectively), and tested by cyclic voltammetry at a scan rate of 20 mV s⁻¹ (shown in Fig. 7(b)). Interestingly, the MoS₂/PANI-2 electrode shows an excellent, symmetric redox CV curve with high current under the curve, compared to the other counterparts. Fig. 7(c) shows galvanostatic discharge curve (GDC) plots for MoS₂/PANI-1, MoS₂/PANI-2 and MoS₂/PANI-3 electrodes at 1 mA cm⁻² constant current density. The non-linear shape of the GDC curves are distinct from the general linear discharge characteristic of the double-layer capacitor (EDLC), which corresponds to the results of the CV test. The MoS₂/PANI-2 electrode shows an extended discharge curve, which is analogous to the CV results. The calculated maximum specific capacitances, from the GDC plots at 1 mA cm⁻² are found to be 485, 812 and 247 F g⁻¹ for the MoS₂/PANI-1, MoS₂/PANI-2 and MoS₂/PANI-3 electrodes, respectively. The MoS₂/PANI-2 electrode shows higher capacitance than that of other MoS₂/PANI-1 and MoS₂/PANI-3 electrodes, and this result concludes that MoS₂/PANI in the 1 : 2 ratio is an appropriate combination for supercapacitor electrodes.

Furthermore, the capacitive behavior of the MoS₂/PANI-2 electrode at different scan rates and discharge curves are evaluated and presented in Fig. 8. The CV curves at different scan rates, ranging from 5 to 200 mV s⁻¹, for the hybrid MoS₂/PANI-2 electrode in aqueous 1 M Na₂SO₄ electrolyte, are shown in Fig. 8(a). The current under the curve increases with the scan rate; i.e., the voltammetry current is directly proportional to the scan rates of CV, and that specifies the capacitive behavior of the electrode.⁴⁶ Furthermore, the shapes of the CV curves do not change with scan rate, suggesting that the electrochemical activity of the electrode is stable throughout the high frequency as well. Similarly, MoS₂/PANI-1 and MoS₂/PANI-3 electrodes show increases in their current densities with increase in scan rates (Fig. S4, see ESI†). Fig. 8(b) shows the galvanostatic discharge curves of the hybrid MoS₂/PANI-2 electrode at different constant current densities from 1 to 6 mA cm⁻². The graph of the calculated specific capacitance with different charging current densities shown in Fig. 8(c) reveals that the specific capacitance decreases with the increase in current densities, which is indicative of less active material access at a higher scan rate, which reduces the effective utilization of material.⁴⁷ The maximum specific capacitance of ∼812 F g⁻¹ is observed for the hybrid MoS₂/PANI-2 electrode at the low current density of 1 mA cm⁻².

Fig. 8 (a) Scan rate dependent (5–200 mV s⁻¹) CV curves of MoS₂/PANI electrodes. (b) Galvanostatic discharge curve (GDC) plots for the MoS₂/PANI electrode within the potential window of 0.0 to 1.0 V, at constant charging current from 1 to 6 mA cm⁻². (c) Graph of specific capacitance against the constant discharging current densities.

For detailed insight into the electrode capacitive behavior with different compositions of MoS₂ and PANI, the electrochemical impedance spectra (EIS) were carried out and are presented in Fig. 9(a). For all three electrodes, EIS measurements were carried out at open circuit potential (OCP) over the frequency range of 10 MHz to 0.01 Hz. All electrodes show a sharp increase of the imaginary part of the EIS at lower frequencies, due to the capacitive behavior of the electrode, with a semi-circular loop at higher frequencies emanating from charge-transfer resistance.⁴⁸ The ideal impedance plot, with vertical line parallel to the imaginary axis, is generally observed for carbon-based materials, such as activated carbon, graphite, CNTs, and graphene. On the other hand, pseudocapacitive materials show inclined impedance with initial semicircles, due to the diffusion and charge transfer resistance (R_ct). The “R_ct” can be calculated from the radius of the initial curvature at higher frequencies⁴⁹ and “R_ct” for the hybrid MoS₂/PANI-2 and MoS₂/PANI-3 electrode is negligible, compared to that of the MoS₂/PANI-1 (3.5 Ω) electrode. The high charge transfer resistance and more inclined impedance line, due to the Warburg diffusion resistance of the bare MoS₂/PANI-1 electrode, restricts its achieving maximum specific capacitance. Interestingly, the electrochemical series resistance (ESR) for MoS₂/PANI electrodes increases with an increase in the PANI amount from 1.8 to 4 Ω. Accordingly, the MoS₂/PANI-2 electrode shows moderate ESR and negligible “R_ct”, leading to the achievement of maximum specific capacitance. The hybrid MoS₂/PANI-2 electrode reveals that high specific capacitance and significant electrochemical properties call for further supercapacitive studies.

Fig. 9 (a) Electrochemical impedance spectrum within the 1 MHz to 10 mHz frequency region, with Nyquist plots for MoS₂/PANI-1, MoS₂/PANI-2 and MoS₂/PANI-3 electrodes. (b) Ragone plot of energy density and power density for the MoS₂/PANI-2 electrode.

A typical Ragone plot of obtained energy and power density for the MoS₂/PANI-2 electrode is shown in Fig. 9(b), which reveals that the maximum energy density (112 W h kg⁻¹) with moderate power density (0.6 kW kg⁻¹) is obtained in the present work. As demonstrated here, the MoS₂/PANI electrode is uniquely advantageous to serve high energy demand. A rough comparison of the present work, with electrodes based on MoS₂/PANI or bare MoS₂ or the composites with other polymers (PPy) fabricated using the conventional binder enriched process, such as the slurry coating technique, is given in Table 1. The 2D MoS₂ and 1D PANI nanowire on SS substrate deposited by the facile EPD method, synergistically exhibits higher supercapacitive performance than some recent reports.^22–28 Such binder enriched electrode processes hinder the supercapacitive performance of the electrodes, as discussed in the introduction. In such a structure, 1D PANI serves as the charge transporter with high surface area to improve the electronic conductivity and utilization of MoS₂ nanosheets as schematically shown in the inset of Fig. 9(b). Herein, we present the effective utilization of 1D PANI nanowires and 2D MoS₂ nanosheets as supercapacitor hybrid electrodes, fabricated by using a binder-free approach (electrophoretic deposition), demonstrating that the 2D layered MoS₂ nanosheets and 1D PANI nanowires composite can be promising electrode materials for energy storage devices.

Table 1 Comparative supercapacitive performance of MoS₂ and conductive polymer composite based electrodes, prepared with and without the binder free approach

Materials	Synthesis methods	Morphology	Cell type	Specific capacitance/electrolyte	Stability	Ref.
MoS2-PPY	Chemical method	Nano plate	Two electrode system	695 F g^−1 (0.5 A g^−1) (KCl)	4000 cycles (85%)	23
MoS2-PANI	Chemical method	Intercalation composite	Three electrode system (Ag/AgCl–Pt)	390 F g^−1 (2.5 mA cm^−2) (Na2SO4)	1000 cycles (60%)	24
MoS2-PANI	Chemical method	Nano needle	Two electrode system	853 F g^−1 (1 A g^−1) (H2SO4)	4000 cycles (83%)	25
MoS2-PEDOT	Chemical bath method	Nano ribbon	Three electrode system (SCE–Pt wire)	405 F g^−1 (1 A g^−1) (H2SO4)	1000 cycles (90%)	28
MoS2-PANI/stainless steel	Electrophoretic deposition method	1D and 2D nanostructured composite	Three electrode system (Ag/AgCl–Pt wire)	812 F g^−1 (1 mA cm^−2) (Na2SO4)	—	This work

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4. Conclusions

A facile strategy has been developed for the large-scale production of binder-free MoS₂/PANI hybrid architectures, using the EPD method, constructed from 2D MoS₂ nanosheets and 1D PANI nanowires. The improved supercapacitive performance of the hybrid MoS₂/PANI emanates from the synergistic cooperation between 1D PANI nanowires and 2D MoS₂ nanosheets, leading to a high specific capacitance of about 812 F g⁻¹. Moreover, the MoS₂/PANI electrodes in aqueous 1 M Na₂SO₄ electrolyte present a high energy density (112 W h kg⁻¹) with decent power density. Such unique multi-dimensional networks of hybrid MoS₂/PANI architectures provide the diffusion path for electrons and ions and allow for efficient charge storage with facilitated charge transport. Thus, the simple, binder free EPD synthetic approach may provide a convenient route for the preparation of large-scale hybrid MoS₂/PANI material electrodes. The enhanced capacitive performance opens up further scope in symmetric, asymmetric, non-aqueous and solid state supercapacitive devices as per the requirements of applications (high energy or power).

Acknowledgements

This work was partially supported by the Priority Research Centers Program (2009-0093823), the Korean Government (MSIP) (No. 2015R1A5A1037668) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), the ICT R&D program of MSIP/IITP (R0101-15-0034), Industrial Strategic Technology Development Program through the Ministry of Trade, Industry and Energy (MOTIE, Korea) (2MR4090), Korea Research Fellowship Program (2015-11-1063) funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea, and the Yonsei University Future-leading Research Initiative.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16078f

‡ These authors contributed equally.

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