High-performance freestanding supercapacitor electrode based on polypyrrole coated nickel cobalt sulfide nanostructures

In the present work, we report the successful fabrication of dandelion-like Nickel–Cobalt Sulfide@Polypyrrole microspheres through the hydrothermal method and its possible application as a binder-free electrode in supercapacitors. This electrode exhibited low charge transfer resistance with a remarkable specific capacitance of 2554.9 F g−1 at 2.54 A g−1, in addition to considerable cycle life stability. Also, an asymmetric device was prepared using NiCo2S4@PPy/NF as positive and rGO/NF as negative electrodes. This asymmetric supercapacitor exhibited a specific capacitance of 98.9 F g−1 at 1.84 A g−1 and delivered an energy density of 35.17 Wh kg−1 at a power density of 1472 W kg−1. Such a remarkable performance can be originated from the synergy effect of NiCo2S4 and PPy and the direct deposition of the composite on the current collector. Our findings suggest the dandelion-like NiCo2S4@PPy as a promising material for making high-performance supercapacitors.


Synthesis of polypyrrole.
was prepared by the chemical oxidative polymerization method. In this method, the monomer was pyrrole, and ammonium persulfate (APS) was used as the oxidizing agent. First, 1.08 mL of pyrrole was dissolved in 100 mL of 1 M hydrochloric acid (HCl) while stirring the solution and placing it in an ice bath (Solution A). In another container, 0.819 mg of ammonium persulfate was dissolved in 100 mL of 1 M HCl, using ultrasonic (solution B). Prior to production of PPy, solution B was placed in an ice bath. In the next step, solution B was quickly and without stirring added to solution A, and the final solution was kept in an ice bath for 3 h until the completion of the polymerization process. The product (black precipitate) was gathered and washed with deionized water and ethanol rigorously and finally dried at room temperature.
Synthesis of nanomaterial NiCo 2 S 4 @PPy. To synthesize NiCo 2 S 4 @PPy nanomaterial on the NF substrate, 5 mg of the as-prepared PPy was dispersed in 50 mL of deionized water. Then 15 mg of NiCo 2 O 4 and 52.3 mg of sodium sulfide monohydrate (Na 2 S.9H 2 O) were added (Solution C). A piece of nickel foam and solution C were transferred into a Teflon-lined stainless steel autoclave and heated at 145 °C for 24 h. After naturally cooling to room temperature, NiCo 2 S 4 @PPy/NF was thoroughly washed with deionized water, and then dried at room temperature.
Characterization. The morphology of the electrode materials was analyzed by scanning electron microscopy with an energy dispersive X-ray accessory (SEM-EDX, VEGA3 TESCAN), in addition to transmission electron microscopy (TEM, Hitachi 200 kV). Phase analysis was studied by the X-ray diffraction method (XRD, D8-advance, Brucker). The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a VG-Microtech Multilab 3000 instrument. Fourier transform infrared spectroscopy (FTIR) was carried out using a Bruker Vector-22 instrument. Porosimetry investigations were performed by N 2 adsorption/desorption using a Belsorp-BELMAX. The pore volumes and surface area of the nanomaterials were measured using the Brauner-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) equations. Furthermore, the Raman spectra were studied using an Ar ion CW laser (1064 nm) and a Raman spectrometer (Model: Rigako, Japan).
Electrochemical measurements. All the electrochemical tests were conducted using an Autolab30 electrochemical workstation (Eco Chemie, the Netherlands) and a Zahner/Zennium (Zahner, Germany) at room temperature. The impedance (EIS) measurements were carried out at the open-circuit potential in 100 kHz to 100 mHz range of frequencies with an AC voltage signal of 10 mV. A conventional three-electrode cell was used for the electrochemical studies of NiCo 2 S 4 @PPy/NF in a 3 M KOH solution. Also, NiCo 2 S 4 @PPy/NF was used as the working electrode, a platinum electrode, and an Ag/AgCl, KCl (sat′d) were employed as the counter and reference electrodes, respectively. Then, active electrode materials were placed on the nickel foam (1 × 1 cm 2 ) by hydrothermal method and used as working electrode. An asymmetric supercapacitor (ASC) was designed with NiCo 2 S 4 @PPy/NF as the positive electrode and, rGO coated on nickel foam (rGO/NF) as the negative electrode with a filter paper separator containing 3 M of KOH electrolyte (denoted as NiCo 2 S 4 @PPy/NF//rGO/ NF). The electrochemical behaviors of NiCo 2 S 4 @PPy in a three-electrode configuration and the ASC device were investigated by cyclic voltammetry (CV), galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy (EIS).

Results and discussion
Characterizations. The fabrication steps of a NiCo 2 S 4 @PPy composite electrode are shown in Fig. 1. In the first step, NiCo 2 S 4 was prepared via the hydrothermal process which ended with a calcination treatment. Simultaneously, Polypyrrole (PPy) was prepared by the chemical oxidative polymerization method. Then, the NiCo 2 S 4 @PPy nanomaterial was deposited on the Ni foam substrate through a facile hydrothermal method.
The morphology of the prepared NiCo 2 S 4 and NiCo 2 S 4 @PPy were probed using the SEM and TEM techniques, and the corresponding results are shown in Figs. 2 and 3. As seen, the SEM image of NiCo 2 S 4 ( Fig. 2a) reveals that the NiCo 2 S 4 microspheres uniformly covered the NF surface. Also, this image shows that the NiCo 2 S 4 flower-like microsphere actually appears in a porous structure with empty spaces between its nanowires and nanoflakes. This structure can facilitate the diffusion of the electrolyte ions to the deeper active sites of the NiCo 2 S 4 nanomaterial and the increased real surface area.
Also, Fig. 2b depicts a typical SEM image of the NiCo 2 S 4 @PPy nanomaterial. As seen in Fig. 2b, the original morphology of NiCo 2 S 4 is maintained after the formation of NiCo 2 S 4 @PPy on the NF surface. As seen in Figs. 2a and 3b, the average radius of the microsphere is smaller than 4 µm in NiCo 2 S 4 and is increased to almost 5 µm in the NiCo 2 S 4 @PPy nanomaterial. Additionally, it can be concluded that the highly accessible surface of NiCo 2 S 4 @ PPy enhances the penetration of the electrolyte ions into the electrode material. In other words, increased accessible surface area and ease of ion and electron diffusion in the porous structure can result in the improved capacitive performance of NiCo 2 S 4 @PPy. Shown in Fig. 3 are TEM images recorded to check the structural and morphological characteristics of NiCo 2 S 4 and NiCo 2 S 4 @PPy. Figure 3a represents the hierarchical structure of NiCo 2 S 4 appearing as many nanowires with an average thickness of almost 15 nm. Also, a higher resolution TEM image of the NiCo 2 S 4 nanowires is provided in Fig. 3b. It is obvious that the nanowire structure is relatively crystalline and its length may be longer than 100 nm. This is in excellent agreement with the SEM results in which NiCo 2 S 4 nanospheres consist of many integrated nanowires (Fig. 2a). During the synthesis of the NiCo 2 S 4 @PPy nanomaterial, it is expected to see the enlargement of the radius of the NiCo 2 S 4 nanowires when they are wrapped with a shell layer of PPy. Figure 3c,d   www.nature.com/scientificreports/ illustrate the NiCo 2 S 4 @PPy nanowires with an amorphous shell of polypyrrole in which the average thickness of the nanowires is almost 20 nm (Fig. 3d). It is noteworthy that the TEM images of NiCo 2 S 4 @PPy support the SEM observations indicate the enlargement of the nanowires when they are covered by PPy. The X-ray diffraction technique was employed to study the structure of NiCo 2 S 4 and NiCo 2 S 4 @PPy and their corresponding results are shown in Fig. 4. As seen in Fig. 4a, a typical XRD plot of NiCo 2 S 4 is displayed in which the diffraction peaks located at 16.5°, 26.8 ∘ , 31.5 ∘ , 38.1 ∘ , 50.4 ∘ , 55.2 ∘ , 58.4, 62.5 ∘ , 65 ∘ and, 68.5 ∘ can be assigned to the respective (111), (220), (311), (400), (511), (440),(531), (620), (533) and (444) planes of the cubic phase of NiCo 2 S 4 (JCPDS 20-0782) 1, [33][34][35] . Also, the NiCo 2 S 4 @PPy composite (Fig. 4b) displays a similar XRD pattern to that of nickel-cobalt sulfide. This is probably due to the thin layer of the amorphous structure of PPy ( Fig. 4c) which does not yield significant characteristic reflection peaks in the recorded pattern of NiCo 2 S 4 @PPy 36 . Thermal gravimetric analysis (TGA) was employed to check the thermal stability of the pure PPy. Figure Sx depicts the TGA curve of pure PPy. Also, the TGA results display three stages of weight loss in the mentioned range. The evaporation of water molecules from the polymer structure is responsible for the first step of weight loss (5.97 percent) up to 150 °C. At 250 °C, the sample loses a significant amount of weight because of the initialization of the PPy structure breakdown. In the third step of weight loss, from 250 to 900 °C, the PPy backbone decomposes completely [37][38][39] .
Moreover, to check the functional groups of the synthesized nanostructure, FT-IR analysis was performed and the corresponding FT-IR spectrum is presented in Fig. 5a. As seen in this figure, the peaks appear at 519.21, 792.85 cm −1 (symmetrical stretching) and 1107 cm −1 (asymmetrical stretching) are attributed to the Ni-S or Co-S vibrations of NiCo 2 S 4 40 . The peak at 934.02 cm −1 could be attributed to the C-H deformation vibration in the -CH=CH − group 41 . In addition, the broad band located at 3411 cm −1 can be assigned to the N-H stretching vibration in the pyrrole rings 42 . To further investigate the nanomaterial structure, Raman analysis was performed on the NiCo 2 S 4 @PPy samples (Fig. S1). The Raman spectrum recorded for pure PPy shows two peaks at 1552 cm −1 and 1340 cm −1 . These peaks can be assigned to the p-conjugated structure and the stretching mode of the ring of the polymer, respectively. Also, a peak appears at 1037 cm −1 which may be attributed to the C-H in-plane deformation. The Raman spectrum of NiCo 2 S 4 @PPy shows clear peaks of PPy, which can be observed at 1552, 1340 and 1037 cm −1 . Additionally, the peaks that appear at 516.3 and 668 cm −1 are the characteristic peaks for the F2g and A1g modes of NiCo 2 S 4 43,44 . X-ray photoelectron spectroscopy (XPS) studies were implemented to obtain a more detailed elemental analysis and the chemical state of the NiCo 2 S 4 @PPy nanomaterial. The survey spectrum of the NiCo 2 S 4 @PPy  Figure 5c-g represent the typical fitted Ni 2p, Co 2p, S 2p, N 2p and, C 1 s peaks of NiCo 2 S 4 @PPy. The XPS spectrum of Ni 2p and Co 2p can be fitted with two spin-orbits and two satellites (shake-up, shows as "sat"). As seen in Fig. 5c, the fitting peaks at 856.39 eV and 873.58 eV can be indexed to Ni 2p 3/2 and Ni 2p 1/2 , respectively 45 . Furthermore, Fig. 5d demonstrates the Co 2p XPS spectrum. The fitting peaks corresponding to 780.7 and 796.2 eV are correlated to Co 2p 3/2 and Co 2p 1/2 , respectively 46,47 . Also, the S 2p XPS spectrum is shown in Fig. 5e. The peaks that appear at 161.5 eV and 163.4 eV are matched to S 2P 1/2 and S 2p 3/2 , respectively 48,49 . Based on the XPS analysis, the chemical composition of NiCo 2 S 4 @PPy contains Ni 2+ , Ni 3+ , Co 2+ , Co 3+ and, S 2− , confirming the previously reported results 33,50 . The N 1 s XPS plot can be deconvoluted into three components, as displayed in Fig. 5f. The fitting peak at the binding energies of 398. Electrochemical studies. To investigate the electrochemical behavior of the NiCo 2 S 4 @PPy nanomaterial, cyclic voltammetry, galvanostatic charge/discharge and, electrochemical impedance spectroscopy were carried out in a three-electrode cell in an alkaline solution of 3 M of KOH. Also, NiCo 2 S 4 @ PPy/NF, Pt and an Ag/AgCl, KCl (sat′d) were used as working, counter and reference electrodes, respectively. Figure 6a exhibits a comparison of the voltammograms for bare NF, NiCo 2 S 4 /NF, and NiCo 2 S 4 @PPy/NF electrodes at a scan rate of 50 mV s −1 within a potential range of -0.2 to 0.6 V. As seen in the Fig. 6a, a pair of redox peaks was appeared for NiCo 2 S 4 /NF and NiCo 2 S 4 @PPy/NF electrodes due to the Faradaic charge storage mechanism for the electrodes. These peaks may be attributed to the reversible redox processes of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ /Co 4+ as follows: As expected, the NiCo 2 S 4 @PPy/NF electrode exhibits much higher specific capacitance than both NiCo 2 S 4 / NF and bare NF electrodes. This fact was also confirmed by GCD measurements (Fig. 6b). The obvious deviation from linear discharge that appears as a plateau, further confirms the Faradaic behavior of the electrode materials. Moreover, the longer discharge time approves the higher specific capacitance for the NiCo 2 S 4 @PPy/NF electrode. This is in a good agreement with voltammetry results. Figure 6c depicts the typical cyclic voltammetry curves of the NiCo 2 S 4 @PPy/NF electrode at various scan rates ranging from 5 up to 100 mV s −1 within a potential range of − 0.2 to 0.6 V (vs. Ag/AgCl). The shape of the CV curves was almost unchanged even at the high scan rate of 100 mV s −1 , indicating the remarkable capacitive behavior and high-rate capability of the NiCo 2 S 4 @PPy electrode material. The galvanostatic charge/discharge tests were correspondingly performed at various current densities ranging from 2.54 to 9.09 A g −1 in a potential range of − 0.1 to 0.45 V (Fig. 6d). At a current density of 2.54 A g −1 , the specific capacitance of the NiCo 2 S 4 PPy/NF electrode was estimated to be 2554.9 F g −1 . The specific capacitance and rate capability at various discharge currents are presented in Fig. 6e and Table S1. It is clear that the specific capacitance was slightly decreased as the current density increased from 2.54 to 9.09 A g −1 . Moreover, the NiCo 2 S 4 @PPy/NF electrode still maintained 72% of its maximum specific capacitance at a (1) www.nature.com/scientificreports/ current of 9.09 A g −1 . The volumetric capacitance was also estimated, and the matching curve is shown in Fig. S3. A volumetric capacitances of 1847.18 F cm −3 at a current of 2.54 A g −1 , and 1387.57 F cm −3 at a current of 9.09 A g −1 were achieved. These findings verify that the NiCo 2 S 4 PPy/NF electrode can be considered as an excellent alternative electrode to use in high-performance supercapacitors. The diffusion controlled and surface capacitive charge storage mechanisms contribute to the total capacitance of NiCo 2 S 4 @PPy/NF. The current dependency on the scan rate, ν, is determined by the following equation in theory 29 : where 'a' and 'b' are adjustable parameters, i is the current and v is the scan rate. When b = 1, the surface capacitive process is the dominant mechanism of charge storage; otherwise, when b = 0.5, diffusion-limited processes govern the current density (battery-type behavior). The b-values of NiCo 2 S 4 @PPy were calculated within the potential window of the observed voltammograms. The b-values near 1.0 within the potential range of − 0.1 and 0.1 V, indicate the pure capacitive charge storage mechanism controlling the current. However, the b-values of 0.65-0.7 were obtained for the potential range of 0.3-0.4 V, implying the contribution of diffusion-controlled and surface capacitive processes. It is noteworthy that NiCo 2 S 4 @PPy/NF exhibits a diffusion-controlled (battery-type) feature, in addition to capacitive charge storage. Thus, the peak current can be considered as the sum of the currents for capacitive processes (k 1 v) and diffusion-limited processes (k 2 v 1/2 ), according to the following equation: The slope and intercept of the lines of v 1/2 against i(V )/v 1/2 can be used to compute k 1 and k 2 (Fig. 6f). The contribution fraction of each capacitance mechanism at different scan rates is displayed in Fig. 6g. It can be concluded that the diffusion-controlled process is the major source of charge storage behavior in NiCo 2 S 4 @PPy/NF. Additionally, the contribution of the capacitive charge storage is increased by increasing the scan rates because of the limited time for ion transport into deeper sites. Therefore, at the higher scan rates, the contribution of the redox processes will decrease. The practical cycle-life stability of the NiCo 2 S 4 @PPy/NF electrode was investigated for 10,000 successive cycles. As shown in Fig. 6h, a remarkable specific capacitance retention of almost 92% after 10,000 cycles demonstrates considerable cycle-life stability. To further check the morphology changes during the long-term cycle stability test, SEM analysis was carried out for the NiCo 2 S 4 @PPy/NF electrode after 2100 cycles. It is obvious that the primary structure is almost well-maintained (shown in Fig. 6i). As a result, positive cooperation of PPy and NiCo 2 S 4 results in the structural integrity of the NiCo 2 S 4 @PPy nanomaterial and improves the cycling performance and stability.
Electrochemical impedance spectroscopy tests were further conducted to inspect the electrical conductivity and ion diffusion properties of the PPy/NF, NiCo 2 S 4 /NF, and NiCo 2 S 4 @PPy/NF electrodes. Figure 7 depicts the Nyquist plots of the electrodes at open circuit potential (OCP). The expanded high-frequency region of the Nyquist plot is shown as inset in Fig. 7, and demonstrates that the NiCo 2 S 4 @PPy/NF electrode has lower R ct compared to the PPy/NF and NiCo 2 S 4 /NF electrodes. Also, at the low-frequency region of these Nyquist plots of the NiCo 2 S 4 @PPy/NF and NiCo 2 S 4 /NF electrodes, diffusion impedance appears. It is an evidence for a www.nature.com/scientificreports/ substantial capacitive behavior of the electrode materials. To numerically compare the electrochemical impedance parameters of the different electrodes, we have used a modified Randles equivalent circuit to fit the experimental EIS results (shown as an inset in Fig. 7). Also, details of the fitting step are shown in Table S4 (supporting  information). As can be seen, both Rs and Rct are smaller for NiCo 2 S 4 @PPy/NF compared with the PPy/NF and NiCo 2 S 4 /NF electrodes (1.12 and 1.64 ohms for NiCo 2 S 4 @PPy/NF reaching 1.25 and 12.37 ohms for pure PPy/NF and 1.84 and 5.32 ohms for the NiCo 2 S 4 /NF counterparts, respectively). These findings confirm the substantial interaction between the NiCo 2 S 4 and PPy constituents of the electrode and the robust contact with NF substrate. Such an excellent interaction provides suitable pathways for electrons and ions in the dandelion-like structure of NiCo 2 S 4 , a remarkable electrical contact, and fast redox reactions of the nanomaterial. It is noticeable that the studied nanomaterials show almost pure capacitive behavior at even high frequencies of 67 and 212 Hz for the NiCo 2 S 4 /NF and NiCo 2 S 4 @PPy/NF electrodes, respectively. The NiCo 2 S 4 @PPy nanomaterial shows superior capacitive behavior compared to the NiCo 2 S 4 /NF, supporting the electrochemical results of cyclic voltammetry and charge/discharge tests 54 . Moreover, the NiCo 2 S 4 @PPy electrode material preserves a significantly shorter relaxation time constant compared to the NiCo 2 S 4 or PPy electrodes. This behavior stems from the substantial electrical conductivity of the nanomaterial electrode due to the notable synergy between NiCo 2 S 4 and PPy in the nanomaterial and enhanced electrical contact between the NiCo 2 S 4 @PPy and NF support without using any insulating polymers as binder.
To further estimate the practical application of NiCo 2 S 4 @PPy/NF, an asymmetric supercapacitor (Fig. 8a) was constructed. Then, the reduced graphene oxide was brushed on a piece of NF (rGO/NF) as the negative electrode, and coupled to NiCo 2 S 4 @PPy/NF as the positive electrode (device notation: NiCo 2 S 4 @PPy//rGO/NF). The NiCo 2 S 4 @PPy//rGO/NF electrode properties of were examined using cyclic voltammetry and galvanostatic charge/discharge tests. Figure 8b displays typical voltammograms of the ASC device in a voltage span of 0 to 1.6 V at a wide range of scan rates (10-100 mV s −1 ). It is obvious that cyclic voltammograms almost keep their quasi-rectangular shape with the increasing scan rate, confirming the perfect capacitive performance and rapid current response of the asymmetric supercapcitor. Figure 8c exhibits the discharge branch of the GCD curves at various currents from 1.84 to 11.04 A g −1 , in a voltage range of 0-1.6 V. The specific capacitance of the ASC device was calculated according to the corresponding GCD cruves. A maximum specific capacitance of 98.92 F g −1 was achieved at a current density of 1.84 A g −1 for the NiCo 2 S 4 @PPy//rGO/NF ASC device. It is notable that the ASC device retained almost 55% of its initial capacitance, when the current increased from 1.84 to 6 A g −1 (Fig. 8d). These results reveal the remarkable rate capability of the NiCo 2 S 4 @PPy//rGO ASC device. The longterm cycle stability as a critical parameter for the real application of a supercapacitor was inspected by voltammetry measurements at the high scan rate of 150 mV s −1 for 1500 cycles. As shown in Fig. 8e, the NiCo 2 S 4 @PPy// rGO/NF device maintains 85% of its primary capacitance after approximately 1500 cycles. Additionally, power and energy densities are the essential parameters to investigate the electrochemical performance: Figure 8f illustrates the Ragone plot (energy against power density) of the asymmetric supercapacitor (ASC). The NiCo 2 S 4 @PPy/NF//rGO/NF exhibits a maximum energy density of 35 Table S2 shows the electrochemical characteristics of the present work compared to the previously reported works. Thus, the NiCo 2 S 4 PPy/NF electrode shows a superior electrochemical performance. Such excellent behavior can originate from the more accessible electroactive spots for the redox reactions and porous structure, which enhance the electron transfer and ion diffusion during the charge/discharge processes. These results reveal that the NiCo 2 S 4 @PPy electrode material has a high electrochemical performance, and it is a promising electrode for supercapacitor applications.

Conclusion
In summary, the NiCo 2 S 4 @PPy/NF electrode was successfully fabricated by a facile hydrothermal method. This nanomaterial exhibited an exceptional specific capacitance of 2554.88 Fg −1 at 2.54 A g −1 discharge current, a remarkable long-term cycling stability (92% of capacitance retention even after 10,000 cycles), and low chargetransfer resistance. An asymmetric supercapacitor based on the NiCo 2 S 4 @PPy/NF as the positive electrode and rGO@NF as the negative electrode was assembled. The as-prepared ASC device can produce a great specific capacitance of almost 98.9 F g −1 at a discharge current of 1.84 A g −1 in a potential range of 0.0 to + 1.6 V. Also, this asymmetric device exhibits notable cycle life stability of 85% after 1500 cycles. Therefore, the NiCo 2 S 4 @PPy/ NF electrode with excellent electrochemical performance, is a suitable candidate to be used in supercapacitors. www.nature.com/scientificreports/