High Performance and Cycling Stability Supercapacitors Employing MnS@Polypyrrole Nanocomposites as Cathode Material

In this study, a nanocomposite is prapared to obtain supercapacitor with high specific capacitance and cycling stability. The nanocomposite is fabricated by the electropolymerization of PPy on MnS, following synthesis of MnS via rapid and simple microwave-assisted method / Bu çalışmada, yüksek özgül kapasitans ve çevrim kararlılığına sahip süperkapasitör elde etmek için bir nanokompozit hazırlanmıştır. Nanokompozit


INTRODUCTION
The rapid development of portable technology and the industrial revolution are rapidly increasing energy demands. The required energy is mainly obtained from fossil fuels, which have a much higher consumption rate. However, fossil fuels consist of carbon that accumulates and causes environmental pollution as well as global warming. Also, other renewable energy sources such as solar, hydro and wind power are unstable and weather dependent. Therefore, dramatic climate X fluctuations, depletion of fossil fuels, environmental impacts and increasing energy demands force researchers to develop energy storage systems.
Electrochemical capacitor as an energy storage is gaining momentum with the growing demand for portable systems and hybrid electric vehicles that require instantaneous high-power density. Compared to secondary batteries, electrochemical capacitors, also known as supercapacitors (SCs), demonstrate higher power density, long life, wide thermal operating range and low maintenance cost [1]. SCs have two energy storage mechanisms: electrochemical double-layer capacitance (EDLCS) and pseudocapacitance. Since electrochemical processes occur both at the surface and in bulk near the surface of the solid electrode, pseudocapacitors exhibit much greater capacitance and energy density than EDLCSs [2]. However, because of redox reactions occur at the electrode, electrodes that exhibit pseudocapacity are prone to swelling and shrinkage during the charge/discharge process, which can lead to poor mechanical stability and insufficient cycle stability. Therefore, the applicability of the pseudocapacity based supercapacitor is strongly dependent on the electrode material. An ideal electrode should have long cycle stability, large active surface area, a uniformly spaced morphology, high electrical conductivity, and rapid ion diffusion [3]. Generally, three types of electrode materials are studied in SCs: i) carbon-based materials, ii) metal oxides/sulphides and iii) conducting polymers [4]. Among these, conducting polymers have been attracted as electrode materials due to their electrochemical behaviours of fast reversible doping and de-doping ability, leading to storing the charges throughout the whole volume [5]. Mostly, because of high conductivity, simple processibility and high chemical stability, polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiopene) (PEDOT) have been used for fabrication of electrodes employing in SCs. Especially, PPy is as an intrinsically conducting polymer has drawn more attention due to its great conductivity, high storage ability, high thermal and environmental durability, excellent redox ability [6]. However, as most conducting polymers, the PPy has a poorer longterm cycle stabilities than metal oxide, carbonaceous and metal sulphides since the redox spots in the polymer backbone are insufficiently stable and the backbone of polymer can be cracked after a few charge/discharge cycles.
Recently, to solve the cycling stability problem and improve capacitance of PPy based SCs, nanocomposite-based electrodes have been designed by combining PPy with other materials such as metal oxides and metal sulphides [6][7][8][9][10]. Metal oxides are promising candidates for use as electrode materials in supercapacitors due to their high theoretical specific capacitance, low cost, and low toxicity. However, due to the low electrical conductivity of these materials (10 -5 -10 -6 S/cm), the specific capacitance values are subject to a high deviation from their theoretical values [11]. Compared to these commonly used electrode materials, metal sulphides are abundant and inexpensive due to the presence of minerals in nature. More importantly, unlike metal oxides, metal sulphides generally exhibit higher electrical conductivity, flexibility, ionic diffusion, and anionic polarization due to the more covalent characteristics of the hard base O -2 ion being replaced by the soft base S -2 /S2 -2 ion [11][12][13][14]. In this regard, metal sulphides not only enhance the electronic properties but also improve the stability of PPy against to swelling and shrinking during the cyclic processes. For example, Peng et al., after synthesizing CuS (copper sulfide) by solvothermal method, obtained CuS/PPy composites at different ratios by in situ polymerization and used them as electrodes in supercapacitor [15]. In the study, the specific capacitance values at 1 A/g current density were recorded as 275, 212 and 427 F/g for pure PPy, CuS and CuS/PPy, respectively. After 1000 cycles, the initial capacity of PPy, CuS and CuS/PPy electrodes based SCs retained as 56%, 81% and 88%, respectively. In another study, Huo et al. synthesized the Co3S4 nanorod structure by hydrothermal method and obtained Co3S4/PPy nanocomposite using in situ polymerization and applied it as an electrode in SC [16]. In the study, the Rct values of Co3S4 and Co3S4/PPy were determined as 0.78 and 0.48 Ω, respectively, indicating that Co3S4 increases ion diffusion and electrical conductivity of resulted composite. Also, because of the synergistic effect between Co3S4 and PPy, the specific capacitance of Co3S4/PPy retained 98% of its initial capacitance value after 1000 cycles at current density of 8 A/g. In another study, Yan et al. prepared  Herein, MnS rectangular prisms were synthesised via microwave-assisted method and coated on Ni foam (NF) by drop casting then MnS@PPy nanocomposite was prepared via electrodeposition of PPy on MnS rectangular prisms. Unlike to hydrothermal and solvothermal method, the MnS rectangular prisms have been synthesised in a very short time (2 h) via microwave-assisted method. Furthermore, electrodeposition of PPy directly on MnS@Ni foam remedied the need for any conductive additives (carbon allotropes etc.) and insulating binders, which reduces the internal resistance and provides faster charge transfer and high adhesion between the active material and the NF. In this research, to increase the electric conductivity, cycle life stability, electroactivity and capacitance of PPy, MnS for the first time have been incorporated into PPy matrix and resulting nanocomposite employed as cathode material for SCs. Electrochemical results reveal that MnS@PPy electrode exhibited a specific capacitance (Cs) of 1102 F/g which is approximately 5.6 times higher than that of the bare PPy (197 F/g). Furthermore, energy density (Ed) of the bare PPy was determined as 4.37 W/kg, by incorporation of MnS into PPy matrix the Ed value increased to 24.5 W/kg. On the other hand, after 1000 charge/discharge cycles, the cycle stability of the bare PPy remained at 72%, while MnS@PPy nanocomposite electrode is 95 %.

Experimental Equipment (Deneysel Ekipman)
To synthesis MnS was used Milestone/FlexiWave Advanced Flexible Microwave Synthesis Platform. To define the phase type and crystalline structure, X-ray diffraction (XRD) patterns of MnS metal sulphide were investigating employing RIGAKU SmartLab. Surface morphology of the samples were investigated using TESCAN MAIA3 XMU scanning electron microscopy (SEM) instrument. Fourier transform infrared (FTIR) spectroscopy measurements of MnS, bare PPy and MnS@PPy were performed using the ATR method of Shimadzu spectrometer over a range from 400 to 4000 cm -1 . Thermal gravimetric analysis (TGA) was performed to examine the thermal properties of the samples. CV curves of samples were recorded using same set up for eelectrodeposition. Electrochemical impedance spectroscopy (EIS) and galvanic charge/discharge (GCD) cycles of bare PPy and MnS@PPy based SCs were recorded employing WonATech ZIVE SP1 potentiostat−galvanostat system with two electrodes.

Synthesis of MnS and MnS@PPy
Nancomposite ( (BULGULAR) In Phase purity and crystallinity of MnS powder was analysed by X-ray diffraction (XRD). As seen in Figure. 1a, the diffraction patterns located at 2 = 29.7 0 , 34.4 0 , 49.4 0 , 58.7 0 ve 61.6 0 angles can be indexed to the (111), (200), (220), (311) and (222) orientations, respectively [18]. All the diffraction patterns agree well with the reported data of cubic phase of α-MnS crystals (JCPDS#88-2223) and no significant impurity peaks were observed [19]. Furthermore, one can see from the sharp and welldefined diffraction peaks, MnS metal sulphide which was synthesized using microwave assisted method presents a good crystallinity. The vibrational properties of MnS, PPy and MnS@PPy were investigated by FTIR spectra as shown in Figure 1b. The peak located at 1235 and 1008 cm -1 in the MnS spectrum can be assigned to the formation of complex sulphur with the active sites in MnS. The peak at 610 cm -1 is ascribed to the Mn-S stretching vibration, indicating that MnS is successfully synthesised with microwave assisted method [9,20]. Moreover, in the spectrum of PPy, the peaks appeared at 1540 and 1453 cm -1 stem from the C-C and C-N stretching vibration of the pyrrole ring. The bands located at 1285, 1170 and 1030 cm -1 are assigned to the C=N bending, C−N stretching and =C−H in-plane vibrations of PPy rings, respectively [21]. The peak at 678 cm -1 is related with C−H deformational vibration-mode of the PPy. In case of MnS@PPy nanocomposite, all the characteristic peaks of PPy can be seen in the spectrum. In addition, a new peak appeared approximately at 600 cm -1 , exhibiting that MnS is incorporated into PPy matrix. Furthermore, the bands seen at 1540, 1453, 1285, 1030 and 678 cm -1 in the spectrum of bare PPy are observed at slightly lower wavenumbers in the spectrum of MnS@PPy nanocomposite at 1521, 1436, 1280, 1022 and 673 cm-1, respectively. The observed shifting in characteristic bands of PPy with incorporation of MnS exhibits the electronic/synergistic interaction working at molecular degrees [21-23]. So, it can be said that MnS@PPy nanocomposite is successfully prepared by electro-polymerization of pyrrole on MnS/NF.

RESULTS
To assess the effects of MnS on thermal properties of PPy, TGA measurements were conducted and the obtained TGA curves are exhibited in Figure 1c. The standard thermograms for bare PPy, MnS and MnS@PPy nanocomposite show a three-step weight loss process as depicted in Figure 1c. The first step weight loss occurred between 28-180 0 C stem from the evaporative loss of water and other volatile impurities [24]. The second step weight loss aroused between 180-385 0 C in bare PPy and MnS@PPy due to gradual thermal decomposition of the polymer chains [25]. The second step weight loss occurred for MnS between 180-550 0C due to the decomposition of sulphur in MnS structure. The third weight loss taken place between 385-1000 0C can be attributed to pyrolysis of the materials. As deduced from TGA plots, bare PPy exhibited around 30.8% weight loss at 385 °C which reduced to 24.5% for MnS@PPy. The reducing in weight loss of nanocomposite indicated that the thermal stability of PPy was significantly improved due to the intervening MnS in the polymer chains [26]. This caused in prolonged degradation of PPy chains, resulting in the increased thermal stability of the nanocomposite.  Figure 2a, atomic percentage of manganese and sulphur elements is 49.2 and 50.8%, respectively. In ideal case, the atomic percentage of Mn and S could be equal, the ratio of Mn/S is 0.97 which is very close to ideal number. Furthermore, X EDS mapping reveal that Mn and S elements homogenously distributed in MnS. EDS results are in good agreement with XRD patterns which are confirming that MnS successfully are synthesised using microwave assisted method in 2 h. As seen in Fig. 2c, the morphology of MnS metal sulphide consists of 500 nm wide cubes and 200 µm long rectangle prisms. In addition, it is seen that there is no formation other than cubic and prismatic structures. As shown in Figure 2d, the morphology of the PPy polymer consists of spherical grains with dimensions of about 400 nm. Moreover, it is observed that the spherical structures are interconnected and densely packed. Figure 2e dedicates that MnS@PPy nanocomposite firmly adhered to NF substrate without insulating adhesive. When Figure 2f is examined, the morphology of the nanocomposite structure consists of both cubic and prismatic MnS structures and spherical grains of PPy polymer. Therefore, it can be said that the MnS@PPy nanocomposite was successfully synthesized using electrochemical polymerization method. Furthermore, incorporation of MnS changed the growth dynamics of PPy, leading that formation of PPy as nano sheets on MnS structures. This indicates that the active surface area of the resulted nanocomposite structure has increased significantly and PPy bounded with MnS rectangular prisms strongly. The enlargement of the surface area provides many advantages for the SC. First, it provides adequate surface contact between the electrode and electrolytes and accelerates the Faraday reactions as well as increases the reaction rate [27]. Second, it makes a large amount of electrolyte accessible for faster mass transfer [28]. Third, increasing the specific surface area promotes high mechanical flexibility, thereby reducing pulverization of electroactive materials during difficult cycle performance [29]. X The electrochemical behaviour of PPy and MnS@PPy nanocomposite was investigated in a 2M KOH aqueous electrolyte with a two-electrode potentiostat-galvanostat system. Cyclic voltammetry (CV), galvanic charge/discharge cycles (GCD), electrochemical impedance spectroscopy (EIS) and cycle stability measurements were performed for each electrode. CV measurements were carried out between 0 and 0.5 V at a scanning rate of 20 mV/s, GCD curves were recorded at different current densities (1A/g, 2A/g, 3A/g, 4A/g and 5 A/g) between 0 and 0.4 V, EIS curves were performed between 100 kHz-0.01 Hz and cycle stabilities of bare PPy and MnS@PPy electrode based SCs were taken as 1000 cycles at a current density of 2A/g. Figure 3 shows the electrochemicql performances of SCs fabricated with bare PPy and MnS@PPy nancomposite. Clearly, a double redox peak is observed in all CV curves over the 0-0.4 V voltage window, indicating pseudocapacitive properties [30]. When Figure 3a is examined, the oxidation and reduction peak current densities of the PPy were 25 and 17 mA/cm2, while by introduction of MnS into the PPy polymer matrix, the peak current densities reached to 121 and 107 mA/cm2, respectively. Moreover, the redox peak positions changed, and the peak-to-peak distance increased significantly compared to PPy. This indicates that MnS dramatically increases the electron transfer rate and speed [31]. Moreover, it indicates that the nanocomposite electrode will show a larger capacitance and lower internal resistance than the bare polymer [32]. Also, compared to PPy, MnS@PPy shows larger CV integrated areas, indicating higher electrochemical capacitance. The remarkable increase in the electrochemical properties of the MnS@PPy nanocomposite compared to the bare PPy is due to the increase in the electrical conductivity of the resulted nanocomposite by acting as a bridge between the PPy chains of the MnS rectangular prisms and the enlargement of the specific surface area by differentiating the surface morphology [33]. Figure 3b-c show the GCD curves of the PPy and MnS@PPy electrodes at different current densities, respectively. During fast charging and discharging, the internal resistance in the discharge curves shows a negligible internal resistance due to diffusion of electrolyte ions. When the current densities are increased, the shape and size of the chargedischarge curve remain the same, indicating stable behaviour of the electrode. The specific capacitance (Csp) of the electrode from the charge-discharge curves was calculated with the following equation.
Here, "I, Δt, m and ΔV represents the discharge current, the discharge time, the weight of the material coated on the NF substrates and potential window, respectively. Furthermore, energy density (Ed) and power density (Pd) of the electrodes were determined according to the fallowing equations.
The determined Csp values decrease with increasing scanning rate. This is because charge and discharge processes occur very quickly at higher scanning rates. Therefore, ions cannot penetrate deep into the electrode, resulting in low specific capacitance. From the GCD curves, the Csp values of the PPy and MnS@PPy electrodes at 1 A/g current density were obtained as 197 and 1102 F/g, respectively. The Csp value was increased approximately 5.6 times with incorporation of MnS into PPy. In addition, the energy (Ed) and power densities (Pd) of PPy and MnS@PPy were determined as 4.37 and 24.5 W/kg and 199.5 and 200 Wh/kg, respectively. These enhancements can be attributed to the increase of the electron transfer rate and speed, as well as the expansion of the surface area. Moreover, it is clear from these results that the specific capacitance of MnS@PPy nanocomposite compared to PPy shows excellent storage capacity even at higher current densities.
As shown in Figure 3d, EIS analyses were performed to further investigate the electrochemical performance of the electrodes. Nyquist plots of all electrodes show a single semicircle in the high frequency domain and a curved line in the low frequency domain. The diameter of the semicircle represents the charge transfer resistance (Rct), which can reflect the permeability of the electrolyte. The linear parts correspond to the Warburg impedance (W), which can represent the ion diffusion resistance. Also, the value at which the curve crosses the x-axis represents the equivalent series resistance (Rs), which includes the internal resistance, the contact resistance of the interface (electrolyte/electrode), and the ionic resistance of the electrolyte [34]. When Figure 3d is examined, it is observed that PPy has higher Rct, Rs and W values compared to MnS@PPy nanocomposite. This shows that the MnS@PPy nanocomposite has excellent electron transfer kinetics, low energy loss, and higher diffusion of the electrolyte on the electrode surface compared to the pure polymer. EIS analysis results support each other with CV and GCD measurements. For practical applications of SCs, capacity retention against number of cycles is an important parameter to consider. As seen in Figure 3e, the specific capacitance of PPy decreased to 72% from its initial value after 1000 cycles, while that of MnS@PPy nanocomposite remain as 95%. The introduction of MnS into the PPy matrix not only increased the electrochemical activity but also significantly increased the cycling stability due to the improvement of stability against to shrinking or swelling.

CONCLUSIONS (SONUÇLAR)
In briefly, rectangular prism shaped MnS metal sulphide was successfully synthesized by microwave assisted method and MnS@PPy nanocomposite was fabricated directly on NF substrate by electropolymerization method. In this way remedied the need for any conductive additives and insulating binders, which reduces the internal resistance and provides faster charge transfer and high adhesion between the active material and the NF. Moreover, because of the introduction of MnS rectangular prisms into the nanocomposite system, the nucleation and growth kinetics of the PPy polymer differed and the specific surface area expanded as well as the charge transfer kinetics and conductivity improved. Electrochemical results show that MnS@PPy electrode exhibited a specific capacitance (Cs) of 1102 F/g which is approximately 5.6 times higher than that of the bare PPy (197 F/g). Furthermore, energy density (Ed) of the bare PPy was determined as 4.37 W/kg, by incorporation of MnS into PPy matrix the Ed value increased to 24.5 W/kg. More importantly, after 1000 charge/discharge cycles, the cycle stability of the bare PPy remained at 72%, while that of MnS@PPy nanocomposite electrode recorded as 95 % due to the improvement of PPy stability against to swelling and shrinking during the charge-discharge process. In the light of the results obtained from current study, it is clearly seen that MnS@PPy structured nanocomposite is a promising candidate for SC applications.

(ETİK STANDARTLARIN BEYANI)
The author of this article declares that the materials and methods they use in their work do not require X ethical committee approval and/or legal-specific permission.

KATKILARI)
Mahir GÜLEN: He conducted the experiments, analyzed the results and performed the writing process.