Hydrothermal Synthesis of Cobalt Ruthenium Sulfides as Promising Pseudocapacitor Electrode Materials

: In this paper, we report the successful synthesis of cobalt ruthenium sulfides by a facile hydrothermal method. The structural aspects of the as-prepared cobalt ruthenium sulfides were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. All the prepared materials exhibited nanocrystal morphology. The electrochemical performance of the ternary metal sulfides was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy techniques. Noticeably, the optimized ternary metal sulfide electrode exhibited good specific capacitances of 95 F g − 1 at 5 mV s − 1 and 75 F g − 1 at 1 A g − 1 , excellent rate capability (48 F g − 1 at 5 A g − 1 ), and superior cycling stability (81% capacitance retention after 1000 cycles). Moreover, this electrode demonstrated energy densities of 10.5 and 6.7 Wh kg − 1 at power densities of 600 and 3001.5 W kg − 1 , respectively. These attractive properties endow proposed electrodes with significant potential for high-performance energy storage devices. with different molar ratios and different reaction times in a 0.5 M H 2 SO 4 aqueous electrolyte using a three-electrode configuration. A series of electrochemical measurements were conducted, including cyclic voltammetry (CV) at various scan rates, galvanostatic charge-discharge (GCD) cycling at various


Introduction
Supercapacitors (SCs) or ultracapacitors, also known as electrochemical capacitors, have attracted intense attention as a principal class of clean and sustainable energy storage devices for modern electronics because of their impressive energy storage potential, with excellent electrochemical reversibility, long lifespan, ultrahigh power density, quick charge/discharge capability, strong safety record, and environmental friendliness. Due to these advantages, SCs hold great promise for next generation energy storage applications, such as regenerative braking for automobiles, uninterruptible power supply systems, and portable electronics [1][2][3].
In general, supercapacitors can be categorized into pseudocapacitors (redox supercapacitors) and electrochemical double layer capacitors (EDLCs) based on their charge storage mechanism and active materials [2−4]. The electrochemical performance of SCs mainly depends on the properties of the active electrode materials; these should possess good electronic conductivity, large specific surface area, high chemical stability, nanoscale topography, and redox chemistry [4−6]. To date, carbon electrodes in EDLCs and metal oxides/hydroxides and conducting polymers in pseudocapacitors have been broadly exploited as promising electrode materials. Carbon based supercapacitors store energy through adsorption/desorption of ions at the double layer interface between the active electrode and the electrolyte [7]. Metal oxides/hydroxides (RuO2, Co3O4, MnO2, NiO, TiO2, Co(OH)2, etc.) [8−12] and conducting polymers (polyaniline, polypyrrole, polythiophene, etc.) [13−15] show much greater specific capacitances than carbon materials by virtue of their rich reversible Faradaic redox reactions, which occur between the electrode material and the electrolyte [16,17]. Carbon materials show lower energy densities and superior cycle stabilities compared to pseudocapacitive materials [18,19]. On the other hand, metal oxides/hydroxides are hampered by their low electrical conductivity and rate capability; and conducting polymers, by their poor cycling stability. These disadvantages currently prevent their use in a wide range of commercial applications [20,21]. So, from a practical point of view, there is an urgent need to develop supercapacitors with high energy density without sacrificing cycle life or rate capability. The key to addressing these challenges is the design and development of advanced electrode materials.
Recently, in the pseudocapacitive materials category, transition metal chalcogenides have gained much interest as potential electrode materials in energy storage applications because of their higher electrical conductivity and multiple available oxidation states, promoting rapid redox reactions compared to their metal oxide or hydroxide counterparts [22−24]. Transition metal sulfides, including cobalt sulfides, nickel sulfides, copper sulfides, ruthenium sulfides, iron sulfides, and molybdenum sulfides, have been employed as electrode materials for energy storage applications [25−28]. Sulfur ions tend to produce flexible nanostructures because of their high electroconductivity and fast charge transport [29]. Amongst these, cobalt sulfide (CoSx), with its various stoichiometric compositions, such as Co9S8, Co3S4, CoS2, and CoS, has been gaining tremendous interest owing to its intriguing physical and chemical properties [30,31]. Ruthenium based materials have also been proven to be the best electrodes for energy storage applications by virtue of their excellent electrochemical properties [29,32,33].
The chemical composition and associated redox properties are factors of primary importance for energy-storage materials. Additionally, many factors affect the rate performance of electrodes, including the morphology, surface properties, microstructure, conductivity, and components of the electrode materials. Currently, ternary metal sulfides have been proposed to overcome some issues with single component metal sulfides or ternary metal oxides, such as rate capability and cycle life [34]. The combination of two different metals can enhance the redox chemistry of the materials and their performance in electrochemical systems when compared with single-metal sulfides. A new ruthenium cobalt oxide (RuCo2O4) material has been prepared by a simple electrodeposition method and demonstrated as an advanced electrode material for supercapacitors [35]. Ternary metal sulfides like NiCo2S4, CuCo2S4, and MnCo2S4 possess much higher electrochemical activity and capacity due to their multiple oxidation states and smaller band gaps, thus allowing for rich redox reactions [36,37]. However, it is still unclear if these materials exhibit any enhancement in electronic conductivity and capacitance as a result.
Metal sulfides have been employed as potential electrode materials in various applications such as batteries, supercapacitors, electro−catalysts, and solar cells [38−42]. Furthermore, various ternary transition metal sulfides have already been studied as efficient electrode materials for supercapacitors [37,43−45]. Particularly, NiCo2S4 has been studied most comprehensively [46−48]. Although ternary metal sulfides have been studied as potential electrode materials for energy storage applications, metal sulfides normally exhibit low rate capability and cycle life stability. To overcome these drawbacks, there is an urgent need to design electrode architectures with good electronic conductivity, plentiful electron transport channels, and robust structural stability [49].
Inspired by the research summarized above, we studied the electrochemical performance of cobalt ruthenium sulfides as advanced electrode materials for supercapacitor applications. The improved electrochemical performance of ternary metal sulfides in terms of energy density, rate capability, and cycling stability from the combination of the two different metal components can enhance the redox chemistry of the materials by efficient synergetic effects. Herein, we report a simple hydrothermal strategy that was employed to prepare cobalt ruthenium sulfides using aqueous solutions of CoCl2·6H2O and RuCl3·nH2O as metal sources and thioacetamide as a sulfur source.
We investigated the electrochemical performance of cobalt ruthenium sulfides synthesized with different molar ratios and different reaction times in a 0.5 M H2SO4 aqueous electrolyte using a threeelectrode configuration. A series of electrochemical measurements were conducted, including cyclic voltammetry (CV) at various scan rates, galvanostatic charge-discharge (GCD) cycling at various applied currents, and electrochemical impedance spectroscopy (EIS) at the open circuit voltage. The effect of reaction time on the electrochemical performance and microstructure of the cobalt ruthenium sulfides was investigated.

Preparation of Cobalt Ruthenium Sulfide Materials
In a typical experiment, 2 mmol of cobalt (II) chloride hexahydrate (475.9 mg), 1 mmol of ruthenium (III) chloride hydrate (207.4 mg), and 8 mmol of thioacetamide (601 mg), PVP (70 mg) were dissolved in 80 mL of H2O. After stirring for 1 h, the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and kept at 160 °C for 24 h. After cooling, the precipitate was vacuum filtered, washed several times with deionized water and ethanol, and then dried at 50 °C for 12 h. The precursor mixture was heat-treated at 650 °C for 2 h under a N2 atmosphere to produce crystalline cobalt ruthenium sulfides (Co2RuS6) as shown in Figure 1. This sample was denoted as Co2RuS6-24. The cobalt ruthenium sulfide samples were prepared through a similar process with different Co/Ru molar ratios of 1:2 and 1:1; these samples are denoted as CoRu2S6 and CoRuS6, respectively. Similarly, cobalt ruthenium sulfide samples were prepared for 36 h and 48 h, denoted as Co2RuS6-36 and Co2RuS6-48, correspondingly. Pristine Co9S8 and RuS2 were prepared following the above procedure in the absence of ruthenium chloride and cobalt chloride hexahydrate, respectively.

Material Characterization
X-Ray photoelectron spectroscopy (XPS) was performed using a Theta Probe electron spectrometer (Thermo Fisher Scientific Co., Waltham, MA, USA). The crystallographic structures of the materials were determined using a Rigaku X-ray diffractometer (XRD; Rigaku, Tokyo, Japan) operated at 40 keV and 15 mA with a Cu Kα radiation source (λ = 1.54 Å). The chemical bonding transitions were analyzed by Fourier transform infrared spectroscopy (FT-IR; Thermo Scientific Nicolet 6700, Waltham, MA, USA) in the frequency range of 4000-400 cm −1 using KBr pellets. The morphologies were examined by high-resolution transmission electron microscopy (HR-TEM; JEOL JEM 2100F, Tokyo, Japan) and elemental analysis was conducted by energy-dispersive X-ray spectroscopy (EDS). Raman spectra of the composites were obtained using a Raman spectrometer (JASCO, NRS-3100, Tokyo, Japan) that was operated at 1.4 mW laser power with an excitation wavelength of 532 nm (green).

Electrode Preparation and Electrochemical Measurements
To prepare the working electrode, active materials were mixed with conductive carbon and polyvinylidene fluoride binder at a mass ratio of 80:10:10. For instance, first polyvinylidene fluoride (PVDF) (10 mg) dissolved in 1-methyl-2-pyrrolidinone (NMP) solvent to prepare solution. We have taken 80 mg active material, 10 mg carbon and grind in mortar for 30 min. Next add this grinded mixture into PVDF-NMP solution and stir for 24 h to obtain the electrode slurry. This slurry coated on stainless-steel substrate with brush in 1 cm × 1 cm area and dried in vacuum at 50 °C for overnight. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed using a potentiostat/galvanostat (Gamry PGSTATZRA Reference 600, Warminster, PA, USA) in an aqueous 0.5 M H2SO4 electrolyte solution at room temperature using a standard three-electrode cell configuration. The working, counter, and reference electrodes were the active material-coated stainless-steel substrate, platinum wire, and an Ag/AgCl electrode, respectively. CV and GCD tests were carried out within the potential range of 0-1 V, while the EIS experiments were conducted in the frequency range from 100 kHz to 0.1 Hz at a 5.0 mV amplitude relative to the open circuit potential.

Results and Discussion
During this synthesis procedure, the released metal ions (i.e., Co 2+ and Ru 3+ ) react with sulfide (S 2− ) ions from H2S, which is generated by the hydrolytic reaction of thioacetamide due to the prolonged hydrothermal treatment, resulting in the formation of ternary metal sulfide nanoparticles. This method avoids the tedious multi-step hydrothermal treatment and solvothermal anionexchange commonly used in previous studies [50]. In the literature, nickel sulfides and cobalt sulfides have also been prepared by using thioacetamide as sulfur source [51]. To confirm the composition of the functional groups on the product obtained from the hydrothermal process followed by heat treatment at 650 °C in N2 atmosphere for 2 h, we analyzed the FT-IR spectra of the samples at different time intervals. The FT-IR spectra of Co2RuS6-24 and Co2RuS6-36 are shown in Figure 3.  In the Raman spectra shown in Figure 4, pristine RuS2 showed characteristic peaks around 377 and 385 cm −1 which can be assigned to Eg and Ag modes, respectively [28]. Pristine Co9S8 showed characteristic peaks at approximately 662, 500, 475, and 188 cm -1 which correspond to the stretching S-S mode and the Eg mode of Co in cobalt sulfides [56]. Similarly, in cobalt ruthenium sulfides (Co2RuS6-24, 36, and 48), the characteristic peaks at 665, 505, 475, and 188 cm −1 are attributed to the stretching S-S mode and Eg mode of Co in cobalt ruthenium sulfides [57,58], and peaks at 375 (Eg) and 385 (Ag) cm −1 correspond to the stretching S-S mode of Ru in cobalt ruthenium sulfide bonds [28]. The above Raman spectra results indicate that the cobalt ruthenium sulfides (Co2RuS6) showed the characteristic modes of both cobalt sulfides and ruthenium sulfides. Figure 5 shows HR-TEM images of the hydrothermally synthesized cobalt ruthenium sulfides. All cobalt ruthenium sulfide materials are irregular in shape, moderately polydisperse, and somewhat aggregated [59].  Figure 5f,g are approximately 20 and 15 nm in size, respectively. These morphology results reveal that in the preparation of Co2RuS6 ternary sulfides, when the reaction time is increased to 48 h, the resulting nanocrystal size decreases slightly. All cobalt ruthenium sulfides formed nanocrystals which were slightly larger than the nanocrystals of pristine RuS2 and Co9S8. Similar to cobalt-iron-pyrites (Co0.625Fe0.375S2) [60], the EDS spectrum confirms the coexistence of Co, Ru, and S elements in the Co2RuS6-36 material with an atomic ratio of 1.92:0.97:5.6 ( Figure 6), which is in good agreement with the stoichiometry of Co2RuS6.   To further characterize the elemental chemical composition and detailed bonding configurations of the samples, XPS measurements were carried out, and Figure 7 demonstrates that the as-prepared samples are mainly composed of Co, Ru, and S elements without any obvious impurities (O peaks originate from surface oxidation of materials). The XPS survey spectra of the cobalt ruthenium sulfides (Figure 7a) confirmed the presence of Co 2p, Ru 3d, Ru 3p, S 2p, and O 1s states. As shown in Figure 7b, two prominent peaks were observed at binding energies of 778.2 and 793.4 eV corresponding to the Co 2p3/2 and Co 2p1/2 spin-orbit peaks, respectively, which confirmed the existence of two kinds of cobalt oxidation states: Co 2+ and Co 3+ [49,61]. The spin-orbit splitting of Co 2p1/2 and Co 2p3/2 is over 15 eV, suggesting the coexistence of Co 2+ and Co 3+ [62]. As shown in Figure 7c, the deconvoluted spectrum of the Ru 3d state shows the presence of two peaks at 280.4 and 284.6 eV, corresponding to the divalent Ru 3d5/2 and Ru 3d3/2 states respectively. The difference between the two peaks (i.e., spin orbit splitting) is 4.2 eV [63]. As shown in Figure 7d, the deconvoluted spectrum of the Ru 3p state consists of two peaks centered at 461.9 eV and 484.4 eV, corresponding to the Ru 3p3/2 and Ru 3p1/2 states respectively [28,29]. The deconvoluted spectrum of sulfur states in cobalt ruthenium sulfide (Figure 7e) shows a main peak centered at 162.3 eV related to the S 2p3/2 state. Figure 7f shows the presence of an XPS peak corresponding to O 1s at a binding energy of 531.3 eV. The above experimental results are in good agreement with previous XPS studies of metal sulfides, confirming the formation of ternary metal sulfides [28,29,49]. Based on the XPS analysis, the near-surface composition of the ternary sulfides is comprised of Co 2+ /Co 3+ , Ru 2+ , and S 2− , which is characteristic of the Co2RuS6 phase. Additionally, the XPS analyses suggest an atomic Co/Ru/S ratio of 1.87:0.91:4.96, which nearly matches stoichiometric Co2RuS6. These results are very similar to results obtained for ternary transition metal sulfides (Co0.67Fe0.33S2), which are used as electrode materials for supercapacitor applications [49].

Electrochemical Performance
Metal sulfides are advanced materials for energy storage applications, such as supercapacitors and lithium-ion batteries. Many transition metal sulfides have attracted great attention as significant pseudocapacitive electrode materials for supercapacitors by virtue of their low electronegativity, good electrical conductivity, rich electrochemical redox sites, and large energy density. Still, their relatively low electrical conductivity and high volume changes severely hamper their use in wider commercial applications.
In this study, we analyzed the electrochemical behavior of all the cobalt ruthenium sulfides using CV (Figures 8a-d). The molar ratio of cobalt in the cobalt ruthenium sulfide microstructure enhances the electrochemical performance. The effect of the preparation time on the structure and electrochemical performance of cobalt ruthenium sulfide was studied. The specific capacitances (Cs) from cyclic voltammetric studies were calculated using the following formula: where Q is the voltammetric charge, ΔV is the potential range, and m is the mass of the active material in the electrode. The Cs values of Co2RuS6-24 (50 F g −1 ) and Co2RuS6-48 (70 F g −1 ) are lower than that of Co2RuS6-36 (95 F g −1 ) at a scan rate of 5 mV s −1 (Figure 8a). Furthermore, the cyclic voltammograms of the Co2RuS6-36 electrode material at different scan rates are demonstrated in Figure 8b. The shape of the cyclic voltammogram changes as the scan rate increases: the potentials of the anodic and cathodic peaks shift to slightly more positive and negative values, respectively, because of electrode polarization [64]. Figure 8c presents cyclic voltammograms of RuS2, CoRu2S6, and CoRuS6 electrode materials at a scan rate of 5 mV s −1 . CoRu2S6 and CoRuS6 electrode samples showed similar cyclic voltammograms which both have lower integral areas than RuS2 electrode thus showed less specific capacitances. The cyclic voltammograms of cobalt ruthenium sulfides feature a pair of redox peaks, indicating that the capacitance is governed by pseudocapacitance and electrochemical double layer capacitance. The specific capacitances of RuS2 and cobalt ruthenium sulfides with different Co/Ru molar ratios and different time intervals are graphically represented in Figure 8d at a scan rate of 5 mV s −1 .  Figures 9a,b show typical GCD curves (2 nd cycles) for Co2RuS6-36 and Co2RuS6-24, respectively, at current densities of 1, 2, 3, and 5 A g −1 in a 0.5 M H2SO4 aqueous electrolyte within a potential range of 0-1 V (vs. Ag/AgCl). The specific capacitances (Cs) were calculated using the following formula: where i is the applied constant current, Δt is the discharge time, ΔV is the potential range, and m is the mass of the electrode material. The symmetry of the charging and discharging characteristics at all current densities reflects the good capacitive behavior of all the electrode materials. The Co2RuS6-36 electrode material showed specific capacitances of 75, 64, 56, and 48 F g −1 at current densities of 1, 2, 3, and 5 A g −1 , respectively (Figure 9a). The specific capacitance of Co2RuS6 electrode material is superior to reported ruthenium oxide materials like bulk RuO2 (49 F g −1 ) [65] and porous RuO2 (50 F g −1 ) [66] and comparatively lower cost. Our reported electrode material demonstrated higher electrochemical performance compared to similar other metal sulfides, such as MoS2 (17.5 F g −1 ) [67], CuS (62 F g −1 ) [68], WS2 (40 F g −1 ) [69], and carbon based materials like thermally reduced graphene oxide (47 F g −1 ) [70]. In addition, 64% of the capacitance was preserved when the GCD rate was increased from 1 to 5 A g −1 , suggesting good rate capability.
The Co2RuS6-24 electrode material showed specific capacitances of 55, 53, 45, and 44 F g −1 at current densities of 1, 2, 3, and 5 A g −1 , respectively (Figure 9b). In addition, 80% of the capacitance was preserved when the GCD rate was increased from 1 to 5 A g −1 , suggesting excellent rate capability. Figure 9c shows typical GCD curves (2 nd cycle) for CoRu2S6 (30 F g -1 ), CoRuS6 (35 F g −1 ), and Co2RuS6 (45 F g −1 ) electrodes, ternary metal sulfides prepared under the same reaction conditions (i.e., a reaction time of 24 h) with different Co/Ru molar ratios at a current density of 3 A g −1 . We can state that electrochemical performances affected by cobalt and ruthenium molar ratios in ternary metal sulfides. But, how it affects is still unclear and needs more research investigations.
The gravimetric energy density (ED) and power density (PD) were calculated from the GCD measurements using the following formulas: where C is the specific capacitance from charge/discharge studies, ΔV is the potential window, and Δt is the discharge time in charge/discharge cycles. The capacitance is constant, but the polarization limits the rate at which the charge may be delivered. This is an entirely kinetic phenomenon and not a thermodynamic one. The Co2RuS6-36 electrode material for supercapacitors showed an energy density of 10.5 Wh kg −1 and power density of 600 W kg −1 at a current density of 1 A g −1 . As the current density was increased from 1 to 5 A g −1 , the Co2RuS6-36 electrode material demonstrated an energy density of 6.7 Wh kg −1 and power density of 3001.5 W kg −1 . The Co2RuS6-24 composite electrode material for supercapacitors showed an energy density of 7.6 Wh kg −1 and power density of 600 W kg −1 at a current density of 1 A g −1 . As the current density was increased from 1 to 5 A g −1 , the Co2RuS6-24 electrode material exhibited an energy density of 6.1 Wh kg −1 and power density of 2745 W kg −1 . Cycle stability is a critical characteristic for practical applications in energy storage devices. Figure 10 shows the cycling performance of the Co2RuS6-36 electrode material at a scan rate of 10 mV s −1 . Significantly, the electrode exhibited good electrochemical stability with 81% retention of its specific capacitance over 1000 cycles.  To further elucidate the electrode kinetics, diffusion behavior, and the electrochemical characteristics of the frequency response behavior of the cobalt ruthenium sulfides, electrochemical impedance spectra were obtained in a frequency range of 100 KHz-0.1 Hz in the 0.5 M H2SO4 aqueous electrolyte. The results are shown in Figure 11 as Nyquist and Bode plots obtained at the open circuit voltage. The Nyquist plots comprise a semicircle in the high-frequency region, which originates from the charge-transfer resistance (Rct), and a vertical line in the low-frequency region (Figure 11a). They reveal the pseudocapacitive behavior and porous structure of the electrode materials. The maximum phase angle was found to be approximately 68° (Figure 11b). The response frequency of the Co2RuS6-36 electrode supercapacitor measured at the open circuit voltage was 1.84 Hz, and the corresponding response time was 0.54 s. The electrolyte solution resistance (Rs) values for the Co2RuS6-24, Co2RuS6-36, and Co2RuS6-48 samples were 2.72 Ω, 2.4 Ω, and 1.65 Ω, respectively. The charge-transfer resistance (Rct) values for the Co2RuS6-24, Co2RuS6-36, and Co2RuS6-48 electrode samples were 1.4, 0.9, and 3.15 Ω, respectively. This faradaic charge transfer resistance is closely associated with electron transfer at the electrode-electrolyte interface. The value of the charge transfer resistance (Rct) is the main part of the resistance of the supercapacitor. If the materials show lower charge transfer resistance, this indicates higher electrical conductivity (i.e., fast response ability of the electrode) [64,71]. Very low values of Rs and Rct indicate a fast charge-discharge process. All the Nyquist plots demonstrate Warburg impedance resistance behavior, which is associated with frequency-dependent ion-diffusion kinetics.

Conclusions
In summary, we have successfully synthesized cobalt ruthenium sulfides through a facile, simple hydrothermal method. Ternary metal sulfides normally possess richer redox-active sites and the formation of cobalt ruthenium sulfides with the synergistic effect of the Co and Ru elements, which have multiple valence states, promotes ions diffusion on the surface of the electrode. This results in improved electrochemical activity and specific capacitance. All the prepared cobalt ruthenium sulfides showed nanocrystal morphology. Therefore, the Co2RuS6-36 electrode material for pseudocapacitors manifested a favorable specific capacitance of 75 F g −1 at 1 A g −1 along with an energy density of 10.5 Wh kg −1 , a power density of 600 W kg −1 , and a good cycling stability with 81% capacitance maintained after 1000 cycles, making this material a promising candidate for energy storage. This facile synthetic process represents an efficient way to prepare electrode materials with enhanced electrochemical properties.