One pot and large-scale synthesis of nanostructured metal sulfides: Synergistic effect on supercapacitor performance

Abstract

Metal sulfides received key interest as an electrode material for storage and conversion of energy. Here, the novel nanostructured N₁₇S₁₈ and (CoNi)₃S₄ materials were synthesized via one-step hydrothermal method, and the synergistic effect of metal ions and electrochemical properties was investigated. A new and simple solution growth technique was employed in this work. The prepared nanopowders were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy techniques. The X-ray diffraction analysis of the prepared nanopowder revealed the formation of cubic phase cobalt nickel sulfides (CoNi)₃S₄ and hexagonal phase nickel sulfides (Ni₁₇S₁₈). Scanning electron microscopy analysis display fibrous, flakes and sheet-like morphology for Co_xS_x, N₁₇S₁₈ and (CoNi)₃S₄, respectively. Fibrous and sheet-like morphology exhibits higher electrochemical performance in supercapacitors. The electrochemical behavior of the amorphous Co_xS_x, crystallite Ni₁₇S₁₈ and (CoNi)₃S₄ modified electrodes was investigated using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge techniques. The specific capacitance of 57 F/g and 31 F/g were obtained for the amorphous Co_xS_x and crystalline (CoNi)₃S₄ powder, respectively. Amorphous Co_xS_x modified electrode retains 76% of initial capacitance after 1000 repeated cycling process. These results of this study suggest that the Co_xS_x and crystalline (CoNi)₃S₄ are appropriate materials for supercapacitor applications.

Keywords

Hydrothermal cobalt nickel sulfides nickel mesh supercapacitor synergistic effect

Introduction

The scarcity of conventional energy resources and the rising concerns on environmental issues are major motivating factors for the development of sustainable and clean energy devices. Supercapacitors received pivotal interest in industrial and research communities owing to their tremendous performance.^1–8 Supercapacitors can be grouped into two types like electrochemical double layer capacitors (EDLC) and electrochemical pseudocapacitors (EPC). The performance of EPC and EDLC depends on the capacitance of working electrode owing to reversible faradic redox reactions and charge separation at electrode/electrolyte interface, respectively.⁹ Much attention has been paid on the EPC as a result of their superior energy density than that of EDLC. Currently, researchers are interested to develop the nanosized transition metal oxides-based materials for several applications like energy and environmental applications.^10–29 Oxide semiconductors and conducting polymers are usually used in pseudo-capacitors.^30–38 For energy storage applications, the utilization of nanosized oxides of semiconductor has certain limitations. Metal oxides particularly, ruthenium (IV) oxide (RuO₂) and manganese (IV) oxide (MnO₂) are extensively studied for supercapacitors due to outstanding electrical conductivity, wide potential window, remarkable reversible features (charge–discharge). However, these materials are limited for real-time applications owing to the short performance rates and high fabrication cost. Particularly, the electrical conductivity of the majority of metal oxides is inadequate.³⁹ Hence, it is essential to develop novel active materials with attractive electrochemical performance for energy storage application, predominantly in supercapacitors.

Recently, metal sulfides received more interest than metal oxides for energy storage applications particularly in supercapacitors due to their superior electronic conductivity, high specific capacitance, excellent redox reversibility, viable to environment and cost effective nature. Thus, metal sulfides are suitable alternates than metal oxides for supercapacitor applications.⁴⁰ Among the materials, molybdenum sulfide (MoS), vanadium sulfide (V₂S), nickel sulfide (NiS) and cobalt sulfide (CoS) have shown better performance in electrochemical applications.^41–44 Although the performance of those metal sulfides is excellent and produces less power density, the binary and ternary sulfides show higher capacitive performance than CoS or NiS owing to their redox reactions and superior electronic behavior ensuing from the synergistic effect of the binary or ternary metal (Co or Ni) ions.⁴⁵ Particularly, nickel cobalt sulfides with phase structures similar to spinal nickel cobalt oxide can exhibit excellent pseudocapacitive behavior owing to the contributions from both the Co and Ni ions with different valence states. Furthermore, the layered spinal structure is accurate for the intercalation/deintercalation of the electrolyte ions equilibrium to the valence of Co or Ni ions during the charge–discharge process. Here, the one-step hydrothermal technique is employed for synthesizing metal sulfides, since it is an adaptable and appropriate technique for the morphology, structure and phase control, particularly for the inorganic materials. To the best of author’s knowledge, nickel cobalt sulfide for supercapacitors application is less reported with similar synthetic procedure and morphological features. Pei et al. developed FeS₂/graphene aerogel composite through hydrothermal method and reported specific capacitance of 313 F g⁻¹ at 0.5 Ag⁻¹. Further, the enhanced capacitance of the materials is mainly due to the flower-like FeS₂ clusters and stable interlinked graphene aerogel.⁴⁶ Li et al. synthesized hollow structured Ni₃S₂ with B-NiS nanorod arrays and displayed specific capacitance of 1158 F g⁻¹ at 2 Ag ⁻¹.⁴⁷ Wang et al. synthesized nickel cobalt sulfide (CoNi₂S₄) via electrodeposition method. Further, the authors reported that the enhanced conductivity and fast charge transfer rate of the fabricated CoNi₂S₄ electrode materials is mainly due to the inclusion of Ni layer. It showed 90.1% retention after 10,000 cycles.⁴⁸ Liu et al. fabricated NiCo₂S₄@CNT nanowire via two-step hydrothermal method and achieved specific capacitance of 2332 F cm⁻³ at 0.04 A cm⁻³. The enhanced capacitance of NiCo₂S₄@CNT is essentially due to the multiple faradic redox reactions, superior ionic transfer channels, bigger surface area and attractive conductivity of electrode.⁴⁹ Akin et al. synthesized Ni_xS_y nanoparticles and Ni₁₇S₁₈/CNT nanocomposites by modified hydrothermal method for hydrogen evolution. Further, the authors utilized NiC_l2.6H₂O, thioacetamide, diethylene glycol as a precursor, and the mixtures were thermally treated at 150°C for 5 h in an autoclave. The ball-like cluster morphology was obtained for Ni_xS_y.⁵⁰ Beka et al. fabricated three-dimensional core/shell structured nickel cobalt sulfide (NiCo₂S₄) through a series of hydrothermal method for supercapacitor applications. Further, the authors used CoCl₂, NiCl₂ and Na₂S as a precursor and carried out thermal treatment at 180°C for 8 h. The nanoneedle-like morphology was obtained for NiCo₂S₄.⁵¹

In this study, the novel nanostructured cobalt, nickel and nickel cobalt sulfides were synthesized by one-step hydrothermal method and their physical and chemical properties were carried out. The electrochemical properties were investigated from the electrochemical workstation. The formation of fibrous and sheet-like morphology of Co_xS_x and (CoNi)₃S₄ exhibits several remarkable electrochemical features for supercapacitors such as rapid electrolyte diffusion and superior electrical conductivity. The present approach is promising for the preparation of metal sulfides in large-scale level and also easy to adopt this method for commercial applications. This report deals with the preparation of cobalt nickel sulfides powder using effortless and inexpensive technique. The present study clearly indicates that the synthesized nickel sulfides and cobalt nickel sulfide nanomaterials are promising candidates for supercapacitor applications.

Experimental

Preparation of materials

The (CoNi)₃S₄ powder was synthesized via a facile hydrothermal route. In a typical experimental route, 0.2 M of cobalt (II) acetate tetrahydrate (C₄H₆CoO₄.4H₂O) and nickel acetate tetrahydrate ((CH₃.COO)₂Ni.4H₂O) were dissolved in 50 ml of double distilled water. Then, 10 ml of equimolar solution of sodium sulfide (Na₂S) was added into the above precursor solution with vigorous stirring and subsequently suspension was obtained. Then, the obtained suspension was transferred into a 50-ml Teflon-lined stainless steel autoclave and heated at 120°C for 10 h. The resultant precipitate was thoroughly washed and dried. Scheme 1 shows the detailed experimental procedure for the synthesis of (CoNi)₃S₄ and binary sulfides (cobalt and nickel sulfides). The single-component sulfides and binary sulfides were synthesized in order to investigate their electrochemical performance. The single-component sulfides such as cobalt sulfide and nickel sulfide were also prepared using the hydrothermal route. For the preparation of cobalt sulfide and nickel sulfide materials, 0.2 M (C₄H₆CoO₄.4H₂O) solution was prepared using 50 ml of double distilled water and kept in a separate beaker. A 0.2 M of ((CH₃.COO)₂Ni.4H₂O) solution was prepared using 50 ml of double distilled water and kept in a separate beaker. Then, 10 ml of equimolar solution of sodium sulfide (Na₂S) was added into the precursor (cobalt and nickel) solutions with vigorous stirring and consequently respective suspension were obtained. Both obtained suspensions were transferred into a separate 50-ml Teflon-lined stainless steel autoclave and heated at 120°C for 10 h. The cobalt sulfide and nickel sulfide samples were obtained after washing and drying the autoclave-treated samples.

Scheme 1.

Schematic representation of synthetic method.

Characterization of materials

The X-ray diffraction (XRD) patterns of all the samples were measured on a (XPERT-PRO) diffractometer with monochromatic CuKα – radiation (λ = 1.5406 Å). Fourier transform infrared spectroscopy (FTIR) of the samples were recorded on a (Thermo Nicolet 380, USA) spectrometer using a KBr pellet technique in the range of 4000–400 cm⁻¹. The field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX) were recorded on a Quanta FEG using an accelerating voltage of 20.00 kV. The electrochemical test was done using a three-electrode cell equipped with an Hg/HgO as reference electrode, platinum foil as a counter electrode and nickel mesh as a working electrode. The electrochemical properties of (CoNi)₃S₄, (Ni₁₇S₁₈) and amorphous Co_xS_x modified electrodes (electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), galvanostatic charge–discharge (GCD)) were examined using an electrochemical work station (biologic – SP 200) at room temperature.

Preparation of electrodes

The electrode modification is a significant process in electrochemical investigations. The working electrodes were prepared by mixing an electro-active material, acetylene black and polytetrafluoroethylene with a ratio of 75:15:10. This mixture was continuously grinded for 10 min in a mortar and then the desired film was achieved. The obtained film was coated on acid-treated Ni mesh with 1 cm² surface area and it is used as a current collector. The electrochemical measurements were carried out with an electrochemical work station and 2 M KOH was used as an electrolyte.

Results and discussion

XRD analysis

Figure 1 shows the XRD pattern of the synthesized nanopowders. Figure 1(a) clearly shows the X-ray amorphous nature of cobalt sulfide (Co_xS_x). The nickel sulfide sample shows (Figure 1(b)) XRD peaks at 45.7° and 34.6° and those peaks could be assigned to (306) and (303) crystalline planes of hexagonal phase of Ni₁₇S₁₈ which is in accordance with the JCPDS card no: 76-2306. XRD pattern (Figure 1(c)) of the cobalt nickel sulfide powder shows the peaks at 38.8° and 33.4° and those peaks may be assigned to (311) and (220) crystalline planes of cubic phase (CoNi)₃S₄ which is also in accordance with the JCPDS card no: 11-0068. The crystallite size of Ni₁₇S₁₈ and (CoNi)₃S₄ samples were calculated using Scherer’s equation. The average crystallite sizes of about 20 and 21 nm are achieved for Ni₁₇S₁₈ and (CoNi)₃S₄ samples, respectively.

Figure 1.

XRD patterns of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ powder and (c) (CoNi)₃S₄ powder.

FTIR analysis

Figure 2 shows the FTIR spectra of amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ samples. Figure 2(a) shows the FTIR spectrum of amorphous Co_xS_x and a peak was observed at 1646 cm⁻¹ due to the bending vibration of absorbed H₂O. Tang et al. reported that the peaks at 1107 and 1630 cm⁻¹ correspond to the bending vibrations of the sulfide groups and bending vibrations of absorbed H₂O, respectively.⁴⁵ Recent report says, the band at 555 cm⁻¹ can be attributed to the Ni-S stretching vibration mode.⁵² Figure 2(b) and (c) shows the peak at around 513 and 508 cm⁻¹ which are assigned to the Ni-S and Co-S stretching vibration mode, respectively. The peaks (Figure 2(a) and (b)) appeared at 613 and 723 cm⁻¹ may be assigned to the Co-S-Co and Ni-S-Ni bending vibration mode, respectively. The appearance of a peak at 1101 cm⁻¹ corresponds to the bending vibrations of the sulfide groups in all the synthesized samples. Figure 2(b) shows the FTIR spectrum of Ni₁₇S₁₈ nanopowder, and a peak observed at 1572 cm⁻¹ is due to the bending vibration of O–H from absorbed H₂O.³⁶

Figure 2.

FTIR spectra of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ nanopowder and (c) (CoNi)₃S₄ nanopowder.

FE-SEM with EDX

Figure 3 shows the FE-SEM images of synthesized metal sulfide powders. Figure 3(a) shows the FE-SEM image of amorphous Co_xS_x powder and it clearly reveals the fibrous morphology. Figure 3(b) and (c) shows the morphological features of Ni₁₇S₁₈ and (CoNi)₃S₄ powders. The FE-SEM observation clearly evinces the formation of flakes and sheet-like structures in the Ni₁₇S₁₈ and (CoNi)₃S₄ samples, respectively. EDX spectra of amorphous Co_xS_x, Ni₁₇S₁₈ and (CoNi)₃S₄ nanomaterials are shown in Figure 4. The percentages of cobalt (Co) and sulfur (S) in the amorphous Co_xS_x are estimated to be 38% and 12%, respectively (Figure 4(a)). The Ni₁₇S₁₈ powder shows (Figure 4(b)) the presence of major elements such as Ni (Nickel) and sulfur. The percentages of Ni and S in Ni₁₇S₁₈ nanopowder materials are estimated to be 45% and 5%, respectively. The percentages of Co, Ni and S in (CoNi)₃S₄ nanopowder are estimated to be 14%, 27% and 6%, respectively (Figure 4(c)). It should be indicated that the presence of both Co and Ni in (CoNi)₃S₄ was observed. The EDX observation clearly reveals the presence of considerable amount of Co and Ni in the synthesized samples.

Figure 3.

FE-SEM images of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ nanopowder and (c) (CoNi)₃S₄ nanopowder with different magnifications.

Figure 4.

EDX spectra of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ nanopowder and (c) (CoNi)₃S₄ nanopowder.

Electrochemical impedance analysis

Figure 5 shows the impedance plots of the amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ coated Ni mesh electrodes in the frequency range of KHz–1 Hz at a bias potential of 100 mV. The intersection of the impedance plots and the real axis (Z′) represents the ohmic resistance of the electrode and the radius of the high frequency loop corresponds to the charge transfer resistance.⁵³ The impedance plots on the negative side of the imaginary axis (Z″), which is corresponding to the inductance may be due to the inductive behaviors of the current collector and external circuit. The R_ct for amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes was obtained from the Nyquist plot of the x-axis. The impedance curves are showing semi-circle in the high frequency region. The ohmic resistance and the charge transfer resistance of the Ni₁₇S₁₈ is larger than the amorphous Co_xS_x, and crystalline (CoNi)₃S₄ modified electrodes, demonstrating that the Co_xS_x and Ni₁₇S₁₈ modified electrodes possess better electronic conductivity and a faster charge transfer rate than the (CoNi)₃S₄ modified electrode. The obtained R_ct values of amorphous Co_xS_x, Ni₁₇S₁₈ and (CoNi)₃S₄ electrodes are 142.8, 230.8 and 564.0 Ω, respectively (Table 1).

Figure 5.

Electrochemical impedance spectra of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ nanopowder and (c) (CoNi)₃S₄ nanopowder-modified nickel mesh electrodes in 2 M KOH.

Table 1.

Resistance values of Co_xS_x, (CoNi)₃S₄ and Ni₁₇S₁₈ materials.

Sample	Resistance (R_ct) value
Co_xS_x	142.8 Ω
Ni₁₇S₁₈	230.8 Ω
(CoNi)₃S₄	564.0 Ω

Characteristics of capacitance

CVs of the amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes in 2 M KOH electrolyte at a scan rate of 25 mV/s is presented in Figure 6(a). The redox has peaks that appeared as a result of the redox reactions of all samples. The CV curve of the amorphous Co_xS_x and crystalline (CoNi)₃S₄ shows a much larger integrated area than that of the Ni₁₇S₁₈ electrode, which reveals the superior electrochemical activity of amorphous Co_xS_x and (CoNi)₃S₄ modified electrodes in KOH electrolyte. It may be possible to achieve better capacitance for the cobalt nickel sulfide-modified electrode due to the beneficial synergistic effect between the Co and NiS. However, the peak current value for (CoNi)₃S₄ modified electrode is slightly lower compared to amorphous Co_xS_x modified electrode because of less surface area of the (CoNi)₃S₄ which affect the capacitance nature. Figure 6(b) to (d) shows the CV curve of amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes at a scan rate of 25–150 mV/s. It is observed from the CV analysis that the increase of current while increasing the scan rate from 25 to 150 mV/s may be attributed to high ionic transport of materials at a higher scan rate. The indicated current-potential response is based on the capacitance and that is mainly owing to the oxidation and reduction reaction of modified electrodes. Scheme 2 shows the electrochemical process for the pseudocapacitive behavior of the active materials. It clearly indicates that the prepared materials have pseudocapacitive nature because electrons created by the electrochemical redox process must be moved to the current collector through the electro-active material layer to produce current. Accordingly, the electronic conductivity of the active materials is important to illustrate the electro-active material left from the current collector.

Figure 6.

(a) CV of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ and (c) (CoNi)₃S₄ modified nickel mesh electrodes in 2 M KOH at a scan rate of 25 mV/s; (b) CV of amorphous Co_xS_x, (c) Ni₁₇S₁₈ and (d) (CoNi)₃S₄ modified nickel mesh electrodes at a various scan rates.

Scheme 2.

Electrochemical process for the pseudocapacitive behavior of the active material.

To further assess the electrochemical behavior of the active materials (amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes), GCD was carried out in a 2 M KOH solution. The GCD curves of amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes (Figure 7) were measured at a current density of 0.5 A/g. The GCD curves reveal the non-linear behavior with increased charge duration. The discharge duration is very short owing to the pseudocapacitive behavior of the modified electrodes, which is in good agreement with CV results. The specific capacitance of the metal sulfide-modified electrodes can be deliberated from the following formula (1)

C_{s} = I Δ t / m Δ V

(1)

where C _s (F/g) is the specific capacitance, I (mA) is the discharge current, Δt (s) is the discharge time, ΔV (V) is the potential window during discharge and m (g) is the mass of the active material within the working electrode. The estimated specific capacitance of the amorphous Co_xS_x, crystalline Ni₁₇S₁₈ and crystalline (CoNi)₃S₄ modified electrodes are 57 F/g, 5 F/g and 31 F/g, respectively. The amorphous Co_xS_x and crystalline (CoNi)₃S₄ modified electrodes have shown long charge and discharge activities compared to crystalline Ni₁₇S₁₈ modified electrode. The better capacitance performance of the amorphous Co_xS_x and crystalline (CoNi)₃S₄ modified electrodes are further confirmed. Figure 8(a) and (b) shows the charge–discharge curves of the amorphous Co_xS_x and crystalline (CoNi)₃S₄ modified electrode at different current densities (0.5–3 A/g). Figure 8(c) shows the specific capacitances of the amorphous Co_xS_x and crystalline (CoNi)₃S₄ modified electrodes, which were calculated based on the charge–discharge curves at different current densities. The anions have acceptable time to transfer between the electrolyte and modified surface^30,34 of the amorphous Co_xS_x and (CoNi)₃S₄ powder at a low current density, resulting in better specific capacitance. The amorphous Co_xS_x have shown superior capacitance behavior; hence we investigated the stability of the amorphous Co_xS_x modified electrode by continuous cycling of more than 1000 times at 1 A/g. The charging and discharging performance of amorphous Co_xS_x modified electrode was carried out and shown in Figure 9 (last 10 cycles, i.e. 991–1000). The estimated capacitance retention of the amorphous Co_xS_x modified electrode is 30 F/g (76%). This result clearly demonstrates that the electro-active material possesses excellent durable electrochemical stability at 1 A/g, which is attributed to the good stability of the active layer with nickel mesh frame. The higher specific capacitance of the amorphous Co_xS_x modified electrode could be owing to the following two factors. First, the improved electronic conductivity provides quick charge transfer in the metal sulfide modified electrode. Second, the effective structure of the amorphous Co_xS_x modified electrode provides superior surface area and more active sites, which can noticeably improve the utilization and the capacitance of active materials.

Figure 7.

GCD of (a) amorphous Co_xS_x, (b) Ni₁₇S₁₈ and (c) (CoNi)₃S₄ modified nickel mesh electrodes in 2 M KOH at 0.5 A/g.

Figure 8.

GCD of (a) amorphous Co_xS_x and (b) (CoNi)₃S₄ modified nickel mesh electrode in 2 M KOH at 0.5 – 3 A/g; (c) specific capacitance of (a) amorphous Co_xS_x, (b) (CoNi)₃S₄ modified nickel mesh electrodes at different current densities.

Figure 9.

GCD of amorphous Co_xS_x modified nickel mesh electrode for last 10 (991st–1000th) cycles recorded in 2 M KOH at 1 A/g.

Conclusions

In summary, we have successfully synthesized nanostructured (CoNi)₃S₄ and Ni₁₇S₁₈ electrode materials via one-step hydrothermal route. The physicochemical properties of the synthesized transition metal sulfide nanopowders were investigated. The cubic and hexagonal crystallite phases were found for (CoNi)₃S₄ and Ni₁₇S₁₈, respectively. Scanning electron microscopy analysis demonstrate fibrous, flakes and sheet-like morphology for Co_xS_x, N₁₇S₁₈ and (CoNi)₃S₄, respectively. Among this morphologies, fibrous and sheet-like morphology displayed higher electrochemical performance in supercapacitors. EIS analysis reveals that the amorphous Co_xS_x modified electrode has low resistance compared to the crystalline Ni₁₇S₁₈ modified electrode. The Co_xS_x electrode material exhibits specific capacitance of 57 F/g in 2 M KOH electrolyte at a current density of 0.5 A/g. The enhanced capacitance of Co_xS_x is mainly due to improved electronic conductivity, rapid electrolyte diffusion and effective structure. Further, Co_xS_x electrode material displayed 76% of capacitance retention after 1000 cycles at 1 A/g and lower resistance.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This article has been written with the financial support of RUSA-Phase 2.0 grant sanctioned vide Letter No. F.24-51/2014-U, Policy (TNmulti-Gen), Department of Education, Government of India, dated: 9 October 2018. The authors gratefully acknowledge the Department of Science and Technology, New Delhi, for the financial support in general and infrastructure facilities sponsored under PURSE 2nd Phase program (Order No. SR/PURSE Phase 2/38 (G) dated: 21 February 2017).

ORCID iD

S Karuppuchamy

C Karthikeyan is presently working as a teaching faculty in the Department of Energy Science, Alagappa University, Karaikudi, India. He earned his PhD in Chemistry at Bharathidasan University, Trichy, India. He is having 9 years of research experience in the field of nanomaterials and bioenergy. His current research interest includes nanomaterial, supercapacitor, solar cells, bioenergy, and photocatalytic materials. He has published more than 25 articles in refereed journals.

R Dhilip Kumar received his PhD in Energy Science from Alagappa University, Karaikudi, India. His research interest includes supercapacitors, nanocomposites and electrocatalysis.

J Anandha Raj is presently working as a Post Doctoral Fellow in the Department of Energy Science, Alagappa University, Karaikudi, India. He received his PhD in Chemistry at Alagappa University, Karaikudi, India.

S Karuppuchamy is currently working as a Professor and head in the Department of Energy Science, Alagappa University, Karaikudi, India. He received the BSc and MSc degree in Chemistry from Madurai Kamaraj University, India. He received his PhD degree in Materials Engineering from Gifu University, Japan. He has carried out his postdoctoral research at Kinki University, Japan. He holds 11 international patents and published more than 130 papers in refereed journals. He has also received several awards for his innovative research work. He has more than 20 years of research experience in the field of nanomaterials and photoelectrochemistry. His current research interests include solar cells, hybrid supercapacitors and photocatalytic materials.

References

Miller

Outlaw

Holloway

BC.

Graphene double-layer capacitor with ac line-filtering performance. Science 2010; 329: 1637–1639.

Pech

Brunet

Durou

, et al. Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nat Nanotechnol 2010; 5: 651–654.

Brezesinski

Wang

Tolbert

, et al. Ordered mesoporous α-MoO₃ with iso-oriented nanocrystalline walls for thin-film pseudocapacitors. Nat Mater 2010; 9: 146–151.

Mai

Yang

Zhao

, et al. Hierarchical MnMoO₄/CoMoO₄ heterostructured nanowires with enhanced supercapacitor performance. Nat Commun 2011; 5: 381–385.

Ania

Khomenko

Raymundo‐Piñero

, et al. The large electrochemical capacitance of microporous doped carbon obtained by using a zeolite template. Adv Funct Mater 2007; 17: 1828–1836.

Reddy

Shaijumon

Gowda

, et al. Multisegmented Au-MnO₂/carbon nanotube hybrid coaxial arrays for high-power supercapacitor applications. J Phys Chem C 2009; 114: 658–663.

Yang

Feng

2d sandwich‐like sheets of iron oxide grown on graphene as high energy anode material for supercapacitors. Adv Mater Weinheim 2011; 23: 5574–5580.

Zhou

Liu

Chen

, et al. Fabrication of Co₃O₄-reduced graphene oxide scrolls for high-performance supercapacitor electrodes. Phys Chem Chem Phys 2011; 13: 14462–14465.

Huang

Chen

Ding

, et al. Nickel–cobalt hydroxide nanosheets coated on NiCo₂O₄ nanowires grown on carbon fiber paper for high-performance pseudocapacitors. Nano Lett 2013; 13: 3135–3139.

10.

Matsui

Yamamoto

Izawa

, et al. Electron transfer behavior of calcined material obtained from a samarium-O–phenylene-S–nickel-S–phenylene-O hybrid copolymer. Mater Chem Phys 2007; 103: 127–131.

11.

Nagalakshmi

Karthikeyan

Anusuya

, et al. Synthesis of TiO₂ nanofiber for photocatalytic and antibacterial applications. J Mater Sci Mater Electron 2017; 28: 15915–15920.

12.

Thamima

Karuppuchamy

Synthesis, characterization and photocatalytic properties of rod-shaped titanium dioxide. J Mater Sci Mater Electron 2016; 27: 458–465.

13.

Santhi

Rani

Karuppuchamy

Degradation of Alizarin Red S dye using Ni doped WO₃ photocatalyst. J Mater Sci Mater Electron 2016; 27: 5033–5038.

14.

Miyazaki

Matsui

Nagano

, et al. Synthesis and electronic behaviors of TiO₂/carbon clusters/Cr₂O₃ composite materials. Appl Surf Sci 2008; 254: 7365–7369.

15.

Matsui

Kira

Karuppuchamy

, et al. The electronic behaviors of visible light sensitive Nb₂O₅/Cr₂O₃/carbon clusters composite materials. Curr Appl Phys 2009; 9: 592–597.

16.

Karuppuchamy

Iwasaki

Minoura

Photoelectrochemical and photocatalytic properties of electrodeposited nanocrystalline titanium dioxide thin films. Vacuum 2007; 81: 708–712.

17.

Karuppuchamy

Ito

Cathodic electrodeposition of nanoporous ZnO thin films from new electrochemical bath and their photoinduced hydrophilic properties. Vacuum 2008; 82: 547–550.

18.

Karuppuchamy

Suzuki

Ito

, et al. A novel one-step electrochemical method to obtain crystalline titanium dioxide films at low temperature. Curr Appl Phys 2009; 9: 243–248.

19.

Karthikeyan

Thamima

Karuppuchamy

Dye removal efficiency of perovskite structured CaTiO₃ nanospheres prepared by microwave assisted method. Mater Today-Proc. Epub ahead of print 15 July 2019. DOI: 10.1016/j.matpr.2019.05.421

20.

Kumar

Karuppuchamy

Facile synthesis of honeycomb structured SnO/SnO₂ nanocomposites by microwave irradiation method. J Mater Sci Mater Electron 2015; 26: 6439–6443.

21.

Matsui

Okajima

Karuppuchamy

The electronic behavior of V₂O₃/TiO₂/carbon clusters composite materials obtained by the calcination of a V(acac)₃/TiO(acac)₂/polyacrylic acid complex. J Alloys Compd 2009; 468: L27–L32.

22.

Karuppuchamy

Brundha

Eco-friendly synthesis of core–shell structured (TiO₂/Li₂CO₃) nanomaterials for low cost dye-sensitized solar cells. Ecotoxicol Environ Saf 2016; 134: 332–335.

23.

Miyazaki

Matsui

Kuwamoto

, et al. Synthesis and photocatalytic activities of MnO₂-loaded Nb₂O₅/carbon clusters composite material. Microporous Mesoporous Mater 2009; 118: 518–522.

24.

Matsui

Saitou

Karuppuchamy

, et al. Photo-electronic behavior of Cu₂O-and/or CeO₂-loaded TiO₂/carbon cluster nanocomposite materials. J Alloys Compd 2012; 538: 177–182.

25.

Karthikeyan

Karuppuchamy

Synthesis of novel CuO-Al₂O₃ catalyst for biodiesel production. Adv Sci Eng Med 2017; 9: 1011–1016.

26.

Kumar

Karuppuchamy

Synthesis and characterization of nanostructured Zn-WO₃ and ZnWO₄ by simple solution growth technique. J Mater Sci Mater Electron 2015; 26: 3256–3261.

27.

Karthikeyan

Karuppuchamy

Transesterification of Madhuca longifolia derived oil to biodiesel using Mg–Al hydrotalcite as heterogeneous solid base catalyst. Mater Focus 2017; 6: 101–106.

28.

Thamima

Karuppuchamy

Biosynthesis of titanium dioxide and zinc oxide nanoparticles from natural sources: a review. Adv Sci Eng Med 2015; 7: 18–25.

29.

Matsui

Santhi

Sugiyama

, et al. Visible light-induced photocatalytic activity of SiO₂/carbon cluster composite materials. Ceram Int 2014; 40: 2169–2172.

30.

Kumar

Andou

Karuppuchamy

Synthesis and characterization of nanostructured Ni-WO₃ and NiWO₄ for supercapacitor applications. J Alloys Compd 2016; 54: 349–356.

31.

Kumar

Andou

Sathish

, et al. Synthesis of nanostructured Cu-WO₃ and CuWO₄ for supercapacitor applications. J Mater Sci Mater Electron 2016; 27: 2926–2932.

32.

Meher

Rao

GR.

Ultralayered Co₃O₄ for high-performance supercapacitor applications. J Phys Chem C 2011; 115: 15646–15654.

33.

Kumar

Andou

Karuppuchamy

Microwave-assisted synthesis of Zn–WO₃ and ZnWO₄ for pseudocapacitor applications. J Phys Chem Solids 2016; 92: 94–99.

34.

Kumar

Karuppuchamy

Microwave mediated synthesis of nanostructured Co-WO₃ and CoWO₄ for supercapacitor applications. J Alloys Compd 2016; 674: 384–391.

35.

Kung

Chen

Lin

, et al. Synthesis of Co₃O₄ nanosheets via electrodeposition followed by ozone treatment and their application to high-performance supercapacitors. J Power Sources 2015; 214: 91–99.

36.

Kumar

Karuppuchamy

Microwave-assisted synthesis of copper tungstate nanopowder for supercapacitor applications. Ceram Int 2014; 40: 12397–12402.

37.

Ghenaatian

Mousavi

Rahmanifar

MS.

High performance hybrid supercapacitor based on two nanostructured conducting polymers: self-doped polyaniline and polypyrrole nanofibers. Electrochim Acta 2012; 78: 212–222.

38.

Sumboja

Wang

Yan

, et al. Nanoarchitectured current collector for high rate capability of polyaniline based supercapacitor electrode. Electrochim Acta 2012; 65: 190–195.

39.

Ikkurthi

Rao

Jagadeesh

, et al. Synthesis of nanostructured metal sulfides via a hydrothermal method and their use as an electrode material for supercapacitors. New J Chem 2018; 42: 19183–19192.

40.

Zhang

, et al. In situ carbon-coating and Ostwald ripening-based route for hollow Ni₃S₄@C spheres with superior Li-ion storage performances. RSC Adv 2016; 6: 101752–101759.

41.

Krishnamoorthy

Veerasubramani

Radhakrishnan

, et al. Supercapacitive properties of hydrothermally synthesized sphere like MoS₂ nanostructures. Mater Res Bull 2014; 50: 499–522.

42.

Feng

Sun

, et al. Metallic few-layered VS₂ ultrathin nanosheets: high two-dimensional conductivity for in-plane supercapacitors. J Am Chem Soc 2011; 133: 17832–17838.

43.

Krishnamoorthy

Veerasubramani

Radhakrishnan

, et al. One pot hydrothermal growth of hierarchical nanostructured Ni₃S₂ on Ni foam for supercapacitor application. Chem Eng J 2014; 251: 116–122.

44.

Justin

Rao

GR.

CoS spheres for high-rate electrochemical capacitive energy storage application. Int J Hydrogen Energy 2010; 35: 9709–9715.

45.

Tang

Chen

, et al. A highly electronic conductive cobalt nickel sulfide dendrite/quasi-spherical nanocomposite for a supercapacitor electrode with ultrahigh areal specific capacitance. J Power Sources 2015; 295: 314–322.

46.

Pei

Yang

Chu

, et al. Self-assembled flower-like FeS₂/graphene aerogel composite with enhanced electrochemical properties. Ceram Int 2016; 42: 5053–5061.

47.

Wang

Xin

, et al. Single-crystal β-NiS nanorod arrays with a hollow-structured Ni₃S₂ framework for supercapacitor applications. J Mater Chem A 2016; 4: 7700–7709.

48.

Wang

Huang

Xiao

, et al. Hierarchical nickel cobalt sulfide nanosheet on MOF-derived carbon nanowall arrays with remarkable supercapacitive performance. Carbon 2019; 147: 146–153.

49.

Liu

Pan

Ding

, et al. In-situ growth of vertically aligned nickel cobalt sulfide nanowires on carbon nanotube fibers for high capacitance all-solid-state asymmetric fiber-supercapacitors. J Energy Chem 2020; 41: 209–215.

50.

Akin

Aslan

Hatay Patir

Enhanced hydrogen evolution catalysis at the liquid/liquid interface by Ni_xS_y and Ni_xS_y/Carbon Nanotube Catalysts. Eur J Inorg Chem 2017; 33: 3961–3966.

51.

Beka

Liu

Nickel Cobalt Sulfide core/shell structure on 3D Graphene for supercapacitor application. Sci Rep 2017; 7: 2105.

52.

Yin

Zhou

Han

, et al. Shape and phase evolution of nickel sulfide nano/microcrystallines via a facile way. J Alloys Compds 2015; 620: 42–47.

53.

Liu

Xing

, et al. Development of novel self-humidifying composite membranes for fuel cells. J Power Sources 2003; 124: 81–89.