Glycine-assisted hydrothermal synthesis of NiCo2S4 as an active electrode material for supercapacitors

Nickel cobalt sulfide (NCS) was synthesized from nickel and cobalt nitrate (NCS-1) by a hydrothermal method. In another method, nickel cobalt sulfide (NCS-2) was also synthesized from hydrothermally synthesized nickel cobalt oxide (NCO). The syntheses of NCS-1 and NCO were conducted in the presence of glycine as a templating agent.

Journal of Science: Advanced Materials and Devices (2019) 376e380 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Glycine-assisted hydrothermal synthesis of NiCo2S4 as an active electrode material for supercapacitors M Sathish Kumar a, b, N Bhagavath a, b, Sudip K Batabyal c, **, Nikhil K Kothurkar a, b, * a Department of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India Center of Excellence in Advanced Materials & Green Technologies (CoE-AMGT), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India c Amrita Center for Industrial Research & Innovation (ACIRI), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India b a r t i c l e i n f o a b s t r a c t Article history: Received March 2019 Received in revised form 27 July 2019 Accepted 10 August 2019 Available online 20 August 2019 Nickel cobalt sulfide (NCS) was synthesized from nickel and cobalt nitrate (NCS-1) by a hydrothermal method In another method, nickel cobalt sulfide (NCS-2) was also synthesized from hydrothermally synthesized nickel cobalt oxide (NCO) The syntheses of NCS-1 and NCO were conducted in the presence of glycine as a templating agent The NCS and NCO samples were thoroughly characterized by different techniques XRD studies showed that both NCO and NCS consisted of the cubic crystal phases Cyclic voltammetry of NCO and NCS revealed that, with an increasing scan rate within the range of 10e100 mV sÀ1, the specific capacitance of the samples reduced The specific capacitance of NCS-2, measured by galvanostatic chargeedischarge, was found to be 675 F gÀ1, which is higher than those of NCO (313 F gÀ1) and NCS-1 (500 F gÀ1) The specific capacitance retention of NCS-2 was 88% over 1000 cycles, indicating the good cyclic stability of the material © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Nickel cobaltite Nickel cobalt sulfide Glycine Pseudocapacitor Supercapacitor Introduction Supercapacitors are an important emerging energy storage technology They have many advantages, including high power density, fast charge and discharge rates, and exceptionally long cycle stability [1,2], as compared to batteries They are suitable for many applications, such as in communication, transportation, consumer electronics and aerospace [2] Supercapacitors can be divided into two categories, namely, electrical double layer capacitors and pseudocapacitors Pseudocapacitors exhibit high specific capacitance, due to the quick reversible faradaic redox reactions from their electrode materials, which could be metal hydroxides, oxides or conducting polymers Among the numerous electrode materials, transition metal oxides, such as NiO [3,4], CuO [5,6], Ni(OH)2 [7], RuO2 [8], Co3O4 [9,10], Fe2O3 [11], and V3O7 [12] offer rich redox reactions and high specific capacitance A few limitations * Corresponding author Department of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India ** Corresponding author E-mail addresses: s_batabyal@cb.amrita.edu (S.K Batabyal), k_nikhil@cb.amrita edu (N.K Kothurkar) Peer review under responsibility of Vietnam National University, Hanoi such as high cost, toxicity and relatively low specific capacitance of RuO2 and the low electron mobility of MnO2, NiO and Co3O4 exist [13e15] NiCo2O4 is a favorable mixed-metal oxide that has been widely investigated in the field of lithium-ion batteries [16], supercapacitors [17,18], and optoelectronic devices [19] for its potential applications It has been reported that, nickel-cobalt binary metal oxides, such as nickel cobaltite (NiCo2O4), possess greater electronic conductivity and electrochemical activity than nickel and cobalt oxides In particular, NiCo2O4 can offer a synergistically greater electrochemical activity as compared to the two singlecomponent oxides [20] The remarkable electrochemical properties of NiCo2O4 have spurred the investigation of spinel NiCo2S4 that shows an even greater electrochemical performance [21] Also, NiCo2S4 has been reported to be an excellent supercapacitor material [22] As compared to NiCo2O4, NiCo2S4 has a much lower optical band gap and a much higher conductivity [21] The substitution of oxygen with sulfur can lead to a more flexible structure due to the lower electronegativity of sulfur This may enable the fabrication of electrodes with better electron transport and mechanical flexibility [22] This paper aims to compare two different synthesis methods of NiCo2S4 (NCS) and to investigate their effects on the properties and performance of the material The first method is the direct hydrothermal synthesis of NCS from the respective metal salts in the https://doi.org/10.1016/j.jsamd.2019.08.005 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M.S Kumar et al / Journal of Science: Advanced Materials and Devices (2019) 376e380 377 presence of glycine The second method is a two-step process In the first step, NiCo2O4 (NCO) was hydrothermally synthesized in the presence of glycine and in the second step, the NCO was converted into NCS by its reaction with sodium sulfide The role of glycine in both the syntheses is in modifying the morphology of the material [20] The NCO and NCS samples synthesized by these two methods were thoroughly characterized and their electrochemical performance was analyzed Experimental 2.1 Materials and chemicals Nickel nitrate hexahydrate (Ni(NO3)2.6H2O) (98%) (Rankem), cobalt nitrate hexahydrate (Co(NO3)2.6H2O) (98%) (Rankem), sodium sulfide (Na2S), and glycine (C8H9NO3) (Nice) were used for synthesizing the samples Fig FTIR spectra of NCO, NCS-1 and NCS-2 2.2 Preparation All the analytical grade reagents were used as purchased In a typical experiment, for the hydrothermal synthesis of NiCo2O4, 0.2 M Ni(NO3)2.6H2O, 0.4 M Co(NO3)2.6H2O and 0.2 M of glycine were added to 30 ml of de-ionized water and stirred until the solution became homogeneous The mixture solution was then poured into a 50 mL Teflon-lined stainless steel autoclave kept in a hot air oven at 200  C for h The final product was washed with DI water several times, filtered and kept in a hot air oven at 100  C The obtained product (nickel cobaltite) was finely ground in a mortar and pestle and named as NCO [14] For the synthesis of NiCo2S4, two methods were followed In the first method, 0.2 M of Ni(NO3)2.6H2O, 0.4 M of Co(NO3)2.6H2O and 0.2 M of glycine were added to 30 ml of deionized water and stirred until the solution became homogeneous The above solution was placed in a 50 mL Teflon lined stainless steel autoclave containing 0.05 M sodium sulfide solution and maintained at 200  C for h [15] The final product was washed with DI water several times, filtered and kept in hot air oven at 100  C The obtained product was NiCo2S4 named as NCS-1 In the second method, the NCO synthesized as described above, was taken along with sodium sulfide 0.05 M and transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 200  C for h The obtained product was washed with DI water several times and dried at 100  C This product was named as NCS-2 NCO corresponds to the CoeO bond vibration Some residual glycine and its degradation products remained in the samples, which were inferred from the presence of the eC¼Oe stretching band at 1635 cmÀ1 and the eC¼Ce stretching bands at 2182 cmÀ1, respectively [15] Fig shows the XRD patterns of NCO, NCS-1 and NCS-2 For NCO, the peaks at 35.2, 36.5, 38.44, 42.69 and 51.57 were indexed to the reflections from the (220), (311), (222), (400) and (422) planes of the cubic phase of NiCo2O4 (JCPDS card no 20-0781) [16,24] Similarly, for NCS-1 and NCS-2, the peaks at 27.30, 30.77, 33.32, 36.87, 38.89, 42.69, 50.98 and 52.46 correspond to the (220), (311), (222), (400), (235), (422), (511) and (440) planes of cubic NiCo2S4 (JCPDS card no 20-0782) [17,25] Due to the appearance of peaks at similar positions, it is difficult to distinguish NCS from NCO in the XRD patterns So the presence of small quantities of oxide in the sulfide samples (NCS-1, NCS-2) cannot be ruled out As compared to NCS-1, NCS-2 shows some shifting of peaks, which might be due to the presence of an interface between the oxide and sulfide The prominent peak corresponding to (235) in NCS-2 is indicative of the crystal growth leading to the petal-like morphology of the material FESEM images of NCO (a), NCS-1 (b) and NCS-2 (c) are shown in Fig NCO showed a flake-like morphology The flakes were roughly 0.6e0.7 mm across and approximately 60e70 nm thick They could be seen to be made of particles ~50e60 nm in size In 2.3 Characterization Thermo-Scientific Nicolet IS10 IR spectrometer was used to measure FT-IR spectra X-ray diffraction (XRD) was characterized by Bruker D8 Advanced instrument ZIVE SP1 instrument was used to carry out cyclic voltammetry (CV) and galvanostatic chargeedischarge measurements FESEM was performed using 30 keV Carl Zeiss Merlin compact and GeminiSEM 300 microscope Results and discussion FTIR analyses of NCO, NCS-1 and NCS-2 samples were conducted, and the results are shown in Fig All the samples showed a broad band located in the range of 3200e3500 cmÀ1, which is attributed to the moisture adsorbed on the samples The peak at 1384 cmÀ1 is attributed to the presence of physisorbed CO2 [23] For NCS-1 and NCS-2, the bands at 631 cmÀ1 (symmetrical stretch) and 1100 cmÀ1 (asymmetrical stretch) correspond to the NieS or CoeS vibrations These groups play an important role in the faradaic reactions of these active electrode materials The peak at 641 cmÀ1 in Fig XRD patterns of NCO, NCS-1, and NCS-2 378 M.S Kumar et al / Journal of Science: Advanced Materials and Devices (2019) 376e380 Fig SEM images of (a) NCO, (b) NCS-1, and (c) NCS-2 some places, the flakes had bunched up to form a somewhat spherical lump of ~2000e2500 mm NCS-1 (Fig (b)) showed a complex morphology consisting of a few irregular structures on the scale of 1e2 mm, in addition to a finer nanostructure consisting of densely-packed nanoscale protrusions NCS-2 showed (Fig (c)) a complex morphology of interconnected lamellae resembling like the petals of a marigold flower The complex morphologies of all three samples are attributable to the presence of glycine as a templating agent, and they are likely to enhance ion-transport leading to good electrochemical performance It should be noted that the lamellae in NCS-2 are the thinnest at approximately 10e20 nm, which provides the highest surface area, leading to a stronger interaction with the electrolyte The connectivity of the nanostructure is also likely to provide good electrical conductivity, leading to good performance So NCS-2, which was produced by the two-step method, involving the synthesis of NCO and the hydrothermal conversion of the NCO to NCS, is expected to exceed the other two samples in electrochemical performance Additional SEM images of NCS-1 and NCS-2 are shown in the Supplementary Information (Figure SI-1) Fig shows the FESEM image (a), overlaid EDS (energy dispersive X-ray spectroscopy) elemental maps for Ni, Co, and S (b), the EDS spectrum (c) and the individual elemental maps (def) for NCS-2 The maps indicate that the above three elements were uniformly distributed throughout the sample, which is in agreement with the XRD data confirming that the sample is NiCo2S4 The atomic ratios of Ni, Co, and S, as measured by EDS, reveal the compound as a sulfur-deficient material because of the presence of oxygen; probably there were some unconverted NCO along with the NCS-2 Maybe the extra interface between oxide and sulfide helped to enhance the capacitance Electrochemical measurements were done with the threeelectrode cell, which consisted of a working electrode, Ag/AgCl as a reference electrode and a platinum counter electrode in 6M KOH electrolyte on the NCO, NCS-1 and NCS-2 samples Fig (a) shows the CV curves of NCO, NCS-1 and NCS-2 at a scan rate of 100 mV sÀ1 NCS-2 possessed a larger included area and consequently a higher specific capacitance as compared to NCO and NCS-1 The shapes of the curves is indicative of a typical faradaic behavior of the materials [17] Well-defined redox peaks were observed in all CV curves, which Fig FESEM image of NCS-2 (a) and its corresponding mapping image (b) EDS analysis (c) and mapping images of cobalt (d), nickel (e) and sulfur (f) M.S Kumar et al / Journal of Science: Advanced Materials and Devices (2019) 376e380 379 Fig (a) Cyclic voltammetry of NCO, NCS-1 and NCS-2 at 100 mV s-À1 scan rate, (b) cyclic voltammetry of NCS-2 at different scan rates, and (c) a plot of the scan rate vs specific capacitance Fig (a) Galvanostatic chargeedischarge analyses of NCO, NCS-1 and NCS-2 at A gÀ1 current density; (b) Galvanostatic charge discharge tests of NCS-2 at different current densities (1e5 A gÀ1); (c) The cyclic stability of NCS-2 over 1000 cycles at A gÀ1 current density are commonly attributable to the M-O/M-O-OH reversible faradaic redox processes; where M represents Ni or Co ion [18] No significant changes in the shape and position of the oxidation and reduction peaks upon increasing scan rates up to 100 mV sÀ1 were observed, which suggests a fast chargeedischarge response in all the NCO, NCS-1 and NCS-2 electrodes (Fig (b)) The CV curves of NCO and NCS-1 are included in Figure SI-2 As expected, with increasing scan rates, (Fig (c)), the specific capacitance decreases due to the accumulation of ions and the thickening of the diffusion layer at the electrodeeelectrolyte interface The redox reactions for this system in an alkaline electrolyte [19] are given below: NiCo2 S4 ỵ OH ỵ H2 O4NiSOH ỵ 2CoSOH þ eÀ CoSOH þ OHÀ 4CoSOH þ H2 O þ eÀ Galvanostatic charge discharge (GCD) studies with the samples on a glassy carbon electrode were carried out using 6M aqueous KOH electrolyte solution in a three-electrode system The synthesized samples acted as a working electrode, silver-silver chloride (Ag/AgCl) was used as a reference electrode and platinum (Pt) was used as a counter electrode, at A gÀ1 current density (Fig 6) The specific capacitance was calculated using the formula [2]: i*Dt  À1  Fg Cs ¼ DV*m (1) where, ‘i’ indicates current (A), ‘Dt’ indicates discharge time (s), ‘DV’ indicates voltage windows (V) and ‘m’ indicates mass (g) Fig (a) shows the GCD curves of NCO, NCS-1 and NCS-2 at a current density of A gÀ1 The triangular shape suggests good reversibility in all three samples NCS-2 showed the highest specific capacitance (675 F gÀ1) among the three due to the favorable ion transport between the electrode and the electrolyte Comparatively, NCS-1 (500 F gÀ1) showed a lower specific capacitance, and NCO (312.5 F gÀ1) showed an even lower value The results suggest that the anion exchange improves the specific capacitance of the material Fig (b) shows the GCD curve of NCS-2 at different current densities (1e5 A gÀ1) At every current density, the triangular shape of the GCD curves was retained Fig (c) shows the cyclic stability of NCS-2 over 1000 cycles at A gÀ1 There was only a 12% loss in the specific capacitance over 1000 cycles and 88% of the specific capacitance retention was achieved The GCD and the cyclic stability results of NCS-1 are shown in Figure SI-3 This study shows that NCS-2 is a material with highly reversible pseudo-capacitance and is a promising active electrode material candidate for use in supercapacitors Conclusion In summary, glycine was used as a templating agent in the hydrothermal syntheses of nickel cobaltite and nickel cobalt sulfide The materials showed complex morphologies with features on the 380 M.S Kumar et al / Journal of Science: Advanced Materials and Devices (2019) 376e380 size scale of tens of nanometers to several micrometers The samples were characterized by different techniques The FTIR studies of nickel cobalt sulfide showed bands at 631 cmÀ1 (symmetrical stretch) and 1100 cmÀ1 (asymmetrical stretch) corresponding to the NieS or CoeS vibrations of nickel cobalt sulfide These groups play an important role in the faradaic redox reaction of the active electrode material XRD analysis confirmed that the samples were the cubic phases of nickel cobaltite and nickel cobalt sulfide, respectively Cyclic voltammetry of both materials showed a typical faradaic behavior with well-defined redox peaks Galvanostatic charge and discharge experiments produced triangular plots, indicating the good reversibility The specific capacitance calculated from the GCD studies for nickel cobalt sulfide was found to be higher than that of nickel cobaltite (312.5 F gÀ1) The nickel cobalt sulfide produced by the two-step method involving an anion exchange outperformed (675 F gÀ1) the directly synthesized nickel cobalt sulfide (500 F gÀ1) in terms of the specific capacitance The two-step nickel cobalt sulfide maintained a specific capacitance retention of 88% over 1000 cycles The superior performance of the two-step nickel cobalt sulfide compared to the directly-synthesized nickel cobalt sulfide and nickel cobaltite is its more complex morphology and lower lamellar thickness This morphology yields a greater interfacial contact between the electrode material and the electrolyte and improves the ion transport across the interface, which, in turn, contributes to its higher specific capacitance Thus, nickel cobalt sulfide, especially when produced by a hydrothermal anion exchange method, is a promising electrode material for supercapacitors Acknowledgements The authors acknowledge Science and Engineering Research Board (SERB) of the Department of Science and Technology (DST), India (Research Grant ECR/2015/000208) and Department of Science and Technology (DST), India for research grant DST/INT/RFBR/P-241 Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.08.005 References [1] M Gidwani, A Bhagwani, N Rohra, Supercapacitors: the near future of batteries, Int J Eng Invent (2014) 22e27 [2] M.S Kumar, K.Y Yasoda, S.K Batabyal, N.K Kothurkar, Carbon-polyaniline nanocomposites as supercapacitor materials, Mater Res Express (2018) 045505 [3] S Kim, J Lee, H Ahn, H Song, J Jang, Facile route to an efficient NiO supercapacitor with a three- dimensional nanonetwork morphology, ACS Appl Mater Interfaces (2013) 1596e1603 [4] S Vijayakumar, S Nagamuthu, G Muralidharan, Supercapacitor studies 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M Hideo, Performance comparison of NiCo2O4 and NiCo2S4 formed on Ni foam for supercapacitor, Compos B Eng 123 (2017) 28e33 [19] L Shen, J Wang, G Xu, H Li, H Dou, X Zhang, NiCo2S4 nanosheets grown