Sanjit Saha

Non-covalent functionalization of reduced graphene oxide using sulfanilic acid azocromotrop and its application as a supercapacitor electrode material

Sulfanilic acid azocromotrop (SAC) modified reduced graphene oxide (SAC-RGO) was prepared by simple non-covalent functionalization of graphene oxide (GO) followed by post reduction using hydrazine monohydrate. Spectral analysis (Fourier transform infrared, Raman and X-ray photoelectron spectroscopy) revealed that successful modification had occurred of GO with SAC through π–π interaction. The electrical conductivity of SAC-RGO was found to be ∼551 S m−1. The capacitive performance of SAC-RGO was recorded using a three electrode set up with 1 (M) aqueous H2SO4 as the electrolyte. The –SO3H functionalities of SAC contributed pseudocapacitance as evidenced from the redox peaks (at ∼0.43 and 0.27 V) present in the cyclic voltammetric (CV) curves measured for SAC-RGO. The contribution of electrical double layer capacitance was evidenced from the near rectangular shaped CV curves and resulted in a high specific capacitance of 366 F g−1 at a current density of 1.2 A g−1 for SAC-RGO electrode. An asymmetric device (SAC-RGO//RGO) was designed with SAC-RGO as the positive electrode and RGO as the negative electrode. The device showed an energy density of ∼25.8 W h kg−1 at a power density of ∼980 W kg−1. The asymmetric device showed retention in specific capacitance of ∼72% after 5000 charge–discharge cycles. The Nyquist data of the device was fitted with Z-view and different components (solution resistance, charge-transfer resistance and Warburg elements) were calculated from the fitted curves.

Band Gap Engineering of Boron Nitride by Graphene and Its Application as Positive Electrode Material in Asymmetric Supercapacitor Device

Nanostructured hexagonal boron nitride (h-BN)/reduced graphene oxide (RGO) composite is prepared by insertion of h-BN into the graphene oxide through hydrothermal reaction. Formation of the super lattice is confirmed by the existence of two separate UV-visible absorption edges corresponding to two different band gaps. The composite materials show enhanced electrical conductivity as compared to the bulk h-BN. A high specific capacitance of ∼824 F g(-1) is achieved at a current density of 4 A g(-1) for the composite in three-electrode electrochemical measurement. The potential window of the composite electrode lies in the range from -0.1 to 0.5 V in 6 M aqueous KOH electrolyte. The operating voltage is increased to 1.4 V in asymmetric supercapacitor (ASC) device where the thermally reduced graphene oxide is used as the negative electrode and the h-BN/RGO composite as the positive electrode. The ASC exhibits a specific capacitance of 145.7 F g(-1) at a current density of 6 A g(-1) and high energy density of 39.6 W h kg(-1) corresponding to a large power density of ∼4200 W kg(-1). Therefore, a facile hydrothermal route is demonstrated for the first time to utilize h-BN-based composite materials as energy storage electrode materials for supercapacitor applications.

Growth of Ni–Co binary hydroxide on a reduced graphene oxide surface by a successive ionic layer adsorption and reaction (SILAR) method for high performance asymmetric supercapacitor electrodes

A simple, additive-free, cost-effective and scalable successive ionic layer adsorption and reaction (SILAR) method is reported to prepare nickel–cobalt binary hydroxide (Ni–Co–BH) on a reduced graphene oxide (RGO) directing template over a macro-porous conductive nickel foam substrate. This green technique is not only considered as fundamental research interest, but also describes the commercial applications of supercapacitors to reduce the electrode fabrication cost. Three different Ni–Co–BH–G (Ni–Co–BH/RGO) composites are synthesised by tailoring the nickel–cobalt ratios. The flower-like 3D framework of Ni–Co–BH–G provides a porous nano-structure to facilitate the charge transfer and ion diffusion. The cathodic peak current density vs. square root of the scan rate slope values of cyclic voltammetry are consistent with specific capacitance (SC) retention (vs. current density) from charge–discharge curves and the diffusion time constant of the Nyquist plot of the Ni–Co–BH–G composites. Taking the advantage of 3D conductive mesoporous open framework, the Ni–Co–BH–G has provided an excellent SC of 2130 F g−1 at 2 A g−1. An asymmetric supercapacitor device is designed with the optimized Ni–Co–BH–G as the positive electrode and concentrated HNO3 treated conducting carbon cloth (CCN) as the negative electrode. An excellent energy density of ∼92 W h kg−1 and a high power density of ∼7.0 kW kg−1 with lifetime stability up to 10000 charge–discharge cycles (capacitance retention ∼ 80%) are provided by the asymmetric device. Four asymmetric devices have been assembled in series, which provided ∼5.6 V charge–discharge potential. The assembled system has powered a 5 V light-emitting diode (LED) successfully.

Hydrothermal synthesis of Fe3O4/RGO composites and investigation of electrochemical performances for energy storage applications

Highly porous nano-structured Fe3O4 particles were successfully prepared on the surface of reduced graphene oxide (RGO) sheets through a one-step hydrothermal method. X-ray diffraction (XRD) and field emission scanning electron microscopy analysis (FE-SEM) confirmed not only the size and porous nature but also the formation of Fe3O4 and Fe2O3-based composites. XRD, FE-SEM and transmission electron microscopy showed the highly crystalline nature of the particles. The reduction of graphene oxide and the formation of a few layers of RGO were confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Electrochemical performances of the Fe3O4/RGO composite were evaluated with two electrode configurations using nickel foam as a material support as well as a current collector. The synergistic effect of RGO and the metal oxide were demonstrated in terms of enhanced energy and power density, excellent electrochemical cyclic stability and low IR drop. The specific capacitance of the Fe3O4/RGO composite was found to be ∼782 F g−1 at a current density of 3 A g−1. The improved electrical conductivity, nanometer scale particle dimension and formation of hierarchical networks with effective redox activity contributed to a remarkable supercapacitor performance.

Band gap modified boron doped NiO/Fe3O4 nanostructure as the positive electrode for high energy asymmetric supercapacitors

A boron doped NiO/Fe3O4 nanostructure was successfully synthesized by a facile one-step hydrothermal method. The boron doping was confirmed from the decreased band gap and increased electrical conductivity of the NiO/Fe3O4 composite. The Nyquist plot of the multimetal oxide was fitted with ZView software for detailed understanding of the effect of concentration of different metal oxides, boron doping and extensive charge–discharge cycles on the electrochemical properties of electrode materials. Very high specific capacitance of ∼1467 F g−1 was achieved as the synergistic effect of low activation energy and short ion diffusion path of the electrode materials. An asymmetric supercapacitor (ASS) was fabricated with the NiO/Fe3O4 composite and thermally reduced graphene oxide as the positive and negative electrode, respectively. The ASS showed a large specific capacitance of ∼377 F g−1 at a current density of 3 A g−1. Furthermore, the ASS showed a large energy density of ∼102.6 W h kg−1 and huge power density of ∼6300 W kg−1 and remained ∼82% stable even after 10 000 charge–discharge cycles. Therefore, a facile hydrothermal method was demonstrated to enhance the electrochemical properties of a multimetal oxide by boron doping for the development of next generation energy storage devices.

Development of high energy density supercapacitor through hydrothermal synthesis of RGO/nano-structured cobalt sulphide composites.

Co9S8/reduced graphene oxide (RGO) composites were prepared on nickel foam substrate through hydrothermal reaction and used directly as supercapacitor electrode. The field emission scanning electron microscopy analysis of the composites showed the formation of Co9S8 nano-rods on the RGO surfaces. The average crystal size of the Co9S8 nano rods grown on the RGO sheets were ∼25-36 nm as calculated from x-ray diffraction analysis. The reduction of graphene oxide (GO) was confirmed by Raman and x-ray photoelectron spectroscopy analysis. The electrical conductivity of the Co9S8/RGO composite was recorded as 1690 S m(-1) at room temperature, which is much higher than that of pure GO further confirming the hydrothermal reduction of GO. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy were investigated to check the electrochemical performances of the Co9S8/RGO composites. The Co9S8/RGO composites supported on nickel foam showed very high specific capacitance (Sc)(1349 F g(-1) at a current density of 2.2 A g(-1)), energy density (68.6 W h kg(-1)) and power density (1319 W kg(-1)) in 6 M KOH electrolyte. The retention in Sc of the composite electrode was found to be ∼96% after 1000 charge-discharge cycles.

In situ preparation of a SAC-RGO@Ni electrode by electrochemical functionalization of reduced graphene oxide using sulfanilic acid azocromotrop and its application in asymmetric supercapacitors

In situ electro-deposition, reduction and functionalization of graphene oxide (GO) with sulfanilic acid azocromotrop (SAC) were carried out through a facile one-step electrochemical method. Nickel foam was used as the anode during the electro-deposition and the aqueous solution of SAC along with GO was used as the electrolyte. The SAC modified reduced GO (RGO) was deposited on the nickel foam (SAC-RGO@Ni) and was directly used as the electrode for capacitive property analysis. The reduction and functionalization of GO were examined by Fourier transform infrared (FT-IR), Raman and X-ray photoelectron spectroscopy (XPS) techniques. The SAC-RGO@Ni provides a very high specific capacitance of ∼1090 F g−1 due to the synergistic effect of double layer capacitance of RGO and the pseudocapacitance of –SO3H functionalities of SAC. An asymmetric supercapacitor (ASC) cell was designed with SAC-RGO@Ni and thermally reduced GO (TRGO) as positive and negative electrodes, respectively. The ASC device exhibits a high effective capacitance of ∼495 F g−1 at a current density of 10 A g−1 and ∼93% of its total discharging time lies in-between 1.5 and 0.75 V. The ASC cell remains stable up to 10 000 charge–discharge cycles. Furthermore, the SAC-RGO@Ni-based ASC device can provide a very high energy density of ∼88.9 W h kg−1 and a large power density of 16 500 W kg−1 ensuring its applicability in high power consumption devices.

Controlled electrodeposition of iron oxide/nickel oxide@Ni for the investigation of the effects of stoichiometry and particle size on energy storage and water splitting applications

Controlled synthesis of nickel/iron multimetal oxides with different stoichiometry and particle sizes was carried out by varying the pH of the reaction medium. Electrodeposited samples grown at different pH values showed a wide range of electrochemical properties such as dissimilar current response and potential window due to the formation of different stoichiometry and surface morphologies. Smaller particle size and higher content of NiO are advantageous due to the creation of a facile diffusion path. Moreover, electrical conductivity as well as series resistance increased for the samples with smaller particle size due to the quantum size effect. The quantum size effect was confirmed from the blue shift of the UV-vis absorbance spectrum. The facile diffusion path lowered the charge transfer resistance and accelerated the reaction rate for water splitting. Furthermore, the quantum size effect shifted the flat-band potential and increased the overpotential in the water splitting reaction. Multimetal oxides exhibited a small overpotential of ∼−0.27 V (corresponding to the current response of 10 mA cm−2) and a small Tafel slope of ∼63 mV dec−1. Finally, an asymmetric supercapacitor (ASC) cell was fabricated with electrodeposited samples, which showed a large potential window of ∼1.6 V along with a high energy and power density of ∼91 W h kg−1 and 7200 W kg−1, respectively. Furthermore, the ASC exhibited very low relaxation time constant (∼1.3 ms) and long stability of ∼83% after 10 000 CD cycles, ensuring the effectiveness of electrodeposited multimetal oxides for energy storage as well as water splitting applications.