Hydrothermal Growth of Hierarchical Ni₃S₂ and Co₃S₄ on a Reduced Graphene Oxide Hydrogel@Ni Foam: A High-Energy-Density Aqueous Asymmetric Supercapacitor

Ni foam@reduced graphene oxide (rGO) hydrogel–Ni₃S₂ and Ni foam@rGO hydrogel–Co₃S₄ composites have been successfully synthesized with the aid of a two-step hydrothermal protocol, where the rGO hydrogel is sandwiched between the metal sulfide and Ni foam substrate. Sonochemical deposition of exfoliated rGO on Ni foam with subsequent hydrothermal treatment results in the formation of a rGO-hydrogel-coated Ni foam. Then second-time hydrothermal treatment of the dried Ni@rGO substrate with corresponding metal nitrate and sodium sulfide results in individual uniform growth of porous Ni₃S₂ nanorods and a Co₃S₄ self-assembled nanosheet on a Ni@rGO substrate. Both Ni@rGO–Ni₃S₂ and Ni@rGO–Co₃S₄ have been electrochemically characterized in a 6 M KOH electrolyte, exhibiting high specific capacitance values of 987.8 and 1369 F/g, respectively, at 1.5 A/g accompanied by the respective outstanding cycle stability of 97.9% and 96.6% at 12 A/g over 3000 charge–discharge cycles. An advanced aqueous asymmetric (AAS) supercapacitor has been fabricated by exploiting the as-prepared Ni@rGO–Co₃S₄ as a positive electrode and Ni@rGO–Ni₃S₂ as a negative electrode. The as-fabricated AAS has shown promising energy densities of 55.16 and 24.84 Wh/kg at high power densities of 975 and 13000 W/kg, respectively, along with an excellent cycle stability of 96.2% specific capacitance retention over 3000 charge–discharge cycles at 12 A/g. The enhanced specific capacitance, stupendous cycle stability, elevated energy density, and a power density as an AAS of these electrode materials indicate that it could be a potential candidate in the field of supercapacitors

Preparation of CTAB-Assisted Hexagonal Platelet Co(OH)₂/Graphene Hybrid Composite as Efficient Supercapacitor Electrode Material

In this paper, our work deals with the preparation of cetyl trimethylammonium bromide (CTAB)-assisted hexagonal platelet β Co­(OH)₂ via a simple hydrothermal process and its graphene-based hybrid composite with improved electrochemical properties. The prepared materials were successfully characterized by FTIR, XRD, FESEM, TEM, and TGA analysis. The materials were subjected to electrochemical analysis by a three electrode system in a 6 M KOH electrolyte in terms of a cyclic voltammetry, cyclic charge–discharge, and electrochemical impedance study. The CTAB-assisted Co­(OH)₂ with hexagonal architecture exhibited the highest specific capacitance of 312 F/g at a 2 mV/s scan rate and 285 F/g at a 2 A/g constant discharge current density. The high utility of pseudocapacitive Co­(OH)₂ is achieved only in its graphene-based hybrid composite, which responded with specific capacitance of 532 F/g at a 2A/g current density and a 94% specific capacitance retention over 500 charge discharge cycles

Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy-Dense, High-Voltage Supercapacitors

A major stumbling block in the development of high energy density graphene-based supercapacitors has been maintaining high ion-accessible surface area combined with high electrode density. Herein, we develop an ionic liquid (IL)–surfactant microemulsion system that is found to facilitate the spontaneous adsorption of IL-filled micelles onto graphene oxide (GO). This adsorption distributes the IL over all available surface area and provides an aqueous formulation that can be slurry cast onto current collectors, leaving behind a dense nanocomposite film of GO/IL/surfactant. By removing the surfactant and reducing the GO through a low-temperature (360 °C) heat treatment, the IL plays a dual role of spacer and electrolyte. We study the effect of IL content and operating temperature on the performance, demonstrating a record high gravimetric capacitance (302 F/g at 1 A/g) for 80 wt % IL composites. At 60 wt % IL, combined high capacitance and bulk density (0.76 g/cm³), yields one of the highest volumetric capacitances (218 F/cm³, at 1 A/g) ever reported for a high-voltage IL-based supercapacitor. While achieving promising rate performance and cycle-life, the approach also eliminates the long and costly electrolyte imbibition step of cell assembly as the electrolyte is cast directly with the electrode material

Preparation of CTAB-Assisted Hexagonal Platelet Co(OH)₂/Graphene Hybrid Composite as Efficient Supercapacitor Electrode Material

In this paper, our work deals with the preparation of cetyl trimethylammonium bromide (CTAB)-assisted hexagonal platelet β Co­(OH)₂ via a simple hydrothermal process and its graphene-based hybrid composite with improved electrochemical properties. The prepared materials were successfully characterized by FTIR, XRD, FESEM, TEM, and TGA analysis. The materials were subjected to electrochemical analysis by a three electrode system in a 6 M KOH electrolyte in terms of a cyclic voltammetry, cyclic charge–discharge, and electrochemical impedance study. The CTAB-assisted Co­(OH)₂ with hexagonal architecture exhibited the highest specific capacitance of 312 F/g at a 2 mV/s scan rate and 285 F/g at a 2 A/g constant discharge current density. The high utility of pseudocapacitive Co­(OH)₂ is achieved only in its graphene-based hybrid composite, which responded with specific capacitance of 532 F/g at a 2A/g current density and a 94% specific capacitance retention over 500 charge discharge cycles

Solid State Flexible Asymmetric Supercapacitor Based on Carbon Fiber Supported Hierarchical Co(OH)_<i>x</i>CO₃ and Ni(OH)₂

Conducting flexible carbon fiber (CF) cloth was used as a substrate for the hydrothermal growth of nickel hydroxide (Ni­(OH)₂) and cobalt hydroxy carbonate [Co­(OH)_<i>x</i>CO₃] with unique hierarchical flowery architecture and then was used as a flexible supercapacitor electrode. In a three-electrode configuration in 6 M KOH aqueous electrolyte, the CF–Ni­(OH)₂ and CF–Co­(OH)_<i>x</i>CO₃ electrode showed the maximum specific capacitance of 789 F/g and 550 F/g, respectively, at 2A/g current accompanied by outstanding cycle stability by retaining 99.9% and 99.5% specific capacitance over 1500 consecutive charge–discharge cycles at 5 A/g. However, the low cell voltage (0.4 V) restricted the respective specific energy to 4.38 and 3.05 Wh/kg at a specific power of 100 W/kg. To overcome the issue, two solid state flexible asymmetric supercapacitors were fabricated using the CF–Ni­(OH)₂ and CF–Co­(OH)_<i>x</i>CO₃ as the anode and sonochemically deposited CNT over carbon fiber as the cathode material in PVA-KOH gel electrolyte. The as-fabricated flexible supercapacitors CF–Ni­(OH)₂//CF–CNT and CF–Co­(OH)_<i>x</i>CO₃//CF–CNT were able to deliver high specific energy of 41.1 and 33.48 Wh/kg, respectively, at high specific power of 1.4 kW/kg accompanied by excellent cycle stability (retaining 98% and 97.6% specific capacitance, respectively, over 3000 charge–discharge cycle at 5 A/g)

High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co₃O₄ Composite Aerogel Electrodes

Modern flexible consumer electronics require efficient energy storage devices with flexible free-standing electrodes. We report a simple and cost-effective route to a graphene-based composite aerogel encapsulating metal oxide nanoparticles for high energy density, free-standing, binder-free flexible pseudocapacitive electrodes. Hydrothermally synthesized Co₃O₄ nanoparticles are successfully housed inside the microporous graphene aerogel network during the room temperature interfacial gelation at the Zn surface. The resultant three-dimensional (3D) rGO-Co₃O₄ composite aerogel shows mesoporous quasiparallel layer stack morphology with a high loading of Co₃O₄, which offers numerous channels for ion transport and a 3D interconnected network for high electrical conductivity. All solid state asymmetric pseudocapacitors employing the composite aerogel electrodes have demonstrated high areal energy density of 35.92 μWh/cm² and power density of 17.79 mW/cm² accompanied by excellent cycle life