Ni(OH)₂ Nanoflowers/Graphene Hydrogels: A New Assembly for Supercapacitors

Abstract

A novel structure of graphene-based hybrid hydrogels was constructed, in which α-Ni(OH) 2 nanoflowers with nanopetals thicknesses of approximately 20 nm were uniformly anchored on a three-dimensional graphene framework. Benefiting from the unique morphological nickel hydroxide nanoflowers and hydrogels, the nickel hydroxide nanoflowers/ graphene hydrogels exhibited great specific capacitances (1 A·g −1 ; 1632 F·g −1 ), great rate capabilities, and longer cycle life (after 1000 cycles, 95.2% capacitance retention) when used as electrodes in supercapacitors. ■ INTRODUCTIONSupercapacitors or ultracapacitors are emerging as the sustainable energy storage systems of the future for varied large-scale applications, mostly in portable electronics, biomedical devices, electric vehicles, and high power applications and so on. 1,2 In comparison with the widely used and common double-layer capacitors, pseudocapacitors (for example, metal oxide, metal hydroxide, and polymers) have higher specific capacitance, as a result of which they are widely explored and considered for supercapacitor applications. 3−5 But, the use of these materials has been disadvantaged by their low cyclic stability and rate capability, due to the low conductivity of these materials.Graphene (GS), has attractive characteristic properties, which makes it an outstanding material compared to many other carbonaceous materials; its excellent characteristic properties of high surface area and attractive electromechanical properties makes it the right material which can be used as a supporting surface for anchoring pseudocapacitive materials to improve both the overall capacitance and cycling stability. Though numerous graphene-based composites that were developed to date have succeeded in effectively improving the overall capacitive performance, many a time, the irreversible and unfavorable restacking of the graphene layers has led to a drastic decrease in the surface area, thereby limiting the free crusade of electron and ions. 6−8 Porosity being one of the vital factors influencing the capacitive performance of electrode materials, graphene-based mesoporous materials, such as graphene-based foams, aerogels, and hydrogels, have been able to attract substantial interest. 9−11 Graphene-based hybrid hydrogels belong to a class of hydrated materials containing a large amount of water, which have received particular attention in the recent times. 12,13 On one hand, the hydrogels have the advantage of being used as electrode materials without a binder, resulting in the reduction in the electrode preparation steps, and also any reduction in the specific capacitance caused by polymer binders used in the slurry preparation for the electrode material. On the other hand, these unique graphene-based hybrid hydrogels are highly hydrophilic in nature and have a high surface area (∼1000 m 2 · g −1 as per methylene blue molecular adsorption studies), allowing fast ion diffusions and electron transport through the hierarchical, porous and conductive graphene hydrogel network...

Hybrid Films of Ni(OH)2 Nanowall Networks on Reduced Graphene Oxide Prepared at a Liquid/Liquid Interface for Oxygen Evolution and Supercapacitor Applications

“…The specific ( C s ) and areal ( C a ) capacitance values of the hybrid films have been calculated from CV curves employing the expression C =(∫ I(V)dV/2mv▵V , where∫ I(V)dV is the integrated area of the CV cycle, m is the mass of the active material (g) or area of the active material (cm 2 ) on the electrode surface, v is the scan rate (V/s) and ▵V is the potential window range in the CV measurement ,,,. Accordingly, the values obtained at a scan rate of 5 mVs −1 are C s =260 Fg −1 & C a =18.2 mF cm −2 for bare Ni(OH) 2 , and C s =1402 Fg −1 & C a =98.12 mF cm −2 for rGO−Ni(OH) 2 .…”

“…The specific capacitance from the chronopotentiometry curves of these hybrid films has been calculated employing the following expression, C s = I d t d /Vm , where I d is the charge/discharge current, t d is the time for a full discharge, V is the potential voltage window and m is the mass of the electrode material. The energy density and power density were calculated according to the expressions, E = 0.5 C s V 2 and P = E / t d , respectively ,,,. At a current density of 0.5 Ag −1 , the energy density observed for rGO−Ni(OH) 2 nanostructured hybrid film ( E =13.6 Wh/kg) is higher than that of the bare Ni(OH) 2 film ( E =11.4 Wh/kg) with a power density of 125 W/kg.…”

“…The energy density and power density were calculated according to the expressions, E = 0.5C s V 2 and P = E/t d , respectively. [22,23,63,29] At a current density of 0.5 Ag À 1 , the energy density observed for rGOÀ Ni(OH) 2 nanostructured hybrid film (E = 13.6 Wh/kg) is higher than that of the bare Ni(OH) 2 film (E = 11.4 Wh/kg) with a power density of 125 W/kg. Figure 8b displays the Nyquist plots of various modified electrodes in the frequency range, 100 kHz to 0.01 Hz.…”

Electrodeposited Stable Binder‐Free Organic Ni(OH)2 Flexible Nanohybrid Electrodes for High‐Performance Supercapacitors

“…The electrodeposition method facilitates inexpensive, easy, binder‐free, and in situ controlled growth of the nanostructured metal hydroxide‐based material over the working electrode. The binder‐free method does not allow usage of expensive binders and minimizes the inactive surface area and extra contact resistance during the electrochemical analysis …”

Two‐Step Deposition/Reduction Synthesis of Porous Lamellar β‐Ni(OH)2/Reduced Graphene Oxide Composites with Large Capacitance for Supercapacitors

“…[14] Wang et al. employed solvothermal reduction to prepare Ni(OH) 2 /graphene hydrogels, which exhibited a great specific capacitance of 1632 F ⋅ g −1 at 1 A ⋅ g −1 and 95.2 % capacitance retention after 1000 cycles . Alberto et al.…”