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All-Solid-State Supercapacitor Based on Graphene Oxide Composite Electrodes
Supercapacitors are promising in increasing energy recycle efficiency when employed with regenerative braking systems in electric vehicles, because of their high power density and long cycle life. However, the low energy density and small operation window of supercapacitors are limiting their applications. Nickel hydroxide-reduced graphene oxide (Ni(OH)2-rGO) composite electrodes exhibit high electrochemical performance in aqueous alkaline electrolyte. Replacing the aqueous electrolyte with a lithium-ion gel polymer electrolyte (GPE), an all-solid-state supercapacitor with widened operation window is demonstrated. After activation, the Ni(OH)2-rGO supercapacitor can successfully operate up to 3 V with battery-like behavior, resulting in a high specific capacity of 85 mAh g⁻¹, specific energy of 36.1 Wh kg⁻¹, and specific power of 77.01 W kg⁻¹. By lowering the upper-limit potential to 2.5 V, the supercapacitor exhibits capacitor-like behavior with a specific capacitance of 6.7 F g⁻¹, specific energy of 12.4 Wh kg⁻¹, and specific power of 6.8 kW kg⁻¹. Further reducing the upper potential limit to 2 V leads to stable operation of the capacitor for over 15,000 cycles with coulombic efficiency of over 95% under the current density of 1.54 A g⁻¹. Our work demonstrates that the Ni(OH)2-rGO supercapacitor with the lithium-ion GPE can work bi-functionally either as a battery-like or a capacitor-like device depending on the operation potential range.
The paper reports preparation and electrochemical study of graphene oxide/samarium sulfide (GO/Sm2S3) composite electrode. The GO/Sm2S3 electrode is prepared using successive ionic layer reaction (SILAR) method. The XRD and FTIR studies show the formation of GO/Sm2S3 composite material on stainless steel electrode. The field emission scanning electron microscopy (FE-SEM) image shows the nano-strips like surface morphology of composite material thin film. The GO/Sm2S3 composite thin film shows superhydrophilic nature with a contact angle of 5°. The surface area of GO/Sm2S3 composite thin film is 31.43 m²g⁻² measured using Brunauer-Emmett-Teller (BET) technique. The electrochemical study of GO/Sm2S3 electrode carried out in 1M Na2SO4 electrolyte exhibits specific capacitance of 360 Fg⁻¹ at 5 mVs⁻¹ scan rate.
In the present work, the electrochemical performance of BiFeO3-graphene composite as electrodes material has been investigated for high power supercapacitor application. The composite material was coated on a stainless steel substrate by drop casting method. The supercapacitor device was made in two electrodes configuration and an aqueous electrolyte of 1 M Na2SO4 was filled between the electrodes. The electrochemical performance was determined using cyclic voltammetery and constant current charging/discharging process. It was observed that the supercapacitor made of BiFeO3-graphene composite electrodes can deliver a power of 18.75 kW kg⁻¹ with maintaining the energy density of 1.9 Wh kg⁻¹. These values are comparable to the reported values in the literature. Surface morphology of the electrode was characterized by scanning electron microscopy. X-ray diffraction was also used for the structural analysis of the electrode.
Electrochemical energy storage technology is based on devices capable of exhibiting high energy density (batteries) or high power density (electrochemical capacitors). There is a growing need, for current and near-future applications, where both high energy and high power densities are required in the same material. Pseudocapacitance, a faradaic process involving surface or near surface redox reactions, offers a means of achieving high energy density at high charge-discharge rates. Here, we focus on the pseudocapacitive properties of transition metal oxides. First, we introduce pseudocapacitance and describe its electrochemical features. Then, we review the most relevant pseudocapacitive materials in aqueous and non-aqueous electrolytes. The major challenges for pseudocapacitive materials along with a future outlook are detailed at the end.
A mechanically assisted solid-state reaction method is employed to prepare graphene/Ni(OH)2 nanocomposites for supercapacitor electrode materials. Morphological analyses reveal that, at a loading 50 wt % Ni(OH)2, nanoparticles with an average size of 10 nm are formed and uniformly dispersed on the surface of reduced graphene oxide sheets functionalized with benzenesulfonate. Electrochemical measurements of the composite material show a high specific capacitance of 1568 F g–1 (based on nickel hydroxide) at a current density of 4 A g–1, significantly higher than that of bare Ni(OH)2 nanoparticles prepared without the use of graphene. This much improved electrochemical performance is enabled by both the well-dispersed Ni(OH)2 nanoparticles that offer large accessible surface area and the hydrophilic functional groups on graphene surface that facilitate electrolyte transport. The scalable solid-state synthesis developed in this work is promising for a green chemical approach to the preparation of supercapacitor electrode materials with high performance.
Two types of double-layer capacitors, based on carbon materials, were analysed: (1) an imaginary nano-capacitor assembled from single graphene sheets, separated by electrolyte layers (thickness of nanometers) and (2) a capacitor based on porous carbons. It has been shown that the maximum specific surface of a porous carbon material which may be used for the construction of a capacitor is ca. 2600 m2 g−1. The maximum energy density of an imaginary double-layer ‘nano-capacitor’, is close to 10 kJ kg−1 at a voltage of U = 1 V (aqueous electrolyte) of ca. 40–45 kJ kg−1 at U ≈ 2.3–2.5 V (organic electrolytes), and at the order of 100 kJ kg−1 at voltages close to 4 V (ionic liquids as electrolytes). The real device consists of porous electrodes and a separator, both soaked with the electrolyte, as well as current collectors. Consequently, the maximum electric capacity expressed versus the mass of the device (ca. 20–30 F g−1), is much smaller than the corresponding value expressed versus the mass of the carbon material (ca. 300 F g−1). In order to obtain the energy density of the device at a level of 100 kJ kg−1 (characteristic for the lead-acid battery), the capacitor with porous carbon electrodes should operate at voltages of ca. 4 V (ionic liquids as electrolytes). However, the specific power density of such a capacitor having an acceptable energy density (ca. 100 kJ kg−1) is relatively low (ca. 1 kW kg−1).
The self-discharge behaviour of supercapacitors is an important factor when considering their suitability for some applications. In this paper, measurements of the self-discharge rates of carbon-based supercapacitors with organic electrolytes are presented and interpreted in terms of two mechanisms. The first is the diffusion of ions from regions of excess ionic concentration formed during the charging of the capacitor and the second is leakage of charge across the double-layer at the electrolyte–carbon interfaces in the capacitor. The dependence of the self-discharge rate on temperature and on the initial voltage across the capacitor is described. q 2000 Elsevier Science S.A. All rights reserved.
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