Electrode of multilayer graphene nanosheet nanocomposite with cobalt oxide

In this work the fabrication and electrochemical characterization of cobalt oxide/ multilayer nanosheets as nanocomposite electrode for large areal capacitance for supercapacitor are carried out. The characterization methods involves in this work are scanning electron microscopy, X-ray diffraction, cyclic voltametry and galvano static charging -discharging methods. Multilayer graphene nanosheet enhances the electrochemical performace of the electrode also has an important role on morphology of the electrode material. Cobalt oxide enhances the properties as an electrode of graphene nanosheet. The specific capacitance of cobalt oxide with multilayer graphene nanocomposite  rises upto 140F/g.

1. Introduction

Need for energy and higher possibility of growing need for energy globally making the promising challenge to get new energy sources and their storage devices. Many attemptes are being done by researchers to find such energy storage device with high capacitance,  high power density, high energy density, longer life cycle etc. This is an attempt to make an electrode of multilayer graphene nanosheet nanocomposite with cobalt oxide. Graphene nanosheet has higher higher electrochemical properties where cobalt oxide enhances its properties in order to rise capacitance value, energy density also power density. This attempt is made to enhance the flexible elctrode properties for its effective use for supercapacitor, as the supercapacitor has already been considered as suitable energy storage system.

Supercapacitor can be categorized in two types based on its energy storage working principles,they are electric double layer capacitor and pseudo-capacitor. Different types of carbon materials are used in RDLCs types in their electrode fabrication while in pseudo- capacitor the different metal oxides are used for electrode. Both the types having different advantages and drawbacks. In the present work the attempt is made to get the advantages of both the types. Choosing cobalt oxide as its having simple preparation, good  chemical stability, applicabolity in electrolysis process and good morphology too. Graphene is a two dimensional allotrope carbon that has already drawn an attension for its futuristic properties for electrode for supercapacitor. The need is making supercapacitor fit in every aspect to meet the present scenario requirement.

2. Experimental details

Graphene and cobalt oxide nano particles were used to make multilayer nanosheet nanocomposite. The composition made to make these powders were 80:20 mass ratio that is 80% cobalt oxide with 20% graphene  to make nanosheet nanocomposite electrode. Here the nanosheet was used to prepare layers over a layer to complete a multilayer electrode to get the favourable effects by increasing the surface area with respect to volume. The tests carried out were XRD for powder  of graphene and cobalt oxide and the electrode prepared was undergone SEM for surface morphology, and the prepared supercapacitor was tested by cyclic voltametry and galvano static charging- discharging method to study the capacitive properties.

3. Results and discussion

Fig.1 shows the XRD spectrum of prepareed sample of CO3O4/ Graphene nanocomposite Fig.2 shows the high magnified image of prepared sample of CO3O4/ Graphene nanocomposite. Here in this micrograph images it can be noticed that there are two types of microstructure namely plates and particles.from the magnified SEM images the it is visible that the creases and crinkles formed is due to the 2D structure. Graphene nanosheet with 2D structure  are also expected to give higher surface to volume ratio. It is also visible that the graphene sheets  and cobalt oxides are independently present and formed the shape togather having size 10μm and 100nm in figure 2(a)and (b). The higher surface to volume ration increases the capacitance of the electrode.

In figure 2(a) it is visible that the cobalt oxide is densely prersent and attached to the graphene sheet. And presence of graphene sheet improves the electrical conductivity of the electrode in order to rise the power density.

Fig.3(a), (b) show the (Cyclic Voltametry) CV curve and capacitance value of the cobalt oxide / graphene nanocomposite electrode. This is the unique method of measuring electrochemical proerties of energy storages devices like electrode for supercapacitor application. In this method of characterization we get the direct value in current  output and specific capacitance value of particular electrode at different voltage and scan rate. The cobalt oxide/ graphene nanosheet nanocomposite electrode was evaluated in electrolyte solution. The redox behavior seen in the CV curve clears the presence of pseudocapacitive behavior thus confirms the presence of CO3O4.

Fig shows CV curve at different scanning rate and in all cases there is an increase in peak current  with the increase in scan rate. except with the lowest scan rate. The highest scanning rate has the highest increase in peak current. But here it is observed that there are bent in the CV curve that shows the unstable electrochemical performance at some scan rate but with rise in scan rate no bent indicates at higher scan rate the electrochemical performance gets stability.

Fig.4 show the curve of (Galvano Static Charge-Discharge) GCD. The time required for charge discharge is lower in CO3O4 as compared to nanocomposite electrode. Curve shows the device was tested and  the voltage was raised upto 0.9V. The linear changes in the charge discharge curve shows that the characteristics of supercapacitor is typical. Fig.4(b) shows Ragone plot for cobalt oxide/ graphene nanosheet nanocomposite based supercapacitor. It shows  energy density and power density on Y axis and X axis. Energy and power density are calculated many times by charging and discharging the device. Energy density was found from the graph that it is ranging from 0.3 to 0.9kW/kg while keeping the energy density from 0.2 to 10 Wh/kg.

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