Graphene like Porous Carbon Sheets Derived from Hibiscus Cannabinus As a Versatile Electrochemical Energy Storage Material
Authors: Nanaji Katchala , Varadaraju U V , Anandan Srinivasan , Rao Tata Narsinga 
Published: February 2020

In recent years, energy storage devices such as supercapacitors (SC) and Li-ion batteries (LIB) have attracted worldwide interest due to their critical role in replacing the conventional fuels in the transportation sector and also owing to their promising electro-chemical characteristics like long cycle life, high energy density, high power density and low toxicity [1]. In LIBs and SCs, carbon electrode material is arguably an important component and plays a pivotal role in the device performance. However, development of low cost carbon electrode materials with improved energy density and long cycle stability are the great challenges to meet the increasing demands of upcoming electric vehicle technology. Various carbon materials, such as activated carbon, mesoporous carbon, graphene and carbon nanotubes have been extensively studied as electrode materials. However, there are certain limitations using these materials for commercial applications due to their inappropriate pore structure, high cost, tedious synthesis conditions etc. [2].
Graphene-like porous carbon sheets (GPCSs) have recently been viewed as the outstanding electrode materials for high rate electrochemical storage devices such as supercapacitors and Li-ion batteries because the 2D porous carbon sheets can integrate the high conductivity as well as high porosity to promote the facile diffusion of both ion transport and electron transfer. Herein, synthesizing graphene like porous carbon sheets from a renewable biomass waste (Hibiscus cannabinus sticks) is of great importance in lowering the production costs and also promoting environmental protection. The relationship between structure of the GPCS-X (X represents the concentration of activation agent) and its electrochemical performance at different concentration of activating agent is explored in detail. The resulting porous carbon sheets possesses excellent textural parameters with a high specific surface area of 2308 m2/g, highly ordered graphitic carbon with a ID/IG ratio of 0.315 which is confirmed by BET, Raman, HR-TEM (Figure 1a and 1b) etc. While being used as supercapacitor application, the GPCS electrode exhibits a high specific capacitance of 240 F/g at 1A/g, excellent rate capability (84% capacity retention at 50 A/g) and also shows outstanding cyclic stability (92% capacitance retention after 25,000 cycles) in aqueous electrolyte. When tested in organic electrolyte with an extended voltage window, the electrode exhibits symmetric charge-discharge profiles as shown in figure 1c. Also, for Li-ion battery application, GPCS electrode material as shown in figure in 1d displayed a high reversible capacity (1050 mAh/g at 100 mA/g), excellent rate performance (230 mAh/g at 5000 mA/g), and good cycling stability (72% capacitance retention after 400 cycles). The excellent electrochemical performance of GPCS electrode material can be attributed to the high electrical conductivity of the graphene network along with high specific surface area. These results demonstrate a facile, low-cost, eco-friendly design of electrode materials for energy storage applications.
Figure 1 (a) FE-SEM and (b) HR-TEM images of GPCS material (c) Charge-Discharge profiles for Supercapacitor application and (d) Rate comparison for Li-ion battery application.

References:

[1] P.G. Bruce, S. A. Freunberger, L. J. Hardwick, J. M. Tarascon, Nature Materials, 2012, 11, 19-29.
[2] K. Nanaji, A. Jyothirmayi, U.V. Varadaraju, T. N. Rao, S. Anandan, Journal of Alloys and Compounds, 2017, 723, 488-497

Figure 1