Coconut Shell-Derived Activated Carbon for High-Performance Solid-State Supercapacitors

Kuan-Ching Lee,Mitchell Shyan Wei Lim,Guan-Ting Pan,Zhong-Yun Hong,Chao-Ming Huang,Siewhui Chong,Timm Joyce Tiong

doi:10.3390/en14154546

Abstract

Coconut shells, low-cost and renewable agro-wastes, were used as a starting material in the synthesis of hierarchical activated carbons via hydrothermal, KOH-activation, and carbonization techniques. The ratio of KOH to hydrochar was varied in a systemic manner to study how it influences the texture and electrochemical behavior of the capacitor. Coconut shell-based carbon coated on nickel foams presented a surface area of 1567 m2 g−1, with micropores as well as mesopores widely distributed. The sample showed superior electrochemical performance, attaining 449 F g−1 at 1 A g−1 in 6 M LiNO3 aqueous solution. The solid-state symmetric supercapacitor device delivered a specific capacitance of 88 F g−1 at 1 A g−1 and a high energy density of 48.9 Whkg−1 at a power density of 1 kW kg−1. At a wide voltage window of 2.0 V, the sample was highly stable during the cycle test, showing a 92% capacitance retention at 2 A g−1 after cycling for 5000 times. The superior performance is due to the sample possessing great BET surface area, a good distribution of pores, and the usage of a suitable electrolyte. This facilitates an electrical double layer that can be deployed for applications to store energy.

Highlights

With greater power densities, as well as being more sustainable compared to secondary batteries, supercapacitors have found wide applications in industrial processes, hybrid electric vehicles, and portable electronics [1,2]
The primary ways to optimize the energy density of electrical double-layer capacitances (EDLCs) are to increase the specific capacitance of the electrode materials and to widen the voltage window
With its low energy requirements and no need for pre-drying biomass, Hydrothermal carbonization (HTC) is a promising process for the preparation of carbon-based electrode materials for EDLCs [12,13]

Summary

Introduction

As well as being more sustainable compared to secondary batteries, supercapacitors have found wide applications in industrial processes, hybrid electric vehicles, and portable electronics [1,2]. The primary ways to optimize the energy density of EDLCs are to increase the specific capacitance of the electrode materials and to widen the voltage window. With its low energy requirements and no need for pre-drying biomass, HTC is a promising process for the preparation of carbon-based electrode materials for EDLCs [12,13]. As a result of a large surface area, a hierarchical porosity, as well as high conductivity, these activated carbons have been reported as better, compared with other agro-waste materials used as electrodes for supercapacitors [20,21,22,23]. A series of KOH-treated, coconut shell-based carbons (CSCKs) were prepared for utilisation in electrodes for efficient and high energy EDLCs. In particular, CSCKs were obtained by hydrothermal carbonization, KOH-activation, and carbonization of coconut shells. This study illustrates that the combination of the hydrothermal carbonization and chemical activation, with appropriate electrolytes, can produce low-cost supercapacitors sustainably for widespread application

Coconut Shell Carbons Preparation

Material Characterizations

Electrochemical Tests

Results

Evaluation of Symmetric Supercapacitor Electrochemical Performance