Elsevier

Electrochimica Acta

Volume 192, 20 February 2016, Pages 251-258
Electrochimica Acta

Natural Cellulose Materials for Supercapacitors

https://doi.org/10.1016/j.electacta.2015.12.138Get rights and content

Abstract

Supercapacitors are becoming important energy storage devices in efficient electrical systems and they are typically fabricated from high surface area carbon materials which are manufactured using complex, eco-destructive processes. An alternative approach is based on the conversion of biomass materials which we explore in this paper. Cellulose from cotton pulp is subject to a two-step carbonisation process at a range of elevated temperatures up 1900 °C. The resultant materials are characterized using a range of physical methods, along with cyclic voltammetry and galvanostatic discharge techniques to measure the specific capacitance of the materials formed. It is shown that the optimum processing temperature is around 1000 °C; at lower processing temperatures, the materials are insufficiently conductive whilst at high carbonization temperatures low capacitance is seen due to a loss of surface area. This arises from the inaccessibility of nanometre size pores which are present in abundance after the lower temperature carbonization steps. Cotton pulp carbonised at 1000 °C showed the highest value of capacitance of 107 F g−1 with excellent stability for 2000 cycles. The electrochemical performance of this material is very competitive to other reported carbon materials and indicates the two-stage carbonisation method described is suitable for converting biomass into high quality carbon-based materials for supercapacitor applications.

Introduction

Energy from sustainable and renewable sources is considered a crucial part of any energy infrastructure which aims to reduce reliance on fossil fuels and address environmental concerns. However, the energy supply from most renewable sources – namely solar, wind, tidal, hydro and thermal – depends heavily on the environmental conditions in which they operate[1]. An unstable supply of energy can be made more stable with the use of energy storage systems, in which excess energy is stored in favourable conditions and released during unfavourable conditions or during periods of high demand. There are many types of energy storage systems based on mechanical, electrochemical, chemical, and electrical principles[2]. Presently, energy storage technologies based on lithium-ion and other advanced secondary batteries are valued for their large energy densities, but are slow to recharge and discharge[1], [3]. To address these problems, electrochemical capacitors (ECs) are being developed with faster discharge rates, higher power and excellent stability, having properties suitable for versatile applications requiring quick bursts of energy[4], [5], [6]. There are two types of ECs with super-capacitor properties, which are categorised by their charge storage mechanisms[7]. First, the electrochemical double layer capacitors (EDLCs), also called super-capacitors, are based on adsorbing/desorbing charged ions from an electrolyte onto highly porous electrodes with high surface area. Second, the pseudo-capacitors which rely on fast and reversible surface redox reactions. Super-capacitor ECs have advantages over other energy storage technologies, including no limitation by electrochemical kinetics, fast energy uptake and delivery, long cycle life, and low sensitivity to temperature[8].

Recently, carbon materials, such as carbon nano-tubes (CNTs) and graphene have been broadly used in structural, thermal, environmental, biomedical, electrical applications, and have been incorporated into energy storage devices including fuel cells and EDLC[9], [10], [11], [12], [13]. They possess the advantages of high surface area, chemical stability, lower resource cost (i.e. carbon versus metals) and high electrical conductivity, with the ability to be manufactured and functionalized into a variety of forms[10], [14]. Ironically, despite being made from an element ubiquitous in nature, the production of most state-of-the-art carbon-based materials often involves complex, eco-destructive, and resource-consuming processes[15], [16], [17]. As suggested by Raccichini[12], the large majority of graphene-like materials are unsuitable for large-scale EDLGs. Instead, green and efficient conversions of abundant and renewable biomass, and waste-biomass, into functional nanomaterials holds the key to sustainable future technologies [17].

In comparison to other chemical or physical treatments used to convert biomass into conductive carbon-based materials, thermal carbonisation is a facile technique requiring relatively low thermal energy input, no complex equipment, no high cost metals and free from chemical pollutants[18], [19]. Wool fibre[20], papers[21], [22], cellulose-based[16], and wood monolith[23], [24] materials have all been carbonised into materials having good conductivity, high surface area, low density, with nano-pore structures. Byrne et al [23], [24]. in their study of wood monolith structure and crystallinity after heat treatment showed that their performance depends on the presence of cellulose micro-fibrils [23], [24]. Zhou et al[4]. synthesised hierarchical N- and O-functionalised carbon materials from cashmere as raw material for electrochemical capacitors. Honeycomb-derived biosensors and super-capacitors were also reported by Seo et. al [17]. These initial studies show that “green” carbon materials have properties suitable for electrical applications, with potential use as EDLCs.

The carbonisation of ever purer forms of cellulose should lead to a better electrical material. In previous work, we carbonised filter paper and obtained high specific capacitance [22]. However, the required carbonisation temperature is as high as 1500 °C and the material is not purely biomass and is relatively expensive so not of practical interest. Cotton pulp (CP) attracted our interests because it is one of the natural carbon fibres, obtaining from abundant inexpensive, environmental friendly natural resources. It is composed of pure cellulose and can be transformed to carbon fibre materials with stable pore structure after heat treatment. Capacitance performance is controlled by specific surface area (SSA) and the hierarchical porous architecture (i.e. micropores and mesopores)[4]; carbonising cotton pulp should lead to a high surface area material and useful pore architecture. In addition, the simple component and structure of cotton pulp provide homogenous properties, which should lead to better reproducibility, stability, and higher performance. In this work, carbonised CP is investigated for the first time as an EDLC material. The influence of carbonisation temperature on the surface area, macro-and micro-structure, electrical properties and the electrochemical performance was carefully examined.

Section snippets

Material synthesis

The cotton pulp (CP) sheet was obtained from a paper manufacturing company comprising ∼100 wt% cellulose. The carbonization of CP was performed in a quartz tube reactor installed in a furnace. Firstly, a pre-treatment step in which CP was heated at 600 °C for 30 min under a N2 atmosphere was performed. Then the sample was transferred to a high vacuum chamber for carbonisation at final temperatures of 1000, 1300, 1500 and 1700 °C for 30 min respectively with a temperature ramping rate of 5 °C min−1.

Material characterisation

The

Material Characterisation

In our synthetic strategy, two steps are involved in the carbonisation procedures, the first involving desorption of volatile material. The thermogravimetric curve of the raw CP sample is displayed in Fig. 1, which shows the weight loss across the temperature range. The main gravimetric loss occurs below 450 °C; which is comparable to other natural biomass materials[30], and arises from a combination of the following: dehydration from the cellulose unit (reported to be around 150-240 °C); thermal

Discussion

The electrochemical experiments described in this work show 1000 °C to be the best temperature to carbonise CP for the purpose of maximizing super-capacitance performance. Conductivity and surface area seem to highly depend on carbonisation temperature, resulting in a capacitance which strongly depends on the carbonisation conditions. The AFM studies showed that carbonisation at high temperatures lead to high conductivity, with a carbonisation temperature of at least 1000°C being required to

Conclusion

In this work, high quality carbon material suitable for super-capacitor applications has been made using cotton pulp. Cotton pulp is a natural source of pure cellulose and it achieves super-capacitor performance after a simple two-stage thermal treatment. The influence of the carbonisation temperature was intensively studied. Cotton pulp carbonised at 1000 °C showed the highest value of capacitance of 107 F g−1 with excellent stability for 2000 cycles. This material benefits from its carbon fibre

Acknowledgement

The authors are grateful for the financial support from the International Collaborative Energy Technology R&D program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (No. NP2014-0001 20138510040050).

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