Nanoporous carbon electrode from waste coffee beans for high performance supercapacitors

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Abstract

Activated carbon was produced from waste coffee grounds by treatment with ZnCl2. Supercapacitor electrodes prepared from this coffee grounds carbon exhibited energy densities up to 20 Wh kg−1 in 1 M H2SO4, and excellent stability at high charge–discharge rates. In a two-electrode cell a specific capacitance as high as 368 F g−1 was observed, with rectangular cyclic voltammetry curves and stable performance over 10,000 cycles at a cell potential of 1.2 V and current load of 5 A g−1. The good electrochemical performance of the coffee grounds carbon was attributed to a well developed porosity, with a distribution of micropores and mesopores 2–4 nm wide, and the presence of electrochemically active quinone oxygen groups and nitrogen functional groups. This work highlights the potential to utilize waste biomass to produce electrode materials for cost-effective energy storage systems.

Introduction

Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors and ultracapacitors, are a promising technology for delivering peak power demands in portable electronic applications, including electric vehicles. Energy storage in EDLCs rely on the accumulation of charge at electrodes purely by electrostatic forces [1], and as no chemical reactions are involved, unlike the case with batteries, high rates of energy delivery, and stable, reversible charge cycling can be achieved. The most common electrode material used in commercial EDLCs is high surface area activated carbon, which is a high cost component [2]. A key challenge is to develop low cost carbons with high energy and power densities.

The specific capacitance of a carbon electrode material is influenced by surface area, but pore size distribution and presence of functional groups are also important. Several recent studies [3], [4], [5], [6] have examined the influence of pore size on EDLC. It was reported by Lin and co-authors [5] that in aqueous electrolytes pores less than 0.5 nm diameter are too narrow for double-layer formation, and pores larger than 2 nm too wide. While mesopores may not contribute a significant number of adsorption sites, other studies [7], [8] have reported that mesopores can facilitate electrolyte transport at fast charge rates. The presence of oxygen and nitrogen functionalities can also enhance the capacitance of a porous carbon material [9] by enhancing the carbon wettability and by pseudo-Faradaic reactions. Carbon pore structure and surface chemistry should be matched to the electrolyte for optimum performance.

Agricultural and food wastes, including various parts of the coffee bean [10], [11], [12], have been widely studied in the production of activated carbons. Activation by ZnCl2 is one method that allows control of mesoporosity in carbons produced from waste biomass [13], [14]. We report the preparation of a nanoporous carbon from waste coffee grounds and the use of this carbon as an electrode material in an aqueous electrolyte EDLC with an energy density up to 20 Wh kg−1.

Section snippets

Activated carbon preparation and characterization

Waste coffee grounds, from our tea room’s domestic espresso machine, were dried at 373 K for 24 h. The dried coffee grounds were mixed for 4 h with a 1:1 weight ratio of ZnCl2 dissolved in a small amount of distilled water. After drying at 373 K the coffee grounds + ZnCl2 mixture was carbonized in a tube furnace under a flow of N2 gas at a heating rate of 5 K min−1 up to 1173 K, and held at this temperature for 1 h. The carbonized sample, referred to as coffee grounds carbon (CGC), was washed in 0.6 M HCl

Porous texture characterization

The adsorption and desorption isotherms of N2 at 77 K on CGC, and Maxsorb, are shown in Fig. 1a. Like the commercial Maxsorb carbon, the CGC displays a Type I isotherm characteristic of a microporous material. The textural properties of CGC are summarized in Table 1. Fig. 1b shows the cumulative pore size distribution of the CGC. Clearly the total pore volume of CGC is much less than that of Maxsorb. However, the CGC has a greater ratio of narrow micropores (<1 nm) to total pore volume. Both

Conclusions

Activated carbon prepared from waste coffee grounds exhibited extraordinary electrochemical capacitance due to a well developed porosity, complemented by pseudo-Faradaic reactions involving oxygen and nitrogen functional groups. These findings highlight the exciting possibility to utilize waste biomass, such as coffee grounds, to produce low cost electrode materials for high performance EDLCs.

Acknowledgements

This research was supported financially by the Australian Research Council. The authors acknowledge the assistance of Dr. Bill Gong who performed the XPS analysis at the Analytical Centre of the University of New South Wales, Sydney.

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