Elsevier

Carbon

Volume 45, Issue 13, November 2007, Pages 2511-2518
Carbon

Electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors

https://doi.org/10.1016/j.carbon.2007.08.024Get rights and content

Abstract

This paper describes the electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes as electrodes in electrical double layer capacitors with organic electrolyte. Onions were formed by vacuum annealing of 5 nm nanodiamond (ND) powder at 1200–2000 °C with the goal to investigate the effect of carbon microstructure on specific capacitance and ion transport. In contrast to micro- or mesoporous activated carbons, the outer surface of carbon onions is fully accessible to electrolyte ions and the size of pores between carbon onions or nanotubes does not depend on the annealing temperature. Charge-discharge measurements revealed a two times decrease in the specific capacitance of onions and nanotubes upon graphitization and formation of polyhedral particles after annealing at 1800 °C and above. However, the capacitance became less current dependant. The carbon onion cells are able to deliver the stored energy under a high current density with a capacitance twice than the one obtained with MWCNT. Electrical measurements and impedance spectroscopy showed about two orders of magnitude increase in conductivity of electrodes and twofold decrease in the equivalent series resistance of the assembled cells after heat treatments of ND. The time constant extracted from the impedance data is around 10 times smaller for ND annealed at above 1800 °C than for activated carbons and is closely approaching the one for MWCNT. This shows that the open structure of carbon onions leads to an increased ability to quickly deliver the stored energy.

Introduction

Electrical double layer capacitors (EDLC) are energy storage devices where the charges are stored at the interface between the electrode and the electrolyte. The electrolyte ions are electrostatically adsorbed on the surface of porous electrodes, typically made of carbon, without involving Faradaic reactions. Those systems are able to deliver more power than a battery and have nearly an infinite cyclability [1], [2], [3]. Additional attractive features include a wide operative temperature range and easy to detect state of charge. Organic electrolytes are currently used in most commercial EDLC due to higher maximum cell voltage as compared to aqueous ones and thus increased power and energy storage densities [4].

Activated carbons are the most widely used electrode materials because they have a high specific surface area accompanied with a high electrochemical stability and relatively high electronic conductivity in organic electrolytes. Until recently it was believed that a pore size two or three times larger than the solvated ion size was appropriate to reach high capacitance values [5], [6]. Nevertheless, the latest studies showed that porous carbons with subnanometer pores and a narrow pore size distribution lead to higher capacitance values than traditional activated carbons even with a solvated ion size larger than the average pore size [7], [8]. However, an important question remains to what extent transport of ions in narrow pores affects the performances of a supercapacitor. Can a material in which ions do not need to diffuse inside long channels and migrate through thousands of bottleneck pores provide a higher charge/discharge rate or a better performance overall? It is difficult to answer these questions because most porous carbons have both micropores, mesopores and outer surfaces of the particles available for ion adsorption and their contributions are difficult to separate. In carbon nanotube electrodes, mainly the outer surface participates in the charge storage [9]. In exfoliated graphites, it is not clear to what extent electrolyte can penetrate between graphene layers and how different will be contribution of basal planes and edges to the total capacitance. The only carbon material in which there are almost no small sub-nanometer pores and all sites on the surface are nearly equal is carbon onion. However, no information on capacitance of carbon onions except a short conference report on their electrochemical behavior in aqueous electrolyte can be found in the literature [10]. The tightly linked texture-porosity properties in activated carbons also hinder the understanding of other fundamental effects, including the influence of the carbon texture on ion migration within pores and their adsorption. Pore size of electrodes prepared from carbon onions, in contrast, primarily depends only on the onion size and is less influenced by the texture or chemistry of the particle surface.

In this work, we investigated the electrochemical behavior of carbon onion-like particles 5–15 nm in size synthesized from nanodiamonds and having a very different pore texture and structure as compared to carbon black and carbon nanotubes. Their open pore texture could be particularly important in organic electrolytes where ion size is larger than those in aqueous electrolytes and the pore size effect could be revealed easier.

Section snippets

Material synthesis and characterization

The ND produced by detonation synthesis was supplied by NanoBlox, Inc. (UD50 grade) and its structure has been described in a previous paper [11]. The as-received ND powder was annealed in a vacuum furnace (Solar Atmospheres, USA) at temperatures of 1200 °C, 1500 °C, 1800 °C and 2000 °C for 2 h under high vacuum (10−5–10−6 Torr). Catalytic chemical vapor deposition (CCVD)-grown MWCNT of 10–20 nm in diameter (Arkema, France) and highly graphitized carbon black powder of ∼40 nm in diameter (PureBlack

Structure and properties

Annealing in vacuum at above 1200 °C results in ∼35% increase of SSA as compared to as-received ND, to 500 ± 40 m2/g (Table 1), presumably due to a decrease in particle density and expansion during phase transformation of diamond to graphite. The slight decrease in SSA observed at 2000 °C could be linked to sintering and coarsening of carbon particles. The pore size distribution (not shown) and the average pore size of ∼6 nm do not change significantly with the annealing temperature. The pore size

Conclusions

A systematic study of the effects of carbon microstructure on ion-electrode interactions has been performed using carbon nanoparticles with outer surfaces accessible to electrolyte and no micropores as electrodes in supercapacitors. We showed that not only pore size but also the defects in the carbon structure affect ion adsorption and migration within the capacitor electrodes. We further observed a reverse correlation between the normalized capacitance and the high frequency/high current

Acknowledgements

The authors would like to acknowledge NanoBlox Inc., Arkema, and Superior Graphite for providing the carbon materials, W.L. Gore & Associates for providing separator membranes; S. Osswald for the electrical conductivity measurements, and J. Chmiola for helpful discussion (both are at Drexel University). The Raman spectrometer used in this work is operated by the Centralized Materials Characterization Facility of the A.J. Drexel Nanotechnology Institute.

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Current address: Georgia Institute of Technology, School of Materials Science and Engineering, Atlanta, GA 30332, USA.

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