Electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors
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.
References (30)
Ultracapacitors: why, how, and where is the technology?
J Power Sources
(2000)- et al.
Principles and applications of electrochemical capacitors
Electrochim Acta
(2000) - et al.
Relationship between the nanoporous texture of activated carbons and their capacitance properties in different electrolytes
Carbon
(2006) - et al.
Electrochemical storage of energy in carbon nanotubes and nanostructured carbons
Carbon
(2002) - et al.
Characterization of nanoporous materials from adsorption and desorption isotherms
Colloid Surf
(2001) - et al.
Diamond nanoparticles to carbon onions transformation: X-ray diffraction studies
Carbon
(2002) - et al.
Novel materials for electrochemical power sources-introduction of PUREBLACK® Carbons
J Power Sources
(2006) - et al.
Electrical resistivity of ultra-dispersed diamonds and carbon-like onions
Chem Phys Lett
(2001) - et al.
KOH activation of carbon nanofibers
Carbon
(2004) - et al.
Electric double layer capacitance of highly pure single-walled carbon nanotubes (HiPCO Buckytubes) in propylene carbonate electrolytes
Electrochem Commun
(2002)
Influence of pore structure on electric double-layer capacitance of template mesoporous carbons
J Power Sources
Influence of carbon nanotubes addition on carbon–carbon supercapacitor performances in organic electrolyte
J Power Sources
On porous electrodes in electrolyte solutions: I. Capacitance effects
Electrochim Acta
The effect of pore size distribution on the frequency dispersion of porous electrodes
Electrochim Acta
Impedance of rough capacitive electrodes: the role of surface disorder
J Electroanal Chem
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Current address: Georgia Institute of Technology, School of Materials Science and Engineering, Atlanta, GA 30332, USA.