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Publication Title | Interconnected Carbon Nanosheets Derived from Hemp for Ultrafast Supercapacitors with High Energy

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Interconnected Carbon Nanosheets

Derived from Hemp for Ultrafast

Supercapacitors with High Energy

Huanlei Wang,†,‡ Zhanwei Xu,†,‡ Alireza Kohandehghan,†,‡ Zhi Li,†,‡,* Kai Cui,‡ Xuehai Tan,†,‡

Tyler James Stephenson,†,‡ Cecil K. King’ondu,†,‡ Chris M. B. Holt,†,‡ Brian C. Olsen,†,‡ Jin Kwon Tak,§ Don Harfield,§ Anthony O. Anyia,§ and David Mitlin†,‡,*

†Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2 V4, Canada, ‡National Institute for Nanotechnology (NINT), National Research Council of Canada, Edmonton, Alberta T6G 2M9, Canada, and §Bioresource Technologies, Alberta Innovates-Technology Futures, Vegreville, Alberta, T9C 1T4, Canada

ABSTRACT We created unique interconnected partially graphitic carbon nanosheets (10 30 nm in thickness) with high specific surface area (up to 2287 m2 g 1), significant volume fraction of mesoporosity (up to 58%), and good electrical conductivity (211 226 S m 1) from hemp bast fiber. The nanosheets are ideally suited for low (down to 0 C) through high (100 C) temperature ionic- liquid-based supercapacitor applications: At 0 C and a current density of 10 A g 1, the electrode maintains a remarkable capacitance of 106 F g 1. At 20, 60, and 100 C and an extreme current density of 100 A g 1, there is excellent capacitance retention (72 92%) with the specific capacitances being 113, 144, and 142 F g 1, respectively. These characteristics favorably place the materials on a Ragone

chart providing among the best power energy characteristics (on an active mass normalized basis) ever reported for an electrochemical capacitor: At a very high power density of 20 kW kg 1 and 20, 60, and 100 C, the energy densities are 19, 34, and 40 Wh kg 1, respectively. Moreover the assembled supercapacitor device yields a maximum energy density of 12 Wh kg 1, which is higher than that of commercially available supercapacitors. By taking advantage of the complex multilayered structure of a hemp bast fiber precursor, such exquisite carbons were able to be achieved by simple hydrothermal carbonization combined with activation. This novel precursor-synthesis route presents a great potential for facile large-scale production of high- performance carbons for a variety of diverse applications including energy storage.

KEYWORDS: biomass . carbon nanosheets . ionic liquid . supercapacitor . energy storage

Electrochemical capacitors (known as ultracapacitors or supercapacitors) based on electrical double layer (EDL) charge accumulation hold promise for a wide range of applications, including por- table electronics, uninterruptable power sources, medical devices, load leveling, and hybrid electric vehicles.1,2 Conventional or- ganic electrolytes used in EDL supercapaci- tors contain a mixture of a solvent and a salt. However, the exclusive use of organic electrolytes may limit the broadening of the supercapacitors' commercial applica- tion base, since solvents such as acetonitrile have issues associated with their flammabil- ity at elevated temperatures, as well as their toxicity and environmental impact. Alterna- tive electrolytes based on solvent-free ionic liquids possess several advantages over or- ganic ones, including higher operating vol- tage windows (>3 V vs ∼2 V), lower toxicity, negligible vapor pressure, and much better

WANG ET AL.

thermal stability.3 Unfortunately, superca- pacitors based on ionic liquids normally perform well only at temperatures near or above 60 C.4,5 The room temperature per- formance, which is an essential prerequisite for most commercial applications, remains poor due to ionic liquid's high viscosity and low ionic diffusivity. Moreover, large cation and anion sizes limit the usefulness of con- ventional microporous activated carbon electrodes since the ions either literally do not fit into pores or become diffusion lim- ited at required scan rates.6,7 It is only with custom tailored eutectic ionic liquids that lower temperature performance may be achieved using carbon nanotubes and car- bon onions.3

Activated carbons,8 templated carbons,9 carbon nanofibers,10 carbon nanotubes,11 carbide-derived carbons,12 and graphene13,14 have been intensively investigated for super- capacitor electrode applications. Among

VOL. XXX ’ NO. XX ’

* Address correspondence to lizhicn@gmail.com, dmitlin@ualberta.ca.

Received for review February 12, 2013 and accepted May 7, 2013.

Published online

10.1021/nn400731g

C XXXX American Chemical Society 000–000 ’ XXXX

A

www.acsnano.org

ARTICLE

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