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Volume 154, 2012
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Graphene-based supercapacitors in the parallel-plate electrode configuration: Ionic liquidsversus organic electrolytes

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Abstract

Supercapacitors with two single-sheet graphene electrodes in the parallel plate geometry are studied via molecular dynamics (MD) computer simulations. Pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4) and a 1.1 M solution of EMI+BF4 in acetonitrile are considered as prototypes of room-temperature ionic liquids (RTILs) and organic electrolytes. Electrolyte structure, charge density and associated electric potential are investigated by varying the charges and separation of the two electrodes. Multiple charge layers formed in the electrolytes in the vicinity of the electrodes are found to screen the electrode surface charge almost completely. As a result, the supercapacitors show nearly an ideal electric double layer behavior, i.e., the electric potential exhibits essentially a plateau behavior in the entire electrolyte region except for sharp changes in screening zones very close to the electrodes. Due to its small size and large charge separation, BF4 is considerably more efficient in shielding electrode charges than EMI+. In the case of the acetonitrile solution, acetonitrile also plays an important role by aligning its dipoles near the electrodes; however, the overall screening mainly arises from ions. Because of the disparity of shielding efficiency between cations and anions, the capacitance of the positively-charged anode is significantly larger than that of the negatively-charged cathode. Therefore, the total cell capacitance in the parallel plate configuration is primarily governed by the cathode. Ion conductivity obtained via the Green-Kubo (GK) method is found to be largely independent of the electrode surface charge. Interestingly, EMI+BF4 shows higher GK ion conductivity than the 1.1 M acetonitrile solution between two parallel plate electrodes.

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Article information


Submitted
02 May 2011
Accepted
11 May 2011
First published
14 Jul 2011

Faraday Discuss., 2012,154, 249-263
Article type
Paper

Graphene-based supercapacitors in the parallel-plate electrode configuration: Ionic liquids versus organic electrolytes

Y. Shim, H. J. Kim and Y. Jung, Faraday Discuss., 2012, 154, 249
DOI: 10.1039/C1FD00086A

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