Journal
JOURNAL OF PHYSICS-CONDENSED MATTER
Volume 26, Issue 28, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0953-8984/26/28/284103
Keywords
electric double layer; mean-field theory; asymmetric ions; differential capacitance; ionic liquids
Categories
Funding
- NSFC [21373118, 21073097]
- Natural Science Foundation of Tianjin [12JCYBJC13900, NCET-10-0512]
- MOE Innovation Team of China [IRT13022]
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The size of ions significantly influences the electric double layer structure of room temperature ionic liquid (IL) electrolytes and their differential capacitance (C-d). In this study, we extended the mean-field theory (MFT) developed independently by Kornyshev (2007 J. Phys. Chem. B 111 5545-57) and Kilic, Bazant, and Ajdari (2007 Phys. Rev. E 75 021502) (the KKBA MFT) to take into account the asymmetric 1:1 IL electrolytes by introducing an additional parameter xi for the anion/cation volume ratio, besides the ionic compressibility gamma in the KKBA MFT. The MFT of asymmetric ions becomes KKBA MFT upon xi = 1, and further reduces to Gouy-Chapman theory in the gamma -> 0 limit. The result of the extended MFT demonstrates that the asymmetric ILs give rise to an asymmetric C-d, with the higher peak in C-d occurring at positive polarization for the smaller anionic size. At high potential, C-d decays asymptotically toward KKBA MFT characterized by gamma for the negative polarization, and characterized by xi gamma for the positive polarization, with inverse-square-root behavior. At low potential, around the potential of zero charge, the asymmetric ions cause a higher C-d, which exceeds that of Gouy-Chapman theory.
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