Towards understanding the effects of van der Waals strengths on the electric double-layer structures and capacitive behaviors
HC Yang and Z Bo and JY Yang and JH Yan and KF Cen, JOURNAL OF POWER SOURCES, 366, 218-225 (2017).
Solid-liquid interactions are considered to play a crucial role in charge storage capability of electric double layer capacitors (EDLCs). In this work, effects of van der Waals (VDW) strengths on the EDL structures and capacitive performances within two representative electrolytes of solvated aqueous solutions and solvent free ionic liquids are illuminated by molecular dynamics simulations. Single crystalline metals with similar lattice constant but diverse VDW potentials are employed as electrodes. Upon enhancing VDW strengths, capacitance of aqueous electrolytes first increases conspicuously by similar to 34.0% and then descends, manifesting a non-monotonic trend, which goes beyond traditiohal perspectives. Such unusual observation is interpreted by the excluded-volume effects stemmed from ion-solvent competitions. Stimulated by predominant coulombic interactions, more ions are aggregated at the interface despite of the increasing VDW potentials, facilitating superior screening efficiency and capacitance. However, further enhancing strengths preferentially attracts more solvents instead of ions to the electrified surface, which in turn strikingly repels ions from Helmholtz layers, deteriorating electrode capacitance. An essentially similar feather is also recognized for ionic liquids, while the corresponding mechanisms are prominently ascribed to the suppressed ionic separations issued from cation-anion competitions. We highlight that constructing electrode materials with a moderate- hydrophilicity could further advance the performances of EDLCs. (C) 2017 Elsevier B.V. All rights reserved.
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