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      A coarse-grained model of room-temperature ionic liquids between metal electrodes: a molecular dynamics study

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          Abstract

          Ion–ion correlations and image charge interactions in RTILs at metal interfaces result in a spontaneous surface charge separation, which is not seen with non-metal electrodes, suggesting the importance of the surface metallicity on RTIL EDLCs.

          Abstract

          Recent mean-field theories predict that room-temperature ionic liquid (RTIL) electric double-layer capacitors (EDLCs) undergo a spontaneous surface charge separation (SSCS) with no applied potential. In this study, we construct a coarse-grained molecular model that corresponds to the mean-field models to directly simulate the behavior of RTILs without invoking mean-field approximations. In addition to observing the SSCS transition, we highlight the importance of the image charge interactions and explore the enhanced in-plane ordering on the electrodes, two effects not accounted for by the mean-field theories. By calculating and comparing the differential capacitance for RTILs confined between perfectly conducting and non-metal electrodes, we show that the image charge interactions drastically improve the energy storage properties of RTIL EDLCs.

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          Most cited references65

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          Fast Parallel Algorithms for Short-Range Molecular Dynamics

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            Thermal Agitation of Electric Charge in Conductors

            H. Nyquist (1928)
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              Relation between the ion size and pore size for an electric double-layer capacitor.

              The research on electrochemical double layer capacitors (EDLC), also known as supercapacitors or ultracapacitors, is quickly expanding because their power delivery performance fills the gap between dielectric capacitors and traditional batteries. However, many fundamental questions, such as the relations between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance have not yet been fully answered. We show that the pore size leading to the maximum double-layer capacitance of a TiC-derived carbon electrode in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid is roughly equal to the ion size (approximately 0.7 nm). The capacitance values of TiC-CDC produced at 500 degrees C are more than 160 F/g and 85 F/cm(3) at 60 degrees C, while standard activated carbons with larger pores and a broader pore size distribution present capacitance values lower than 100 F/g and 50 F/cm(3) in ionic liquids. A significant drop in capacitance has been observed in pores that were larger or smaller than the ion size by just an angstrom, suggesting that the pore size must be tuned with sub-angstrom accuracy when selecting a carbon/ion couple. This work suggests a general approach to EDLC design leading to the maximum energy density, which has been now proved for both solvated organic salts and solvent-free liquid electrolytes.
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                Author and article information

                Contributors
                (View ORCID Profile)
                (View ORCID Profile)
                Journal
                PPCPFQ
                Physical Chemistry Chemical Physics
                Phys. Chem. Chem. Phys.
                Royal Society of Chemistry (RSC)
                1463-9076
                1463-9084
                May 18 2022
                2022
                : 24
                : 19
                : 11573-11584
                Affiliations
                [1 ]Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, 91125, USA
                Article
                10.1039/D2CP00166G
                30d91b74-dc80-4b64-b9d1-c8014f5d4f70
                © 2022

                http://rsc.li/journals-terms-of-use

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