Reliability of Constant Charge Method for Molecular Dynamics Simulations on EDLCs in Nanometer and Sub-Nanometer Spaces

JY Yang and Z Bo and HC Yang and HL Qi and J Kong and JH Yan and KF Cen, CHEMELECTROCHEM, 4, 2486-2493 (2017).

DOI: 10.1002/celc.201700447

The constant charge method (CCM) and constant potential method (CPM) are two major techniques to apply electric charges on electrodes for molecular dynamics (MD) simulations on electric double-layer capacitors (EDLCs). Compared with CCM, CPM is more realistic since the influence of electrode polarization is taken into account, although computationally more expensive. In this work, the reliability of CCM for MD simulations on EDLCs in nanometer and sub-nanometer spaces is investigated. Particular attention is paid to the comparison of CCM and CPM for modeling EDLCs in terms of ion concentration and EDL structures in such nano/sub-nano confined spaces, which are widely found in graphene-based materials. The deviation for ion concentration calculated from CCM and CPM was found to be significant in both neutral and charged confined spaces (e.g., deviation ratio of 40.27% and 11.18% for Cl- ions in neutral and charged 9 angstrom space, respectively), which is different from previous observations in bulk solutions. This result could be attributed to the strong electrode polarization in nano/sub-nano confined spaces. Although CCM and CPM yielded essentially similar EDL structures, the time evolution of electrode charge with a potential difference could be conducted only with CPM. The findings of the current work could provide useful insights for choosing appropriate methods for MD simulation on EDLCs in nano/sub-nano confined spaces.

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