Ion-ion correlations across and between micropores in carbon electrodes*

Graphene nanostructures can be the solution for maximizing the capacitance and improving the performance of electrical double-layer capacitors [1]. Also, ionic liquids (IL) can be considered for optimizing these devices owing to some of their unique properties [2]. The combination of both is expected to be key for the development of supercapacitors with high energy density and long cycling life [3].
In this work, we performed MD simulations of a common IL confined between two types of nanoporous carbons with very different features. This IL was represented by a coarse-grained model with reduced charges [4], and the electrodes were maintained at constant potential. The first type of electrode consists in a carbide-derived carbon with a highly disordered structure [5], whereas the second one is a highly-ordered nanoporous electrode consisting of 6 graphene planes that are randomly perforated and separated by an interlayer distance which was systematically varied between 7 and 10 Å [6, 7]. The former was observed to be a better candidate for supercapacitor applications. In particular, we show that disordered materials are needed to mitigate the importance of the ion-ion correlations between the pores that arise when the IL is strongly confined between highly-ordered electrodes, which has important consequences for the performance of supercapacitors.

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[2] E. Kowsari, Editor: Prof. S. Handy, Chapter 19, ISBN: 978-953-51-2122-0 (2015).
[3] R. D. MacFarlane et al., Energy Environ. Sci., 7, 232 (2014).
[4] C. Merlet et al., J. Phys. Chem. C, 115, 16613 (2011).
[5] C. Merlet et al., Nat. Mater., 11, 306 (2012).
[6] T. Méndez-Morales et al., J. Chem. Phys., 148, 193812 (2018).
[7] T. Méndez-Morales et al., Energy Storage Mater., 17, 88 (2019).