Phosphonium-based ionic liquids with short alkyl chains present low viscosity besides their relative high electrochemical stability. These properties make them good candidates for electrolytes of electrochemical double-layer capacitors (EDLC). We performed molecular dynamics (MD) simulations of (2-methoxyethyl)­triethylphosphonium [P222,2O1] bis­(trifluoromethanesulfonyl)­imide [NTf2] ionic liquid confined in planar and nanoporous graphene electrode. The electrodes were simulated with a constant potential model, which allows the carbon charges to fluctuate. In spite of the ether function in the longer chain of phosphonium, the ions are organized in layers of alternated charge close to the surface of planar electrodes. The differential capacitance on the negative electrode is lower than in the positive electrode, which reflects the larger size of phosphonium cations. In nanoporous carbons, inside the pores of 8.2 Å, there is a monolayer of ions, whereas in larger pores (12 Å) there are one layer of N atom of anion and two layers of P atom of phosphonium cations. With both porous electrodes, the ions of the same charge are mostly adsorbed in front of each other across the graphene plane due to high image charges of carbon atoms of the electrode in between the ions. In the electrode of narrower pore, the capacitance varies with the applied voltage, which impacts the overall energy density of the electrode.

Electric double-layer capacitors store energy because of the adsorption of ions on the surface of electrodes. A realistic model to describe the electrolyte–electrode interface is based on the constant potential method that allows the electrode charges to fluctuate in order to try to mimic the polarization of metallic electrodes [J. Phys. Chem. Lett. 2013, 4, 264–268]. We performed molecular dynamics simulations of graphene oxide (GO) electrodes using the constant potential model comparing carefully the interface structure, polarization, and charging processes of an ionic liquid with the respective properties calculated for graphite electrodes. The layered structure of the ions at the electrode–electrolyte interface is less organized in comparison with that observed for graphite electrodes, which reduces overscreening. With regard to performance in terms of energy storage, graphite performs better than GO in a wide range of applied voltages. The charging dynamics of GO is slower at low applied voltages. At high voltages, the stronger electrostatic interactions between the charged electrode and electrolyte prevail, allowing for similar charging times for both supercapacitors.

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