Molecular Dynamics Simulations of Ether-Modified Phosphonium Ionic Liquid Confined in between Planar and Porous Graphene Electrode Models

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 [P-222,P-201] his(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 angstrom, there is a monolayer of ions, whereas in larger pores (12 angstrom) 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.