Potential-Dependent Structure of the Ionic Layer at the Electrode Interface of an Ionic Liquid Probed Using Neutron Reflectometry

Neutron reflectometry (NR) has been performed at the electrochemical interface between a Nb film and an ionic liquid, trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate ([THTDP+][PFPB-]). THTDP+ and PFPB- have large sizes and a large difference in the scattering length density, both of which are suitable to sensitively probe the potential-dependent structure of the electric double layer (EDL) at the electrode interface of ionic liquids using NR. The neutron reflectivity profiles as a function of momentum transfer have shown a clear potential dependence that is ascribable to the ionic composition change in the EDL: the accumulation of PFPB- and the depletion of THTDP+ at positive potentials and vice versa at negative potentials. The analysis of the reflectivity profiles has revealed that the EDL is composed of one ionic layer as in the Helmholtz model and is fully occupied by either THTDP+ or PFPB- at potentials far from the potential of zero charge. Further electrochemical analysis suggests that the differential capacitance decreases under fully occupied conditions, which agrees with the camel or bell-shaped behavior of the differential capacitance predicted previously due to ion crowding in the EDL in ionic liquids.