Grand canonical Monte Carlo simulation results are reported for an electric double layer formed by a planar electrode, spherical anions, and dimer cations. The dimer cation consists of two tangentially tethered hard spheres: one is charged and the diameter is fixed at 0.425 nm, while the other is neutral and its diameter can vary and take the following values: 0.0, 0.170, 0.255, 0.340, 0.425, 0.510, 0.680 nm. The ion charge numbers(valencies) are +1 and -1. The ions are immersed in a solvent mimicked by a dielectric continuum medium whose relative permittivity is 8r. The electrolyte concentration is 1 M, and the temperature is 298.15 K. The results for the singlet distribution and the mean orientation functions of charged dimers at the negative electrode show that the large diameter of neutral spheres stabilizes the perpendicular orientation of dimers with charged spheres closer to the electrode. The particle size-asymmetry further leads to a rich double layer structure and asymmetric shaped differential capacitance curves about zero surface charge, similar to that seen in an earlier study involving asynimetric ion valency but symmetric particle size. For a negatively charged electrode and with increasing diameter of the neutral sphere the absolute value of the mean electrostatic potential increases and the differential capacitance decreases. A model of electric dOuble layer is proposed in which two layers of charged spheres are separated by a layer of neutral ones. (C) 2016 Elsevier Ltd. All rights reserved.