The electrical double-layer structure and capacitance in room temperature ionic liquids at electrified interfaces were systematically studied with use of the self-consistent mean-field theory. The capacitance curve departs from symmetry with respect to the point of zero charge when unequal ion-size is implemented or when specific adsorption of ions is introduced. For the case of unequal ion-size, the shape of the capacitance curve is strongly determined by the size of the counterion and only weakly influenced by the co-ion size. When present, specifically adsorbed ions would change the capacitance within a limited range of applied potential from the point of zero charge, which itself varies with the strength of specific adsorption.