Upon an increment in the magnitude of 𝜎 free , water molecules at the IHP could reduce the potential drop across the EDL via rearranging their orientations. Consequently, 𝐶 dl , defined in Eq.( ), is increased due to water polarization effects, as seen in the comparison between the full model and the model without water effects. Equivalently, water molecules at the IHP contribute a negative capacitance in series with the GCS capacitance, as pointed out earlier by Huang et al. and Le et al. The two peaks of the 𝐶 dl curve become sharper and move closer to the pzc, compared to their counterparts of the model without water effects. The two peaks signify saturation of water polarization effect. Mathematically, it corresponds to 〈cos α w 〉 = 1 or -1 depending on the sign of 𝜎 free .