To locate the point of zero electrode charge (PZEC), we have applied a series of external potentials to the graphene oxide/electrolyte system. Results of this "titration" procedure are shown in Fig. . Comparing PiNet and PiNet2, the PZECs are almost identical and about -0.30 V. The corresponding electrostatic potential profile of the system is shown in Fig. . Despite having almost the same PZECs, the contributions from two sides of electrode to the overall potential drop are not the same between PiNet and PiNet2. This raises the question whether this difference led by PiNet2 contributes to the capacitance asymmetry which are often found in the aqueous electrolyte solutions and suggested to correlate with water adsorption energy. In Fig. , the capacitance as it changes with hydroxyl surface coverage for the PiNet and PiNet2 models as well as the electrostatic potential profiles for the 20% surface coverage case are shown. Despite the fact that the potential profiles in Fig. seems to shift up by about 80 mV when switching from PiNet to PiNet2, the resulting capacitance remains almost the same. It should be pointed out that the Helmholtz capacitance reported here appear to be half of that reported previously. This is due to a missing factor of 2 in the conversion instead of any physical difference in the simulation systems. Nevertheless, the main conclusion remains the same that the proton charge can increase the Helmholtz capacitance at PZEC significantly, which becomes comparable in magnitude with respect to those found in the metal-oxide based systems for protonic double layer. Comparing electrostatic potential profiles of the charged system of PiNet and PiNet2 respectively with those of the corresponding neutral system, it is found that there is a constant shift in two cases as a result of the differing base charges. Therefore, this shift simply cancels out when computing the capacitance because only the potential difference between the neutral and the charged surfaces matters. This highlights the fact the Helmholtz capacitance is a response property.