Differential capacitance measurements. The differential capacitances 𝐶 & were measured by potentiostatic impedance spectroscopy (PEIS). The PEIS measurements were conducted at -0.1 ~ 1.0 V using a Gamry Interface 1010E Potentiostat within the frequency range between 20 kHz and 1 Hz and 10 mV amplitude. The obtained Nyquist plots were fitted by Zview software with an equivalent circuit of 𝑅 F -(𝑅 JH 𝐶𝑃𝐸) (Fig. ) , where 𝑅 F and 𝑅 JH are the solution resistance and the charge transfer resistance, respectively. The capacitive effects of EDL were modeled using a constant phase element (CPE) to offset the influence of the distributed time constant. The impedance of CPE is 1/𝑌 * (𝑗𝜔) , where 𝑌 * is the CPE coefficient and the exponent, 𝑛, is in the range of 0 ≤ 𝑛 ≤ 1. The equivalent capacitance (𝐶 7KLDM ) was calculated using the method reported by Brug et al. :