where 𝑃 is the pressure (1 atm), 𝑇 is the temperature (296 K), 𝑅 is the general gas constant, 𝑓 𝑖 and 𝑓 𝑒 are the influent and effluent gas flow rates, 𝑚 𝑡𝑜𝑝 is the mass of top electrode, A is the area of electrodes, V is the volume of electrodes, and t is the total charging time (including the 30 min holding step).