This study investigated essential parameters of capacitive energy extraction based on double-layer expansion (CDLE) device. The parameters, in particular, were the porous electrode pore size diameter and the operating flow rate. Understanding both parameters will help to improve the performance of the CDLE device for practical application. CDLE cycle was performed under different flow rates using three different samples with different average pore sizes. The data were analyzed based on the Gouy-Chapman-Stern (GCS) theory. It was found that higher energies were extracted with smaller average pore size, because it increases the number of transferred charges, due to the decrease in the diffusion distance of ions. Consequently, the redistribution of ions is also likely to occur and accelerates the rate of increase in the voltage. In addition, the energy extracted increases with the external voltage. Higher external voltage increases the charge transfer and reduces the differences between the voltage rise and the voltage drops during the solution exchange process. It was also found that the extracted energies in all the cycles were identical at different flow rates owing to no change in electric double layer (EDL) formation. However, as the flow rate increases, ions diffusion was accelerated, which improve power production. (C) 2020 Elsevier Ltd. All rights reserved.