Pore-Size Effect on Capacitive Energy Extraction from the Salinity Gradient with Porous Electrodes

Abstract

The microscopic mechanisms through which the pore size of electrodes influences the extraction of salinity gradient energy via a capacitive double-layer expansion method remain not yet fully understood. Herein, we elucidate the relationship between the extracted energy and the pore size in the porous electrode using classical density functional theory. The influence of the pore size on energy, capacitance, and energy density is systematically explored, and a nonmonotonic relationship between the extracted energy and pore size is put forward, which indicates that maximum energy extraction can be attained with an optimal pore size. Moreover, we find that the optimal pore size grows when the operating voltage is enhanced, and the corresponding extraction energy is also raised. Further analysis reveals that the nonmonotonic dependence of energy extraction on the pore size stems from the distinct ion adsorption within nanoscale pores immersed in seawater versus freshwater solutions, and the optimal pore size can be determined empirically by the pore width at which ion adsorption onto the electrode surface in seawater just reaches its saturation. This study not only discloses the microscopic mechanism of pore size influencing energy extraction but also proposes a feasible approach to determining the optimal pore size.