A full-factor model for nano-confined osmotic energy conversion with ionic volume and dielectric effects

Salinity-gradient energy, as a crucial renewable energy, can be efficiently and directly transduced into electricity through nanofluidic osmotic energy conversion. Nanofluidic osmotic power generation technology is based on nanochannels and employs the “gate effect” of the electrical double layer in nanochannels to generate an ion current. In the conventional partial-factor model for osmotic energy conversion (PF-OEC), the ion is assumed to be a point charge, and the ionic volume and dielectric effects are neglected. The ion concentrations at charged surfaces are overestimated during osmotic power generation. In this study, a full-factor model for osmotic energy conversion (FF-OEC) is established to overcome the point-charge assumption of the conventional model. The steric force, Born force, and dielectrophoretic force are fully incorporated into the model to satisfy the nano-confined scenarios. Based on FF-OEC, a numerical model using the finite element method is further constructed to unravel the ionic volume and dielectric effects. The feasibility of FF-OEC is experimentally verified by measuring the osmotic current and diffusion potential of four monovalent and divalent cations. The full-factor model can capture a more physically realistic electrical double-layer structure, which is more uniform compared to the conventional model. PF-OEC is applicable to nanochannels with large radii (larger than or equal to 5 nm) and low surface charge densities (lower than or equal to 0.04 C m⁻²). FF-OEC is applicable to nanochannels with various radii and surface charge densities. When the radius decreases or the absolute value of surface charge density increases, PF-OEC underestimates power generation performance, and FF-OEC is required to overcome the underestimation of performance in nano-confined channels. This study enriches the theory of ion-selective transport in nano-confined channels and provides a basis for the device design under multiple environments.