Enhancing the osmotic energy conversion of a nanoporous membrane: influence of pore density, pH, and temperature.

Salinity gradient power, which converts Gibbs free energy of mixing to electric energy through an ion-selective pore, has great potential. Towards practical use, developing membrane-scaled nanoporous materials is desirable and necessary. Unfortunately, the presence of a significant ion concentration polarization (ICP) lowers appreciably the power harvested, especially at a high pore density. To alleviate this problem, we suggest applying an extra pressure difference ΔP across a membrane containing multiple nanopores, taking account of the associated power consumption. The results gathered reveal that the application of a negative pressure difference can improve the power harvested due to the enhanced selectivity. In addition, if the pore density of a membrane is high, raising its pore length is necessary to make the energy harvested economic. For example, if the pore length is 2000 nm and the pore density is 2.5 × 109 pores per cm2, an increment in the power density of 213 mW m-2 can be obtained by applying ΔP = -1 bar at pH 11 and 323 K, where a net positive power density can be retrieved. The performance of the system considered under various conditions is examined in detail, along with associated mechanisms.

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores.

Ample studies have shown the use of nanofluidics in the ionic diode and osmotic power generation, but similar ionic devices performed with large-sized mesopores are still poorly understood. In this study, we model and realize the mesoscale ionic diode and osmotic power generator, composed of an asymmetric cone-shaped mesopore with its narrow opening filled with a polyelectrolyte (PE) layer with high space charges. We show that, only when the space charge density of a PE layer is sufficiently large (>1×106 C/m3), the considered mesopore system is able to create an asymmetric ionic distributions in the pore and then rectify ionic current. As a result, the output osmotic power performance can be improved when the filled PE carries sufficiently high space charges. For example, the considered PE-filled mesopore system can show an amplification of the osmotic power of up to 35.1-fold, compared to the bare solid-state mesopore. The findings provide necessary information for the development of large-sized ionic diode and osmotic power harvesting device.