Reverse electrodialysis in conical-shaped nanopores: salinity gradient-driven power generation

Abstract

Numerical simulations based on the full Poisson–Nernst–Planck (PNP) equations are performed to investigate the reverse electrodialysis (RED) in negatively charged conical nanopores. The simulations consider three different salts (i.e., KCl, NaCl and LiCl) and examine the effects of the concentration gradient, surface charge density and diffusion coefficient on the current–voltage characteristics and energy conversion efficiency of the RED system. When diffusion takes place from the base side to the tip side, the transference number of counter-ion (t₊) can be enhanced due to an addition in the EDL overlap near the tip side. It results in the higher energy conversion efficiency of RED than the salt ion diffusion from the tip side to the base side, i.e., the RED performance of the conical nanopore is dependent on the direction of salt concentration gradient. In addition, the results show that the conversion efficiency increases as the diffusion coefficient of the positive ions (D₊) approaches that of the negative ions (D₋). For D₊ < D₋, the diffusion current and diffusion potential in negatively charged conical nanopores show negative on the current–voltage characteristics and the conversion efficiency increases as the concentration gradient is increased at small surface charge density. Finally, the calculated maximum power conversion efficiency reaches 45%.