Electrokinetic energy conversion and desalination of an asymmetric nanopore with applied temperature gradients

The electrokinetic energy conversion (EKEC) and desalination performance of a cation-selective funnel-shaped nanopore are investigated theoretically through a continuum description. The effects of pore geometry, characterized by a symmetric factor between the conical and the cylindrical regions, are considered along with pressure and temperature gradients. A desalination index is proposed to evaluate the performance of desalination, examining the trade-off between the ion rejection rate and the volumetric flow rate. It is found that applying a positive (or negative) ∆T relative to ∆P lowers (or raises) the rejection rate, as influenced by the ion selectivity. In addition, the rejection rate at a positive pressure difference (∆P>0; flow coming from tip to base) is slightly larger than that at ∆P<0 because the former has a stronger inlet electric field. Based on the desalination index, a positive ∆T manifests a better desalination performance than a negative ∆T, in contrast to the EKEC case, which displays better energy efficiency under a negative ∆T than under a positive ∆T. The smaller diffusivity of Na+ compared with K+ makes the power density and efficiency of NaCl better than those of KCl. Meanwhile, the more significant thermal diffusion for NaCl impacts its rejection rate more appreciably.