Energy Harvesting from Electrokinetic / Pressure-Driven Flows with Steric Effect and Reverse Electrodialysis in Nanospace

Abstract

As the channel size approaches the thickness of the charged layer (typically 10~100 nm) the resulting molecular and non-equilibrium effects are markedly different from those observed in larger channels and have a significant effect on the transport behavior of solutes and solvents As a result the problem of modeling fluidic behavior at the nanoscale has attracted increasing interest in recent years In the first part of this thesis simulations based on a Poisson-Boltzmann model suitably modified to take account of the steric (i e finite ion-size) effect are performed to investigate the ionic transport phenomenon within two nanofluidic confinements namely a nanoslit and a nanotube for the case of both electrokinetic and pressure-driven flows The results show that for all values of the electrolyte concentration the streaming conductance increases when the steric effect is taken into account; particularly under high surface charge density conditions In addition it is shown that the steric effect amplifies the net charge density in both confinements The enhancement in the streaming conductance is particularly pronounced in the nanotube since the geometry effect of the nanotube results in a greater amplification of the net charge density Finally it is shown that for both nanofluidic confinements the contribution of electroosmotic flow (EOF) to the electrical conductance increases when the finite ion size is taken into account In the second part of the thesis numerical simulations based on the full Poisson-Nernst-Planck (PNP) equations are performed to investigate the reverse electrodialysis (RED) phenomenon in negatively-charged conical nanopores The simulations consider three different salts namely 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 It is shown that when diffusion takes place from the base side of the nanopore to the tip side the transference number of counter-ions ( ) is enhanced due to an overlapping of the electrical double layer (EDL) in the tip region As a result 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 also increases as the diffusion coefficient of the positive ions ( ) approaches that of the negative ions ( ) For the diffusion current and diffusion potential in the negatively-charged conical nanopore show negative on the current-voltage characteristics Moreover the conversion efficiency also increases with an increasing concentration gradient given a small surface charge density For the RED device considered in the present research the maximum power conversion efficiency is found to be around 45%

Date of Award	2015 Mar 20
Original language	English
Supervisor	Ruey-Jen Yang (Supervisor)