Enhanced Salt Removal by Unipolar Ion Conduction in Ion Concentration Polarization Desalination.

Rhokyun Kwak,Lan Chen,Jongyoon Han,Van Sang Pham,Bumjoo Kim

doi:10.1038/srep25349

Abstract

Chloride ion, the majority salt in nature, is ∼52% faster than sodium ion (DNa+ = 1.33, DCl− = 2.03[10−9m2s−1]). Yet, current electrochemical desalination technologies (e.g. electrodialysis) rely on bipolar ion conduction, removing one pair of the cation and the anion simultaneously. Here, we demonstrate that novel ion concentration polarization desalination can enhance salt removal under a given current by implementing unipolar ion conduction: conducting only cations (or anions) with the unipolar ion exchange membrane stack. Combining theoretical analysis, experiment, and numerical modeling, we elucidate that this enhanced salt removal can shift current utilization (ratio between desalted ions and ions conducted through electrodes) and corresponding energy efficiency by the factor ∼(D− − D+)/(D− + D+). Specifically for desalting NaCl, this enhancement of unipolar cation conduction saves power consumption by ∼50% in overlimiting regime, compared with conventional electrodialysis. Recognizing and utilizing differences between unipolar and bipolar ion conductions have significant implications not only on electromembrane desalination, but also energy harvesting applications (e.g. reverse electrodialysis).

Highlights

In conventional electrochemical desalination systems such as electrodialysis (ED)(Fig. 1a)[1,2,3] and capacitive deionization (CDI)[4,5], both cations and anions are removed from opposite sides either by ion exchange membranes (IEMs) or ion selective electrodes
By determined current utilization (CU), energy efficiency improves as V* is decreased, at the cost of slower ion removal at low driving electric field; slower ion removal requires lower operating flow rate or larger system. This trade-off between production capacity and energy efficiency is universal, and can be found in other desalination methods (e.g. applied pressure vs. energy in reverse osmosis)[6]. Escaping from this trade-off issue, in this paper, we show that one can achieve higher salt removal ratio in ion concentration polarization (ICP) desalination than in ED by relying on unipolar ion conduction with only cation exchange membranes (CEMs) (Fig. 1b)
As per directions of the membranes’ biased transport, ion depletion occurs on the anodic side of the CEM (anion exchange membrane (AEM)) (Fig. 1)

Summary

Introduction

In conventional electrochemical desalination systems such as electrodialysis (ED)(Fig. 1a)[1,2,3] and capacitive deionization (CDI)[4,5], both cations and anions are removed from opposite sides either by ion exchange membranes (IEMs) or ion selective electrodes. This trade-off between production capacity and energy efficiency is universal, and can be found in other desalination methods (e.g. applied pressure (flow rate) vs energy in reverse osmosis)[6] Escaping from this trade-off issue, in this paper, we show that one can achieve higher salt removal ratio in ion concentration polarization (ICP) desalination than in ED by relying on unipolar ion conduction with only cation exchange membranes (CEMs) (Fig. 1b). As per directions of the membranes’ biased transport, ion depletion occurs on the anodic (cathodic) side of the CEM (anion exchange membrane (AEM)) (Fig. 1) In this scenario, the strength of ion depletion on the CEM and the AEM are different, because cation and anion diffusivities are different[10,11,12]; these CEMs and AEMs are not contribute to desalination. The analytical solution and microscopic observation by simulation offer a solid support to this explanation (SI Section 1,2)

Methods

Results

Discussion

Conclusion