Capillarity Based Ion Concentration Polarization for Desalination and Energy Harvesting

Abstract

Ion Concentration Polarization (ICP) phenomenon is a traditional electrochemical ion transportation process near perm-selective membrane and appears as a steep concentration gradient under d.c. bias. The major function of ICP is an active ion control by an external electric field so that it is significantly useful to study the new ion transportation through nanoporous junction (or membrane) and develop novel engineering applications such as biomolecular preconcentrator and electro-desalination. Currently, the inherent capillarity of the nanostructure was demonstrated to initiate the same ICP phenomenon instead of an external power source so that a power-free desalination was achieved. This capillarity ion concentration polarization (CICP) device is shown to be capable of desalting an ambient electrolyte more than 90% without any external electrical power sources (Figure A). Theoretical analysis for both static and transient conditions are conducted to characterize this phenomenon. These results indicate that the CICP system can offer unique and economical approaches for a power-free water purification system in resource limited settings. As a reverse process of CICP, the chemical to electrical energy conversion system was successfully demonstrated as well. Since a convective flow near the perm-selective membrane plays a deterministic role for the ion transportation in both CICP and ICP mechanism, micro-pillar structures that enhance the convective flow around the membrane would accelerate the ion flux leading higher electrical current with the micro-pillar structure (Figure B). These microfluidic visualization platforms are significantly useful in both desalination and energy harvesting applications since it would provide a differential shell of such macroscopic systems that most of important ion exchanges occur microscopically. Figure A. Desalination using CICP mechanism. Figure B. current – time relation in ICP energy harvesting with and without micro pillar structures. Figure 1