Here, we examine electromembrane systems for low-concentration desalination applicable to ultrapure water production. In addition to electrodialysis and ion concentration polarization (ICP) desalination, we propose a recovery-reduced ICP strategy for reducing the width of the desalted outlet for a higher salt removal ratio (SRR). The correlation between conductivity changes and thickness of the ion depletion zone is identified for electrodialysis, ICP_H (1:1), and ICP_Q (3:1) with a low-concentration feed solution (10 mM, 1 mM, 0.1 mM NaCl). Based on the experimental results, the scaling law and SRR for the electroconvection zone are summarized, and current efficiency (CE) and energy per ion removal (EPIR) depending on SRR are also discussed. As a result, the SRR of electrodialysis is mostly around 50%, but that of recovery-reduced ICP desalination is observed up to 99% under similar operating conditions. Moreover, at the same SRR, the CE of recovery-reduced ICP is similar to that of electrodialysis, but the EPIR is calculated to be lower than that of electrodialysis. Considering that forming an ion depletion zone up to half the channel width in the electromembrane system typically requires much power consumption, an ICP strategy that can adjust the width of the desalted outlet for high SRR can be preferable.

This study demonstrates a compact, modular platform that simultaneously generates freshwater and hydrogen gas by leveraging nanoelectrokinetic ion concentration polarization through cation-exchange membranes (CEMs). Using a microfluidic platform, we demonstrate real-time concurrent hydrogen production at the cathode and freshwater production adjacent to the anodic side of CEM. Ion removal is confirmed by fluorescence imaging and ion chromatography, while gas generation is validated using pH indicators. To evaluate practical performance, the device is scaled-up to a mesoscale platform, enabling quantitative measurement of purification efficiency and hydrogen output across varying parameters such as applied currents and source water concentration, etc. We demonstrate that approximately 10% of input electrical energy is recovered as hydrogen, improving overall energy efficiency and enabling the possibility of periodically substituting externally supplied power with internally produced hydrogen. At higher applied currents, hydrogen ions transport through CEM is favored over sodium ion transport, boosting hydrogen production efficiency and device durability. This energy-recoverable purification platform reduces infrastructure needs and suits for decentralized and portable applications in resource-limited settings.

The development of an electroconvective vortex explains the induction of overlimiting current in high voltage applications. However, an in-depth understanding of the dynamics remains challenging because the electrohydrodynamic behavior is unstable. Here, we show how the vortex develops and actively control this using a one-dimensional (1D) grid-patterned ion exchange membrane (IEM). We employed ultraviolet nanosecond laser ablation to fabricate micro-scale (100 𝜇m) non-conductive 1D grid patterns on a commercial IEM. Vortex growth and the electrical responses were simultaneously monitored to derive correlations between the vortex dynamics and ion transport. The rate of electric potential fall increased rapidly during vortex development but decreased more gradually on transverse merging of the vortices. When shear flow was present, the modified scaling law was redefined via dimensionless number analysis of the electroconvective flow by the length of the impermeable 1D grid pattern and the applied voltage. The overlimiting currents and the desalination performances of 1D grid-patterned IEMs were successfully demonstrated. Based on these insights into the electroconvective vortex, we show that the electrical heterogeneity of the IEM actively controls the vortex and enhances desalination performance.

The electromembrane process, which has advantages such as scalability, sustainability, and eco-friendliness, is used in renewable energy fields such as fuel cells and reverse electrodialysis power generation. Most of the research to visualize the internal flow in the electromembrane process has mainly been conducted on heterogeneous ion exchange membranes, because of the non-uniform swelling characteristics of the homogeneous membrane. In this study, we successfully visualize the electroconvective vortices near the Nafion homogeneous membrane in PDMS-based microfluidic devices. To reinforce the mechanical rigidity and minimize the non-uniform swelling characteristics of the homogeneous membrane, a newly developed swelling supporter was additionally adapted to the Nafion membrane. Thus, a clear image of electroconvective vortices near the Nafion membrane could be obtained and visualized. As a result, we observed that the heterogeneous membrane has relatively stronger electroconvective vortices compared to the Nafion homogeneous membranes. Regarding electrical response, the Nafion membrane has a higher limiting current and less overlimiting current compared to the heterogeneous membrane. Based on our visualization, it is assumed that the heterogeneous membrane has more activated electroconvective vortices, which lower electrical resistance in the overlimiting current regime. We anticipate that this work can contribute to the fundamental understanding of the ion transport characteristics depending on the homogeneity of ion exchange membranes.