Sungil Jeon

Ion storage and energy recovery of a flow-electrode capacitive deionization process

The ion storage and extraction (or the ion charge and discharge) of a continuous capacitive deionization system were investigated using novel flow-electrode capacitive deionization (FCDI). The flow-electrode, charged by constant voltage, generated about 20% of the supplied energy in an FCDI cell during constant current discharge with NaCl solution (concentration: 35.0 g L−1).

Flow-Electrode Capacitive Deionization Using an Aqueous Electrolyte with a High Salt Concentration

Flow-electrode capacitive deionization (FCDI) is novel capacitive deionization (CDI) technology that exhibits continuous deionization and a high desalting efficiency. A flow-electrode with high capacitance and low resistance is required for achieving an efficient FCDI system with low energy consumption. For developing high-performance flow-electrode, studies should be conducted considering porous materials, conductive additives, and electrolytes constituting the flow-electrode. Here, we evaluated the desalting performances of flow-electrodes with spherical activated carbon and aqueous electrolytes containing various concentrations of NaCl in the FCDI unit cell for confirming the effect of salt concentration on the electrolyte of a flow-electrode on desalting efficiency. We verified the necessity of a moderate amount of salt in the flow-electrode for compensating for the reduction in the performance of the flow-electrode, attributed to the resistance of water used as the electrolyte. Simultaneously, we confirmed the potential use of salt water with a high salt concentration, such as seawater, as an aqueous electrolyte for the flow-electrode.

Surface-modified spherical activated carbon for high carbon loading and its desalting performance in flow-electrode capacitive deionization

We have synthesized a new type of activated carbon (AC) containing ion-selective functional groups, trimethylammonium (AC-N) for anodes and sulfonate (AC-S) for cathodes, for high carbon loading of flow-electrodes. The AC-N and AC-S were partially covered with a 50 nm-thick polymer layer and their surfaces became more hydrophilic than that of bare AC. In the case of bare AC, the maximum carbon concentration in the flow electrodes was 10%, while in the case of the surface-modified AC (AC-N and AC-S), it increased to a maximum of 35% and decreased the viscosity due to the electrostatic repulsion. Moreover, with the increase in carbon concentration, the salt removal efficiencies were improved from 8.2% to 27.7%. This increase in efficiency was attributed to the formation of percolating networks, which occurred because of high carbon loading. The resulting improvement in electronic conductivity at higher loading led to a higher current, and thus an improved salt removal efficiency. Therefore, we expect that the surface-modified AC electrode can be used as a dispersant for hydrophobic AC particles in aqueous solution, as well as in flow electrodes to improve desalting performance in FCDI systems.

Tailoring the surface pore size of hollow fiber membranes in the TIPS process

A new method was used to tailor the surface pore size of PVDF hollow fiber membranes in the TIPS process by the co-extrusion of different solvents at the outer layer of the extruded polymeric solution. In this method, the membrane properties can be controlled over a wide range of mean pore size (from 24 to 600 nm), water permeability (from 3 to more than 4000 L m−2 h−1 bar−1), and acceptable particle rejection (from 50 to 500 nm), with only a slight change in the membrane mechanical strength. Ternary interaction between the extruded solvent, polymer, and diluent in the polymer dope solution played a major role in controlling the hollow fiber membrane properties. At the interface of the extruded solvent and polymeric solution, segregation of the diluent or PVDF molecules near the outer skin layer of hollow fiber membrane strongly affected the mean pore size and filtration performance over a wide range, from MF membranes to typical UF membranes. Considering the competitive solubility difference between the extruded solvent and the diluent in the polymeric solution and that of PVDF, a road map has been shown for the selection of a satisfactory extruded solvent to obtain membranes with desired properties.

Poly(vinylidene difluoride)/poly(tetrafluoroethylene-co-vinylpyrrolidone) blend membranes with antifouling properties

To inhibit fouling phenomenon in membrane process, a new amphiphilic copolymer, poly(tetrafluoroethylene-co-vinylpyrrolidone) (P(TFE-VP)), was blended with poly(vinylidene difluoride) (PVDF) to fabricate a series of antifouling membranes via non solvent induced phase separation (NIPS) method. The effect of copolymer blend ratios and TFE/VP ratios on membrane properties were evaluated, and the stability of P(TFE-VP) in PVDF membrane was studied. The membrane morphology was controlled by adjusting polymer concentration in dope solution, such that all membranes have similar pore size and density, as well as pure water permeability. In evaluating the effect of TFE/VP ratios, the content of VP in dope solutions was also adjusted to allow a fair comparison. We found that for P(TFE-VP) with a higher VP content, adsorption of BSA on polymer film was negligible. Higher blend ratios of this copolymer resulted in higher surface VP content and better hydrophilicity, but antifouling performance ceased to improve when blend ratio was larger than 1:9 (copolymer:PVDF). Meanwhile, a lower VP content in copolymer resulted in inferior hydrophilicity and severe fouling of the blend membranes. It was also proved that comparing with PVP homopolymer, P(TFE-VP) had satisfying stability inside PVDF membrane.

1.7 PVDF Hollow Fibers Membranes

With the fast growth of population during last century, serious global issues raised that global warming and potable water shortage are the major ones that challenge humankind life. During last decades, membrane technology has developed very quickly and attracted much attention as a great potential for solving humankind problems. Poly(vinylidene fluoride) (PVDF) hollow fiber is one of the technologies that are already well industrialized. In this chapter, preparation, modification, and application of PVDF hollow-fiber membrane are summarized. In preparation part, we especially focus on the thermally induced phase separation and nonsolvent-induced phase separation. These two methods widely have been applied for membrane preparation in industrial scale. For both methods, phase diagram, kinetic effects, and preparation parameters are discussed in detail. In some cases, the results of flat-sheet membranes which expected that could be potentially applied for hollow-fiber membrane preparation in the future are also included. Since virgin PVDF membrane comes with some drawbacks, modification for PVDF membranes (making it more hydrophilic and hydrophobic) is studied. Finally, the important application of PVDF membrane is summarized.

One-step fabrication of polyamide 6 hollow fibre membrane using non-toxic diluents for organic solvent nanofiltration

We developed new polyamide 6 hollow fibre membranes using a green process to fabricate cutting-edge “organic solvent nanofiltration” membranes by one-step spinning process for organic solvent separation. This economic and sustainable membrane showed good rejection and durability performance in various organic solvents.

Tailoring both the surface pore size and sub-layer structures of PVDF membranes prepared by the TIPS process with a triple orifice spinneret

In this study, the surface and sub-layer structures of poly(vinylidene fluoride) (PVDF) membranes were effectively tailored by extruding different types of solvents at the outer layer of the polymer solution with a triple-orifice spinneret using the thermally induced phase separation (TIPS) process. The segregation of the diluent or PVDF chains to the interface between the extruded solvent and polymer solution was exploited to tailor the membrane surface structure. In contrast, the diffusion of extruded solvents having good compatibility with PVDF into the polymer solutions changed the phase separation mechanism and resulted in the formation of a novel composite-like structure (spherules connected by the bicontinuous network structure) in the sub-layer of the membrane. This membrane structure enhanced the permeation stability drastically. The membrane properties and structures are summarized based on the change in the competitive ternary interactions between the polymer, diluent, and extruded solvent, and could be used as a guide for selecting appropriate solvents to design and tailor membranes with desired structures and properties.

Dual Superlyophobic Aliphatic Polyketone Membranes for Highly Efficient Emulsified Oil–Water Separation: Performance and Mechanism

Efficient treatment of difficult emulsified oil-water wastes is a global challenge. Membranes exhibiting unusual dual superlyophobicity (combined underwater superoleophobicity and underoil superhydrophobicity) are intriguing to realize high-efficiency separation of both oil-in-water and water-in-oil emulsions. For the first time, a robust polymeric membrane demonstrating dual superlyophobicity to common apolar oils was facilely fabricated via a simple one-step phase separation process using an aliphatic polyketone (PK) polymer, thanks to a conjunction of intermediate hydrophilicity and re-entrant fibril-like texture upon the prepared PK membrane. Further chemical modification to improve surface hydrophilicity slightly can enable dual superlyophobicity to both apolar and polar oils. It is found that a nonwetting composite state of oil against water or water against oil was obtainable on the membrane surfaces only when the probe liquids possess an equilibrium contact angle (θow or θwo) larger than the critical re-entrant angle of the textured surfaces (73°), which can explain the existences of dual superlyophobicity and also the nonwetting to fully wetting transitions. A simple design chart was developed to map out the operational windows of material hydrophilicity and re-entrant geometry, that is, a possible zone, to help in the rational design of similar interfacial systems from various materials. Switchable filtrations of oil-in-water and water-in-oil nanoemulsions were achieved readily with both high flux and high rejection. The simplicity and scalability of the membrane preparation process and the well-elucidated underlying mechanisms illuminate the great application potential of the PK-based superwetting membranes.