Jae-Hwan Choi

Pore size characterization of cation-exchange membranes by chronopotentiometry using homologous amine ions

Abstract Pore size of Neosepta CMX (Tokuyama Soda, Japan), Selemion CMV (Asahi Glass, Japan) and heterogeneous HJC cation-exchange membrane (Hanguk Jungsoo, Korea) was characterized by chronopotentiometry in NH 4 Cl and in other amine chloride solutions — simple homologous amine chloride, R–NH 3 Cl, where R=CH 3 , CH 2 CH 3 , (CH 2 ) 2 CH 3 , (CH 2 ) 3 CH 3 , (CH 2 ) 4 CH 3 , and (CH 2 ) 5 CH 3 . The chronopotentiometric curves obtained in the CMX and CMV membranes showed a steep inflection while diffusive in the heterogeneous HJC membrane. In all membranes, the inflections of chronopotentiometric curves became diffusive with the enlargement in molecular size of transporting ions. Assuming that the ion-exchange membrane consisted of conducting and non-conducting phases, Sand’s equation was modified in this study. Using the modified equation, the fraction of inert material contained in the membrane and the pore size were determined by comparing the theoretical and experimental transition times. The pore size of the membranes, determined by the chronopotentiometry method, correlated with membrane conductivity. It was considered that the diffusive inflection in the chronopotentiometric curve of the heterogeneous membrane was caused by the delay in the formation of the concentration gradient. This delay, in turn, was due to a high fraction of non-conducting region.

Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties

Structural change of an ion-exchange membrane under a high electric field was investigated by comparing water dissociation and the FTIR spectra between the virgin membrane and that used at an overlimiting current density. From a series of water dissociation experiments at overlimiting current densities, it was observed that water dissociation in an anion-exchange membrane used at an overlimiting current density was higher than that in a virgin membrane at the same current density. The FTIR study revealed that the tertiary amine groups are formed from the quaternary ammonium groups on the anion-exchange membrane surface where ion depletion occurs under the influence of the applied strong electric field. The occurrence of increased water dissociation is considered to be caused by the protonation and deprotonation of the tertiary amine groups in the anion-exchange membrane. On the other hand, there was no structural change for the cation-exchange membrane under the electric field investigated in this study, which is coincident with the results of water dissociation experiments for the CMX membrane. In addition, we found that membrane resistance, permselectivity, and plateau length of the current-voltage curve were affected by the converted tertiary amine groups depending on the solution pH.

Recovery of lactic acid from sodium lactate by ion substitution using ion-exchange membrane

Abstract Electrodialysis experiments were carried out to achieve efficient production of lactic acid from sodium lactate. Conventional electrodialysis (CED) consisting of cation- and anion-exchange membrane and ion substitution electrodialysis (ISED) consisting of only cation-exchange membrane were performed and the results were compared in terms of lactic acid loss and the ratio ( η I ) of sodium ions transported to the current supplied. While both electrodialysis operations removed over 95% of sodium ions from the feed solution, the CED operation, however, resulted in a considerable loss of lactic acid, whereas there was no such loss in the ISED operation. Furthermore it was found that the ratio of sodium ions transported to the current supplied was above 1.5 during the early stage of operation in the ISED experiment, indicating that the sodium ions are transported not only by electric force but also by concentration gradient in the ISED operation. By comparing the results of the two electrodialysis operation modes, it was found that the ISED operation is advantageous for the production of lactic acid from sodium lactate in terms of preventing lactic acid loss. In the ISED operation, however, process efficiency decreased with the accumulation of sodium ions in the acid compartment because sodium ions in the acid solution are transported back to the feed stream. This finding suggests the desirability of using a proton permselective cation-exchange membrane, which is preferentially permeable to proton ions among the various cations present in the solution.

An electrodialysis model for determination of the optimal current density

In light of broad use of the electrodialysis (ED) processes, it has become crucial to minimize its operating costs. The total cost of the ED process consists of energy, investment and maintenance costs. Energy cost increases linearly with the current density while the investment cost decreases with current density, resulting from the membrane cost. This trend implies that current density is a design parameter that has considerable influence on the process cost. In this study, we performed an overall cost assessment of an ED process as a function of current density and determined an economically optimal current density for the batch electrodialysis of NaCl using a rigorous model. Salt concentration and energy consumption predicted by the model were in good agreement with the experimental results. Further, membrane area and power consumption were calculated as a function of current density using the proposed procedure. The overall cost assessment shows that the selection of an optimal current density enabled to significantly reduce the ED process cost.

All-solid-state proton conductive membranes prepared by a semi-interpenetrating polymer network (semi-IPN)

Novel all-solid-state ion conductive polymer membranes were synthesized for anhydrous high temperature fuel cells. Ion-conductive monomers (ethylene glycol methacrylate phosphates, phosmers) were employed for fixing ion-conductive moieties in the matrix polymer (sulfonated poly(arylene sulfone), sPAS). Inorganic materials, acid-doped titanium oxide and tin indium phosphate were introduced to form anhydrous polymer composite membranes. As a result, poly(phosmer)s significantly improved both the proton conductivity and flexibility of the composite membranes. Sulfonated poly(arylene sulfone)–poly(phosmer)–tin indium phosphate composite membranes showed the highest ion-conductivity value under anhydrous conditions due to the dipolar interaction of poly(phosmer) with the matrix polymer and the inorganic material. The composite membranes exhibited a drastic enhancement in performances of anhydrous high temperature fuel cells compared to those of Nafion-based composite membranes. The fuel cell performances indicate that the poly(arylene sulfone)–poly(phosmer)–inorganic particle composites could be applicable for anhydrous high temperature fuel cells as promising polymer electrolyte membranes.

Preparation of stable polyethylene membranes filled with crosslinked sulfonated polystyrene for membrane capacitive deionization by γ-irradiation

Qian Qiu, Ji-Hoon Cha, Young-Woo Choi, Jae-Hwan Choi, Junhwa Shin, and Youn-Sik Lee* 1Division of Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National University, 567 BaekjeDaero, Deokjin-gu, Jeonju, Jeonbuk 54896, Korea 2Hydrogen & Fuel Cell Center for Industry, Academy, and Laboratories, New & Renewable Energy Research Division, Korea Institute of Energy Research, Buan, Jeonbuk 56332, Korea 3Department of Chemical Engineering, Kongju National University, 1223-24, Cheonan-daero, Seobuk-gu, Cheonan, Chungnam 31080, Korea 4Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongup-si, Jeonbuk 56212, Korea