Masao Sudoh

Water permselectivity in the pervaporation of acetic acid–water mixture using crosslinked poly(vinyl alcohol) membranes

Abstract The states of water absorbed in the crosslinked poly(vinyl alcohol) (PVA) membranes was studied thermally using differential scanning calorimetry (DSC). It was observed that water absorbed in the membranes was distinguished into free water, freezing bound water and non-freezing bound water. As the crosslinking time increased, the non-freezing bound water, the freezing bound water and the free water contents decreased. The bound water acted as an effective plasticizer for the polymer due to its interaction with the polymer. In the pervaporation of acetic acid–water mixture using the crosslinked PVA membranes, the decrease in the acetic acid and water permeation with increasing crosslinking time was correlated with the crosslinked structure of the membranes and the decrease in the bound water content in the polymer which led to a decrease in the plasticization effect. As a result, the separation factor increased with increasing crosslinking time.

Effects of thermal and concentration boundary layers on vapor permeation in membrane distillation of aqueous lithium bromide solution

The permeation flux of water vapor in membrane distillation is affected by membrane properties, the difference in the vapor pressure between the opposite sides of the membrane, and the operational conditions. Since the vapor pressure is a function of the concentration and the temperature, the vapor flux through a membrane in decreased with increasing concentration and decreasing temperature of the salt solution. Permeation experiments with a PTFE membrane (80 μm thick, 0.2 μm pore diameter, 0.75 porosity, 2.38 × 10−3 m2 effective surface area) were conducted by using a batch cell, which had two reservoirs stirred by magnetic bars, separated by a membrane, and thermostated by hot water and cool water circulated in respective jackets. The temperature of the hot water was ranged from 308 to 373 K, and the temperature of the cool side was kept at 288 K. The stirring rate was varied from 200 to 800 rpm. The concentration of aqueous lithium bromide solution was ranged within 0 to 55 wt%. The water flux was obtained by the moving rate of the water meniscus in the capillary connected to the sealed cell. The permeate flux was affected by the thermal and concentration boundary layers. The analogy analysis for the thermal and concentration boundary layers was applied to the permeation mechanism, and well explained the effects of the stirring rate and the difference in the temperature on the permeation flux. The thickness of the thermal boundary layer was found to be larger than that of the concentration boundary layer, and both layers were hardly negligible under the conditions of this work.

Development of Trickle‐Bed Electrolyzer for On‐Site Electrochemical Production of Hydrogen Peroxide

A practical‐scale cell with a trickle‐bed electrode was developed for on‐site electrochemical production of hydrogen peroxide by the cathodic reduction of oxygen. Terminal voltage and current efficiency were affected by the dispersions of liquid and oxygen gas in the trickle‐bed cathode and by the current density. Current efficiency was improved by obtaining uniform dispersions of electrolyte and oxygen in the trickle‐bed cathode. A current efficiency of 97.4% was obtained at a cell voltage of 2.1 V and a current density of . To save power, the temperature of the electrolyte was not controlled, and exothermic reactions increased the temperature of the anolyte recovered at the outlet from the inlet temperature of 37–44°C. The temperature of the catholyte rose simultaneously from an inlet temperature of 25–44°C. The crystallization of sodium peroxide within the cathodes was prevented by controlling peroxide concentration, and the maximum concentration of hydrogen peroxide in a 5.0% caustic soda solution was 2.1%.

Degradation Evaluation of Gas-Diffusion Electrodes for Oxygen-Depolarization in Chlor-Alkali Membrane Cell

The cathodic properties of the oxygen reduction at gas-diffusion electrodes were examined by an ac impedance analysis using an equivalent circuit in order to improve the stability of the gas-diffusion electrode in 30% concentrated alkaline solution at the temperature of 353 K. The effect of the wettability and penetration of the electrolyte into the gas-diffusion electrode was estimated by the impedance parameters of the charge-transfer resistance and the double-layer capacitance. Comparing the electrodes using Ag and Pt catalysts for different lifetimes, the Ag-loaded prototype electrode showed a higher performance than the Pt-loaded electrode. The cathode potential increased in a negative direction and the diameter of the Nyquist plots also increased for the longer used electrode. The surfaces of the gas-diffusion electrodes were observed and analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. The thickness of the longer used electrode decreased because of the degradation. Carbon, polytetrafloroethylene, and silver were deformed by the damage of the electrode surface, and the peaks of sodium and oxygen were observed.

Impedance Analysis of Gas‐Diffusion Electrode Coated with a Thin Layer of Fluoro Ionomer to Enhance Its Stability in Oxygen Reduction

The kinetics of oxygen reduction on gas-diffusion electrodes has been extensively investigated, but quantitative data on the electrode characteristic or the mechanism of the electrode degradation are not sufficient. Therefore, electrode characteristics were evaluated by the change of the composition of the electrode, polytetrafluoroethylene content, or catalyst species of Ag or Pt to determine quantitatively the effect of the electrolyte penetration. In addition, a Nafion thin film or a plasma-polymerized film was coat-ed on the surface of the electrodes to improve the stability of the electrode. The cathodic properties of oxygen reduction on the gas-diffusion electrode were examined by the ac impedance method. The effect of the penetration of the electrolyte into the gas-diffusion electrode was determined quantitatively by using the charge-transfer resistance and the double-layer capacitance, corresponding to catalyst consumption and wettability in the interafacial area of the electrode in highly concentrated alkaline solution at high temperature. From the acceleration test with ELAT (E-TEK Co.) electrodes, the charged thin film was found to minimize the penetration of alkaline solution into the electrode.

Mathematical Modeling of the Sodium/Iron Chloride Battery

A mathematical model of the sodium/iron chloride battery containing a molten AlCl{sub 3}-NaCl electrolyte is presented. A cylindrical cell consisting of a positive iron electrode, an electrolyte reservoir, a separator, and a negative sodium electrode is considered. The analysis uses concentrated-solution theory within the framework of a macroscopic porous electrode model. The effects of the state of discharge, the cell temperature, the precipitation and dissolution rates of NaCl, and the current density on the current-potential relation during the discharge and charge cycles are discussed. The major influences on battery performances are changes in porosity and component volume fractions during cycling.

Generation Performance of Gas-Feed Direct Methanol Fuel Cell

Abstract The modification effect of Nafion on the generation performance at different temperatures and methanol concentrations was investigated. The direct methanol fuel cell performances and electrochemical properties of the DMFC system using as-received Nafion117, a modified Nafion membrane, and using the Nafion117 MEA preparation by spray treatment on the surface of each catalyst layer for conductivity improvement. The open circuit voltage using the modified Nafion membrane was higher than that using Nafion117 at the cell temperature of 343–383 K and methanol concentration of 1.5–10 kmol/m3. The spray treatment of MEA was effective and improved the short circuit current up to 461 mA/cm2 at a 5 kmol/m3 methanol concentration in comparison with no treatment of MEA because of the low interfacial resistance. The power density of 75 mW/cm2 (no treatment Nafion117 MEA:40 mW/cm2) was obtained. The method of the spray treatment was found to be very effective for the DMFC system. The cell performance of Nafion117 MEA increased with the methanol concentration because of the reduction of the concentration overvoltage.

Modification Effect of Proton-Exchange Membrane on Methanol Permeation and Proton Conductivity for Direct Methanol Fuel Cell

Abstract The Nafion membranes modified with a long chain counter ion, (C8H17)4N+ or (CH3)4N+, and sandwich-type modified Nafion membranes were prepared as proton conducting membranes (PEM) for a direct methanol fuel cell (DMFC). We evaluated the methanol permeability, ionic conductivity, ion cluster diameter, ion exchange capacity and water content. The ion cluster diameter of the modified Nafion membranes was determined by small angle X-ray diffraction (SAXRD) measurements, and decreased in comparison with Nafion 117. The methanol crossover flux decreased to less than 10% that of Nafion 117 with the decreasing ion cluster diameter. For the sandwich-type membrane (Octyl-s1), the methanol crossover flux was 46% that of Nafion 117 and the ionic conductivity was 4.2 S/m.

Performances of Fuel-Cell-Type CO Sensors Using Each of Polybenzimidazole and Nafion Membranes

This study describes a new CO sensor based on a proton exchange membrane for combustion applications over 100°C. One of the promising high-temperature polymers is based on the phosphoric acid doped polybenzimidazole (PBI) polymer. Output at less than 300 ppm CO was clearly detected using the PBI at around 200°C. Based on the polarization curves, the current density approximately linearly decreased with the decreasing voltage. It was found that the PBI had a high CO to H 2 selectivity in the combustor exhaust. The contributions of the anode and cathode electrodes to the output voltage are revealed using a normal hydrogen electrode. Sensor performance using the PBI was less influenced by the humidity than when using Nafion.

Design of Direct Methanol Alkaline Fuel Cell with Anion Conductive Membrane Prepared by Plasma Polymerization

Introduction Direct methanol fuel cells (DMFCs) may be expected to be power sources of mobile applications instead of primary or secondary batteries, because of potentially high energy density. However, the development of DMFCs has been hampered due to several serious problems: slow electrode-kinetics, CO poisoning of Pt catalyst at low temperature, methanol crossover and high cost of the membrane, catalyst and so on. In direct methanol alkaline fuel cells (DMAFCs), OH transports across the membrane, against the methanol cross over (MCO). Then MCO is expected to decrease by using anion conductive membrane. It approaches for the development of DMAFCs has been proposed. Plasma polymerization is expected to give highly cross-linked, uniform, thin and also chemically and thermally stable organic-membranes on various substrates. In this study, thin anion exchange membranes were prepared by plasma polymerization for use as an electrolyte in DMAFCs. The chemical properties of the membranes and DMAFC performance were measured. The objective of this work is to fabricate thin anion conductive membrane, and to adopt for AFCs system. These membranes property were compared with commercial membranes (Nafion117, Nafion112, AHA).