Tatsuhiro Okada

Ion and water transport characteristics of Nafion membranes as electrolytes

Transport characteristics of Nafion membranes, that have been published earlier, are re-evaluated. It is found that the specific conductivity of the membranes is not only determined by the mobility of the ions, but largely also by the interaction of ions with water and with microscopic membrane channel structures. Similarly, the water transference coefficient, defined as the number of moles of water transported per Faraday through the membrane, is governed by two effects: an electrostatic effect between ion and water dipoles, and an effect due to the size of the cation. Contributions to electro-osmotic water transfer are water of hydration to cations and hydrodynamically pushed water molecules. The size of the ion compared to the channel diameter, has a major impact on the electric conductivity, but also on water transport. It is shown that hydrophilic cations can promote an enlarged hydrophilic domain in the membrane, that is accompanied by a lower membrane resistance. Criteria for designing high performance ion conducting membranes are given based on this basis.

Transport and equilibrium properties of Nafion® membranes with H+ and Na+ ions

Abstract Transference coefficients, ionic conductivities and equilibrium properties are reported for Nafion® 117 membranes containing a mixture of two cations and water. The membranes were equilibrated in 11 kinds of aqueous solutions of HCl and NaCl, with mole fractions of HCl between 0 and 1, all having cCl−=0.03 kmol m−3. Some experiments were repeated for Nafion® 112 and 115 membranes. By electron probe microanalysis (EPMA), it was found that Nafion® 117 membranes have a slightly higher affinity for Na+ than for H+. The water content of the membrane was determined gravimetrically. It decreased from 20.6±0.1 to 18.4±0.2 molecules water per cationic site, as the membrane changed from the pure H-form (HM) to the pure Na-form (NaM). The water transference coefficient, tH2O, obtained by streaming potential measurements, increased in Nafion® 117 from 2.5±0.1 to 9.2±0.1 as the mole fraction of Na+ in the membrane, xNaM, increased from 0 to 1. Most of the increase occurred for xNaM>0.5. The transference number of H+ in the membrane, tH+(m), which was determined by an improved emf-method, showed a rapid decrease for xHM

Simulation for water management in membranes for polymer electrolyte fuel cells

Water management in membranes for polymer electrolyte fuel cells during their operational conditions is considered theoretically. Using a linear transport equation based on the diffusion of water and the electroosmotic drag, analytical solutions for water concentration profiles in the membrane are obtained from which membrane resistance overvoltage and other characteristic values are calculated. Specific parameters of the membranes such as water transference coefficient tH2O, water permeability Lp, specific membrane conductivity κ etc., at cell operating temperatures (50 to 80°C) have been obtained from the experiment, and used as input parameters to the analytically derived expressions for water balance calculations. Hydration states of the membrane are simulated for various current densities at the fuel cell operation conditions. The effects of several operational factors of fuel cells on the membrane water content are discussed systematically, among which the membrane thickness and humidification conditions are shown to be the most significant. Contamination of the membrane by foreign impurities turned out to cause a serious problem of the water depletion at the anode side of the membrane. For the purpose of testing the validity of the method, the net water flux and the change in electric resistance inside the membrane are calculated extensively and compared with reported experimental results. The present method turned out to be fairly satisfactory for predictive water management, in spite of its simplicity of the simulation procedure.

Theory of water management at the anode side of polymer electrolyte fuel cell membranes

Abstract The behavior of the water transport and the water concentration profile at the anode side of the membrane in a polymer electrolyte fuel cell is considered theoretically. A linear transport equation based on the diffusion of water and electroosmotic water drag was taken into account. Two kinds of boundary value problem were treated: (1) for semi-infinite boundary conditions, analytical solutions for water concentration at a given time and location were obtained, under either humidified or non-humidified conditions at the anode side of the membrane; (2) for finite boundary conditions, the steady-state water concentration profile was obtained with two kinds of boundary condition at the cathode side. The effect of various fuel cell operation and membrane state parameters on the water concentration was evaluated in a systematic way. Among those parameters tested, the current density and water penetration parameters are essential in determining membrane water content. Membrane thickness and the diffusion coefficient of water are important parameters in the case of finite boundary conditions. It turned out that contamination of the membrane by foreign impurities (e.g. NaCl), even on the surface, will cause a serious effect on the water depletion at the anode side. Water supply from the anode side of the membrane results in a considerable improvement of the membrane state with regard to water depletion. Although the method was fairly simplified in comparison with the most detailed analysis of membrane water content, it gave explicit and easy-to-handle equations for the purpose of quantitative or semi-quantitative evaluation of the membrane water management in polymer electrolyte fuel cell operations.

Oxygen Reduction Characteristics of Heat‐Treated Catalysts Based on Cobalt‐Porphyrin Ion Complexes

Oxygen reduction characteristics of graphite electrodes modified with aggregated cobalt-porphyrins heat-treated at various temperatures and then impregnated in Nafion polymer were investigated systematically. The aggregated cobalt-porphyrin compound was adsorbed on graphite powder and then heat-treated at various temperatures ranging from 200 through 1200 °C. The catalysts were evaluated for electroreduction performances of oxygen on modified electrodes in sulfuric acid solutions. The electrocatalytic performances of catalysts as measured in rotating ring-disk electrodes showed that the most effective catalytic activity for oxygen reduction was attained for the aggregated cobalt-porphyrin compounds on graphite powder heat-treated at temperatures between 600 and 800 °C. The surface concentration of Co and N as measured by X-ray photoelectron spectroscopy increased as the heat-treatment temperature was increased up to 800 °C. The electrochemical performance of pyrolyzed cobalt-porphyrin catalysts changed in parallel with the surface concentration of Co and N. In the temperature range 600-800 °C, it appeared that the increased catalytic activity originated from the well-dispersed Co-N 4 moiety or from fragments of the original molecules still retaining the cobalt bound to nitrogen atoms. In the higher temperature region, Co-N 4 bonds were no longer detected, and the presence of cobalt in the metallic states (β-Co) in the catalysts was confirmed by X-ray diffraction analysis. From the results of the stability tests, the pyrolyzed cobalt porphyrin electrode systems were found to be more stable than the nonheat-treated catalysts.

Theory for water management in membranes for polymer electrolyte fuel cells. Part 1. The effect of impurity ions at the anode side on the membrane performances

Abstract Performance degradation in membranes for polymer electrolyte fuel cells was discussed theoretically for the case where the membrane is contaminated with foreign impurity cations at the cathode side. Water transport in a two-cation system membrane was considered by assuming an ‘infected zone’ of finite thickness. Four kinds of boundary value problems were solved, and analytical formulae derived for the water concentration profile across the membrane. The water content in the membrane, the net water flux and the membrane resistance overvoltage were calculated systematically as functions of several relevant parameters in fuel cell operations. Localized contamination at the cathodexa0∣xa0membrane interface turned out to be even more serious than the uniform contamination of the membrane or localized contamination at the anode side. It is noted that special caution should be directed in order to avoid the membrane contamination, especially at the cathode side, because contaminant will easily enter from the air stream through the cathode compartment of a fuel cell.

Study of Pt Electrode/Nafion Ionomer Interface in HClO4 by In Situ Surface-Enhanced FTIR Spectroscopy

Pt electrode/Nafion interface has been characterized in HCIO 4 aqueous solutions using surface-enhanced infrared absorption spectroscopy (SEIRAS) with the aim of investigating the structure of the interface which plays a crucial role in the performance of polymer electrolyte fuel cells (PEFCs). A potential dependent band was found around 1100 cm - 1 and assigned to the symmetric vibration of the -SO - 3 groups of the ionomer membrane. Electric field driven orientation/adsorption of the ionomer membrane at the interface was suggested from the potential dependence of the band intensities of -SO - 3 groups. It was inferred that the -SO - 3 groups act like counterions at the Pt/ionomer interface to form the electric double layer.

Ion and Water Transport Characteristics in Membranes for Polymer Electrolyte Fuel Cells Containing H + and Ca2 + Cations

The effect of contamination by Ca{sup 2+} ions in proton conductive membranes for polymer electrolyte fuel cells was investigated systematically. Ion and water transport characteristics of Nafion membranes, which were equilibrated with 0.02 to 0.03 kmol/m{sup 3} of HCl/CaCl{sub 2} mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that Ca{sup 2+} has much higher affinity than H{sup +} to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site H{sub 2}O/SO{sub 3}{sup {minus}}, decreased about 19% from 21 for H-form membrane to 17 for Ca-form membrane. The water transference coefficient was obtained from streaming potential measurements of Nafion 115 membranes of various H{sup +}/Ca{sup 2+} cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca{sup 2+} content in the membrane increased, especially when the equivalent fraction of H{sup +} in the cationic exchange sites x{sub HM} became less than 0.5. Ionic transference numbers for H{sup +} in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H{sup +} became less than 0.5. Membrane conductivity changed linearlymorexa0» with H{sup +} composition in the membrane. In strong contrast to the interaction mode between H{sup +} and Ca{sup 2+} cations during ionic conduction, which appeared almost independent, a certain extent of interference was observed among water molecules as they were carried along by cations in the membrane. It was predicted that if Ca{sup 2+} ions enter the fuel-cell membrane, they cause serious effects to membrane drying and result in deterioration of fuel-cell performance. The advantage of this methodology in the study of transport characteristics of fuel-cell membranes is stressed due to ease and accuracy of measurements.«xa0less

Simultaneous determination of the concentration of methanol and relative humidity based on a single Nafion(Ag)-coated quartz crystal microbalance

Abstract A quartz crystal microbalance (QCM) coated with Nafion ® film recast from Nafion ® (Ag) complex solution has been used to investigate the interaction between methanol, water and Nafion ® (Ag) and to determine simultaneously the concentration of methanol and water (relative humidity). The frequency shift caused by methanol and water adsorbed onto Nafion ® (Ag) was in the order: water>methanol, due to larger association constant of water than methanol. Binding rate constant analysis showed that methanol demonstrates larger binding and dissociation rate constants than water due to larger vapor pressure of methanol. Molecular mechanics calculation gave binding energy change between Nafion ® (Ag) and methanol or water molecules. Artificial neural networks (ANNs) method was utilized to process the frequency response data of the single piezoelectric crystal at different times in the light of different adsorption dynamics of methanol and water on the Nafion ® (Ag) coated QCM. Concentrations of methanol and water can be estimated simultaneously with the ANNs.

Pumping effects in water movement accompanying cation transport across nafion 117 membranes

Streaming potential study showed that the water transference coefficients of Nafion 117 equilibrated with solutions of aliphatic ammonium chlorides which have both hydrophilic and hydrophobic parts increased with the increase in the size of aliphatic ammonium cations, which was definitely opposite to the trends observed in the case of alkali and alkaline earth metal cations. The result revealed the fact that the pumping effect of the cations indeed exists and would be the dominant contribution in the case of hydrophobic cations. In the case of hydrophilic cations, cations together with strongly hydrated water molecules pump loosely-bound or peripheral water molecules.