ABSTRACT
TATSUHIRO OKADA Department of Polymer Physics, National Institute of Materials and Chemical Research, Ibaraki, Japan
MAGNAR OTTpY The Membrane Research Group, Telemark Technical Research and Development Center, Porsgrunn, Norway
I. Introduction 456
II. The Equilibrium State 458 A. The membrane polymer and the membrane water 459 B. Ionic distributions over a quasi-anion lattice 460
III. Irreversible Thermodynamic Description 461 A. Coupled transports in the bulk membrane phase 461 B. Further comments on surface heat effects vs. bulk heat
effects 464
IV. Experimental Designs 466 A. Membrane conductivity from a stack method 466 B. Ion transference numbers from a stack method 468 C. Water transference numbers from streaming potentials 469 D. The transported entropy from Seebeck coefficients 471 E. Thermal osmosis experiments 472
V. Ionic Transport 472
VI. Water Transport 474 A. The water transference number 474 B. Water permeability 477
VII. Entropy Transport by Transport of Ions and Water 477 455
VIIL Conclusions 478 List of Symbols 478 References 479
Water, ion and entropy transport are described in two ion-exchange membranes, the homogeneous cation exchange membrane CR61 AZL 389 from Ionics and the inhomogeneous Nafion 117 membrane from DuPont. Irreversible thermodynamics is used to describe transports and design experiments. Transport properties are interpreted on the background of equilibrium data. While the water content of the Ionics membrane is almost constant, the water content of the Nafion membrane varies largely with membrane cation(s). There are two types of water in the Nafion membrane: one type that does not freeze, and one type that freezes. Ions distribute according to regular solution theory in both membranes. The equilibria for water and ion exchange are slow reactions; they are determined by small self-diffusion coefficients. The electric mobility of an ion in the membrane is smaller than in dilute water solutions; by one order of magnitude in the Ionics membrane. The electric mobility of a cation in a mixture with protons is constant, confirming small interactions between the proton, the other ion and the charged polymer chain. Interactions between two different alkali cations were quantified. A characteristic amount of water follows the bulk transport of a monovalent cation, also when it is in a mixture with another monovalent cation. This model has no microscopic analogue. The number of water molecules associated with reversible transport of an ion depends on the field strength of the ion, but seems uncorrelated with the water content in the membrane. The water permeability is larger than expected from self diffusion, and increases with increasing water content. The membrane surface is the source or sink for entropy changes during transport. Data relevant for fuel cell modeling are given. One may expect jumps in intensive variables at the membrane surface for the combination of large membrane fluxes and small thermal and electrical conductivities.
