Ion and Pore Fluid Transport Properties of Nafion Membrane Separating Two Electrolyte Solutions. Part.II. Kinetics of the Ru (2,2-bipyridine)2/3 Ion Transfer
Abstract
Ion exchange and impedance measurements were used to investigate the ion and pore fluid transport properties of the as-received Nafion(R) membrane (Nafion(R) 117, 0.181 mm in thick) in the presence of the Ru(bpy)(3)(2+) cation (bpy = 2,2'-bipyridine). Very slow transport of Ru(bpy)(3)(2+) across the membrane is reflected in both the ion exchange rate and the impedance of the membrane located between two electrolyte solutions.
The initial rate of exchange of proton by Ru(bpy)(3)(2+) in the H form Nafion(R) corresponds to a diffusion coefficient of 7.3 x 10(-14) cm(2) s(-1). At higher Ru(bpy)(3)(2+) loadings, the ion transport rate decreases, so that the 100% limit is not reached in a reasonable period of time.
Impedance behaviour can be understood on the basis of the theoretical model previously developed and applied to the transport of the alkali metal cations and proton. However, a coupling between the kinetic and transport impedances is manifested here by an increased Warburg coefficient of the membrane impedance, which is predicted to correspond to the low ratio of rates of the pore fluid flow inside the membrane and the interfacial ion transfer.
The latter process is characterized by an apparent rate constant of about 10(-3) cm s(-1). Ion and pore fluid transport properties of Nafion(R) are likely to be related to its morphology, e.g. to the diameter of pores.
An analysis of the membrane capacitance suggests that the equilibrium potential difference across the Nafion(R) membrane is concentrated in the compact layer at the membrane solution interfaces, which can be represented as the surface dipole formed by the ionized fixed sites on the polymer surface and the counterions from the solution phase.