Activity of Nafion

The activity of protons and water content in Nafion films are considered for different electrolytes and electrolyte concentrations in the solution.  A model of activity effects in Nafion will allow optimization of operating conditions for systems such as fuel cells and sensors where proton exchange membranes and films significantly impact system response.  Cyclic voltammetry (CV) experiments were undertaken for Nafion films on electrodes with tris(2,2ʹ–bipyridyl)dichloro–ruthenium(II) hexahydrate (Ru(bpy)₃²⁺) as the redox probe.  The probe was equilibrated into Nafion from the electrolyte solution that contain different inert electrolytes at various concentrations.  The glassy carbon working electrode was modified with a recast Nafion film made by drop casting Nafion suspension and then evaporation of the casting solvents.  Terahertz spectroscopy experiments are used to find the dielectric constant of the recast Nafion film.  The dielectric constant in Nafion is needed to calculate activity coefficients in Nafion.

From the data obtained at the Nafion modified electrodes, i_p (i_p is peak current) varies with inert electrolyte cation.  For plots of i_p versus v^1/2 (v is scan rate), inert electrolytes with the same cation had similar slopes where the observed peak currents ranked as H⁺ > Na⁺ > K⁺.  Cations with smaller hydration energies are more easily exchanged into Nafion.  The redox probe had no voltammetric response with high concentrations of the smaller cation electrolytes, consistent with little probe in the film.  However, even for small cation electrolytes, at sufficiently low electrolyte concentrations, Ru(bpy)₃²⁺ yields a voltammetric response. From the slopes of i_p versus v^1/2, C_film D_film^1/2 is found, where C_film and D_film are the concentration and the diffusion coefficient of Ru(bpy)₃²⁺  in the film. C_film D_film^1/2 does not change with the electrolyte concentration.  Convolution will further refine determination of C_film D_film^1/2 and the dielectric constant will allow further evaluation of activity effects in Nafion.