Nafion Film Investigations Using Ferricyanide As a Probe Molecule

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
M. K. Reynolds and D. W. Paul (University of Arkansas)
Nafion membranes are a common choice in proton exchange membranes used in hydrogen-oxygen fuel cells. Their purpose is to separate the fuel and the oxidant within the cell, and allow protons from hydrogen oxidation to migrate through the Nafion membrane to the cathode. The electrons from the oxidation are thermodynamically pushed to the cathode through an external circuit where the current is driven through a load. One major issue with proton exchange membranes is their durability. Degradation can occur mechanically and chemically. In this work, a probe molecule, ferricyanide with hydrodynamic voltammetry was used to determine tears or imperfections in Nafion membranes cast over gold electrodes. Ferricyanide is negatively charged and therefore should not enter a Nafion membrane, any recorded red/ox signal indicates that the molecule has gained access to the bare electrode through an imperfection in the membrane. Hydrodynamic voltammetry was used to control the thickness of the solution diffusion layer thickness so that diffusion through the solution could be discriminated from diffusion processes in the membrane. During the work we also discovered, as others had, that negatively charged species1 and ferricyanide specifically2 can permeate into Nafion, albeit at a very slowly. This permeation rate is too slow to be recorded on the short time-scale of a hydrodynamic voltammetry experiment. However, if ferricyanide was included in the solution used to hydrate the membrane, a ferricyanide red/ox signal was seen indicating that approximately 0.4 mM had become trapped in the membrane (Figure 1). Moreover, only hydrated membranes containing ferricyanide, provide enough conductivity through the membrane to detect ferricyanide in solution (Figure 2). The signals obtained through these membranes were evaluated with a combination of the Koutecky-Levich equation3 and permeability equations from the Gough group4.

1. Unnikrishnan, E. K.; Kumar, S. D.; Maiti, B., Journal of Membrane Science 1997, 137, 133-137.

2. DeWulf, D.; Bard, A. J., J. Macromol. Sci., Chem. 1989, A26 (8), 1205-9.

3. Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications. 1980; p 718 pp.

4. Gough, D. A.; Leypoldt, J. K., Analytical Chemistry 1979, 51 (3), 439-444.