Oxygen Reduction at Nafion Film‐Coated Platinum Electrodes: Transport and Kinetics

Abstract

The primary objectives of the work described in this paper were to evaluate the concentration and diffusion coefficient of O2 in a commercial perfluorosulfonate ionomer (Nafion) and to assess the effect of a film of this ionomer on the rate of O2 reduction at a substrate Pt electrode surface. Electrochemical experiments at Nafion film‐coated electrodes were used to obtain these data. A high temperature film‐casting procedure, which has been shown to yield high quality solution‐cast Nafion films, was used to prepare the Nafion‐coated electrodes. The Nafion film‐coated electrodes were equilibrated with and exposed to 0.7M H3PO4 during the electrochemical experiments; it was of particular interest to compare the diffusion, solubility, and kinetic data obtained at the film‐coated electrode to analogous data obtained at a bare electrode, exposed to this electrolyte. We have found that the diffusion coefficient is lower in Nafion than in the electrolyte solution but that the solubility and oxygen reduction rate are higher.

Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics

“…Ink suspension of high surface area carbon supported Pt catalyst was prepared by dispersing Pt/C (46.6 wt%, Tanaka Kikinzoku International, Inc.) into DI water to achieve a concentration of 0.1 mg Pt /mL followed by ultrasonication for 1 h to obtain a uniform suspension. Thin-film Pt/C electrodes were prepared by pipetting different amounts of ink suspension onto pre-polished glassy carbon electrodes to achieve a final loading of 4 Electrochemical measurements.-The electrochemical measurements were performed in a glass cell for rotating electrodes (PINE Research Instrumentation), with a double junction silver/silver chloride (Ag/AgCl) electrode as the reference electrode, a Pt wire as the counter electrode and a 5 mm diameter polycrystalline Pt disk or a 5 mm diameter glassy carbon as the working electrode using a multichannel potentiostat (Princeton Applied Research). Cyclic voltammograms (CV) of Pt disk and Pt/C were recorded between ∼0.03 to 1.1 V vs. RHE in Ar-saturated electrolytes at a scanning rate of 50 mV/s.…”

Kinetic Study of the Hydrogen Oxidation Reaction on Membrane Coated Electrodes. Part II: Applications~!2009-10-21~!2010-02-10~!2010-04-20~!

“…The model takes into account not only the diffusion process in each layer but also the transfer of molecular hydrogen through the corresponding interfaces. This is a limitation to the hydrogen diffusion from the bulk solution to the reaction plane, which had not been taken into account in previous analysis [2][3][4][5][6]. Starting from this model and on the basis of the kinetic treatment previously developed [7][8][9][10], the expressions for the current-potential dependence for the hor on electrodes covered by n polymeric layers were obtained.…”

Kinetic Study of the Hydrogen Oxidation Reaction on Membrane Coated Electrodes. Part I: Theoretical Aspects

“…For kinetic studies, the electrocatalysts are usually covered by a protonic conductor membrane to reproduce the conditions observed in the fuel cell. In most cases a unique membrane was used [4][5][6][7][8][9][10], but recently new arrangements with a multilayer membrane were proposed [11][12][13]. However, the formalism for an appropriate interpretation of these results has not been developed yet.…”

“…Consequently, for the interpretation of the experimental current-potential curves, the diffusion process inside the membrane must be considered. This has been carried out through an extension of the Levich-Koutecky equation, incorporating a term corresponding to the diffusion in the membrane film [5,8],…”