Voltage-concentration relations with enhanced slopes in electrochemical cells based on Nafion (R)

In electrochemical cells based on Nafion (R), exposed to a humid atmosphere that contains oxygen, hydrogen or a mixture of oxygen and hydrogen, the op en circuit voltage, V-oc, depends linearly on the logarithm of the concentr ations [H-2] and [O-2] in the gas phase. We show, experimentally, that the slope of these relations is significantly higher than in a concentration ce ll based on a free proton conductor. This was explained before to be due to a mixed potential formed by the oxidation of hydrogen. However, this expla nation cannot be applied when one of the reacting gases H-2 or O-2 is missi ng. We suggest a different explanation that involves reactions that are in equilibrium and others totally blocked. With this mixed equilibrium-non-equ ilibrium approach (MENEA), we are able to interpret all the experimental re sults obtained under different conditions. in the analysis of these experim ents, we always find a water transference coefficient m similar to 3, as ex pected for the species H+. (H2O)(m) that transports protons in highly humid ified Nafion (R). The explanations are consistent with (a) Ag and Pd cataly zing the absorption of water by Nafion (R), (b) Pt and Pd catalyzing the re action of oxygen and hydrogen with charged complexes in Nafion (R), and (c) in the absence of oxygen, Ag, Pt and Pd forming an oxide with water. (C) 2 001 Elsevier Science B.V. All rights reserved.