Voltage–concentration relations with enhanced slopes in electrochemical cells based on Nafion®

In electrochemical cells based on Nafion®, exposed to a humid atmosphere that contains oxygen, hydrogen or a mixture of oxygen and hydrogen, the open circuit voltage, Voc, depends linearly on the logarithm of the concentrations [H2] and [O2] in the gas phase. We show, experimentally, that the slope of these relations is significantly higher than in a concentration cell 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 explanation cannot be applied when one of the reacting gases H2 or O2 is missing. We suggest a different explanation that involves reactions that are in equilibrium and others totally blocked. With this mixed equilibrium–non-equilibrium approach (MENEA), we are able to interpret all the experimental results obtained under different conditions. In the analysis of these experiments, we always find a water transference coefficient m∼3, as expected for the species H+·(H2O)m that transports protons in highly humidified Nafion®. The explanations are consistent with (a) Ag and Pd catalyzing the absorption of water by Nafion®, (b) Pt and Pd catalyzing the reaction of oxygen and hydrogen with charged complexes in Nafion®, and (c) in the absence of oxygen, Ag, Pt and Pd forming an oxide with water.