Water transport in fuel cell membranes measured by laser interferometry

Abstract

(cont.) The coefficients of electro-osmotic drag were found to increase with the increasing water content, which indicates that the Grotthuss mechanism of proton transfer is not active in the membranes with low water content. The successful evaluation of these coefficients has not only showed the validity of this new technique, which is confirmed by the agreement between the measured coefficients and the models proposed to explain the transport of water within Nafion® membranes, but also provided useful information to describe and understand the water transport mechanism in Nafion® membranes. Finally, the application of the laser interferometry to the membranes within a fuel cell has demonstrated the feasibility of the technique as an in-situ tool for monitoring the water content, which can be used for optimizing the designs of fuel cell components.

Proton exchange membrane (PEM) fuel cells are considered as one of the most promising clean energy conversion devices. The strong dependency of the proton conductivity of Nafion® membranes, which are the most widely used PEM, on the water content and the large amount of swelling due to water uptake makes the understanding of the water transport mechanism in these membranes crucial for estimating the performance and the durability of the fuel cells and developing a better system. In this regard, a laser interferometry technique to visualize the water content of Nafion® membranes with temporal and spatial resolutions has been developed on the basis of the linear relationship between the optical path length and the water content. The technique was applied 1) to evaluate the fundamental properties of the water transport, the coefficients of diffusion and electro-osmotic drag, and 2) to monitor a Nafion® membrane assembled in a fuel cell. The procedures for evaluating the transport properties rely on the temporal and spatial derivatives of the water content obtained from the laser interferometry and the use of the governing equations for the transport phenomena, which allow the calculation of the coefficients of diffusion or electro-osmotic drag as a function of the water content from a single experiment. The measured chemical diffusion coefficients showed a strong dependency on the compressive stress applied on the membrane, suggesting that a spatially non-uniform distribution of the diffusion process or the water content can develop during the fuel cell operation.