A method for measuring in-plane effective diffusivity in thin porous media

Abstract

A new experimental technique for measuring the in-plane components of the effective diffusivity tensor of thin porous materials is presented. The method is based on the transient diffusion of oxygen from air into a porous sample initially purged with nitrogen. The oxygen concentration is measured at a fixed location in the sample with time and the response is fitted to an analytical solution of Fick’s law for one-dimensional, transient diffusion. As validation, it was confirmed that this method reproduced the theoretical value of oxygen diffusivity in nitrogen within 1% when no sample is present. Effective diffusion coefficients were measured for a variety of thin fibrous graphite paper materials typically used in fuel cell electrodes. The sample holder was designed to allow varying degrees of compression, thereby changing the porosity and tortuosity of the material. As expected the effective diffusivity drops with compression, not only due to a decrease in porosity but also to a large increase in tortuosity. The present method provides accurate, fast, and repeatable measurements, is applicable to electrically conductive materials where brine conductivity is difficult to interpret, uses a simple sample holder, an off-the-shelf oxygen sensor, and involves only air and nitrogen gas. The obtained values were in excellent agreement with comparable results in the literature, yet with a much more direct method.