Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode

Abstract

Gas permeability, the product of gas diffusivity and Henry's gas-absorption constant, of ionomer membranes is an important transport parameter in fuel cell and electrolyzer research as it governs gas crossover between electrodes and perhaps in the catalyst layers as well. During transient operation, it is important to divide the gas permeability into its constituent properties as they are individually important. Although transient microelectrode measurements have been used previously to separate the gas permeability into these two parameters, inconsistencies remain in the interpretation of the experimental techniques. In this work, a new interpretation methodology is introduced for determining independently diffusivity and Henry's constant of hydrogen and oxygen gases in ionomer membranes (Nafion 211 and Nafion XL) as a function of relative humidity using microelectrodes. Two time regimes are accounted for. At long times, gas permeability is determined from a twodimensional numerical model that calculates the solubilized-gas concentration profiles at a steady state. At short times, permeability is deconvoluted into diffusivity and Henry's constant by analyzing transient data with an extended Cottrell equation that corrects for actual electrode surface area. Gas permeability and diffusivity increase as relative humidity increases for both gases in both membranes, whereas Henry's constants for both gases decrease with increasing relative humidity. In addition, results for Nafion 211 membranes are compared to a simple phase-separated parallel-diffusion transport theory with good agreement. The two-time-regime analysis and the experimental methodology can be applied to other electrochemical systems to enable greater precision in the calculation of transport parameters and to further understanding of gas transport in fuel cells and electrolyzers.

A Numerical Assessment of Mitigation Strategies to Reduce Local Oxygen and Proton Transport Resistances in Polymer Electrolyte Fuel Cells

“…This range agrees with previous experimental data reported for the oxygen diffusivity in bulk Nafion membranes, even though there is a large variability among authors (three orders of magnitude) [35]. The large fluctuation of the diffusivity in bulk Nafion can be ascribed to conditioning, substrate interaction, and confinement of membranes [46][47][48]. The oxygen diffusivity considered here is a good approximation, which leads to realistic values of the local oxygen transport resistance.…”

A numerical Assessment of Mitigation Strategies to Reduce Local Oxygen and Proton Transport Resistances in Polymer Electrolyte Fuel Cells

“…This range agrees with previous experimental data reported for the oxygen diffusivity in bulk Nafion membranes, even though there is a large variability among authors (three orders of magnitude) [25]. The large fluctuation of the diffusivity in bulk Nafion can be ascribed to conditioning, substrate interaction and confinement of membranes [36][37][38]. The oxygen diffusivity considered here is a good approximation, which leads to realistic values of the local oxygen transport resistance.…”

A One-Dimensional Model of a PEM Fuel Cell with the Cathode Catalyst Layer Hydrophobically Treated for Water Management

“…Several of the issues raised in this work will be addressed in the future, including the followings: (1) the temperature-gradient-driven water transport will be added to the pressure-gradient-driven water transport in the cathode GDM, (2) the appropriately measured capillary pressure correlation of MPL and CL, (3) the relationship between oxygen diffusivity/solubility and water content of Nafion® will be investigated (some of this work is undergoing by other group 61 ), and (4) the depth of the hydrophobic region in the ultrathin Nafion® layer, which is underway through a collaboration with a group at NIST.…”