Measuring Oxygen, Carbon Monoxide and Hydrogen Sulfide Diffusion Coefficient and Solubility in Nafion Membranes

TL;DR

This study develops a diffusion cell to measure gas transport properties in Nafion membranes, estimating diffusion coefficients and solubility for O2, CO, and H2S, and analyzing their temperature and pressure dependence.

Contribution

It introduces a novel electrochemical method to accurately estimate gas diffusion and solubility in Nafion membranes considering water flux effects.

Findings

Permeability increases with temperature for all gases.

Diffusion coefficients are invariant with pressure in Nafion 117.

Henry's law describes oxygen solubility increase with pressure.

Abstract

A Devanathan-Stachurski type diffusion cell made from a fuel cell assembly is designed to evaluate the gas transport properties of a proton exchange membrane as a function of cell temperature and gas pressure. Data obtained on this cell using the electrochemical monitoring technique (EMT) is used to estimate solubility and diffusion coefficient of oxygen (O2), carbon monoxide (CO) and hydrogen sulfide (H2S) in Nafion membranes. Membrane swelling and reverse-gas diffusion due to water flux are accounted for in the parameter estimation procedure. Permeability of all three gases was found to increase with temperature. The estimated activation energies for O2, CO and H2S diffusion in Nafion 112 are 12.58, 20 and 8.85 kJ mol^-1, respectively. The estimated enthalpies of mixing for O2, CO and H2S in Nafion 112 are 5.88, 3.74 and 7.61 kJ mol^-1, respectively. An extensive comparison of transport properties estimated in this study to those reported in the literature suggests good agreement. Oxygen permeability in Nafion 117 was measured as a function of gas pressures between 1 and 3 atm. Oxygen diffusion coefficient in Nafion 117 is invariant with pressure and the solubility increases with pressure and obeys Henry's law. The estimated Henry's constant is 3.5 x 10^3 atm.