Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode

Gas permeability,the product of gas diffusivity and Henry'sgas-absorption constant, of ionomer membranes is an important transportparameter in fuel cell and electrolyzer research as it governs gascrossover between electrodes and perhaps in the catalyst layers aswell. During transient operation, it is important to divide the gaspermeability into its constituent properties as they are individuallyimportant. Although transient microelectrode measurements have beenused 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 determiningindependently diffusivity and Henry's constant of hydrogenand oxygen gases in ionomer membranes (Nafion 211 and Nafion XL) asa function of relative humidity using microelectrodes. Two time regimesare accounted for. At long times, gas permeability is determined froma two-dimensional numerical model that calculates the solubilized-gasconcentration profiles at a steady state. At short times, permeabilityis deconvoluted into diffusivity and Henry's constant by analyzingtransient data with an extended Cottrell equation that corrects foractual electrode surface area. Gas permeability and diffusivity increaseas relative humidity increases for both gases in both membranes, whereasHenry's constants for both gases decrease with increasing relativehumidity. In addition, results for Nafion 211 membranes are comparedto a simple phase-separated parallel-diffusion transport theory withgood agreement. The two-time-regime analysis and the experimentalmethodology can be applied to other electrochemical systems to enablegreater precision in the calculation of transport parameters and tofurther understanding of gas transport in fuel cells and electrolyzers.