Oxygen mass transport parameters in ionomer films under controlled relative humidity

Date created: 
Polymer electrolyte fuel cells
Oxygen mass transport

Mass transport parameters are determined for the oxygen reduction reaction (ORR) at the electrochemical interface of a platinum microdisk electrode and five different series of polymer electrolyte membranes. The series included proton exchange polyfluorosulfonic acid (PFSA) based membranes (Nafion 117, Nafion 211), films cast from PFSA dispersions (DE2020), anion exchange quaternary ammonium based membranes (FAA-3), and films cast from hexamethyl-p-terphenyl polymethylbenzimidazole (HMT-PMBI) dispersions. The membranes differ in chemical structure, morphology, and water content controlled by relative humidity. The series of materials were investigated over a range of temperatures (50-70 °C) and relative humidities (30-98% RH) using a solid state electrochemical cell. Cyclic voltammetry yielded the potentials where ORR is mass transport limited, as well as the electrochemically active surface area of the platinum microdisk electrode. Chronoamperometry was performed at mass transport limiting potentials, where fitting the current transients to analytical models (Cottrell/Shoup-Szabo) allowed for the calculation of oxygen diffusion coefficient (Db), solubility (cb), and permeability (Db*cb) for the applied environmental conditions. A numerical model is also presented which highlights constraints in using the analytical models to determine mass transport parameters when the inherently-assumed infinite electrolyte thickness is not present. During chronoamperometric measurements, where the potential applied results in the generation of liquid water at the membrane/electrode interface (ORR), a reversible time-dependent behaviour was observed where Db and Db*cb increased over time to plateau values. The time-dependency responds to changes in relative humidity and is reversible, where mass transport parameters shift to a vapour equilibrated state over long periods of time. It is suggested that the electrochemically generated liquid water at the membrane/platinum interface during oxygen reduction results in a morphological change over repeated perturbations in the form of chronoamperometric analysis. The presence of interfacial liquid water causes hydrophilic channels, which are not present in substantial amounts at the interface in the vapour equilibrated-state, to reorient toward the surface. The increase in water-filled channels at the interface can explain the increase in Db and Db*cb, which are dependent on water content. Oxygen mass transport parameters for both proton and anion exchange membranes are reported as a function of relative humidity. In order to perform electrochemical measurements at < 70% RH (at 50 °C) for perfluorosulfonic acid membranes and for all conditions for anion exchange membranes, a modified twoelectrode setup was employed and compared to a three-electrode configuration. The oxygen diffusion coefficient is observed to depend on the water content. A lower relative humidity resulted in lower values of Db; significantly so for alkaline anion exchange samples compared to their acidic counterparts. cb was observed to exhibit an inverse relationship, which increases with decreasing relative humidity. The decrease in Db as the relative humidity was lowered was larger than the increase in cb, which lead to a decrease in Db*cb as the relative humidity was lowered for all membranes.

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This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Senior supervisor: 
Steven Holdcroft
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.