Separation Method for Oxygen Mass Transport Coefficient in Gas and Ionomer Phases in PEMFC GDE

Abstract

A novel method to determine the oxygen mass transport coefficient and its separation into gas and ionomer contributions was developed and validated. The method is based on the use of a limiting current density distribution mathematical model and different diluent gases with varying molecular weights. A linear relationship between the inverse overall mass transport coefficient and the diluent molecular weight was revealed. Because the use of different gas diluents with different molecular weights only affects the mass transport coefficient in the gas phase, an extrapolation to a zero diluent weight gives the O 2 mass transport coefficient in the ionomer phase. The gas phase mass transport coefficient is then calculated using the additive relation between the overall mass transport coefficient and its components. The O 2 mass transport coefficient in the electrolyte phase was found to be equal to 0.01284 m s −1 , while in an N 2 diluent, the coefficient is 0.02727 m s −1 . Effects of selected operating conditions (temperature, O 2 concentration) and membrane/electrode assembly materials (cathode Pt loading and gas diffusion layer) on the O 2 mass transport coefficient were studied and discussed.

Liquid water saturation and oxygen transport resistance in polymer electrolyte membrane fuel cell gas diffusion layers

“…Several recent studies [74][75][76][77][78] have focused on measuring the individual transport resistance contributions of the fuel cell layers by performing limiting current measurements with varied pressures, GDL thicknesses, cathode balance gases (also known as oxygen "diluents"), and RH levels. Baker et al [77] determined the relative contributions of the channels, GDL substrate, MPL, and catalyst layer to total transport resistance by manipulating GDL and MPL thicknesses and the operating pressure.…”

“…In the study by Nonoyama et al [75], the ionomer layer contribution to transport resistance was isolated by varying the cell temperature, since the impact of temperature on ionomer diffusion coefficient is much larger than the impact of temperature on molecular and Knudsen diffusion. Reshetenko et al [76] also used a limiting current-based methodology to identify the catalyst layer ionomer diffusion contribution.…”

“…Depending on the chosen experimental conditions and on the GDL materials and catalyst loadings, the GDL has been found to contribute between 32% and 61% of the total oxygen transport resistance in the fuel cell [75][76][77][78] in when there is no liquid water present. At low RH, the catalyst layer was generally the largest contributor to the oxygen transport resistance, owing to a decrease in oxygen diffusivity through the ionomer layer when the ionomer was poorly hydrated [78].…”

“…At low RH, the catalyst layer was generally the largest contributor to the oxygen transport resistance, owing to a decrease in oxygen diffusivity through the ionomer layer when the ionomer was poorly hydrated [78]. In references [75][76][77][78], however, the limiting current experiments were performed exclusively in low current-density conditions to avoid liquid water condensation in the GDL. Water accumulation in the cathode GDL of a PEM fuel cell reduces the available pore space for oxygen transport, thereby reducing the effective diffusivity of oxygen through the material [33,79,80].…”

“…The second component is the transport resistance in the catalyst layer, wherein oxygen gas navigates small pore openings and permeates through a thin film of ionomer [104][105][106] to reach the catalyst surface. Oxygen transport in the catalyst layer is dominated by Knudsen diffusion and dissolution and diffusion through the ionomer thin film [75,76,78,104]; therefore the transport resistance in this layer is primarily independent of pressure. Between the CL and the flow field is the GDL ( ).…”