RESUMO
Mass transport of oxygen through an ionomer contained within the cathode catalyst layer in an anion exchange membrane fuel cell is critical for a functioning fuel cell, yet is relatively unexplored. Moreover, because water is a reactant in the oxygen reduction reaction (ORR) in alkaline media, an adequate supply of water is required. In this work, ORR mass transport behavior is reported for methylated hexamethyl-p-terphenyl polymethylbenzimidazoles (HMT-PMBI), charge balanced by hydroxide ions (IEC from 2.1 to 2.5 mequiv/g), and commercial Fumatec FAA-3 membranes. Electrochemical mass transport parameters are determined by potential step chronoamperometry using a Pt microdisk solid-state electrochemical cell, in air at 60 °C, with relative humidity controlled between 70% and 98%. The oxygen diffusion coefficient (DbO2), oxygen concentration (cbO2), and oxygen permeability (DbO2·cbO2) were obtained by nonlinear curve fitting of the current transients using the Shoup-Szabo equation. Mass transport parameters are correlated to water content of the ionomer membrane. It is found that the oxygen diffusion coefficients decreased by 2 orders of magnitude upon reducing the water content of the ionomer membrane by lowering the relative humidity. The limitation of the Shoup-Szabo equation for extracting ORR mass transport parameters using thin ionomer films was evaluated by numerical modeling of the current transients, which revealed that a significant discrepancy (up to 29% under present conditions) was evident for highly hydrated membranes for which the oxygen diffusion coefficient was largest, and in which the oxygen depletion region reached the ionomer/gas interface during the chronoamperometric analysis.
RESUMO
The hydrogen pump technique has been shown to be an effective method to measure the effective protonic conductivity of intermediate layers (ILs) that mimic the catalyst layers used in proton exchange membrane fuel cells and electrolyzers. It has been hypothesized, however, that the technique is limited to testing ILs that are inactive during the hydrogen reaction as proton transport through the ionomer in the layer can be bypassed by transferring the charge to the electronic phase via the reaction. This work uses numerical modeling, supported by experimental testing, to investigate the impact of IL hydrogen reaction activity, thickness, and electronic conductivity on the prediction of the IL protonic conductivity. A transient, 2-D, through-the-channel model is developed and implemented using the finite element method to predict the performance of hydrogen pump cells and perform electrochemical impedance spectroscopy. It is shown both numerically and experimentally that for iridium black and for platinum-/carbon-based ILs, the protonic phase is almost entirely bypassed, reducing the overall cell resistance and making the determination of the true conductivity difficult. The model can be used to provide an estimate of the resistance of the active layers, which is not possible using only experiments. In addition, the interfacial contact resistance between the membrane and the catalyst layers is determined using the high-frequency resistance, and the alternating current method for the hydrogen pump is studied to determine the accuracy of the method. Finally, further insights are provided through a breakdown of the resistances of each phase, as well as the potential profiles, in an active IL, and through parametric studies on the impact of varying the IL activity, thickness, and electronic conductivity.