Good Practices and Limitations of the Hydrogen Pump Technique for Catalyst Layer Protonic Conductivity Estimation

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.