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 effectivemethodto measure the effective protonic conductivity of intermediate layers(ILs) that mimic the catalyst layers used in proton exchange membranefuel cells and electrolyzers. It has been hypothesized, however, thatthe technique is limited to testing ILs that are inactive during thehydrogen reaction as proton transport through the ionomer in the layercan be bypassed by transferring the charge to the electronic phasevia the reaction. This work uses numerical modeling, supported byexperimental testing, to investigate the impact of IL hydrogen reactionactivity, thickness, and electronic conductivity on the predictionof the IL protonic conductivity. A transient, 2-D, through-the-channelmodel is developed and implemented using the finite element methodto predict the performance of hydrogen pump cells and perform electrochemicalimpedance spectroscopy. It is shown both numerically and experimentallythat for iridium black and for platinum-/carbon-based ILs, the protonicphase is almost entirely bypassed, reducing the overall cell resistanceand making the determination of the true conductivity difficult. Themodel can be used to provide an estimate of the resistance of theactive layers, which is not possible using only experiments. In addition,the interfacial contact resistance between the membrane and the catalystlayers is determined using the high-frequency resistance, and thealternating current method for the hydrogen pump is studied to determinethe accuracy of the method. Finally, further insights are providedthrough a breakdown of the resistances of each phase, as well as thepotential profiles, in an active IL, and through parametric studieson the impact of varying the IL activity, thickness, and electronicconductivity.