Determination of Oxygen Transport Resistance through Limiting Current Analysis in Hydroxide Exchange Membrane Fuel Cells

Abstract

Development of Hydroxide Exchange Membrane Fuel Cells (HEMFC) is motivated by the promise of significantly reduced overall module costs compared to Proton Exchange Membrane Fuel Cells (PEMFC). By transitioning from a low-pH to a high-pH environment, less expensive materials become stable1. Specifically, platinum-group metal loading can be greatly reduced or potentially eliminated from the catalyst formulations2, and the bipolar plates can be manufactured more cost effectively. However, HEMFCs are currently in the early stages of development, and improvements in performance and durability are needed.A key challenge in the design of PEM and HEM fuel cells is maximizing O2 transport from the inlet air stream to the triple phase boundary in the cathode catalyst layer. O2 transport resistance causes voltage losses at high current densities, limiting maximum power density. Limiting current analysis has been used extensively to experimentally determine O2 transport resistance in PEMFCs, and we demonstrate that this technique is valid for HEMFCs and produces similar results. Our analysis quantifies the individual O2 transport resistance contributions from several factors: molecular diffusion through the gas diffusion layer (GDL), Knudsen diffusion through the microporous layer (MPL) and catalyst layer, and diffusion through ionomer in the catalyst layer. Importantly, PEMFC and HEMFC have a different water balance. In PEMFC, an MPL is added to relieve the cathode GDL of flooding but also contributes to O2 transport losses. Conversely, in HEMFCs, the cathode dries-out and the anode floods; therefore, the MPL is an unnecessary component on the cathode GDL3. We report that the elimination of the MPL significantly decreases the O2 transport resistance and improves the performance under air, especially at high current density.References Setzler, B. P., Zhuang, Z., Wittkopf, J. A. & Yan, Y. Activity targets for nanostructured platinum-group-metal-free catalysts in hydroxide exchange membrane fuel cells. Nat. Nanotechnol. 11, 1020–1025 (2016).Wang, J. et al. Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells. Nat. Energy 4, 392–398 (2019).Kaspar, R. B., Wittkopf, J. A., Woodroof, M. D., Armstrong, M. J. & Yan, Y. Reverse-Current Decay in Hydroxide Exchange Membrane Fuel Cells. J. Electrochem. Soc. 163, F377–F383 (2016). Figure 1