Optimization of the Cathode Catalyst Layer Composition of a PEM Fuel Cell Using a Novel 2-Step Preparation Method

Abstract

For good performance and high durability PEM fuel cells run at high water saturation levels. However, excess liquid water generated by the oxygen reduction reaction at the cathode can block pores in the catalyst layer so that reactant gases can't access the active catalyst sites. Thus, to prevent electrode flooding, the optimal catalyst layer structure has to provide channels for gas and liquid water transport, while maintaining high ionic and electronic conductivity at the same time. In detail the catalyst layer contained Nafion® as the ionic component to extend the three-dimensional reaction zone of the electrode, and needed Teflon® to provide continuous hydrophobic pathways for reactant gas transport. A simple intermixing process of the components doesn't allow optimal placement of Nafion® and Teflon® within the catalyst layer leading to coverage of active catalyst sites by Teflon®. By means of a two-step process the formation of the catalyst ink was separated into two parts. In the first step, a mixture of Nafion® ionomer and catalyst particles was annealed to form ionomer coated catalyst particles. In the second step, these ionomer coated catalyst particles were mixed with nano-sized Teflon® particles and additional Nafion® ionomer, which was needed to crosslink the ionomer coated catalyst agglomerates. Since the catalyst particles have been covered by Nafion® ionomer before Teflon® was added, active sites were not blocked by Teflon®, which could be placed into void spaces between the catalyst clusters to form continuous hydrophobic pathways for gas transport. To determine the optimal composition for the catalyst ink in the two-step process, a matrix study with 16 different catalyst compositions was developed, and electrodes prepared from this matrix were tested in a fuel cell using the operating conditions chosen from this study. From this test two regions of catalyst composition that resulted in electrodes with good fuel cell performance were identified. The best performing fuel cells with peak powers of 0.5 W/cm² were obtained with cathode catalyst layer with a composition of Nafion®:Teflon®:C of 1.375:0.375:1 and 0.875:0.875:1, respectively. A comparison study of a two-step and one-step prepared catalyst was also done to characterize the effect of air flow rates with the different catalyst layer structures. This catalyst composition study for the two-step process resulted in the following understandings. First, an adequate amount of Nafion® is needed to provide ionic conduction within the catalyst layer and extend the 3-D reaction zone. Too little Nafion® resulted in poor ionic conductivity and too much Nafion® led to high liquid water entrapment, because of its high hydrophilicity, and poor oxygen diffusion. Second, an adequate amount of Teflon® was needed to provide continuous hydrophobic pathways within the catalyst layer for gas transport. The amount of Teflon® depended greatly on the Nafion® content, which determined the void volume available. While too little Teflon® didn't result in continuous hydrophobic pathways, too much Teflon® resulted in separation and isolation of reactive particles agglomerates and poor electronic conductivity. In general the two-step approach led to better performing catalyst layers which were less sensitive to liquid water flooding. This was even more evident at lower air flow rates where liquid water flooding is more severe. The better performance was attributed to the more ordered catalyst layer structure. Future work should confirm this finding in the whole composition range and have a closer look on the annealed catalyst particles and the final micro-structure of the catalyst layer.