Optimization of Catalyst Layer Properties for High Temperature Polymer Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells have been demonstrated as a promising technology in meeting some of the challenges faced with shifting towards a clean and renewable energy economy. High Temperature PEM (HTPEM) fuel cells provide an attractive alternative to low temperature PEM fuel cells with respect to fuel flexibility, simplified cooling system, balance of plant and higher value waste heat without reactant humidification (1). Furthermore, the higher operating temperature range of 140-180 °C dramatically increases the tolerance of the noble metal catalysts to impurities in the reactants at both the anode and cathode. To date, most of the success in the field of HT-PEM fuel cell development has been realized through the implementation of a phosphoric acid-doped polybenzimidazole (PBI) membrane electrolyte [1]. The use of such a material allows for operating temperatures in the range of 140-180 °C.

We have previously presented work regarding the effect of m-PBI molecular weight on membrane oxidative stability and MEA performance [2,3]. In the present work, we seek to provide an update regarding challenges related to the optimization of the properties of the catalyst layers, especially the properties of the cathode as this have a significant impact on the performance and durability of HTPEM fuel cells. The performance and Pt utilization in the cathode is highly depending on parameters like porosity, pore size distribution, hydrophilic/hydrophobic properties, catalyst and acid content. In this communication we will focus on the performance of two different catalysts by varying the hydrophobic/hydrophilic properties and the Pt loading..

The hydrophobic/hydrophilic properties are modified by addition of PTFE (increased hydrophobicity), PBI (increased hydrophilicity) or “binderless” (neutral) to the cathode. The loading is varied by adding more catalyst to the catalyst layer and thus making it thicker. The two different catalysts are Pt/C and PtCo/C. Figure 1 shows examples of the performance during the first ~100 hours of operation for a HTPEM MEAs using hydrophilic cathodes.

Figure 1: Performance of HTPEM MEAs using cathodes with increased hydrophilicity and Pt/C catalyst. Pt loading on anode and cathode: 0.9 mg/cm2 on anode. λH2 and λair  was 1.5 and 2.5, respectively. Temperature: 160 °C.

The performance during the first ~100 hours of operation of HTPEM MEAs using these hydrophilic, hydrophobic and binderless cathodes is evaluated under the hydrogen/air mode at 160 °C and the results will be presented. The correlation of the MEA performance with the catalyst layer structures is explored and further optimization is outlined in order to achieve high performance while reducing the Pt loading.