The Research for the Cathode Catalyst Layer to Enhance the Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

This thesis describes the analysis of the cathode catalyst layer (CCL) of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). This study examines the resistance analysis of the commercial cathode by the distribution of relaxation time (DRT) method of electrode half-cell. The five peaks were observed in the DRT plot separate the resistances by frequency: Peak 1and Peak 2 (0.1 – 102 Hz) represent the charge transfer resistance of oxygen reduction reaction, and Peak 3 (102 – 103 Hz) represents the resistance for proton conductive loss. Peak 0 (< 0.1 Hz) appears due to produced H2O between electrolyte and cathode GDE and Peak 4 (103-104 Hz) appeared from the MEA is assigned to the anode reaction. The DRT analysis indicated that the phosphoric acids in the CCL of the HT-PEMFC enhance proton conduction to improve reaction kinetics (positive effect) bu poison the Pt electrocatalyst and limit the oxygen transport (negative effect). To mitigate the mass transport issue, hydrophobic polymers such as polytetrafluoroethylene (PTFE) are used in the CCL using polyvinylidene fluoride (PVDF) as a binder. Microscopy and contact angle analysis indicated that the PTFE dispersed evenly on the carbon surface and CCL became mor hydrophobic as PTFE content increased. A maximum MEA performance was dbtaied with 6% PTFE. Since the hydrophobic PTFE repels the phosphoric acid within the CCL, the additive including the PTFE made the oxygen passage. Finally, polymers containing phosphonic acid groups were used as a binder. The cathode using the phosphonic acid polymers showed lower performance than the cathode using the PVDF suggesting the hydrophobicity is the predominating factor for HT-PEMFC performance. This is because the commercial membrane used in this research has a high amount of phosphoric acid. This research suggests that advanced membranes with a lower amount of phosphoric acid may perform better with phosphonic acid ionomers.