Evaluation of Ionomer Distribution on Electrocatalysts for Polymer Electrolyte Fuel Cells by Use of a Low Acceleration Voltage Scanning Electron Microscope

Abstract

The qualitative evaluation of the dispersion of prefluorosulfonic ionomer (PFSI) with different ionomer/carbon mixing ratios (I/C) using electron microscopy was carried out without the use of a stain treatment. Both low acceleration voltage transmission electron microscopy (LAV-TEM) and ultralow acceleration voltage scanning electron microscopy with a retarding method (ULV-SEM) use a characteristically low acceleration voltage, which allows the selective examination of the sensitive ionomer morphology. The high-performance charge-coupled device enables one to obtain high contrast ionomer images without the use of lead or cesium staining, which could otherwise result in morphological changes during these pre-treatments. The electrochemically active surface area of the polymer electrolyte fuel cell using Pt/GCB increased with increasing PFSI content and saturated at an ionomer/carbon weight ratio (I/C) of 1.2, where full coverage of the ionomer was detected by LAV-TEM. The ULV-SEM images showed the obvious occlusion of the primary and secondary pores of the Pt/GCB catalyst layers above I/C = 1.2. The nitrogen gas adsorption measurement, carried out by use of quenched solid-density-functional theory analysis, also supported the occlusion of the primary and secondary pores of the Pt/GCB catalyst layers above I/C = 1.2.