Measurement and Analysis of Gas Transport Properties in Catalyst Layers of Polymer Electrolyte Fuel Cells with Different Ionomer to Carbon Ratio

Abstract

Catalyst layers of polymer electrolyte fuel cells are electrochemical reaction fields with complicated porous structures. Catalyst layers are composed of carbon particles supporting platinum catalyst (Pt/C), ionomers and micropores. Gas transport properties in catalyst layers are key factors for improving cell performance. It is necessary to clarify the relationship between the gas transport properties and the microstructure. The authors reported that ionomer in the catalyst layer affects the porous structure and resultant cell performance [1]. Therefore, the objective of this study is to investigate the effects of ionomer to carbon ratio (I/C) of the catalyst layer on the porous structure formation and gas transfer.In this study, gas transport properties in catalyst layers with different I/C were evaluated by the measurement of gas transfer through thin porous media operated under a micro-channel apparatus (Gas-TOUCA) [2]. The porous structures of catalyst layers were evaluated by the nitrogen adsorption method and the observation using a Field-emission scanning electron microscope (FE-SEM).Typical experimental results of the measurement of Gas-TOUCA are shown in Figure 1. The measurement was carried out for the catalyst layers under the four conditions of I/C 0.1, 0.3, 0.5 and 1.0. The effective diffusion coefficient in the catalyst layer with I/C 0.5 was the largest, and that in the catalyst layer with I/C 1.0 was the smallest of the four conditions. The tortuosity factor of the catalyst layers with I/C 0.1, 0.3 and 1.0 was approximately 3. However, the tortuosity factor of catalyst layers with I/C 0.5 was approximately 2. Segregation of ionomer was observed in the catalyst layers with I/C 0.5 and 1.0 by SEM. On the other hand, there was no ionomer segregation in the catalyst layers with I/C 0.1 and 0.3, and most of the ionomer could have adsorbed on the Pt/C particles. Ionomer works as a dispersant in electrode slurries [3]. Less amount of ionomer leads to large attractive interaction of particles in the slurry, whereas an excessive amount of ionomer leads to deposition to pores in the catalyst layer. Therefore, the experimental results indicated that there is an optimum I/C ratio for gas transport.