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A long-term operation test of PEFCs was conducted for single cells having an effective area of 25 cm2. The decay rate of the cell voltage was relatively large in the beginning, decreased to about 4 µV h−1 from 3000 to 6000 h, and about 7 µV h−1 in the later stage from 9000 to 12000 h. In order to clarify the cause of the voltage decay, materials composing membrane electrode assembly (MEA) were analyzed by disassembling a cell every 3000 h. After operation, changes of materials, such as particle size change of Pt-Co cathode catalyst, loss of ionomer layers on Pt-Co/C, decrease in thickness of the polymer electrolyte membrane, pore size changes of the cathode, were observed. Electrochemical active surface area of the electrode showed a decreasing tendency. Based on these results, we focused on the improvement of cathode, and developed a “four-layer electrode catalyst” which consisted of Pt-Co alloy particle, carbon support, ionomer and fluorocarbon resin. A test cell using this new type cathode exhibited better performances than a cell using conventional cathode in the durability test under low humidity or low temperature conditions.

Water management of the catalyst layer in a polymer electrolyte membrane fuel cell was improved by focusing on the three-phase boundary. To fabricate the cathode catalyst layer, carbon-support with catalyst and fluorine resin were mixed and heat-treated at the melting temperature of fluorine resin. The transmission electron microscopy images of this electrode catalyst showed that the surfaces of the catalyst particle and of the carbon-support were partly coated by thin film of melted fluorine resin. Ionomer electrolyte material was added to this electrode catalyst, and an electrode catalyst with a four-layer structure was fabricated. The durability at 50°C of a single cell using the four-layer electrode catalyst as the cathode was examined. The deterioration rate was lower than that using the conventional three-layer electrode catalyst. In addition, a load change test at 50°C, an OCV test at 50°C, and a durability test at room temperature gave good results.