Measurement of Time-Dependent Changes and Spatial Distributions of Current Density in PEFC and Water-Content of PEM Using a Planer-Surface Coil as an NMR Signal Detector (Second Report: Effect of Relative Humidity and Utilization Ratio of Fuel Gas on Current Density Distribution)

Abstract

To increase the electric power density of a polymer electrolyte fuel cell (PEFC), it is necessary to increase current density flowing through the membrane electrode assembly (MEA) in the whole PEFC. And this is controlled by the conditions of fuel gas such as flow rate and humidity. We measured the spatial distribution and time dependent changes of current density in the PEFC and the water content in PEM using Nuclear-Magnetic-Resonance (NMR) sensors in order to determine the effect of humidity and the utilization ratio of fuel gas. Our experimental results showed that the current density was uniformly distributed from the inlet to the outlet of the fuel gas when the flow rate was high and the humidity of the fuel gas was 70 %RH. On the other hand, when the flow rate was low and the humidity of the fuel gas higher (85 %RH), the spatial distribution of current density and water content in the PEM was non-uniform. We determined the relationship between the transfer phenomenon of water in the PEFC and this non-uniformity of water content and current density. In addition, a map illustrating the behavior of water content in the PEM and current density under electric-generating condition of the PEFC was made showing the relationship between relative humidity and utilization ratio of fuel gas supplied to the PEFC.