In-Situ Measurement and Analysis of Water Movement in PEFCs

Polymer electrolyte fuel cells (PEFCs) are promising devices for power generation, portable and automotive applications. For the next generation of PEFCs, it is promising to operate the fuel cell at high temperature and low humidity conditions. Especially, water movement at low humidity condition plays an important role to achieve good performance and it is necessary to control appropriate humidification in both electrodes. In general, proton conductivity of polyelectrolyte decreases under low humidity condition. On the other hand, generated liquid water inhibits the oxygen diffusion by blocking a pore of the catalyst layer. These phenomena lead to the significant decrease in cell performance. Hence appropriate control of the water movement through polyelectrolyte membrane in PEFCs will lead to the improvement of the cell performance.

Water movement inside the electrolyte membrane is caused by two phenomena: water diffusion driven by water activity gradient between two side of the membrane and electro-osmotic water dragged by hydrogen ion from the anode to the cathode. The electro-osmotic coefficient, which is one of the physical properties of the membrane, greatly reflects the behavior of water movement in the electrolyte membrane. Although some researchers report to measure the electro-osmotic coefficient, there are scarce measurements under operating condition of fuel cell. In this research, the objective is to construct the measurement system of electro-osmotic coefficient under the in-situ cell operating condition. The system for measurement of water movement is shown in Figure 1. The JARI standard MEA was used in this measurement and the cell temperature was kept at 80°C. The gas relative humidity (RH) was controlled by a precise humidifier that has reflux-type saturator with the precision of the dew point (± 0.2°C). Humidified (anode 40%RH, cathode 20%RH) hydrogen and air were supplied to the anode and cathode, respectively, at a flow rate of 500 mL/min. Then dew point of each electrode was measured by mirror cooling dew-point meter with the precision of the dew point (± 0.2°C) and the diffusion constant was calculated. To calculate the electro-osmotic coefficient, the same measurement of dew point was conducted on condition that the current density was kept at 0.05 A/cm². By subtracting the amount of estimated diffusion water from total movement of water, the electro-osmotic coefficient was estimated. Mass balance of water was confirmed in each measurement and the electro-osmotic coefficient was estimated as 1.6. This value is similar to the reported values^[1]. Therefore, measurement system of the electro-osmotic coefficient under cell operating condition was constructed. The dependence of water content of electrolyte membrane and current density of the electro-osmotic coefficient should be evaluated in future work.

Acknowledgement

The authors thank Dr. Yoshiyuki Hashimasa and Dr. Takahiro Shimizu of Japan Automobile Research Institute for providing the JARI standard MEA.

Reference

[1] S. Ge, B. Yi, and P. Ming, J. Electrochem. Soc., 153, A1443 (2006).