Mikio Wada

Measurement of the Potential of Bipolar Plates Inside a PEFC During Operation

Commercialization of metal bipolar plates for polymer electrolyte fuel cells (PEFCs) requires excellent corrosion resistance and low cost. It is thus important to have an accurate grasp of the corrosion environment. In this study, we developed equipment that performs realtime measurement of the potential and pH on the surface of bipolar plates during power generation. Figure 1 shows the main components of the device used to measure the pH and the potential of the bipolar plates. A water-permeable electrical insulation sheet was inserted between the gas diffusion layer and electrodes and pH probe so the probes could stay in contact with water. The reference electrodes that create minimal leakage of the internal filling solution were used because these can minimize contamination. Also ion sensitive field-effect transistors (ISFETs) were adopted as the pH probe, because these do not need the filling electrolyte. Figure 2 shows one example of behavior of the pH during power generation in the cell. The pH value on the anode side was about 6, and on the cathode side about 5. These values did not depend on the cell voltage. On the other hand, measuring the potential of the bipolar plates showed that the highest cathode potential was at the time of OC. Thus, the most corrosive environment was found to be on the cathode side at the time of OC. Moreover, it became clear that the potential on the surface of the bipolar plates was relatively low compared with that of the catalyst layer. The reason for this is that the pH on the surface of the bipolar plates is higher than the pH of the electrolyte in the catalytic layer. To verify the measurement results with this potential, Figure 3 shows the correlation of cell voltage and the measured difference in potential the cathode and anode sides. The potential difference at the time of OC becomes equal to the cell voltage but when a load is placed on the cell, the potential difference and cell voltage tend not to agree. It is believed that this resulted because a liquidjunction potential was created due to a voltage drop from the internal resistance of the cell and the pH difference of the water on the anode and cathode sides. For example, with a cell voltage of 0.5 V, the liquid-junction potential was estimated to be approximately 0.25 V, nearly agreeing with the measured value. These results confirmed that the potential and pH on the surfaces of the bipolar plates during power generation could be measured by installing a reference electrode and a pH probe in the cell. This method was applied and the corrosion resistance of the material was investigated during power generation. Figure 1. Schematic diagram of bipolar plate potential measurement