Yasuo Yokouchi

Development of an eight-channel NMR system using RF detection coils for measuring spatial distributions of current density and water content in the PEM of a PEFC ☆

The water generation and water transport occurring in a polymer electrolyte fuel cell (PEFC) can be estimated from the current density generated in the PEFC, and the water content in the polymer electrolyte membrane (PEM). In order to measure the spatial distributions and time-dependent changes of current density generated in a PEFC and the water content in a PEM, we have developed an eight-channel nuclear magnetic resonance (NMR) system. To detect a NMR signal from water in a PEM at eight positions, eight small planar RF detection coils of 0.6 mm inside diameter were inserted between the PEM and the gas diffusion layer (GDL) in a PEFC. The local current density generated at the position of the RF detection coil in a PEFC can be calculated from the frequency shift of the obtained NMR signal due to an additional magnetic field induced by the local current density. In addition, the water content in a PEM at the position of the RF detection coil can be calculated by the amplitude of the obtained NMR signal. The time-dependent changes in the spatial distributions were measured at 4 s intervals when the PEFC was operated with supply gas under conditions of fuel gas utilization of 0.67 and relative humidity of the fuel gas of 70%RH. The experimental result showed that the spatial distributions of the local current density and the water content in the PEM within the PEFC both fluctuated with time.

Measurement of current-density in PEFC with NMR sensors

In order to improve the power generation performance of polymer electrolyte fuel cells (PEFC), it is necessary to maintain high current density over the whole area of the membrane electrode assembly (MEA) that includes the additional Pt-carbon particles loaded on the Polymer Electrolyte Membrane (PEM) as an electrocatalyst layer. However, the current density generated at the MEA is distributed unevenly due to a lack of hydrogen, flooding, and so on. Therefore, achieving a higher current density in a PEFC requires monitoring the local current density. The local current density in a PEFC can be measured by the frequency shift of the NMR signal received from planar surface coils inserted into the PEFC as sensors. This method is based on the relationship that the spatial gradient of the frequency shifts in NMR signals along the MEA is proportional to the magnetic field strength induced by local current density. In this study, two kinds of MEA were used. One MEA was with the platinum catalyst applied to an area of 50 mm * 50 mm. The other MEA was with platinum catalyst over half this area. The distributions of the frequency shift of NMR signals in PEFCs using these two MEAs were measured. These measured distributions of the spatial gradients were in agreement with those obtained by the theoretical-analysis of magnetic fields in PEFC. The spatial distributions of current density generated in PEFCs were obtained from the spatial gradients and theoretical result.© 2011 ASME