Method to analyze the spatial current distribution in an operating PEFC based on NMR mesurement using small planer surface coils

In order to measure the local current density generated in a polymer electrolyte fuel cell (PEFC), nuclear magnetic resonance (NMR) signals were acquired by a coil inserted in a PEFC with a power generation area of 50 mm × 50 mm. Eight small planar surface coils with inner diameters of 0.6 mm were inserted between the gas diffusion layer and the membrane electrode assembly of the fuel cell. Changes in the frequency of the NMR signals due to local currents generated in the PEFC were recorded and analysed. To compute the spatial distribution of these local currents from the measured frequency shifts, it is necessary to use an inversion analysis based on the Biot-Savart law, which connects current and magnetic field strength. An algorithm developed in this study was used to quickly minimize the difference between the frequency shift calculated from an assumed current distribution and that measured experimentally. The algorithm uses the dependence of frequency shift on local current based on the equations of electricity and magnetism. In this paper we describe the derivation of the proportionality relation and the inversion algorithm. The computation time required for the inversion using the developed algorithm was a few tens of seconds. Using the developed algorithm, the spatial distribution of local current density generated in the PEFC was analysed.