Local Water-content Measurement of Polymer Electrolyte Membrane for Fuel Cell Applications Using Planar Surface Coil as a NMR Signal Detector

Abstract

A real-time monitorinsg method of local water content in polymer electrolyte membrane (PEM) is required to achieve higher current density operation of a polymer electrolyte fuel cell. Based on the T2 relaxation measurement of nuclear magnetic resonance (NMR), the local water-content sensor consisting of the planar surface coil of 0.8 mm outside diameter and a permanent magnet of 1.0 Tesla was developed. In this research, T2 (CPMG) relaxation time of PEM was measured by Carr-Purcell-Meiboom-Gill (CPMG) method. The measurement time of CPMG method is 1.1 s. It is found from the result using the solenoidal coil that the T2 (CPMG) relaxation time of the membrane at 75°C increases with increasing a vapor concentration in N2 gas.The accuracy of T2 (CPMG) measurement by two small planar coils of 0.8 mm and 2.0 mm outside diameters was evaluated using CuSO4 sulution with the concentrations of 0.1 and 1.2 mmol/l as standard samples. The average and coefficient of variation of T2 (CPMG) measured by the planar surface coils were evaluated by comparing with the T2 (CPMG) STAND measured by the standard solenoidal coil. In the case of 2.0 mm planar coil, the average of T2 (CPMG) D=2.0 mm was equal to T2 (CPMG) STAND within the accuracy of 6%, and the coefficient of variation was in the range of 0.11 to 0.13. In the case of 0.8 mm planar coil, the average of T2 (CPMG) D=0.8 mm was longer than T2 (CPMG) STAND by the factor of 1.32 to 1.54 due to inhomogeneous perturbing magnetic field exposed from the small planar coil, and the coefficient of variation was in the range of 0.14 to 0.16. Furthermore, T2 (CPMG) measurement of PEM by the planar surface coils was performed. Using the planar surface coil and the obtained relationship between T2 (CPMG) and water content of PEM, the developed monitoring system can measured local water content in the membrane during 1.1 s.