Tracer-desorption H-1 NMR measurement of diffusion coefficients in polyphosphazene ion-exchange membranes

The growth of an NMR signal due to diffusion of solute out of a thin membra ne has been investigated for the measurement of diffusion rates under condi tions that are not suitable for accurate application of pulsed field gradie nt methods. Such conditions include those where the internal solute signal is either too small or too broad to be utilized or where the diffusion rate is too slow to yield observable attenuation of the NMR signal with commonl y available gradient strengths or possible gradient application times. The method was used to measure the mutual diffusion coefficient of methanol in a series of polyphosphazene-based cation-exchange membranes, which are curr ently being examined for possible use in direct liquid methanol fuel cells. Diffusion coefficients at 25 degreesC were in the range of 8.0 x 10(-8) to 4.0 x 10(-7) cm(2)/s for methanol concentrations of 1.0-5.0 M and were sig nificantly smaller than those reported for a Nafion perfluorosulfonic acid membrane. Successful application of the tracer-desorption NMR approach requ ires that the external solute signal be observable in a single pulse and th at the membrane desorption rate not be too fast (the diffusion coefficient should be < 10(-6) cm(2)/s).