Tracer-Desorption ~1H NMR Measurement of Diffusion Coefficients in Polyphosphazene Ion-Exchange Membranes

摘要

The growth of an NMR signal due to diffusion of solute out of a thin membrane has beeninvestigated for the measurement of diffusion rates under coonditions that aer not suitable for accurate application of pulsed field gradient 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 commonly 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-excahnge membranes, which are currently being examined for possible use in direct liquid methanol fuel cells. Diffusion coefficients at 25 deg C 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 wer significantly smaller than those reported for a Nafion perfluorosulfonic acid membrane. Successful applicationof the tracer-desorption NMR approach requires that the external solute signal be observable ina single pulse and that the membrane desorption rate not be too fast (the diffusion coefficient should be < 10~(-6) cm~2/s).