High Pressure NMR Investigation of 3M Fuel Cell Ionomers

Nuclear Magnetic Resonance (NMR) measurements were obtained for polymer electrolyte membranes (PEM) provided by 3M Corp. through the courtesy of Dr. Steve Hamrock.  These membranes have potential for use within PEM Fuel Cells (PEMFCs) and can operate at somewhat lower relative humidity than NAFION. NMR spin-lattice relaxation (T₁) and static gradient self-diffusion measurements were obtained as a function of applied hydrostatic pressure for three different equivalent weight (EW) membranes at several values of water content.  T₁ measurements allow us to study the rotational molecular motion of water within the membranes, whereas diffusion directly probes translational motion (1).  By measuring both T₁ and self-diffusion, possible relationships between rotational and translational motion within the membranes can be observed and effects due to water content or EW can be found.

The membranes studied had EWs of 700, 825, and 1000, and each was prepared with both 10% and 20% water content (wt. %). Measurements were conducted at several temperatures. For T₁ measurements, we used deuteron NMR in membranes equilibrated with D₂O because of the high sensitivity of nuclear quadrupole relaxation to rotational motions (2). In order to measure diffusion (using proton NMR on samples equilibrated with H₂O) in samples contained within a high pressure Cu/Be cell, it was necessary to use a spin-echo pulse sequence in the static gradient in the fringe field of a 7.3 T superconducting magnet.  Figure 1 displays isothermal variable pressure T₁ results obtained for the 700 EW membrane with 20 wt. % D₂O. It can be seen that T₁ values increase with temperature and decrease with pressure.

The results for different membranes also suggest that values for T₁ increase with higher membrane EW and with higher D₂O content.  It is found that there is an increase in activation volume, Δv, with lower water content and lower temperatures, which was calculated using

                                     Δv = -kT[lnT₁/P]_T                              [1]

Table 1 displays the activation volume of each membrane at various temperatures with 20 wt. % D₂O.

Measurements of water self-diffusion as a function of pressure for each sample at different water content are underway.  Diffusion activation volumes will thus be obtained and compared with those for deuteron spin-lattice relaxation. Such comparison will shed light on the relationship between water translational and rotational motion inside the ionomer, and hopefully lead to insight into the proton transport mechanism.

REFERENCES

1. Jayakody, JRP, Stallworth, PE, Mananga, ES, Farrington-Zapata, J, and Greenbaum, SG.  J. Phys. Chem. B 108, 4260 (2004).

2. Fontanella, JJ, Wintersgill, MC, Chen, RS, Wu, Y, and Greenbaum, SG.  Electrochimica Acta 40, 2321 (1995).

This work was supported by the Office of Naval Research.