(294d) Determination of Electroosmotic Drag and Proton Conduction Mechanism In Proton Exchange Membranes for Use In Low Temperature PEMFCs

One of the major barriers to the commercialization of proton exchange membrane fuel cells is durability. During operation, the electrochemically active surface area (ECA) of the cathode catalyst layer decreases primarily due to platinum dissolution and diffusion on both the nanometer and micrometer scale [1]. Nanoscale diffusion results in Ostwald ripening and loss of ECA due to an increase in the average particle size. Microscale diffusion results in precipitation of platinum particles in the membrane as platinum ions are reduced by crossover hydrogen [2]. Models of the catalyst degradation process have shown that the rate of ECA loss is dependent on the mobility of platinum ions in the membrane [3]. Therefore, a membrane with low platinum ion mobility that maintains high proton mobility is desired.

There are two mechanisms of proton conduction in proton exchange membranes: the vehicle mechanism and the Grotthuss (proton-hopping) mechanism. Because the Grotthuss mechanism is exclusive to protons, a membrane that conducts protons primarily by the Grotthuss mechanism is expected to have a lower platinum ion mobility. The relative importance of both proton conduction mechanisms can be determined by measuring the electroosmotic drag coefficient. The vehicle mechanism drags some water molecules with each proton, while the Grotthuss mechanism results in very little water transport.

The electroosmotic drag coefficient was measured as a function of water activity in Nafion 112, poly(phenylene sulfone) (PS) (IEC=1.0 meq/g) [4], and mixed H⁺/Co²⁺ form Nafion 117 membranes by the concentration cell method described by Fuller and Newman [5]. Both membranes were found to have a drag coefficient dependent on the hydration state of the membrane. The drag coefficient was constant when the water vapor activity (a_w) was below 0.75. In the range 0.75<a_w<1.0, the drag coefficient was also constant, but approximately twice as high as in the low humidity regime. In Nafion, a_w=0.75 corresponds to a water content of 6 moles H₂O per mole SO₃^- [6]. The drag coefficient of Nafion 112 was 1.1 for a_w<0.75 and 2.4 for a_w>0.75, while the drag coefficient of the PS membrane was 0.55 for a_w<0.75 and 0.95 for a_w>0.75. The Nafion 117 membrane in a mixed Co²⁺/H⁺ form had a drag coefficient of 2.0 for a_w<0.75 and 4.8 for a_w>0.75. When alloy catalysts are used, unstable metals like cobalt may have a high initial rate of dissolution, causing a change in the transport properties of the membrane. The results indicate that the Grotthuss mechanism makes a larger contribution to proton conduction in PS membranes than in Nafion membranes. This is expected to lead to a higher ratio of proton to platinum mobility in PS compared to Nafion.

References

1.            Ferreira, P.J., et al., Instability of Pt/C electrocatalysts in proton exchange membrane fuel cells - A mechanistic investigation. Journal of The Electrochemical Society, 2005. 152(11): p. A2256-A2271.

2.            Bi, W., G.E. Gray, and T.F. Fuller, PEM fuel cell Pt/C dissolution and deposition in nafion electrolyte. Electrochemical and Solid State Letters, 2007. 10(5): p. B101-B104.

3.            Bi, W., et al., The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell. Electrochimica Acta, 2009. 54(6): p. 1826-1833.

4.            Schuster, M., et al., Highly Sulfonated Poly(phenylene sulfone): Preparation and Stability Issues. Macromolecules, 2009. 42(8): p. 3129-3137.

5.            Fuller, T.F. and J. Newman, Experimental-Determination of the Transport Number of Water in Nafion-117 Membrane. Journal of The Electrochemical Society, 1992. 139(5): p. 1332-1337.

6.            Springer, T.E., T.A. Zawodzinski, and S. Gottesfeld, POLYMER ELECTROLYTE FUEL-CELL MODEL. Journal of The Electrochemical Society, 1991. 138(8): p. 2334-2342.