Mechanism of Perfluorosulfonic Acid Membrane Chemical Degradation under Low RH Conditions

Chemical degradation of perfluorosulfonic acid (PFSA) membranes of proton exchange membrane (PEM) fuel cells is a well-known process which can limit PEM fuel cell lifetimes.¹ Chemical degradation can weaken membranes to the point where they no longer provide an effective barrier to reactant gas permeation or electrical shorting. The chemical degradation of PFSA-based PEMs is initiated and propagated by the actions of a variety of oxidants formed during fuel cell operation including hydroxyl radical, hydroperoxyl radical and hydrogen peroxide. Of these oxidants, only hydroxyl radical is capable of abstracting hydrogen atoms from carboxylic acid end groups that are the reactive functional groups of a degrading PFSA.  Recently, the use of active, reversible hydroxyl radical scavengers such as Ce³⁺ and Mn²⁺ have increased the stability of PFSA membranes by as much as 1000-fold relative to unmitigated membranes.²Despite the durability improvements offered by these hydroxyl radical scavengers, there is evidence that PFSA membranes are susceptible to chemical damage by low humidity hydrogen peroxide vapor.

In order to probe the sensitivity of PFSA membranes to highly damaging, low RH oxidative reaction conditions, we have developed a highly versatile ex-situ H₂O₂ vapor test that allows the degradation mechanism to be probed as a function of RH.³ During a typical vapor test, H₂O₂ (30 ppm) is delivered to a bare membrane over three, 20-hour segments with RH values of 95, 23 and 95%.  The rates of chemical degradation are monitored by quantifying fluoride evolution rates (FER) as a function of time. FER values greatly increase (> 20x) as the RH decreases from 95 to 23% in the second segment of the experiment. Upon returning to 95% RH, FER values are enhanced 3-9x relative to the initial conditions (Figure 1). The elevated FER values of the final step are associated with chain scission processes that occur during the low RH excursions. Indeed, the increased rates of chemical degradation in the third test segment have been correlated with increased carboxylate concentration within the membrane after vapor testing.

The increased carboxylic acid concentration is a clear indication that PFSA main chain scission processes occur during the very low RH phase of the H₂O₂ vapor test. In recent years, several chain scission mechanisms have been proposed including cleavage of side chain ether linkages⁴ and C-S bond rupture either directly or after sulfonyl radical formation.¹

In order to distinguish among the hypothesized side chain degradation mechanisms leading to main chain backbone scissions, we have conducted a series of H₂O₂ vapor experiments on fully Na⁺-exchanged Nafion^® NRE212CS (Figure 1). Successful execution of these experiments required careful exclusion of atmospheric CO₂ to prevent protonation of carboxylate groups. After treatment of the Na⁺ NRE212CS with H₂O₂ vapor, subsequent protonation of the membrane followed by a second H₂O₂ vapor exposure reveals that no chain scission occurs during exposure of Na⁺ Nafion^®to the highly aggressive hydrogen peroxide vapor. The results suggest that under very low RH conditions, the protonated sulfonic acid groups may participate in side chain reactions leading to main chain scissions.

References

 

1. C. S. Gittleman, F. D. Coms, Y-H. Lai, Membrane Durability: Physical and Chemical Degradation, in Polymer Electrolyte Fuel Cell Degradation, pp 15-88, M. M. Mench, E. C. Kumbur, T. N. Veziroglu, eds, Academic Press, 2012
2. F.D. Coms, H. Liu, J.E. Owejan, ECS Trans, 16 (2008) 1735-1747.
3. F.D. Coms, H. Xu, T. McCallum, C. Mittelsteadt  ECS Trans,  50(2) (2013) 907-918
4. L. Ghassemzadeh, S. Holdcroft, J. Am. Chem. Soc, 2013, 135, 8181-8184.