Chemistry and Degradation Mitigation Effect of Cerium Oxide in Polymer Electrolyte Membranes

One of the main obstacles to wide-spread commercialization of polymer electrolyte membrane fuel cells (PEMFC) is the short operational lifetime of perfluorosulfonic acid (PFSA) membranes used in these devices.  Various radical oxygen species are considered to be the main degrading species for PFSA membranes.[1]

Radical scavenging properties of cerium oxide Ce(III)/Ce(IV) redox couple in the biological environment are well-known and hence its application in PEMFC, as a degradation mitigator, has received considerable attention.[2, 3]  In this work, two cerium oxide nanoparticle formulations with an order of magnitude difference in average size (2-5nm and 20-150nm) were added to 1100 EW PFSA membranes in 0.5, 1.0 and 2.0 wt% loading.   We will discuss that ceria, in the acidic environment of heated/humidified membranes, is reduced to Ce(III) ions which negatively impacts proton conductivity. Both liquid and gas Fenton tests were conducted.  These tests showed that the fluoride emission is reduced by an order of magnitude with the polymer end groups as the main point of attack for liquid Fenton tests and additional side-chain attack for gas Fenton tests.  The extent of durability improvement is found to be independent of cerium oxide particle sizes.[4] 

The membranes were subjected to 94 and 500 h open-circuit voltage hold accelerated durability tests. The open-circuit voltage decay rate was shown to be decreased by half and seven-fold for the 94 h and 500 h tests respectively.   In 500 h tests, ceria-containing MEAs demonstrated  at least two orders of magnitude reduction in fluoride emission rates with minimal change in the integrity of the MEA while the baseline MEA underwent catastrophic failure.  In the presence of cerium oxide nanoparticles, the platinum deposition in the membrane was found at previously reported distance from cathode; however, the platinum particles showed an increase in size and caused a broadening with particles reaching further into the membrane.[5]

[1]     J.M. Fenton, M.P. Rodgers, D.K. Slattery, X. Huang, V.O. Mittal, L.J. Bonville, H.R. Kunz, Membrane Degradation Mechanisms and Accelerated Durability Testing of Proton Exchange Membrane Fuel Cells, ECS Trans. 25 (2009) 233-247.

[2]     P. Trogadas, J. Parrondo, V. Ramani, Degradation mitigation in polymer electrolyte membranes using cerium oxide as a regenerative free-radical scavenger, Electrochemical and Solid State Letters 11 (2008) B113-B116.

[3]     F.D. Coms, H. Liu, J.E. Owejan, Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions, ECS Trans. 16 (2008) 1735-1747.

[4]     B.P. Pearman, N. Mohajeri, D.K. Slattery, M.D. Hampton, S. Seal, D.A. Cullen, The chemical behavior and degradation mitigation effect of cerium oxide nanoparticles in perfluorosulfonic acid polymer electrolyte membranes, Polymer Degradation and Stability 98 (2013) 1766-1772.

[5]     B.P. Pearman, N. Mohajeri, R.P. Brooker, M.P. Rodgers, D.K. Slattery, M.D. Hampton, D.A. Cullen, S. Seal, The degradation mitigation effect of cerium oxide in polymer electrolyte membranes in extended fuel cell durability tests, Journal of Power Sources 225 (2013) 75-83.