Determination of Ce3+ and Co2+ Diffusion Coefficients in NR211 Membrane
Authors: Coms Frank D. , McQuarters Ashley B 
Published: February 2020

The proton exchange membrane fuel cell (PEMFC) is a promising clean energy propulsion system for automobiles and other vehicles. A critical component of the PEMFC is the polymeric ionomer membrane which enables proton conduction and provides a crucial barrier to both reactant gas crossover and electrical shorting. Many fuel cell systems employ thin (12-25 µm) perfluorosulfonic acid (PFSA) based membranes due to their superior mechanical and chemical stability properties. The primary product of a PEMFC is water, generated via the four-electron oxygen reduction reaction. However, smaller amounts of highly oxidizing species such as hydroxyl radical (•OH), and hydrogen peroxide (H2O2) are generated either chemically or electrochemically. These, and other, oxidizing species can react with non-fluorocarbon end groups or with acid side chain functional groups to degrade and weaken the polymeric membrane until causing a physical breach. In the absence of effective chemical mitigation, proton exchange membranes will survive no more than a few hundred hours of operation under typical automotive duty cycles involving large humidity and potential variations. Fortunately, PFSA lifetimes can be extended up to and beyond 10,000 hours of operation using •OH redox quenchers such as Ce3+ and Mn2+ that efficiently reduce •OH to H2O. The remarkable effectiveness of these redox quenchers is due to their high competitive activity toward •OH reduction and the rapid and benign reduction of the oxidized metal to the reduced form via reaction with ubiquitous H2O2.
Within an ionomer, cations, including Ce3+, associate (ion pair) with the negatively charged sulfonate groups to varying degrees and have been observed to diffuse in-plane on the centimeter length scale during ten to100 hours of fuel cell operation. More rapid migration of cations into the electrode layers has been observed over the µm length scale of MEA thickness. The movement of cations within the MEA is governed by local water content (λ = n H2O/-SO3
-), water content gradients (Δλ) and electrical potential gradients. The observed in-plane movement of Ce3+ and other cations such as Co2+ can lead to large cation concentration gradients within a fuel cell MEA. This redistribution of cations can have a significant impact on cell performance and mitigation of chemical degradation processes. Accordingly, there is great interest in understanding and quantifying the factors that govern cation movement to optimize fuel cell efficiency and durability.
We will report the in-plane isotropic diffusion coefficients of Ce3+ and Co2+ within NRE211 membrane over a wide range of uniform temperature and relative humidity values. The cation concentrations have been monitored using a variety of analytical techniques including FTIR, x-ray fluorescence and UV spectroscopy (Figure 1). The derived diffusion coefficients show a strong dependence on both temperature and relative humidity value (water content) and are consistent with the macroscale redistribution of cations seen in operating fuel cells. Quantitative structure activity relationships and cation movement under RH gradients will also be discussed.

Figure 1