Investigation of Ce3+ Mobility in NR211 Under RH Gradients

Abstract

A proton exchange membrane fuel cell (PEMFC) is an attractive candidate for use in transportation and stationary applications as a clean energy source. A key component of a PEMFC is the polymeric ionomer membrane that serves as a proton conductor and prevents reactant gas crossover and electrical shorting. A widely used class of polymeric ionomers are perfluorinated sulfonic acid (PFSA) type ionomers because of their overall superior chemical and mechanical properties. The primary product of an operating fuel cell is water, but there are also small quantities of hydroxyl radical (•OH) and hydrogen peroxide (H2O2) produced from chemical and electrochemical reactions. These species react with non-fluorocarbon atoms or the sulfonic acid moiety to degrade the polymeric ionomer. To mitigate the chemical degradation of the polymeric ionomer, redox active cations such as Ce3+ and Mn2+ can be incorporated in membrane electrode assemblies (MEAs). These cations react at kinetically sufficient rates with •OH to form water. The resulting oxidized cation is subsequently reduced by H2O2 to complete the redox cycle. Within the polymeric ionomer, these cations associate (ion pair) with negatively charged sulfonate (SO3 -) groups. At the same time, cation complexation of the SO3 - groups modulates the local water content (l = nH2O/SO3 -) in the membrane. During fuel cell operation over a period of ten to 100 hours, cations are observed to diffuse on a centimeter length scale. The redistribution of cations can impact the fuel cell performance and mitigation of chemical degradation reactions. The movement of cations within an MEA is dictated by water content (l), water content gradients (Dl), and potential gradients. Therefore, the optimization of fuel cell performance and durability requires understanding how these factors impact cation movement. In this presentation, we will report isotropic diffusion coefficients of Ce3+ and Co2+ within a NR211 membrane over an array of uniform temperature and relative humidity values. The cation concentrations were monitored using FTIR, x-ray fluorescence, and UV-vis spectroscopies. The derived diffusion coefficients from these data show a strong dependence on relative humidity and temperature. These mobility rates are also consistent with redistribution of cations in operating fuel cells. In addition, cation movement under RH gradients will be presented. In this work, a dual-RH chamber apparatus was employed to study cation mobility rates over a range of RH chamber values and temperatures. For example, uniformly Ce3+-doped NR211 membranes clearly exhibit cation migration from the high to the low RH chamber (Figure 1a) and the mobility of cations in the low RH chamber is greatly reduced. Much additional experimentation with Ce3+ patch-doped NR211 strips (Figure 1b) has been conducted to elucidate the influence of water gradients on the rate and extent of cation mobility. Finally, a 1-D in-plane Ce3+ transport model that was developed from these data will be discussed. Figure 1