Molecular Weight Dependent Ionomer Volume Expansion and Its Effect on the Performance of PEM Fuel Cell Electrodes

Abstract

Within the past years, proton exchange membrane (PEM) fuel cells have become more and more attractive due to their potential for the transition towards an environmentally friendly hydrogen economy. Especially by reducing the platinum catalyst loading, significant system cost reductions could be achieved, but low Pt loadings still lead to unassigned voltage losses during operation.[1,2] In order to overcome those losses, well-designed catalyst layers with optimized ionomer content and distribution are indispensable. The used perfluorosulfonic acid (PFSA) ionomers for PEM fuel cells are commonly characterized by the chemical structure of the ionomer and the equivalent weight (EW), which strongly affects the proton conductivity.[3,4] Current research focusses on the synthesis of modified ionomers with higher oxygen permeability to reduce mass transport losses, but also investigates the influence of the ionomer’s molecular weight (MW).[1,5,6] A high MW minimizes the water uptake of the ionomer/membrane in liquid water, which is desired for the use in PEM fuel cells to reduce mechanical stress during relative humidity (RH) cycling.[5] The exact MW of an ionomer is not easily accessible, but can be estimated by measuring the melt flow index (MFI), whereas a high MW is commonly reflected by a lower MFI if the ionomer chemistry and EW are the same.Within this study, a 3M ionomer modified to have a high MFI of 156 g/10 min (measured at 265 °C with a mass of 5 kg) is investigated and compared to a 3M standard ionomer with a low MFI of 6 g/10 min. Both PFSA ionomers have the same chemical structure and a similar EW of ~800. The significantly higher MFI of the modified ionomer (i.e., comprising a lower MW) is clearly reflected by a ~2 times higher water uptake compared to the 3M standard ionomer (measured for solution-cast membranes in liquid water at 80 °C). To evaluate the impact of the MFI within the cathode electrode, 5 cm² active area membrane electrode assemblies (MEAs) with these two ionomers at various ionomer/carbon (I/C) mass ratios in the cathode electrode (based on a 40 wt.% Pt/Vulcan catalyst at loadings of 0.11 mgPt/cm²) are manufactured and electrochemically characterized. At sufficiently high I/C ratios and under humid operation (i.e., I/C = 0.90 and 90 % RH), differential flow H2/air polarization curves of the MEAs reveal that the higher liquid water uptake of the 3M high-MFI ionomer can be correlated with a lower cell voltage (see Figure 1). Contrary to the MEAs with the 3M low-MFI standard ionomer, the larger ionomer swelling in MEAs with the 3M high-MFI ionomer reduces the void volume fraction in the electrode structure during operation. Most pronounced at high current densities, where oxygen mass transport plays an important role, the MEAs containing the 3M high-MFI ionomer show a significant voltage loss of ~50 mV at 2.5 A/cm² compared to the MEAs with the 3M low-MFI ionomer. This study demonstrates that the effect of ionomer volume expansion upon changes in the MFI and thus the MW might not be crucial for the MEA performance when using electrodes with a low I/C ratio, but is critical when aiming for electrode compositions with a high ionomer content, in which case a low-MFI (i.e., a high MW) ionomer should be favorable.