Proton conducting hydrocarbon membranes: Performance evaluation for room temperature direct methanol fuel cells

The methanol permeability, proton conductivity, water uptake and power densities of direct methanol fuel cells (DMFCs) at room temperature are reported for sulfonated hydrocarbon (sHC) and perfluorinated (PFSA) membranes from Fumatech®, and compared to Nafion® membranes. The sHC membranes exhibit lower proton conductivity (25-40 mS cm−1 vs. ∼95-40 mS cm−1 for Nafion®) as well as lower methanol permeability (1.8-3.9 × 10−7 cm2 s−1 vs. 2.4-3.4 × 10−6 cm2 s−1 for Nafion®). Water uptake was similar for all membranes (18-25 wt%), except for the PFSA membrane (14 wt%). Methanol uptake varied from 67 wt% for Nafion® to 17 wt% for PFSA. The power density of Nafion® in DMFCs at room temperature decreases with membrane thickness from 26 mW cm−2 for Nafion® 117 to 12.5 mW cm−2 for Nafion® 112. The maximum power density of the Fumatech® membranes ranges from 4 to 13 mW cm−1. Conventional transport parameters such as membrane selectivity fail to predict membrane performance in DMFCs. Reliable and easily interpretable results are obtained when the power density is plotted as a function of the transport factor (TF), which is the product of proton concentration in the swollen membrane and the methanol flux. At low TF values, cell performance is limited by low proton conductivity, whereas at high TF values it decreases due to methanol crossover. The highest maximum power density corresponds to intermediate values of TF.