Effect of Nafion ionomer and catalyst in cathode layers for the direct formic acid fuel cell with complex capacitance analysis on the ionic resistance

Using platinum (Pt) black and carbon-supported Pt (Pt/C) as cathode catalysts, membrane-electrode assemblies (MEAs) were fabricated with various Nafion ionomer content, and their direct formic acid fuel cell (DFAFC) performances were investigated. In MEAs incorporating Pt black catalysts, the current density at 0.6 V was highest at ionomer/catalyst volume ratio of 1.0, which was consistent with the electrochemical active area (EAS) variation measured by cyclic voltammetry. However, the current density measured at 0.3 V, the cell performance increased with Nafion ionomer content, especially at low ionomer loading, indicating that proton transport rate played an important role. The variation in ionic resistance (R_ion) of cathode layers with Nafion ionomer content was experimentally confirmed by using the complex capacitance analysis of impedance data implemented with nitrogen (cathodes)/hydrogen (anodes) atmosphere. For Pt/C, the layer thickness and EAS of cathode were larger than those of MEA cathode using Pt black; and the current densities at 0.6 V were lower than those of Pt black, suggesting that smaller fraction of EAS was utilized.