Influence of Equivalent Weight of Ionomer on Proton Conduction Behavior in Fuel Cell Catalyst Layers

Although the properties of Nafion membranes (10 to 100s mu m thick) have been well studied for decades, understanding of nano-sized Nafion ionomer is rather limited, which hampers the further improvement of fuel cell catalyst layers. In this work, ultrathin Nafion film on SiO2 model substrate is prepared via self-assembly method to represent the ionomer in fuel-cell catalyst layers, which allows the accurate characterization of the proton conductivity and microstructure of the ionomer. Proton conductivity measurements of Nafion films with different ionomer equivalent weights demonstrate that the ionomer with lower equivalent weight possesses higher proton conductivity. The measured catalyst layer electrolyte resistances decrease with decreasing ionomer equivalent weights, as expected. It is found that the confinement environment of Nafion ionomer results in a loss of microphase separation, which can be alleviated by lower equivalent weight ionomer that in turn improves the ion mobility, and thus promoting proton conduction. (C) 2019 The Electrochemical Society.