Investigation of self-humidifying anode in polymer electrolyte fuel cells

Abstract

A new design of self-humidification for fuel cells was proposed for polymer electrolyte fuel cell. A conventional hydrophobic electrode protonized by Nafion–silica suspension as the anode in the fuel cell has achieved a significant self-humidifying effect. The thin film formed in the immediate vicinity of the catalyst sites using Nafion–silica suspension plays the role of not only transporting protons from active sites in catalyst layer to the membrane but also retaining the water for wetting the electrolyte component in the catalyst layer. The morphologies and particle distributions of Nafion and silica with different silica weight percentage to Nafion (0%–10%) were determined by transmission electron microscopy (TEM). The results showed that the nano-sized silica particles were homogeneously dispersed and embedded in the Nafion matrix to form the Nafion–silica nano-composite which functions the proton conduction in the catalyst layer. However, the network structure and some agglomerations of silica particles were observed with the high percentage of silica used. The addition of silica in the Nafion matrix could improve the water absorption of the Nafion material with the support of water uptake measurement. The water for moisturizing the anodic catalyst layer comes from the back-diffusion of water molecules from the cathode and, to a much lesser extent, the oxidation of hydrogen by the crossover oxygen at the anode, which was verified by using various thicknesses of Nafion membranes. The Nafion–silica composite also exhibits the improvement on the proton conductivity in the ambient condition with RH of 75% at the room temperature. The optimal performance of PEFC was obtained with the 6% of silica at 60 ∘ C using hydrogen and air without external humidification. This study also confirmed that using very thin membrane with impregnated Nafion–silica suspension in the anode catalyst layer can effectively improve the performance of PEFC due to the self-humidification design.