Effect of Short-Term Thermal Annealing on the Nanostructure of Nafion Thin Films on Platinum Substrates

At the cathode of a polymer electrolyte fuel cell (PEFC), protons transported from the anode react with the oxygen gas at the interface of a platinum (Pt) catalyst and an ionomer thin film, where protons pass through a water network in the thin film and the oxygen permeates through it. This implies that the conductivity and permeability of the film, which are influenced by the nanostructure of the film, can be the rate-limiting factors for the reaction. In this study, the effect of short-term thermal annealing, which imitates the industrial decal process and results in high gas permeability, on the nanostructure of a Nafion thin film coated on a Pt layer on a substrate was examined using a combination of grazing-incidence small-angle X-ray scattering (GI-SAXS), neutron reflectometry (NR), and positron annihilation spectroscopy (PAS). The structural evolution during thermal annealing was observed using GI-SAXS and NR analyses. An initial isotropic water network structure was transformed into a transient homogeneous structure after 20 min of thermal annealing, and a stable multilayered interfacial structure was induced at the interface with the Pt substrate in the out-of-plane direction after 4 h of long-term annealing. A significant increase in the free volume size of the Nafion thin film was observed using PAS analysis after 20 min of thermal annealing, which could increase the oxygen permeability a few times and one order of magnitude more than the initial state and equilibrium state after annealing, respectively. These results indicate that the transient disordered structure of the Nafion thin film may be the origin of the oxygen permeability enhancement by the decal process, in which the structure is quenched during the transformation from the initial metastable state to the final stable state during short-term thermal annealing.