Does an Ionomer Penetrate a Carbon Mesopore? Free-Energy Analysis Using Molecular Dynamics Simulations
Nobuaki Kikkawa, Ryosuke Jinnouchi
Abstract
Mesoporous carbon support enhances the catalytic activity and gas diffusivity of the cathode catalyst layers of polymer-electrolyte fuel cells. The key role of the mesopores is believed to be their suppression of catalyst poisoning by the ionomers and reduction of the oxygen diffusivity near the catalysts. However, because of the difficulty to observe the ionomer distributions directly, it is unclear whether the ionomer penetrates the mesopores. Here, using molecular dynamics simulations of ionomer penetration into a seamless carbon mesopore, we show that the penetrability depends upon the pore size and the type of solvent. Free-energy profiles obtained from the simulations indicate that an ionomer in a water solvent easily invades a mesopore with a diameter >2 nm, while in a mixed water–ethanol solvent, it cannot do so. Structural differences between the two solvents suggest that three factors affect penetrability. Our results suggest that the ionomer distribution can be controlled by optimizing the solvent composition in the catalyst ink.