Behaviour of water molecules in Nafion 117 for polymer electrolyte membrane fuel cell by molecular dynamics simulation

Proton exchange membranes play a critical role as electrolytes for proton transports in polymer electrolyte membrane fuel cells. A membrane, such as Nafion 117, consists of a polytetrafluoroethylene backbone and side chains that terminate with sulfonate groups (). During operation of fuel cells, membranes become preferentially hydrated by absorbing water needed for effective proton conduction. Water management and movement, therefore, are extremely important for the efficient operation of the fuel cells. In this paper, we set up the molecular models for hydrated Nafion 117 and perform molecular simulations for various temperatures and monomer numbers to analyse the motion of water and hydronium molecules. Diffusion coefficients estimated from the mean-square displacements agree well with the experimental estimation. The distribution and structure of water molecules in Nafion 117 are analysed using radial distribution functions and Voronoi tessellation. The result shows that the distribution of water molecules in the Nafion membrane is quite close to that of hexagonal ices but quite deviated from that of pure water molecules.