The Nature of Proton Conduction in a Polymer Electrolyte Membrane, Nafion

Proton transport in the aqueous environment has attracted much attention for more than a century. It is well known that protons are transported by the Grötthuss mechanism (proposed two hundred years ago), which is sometimes referred to by terms such as proton jumping and proton hopping. However, the proton transport mechanisms in constrained systems such as biological membranes, electrolyte membranes and minerals are rather vague, despite the fact that proton transport plays an important role in the functionality of such systems. The focus of study in this paper is a polymer electrolyte membrane, which is placed between the anode and cathode in polymer electrolyte fuel cells (PEFCs). Detailed knowledge of the proton transport mechanisms in these membranes is extremely important because the generation of power by fuel cells is limited by proton conduction. The nature of the proton conduction in such a constrained environment is also very interesting from a purely scientific point of view. Nafion is a membrane widely used in PEFCs due to its very good proton conductivity. Several mechanisms of proton transport have previously been proposed in the literature, but have not yet yielded precise descriptions on the atomistic level; various details of these mechanisms are still unclear and controversial. We believe that this lack of consensus regarding the proton conduction mechanisms results mainly from a lack of direct and unambiguous information provided by experimental studies. However, such an investigation may be more suited to highly sophisticated molecular simulations. Therefore, in the present paper, we have applied first-principles molecular dynamics simulations to investigate the nature of the proton conduction in Nafion.