Revisiting Ion Transport in Nafion

Nafion is a charged polymer of significant interest as a fuel cell membrane due to rapid and selective transport of protons. Small molecule transport in Nafion has been explored in considerable detail over the past several decades and draws continued interest. However, this literature has evolved somewhat independently from other fields studying charged polymers (e.g., polyelectrolytes and water purification), with different models applied among these fields. Moreover, many Nafion studies focus on proton transport, while most studies of charged polymers in the membrane community focus on ions other than protons, given the importance of such ions to desalination. This presentation details experimental measurements and theoretical modeling of ion transport in Nafion, connecting teachings from several communities interested in transport through polymers. The importance of counter-ion condensation and ion speciation (i.e., ion pairing) on ion transport in charged polymers are highlighted. The development and applicability of a thermodynamic model, accounting for these phenomena, to predict ion sorption in Nafion without adjustable parameters is discussed. Together, these results provide critical fundamental insights for designing highly selective membrane materials.