Modelling of morphology and proton transport in PFSA membranes

Abstract

Computational modelling studies of the structure of perfluorosulfonic acid (PFSA) ionomer membranes consistently exhibit a nanoscopic phase-separated morphology in which the ionic side chains and aqueous counterions segregate from the fluorocarbon backbone to form clusters or channels. Although these investigations do not unambiguously predict the size or shape of the clusters, and whether or not the channels percolate the matrix or if the connections between them are more transient, the sequence of co-monomers along the main chain appears strongly to influence the domain size of the ionic regions, with more blocky sequences giving rise to larger domain sizes. The fundamental insight that substantial rearrangement of the sulfonic acid terminated side chains and fluorocarbon backbone takes place during swelling or shrinkage is borne out by both molecular and mesoscale simulations of model PFSA polymers, along with ab initio electronic structure calculations of minimally hydrated oligomeric fragments. Molecularlevel modelling of proton transport in PFSA membranes attests to the complexity of the underlying mechanisms and the need to examine the chemical and physical processes at several distinct time and length scales. These investigations have revealed that the conformation of the fluorocarbon backbone, flexibility of the sidechains, and degree of aggregation and association of the sulfonic acid groups under minimally hydrated conditions collectively control the dissociation of the protons and the formation of Zundel and Eigen cations. The former appear to be the dominant charge carriers when the limiting water content allows only for the formation of a contact ion pair with the tethered sulfonate anion. As the water content increases, solventseparated Eigen ions begin to appear, indicating that the dominant mechanism for diffusion of protons occurs over a region approximately 4 Å away from the sulfonate groups. Finally, both the vehicular and Grotthuss shuttling mechanisms contribute to the mobility of the protons but, surprisingly, they are not always correlated, resulting in a lower overall diffusion coefficient. In summary, as the preceding observations indicate, the state of computational modelling of PFSA membranes has progressed sufficiently over the last decade to enable its use as a powerful predictive tool with which to guide the process of designing novel membrane materials for fuel cell applications.

A Theoretical Model of Overpotential at Interfaces in Polymer Electrolyte Fuel Cells

“…However, in recent decades, microscale analyses have been conducted in which proton behavior is modeled by atomic interactions. First-principles electronic-structure calculations have been performed to investigate proton transfer through electrolyte membranes [11]. Herein, overpotentials in PEFCs are evaluated using a theoretical model in which inelastic collisions of protons at electrode-electrolyte interfaces give rise to energy losses.…”

“…In addition, those models disregard atomistic-scale phenomena. On the other hand, recent reports [8]- [11] have emphasized the importance of understanding proton conduction mechanisms by focusing on the behavior of discrete charged particles. Atomistic-scale investigations that exploit recent advances in nanoscale fabrication techniques are expected to clarify the mechanisms of various phenomena in PEFCs, resulting in breakthroughs.…”

Untitled

“…The presence of negative potential at the interface can be associated with the excess of negative charge there: we can assume that Nafion is charged negatively thanks to protons coming off the Nafion interface to the bulk of water. Indeed, it is known (see, e.g., (Kreuer and Membr 2001;Elliot and Paddison 2007)), that in the process of Nafion swelling in water, the channels filled with water and enriched with protons (or, better, hydronium ions) are formed inside the Nafion matrix; the diameter of those channels is 2 -3 nm. The excess of positive charges inside the channels is assumed to be compensated by the presence of negative charges localized at the channel boundaries; these negative surface charges are just ionized sulfonate groups SO3 -according to the reaction of surface dissociation [OCF 2 C-…”

Modeling and Simulation for Fuel Cell Polymer Electrolyte Membrane

“…Equations (15)(16)(17)(18) with T in Kelvin were used for diffusion in the Nafion phase [26] and water phase [27]. Water diffusion D'…”

“…However, since DPD, in principle, allows for the crossing of polymer chains, serious care should be taken when directly applied to study dynamic phenomena. Molecular simulation along with ab initio calculation was applied to proton transport phenomena [15]. This multi-scale treatment is a straightforward approach, but evaluating long-range motion through the water cluster network for various membranes at several hydration levels needs a lot of computation time.…”