Ab Initio Molecular Dynamics Study of the Grotthuss Mechanism for Hydroxide Ions in a Homogenous Anion-Exchange Membrane Used in Fuel Cells

Abstract

Development of anion-exchange membrane fuel cells (AEMFCs) is of great interest among scientific community because of their potential of commercialization at low cost for their use in automotive transportation, portable batteries and power generators [1,2]. However, they currently show serious losses in efficiency associated principally with the low ionic conductivity of the polymeric membranes used as electrolytes [1,3,4]. In order to improve ionic conductivity, different theoretical approaches have been proposed to represent the transport phenomena involved in the mobility of hydroxide ions through hydrated anion-exchange membranes [1,4,5]. However, the characteristics of the mechanisms responsible of that transport are not well known. Particularly, the mechanism responsible of the major contribution to hydroxide mobility in aqueous media, called structural diffusion or Grotthuss mechanism is not completely understood in different systems [1,4]. Although theoretical studies have described how the Grotthuss mechanism takes place in aqueous media and which are its characteristics [6], currently it has not been made extensions of those studies to describe the Grotthuss mechanism in hydrated anion-exchange membranes, taking into account structural aspects of the polymer such as its hydration degree and type of cationic functional group [1]. The mentioned conceptual gaps hinder the improvement of the anion-exchange membranes for engineer and technological applications, considering this from a theoretical point of view. For that reason, the objective of this research is to determine and describe the characteristics of the Grotthuss mechanism for hydroxide ions in the hydrated quaternized polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (QSEBS) membrane for different humidification values. This will be carry out by means of Car-Parinello Ab Initiomolecular dynamics (AIMD), which is considered the most suitable simulation technique in terms of prediction and accuracy given the characteristic length and time scales of the Grotthuss mechanism, and also taking into account that to date there are no formal studies in this topic. From the simulations carried out in this study, qualitative descriptions are derived for the Grotthuss mechanism for hydroxide ions through hydrated QSEBS membrane. Also, radial distribution functions, mechanism characteristic times and diffusion coefficients associated with it are also estimated and analyzed. It is expected that the new understandings derived from this research could be used to propose the structural characteristics of a certain polymeric material which maximize the ion mobility due to Grotthuss mechanism and therefore its ionic conductivity [1], so making a strong contribution to the improvement of the efficiency of AEMFC.