Molecular-Level Modeling of the Structure and Wetting of Electrode/Electrolyte Interfaces in Hydrogen Fuel Cells

Abstract

Molecular dynamics (MD) simulations were performed to investigate the structural and dynamical behavior of water and hydronium ions at the electrode/electrolyte interface of hydrogen polymer electrolyte membrane (PEM) fuel cells. Specifically, we have studied the hydrated Nafion membrane, humidified for four different water contents, 5, 10, 15, and 20%, at 300 K. We analyzed the three-phase interface where the hydrated PEM is in contact with the vapor phase and with either the catalyst surface (platinum in this paper) or the catalyst−support surface (graphite in this paper). These molecular simulations represent portions of interfaces that exist within the PEM fuel cells. We observed significant wetting of the catalyst surface by a mixture of polymer, water, and hydronium ions but not beyond a monolayer. We observed virtually no wetting of the graphite surface. On the catalyst surface, the degree of wetting of the catalyst surface depends strongly on the level of membrane humidity. The pair correlation functions indicate that the water molecules adsorbed in a monolayer on the catalyst surface form small domains of ordered structures, which are bound by fragments of Nafion on the surface. The diffusion of protons from the catalyst surface into the membrane must proceed across this highly inhomogeneous surface.

Effect of Pt nano-particle size on the microstructure of PEM fuel cell catalyst layers: Insights from molecular dynamics simulations

“…Although atomistic models are widely used to investigate ion transport in bulk membrane, there have been relatively few studies of composite catalyst layer using this approach. These include the recent work of Liu et al [17] and Selvan et al [18] who were able to show how structural and transport properties of water and hydronium ions at membrane/vapor interface are related to membrane water content. Lamas and Balbuena [19] examined the role of Nafion content in the CL.…”

Molecular‐Level Investigation of Critical Gap Size between Catalyst Particles and Electrolyte in Hydrogen Proton Exchange Membrane Fuel Cells

“…The model of Nafion, water and the hydronium ions used in this work is identical to that of our previous work to model hydrated Nafion [5][6][7]. The CF 3 , CF 2 and CF in Nafion are modelled as united atoms to reduce computational costs.…”

“…The number of each type of molecule used in the simulation is provided in Table 1. The amount of the hydrated membrane material used in this work is the same as that of our previous three-phase interfacial system [7]. The dimensions of each system are provided in Table 2.…”

“…The initial conditions of the systems took an equilibrated configuration of Nafion/vapour/graphite surface from the previous work [7] and replaced the graphite carbon atoms with the combination of graphite atoms and catalyst particles, with the same unit cell of graphite on each side of Pt surface. With the new solid surface in place, we allow the full system to equilibrate.…”

“…This modelling work is a continuation of a molecular dynamics study that has investigated the structure and transport properties in hydrated Nafion [5], at the interface between Nafion and a water vapour [6], at the three-phase interface between Nafion, water vapour and a catalyst surface (modelled as [100] Pt) [7], and at the three-phase interface between Nafion, water vapour and a carbon support surface (modelled as graphite) [7]. In the previous work, the catalyst surface or the support surface was immediately adjacent to a nominally planar hydrated Nafion surface.…”