Effects of Ionomer Film Thickness and Water Content on Oxygen Permeation Properties of PEFC Cathode Catalyst
Authors: Hikida Kengo , Suzuki Hiroto , Tokumasu Takashi 
Published: December 2024

This study explores the effects of film thickness and water content on the oxygen permeation properties of ionomers in the cathode catalyst layer of Polymer Electrolyte Fuel Cells (PEFCs) using Molecular Dynamics (MD) simulations. The aim is to understand in detail how these parameters influence the efficiency of oxygen transport within the ionomer of the catalyst layer, which is important for optimizing the performance of PEFCs.

In this study, we adopt the MD simulation, which computes the time evolution of the positions and velocities of particles based on Newton's equations of motion.

In this study, Nafion, commonly used in PEFC, served as the ionomer material, employing a simplified model with 10 sulfonic acid groups per chain to lower computational costs, yet reflecting its actual structural and transport properties[1]. For the oxygen molecule, a model in which the intramolecular force is represented by the Morse potential is used. For the water molecule, the F3C model in which the oxygen and hydrogen atoms move independently and are combined with BOND and ANGLE potentials to form a molecule was used.

The number of Nafion chain in the ionomer was changed to 14, 25, and 35 to control the film thickness. From here, the difference of thickness is expressed by Thin, Medium, and Thick. The thicknesses correspond to 3.5 nm, 5.0 nm, and 7.0 nm, respectively. The water content was varied with λ=3, 7, and 11 to investigate the effect of water content in the ionomer on oxygen permeability. Water content is the ratio of the sum of water molecules and hydronium ions to the number of hydrophilic groups in the polymer.

In this study, the oxygen permeability of the ionomer membrane was determined using equation (1). The number of atoms in a water cluster that contains the most atoms was determined as the maximum cluster size to analyze the extent to which water clusters expand. Furthermore, a group of water molecules and hydronium ions, in which the distance between the oxygen atoms within the water molecules and hydronium ions is within 3.5 Å, has been defined as a water cluster. 3.5 Å is the end value of the first peak in the Radial Distribution Function (RDF) between the oxygen atoms of the water molecule in the Nafion bulk film. RDF is a function that shows the average density of particles as a function of distance from a specific point. The density distribution of Nafion and water vertical to the platinum interface was calculated to analyze the layer structure of the film.

Fig. 1 shows the film thickness dependence of maximum cluster size for each water content condition. The vertical axis of the graph is the common logarithm of the maximum cluster size. For water content λ=3, the maximum cluster size is the highest when the film thickness is Medium. However, for water contents λ=7 and 11, the maximum cluster size increases as the film thickness increases.

Fig. 2 shows the film thickness dependence of oxygen permeability for each water content condition. For a water content of λ=3, the oxygen permeability reaches a minimum value when the film thickness is medium, and for water contents of λ=7 and 11, the oxygen permeability decreases as the film thickness increases. Moreover, oxygen permeability decreases with increasing water content across different film thicknesses.

Fig. 3 shows the vertical density distribution of water molecules and Nafion at the Pt interface for each film thickness condition at a water content of λ=3. In this graph, the water density has a second and third peak in addition to the first peak above the Pt interface only when the film thickness is Medium, suggesting that a thick layer of water formed near the interface hinders the permeability of oxygen. In addition, when the film thickness is Thick, there are two peaks in water density at the top of the film, suggesting that a layer of water has formed at the top. However, the water layer formed in a thicker film is considered to be more mobile than that on the hydrophilic platinum surface, the effect of the water layer on oxygen permeability is considered to be smaller than that of the water layer formed in the intermediate film thickness.

From the above results, designing the catalyst layer to avoid forming water layers near the platinum interface that hinder oxygen permeability is important.

Refarence:

[1]Venkatnathan A, Devanathan R, Dupuis MJournal of Physical Chemistry B (2007) 111(25) 7234-7244

Figure 1