Abstract ▼ AI-Helper

Polymer electrolyte membrane fuel cells (PEMFCs) are expected to be useful as energy convertors
for transportation and stationary applications, due to its high energy efficiency, low emissions and low
working temperature. However, its high production cost and poor long-term durability impede...

Polymer electrolyte membrane fuel cells (PEMFCs) are expected to be useful as energy convertors
for transportation and stationary applications, due to its high energy efficiency, low emissions and low
working temperature. However, its high production cost and poor long-term durability impede the
widespread commercialization process. A significant portion of the total cost is the cost of the membrane
electrolyte assembly (MEA). The performance of the cathode catalyst layer in the MEA is a major
controlling factor of the ultimate cell performance. Therefore, numerous studies on the understanding
of the morphology and characteristics on catalyst layer have been carried out. However, the morphology
and accompanying properties about the distribution of ionomer and dispersion solvent on the catalyst
layer still involve many unknown aspects. In the present study, a molecular-level morphological insight
is gained by means of molecular dynamics (MD) simulation. For controlling the distribution of Nafion
ionomer and dispersion solvent on catalyst layer, we alter two key factors: (i) the type of dispersion
solvent (dipropylene glycol (DPG), water and DPG/water mixture) and (ii) the surface properties of
Pt/C catalyst. Depending on the type of dispersion solvent, the dispersion state of Nafion ionomer was
changed. As increasing the ratio of dipropylene glycol (DPG) in dispersion solvent, the Nafion ionomer
was well distributed in the dispersion solvent. Moreover, we find that the alteration of interfacial components in contact with the surface of catalyst layer take place by changing the surface property of
catalyst.
Next, we focused on scrutinizing the ionomer/Pt catalyst interface by tailoring a surface property
of hydrophilic Pt catalyst with a hydrophobic molecular masking, with the object of alleviating the
ionomer coverage on the Pt surface. The nature of catalyst surface in fuel cell catalyst layer plays an
important role in the ionomer distribution. In this study, molecular dynamics (MD) simulation was
utilized to understand the effect of the surface property of Pt catalyst on the ionomer distribution by
switching the catalyst surface from hydrophilic to hydrophobic nature. From equilibrium structures and
radial distribution functions (RDFs), we find that the surface properties alter the distribution of the
ionomer and dispersion solvent on the surface. For the Pt surface system, the water molecule and
sulfonic acid group of the ionomer were mainly situated in close proximity to the Pt surface. It is because
the water molecule and sulfonic acid group possess hydrophilic properties, and are strongly interacted
with the hydrophilic Pt surface. On the contrary, in case of the alkylthiol (C18H37S) – modified Pt surface
system, the DPG molecule and the backbone part of Nafion ionomer were distributed in the immediate
proximity of the alkylthiol-modified Pt surface owing to the favorable hydrophobic – hydrophobic
interaction. In addition, the accompanying transport properties of water molecules are varied by the
nature of catalyst surface. The results of the calculated diffusion coefficients show that the water
molecules in alkylthiol-modified Pt surface system diffuse faster than pure Pt surface, and the diffusion
constant is gradually decreasing as an decrease in the content of DPG/water dispersion solvent among
the Nafion solution.
Finally, in order to make a uniform distribution of ionomer coverage at the both Pt cataylst and
carbon support, the functional groups were introduced on the carbon support. Here molecular dynamics
(MD) simulation was utilized to provide molecular insight into structural change for ionomer
distribution on carbon support surface by chemical functionalization. According to the interfacial
binding energies, the introduction of functional group into the carbon support led to enhance the molecular interaction between the ionomer solution and the carbon support surface, with binding
enhancing in the following order: hydroxyl-functionalized (-523.86 ± 9.04kcal/mol) < epoxyfunctionalized (-476.59 ± 7.84 kcal/mol) < amine-functionalized (-473.04 ± 6.82 kcal/mol) < pristine
carbon support surface (-455.28 ± 3.90 kcal/mol). Therefore, we concluded that the functionalization
of carbon support led to more uniform distribution on both of surfaces for Pt catalyst and carbon support
through the improvement for the interaction between the carbon support and Nafion solution. From the
snapshots of equilibrium structures, z-density plots and radial distribution functions (RDFs), we
observed the significant structural changes of Nafion solution at the different carbon support surfaces.
For the hydrophilic surfaces such as hydroxyl-, epoxy- and amine-functionalized surfaces, a large
portion of hydroxyl groups of water and IPA molecules were mainly distributed on the surfaces. In
addition, the sulfonic acid groups of side chain were also located in the proximity on the hydrophilic
surfaces except for the epoxy-functionalized surface. In case of the epoxy-functionalized surface, the
negatively charged epoxy-functionalized surface unfavored the negatively charged sulfonic acid groups
by the repulsive electrostatic interaction. On the other hands, the hydrophobic pristine carbon surface
mostly interacted with the hydrophobic backbones of Nafion ionomer, and IPA molecules which were
relatively more hydrophobic solvent compared to water molecules.
These findings can provide new insights into designing the optimized interface structure of catalyst
layer, with excellent performance of PEMFC.

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