Interfacial Distribution of Nafion Ionomer Thin Films on Nitrogen-Modified Carbon Surfaces

Abstract

Chemically modified carbon supports for the cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) show considerable promise for boosting the oxygen reduction reaction. This study evaluated the ionomer distribution of Nafion ionomer thin films on nitrogen (N)modified carbon surfaces along their depth direction. Neutron reflectivity (NR) measurements performed using the double-contrast technique with H 2 O and D 2 O revealed that the introduction of N functional groups to carbon thin films promoted ionomer adsorption onto the surface under wet conditions (22 °C, 85% relative humidity). Molecular dynamics (MD) simulations conducted to verify the origin of the robust contact between the ionomer and N-modified carbon surface revealed an ionomer adsorption mechanism on the N-modified carbon surfaces, which involved Coulomb interactions between the positively charged carbon surface and the ionomer side chains with negatively charged sulfonic acid groups. The positive surface charge, which was determined using the contents of the N functional groups estimated by X-ray photoelectron spectroscopy, was found to be sufficient as an impetus for ionomer adsorption. This strategy involving NR measurements and MD simulations can provide insights into the solid−ionomer interfacial structures in a cathode catalyst layer and can therefore be extensively employed in studies on PEFCs.

Insight into Oxygen Transport in Solid and High-Surface-Area Carbon Supports of Proton Exchange Membrane Fuel Cells

“…The carbon-dense layer region (∼0.75 nm in thickness) exhibits a remarkably higher density due to the interactions with the carbon support, which is approximately 1.5 times higher than that in the bulk region. Experiments results revealed that Nafion self-assembles through its hydrophobic backbones on the graphitic surfaces of carbon supports due to the attractive interactions, thus forming the thin adsorption layer on the carbon surface with a significantly higher density. − For the corresponding oxygen distribution in Figure c, it is highly concentrated in the carbon-dense layer regions of both SC and HSC supports, which suggests that those regions are capable of accumulating oxygen. Besides, it is noteworthy that oxygen accumulation also exists above the carbon-dense layer of HSC (about 0.75–1.5 nm in the z direction), while it is not conspicuous on the SC support as oxygen can be gradually consumed during the diffusion process.…”

Impacts of Pt/Carbon Black Catalyst Surface Hydrophilicity on Ionomer Distribution and Durability during Water-Generating Load Cycling of Polymer Electrolyte Fuel Cells

“…They are amphiphilic materials with a backbone consisting of hydrophobic perfluorinated groups and hydrophilic side chains containing fluorocarbons, ether groups, and sulfonic acid at the ends. Although there is controversy regarding the adsorption morphology of ionomers on the catalyst surface, ,, side-chain adsorption onto Pt oxide is preferable; ,, the sulfonic acid can interact with hydrophilic groups via hydrogen bonding and electrostatic interactions with the functionalized support. , Thus, we considered that the dispersion and adsorption morphology of ionomers might have a great deal to do with the functional group and hydrophilic information obtained above.…”

“…It remains ambiguous as to which component of the catalyst surface, be it Pt or carbon, predominantly attracts the ionomer in each MEA. Nevertheless, several studies highlighting interactions between ionomers and carbon functional groups ,,, suggest that in the case of K-2, the ionomer has a greater tendency to adsorb on the carbon surface than is the case for K-3, attributed to the presence of a higher number of hydrophilic groups which can strongly interact with the −SO 3 – moiety. This tendency could potentially counteract Pt specific adsorption.…”

“…The ionomer’s sulfonic acid functional group generally interacts strongly with Pt or Pt oxide. , Furthermore, numerous polar functional groups, such as −OH, −NH 2 , and −COOH, exist on the carbon surface , and can easily interact with the −SO 3 – of the ionomer . Also, because the −SO 3 – moiety has ionic charge, electrostatic interactions can also be considered with the functionalized support. − Therefore, it is necessary to consider the balance between the surface conditions of Pt and carbon to discuss the dispersion of the ionomer on the catalyst surface. Although the manufacturing process naturally introduces most functional groups on the carbon surfaces, it is possible to introduce additional functional groups through specific chemical treatments. , We hypothesized that good ionomer dispersibility could be achieved by carefully regulating the catalyst surface state.…”

Comprehensive Molecular Dynamics Study of Oxygen Diffusion in Carbon Mesopores: Insights for Designing Fuel-Cell Catalyst Supports

“…The procedure of generating the infinite chain ionomer was mainly based on previous work. 51 First, a linear straight ionomer chain with n monomer monomers infinitely connected by a periodic boundary was created, indicating that only one periodic ionomer chain exists in the system. We considered that the artifact of this approach is small, as the intertwined structure of multiple ionomers does not largely change in the time scale of the MD simulation.…”