Effect of Substrate Surface Charges on Proton Conduction of Ultrathin Nafion Films

Abstract

A potential approach to enhance the suppressed proton conductivity of nanoscale ultrathin Nafion films is to adjust the ionomer structure via regulating the catalyst–ionomer interaction. To understand the interaction between substrate surface charges and Nafion molecules, self-assembled ultrathin films (∼20 nm) were prepared on the SiO2 model substrates, which were modified with silane coupling agents to carry either negative (COO–) or positive (NH3 +) charges. Specifically, the surface energy, phase separation, and proton conductivity were investigated by contact angle measurements, atomic force microscopy, and microelectrodes to illuminate the relationship between the substrate surface charge, thin-film nanostructure, and proton conduction. Compared to electrically neutral substrates, ultrathin films formed faster on the negatively charged substrate with an 83% increase in proton conductivity but formed more slowly on the positively charged substrate, with proton conductivity decreased by 35% at 50 °C. The surface charges interact with sulfonic acid groups of Nafion molecules to alter molecular orientation, resulting in different surface energies and phase separation, which are responsible for proton conductivity variation.

Engineering Ionomer–Substrate Interface to Improve Thin-Film Proton Conductivity in Proton Exchange Membrane Fuel Cells

“…This low hydrophilicity and unique silane structure minimized the substrate-induced entrapment of Nafion chains and water molecules, thereby improving proton conduction in the Nafion films. These findings align with previous research showing that introducing hydrophobic or electrostatically repulsive functionalities can prevent specific adsorption of −SO 3 – ions of Nafion onto catalysts and improve proton transport and ORR activity. ,− …”

“…If silane is suppressing the lamella formation by letting a lower number of chains reach the substrate interface, the ionomer–substrate interaction next to the substrate (Figure g, right) can be milder. This weakened interfacial interaction can be a major reason behind the improved conductivity , in ionomer films on silane. Literature on block copolymer thin films also supports how lamella weakens the ionic conductivity in the direction the lamella grows. ,, Therefore, the disruption of the lamella and significant improvement in through-plane proton conductivity in our case (Figure f) complemented each other.…”

“…To address these interfacial weak ion and gas transport issues within ionomer thin films, attempts have been made to alter the structure and chemistry of catalysts ,, and ionomers. ,,, Interfacial restructuring is emerging as a promising approach to negate interfacial chain pinning and its detrimental effects (catalyst poisoning and slow ORR). For example, modifying the electrocatalysts with an inert skin layer, rendering substrates with hydrophobic ,− /electrostatically repulsive moieties, or favorably orienting the lamellar structure , weakened the specific adsorption of −SO 3 – anions of Nafion chains onto the substrate , and increased the overall proton conductivity as well as ORR activity . When ionic liquids (ILs), − IL-modified block copolymers, , or peptides , were present at the catalyst active sites, phase segregation and specific adsorption of ion-conducting groups of ionomers on Pt altered in a manner which favored ORR and O 2 transport in some cases.…”

“…From these stemmed the idea that a porous or brush-like/spider-web-like architecture adjacent to a substrate may prevent the surface-parallel alignment and anchoring of ionomers and facilitate proton conduction. Moreover, substrate hydrophobicity ,− and the number of surface-interacting functional groups of ionomers (−SO 3 H in Nafion) localized next to a substrate may determine the extent to which water and ionomer chains can be confined to the substrate . These observations hinted that through interfacial engineering we can manipulate, understand, and make ionomer self-assembly, ionomer chain orientation, distribution of ionomeric functional groups, and mechanical properties of ionomer thin films more favorable for interfacial- and across-the-film proton conduction.…”

“…The surface-parallel ionomer chains adsorbed onto catalyst particles also made ORR active sites inaccessible to O 2 and caused performance loss of PEMFCs. 10,43,30,44,45 To address these interfacial weak ion and gas transport issues within ionomer thin films, attempts have been made to alter the structure and chemistry of catalysts 46,26,47 and ionomers. 10,31,37,48 Interfacial restructuring is emerging as a promising approach to negate interfacial chain pinning and its detrimental effects (catalyst poisoning and slow ORR).…”