Advancing ionomer design to boost interfacial and thin-film proton conductivity via styrene-calix[4]arene-based ionomers

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

“…In this next-to-substrate region, ionomer chains preferentially orient their backbones parallel to the substrate, while their side chains (−SO 3 H-terminated) face the substrate. ,− This orientation favors interfacial interactions among silanol (−SiOH) groups of substrate, water, and −SO 3 H groups of Nafion chains lying flat on the surface. As a result, ionomer chains and water get pinned to the substrate, ,,− lose mobility, ,,, and stiffen the films. ,,,, Interfacial effects around the substrate can also cause a distribution of glass transition temperature, − polymer chain dynamics, , mobility, , hydration, , and ion-conduction environment across ionomer films. ,,,− For example, using confocal laser scanning microscopy (CLSM), we are the first to reveal that ionic conduction across ionomer thin films varies along the depth of the film and is highly interface dependent. ,, Precisely, proton conduction of Nafion thin films on unmodified glass substrates was extremely weak over a broad region next to the substrate interface and then gradually ramped up as we approached the air interface . CLSM also identified a stiffened region next to the substrate within these Nafion films .…”

“…While PEMFC holds great promise, its power performance is impacted by the sluggish oxygen reduction reaction (ORR) at the cathode of the cell. To improve the ORR kinetics, we must overcome O 2 and proton transport limitations at the catalyst interfaces of PEMFC cathodes. − The current benchmark ionomer, Nafion conducts protons poorly in subμm thick films, − interfacing catalyst particles on cathode where ORR occurs. Thus, understanding and appropriately engineering the interface between the catalyst and sub-μm-thick ionomer layers on electrodes are critical to overcoming proton transport limitation.…”

“…A similar conclusion was drawn by others ,, as the highly oriented lamella of ionomers or block copolymers near the electrode interface suppressed the through-plane ionic conductivity in thin films. The surface-parallel ionomer chains adsorbed onto catalyst particles also made ORR active sites inaccessible to O 2 and caused performance loss of PEMFCs. ,,,, …”

“…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 .…”

“…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.…”