Surface-Initiated Growth of Ionomer Films from Pt-Modified Gold Electrodes

Abstract

The ability to chemically wire ionomer films to electrode surfaces can promote transport near interfaces and impact a host of energy-related applications. Here, we demonstrate proof-of-concept principles for the surface-initiated ring-opening metathesis polymerization (SI-ROMP) of norbornene (NB), 5-butylnorbornene (NBH4), and 5-perfluorobutylnorbornene (NBF4) from Pt-modified gold substrates and the subsequent sulfonation of olefins along the polymer backbones to produce ultrathin sulfonated polymer films. Prior to sulfonation, the films are hydrophobic and exhibit large barriers against ion transport, but sulfonation dramatically reduces the resistance of the films by providing pathways for proton diffusion. Sulfonated films derived from NBF4 and NBH4 yield more anodic potentials for oxygen reduction than those derived from NB or unfunctionalized electrodes. These improvements are consistent with hydrophobic structuring by the fluorocarbon or hydrocarbon side groups to minimize interfacial flooding and generate pathways for enhanced O(2) permeation near the interface. Importantly, we demonstrate that the sulfonated polymer chains remain anchored to the surface during voltammetry for oxygen reduction whereas short-chain thiolates that do not tether polymer are removed from the substrate. This approach, which we extend to unmodified gold electrodes at neutral pH, presents a method of cleaning the ionomer/electrode interface to remove molecular components that may hamper the performance of the electrode.

Composite Fluorocarbon Membranes by Surface-Initiated Polymerization from Nanoporous Gold-Coated Alumina

“…To provide charge transport to the membrane, we performed sulfonation of the pNBF6 film with acetyl sulfate in order to introduce sulfonic acid and hydroxyl groups in the backbone of the polymer film and promote ion transport through the membrane. 29,30 Results from this work confirm successful integration of the pNBF6 throughout the membrane or as a selective skin initiated only from the NPGL layer.…”

Surface-Initiated Polymerization of 5-(Perfluoro-n-alkyl)norbornenes from Gold Substrates

“…The CF 2 stretching modes absorb strongly from 1100 to 1400 cm −1 . Because of the helical structure of fluoroalkyl chains, two types of CF 2 stretching peaks are expected in the IR: those lying along the helical axis (ν ax (CF 2 ), 1300−1400 cm −1 ) and those perpendicular to the helical axis (ν pd (CF 2 ), 1100−1300 cm −1 ). − The ratio of ν pd (CF 2 ) to ν ax (CF 2 ) absorbance for the film, relative to the same ratio for isotropic orientation of drop-cast pNBF n films, provides information on the orientation of the fluorocarbon side chains in the polymer film relative to the surface normal. RAIRS utilizes inherent surface selection such that the intensity for a given mode in the IR spectrum is proportional to the square of the component of its dynamic dipole moment oriented along the surface normal.…”

“…26 However, negligible film loss was observed during the sulfonation of pNBF4 over a 24 h exposure to acetyl sulfate. 32 The improved stability imparted by the fluorocarbon side chains is likely the result of diluting olefin functionality within the film to inhibit reactions such as coupling and backbiting that are known to cleave chain fragments. [33][34][35] For n g 4, the fluorocarbon chain consists of g77% of the mass of the monomer repeat unit.…”

Surface-Initiated Ring-Opening Metathesis Polymerization of Pentadecafluorooctyl-5-norbornene-2-carboxylate from Variable Substrates Modified with Sticky Biomimic Initiator

“…Ring-opening metathesis polymerization (ROMP) offers an effective way in polymerization of norbornene monomers with large side chain substitutes. Recently, Jennings et al successfully employed ROMP to graft partially fluorinated polymers on gold substrates using Grubbs second generation catalyst. , In this article, we perform surface-initiated ring-opening metathesis polymerizations (SI-ROMP) of pentadecafluorooctyl-5-norbornene-2-carboxylate (NCA-F 15 ) from catecholic initiator. The good adhesion of the catecholic initiator onto variable metal substrates allows modification of these substrates with chemically bonded low surface energy polymer coatings.…”