Behavior of Channel Water and CF₂H Side-Chain Terminal Groups in Swollen Nafion Polymer Electrolyte Membranes after Thermal Treatment

Abstract

Polymer electrolyte membranes (PEMs) are representative systems for the study of the proton conduction mechanism and water dynamics in nanopores/channels. Our 1H nuclear magnetic resonance data for Nafion PEMs, which are subjected to thermal degradation and then swollen in water, indicate that (1) water is present next to the side chains even after the removal of the SO3H groups, (2) longer heat-treatment depletes more SO3H groups and produces more CF2H groups, (3) the water near the side chains allows for the liquid-like motion of the CF2H groups, and (4) the motion correlates well with the content and dynamics of water in the channels. As the thermal degradation progresses, the Nafion membranes lose their ionic and hydrophilic nature due to the conversion of CF2SO3H groups to CF2H groups. In addition, our results demonstrate that increasing channel hydrophobicity leads to increased water dynamics in the channels.

Nanometer-Scale Water Dynamics in Nafion Polymer Electrolyte Membranes: Influence of Molecular Hydrophobicity and Water Content Revisited

“…The overall results indicate that both 4-amino-TEMPO and TAD enhance the hydrophobicity of the channel walls in Nafion membranes and that water moves faster near the hydrophobic surfaces even on the nanometer scale. This faster movement of water over hydrophobic surfaces also supports the findings of previous reports. , …”

“…This faster movement of water over hydrophobic surfaces also supports the findings of previous reports. 17,22 The D w values of hydrated Nafion samples measured by PFG-NMR gradually increased about 20% as λ varied from 7 to 15. 23 These are spatially averaged D w values because PFG-NMR does not spatially differentiate D w values.…”

Upcycling aromatic polymers through C–H fluoroalkylation

“…We hypothesized that the lower bond dissociation free energy (BDFE) of the carbon–bromine bond would enable selective functionalization through radical chemistry. The difluoromethyl group is of emerging interest in medicinal chemistry due to its ability to act as a hydrophobic hydrogen-bond donor,45–48 but its potential has rarely been studied in polymer science 49–52. Bromine-atom abstraction initiated by azobisisobutyronitrile (AIBN) in the presence of tributyltin hydride as a hydrogen atom transfer reagent led to quantitative conversion of the bromodifluoromethyl group to a difluoromethyl group.…”

Overhauser dynamic nuclear polarization (ODNP)-enhanced two-dimensional proton NMR spectroscopy at low magnetic fields

“…Another recent example is the study of water diffusion in polymer membranes such as Nafion. Water located inside the membrane (channel water) has a higher chemical shift of about 5.5 ppm, while residual water has a chemical shift of 4.7 ppm (Kim et al, 2016;Überrück et al, 2018). To study the diffusion behavior using ODNP spectroscopy high-resolution techniques are required in combination with multidimensional NMR methods.…”

“…Once the liquid-state NMR spectrum is recorded, the laser is switched off, the sample freezes, and it can be repolarized (Joo et al, 2009(Joo et al, , 2006Sharma et al, 2015). Alternatively, the liquid sample can be polarized at low magnetic fields (e.g., 0.35 T) and either rapidly shuttled into a highfield NMR spectrometer (e.g., 14.2 T) (Krahn et al, 2010;Reese et al, 2008Reese et al, , 2009 or transferred to the high-field magnet using a pump (Dorn et al, 1989(Dorn et al, , 1988Liu et al, 2019). However, the polarization decreases during the trans-fer time, compromising the sensitivity gain achieved by the Overhauser dynamic nuclear polarization (ODNP) process.…”