Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

Abstract

Nafion, the most widely used polymer for electrolyte membranes (PEM) in fuel cells, consists of fluorocarbon backbones and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. While the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. Using molecular spin probes that selectively partition into heterogeneous regions of PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels compared to within the water channels. The concept that surface chemistry at the (sub-)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEM.

Nanoscale Distribution of Sulfonic Acid Groups Determines Structure and Binding of Water in Nafion Membranes

“…Molecular dynamics (MD) simulations have suggested that nanoscale physicochemical properties,f or example,t he size [4a, 8] and chemical constitution [5] of water channels and hydrophobic domains,p lay ac rucial role for the overall physicochemical properties of PEMs,i na ddition to the macroscale membrane chemistry and pore connectivity of water channels.H owever, direct experimental proof relating nanoscale chemical constitution with membrane physical chemistry for Nafion is scarce. [5,6] Small-angle scattering experiments (SAXS/SANS) have shown the radius of cylindrical water channels to be 5-20 % smaller in extruded than in solution-cast hydrated Nafion of 1100 gE W. [7c, 16] It follows that the circumference of an idealized, cylindrical channel (as well as the channel surface area) is accordingly 5-20 %l arger for N212 compared to N117. Thus,t he area-to-volume (A/V)r atio of the channel surface is 5-20 %larger for N117, assuming the total length of the channels is the same and much larger than the channel radius in both membranes.R ecall that the spectra from the two fully hydrated membranes (Figure 1) show the same concentrations of Nafion (SO 3 À and COC moieties) and water (integrated OH) per CF 2 group within the focal volume.…”

“…[4] In addition to chemical composition and membrane topology, the nanoscale chemical constitution of water channels,that is, how functional head groups are distributed across them, should also directly affect membrane-water chemistry,a nd therefore transport properties,i nP EMs; [5] however, experimental proof of this effect is scarce. [5,6] In this work, we quantify chemical interactions between water and Nafion in two membranes with the same chemical composition (and equivalent weight, EW) that have been cast in different ways by using nonlinear Raman spectroscopy. Our results show that the molecular environment in the nanochannels of these two compositionally identical, but differently processed Nafion membranes is distinct under fully hydrated conditions,w here proton conductivity is maximized.…”

Probing the neutralization behavior of zwitterionic monomer-containing dental adhesive

“…Han et al . have investigated the water- and proton-diffusion in polymer electrolyte membrane, and their results indicated that the surface chemistry (sulfonic acid, and fluorocarbon) directly influenced proton transport [48]. Although the polymer material used in our experiment was different from Han’s, the enhanced rate of neutralization associated with the experimental sample containing MPC indicated that proton diffusion was promoted.…”

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

“…Nafion serves as the archetype polyelectrolyte,d ue to its commercial availability and widely investigated properties. [15,16] An instructive image of nanoscale segregation in amodel Nafion film simulated with the coarse-grained dissipative particle dynamics [14] is given in Scheme 1. Upon hydration, Nafion nanosegregates into hydrophobic and hydrophilic subphases.…”

“…[14] Thel atter represents ac ontinuous 3D network of water domains of the nanometer size. [15,16] An instructive image of nanoscale segregation in amodel Nafion film simulated with the coarse-grained dissipative particle dynamics [14] is given in Scheme 1. Thec ontinuity and restricted geometry of the hydrophilic subphase allows for uniform nucleation and controlled growth of MONP.T his approach, that has been earlier demonstrated with examples of Fe 2 O 3 , [17] ZnO, [18,19] and from zirconium phosphate, [20,21] has particular advantages over other efforts to incorporate MONP into Nafion, which have largely focused on traditional techniques to fabricate the nanoparticles beforehand then incorporating them into the polymer,such as dropcasting with apolymer resin via the doctor blade method [22] or with the use of organic salts as the precursor.T hese traditional methods suffer from the disadvantage of larger than desired MONP, and the need for an additional capping agent.…”