Fluorine-Free Precise Polymer Electrolyte for Efficient Proton Transport: Experiments and Simulations

Abstract

Designing polymers with controlled nanoscale morphologies and scalable synthesis is of great interest in the development of fluorine-free materials for proton exchange membranes in fuel cells. This study focuses on a precision polyethylene with phenylsulfonic acid branches at every 5 th carbon, p5PhSA, with a high ion-exchange capacity (4.2 mmol/g). The polymers selfassemble into hydrophilic and hydrophobic co-continuous nanoscale domains. In the hydrated state, the hydrophilic domain, comprised of the polar sulfonic acid moieties and water, serves as a pathway for efficient mesoscopic proton conductivity. The morphology and proton transport of p5PhSA are evaluated under hydrated conditions using in situ X-ray scattering and electrochemical impedance spectroscopy techniques. At 40 °C and 95% relative humidity, the proton conductivity of p5PhSA is 0.28 S/cm, which is four times greater than Nafion™ 117 under the same conditions. Atomistic molecular dynamics (MD) simulations are also used to elucidate the interplay between the structure and the water dynamics. The MD simulations show strong nanophase separation between the percolated hydrophilic and hydrophobic domains over a wide range of water contents. The percolated hydrophilic nanoscale domain facilitates the rapid proton transport in p5PhSA and demonstrates the potential of precise hydrocarbon-based polymers as processible and effective proton exchange membranes.

Understanding ion diffusion in anion exchange membranes; effects of morphology and mobility of pendant cationic groups

“…Also, an in-depth literature review of the recent works on similar systems shows that equal or even shorter polymer chain lengths are typically used in MD simulations. 25,38–40 Simulations of hydrated membranes have been performed at different (intermediate) hydration levels in AEMs ranging from λ = 2 to 15, where λ is the number of water molecules per fixed ionic (quaternary ammonium) group. λ can be converted to water content (wt%) using eqn (1).where wt% H 2 O is the weight percent of water, IEC is ion exchange capacity in mequiv g −1 and M water is the molecular weight of water in g mol −1 .…”

“…Since we are not interested in the diffusive dynamics of the chains but rather in the diffusive dynamics of water and anion, and since the polymer solution structure does not depend much on chain length in the so-called semi-dilute solution regime 37 , the chain length chosen by us should allow us to draw meaningful conclusions from our simulations. Also, an in-depth literature review of the recent works on similar systems shows that equal or even shorter polymer chain lengths are typically used in MD simulations 25,[38][39][40] in which wt% is the weight percent of water, IEC is ion exchange capacity in (mequiv/g) and M water is the molecular weight of water (g/mol). λ = 0 is also used to calculate the density of dried membranes.…”

Ordered Nanostructures in Thin Films of Precise Ion-Containing Multiblock Copolymers

“…Recent advances in polymer chemistry have enabled the precise segmentation of functional groups in polyethylene-based copolymers in contrast to the randomly distributed sulfonic acid groups of Nafion. 21 − 23 Notably, sulfonic acid groups placed at every 21st carbon of polyethylene blocks form well-defined ionic layers that achieve comparable proton conductivity to commercial Nafion. 24 In addition, strictly alternating multiblock copolymers composed of polyethylene and sulfosuccinate ester blocks with metal counterions (Li + , Na + , and Cs + ) have shown various ordered ionic aggregate morphologies, including layered, double-gyroid, and hexagonally packed cylinders.…”

Superionic Li-Ion Transport in a Single-Ion Conducting Polymer Blend Electrolyte

“…This phenomenon has been reported in computational and experimental studies of multiple sulfonated proton-conducting polymers swollen with water. 44,45 In those studies, the presence of water lowers the electron density of the sulfonated polar domains, to values similar to that of the nonpolar backbone, leading to the disappearance of the aggregate peak, even though there is still nanophase separation. We suspect a similar behavior in p5PhTFSI−Li and PEO blends, where 1000 g mol −1 PEO interacts with the ions to swell the polar domains, decreasing the electron density of the ions and reducing the overall electron density contrast in the blends as a function of PEO content.…”