# Long Alkylene Spacers Promote Structural Ordering and Proton Transport in Phosphonic Acid–Based Polymer Electrolyte Membranes

## TL;DR

This study shows that longer alkylene spacers in polymer membranes improve proton transport and structural order, leading to better performance in high-temperature fuel cells.

## Contribution

The novel use of eight-carbon alkylene spacers in phosphonic acid-based polymers enhances proton conductivity under low humidity and high temperature.

## Key findings

- soPA membranes achieved 4.4 and 7.5 mS cm–1 conductivity at 120 °C under 20% and 40% RH, respectively.
- The longer alkylene spacers in soPA promote nanophase separation and greater molecular ordering.
- The flexibility of longer spacers allows more efficient proton transfer between phosphonic acid groups.

## Abstract

Next-generation polymer electrolyte fuel cells (PEFCs)
require
polymer electrolyte membranes (PEMs) capable of operating at temperatures
above the boiling point of water (100 °C) and under low-humidity
below 40% RH. In this study, we synthesized poly­(8-(p-styryl)-1-octanephosphonic acid) (soPA), a polymer bearing phosphonic
acid groups on the side chains connected via eight-carbon alkylene
spacers. soPA was insoluble in water and formed a highly oriented
lamellar phase-separated nanostructure with a 2.9 nm domain spacing,
consisting of a hydrophobic phase formed by the alkylene spacers and
the polystyrene backbone, and a hydrophilic phase containing the phosphonic
acid groups. Despite its lower acid group density compared with poly­(4-(p-styryl)-1-butanephosphonic acid) (sbPA) with shorter four-carbon
spacers, the soPA membrane exhibited higher conductivities than sbPA.
For instance, soPA achieved conductivities of 4.4 and 7.5 mS cm–1 at 120 °C under 20% and 40% RH, respectively,
values which are approximately 4 and 2.9 times higher than those of
sbPA. These enhanced conductivities of soPA can be attributed to the
higher morphological and molecular ordering induced by the nanophase
separation as well as the greater freedom of motion of the phosphonic
acid groups provided by the flexibility of the longer alkylene spacers,
which facilitate more efficient proton transfer between them.