Long Alkylene Spacers Promote Structural Ordering and Proton Transport in Phosphonic Acid-Based Polymer Electrolyte...

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Science

Background:

• Next-generation polymer electrolyte fuel cells (PEFCs) demand polymer electrolyte membranes (PEMs) for high-temperature (>100 °C) and low-humidity (<40% RH) operation.
• Current PEMs face limitations in performance under these demanding conditions, hindering fuel cell advancements.

Purpose of the Study:

• To synthesize and characterize a novel phosphonic acid-containing polymer, poly-(8-(p-styryl)-1-octanephosphonic acid) (soPA).
• To evaluate the performance of soPA-based membranes for high-temperature and low-humidity PEFC applications.
• To understand the structure-property relationships governing proton conductivity in soPA.

Main Methods:

• Synthesis of soPA polymer with phosphonic acid groups on side chains via eight-carbon alkylene spacers.
• Characterization of soPA's nanostructure, including phase separation and domain spacing (2.9 nm).
• Measurement of proton conductivity of soPA membranes at elevated temperatures (120 °C) and low relative humidity (20-40% RH).

Main Results:

• soPA formed a highly oriented lamellar nanostructure with distinct hydrophobic and hydrophilic phases.
• Despite lower acid group density, soPA membranes exhibited significantly higher proton conductivities than those with shorter spacers (sbPA).
• soPA achieved conductivities of 4.4 and 7.5 mS cm^-1 at 120 °C under 20% and 40% RH, respectively, outperforming sbPA.

Conclusions:

• The enhanced performance of soPA is attributed to superior morphological and molecular ordering from nanophase separation.
• Increased flexibility and freedom of motion of phosphonic acid groups due to longer alkylene spacers facilitate efficient proton transfer.
• soPA represents a promising material for developing advanced PEMs for high-performance fuel cells operating under challenging conditions.