Ladder-type sulfonated poly(arylene perfluoroalkylene)s for high performance proton exchange membrane fuel cells

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Chemistry

Background:

• Proton exchange membranes (PEMs) are critical components in fuel cells.
• Developing PEMs with superior proton conductivity, mechanical stability, and durability is essential for efficient fuel cell operation.
• Existing PEMs often face limitations in performance under varying humidity and temperature conditions.

Purpose of the Study:

• To synthesize novel sulfonated poly(arylene perfluoroalkylene)s with a sulfone-bonded ladder structure (SPAF-P-Lad) for advanced PEM applications.
• To investigate the physicochemical properties, including solubility, molecular weight, ion exchange capacity (IEC), proton conductivity, and mechanical stability, of the synthesized SPAF-P-Lad membranes.
• To evaluate the fuel cell performance and long-term durability of the SPAF-P-Lad membranes, particularly under challenging operating conditions like low humidity.

Main Methods:

• Synthesis of SPAF-P-Lad membranes via sulfonation of precursor SPAF-P polymers using oleum.
• Characterization of membrane properties: solubility, molecular weight (Mn, Mw), IEC, glass transition temperature (Tg), Young's modulus, and yield stress.
• Proton conductivity measurements and comparison with precursor membranes.
• Fuel cell performance testing, including current-voltage characteristics under low humidity with air.
• Long-term durability assessment using an open circuit voltage (OCV) hold test for 1000 hours.

Main Results:

• SPAF-P-Lad membranes exhibited excellent solubility and high molecular weights (Mn: 145.4-162.9 kDa, Mw: 356.9-399.1 kDa), enabling the formation of bendable membranes with IECs of 1.76-2.01 meq. g-1.
• These membranes showed significantly higher proton conductivity compared to precursor SPAF-P membranes due to enhanced water affinity.
• SPAF-P-Lad membranes demonstrated superior mechanical stability and tensile properties (Young's modulus: 0.51-0.59 GPa, yield stress: 23.9-29.6 MPa) compared to SPAF-P membranes (Tg: 72-90 °C, Young's modulus: 0.08-0.42 GPa, yield stress: 5.7-15.1 MPa).
• The SPAF-mP-Lad membrane achieved a higher current density (0.56 A cm-2 at 0.5 V) under low humidity with air than the SPAF-pP membrane (0.46 A cm-2).
• The SPAF-mP-Lad membrane maintained stability during a 1000-hour OCV hold test, with minimal voltage decay (70 μV h-1).

Conclusions:

• The novel SPAF-P-Lad membranes possess a promising combination of high proton conductivity, excellent mechanical strength, and good durability.
• These ladder-structured polymers represent a significant advancement in PEM technology for fuel cells, particularly for operation under demanding conditions.
• The observed minor degradation under accelerated testing suggests high potential for long-term application in fuel cells.