Highly sulfonated poly ether ether ketone chelated with Cu²⁺ as a proton exchange membrane at sub-zero temperatures

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Science

Background:

• Improving proton conductivity of proton exchange membranes (PEMs) at low temperatures is crucial for expanding fuel cell applications.
• Sulfonated poly ether ether ketone (SPEEK) membranes offer high proton conductivity but suffer from poor mechanical strength and high swelling.
• Low temperatures significantly impede the performance of conventional PEMs due to reduced ion mobility and water freezing.

Purpose of the Study:

• To develop novel SPEEK-based membranes with enhanced low-temperature proton conductivity and mechanical stability.
• To investigate the effect of copper ion (Cu2+) coordination on the properties of SPEEK membranes.
• To evaluate the potential of these modified membranes for sub-zero fuel cell applications.

Main Methods:

• Preparation of SPEEK membranes with varying sulfonation degrees.
• Fabrication of Cu2+-coordinated SPEEK (SPEEK-Cu) membranes via chelation.
• Characterization of membrane properties including ion exchange capacity, water uptake, mechanical strength, and proton conductivity.
• Quantification of water states using low-temperature differential scanning calorimetry (LT-DSC).
• Performance evaluation in fuel cells at sub-zero temperatures.

Main Results:

• Highly sulfonated SPEEK membranes exhibited high proton conductivity (0.074 S/cm at -25°C) but poor mechanical properties.
• SPEEK-Cu membranes retained high -SO3H content, showing improved mechanical strength and dimensional stability compared to pristine SPEEK.
• SPEEK-Cu membranes achieved a proton conductivity of 0.054 S/cm at -25°C and a maximum fuel cell power of 0.42 W/cm² at -10°C.
• LT-DSC analysis revealed that bound water in SPEEK-Cu membranes contributes to anti-freezing performance, retarding performance decay at sub-zero temperatures.

Conclusions:

• Copper ion coordination effectively enhances the mechanical properties and dimensional stability of SPEEK membranes while maintaining good proton conductivity.
• The SPEEK-Cu membranes demonstrate superior low-temperature performance and anti-freezing characteristics, outperforming other reported materials.
• Composite membranes like Cu2+-SPEEK hold significant promise for practical sub-zero fuel cell applications.