Recent Progresses and Challenges to Determine Properties of Sulfonated Polyether Ether Ketone Based Electrolytes for Direct Methanol Fuel Cell Applications

This review focuses on fillers, modifications, and methods used in the preparation and development of sulfonated poly(ether ether ketone) (sPEEK) membranes, specifically for direct methanol fuel cell (DMFC) applications as proton exchange membranes in recent years. The primary objective is to evaluate recent advancements by emphasizing key characteristics such as water uptake and swelling capacity, ionic conductivity, methanol permeability, and single cell polarization tests. Additionally, the review aims to provide insights for future researchers by discussing the preparation processes of electrolytes. It presents basic characterizations of membrane electrolytes, including evaluations of the sulfonation degree and ion exchange capacities of sPEEK. High performance of membrane electrolytes is essential for commercialization and to compete with established membranes like Nafion®, which has a perfluorosulfonic acid structure. Therefore, the review also covers detailed characterization methods for assessing long-term stability when available in the related studies. Numerical results and indicators are categorized and tabulated for easy interpretation and comparative analysis.