Phosphoric Acid Doped Melamine-Cross-Linked Sulfonated Poly(Ether Ether Ketone) for Direct Methanol Fuel Cells

The key to achieving high power density and a long operational lifetime in direct methanol fuel cells (DMFCs) lies in the development of proton exchange membranes exhibiting both high proton conductivity and superior chemical stability. In this study, sulfonated poly(ether ether ketones) (SPEEKs) were modified using various ratios of melamine (Mel), followed by dipping in phosphoric acid (PA), to yield SPEEK-Melx/PA membranes. The primary advantage of this approach is that the alkaline sites on melamine can immobilize PA molecules through acid–base interactions, while the resulting ionic cross-links enhance both thermal and mechanical stability. Furthermore, the proton conductivity at the acidic sites is increased due to acid–base pairing, forming an effective pathway for proton migration. Notably, the SPEEK-Melx/PA membranes demonstrated a mechanical strength exceeding 28.4 MPa and an elongation at break of over 200% under hydrated conditions, along with improved thermal stability compared to unmodified SPEEK. A maximum proton conductivity of 259.01 mS cm–1 was achieved. In DMFC testing, the membrane exhibited a peak power density of 69.74 mW cm–2 and retained 94.6% of its initial voltage after a 100-h durability assessment. These results surpassed the performance of both Nafion and unmodified SPEEK membranes. The findings suggest that melamine-cross-linked SPEEK membranes exhibit significant potential for practical application in DMFC systems.