Design and development of nucleobase modified sulfonated poly(ether ether ketone) membranes for high-performance direct methanol fuel cells

Abstract

The development of low-cost and high-performance proton exchange membranes (PEMs) for direct methanol fuel cell (DMFC) applications is a promising solution to the energy shortage. In this work, a series of sulfonated poly(ether ether ketone)s (SPEEKs) modified with two nucleobases, adenine (A) and cytosine (C) with different degrees of substitution, SPEEK-A_x and SPEEK-C_x were successfully synthesized. The basic sites of A and C can immobilize phosphoric acid (PA) molecules through acid–base interaction to fabricate PA-doped SPEEK membranes. In addition, the formation of ionic crosslinking caused by basic sites enhances the thermal stability and mechanical strength of the PA-doped membranes. As a result, the SPEEK-A_x/PA and SPEEK-C_x/PA membranes possess satisfactory strength (>23.5 MPa) even if their elongation at break remained at a very high level (>200%) in the hydrated state. These PA-doped membranes exhibited ultra-high proton conductivities (all beyond 220 mS cm⁻¹) at 80 °C, while the conductivity of SPEEK was only 43.8 mS cm⁻¹ with similar IEC values. This may be attributed to the acid–base pairs between the acidic sites (–SO₃H and H₂PO₄⁻) and basic sites, which can serve as efficient transportation paths for proton migration. Furthermore, the ionic crosslinked structure decreased the methanol crossover of the membranes. The SPEEK-C₅₀/PA membrane possesses outstanding DMFC performance with a maximum power density of 141.7 mW cm⁻² when fed with 2 M methanol at 80 °C, more than twice as much as that of SPEEK (67.2 mW cm⁻²) and Nafion 115 (64.1 mW cm⁻²), which outperforms the majority of previously reported Nafion and SPEEK membranes. Therefore, the prepared nucleobase modified PEMs showed promising potential for high-performance DMFC application.