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

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 degrees 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-C-50/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 degrees 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.

Automated summary

In this study, low-cost and high-performance proton exchange membranes (PEMs) modified with two nucleobases were successfully synthesized. The membranes showed enhanced thermal stability, mechanical strength and proton conductivity due to acid-base interaction and ionic crosslinking. They also exhibited suppressed methanol crossover, leading to outstanding performance in direct methanol fuel cell (DMFC) applications.