High methanol tolerant proton exchange membranes based on novel coupling-type sulfonated poly(phenylquinoxaline) for direct methanol fuel cells

Coupling-type sulfonated poly(phenylquinoxaline) (c-SPPQ) bearing both side chain and main chain sulfonic acid groups were synthesized through the post-sulfonation process. The proton exchange membranes prepared with c-SPPQ showed good performance, such as low methanol permeability, high methanol tolerance and good mechanical properties due to the acid-base interaction between the sulfonic acid and phenylquinoxaline groups. The dimensional change of c-SPPQ-3 in the length direction was 33% at 30 °C in 45 wt% methanol solution, demonstrating the stability of membrane electrode assembly against shearing force. In contrast, NR212experienced significant swelling. The methanol flux of c-SPPQ-3 was 45 mg cm⁻² h⁻¹ for at 30 °C with a methonal concentration of 10 wt%. This was almost 10 times lower than that of NR212. With a 20 wt% methanol feed concentration at 75 °C, the maximum power density of c-SPPQ-3 was 88 mW cm⁻², 1.8 times higher than that for NR212 (48 mW cm⁻²). This was attributed to the reasonably high proton conductivity and low methanol permeability of c-SPPQ-3. When the methanol feed concentration was further increased to 50 wt%, the maximum power density of c-SPPQ-3 slightly decreased to 60 mW cm⁻², while that of NR212 was negligible. With such superior performance, c-SPPQ shows excellent promise for meeting the requirements of direct methanol fuel cells.