Main-chain densely sulfonated poly(phenylquinoxaline) PEMs: Achieving high power density in DMFCs via microphase methodology

Abstract

A series of sulfonic acid groups densely on main chain in random (R-SPPQs) and multiblock (MB-SPPQs) form polyphenylquinoline proton exchange membranes (PEMs) with acid-base ion crosslinking were synthesized. The relationship between membrane selectivity of proton conductivity (σ)/methanol permeability (P_MeOH) and microstructure is investigated. R-SPPQs PEMs benefit from acid-base ion crosslinking. Ion exchange capacity (IEC) changing from 1.73 to 2.40 meq g⁻¹, P_MeOH increases from 1.89 × 10⁻⁷ to 3.15 × 10⁻⁷ cm² s⁻¹, approximately 1/10 that of NR212. R-SPPQ-5 shows the highest relative membrane selectivity (RS) of 5.5. Its maximum power density (W_max) of 100 mW cm⁻² exceeds that of NR212 (70 mW cm⁻²). Low repetitive structural units of MB-SPPQ-0505 with unconnected hydrophilic microphase-separation structure exhibit promoted σ of 103 mS cm−1 at 60 °C, outperforming R-SPPQ-4 with similar IEC by 45 %. The P_MeOH value of 4.7 × 10⁻⁷ cm² s⁻¹ represents a 66 % increase over that of R-SPPQ-4, with a slight decrease in RS to 4.4. Its W_max is 103 mW cm⁻². This stems from the combination of low internal resistance and low P_MeOH. There's no significant attenuation in battery performance in the 50-h durability test, indicating promising potential application prospects in direct methanol fuel cells. Increased repetitive structural units of MB-SPPQ-1010 with interconnected microphase-separated structure, resulting in a further 17 % increase in σ than MB-SPPQ-0505 at 60 °C. Whereas P_MeOH is elevated to 10.4 × 10⁻⁷ cm² s⁻¹, resulting in a decrease in RS to 2.7. Its W_max drops to 65 mW cm⁻².