Rational design of comb-shaped poly(arylene indole piperidinium) to enhance hydroxide ion transport for H2/O2 fuel cell

Abstract To produce anion exchange membranes (AEMs) possessing high conductivity and chemical stability, we propose a ternary copolymerization strategy to prepare comb-shaped poly(arylene indole piperidinium) copolymers with different lengths of side chain by the design of alkyl-functionalized isatin comonomers in the acid-catalyzed Friedel-Crafts polycondensations of N-methyl-4-piperidone and p-terphenyl. These polymers have a characteristic feature of hydrophobic side chain separately attached onto aryl-ether free backbones to induce microphase separated morphology and stable piperidinium cation within the backbone to ensure the alkaline stability. PITP-C10Q85 membrane with C10 side chain showed the highest hydroxide conductivity of 134.5 mS/cm at 80 °C due to the formed microphase separated morphology. Alkaline stability testing in 1 M NaOH at 80 °C demonstrated that 80% of initial conductivity was retained for PITP-C10Q85 membrane after 1200 h of testing, owning to the chemical degradation of piperidinium cation in comb-shaped membranes via Hoffman elimination and nucleophilic substitution reaction. By assembling in alkaline fuel cells, PITP-C10Q85 membrane delivered a peak power density of 445 mW/cm2 at a current density of 870 mA/cm2 at 60 °C. Meanwhile, in-situ stability of PITP-C10Q85 membrane demonstrated a voltage decay rate of 3.67 mV/h over 75 h of operating at 300 mA/cm2, and post-cell analysis of the aged membrane revealed that predominant nucleophilic substitution reaction was found for the degradation of PITP-C10Q85 membrane, being different from the degradation mechanisms in the ex-situ stability tests. The above results manifested that regulating the alkyl side chains on the backbone directly is a promising strategy to achieve high performance AEMs with favorable morphology, high conductivity and alkaline fuel cell performance.