Multi-block sulfonated poly(arylene ether nitrile) polymers bearing oligomeric benzotriazole pendants with exceptionally high H2/O2 fuel cell performance

Abstract

Aromatic ionomers have emerged as promising alternatives to perfluorosulfonic acid polymers to be applied as proton exchange membranes in fuel cells. However, the paradox between the ion conductivity and stability is still a challenge precluding the commercialization of aromatic ionomers. In this paper, we report the design of a novel sulfonated poly(arylene ether nitrile) multi-block structure by introducing a key component, alkyl benzotriazole (Bt) side chains, into the hydrophobic segments. The modified structure could facilitate polymer phase-separation and generate self-standing films with excellent mechanical properties, and it effectively suppresses the excessive swelling of the membrane owing to strong electrostatic interactions between the Bt chains and sulfonic acid groups. Moreover, the Bt unit could act as both a proton acceptor and proton donor, causing a dramatic increase in the ion conductivity of the membrane. The most optimal membrane possesses an ion-exchange capacity of 2.15 meq g−1 and exhibits a weaker relative humidity (RH) dependence and higher proton conductivity than the commercial Nafion 212 over the entire RH range. Remarkably, the maximum power output of the fuel cell based on the most optimal membrane reaches 1090, 856, and 451 mW cm−2 at 95%, 70%, and 30% RH, respectively, which are more than 2 times higher than those of the non-Bt analogue. Further, the current densities (I0.6) ranging up to 1500 and 1000 mA cm−2 (0.6 V) at 95% and 70% RH are both much higher than those of the Nafion. Our study provides a novel methodology for the design of aromatic ionomer structures with excellent performances for practical fuel cell application.