No abstract

The crucial impact of the elevated degree of branching (DB) on improving the properties of proton exchange membranes (PEMs) has been demonstrated, yet PEMs (employing the B 3 monomer as the branching agent) with DB values exceeding 10% have not been reported. Herein, we report on the synthesis of a series of highly branched sulfonated poly(arylene ether ketone sulfone) copolymers bearing a 12.5% DB value for PEMs, namely, B-12.5-SPAEKS-x (where x represents the molar ratio of the monomer attaching the sulfoalkyl chain to the employed bisfluoride monomers). To accomplish this, a triphenol derivative was designed as the branching agent to reduce the possibility of macroscopic cross-linking during polymerization, and the flexible sulfoalkyl side chains were grafted to the polymer backbone to enhance the chain entanglement of the copolymers. Among all of the fabricated membranes, the B-12.5-SPAEKS-graft-40 membrane exhibited comparable proton conductivity (118.6 mS cm −1 ) and swelling change (20.7%) with those of Nafion 117 in a fully hydrated state at 80 °C, realizing the trade-off between the proton conductivity and dimensional stability. Furthermore, the proton conductivity of the B-12.5-SPAEKS-graft-50 membrane was evidently superior to that of Nafion 117 in both the fully hydrated state and high relative humidity conditions; therefore, a higher maximum power density of 529.5 mW cm −2 was achieved in the H 2 /air fuel cell.

Branched sulfonated polymers present considerable potential for application as proton exchange membranes, yet investigation of branched polymers containing sulfonated branched centers remains to be advanced. Herein, we report a series of polymers with ultradensely sulfonated branched centers, namely, B-x-SPAEKS, where x represents the degree of branching. In comparison with the analogous polymers bearing sulfonated branched arms, B-x-SPAEKS showed a reduced water affinity, resulting in less swelling and lower proton conductivity. The water uptake, swelling ratio (in-plane), and proton conductivity of B-10-SPAEKS at 80 °C were 52.2%, 57.7%, and 23.6% lower than their counterparts, respectively. However, further analysis revealed that B-x-SPAEKS featured significantly better proton conduction under the same water content due to the formation of larger hydrophilic clusters (∼10 nm) that promoted efficient proton transportation. B-12.5-SPAEKS exhibited a proton conductivity of 138.8 mS cm–1 and a swelling ratio (in-plane) of only 11.6% at 80 °C, both of which were superior to Nafion 117. In addition, a decent single-cell performance of B-12.5-SPAEKS was also achieved. Consequently, the decoration of sulfonic acid groups on the branched centers represents a very promising strategy, enabling outstanding proton conductivity and dimensional stability simultaneously even with low water content.