An ideal proton exchange membrane for fuel-cell application should possess excellent dimensional stability, high ionic conductivity and inexpensiveness. Introducing cross-links into the polymer structures is a popular strategy to alleviate the tradeoff between swelling ratio and ionic conductivity of high ion exchange capacity proton exchange membranes. However, the cross-links in these membranes will block the transport of H+, resulting in unfavorable decrease of conductivity. Here we report a systematic study of acSPEEK-S-x membranes with alkyl chain attached ionic cross-links to demonstrate how the unique cross-links can improve both the conductivity and swelling behavior of the materials. Compared to the conventional SPEEK, in addition to high anti-oxidative stability, the ionic conductivity and swelling behavior of the acSPEEK-Sx samples are significantly improved. A high proton conductivity of 101.2 mS cm(-1) and a low swelling degree of 15.0% are observed in the acSPEEK-S-6 membrane at the temperature of 80 degrees C. Based on the high-performance acSPEEK-S6 sample, a peak power density of 750 mW cm(-2) is achieved at 60 degrees C, which is similar or superior to those of Nafion 112 (710 mW cm(-2)) under the same measurement conditions. (c) 2018 Elsevier Ltd. All rights reserved.