Sulfonated polyimide proton exchange membranes with graphene oxide show improved proton conductivity, methanol crossover impedance, and mechanical properties - Chen, Jyh-Chien.; Cheng, Ming-Yao.; Hwang, Bing-Joe.; Kao, Kuei-Yu.; Rick, John.; Shen, Wei-Chung.; Tseng, Chi-Yung.; Ye, Yun-Sheng.