A novel integrally thin skinned asymmetric proton exchange membrane (ITSA-PEM) is proposed to disentangle the typical conductivity-selectivity dilemma in PEMs for vanadium flow batteries (VFBs). The membrane is successfully fabricated by a porogen-leaching-out method. It consists of a porous sublayer and an ultrathin skin layer, which is defect-free verified by the high H-2/N-2 separation factor of 64.9. The degree of sulfonation (DS) of PEM is reduced to extremely low (DS = 36.3%) to suppress swelling, and numerous interconnected pores are introduced to facilitate proton transfer. Low swelling ratio and defect-free skin layer lead to undetectable vanadium permeation. Meanwhile, the area resistance of ITSA-PEM is dramatically lowered to 2.1 Omega cm(2) from 5.4 Omega cm(2) of the dense PEM. Therefore a membrane with both improved proton conductivity and ion selectivity is obtained. Low DS also equips the membrane with sufficient mechanical strength and enhanced thermal stability. The VFB assembled with ITSA-PEM displays high energy efficiencies (EE: 75.6-90.2%) over a current density of 20-80 mA cm(-2), much superior to those of Nafion 211 (EE: 55.9-73.4%). It also shows favorable stability and slow capacity decay rate during cycling test over 50 cycles. (C) 2017 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.