Due to their preferential cation transport, dense cation exchange membranes like Nafion membranes are unsuitable for Zn/Br redox flow batteries which require bi-ionic transport of Zn2+ and Br- ions. This work shows that scaling the water cluster size of Nafion membranes by a pre-hydration treatment can achieve not only a high ionic conductivity but also a bi-ionic transport property. Small angle X-Ray scattering, diffusion cell, and electrochemical analysis verify that by increasing the pre-hydration temperature, the water clusters are expanded, resulting in an increase of water uptake, ionic conductivity and the anion transference number. The bi-ionic transport and low area specific resistance induced by the pretreatment enable the successful operation of a Zn/Br redox flow battery with a NRE-212 membrane. Compared with a conventional porous membrane, the properly treated Nafion membrane has an 11.3% higher energy efficiency indicating that the dense structured ion exchange membrane can be used for Zn/Br flow batteries by scaling the water cluster size.