In this work, we present a simple and cost-effective method to form a dual-scale porous electrode by KOH activation of the fibers of carbon papers. The large pores (similar to 10 mu m), formed between carbon fibers, serve as the macroscopic pathways for high electrolyte flow rates, while the small pores (similar to 5 nm), formed on carbon fiber surfaces, act as active sites for rapid electrochemical reactions. It is shown that the Brunauer-Emmett-Teller specific surface area of the carbon paper is increased by a factor of 16 while maintaining the same hydraulic permeability as that of the original carbon paper electrode. We then apply the dual-scale electrode to a vanadium redox flow battery (VRFB) and demonstrate an energy efficiency ranging from 82% to 88% at current densities of 200-400 mA cm(-2), which is record breaking as the highest performance of VRFB in the open literature. (C) 2016 Elsevier B.V. All rights reserved.