We demonstrate application of a commercial nanoporous polyvinyl chloride (PVC)/silica separator in an all-vanadium redox flow battery (VRB) as a low-cost alternative to expensive Nafion membranes. This hydrophilic separator is composed of silica particles enmeshed in a PVC matrix that creates unique porous structures. These nano-scale pores with an average pore size of 45 nm and a porosity of 65% serve as ion transport channels that are critically important for flow battery operation. The VRB flow cell using the PVC/silica separator produces excellent electrochemical performance in a mixed-acid VRB system with average energy efficiency (EE) of 79% at the current density of 50 mA.cm(-2). This separator affords the VRB flow cell with excellent rate capability with its EE higher than that of Nafion membrane at current densities above 100 mA.cm(-2). With this separator, the EE of the VRB flow cell exhibits great tolerance to temperature fluctuations in the typical operational temperature range of the mixed-acid VRB system. More importantly, the flow cell using the separator demonstrates an excellent capacity retention over cycling, which enables the VRB system to operate in the long term with minimal electrolyte maintenance. (C) 2013 The Electrochemical Society.