We have demonstrated a new ferri/ferrocyanide - polysulfide (Fe/S) flow battery, which employs less corrosive, relatively environmentally benign neutral alkali metal ferri/ferrocyanide and alkali metal polysulfides as the active redox couples. A cobalt nanoparticle -decorated graphite felt was used at the polysulfide side as the catalyst. Excellent electrochemical performance was successfully acquired in the Fe/S flow cells with high cell efficiencies (99% coulombic efficiency and similar to 74% energy efficiency) and good cycling stability over extended charge/discharge operations. The positive half-cell appears to be the performance - limiting side in the Fe/S flow battery determined by using a carbon cloth probe. The inexpensive redox materials and possibly cell part materials can lead to reduced capital cost, making the Fe/S flow battery a promising cost-effective energy storage technology candidate. (C) 2015 The Electrochemical Society. All rights reserved.