Polysulfide is a promising redox couple employed in various aqueous redox flow batteries (RFB) for large scale electrochemical energy storage due to its high solubility and low cost. However, polysulfide has much lower rate constant in comparison with other commonly used redox couples in RFB. Therefore, efficient electrocatalysis is a key to achieving high performance for polysulfide-based RFB. Here we first systematically investigated the electrocatalytic activity of various pyrite phase metal disulfides (FeS₂, CoS₂, NiS₂) as thin film electrodes towards polysulfide redox reactions under steady state condition. Then we further demonstrated a high-performance polysulfide/ferrocyanide RFB using optimized high surface area NiS₂ nanostructures grown on carbon paper as the electrode. The NiS₂/carbon paper electrode showed a high exchange current density of 9.3 mA cm^-2 and was able to achieve a current density of 38.6 mA cm^-2 at an over potential of 100 mV. An aqueous polysulfide/ferrocyanide RFB employing NiS₂/carbon paper as anode and plain carbon paper as cathode was further evaluated by constant current cycling test and electrochemical impedance spectroscopy to demonstrate high charging-discharging cycling performance and low charge transfer impedance.