Correlating Microstructure and Activity for Polysulfide Reduction and Oxidation at WS2Electrocatalysts

The polysulfide reduction and oxidation activity of WS2 electrocatalysts was studied. This was undertaken with a specific view to improve the efficiency of the polysulfide-bromine redox flow battery, for large scale energy storage. Using data from the literature, it is estimated that the catalysts would require exchange current densities of ∼0.7 to 1 mA cm−2 true microscopic surface area to enable them to become economically viable. The experiments were performed upon three different forms of WS2: platelet like particles of the 2H polytype of WS2 (2H-WS2), ball milled WS2 and inorganic fullerene-like WS2 (IF-WS2). The catalysts were characterized ex-situ using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and N2 gas adsorption methods. Electrochemical measurements were performed at 35°C, in aqueous solutions of 1.8 M Na2S2.47 and 1 M NaOH, simulating the operating conditions of a half-charged polysulfide-bromine redox flow battery. The catalyst activity increased in the following order: IF-WS2 < 2H-WS2 < ball milled WS2. We attempt to rationalize this trend on the basis of the morphological features observed ex-situ. The exchange current density of the ball milled WS2, at ∼0.012 mA cm−2, falls short of the estimated target by a factor of ∼60 to 70.