Correlating Microstructure and Activity for Polysulfide Reduction and Oxidation at WS₂Electrocatalysts

Abstract

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.

Understanding Heterogeneous Electrocatalysis of Lithium Polysulfide Redox on Pt and WS2Surfaces

“…It is widely known that the unsaturated coordinated planes (active sites) and d-band characteristic present in transition metal/dichalcogenides are responsible for catalytic activity and is proved in several electrochemical reactions. Similarly, the aqueous PS redox process is activated by using catalytic surface in solar cell , and flow battery − applications which support the surface involved electrocatalysis of sulfur redox process in the Li–S cell.…”

Electrocatalysis of Lithium Polysulfides: Current Collectors as Electrodes in Li/S Battery Configuration

“…It is well known that the insulating nature of polysulfides causes poor reaction kinetics and hence influences overall redox process. On the other hand, use of electrocatalytic electrodes has found to enhance the reaction kinetics of aqueous polysulfides in photoelectrochemical solar cells 30 31 32 33 34 and redox flow cells 35 36 37 38 39 . However, to the best of our knowledge, there have been no reports on utilizing electrocatalysis concept in non-aqueous polysulfides redox reactions.…”

Hydrophilic tannic acid-modified WS2 nanosheets for enhanced polysulfide conversion in aqueous media

“…The specific surface area of ta-WS 2 was 75.5 m 2 g −1 , while the pristine WS 2 merely possessed a surface area of 15.6 m 2 g −1 . Because the catalyst activity was directly proportional to the number of edge planes at the catalyst surface for polysulfide redox reactions [38], the ta-WS 2 with the higher specific surface area accompanied more active sites or edge planes, which greatly improved the catalytic performance in the polysulfide system. Also, the smaller size of ta-WS 2 is beneficial to form effective electron pathways from conductive framework to the electrocatalyst surface, which results in the lower charge transfer resistance and ensures the higher utilisation of polysulfides.…”

“…These investigations have shown that the surface chemistry of WS 2 is closely related toward high catalytic activity or high ion storage performance. More importantly, different forms of WS 2 as the electrocatalysts for polysulfide reduction and oxidation were studied, which reported that the catalyst activity was directly proportional to the number of edge planes at the catalyst surface [38]. Furthermore, transition metal dichalcogenides, owing to their strong chemical affinity with polar polysulfide species, are found to play an anchoring effect in the organic electrolyte-based lithium-sulfur batteries, which can introduce intimate contact with polysulfides and thereby facilitate the polysulfide redox reactivity [26,39,40].…”