Polysulfide-Based Aqueous Redox Flow Batteries Enhanced by Carbon Electrodes with S8/Sx2– Redox Pairs and Hydrophilic Carbon Nanocuboids

Abstract

Polysulfide-based aqueous redox flow batteries (PS-ARFBs) are a viable alternative for energy storage owing to their impressive theoretical capacity, inherent safety features, low operating costs, and cost-effective design. However, the primary challenges facing PS-ARFBs are slow kinetics and limited cycle life, which significantly impede their practical applications. To overcome these obstacles, we have developed an innovative functional electrode (KB/S-HCN-2:1-CF) that integrates S₈/S_x^2– redox pairs (KB/S) with hydrophilic carbon nanocuboids (HCNs) as electrocatalysts. This design enhances the redox kinetics of polysulfides and optimizes sulfur utilization. Remarkably, the KB/S-HCN-2:1-CF electrode reduces the overpotential of a polysulfide-ferri/ferrocyanide (S–Fe) redox flow battery from 1110 to 237 mV at a current density of 40 mA cm^–2. Furthermore, an S–Fe flow cell equipped with this modified electrode demonstrates an increased initial capacity of 268.9 mAh at 40 mA cm^–2 at a lower S_x^2– concentration and an improved energy efficiency of nearly 10%. Particularly, a plausible explanation for the roles of S₈ and HCNs in promoting the reduction of polysulfides has been proposed, as confirmed by DFT methods and ex-situ UV–vis spectroscopy in polysulfide electrolytes. This study offers a promising approach to the challenges faced by PS-ARFBs, paving the way for high-capacity and long-lasting performance.