SILAR-Deposited Metal Chalcogenides Promoting Polysulfide Conversion in Sulfur-Based Redox Flow Batteries

Rechargeable flow batteries (RFBs) are leading the way as possible candidates for low-cost, long-duration energy storage applications (>10 hours), due to their unique capability to decouple power and energy. The state-of-the-art in RFB technologies is represented by the most advanced all-vanadium chemistry (VRFB), but the limited availability and cost of the raw materials hinder their diffusion. The main objective of this work is to explore innovative electrolytes for next-generation cost-effective redox flow batteries. Sulfur can fulfill the requirement of high specific capacity (1672 mAhg-1), has the lowest cost per unit charge among reactive materials (52 Ah $-1) [1], however the well-known sluggish kinetics of sulfur-based electrolytes require the use of electrocatalysts to enhance the power output. Cobalt sulfide (CoS2) is the most used catalyst for polysulfide conversion but concerns about cobalt avaibility and price volatility make it a critical material to replenish [2]. SILAR technique is a simple and scalable method to grow semiconducting thin films and in this work is used to graft metal chalcogenides on carbon felt fibres as polysulfide catalytic materials [3], [4]. In this study, to avoid degradation issues related to the most commonly used highly alkaline polysulfide solutions ([OH-] > 3M), sulfur was dissolved in KCl supporting electrolytes. To demonstrate the operability of the proposed polysulfide anolyte, a custom made electrochemical cell was used to independently investigate the electrochemical activity and stability of the electrode under real conditions. The design of the cell allows the characterization of the anolyte and catalyst directly on the carbon felt, both under static and flowing conditions, thereby avoiding characterization on usually used ideal and unrepresentative substrates of the system, such as glassy carbon electrodes. The electrochemical activity of the SILAR-deposited CuS catalyst is studied at different loadings to determine the correlation between the increase in electrochemical activity with the amount of catalyst deposited. Scanning electron microscope (SEM) images were used to demonstrate the morphology and uniformity of deposition, XRD and EDS distribution maps to evaluate phase and elemental distribution. The electrolyte stability and catalyst degradation in time were investigated through charge/discharge tests of a polysulfide/ferricyanide RFB cell. Overall, the study provides significant insights into the development of high-performance aqueous polysulfide-based batteries. Based on our findings, sulfur has great potential as a cost-effective anolyte candidate for integration into redox flow batteries. References: [1] J. Liu et al., ‘Sulfur-Based Aqueous Batteries: Electrochemistry and Strategies’, J. Am. Chem. Soc., vol. 143, no. 38, pp. 15475–15489, Sep. 2021, doi: 10.1021/jacs.1c06923. [2] M. Cheng et al., ‘Polysulfide Catalytic Materials for Fast-Kinetic Metal–Sulfur Batteries: Principles and Active Centers’, Adv. Sci., vol. 9, no. 2, p. 2102217, 2022, doi: 10.1002/advs.202102217. [3] H. Soonmin, ‘Recent Advances in the Growth and Characterizations of SILAR-Deposited Thin Films’, Appl. Sci., vol. 12, no. 16, Art. no. 16, Jan. 2022, doi: 10.3390/app12168184. [4] S. Palchoudhury, K. Ramasamy, J. Han, P. Chen, and A. Gupta, ‘Transition metal chalcogenides for next-generation energy storage’, Nanoscale Adv., vol. 5, no. 10, pp. 2724–2742, May 2023, doi: 10.1039/D2NA00944G.