Towards the Development of a Polysulfide-Polyiodide Redox Flow Battery

As we move towards a reliance on renewable energy, low-cost energy storage systems will play a pertinent role in their integration into the existing power grid. Redox flow batteries (RFBs) are an electrochemical energy storage system that have recently received increased attention due to their separation of energy and power units, durability, and scalability. With current commercial RFBs having high chemical costs, efforts have gone into finding more cost-effective active species. In this work, we demonstrate preliminary steps in developing a polysulfide-polyiodide redox flow battery. Sodium polysulfide and sodium iodide are first characterized by cyclic voltammetry, showing a reduction potential of -0.40V and 0.48V vs Ag/AgCl respectively, giving a theoretical cell voltage of 0.88V. The species were shown to not react upon mixing and as such are appropriate for a full-cell without significant concern for separator design. The polysulfide-polyiodide system demonstrates cycling capability in constant current flow cell experiments, able to run for over 20 cycles. However, it was found that polysulfide demonstrates significant kinetic polarization. Thus, the need for a catalyst-modified electrode on the polysulfide side was made clear. Nickel polysulfide was shown to have the lowest overpotential out of the catalysts tested and is a promising catalyst for use in future full-cell testing. Additionally, graphene is tested as a catalyst for the polyiodine, showing 25% increase in capacity. Future work on the system will be performed to demonstrate a high concentration system using the catalysts demonstrated here.