Decoupling Redox Actives Solubility and Energy Density in Redox Flow Batteries Using Supersaturated, Phase-change Electrolytes

Abstract

Redox flow batteries (RFBs), with their characteristic decoupling of power density and energy capacity, are scalable energy storage systems that can help smooth out the intermittency of solar and wind power on the electric grid. Amongst the many different redox couples considered in RFBs, the cerium (Ce) redox couple, Ce(IV)/Ce(III), is particularly attractive due to its high standard potential (1.61 V vs. standard hydrogen electrode, SHE) and the relative abundance of Ce in the Earth crust (as abundant as lead). Ce-based RFBs such as the Zn-Ce system have been scaled-up and commercialized but meeting ever lower energy storage cost targets (<$50/kWh) requires significantly higher Ce electrolyte energy density. Herein, in a departure from previous studies which utilize acid-supported Ce electrolyte solutions (e.g., sulfuric acid), we have developed a new Ce electrolyte with ammonium sulfate (AS) working as the supporting electrolyte.The solubility of Ce(IV) was enhanced to 1.23 M by optimizing the ratio between Ce salt and AS. This ~40% increase in energy density was found to be a function of the solution chemistry with complementary effects of Ce(IV) hydrolysis with water and complexation with anions. This costeffective Ce electrolyte was paired with the titanium (Ti) redox couple to demonstrate a high energy density, low-cost Ce-based RFB.