Decoupling the solubility of redox active species and the energy density of 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. Among 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 Earth's 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 kW h⁻¹) requires significantly higher Ce electrolyte energy density. Herein, in a departure from previous studies that utilize acid-supported Ce electrolyte solutions (e.g., sulfuric acid), we have developed a new phase-change Ce electrolyte with ammonium sulfate (AS) as the supporting electrolyte. The solubility of Ce(IV) was enhanced to 1.23 M by optimizing the ratio between the Ce salt and AS. This ∼40% increase in energy density was found to be a function of solution chemistry, with complementary effects of Ce(IV) hydrolysis with water and complexation with anions. This cost-effective Ce electrolyte was paired with the titanium (Ti) redox couple to demonstrate a high-energy-density, low-cost Ce-based RFB. During cycling, part of the Ce was precipitated to form a slurry in the electrolyte. This slurry was succesfully cycled and reoxidized to the more soluble Ce(IV) form. 20 cycles (75 minutes per cycle) of the phase-change Ti–Ce RFB was demonstrated with a final energy efficiency > 70%.