A novel energy storage system incorporating electrically rechargeable liquid fuels as the storage medium

We propose a novel concept of energy storage that incorporates electrically rechargeable liquid fuels made of electroactive species, known as e-fuels, as the storage medium. This e-fuel energy storage system comprises an e-fuel charger and an e-fuel cell. The e-fuel charger electrically charges e-fuels, while the e-fuel cell subsequently generates electricity using charged e-fuels whenever and wherever on demand. The e-fuel energy storage system possesses all the advantages of conventional hydrogen storage systems, but unlike hydrogen, liquid e-fuels are as easy and safe to store and transport as gasoline. The potential e-fuel candidates have been identified to include inorganic electroactive materials, organic electroactive materials, and suspension of solid electroactive materials. In this work, we demonstrate an example e-fuel energy storage system for large-scale energy storage using inorganic e-fuels composed of V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples, and compare the performance of the e-fuel energy storage system with that of existing technologies. Results show that our e-fuel charger achieves a charge efficiency of as high as ∼94%, while the e-fuel cell is capable of delivering a peak power density of 3.4 W cm^-2, which is 1.7 times higher than that of hydrogen fuel cells. More excitingly, the e-fuel energy storage system exhibits a round-trip efficiency of 80.0% and an electrolyte utilization of 83.0% at an ultra-high discharge current density of 1,000 mA cm^-2, which are 19.9% and 67.3% higher than those of conventional vanadium redox flow batteries. This unprecedented performance allows a 27.0% reduction in the capital cost of the e-fuel energy storage system compared with that of vanadium redox flow batteries.