Flow battery is a promising energy storage technology for facilitating utilization of renewable resources. While new types of flow batteries have been explored toward high energy density, hampering the power density due to high electrolyte viscosity and sluggish reaction kinetics. Discovery of an aqueous electrolyte with multi-electron transfer reaction is thus favorable for both high energy and power densities due to its multiple charge stored at the same concentration. Both criteria are crucial to improve the flexibility of cell design and widen the application potential. Herein, bismuth is pioneered as negative electrolyte (negolyte) for hybrid flow battery owing to its three-electron reaction and the significantly increased solubility in methanesulfonic acid. In conjunction with cerium electrolyte, a volumetric energy density of 90 Wh L −1 is achieved and simultaneously a high power density of 295 mW cm −2 at 90% state-of-charge is demonstrated using low-cost carbon electrode. Furthermore, a high volumetric capacity of 120 Ah L −1 is reached via adopting graphite felt, which is 100% of the theoretical specific capacity of 1.5 M bismuth negolyte.
A bimetallic thermally regenerative ammonia-based battery (B-TRAB) using inexpensive materials efficiently converts low-grade thermal energy into electricity with high power density. Through thermal regeneration of the anolyte, the B-TRAB system realized high-voltage discharge (1.38 V) with redox couples [Cu(II)/Cu and Zn(NH3)42+/Zn] and low-voltage charge (0.72 V) with Zn(II)/Zn and Cu(NH3)42+/Cu. Adding 2 M NH3 to the anolyte of a Cu/Zn-TRAB produced a peak discharge power density of 525 W m−2-electrode (120 W m−2-membrane), and the voltage (or current) and peak power could be doubled by connecting two cells in series (or parallel). In successive regeneration cycles, a maximum power density of 115 W m−2-electrode (26 W m−2-membrane) and a net energy density of 714 W h m−3 were produced at a constant current density of 100 A m−2. The power density was enhanced to 515 W m−2-electrode (118 W m−2-membrane) and maintained in successive cycles by adding acid to the regenerated catholytes, with a net energy density of 299 W h m−3 (discharged at a constant load of 12 Ω and charged at a constant current density of 100 A m−2). A thermal-electricity conversion efficiency of 0.95% (10.7% relative to the Carnot efficiency) was achieved.
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