A bimetallic thermally regenerative ammonia-based battery for high power density and efficiently harvesting low-grade thermal energy

Abstract

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.