A thermally regenerative ammonia-based battery for efficient harvesting of low-grade thermal energy as electrical power

Abstract

Thermal energy was shown to be efficiently converted into electrical power in a thermally regenerative ammonia-based battery (TRAB) using copper-based redox couples [Cu(NH 3 ) 4 2+ /Cu and Cu(II)/Cu].Ammonia addition to the anolyte (2 M ammonia in a copper-nitrate electrolyte) of a single TRAB cell produced a maximum power density of 115 AE 1 W m À2 (based on projected area of a single copper mesh electrode), with an energy density of 453 W h m À3 (normalized to the total electrolyte volume, under maximum power production conditions). Adding a second cell doubled both the voltage and maximum power. Increasing the anolyte ammonia concentration to 3 M further improved the maximum power density to 136 AE 3 W m À2 . Volatilization of ammonia from the spent anolyte by heating (simulating distillation), and re-addition of this ammonia to the spent catholyte chamber with subsequent operation of this chamber as the anode (to regenerate copper on the other electrode), produced a maximum power density of 60 AE 3 W m À2 , with an average discharge energy efficiency of $29%(electrical energy captured versus chemical energy in the starting solutions). Power was restored to 126 AE 5 W m À2 through acid addition to the regenerated catholyte to decrease pH and dissolve Cu(OH) 2 precipitates, suggesting that an inexpensive acid or a waste acid could be used to improve performance.These results demonstrated that TRABs using ammonia-based electrolytes and inexpensive copper electrodes can provide a practical method for efficient conversion of low-grade thermal energy into electricity. Broader contextThe utilization of waste heat for power production would enable additional electricity generation without any additional consumption of fossil fuels. Thermally regenerative batteries (TRBs) allow a carbon neutral approach for the storage and conversion of waste heat into electrical power, with potentially lower costs than solid-state devices. Here we present a highly efficient, inexpensive, and scalable ammonia-based TRB (TRAB) where electrical current is produced from the formation of copper ammonia complex. The ammonia can then be captured and concentrated by distillation of the anolyte, allowing recharge of the system. The voltage created by ammonia addition in the anolyte results in copper deposition onto the cathode, and loss of copper from the anode. However, by reversing the function of electrodes in the next cycle, there is no net loss of copper. With a 3 M anolyte ammonia, a TRAB produced the highest power density ever obtained for an aqueous-based, thermoelectrochemical system, of 136 AE 3 W m À2 . This power density was substantially higher than those produced using salinity gradient energy technologies based on generating salty and less-salty solutions using waste heat. This TRAB technology therefore represents a new and promising approach for efficient harvesting of low-grade waste heat as electrical power.

Electrical power production from low-grade waste heat using a thermally regenerative ethylenediamine battery

“…This alternating cycle of electrode dissolution/deposition allows the Cu electrodes to be operated in closed loop cycles as long as the mass lost and gained on the electrodes is balanced in each cycle. While the cathodic deposition of copper is efficiently regained from the current (i.e., the increase in mass of the electrode is equal to the expected value based on the number of electrons transferred), the conversion of anode copper into current of TRABs is only 35% (i.e., approximately three times as much copper dissolves from the electrode as would be predicted based on the numbers of electrons transferred) [17][18][19]. This low conversion of anode copper into current (i.e., anodic columbic efficiency, ACE) of TRAB would limit its use in closed-loop cycles as there would be a net loss of anode copper into solution for each cycle, eventually requiring the electrodes to be replaced.…”

“…The cells used for all tests were constructed as previously described for TRABs [17,22]. The cells consisted of a cathode and an anode chamber, each 4 cm long and 3 cm in diameter, separated by an anion exchange membrane (AEM; Selemion AMV, Asashi Glass, Japan) with a projected surface area of 7 cm 2 .…”

“…To monitor the electrode potentials, two Ag/AgCl reference electrodes (þ0.211 V vs. SHE; RE-5B; BASi) were inserted 1 cm a way from each electrode (2 cm away from the membrane). To facilitate the mass transfer of ions to the electrode, the catholyte was mixed using a magnetic stirrer (6.4 x 15.9 mm; VWR) at 600 rpm [17]. The electrolytes were prepared by dissolving 0.1 M of Cu(NO 3 ) 2 (Sigma-Aldrich) in deionized water with 5 M NH 4 NO 3 as the supporting electrolyte to increase conductivity.…”

“…An alternative approach to convert waste heat to electricity, called a thermally regenerative battery (TRB) based on using ammonia (TRAB), was recently shown to be capable of producing a significantly higher power density of ~80 W m -2 -electrode area, with a Carnot thermal-electric conversion efficiency (6.2%) that was greater than previous systems [17]. Unlike TESs which rely on reversible redox couples, TRBs operate using chemical potentials obtained by adding ligands into a metal salt solution [17][18][19].…”

“…Unlike TESs which rely on reversible redox couples, TRBs operate using chemical potentials obtained by adding ligands into a metal salt solution [17][18][19]. In a TRAB, copper ammine complexes are produced when ammonia is used as the ligand in the anolyte, but not in the catholyte.…”