Power and energy capacity tradeoffs in an all-aqueous copper thermally regenerative ammonia battery

Thermally regenerative ammonia batteries (TRABs) can provide energy storage and produce electrical power from low-grade waste heat instead of electricity. The use of all-aqueous copper-based electrolytes has recently produced higher power densities than those achieved using previous TRAB approaches based on reversible metal deposition and dissolution processes, but further gains are possible in power and energy density. We investigated the limitations of power and energy density and how they are impacted by the electrolyte composition and discharge currents. By increasing the ammonia concentration from 1 to 5 M, the power density of the battery increased from 11.2 to 28.5 mW cm⁻², but the energy density decreased from 0.56 to 0.31 Wh L⁻¹. Increasing discharge current densities from 4 to 12.5 mA cm⁻² increased the average power density during discharge from 2.4 to 5.9 mW cm⁻² without appreciable losses in energy density. Increasing the copper concentration from 0.1 to 0.5 M increased both energy density to 2.15 Wh L⁻¹ and energy efficiency to 2.2% but did not substantially impact the power density. These results represent the highest performance metrics achieved for a low-grade waste heat to electricity system.