High Power Thermally Regenerative Ammonia-Copper Redox Flow Battery Enabled by a Zero Gap Cell Design, Low-Resistant Membranes, and Electrode Coatings

Thermally regenerative batteries (TRBs) is an emerging platform for extracting electrical energy from low-grade waste heat (T < 130 °C). TRBs using an ammonia-copper redox couple can store waste-heat energy in a chemical form that can be later discharged to electrical energy upon demand. Previous thermally regenerative ammonia battery (TRAB) demonstrations suffered from poor heat to electrical energy conversion efficiency when benchmarked against thermoelectric generators (TEGs). In this work, we report the highest power density to date for a TRAB (280 W m^–2 at 55 °C) with a 5.7× improvement in power density over conventional TRAB designs. Notably, the TRAB was configured similar to a redox flow battery setup, which is termed here an ammonia flow battery (AFB). The substantial improvement in the AFB power density translated to thermal efficiency (η_th) values as high as 2.99% and 37.9% relative to the Carnot efficiency (η_th/C). These values correspond to an 87.6% improvement in η_th value over conventional TRAB designs and the highest reported η_th/C for low-grade waste heat recovery using TRABs. The excellent performance of the AFB was ascribed to a zero gap design, deploying a low-resistant, inexpensive anion exchange membrane (AEM), and implementing a copper ion selective ionomer coating on the copper mesh electrodes. The high-power AFB in this report represents a significant milestone in harvesting low-grade waste heat.