Enhancing thermally regenerative battery performance by mitigating ammonia crossover

Abstract

The application of low-grade heat sources (<130 °C) for energy conversion is crucial in various industries facing rising energy demands. Thermally regenerative batteries (TRBs) have emerged as a promising solution for converting heat into electricity while also enabling energy storage. However, ammonia crossover and self-discharge considerably compromise the long-term stability and efficiency of ammonia-based TRBs. In this study, a buffer chamber is introduced to mitigate ammonia crossover, improve system stability, and extend the discharge duration. Experimental results demonstrate that the buffer system effectively reduces ammonia permeation into the catholyte, minimizes pH fluctuations, and enhances overall performance. In the present study, the maximum power density of 53.1 W/m² was obtained. The discharge period was extended to 800 min from 330 min with the buffer system, resulting in stable total energy output. In this case, although the power density decreased, the addition of the chamber increased the total energy output by 0.201 kWh/m² compared to the 330 min operation case. The highest heat-to-electric conversion efficiency achieved was 1.18 % using a Cu(BF₄)₂/NH₄BF₄ electrolyte pair. Additionally, a Z index based on concentration gradients was developed to assess TRB efficiency, offering a more accurate evaluation metric than conventional thermoelectric figures of merit. These findings suggest that the integration of a buffer chamber and the optimization of electrolyte compositions can significantly enhance TRB performance.