Penn State researchers develop thermally regenerative ammonia battery (TRAB) for efficient waste heat recovery

8 December 2014

Researchers at Penn State University have demonstrated the efficient conversion of low-grade thermal energy into electrical power using a thermally regenerative ammonia-based battery (TRAB). A paper on their work is published in the RSC journal Energy Environmental Science.

The battery uses copper-based redox couples [Cu(NH₃)₄²⁺/Cu and Cu(II)/Cu]. Ammonia addition to the anolyte (the electrolyte surrounding the anode) of a single TRAB cell produced a maximum power density of 115 ± 1 W m⁻² (based on projected area of a single copper mesh electrode), with an energy density of 453 Wh m⁻³ (normalized to the total electrolyte volume, under maximum power production conditions).

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Adding a second cell doubled both the voltage and maximum power. Increasing the anolyte ammonia concentration from 2M to 3 M further improved the maximum power density to 136 ± 3 W m⁻².

The battery will run until the reaction uses up the ammonia needed for complex formation in the electrolyte near the anode or depletes the copper ions in the electrolyte near the cathode. Then the reaction stops.

To “recharge”, the TRAB uses low-grade waste heat from an outside source. The researchers distill ammonia from the effluent left in the battery anolyte and then recharge it into the original cathode chamber of the battery. The chamber with the ammonia now becomes the anode chamber and copper is re-deposited on the electrode in the other chamber, now the cathode, but formerly the anode. The researchers switch ammonia back and forth between the two chambers, maintaining the amount of copper on the electrodes.

The concept of the TRAB. Zhang et al. Click to enlarge.

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 ± 3 W m⁻², with an average discharge energy efficiency of ~29% (electrical energy captured versus chemical energy in the starting solutions).

Power was restored to 126 ± 5 W m⁻² 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.

The researchers note that the current thermally regenerative ammonia battery is not optimized, so that further work could both produce more power and reduce the cost of operating the batteries.

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.

Other researchers on this project were Jia Liu, postdoctoral fellow and Wulin Yang, graduate student, both in environmental engineering. The researchers have filed a preliminary patent on this work.

The King Abdullah University of Science and Technology supported this work.

Resources

• Fang Zhang, Jia Liu, Wulin Yang and Bruce E. Logan (2015) “A thermally regenerative ammonia-based battery for efficient harvesting of low-grade thermal energy as electrical power,” Energy Environ. Sci. doi: 10.1039/C4EE02824D