Researchers from UNIST and Nanyang Technological University have developed a thermal regenerative electrochemical cycle (TREC) system that provides a new way to convert low-grade heat into usable energy. This system, enhanced by understanding the role of structural vibrational modes, particularly in water molecules, shows potential for improving energy conversion at small temperature differences. Such advances in TREC systems could revolutionize the use of low-grade heat in wearable technology and secondary batteries.
The breakthrough TREC system developed by a team of researchers exploits structural vibration patterns to efficiently convert low-grade heat into energy. This advancement could transform energy conversion in wearable technology and secondary batteries.
A research team co-led by Professor Hyun-Wook Lee and Professor Dong-Hwa Seo of the School of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST), in collaboration with Professor Seok Woo Lee of Nanyang Technological University in Singapore, has achieved a major breakthrough in utilizing low-grade heat sources (<100°C) for efficient energy conversion. Their pioneering work focuses on developing efficient thermal regenerative electrochemical cycle (TREC) systems capable of converting small temperature differences into usable energy.
Figure 1. Schematic showing the different mechanisms of the battery and TREC system. While the battery system (left) loses some stored energy as unusable energy, the TREC system (right) can convert low-grade waste heat energy into electrochemical energy during the battery cycle. Image source: UNIST
Traditional energy harvesting systems face challenges in efficiently utilizing low-grade heat sources. However, TREC systems offer an attractive solution because they integrate battery functionality with thermal energy harvesting. In this study, the research team delved into the role of structural vibration modes in enhancing the efficacy of TREC systems.
By analyzing how changes in covalent bonds affect vibrational modes (specifically affecting structural water molecules), the researchers found that even trace amounts of water can induce strong structural vibrations in the A1g stretching mode of cyanide ligands. These vibrations contribute significantly to the large temperature coefficient (ɑ) within the TREC system. Based on these insights, the team designed and implemented an efficient TREC system using sodium ion water electrolyte.
Figure 2. TREC principle and the influence of water molecules in the PBA structure. (Top) Effect of removing water molecules on CuHCFe structure and covalent changes (-ICOHP/eV). The average -ICOHP values for Cu─N and Fe─C bonds and the SD of the -ICOHP values for 6 Fe─C bonds are given. (Center) Effect of water molecules on the stretching vibrational modes of cyanide ligands. (Bottom) d) Power harvested by TREC full cell and half cell. The low and high temperatures are 10 and 60°C respectively. Based on O/Cu-x, the current density of the full cell is set to 0.5C (30mAg−1). Image source: UNIST
"This study provides valuable insights into how structural vibration modes enhance the energy harvesting capabilities of TREC systems," explained Professor Hyun-Wook Lee. "Our results deepen our understanding of the intrinsic properties of Prussian blue analogues that are modulated by these vibrational modes, opening up new possibilities for improved energy conversion."
The potential applications of the TREC system are very wide, especially in wearable technology and other devices where small temperature differences exist. By efficiently capturing low-grade heat and converting it into usable energy, TREC systems offer a promising avenue for developing next-generation secondary batteries.