Engineering the Electrochemical Temperature Coefficient for Efficient Low-Grade Heat Harvesting

Abstract

Low-grade heat to electricity conversion has shown a large potential for sustainable energy supply. Recently, the low-grade heat harvesting in the thermally regenerative electrochemical cycle (TREC) is a promising candidate with high energy conversion efficiency. In this system, the electrochemical temperature coefficient (alpha) plays a dominant role in efficient heat harvesting. However, the internal factors that affect are still not clear and significant improvements are needed. Here, alpha of various Prussian Blue analogues (PBAs) is investigated and their lattice change during cation intercalation is monitored using the ex situ X-ray diffraction (XRD) method. For the first time, it is found that alpha is highly related to the lattice parameter change. Large lattice shrinkage exhibits a large negative alpha, while lattice expansion is corresponding to a positive alpha. These are mainly attributed to the different phonon vibration entropy changes upon cation intercalation in various PBAs. Especially, purple cobalt hexacynoferrate delivers the largest alpha of -0.89 mV K-1 and enables highly efficient heat conversion efficiency up to 2.65% (21% of relative efficiency). The results of this study provide a fundamental understanding of temperature coefficient in electrochemical reactions and pave the way for designing high-performance material for low-grade heat harvesting.