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 (α) plays a dominant role in efficient heat harvesting. However, the internal factors that affect α are still not clear and significant improvements are needed. Here, α 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 α is highly related to the lattice parameter change. Large lattice shrinkage exhibits a large negative α, while lattice expansion is corresponding to a positive α. These are mainly attributed to the different phonon vibration entropy changes upon cation intercalation in various PBAs. Especially, purple cobalt hexacynoferrate delivers the largest α 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.of α (negative/positive). These phenomena are in good consistent with our theoretical calculation. In particular, purple cobalt hexacynoferrate (CoHCFe-pp) shows the largest α along with the largest lattice parameter change. Therefore, we selected CoHCFe-pp as the electrode material in TREC for low-grade heat harvesting and achieved a large heat-to-electricity conversion efficiency of 2.65% in the temperature range of 10-50 °C, which is as high as 21% of theoretical limitation.

Efficient Low‐Grade Heat Harvesting Enabled by Tuning the Hydration Entropy in an Electrochemical System

“…In our previous study, we tuned ΔS intr by changing the nitrogen-coordinated transition metal in PBAs. [22] In this work, we focused on exploring the effect of cationic transfer on ΔS it . One of the dominant factors determining ΔS it is the different hydration state of the cations in aqueous electrolyte compared to that in electrode material.…”

“…In our previous study, we tuned Δ S intr by changing the nitrogen‐coordinated transition metal in PBAs. [ 22 ] In this work, we focused on exploring the effect of cationic transfer on Δ S it .…”

“…To date, several typical conversion systems have been explored, such as solid-state thermoelectric (TE), [3][4][5][6][7][8][9][10] liquidbased thermogalvanic cell (TGC), [11][12][13][14] thermally regenerative ammonia-based battery (TRAB), [15][16][17] and thermally regenerative electrochemical cycle (TREC). [18][19][20][21][22] Among these technologies, TREC harvests thermal energy by charging and discharging the electrochemical cell at different temperatures. Then, the change of the output voltage of the cell for the temperature allows less charging voltage (energy) than discharging voltage (energy) in the cycle and presents a higher energy conversion efficiency (η e ).…”

A Flexible Bilayer Actuator Based on Liquid Crystal Network and PVDF–TrFE for Low‐Grade Waste Heat Harvesting

“…This dynamic property greatly benefits the film acting as an energy generator of low‐grade waste heat, rendering it a great advantage over the other low‐grade heat recovery devices. [ 8,10,30 ]…”

“…[ 6–8 ] Although thermoelectrics are not easily combined with portable and miniaturized devices because of their inherent shape and heavy weight. [ 9,10 ] Furthermore, the thermoelectric devices based on Seebeck effect suffer from small Seebeck coefficient ( S ≈ 0.2 mV K −1 ), [ 8 ] which greatly limits their harvesting efficiency. The other is to employ pyroelectric materials.…”