Graphical Special K : Low-cost K 4 Fe(CN) 6 was studied as a cathode for potassium-ion batteries, which exhibit a high voltage of approximately 3.6 V and long lifespan of 400 cycles with an average capacity decay of 0.063 % per cycle. These are endowed by the highly reversible [Fe(CN)] 3−/4− redox couple for charge compensation, with enhanced ionic and electrical conductivity. Potassium-ion batteries (KIBs) are of interest for large-scale electrical energy storage, owing to the abundance of K resources and potential high energy density. Low-cost cathodes with high performance are crucial for KIBs. Herein, K 4 Fe(CN) 6 is shown to be a low-cost and high-voltage cathode for KIBs. It can deliver a high voltage of approximately 3.6 V and a discharge capacity of 65.5 mAh g −1 with a lifespan of 400 cycles of discharge and charge. This is attributed to the strong σ bonds between C atoms and Fe and to the reduced particle size and good contact with conductive carbon brought about by ball milling, which benefit both the K + ion and the electronic conduction. The [Fe(CN) 6 ] 3−/4− redox couple is found to be responsible for charge compensation upon reversible extraction/insertion of K + from/into K 4 Fe(CN) 6 . The high voltage and stability of K 4 Fe(CN) 6 will make it a promising low-cost cathode for KIBs and encourage more investigations into high-performance cathode materials.
Emerging thermocells exhibit a significant potential in low-grade heat harvesting owe to their high mV-scale thermopower, low cost, and high robustness. Fundamentally, three effects contribute to the thermopower enhancement of liquid thermocells, i.e. original concentration effect, solvation effect, and normalized concentration gradient effect. Yet, the contribution of the solvation effects and the normalized concentration gradient in organic solvents remains a subject of debate. In this paper, we integrate experiments, spectral analysis, and electrochemical theory to demonstrate that the normalized concentration gradient dominates the enhancement of thermopower in the investigated organic solvents, whereas the solvation effect is weak. Further, the introduction of ethanol results in a thermosensitive solubility of K 4 Fe(CN) 6 and a minimal concentration at the cold end, which establishes a huge substantial normalized Fe(CN) 6 4- concentration gradient and increases the Seebeck coefficient to 3.4 mV/K. Simultaneously, the precipitation resulting from the introduction of an organic solvent notably diminishes the heat convection within the system. As a result, the optimized TC-35EtOH system yields a ZT value (0.094) and Carnot-relative efficiency (2.52 %) that are 7 times greater than those of its initial counterpart. Additionally, the system’s high power density of 3.64 W/m 2 suggests its potential for harnessing low-grade heat waste energy.
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