Development of direct thermal charging cell for low-grade heat harvesting based on Prussian blue analogue anode

Abstract

Efficient recovery of the dissipated thermal energy, especially the low-grade waste heat (< 100 °C), is of great significance in relieving the stress of the fast-growing fossil fuel demand and the climate change caused by greenhouse gas emissions. Intensive research has been conducted on liquid-based waste heat recovery technologies to enhance the thermopower (α), output power, and energy conversion efficiency (ηE). Recently, a direct thermal charging cell (DTCC) was proposed to convert heat into electricity under isothermal heating operation without building a thermal gradient or thermal cycles. DTCC was initially designed with a graphene oxide (GO) cathode, a polyaniline (PANI) anode, and an electrolyte-containing Fe2+/Fe3+redox couple. This dissertation focuses on the improvement of DTCC by replacing the anode with nickel hexacyanoferrate (NiHCFeII), one of the Prussian blue analogues (PBAs). Moreover, PBAs are investigated in regard to the effect of experimental conditions on their electrochemical behavior and possibility as the anode of DTCC. The newly designed DTCC consists of GO cathode, NiHCFeII anode, and KCl electrolyte, called DTCCNCF. The α of DTCCNCF comes from the thermopseudocapacitive effect from GO and the entropy change of NiHCFeII. NiHCFeII as the anode improves the discharge capacity and allows the device to be recycled via simple chemical treatment after a long-term operation, realizing green recycling. Besides, The Fe2+/Fe3+-based redox electrolyte in the original DTCC is replaced by a neutral, safe, and environmentally friendly KCl solution, avoiding the risk of environmental pollution caused by leakage. DTCCNCF achieves an ηE of 3.59% (26.2% of Carnot efficiency) at 70 °C, leaping to the forefront of existing thermoelectrochemical and thermoelectric systems in a low-grade heat regime. The developed system has the advantages of green technology, flexibility, cost-effectiveness, and scalability, making it promising in practical applications for direct energy conversion from low-grade heat sources. This work paves the way to upgrade DTCC via selecting a PBA anode with alternative α and capacitance. Low-vacancy NiHCFe was synthesized by slowing down the crystallization rate. When low-vacancy NiHCFe was tested as the anode of DTCCNCF, the ηE of DTCCNCF was further enhanced to 4.02% at 70 °C (29.4% of Carnot efficiency) due to the lower overpotential. Zeolitic/coordinated water occupying the vacancies can be extracted at high temperatures or when discharged for the first time, resulting in unstable performance. ACoFe(CN)6 is also proved to be a promising material as the electrode of DTCC with a large positive α, while the alkali metal ion A+ has a significant impact on the thermal voltage response.