A nickel hexacyanoferrate based thermo-electrochemical device For efficient heat-to-electricity conversion

Abstract

Effective and reliable ways to generate renewable energy is crucial for reducing global carbon emission in the ongoing battle against the climate crisis. Currently, low-temperature waste heat accounts for more than half of the rejected waste thermal energy produced in the United States. Traditional waste heat recovery methods such as steam cycle and thermoelectrics fall short at low temperatures due to uneconomically low conversion efficiency. The electrochemical conversion of heat to electricity, or thermogalvanic energy conversion, had been investigated for decentralized low-temperature applications. Traditional thermogalvanic cells were capable of harvesting thermal energy from spatial temperature gradients similar to thermoelectric plates. Lately, novel thermogalvanic devices had also been devised to harvest energy from cyclical temperature fluctuations through a technique known as Thermally Regenerative Electrochemical Cycle (TREC). In particular, the charging-free TREC cell could passively generate energy through no other external input than ambient temperature fluctuations. Thermogalvanic cells typically suffered low conversion efficiency and low open-circuit voltage due to a plethora of limitations. The motivation of this work was therefore to construct a highly-efficient thermogalvanic cell that could also produce high potential for practical applications. In this work, a charging-free TREC thermogalvanic cell based on Nickel Hexacyanoferrate was conceptualized, designed, and built. Owing to NiHCF's competitive temperature coefficient and gravimetric capacity of -1.0 mV/K and 60 mAh/g, the resultant charging-free cell achieved a full-cell temperature coefficient of -2.0 mV/K and a conversion efficiency of 9.33% relative to the Carnot limit. Furthermore, the practicality and manufacturability of the cell was verified through electronic integration testing and flexible cell fabrication.