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New cell efficiently converts waste heat to energy

Waste heat. (Reference image by Stefan Gara, Flickr).

Scientists at Korea’s Daegu Gyeongbuk Institute of Science and Technology have designed a novel hybrid thermoelectrochemical-concentration cell that outperforms similar state-of-the-art devices and shows promise in utilizing waste heat to generate electricity.

In a paper published in the Chemical Engineering Journal, the researchers point out that the cell opens the door to commercially feasible energy harvesting systems that could power IoT devices and sensors by leveraging thermal energy.

The article also explains that the conversion of a temperature difference into electricity is already possible through thermoelectrochemical cells (TECs). These devices can leverage waste heat to sustain a reduction-oxidation (redox) reaction that, in turn, produces electricity.

The problem is that, so far, TECs are lacking commercial implementations due to their low energy conversion efficiency, lacklustre output power, and costly fabrication. 

Seeing these difficulties as opportunities, the DGIST researchers came up with a breakthrough in energy conversion that would make TECs viable for untethered low-power devices.

Led by Professor Hochun Lee, the group combined the operating principle of TECs with that of concentration galvanic cells, creating a hybrid thermoelectrochemical-concentration cell (TCC). Although TCCs are not a new concept, the design put forward by the team overcomes some critical limitations of existing TECs.

The TCC reported in this study is based on redox reactions involving iodine ions (I−) and triiodide (I3−). Different from what happens in conventional TECs, these reactions occur in a non-aqueous carbonate solution that uses dimethyl carbonate (DMC) as a solvent. This particular selection of materials creates a peculiar effect.

As the temperature of the hot side increased beyond 40°C, the DMC reacted with I− to produce a porous, gel-like layer of Li2CO3 near the hot electrode that helped maintain a large difference in the concentrations of I− and I3− throughout the cell, greatly boosting its performance.

“Our hybrid cell demonstrates a remarkable thermal conversion efficiency (5.2%) and outperforms the current best n-type TECs,” Lee said in a media statement. “In addition, the simple structure and fabrication process of our TCCs offer a practically feasible platform for thermal energy harvesting.”

In Lee’s view, further studies will be needed to refine this unprecedented approach to TCC design and, hopefully, achieve the goal of connecting multiple TCCs in series to reach commercially acceptable capabilities.

“IoT-connected societies will require economic and autonomous power sources for their IoT devices and sensors, and we believe TECs will be the ideal candidate to meet their needs,” Lee said.