Thermocell promises clean, continuous energy

ACES researchers are refining a clean energy device called a thermocell, which coverts heat directly into energy and can maintain a charge as long as one electrode is hot and the other is cold.

What is a thermocell?

In a nutshell, this device generates clean energy from waste heat.

A thermocell is similar to a battery in that it consists of two electrodes in contact with an electrolyte.

The advantage of the thermocell is that the device never needs to be recharged – energy conversion is instant and continuous as long as one electrode is heated and the other is kept cold.

Where could they be used?

When you think of a cell sitting on a hot pipe while being exposed to cold air, you realise the potential applications of the device are limitless – you could use a thermocell to recover the energy lost as heat from a car engine, or you could use it harvest geothermal energy.

There are lots of waste heat sources, many of them continuous, which could be harvested as useful energy sources.

Where is the science up to?

Unfortunately, progress in this field to date is limited and the fundamental science and inner workings of the cell are poorly understood. Thermocell development is a core-funded research activity in ACES that recognises the urgent need for the sustainable generation of energy.

Our research tries to understand the science behind the chemical reaction occurring in the thermocell and the effect of different electrolytes. In a typical electrochemical device like a battery, the electrolyte would be an organic solvent with salts dissolved to provide charged ions. However, these traditional and widely used electrolytes have issues of flammability and sometimes toxicity.

We are trying to find safer, alternative electrolytes such as ionic liquids. Ionic liquids are salts that are liquid at room temperature. The charged ions needed for the thermocell are present in a liquid form eliminating the need for organic solvents. Additionally, ionic liquids have high boiling points, are non-volatile and non-flammable making them great candidates for use in the thermocell.

What have you found so far?

Our team recently had a paper published in Faraday Discussions which examined the effect the unique chemical structure of selected ionic liquids had on the performance of the thermocell.

As a result of these studies, we achieved the highest Seebeck coefficient (a measure of voltage produced per degree by a device) published so far for ionic liquid-based electrolytes.

In this paper, we also showed the benefit of adding propylene carbonate, a high boiling molecular solvent, to the ionic liquid. This increases the power outputs of the device, and enables us to use ionic liquids that are solid at room temperature or that do not dissolve the redox couple well in their neat state.

Where to next?

Going forward, we plan to continue our investigation of ionic liquid electrolytes and the relationship between their structure and the performance of the device. By the end of this project, we hope to draw conclusions which will help our future choices of ionic liquids for this application. There are presently hundreds of ionic liquids, and new ones being synthesised on regular basis. Being able to choose the ‘good’ ones by just looking at the structure would be a big step forward in this field.

Read more about the ACES Energy Program.

Pictured from left, researchers Danah Al-Masri and A/Prof. Jenny Pringle