Stable and eco-friendly thermoelectric material discovered

A thermoelectric metal oxide film with a thermoelectric figure of merit of ~0.55 at 600°C has been discovered, opening new avenues towards the widespread use of thermoelectric converters. It has been described in the journal ACS Applied Materials & Interfaces.

Waste heat is a very promising source of energy conservation and reuse, by means of converting this heat into electricity — a process called thermoelectric conversion. Commercially available thermoelectric conversion devices are synthesised using rare metals — but while these are quite efficient, they are expensive and, in the majority of cases, utilise toxic materials.

One alternative is the use of oxide-based thermoelectric materials, but there is a lack of evidence for their stability at high temperatures. A team led by Professor Hiromichi Ohta from Hokkaido University has now synthesised a barium cobalt oxide thermoelectric converter that is reproducibly stable and efficient at temperatures as high as 600°C.

Thermoelectric conversion is driven by the Seebeck effect: when there is a temperature difference across a conducting material, an electric current is generated. However, efficiency of thermoelectric conversion is dependent on a figure called the thermoelectric figure of merit ZT. Historically, oxide-based converters had a low ZT; recent research has revealed that many candidates have high ZT, but their stability at high temperatures was not well documented.

Ohta’s group has been working on layered cobalt oxide films for over two decades. In this study, the team sought to examine the thermal and chemical stability of these films, as well as measure their ZT values, at high temperatures. They tested cobalt oxide films layered with sodium, calcium, strontium or barium, analysing their structure, resistivity and thermal conductivity.

They found that, of the four variants, the barium cobalt oxide layered film retained its stability in terms of structural integrity and electrical resistivity at temperatures as high as 600°C. In comparison, the sodium and calcium cobalt oxide films were only stable until 350°C, and the strontium cobalt oxide film was stable up to 450°C. The ZT of the barium cobalt oxide film increased with the temperature, reaching ~0.55 at 600°C — comparable to some commercially available thermoelectric converters.

“Our study has shown that barium cobalt oxide films would be excellent candidates for high-temperature thermoelectric conversion devices,” Ohta said. “In addition, they are environment friendly, giving potential for wide deployment.”

Image credit: Hiromichi Ohta.

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