Electricity can be easily converted into heat—every electric cooker does it. But is the opposite also possible? Can heat be converted into electricity—directly, without a steam turbine or similar detours?

Physicist Thomas Seebeck answered this question with a clear “yes” over 200 years ago. He was able to show that certain materials, known as thermoelectrics, generate electricity when heated on one side and cooled on the other. A temperature difference creates electrical energy—without the need for mechanical generators. This is now known as the Seebeck effect.

Such thermoelectric generators are very practical when small amounts of electrical energy are required. They are used in space missions, for example. Unfortunately, however, the thermoelectric materials known to date are not efficient enough to replace conventional power plants on a large scale.

The working group led by Prof. Andrej Pustogow at the Institute of Solid State Physics at TU Wien is therefore investigating new materials with improved thermoelectric properties. Now, using a new trick, they have succeeded in significantly improving the performance of thermoelectrics.

The paper is published in the journal Physical Review X.

More heat—more mobility

“Despite a century of intensive research on semiconducting materials, there have been no significant advances since the discovery of bismuth telluride compounds in the 1950s that would have led to widespread, everyday use of the technology,” says Pustogow. “We have now taken a major step forward—with metallic materials that have not been in the focus of attention in this area until now.”

The Seebeck effect is based on the fact that the mobility of positive and negative charge carriers depends on the material on the one hand, but also on the temperature on the other. “Let’s assume we have a semiconductor in which only negative electrical charges can move,” says Pustogow.

“At first, they are evenly distributed throughout the material, which is electrically neutral everywhere. However, if one side is heated and the other cooled, the negative charge carriers move faster and further on the hot side, so there will be less negative charge there than on the cold side.” This creates a voltage difference from which electrical energy can be obtained.

In most metallic materials, both positive and negative charge carriers can move. This means that both types of mobile charge carriers tend to be found more on the cold side rather than the hot side. “Plus and minus balance each other out, so no voltage is generated in this way,” says Pustogow.

“That’s why metallic materials were hardly considered in connection with the thermoelectric effect. It was thought that they were not suited for this purpose. However, we have now been able to show that metals can indeed be excellent thermoelectrics.”

Different speeds—traffic jam of charge carriers

The crucial trick is to ensure that positive and negative charge carriers move at different speeds. “You can imagine the movement of the charges as if they were on a highway,” explains Pustogow.

“The positive charges flow on the left lane and the negative charges on the right lane. By creating a traffic jam on the left lane, the positive charges get stuck, while the negative charges flow unhindered on the right lane.” This way, excellent thermoelectrics can be obtained, even if they have both positive and negative charge carriers.

The “traffic jam” is created by incorporating additional immobile charge carriers into the material. The team was able to demonstrate that this works with certain nickel-gold alloys as early as 2023. “Now we have found a significantly cheaper alternative without gold in a compound of nickel and indium,” says Fabian Garmroudi, first author of the study.

A geometry reminiscent of Japanese basket weaving

In their search for new—and above all cheaper—alternatives, the researchers came across so-called kagome metals. The term “kagome” originally comes from Japanese and refers to woven bamboo baskets with a special pattern of hexagons and triangles that touch at their edges.

“Surprisingly, there are materials in nature in which the atoms arrange themselves in exactly this pattern. We call this ‘geometrical frustration.” For example, it turns out that charges can become extremely immobile and are trapped within the Kagome star,” explains Garmroudi.

Golden prospects—even without gold

The researchers have now been able to show that this Kagome geometry can lead to an extremely large Seebeck effect—considerably larger than in previously used nickel-gold alloys. While the negative charges flow unimpeded in a Kagome metal, the accumulation of positive charges at room temperature enables very high efficiency: the novel thermoelectrics may even exceed commercially available bismuth telluride thermoelectrics.

“With these Kagome metals, we have struck gold, and we are now systematically improving their thermoelectric properties with our expertise in tuning geometrical frustration,” says Pustogow, whose team at TU Wien has been studying frustrated materials for years.