Extraordinary New Material Converts Waste Heat Into Energy

Abstract Energy Generation Concept

Purified tin selenide has extraordinarily high thermoelectric performance.

Perseverance, NASA’s 2020 Mars rover, is powered by something very desirable here on Earth: a thermoelectric device, which converts heat to useful electricity.

On Mars, the heat source is the radioactive decay of plutonium, and the device’s conversion efficiency is 4-5%. That’s good enough to power Perseverance and its operations but not quite good enough for applications on Earth.

A team of scientists from Northwestern University and Seoul National University in Korea now has demonstrated a high-performing thermoelectric material in a practical form that can be used in device development. The material — purified tin selenide in polycrystalline form — outperforms the single-crystal form in converting heat to electricity, making it the most efficient thermoelectric system on record. The researchers were able to achieve the high conversion rate after identifying and removing an oxidation problem that had degraded performance in earlier studies.

The polycrystalline tin selenide could be developed for use in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings. A key application target is capturing industrial waste heat — such as from power plants, the automobile industry and glass- and brick-making factories — and converting it to electricity. More than 65% of the energy produced globally from fossil fuels is lost as waste heat. 

Tin Selenide Pellet

Purified tin selenide shown in pellet form. The material has extraordinarily high thermoelectric performance. Credit: Northwestern University

“Thermoelectric devices are in use, but only in niche applications, such as in the Mars rover,” said Northwestern’s Mercouri Kanatzidis, a chemist who specializes in the design of new materials. “These devices have not caught on like solar cells, and there are significant challenges to making good ones. We are focusing on developing a material that would be low cost and high performance and propel thermoelectric devices into more widespread application.”

Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences, is a co-corresponding author of the study. He has a joint appointment with Argonne National Laboratory.

Details of the thermoelectric material and its record-high performance were published on August 2, 2021, in the journal Nature Materials

In Chung of Seoul National University is the paper’s other co-corresponding author. Vinayak Dravid, the Abraham Harris Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering, is one of the study’s senior authors. Dravid is a long-time collaborator of Kanatzidis’.

Thermoelectric devices are already well defined, says Kanatzidis, but what makes them work well or not is the thermoelectric material inside. One side of the device is hot and the other side cold. The thermoelectric material lies in the middle. Heat flows through the material, and some of the heat is converted to electricity, which leaves the device via wires.

The material needs to have extremely low thermal conductivity while still retaining good electrical conductivity to be efficient at waste heat conversion. And because the heat source could be as high as 400-500 degrees Celsius, the material needs to be stable at very high temperatures. These challenges and others make thermoelectric devices more difficult to produce than solar cells.

‘Something diabolical was happening’ 

In 2014, Kanatzidis and his team reported the discovery of a surprising material that was the best in the world at converting waste heat to useful electricity: the crystal form of the chemical compound tin selenide. While an important discovery, the single-crystal form is impractical for mass production because of its fragility and tendency to flake.

Tin selenide in polycrystalline form, which is stronger and can be cut and shaped for applications, was needed, so the researchers turned to studying the material in that form. In an unpleasant surprise, they found the material’s thermal conductivity was high, not the desirable low level found in the single-crystal form.

“We realized something diabolical was happening,” Kanatzidis said. “The expectation was that tin selenide in polycrystalline form would not have high thermal conductivity, but it did. We had a problem.” 

Upon closer examination, the researchers discovered a skin of oxidized tin on the material. Heat flowed through the conductive skin, increasing the thermal conductivity, which is undesirable in a thermoelectric device. 

A solution is found, opening doors

After learning that the oxidation came from both the process itself and the starting materials, the Korean team found a way to remove the oxygen. The researchers then could produce tin selenide pellets with no oxygen, which they then tested.

The true thermal conductivity of the polycrystalline form was measured and found to be lower, as originally expected. Its performance as a thermoelectric device, converting heat to electricity, exceeded that of the single crystal form, making it the most efficient on record.  

The efficiency of waste heat conversion in thermoelectrics is reflected by its “figure of merit,” a number called ZT. The higher the number, the better the conversion rate. The ZT of single-crystal tin selenide earlier was found to be approximately 2.2 to 2.6 at 913 Kelvin. In this new study, the researchers found the purified tin selenide in polycrystalline form had a ZT of approximately 3.1 at 783 Kelvin. Its thermal conductivity was ultralow, lower than the single-crystals.

“This opens the door for new devices to be built from polycrystalline tin selenide pellets and their applications explored,” Kanatzidis said.

Northwestern owns the intellectual property for the tin selenide material. Potential areas of application for the thermoelectric material include the automobile industry (a significant amount of gasoline’s potential energy goes out of a vehicle’s tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers).

Reference: “Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal” by Chongjian Zhou, Yong Kyu Lee, Yuan Yu, Sejin Byun, Zhong-Zhen Luo, Hyungseok Lee, Bangzhi Ge, Yea-Lee Lee, Xinqi Chen, Ji Yeong Lee, Oana Cojocaru-Mirédin, Hyunju Chang, Jino Im, Sung-Pyo Cho, Matthias Wuttig, Vinayak P. Dravid, Mercouri G. Kanatzidis and In Chung, 2 August 2021, Nature Materials.
DOI: 10.1038/s41563-021-01064-6

The research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2020R1A2C2011111), Nano·Material Technology Development Program through the NRF grant funded by the Korean Government (NRF-2017M3A7B4049274 and NRF-2017M3A7B4049273) and the Institute for Basic Science (IBS-R009-G2).

15 Comments on "Extraordinary New Material Converts Waste Heat Into Energy"

  1. So… Something LIKE it is already on Mars. Nothing new under the Martian Sun.

  2. So, here’s a plan. Let the material work in an automatic machine to work against global CO2, taking heat from heat domes to use in creating energy to sequester excess CO2 deep inside the earth (like where the fossil fuels used to be). Eventually the climate will cool, then we can use the material to trap the heat from other unwanted sources (like forest fires), and use the energy to cool fusion reactors or other sources of heat used in supporting our infrastructures.

  3. Guy W Hubert | August 4, 2021 at 5:08 am | Reply

    So in this article telling us how efficient this wonderful new material is, you never told us how efficient it is.

    • Terry The Wonderbread King | August 4, 2021 at 7:33 am | Reply

      After they told me the efficiency of the material in the rover percentage wise, I was waiting for a percentage in the new material. Instead they switched up to some ZT number(useful to know exist)that I have no idea how to convert to a percentage…😒 Give me some to compare dammit.

  4. Terry The Wonderbread King | August 4, 2021 at 7:27 am | Reply

    I wish these people would reach out to companies, with innovative people like Elon Musk, or even regular people that work in their, youtube inventors and the like, so they can do something with the discovery. I have noticed that these assholes in a lab get excited about a new discover, and will write a paper, but dont work with, or tell nobody beyond that. We got some ultra creative people out here, we should be further than we are.

    • Few corporations want to develop risky new ideas that may or may not produce profits in the future. As their bottom line is profits for their investors and officers, and to raise their stock prices to enrich their ownership, which holds the most shares, they would rather develop a proven resource which guarantees a profit, than a “pie in the sky” scheme that may never work out. At least those folks that you deem to be ‘assholes’ are bringing new ideas to the forefront, where they may may be developed when all other methods of previously creating energy are no longer possible. Remember, Tesla’s alternating current generators may have taken many more years to become our viable source of electricity, if he hadn’t released a struggling Westinghouse from the payments he owed on the patents Tesla held on the devices. In the end, J.P. Morgan threatened a hostile stock takeover of Westinghouse’s company, which forced him to sell out to the multimillionaire, who formed G.E. shortly thereafter. I ask you: “What great new ideas have you helped to define or create lately?”

  5. This says that the Current efficiency is 4-5%, and that this new material”breaks the records”.

    Did it say what the new record is?

  6. systemBuilder | August 4, 2021 at 10:51 am | Reply

    Useless article because it doesn’t list the overall percent efficiency improvements achieved!

  7. The efficiency seems to be.
    ZT increase from 2.4 (average 2.6 to 2.2) to ZT off new polycrystalline tin selenide namely 3.1. That should be a 29% increase (3.1 : 2.4 = 1.29).

    % ZT increase from 4.5%(average of 4-5%) to (4.5 x 1.29) equals 5.8% efficiency in generating electricity for the new polycrystalline tin selenide.

    Or am I missing something ? (I am not factoring in the thermal conductivity difference between the to materials).

  8. This is science by press release and doesn’t provide any useful information like how efficient this supposedly wonderful discovery is.

  9. Converts energy into…energy.


  10. I did a little digging as I was frustrated too! The actual efficacy would be a factor of the index at a temperature difference between the two sides. In the case of capturing heat from decaying uranium hotter conversion with better insulation would be exponentially better at extracting the energy at a given index. What’s on the rover has an index of 2.7 at 700 kelvin and not very much insulation. A 3.1 with great insulation factors (looks like .25 of other materials) at the same temperature delta, your talking at least 3-4x the efficacy. Something like 20% in that application.

  11. Grahame Hadden | August 9, 2021 at 3:47 am | Reply

    The figure of merit is meaningless. Also having to work a 500 Celsius is useless for most applications. What is being used on Mars is a Peltier device that has an efficiency of 10%. That means for every 100 watts generated by the atomic pile only 10 watts is converted to electricity. The efficiency rating is totally dodged in this article indicating dodgy science. If the material is extracting electricity from something that is at 700 kelvin it needs to be reducing the final temperature significantly. The efficiency is not har to calculate, so why avoid it???

  12. schinud omsub | August 9, 2021 at 6:26 pm | Reply

    Somebody should put this new thermalelectric material into some of the people mouths here and let the hot air generates electricity, and becomes constructive for once. It would make the world a better place to live.

    The key application here is “waste heat”. If the efficiency rate is very high, they would be talking about inventing an amazing new generator that is superior to anything else ever exists.

    Human activities have caused our earth to have EXCESS HEAT that would end all lifeforms here. This material may help bringing back the thermal balance.

  13. Let’s put this stuff in death valley | October 30, 2021 at 12:13 pm | Reply

    I hear: materiel that Converts heat to power. I also hear the earth is getting too hot. Me thinks: Stuff mountains of this stuff in Death Valley, and plug in a few thousand power cords. Cooler earth + elcectricty.

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