Researchers in Korea and the US have created the most efficient thermoelectric material to date. The material, polycrystalline tin selenide, boasts a heat-to-electricity conversion efficiency of nearly 20%, and could be used in devices that capture waste heat from industrial power plants as well as heat generated by combustion engines in cars, ships and tankers.
Over 65% of the energy produced worldwide is lost as waste heat. Thermoelectric power generators, which are semiconductor-based electronic devices, can turn this heat into electricity via the Seebeck effect. However, to do this well, they need to be made from materials that have an extremely low thermal conductivity while also being good electrical conductors. This is a tricky combination to achieve, especially since the devices may also be exposed to heat sources as hot as 400-500 °C, and thus need to be robust to high temperatures.
Poor mechanical properties
In 2014, a team led by chemist and materials designer Mercouri Kanatzidis at Northwestern University discovered that single-crystal tin selenide (SnSe) was better at converting waste heat to useful electricity than any other known material, with a record-high thermoelectric figure of merit, ZT, of 2.6 at 924 K. Subsequently, another research group found that the ZT of bromine-doped n-type single SnSe crystals was even higher, reaching 2.8 at 773 K. However, the single-crystal version of SnSe is tricky to synthesize and has poor mechanical properties, so it cannot be mass produced.
The polycrystalline form of SnSe, in contrast, is a simple, binary, inexpensive and Earth-abundant material with good mechanical properties. Its conduction properties should also be in its favour: polycrystalline samples are generally understood to have lower lattice thermal conductivities than single crystals thanks to the additional scattering of phonons (vibrations of the crystal lattice) at crystal grain boundaries. Surprisingly, though, other researchers found that the lattice thermal conductivity for polycrystalline SnSe was higher than for the corresponding single-crystal version.
Removing the “skin”
The Northwestern team’s initial measurements of polycrystalline SnSe proved similarly disappointing. However, when the researchers took a closer look at their sample, they found that a thin layer of oxidized tin had formed on its surface. This heat-conducting “skin” is 150 times more thermally conductive than tin selenide itself, so Kanatzidis and In Chung of Seoul National University developed a new synthesis technique to minimize its presence. Their approach involved reducing the tin starting material as well as the tin selenide compound using hydrogen and argon and then annealing the ensemble to high temperatures.
Inexpensive thermoelectric material works at room temperature
The resulting material has a heat-to-electricity conversion efficiency of nearly 20%, with a ZT of 3.1 at a temperature of 783 K. This is far higher than any other bulk thermoelectric system studied to date, which is good news since the commercialization of thermoelectric technology has been seriously limited by low ZTs and the presence of toxic elements like lead (Pb) or rare ones like telluride (Te). Indeed, the ZT of most thermoelectric materials is less than 2, while that of PbTe is around 2.5.
The material in this work, which the team describes in Nature Materials, is a p-type material. Since thermoelectric devices require a pair of p- and n-type materials with similar thermoelectric properties, members of the team say they now hope to develop an n-type counterpart.
