High-efficiency thermoelectric materials: new insights into tin selenide
SnSe is a highly layered orthorhombic structure. SnSe undergoes a phase transition of second order at 500°C with an increase of the crystal symmetry from space group Pnma (left) to Cmcm (right). © HZB
Tin selenide might considerably exceed the efficiency of current record holding thermoelectric materials made of bismuth telluride. However, it was thought its efficiency became enormous only at temperatures above 500 degrees Celsius. Now measurements at the BESSY II and PETRA III synchrotron sources show that tin selenide can also be utilised as a thermoelectric material at room temperature – so long as high pressure is applied.
The thermoelectric effect has been known since 1821: with certain combinations of materials, a temperature difference generates an electric current. If one end of the sample is heated, for example using waste heat from a combustion engine, then part of this otherwise lost energy can be converted into electrical energy. However, the thermoelectric effect in most materials is extremely small. This is because to achieve a large thermoelectric effect, heat conduction must be poor, whereas electrical conductivity must be high. However, heat conduction and electrical conductivity are almost always closely associated.
For this reason, the search for thermoelectric materials concentrates on compounds with special crystalline structures such as bismuth telluride (Bi2Te3). Bismuth telluride is one of the best thermoelectric materials known to date. However, both bismuth and tellurium are rare elements, which limit their large-scale use. So the search continues for suitable thermoelectric materials among more abundant non-toxic elements.
Six years ago, a research team from the USA (https://www.nature.com/articles/nature13184) discovered that tin selenide above 500 degrees Celsius can convert about 20 per cent of heat into electrical energy. This is an enormous efficiency and considerably exceeds the value for bismuth telluride. In addition, tin and selenium are abundant.
This extremely large thermoelectric effect is related to a phase transition or re-arrangement of the crystal structure of tin selenide. The crystal structure of tin selenide consists of many layers, similar to filo or puff pastry. At 500 degrees Celsius, the layers start to self-organise and the heat conduction decreases, while charge carriers remain mobile. The efficiency of the thermoelectric effect in this crystallographic orientation of tin selenide has not been exceeded by any other material to date.
High pressure works
An international team led by Dr. Ulrich Schade at the HZB has now comprehensively examined samples of tin selenide with the aid of infrared spectroscopy at BESSY II and hard X-rays at PETRA III. The measurements show that the desired crystal structure is produced by either high temperature at normal pressure or high pressure (above 10 GPa) at room temperature. The electronic properties also change from semiconducting to semi-metallic in the high-temperature structure. This fits the predictions from theoretical calculations of the model and also from band-structure calculations.
“We are able to explain with our data and our calculations why tin selenide is such an outstanding thermoelectric material over a wide temperature and pressure range”, says Schade. Further development work will be necessary to guarantee long-term stability, for example, before thermoelectrical devices based on tin selenide really come onto the market, though. Then tin selenide might become an economical and readily available alternative to bismuth telluride.
Published in Physical Chemistry Chemical Physics (2019):
“Effects of temperature and pressure on the optical and vibrational properties of thermoelectric SnSe.” Ilias Efthimiopoulos, Matthias Berg, Annika Bande, Ljiljana Puskar, Eglof Ritter, Wei Xu, Augusto Marcelli, Michele Ortolani, Martin Harms, Jan Mueller, Sergio Speziale, Monika Koch-Mueller, Yong Liu, Li-Dong Zhao, and Ulrich Schade.
DOI: 10.1039/C9CP00897G
- BESSY II: How intrinsic oxygen shortens the lifespan of solid-state batteriesAlthough solid-state batteries (SSBs) demonstrate high performance and are intrinsically safe, their capacity currently declines rapidly. A team from the TU Wien, Humboldt-University Berlin and HZB has now analysed a TiS₂|Li₃YCl₆ solid-state half-cell in operando at BESSY II using a special sample environment that allows for non-destructive investigation under real operating conditions. Data obtained by combination of soft and hard X-ray photoelectron spectroscopy (XPS and HAXPES) revealed a new degradation mechanism that had not previously been identified in solid-state batteries. They have gained some surprising insights, particularly regarding the harmful role played by intrinsic oxygen. This study provides valuable information for improving design and handling of such batteries.
- Spintronics at BESSY II: Real-time analysis of magnetic bilayer systemsSpintronic devices enable data processing with significantly lower energy consumption. They are based on the interaction between ferromagnetic and antiferromagnetic layers. Now, a team from Freie Universität Berlin, HZB and Uppsala University has succeeded in tracking, for each layer separately, how the magnetic order changes after a short laser pulse has excited the system. They were also able to identify the main cause of the loss of antiferromagnetic order in the oxide layer: the excitation is transported from the hot electrons in the ferromagnetic metal to the spins in the antiferromagnet.
- Electrocatalysts: New model for charge separation at the solid-liquid interfaceHydrogen is at the heart of the transition to carbon neutrality, as both an energy carrier and a reagent for green chemistry. However, large-scale production of hydrogen via electrolysis, as well as the production of many other chemical products, requires significantly cheaper and more efficient catalysts. A precise understanding of the electrochemical processes that take place at the interface between the solid catalyst and the liquid medium is highly useful for developing better electrocatalysts. In the journal Nature Communications, an European team has now presented a powerful model that determines charge separation at the interface, the formation of the electric double layer and local electric potential variations, and the resulting influence on the catalytic activity.