Recently, scientists from the Shanghai Institute of Ceramics (China), Northwestern University (US), and Justus-Liebig-University Giessen (Germany) have successfully proposed a strategy for stable use of liquid-like thermoelectric materials that have superior performance in the application of industry waste heat recovery. Based on a deep understanding of atom migration and deposition of liquid-like ions, electron-conducting but ion-blocking barriers are used to stabilize such high performance liquid-like thermoelectric materials even when worked at a high electric field and/or large thermal gradient. This work opens the possibility of reconsidering the application of liquid-like thermoelectric materials in industry. The related work is published in Nature Communications (DOI:10.1038/s41467-018-05248-8.) and Journal of Inorganic Materials (Vol.32, 2017, 1337-1344).

Physical and chemical processes of ion migration and metal deposition in liquid-like materials [Image by Shi Xun]

Thermoelectric technology, as a clean energy conversion technique, has the ability to freely convert energy between heat and electricity. It can collect industrial waste heat such as from automobile exhaust gas or steel plants and then convert it to useful electricity. The conversion efficiency of a thermoelectric material is dependent on its thermoelectric figure of merit (zT). Reducing material’s lattice thermal conductivity can improve zT, but has a limitation in that the minimum value is equal to that in the material’s glass state. In 2012, Prof. Chen Lidong and Prof. Shi Xun at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, first proposed the idea of using liquid-like ions to achieve lattice thermal conductivity below the limit value of a glass state. Since then, a large family of liquid-like thermoelectric materials (Cu_2-δX (X = S, Se, Te), Cu₅FeS₄, Ag₉SnSe₆, etc) within the concept of “Phonon-liquid electron-crystal” has been successively reported with ultrahigh zTs comparable with or higher than the state-of-the-art thermoelectric materials (Nat. Mater. 2012, Adv. Mater. 2013&2014&2015&2017, Energ. Environ. Sci. 2014&2017, npj Asia Mater. 2015, etc.). In combination of non-toxic and earth-abundant components, these high performance liquid-like materials show a great potential in industry applications. However, the presence of mobile ions inside the lattice strongly affects the stability of these liquid-like materials. Under an external electric field or thermal gradient, the mobile ions are prone to deposit on the surface at the cathode to form metals, leading to poor stability. Thus, metal deposition caused by ion flux must be restricted to improve the stability of thermoelectric devices consisting of liquid-like thermoelectric materials before used in any industrial application.

The scientists found that the mobile ions in liquid-like thermoelectric materials can migrate from places with high electrochemical potential to those with low electrochemical potential when a directional force or field is applied to the material. Such orientated migration behavior induces the formation of an ion concentration gradient along the sample. However, the metal deposition only occurs when the chemical potential of mobile ions at the cathode is equal to the chemical potential of pure metal. Thus, each liquid-like material has a threshold for metal deposition. When this threshold is not achieved, liquid-like materials reach a steady-state condition which is similar to other traditional thermoelectric materials. This threshold for metal deposition can be interpreted by the maximal chemical potential difference (called the critical chemical potential difference) that can be sustained by the material. The critical chemical potential difference is an intrinsic physical parameter, which is only dependent on the material’s chemical composition and environment temperature.

In order to prove the existence of threshold for metal deposition in experiments, the scientists built a home-made instrument to characterize the stability of liquid-like thermoelectric materials. By measuring the relative resistance (or Seebeck coefficient) variation, the critical chemical potential difference for a series of Cu_2-δ(S,Se)-based liquid-like thermoelectric materials was determined, with the value ranging from 0.02 to 0.12 V. The larger critical chemical potential difference indicates the material has higher stability. In the isothermal condition, the critical chemical potential difference increases with increasing Cu deficiency or environment temperature. In the non-isothermal condition with thermal gradient, the critical chemical potential difference is also related with the direction of the thermal flux flowing through the material. When the directions of the thermal flux and current are the same, the critical chemical potential difference is smaller than the value when they are opposite.

Based on an understanding of the mechanism of ion migration and deposition, the scientists proposed that the stability of liquid-like thermoelectric materials can be substantially improved via introducing electron-conducting but ion-blocking barriers inside the material. These barriers limit the ion movement, but allow the free movement of the electrons or holes yielding negligible effect on the thermoelectric performance. Engineering the number of barriers can tune applied voltages (high current densities) below the critical chemical potential even under high electric field and/or large thermal gradient. Experimentally, this strategy has been successfully confirmed in multi-segmented Cu_1.97S material. Beyond thermoelectrics, this work also provides approaches to engineer other mixed ionic/electronic conductors, such as perovskite photovoltaic materials.

Reference：

Suppression of atom motion and metal deposition in mixed ionic electronic conductors, https://www.nature.com/articles/s41467-018-05248-8

Measuring ionic conductivity in mixed electron-ionic conductors based on the ion-blocking method, http://www.jim.org.cn/CN/abstract/abstract13726.shtml

For more information, please contact:

Prof. Shi Xun, Dr. Qiu Pengfei, Prof. Chen Lidong

E-mail:

xshi@mail.sic.ac.cn

qiupf@mail.sic.ac.cn

cld@mail.sic.ac.cn

Source: Shanghai Institute of Ceramics, Chinese Academy of Sciences