Sandia National Laboratories researchers, with colleagues at the University of Virginia, have made the first measurements of thermoelectric behavior by a nanoporous metal-organic framework (MOF), a development that could lead to an entirely new class of materials for such applications as cooling computer chips and cameras and energy harvesting. “These results introduce MOFs as a new class of thermoelectric materials that can be tailored and optimized,” said Sandia physicist François Léonard.
This work, published in a paper in the journal Advanced Materials, builds on previous research in which the Sandia team realized electrical conductivity in MOFs by infiltrating the pores with a molecule known as tetracyanoquinodimethan (TCNQ), as described in a 2013 paper in Science (Talin et al. 2013). (Earlier post.)
Molecular cluster calculations indicate that TCNQ can bind to two Cu(II) dimer units within the MOF pores, and a partial charge transfer of ≈0.3 e− between the TCNQ and the framework was estimated by infrared and Raman spectroscopic measurements. Temperature dependent conductivity measurements indicated hopping carrier transport with a very low activation energy of ≈0.04 eV. The creation of an electrical conductor from the insulating Cu3 (BTC)2 framework immediately raises the question: could this conducting Guest@MOF material be technologically useful? One application for which the unique combination of properties of conducting MOFs and Guest@MOFs could be advantageous is thermoelectricity.—Erickson et al.
The Sandia team found that not only is the TCNQ-filled MOF material thermoelectric but the efficiency of its temperature conversion approaches that of the best conducting materials such as bismuth telluride, said Sandia senior scientist Mark Allendorf.
The researchers also gained a fundamental understanding of the charge transport properties of these novel materials that furthers the long-range goal of molding MOFs into electronic and optoelectronic devices.
MOFs have a crystalline structure consisting of rigid organic molecules linked together by metal ions. The hybrid of inorganic and organic components produces an unusual combination of properties: nanoporosity, ultra-large surface areas and remarkable thermal stability, which are attractive to chemists seeking novel materials. The empty space framed by the organic molecules and metal ions can be filled with practically any small molecule a chemist chooses.
We describe this concept as Guest@MOF, with the guest being practically any molecule small enough to fit in the MOF pores. The great thing about chemistry is you can synthesize a wide variety of molecules to be inserted inside a MOF to change its properties. In optimizing materials, this gives you a lot of knobs to turn.—Alec Talin, a materials scientist at Sandia
The researchers had to devise a method to measure the thermoelectric properties of TCNQ@MOF, where TCNQ was the guest molecule. They created a thermoelectric device by connecting Peltier heaters and coolers to each end of a thin film of TCNQ@MOF to generate a tiny temperature gradient. They then accurately measured the temperature gradient with an infrared camera while simultaneously measuring the generated voltage. From these data they obtained the Seebeck coefficient—i.e., the measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference as induced by the Seebeck effect.
Patrick Hopkins, an assistant professor of mechanical engineering at the University of Virginia, and his graduate student Brian M. Foley used a laser technique to measure the thermal conductivity.
The resulting measurements showed great promise. TCNQ@MOF has a high Seebeck coefficient and low thermal conductivity, two important properties for efficient thermoelectricity. The Seebeck coefficient was in the same range as bismuth telluride, one of the top solid-state thermoelectric materials.
The measurements also captured data that has advanced the team’s fundamental understanding of the TCNQ@MOF electronic structure. Sandia physicist Catalin Spataru and materials scientist Mike Foster conducted detailed electronic structure calculations of TCNQ@MOF and Sandia materials scientist Reese Jones performed thermal conductivity simulations.
The simulations allowed the researchers to verify the source of the charge transport and establish that TCNQ@MOFs is a p-type material. Applications such as transistors and diodes require semiconductors of both p-type and n-type.
We’re now looking for a molecule that in combination with a MOF creates an n-type semiconductor with similar properties to TCNQ@MOF. Once we find that, we’ll be at the early stage of creating a full thermoelectric device.—François Léonard
The researchers are now improving the thermoelectric efficiency of TCNQ@MOF. One avenue is to change the MOF films from the polycrystalline structures used in the initial research to single-crystal.
A unified structure should conduct electricity better. However, we believe the interfaces between the polycrystal grains contribute to the low thermal conductivity. So the best energy conversion efficiency will likely be achieved by balancing these two parameters.—Sandia chemist Vitalie Stavila
The researchers are also turning their thermoelectric measurement technique to other MOFs and materials, such as carbon nanotube thin films.
Erickson, K. J., Léonard, F., Stavila, V., Foster, M. E., Spataru, C. D., Jones, R. E., Foley, B. M., Hopkins, P. E., Allendorf, M. D. and Talin, A. A. (2015), “Thin Film Thermoelectric Metal–Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity” Adv. Mater. doi: 10.1002/adma.201501078
A.A. Talin, A. Centrone, A.C. Ford, M.E. Foster,V. Stavila, P. Haney, R.A. Kinney, V. Szalai, F. El Gabaly, H.P. Yoon, F. Léonard and M.D. Allendorf (2013) “Tunable electrical conductivity in metal-organic framework thin-film devices,” Science doi: 10.1126/science.1246738