Thermoelectric devices convert heat to electricity and have no moving parts, making them extremely attractive for cooling and energy harvesting applications. Thermoelectric metal-organic frameworks (MOFs) could take these advantages a step further with improved performance, smaller size and flexible designs. MOFs have a crystalline structure that resembles molecular scaffolding, consisting of rigid organic molecules linked together by metal ions.
Sandia researchers have made the first measurements of thermoelectric behavior of a nanoporous 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. Their results were published in “Thin Film Thermoelectric Metal–Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity,” which appeared April 28 online in Advanced Materials. This work builds on previous research in which the Sandia team realized electrical conductivity in MOFs by infiltrating the pores with a molecule known as tetracyanoquinodimethane, or TCNQ, as described in a 2014 article in Science.
“The fact that a TCNQ-filled MOF conducts electricity quite well made us hopeful that we’d also see thermoelectricity, but it was by no means a given,” said Sandia senior scientist Mark Allendorf (in Sandia’s Transportation Energy Center). “We found that not only is the material thermoelectric but also the efficiency of its temperature conversion approaches that of the best conducting materials like bismuth telluride.”
The hybrid of inorganic and organic components produces an unusual combination of properties: nanoporosity, ultralarge 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 is what truly sets MOFs apart—empty space that can be filled with practically any small molecule a chemist chooses. “The great thing about chemistry is you can synthesize a wide variety of molecules to be inserted inside a MOF to change its properties,” explained Sandia materials scientist Alec Talin (in Sandia’s Materials Physics Dept.). In optimizing materials, this gives you a lot of knobs to turn.”
MOFs are so new—they were only discovered in 1999—that researchers often find themselves on the frontier of science with few established tools or even a clear understanding of the material’s fundamental properties.
François Léonard (also in Sandia’s Materials Physics Dept.), Talin, and Kristopher Erickson (in Sandia’s Special Programs Dept.), accurately measured the temperature gradient with an infrared camera while simultaneously measuring the generated voltage. From these data they obtained the voltage per unit of temperature change, known as the Seebeck coefficient. Patrick Hopkins, at the University of Virginia, and his graduate student Brian Foley used a laser technique to measure the thermal conductivity. The resulting measurements showed great promise.
A TCNQ-filled 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 next step is how do we make it better?” said Allendorf. “The energy conversion is not competitive yet with solid-state materials, but we think we can improve that with better electrical conductivity.”
Once thermoelectric MOFs realize sufficient energy-conversion efficiency, they could begin replacing existing cooling methods in devices where compactness and weight are priorities.
- Cameras mounted on satellites—they require constant cooling to function properly.
- Laptop computers, smartphones, and other portable electronics—replacing the fans with thermoelectric MOFs could reduce the weight and the number of moving parts that will eventually wear out.
Energy-harvesting thermoelectric devices capitalize on wasted heat to generate power. A thermoelectric device near a car engine or exhaust system could capture that wasted heat to generate power for the car’s electronics. Thermoelectric devices could also be used to provide localized cooling for passenger comfort.
Read the Sandia news release.