This is an illustration of the novel thermoelectric device made using films of pencil graphite and a conductive co-polymer paint. Image: HZB.
This is an illustration of the novel thermoelectric device made using films of pencil graphite and a conductive co-polymer paint. Image: HZB.

The thermoelectric effect was first discovered almost 200 years ago by Thomas Seebeck. If two different metals are brought together, then an electrical voltage can develop if one metal is warmer than the other, allowing residual heat to be partially converted into electrical energy.

Residual heat is a by-product of almost all technological and natural processes, from power plants to every household appliance to the human body. It is one of the largest underutilized energy sources in the world – and usually goes completely unused.

Unfortunately, as useful as it could be, the thermoelectric effect is extremely small in ordinary metals. This is because metals not only have a high electrical conductivity but a high thermal conductivity as well, meaning differences in temperature disappear immediately. Thermoelectric materials need to have a high electrical conductivity but a low thermal conductivity.

Thermoelectric devices made of inorganic semiconductor materials such as bismuth telluride are already being used today in certain technological applications. However, such material systems are expensive and so their use only makes economic sense in certain situations. Flexible, non-toxic, organic thermoelectric materials based on carbon nanostructures are also being investigated for use in the human body.

Now, in a paper in ACS Applied Materials & Interfaces, a team of researchers led by Norbert Nickel at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany has shown that the effect can be obtained much more simply. Using a normal HB-grade pencil, the researchers first color a small area of ordinary photocopy paper, producing a graphite film. They then apply a transparent, conductive film of co-polymer paint (PEDOT:PSS) onto a second piece of paper. Both pieces of paper are used to connect hot and cold materials.

The pencil traces on the paper can deliver a voltage comparable to other far more expensive nanocomposites currently used as flexible thermoelectric elements. And this voltage can be increased tenfold by adding some indium selenide to the pencil graphite.

The researchers investigated these graphite and co-polymer films using a scanning electron microscope and Raman scattering at HZB. "The results were very surprising for us as well," explains Nickel. "But we have now found an explanation of why this works so well: the pencil deposit left on the paper forms a surface characterised by unordered graphite flakes, some graphene and clay. While this only slightly reduces the electrical conductivity, heat is transported much less effectively."

These simple constituents might be able to be used in the future to print thermoelectric components onto paper that are extremely inexpensive, environmentally friendly and non-toxic. Such tiny and flexible components could also be applied directly to the body, using body heat to operate small devices or sensors.

This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.