Hidden talents: Converting heat into electricity with pencil, plastic and paper

Thermoelectric materials can use thermal differences to generate electricity. Now there is an inexpensive and environmentally friendly way of producing them with the simplest of components: a normal pencil, photocopy paper, and conductive paint are sufficient to convert a temperature difference into electricity via the thermoelectric effect. This has now been demonstrated by a team at the Helmholtz-Zentrum Berlin.

The thermoelectric effect is nothing new – it was discovered almost 200 years ago by Thomas J. Seebeck. If two different metals are brought together, then an electrical voltage can develop if one metal is warmer than the other. This effect allows residual heat to be partially converted into electrical energy. Residual heat is a by-product of almost all technological and natural processes, such as in power plants and every household appliance, and the human body as well. It is one of the largest underutilised energy sources in the world - and usually goes completely unused.

A team led by Prof. Norbert Nickel at the HZB has now shown that the effect can be obtained much more simply: using a normal HB-grade pencil, they covered over a small area in pencil on ordinary photocopy paper. As a second material, they applied a transparent, conductive co-polymer paint (PEDOT: PSS) onto the surface.

What transpires is that the pencil traces on the paper deliver a voltage comparable to other far more expensive nanocomposites that are currently used for flexible thermoelectric elements. And this voltage could be increased tenfold by adding some indium selenide to the graphite from the pencil.

The researchers investigated graphite and co-polymer coating films using a scanning electron microscope and spectroscopic methods (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.”