Understanding of the thermoelectric effect is not new — two different metals brought together can develop an electrical voltage if one is warmer than the other. This effect allows residual heat, which is vastly underutilized, to be partially converted into electrical energy.

However, in addition to high electrical conductivity, metals also possess high thermal conductivity, so that temperature differences dissipate rapidly. Thermoelectric devices composed of inorganic semiconductor materials such as bismuth telluride are already in use but are costly for many applications.

Researchers from Helmholtz-Zentrum Berlin, Germany, put pencil to paper to consider the use of inexpensive materials with low-thermal and high-electrical conductivity for this purpose. The materials they selected: pencil and paper.

How can such commonplace objects be manipulated to generate voltage? An HB-grade, or no. 2, pencil was applied to the surface of ordinary photocopy paper, after which a transparent, conductive co-polymer paint was applied. 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 explain that the pencil deposit left on the paper forms a surface characterized 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 find future use in printing thermoelectric components onto paper that are extremely inexpensive, environmentally friendly and non-toxic. Such tiny, flexible components could also be used directly on the body and could harness body heat to operate small devices or sensors.

The research is published in ACS Applied Materials & Interfaces.