The resulting material has a higher thermal stability compared to other thermoelectric materials based on synthetic polymers, which allows it to reach temperatures of 250 °C and be wrapped around pipes or components. In addition, the device does not use toxic elements, and the cellulose can easily be recycled, since it can be degraded by an enzymatic process converting it into glucose, while recovering the carbon nanotubes, which are the most expensive element of the device. The thickness, around 10 microns, colour and transparency of the material can also be controlled.
“Instead of making a material for energy, we cultivate it” said Mariano Campoy-Quiles, a researcher at the Institute. “Bacteria, dispersed in an aqueous culture medium containing sugar and carbon nanotubes, produce the nanocellulose fibers that end up forming the device, in which the carbon nanotubes are embedded.”
arbon nanotubes were chosen for their dimensions: “Thanks to their nanoscale diameter and their few microns in length, carbon nanotubes allow, with very little quantity (in some cases up to 1% ), to obtain electrical percolation, i.e. a continuous path where the electrical charges can travel through the material, allowing cellulose to be conductive,” said Campoy-Quiles. “The dimensions of carbon nanotubes are similar to those of cellulose nanofibres, which results in a homogeneous dispersion. In addition, the inclusion of these nanomaterials has a positive impact on the mechanical properties of cellulose, making it even more deformable, extensible and resistant, ” said team member Anna Roig.
These devices could be used to generate electricity from residual heat to feed sensors in the field of the Internet of Things, Agriculture 4.0 or Industry 4.0. “In the near future, they could be used as wearable devices, in medical or sports applications, for example. And if the efficiency of the device was even more optimized, this material could lead to intelligent thermal insulators or to hybrid photovoltaic-thermoelectric power generation systems,” said Campoy-Quiles. “With the high flexibility of the cellulose and to the scalability of the process, these devices could be used in applications where the residual heat source has unusual forms or extensive areas, as they could be completely covered with this material,” said Roig.
The energy conversion is roughly a tenth that of pure sheets of carbon nanotubes (CNT) but sonsiderably cheaper to manufacture. The highest measured power factor is of the order of 20 μW m−1 K−2 with a CNT loading of about 6 per cent by weight.
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