It’s always funny how one material or composite that you never associate with electronics pops up as part of research for one purpose and then comes up again soon after for a totally different area. In this column in November, I wrote about how researchers from Brown University and the University of Maryland had used cellulose nanofibrils, which are derived from wood, combined with copper, to form a material with an ion conductivity that is 10 to 100 times better than other polymer ion conductors. The new material can be used as a solid battery electrolyte or as an ion-conducting binder for the cathode of an all-solid-state battery. The material is thin and flexible and solves the brittleness problems found in previous solid state electrolytes.
Now, a second use of cellulose has been proposed by researchers for use in the power industry. Researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC) have developed a paper that converts waste heat to electricity, also using a cellulose material. Instead of using plant based cellulose, the researchers produced their own using bacteria and combined it with small amounts of a conductor nanomaterial. The paper device created from the cellulose material could be used to generate electricity in wearables, in medical and sports applications, and as intelligent thermal insulation. Other applications for the new devices include generating electricity from residual heat to feed sensors in the Internet of Things, Agriculture 4.0 or Industry 4.0.
“The new device is composed of cellulose, produced in the laboratory by bacteria, with small amounts of a conductor nanomaterial, carbon nanotubes, using a sustainable and environmentally friendly strategy” explains Mariano Campoy-Quiles, researcher at the ICMAB. "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. Due to 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."
Bacteria dispersed in an aqueous culture medium containing sugars and carbon nanotubes produce the nanocellulose fibers that will end up forming the device in which the carbon nanotubes are embedded. "We obtain a mechanically resistant, flexible and deformable material, thanks to the cellulose fibres, and with a high electrical conductivity, thanks to the carbon nanotubes," explains Anna Laromaine, a researcher in the study.
The new material has a higher thermal stability compared to other thermoelectric materials based on synthetic polymers, which allows it to reach temperatures of 250 °C. 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. Carbon nanotubes have been chosen for their dimensions: with a nanoscale diameter and a few microns in length, carbon nanotubes allow the material to offer a continuous path where the electrical charges can travel through the material, allowing the cellulose to be conductive and a thermal insulator at the same time. The dimensions of the carbon nanotubes are similar to those of cellulose nanofibres, which results in a homogeneous dispersion. The inclusion of these nanomaterials has a positive impact on the mechanical properties of cellulose, making it even more deformable, extensible and resistant.
The “Farming thermoelectric” study has been published in the Energy & Environmental Science journal.
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