Earlier this year, we discussed how e-textiles are an increasingly common area of research, stymied by power supply concerns. Now, researchers from Rice University have created woven nanotube fibers that turn heat into power—a discovery they hope will promote the commercialization of energy harvesting e-textiles. 

This new research relies on the production of something called textile-based thermoelectric generators (T-TEGs).

The researchers first weaved carbon nanotubes into thread-like fibers. Next, they sewed these fibers into fabric to act as a thermoelectric generator. Image used courtesy of Jeff Fitlow and Rice University

T-TEGs are capable of converting body heat into electrical energy. In contrast to conventional thermoelectric generators made from materials such as bismuth telluride (Bi2Te3) and lead telluride (PbTe), novel T-TEGs have air permeability, high flexibility, and good thermal and electrical resistance. They are also more cost-effective to manufacture.

How might these T-TEGs be yet another key to widespread adoption of e-textiles?

1-D, 2-D, and 3-D T-TEGs 

To answer this question, it's first important to make a distinction between three types of T-TEGs: 1-D, 2-D, and 3-D. These three dimensions result from different materials used in the fabrication process. 

While a 1-D T-TEG is made of organic materials like yarn, filament, or fiber, a 2-D T-TEG has a combination of fabrics and organic or inorganic thermoelectric materials. 3-D T-TEGs are made when inorganic thermoelectric ingots (such as bismuth telluride alloys) are woven into fabrics. 

Textile-based thermoelectric generator finds application in a wide range of areas including passive sensing. Image used courtesy of Wiley Online Library

The manufacturing process of a 1-D T-TEG first involves spinning techniques such as wet spinning to make thermoelectric materials. Using this technique, manufacturers can create polymer-based fibers by dispersing polymer in a solvent while immersing the spinneret in a coagulation bath to solidify the fibers. The corresponding TEG is then fabricated by connecting the thermoelectric fibers electrically in series and thermally in parallel with electrodes.

Common materials used to fabricate the thermoelectric fibers or yarns include carbon nanotubes, graphene, and the composite of polyethylene glycol/carbon nanotube. 2-D T-TEGs employ fabrics with high flexibility, such as cotton, to be coated on thermoelectric thin films.

What Factors Influence the Power Output of a T-TEG?

Just how much power can T-TEGs produce? The Rice researchers found that the maximum output power generated by a T-TEG is inversely proportional to its internal resistance. Generally, the output power of T-TEG is majorly influenced by a number of factors including its structural design, environmental conditions, and materials used in the fabrication.

One of the Rice researchers presenting a cotton fabric woven with carbon nanotube fibers, which can transform heat into energy that powers an LED. Image used courtesy of Jeff Fitlow and Rice University

T-TEG developers aim for a high thermoelectric power factor, which is useful in energy harvesting applications. In general, conventional inorganic thermoelectric materials usually demonstrate better power factor performance than organic thermoelectric materials.

For instance, while bismuth telluride alloys exhibit a power factor of approximately 4.5 mW m-1 K-2, a composite of p-type polyethylene glycol/carbon nanotube (developed through a wet spinning method) yields a high power factor of 378 µW m-1 K-2.

Rice University Yields a Giant Power Factor

The Rice University researchers fabricated a 1-D T-TEG using double-wall carbon nanotube (or DWCNT) fibers. These fibers are characterized by high electrical conductivity, high density, and a low level of impurities. What’s more, they have wall diameters in microscopic size.

As noted earlier, spinning techniques can be used to create the thermoelectric material used to fabricate 1-D T-TEGs. The Rice researchers used such a method to spin the carbon nanotubes in chlorosulfonic acid (CSA).

The researchers reported the highest recorded power factor of 14±5 mW m-1 K-2 when the carbon nanotube-based thermoelectric fibers were chemically doped. This feat was achieved during the spinning procedure to tune the Fermi Energy close to a 1D Van Hove singularity (VHS).

Schematic of the carbon nanotube-based textile-based thermoelectric generator. Image used courtesy of Nature

As a small-scale experiment, it provided enough energy to power up a light-emitting diode (LED). The researchers, however, noted that their carbon nanotube-based thermoelectric material may find use beyond energy harvesting applications; they also see it being effective for active cooling when developers require a large power factor and thermal conductivity.