Flexible Thermoelectric Generators based on Template- Assisted Electrodeposition of Bismuth Telluride inside Commercial Filters

Miniaturized thermoelectric generators utilizing bismuth telluride (n-type) and tellurium (p-type) semiconductors have been fabricated within commercial polyester filters. Employing electrochemistry for fabrication for both the electrical contacts and the active thermoelectric material, we create an intricate network of nanostructured bismuth telluride or tellurium, which is electrically connected to the top and bottom electrodes. The control over the electrochemical process allows us to maintain the flexibility of the commercial filter in the final device. It is mandatory to be flexible in applications such as harvesters of body heat to power wearable devices, where adaptability to human skin is essential.<br/><br/>Electrochemical deposition is a cost-effective and scalable fabrication method, that does not use high vacuum or expensive facilities, but has the advantage of allowing conformal growth of the material when a template is used. Given that the commercial filters have hollow structures in the order of tens to hundreds of nanometers in diameter and microns in length, the obtained material will be nanostructured with those dimensions. Our work demonstrates enhanced thermoelectric performance compared to electrodeposited thin films due to the nanostructure [1]. Also, with the control of the different parameters during the electrochemical deposit (temperature, applied voltage, etc.), the stoichiometry, morphology, and crystallographic orientation can be tailored. Once the samples were grown, scanning electron microscopy images, X-Ray diffraction images, and Raman spectroscopy were used to get complete information on the material and optimize the filling of the template, the material composition (which is crucial in the case of bismuth telluride), and crystal orientation to have the best performing material for applications in the out-of-plane direction, that is, with the temperature gradient applied in the direction of the thickness of the template. Once these first parameters were optimized, and a high filling ratio of nanostructures oriented along the [110] direction with stoichiometric composition was obtained [2], the transport properties were also measured (Seebeck coefficient and electrical conductivity) to fine-tune and optimize the material.<br/>For the final thermoelectric generator, which uses the gold electrode evaporated in one of the faces of the filter to perform template-assisted electrochemical deposition as the bottom electrode, a top electrical contact needs to be implemented. This has been made by electrochemically depositing nickel on top of the previously grown bismuth telluride or tellurium nanostructure. In this way, the electrical contact between the different nanostructures of thermoelectric material and the top electrode is granted, and a macroscopic electrode is formed on the top of the filter, which simplifies the use of the generator.<br/>Finally, the optimization of the geometry of the thermoelectric generators was firstly studied by simulations carried out with COMSOL MULTIPHYSICS and then implemented in actual devices. For the moment, thermoelectric generators consisting of 2 or 4 pairs of legs fabricated inside the commercial filter have been implemented.<br/><br/><br/><b>References</b><br/>[1] A Ruiz-Clavijo et al., <i>ACS Applied Energy Materials </i><b>4</b>, 13556 (2021).<br/>[2] A. Ruiz-Clavijo <i>et al. </i><i>Nanomaterials </i><b>8</b> 345 (2018).