Diameter-Dependent Carbon Nanotube Hydrogel Formed with Tannic Acid and Its Application in Thermoelectric Power Generation

In this study, we discovered a new diameter-dependent carbon nanotube (CNT) hydrogel composed exclusively of CNTs and tannic acid (TA). Accordingly, we first examined the relationship between the concentrations of CNTs and TA, as well as the CNT diameter, and whether gelation occurred. As a result, we found that when the TA concentration was fixed at 0.15 wt%, the threshold CNT concentration required for gelation was 0.05 wt%, which was lower than the values reported for previously known CNT hydrogels. We also determined that a TA to CNT weight ratio of 2-3 is critical for gelation. Furthermore, we found that subjecting the CNT dispersion to hydrothermal treatment at 160 °C, followed by freezing and ambient drying, produced a CNT aerogel that retained its 3D structure. Then, we evaluated the thermoelectric properties (electrical conductivity and Seebeck coefficient) of the resulting CNT hydrogel and aerogel under a temperature gradient for application. Both materials exhibited stable and reproducible electromotive force, and the measured Seebeck coefficients were comparable to those of conventional CNT-based thermoelectric materials. These findings demonstrate that 3D thermoelectric materials can be readily fabricated from CNT dispersions via simple processes and highlight the potential of these materials for future applications in energy-harvesting devices.