Ink-Based 3D Printing of Selenium-Free Bismuth Telluride Thermoelectric Materials for Enhanced Waste Heat Energy Harvesting

The thermoelectric (TE) effect, which converts heat to electrical energy or vice versa, offers a promising strategy for waste heat recovery and sustainable energy solutions. Among TE materials, bismuth telluride (Bi₂Te₃) alloys demonstrate high figure of merit (ZT) values near room temperature, indicating significant potential for TE applications. Typically, Bi_2-xSb_xTe₃ exhibits p-type behavior and Bi₂Se_xTe_3-x exhibits n-type behavior. However, n-type material exhibits relatively low ZT value compared to their p-type counterparts (p-type: ZT ~ 1.8; n-type: ZT ~ 0.8). This discrepancy is particularly pronounced due to the instability of the highly volatile Se atom in n-type alloys. To mitigate this problem, researchers have developed Se-free n-type alloys comparable to p-type materials.
Meanwhile, the conventional process for fabricating Bi₂Te₃ faces not only shape limitations and the complexity of the process, but the highly anisotropic structure of Bi₂Te₃ also induces anisotropic thermal and electrical conductivity, which makes it difficult to achieve high ZT values. In contrast, 3D printing technology enables the creation of complex geometries, minimizes material consumption, and reduces construction time. However, in ink-based 3D printing commonly used for TE materials, the organic binders in the ink can sometimes lead to high porosity and organic residues, resulting in reduced performance compared to bulk TE materials.
To address these issues, we performed fabrication of n-type Bi_1.7Sb_0.3Te₃ (BST) using an ink-based 3D printing process and incorporation of edge-oxidized graphene (EOG). The ink was synthesized from BST powder, glycerol, and EOG, without using any organic binder. The binder-free ink minimized pore formation and allowed the added EOG to connect separated grains. Additionally, the stable selenium-free component allows long-term pressureless sintering for 3D printed architectures. These resulted in a ZT value (~0.7), similar to that of commercial bulk n-type Bi₂Te₃ materials. Furthermore, a single-element device composed of 3D printed EOG-BST exhibited a twofold enhancement in performance compared to pure-BST, demonstrating the ability of 3D printing process to manufacture selenium-free n-type Bi_1.7Sb_0.3Te₃.