Geometric optimization of Cu₂Se-based thermoelectric materials for enhanced power generation

Global energy consumption is increasing rapidly, creating an urgent need for sustainable and renewable energy sources to mitigate fossil fuel depletion and environmental pollution. Among various renewable sources, waste heat plays a critical role, as it is generated by numerous systems such as automobiles, ships, power plants, and industrial facilities like oil refineries and steel plants. Thermoelectric (TE) power generation offers a promising solution for efficiently recovering waste heat by enabling the direct conversion of heat into electricity without environmental impact. The energy conversion efficiency of TE devices depends on both the performance of the TE material and the temperature difference across the device. Maximizing this temperature gradient and enhancing energy conversion efficiency require careful design at the device level, especially for TE generators. In conventional cuboid TE leg-based devices, key design parameters include the length, aspect ratio, and fill factor of the cuboid legs.

Another strategy involves optimizing non-cuboid geometries for individual TE legs, providing an additional layer of control over thermal transport and facilitating larger temperature differences compared with cuboid designs. While several simulation studies have explored non-cuboid TE leg geometries, experimental validation combined with comprehensive design for power generation has been limited. This limitation arises from the difficulty of fabricating complex geometries using conventional metallurgical processes.

Recently, 3D printing has emerged as a promising method for manufacturing TE materials, allowing customization of TE legs with specific geometries. However, this technology is still under development, and significant advancements are needed to meet the manufacturing standards required for TE materials. These standards include maintaining structural integrity and achieving high TE figure of merit (ZT) values in printed products. Unfortunately, the efficiencies reported for 3D-printed TE materials are generally lower than those obtained through traditional methods, such as hot pressing or melting. Improving the TE properties of these materials requires precise engineering of atomic defects and microstructures during fabrication. However, in 3D printing, adding functional components or altering microstructures can negatively impact the rheological properties of the particle-based inks, reducing the printability of the 3D structures.

In this study, we present a design strategy to identify the optimal geometry for high-temperature power generation in Cu2Se using 3D finite element model (FEM) simulations, followed by experimental validation through extrusion-based 3D printing. Cu2Se is considered an excellent high-temperature TE material due to its high efficiency, resulting from its extremely low thermal conductivity. We conducted numerical simulations to evaluate the power generation capabilities of eight different Cu2Se leg geometries under various conditions, eventually identifying and optimizing the hourglass geometry for superior performance. Additionally, controlled liquid-phase sintering and selenium evaporation during the fabrication of the printed Cu2Se allowed the formation of high-density stacking faults (SFs) and dislocations. These defects reduced the thermal conductivity of Cu2Se, enhancing the ZT value up to 2.0 at 950 K.

To validate our designs, we fabricated and tested the power-generating performance of the 3D-printed Cu2Se legs. The hourglass-shaped TE leg demonstrated significantly higher power output and efficiency compared to the conventional cuboid-shaped leg, providing experimental validation for our approach. This work highlights the potential of optimized geometries in maximizing the performance of TE devices, offering a new pathway for enhancing the power generation capabilities of TE materials.