High-entropy Fe$_2$VAl-based thermoelectric modules with improved conversion efficiency
Thermoelectric (TE) materials enable the direct conversion of heat into electricity and are attractive for sustainable energy applications. For practical deployment, TE materials must combine high efficiency with low cost, non-toxicity, and scalability. In this work, we optimize the TE performance of low-cost and robust Fe$_2$VAl-based full-Heusler compounds through high-entropy engineering: a synergistic combination of heavy-element doping and controlled off-stoichiometry results in substitutional disorder on all lattice sites, triggering one of the lowest lattice thermal conductivities, $\kappa_\text{L}\sim2.3$ W m$^{-1}$ K$^{-1}$, reported so far for full-Heusler systems. The resulting materials exhibit improved values of the average figure of merit $zT_\text{ave}\approx 0.3$ from $300-500$ K. To demonstrate reproducibility and technological relevance, a full TE module (TEM) based on the optimized alloys was fabricated and characterized. Scaled-up material batches were synthesized by hot pressing, exhibiting TE properties in excellent agreement with laboratory-scale samples, with only slightly increased resistivities in absence of post-annealing treatments. Owing to the excellent mechanical workability of Fe$_2$VAl-based materials, the TEM legs were directly brazed onto copper electrodes, enabling robust module fabrication. A ($6\times 6$)-leg TEM was assembled and systematically characterized. The device exhibits the highest output power and one of the highest conversion efficiencies reported to date for Fe$_2$VAl-based generators over the broad temperature range of $300-673$ K, underscoring the potential of this material system for scalable TE energy harvesting.