Realizing Ultrahigh Conversion Efficiency of ≈9.0% in YbCd₂Sb₂/Mg₃Sb₂ Zintl Module for Thermoelectric Power Generation

Recently, YbCd₂Sb₂-based Zintl compounds have been widely investigated owing to their extraordinary thermoelectric (TE) performance. However, its p orbitals of anions that determined the valence band structure are split due to crystal field splitting that provides a good platform for band manipulation by doping/alloying and, more importantly, the YbCd₂Sb₂-based device has yet to be reported. In this work, single-phase YbCd_1.5Zn_0.5Sb₂ is successfully obtained through precise chemical composition control. Then, YbMg₂Sb₂-alloying increases the cationic vacancy defect formation energy and further optimizes carrier concentration. Moreover, the band structure of YbCd_1.5Zn_0.5Sb₂ is subtly manipulated, and the underlying mechanism is experimentally explored. Combined with the reduced lattice thermal conductivity, a high peak ZT value of ∼1.43 at 700 K is obtained for YbCd_1.425Zn_0.475Mg_0.1Sb₂. Subsequently, choosing Fe₉₀Sb₁₀ as the diffusion barrier layer and adopting the transient liquid phase bonding technique, for the first time, it is demonstrated that YbCd₂Sb₂/Mg₃(Sb, Bi)₂ TE module with an ultrahigh conversion efficiency of ≈9.0% at a heat difference of 430 K. More importantly, this module displays good thermal stability. This work paves the way for YbCd₂Sb₂ materials and devices in mid-temperature heat recovery.