Enhanced thermoelectric performance in n-type NbFeSb-based half-Heusler alloys via Co-Cr dual-donor co-doping

NbFeSb-based half-Heusler (HH) alloys have emerged as a promising class of p-type medium- and high-temperature thermoelectric (TE) materials, owing to their outstanding thermostability, electrical, and mechanical properties. However, the inferior n-type electron transport properties of NbFeSb-based alloys pose a critical challenge for developing practical thermoelectric devices, particularly due to the stringent requirement of thermal expansion compatibility between p-n legs. Herein, we construct a series of n-type Nb 1- y Cr y Fe 1- x Co x Sb ( x = 0.04, 0.06, and 0.08; y = 0, 0.01, 0.03, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) HH alloys. Specifically, Co doping not only induces a p-type to n-type transition in NbFeSb but also achieves a remarkable enhancement in power factor for NbFe 0.94 Co 0.06 Sb. Furthermore, Cr doping results in an order-of-magnitude refinement in grain size, with the increased grain boundaries effectively scattering low-frequency phonons. Edge dislocations, nano-precipitates, and multi-scale grains enhance the full-wavelength phonons scattering. As a result, a peak zT of Nb 0.97 Cr 0.03 Fe 0.94 Co 0.06 Sb increases from 0.006 for pristine NbFeSb to 0.28 at 923 K. This work demonstrates a dual-donor co-doping strategy to suppress κ L and optimize TE performance in n-type NbFeSb HH alloys.