Simultaneously Enhanced Thermoelectric and Mechanical Properties in Mg3(Sb,Bi)2 by MXene Compositing for Automotive Waste Heat Recovery

In this work, high-efficiency, lightweight, and robust Mg₃(Sb,Bi)₂-based thermoelectric materials were developed for a series hybrid electric vehicle powered by synthetic fuels (e-fuels) with a lean-burn spark-ignition (SI) internal-combustion engine to reduce greenhouse gas emissions. Our model simulation has demonstrated that the lean-burn process dramatically lowers the waste heat temperature to ∼450 K. By compositing Mg₃(Sb,Bi)₂-based materials with electrically conductive layered MXeneTi₃C₂T_x, we successfully enhanced the n-type thermoelectric figure of merit zT in the 300–500 K range as well as its room-temperature compressive strength. Scanning transmission electron microscopy images revealed MXene located at the grain boundaries of Mg_3.2SbBi_0.99Te_0.01, preventing grain growth. The reduced grain size decreased the lattice thermal conductivity, while simultaneously improving the compressive strength. The MXene compositing also improved the chemical homogeneity, decreasing the electrical resistivity. A zT ∼ 1.2 at 473 K was obtained for n-type Mg_3.2SbBi_0.99Te_0.01 + 1.0 wt% MXene. Simulations of the power generation characteristics of thermoelectric modules based on n-type Mg_3.2SbBi_0.99Te_0.01 + 1.0 wt% MXene (with bismuth telluride p-type legs) showed a maximum conversion efficiency η_max ∼7.9% for 473 K on the hot side and 293 K on the cold side. Using this module for waste heat recovery on the exhaust pipe of the automotive lean-burn SI internal-combustion engines, thermal efficiency under steady-state conditions could be improved by 0.7% and fuel efficiency under WLTC (Worldwide Harmonized Light Vehicles Test Cycle) conditions by 1.4% according to the simulation.