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 MXeneTi3C2Tx, 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 Mg3.2SbBi0.99Te0.01, 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 Mg3.2SbBi0.99Te0.01 + 1.0 wt% MXene. Simulations of the power generation characteristics of thermoelectric modules based on n-type Mg3.2SbBi0.99Te0.01 + 1.0 wt% MXene (with bismuth telluride p-type legs) showed a maximum conversion efficiency ηmax ∼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.