Violation of the Wiedemann–Franz Law and Ultralow Thermal Conductivity of Ti₃C₂T_<i>x</i> MXene

The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, and electromagnetic interference shielding. While their optical, electronic, and electrochemical properties have been widely studied, information on the thermal properties of MXenes is scarce. In this study, we investigate the heat transport properties of Ti₃C₂T_<i>x</i> MXene single flakes using scanning thermal microscopy and find exceptionally low anisotropic thermal conductivities within the Ti₃C₂T_<i>x</i> flakes, leading to an effective thermal conductivity of 0.78 ± 0.21 W m^-1 K^-1. This observation is in stark contrast to the predictions of the Wiedemann-Franz law, as the estimated Lorenz number is only 0.25 of the classical value. Due to the combination of low thermal conductivity and low emissivity of Ti₃C₂T_<i>x</i>, the heat loss from it is 2 orders of magnitude smaller than that from common metals. Our study explores the heat transport mechanisms of MXenes and highlights a promising approach for developing thermal insulation, two-dimensional thermoelectric, or infrared stealth materials.