Defect Engineering for Flexible n-Type Mo₂TiC₂T_<i>x</i> <i>o</i>-MXene Thermoelectric Efficiency Enhancement

With the rapid advancement of wearable electronics, the demand for efficient portable power supplies has become increasingly urgent. Thermoelectric materials, which can directly convert heat, such as body heat, into electricity, offer a promising avenue for sustainable energy supplementation. However, achieving a high thermoelectric performance in flexible materials suitable for body heat harvesting remains a significant challenge. Here, we introduce a strategy for synergistically tuning surface oxygen defects and optimizing microstructures in low-dimensional semiconductor materials, resulting in flexible, ammoniated dual-transition metal carbide <i>o</i>-MXene N-Mo₂TiC₂T_<i>x</i> with enhanced properties. Theoretical and experimental analyses reveal that high-temperature ammoniation produces a low-oxygen-functionalized surface, reduces interlayer spacing, and minimizes defect density, thereby significantly increasing the electrical conductivity. Nitrogen atoms incorporated at the nanosheet terminals further increase the electron density near the Fermi level, resulting in an enhanced Seebeck coefficient. Consequently, N-Mo₂TiC₂T_<i>x</i> films treated at 900 K achieve an electrical conductivity of 1.03 × 10⁴ S m^-1, a Seebeck coefficient of -27.8 μV K^-1, and a power factor of 7.99 μW m^-1 K^-2 at room temperature, nearly 1.2-fold higher than that of untreated materials, while retaining excellent flexibility. Moreover, a wearable thermoelectric generator constructed from these N-Mo₂TiC₂T_<i>x</i> films produces a voltage of 1.4 mV under a temperature gradient of approximately 12 K between human skin and ambient air, underscoring its excellent capacity for harvesting low-grade thermal energy. These findings establish a paradigm for the development of flexible, high-performance thermoelectric materials, paving the way for next-generation wearable and industrial energy applications.