Thermally Chargeable Supercapacitor with 3D Ti₃C₂T_x MXene Hollow Sphere Based Freestanding Electrodes

Thermally chargeable supercapacitors (TCSCs) are emerging and promising devices that could convert thermal energy spontaneously existing in nature into electricity and then store energy for further utilization. Herein, this work reports on the freestanding 3D Ti₃C₂T_x MXene-based TCSC, where lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), poly(ethylene oxide) (PEO), and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) serve as gel electrolyte. Different from the semiconductor-based thermoelectric materials, the fabricated Ti₃C₂T_x TCSC exhibits a high Seebeck coefficient of 78.4 mV K⁻¹. Moreover, when applying a temperature difference (ΔT) of 5.8 K between the cold and hot sides, the Ti₃C₂T_x TCSC devices provide a stable high voltage of 400.6 mV. The charging/discharging cycles of the Ti₃C₂T_x TCSC devices upon periodic ΔT of 3.3 and 5.8 K demonstrate their excellent stability. To realize the practical application, the four Ti₃C₂T_x TCSCs devices connected in series are charged by supplying a ΔT of 5.8 K and used to power a digital timer, demonstrating the possibility and application of the TCSCs in self-powered integrated electronics.