Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance.

2D transition-metal carbides and nitrides, known as MXenes, have displayed promising properties in numerous applications, such as energy storage, electromagnetic interference shielding, and catalysis. Titanium carbide MXene (Ti C T ), in particular, has shown significant energy-storage capability. However, previously, only micrometer-thick, nontransparent films were studied. Here, highly transparent and conductive Ti C T films and their application as transparent, solid-state supercapacitors are reported. Transparent films are fabricated via spin-casting of Ti C T nanosheet colloidal solutions, followed by vacuum annealing at 200 °C. Films with transmittance of 93% (≈4 nm) and 29% (≈88 nm) demonstrate DC conductivity of ≈5736 and ≈9880 S cm , respectively. Such highly transparent, conductive Ti C T films display impressive volumetric capacitance (676 F cm ) combined with fast response. Transparent solid-state, asymmetric supercapacitors (72% transmittance) based on Ti C T and single-walled carbon nanotube (SWCNT) films are also fabricated. These electrodes exhibit high capacitance (1.6 mF cm ) and energy density (0.05 µW h cm ), and long lifetime (no capacitance decay over 20 000 cycles), exceeding that of graphene or SWCNT-based transparent supercapacitor devices. Collectively, the Ti C T films are among the state-of-the-art for future transparent, conductive, capacitive electrodes, and translate into technologically viable devices for next-generation wearable, portable electronics.