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 (Ti3 C2 Tx ), in particular, has shown significant energy-storage capability. However, previously, only micrometer-thick, nontransparent films were studied. Here, highly transparent and conductive Ti3 C2 Tx films and their application as transparent, solid-state supercapacitors are reported. Transparent films are fabricated via spin-casting of Ti3 C2 Tx 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-1 , respectively. Such highly transparent, conductive Ti3 C2 Tx films display impressive volumetric capacitance (676 F cm-3 ) combined with fast response. Transparent solid-state, asymmetric supercapacitors (72% transmittance) based on Ti3 C2 Tx and single-walled carbon nanotube (SWCNT) films are also fabricated. These electrodes exhibit high capacitance (1.6 mF cm-2 ) and energy density (0.05 µW h cm-2 ), and long lifetime (no capacitance decay over 20 000 cycles), exceeding that of graphene or SWCNT-based transparent supercapacitor devices. Collectively, the Ti3 C2 Tx 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.