Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors—in particular, pseudocapacitors1, 2. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium3. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre4, 5; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films6. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the ‘MXene’ family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a ‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre7, 8. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide ‘clay’ have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report8, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Processing of MXene clay. (117 KB)
a, Dried and crushed powder. b, c, Hydrated clay is plastic and can be readily formed and moulded. d, Demonstration of films produced in the roller mill. e, f, Rolled freestanding film being lifted off Celgard membranes.
- Extended Data Figure 2: SEM images. (204 KB)
a, Multilayer MXene particle. b, Cross-section of rolled Ti3C2 film, showing shearing that is most probably responsible for the loss of the 60° angle peak in the XRD pattern.
- Extended Data Figure 3: Contact angle. (97 KB)
Digital image showing contact angle of a water droplet on rolled MXene film, indicating its hydrophilic surface.
- Extended Data Figure 4: TEM characterization of dispersed Ti3C2Tx flakes. (334 KB)
a, Representative TEM image showing the morphology and size of a large single-layer Ti3C2Tx flake. Note folding on all sides of this large flake. b, The lateral size distribution of the dispersed Ti3C2Tx flakes. c–e, Representative TEM images showing single-layer (c), double-layer (d) and triple-layer (e) flakes. f, Statistical analysis of layer number distribution of dispersed Ti3C2Tx flakes. Note that the fractions of double- and few-layer flakes are overestimated owing to inevitable restacking and edge folding of single-layer flakes during preparation of samples for TEM analysis. Edge folding is clearly seen in a. An example of restacking is shown in Extended Data Fig. 5.
- Extended Data Figure 6: Gravimetrically normalized capacitance. (458 KB)
Cyclic voltammetry profiles at different scan rates for 5-µm-thick (a), 30-µm-thick (b) and 75-µm-thick (c) electrodes in 1 M H2SO4. d, Gravimetric rate performances of rolled electrodes, 5 µm thick (black squares), 30 µm thick (red circles) and 75 µm thick (blue triangles).