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, pseudocapacitors 1,2 . 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 premium 3 . The best volumetric capacitances of
carbon-based electrodes are around 300 farads per cubic centimetre 4,5 ;
hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500
farads per cubic centimetre with great cyclability, but only in thin
films 6 . Recently, electrodes made of two-dimensional titanium carbide
(Ti 3 C 2 , a member of the ‘MXene’ family), produced by etching
aluminium from titanium aluminium carbide (Ti 3 AlC 2 , a
‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to have
volumetric capacitances of over 300 farads per cubic centimetre 7,8 .
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’ havevolumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report 8 , and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.