Swellable clay-like two-dimensional titanium carbide material can expanded by hydrating and shaped or rolled out into solids or multilayer stacks with volumetric electrical capacitance almost twice that of the best previous material, according to US researchers. [Barsoum et al, Nature, 2014, online]
Writing in the journal Nature, Michel Barsoum of Drexel University, Philadelphia, Pennsylvania, and colleagues explain how batteries for portable applications, such as electric vehicles and mobile gadgets have high electrical density but all suffer from slow charging times whereas capacitors can be charged and discharged rapidly but suffer from low energy densities. The development of fast and powerful energy storage devices with high capacity per unit volume is high on the materials science agenda.
Much of the focus of recent research has been on carbon-based materials where capacities as high as 300 farads per cubic centimeter (Fcm-3) have been achieved. Other materials, such as hydrated ruthenium oxide can be charged up to 1000-1500 Fcm-3, but ruthenium is an expensive metal. This group has previously demonstrated that titanium carbide (a member of the ‘MXene’ family of materials), can be made by etching aluminum from titanium aluminum carbide using concentrated hydrofluoric acid. The resulting product has a capacitance on par with the best carbon-based materials. Illustration courtesy of team member Maria Lukatskaya.
Barsoum and colleagues reasoned that fine-tuning the synthesis for this material might allow them to control the end product more effectively and to boost capacitance significantly. They have now demonstrated that they can produce two-dimensional titanium carbide with the assistance of lithium fluoride and hydrochloric acid to produce a hydrophilic material that swells when hydrated and shrinks when dried. The product is as mouldable as modeling clay or as rollable as dough and so can be shaped or formed into films tens of micrometers thick. Their tests on this material show excellent cyclability and rate performance, as well as the aforementioned high capacitance per unit volume at 900 Fcm-3, or 245 Fg-1. They add that the preparative route is much faster than other approaches as well as avoiding harmful concentrated hydrofluoric acid.
The team suggests that this "first generation" demonstration is only the beginning for non-oxide two-dimensional materials. Given the high capacitance and malleability of the material, with further work they anticipate significant development to even higher capacity with the potential to mould the materials to fit various device form factors as well as provide electric vehicles with a low volume, high-energy storage system in the future.
"We are interested specifically in the mechanisms causing the swelling behavior in the MXenes," Barsoum told Materials Today. "It is well understood for various types of clay-like materials (for example, charge-balancing cations in clays like kaolinite, or anions in layered double hydroxides), but we are still trying to pin down what exactly is the driving force for the volume expansion with water in titanium carbide." He adds, "The ultimate goal is to double or triple our current capacity numbers."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".