"Capacitive charge storage offers a number of desirable properties when compared with conventional batteries, including charging within seconds, long-term cycling stability and the ability to deliver up to ten times more power than batteries," the team explains.

These features are desirable for a range of applications, in electric vehicles as regenerative braking devices that capture the otherwise lost energy of a vehicle as the brakes are applied and storage for renewable energy supplies such as wind and wave power which can come in short bursts. Conventional rechargeable batteries often charge too slowly to be useful in these contexts. Other applications of capacitors, and more specifically pseudocapacitors that combine features of both rechargeable battery and standard capacitor are in mobile digital technologies and flash photography.

The team has now demonstrated that mesoporous films of iso-oriented molybdite (a-MoO3) show impressive charge storage behavior because of a combination of the atomic scale structures and the nanoscale structure of the films.  They show this by comparing the crystalline mesoporous films  to other forms of molybdenum(VI) oxide, such as mesoporous amorphous materials or crystalline materials that are not porous. Moreover although the amorphous mesoporous and crystalline forms demonstrate redox pseudocapacitance, the layered crystalline form is also amenable to electrical charging involving lithium ion insertion, whereby the ions are intercalated between the layers of a-MoO3.

Previously, the transfer of metal ions into transition metal oxide materials has proved to be rather slow. Although molybdite is an electroactive 2D layered material that can accommodate lithium ions (three for every two molybdenum ions), its modest kinetics and poor cycling behaviour have precluded its use in batteries.

In mesoporous form, however, lithium insertion takes place so rapidly, that it can be considered a pseudocapacitance process. This phenomenon offers the potential for more rapid charging as well as meaning higher power densities might be achievable that are on a par with rechargeable lithium-type batteries rather than standard capacitors.

"The potential for practical advances in energy storage using these materials and these general design rules are exciting," the researchers conclude.