Electrochemical energy-storage devices such as Li-ion batteries (LIBs) and electrochemical supercapacitors (ECs) are currently the best option for powering portable electronics. Even better would be to combine the two types of device into one multifunctional electrode that combines the high energy density and capacity of Li-ion batteries with the high power density of supercapacitors. Capacitors are devices that store electric charge but ECs can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied.

Until now, researchers have mainly studied carbon materials such as nanotubes and graphene nanoribbons for LIB anodes and ECs. However, they are far from ideal because carbon only has a theoretical storage capability of around 370 mAh/g in LIB anodes and less than 150 F/g in ECs. Transition metal oxides, in particular Fe2O3 (which has a high storage capability of 1005 mAh/g in LIB anodes and more than 1340 F/g in ECs), are good alternatives but again there is a problem in that these structures expand and contract too much during the charge/discharge cycles of a battery, which limits their use too. The fact that most transition-metal oxides conduct electricity poorly is also a big drawback.

Carbon-onion coating

One way of restricting the volume changes in these oxides and enhancing their electronic conductivity is to passivate them using a coating made of “carbon onions” (carbon nanoparticles consisting of concentric graphite-like shells).

James Tour and colleagues have now done just this for electrodes made from tantalum oxide nanotubes. The researchers coated the surface of the tubes with Fe2O3 nanoparticles, themselves coated with carbon-onions. The carbon-onion layers act as microelectrodes to separate the two different metal oxides and form a nanoscale 3D sandwich structure (see figure). The result is that space-charge layers form at the boundaries between the two oxides that can then store energy thanks to charge separation.

Multifunctional electrode

The 3D nanostructured films are excellent LIBs (800 mAh/cm3) and good supercapacitors too (capable of storing more than 18 mF/cm2). “The fact that the two devices can be assembled onto the same electrode is promising for next-generation hybrid energy storage and delivery,” says team member Yang Yang. “Ta2O5 nanotubes and carbon-coated Fe2O3 nanoparticles are electrochemically active for both LIBs and supercapacitors and the carbon coating also makes the electrodes more conductive than if the Ta2O5 nanotubes were left bare,” he told nanotechweb.org. “The electrode actually becomes multifunctional thanks to us using both materials.”

According to the team, the devices might find use in wearable energy devices, including smart textiles. They might also be ideal as air batteries.

The researchers describe their electrochemical energy storage devices in ACS Nano DOI: 10.1021/nn502341x.