Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte–electrode interface when voltage is applied.

Supercapacitors, often used to bridge the gap between conventional supercapacitors and rechargeable batteries, are usually made from carbon materials or transition metal oxides and have high power densities and long-term cyclability. However, these devices can be expensive and researchers are busy looking for alternative materials from which to make them.

Anodization process makes porous layer

Now, a team led by James Tour is saying that Ni(OH)2 might just fit the bill here. The researchers produced their 3D thin film by first growing nanoporous NiF2/NiO layers on Ni foil by a technique called anodization. These layers convert into Ni(OH)2 after hydrothermal treatment.

“The anodization process makes the porous layer (which is the active layer in the finished device) attach strongly to the Ni foil substrate (which is the current collector) so we do not need any binder or other additives to coat the active materials onto the current collector,” explains team member Yang Yang. This simplifies things – and of course, reduces cost.

Enhanced capacitance thanks to lots of active sites

Working within a wide voltage window of 1.6 V in an expensive electrolyte of 6 M potassium hydroxide, the device delivers a high capacitance of 192 F/g, an energy density of 68 Wh/kg and a power density of 44 kW/kg – and this is over 10,000 charge-discharge cycles, he adds. "Our devices show greatly enhanced capacitance thanks to them forming lots of active sites in the porous structure," he says.

The device might be used in applications such as Li-ion batteries and water splitting, Tour tells

The team says that it is now busy building hybrid devices capable of generating energy and storing it in the same platform using the same materials.

The current work is reported in ACS Nano.