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Recent developments in supercaps

Posted: 21 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Supercaps? nanomaterials? electrochemical double layer capacitors? EDLCs? photovoltaic energy?

Due to environmental, cost, security and efficiency reasons, we are moving further away from fossil fuels and closer to electricity, fundamentally altering the way we use energy. But electricity is easier to generate than it is to store, and with little progress having been made in battery technology, we face the challenge to find a cleaner, higher-power, energy-storage device. Supercaps are stepping up to that challenge by increasing energy density with new nanomaterials that enlarge electrode surface area and thus boost their ability to hold an electric charge. These advances in materials science/production methods and a thinner form factor for an optimal physical fit are equipping supercaps to be players in peak power management and energy storage at all levels from embedded electronics in portable devices to grid-level storage where they can complement batteries for better efficiency.

The beauty of a supercap as a power source is that, as it transfers and stores a charge, an electrostatic or physical effect takes place rather than a chemical reaction as occurs in a battery. Because that physical effect is reversible, a supercap can charge and discharge quickly, over and over again. But energy storage is directly proportional to a supercap's capacitance, which, in turn, is proportional to its plate or electrode surface area to which charging particles cling. Electrode surface area also determines current-carrying capability.

Nanomaterials extend charge-carrying surface area
Because a supercap's ability to hold a charge per unit weight has been small, it's been a niche player until now. But new materials are adding energy density to a supercap's technical advantageshigh power density, fast charge and discharge, long lifetime, tolerance to a wide temperature range, reliability and being maintenance-free. Traditional supercaps or EDLCs (electrochemical double layer capacitors) use activated carbon, a porous, amorphous material. Nanomaterials such as carbon nanotubes, graphene-based electrodes and carbide-derived carbon are adding heft to this emerging class of supercaps.

Vertically aligned, single-walled carbon nanotubes, hexagons bound and tied into a tube, have a crystal structure and physical properties that lead to an enlarged electrode surface area vs. activated carbon. Each carbon nanotube is a single nanometre in diameter. Graphene has a similar atomic structure to carbon nanotubes but differs in being a flat sheet of carbon atoms, connected into hexagons like a honeycomb lattice. In being flat, it's similar to silicon so that engineers can process graphene with some of the familiar techniques they use for silicon.

Coating carbon materials with 2- to 5-nm particles of silicon dioxide, also known as silica or nano sand, is another method being used to beef up capacitance. The nanoparticles self-assemble and can be less costly than potentially expensive carbon nanotubes and graphene. Also being developed is a hemp-based alternative to graphene. It can't match graphene in performance but is said to match energy at a fraction of the price. Exploration of higher temperature and voltage performance using new electrolyte materials such as ionic liquid is also under way.

A second major approach, also counting on new electrode materials to boost a supercap's energy density, is the production of asymmetric or hybrid supercaps that have one battery electrode and one supercap electrode. One combination is nanoporous nickel hydroxide and activated carbon, which together increase energy density and can translate to smaller size. Another is the use of laser-scribed graphene for its conductivity and manganese dioxide, used in alkaline batteries. Also being embraced is a method that forms one electrode using nanoporous metal oxide with a liquid crystal templating technology. Ruthenium oxide doped supercaps have been produced for specialised high energy and peak power delivery applications for a number of years. Hybrid supercaps with lithium doped to a carbon-based material of the negative electrode also target a significant increase in energy density and are known as lithium-ion capacitors (LICs).

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