The breakthrough of alternative energy sources and electric driving is currently still hindered by a persistent bottleneck: energy storage. There is not yet a material that can store as much useful energy as a full tank of petrol. That now appears to be changing with the discovery of a super material by researchers at the Australian National University.
Super capacitors as an alternative to the battery
Broadly speaking, there are three ways to store electrical energy. The first way is the well-known rechargeable battery or accumulator. There is a chemical reaction taking place at two poles, which produces a surplus of electrons at one pole versus a shortage of electrons at the other pole. This creates an electric current. The advantage is that a relatively large amount of energy can be stored per kilogram: the reason that there are batteries in electric cars. The disadvantage is that charging takes an excruciatingly long time and the necessary losses occur, because the charged ions have to struggle through the viscous liquid.
The second way is to store the energy in a magnetic field of an enormous electric coil. This requires a gigantic current through a conductor that produces no losses, in other words a superconductor. The results of this SMES, super conducting electric storage, are not great yet, because the cooling of a superconductor consumes up to a few dozen kelvins of energy, the energy density is only small and superconducting materials are expensive. The advantage, however, is that the power can be absorbed and delivered very quickly, in fractions of seconds, with little loss.
The third method is to store electrical charge. This is done by bringing two conductive plates close together and putting them under tension. Electrons now accumulate in the negatively charged plate and positively charged 'holes' (missing electrons) on the positively charged plates. The larger the surface of the plates, the closer the plates are to each other and the higher the voltage, the more energy they store. Capacitors, as these devices with built-in charged plates are called, are an indispensable part of most electrical circuits. The electrical energy stored in them is minimal, but with the development of supercapacitors they are developing as a serious alternative to batteries: the best supercapacitors perform at approximately 10% of the energy density of batteries. Like SMES, capacitors charge and discharge quickly (fractions of seconds) with little loss. However, some types of capacitors have difficulty holding their charge, because plates located close to each other easily short-circuit.
How does the ideal capacitor work?
In order to store as much energy as possible, the capacitor must meet three requirements.
- The plates should be as close to each other as possible; the capacity is in fact inversely proportional to the distance between the plates. In practice, quantum tunneling and breakdown (short-circuiting) become a problem when distances are too small. Partly for this reason, the minimum distance between plates is a few atomic thicknesses.
- The plates must have the largest possible surface area per gram. The thoughts then naturally turn to graphene, the revolutionary material of an atomic layer thickness. An alternative consists of “plates” which are basically intertwined tree-like networks.
- The plates must be leak-free: there must be no leaks or short circuits.
- The dielectric constant must be high. This means that for a given plate distance, plate size and stress a lot of charge is stored. For example, the dielectric constant for air is 1.
- The capacitor must be tolerant over a wide temperature range.
How does this new material get such a large capacity?
The discovered material, based on the abundantly available titanium oxide, works on the basis of molecular defective dipoles, ie structures on an atomic scale. A dipole is a combination of a positive and negative charge opposite each other. This gives the material an extremely large dielectric constant that is four orders of magnitude (10 ^ 4, around a factor of ten thousand) or more above normal values. This means that the material can store more than ten thousand times as much energy as a 'normal' capacitor. Unfortunately, no estimates of energy densities and energy densities are known from the article or accompanying documents, but if we assume a few electron volts per atomic group, 1-10 MJ per kg seems a reasonable estimate. This is close to the energy density of the very best batteries, making it an interesting material for energy storage. Since the metal titanium is one of the most abundant atomic elements on Earth (0.63% of all matter is titanium) this could be the breakthrough we are all waiting for.
W. Hu et al., Electron-pinned defect dipoles for high-performance colossal permittivity materials, Nature Materials (2013)
Patent: Giant dielectric constant material
Increased capacity, ANU (2013)