Electricity on tap
(appeared on 15th Jan 2014)

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Storing solar or wind power for use when needed may become practicable and economical, says S.Ananthanarayanan.

The rechargeable battery has been used mainly to pack electricity to carry along, in the motor car or the cell phone, for instance, or to a remote place. But another area for its use is to collect electricity that is generated from sunlight or from the wind, for use when it is dark or cloudy or when the air is still. This capacity, at a sufficiently large scale, would help us use the sun and wind as alternatives to the continuous power supply from coal, oil or nuclear power plants.

Conventional chargeable batteries have the limitation of rapidly getting ‘fully charged’ and also that they work at peak power only for a short while. An alternative has been the flow battery, where electricity is stored not as electric tension between components within the battery, as is usual, but through changes in a liquid that flows through the battery and can be separately stored. The battery is then used by letting the ‘charged’ liquid flow back through the battery, to give up energy as steady electric current. And the amount of power that can be stored depends only on how much liquid can be stored and the battery never gets ‘fully charged’. The problem with flow batteries has been that the materials used for flow batteries are scarce and expensive. Brian Huskinson, Michael P. Marshak, Changwon Suh, Süleyman Er, Michael R. Gerhardt, Cooper J. Galvin, Xudong Chen, Alán Aspuru-Guzik, Roy G. Gordon and Michael J. Aziz, at Harvard, USA and Eindhoven, the Nederlands, report in the journal, Nature that they have developed an alternative class of materials based on freely accessible organic molecules.

The battery

The simple cell, like the dry cell we use in the electric torch, uses the energy stored within chemicals to generate electricity as they change into a less energetic form. The rechargeable cell, like we find in the car battery, creates a chemical change in the acid between the lead plates, when it is charged by passing electricity through the battery cells. This change leaves the lead plates oppositely charged and connecting the plates with a wire would let electricity flow, at the same time undoing the chemical change inside the cell. But the voltage to which the cell can be charged how much energy can be stored depend on the size of the plates and the volume of the cell, and are limited and fixed. Lead acid batteries are thus not convenient or economical for storage of large quantities of electricity that is needed for homes or industry.

The flow battery overcomes the size limitation of the conventional storage battery. In the normal battery, the liquid separating the electrodes stays within the cell and supports the voltage difference between the plates. But in the flow battery, the energy supplied during charging is taken up by the liquid around an electrodes and the liquid is continuously replaced. The liquids that have been raised to different energetic forms can thus be stored in large containers and any amount of electricity can be pumped in and stored.

A common and successful form of flow cell uses the vanadium redox process, where the property of atoms of the metal, vanadium, to exist in combination with different numbers of oxygen atoms is made use of. The liquid circulated is usually a solution of vanadium pentoxide, V2O5, where two atoms of vanadium combine with five atoms of oxygen, in sulphuric acid. The liquid is passed through two compartments, which contain the two electrodes and which are separated by a membrane. During the charging cycle, the negative half-cell adds electrons to the vanadium ions, while the other half cell captures electrons. The ions thus change form, while positively charged hydrogen ions are exchanged through the membrane, to balance the total charge. During discharge, when the flow is reversed, the liquid in the negative half-cell pushes its extra electron on to the electrode, to flow to the other electrode as a current. The other electrode, in turn, restores the captured electron to the vanadium ions with the charges balanced by reverse exchange through the membrane.

The battery is hence charged by energizing the electrodes and drawing away the electrolyte at a controlled rate. Running the electrolyte back through the cell then provides a typical 1.41 volts at 25°C. The working of this kind of cell needs this metal-like property of being able to readily assume and retain energized forms. The vanadium based cell displays very quick response time and has advantages of very little loss of vanadium, long operation life and large capacity, and is widely used for storage and use of electricity from intermittent sources.

But the great problem with this class of flow batteries is that the metals that are suitable, called electro-active materials, which can form liquid solutions, and also precious metal catalysts that are needed, are scarce and expensive. In fact, it is estimated that all the known vanadium deposits may be able to help create energy sources only for a fraction of our energy needs. It is in this context that the techinque now reported, using an organic, and freely available electro-active material, is of great interest.

Metal free flow battery

The work reported uses the chemical properties of a family of molecules called quinones. A carbon based compound called anthraquinone disulphonic acid, or AQDS, a form of quinone that is commonly found in the plant, rhubarb, carries two units of electrical charge, which is better than usual materials, and is able rapidly take on and give these charges on carbon electrodes in a sulphuric acid medium. This quinine/hydroquinone change in one half cell, combined with a bromine/bromide (ie, combined with hydrogen and hence charged) cycle in the other half cell is found to have good power yield and 99% preservation of the reagents in each cycle.

The fact that the quinone compound is an organic molecule also allows synthetic fine-tuning of the electro-active properties, like the working voltage, and also the solubility of the material. And then, as a material consisting of only abundant elements, of carbon, sulphur, hydrogen and oxygen, it can be manufactured on a large scale or even sourced through natural processes. As a simple working process on carbon electrodes, there is no need for expensive catalysts. The carbon electrodes also suppress creation of hydrogen ions and this, with the large size of the quionone molecule, protects against ‘leakage’ of charge across the membrane.

Over all, the use of AQDS is found by the authors of the paper to “represent a new and promising direction for cost effective large scale energy storage.”

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