ElectriStor™ — The Science
Let’s explore the science behind how the ElectriStor™ liquid energy storage system works.
The basis of our Generation 1 system is the vanadium redox chemistry. Below is an instructional video and a description of how this chemistry allows us to store energy. You may find it useful to view the video first, and then read more about what you just saw in the video, in the description that follows.
Strictly speaking, a battery is an assembly of cells, electrically connected in series or in parallel, but common usage treats cells as batteries, e.g., as the individual alkaline AA or AAA cells, called batteries, used in many consumer devices. Inside all battery cells are a positive electrode and a negative one. Each of the electrodes is physically connected to a material that conducts electronic current, so that electrons can flow into or out of the cell and from there into or out of the electrodes. The surface of each electrode is in intimate contact with an electrolyte, often but not always a liquid, in which electric current can flow by movement of ions, not electrons. You may recall that ions are like atoms or molecules, but bear a positive charge or a negative charge. When electrons flow into or out of the electrode, a reaction occurs at the interface between the electrode surface and the electrolyte; this process is called an electrochemical reaction. Electric current exactly equal to the external electronic current then flows through the cell from one electrode to the other, carried by the flow of ions through the electrolyte. If a given electrode is losing electrons to the external circuit, the electrochemically active material in that electrode is said to be oxidized; if it is receiving electrons from the external circuit, it is said to be reduced. Each of these processes must occur simultaneously in the charging or discharging of a battery, so the term redox is used as a short form for “reduction-oxidation,” for the net result of the electrochemical reactions occurring in a cell.
In nearly all conventional batteries, the active materials in the electrodes are solids. The type of battery which has come to be called a redox flow battery or RFB differs from conventional batteries in that the active materials are dissolved in a liquid electrolyte and are circulated through or past the electrodes in the cell. We draw a distinction between RFBs and batteries in which at least one of the active materials is plated out of solution as a solid, which we call hybrid flow batteries. An example is the commercially available zinc bromine battery. Each electroactive material can exist in an oxidized state and a reduced state. During charging or discharging the battery, an electric current flows to the positive and negative poles of the battery. In the all-vanadium system, which is the basis of WattJoule’s Gen I, the discharge reactions that occur at the two electrodes are shown below. The symbol e⁻ represents the negatively charged electron. Each reaction is reversed for the charge process.
At the negative electrode during discharge:
At the positive electrode during discharge:
The total cell reaction for discharge is the sum of these two cell reactions. The negative electrode reaction occurs at the surface of the negative electrode, as the mixture of charged and discharged ions in the negative electrolyte circulates through the cell and returns in a loop to the external negative electrolyte tank. The same holds for the positive electrode reaction. The two electrolytes are prevented from mixing inside the cell by a specialized separator through which ions can travel to complete the internal circuit. The net result of the reactions at the electrodes and the circulation of electrolyte from the storage tanks through the cell is that the chemical energy of the ions that comprise the negative and positive active materials is transformed into electrical energy that is provided to an external circuit. In short, we say that a redox flow battery stores energy in liquids.