1. All-vanadium redox flow battery
Vanadium redox stores electrolyte components separately and transports them between cells. In an all-vanadium redox flow battery, the charge is stored in this solution, and when the solution is fully discharged, the battery is recharged. The energy of a battery depends on the volume of the stored solution, and its available power (current) depends on the size of the battery pack that makes up the electrodes.
The electrolyte in an all-vanadium redox flow battery contains vanadium. The University of New South Wales has patented a vanadium-containing sulfuric acid solution battery. The electrolyte in the positive half of the battery contains VO2+ and VO2+ ions, while the electrolyte in the negative half of the battery contains V3+ and V2+ ions.
When the all-vanadium redox flow battery is being charged, the VO2+ ions of the positive electrode are converted into VO2+ ions, and the electrons leave the positive electrode of the battery at this time. Likewise, at the negative electrode, the introduced electrons convert V3+ ions into v2+ ions. The capacity of the battery can be determined by monitoring the state of the charged electrolyte. The discharge process is the opposite.
Since the solution is the charge, it is theoretically possible to discharge one battery and charge the other, and then just exchange the solution. In practice this is impractical in this field.
Assume that during discharge, the voltage of each cell is 1.41V. Therefore, a 12V battery will need to consist of about 9 cells.
All vanadium redox flow batteries have the following advantages:
(1) The capacity is infinite as it only depends on the volume of solution stored.
(2) They can provide large current quickly and can provide overload current in a short time (according to the University of New South Wales, 400% of the rated current can be passed in 10s).
(3) It can be charged at a high rate.
(4) If there is no discharge for a long time, the solution will not be destroyed.
(5) If the positive and negative solutions are inadvertently mixed, no permanent damage will occur.
(6) The battery does not require an equalized overcharge cycle.
(7) The solution has an infinite life, so the replacement cost is low.
All-vanadium redox flow batteries have the following disadvantages:
(1) The need to use a pump in the battery makes this battery more complicated than other types of batteries. This can be a hindrance to their use in remote stand-alone systems.
(2) Lower energy to volume ratio.
2. Zinc-bromine batteries
A zinc-bromine battery operates in a similar way to an all-vanadium redox flow battery. It also has an electrolytic cell that stores the positive and negative electrode solutions. The battery is located between the two plates, and the charge is also stored in the electrolyte.
In a zinc-bromine battery, an aqueous solution of zinc bromide and a quaternary ammonium salt is pumped through the battery pack. During charging, zinc metal is plated onto the electrodes, and the released bromine reacts with the quaternary ammonium salt to produce a thick oil-like complex. During discharge, the zinc redissolves, returning the electrode to its original state.
The electrolyte consists of zinc bromide salt dissolved in water. During charging, metallic zinc separates from the electrolyte solution and coats the negative plate, while bromide converts to bromine on the positive surface and is immediately stored in the electrolyzer as a safe chemically complex organic phase (a thick oil-like complex).
When discharging, the reaction process of the negative terminal is
Zn↔Zn2+(aq)+2e–
While at the positive extreme, bromine is converted to bromide by the process of
Br2(ag)+2e–↔2Br–aq
Therefore, the overall reaction equation is
Zn+Br2(aq)↔2Br–(aq)+Zn2+(aq)
The voltage per cell is about 1.67V, so a 12V battery typically requires 8 banks.
The advantages of zinc bromide batteries are as follows:
(1) Higher energy density than all-vanadium redox flow batteries.
(2) The capacity is infinite as it only depends on the volume of the solution stored.
(3) If there is no discharge for a long time, the solution will not be destroyed.
(4) The electrode is not affected by the reaction and will not corrode.
The disadvantages of zinc bromide batteries are:
A pump is required in the battery, making it more complicated than other types of batteries. This can be a hindrance to their use in remote stand-alone systems.