1.2.3 Zinc-cerium redox system
Zinc-cerium batteries are a type of redox flow battery first proposed by Plurion Inc.
(UK) in 2000s. Negative side half cell contains zinc electrolyte, positive side half cell contains cerium electrolyte and electrolytes which are circulated while operation are stored in two separate reservoirs. Negative and positive half cell electrolyte compartments are separated by a Nafion cation exchange membrane. Due to the high standard electrode potentials of both cerium and zinc redox reactions in aqueous media the open-circuit cell voltage is significantly high as 2.43 V [7]. Compared to other developed flow battery systems, this system has the highest power density per electrode area and cell voltage. Also supporting electrolyte which is made up of Methanesulfonic acid is used. During charging zinc is electroplated at the negative electrode and redox reactions of Ce(III)/Ce(IV) occur at the positive electrode.
Furthermore, this system is classified as a hybrid flow battery.
1.2.4 Polysulphide bromide battery (PSB)
A polysulphide bromide battery is a flow battery in which sodium polysulphide and sodium bromide are used to prepare electrolyte salt solution. The Sodium ions are allowed to pass through the membrane to maintain the electroneutrality of the cell. Even though this technology is used to be environmentally friendly, there is possibility that toxic bromine vapour might be released if any accident occurs[8].
There are engineering technical difficulties need to be solve for the commercial use.
1.2.5 Soluble lead-acid battery
This type of flow battery is based on the electrode chemical reactions of lead(II) in methanesulfonic acid solution. The Pb(II) is highly soluble in the aqueous acid electrolyte, therefore this system differs from conventional lead acid battery. It also differs from other redox flow batteries because there is no need of membrane or separator and it needs only a single electrolyte, therefore it considerably minimizes the cost and design complexity of the batteries [10]. During charging process due to electrode reactions soluble species convert into a solid phase, while discharging process dissolution takes place in the electrode. The process of deposition and dissolution of lead due to reaction should be fast which results in no overpotential.
1.2 Types of flow batteries 9 If there is overpotential takes place then hydrogen evolution bubble formation may occur which results in reducing the capacity of the cell.
1.2.6 Vanadium-bromine redox system
Long ago the research on vanadium-bromide redox flow system was started, due to lack of technology development most of the researchers have recently shown interest.
The vanadium bromide redox flow cell uses a Br/Br3 couple in the positive half- cell and vanadium V(II)/vanadium V(III) couple in the negative half cell, using a mixture of hydrobromic acid, vanadium bromide and hydrochloric acid as the electrolyte in both half-cells. The two V /Br half-cell electrolytes are separated by an ion exchange membrane. TheV /Br cell uses the same elements in both half-cell solutions, therefore there are no problems of cross contamination. At the positive half cell no electrochemical reaction takes place for the Vanadium-bromine redox system, i.e., the oxidation of V(IV) to V(V) does not occurs while cell charging. The V/Br redox system has all the advantages of the original vanadium redox flow cell, but the higher solubility of vanadium bromide has the potential to produce higher energy densities [11]. The possibility of using vanadium V(III)/V(IV) solutions and the excess bromide ions in the positive half-cell lead to increase in the energy densities approximately doubled.
1.2.7 All-vanadium redox flow battery (VRFB)
The all-vanadium redox flow battery first developed by Maria Skyllas-Kazacos and co-workers at the University of New South Wales, Australia. It has two half cells, encompasses electrolyte using vanadium redox couples, so there is no problem of self- discharge of vanadium ions through the membrane [9]. The vanadium is available in four different oxidation states, this property of vanadium utilizes to make a battery that has just one electroactive element instead of two. These flow batteries are more suitable for large stationary energy storage applications. A VRFB includes number of assembled power cells, each of which contains two half cells that are separated by proton exchange membrane or separator. The reduction and oxidation electrochem- ical reactions occurs in the porous electrodes from which the current is collected.
The other VRFB components include pipes and pumps so that the electrolyte can flow from the external tanks to the stack. The electrolyte is prepared by dissolving
vanadium pentoxide into the dilute sulphuric acid solution. The VRFB electrolyte is reusable [12] and has an indefinite life span. According to Skyllas-Kazacos and co-researchers the VRFB system is not damaged by repeated total discharge or fluc- tuating power demand or charge rates as high as the maximum discharge rates.
VRFB system can be charged to ensure that gassing side reactions is eliminated during the high charge rates associated with rapid charging cycles. The VRFB cells can be overdischarged and overcharged within the capacity of the electrolytes and can be cycled from any state of charge or discharge, without permanent damage to the electrolytes or cells [13].