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Batteries have been around for a long time and new ones are constantly being developed in academia or industry. This article takes a look at redox flow batteries and the applications they are commonly used in.
What are Flow Batteries?
A flow battery is an electrochemical cell which uses two different solvents systems. Each system has a different active chemical component (metallic salt) dissolved into the solvent (electrolyte). Flow batteries are often considered to be a cross between a rechargeable battery and a fuel cell, and can be operated as either.
The flow battery operates by pumping the electrolyte solutions (known as the catholyte and the anolyte) through a core region containing the cathode and the anode. The two are separated by a selective ion membrane. It is the ion exchange generated between the two electrodes in this region that creates an electrical potential and generates electricity.
One major advantage of using flow batteries, is their ability of almost instantaneous recharging. This is achieved in flow batteries by replacing the electrolyte liquid and simultaneously recovering the spent material- which can then be re-energized.
There are many types of flow battery, including redox flow batteries. The main difference between traditional rechargeable batteries (without bulk flow) and flow cells is that the energy is stored in the flow cells for each electrode, whereas it is stored in the electrode itself in standard batteries.
Many flow batteries work on the premise of the following half reactions during the discharge cycle:
Anode Compartment: An+1 – e– → An
Cathode Compartment: Cn+1 + e– → Cn
In flow batteries, charge neutrality is always maintained by the membrane and the different compartments allows the power and energy components to be scaled independently of each other. The capacity in flow cell batteries is determined by the amount of electrolyte and the concentration of active ions in the electrolyte solution, whereas the power is a function of the electrode area within the cell.
Similar to Li-ion batteries, flow batteries can be stacked in series to increase the voltage and meet the requirements for certain applications. However, the electrolyte tanks remain external to the battery system.
What is a Redox Flow Battery?
Redox flow batteries were first developed in the 1970s by NASA for use in the space program and have since become a large market, with the expectation that the market will be worth at least $4 billion in 10 years’ time. The name originates from the redox reactions (i.e. oxidation and reduction) that take place to generate an electrical current.
Whilst redox flow batteries are known to be inferior to Li-ion batteries in terms of power output (and energy density), they are superior in terms of their life cycle safety, payback time (despite larger upfront costs), capacity retention and reliability- especially for stationary applications. Redox flow batteries are cost-effective against Li-ion batteries in another sense, in that they retain most of their initial value because their core components are easier to recycle than other batteries.
Whilst many different types of redox flow cell batteries are used today, common systems include vanadium-vanadium (different oxidation states of vanadium), iron-chromium and zinc-bromide redox flow cells.
During the discharge mechanism in redox flow batteries, an electron is released through an oxidation reaction on the anodic side of the cell. The electron then passes through an external circuit and is accepted at the cathode side of battery via a reduction reaction. The discharge process involves the facilitation of a higher chemical potential state to a lower potential state.
The charging mechanism reverses both the direction of the current and chemical reactions. In redox flow cells, hydrogen ions (H+) are exchanged between the two half-cells to maintain charge neutrality.
The general half reaction mechanism scheme for a redox flow cell looks like this (C=Catholyte, A=Anolyte):
Discharge:
C3+ + e– à C2+ (Reduction)
A2+ à A3+ + e– (Oxidation)
Charge:
C2+ àC3+ + e– (Oxidation)
A3+ + e– à A2+ (Reduction)
There are two types of redox flow batteries- true redox flow batteries and hybrid redox flow batteries. Flow batteries are classed as ‘true’ when all of their chemical active species are fully dissolved, at all times. Examples include vanadium-vanadium and iron-chromium systems.
For a flow battery to be categorized as a hybrid cell, at least one of the chemical constituents has to be plated as a solid during the charge cycle. Zinc-bromide and zinc-chloride redox flow batteries are examples of hybrid cells.
The vanadium redox flow cell was the first of its kind and utilizes vanadium in two different oxidation states for the anolyte and catholyte, commonly V2+/V3+ and VO2+/VO2+ (i.e. V5+/V4+), respectively. Vanadium-vanadium flow cells commonly operate around 10-40 °C and exhibit a cell voltage of 1.4-1.6 V, power densities in the 100’s mWcm-2 and a DC-DC efficiency of 60-80%.
The iron-chromium redox flow cell utilizes iron (Fe3+/Fe2+) and chromium (Cr2+/Cr3+) ions to generate a current. Iron-chromium systems have a higher operating temperature than vanadium systems (40-60 °C) and possess a cell voltage of 1.18 V, power densities between 70-100 mWcm-2 and a DC-DC efficiency of 70-80%.
Zinc-bromide hybrid cells use zinc metal alongside either Br2 or Br3 molecules to create a redox flow system. Zinc-bromide cells are not as widely used compared to some other cells despite being older than many. They were initially produced by Exxon, but the toxic nature of the bromine and maintaining a stable organic-bromine complex is a key challenge (and often active cooling systems are required to maintain the stability).
Despite these challenges, more tests are being done on these cells because they offer one of the highest cell voltages and the reaction releases two electrons per zinc atom. However, unlike many other redox flow cells, the energy and power are not fully decoupled and the energy storage capacity of the system is dependent upon both the size of the electrode area and the electrolyte reservoir.
Applications of Redox Flow Batteries
One of the most notable applications currently being developed, is that of the largest battery in the world. Being produced in China, it has a projected power of 200 MW / 800 MWh and is going to be completely powered using redox fuel cells.
It is thought that if this project in China is successful, it will not only be produced across China, but also replicated around the world. There are also many other applications being thought about and developed, including, in electric vehicles, large-scale energy storage applications and renewable energy grids.
Sources and Further Reading
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