There is a desire to transition towards clean, sustainable energy to protect the planet and reverse the effects of climate change. The increasing use of renewables is encouraging, but these can produce so much more energy than we can use at any moment; what is needed is a way to store the excess until required. One promising energy storage technology is redox flow batteries, a type of electrochemical energy storage device that can be used like a fuel cell or a rechargeable battery. They have great potential for energy storage thanks to their flexible system design and ease of scaling up.
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What are Redox Flow Batteries?
Redox flow batteries (RFB) convert chemical energy into electrical energy through reversible oxidation and reduction of working fluids, employing heterogeneous electron transfer i.e., electrons relocate from one atom or molecule to another.
An RFB system consists of three main components: energy storage tanks, an electrochemical cell, and a flow system. They have a unique ability to decouple power and energy based on their architecture, so conversion and storage occur separately. Energy conversion occurs in two half cells separated by an ion-permeable membrane to ensure the ion mix as little as possible.
In an RFB, charging and discharging can take place in the same cell, with the cell’s energy determined by the amount of storage medium or electrolyte, and the power – which can be scaled separately - determined by the cell size and the number of cells.
RFBs offer technical advantages over other conventional rechargeables, including different liquid tanks and near-limited longevity, but comparatively, they are not as powerful and necessitate complex electronics.
Most RFBs are based on carbon electrodes but there has been much research into alternative materials. Since 2000, the number of scientific publications and commercialization efforts have risen significantly, although only a small handful of these will be commercially relevant.
Improving Materials in Batteries
Vanadium redox flow batteries (VRFB) were the first commercial RFB, originally marketed in the 1980s. They offer several advantages over other chemistries, despite their limited power and energy density.
VRFBs work like standard RFBs except vanadium is used at both electrodes. This prevents cross-contamination and results in unparalleled cycle lives (between 15,000 and 20,000) and a record levelized cost of energy (a measure of the average net cost of electricity generation for a plant over its lifetime).
Research continues to improve their efficiency and increase the defined current and power density of these RFBs. It has focused on electrolytes able to increase active material concentration and energy density, membranes with higher proton conductivity, and lower ion crossover. Other problems to overcome include reducing the bulk of the RFB, and sourcing vanadium, a strategic material with limited availability.
Zinc-bromine flow batteries were used in early electric vehicles in the 1980s. They are the most common hybrid flow battery, employing zinc at the negative electrode and an aqueous solution of zinc bromide as the electrolyte. They have no cycle life limitation because the electrolyte does not suffer from aging effects.
Zinc-bromine RFBs promise high specific energy and cell voltage in theory, but practically these are much lower than expected. They also suffer zinc deposition on the negative electrode, which leads to dendrite growth and cell failure over time. They are also expensive as they require sequestering and complexing agents to prevent toxic bromine vapor emissions.
Zinc-polyiodide RFBs claim to be safer as they do not contain any acidic electrolytes and are non-flammable. They do, however, suffer zinc buildup at the negative electrode, reducing its efficiency. The zinc forms dendrites which limit the cell’s power density, although this can be somewhat solved by adding alcohol to the electrolyte.
The Future of Redox Flow Batteries
RFBs are most certainly key to sustainable stationary energy storage for the future, but there are still improvements to be made. The main focus is on durability and cost and although VRFBs are the leaders in terms of market infiltration and performance, there is a need to find alternatives because of the scarcity of the transition metal.
Research is focused on new redox chemistries and new cell configurations. Alternatives may include aqueous organic pure flow batteries such as vanadium-oxygen RFB, in which the positive electrode is replaced with an air electrode, or vanadium-bromine flow batteries, with the aim to improve on the limited energy density of VRFB.
While both have only been tested on a small scale, they are promising, as are the many other chemistries under investigation, including those using no vanadium, such as hydrogen bromine flow batteries.
Whichever route is chosen, alternative candidates for stationary storage must be cost-effective to allow for the integration of a larger proportion of renewables into the electricity grid. They must be safe and affordable, and capable of seamlessly replacing the lithium-ion batteries that currently dominate.
References and Further Reading
Noak, J. et al. (2020) Redox flow batteries for renewable energy storage, Energy Storage News - https://www.energy-storage.news/blogs/redox-flow-batteries-for-renewable-energy-storage. Accessed 21 June 2021.
Sánchez-Díez, E. et al. (2021) Redox flow batteries: Status and perspective towards sustainable stationary energy storage, Journal of Power Sources - https://www.sciencedirect.com/science/article/pii/S0378775320311083. Accessed 21 June 2021.
Chen, R. et al. (2017) Redox Flow Batteries: Fundamentals and Applications, Redox – Principles and Advance Applications - https://www.intechopen.com/books/redox-principles-and-advanced-applications/redox-flow-batteries-fundamentals-and-applications. Accessed 21st June 2021.