Scientists
have found a way to push zinc–bromine flow batteries to the next level. By
trapping corrosive bromine with a simple molecular scavenger, they were able to
remove a major barrier to the performance and lifespan of flow batteries.
By adding a sodium sulfamate (SANa) scavenger to the electrolyte, the
researchers were able to trap the bromine (Br₂) produced during battery operation by converting it
into a brominated amine. This simple change delivered a twofold payoff: it
extended the battery life by reducing the corrosive Br₂ and made the batteries much safer for both the
environment and the people maintaining them.
After comparing it with traditional
bromine-based flow batteries, they found that while the conventional systems
showed a sharp drop in performance after only 30 cycles due to corrosion, the
newer flow battery designs operated for over 700 cycles—nearly 1,400
hours—without significant performance loss.
The findings are published in Nature Energy.
Bromine acts as both the boon and bane
Flow batteries are rechargeable
systems that store energy in liquid electrolytes held in external tanks, making
them uniquely scalable and safe for renewable energy applications.
Zinc–bromine
flow battery variants are particularly gaining traction due to their high energy
density and low-cost materials, positioning them as potential alternatives to
traditional rechargeable batteries. These batteries store and deliver
electricity by pumping liquid solutions from external tanks through a central
reaction unit.
During charging, zinc ions plate
out on the negative electrode as solid metal, while bromide ions are oxidized
to bromine at the positive electrode. These reactions reverse the dissolution
of zinc back into the liquid, and bromine reverts to bromide, releasing stored
electrical energy.
However, today's bromine-based flow
batteries struggle with short lifetimes. The bromine released during charging
eats up key parts like electrodes, pipes, and even the storage tanks. On top of
that, bromine is highly volatile and toxic, so even small leaks can pollute the
air, irritate sensitive tissues, and affect the nervous system.
After several attempts to find a perfect antidote to the problem, researchers discovered that sodium sulfamate or SANa functioned as a chemical trap and altered how bromine behaves within the battery.
SANa
initiated a disproportionation reaction—a redox process in which a single
substance is both oxidized and reduced—splitting the bromine gas and binding to
it to form a mild product, N-bromo sodium sulfamate. This reaction reduced the
concentration of free-floating bromine to an acceptable level of ~7 millimoles
per liter (mM).
The scavenger's action of binding to
bromine had a beneficial reaction: a two-electron transfer phenomenon, which
enabled the battery to store significantly more energy. The flow battery with
added SANa achieved an energy density of 152 Wh/l, compared with 90 Wh/l for
conventional versions.
To test the battery's limits, the team
designed a 5 kW stack comprising 30 individual battery cells connected in
series. They found that the newly designed flow battery achieved over 700
stable cycles. The redesigned flow battery delivered more than 700 stable
cycles and did so at a much lower cost, since it no longer required expensive
corrosion-resistant membranes, pumps, or storage tanks.
The researchers are hopeful that insights from this study can make it feasible to design mild, long-life, and high-energy-density Br-based batteries for grid-scale applications.
Source: Unlocking corrosion-free Zn/Br flow batteries for grid-scale energy storage
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