A team of researchers at the Georgia Institute of Technology have developed a more compact flow battery cell configuration that reduces the size of the cell by three-quarters, and correspondingly reduces the size and cost of the entire flow battery.The innovation could revolutionise how everything from major commercial buildings to residential homes are powered.
‘The advantage of a coal power plant is it’s very steady,’ said Nian Liu, an assistant professor at the Georgia Institute of Technology. ‘If the power source fluctuates, as it does with clean energy, it makes it more difficult to manage, so how can we use an energy-storage device or system to smooth out these fluctuations?’
Flow batteries offer a potential solution. In a flow batteries, electrolytes flow through electrochemical cells from storage tanks. Existing flow battery technologies cost more than US$200/kilowatt hour and are hence too expensive for practical application; however, the new cell technology developed in Liu’s lab in the School of Chemical and Biomolecular Engineering (ChBE) could bring that cost down significantly.
Flow batteries get their name from the flow cell in which electron exchange happens. Their conventional design, the planar cell, requires bulky flow distributors and gaskets, increasing size and cost but decreasing overall performance. The cell itself is also expensive. To reduce the footprint and cost, the researchers focused on improving the flow cell’s volumetric power density (W/L-of-cell).
They turned to a configuration commonly used in chemical separation – a sub-millimetre, bundled microtubular (SBMT) membrane – made of a fibre-shaped filter membrane known as a hollow fibre. This innovation has a space-saving design that can mitigate pressure across the membranes that ions pass through without needing additional support infrastructure.
‘We were interested in the effect of the battery separator geometry on the performance of flow batteries,’ said Ryan Lively, a professor in the ChBE. ‘We were aware of the advantages that hollow fibres imparted on separation membranes and set out to realise those same advantages in the battery field.’
Applying this concept, the researchers developed an SMBT that reduces membrane-to-membrane distance by almost 100 times. The microtubular membrane in the design works as an electrolyte distributor at the same time, without the need for large supporting materials. The bundled microtubes create a shorter distance between electrodes and membranes, increasing the volumetric power density. This bundling design is the key discovery for maximising flow batteries’ potential.
To validate their new battery configuration, the researchers used four different chemistries: vanadium, zinc-bromide, quinone-bromide and zinc-iodide. Although all of the chemistries were found to be functional, two were most promising. Vanadium was the most mature chemistry, but also less accessible, and its reduced form is unstable in air. They found that zinc-iodide was the most energy-dense option, making it the most effective for residential units. Zinc-iodide offered many advantages, even compared to lithium: it has less of a supply chain issue and can also be turned into zinc oxide and dissolve in acid, making it much easier to recycle. This electrochemical solution, combined with the unique shape of the flow battery, proved to be more powerful than conventional planar cells.
‘The superior performance of the SMBT was also demonstrated by finite-element analysis,’ said Xing Xie, an assistant professor in the School of Civil and Environmental Engineering. ‘This simulation method will also be applied in our future study for cell performance optimisation and scaling up.’
With zinc-iodide chemistry, the battery could run for more than 220 hours, or to more than 2,500 cycles in off-peak conditions. It could also potentially reduce the cost from US$800 to less than US$200 per kilowatt hour by using recycled electrolyte.
The researchers are already working on commercialisation, focusing on developing batteries with different chemistries and scaling up their size. Scaling will require coming up with an automated process to manufacture a hollow-fibre module, which is currently done manually, fibre by fibre. They eventually hope to deploy the battery in Georgia Tech’s 1.4-megawatt microgrid in Tech Square, a project that tests microgrid integration into the power grid and offers a living laboratory for professors and students.
The SBMT cells could also be applied to different energy storage systems, such as electrolysis and fuel cells. The technology could even be strengthened with advanced materials and different chemistry in various applications.
‘This innovation is very application driven,’ Liu said. ‘We have the need to reach carbon neutrality by increasing the percentage of renewable energy in our energy generation, and right now, it’s less than 15 per cent in the USA. Our research could change this.’
The research has been published in the Proceedings of the National Academy of Sciences.