Symmetrical flow battery may strike right balance for grid-scale storage
As useful and impressive as current battery technology is, versions that store renewable energy for grid-scale use may look vastly different to those inside today's phones and electric vehicles. One promising technology is flow, or redox flow, batteries, which store energy in fluids inside tanks that could be upsized to satisfy energy demands as they increase. A new symmetrical design takes us a step closer to unlocking their potential, and leans on more environmentally friendly materials while it's at it.
Because the energy provided by renewable sources is intermittent by nature, using it for grid-scale applications will require large-scale storage solutions. Flow batteries are an attractive proposition because the problem can be tackled by storing liquid electrolytes in tanks for months at a time, with chemical energy being converted into electricity when the fluid is passed through a special membrane in between two tanks.
Conventional designs use a scarce and expensive metal called vanadium as the basis for the electrolyte solution, which raises questions over their viability as a long-term solution. Scientists working in this space are increasingly demonstrating the potential of greener and cheaper alternatives, finding inspiration in everything from shrimp shells, to saltwater, to candles.
Scientists at the University of Groningen in the Netherlands have conceived a different kind of flow battery that not only uses an organic molecule in place of vanadium, but takes on a symmetrical form. The two tanks in a flow battery generally hold fluids with different compositions, but scientists have made inroads with symmetrical designs by using hybrid molecules that serve the purposes of both fluids, though these quickly compromise its performance.
"The drawback of this approach is that only one part of the molecule is used on either side," said Edwin Otten, Associate Professor of Molecular Inorganic Chemistry at the University of Groningen. "And, during use, reactive radicals appear that degrade over time. This makes stability a problem."
Otten and his team were on the hunt for a molecule that could solve this stability issue and both accept and donate electrons to effectively do the job of two molecules, negating the need for a hybrid approach. They believe they have found the answer in what's called a Blatter radical, which is a bipolar organic compound with intrinsic stability. The compound was put to use in a small electrochemical cell, where the scientists proved its viability over 275 charge and discharge cycles.
"We need to bring this up to thousands of cycles; however, our experiments are a proof of concept," said Otten. "It is possible to make a symmetrical flow battery that has good stability."
The scientists say the Blatter radical molecule is relatively simple to make and scaling up production for industry use is possible, though first they'll need to create a water-soluble version of it for use in flow battery tanks, and then conduct larger scale tests.
"The crucial test is to see whether our compounds will be stable enough for commercial applications," said Otten.
The research was published in the Journal of the American Chemical Society.
Source: University of Groningen