Storing renewable energy is just as important as generating it, and flow batteries might be one of the most promising ways to do that. While there are plenty of hurdles to jump over in perfecting the tech, a team of Harvard engineers has been making strides over the past few years with organic flow batteries, and has now tested a new molecule that makes for the longest-lasting, high-performance organic flow battery so far.
Flow batteries are built with two liquid electrolytes that are stored in external tanks and piped into the cell as needed. During charging and discharging, they pass electrons back and forth through a membrane in the cell, and their storage capacity and power output can be tweaked by changing the size of the tanks and membrane, respectively.
Traditionally, the best results from this type of battery came from electrolytes of vanadium and bromine dissolved in acid, but these chemicals can be costly and caustic. Organic alternatives to vanadium were found in the form of quinones, molecules similar to those used in plants and animals to store energy.
In 2014, the Harvard team started experimenting with over 10,000 types of quinones, gradually figuring out which ones performed best. Later, they replaced bromine with ferrocyanide and switched the acid for an alkaline mixture, then singled out a modified version of vitamin B2 as a particularly useful quinone. And finally, last year they tweaked the recipe to run on neutral water.
The problem was, these designs didn't quite pack the energy punch that's required for a practical flow battery. Now, the researchers have modified a quinone to make a new organic molecule that balances lifespan with performance, creating what they claim is the longest-lasting, high-performance organic flow battery built so far. The key ingredient has been dubbed the "Methuselah molecule," named after the Biblical old codger who apparently celebrated almost a thousand birthdays – if the Old Testament is to be believed.
"In previous work, we had demonstrated a chemistry with a long lifespan but low voltage, which leads to low energy storage per molecule, which leads to high cost for a given amount of energy stored," says Michael Aziz, co-lead author of the study. "Now, we have the first chemistry that has both long-term stability and comes in at more than one volt, which is commonly considered the threshold for commercial deployment. I believe it is the first organic-based flow battery that meets all of the technical criteria for practical implementation."
In tests on the molecule, Methuselah was shown to have a fade rate of less than 0.01 percent per day, and under 0.001 percent over each charge and discharge cycle. That means it would degrade by less than three percent each year, and could run for tens of thousands of cycles.
Methuselah also dissolves easily into a weak alkaline electrolyte. That helps it pack a higher energy density and reduces the overall cost of materials, since the walls and membrane don't need to be particularly corrosion-resistant.
"This important work represents a significant advance towards low cost, long duration flow batteries," says Imre Gyuk, Director of the US Department of Energy's storage program. "Such devices are needed to allow the electric grid to absorb increasing amounts of green but variable renewable generation."
The research was published in the journal Joule.
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