While there has been a global drive for cleaner sources of energy, a major hurdle to bringing about this change has been energy storage. After all, what is the use of generating large amounts of energy if you can't store it for when you need it? To address this problem, researchers from the John A. Paulson School of Engineering and Applied Sciences (SEAS) at Harvard have developed a flow battery that could run for more than a decade .
Flow batteries work by storing energy in the liquid solutions of two separate tanks, a positive and a negative terminal. The amount of energy stored is only limited by the size of the tanks themselves. These batteries are fully rechargeable, and the electroactive material that is found within electrodes in conventional disposable batteries is actually part of the electrolyte solution . When the fluids are pumped through the cell, oxidation/reduction occurs and an electric potential is formed . One problem with flow batteries is that they lose storage capacity after charge-discharge cycles. This means the battery requires continual maintenance to operate. Researchers at Harvard were able to address this issue by changing the molecular structures of compounds in the electrolyte solutions and made them soluble in water. The result: a battery that only loses one percent of capacity per 1000 cycles.
Before setting out on improving the flow battery, the researchers had to determine why molecules kept degrading quickly in neutral cells. To do this, they examined how viologen in the negative electrolyte decomposed. Postdoctoral fellow, Eugene Beh, was able to modify the molecule's structure to help it endure more charge cycles. For the positive electrolyte solution, the group looked at ferrocene. Ferrocene can store charge well, but is insoluble in water. The researchers approached the ferrocene like they did with viologen to turn it into a highly soluble molecule that could be cycled safely. Thus, a new class of cost-effective flow batteries is possible. A third of the cost of existing batteries is due to the hostile chemical environment within the battery . A safer battery means cheaper material can be used to encase it.
While there is still time before the flow battery is widely adapted, it has a promising future in the energy storage industry. Compared to the flow battery, the widely known “disposable” battery can only be used once. This is because there is an irreversible change in the battery's composition after usage . Lead researcher Michael Aziz stated that Lithium ion batteries couldn't come close to surviving the 1000 complete charge-discharge cycles a flow battery could achieve . The battery is safe because the electrolytes are dissolved in water. “If it spilled on the floor, it wouldn’t eat the concrete,” assured Roy Gordon, another head researcher. As a result, the batteries could be built out of cheaper material, lowering the cost of production. This is essential as it helps get the flow battery to the $100 per kilowatt-hour price point. The Department of Energy set this number as a goal because it would place stored solar and wind energy on level ground with power from traditional power plants [1, 3].
The flow battery could have a great impact on the energy storage sector for not just economical reasons, but also pave the way for safer, more powerful batteries.