A new type of flow battery with the ability to store energy in organic molecules that are dissolved in neutral pH water has been developed by scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). The new technology not only ensures a non-corrosive, non-toxic battery with a remarkably longer service life but would also decrease the production cost significantly.
Michael Aziz, the Gene and Tracy Sykes Professor of Materials and Energy Technologies, and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science headed the study, which was published in the journal ACS Energy Letters.
In the case of flow batteries, energy is stored in liquid solutions contained in external tanks. Hence the size of the tanks is directly proportional to the amount of energy stored. Although flow batteries are the preferred storage solution for intermittent, renewable energy such as wind and solar energy, the energy storage capacity of prevalent flow batteries is mostly impaired after numerous charge-discharge cycles, thus necessitating periodic maintenance of the electrolyte to ensure capacity restoration.
The Harvard researchers were able to design a battery that lost only 1% of its capacity per 1000 cycles. They achieved this by carrying out modification of the structures of molecules constituting the negative and positive electrolyte solutions, and rendering them water soluble.
Lithium ion batteries don’t even survive 1000 complete charge/discharge cycles.
Because we were able to dissolve the electrolytes in neutral water, this is a long-lasting battery that you could put in your basement. If it spilled on the floor, it wouldn’t eat the concrete and since the medium is noncorrosive, you can use cheaper materials to build the components of the batteries, like the tanks and pumps.
The reducation in the cost is very important because the aim of the Department of Energy (DOE) is to develop a battery with the ability to store energy for less than $100 per kilowatt-hour. Such lower costs will render stored solar and wind energy competitive when compared to energy generated from conventional power plants.
If you can get anywhere near this cost target then you change the world. It becomes cost effective to put batteries in so many places. This research puts us one step closer to reaching that target.
This work on aqueous soluble organic electrolytes is of high significance in pointing the way towards future batteries with vastly improved cycle life and considerably lower cost. I expect that efficient, long duration flow batteries will become standard as part of the infrastructure of the electric grid.
Imre Gyuk, Director of Energy Storage Research at the Office of Electricity of the DOE
According to Eugene Beh, first author of the paper and a postdoctoral fellow, the main aim in developing the battery was first to interpret the reason for rapid degradation of the previous molecules in neutral solutions. Beh first discovered the decomposing mechanism of the molecule viologen in the negative electrolyte, which enabled him to alter its molecular structure, thus making it more resilient.
Next, the researchers concentrated on ferrocene (a molecule popular due to its electrochemical properties) in the case of the positive electrolyte.
Ferrocene is great for storing charge but is completely insoluble in water. It has been used in other batteries with organic solvents, which are flammable and expensive.
However, the researchers functionalized ferrocene molecules in a manner similar to viologen, thus rendering an insoluble molecule into a readily soluble molecule that can be cycled in a stable manner.
Aqueous soluble ferrocenes represent a whole new class of molecules for flow batteries.
In particular, the neutral pH will aid in reducing the cost of the ion-selective membrane separating the two sides of the battery. High-cost polymers (costing nearly one-third of the overall device cost) are used in a majority of prevalent flow batteries to withstand the vigorous chemical reactions that occur in the battery. When salt water is chiefly used on both sides of the membrane, low-cost hydrocarbons can be used as a substitute for expensive polymers.
Diana De Porcellinis, Rebecca Gracia, and Kay Xia coauthored the paper. The Office of Electricity Delivery and Energy Reliability and the Advanced Research Projects Agency-Energy, both part of the DOE, supported the research.
The researchers, with the aid of the Harvard’s Office of Technology Development (OTD), are working in collaboration with various companies to develop the technology for industrial applications as well as to refine the interactions between the electrolyte and the membrane. A portfolio of pending patents on advancements in flow battery technology has been filed by the Harvard OTD.