It is widely accepted that this century will need to see a great transition in the energy markets, away from finite supplies of fossil fuels and towards genuinely renewable forms of energy. One of the problems with this change in infrastructure is the intermittency of supply from the significant renewables: wind and solar power.
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Even though technological advances have meant that wind and solar are often the cheapest forms of electricity, intermittency may prevent them from supplying a dominant fraction of global electricity – at least, not without storage. With the price of solar photovoltaics and wind power continuing to drop precipitously, it is now energy storage and resilience of the grid that are the primary unsolved problems to making a renewable future into reality.
Energy storage solutions are diverse and vary from flywheels – where the energy is stored in a rotating wheel due to the conservation of angular momentum – to compressed air energy storage. Most of the technological innovation focuses on batteries.
While Tesla’s electric cars are powered by lithium-ion batteries, they have their disadvantages; they remain expensive, have yet to be deployed at the vast scales required, and most of all, suffer from degradation across charge cycles. This is familiar to anyone who owns a mobile phone powered by a lithium-ion battery - over the years, the maximum capacity of the battery declines with each charge cycle.
Many have suggested that redox flow batteries might be the solution; the concept here is to pump chemical solutions of opposite charge across electrodes, generating power for the electrical grid. A membrane separates two chemical components.
Since there are no solid-to-solid phase transitions, flow batteries boast a much longer lifetime than lithium-ion batteries, and they can operate at much higher power and current flow densities – so they’re useful for all kinds of electrical grid applications, including load balancing and peak shaving as well as energy storage. The longer lifetime means that the system is more reliable and reduces the cost of replacing the storage component. In addition, since you recharge them simply by replenishing the electrolyte, they could be refueled as fast as a car is today; a major advantage for certain kinds of electric vehicle.
Now new research from the University of Buffalo and the University of Rochester, published in the journal Chemical Science, has potentially found a promising compound that could be used in these redox flow batteries. It consists of modified metal-oxide clusters, or polyoxometalates, which are a particular type of molecule that have useful electroactive properties; the modification makes them twice as effective for use in redox flow batteries. The research was led in Dr. Ellen Matson’s lab in the University of Rochester, in collaboration with Dr. Timothy Cook at the University of Buffalo and using a cluster developed in the lab of German chemist Johann Spandl.
"Energy storage applications with polyoxometalates are pretty rare in the literature. There are maybe one or two examples prior to ours, and they didn't really maximize the potential of these systems," says first author Lauren VanGelder, a third-year Ph.D. student in Matson's lab and a UB graduate who received her BS in chemistry and biomedical sciences.
Initial studies of the cluster met with mixed results; although the cluster had useful properties, it lacked stability. However, by replacing the compound's methanol-derived methoxide groups with ethanol-based ethoxide ligands, stability was enhanced, and the amount of energy that a battery could potentially store was doubled.
We carried out a series of experiments to evaluate the electrochemical properties of the clusters. Specifically, we were interested in seeing if the clusters were stable over the course of minutes, hours, and days. We also constructed a prototype battery where we charged and discharged the clusters, keeping track of how many electrons we could transfer and seeing if all of the energy we stored could be recovered.
These experiments let us calculate the efficiency of the device in a very exact way, letting us compare one system to another. Because of these studies, we were able to make molecular changes to the cluster and then determine exactly what properties were effected.
Dr. Timothy Cook
What's really cool about this work is the way we can generate the ethoxide and methoxide clusters by using methanol and ethanol. Both of these reagents are inexpensive, readily available and safe to use. The metal and oxygen atoms that compose the remainder of the cluster are earth-abundant elements. The straightforward, efficient synthesis of this system is a totally new direction in charge-carrier development that, we believe, will set a new standard in the field.
Dr. Ellen Matson