CLEVELAND—Among the enduring challenges of storing energy—for wind or solar farms, or backup storage for the energy grid or data centers—is batteries that can hold large amounts of electricity for a long time.
In addition to having a large capacity—potentially enough to power a neighborhood or small city for days or weeks—ideally these batteries would be safe, affordable and environmentally harmless.
With an eye toward meeting those benchmarks, researchers at Case Western Reserve University are developing novel electrolytes—fluids that can conduct ions—for rechargeable flow batteries.
Flow batteries function like extra-large fuel tanks
Imagine a car with an ordinary fuel tank capacity of a 400-mile trip. Without changing the engine, you could double the length of the trip by doubling the tank's capacity. The same is true for flow batteries: the "engine" doesn't change; the size of tanks holding the active ingredients determines the amount of energy that could be stored.
The Case Western Reserve team recently demonstrated a new electrolyte for flow batteries that is less volatile, meaning it is harder to evaporate or catch fire, and allows a new type of conductivity. The electrolyte structure allows protons—hydrogen atoms that carry a positive electric charge—to "bounce" from molecule to molecule like the ricochet off a billiard ball.
The research, published in the journal Proceedings of the National Academy of Sciences by researchers at the university's Breakthrough Electrolytes for Energy Storage Systems Energy Frontier Research Center (BEES2 EFRC), describes a new type of electrolyte with a special structure. These electrolytes improve the flow of electrons, likely because they are highly effective at conducting protons.
These electrolytes enable a completely new battery design and open more possibilities for developing safe large-scale energy storage technologies.
BEES2 researchers also believe their electrolytes could benefit other types of electrochemical technologies, such as electrocatalysis—a process that produces chemicals without requiring high pressure or temperatures.
"We have accepted the fact that these fluids need to be thick for safety reasons," said lead researcher Burcu Gurkan , Kent Smith Professor II of chemical and biochemical engineering at the Case School of Engineering and director of the BEES2 EFRC. "But instead of forcing large charged particles to push through that thick fluid, we're letting tiny hydrogen ions hop from molecule to molecule to make their way to the electrode."
How they differ from conventional batteries
Conventional lithium-ion batteries, like those in cell phones and laptop computers, move a lithium ion through an organic electrolyte, storing the lithium at the opposite electrode, and moving it back when the battery is discharged. The volatile electrolyte is prone to catching fire if the battery overheats, making them unsuitable for large-scale storage.
The researchers created a novel electrolyte that allows protons—hydrogen ions—to jump from one bond to another—rather than physically moving through the liquid, Gurkan said. They extensively characterized the electrolytes using a variety of techniques and utilized computational modeling to understand the mechanism.
"That type of conductivity is not affected as much by the viscosity—or thickness—of the solution," said the study's coauthor Robert Savinell , the George S. Dively Professor of Engineering and founding director of BEES EFRC. "It allows protons to conduct easily while the fluid remains non-volatile and safe."
Legacy and future steps
Gurkan said the technology from their research, supported by the U.S. Department of Energy, is still being developed.
"It's not at the stage where we can just march with this idea and make the flow battery," she said. "This doesn't yet have the chemical solubility we need for the density of energy storage we want. That's one of the next challenges we need to solve."
The electrolyte research at BEES2 EFRC builds on a distinguished 50-year legacy of an intentional focus on electrochemistry and electrochemical engineering at Case Western Reserve, bridging the College of Arts and Sciences and Case School of Engineering.
The research involved collaborators from: New York University, City University of New York, University of Tennessee, University of Illinois Urbana-Champaign, University of Sheffield, Rutherford Appleton Laboratory and the European Synchrotron Radiation Facility.
