A non-flammable flow battery electrolyte uses proton-hopping to conduct electricity safely

Storing energy from wind and solar farms, powering a neighborhood through a three-day grid outage, providing backup capacity for data centers - all require batteries that can hold large amounts of electricity for extended periods, operate safely, and do so at a cost that makes them viable at scale. Lithium-ion batteries, ubiquitous in consumer electronics and electric vehicles, meet some of these criteria but fail critically on others: their organic electrolytes are volatile and prone to fire, making them unsuitable for the megawatt-hour scale storage that grid applications require.

Flow batteries offer a different architecture. Rather than storing energy in fixed electrode materials, they pump liquid electrolytes - fluids containing energy-carrying ions - through an electrochemical cell. The amount of energy stored depends on the size of the tanks holding those liquids, not the size of the cell itself. Doubling energy capacity means doubling tank volume, not redesigning the entire battery. Researchers at Case Western Reserve University have now demonstrated a new electrolyte for flow batteries that addresses a fundamental tension in the field: the tradeoff between safety and conductivity.

The proton-hopping mechanism

Conventional flow battery electrolytes that use water-based or organic solvents face a core problem: making them safe - less volatile, less flammable, more chemically stable - typically means making them more viscous, and higher viscosity slows the movement of ions through the fluid, reducing conductivity and power output.

The Case Western Reserve team, working at the Breakthrough Electrolytes for Energy Storage Systems Energy Frontier Research Center (BEES2 EFRC), took a different approach. Their electrolyte allows protons - hydrogen ions carrying a positive charge - to conduct electricity not by physically moving through the fluid, but by hopping between molecular bonds. The proton reaches the electrode not by swimming across the electrolyte, but by passing a baton from one molecule to the next in a chain.

"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 Case School of Engineering and director of the BEES2 EFRC. "But instead of forcing large charged particles to push through that thick fluid, we are letting tiny hydrogen ions hop from molecule to molecule to make their way to the electrode."

This type of conductivity - called Grotthuss mechanism after the 19th-century chemist who first described it for water - is less sensitive to fluid viscosity than conventional ionic conductivity. A thick fluid that would slow a conventional ion to a crawl can still support rapid proton hopping if the molecular architecture is designed correctly. The result is an electrolyte that can be made non-volatile and safe without the conductivity penalty that has constrained previous safety-optimized designs.

How it differs from conventional batteries

Conventional lithium-ion batteries move a lithium ion through an organic electrolyte, storing the lithium at the opposite electrode. The organic electrolyte is volatile and prone to ignition if the battery overheats - which explains why lithium-ion fires are a serious safety concern in large-scale applications. The Case Western team electrolyte replaces this mechanism with a proton-conducting system that remains stable at higher temperatures and does not carry the same ignition risk.

"That type of conductivity is not affected as much by the viscosity of the solution," said study co-author 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."

The research team extensively characterized the electrolytes using multiple analytical techniques and used computational modeling to understand the hopping mechanism at the molecular level. This combination of experimental and computational approaches provides a mechanistic foundation for further design work rather than an empirical result without explanation.

Current status and next challenges

The technology is at an early stage of development. The electrolyte has demonstrated the proton-hopping conductivity mechanism and has enabled a new battery design concept, but it does not yet meet the energy density requirements for practical large-scale deployment.

"It does not yet have the chemical solubility we need for the density of energy storage we want. That is one of the next challenges we need to solve," said Gurkan. Increasing the concentration of energy-carrying species in the electrolyte while maintaining the structural features that enable proton hopping is the central engineering problem the team is working to address.

The researchers also believe their electrolyte could benefit electrocatalysis - processes that produce chemicals without requiring high pressure or temperature - though this application is further from commercialization than the flow battery work.

The research was supported by the US Department of Energy and 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. The results were published in Proceedings of the National Academy of Sciences.