Are “liquid batteries” the future of renewable energy storage?

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As California rapidly transitions to renewable fuels, it needs new technologies that can store energy for the power grid. Solar energy declines at night and declines in winter. Wind energy ebbs and flows. As a result, the state relies heavily on natural gas to smooth out the ups and downs of renewable energy.

“The electrical grid consumes energy at the same rate as you produce it, and if you’re not using it and you can’t store it, you have to throw it away,” said Robert Waymouth, Robert Eckles Swain Professor of Chemistry in the College of Humanities and Sciences.

Waymouth leads a Stanford team investigating a new renewable energy storage technology: liquid organic hydrogen carriers (LOHC). Hydrogen is already used as a fuel or as a means of generating electricity, but it is difficult to store and transport.

“We are developing a new strategy for the selective conversion and long-term storage of electricity in liquid fuels,” said Waymouth, senior author of a study detailing this work in the journal Journal of the American Chemical Society. “We also discovered a novel, selective catalytic system for storing electricity in liquid fuel without producing hydrogen gas.”

Liquid batteries

Batteries used to store electricity for the grid – as well as batteries for smartphones and electric vehicles – use lithium-ion technologies. Due to the scale of energy storage, researchers continue to look for systems that could complement these technologies.

According to the California Energy Commission: “From 2018 to 2024, California’s battery capacity increased from 500 megawatts to more than 10,300 MW, with an additional 3,800 MW planned to be available by the end of 2024. The state predicts 52,000 MW of battery storage will be needed by 2045.”

Candidates include LOHCs that can store and release hydrogen using catalysts and elevated temperatures. One day, LOHC cells will be able to function widely as “liquid batteries”, storing energy and, when necessary, effectively releasing it in the form of usable fuel or electricity.

The Waymouth team is investigating isopropanol and acetone as components of hydrogen energy storage and release systems. Isopropanol – or rubbing alcohol – is a high-density liquid form of hydrogen that can be stored or transported using existing infrastructure until it is used as fuel in a fuel cell or the hydrogen is released for carbon-free use.

However, methods of producing isopropanol using electricity are inefficient. Two protons from water and two electrons can be converted into hydrogen gas, and then a catalyst can produce isopropanol from this hydrogen. “But hydrogen is not needed in this process,” Waymouth said. “Its energy density per unit volume is low. We need a way to make isopropanol directly from protons and electrons, without producing hydrogen gas.”

Daniel Marron, the study’s lead author and who recently completed his PhD in chemistry at Stanford University, outlined how to solve this problem. He developed a catalytic system that allows the combination of two protons and two electrons with acetone to selectively produce LOHC isopropanol, without producing hydrogen gas. He did this using iridium as a catalyst.

The key surprise was that the magic ingredient was cobaltocene. Cobaltocene, a chemical compound of the base metal cobalt, has long been used as a simple reducing agent and is relatively inexpensive. The researchers found that cobaltocene is extremely effective when used as a cocatalyst in this reaction, directly supplying protons and electrons to the iridium catalyst, rather than releasing hydrogen gas as previously expected.

Fundamental future

Cobalt is already a common material in batteries and is in high demand, so the Stanford team hopes the new understanding of cobaltocene’s properties will help scientists develop other catalysts for the process. For example, scientists are exploring more common base metal catalysts such as iron to make future LOHC systems more affordable and scalable.

“It’s basic science, but we think we have a new strategy for storing electricity more selectively in liquid fuels,” Waymouth said.

It is hoped that as this work develops, LOHC systems will be able to improve energy storage for industry and energy sectors, or for individual solar or wind farms.

Despite all the complicated and demanding work behind the scenes, as Waymouth summarized, the process is actually quite elegant: “When you have excess energy and there is no demand for it on the grid, you store it as isopropanol. When you need energy, you can return it as electricity.”

Reference: Marron DP, Galvin CM, Dressel JM, Waymouth RM. Cobaltocene-mediated catalytic hydride transfer: electrocatalytic hydrogenation strategies. J Am Chem Soc. 2024:jacs.4c02177. doi: 10.1021/jacs.4c02177

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