Redox-Flow Cell Stores Renewable Energy as Hydrogen

A hybrid energy system combines a redox-flow battery with a water electrolyzer for low-cost grid storage

3 min read

Nel Hydrogen bench test
Bench test set-up at Nel Hydrogen.
Photo: Nel Hydrogen

When it comes to renewables, the big question is: How do we store all that energy for use later on? Because such energy is intermittent in nature, storing it when there is a surplus is key to ensuring a continuous supply—for rainy days (literally), at night, or when the wind doesn’t blow.

Using today’s lithium-ion batteries for long-term grid storage isn’t feasible for a number of reasons. For example, they have fixed charge capacities and don’t hold charge well over extended periods of time. 

The solution, some propose, is to store energy chemically—in the form of hydrogen fuel—rather than electrically. This involves using devices called electrolyzers that make use of renewable energy to split water into hydrogen and oxygen gas. 

“Hydrogen is a very good carrier for this type of work,” says Wei Wang, who is the chief scientist for stationary energy storage research at the Pacific Northwest National Laboratory in Washington. It’s an efficient energy carrier, and can be easily stored in pressurized tanks. When needed, the gas can then be converted back into electrical energy via a fuel cell and fed into the grid.

But water electrolyzers are expensive. They work under acidic conditions which require corrosion-resistant metal plates and catalysts made from precious metals such as titanium, platinum, and iridium. “Also, the oxygen electrode isn’t very efficient,” says Kathy Ayers, vice-president of R&D at Nel Hydrogen, an Oslo-based company that specializes in hydrogen production and storage. “You lose about 0.3 volts just from the fact that you’re trying to convert water to oxygen or vice versa,” she says. Splitting a water molecule requires an applied voltage of 1.23 V.

In a bid to overcome this problem, Nel Hydrogen and Wang’s team at Pacific Northwest joined forces in 2016, after receiving funding from the U.S. Department of Energy’s Advanced Research Projects Agency-Energy. The solution they’ve come up with is a fuel cell that acts as both a battery and hydrogen generator.

“We call it a redox-flow cell because it’s a hybrid between a redox-flow battery and a water electrolyzer,” explains Wang.

Nel Hydrogen's reversible fuel cell systemNel Hydrogen's reversible fuel cell system.Photo: Nel Hydrogen

A redox-flow battery, in essence a reversible fuel cell, is typically made up of a positive and negative electrolyte stored in two separate tanks. When the liquids are pumped into the battery cell stack situated between the tanks, a redox reaction occurs, and generates electricity at the battery’s electrodes.

By comparison, the new invention has only one electrolyte, comprised of an iron salt (rather than the more commonly used vanadium) dissolved in acid. When hydrogen ions react with the iron salt (Fe2+), hydrogen gas is produced at the platinum-coated carbon cathode in the battery stack.

“We introduce iron as a middleman, so we can separate electrolysis into two reactions,” says Wang. Doing so allows one to control where and when to reverse the reaction to produce electrical energy to supply to the grid. “The system gives you flexibility... you could do the regeneration during evening time when electricity prices are at a peak,” he says.

Regenerating Fe2+ in the reverse reaction also allows for the continuous production of hydrogen gas, he says. “And because the hydrogen-iron cell uses about half the voltage of a traditional electrolyzer, you can generate hydrogen at a much cheaper cost if you do everything right.”

It also helps that iron is much cheaper and more abundant compared with vanadium.

Qing Wang, a materials scientist at the National University of Singapore, sees another benefit. “If you care more about purity and want to have ultra-pure hydrogen, then maybe it’s a good solution,” he says. Cross-contamination can sometimes occur during electrolysis because the hydrogen and oxygen gases produced are so small that they are able to traverse the membrane separator. 

The new redox-flow cell performed well in lab tests, exhibiting a charge capacity of up to one ampere per square centimeter, a ten-fold increase over normal flow batteries. It was also able to withstand “several hundred cycles” of charging, which has never been demonstrated before in hydrogen ion flow batteries, says Wang, who has a number of patents for the invention, with a few more pending.

While the PNNL team experimented on a single cell measuring 10 square centimeters, Ayers and her colleagues at Nel Hydrogen proved that the technology could work even when scaled up to a five-cell stack measuring 100 square centimeters. They plan to spend the next few months fine-tuning the system and eliminating kinks, such as how to minimize damage to the pumps caused by the acidic electrolyte, before commercializing it. 

This post was updated on 20 April 2020. 

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