By Kyle Proffitt
July 9, 2024 | A group of scientists working at Oak Ridge National Laboratory (ORNL) has reported major advances in the area of grid storage batteries. The key differentiator for these batteries is that instead of only storing energy for use when renewable sources are insufficient, they have an extra trick—they also capture and fix carbon dioxide. Two recent studies detail the work, one published and one under review.
Battery Power Online spoke with Ilias Belharouak, Corporate Fellow and Head of Electrification Section at ORNL, to better understand how these batteries work and how they might fit into the renewable energy ecosystem. The idea has a number of advantages relative to conventional batteries. “You can actually power other circuitry or other systems through this battery while you’re capturing CO2,” he said. “At the same time, you’re making product out of the CO2 as a solid… it precipitates, you can remove it, and you can sell it”. And the price tag of the entire setup is a major asset. “The whole system is dirt cheap,” Belharouak added.
Two Leading Designs
The batteries that ORNL is developing come in two primary forms at this point. The first is a Na-CO2 design; it was described in work published in May in Journal of Power Sources (DOI: 10.1016/j.jpowsour.2024.234643). For this battery, sodium metal acts as the anode, and is housed in a sealed compartment surrounded by conductive organic electrolyte. A NASICON membrane allows sodium ions to pass into and out of this chamber. For the cathode, a low-cost mixed metal (iron and nickel) hydroxide is used, and is immersed in a basin of saltwater that acts as electrolyte. The idea is that plain seawater could eventually be used.
When CO2 is bubbled into this solution, it forms some carbonic acid, which then reacts with dissolved sodium ions in a reaction catalyzed by the iron-nickel electrode. This effectively creates a demand for electrons to complete the reduction of water to hydrogen gas, and these electrons are supplied externally in a flow from anode to cathode—energy that can be harnessed to power devices immediately. Sodium ions from the anode migrate through electrolyte toward the cathode, replacing those used to form sodium bicarbonate and maintaining electrical neutrality.
When charging the cell (presumably from a renewable energy resource), the sodium ions return to the anode, but CO2 is not recreated, unlike in some other metal-CO2 battery designs. “You’re not going to decompose the sodium carbonate,” Belharouak said. Instead, “you’re just going to oxidize the water inside that aqueous solution.” The catalyst also participates in promoting this oxygen evolution. Belharouak summarizes the novel approach: “That’s the beauty of the system… you’re not reversibly putting CO2 back in the atmosphere, you’re capturing it.”
The sodium bicarbonate that is formed during discharge precipitates, and it can be removed for use in other applications. However, it can also clog the electrode and eventually lead to cell deactivation. A major discovery of the ORNL group was that they could overcome and even reverse deactivation simply by extending the charge time. “If you do this slow charging and discharging process you can actually reactivate the electrode to its initial or pristine stage.” It’s also possible to just slow the charge relative to discharge and reactivate the cell over a few cycles. “The cell will become like brand new, basically,” Belharouak said. This is a unique situation in comparison with a lithium-ion battery, where it is much more difficult to recover lost cell activity, and in this case, the Na-CO2 cell can remain operational even while it is being restored.
Battery Design Two
The ORNL researchers have taken the idea a step further by creating a second battery type using aluminum, and this work is currently under review for publication. ORNL shared a pre-print manuscript with Battery Power Online. These Al-CO2 batteries work according to the same general principles, but they have an additional advantage. Because it is less reactive, the aluminum does not need a separate chamber. It can sit in the same open reservoir, reducing design complexity. A simple porous plastic separator is used to prevent a short circuit. Aluminum is easier to acquire and cheaper than sodium. Belharouak says that the strategy could also work with zinc or magnesium.
The same saltwater solution for Na-CO2 cells is used with the addition of some potassium hydroxide, and the same iron-nickel hydroxide catalyst facilitates the reactions. When CO2 is bubbled in, aluminum ions from the anode will “migrate into proximity to the cathode to form aluminum sodium carbonate products that will fall to the bottom of the basin,” according to Belharouak. For at least some of the conditions used with Al-CO2 batteries, the byproducts did not clog the electrode, and therefore no specialized cycling protocol should be needed.
Perhaps most importantly, these cells can operate for 10 hours and beyond. “It can be even for 48 hours of reaction,” Belharouak said. This seems well equipped to provide energy when the sun is down or the wind is calm. The cells have also been shown to cycle stably for more than 500 hours.
Carbon Capturing Credentials
In the realm of CO2 capture, some technologies pull the gas directly from the air. However, direct air capture requires large energy investment because of the relatively low (0.04%) ambient concentration. Capturing CO2 more directly at a power station or cement plant is easier but still needs energy. The aqueous metal-CO2 batteries developed at ORNL currently work by CO2 infusion and do not scavenge CO2 from the atmosphere. However, compared to CO2 capture technologies that only take energy, these batteries have a major advantage of providing energy while they capture. “With a regular carbon fixation process, there is no electrochemistry. Once you capture, you’re done, and it involves a lot of energy consumption, because you need to power those systems to capture CO2,” Belharouak explained.
Where Do You Put Them?
Given that these batteries perform two somewhat unrelated tasks, one may wonder where they’re best placed. “You may think of these being deployed in a place where CO2 is being generated substantially and being captured,” Belharouak said. But again, “these electrons that are being generated through the electrochemical process can actually go and power certain functionalities within that system that generates CO2. You’re getting that advantage of capturing and actually powering other stuff”. As to how high the concentration of CO2 needs to be, he also said that “the reaction could be enabled with just 10%, which is more than enough.” Belharouak says the batteries are not limited to power-plant-type installations. “It can actually be even off-grid.” In such a scenario, the CO2 would likely need to be purchased, but this may still make sense so long as the batteries are sufficiently inexpensive and provide energy over an extended period.
Cheap and Replaceable
These batteries are “not geared for an application that requires high energy density, such as an EV,” Belharouak said. The big idea is that by creating them as open or mostly open systems with low-cost components, they can be produced at scale, and parts can be replaced quite easily. Compared with typical grid storage batteries using lithium iron phosphate (LFP) cathodes, they are probably half the energy density, Belharouak said, but that’s acceptable given all the other attributes. Again, this includes a lack of reliance on critical materials—no cobalt, no lithium, no precious metal catalysts. And because they’re inexpensive and open systems, they really don’t need to last forever.
Belharouak explained how this might play out. “Let’s think about a scenario here where we need to replace the whole thing. It does not take much to replace a basin, to replace that aqueous solution… it’s just salt in water. The electrodes are very inexpensive.” “At some point the catalyst will age, will fade,” Belharouak said, but “because they’re inexpensive, we can afford replacement or swapping with new electrodes.” And the architecture makes this simple. “You don’t need to open the system. You just remove the electrode… you continue the reaction, no issues at all.”
How Soon?
For now, the batteries are small, just glass bottles. “We have reduced the system to practice in a small prototype,” Belharouak said. As a national laboratory, ORNL does not commercialize technologies directly; their goal is to provide proof of concept and help promote commercialization and adoption. Belharouak says the main concerns now are optimizing and understanding different aspects of the system, such as how different climates may affect performance, or how components will hold up over time in salt solutions and resist corrosion. Ultimately, Belharouak says they are “moving slowly, through some industry collaboration, to a larger scale.”
