What were the main problems holding iron-air batteries back?
Iron-air batteries have long been promising for grid-scale energy storage because iron is abundant, cheap, and non-toxic, and the batteries have high energy density. But two major issues kept them from being practical: the iron electrode would gradually become coated with a non-conductive layer (passivation), and hydrogen gas would be released during charging, wasting energy and degrading the battery. These problems caused the battery's capacity to fade quickly, making it unreliable for long-term use.
A 2023 study [1] confirmed that passivation happens when a layer of iron(II) hydroxide forms on the electrode during discharge, which then cannot be reduced back to iron. The researchers found that electrodes with a large surface area and small pore size resisted this deactivation better, retaining nearly 75% of their initial capacity after 100 cycles. While that's an improvement, losing a quarter of capacity in 100 cycles is still far from practical for a grid battery that needs to last thousands of cycles.
How are researchers solving these problems now?
Two different approaches have shown dramatic improvements. The first uses an ionic liquid (a salt in liquid form) added to the battery's electrolyte. A 2021 study [2] found that adding a specific ionic liquid (1-ethyl-3-methylimidazolium L-(+)-lactate) suppressed hydrogen release by over 97% and prevented passivation by forming iron(II) lactate instead of iron oxides. The result was a battery that retained over 94% of its capacity after 1,000 cycles, with a nominal capacity of 0.416 Ah per gram of iron. That's a huge leap from the 100-cycle, 75% retention seen in the earlier study.
The second approach modifies the iron electrode itself by adding sulfur. A 2022 study [3] created sulfur-modified iron oxide electrodes that suppressed both hydrogen evolution and passivation. The best electrodes lost less than 0.5% of their capacity per cycle and operated for over 400 hours of continuous charging and discharging. Both studies [2] and [3] converge on the same conclusion: controlling the chemical reactions at the electrode surface is the key to making iron-air batteries practical.
So how close are we to actual climate impact?
These lab results show that the fundamental science is now in place to make iron-air batteries work reliably. The 94% capacity retention over 1,000 cycles [2] and the 400-hour stable operation [3] are exactly the kind of performance needed for grid storage—storing solar and wind power for when the sun isn't shining or the wind isn't blowing. Because iron is cheap and abundant, these batteries could be far more affordable than lithium-ion for large-scale storage, which would directly help decarbonize the electricity grid.
However, these are still lab-scale demonstrations. The next steps are scaling up from small coin-cell or single-electrode tests to full-sized batteries, and then to commercial manufacturing. The 2023 study [1] explicitly says that understanding electrode properties is key to "scale the technology up and optimize the cell formulations." So while the path is now clear, it will likely take another 5–10 years before iron-air batteries are deployed at the scale needed to meaningfully impact climate change.
About These Sources
This answer is built on 3 peer-reviewed studies — published from 2021 to 2023, 1 in Q1 journals, collectively cited 72 times — selected as the most relevant from 3 studies that passed quality screening, drawn from 44 papers retrieved from a database of over 500 million.
Sources used in this answer
Investigation of the properties influencing the deactivation of iron electrodes in iron-air batteries
This 2023 study found that iron electrode stability is highly correlated with surface area and pore size; the best electrode retained nearly 75% of capacity after 100 cycles, and passivation was linked to a non-conductive iron(II) hydroxide layer.
Improved battery capacity and cycle life in iron-air batteries with ionic liquid
This 2021 study showed that adding an ionic liquid (EML) to the electrolyte suppressed hydrogen release by over 97% and prevented passivation, achieving over 94% capacity retention after 1,000 cycles with a capacity of 0.416 Ah/g at C/5.
Iron Electrodes Based on Sulfur-Modified Iron Oxides with Enhanced Stability for Iron–Air Batteries
This 2022 study demonstrated that sulfur-modified iron oxide electrodes suppressed hydrogen evolution and passivation, with the best electrode losing less than 0.5% capacity per cycle and operating for over 400 hours of continuous cycling.