Air Electrodes
Metal–air batteries, including zinc–air, aluminum–air, and lithium–air systems, have attracted enormous interest from scientists and engineers for a simple reason: theoretical energy density.
On paper, they look unbeatable.
Lithium–air: ~13,300 Wh/kg
Aluminum–air: ~8,100 Wh/kg
Zinc–air: ~1,086 Wh/kg
That’s far beyond what conventional lithium-ion batteries can offer.
But theory and reality part ways quickly.
The Core Problem: Energy Efficiency, Not Energy Density
During discharge, the metal anode oxidizes while oxygen is reduced at the air cathode via the oxygen reduction reaction (ORR).
During charge, the process reverses through the oxygen evolution reaction (OER).
The problem is that both reactions are inefficient.
Metal–air batteries suffer from large voltage losses, meaning there’s a big gap between:
the voltage you put in during charging, and
the voltage you get out during discharge.
That gap is called overpotential, and it represents energy that turns into heat instead of useful work.
The result: astonishing theoretical energy density + disappointing real-world efficiency.
So what is an air electrode, really?
An air electrode is not a magical membrane that “just breathes oxygen.”
It’s a porous, deliberately engineered structure that has to do several hard things at once: move oxygen gas to the right places, allow liquid electrolyte to wet those same regions, conduct electrons efficiently, and keep its catalyst alive under highly reactive conditions.
And it has to do all of that while operating in very different electrochemical environments depending on whether the battery is charging or discharging.
That last part matters more than most explanations admit. The same piece of hardware is being asked to behave like two different electrodes, at different voltages, under different chemical stresses, often using the same materials. Most of the real problems with air electrodes start there.
Why ORR and OER get separated
On paper, oxygen reduction (ORR) and oxygen evolution (OER) look like reverse reactions. In practice, they are not mirror images at all.
They prefer different catalysts. They degrade materials in different ways. And they operate most efficiently at different voltages and surface chemistries.
A “bifunctional” air electrode is one that claims to do both. In reality, that almost always means compromise. You can tune an electrode to survive the brutal conditions of OER, but you usually pay for it with poorer ORR kinetics. You can optimize for fast ORR, but those same materials often corrode, oxidize, or fall apart under OER.
Many so-called bifunctional electrodes quietly acknowledge this by using layered structures, mixed catalysts, or composite architectures that separate the functions anyway—just without saying so explicitly.
Form Energy’s choice to use separate air electrodes isn’t exotic. It’s pragmatic. It reflects something the literature has shown for decades: the optimal ORR electrode and the optimal OER electrode are not the same object.
Why long-duration storage loves this architecture
For long-duration energy storage, the priorities are different.
Efficiency still matters, but cycle life, cost, and durability matter more. Slower charge and discharge rates are acceptable. System-level complexity can be traded for electrodes that last longer and fail more gracefully.
Separating ORR and OER makes that trade easier. Each electrode can be optimized for the job it actually does. Catalyst choices become simpler. Lifetime management improves. Fewer compromises get baked into every cycle.
This kind of architecture makes little sense for electric vehicles, where power density and simplicity dominate. For grid storage, it makes a lot of sense.
Where the hype comes from
Metal–air batteries look incredible on paper.
They dominate Ragone plots. They shine in theoretical comparisons. They make for excellent pitch-deck slides.
What those plots rarely show is round-trip efficiency, catalyst replacement costs, degradation under sustained OER, or what it takes to manufacture these electrodes reliably at scale. The gap between theoretical energy density and usable, economical systems is where most of the hype lives.
The real engineering friction
Scaling air electrodes isn’t about discovering a magic catalyst. It’s about surviving reality.
You need catalysts that don’t dissolve or restructure under OER. You need carbon supports that don’t corrode away. You need electrolytes that stay where they’re supposed to be. You need gas diffusion layers that work consistently across square meters, not just in coin cells. And you need to maintain stable triple-phase boundaries for years, not weeks.
What would actually need to change to scale
Real progress would require genuinely durable OER catalysts that don’t poison ORR performance, electrode architectures that tolerate long idle periods without drifting, manufacturing methods that control porosity and wetting at scale, and perhaps most importantly honest system-level efficiency targets.
The Bottom Line
Air electrodes aren’t fake.
They’re just much harder than the pitch deck implies.
And once you accept that ORR and OER are fundamentally different problems, using two air electrodes starts to look less like a quirk and more like good engineering.
References
Liu et al., Chemical Communications, 2025. https://doi.org/10.1039/D5CC03685B
Timofeeva et al., Current Opinion in Electrochemistry, 2023. https://doi.org/10.1016/j.coelec.2023.101246
Liu et al., Energy Storage Materials, 2020. https://doi.org/10.1016/j.ensm.2019.12.011
Rajore et al., Journal of Power Sources, 2024. https://doi.org/10.1016/j.jpowsour.2024.235101