[fuzzfactor]

For each kilo of iron you would like to extract electricity from by oxidizing it using ordinary air, you will need to process about 1.85 kilos of air.

This is the amount of air that contains the 0.43 kilos of oxygen that will be needed to effect the oxidation reaction by combining with the iron.

This much air will have to be processed at a rate and in such a way that the oxygen contained in the air will be efficiently made available for the iron to react with, at the desired rate of energy generation expected from a given mass of iron.

On the positive side, air is pretty light so it might not seem to require as much heavy lifting as the iron content, on the negative side air is pretty thin and kilos of air take up lots of space.

If something like this were to be scaled to absurd enough sizes, you never know what could happen.

Unlike huge wind turbines which can end up knocking birds out of the sky, a massive operation to remove oxygen from the air might just suffocate them from a distance.

[tim333]

I guess maybe rusting is slow so the battery takes hours to discharge even if you short it?

[fuzzfactor]

Not exactly.

There would be just as many sparks as there would from any other battery capable of delivering the same amperage at the same voltage level.

Oxidation of a given mass of iron may occur over a wildly variable amount of time depending on conditions.

This is where the implementation comes in, the rate of oxygen absorption when discharging may or may not be a limiting factor, considering other things like the working nature of the electrolyte, and the physical structure of the metallic iron in contact with the electrolyte.

In general the more surface area of a metal electrode of given mass, the more current it will be able to deliver at one time.

Rusting does not have to be slow, a good demo is to machine or polish plain iron (not steel) to a clean smooth rust-free appearance where it's pure bare metal. Bottom of a cast iron skillet works. Under ambient conditions of temperature and humidity it can retain its mirror-like appearance for weeks or months, but eventually will get a brown coating of surface rust. Sooner if the humidity is higher, and much sooner if temperature fluctuations have caused any humidity to condense at times. Or you can put the freshly polished metal out in the sun for a few minutes to heat up, add a drop of water, let it sit level and watch it rust before your eyes.

And that's plain water, which is actually not an electrolyte, since electrolytes' major property is to conduct electricity, versus very pure water whose quality is measured by its lack of conductivity.

The drop of water on the iron surface is not exactly a common catalyst either even though it increases the rate of reaction between the iron and the oxygen in the air. Water there is mainly the solvent that makes it more possible.

In the article water is considered the "electrolyte" but I would say it's more like the main low-cost ingredient that turns into electrolyte once you start putting electrodes in the water, and naturally changes constituency as discharging and charging take place. On contact some iron ions will dissolve in the water[0] and it will start to become capable of conducting electricity before too long. Like any other battery not only the electrode structure but also the electrolyte must be robust enough to carry the entire amount of current expected to be delivered.

[0] This happens without any acid being intentionally added, but when working there needs to be a fairly high concentration of ions in the electrolyte to support high conductivity. Conventionally not only the metal ions but also acid or alkali ions in the electrolyte contribute together.