1 milliliter = 1 cm^3

237 ml = 1 cup

EDIT: Nevermind regarding the unit confusion. I'm still learning how to read.

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> The test device was powered solely by sunlight, and although it was a small proof-of-concept device, if scaled up its output would be equivalent to more than a quarter-liter of water per day per kilogram of MOF, the researchers say.

...

> The next step, Wang says, is to work on scaling up the system and boosting its efficiency. “We hope to have a system that’s able to produce liters of water.” These small, initial test systems were only designed to produce a few milliliters, to prove the concept worked in real-world conditions

[Edited]

It could be any, if the device depends on area. The more area you use, the more water you get. Weather report often uses litres per square metres. One meter high per square metre is a thousand litres, so one mm would be a litre.

Edit: in the article I read: These small, initial test systems were only designed to produce a few milliliter.

So it's much less :-)