Is "Air-gen" really possible, and how is it different from perpetual motion? "Air-gen", as seen in Nature (See here and here), supposedly extracts net positive energy from ambient air at equilibrium.  
Is this really possible?  Doesn't this violate a law of thermodynamics like perpetual motion machines?
How is "Air-gen" fundamentally different from Maxwell's Demon and the Brownian Ratchet?
 A: I only read a couple of web articles, after you asked the question. But, it seems to me that the researchers give no explanation for the phenomenon and their conclusion that it is extracting energy from an ambient atmosphere at equilibrium seems entirely unjustified. They are jumping the gun. This requires much more careful checking. 
What they note is that if you have a sheet of this material in a warm atmosphere with ambient light and water, it generates a sustained current. This does not mean that the energy is extracted from the humidity - it could equally well mean that the sustained voltage is triggered by the water molecules, but the power is provided by the photons, and that this is just some kind of solar panel.
I give the following only as a possible mechanism given what I have read so far, which is not very much, but I think it gives an idea of how the phenomenon could occur.
Hypothetical conjecture on my part ....
Suppose the function of the material in the bacterium is to be a membrane that collects water into a chamber. It might contain molecules that are in a state that will attach to a water molecule and then distort in a manner to push the water molecule through the membrane. This type of behaviour is common in cytology. This could result in a charge separation. If left at this point, the charge would be canceled by attraction of ions. But, perhaps the molecule is then hit by a photon that releases the water molecule and causes transporter molecule returns to its original state. 
What you need for this is a molecule that is in a lower energy state when it is hugging a water molecule, and which will release the water molecule when struck by light, and that is polar when it is hugging the water molecule.
This is a solar panel that needs to be wet to work. But, the water is just part of the operating conditions and does not provide the energy. 
