Minor corrections: It is not endless (the generation needs constant water supply), and by charcoal I mean nano-structured carbon layers. Still, it is impressive to see how a simple physics phenomenon could give rise to an important application: producing electrical energy. A group of scientists from China published a paper on the topic in Nature Nanotechnology Letter last month.
Water is a molecule composed of 2 Hydrogen (H) and 1 Oxygen (O) atom, and collectively they behave slightly ionic – there are H+ and OH- ions in the system, for example a cup of water. When the water on the carbon surface evaporates, it will induce a force to pull water through the tiny channels in the carbon layers. An usual piece of carbon is hydrophobic, meaning it repels water and stops the action pretty much.
The scientists found out that by treating the carbon to heat and plasma, the surface will be a mixture of carbon and oxygen compounds, and turns into hydrophilic surface. That is, water-loving oxidised carbon. Hence, the water gets pulled through the channels and evaporates at the other end at a steady rate, provided the vapour pressures at both sides don’t change.
How does it produce electricity then? Remember we mentioned earlier: the water contains ions, and a stream of water in motion is a current, carrying minuscule but measurable electrical charge. Ta-da, we produced electricity!
The scientists further found out the voltage produced can reach up to 1 V (high enough to light up an LED), and can be turned on and off by opening and closing the box in which the experiment is contained. This cheap, controllable way of producing electricity from evaporation of water could lead to very practical uses in real life, such as power generation at rural areas or places with little sunlight.