It doesn't scale though that is why it has been forgotten. You can also get free electricity from plants but again it doesn't scale. There is a device that you put near the roots of plants and it harvests electricity enough to power a light or charge a phone but again it doesn't scale.

You can get energy from water if you split the oxygen and hydrogen atoms but you end up using more energy than what you get out. That is why it is not worth it to use hydrogen as fuel at least for now.