Technology

Essentially if you combine hydrogen with oxygen, you get water. This chemical reaction happens naturally when the two are exposed to each other and produces heaps of energy which can easily be controlled and used for anything else.

One way to produce hydrogen is to take water, and heaps of energy, and "split" the water into hydrogen and oxygen. You can just release the oxygen into the air (since you'd be making too much to sell it).

The cost largely comes down to where you get your energy from. As solar gets more and more widely deployed, some countries now have more energy during the day than they can use - the price of power in those countries is not just close to zero sometimes it's negative. The grid will literally pay you to use the electricity during peak production. Since it's cheaper to provide power than shut down infrastructure that will be needed again in a few hours.

At that point, all you need to produce hydrogen is water. The power is free. And it doesn't need to be pristine water either - ocean water is fine.

Hydrogen itself is perfectly clean - it produces water or steam. The debate over wether or not it's "clean" is all about the energy used to produce it and that is changing as our electricity grid moves to zero emission power sources. One of them being hydrogen — which is a great way to fill in gaps when solar and wind aren't producing power.