For decades, hydrogen has held promise as a revolutionary tool in the clean energy transition. It can be a fuel and energy carrier. The hydrogen fuel cell was once seen as a breakthrough technology, basically a battery that could be continually refueled – “you put in hydrogen and oxygen and you get electricity, heat, and water out. So the emissions are just water,” says Joe Romm, author of “The Hype About Hydrogen.” When made from renewable energy, hydrogen can be a carbon-free fuel. But it has other challenges.