Hydrogen.

In green technology circles, just mentioning the word is enough to start an argument.

Hydrogen could become a ubiquitous source of electricity and heat, say advocates. Plus, it’s somewhat prevalent. Hydrogen remains the most abundant element in the universe and every molecule of water contains two atoms of hydrogen.

Some car makers -- Toyota, Honda, Mercedes -- argue that hydrogen fuel cells will ultimately wind up in vehicles, and thereby reduce both the weight and cost of these cars. Hydrogen cars would also eliminate the pain of charging. It takes three to eight hours to charge car batteries; filling a car’s tank with hydrogen takes five minutes.

Now let’s cue to ugly reality. While hydrogen fuel cells don’t generate carbon emissions directly, producing hydrogen today is a dirty business. Most chemical companies make it by breaking up methane molecules: every kilogram of hydrogen produced through this process results in 9.3 kilograms of carbon dioxide, which typically gets released into the atmosphere.

Pulling hydrogen from water molecules requires, in most circumstances, tremendous amounts of energy. Some have suggested that the only way to make it economical is to harness the waste heat from nuclear power plants. But that would require more power plants.

Fuel cells -- which depend on expensive materials like platinum -- remain in the experimental stage, as well. I’ve driven a few hydrogen-powered cars. They are the best-driving, smoothest-handling cars I’ve ever experienced. I asked a company representative how much they cost. “Probably a million,” she estimated. The Hydrogen Highway lead to one place: a dead end.

But back to the advocates. Hope springs eternal. At MIT, Daniel Nocera has identified a chemical catalyst that can reduce the amount of energy required to strip hydrogen from water molecules. Others, like SignaChem, have come up with dry chemical pellets: put them in water and hydrogen is created.

Hydrogen advocates -- and there are fewer of them now than there were even three years ago -- readily admit the technology has not lived up to its promise. Virtually all of the experience to date seems to bolster the case of the critics.

But still, the tantalizing possibility beckons.

Maybe…or maybe not.

What do you think?