It's a gas

Washington, D.C. now has a gas station that actually dispenses a gas. California Yankee notes that a new hydrogen-dispensing gas station has been built in the area, and Shell Hydrogen and General Motors hopes this will be the beginning of a hydrogen economy. Right now, there are six hydrogen-powered vehicles in the DC area, so the station could have one bay dedicated to each vehicle, and maybe even a few left over.

Hydrogen sounds like a really neat way to go. You burn it, and the exhaust is water vapor. No problem.

Of course, there are a few problems:

First, there aren't any hydrogen wells. You can't obtain pure hydrogen on this planet in large quantities without doing a lot of work to pull it out of other molecules.

Petroleum products and other fossil fuels can be removed from the ground and burned. At most, a small amount of processing is required. But what we're doing when we burn fossil fuels is releasing solar energy that was stored in the molecules millions of years ago.

Likewise, if we use nuclear fission, we're "burning" uranium, thorium, plutonium, or other heavy atoms. Here, we're releasing solar energy that was stored in those atoms in the cores of other suns, billions of years ago.

In the case of hydrogen, most of what's available is either in the form of hydrocarbons (fossil fuels, again) or in compounds that are already at their lowest energy. In order to break the hydrogen loose, we have to add energy.

Some fans of hydrogen power point to the water that covers three quarters of the planet's surface, and observe that it's 22% hydrogen by weight. Well, big deal. It takes energy to pull the hydrogen loose, and you're bound to get less energy back when you burn it, due to the second law of thermo, if nothing else. Hydrogen from sea water is, at best, a storage battery. Unless we have some other form of cheap energy to split water, there's no real point in using it.

When we burn hydrogen, we normally think of the only result being water vapor. I haven't examined the specs for hydrogen burning engines in any detail, but I'll just bet they still produce photochemical smog.

When I took physical chemistry, decades ago, I learned about reaction equilibria. A lot of reactions will go from the reagents to the product, until all the reagent is used up. Mix hydrogen and oxygen gases, add a spark, and when the dust settles, all available hydrogen and oxygen will have combined to form water vapor. Hydrogen molecules and oxygen molecules are in a higher energy state than water molecules are. When you rearrange the atoms in the first to form the second, you wind up with a bunch of energy left over. In order to split the water into separate components, you have to put that energy back in somehow.

In some reactions, the energy difference isn't very large, and there can be enough energy floating around in the environment to drive the reaction "uphill", at least in part. When the dust settles, we might find that, say, 90% of the reagents have combined to form the product, and the rest stay behind as un-reacted reagents. In reality, what happens is that the reagents are constantly combining to make the product, but at equilibrium, for every ten molecules that form, one comes apart to make the original reagents.

We can calculate this equilibrium, and make predictions about how it will shift if we add more of one chemical or another.

In many (if not most) cases, this equilibrium point changes with temperature. In some cases, the hill "flattens out", so that instead of the reaction going 90% of the way to completion, it may only go 60% of the way. In some cases, the equilibrium point changes so the reaction tends to go the other way.

At high temperatures, nitrogen and oxygen will combine to form "oxides of nitrogen" -- nitrous oxide, and maybe a little nitric oxide thrown in for flavor. This is a major component of smog, and it is highly sensitive to the temperature inside the combustion chamber of an engine.

At low temperatures, such as those a lot closer to room temperature, oxides of nitrogen are much less likely to form, and in fact, their decompositon into oxygen and nitrogen is strongly favored. Of course, since chemical reactions go much more slowly at low temperatures, you need something to speed them up. Something like a catalytic converter. However, once the catalyst is present, the reaction runs very nicely, and oxides of nitrogen fall apart into oxygen and nitrogen. The energy from this reaction is dumped into the environment, which is why catalytic converters heat up – sometimes enough to set brush on fire.

(And that, by the way, is my main problem with Larry Niven's stories set on Mars. The Martian atmosphere is made up of nitrous oxide in his stories. In some of his stories, the crews in domes or tents have to worry about running out of breathing air. It's a pity no one thought to pack along a catalytic converter and pump Martian air through it. They'd wind up with an oxygen-nitrogen atmosphere, and it would be warmed up, too.)

In any event, a hydrogen engine will probably not be fed pure oxygen. Instead, it will mix hydrogen with air, which is 78% nitrogen. Not all the oxygen will combine with hydrogen, and at high temperatures, some of it will combine with nitrogen to make smog.

Hydrogen may turn out to be very useful, but I don't see it as a cure-all.