Scientific Method / Science & Exploration

Hydrogen powered cars may be fueled by stored ammonia

Burning or splitting ammonia may be key to the rise of the hydrogen economy.

Ammonia may not explode, but it's hazardous enough that it needs to be approached with this sort of protective gear.




However, if you choose to go with ammonia, its difficult to burn directly—it is preferable to come up with a process that splits it into hydrogen and nitrogen gases before combustion. Normally, this is done using a catalyst containing ruthenium, a rare and relatively expensive metal. Take the money you saved on the tank and throw it into the catalyst. Not a great solution.

But over a hundred years ago, chemists discovered that ammonia can be cracked by sodium amide (NaNH2). It does this by cyclically decomposing to produce sodium metal, nitrogen, and hydrogen. The hydrogen and nitrogen form stable gases, leaving the highly reactive sodium metal to react with ammonia, reforming sodium amide. This cycle is stable, fast, and, it seems, efficient.

Sodium amide is made from cheap and abundant elements, making it a very desirable catalyst. But it has been ignored because it's also difficult to work with—my general rule of thumb is that if a compound contains a metal, nitrogen, and not a lot else, then I leave the lab quietly and hide in a nearby bunker.* Sodium amide has a low melting point and is quite reactive, meeting all my expectations. In this case, however, those brave researchers in years past found that sodium amide decomposes ammonia efficiently before it (a) melts and (b) explodes reacts vigorously.

The current research set out to show that sodium amide is a viable catalyst by examining reaction efficiencies in various configurations. First, the researchers showed that sodium amide performs as well as a ruthenium-based catalyst—it doesn't need to be better, just good enough. The rest was then details: how do you make a supporting structure that holds the sodium amide, exposes lots of surface area to the ammonia, and doesn't restrict flow? They tried supporting nickel foams and wools. The wool performed slightly better at higher flow rates, while the foam was able to provide a more pure flow of hydrogen.

Ammonia also contains a significant amount of energy and could simply be burnt. The researchers discussed this, stating that an intermediate solution would be to use the sodium amide reactor to overcome the problem that ammonia is rather difficult to ignite. The reactor provides some percentage of hydrogen in an ammonia flow. The hydrogen is used to spark ignition, and the remaining ammonia provides the bulk of the energy. Over time, as we make more and more efficient catalytic structures, the fuel mix in later engines would shift toward hydrogen. Even later still, combustion engines might be replaced by fuel cells and electric motors.

The nice thing about this path is that it's gradual. We don't need the perfect catalytic converter and fuel cell to get started. It also uses materials that are readily available. Ammonia is a bulk product already, the sea is full of sodium, and the air is full of nitrogen.

The bad news is the ammonia itself—it's a toxic gas. Gasoline, for all its problems, is a hard-to-burn liquid. In a car accident, even if the fuel tank bursts, a fast explosion is unlikely. And because gasoline is a liquid, you're not going to breathe much of it in. Ammonia may be just as unlikely to explode, but that won't matter, because you'll be breathing it in if the tank cracks, and it will turn any water it comes into contact with (such as the stuff on the surface of your lung cells) into a strongly basic solution.

Nevertheless, we clearly know how to handle gas-powered vehicles safely: cars powered by liquified natural gas and compressed natural gas have been around for a long time now. Very few explosions or deaths from gas inhalation mean that we must be doing something right.

Journal of the American Chemical Society, 2014, DOI: 10.1021/ja5042836

* Yes, I know azides are not amides.

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