How do Hydrogen Cars Work
Switching to vehicles that use hydrogen instead of gasoline as fuel would have several benefits. Chief among these benefits: a significantly reduced dependence on an ever decreasing, and sometimes volatile, oil supply and reduced pollutant emissions.
There are several obstacles that must be overcome before we can get there. Some of them are on the society side; what manufacturer will build a hydrogen car when no hydrogen refueling infrastructure exists? But who would build a hydrogen infrastructure if there are no cars to use it? And how should we produce all this hydrogen?
But this is an article about the cars themselves, so let's talk about their needs. Basically, each car will need some way to store the hydrogen, and some way to use it to gain locomotive power.
1. STORING HYDROGEN
Hydrogen gas has an energy density of 143 MJ/kg, versus 46 MJ/kg for gasoline. This means that even if a vehicle could only use the hydrogen with the same efficiency that we can use gasoline, (and we can in fact do better), you would still need to carry a much smaller mass of hydrogen than gasoline to move your car the same distance. That's great news!
The bad news is that hydrogen is a gas (that is, a gas like oxygen, not a gas like gasoline!) at room temperature and the amount we would need occupies a much greater volume at normal pressures than liquid gasoline. Specifically, to get the energy in one gallon of gasoline we would need 380 standard cubic feet of hydrogen gas. For comparison, your car might have around 100 cubic feet of passenger volume total. So we need to get a little creative to store all the hydrogen we need. Here are three possibilities.
1a. Compressed gas storage
We've said that hydrogen gas takes up 380 "standard" cubic feet meaning this is the volume occupied at normal temperatures at atmospheric pressure. One thing we could do to reduce this is compress the gas, raising its pressure and decreasing the volume it must occupy. In fact, this is usually how hydrogen gas is stored in research labs today.
On the bright side this is a very simple storage solution, and refueling would be easy. This is the solution prototype vehicles available now have tended to use. However, we would end up sacrificing part or all of the hydrogen weight advantage as a result of the heavy tanks that must be used to store the very high pressure hydrogen.
Hydrogen is also the smallest molecule around, and thus especially prone to leaking out of seals. Any leaks that do occur while the vehicle is in an enclosed space could result in an explosion hazard as hydrogen gas builds up in the space. There is also always the possibility that the storage tank could be damaged in an accident with catastrophic consequences; however they would probably be the strongest part of a vehicle.
1b. Liquid hydrogen
We can avoid the problems of high pressure gas entirely by storing hydrogen as a liquid. This is one of the fuels used by the space shuttle. To get the same amount of energy stored in one gallon of gasoline we would need 1.27 gallons of liquid hydrogen; however, the hydrogen would only weight 1/8 as much.
All the problems with this method are the result of the fact that liquid hydrogen is really, really cold. It's -423 degrees Fahrenheit, to be exact. So special, very well insulated tanks would be needed to hold it both in your car and at the refueling station. Furthermore, since you could never insulate the tank perfectly, there would always be some boil-off, meaning you would be slowly losing fuel, even if your car was just sitting in the driveway.
1c. Metal hydride storage
A third option is to store the hydrogen chemically, in solids. For example, lithium borohydride (LiBH4) is a solid at room temperature that contains, in each molecule, four atoms of hydrogen. The hydrogen can be released by heating the material, or by reacting it with another chemical, such as water.
The good news is that these solids can pack the hydrogen extremely tightly. Many of them actually offer a higher hydrogen density than liquid hydrogen. They also eliminate the dangers of high pressures, or the expense of constant boil-off.
On the downside, metal hydrides do add additional weight to the fuel tank. (For example, lithium borohydride contains lithium and boron that is never used.) Refueling may also be difficult, because high temperatures and high hydrogen pressures are often required to drive the chemical reaction backwards and reproduce the original material after the hydrogen has been used. (It may also take a long time.) If small amounts of impurities are introduced each refueling cycle, this can also degrade the performance of the material over time.
2.USING HYDROGEN
After we figure out how to store the hydrogen, there are essentially two ways we can use it. In similar fashion to your current vehicle, we can burn it in a hydrogen combustion engine. Alternatively, we can use it in a fuel cell.
2a. Hydrogen combustion engine
The hydrogen combustion engine burns fuel just like the gasoline engine in your car. Since hydrogen contains no carbon, however, the only product of its combustion with oxygen gas is water. If we burn the hydrogen in the atmosphere, which also contains nitrogen, some of the so-called "NOx" pollutant gases may be produced as a result of the heat of the reaction. Engines burning hydrogen are generally slightly more efficient than those burning gasoline.
2b. Hydrogen fuel cell
A perhaps more popular, and still more efficient option is the fuel cell. A fuel cell consists of something like an anode which we pass hydrogen gas over, and a cathode which oxygen gas is passed over, separated by a semi-permeable membrane. (If you don't know what anode and cathode mean, just hang with me.) At the anode, the hydrogen, which normally consists of a proton being orbited by an electron, is split apart into bare protons and bare electrons.
The protons are able to pass through the membrane and head toward the cathode, but the electrons are blocked and must travel around it, through an electrical circuit that will contain the vehicle engine. When the electrons finally arrive at the cathode they recombine with the protons and the oxygen to form water, the only waste product of this system. There are several variations on this theme, but you have the basic idea.
The biggest benefit of the fuel cell is efficiency. Much less energy is lost as heat. While the typical gasoline engine might convert 20% of the liberated energy into mechanical work (running the engine), for a fuel cell we could theoretically get something more like 80%. (In practice the number has been smaller, but still much higher than what we get with gasoline engines.) While each fuel cell can only produce a small voltage or current, they can be "stacked" to increase that limit to whatever we need.
On the down side, fuel cells can be quite expensive. Many of them, for example, use platinum catalysts, and platinum is currently over $1000/ounce. They are also often very sensitive to carbon monoxide poisoning, which can degrade their performance over time.
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For more information on the chemistry of hydrogen storage and fuel cells, see the book "Introduction to Hydrogen Technology" by Roman J. Press and others. Be warned, however, that the first edition has many typos.
