When hydrogen burns, the primary product of combustion is water. Compared to fossil fuels like coal, oil and natural gas, it seems like a perfect “fuel” to power our vehicles. No greenhouse gases like CO2 are produced to heat up the planet, nor are the other toxic gases like hydrogen sulfide and sulfur dioxide that cause the acid rain that is destroying forests all over the planet. Some small quantities of nitrogen oxides result from burning hydrogen, but much less than from fossil fuels.
Hydrogen is not really a fuel. It is an energy “carrier” like electricity. Fuels are mined or pumped from the earth’s crust. Hydrogen is not generally present in nature as an element. It is chemically very reactive, bonding readily with other elements. It takes a lot of energy to separate it, more than can be obtained by burning it…or combining it with oxygen in a fuel cell to produce electricity. Even so, it is preferable to fossil fuels, especially if clean energy sources are used to produce it.
There are two primary methods for producing hydrogen: Electrolysis of water and steam-methane reforming. Electrolysis is performed by passing an electric current through water. So it takes electrical energy…a lot of it…to produce hydrogen this way. Steam-methane reforming uses steam to separate the four hydrogen atoms from the carbon atom in natural gas (methane). A by-product of this process is carbon dioxide, a greenhouse gas, so this is a less desirable method for producing hydrogen.
There are some problems in using hydrogen to power vehicles. The energy density of hydrogen is very high…three times higher than gasoline. But hydrogen is very light. Even in liquid form, a gallon of hydrogen has only about a quarter of the energy of a gallon of gasoline. So a hydrogen tank in a car would have to be four times larger than a gasoline tank. If the hydrogen is stored as a compressed gas, a large and heavy high-pressure storage tank is required. If the hydrogen is stored as a liquid, that tank must be insulated very well because the liquid hydrogen is stored at -273 degrees Celsius. Does this sound practical for a vehicle?