A hydrogen fuel-cell vehicle (HFCV for short) uses the same kind of electric motor to turn the wheels that a battery-electric car does. But it's powered not by a large, heavy battery but by a fuel-cell stack in which pure hydrogen (H2) passes through a membrane to combine with oxygen (O2) from the air, producing the electricity that turns the wheels plus water vapor. What this means is that a fuel-cell vehicle is technically a series hybrid, which is why they are sometimes classified as fuel-cell hybrid electric vehicles (FCHEV).
To scientists, hydrogen isn't actually a fuel but an energy carrier. Ignore that distinction, though, because HFCV drivers refill their vehicles' carbon-fiber high-pressure tanks at "hydrogen fueling stations" very similar in concept to the old reliable gas station, with a similar five-minute refueling time.
You may hear that hydrogen is the most common element in the universe. At the atomic level, that's true—but hydrogen is never found in its pure state. It's always combined with other elements. Its strong propensity to bind with anything in sight makes it a good energy carrier. Creating pure hydrogen for vehicles requires using a great deal of energy to "crack" a compound like natural gas (CH4) into pure H2, with CO2 as a byproduct. (Most hydrogen today is derived from fossil fuels like natural gas.) Run through a fuel cell, the hydrogen immediately gives back that energy, in the form of electricity, as soon as it combines with oxygen. Out of the exhaust pipe comes only water vapor (H2O).
A hydrogen fuel cell converts potential chemical energy into electrical energy using a proton exchange membrane (PEM) that uses hydrogen gas (H2) and oxygen (O2). However, since oxygen is readily available in the atmosphere, the fuel cell only needs to be supplied with the hydrogen required to power the vehicle.
Hydrogen fuel cells are made up of a negatively charged cathode and a positively charged anode which are put in contact with an electrolyte. The electrolyte is the proton exchange membrane, a specially treated material. Hydrogen gas enters the fuel cell on the anode side and is forced through the catalyst by pressure. The PEM only conducts positively charged ions, while blocking the electrons. The anode conducts the electrons, which have been freed from the hydrogen molecules, through an external circuit. These electrons provide the power to drive the electric motor, light bulbs, and so forth.
Meanwhile, oxygen is forced through the catalyst from the cathode side, where the negative charge of the atoms attracts the hydrogen atoms that have been pushed through the external circuit, before the hydrogen ions and the oxygen recombines to form water.
The following hydrogen fuel cell equation shows the process:
O2 + 4H+ + 4e– → 2H2O
2H2 → 4H+ + 4e–
2H2 + O2 → 2H2O (net reaction)
Hydrogen fuel cells vary and use different materials for the catalyst, mainly platinum nanoparticles. These nanoparticles face the PEM and the catalyst is rough and porous so as to expose the maximum surface area to the hydrogen or oxygen.
The fuel cells are placed together in stacks. The stacks are embedded in a module including fuel, water and air management, and coolant control hardware and software.
Hydrogen fuel cells offer both advantages and disadvantages compared to traditional engines. Fuel cells are not only more reliable due to a lack of moving parts, but they are more efficient too. This greater efficiency is because the chemical potential energy is converted directly into electrical energy rather than having to first be converted into heat and then again for the mechanical work – which is known as the ‘thermal bottleneck.’ Exhaust or tailpipe emissions from hydrogen fuel cell electric vehicles (FCEV) are also cleaner than from traditional internal combustion engines, as they emit just water and some heat, rather than the plethora of greenhouse gases associated with traditional combustion engines.
However, there are a number of challenges with hydrogen fuel cells, including being expensive to produce. This is primarily due to the expense of the rare substances, such as platinum, required for the catalyst. The earliest fuel cell designs also struggled to perform at low temperatures, but later modifications to the technology have ensured that this has now been addressed. The service life of fuel cells is also now comparable to that of other vehicles, with a PEM expected to last for 7,300 hours under cycling conditions.
With hydrogen fuel a specialized commodity for the general public, the small network of retail stations naturally charges high prices. To quote the California Hydrogen Business Council, “Currently, a kilogram of hydrogen costs between $10 and $17 at California hydrogen stations, which equals about $5 to $8.50 per gallon of gasoline” to cover the same distance. (A Toyota Mirai hydrogen car holds about five gallons of hydrogen.)
To offset this disadvantage, Honda, Hyundai, and Toyota have all offered their lessees and buyers free hydrogen fuel for various periods. Each manufacturer has a slightly different offer: A Toyota Mirai comes with up to $15,000 of complimentary hydrogen, while a Hyundai Nexo includes the same $15,000 over a three-year lease or up to six years of ownership.
After those offers expire, however, the driver is on their own. And if hydrogen can be compared to gasoline at $5 to $8.50 a gallon, note that charging an EV overnight usually equates to gasoline at just $1 to $2 a gallon.
HFCVs are widely considered as safe as any other car; since the high-pressure tanks are designed to survive even the highest-speed crashes without leaking or breaching. While hydrogen skeptics routinely cite the Hindenburg explosion of 1937, the hydrogen tanks and their hardware would likely survive even if the rest of the car were destroyed in a crash. No injuries or deaths specific to the hydrogen components have been recorded in the relatively small number of HFCVs sold to date.