Hydrogen and Fuel Cell Vehicles

Fuel cell cars are an emerging technology. At present, numerous prototypes are being developed, some are currently being tested on the roads and a few manufacturers claim to be close to launching production models.

How do Fuel Cell Vehicles Work?

Fuel cell vehicles are essentially an advanced form of electric vehicle, but with a fuel cell instead of a normal electric battery.

When fully charged, a normal electric battery contains a finite quantity of electrical potential energy. Once all this energy has been used – for example, to power the car – the battery will be empty and will need to be re-charged. It is also possible to charge the battery whilst it is in use, by attaching a generator or alternator to convert another form of energy into electrical energy and transfer it into the battery.

Fuel cells, on the other hand, produce electrical energy through an electro-chemical reaction that releases heat and water. A fuel cell is similar to a normal battery cell, with anode, cathode and electrolyte. For the electrical current to be generated, it is necessary to maintain a continual supply of fuel and oxygen into the fuel cell. Thus the fuel cell produces electricity but does not store it; as soon as the fuel supply is interrupted, the current stops.

Fuel Cell Development

Road vehicles are by no means the only application of fuel cells; they are also used for power generation and other industrial uses. Researchers are still exploring different combinations of materials for electrodes, electrolyte and fuel. The type that seems best suited to vehicular use is called the Proton Exchange Membrane Fuel Cell (PEMFC). Whereas other fuel cells use liquid acid or alkali electrolytes, PEMFCs use a solid polymer material, and are able to operate at relatively low temperatures; they therefore have the advantage of shorter start-up times, and are considered intrinsically safer.

Hydrogen as an Automotive Fuel

Hydrogen is viewed as the ideal fuel for fuel cells because it ionises easily, is highly combustible and has high energy content. However, to use hydrogen as an automotive fuel, one challenge that has to be overcome is storage. Whilst 1kg of hydrogen contains as much energy as 2.8kg of petrol, this 1kg of hydrogen takes up far more space because of its much lower density. It has been calculated that if petrol and hydrogen containing equal amounts of energy are stored at room temperature, the volume of hydrogen will be 800 times greater.

There are three ways of reducing the volume of hydrogen. It can be compressed, but high pressure is required. It can be liquefied using cryogenic techniques, but a lot of energy is used in achieving the low temperature required (-253 degrees Celsius). Or metal hydrides can be used; these are metal alloys that will absorb hydrogen under pressure and release it again when they are heated and the pressure is reduced. This is a less hazardous storage method but would add considerable weight to a vehicle.

Another answer is not to store hydrogen at all, but instead to store a liquid fuel that is rich in hydrogen, and reform it onboard the car. Possibilities are petrol, natural gas, ethanol, propane and methanol. All except methanol require processing temperatures of 600 degrees Celsius or above; methanol can be reformed at 260 degrees Celsius, so has a clear advantage. However, since they are all fossil fuels this method is not as environmentally acceptable as using pure hydrogen, firstly because they would inevitably produce unwanted emissions, albeit probably quite low levels, and secondly because they are non-renewable. It is however possible that biomethanol could be used.

Fuel Cell Vehicles in Operation

This is a developing technology and there are many unknown factors, including likely costs to the end user. The cars themselves would not necessarily look different or be more complicated to operate. Performance is likely to be comparable: one recently-unveiled prototype has a top speed of 100mph and a 0 to 60 time of 12.8 seconds. Depending on whether the primary fuel is a hydrogen-rich crude-oil based fuel or pure hydrogen, refuelling a fuel cell car need not be any different to the way motorists currently fill up with petrol or Autogas.

Since fuel cells power the vehicle by electricity, they have all the same advantages of EVs: they are silent, they use almost no energy whilst stationary in traffic, transmission of power to the wheels is simple, and regenerative braking can be used to recover and recycle energy. With hydrogen as the primary energy source there will be no greenhouse gas emissions during use, and if the hydrogen is generated by a renewable energy source the entire cycle can be pollution-free. Though time alone will tell, many people believe fuel cells will be the energy of the future.