Since Gottlieb Daimler first rolled off in a motorized carriage in 1886, the mainstream automobile has always been powered by hydrocarbons (petrol, methanol).  Refinements of the internal-combustion engine (ICE) have improved efficiency and lessened emissions to a level only a few percent of those early engines.  Now, more than a hundred years later, the company he co-founded, Mercedes-Benz, has just started operations of the first global fleet of fuel cell cars: sixty A-Class cars in five cities.  The payoff, in the form of commercial availability of zero-emissions cars and trucks, is expected in 15 years.

For some companies, the interim is hybrid powerplants, with an electric motor doing most of the stop-and-go duty in cities, with an ICE kicking in at higher speeds and to recharge the batteries powering the motor.

Fuel cell vehicles are a different matter altogether.  For one, they don’t burn hydrocarbons, or even hydrogen. There have been experimental internal-combustion cars that use hydrogen as their fuel, but they still emit potentially poisonous gases and their efficiency is only half that of the fuel cell. 

A fuel cell works by combining stored hydrogen with oxygen in the air, generating an electric current and, as a by-product, pure water.  The most commonly-used fuel cell in cars is the Proton-Exchange Membrane. Think of it as a sandwich, with the thin plastic-film PEM as the ham and a catalyst coating as the mayonnaise.   The bread slices are positively and negative charged plates into which gas ducts have been milled.  Through these ducts, hydrogen flows on one side and air containing oxygen on the other.  The catalyst breaks down the hydrogen atoms into protons and electrons.  The protons can penetrate the membrane but the electrons cannot.  As a result, a voltage builds up between the two electrodes.  Join the two electrodes and you generate a direct current.  Each fuel cell generates only a small amount of current, around 0.15 kW, so a stack of these cells are put together to generate enough power to drive a vehicle.

What about the source of the hydrogen, then?  It’s extracted from water, using hydrolysis, and that takes energy.  Positive and negative plates are dipped into water, and a current is passed through them.  This splits the water into hydrogen and oxygen.  Essentially, it's the reverse of the fuel cell process.

Pollution-wise, there's a right way and a wrong way to extract hydrogen. The wrong way is to use electricity generated from gas and coal.  Unfortunately, most of today's hydrogen is produced this way.  The right way to do is to use renewable energy sources: solar, wind, and geothermal.  Iceland, one of the first countries to commit to a hydrogen economy, uses its considerable geothermal resources to extract the element.  Compare that to hydrocarbons, where, no two ways about it, storage and use in a vehicle will inevitably result in some sort of pollution.

Despite the imagery of the Hindenburg catching fire (actually due to the fabric of the airship’s outer skin and a new protective coating, not the hydrogen), the element, properly handled, is as safe as petrol or propane.  The hydrogen is stored in high-pressure tanks beneath the floor, but multiple crash tests have proved them safe. 

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