Battery electric vehicles and plug-in hybrids may be the hot commodity in the green automotive game right now, but if you ask many of the engineers and executives in the auto industry about the best long-term solution for eliminating vehicle pollution, their answer is likely to be an electric vehicle powered by a hydrogen fuel cell. A decade ago, fuel cell vehicles were expected by many to be the next big thing, and plenty of car companies threw around 2010 as a target for commercial viability. Well, 2011 models are in dealer showrooms now, and there aren't any powered by fuel cells to be had. Unfortunately, continued high costs and the challenge of building a hydrogen-fueling network have slowed progress. Nonetheless, automakers have continued development and field-testing of fuel cell vehicles, most notably Honda's FCX Clarity, which Honda began leasing to specially selected customers in mid-2008.

But perhaps an even bigger roadblock on the highway to hydrogen is one that the car companies haven't even begun to address: Most people don't have the slightest idea what a fuel cell is or how it works.

So let's explain.

A fuel cell is a device that produces electricity from a chemical reaction. In the fuel cell, two or more chemicals are combined, which forms a different compound and release electrons in the process. Fuel cells can operate using a variety of chemical sources but hydrogen and oxygen are the most common because the only byproduct besides electricity is water.

To understand the fuel cell on a deeper level, we need to start with a bit of basic chemistry. The smallest unit of any physical matter is the atom, which is made up of three types of particles: positively charged protons, negatively charged electrons and uncharged neutrons. In its normal state, an atom has an equal number of protons and electrons, making the overall charge neutral. The simplest atom of all is hydrogen, which consists of one proton, one electron, and no neutrons. Oxygen is more complex, containing eight of each.

In its simplest form, a fuel cell consists of a five-layered sandwich, with two sets of grooved "flow plates" and platinum-covered, graphite electrodes on either side of a plastic membrane. One electrode is positively charged (called the "cathode") while the other is the negatively charged "anode." Hydrogen gas is fed into the grooves on one side of the sandwich where it flows through the anode. The platinum acts as a catalyst and causes the hydrogen atoms to split into individual electrons and protons.

On the other side of the sandwich, air (containing oxygen) is fed into the grooves. The membrane only allows the positive protons to pass through it, while the electrons are conducted into a separate circuit outside of the fuel cell. That free flow of electrons through the circuit is the electric current that can be used to power anything from a light bulb to a motor or even charge a battery.

In a fuel cell system that external circuit eventually brings the electrons back around to the opposite flow plate where they meet back up with the protons and oxygen atoms in the air. Each oxygen atom then combines with two protons and two electrons to form a water molecule, which drains out through the grooves of the flow plate as the only "waste" product.

If all of this is hard to follow, check out the diagram.

Each individual sandwich of membrane, electrodes and flow plates makes a single cell which typically doesn't produce enough voltage or current to be useful for anything but the smallest devices. In order to produce enough electrical power to drive a car, several hundred cells are stacked up and wired together in series. The output of the stack can be adapted to different applications by just adding or removing cells.

Until now, fuel cell stacks have been built by hand with individually machined components for the housing and plumbing. Automakers like Toyota, Honda, Hyundai, General Motors and Mercedes-Benz are all developing fuel cell stack designs that are optimized for mass production with more integration and reduced platinum requirements to bring the cost down. Toyota has promised an affordable series production fuel cell vehicle by 2015 and Hyundai has plans to start selling a fuel cell vehicle in its home market of South Korea by 2012.

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