What is a fuel cell? PH Explains
A brief introduction to the power source of modern fuel cell electric vehicles
A fuel cell is a device that uses electrochemical reactions, like a battery, to produce electricity. Unlike a battery, however, fuel cells require a constant supply of fuel and oxidiser in order to create power.
In automotive applications, a 'Proton Exchange Membrane' fuel cell - alternatively referred to as a 'Polymer Electrolyte Membrane' fuel cell - is typically used. These consume oxygen, sourced from ambient air, and hydrogen. A reaction between the two then takes place in the cell, producing electricity.
Many cells are then combined into what is known as a 'fuel cell stack', the output from which can be used to power the motor of an electric vehicle and charge its battery. More prominently, the only by-products from a PEM fuel cell are heat and water. This means that there are no harmful local emissions, making the PEM fuel cell ideal for automotive use.
Using fuel cells also has some advantages over pure electric powertrains, which is why some manufacturers have persisted with them. For example, the output of a fuel cell does not degrade excessively in cold conditions. The hydrogen tanks in the vehicle are also comparatively quick to fill from a storage system, enabling zero-local emissions motoring without the need for long charging times.
How does a hydrogen fuel cell work?
Inside a fuel cell is a special membrane, called the proton exchange membrane, which splits the cell in half. Each side of the membrane is coated with a layer of catalyst and a layer of conductive material, which forms an electrode. These electrodes, on either side of the membrane, are connected by a circuit.
Gases cannot pass through the membrane but, as the name suggests, protons - positively charged subatomic particles - can get through. On either side of the PEM are plates, which contain milled passages that press up against the membrane. Hydrogen is pumped through one side, while oxygen-containing air is pumped through the other.
Each molecule of hydrogen contains two atoms - which themselves contain a proton and an electron. When the hydrogen molecules hit the catalyst, a reaction occurs that splits the molecule into two protons - positively charged hydrogen ions - and two electrons.
On the other side of the membrane, when a molecule of oxygen encounters the catalyst, it splits into two negatively charged oxygen ions. This negative charge attracts the protons from the other side, drawing them through the membrane. The electrons, which cannot follow them, have to instead pass through the electrical circuit between the two conductive elements on either side of the membrane.
These electrons, as they flow through the wire between the elements, generate an electrical current - which can be used to power the car or charge its batteries. The protons which have passed through the membrane then react with the oxygen. This process, aided by the catalyst, produces heat and water.
Couple lots of fuel cells together, into a large stack array, and you end up with a powerful source of electricity which can be used to charge batteries and drive electric motors. The Toyota Mirai FCEV has 370 cells in its fuel cell stack, for example.
A brief history of hydrogen fuel cells
Welsh scientist Sir William Grove and German-Swiss physicist Christian Friedrich Schönbein independently invented fuel cells in 1839. Development was slow but, as chemical and material technology advanced, more effective fuel cells began being demonstrated in the early 1930s.
The space race prompted a surge in development in the 1950s, as NASA settled on the fuel cell as an ideal power source for many spacecraft. Numerous companies sought to capitalise on the new technology, including the likes of GM - which, in 1966, unveiled its fuel-cell powered 'Electrovan'.
Prohibitive costs meant that fuel cells would stay in trial applications throughout the 1970s and 1980s. In the 1990s, however, many manufacturers began turning a more serious eye to hydrogen power as environmental concerns and the demand for cleaner vehicles began to increase. Toyota, for example, unveiled a prototype fuel cell electric vehicle - FCEV - in 1996.
It was Honda that would ultimately launch the first production FCEV, in the form of the FCX Clarity, in 2008 - but only a small number were produced and they could only be leased by customers. In 2014, Hyundai then unveiled the ix35 FCEV and Toyota rolled out its Mirai. A new version of the Clarity, built in larger numbers, would also arrive in 2017.
Sales of hydrogen fuel cell-equipped vehicles remain few and far between, though, due to limited refuelling infrastructure and the high cost of the technology. Ever-improving battery technology has also greatly reduced both customer and manufacturer interest in the comparatively complicated and costly fuel cell systems.
That said, as mentioned previously, the set-up does have its advantages - such as being comparatively quick to refuel and not prone to issues in challenging climates. Consequently, FCEVs may prove ideal for certain markets and users.
Furthermore, the increasing availability of renewable energy makes the production of hydrogen - which can be stored or transported elsewhere for later use - more viable. Alternative methods of shipping are also being developed, such as using ammonia to transport hydrogen to conversion systems at refuelling sites, in order to overcome the challenges of handling and transporting bulk hydrogen.
I suspect it may not be quite as efficient as a fuel cell, but wouldn't it be a lot more fun?
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