What are fuel cells?

Car manufacturer Lotus has demonstrated a black cab powered by hydrogen fuel cells, which they hope will be just one of a fleet operating during the London Olympics in 2012. But what are fuel cells, and how do they work?

 Generating power

Much like battery-powered electric cars, those that run on hydrogen use electricity to drive the wheels. But rather than storing that electricity, with batteries sometimes taking many hours to recharge, fuel cells generate power onboard by reacting hydrogen and oxygen in the presence of an electrolyte – a non-metallic conductor in which electrical flow is carried by the movement of ions. Most of them are “proton-exchange membrane” cells. They function as follows:

Hydrogen gas is delivered to a negatively charged anode on one side of the cell while oxygen is channeled to a positively charged cathode on the other side.

At the anode, a catalyst – often platinum – knocks the hydrogen atoms’ electrons off, leaving positively charged hydrogen ions and free electrons.

A membrane placed between the anode and cathode only allows the ions to pass through. The electrons must travel along an external circuit – generating an electric current.

At the cathode, the electrons and the hydrogen ions combine with oxygen to make water, which then flows out of the cell.

A fuel cell takes in hydrogen and oxygen and produces electricity, with water and heat as byproducts.

Aren’t they expensive?

Two things have prevented major production of hydrogen-powered cars until now: the cost, and producing the hydrogen in the first place. Until recently, the platinum catalyst that splits the hydrogen into an ion and an electron has been prohibitively expensive.

Up to a few years ago, hydrogen fuel cells cost around $1000 for every kilowatt of power they generated – or around $100,000 per car. There were various avenues of research into how to bring the cost down, including work at Lawrence Berkeley Laboratories on replacing the platinum catalyst with a platinum-nickel alloy that was 90 times more efficient.

By last year, US Department of Energy reported that it had got the cost down to $61 per kilowatt – far closer to the target cost of $30. One further possibility being explored by Ballard Power Systems is enhancing the platinum with carbon silk. This is expected to bring a 30% reduction in cost with no loss of performance.

Where does the hydrogen come from?

As free hydrogen doesn’t occur naturally on Earth in any large quantity, it has to be extracted from water. This is normally achieved by reforming natural gas or by passing a current through water, splitting it into its components, hydrogen and oxygen.

This has led some to question fuel cells’ credentials as an environmentally friendly power source – they’re only as green as the electricity that makes the hydrogen, which often comes from burning fossil fuels.

But some have suggested that hydrogen can instead by produced at home by using sunlight to split water with the help of a catalyst, titanium dioxide – the white in white paint. With more centralised production, a large infrastructure of hydrogen refueling stations would be required if vehicles powered by fuel cells are to become used more widely than just by demonstration taxis.

The first commercial hydrogen refueling station was opened in Iceland in 2003. Japan has a number of filling stations and they are becoming more common in the US – however this may change now that the Obama administration has cut off public funding for the development of hydrogen vehicles.

The first hydrogen station in the UK – in Hornchurch, northeast London – closed in 2007 after the end of a three-year trial of hydrogen-powered buses. Others have since opened up in London to support a larger trial of buses, and in Birmingham.

Find out more about fuel cells with physics.org

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