They run on fuel and air but they're not internal combustion engines. They produce electrical current but they're neither batteries nor generators. So what exactly is a fuel cell?
One of the most exciting applications of spongy nickel catalyst is in the area of hydrogen fuel cells. A fuel cell is an electro-chemical converter; that is it takes a fuel and an oxidant and produces electricity together with heat and exhaust chemicals. There are many different designs of fuel cell and not all of them run on hydrogen. Some designs are powered by hydrocarbons such as methane and diesel or by alcohols. Others use exotic oxidants such as chlorine. Hydrogen fuel cells are particularly environmentally attractive since they use hydrogen as a fuel, oxygen from the air as an oxidant and so as a result the exhaust is pure water. Consequently they are ‘locally non-polluting’
Even within the category of hydrogen fuel cells there are many different types, each with different operating temperatures, peak efficiencies maximum loads and designs. The information here relates to the alkali fuel cells that are being developed as part of the IMPRESS project. Do not be surprised if you encounter different designs that operate in fundamentally different ways and uses different materials.
An image of an IMPRESS fuel cell is shown next to an animation of its operation.
The fuel store contains the supply of hydrogen, adsorbed on to the metal hydride surface. This results in large quantities of hydrogen being stored in a small volume but at quite low pressure. A standard external hydrogen store can hold 230 litres of hydrogen (measured at STP) in a container approximately the size of a soft drinks can at a pressure of three bar (three atmospheres). The unit weighs just 2.5 kg.
Hydrogen released from the fuel store reacts with the spongy nickel catalyst thus:
The hydrogen ions remain adsorbed on the spongy nickel catalyst surface whilst the electrons are free to flow through an external circuit in order to drive the load.
Oxygen from the air reacts with a separate catalyst thus:
The electrons are drawn from the external circuit and the water molecules are drawn from the electrolyte which contains a potassium hydroxide gel. The electrolyte gel supports the transport of K+ and OH- ions as well as water.
Back at the hydrogen side the hydroxyl ions react with the adsorbed protons to produce water, thus:
Given that a standard external fuel store can contain 230 litres of hydrogen (measured at STP) and that this amount of fuel will provide 40 Ah (ampere hours) of charge at 12 V calculate the energy efficiency of the fuel cell. What assumptions (if any) have you made in reaching this figure?
The fuel cell has been around for a very long time; the theory was first published in 1838 and the first working fuel cell was presented seven years later. However these early cells were more demonstrations of scientific theory than economically viable products. When fuel cells became commercially available in the 1960s the high cost meant that they were limited to very specialist areas. For example, the electrical power for the Apollo 11 spacecraft that carried Neil Armstrong, Buzz Aldrin and Michael Collins to the Moon in 1969 was produced by a bank of fuel cells.
One reason for the high cost of fuel cells has been that the catalyst used to oxidise the hydrogen has traditionally been made of platinum, an extremely expensive metal. The total annual world production of the metal is only a little over 200 tonnes. Put that in context, if you poured the entire supply of platinum produced in the last ten years in to an olympic sized swimming pool it would only form a layer 7.5 cm thick. Environmentally speaking, platinum also has a large impact since a great deal of ore has to be extracted in order to produce a small quantity of the pure metal, typical concentrations in a good deposit are as low as 0.5 ppm.
Despite this, hydrogen fuel cells are already being widely used. For example the European Union is funding a trial of fuel cell powered buses in several major European cities including London, Amsterdam, Madrid, Stuttgart and Stockholm. They have proved to be very popular and many of the trial cities have already committed themselves to purchasing more of these buses in the near future.
Fuel cells can be made in to compact, light weight units and the fact that they have no moving parts leads to very high reliability. This makes them ideal for powering equipment in remote locations. In Finland for example many families own small cabins on the shores of the numerous lakes, far from the nearest roads and power supply. To operate a diesel powered generator here, producing noise and exhaust fumes would largely defeat the object of the isolated rural retreat. Many Finns use hydrogen fuel cells of different sizes to power their cabins, boats and computers. It is even possible to purchase a fuel cell ‘boosted’ bicycle. All of these fuel cells operate silently and of course produce only water vapour as their exhaust.
As fuel cells become more compact and a distribution system for hydrogen becomes a reality we can expect to see fuel cells being more widely used. Iceland is already phasing in fuel cell powered public transport and working on eliminating fossil fuels from private transport in the near future. If you visit the London Science Museum you can see a scooter powered by an IMPRESS hydrogen fuel cell pack. The near future should also bring micro-fuel cells to power lap top computers and mobile phones.
If hydrogen can be piped in to houses then you can expect the home of the future to be powered by electricity generated from large, fixed fuel cells. Any waste heat generated could be used immediately to heat the house or stored for later use
The limiting factors to adoption are currently the high purchase price of the fuel cells and availability of a widespread hydrogen distribution system. The development in the IMPRESS project of spongy nickel as a replacement for the current expensive platinum catalyst should contribute to a dramatic reduction in cost and so drive the development of an effective distribution system.