Polymer electrolyte membrane fuel cell

Generally fuel cells are classified by their electrolyte. One of the most common types of cell is the polymer electrolyte membrane fuel cell. Sometimes called the proton exchange membrane fuel cell (PEMFC in either case), it is also referred to as the solid polymer fuel cell. This is one of the most common types of cell, being appropriate for both vehicle and static application. Of all the cells in production it has the lowest operating temperature of 80°C. The cell consists of an anode and a cathode separated by an electrolyte, in this case usually Teflon. Both the anode and cathode are coated with platinum which acts as a catalyst. Hydrogen is fed to the anode and an oxidant (oxygen from the air) to the cathode. The catalyst on the anode causes the hydrogen to split into its constituent protons and electrons. The electrolyte membrane allows only protons to pass through to the cathode, setting up a charge separation in the process. The electrons pass through an external circuit creating useful energy at around 0.7 volts then recombining with protons at the cathode to produce water (see Fig. 7.2).

To build up a useful voltage, cells are stacked between conductive bipolar plates, usually graphite, which have integral channels to allow the free flow of hydrogen and oxygen (see Fig. 7.3).

The electrical efficiency of the PEMFC is 35% with a target of 45%. Its energy density is 0.3kW/kg compared with 1.0kW/kg for internal combustion engines.

One problem with the PEMFC is that it requires hydrogen of a high degree of purity. Research activity is focusing on finding cheaper and more robust catalysts as well as more efficient ion exchange polymer electrolytes.

Originally the PEMFCs were conceived for vehicle application. Now they are being developed to supply single homes or housing estates with electricity and heat. Approximately the same amount of heat and electricity are generated. Initially the hydrogen fuel will be obtained from reformed natural gas supplied through existing networks. Rather optimistically the journal New Scientist in its editorial (25 November 2000) predicted that 'Within a couple of years, fuel cells will provide heat and power for homes and offices'. It goes on to suggest that cheap gas will enable fuel cells 'to undercut today's combination of

heating boiler and mains electricity'. It is more likely that 2008 will be the date by which fuel cell combined heat and power will start to penetrate the domestic market.

However, it has already penetrated the sphere of transport. Buses in the city of Vancouver have been operating Ballard PEMFCs since 1993. The cells deliver 125 h.p. and the buses have a range of 100 miles. The Chicago Transit Authority hopes to substitute this technology for all its 2000 buses. London has a fleet of six fuel cell buses as a demonstration project.

California was the pioneer in placing restrictions on fossil fuel vehicles. It planned that 10% of cars in the state would be powered by hydrogen by 2004 but it had to capitulate to the massed ranks of the automotive lobby.

Italy has an accelerating problem of pollution from vehicles. The Lombardy region is particularly affected with its capital Milan experiencing smog levels five times the legal limit. The result is that the regional government was considering a ban on the sales of all petrol and diesel cars after 2005. It is unlikely that there will, in the near to medium-term future, be a network access to pure hydrogen for vehicles. In the interim it is most likely the gas will be catalysed from methanol.

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