Proton exchange membrane fuel cell

Sometimes called the polymer electrolyte 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, 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 and heat (Figure 13.1).

To build up a useful voltage cells are stacked between conductive bi-polar plates, usually graphite, which have integral channels to allow the free flow of hydrogen and oxygen (Figure 13.2).

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

Figure 13.2

Fuel cell stack

Future Outlook Hydrogen Fuel Cells

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.

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