The idea of converting fuel into electricity by an electrode-electrolyte system originated in the 19th century (Grove, 1839). The basic principle behind a hydrogen-oxygen fuel cell was described in section 4.1.6. The first practical applications were in powering space vehicles, starting during the 1970s.
Developed for stationary power applications, the phosphoric acid cells use porous carbon electrodes with a platinum catalyst and phosphoric acid as electrolyte, and feed hydrogen to the anode, with anode and cathode reactions given by (4.72) and (4.73). The operating temperature is in the range 175-200°C and water is continuously removed.
Alkaline cells use KOH as electrolyte and have anode and cathode reactions of the form
These cells operate in the temperature range 70-100°C but specific catalysts require maintenance of fairly narrow temperature regimes. Also, the hydrogen fuel must have a high purity and notably not contain any CO2. Alkaline fuel cells have been used extensively on spacecraft and recently for road vehicles (Hoffmann, 1998a). Their relative complexity and use of corrosive compounds requiring special care in handling make it unlikely that the cost can be reduced to levels acceptable for general-purpose use.
The third fuel cell type in commercial use is the proton membrane exchange (PEM) cell. It has been developed over a short period of time and is considered to hold the greatest promise for economic application in the transportation sector. It contains a solid polymer electrolyte membrane sandwiched between two electrodes. The membrane material may be poly-perfluorosulphonic acid. A platinum or Pt-Ru alloy catalyst is used to break hydrogen molecules into atoms at the anode, and the hydrogen atoms are then capable of penetrating the membrane and reaching the cathode, where they combine with oxygen to form water, again with the help of a platinum catalyst. The anode and cathode reactions are again (4.72) and (4.73), and the operating temperature is 50-100°C (Wurster, 1997). Figure 4.108 shows a typical layout of an individual cell. Several of these are then stacked on top of each other. This modularity implies that PEM fuel cells can be used for applications requiring little power (1 kW or less). PEM cell stacks are dominating the current wealth of demonstration projects in road transportation, portable power and special applications. The efficiency of conversion for the small systems is between 40% and 50%, but a 50 kW system has recently shown an efficiency near 60%. As indicated in Fig. 4.109, an advantage of particular importance for automotive applications is the high efficiency at part loads, which alone gives a factor of two improvement over current internal combustion engines.
anode membrane — cathode
Was this article helpful?
Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.