Electrical Engineering ⇒ Topic : Fuel Cells
Whenever a fuel such as wood, coal or natural gas burns, it combines with oxygen to create a new substance. The chemical reaction is called oxidation and is accompanied by the release of a large amount of energy in the form of heat. The heat may be used to produce steam, which in turn can drive a turbogenerator set. Unfortunately, the efficiency is very low (about 20%) when heat is converted into electrical energy in this way. Can we permit oxidation to take place without actually burning the fuel? This is what is done in a fuel cell. The energy of oxidation appears in the form of electrical energy at the electrodes of the fuel cell. The efficiency of a fuel cell is more than 40%. Clearly, this is an improvement compared to the efficiency obtained by burning the fuel.
Hydrogen-oxygen fuel cell. When hydrogen burns, it always combines in definite proportions with oxygen. For complete combustion, 1 kg of hydrogen consumes 8 kg of oxygen. In the process about 120 x 106 J of heat is released and the resulting product is water. In hydrogen-oxygen fuel cell, oxidation is permitted to take place without actually burning hydrogen. The energy of oxidation is converted into electrical energy.
Fig. (a) shows the various parts of a hydrogen-oxygen fuel cell. It consists of two platinum electrodes A and B immersed in a solution of H2SO4 (electrolyte). The hydrogen gas is continuously supplied to **electrode A and oxygen gas to electrode B. An electrical load is connected across the terminals.
Working. When hydrogen gas touches the electrode A, a special reaction takes place, causing every molecule of hydrogen to break up into 2 electrons and 2 positive hydrogen ions. The hydrogen ions move into the electrolyte and slowly travel towards electrode B. The electrons, on the other hand, are captured by electrode A and flow through the load and on towards electrode B. The oxygen molecules that touch electrode B capture electrons coming via the load, thereby becoming negative oxygen ions. These oxygen ions move into the electrolyte and combine with hydrogen ions to form water. Water is periodically removed to prevent contaminating the fuel cell. It may be seen that oxidation takes place inside the electrolyte where hydrogen and oxygen ions combine. However, no energy is released during the process; all the energy of oxidation appears as electrical energy at the electrodes. In fact, the electrons released at the electrode A (i.e. fuel terminal) are recaptured at the electrode B (i. e. oxygen terminal) with the result that a steady current flows through the load. Ideally, the electric power supplied to the load is equal to the thermal power that would be released if the fuel were burnt. There are, however, some losses but the efficiency of even a small fuel cell is more than 40%.
The voltage of a hydrogen-oxygen fuel cell is about 0.8 V. However, the current supplied by such a cell is very high of the order of kA. Hence fuel cells, like batteries are basically high current, low voltage d.c. devices. Higher voltages are obtained by connecting cells in series.
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