| Energy has become a basic need of the modern society in all imaginable dimensions. Carbon, petroleum and natural gas are the main sources of energy. The worldwide concern on the environmental consequences and irreversible extinction of fossil fuels has led to the development of fuel cells. Various types of Fuel Cells include Hydrogen Fuel Cells, Proton Exchange Membrane Fuel Cells, Methanol Fuel Cells, Alkaline Fuel Cells, Phosphoric Acid Fuel Cell and Solid Oxide Fuel Cells.; Alkaline Fuel Cells were the first fuel cells that were being used by NASA for a very long time on space missions. These cells can achieve power generating efficiencies of up to 70 percent. They use alkaline Potassium Hydroxide as the electrolyte. Alkaline Fuel Cell's high performance is due to the rate at which chemical reactions take place in the cell. Until recently they were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility. Alkaline Fuel Cells are, however, the cheapest of fuel cells to manufacture.; The existing descriptions for the current-voltage characteristics of a fuel cell are based on classical thermodynamics, which treats the cell as a simple reversible chemical reaction as A + B ↔ C + D. The cell voltage is given through the Nernst Equation in terms of the concentrations of the reacting species. However, cell current can only be expressed through voltage losses due to charge and mass transport, which are termed "polarizations". The main component of the cell current is controlled by the diffusion gradients at the two porous electrodes and is expressed through Fick's Law.; For a fuel cell consisting of the negative anode and positive cathode separated by an ion conducting medium (electrolyte), into which two reacting gas species are introduced, the open circuit voltage is generally derived from the Nernst equation(which is explained in detail in the further chapters). According to the equation the actual potential difference of the cell is expressed in terms of change in Gibbs free energy. In addition to that the Nernst-based model of a fuel cell also implies that it is a constant voltage electrical device. On this basis, the fuel cell would deliver an unlimited current at a constant voltage until the intervention of some other physical mechanism. The Nernst model provides no description of the current transport mechanisms in the fuel cell, especially through the conducting medium. It is simply and purely a dynamic equilibrium model.; The Electronic model being proposed would try to explain about a device which would more likely act as a constant current device rather than a constant voltage device. The proposed model has a number of salient features which avoid the pitfalls that occur when using the thermodynamic model. By using the proposed model to describe the energy states in the conducting medium and the electrodes, the potential barriers that are formed at the interfaces between the conducting medium and the electrodes can be described. The end result would be a description of the fuel cell which should more readily allow for understanding what changes can be made to improve the efficiency. |
