Fuel Cell

A fuel cell has two electrical terminals called electrodes. In this fuel cell, hydrogen gas is fed through the negative electrode into a solution called an electrolyte. The hydrogen reacts with electrically charged hydroxyl ions, each consisting of a hydrogen atom and an oxygen atom. This reaction forms water. During the reaction, the hydrogen gas gives up electrons. The electrons flow around a circuit to the positive electrode, lighting the bulb in the process. At the positive electrode, oxygen gas reacts with these electrolyte, renewing the ions. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram by Linda Kinnaman.

Fuel cells have certain advantages over batteries, which operate in a similar way. One major advantage is that a fuel cell's electrodes (electric terminals) remain largely unchanged during its operation, but a battery's electrodes can be gradually used up.

Fuel cells are more efficient than conventional electric generators, which convert heat energy to electrical energy. Conventional generators cannot convert all the heat that is sent to them, so some of this heat is wasted. Fuel cells produce less waste heat.

Fuel cells are extremely expensive, however, so they are used only for applications in which cost is of secondary importance. For example, fuel cells produce electrical energy for the United States space shuttles.

In a fuel cell, the fuel and the oxidizer take part in electrochemical reactions at two separate electrodes. Another substance, the electrolyte, also contributes to these reactions. In most fuel cells, the electrolyte is a liquid solution.

Electric current (a flow of electrical charge) occurs both outside and inside the cell. The outside current flows through an electric circuit that leads from one electrode to the other. This circuit contains one or more devices such as motors and lights that use electrical energy produced by the cell. The inside current flows through the electrolyte.

Chemicals. In the most common type of fuel cell, the fuel is hydrogen gas, the oxidizer is oxygen gas, and the electrolyte is potassium hydroxide dissolved in water. The potassium hydroxide is in the form of two kinds of ions (electrically charged atoms or groups of atoms): (1) potassium ions, each consisting of a single potassium atom and carrying a positive charge; and (2) hydroxyl ions, each made up of a hydrogen atom and an oxygen atom and carrying a negative charge.

Operation. From pressure containers located outside the cell, hydrogen gas is fed through one of the electrodes into the electrolyte, and oxygen gas is fed through the other electrode into the electrolyte. At the first electrode, hydrogen reacts electrochemically with hydroxyl ions, forming water and releasing electrons. Electrons carry a negative charge, so the electrode becomes negatively charged. This charge causes electrons to flow from the first electrode, through the outside circuit, to the second electrode. This flow of electrons is the outside current.

Electrons that arrive at the second electrode due to the outside current take part in an electrochemical reaction involving water and the oxygen gas that is sent to this electrode. This reaction produces hydroxyl ions, which flow through the electrolyte to the first electrode. This flow of negatively charged hydroxl ions is the inside current.

These reactions at the electrodes continue as long as hydrogen and oxygen are supplied to the electrodes. The cell also produces water, which is removed.

Contributor: Ralph E. White, Ph.D., Prof. and Chairman, Department of Chemical Engineering, Univ. of South Carolina.

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