TABLE OF CONTENTS Fuel cells and batteries are electrochemical devices that convert chemical energy directly into electrical energy without combustion. In an internal or external combustion engine, electrons are transferred from the fuel directly to the oxidizer, releasing the stored energy as heat. Converting this heat into mechanical energy is subject to the limitations of the Carnot cycle.
In a fuel cell or battery, the fuel releases electrons to an electrode (the anode). These electrons then travel through an external circuit (or load), doing useful work, until they reach the cathode of the fuel cell or battery, where they combine with the oxidizer. The movement of some ions through the electrolyte that connects the anode and the cathode completes the electrical circuit.
In batteries, the fuel, as well as the oxidizer, is stored inside the battery. When all the fuel is consumed, the battery has to be recharged or replaced. In a fuel cell, the fuel and the oxidizer are stored external to the cell. Theoretically, the fuel cell can, therefore, operate indefinitely as long as fuel and oxidizer are supplied.
There are a number of different types of fuel cells based on the electrolyte used, which in turn determines the operating temperature. The current discussion is limited to fuel cells using a fluorinated ionomer as the electrolyte (PEM fuel cells), with a typical operating temperature of 60 120 C. However, other electrolytes should be briefly mentioned, in order of increasing operating temperature (given in parenthesis): Phosphoric acid (160 220 C), molten carbonate (600 800 C) and...
