Fuel cells are electrochemical devices that convert chemical energy from the reactants directly into electricity and heat. The device consists of an electrolyte layer in contact with a porous anode and cathode on either side. An illustration of a fuel cell with reactant/product gasses and the ion conduction flow directions through the cell is shown in Figure 1.
Figure 1: A single PEM fuel cell configuration
In a standard fuel cell, gaseous fuels are fed continuously to the anode (negative electrode), while an oxidant (oxygen from the air) is fed continuously to the cathode (positive electrode). Electrochemical reactions take place at the electrodes to produce an electric current. Some of the advantages of fuel cell systems are:
• A high operating efficiency that is not a function of system size.
• A highly scalable design.
• Several types of potential fuel sources are available.
• Zero or near-zero greenhouse emissions.
• There are no moving parts in the fuel cell stack, which provides reliable, vibration-free operation. (There may be pumps or compressors in some fuel cell plant subsystems).
• Nearly instantaneous recharge capability when compared to batteries.
Some of the limitations common to all fuel cell systems include:
• Cost-effective, mass produced pure hydrogen storage and delivery technology.
• Fuel Reformation technology may need to be considered if pure fuel is not used.
• Fuel cell performance may gradually decrease over time due to catalyst degradation and electrolyte poisoning if pure fuel is not used.
Comparison with Batteries
Fuel cells are like batteries, but also have some significant differences. Both technologies are electrochemical devices that produce energy directly from an electrochemical reaction between the fuel and the oxidant. Some of the unique characteristics of a battery include:
1. It is an energy storage device.
2. The maximum amount of available energy is based on the amount of chemical reactant stored in the battery itself.
3. A battery has the fuel and oxidant reactants built into itself (onboard storage), in addition to being an energy conversion device.
4. In a secondary battery, recharging regenerates the reactants. This involves putting energy into the battery from an external source.
The fuel cell is an energy conversion device that can produce electrical energy as long as the fuel and oxidant are supplied to the electrodes. Figure 2 shows a comparison between a fuel cell and battery.
Figure 2. Comparison of a fuel cell and a battery
The lifetime of a primary battery is limited due to the following:
1. The battery stops producing electricity when the chemical reactants stored in a battery runs out.
2. When a battery is not being used, a very slow electrochemical reaction takes place that limits the lifetime of the battery.
3. The battery life is dependent on the lifetime of the electrode.
In comparison, a fuel cell is an energy conversion device where the reactants are supplied. The fuels are stored outside the fuel cell. A fuel cell can supply electrical energy as long as fuel and oxidant are supplied. Also, no “leakage” occurs in a fuel cell, and no corrosion of cell components occurs when the system is not in use.
Comparison with Heat Engine
A heat engine also converts chemical energy into electric energy, but through intermediate steps:
1. The chemical energy is first converted into thermal energy through combustion;
2. Thermal energy is then converted into mechanical energy by the heat engine; and
3. Finally, the mechanical energy is converted into electric energy by an electric generator.
This multistep energy process requires several devices to generate electricity. The maximum efficiency is limited by Carnot’s law because the conversion process is based upon a heat engine, which operates between a low and high temperature. The process also involves moving parts, which implies that they wear over time. Regular maintenance of moving components is required for proper operation of the mechanical components. Figure 3 shows a comparison between a fuel cell and a heat engine/electrical generator.
Figure 3. Comparison of a fuel cell to a heat generator
Since fuel cells do not have any moving parts during operation, they are more reliable than heat engines and have less noise. This results in lower maintenance costs, which make them especially advantageous for space and underwater missions. Electrochemical processes in fuel cells are not governed by Carnot’s law; therefore, high operating temperatures are not necessary for achieving high efficiency. Also, the efficiency of fuel cells is not strongly dependent on operating power. It is their inherently high efficiency that makes fuel cells an excellent option for a broad range of applications, including automobiles, buses, distributed electricity, and portable systems.