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Parts of a Fuel Cell

Polymer electrolyte membrane (PEM) fuel cells are the current focus of research for fuel cell vehicle applications. PEM fuel cells are made from several layers of different materials, as shown in the diagram. The three key layers in a PEM fuel cell include:

Other layers of materials are designed to help draw fuel and air into the cell and to conduct electrical current through the cell.

Membrane Electrode Assembly

The electrodes (anode and cathode), catalyst, and polymer electrolyte membrane together form the membrane electrode assembly (MEA) of a PEM fuel cell.

  • Anode. The anode, the negative side of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so they can be used in an external circuit. Channels etched into the anode disperse the hydrogen gas equally over the surface of the catalyst.

  • Cathode. The cathode, the positive side of the fuel cell, also contains channels that distribute the oxygen to the surface of the catalyst. It conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.

  • Polymer electrolyte membrane. The polymer electrolyte membrane (PEM)—a specially treated material that looks something like ordinary kitchen plastic wrap—conducts only positively charged ions and blocks the electrons. The PEM is the key to the fuel cell technology; it must permit only the necessary ions to pass between the anode and cathode. Other substances passing through the electrolyte would disrupt the chemical reaction.

The thickness of the membrane in a membrane electrode assembly can vary with the type of membrane. The thickness of the catalyst layers depends upon how much platinum (Pt) is used in each electrode. For catalyst layers containing about 0.15 milligrams (mg) Pt/cm2, the thickness of the catalyst layer is close to 10 micrometers (μm)—less than half the thickness of a sheet of paper. This membrane/electrode assembly, with a total thickness of about 200 μm (or 0.2 mm), can generate more than half an ampere of current for every square centimeter of assembly area at a voltage of 0.7 volts, but only when encased in well-engineered components—backing layers, flow fields, and current collectors.

Catalyst

All electrochemical reactions in a fuel cell consist of two separate reactions: an oxidation half-reaction at the anode and a reduction half-reaction at the cathode. Normally, the two half-reactions would occur very slowly at the low operating temperature of the PEM fuel cell. Each of the electrodes is coated on one side with a catalyst layer that speeds up the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM. Platinum-group metals are critical to catalyzing reactions in the fuel cell, but they are very expensive. DOE's goal is to reduce the use of platinum in fuel cell cathodes by at least a factor of 20 or eliminate it altogether to decrease the cost of fuel cells to consumers.

Hardware

Magnified image of catalyst in contact with the solid polymer electrolyte membrane of a fuel cell. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.

The backing layers, flow fields, and current collectors are designed to maximize the current from a membrane/electrode assembly. The backing layers—one next to the anode, the other next to the cathode—are usually made of a porous carbon paper or carbon cloth, about as thick as 4 to 12 sheets of paper. The backing layers have to be made of a material (like carbon) that can conduct the electrons that leave the anode and enter the cathode. The porous nature of the backing material ensures effective diffusion (flow of gas molecules from a region of high concentration to a region of low concentration) of each reactant gas to the catalyst on the membrane/electrode assembly. The gas spreads out as it diffuses so that when it penetrates the backing, it will be in contact with the entire surface area of the catalyzed membrane.

The backing layers also help in managing water in the fuel cell; too little or too much water can cause the cell to stop operating. Water can build up in the flow channels of the plates or can clog the pores in the carbon cloth (or carbon paper), preventing reactive gases from reaching the electrodes.

The correct backing material allows the right amount of water vapor to reach the membrane/electrode assembly and keep the membrane humidified. The backing layers are often coated with Teflon™ to ensure that at least some, and preferably most, of the pores in the carbon cloth (or carbon paper) do not become clogged with water, which would prevent the rapid gas diffusion necessary for a good rate of reaction at the electrodes.

Pressed against the outer surface of each backing layer is a piece of hardware called a bipolar plate that typically serves as both flow field and current collector. In a single fuel cell, these two plates are the last of the components making up the cell. The plates are made of a lightweight, strong, gas-impermeable, electron-conducting material—graphite or metals are commonly used even though composite plates are now being developed.

Photograph of the bipolar plates that serve as both flow field and current collectors in PEM fuel cells. The plates are made of a lightweight, strong, gas-impermeable, electron-conducting material-graphite or metals are commonly used, although composite plates are now being developed.

The first task served by each plate is to provide a gas "flow field." Channels are etched into the side of the plate next to the backing layer. The channels carry the reactant gas from the place where it enters the fuel cell to the place where it exits. The pattern of the flow field in the plate (as well as the width and depth of the channels) has a large impact on how evenly the reactant gases are spread across the active area of the membrane/electrode assembly. Flow field design also affects water supply to the membrane and water removal from the cathode.

Each plate also acts as a current collector. Electrons produced by the oxidation of hydrogen must (1) be conducted through the anode, through the backing layer, along the length of the stack, and through the plate before they can exit the cell; (2) travel through an external circuit, and (3) re-enter the cell at the cathode plate. With the addition of the flow fields and current collectors, the PEM fuel cell is complete; only a load-containing external circuit, such as an electric motor, is required for electric current to flow.