Hardware

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) prevent 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, 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.