TABLE OF CONTENTS The heart of a fuel cell is a polymer, proton-conductive membrane. On both sides of the membrane there is a porous electrode. The electrodes must be porous because the reactant gases are fed from the back and must reach the interface between the electrodes and the membrane, where the electrochemical reactions take place in the so-called catalyst layers, or more precisely, on the catalyst surface. Technically, the catalyst layer may be a part of the porous electrode or part of the membrane, depending on the manufacturing process. The multilayer assembly of the membrane sandwiched between the two electrodes is commonly called the membrane electrode assembly or MEA. The MEA is then sandwiched between the collector/separator plates "collector" because they collect and conduct electrical current, and "separator" because in multicell configuration they separate the gases in the adjacent cells. At the same time, in multicell configuration they physically/electrically connect the cathode of one cell to the anode of the adjacent cell, and that is why they are also called the bipolar plates. They provide the pathways for flow of reactant gases (socalled flow fields), and they also provide the cell structural rigidity.
The following processes take place inside the fuel cell (the numbers correspond to those in Figure 4-1):
Gas flow through the channels; some convective flows may be induced in the porous layers.
Gas diffusion through porous media.
Electrochemical reactions, including all the intermediary steps.
Proton transport through proton-conductive polymer membrane.
Electron conduction through electrically conductive cell components.
