Hydrogen molecules first enters the hydrogen electrode (called the anode) of the fuel cell (step 1). The hydrogen molecules then react with the catalyst coating the anode, releasing electrons to form a positively charged hydrogen ion (step 2). These ions cross the electrolyte and reach the oxygen at the second electrode (called the cathode) (step 3). The electrons, however, cannot pass the electrolyte. Instead, they flow into an electrical circuit, generating the power of the fuel cell system (step 4). At the cathode, the catalyst causes the hydrogen ions and electrons to bond with oxygen from the air to form water vapor, which is the only byproduct of the process (step 5).
