The proton exchange membrane fuel cell (PEMFC) is a type of fuel cell that operates as the "reverse" of water electrolysis. A single cell consists of an anode, a cathode, and a proton exchange membrane. The anode is where the hydrogen fuel is oxidized, and the cathode is where the oxidant is reduced. Both electrodes contain catalysts to accelerate the electrochemical reactions. The proton exchange membrane serves as the electrolyte. During operation, it functions as a direct current power source, with the anode as the negative terminal and the cathode as the positive terminal.