Electrons circulate in an external

Electrons circulate in an external electric circuit under a potential difference. The electric potential generated in a single unit is about 0.9V. To achieve a higher voltage, several membrane units need to be connected in series, forming a fuel cell stack. The electrical power output of the fuel cell is about 60% of its energy generation, the remaining energy is released as heat. Generally, oxygen is fed to the cathode as an air stream; in practical systems, an excess of oxygen is fed to the cathode to avoid extremely low concentration at the exit. Frequently, a 50% or higher excess with respect to the stoichiometric oxygen is fed to the cathode. For the anode, instead, it is not typically the stoichiometric ratio, but rather the amount of hydrogen converted to the fuel cell as a percentage of the feed that is specified. This amount is named as the hydrogen utilization factor Uf; when pure hydrogen is fed to the PEMFC, this factor can be assumed equal to unity.