Proton exchange Membrane (PEM) fuel cells, also known as Polymer exchange
membrane fuel cells typically operate on pure (99.999%) hydrogen fuel. The
PEM fuel cell combines the hydrogen fuel with the oxygen from the atmosphere
to produce Water, heat (up to 90°C) and electricity.
How it Works
Fuel cells typically utilise platinum based
catalysts on the Anode to split the Hydrogen into positive ions (protons)
and negative electrons. The ions pass through the
membrane to the cathode to combine with oxygen to produce water. The
electrons must pass round an external circuit creating a current to rejoin
the H2 ion on the cathode. Chemical Equation:
Anode: 2H2 »» 4H+ + 4e-
O2 + 4H+ + 4e- »» 2H2O
Each cell produces approximately 1.1 volts, so to reach the required
voltage the cells are combined to produce
stacks. Each cell is divided with
bipolar plates which while separating them provide a hydrogen fuel distribution
channel, as well as a method of extracting the current.
PEM fuel cells are considered to have the highest energy density of all
the fuel cells, and due to the nature of the reaction have the quickest
start up time (less than 1 sec) so they have been favoured for applications
such as vehicles, portable power and backup power applications.
The intolerance of the catalysts to impurities such as carbon monoxide
has led to developments of
high temperature membranes which operate at 150°C +. This enables the
catalysts to tolerate greater impurities in the hydrogen supply.
Image source - http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html
Courtesy: SGL Carbon