What Is a Proton Exchange Membrane?

A proton exchange membrane is a super thin sheet of specialized polymer that acts as a selectively permeable membrane for proton movement in a single direction. In order to achieve this type of selective permissibility, the membrane is constructed from a polymer type known as ionomers. Under acceptable conditions, this proton exchange membrane or polymer electrolyte membrane (PEM) will only permit the passage of protons, while completely blocking access to other ions and gaseous molecules like hydrogen or oxygen.

Ionomers and Their Role in PEM

• The semipermeable nature of this membrane is a result of its combination of units. The majority consist of electrically neutral recurring units, while the rest are charged units. As a result, these specific polymers or ionomers show a change in viscosity with an increase in temperature. The ionomers used for PEM can either be manufactured by fusing special materials into a polymer matrix or by taking pure polymers. A perfluorinated ionomer used extensively for PEM production is Nafion. The rights to formulation of Nafion are solely DuPont's.

Role of PEM in Fuel Cell Technology

• Proton exchange membrane forms an integral part of today's fuel cell technology and, if successful, can power our homes in the days to come. Its fundamental action in the production of electrons within the cell is to maintain form and selectivity so as to avoid cross movement of gases. In fact, the technology is named proton exchange membrane fuel cell (PEMFC) after the semipermeable membrane. The basic construction can be distributed into four main sections.

Four Components of PEMFC

• The negative electrode or anode in the cell controls the movement of electrons once they are unshackled form the hydrogen molecules. The positive electrode or cathode has an etched surface in order to allow greater access of oxygen to the catalyst. The catalyst is a special material like platinum that does not take part in the reaction process but indirectly regulates the process. Finally, the PEM acts as an electrolyte, apart from being a selective medium during the reaction.

The Oxidation and Reduction Processes

• Hydrogen gas enters through the anode, and its pressurized state pushes it on to the catalyst, which in turn helps to ionize the hydrogen atom by releasing electrons from it. These electrons run the electric motor. Upon completion of the task, they are returned to the cathode. At the other end, oxygen gas is passed on to the catalyst through the cathode. Ionization of oxygen attracts the already positively ionized hydrogen to travel through the PEM to the other side to combine with the oxygen ions and form water.

Fuel Cell Parameters and Stability

• A typical electrolytic reaction generates a small potential of roughly 0.7 volts. In order to increase the electric potential to a practical value, fuel cells are combined together to form a stack. The connection between two fuel cells is established via conduction plates or bipolar plates. Typically, these plates go through a lot of oxidation and reduction reactions on either side. Therefore, the plates are made from graphite and other composites to maintain stability over time.