Membranes for PEM Fuel Cells

For at least two decades, fuel cells have been seen as a promising solution to energy problems, although they have recently become more of a link in an energy conversion or management chain than a universal solution. In principle, a fuel cell converts the energy of a fuel (e.g. hydrogen, light alcohol, hydrocarbons) into electricity, heat and water – and carbon dioxide – in the presence of an oxidizer such as oxygen from the air.

Fuel cells can be used for stationary applications (e.g. in the home, or to power small portable electrical appliances), or on-board for transport, i.e. for electric traction. Depending on the temperature level at which energy is converted, several fuel cell technologies are available, the best-known of which are polymer electrolyte membrane fuel cells (PEMFC), at temperatures below 200°C, and solid oxide fuel cells (SOFC), generally above 600°C. Compared with solid oxide fuel cells, membrane fuel cells offer a number of advantages, including shorter response times to load, lower toxic gas emissions, and significantly lower thermomechanical and corrosion stresses. The downside is a lower electrical efficiency than that offered by high-temperature cells, and greater sensitivity to any pollutant gases contained in the fuel or oxidizer.

The fuel is oxidized at the anode into different species depending on its nature, but in all cases the oxidation produces protons which are transferred to the cathode through the membrane inserted between the two electrodes: here lies an important property of membranes, i.e. enabling this transfer with the lowest possible (ionic) resistance to limit the voltage losses of ohmic-type cells. The membrane used for proton transport is said to be cationic. Some membrane cells work with anionic membranes that ensure the passage of hydroxide anions formed at the cathode to the anode: these membranes are called anionic membranes. In this article, only the case of cationic membranes is discussed, but many of the points presented here are applicable to anionic membranes.

This article describes the different types of membranes used in PEMFC-type batteries, as well as their operating principle: in all cases, this component must make it as easy as possible for protons generated at the anode by hydrogen oxidation to be transferred to the cathode under the action of the electric field created by the polarization of the electrodes. Membranes are not the site of electric charge generation – as are – electrodes, but a medium that can be physically assimilated to a polyelectrolyte gel, enabling the transfer of protons from the anode to the cathode, the current generated by the cell being directly proportional to this flow of protons according to Faraday's law.

After a brief review...