31.16: Membrane

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Dupont’s Nafion™ ion exchange membrane forms the basis of the proton exchange membrane fuel cell. Each company involved in the development of PEMFCs may have their own variation on Nafion, however, they’re all based on the same sulphonated fluoropolymers and Nafion remains something of an industry standard in membranes, to which all others are compared (although it is not always most suitable). Nafion is a polymer based on PTFE (polytetrafluoroethylene).

PTFE

Nafion is essentially PTFE containing a fraction of pendant sulphonic acid groups. (Nomenclature: “sulphonic acid group” usually refers to the un-dissociated SO₃H group, where as “sulphonate” refers to the ionised SO₃^– group after the proton has dissociated). The ion containing fraction is normally given in terms of equivalent weight (i.e. number of grams of dry polymer per mole of acidic groups). The useful equivalent weight for Nafion ranges from 800‑1500 g mol^–1.

Nafion structure (Left) and a fluoropolymer (Right), made by DOW chemical company, also used in PEMFCs.

The length of and the precise nature of the side chains vary between different brands of polymer. Common to all is the PTFE based fluorocarbon “backbone” of the polymer that has several desirable properties:

PTFE is hydrophobic - this means the hydrophilic sulphonate groups are effectively repelled by the chains and cluster together.

PTFE is extremely resilient to chemical attack – the environment within the membrane is hostile and very acidic. Hydrocarbon-based polymers would tend to degrade rapidly.

PTFE is a thermoplastic with high mechanical strength – meaning very thin membranes can be produced, reducing the thickness of each cell and increasing the power density of the stack.

Transport through the membrane

The animation below demonstrates schematically the mechanism of proton transport in the proton exchange membrane.

In reality, the protons would be strongly associated with water molecules and transported in the form of H₃O_⁺ hydronium ions, or even higher order cations. The Zundel (H₅O₂_⁺ - basically a protonated water dimer) and the Eigen (H₇O_3⁺) cations are thought to be particularly important in transfer of protons from one hydronium to another.

Points to note:

Sulphonic acids are highly acidic (pKa ~ –6 in Nafion) meaning they have a high tendency to dissociate into anions and protons (the effect of the aliens’ blood in the “Alien” films was produced with chlorosulphonic acid). It is of course, these protons that act as the charge carriers through the membrane.

In order for the polymer to conduct H_⁺ it must be hydrated to the correct degree, in order to promote dissociation of ionic groups and provide a mechanism for proton transport. Proton conductivity is strongly dependant on the water content of the membrane. The water in the membrane is localised to the hydrophilic groups, where the protons dissociate and are transported in a vehicular manner (by diffusion of hydrated protons) and also structurally (via proton transfer between hydrated clusters).

Typical PEMs have conductivity in the order of 0.01–0.1 S cm^–1 at 80–90 °C, which is a far lower temperature than other solid-state (usually ceramic) electrolytes.