How Does Ion Conduction Work in PEMFC and Proton Conductors?

[avocadogirl]

I'm having some trouble grasping what it means for something to conduct ions. A material is ion-conducting, transporting hydrogen ions (protons, essentially), but electronically insultating? For something to conduct current, it is ultimately nothing more than a flow of charge and, I recall something, vaguely, about Franklin's misnomer in establishing that current is technically "the flow of the absence of charge," but, I guess I'm struggling with what it is to conduct a current of positive charge? I feel like the explanation is going to be shamefully obvious but, it is just not sinking in.

Thank you.

 

[Topher925]

I feel like the explanation is going to be shamefully obvious but, it is just not sinking in.

Actually, it is definitely not shamefully obvious. The mechanisms behind ion conduction through solid electrolytes is anything but well understood and is still a very active area of research in physical chemistry.

In order for something to be non-electrically conductive it needs to have strongly bound electrons in its outer bands. Since most popular solid electrolytes, i.e. Nafion, Flemion, Gore Select, have perfluorinated backbones (F-C bonds) they are very electrically insulative.

As for ion conduction, its a bit complicated. There are many models based on statistical mechanics and molecular kinetics but the most widely accepted one is the "cluster model". With Nafion, you have a perfluorinated backbone with sulfonated side chains, which makes the material very hydrophilic. When the material is humidified and water is absorbed, these sulfonated chains group together and form "clusters" which develop pathways in between each other. When you introduce a proton into this material, you form a hydronium molecule (H₃O) which is very weekly attracted to the sulfonated side chains. The hydronium molecule will migrate across the material by "hopping" from one sulfonated cluster to another until reaches the end of the road so to speak.

The phenomenon in which the ion brings water with it is known as "electro-osmotic drag" and does not happen in all solid electrolytes. For example, acid-base complexes do not have any EO drag aren't as dependent on humidity as perfluorinated electrolytes are. They work by a different transport mechanism which I, nor I think anyone else really fully understands. Ion conductive materials is a very active field of research and IMO is a technology still in its infancy.

BTW, Nafion which is the most popular and well performing ion conductor was invented almost 40 years ago. No one has yet to create a better performing solid electrolyte since Dupont did way back in the 70s.