Proton conduction mechanism hydrophilic sulfonic acid group

Sulfonated polyimides (SPI, Fig. 11.11) are potential candidates for proton exchange membranes and direct methanol fuel cells (DMFC) because the polyimide forms a network structure to control methanol permeability, and if sulfonic acid groups are introduced in the polymer chain, then it becomes hydrophilic and facilitates proton conduction. Polyimide is mechanically and thermally stable and chemically resistant [77]. 

Due to the fact that, in SPEEK, the separation into hydrophilic and hydrophobic domains is less pronounced (Rikukawa and Sanui 2000), the hydrophobic/ hydrophilic domain difference is smaller, the backbone is less hydrophobic, and the sulfonic acid functional group is less acidic. Therefore, SPEEK is less polar, its conductivity is lower, and the flexibility of the polymer backbone is less. In order to improve the proton conductivity, the DS needs to be higher. However, this will also increase the methanol permeability and other problems of high swelling and loss of mechanical properties will arise. As a result, in order to overcome these problems. 

The proton conductivity plays an important role in determining fuel cell performance. Therefore, it is very necessary to investigate the membrane conduction mechanism. For the pristine SPEK membrane, the proton conduction is dependent on water. The hydrophobic domain (polymer backbone) provides the morphological stability and prevents the membrane from dissolving in water. The sulfonic acid functional groups aggregate to form hydrophilic domains that are hydrated in the presence of water. And the connected hydrophilic domain is responsible for the transport of protons and water. 

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