Proton conduction mechanism hydrophobic polymer

Hydrated Acidic Polymers. Hydrated acidic polymers are, by far, the most commonly used separator materials for low-temperature fuel cells. Their typical nanoseparation (also see Section 1) leads to the formation of interpenetrating hydrophobic and hydrophilic domains the hydrophobic domain gives the membrane its morphological stability, whereas the hydrated hydrophilic domain facilitates the conduction of protons. Over the past few years, the understanding of the microstructure of these materials has been continuously growing, and this has been crucial for the improved understanding of the mechanism of proton conduction and the observed dependence of the conductivity on solvent (water and methanol) content and temperature. 

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. 

By using hydrophobic fluorinated polymer backbones, the phase separation can be made more distinct in phosphonated membranes, which in turn can enhance the proton conductivity. DesMarteau and co-workers have studied the proton transport characteristics of model perfluoroacid compounds functionalized with phosphonic, phosphinic, sulfonic, and carbo)q lic acids. The results indicated that the proton transfer in phosphonic and phosphinic acids occurs via structural diffusion rather than by a vehicle mechanism. The findings suggested that fluoroallqrlphosphonic and -phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors. 

Membranes based on aromatic poly(ether ether ketone) (PEEK) seem very promising. In addition to their low methanol permeability, they have good mechanical properties, and high thermal stability, and the proton conductivity can be controlled through the degree of sulfonation. The main difference between Nafion and SPEEK membranes, which makes the latter less permeable to methanol, can be attributed to the difference in their microstructure. In SPEEK membranes, there is less pronounced hydrophiUc/hydrophobic separation as compared with Nafion membrane, and the flexibility of the polymer backbone of SPEEK produce narrow proton channels and a highly branched structure that baffles the transfer of methanol [18]. Thus, SPEEK membranes have lower electro-osmotic drag and methanol permeability values. 

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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