Protogenic groups

Steininger, H., Schuster, M., Kreuer, K. D., Kaltbeitzel, A., Bingol, B., Meyer, W. H., Schauff, S., Brunklaus, G., Maier, J. and Spiess, H. W. 2007. Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group A progress report. Physical Chemistry Chemical Physics 9 1764-1773. 

Schuster M, Rager T, Noda A, Kreuer KD, Majer J (2005) About the choice of the protogenic group in PEM separator materials for intermediate temperature, low humidity operation a critical comparison of sulphonic acid, phosphonic acid and imidazole functionalized model compounds. Fuel Cells 5 355-365... 

Figure 23.2 Schematic representation of the nanostructures of (a) hydrated acidic ionomers such as Nafion, (b) complexes of an oxo-acid and a basic polymer such as PBI-n H3PO4 and (c) proton solvents fully immobilized via flexible spacers (in this particular case the proton solvent (phosphonic acid) also acts as a protogenic group). Note, that there are different types of interaction between the polymeric matrices (green) and the liquid or liquid-like domains (blue). The protonic charge carriers (red) form within the liquid or liquid-like domain, where proton conduction takes place.

It should be mentioned, that the transport properties of phosphonic acid, which has recently been used as protogenic group in fully polymeric proton conductors (see Section 23.4.2), seem to be similar to these of phosphoric acid... 

Connectivity in the conduction of protons encompasses hydrogen bonding of the of the protogenic groups to one another and/or water molecules. On... 

Cooperativity in the conduction of protons in hydrated PEMs encompasses effects including the mobility of protons via a flux of water molecules, the amphotericity (i.e. the ability to act as both a Lowry-Bronsted proton donor and acceptor) of the protogenic groups, and the motion of either the proto-genic group or side chain that facilitates the hand off or net transport of a proton. In this section we exclude our discussion to only the latter flexibility of the side chains of PFSA membranes. 

Kreuer KD (2005) Choosing the protogenic groups in PEMS for high temperature, low humidity operation. Conference on advances in materials for proton exchange membrane fuel ceU systems. Pacific Grove, 20-23 Feb 2005... 

The electrochemical reduction of molecular oxygen through a four electron transfer at the cathode of a fuel cell. The CF2SO3H group is the protogenic group on ionomers and membranes utilized in catalyst layer and electrolyte in a fuel cell. 

Paddison SJ, Kreuer KD, Maier J (2006) About the choice of the protogenic group in polymer electrolyte membranes ab initio modelling of sulfonic acid, phosphonic acid, and imidazole functionalized alkanes. Phys Chem Chem Phys 8 4530-4542... 

Another approach to providing ionomers with lower EW and suitable mechanical and solubility properties is to have more than one acidic proton per side chain. If the side chain has additional protogenic groups, a low EW ionomer can be prepared having a higher degree of backbone crystallinity, and hopefully increased stability toward liquid water. One way to prepare such ionomers is to include a highly acidic bis-sulfonyl imide acid in the side chain. Such materials were prepared by Desmarteau (Polymers 7a,b) and more recently at 3M (Polymers 8 and 9) [48, 49, 51]. The structures of some of these materials are shown in Fig. 17.9. 

The relationship between the number of TFE units that form the backbone crystallites and EW is shown in Fig. 17.10. The slope of each line gives the EW of the ionomer/the ratio of TFE units to protons in the polymer, and the intercept is the MW of the acid functional monomer/the number of protons. This shows the utility of having multiple protogenic groups on each side chain in providing polymers having high crystallinity and low EW. 

Two inorganic acids - phosphonic and phosphoric acid - have been in the focus of several theoretical studies in connection with fuel-cell membranes. Phosphoric acid is used as a proton-conducting electrolyte, whereas phosphonic acid is used as a protogenic group in proton-conducting polymers (e.g. in polyvinyl phosphonic acid). 

FLUORINATED lONOMERS AND lONOMER MEMBRANES CONTAINING THE BIS[(PERFLUOROALKYL) SULFONYL]IMIDE PROTOGENIC GROUP... 

Steininger H, Schuster M, Kreuer KD et al (2007) Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group a progress report. Phys Chem Chem Phys 9 1764-1773... 

For a polyphosphazene to be used as the membrane material in a fuel cell, it must possess some kind of protogenic functionality, to allow the polymer to conduct protons. The most widely used protogenic group is the sulfonic acid moiety which can be incorporated into the macromolecular chain either at the polymer synthesis step (Fig. 4a) or later, during postsulfonation (Fig. 4b). While the postsulfonation reaction is generally easier to carry out, it is the direct synthesis of the functionahzed polyphosphazene that should give better control over the final material properties and minimize imwanted side... 

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