Doping acid uptake

Crosslinked PBI membranes have been proposed for improved mechanical strength and chemical stability. Li et al. [428] used p-xylene dibromine as cross linker agent and obtained 13 % crosslinked PBI with conductivities close to 100 mS.cm at 180 °C and high acid doping (2a = 7.75). Xu et al. [429], synthesized crosslinked PBI by condensation of 1,3,5 benzenetricarboxylic acid (BTA) and 3,3 -diaminobenzidine (DAB). The membranes exhibited high acid uptake and a conductivity of 64 mS.cm at 170 °C under dry conditions. 

In summary, ABPBI membranes seems to have similar or higher proton conductivity than PBI membranes, under similar conditions of acid doping, humidity and temperature, as expected from the acid uptake and acid dissociation differences discussed above. 

As mentioned in the previous sections, the electrolyte materials are doped with H3PO4 in order to assure high proton conductivity. Acid uptake is a result of the interactions of the polar basic pyridine group with phosphoric acid. The pyridine ring can react and be protonated by H3PO4 [7, 36, 37] as illustrated in Fig. 5.13a. These specific interactions were surveyed and demcaistrated by means of FT-Raman spectroscopy. Spectra of Copolymer I, pristine and after gradual doping with phosphoric acid are depicted... 

The water and phosphoric acid uptake cause dimensional changes of the membrane (swelling). The swelling is normally calculated on the dry undoped membrane volume basis according to (6.15), where Uvmdopcd and Udoped are the volume of the undoped and doped membrane, respectively. 

The thermodynamic data from the adsorption isotherms can be combined successfully with spectroscopic data from Raman investigations. From the changes of characteristic Raman bands as a function of doping degree 6, two distinguishable stages of protic electrolyte (phosphoric acid) uptake can be identified ... 

HT-PEM fuel cells operate with phosphoric acid doped polymer membrane as electrolyte. The acid is physically adsorbed to the membrane. The phosphoric acid distribution within the fuel cell components, such as membrane, catalyst layers, microporous layer, gas diffusion layers, and bipolar plates, is known to be a critical parameter for performance and life time of this type of fuel cells [10]. There are no defined specifications about phosphoric acid uptake of the bipolar plate because its impact on the fuel cell performance strongly depends on several parameters and always has to be considered in a context of the overall fuel cell design. 

A sulfonated polybenzimidazole that has all the desired properties of conductivity and thermal stability to be used at 150-200 °C is sulfonated ABPBI prepared by direct sulfonation of a previously cast membrane, followed by phosphoric acid doping as detailed in Fig. 3.34. Sulfonation of ABPBI reduces the phosphoric acid uptake at low doping bath concentrations, but at high bath concentrations the sulfonated ABPBI absorbs more acid than the nonsulfonated ABPBI (Fig. 3.35), and the phosphoric acid uptake in these concentrated baths increases with the sulfonation degree (Fig. 3.36). As it could be expected, this higher doping level leads to an increase in conductivity as shown in Fig. 3.37 [382,396]. 

A PBI solution with an inherent viscosity of 1.2 dL/g in cone. H2SO4 was used to prepare the membranes through solution-casting in a vacuum oven at 80-90 °C. The film formed was subsequently peeled off and treated with water at 60 °C for one week to remove the residual solvent completely. The film was then dried at 100 °C under vacuum for two days and the doping was carried out by keeping the membranes immersed in 88% H3PO4 solution for 72 h and subsequently drying them at 100 °C for two days. The phosphoric acid uptake was about 13 moles per repetitive unit. 

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