Acid doped basic polymers

PBl is today considered the most interesting basic polymer for preparation of acid-base membranes, especially if doped with phosphoric acid [24]. In Fig. 3.3 the scheme of the commercial PBl known as Celazole is shown. 

Figure 23.10 Proton conductivity of a few prototypical proton conducting separator materials Nafion as a representative of hydrated acid ionomers (see also Fig. 23.2(a) [43, 78], a complex of PBI (polybenzimidazole) and phosphoric acid as a representative of adducts of basic polymers and oxo-acids (see also Fig. 23.2(b)) [16], phosphonic acid covalently immobilized via an alkane spacer at a siloxane backbone (see also Fig. 23.2(c)) [127], the acid salt CsHSO, [125] and an Y-doped BaZrOj [126].

The spectrum shown in Fig. 5.5a is for the doped BaO-polyaniline sample, which exhibits absorption bands at 420 nm and 780 nm, characteristic of Bronsted acid-doped polyaniline. The spectrum shown in Fig. 5.5b is for the same sample after treatment with a NH4OH 1.0 mol dm solution. As can be seen, the 420 nm and 780 nm absorption bands remain at the same positions, even after treatment in basic solution. This is not observed for the free polymer, for which a reaction with base promotes deprotonation, which causes the appearance of an absorption band at 625 nm. 

To devise polymeric systems that can conduct protons with little or no water, in addition to possessing good chemical and electrochemical stability above 100 °C, is very challenging and has been the focus of intensive research over the last decade. One of the most successful strategies has been to replace the water with less volatile compounds. For example, strong 0x0 acids, e.g. phosphoric acid or sulfuric acid, have been complexed with basic polymers such as polybenzimidazole (PBI). PEMs based on PBIs doped with phosphoric acid have been used successfully at temperatures well above 100 °C under very dry conditions. However, there is still a risk that the acid will be partly leached out, especially at low temperature and high humidity. 

In the vast majority of cases, phosphoric add has been used as acid dopant for PBI or other basic polymers. Early research, however, also considered doping with other adds, including sulfuric and perchloric acids (Xing and Savadogo, 1999). More recently, and with the particular objective of avoiding leaching of the addic component from the membrane in the presence of a liquid feed or, polymerised organic derivatives of phosphoric add have... 

Several materials have been proposed and commercialized as electrolytes for HT-PEFCs. As introduced before, the main polymers used materials from the PBI family and the Advent tetramethyl pyridine sulfone (TPS) family, both being basic polymers allowing chemical interaction with mineral acids (e.g., phosphoric acid) see Fig. 1. Differences can be fotmd both in the chemistry and in the synthetic process especially among the different PBIs, yielding different physicochemical properties such as glass transition temperatures, mechanical stabilities, proton conductivities, and achievable phosphoric acid doping levels (defined as either the ratio of phosphoric acid molecules per polymer repeat unit or the weight ratio of polymer and included phosphoric acid) for a summary, see Table 1. 

PAni is a special conducting polymer since its doped state can be controlled by acid-base reactions. This effect is widely used to detect acidic and basic gases, in particular ammonia gas, because when exposed in ammonia gas, PAni undergo dedoping by deprotonation (Jin et al. 2001 Hu et al. 2002 Bai and Shi 2007). The mechanism of this interaction is shown in Fig. 3.10. 

Another strategy for the improvement of the thermal, dimensional, mechanical, and electrochemical stability is the application of acid-base concepts to H3P04-doped high-T basic polymers. The acid-base blend concept comprises blending of a basic polymer with an acidic polymer, where ionical acid-base cross-links are formed by proton transfer from the acidic group to the basic group (see Fig. 4.1). 

The idea to use the pyridine units as basic moieties in a rigid aromatic polymer was firstly demonstrated by our group in 2003 [7]. In this first approach, despite the use of the pyridine main chain units, phenyl phosphinoxide moieties were also included in the polymer structure both to improve solubility and to create more interaction sites with the phosphoric acid doping agent. New monomers were prepared using... 

Basically, PBI-based membranes show high proton conductivities when properly doped with strong acids such as H3PO4 (PA) [5,8]. PBI is, in fact, a basic polymer, which chemically dissociates the PA with an equilibrium constant has been calculated to be 1.17 x 10 [9],... 

Savinell and co-workers, [105-107], who have principally studied phosphoric acid doped polybenzimidazole (PBl). Similar systems have been reported by He and co-workers [108], who in addition report the conductivities of PBl based membranes doped with PTA and zirconium hydrogen phosphate [108], Acid-base interactions in entirely polymeric systems have been reported by Kerres and co-workers [102], who prepared and stndied several membranes prepared by blending polymers with acidic (snlfonated-PEEK, sulfonated polyethersulfone) and basic (polybenzimidazole, poly-vinylpyridine) characteristics. Selected acid-base polymer systems are discnssed in the following. 

Other PBIs CaroUo et al. [ 109] synthesized a series of PBls with different numbers of N-atoms, that is, different basicities, including poly(2,2 -(2,6-naphthalin)-5,5 -bibenzimidazole) (naphthalin-PBI) (Fig. 4.6d) and poly(2,2 -(2,6-pyridin)5,5 -bibenzimidazole) (Py-PBI) (Fig. 4.6b). They found that both the acid doping level and the proton conductivity remarkably increased with the membrane molecular weight and basicity, which depend on the amount of NH groups as well as on their position in the polymer backbone. In addition to Py-O-PBI (Fig. 4.6c), OO-PBI (Fig. 4.6e) was synthesized by a standard polycondensation reaction of 3,3, 4,4 -tetraaminodiphenyl... 

Figure 29.8 Structures of two basic polybenzitnidazoles (a) FBI and poly(2,5-benzimidazole), also known as AB-PBI and (b) schematic description of polymer matrix-assisted proton transport in phosphoric acid doped FBI.

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