Acid doping level

Variation of the conductivity of polymer complexes PBI/H3PO4 as a function of water vapour activity, temperature and acid doping level was studied [183]. It was shown that the conductivity of heavily-doped complexes (500 mol %) is nearly twice as high as that of the film doped to 338 mol % at the same temperature and humidity. For instance, the conductivity of PBI doped with 500 mol % H3PO4 (5H3PO4 molecules per repeating unit of PBI) is 3.5 X 10 S cm at 190 °C and water vapour activity of 0.1. 

More interesting results were obtained by Xiao et al. [200] using the PPA sol- gel process shown in Fig. 6.11. This procedure leads to very high acid doping levels (2a > 10-20), never reached by PBl membranes prepared with standard casting procedures. The conductivity of a membrane prepared with the PA A sol-gel method and 2a 16, raises from 15 mS.cm at 20 °C up to 270 mS.cm at 200 °C. The last value is comparable with those observed for the best proton conducting Nafion/inorganic composite membranes at temperatures above 100 °C. 

A partially sulfonated PBI can be prepared from 3,3 -diaminobenzidine, isophthalic acid, and 5-sulfoisophthalic acid with poly(phosphoric acid) [8]. The reaction takes place at 220 °C for 25 h. In the course of the reaction, the color of the solution changes from ocher to dark brown. Afterwards the polymer is precipitated in water and dried in vacuo. In the course of the preparation of a fuel cell membrane, the poly(phosphoric acid) is hydrolyzed into phosphoric acid due to moisture in air so that the polymer membrane has an acid doping level of 2000. 

Poly(2,2 -( 1,4-phenylene)5,5 -bibenzimidazole) can be obtained under certain conditions of polymerization as a high-molecular-weight species [29]. The polymer solutions can be used directly for phosphoric acid doped PBI membranes. Such membranes show high phosphoric acid doping levels. At 160 °C a high carbon monoxide tolerance for fuel cells is observed. 

Figure 2. (a) Weight increases of FBI membranes after being immersed in phosphoric acid of different concentrations at room temperature, (b) Acid doping level of FBI membranes as a function of the acid concentration at room temperature. Reprinted from Q. Li, R. He, R. W. Berg, H. A. Hjuler, N. J. Bjerrum, Water uptake and acid doping of polybenzimidazoles as electrolyte membranes for fuel cells. Solid State Ionics, 168 (2004) 177-185, Copyright (2004) with permission from Elsevier. 

Figure 14. (a) Doping level dependence of ionic conductivity of copolymer III50 at room temperature. (b) Temperature dependence of ionic conductivity of acid doped Copolymers Ileo (-A-) (acid doping level 190 vrt.%), Vso (- -) (acid doping level 200 wt.%), and VI90 (- -) (acid doping level 270 wt.%.) Anhydrous conditions.Copolymers are shown in Fig. 10. 

A more clear view can be seen in Fig. 31 where the polarization resistance and cell voltage at 200mA/cm is plotted against P02 for two different phosphoric acid doping levels at the electrochemical interface. 

The DMAc-cast membrane was doped with various concentrations (30, 40, 50, and 60wt%) of aqueous H3PO4 (PA) solution or 14(M) PA solution or 9(M) PA solution. Equilibrium was reached after 3 days at room temperature. The acid doping levels were expressed as moles of PA per mole of PBI repeat unit. Cross-linked PBI membranes require either covalently or ionically higher acid concentrations [97]. 

Cheddie and Munroe presented a one-dimensional model (across the MEA), where the effects of gas solubility are taken into account and a simple submodel for the blocking of catalyst surface sites by adsorbed phosphate spedes is incorporated [22]. The conductivity of the electrolyte and the solubihty of hydrogen and oxygen are taken into account as functions of temperature. The numerical solution shows very good agreement with experimental results. The influence of the phosphoric acid doping level is discussed. One major conclusion of the model is that only 1% of the catalyst surface is utilized in fuel-ceU operation. 

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. 

Table 13.2 Percent composition, acid doping level, and proton conductivity data for various p-PBI-block-AB-PBI membranes [25]...

Fara-PBI/AB-PBI (mole ratio, x/y) Acid doping level (PA/2 benzimidazole) Proton conductivity (S/cm 160°C) Membrane composition %) ... 

Using a Pt anode electrode and a Pt-alloy cathode electrode, polarization tests were performed on the homopolymer 20H-PBI MEA (Fig. 13.13). The homopolymer produced a voltage of 0.69 V using a load of 0.2 A cm at 180°C and H2/air this is greater than the 0.663 V produced by p-PBI under the same conditions. The high acid doping level and the membrane chemistry significantly contribute to the excellent performance of the 20H-PBI membrane. Overall, the fuel cell performance of 20H-PBI is comparable to that of p-PBI. 

SI in the following, see Fig. 4.5) [43]. Blend compositions (in wt%) from 100 Bl/0 SI up to 50/50 were investigated. It was noted that the acid doping level decreased with decreasing PBI content which can be traced back to both the ionical cross-links by interactions of with Bl and by decreasing Bl proportion in the blend for the 50/50 blend, a maximum ADL of 8.5 with respect to the PBI component was reached, while the pure Bl maximal ADL was 16. The ionic conductivity of the blend membranes was measured in dependence of temperature, acid... 

The equilibrium hydration level depends both on temperature and on steam partial pressure. The water generated at the cathode and the extent to which it equilibrates in the membrane may vary between the different polymer electrolytes, then-acid doping level, and the preparation method of the electrolyte, e.g., the solvent used for the... 

Two processes take place. Water is absorbed due to the hydrolysis of the pyrophosphoric acid according to the reverse of (5.1), up to the point that A = 0, showing that the dimerization of phosphoric acid is a fully reversible process. As expected, the equilibrium constant of phosphoric acid dimerization increases with temperature. Further increase in Pn o resulted in solubility of free water in the HsPOVpolymer system. There is a similar dependence on PE2O for samples used sharp increase at low / h 0 and thereafter linear variation of A. An increase of the acid doping level of the polymers results in enhanced ability to absorb water. 

An increase in evaporation rate is observed upon increasing steam partial pressure and/or acid doping level. 

The term acid doping level (ADL) is widely used and is defined as the number of phosphoric acid molecules per polymer repeat unit and can be calculated according to (6.14), where Mpbi is the molar mass of the polymer repeat xmit and MpA is the molar mass of PA. 

High molecular weight polymers are desirable in order to obtain membranes with adequate mechanical strength, especially at high acid doping levels. However, the IV of/mPBI prepared by melt condensation must be kept low in order to avoid the polymer being insoluble or infusible due to side reactions. For example, the... 

Fig. 9.7 Proton conductivity at different temperatures and partial pressures of water vapor for mPBI membranes with phosphoric acid doping levels of 3.38 and 5.01. Data compiled fr

Oono Y, Sounai A, Hori M (2009) Influence of the phosphoric acid-doping level in a polybenzimidazole membrane on the cell performance of high-temperature proton exchange membrane fuel cells. J Power Sources 189 943-949... 

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