Doping degree

As it has been mentioned in Chapter 1, application of various additives to the surface of adsorbent is effective from the stand-point of obtaining the required selectivity to a specific type of active particles. Thus, doping the surface of ZnO by zinc made it possible to reduce the sensitivity of such sensors to H-atoms and, vice versa, increase to O-atoms. The highest selectivity is obtained at a specific doping degree [5]. 

Fig. 3.18. Kinetics of conductivity of ZnO film during adsorption of methyl radicals CH3 at room temperature depending on the degree of preliminary alloying of the surface by titanium atoms. 1 - Blank experiment with a clean (Ti-atom free) film (O - before doping - after heating of alloyed film at 350 C, i. e. after the film has been regenerated) 2-5 - Experiments with doped films. Doping degree increases in the following row 2<3<4<5.

We have doped sulfur lignin and sodium lignosulfonate in vapor-phase (iodine, bromine, and ammonia) and in liquid phase (sodium and ferrichlo-ride). The conductivity mainly depends on the nature of the dopant ion and the doping degree. Doping can be monitored by IR-spectroscopy. The intensities of the peaks decrease, and the fine structure vanishes, when the... 

Figure 3. a, the effect of doping degree on IR spectra (bromine) b, ESR-spectra of sulfur lignin doped with bromine. 

The more traditional approach has already been used in anodic electrocrystallization processes to produce nanocompositions and superlattices of mixed Ti-Pb oxides [341-347]. With HTSC materials, initial steps have been made in this direction in studies on the electrochemical deposition of conductive polymers on the surface of microband YBCO electrodes [28,50,433]. In the resulting composition, the reversible transition from the HTSC/metal junction (at the high doping degree of the polymer) to the HTSC/semiconductor junction has been achieved. The properties of these compositions allow one to control the shift over a wide interval. 

There slxqFTIR spectra of synthesized samples of individual PAn, nano-TiO -zS and their composites on the Fig. 2. The form of FTIR spectrum 7 in the 400-4000 cm interval and set observed characteristic bands, namely at 3400-2880, 1560, 1470, 1290, 1110 and 793 cm , corresponds to polyaniline [28], In particular, the peak at 3400-2880 cm corresponds to so-called H-band which is a visualization of H-bonding imine- (-NH-) and protonated imine-group (-NH+-) polyaniline macromolecules between itself [46. Two characteristic bands at the 1560 and 1470 cm are connected with vibrations of quinoid and benzenoid rings and they are characteristic features of the polyaniline. Intensive peaks at 1290, 1110 and 793 cm", corresponds to the emeraldine salt and indicated on the high doping degree of PAn [46. ... 

A higher energy density can be reached by an increase in the doping degree and... 

Several authors have determined methanol permeability for PBI [12, 192, 197, 280,413], methyl and ethyl-PBI [197], and fluorinated PBI [198,414], for different acid doping degree. The reported permeability coefficients are summarized in Fig. 6.33, where the A values are indicated. 

The permeability coefficients of PBI membranes, even those with high doping degrees, indicate than they are more efficient methanol barrier than Nafion. For instance, Pr = 0.01 for undoped PBI, and = 0.040—0.036 for PBI with 2a =1.9 in the range of temperature 30-90 °C. This is not only a result of the lower methanol uptake of methanol by PBI, but it is certainly due to a reduced diffusion coefficient of methanol in this polymer. 

The proton conductivity of PBI and modified PBI membranes has been studied under different conditirms of membrane preparation, acid doping degree, temperature, and water activity. The results for PBI membranes prepared by casting from dimethyl acetamide (DMA) and other solvents (NMP N-Methyl pyrrolydOTie, DMSO dimethyl sulfoxide, TFA, trifluoroacetic acid) are summarized in Table 6.6. 

An isomer of ABPBI which contains head-to head and tail-to-tail benzimidazole sequences, was recently synthesized [449] and membranes were prepared by using the PPA sol- gel process (see Fig. 6.11). The membranes, like those prepared with PBI using this method PBI [200], have a much higher doping degree and their conductivities are above 200 mS.cm at 180 °C, even without humidihcatimi. Proton conductivities above 200 mS.cm were also reported for commercial crosslinked ABPBI membranes by Fumatech [425, 447], at 120 and 140 °C and partial humidification. ABPBI/MMA membranes exhibit only modest conductivities [419, 445], while an ABPBI/PVPA composite [450], which is the equivalent to the commercial PBI-based Celtec V by BASF Fuel Cells shows a poor conductivity. 

An important parameter that characterizes the oxidized (or more generally doped) polymer is the ratio between the number of charges in the polymer and the number of monomer units. This ratio is called the doping level or doping degree. Let the general stoichiometry of the polymer formation be... 

The three most important methods for determining the doping degree are... 

Membrane FT mass loss after 48 h (wt%) FT mass loss after 120 h (wt%) ADL PA doping degree PA swelling (x direction) (%)... 

Fig. 4.9 Fuel cell test of an S11B4 blend membrane, PA doping degree 270 wt% [74]. Reproduced with permission of Elsevier...

Fig. 4.26 Proton conductivities of the 1921C at 131 and 199 % PA doping degree, respectively, at 20 % RH...

As indicated in Fig. 4.26, with a PA doping degree increase from 131 to 199 %, an increase of proton conductivities of up to a factor of 9 is observed. 

In analogy to the fractional coverage d of the original BET model, a doping degree A/ x/A rpu is introduced, which relates the uptake of electrolyte molecules to the number of repeat units of the polymer. By inserting (8.18), (8.19) and (8.20) one obtains. 

Fig. 8.11 FT-Raman spectra (left) of pristine m-PBI bottom) and doped with H3PO4 at different doping degrees 6 (25 °C), taken from Daletou et al. [81]. In the rectangle at 910 cm the Raman signal of buLk-Iike H3PO4 is marked by an up arrow. Adsorption isotherm right) for H3PO4 by m-PBI (25 °C, see also Fig. 8.10).

Fig. 8.13 Intensities of the Raman peaks at 1570 cm for m-PBI left, Daletou et al. [81]) and at 1577 cm for Fumapem AM-55 right, own measurements) as a function of the H3PO4 doping degrees 6. The peak intensities are normalised to the Raman peak at 1000 cm (m-PBI) and 960 cm (Fumapem AM-55). The corresponding...

Raman data is shown in Figs. 8.11 and 8.12. The red dashed lines mark the doping degree 0 equivalent to full protonation of the polymer chains, i.e. 0 = z in case of a protic electrolyte transferring only one proton per molecule... 

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