Hydrogen permeation polarization

The effect of a promoter varies with its concentration. Figure 1 shows the rate of hydrogen permeation through a steel membrane under cathodic polarization in the presence of Group VB and VIB elements [12]. In most cases, the promoters achieve their maximum effect at relatively low concentrations. Because of hydrolysis and other secondary reactions, higher concentrations often lead to the deposition of insoluble products that can inhibit hydrogen entry. 

Defining Concentration Polarization Coefificient According to Hydrogen Permeation Driving Force... 

All of the considerations made in the previous section lead to the necessity of defining a concentration polarization coefficient that could be directly connected to specific permeation quantities.Here, this approach will be direct to hydrogen permeation through Pd-based membranes. 

In particular, by comparing the performance of the same membrane in different configurations, the authors showed that in tubular configuration the extent of concentration polarization is the limiting step for hydrogen permeation (as also indicated above), while with the same membrane used in micro-channel configuration the concentration polarization effect can be completely neglected [27, 57]. 

In another work [58], the authors studied the influence of CO and CO2 (components always present in reforming reactions) on the permeation of hydrogen. Although concentration polarization is not a problem, at low temperatures CO preferentially absorbs on the Pd surface (as found for all kind of configurations) depleting the hydrogen permeation rate. 

Kara, S., Sakaki, K. and Itoh, N. (1999) Decline in Hydrogen Permeation Due to Concentration Polarization and CO Hindrance in a Palladium Membrane Reactor. Industrial and Engineering Chemistry Research, 38,4913 918. 

Chen, W. H., Syu, W. Z., Hung, C. I. (2012). Numerical characterization on concentration polarization of hydrogen permeation in a Pd-based membrane tube. International Journal of Hydrogen Energy, 36, 14734—14744. 

The V-I characteristic of a single cell A is shown in Fig. 14.8(a). Although the power density has not reached the target, it is proved that the HMFC concept works without any serious problems. The cell resistance was divided into an IR resistance and a polarization resistance by the AC impedance method. The composition of cell resistance is shown in Fig. 14.8(b). The polarization resistance is further analyzed by an AC impedance method because it is difficult to introduce a reference electrode to a fuel cell with a thin-film electrolyte [12]. AC impedance spectra were measured for various Ph2 of anode side and Pq2 of the cathode side to determine the polarization (Fig. 14.9). In all conditions, two polarization semicircles were seen. One semicircle has a peak around 3 kHz (Ra), and the other has a peak around 30 kHz (Rb). Both of these are affected by Po2 of cathode gas, and none of these is affected by Ph2 of anode gas. So, both Ra and Rb correspond to the cathode polarization, and the anode polarization is very small compared to the cathode polarization. As mentioned, Pd has very high hydrogen permeation capability, and it is reasonable that Pd has high activity as a fuel cell anode. The Po2 dependence of Ra and Rb is analyzed... 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

X-ray crystallographic studies (59) have defined the conformations and hydrogen bonding of the tetracyclines under nonpolar and polar conditions. These are shown ia Figure 3. It is beheved that the equiUbrium between the 2witterionic and nonioni2ed forms is of importance for the broad-spectmm antibacterial activity, membrane permeation, and pharmacokinetic properties. 

Increase in polarity, polarizability, and hydrogen-bonding ability leads to a decrease in BBB permeation rate. Compound size as measured by the McGowan volume leads to an increase in the permeation rate. 

Descriptors of polarity such as hydrophilic regions, capacity factors, and hydrogen-bonding are inversely correlated with BBB permeability. Diffuse polar regions are acceptable for BBB permeation as compared to dense and localized polar regions. 

Compared to hydrocarbonaceous silica RPC sorbents, not as much commitment has been made to the development of bonded, polar-phase sorbents suitable for the high-performance chromatographic separation of peptides. Due to polar, notably hydrogen bonding, interactions between the peptide and the hydrophilic surface of the sorbent useful selectivity effects can, however, be achieved. In fact, at least two types of separation mechanisms can be identified with bonded polar-phase sorbents. In the first mode, the peptides do not interact per se with the bonded polar-phase sorbent but, rather, are separated on the basis of their ability to permeate into the pores and elute in order of their hydrodynamic volume. In this mode, peptides are separated by steric exclusion effects, with the retention (in terms of elution volume, Ve) of a partial retained peptide, Pb described by the following relationships ... 

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