Oxygen, diffusion coefficient Solubility

The effects of equivalent weight (FW = g polymer/mol SO3H) and water content on diffusion coefficient, solubility, and permeability of oxygen for fully hydrated BAM, S-SEBS, ETFE- -PSSA, Nafion 117, and BPSH membranes have been studied. It has been found that the diffusion coefficients of all the studied membranes decrease with increasing EW, while the solubility correspondingly increases. These trends are the same as found in... 

The experimental data of oxygen solubility (Cqj) and diffusion coefficient (Dqj) in Nafion 211 membrane, as an example, are listed in Table 1.11. Note that the water content in Table 1.11 can be expressed as A, which is defined as hydration number. For a clearer observation, based on Table 1.11, Figure 1.11 shows both O2 solubility and diffusion coefficient as a function of water content. It can be seen that oxygen diffusion coefficient is proportional to water content, and oxygen solubility is not very sensitive to the water content. 

Figure 1.11 Diffusion coefficient, solubility, and permeability of oxygen vs water content for Nafion 211, measured at 100% RH, 30 psi O2 pressure. ...

Haug AT, White RE. Oxygen diffusion coefficient and solubility in a new proton exchange membrane. J Electrochem Soc 2000 147(3) 980—3. 

Fig. 3. (a) Oxygen diffusion coefficient of 98L-PLA films as a function of water activity (b) solubility coefficient of 98L-PLA films as a function of water activity. 

Using the same techniques, Parthasarathy et al. determined the impact of temperature on the oxygen diffusion coefficient and solubility in Nafion [32]. They... 

Table 5. Diffusion and Solubility Coefficients for Oxygen and Carbon Dioxide in Selected Polymers at 23°C, Dry ...

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

The effect of temperature is complex since there are two conflicting factors, (a) a decrease in the oxygen concentration which results in a decrease in and (b) an increase in the diffusion coefficient that increases about 3% per degree K rise in temperature. In a closed system from which oxygen cannot escape there is a linear increase in rate with temperature that corresponds with the increase in the diffusion coefficient. However, in an open system although the rate follows that for the closed system initially, the rate starts to decrease at about 70°C due to the decrease in oxygen solubility, which at that temperature becomes more significant than the increase in the diffusion coefficient see Section 2.1). 

Neither is it connected with oxygen solubility and diffusion, since oxygen solubility in deformed polymers decreases by 5-10 times, and diffusion coefficient lessens significantly. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

Table I. Oxygen and water vapour permeabilities (P02 PH2O) with mean deviation (m.d.), water solubility (S1J20) and diffusion coefficient (Djj o) at 21°C.

Transport properties of oxygen are better in the fluorinated polymer than in aqueous electrolytes. Although the diffusion coefficient is lower than in water, the oxygen solubility is largely enhanced so that the transport parameter D + c02 is higher than in water (160, 161). 

We remember that minority point defect concentrations in compounds depend on the activity of their components. This may be illustrated by the solubility of hydrogen in olivine since it depends on the oxygen potential in a way explained by the association of the dissolved protons with O" and O- as minority defects [Q. Bai, D. L. Kohlstedt (1993)]. Similarly, tracer diffusion coefficients and mobilities of Si and O are expected to depend on the activity of Si02. The value (0 lnDf/0 In aSio2)> = Si and O, should give information on the disorder type as discussed in Section 2.3. 

The diffusion and solubility coefficients for oxygen and carbon dioxide in selected polymers have been collected in Table 2. 

Reasonable prediction can be made of the permeabilities of low molecular weight gases such as oxygen, nitrogen, and carbon dioxide in many polymers. The diffusion coefficients arc not complicated by the shape of the penneant, and the solubility coefficients of each of these molecules do not vary much from polymer to polymer. Hence, all that is required is some correlation of the penneant size and the size of holes in the polymer matrix. Reasonable predictions of the pemieabilides of larger molecules such as flavors, aromas, and solvents are not easily made, The... 

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