Membrane contamination

The membrane contaminants sources and general mechanisms are described in chapter 2. Chapter 5 provides a discussion on membrane properties and degradation mechanisms, and a more detailed discussion on membrane contamination sources, mechanisms, and mitigation. 

The main contaminants for the membrane are cationic species, such as metal ions, which may come from contaminated air and fuel streams when moisture is present, metal fuel cell components, balance-of-plant components, or nonmetal contaminated component materials. Other organic and inorganic materials can also contaminate the membrane, but the effects of these are less well documented. Component materials supplying contaminants may include the platinum catalyst or alloying metals, such as ruthenium or cobalt, which may leach out into the membrane the raw material source for the carbon materials (in the catalyst support, microporous layer, gas diffusion layer, or plate materials) may also have inherent metal or other chemical impurities and seal and gasketing materials, such as silicone, can decompose and contaminate the membrane. All of the membrane contaminants can also impact the ionomer materials present in the catalyst layers. 

The sulfonic acid groups in the proton exchange membrane have a high affinity for many cationic species. Replacement of the protons by the contaminant cations will result in reduced conductivity of the membrane and performance loss. In addition, the membrane morphology and structure can be affected. An additional degradation issue associated with contaminant metal ions is the occurrence of Fenton s reactions to produce peroxy and hydroperoxy radicals, which in turn will chemically attack the membrane polymer structure. This mechanism is described in section 1.72.1 of this chapter and in chapter 5. The most common Fenton s metal of concern commonly found in the MEA is iron. 

Of the most common air and fuel gaseous contaminants, ammonia is the most significant membrane contaminant, as the cationic species Nff + will interfere with the proton conduction mechanism and result in decreased membrane conductivity. Other gaseous contaminants may also affect the membrane and ionomer functionality through pH effects and decomposition products these are discussed in chapter 5. 

---

Health and Safety Factors, Toxicology. Phosphoms trichloride severely bums skin, eyes, and mucous membranes. Contaminated clothing must be removed immediately. Vapors from minor inhalation exposure can cause delayed onset of severe respiratory symptoms after 2—24 h, depending on the degree of exposure. Delayed, massive, or acute pulmonary edema and death can develop as consequences of inhalation exposure. 

To avoid membrane contamination, wear gloves during all the... 

Figure 2. Subtractive proteomics. It is impossible to purify NEs to homogeneity because of the many connections to both the nucleoplasm and the cytoplasm. Thus, biochemically purified NEs are expected to be contaminated with chromatin and cytoskeletal proteins and with vesicles from organelles such as mitochodria and ER. In contrast, some of these expected contaminants can be purified free of NE contamination. One such contaminant is ER, which can be isolated as microsomes. Another is mitochondria, which has a well characterized protein complement. Therefore NE and microsomal membrane fractions are separately isolated and analyzed for protein content by MudPIT. All proteins appearing in both fractions are removed from the NE dataset because they could be due to ER vesicles sticking to the isolated nuclear NEs. Similarly, known mitochondrial proteins are removed. Because ER and mitochondria are the only expected membrane contaminants of NEs, all remaining integral membrane proteins in the NE fraction should be NE-specific in theory. After prediction by computer algorithm for membrane-spanning segments, an in silica purified NE transmembrane protein list is obtained. A limitation of this approach is that it discounts any proteins that are found both within the ER and the NE membranes (e.g. solid black triangles).

Convenience membrane replacement and easy cleaning of surface contamination High energy consumption per unit amount of liquid treated High feed flow rate helps reduce the membrane contamination Low packing density of the module Simple pretreatment of feed liquid... 

Since the imembrane usually presents the largest surface area of material in commercial filters, it is from membrane contamination during manufacture that... 

Integrating the selective membrane in a critical enviromnent as chemical reaction one (high temperature and pressure, presence of membrane contaminants), i.e., imposing the integrated membrane reactor configuration, is a challenging issue. Nowadays, only laboratory scale prototypes have been constructed, and even if promising performance has been obtained, the reactor scale-up problems have stiU not be properly faced. 

These results clearly indicate that the bulk of the in vitro sialic acid incorporation into endogenous gangliosides and sialo-glycoproteins was in retina membranes other than ROS. Centrifugation of ROS free membranes in sucrose density gradients resulted in the isolation of the more active membranes for the transfer of H-NeuNAc from CMP- H-NeuNAc, which were collected at the junction between the sucrose layers of densities 1.10 - 1.11. The labelled lipids and proteins found in the ROS membranes after they were incubated with CMP- H-NeuNAc could arise from Pla membranes contaminating the crude ROS fraction during fractionation of retinas. 

Membrane contamination can be minimized by the high feed flow rate. 

Membrane Contaminants/Impurities and Their Basic Chemistry.73... 

Membrane conductivity in PvEM fuel cell systems decreases with increasing NH3 concentration, as reported by Uribe et al. [84,86]. The membrane contamination process can be described by the following reactions [85] ... 

The foreign cationic ions, such as alkali metals, alkaline earth metals, transition metals, and rare earth metals, can cause membrane contamination and then fuel cell performance degradation. The sources of these cationic ions are as follows ... 

In future work, more contamination modeling is needed, especially to account for the effect of different types of contaminants, such as anion and cation contaminants, on cell performance. In reality, of course, multiple contaminants are often the case, so a multicontaminant model needs to be developed that will take into accoimt both anode and cathode contaminants, as well as membrane contamination. 

In this chapter, where possible, we will use the definitions contained in Electrochemical Systems, 3 ed. (Newman and Thomas-Alyea [25]). We will discuss membrane contaminant models in one space dimension and time, which is sufficient to highlight all the major effects, but is readily extendable to two or three dimensions. In general, positive ionic current density, denoted by / in A/cm, will flow from left to right. We denote distance by 2 in absolute units (cm) and by x in scaled units. If L is the membrane thickness, X = z/L. 

---