MEMBRANE DEPOSIT MORPHOLOGY STUDIES

Finally, membrane morphology studies gave insights into the nature of the fouling layers formed. More deposition is observed for the largest compound, IHSS HA. The amount of deposit increased with calcium concentration. Further, calcium concentration also influences the structure of the formed layer. The anticipated solubility effects are confirmed, as precipitates of calcium-organic compounds and calcium carbonates are visible. 

Schafer et al. [35] studied the role of concentration polarization and solution chemistry on the morphology of the humic acid fouling layer. Irreversible fouling occurred with all membranes at high calcium concentrations. Interestingly, it was found that the hydrophobic fraction of the humic acids was deposited preferentially on the membrane surface. This result is similar to the work of Ridgway et al. [31], who showed that the hydrophobic interaction between a bacterial cell surface and a membrane surface plays a key role in biofilm formation. The formation of two layers, one on top of the other, was also observed by Khatib et al. [36]. The formation of a Fe-Si gel layer directly on the membrane surface was mainly responsible for the fouhng. 

Perovskite-structured membranes, in the form of thin films supported on porous ceramic or metal substrates, have been studied extensively in the past decade. Thin films offer several advantages including reduced material cost, improved mechanical strength and possibly higher H2 flux. Chemical vapor deposition (CVD) [99], electrochemical vapor deposition (EVD) [100] and sputtering [101] represent typical methods. However, dense films have been difficult to obtain by these methods. It was found that the continuity and gas-tightness of the deposited films were very sensitive to the morphologies and pore size of substrates. 

We initially studied the polymer grafting employing model substrates, nanothick PET fibns deposited on a silicon wafer, to determine the optimal conditions for the surface modification of the membranes. Figure 1 shows the morphology of the PET films after annealing (140 C for 3 hours). The film imiformly covers the wafer at both micro- and nano-levels (Figure la). After the... 

Porosified silicon membranes of defined thicknesses were first studied in the 1990s and have now been realized by electrochemical anodization, micromachining techniques, and the annealing of ultrathin deposited films. The three fabrication routes produce very different morphologies and levels of porosity. A variety of applications have been explored for both macroporous and mesoporous membranes and these are also surveyed. Wholly microporous membranes in silicon, where all pores have diameters less than 2 nm, have not been achieved to date. 

Nafion membranes were modified by the in situ electrodeposition of polypyrrole inside the membrane pores on the anode side only, in order to prevent the crossover of methanol in the direct methanol fnel cell (DMFC) [86]. The modified membranes were studied in terms of morphology, electrochemical characteristics, and methanol permeability. FTIR and SEM confirmed the presence of the polypyrrole on the anode side of the Nafion membrane. SEM showed the polymer to be present both on the membrane surface and inside the membrane pores. It was found to be deposited as small grains, with two distinct sizes the smallest particles had a diameter of around 100 nm, whereas the larger particles had diameters of around 700 ran. Methanol permeability was determined electrochemically and was shown to be effectively reduced. Controlling the phase through blend or block co-polymer will be a good approach in PEM development for satisfying requirement in various DMFC systems. 

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