Oxygen transport composite membranes

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

One promising approach to facilitated transport pioneered by Nishide and coworkers at Wasada University is to chemically bind the oxygen carrier to the polymer backbone, which is then used to form a dense polymer film containing no solvent [28], In some examples, the carrier species is covalently bonded to the polymer matrix as shown in Figure 11.29(a). In other cases, the polymer matrix contains base liquids which complex with the carrier molecule through the base group as shown in Figure 11.29(b). Because these films contain no liquid solvent, they are inherently more stable than liquid membranes and also could be formed into thin films of the selective material in composite membrane form. So far the selectivities and fluxes of these membranes have been moderate. 

In the case of oxygen transport the best prospects at this moment are the use of metal-oxide composites with high electronic conductivity, or separation with perovskite-derived membranes as reported by Balachandral et al. [22]. These latter membranes are thick (0.5-1.0 mm) and have long-term stability at high temperature. 

Barton, S.C. (2005) Oxygen transport in composite biocathodes. Proton Conducting Membrane Fuel Cells III, Proceedings, 2002 (31), 324—335. 

Recently, Figoli et al. [15] reported the use of polymerized bicontinuous microemulsion (PBM) membranes as nanostructured liquid membranes for facilitated oxygen transport. The final bicontinuous microemulsion consisting of an interconnected network of water and oil channels, stabilized by the interfacial surfactant film, in which the oil (monomer) channels were polymerized to form the polymeric matrix of the liquid membranes (Fig. 7.6) and the channel width (pore size) of the membranes could be tuned between 3 and 60 nm by adjusting the composition of the cosurfactant, while the water phase remained unchanged and it was the solvent for the novel oxygen carrier. 

CaTij Fe 03 materials are of great interest for membrane applications for a number of reasons. They have great chemical stability and an almost composition and temperature independent thermal expansion coefficient of 12 x 10 1/K. In addition, its cost is much lower compared to that of BSCF and LSCF. However, a serious drawback is its poor oxygen transport capability. Its oxygen fiux is about 0.02 ml/cm /min, around two orders of magnitude lower than that of BSCF. 

Equations 47-49 describe variations of parameters along the y coordinate of the catalyst layer (y = z/lc 1), where z is the catalyst layer thickness coordinate, y = 0 specifies the catalyst layer/gas interface, and y = 1 specifies the catalyst layer/ionomeric membrane interface (see Fig. 44), in which Rc (= /Ci/cr) is the protonic resistance through a unit cross-sectional area of the catalyst layer and 7d (=nFDC /lc ) is a characteristic diffusion-controlled current density in the catalyst layer. The thickness of the catalyst layer disappears from the equations by introducing Rc, ax, and I ). The experimental variables considered include the overpotential 1], the current density /, and the oxygen concentration C, when pox = 1 atm at the catalyst layer/gas interface. The O2 partial pressure, pox, at the catalyst layer/backing layer interface is determined, in turn, by the cathode inlet gas stream composition and stoichiometric flow rate and by the backing layer (GDL) transport characteristics. 

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