Catalyst layer electron transport effects

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

Fig. 44 Schematic of regions considered in comprehensive modeling of the PEFC air electrode. From left to right gas flow channel, GDL, cathode catalyst layer. Oxygen in transported through the porous GDL by gas-phase diffusion through an inert mixture of nitrogen and water vapor. The catalyst layer is described in terms of effective transport characteristics of gas, protons, and electrons [13].

The ammoxidation of substituted toluenes over differently prepared (NH4>2(V0)3(P207)2-and V0(P03)2-phases as well as over (VO)2P207 has been studied by catalytic and in situ-ESR measurements. For effective catalytic performance at least two structural properties were found to be essential i) Closely neighbouring centres must be exposed at the surface which enable the simultaneous adsorption and conversion of the substrate and ii) the catalyst structure must contain building blocks of exchange-coupled ions e. g. in the form of chains or layers which support the electron transport during the redox process. 

NADH to ferricyanide, which proceeds effectively in aeorbic conditions, stops completely in anaerobic conditions. It could be supposed that in anaerobic conditions no peroxides necessary for electron transport are formed. Thus, it could be assumed that one of FMN s functions is that it maintains in mitochondrial membrane a certain optimal concentration of peroxide radicals, which are needed as catalysts of the electron transport reaction during respiration. It was shown that the formation of peroxides with the participation of FMN disappears in acid media. On the other hand, when peroxides are formed the layer adjacent to the membrane becomes enriched in protons. Hence, it follows that the peroxide formation process might possibly be a self-regulating one, the rate of which cannot rise endlessly. This circumstance once more substantiates the supposition that this process might play a very important functional role in membrane redox reactions. 

The presence of a so-called microporous layer at the interface with the catalyst layer has a positive effect on the contact resistance. For carbon black-based catalyst layers, it is usually assumed that electron transport losses are negligible. This may not be the case when less well-conducting oxide or carbide supports are considered. 

An ideal GDL is required to effectively transport the gas reactants to flie catalyst layers, have low electronic resistance, have a surface that enhances good electronic contact, and have proper hydrophobicity for each application. In particular, for the highest power output when the fuel cell is operated at a relatively high current density, a higher flux of gas feed is needed, which requires the ideal GDL to effectively transport reactant gases to flic catalyst surfaces at a high rate [33]. 

The preparation methods of gas diffusion electrodes, short GDE, have strong influence on the properties of the electrode. Within the manufacturing process, for example, the formation of electron conducting pathways through the electrode, the formation of pores to get a high gas distribution, and the effective connection of the electrolyte with proton and product water transport channels can get controlled. All this parameters are very important to get a good three phase boundary zone inside the catalyst layer and with this a working MEA [3]. 

Overall, including all the detrimental factors in catalyst utilization, it is quite likely that far less than 20% of the catalyst is effectively utilized for reactions. Ineffectiveness of catalyst utilization is a major downside of random three-phase composite layers, which are, nevertheless, the current focus in CCL development. Obviously, there are enormous reserves for improvement in these premises. The alternative could be to fabricate CCLs as extremely thin, two-phase composites 100-200 nm thick), in which electroactive Pt forms the electronically conducting phase, eventually deposited on a substrate. The remaining volume should be filled with liquid water, as the sole medium for proton and reactant transport. 

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