Gradient catalyst layer

It is known that the catalyst layer is far from uniform, especially in the case of a gradient catalyst layer. Thus, profiling properties, such as conductivity, in the catalyst layer are important. Both an electronic conductor (carbon) and an ionic conductor (Nafion ) exist in the catalyst layer, which can be considered a conductive polymer. The conductive polymer electric circuit model has been applied to the catalyst layer, and an ionic conductivity profile was obtained [8], as shown in Figure 4.33. 

Temperature gradient among reactor wall, catalyst-layer, and reactant solution under superheated liquid-film conditions. 

Steep temperature gradients inside the catalyst layer will enhance the bubble formation and bring about efficient product desorption and effective regeneration of vacant active sites consequently. There irreversible processes are followed by another irreversible act of bubble detachment from the surface. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

In the laboratory experiments, DOC monolith samples (length 7.5 cm, diameter 1.4 cm) with rather thin catalyst layer coating ( 25 pm) were employed to minimize the internal diffusion effects. The samples were placed into a thermostat to suppress the formation of temperature-gradients along the channels. In the course of each experiment, the temperature of the inlet gas and the monolith sample was increased at a constant rate of /min within the range of 300-800 K. The exhaust gases at the inlet of the converter were simulated by synthetic gas mixtures with defined compositions and flow rates (cf. individual figure captions all gas mixtures contained 6% C02 and 6% H20). 

The fact that ATR-IR spectroscopy uses an evanescent field and therefore probes only the volume very close to the IRE has important consequences for its application in heterogeneous catalysis, in investigations of films of powder catalysts. The catalyst particle size and packing affect the size of the detectable signals from the catalyst and bulk phase. Furthermore, if the catalyst layer is much thicker than the penetration depth of the evanescent field, diffusion of reactants and products may influence the observed signals. In fast reactions, gradients may exist within the catalyst layer, and ATR probes only the slice closest to the IRE. 

For an exothermic reaction the gas now entering at O will be heated as it flows over the hot bed, and the bed is cooled. Reaction then occurs as the E end is approached, and the gas leaves with the appropriate conversion at E. The O end of the reactor now has an axial temperature gradient resulting from the passage of the cool gas. At an appropriate moment the reactive gas is switched back to E. The feed is now heated by the hot catalyst and reacts, but further down the bed it encounters the cold catalyst layers, where its conversion continues to increase for a reversible exothermic reaction such as the oxidation of SO2. Before the temperature at O... 

Gas-diffusion electrodes with these catalysts are prepared by a few methods metalceramic porous hydroclosing layer used to work with enhanced pressure, as well as to work on air hydroclosing layer is hydrophobic Teflon or hydrophobic soot. Thus three-phase boundary (reaction s zone) created by pressure difference between electrolyte and gas hollow or by gradient of layer s wetting. Nickel net 400 micron is current conductor for all types of electrodes. 

While the model presented applies to a wide range of PEM fuel cell operational regimes there are some caveats to its application. We assume the flux of gas and heat out of the catalyst layer scale with the current density. This would not be the case, for example, with a dry anode feed and a wet cathode feed, which would generate water transport independent of the current level. We assume there is no lateral pressure gradient imposed in the x direction across the GDL such as would arise in interdigitated or serpentine flow fields we consider straight flow fields. 

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