Equilibrium catalyst layers

Our "superheated liquid-film concept" stands on the thermodynamic basis of (1) equilibrium shifts due to reactive separation under boiling and refluxing conditions and (2) irreversible processes of heat flows through the catalyst layer as well as bubble formation from the catalyst surface. 

DC, the product of the effective diffusion coefficient of oxygen (cm2 s-1), and the equilibrium oxygen concentration (moles cm-3 atm-1) in the catalyst layer at the interface with the backing (GDL),... 

The comparative data of the cell performances obtained in methanol/air (Fig. 7) shows that the best performance was achieved for cell 3 with carbon supported catalyst. As it is shown in Fig. 7, the performance of the cell 3 improved with time, which was not observed for the cells 1 and 2 tested under similar conditions. This effect can be explained by the presence of a higher concentration of Nafion in the Pt/C cathode catalyst layer, which in this case probably needs time to approach equilibrium and humidification. 

Figure 1.2 shows the thermodynamic equilibrium for mobile platinum versus potential for an acidic solution in equilibrium with an exposed platinum surface and a lower branch of equilibrium concentration at potentials more positive than approximately 1.1 volts relative to a hydrogen electrode, where the plotted concentration denotes the concentration of mobile species in equilibrium with a platinum oxide layer covering the surface. This shows that excursions to higher potential can rapidly increase the rate of platinum dissolution prior to passivation of the surface. Once the surface is passivated, the dissolution stops and redeposition can occur, albeit incomplete redeposition, as the platinum, once rendered mobile, is free to redeposit on larger particles or diffuse away from the catalyst layer altogether [32]. 

Water in the cathode catalyst layer may exist in multiple states such as absorbed in the ionomer, as vapor, and as sohd phases at sub-freezing conditions. Assuming phase equilibrium, solid water can emerge when the vapor pressure reaches the saturated level. Before that, most water produced from the ORR can be absorbed in the ionomer, which hereby can be defined as the first stage. The second stage is characterized by solid production and ice volume growth within the catalyst layer. 

Figure 5.12 Ratio of calculated conversion of methane steam reforming to equilibrium conversion for various space velocities and thicknesses ofthe catalyst layer the gas flow was assumed to take place in a 50-p,m high gap above the catalyst [385].

At the nanoscale describes the stmctuial changes of the electrochemical double layer surrounding the catalyst and caibon nanoparticles during the degradation process. These nanoscale models consist of a non-equilibrium compact layer sub-model describing the competitive adsorption of the intermediate reaction species, of the parasitic water molecules and oxide formation on both catalyst and caibon, and of a non-equilibrium diffuse layer sub-model in the electrolyte, describing the transport (electro-migration and diffusion) of protons and metallic ions (produced by the catalyst dissolution) close to the nanoparticles. 

A cell that is operated infinitesimally close to electrochemical equilibrium (or open circuit conditions) will not produce any useful power output. To produce a significant power output, sufficient to propel a vehicle, for instance, the cell must be operated at a current density on the order of 1 A cm . Under load, the value of the current density jo of fuel cell operation determines the power output. The current density is directly related to reaction rates at catalyst layers, as well as flows of electrons, protons, reactants, and product species in the cell components. Each of these processes contributes to irreversible heat losses in the cell. These losses diminish the amount of electrical work that the cell could perform. 

In one-dimensional electrode modeling, (x) denotes the metal phase potential and (x) the electrolyte phase potential. The gradient of the metal (carbon) phase potential drives the electron flux, while protons move along the potential gradient of the electrolyte (ionomer) phase. At equilibrium, these gradients are zero and the potentials in the distinct phases are constant, (p (x) = and O (x) = 4) . The potential distribution of a working PEFC with porous electrodes of finite thickness is shown in Figure 1.9. For illustrative purposes, a simple assembly of anode catalyst layer, PEM and cathode catalyst layer is displayed. 

In the cathode catalyst layer, the metal phase potential 4> must be lowered relative to its value at equilibrium in order to enhance the rate of the ORR. The true cathode overpotential tjorr is thus negative (Figure 1.9). Note that in many cases, it is convenient to work with the positive ORR overpotential, which is rioRR = -rjoRR-The anode overpotential has a positive value. For example, if the anode is grounded (( = 0), O is negative, while tjhor = 0 - 0 is positive. 

Absolute values of potentials in metal and electrolyte phases do not matter besides, they cannot be measured. For the determination of catalyst layer local overpotentials, it only matters by how much the local values of and deviate from their equilibrium values. 

The extracted parameters are vital for rationalizing mechanisms and amounts of water fluxes in PEFCs. The model could be applied for the analysis of sorption data at varying PEM thickness and equilibrium water content. Experiments running at varying T would provide activation energies of the vaporization-exchange rate constant and bulk transport coefficients. Similar modeling tools can be developed for the study of water sorption and fluxes in catalyst layers. They can be extended, furthermore,... 

Example 5.15 Effect of Microporous Layer Often, to enhance water transport, a special highly hydrophobic coating between the DM and catalyst layer will be used, called the microporous layer (MPL). This layer has an average pore size somewhere between that of the catalyst layer and the macro-DM. Consider a bilayered DM coated with an MPL. In equilibrium, there will be a liquid saturation jump at the interface between the MPL and the DM, because of the discontinuity in pore sizes. That is, in order to have a liquid phase pressure balance at the MPL-DM interface, as required for equilibrium, the saturation in the MPL will be lower, since the pore size is lower, increasing the capillary pressure. Assume... 

Because hydrolytic reactions are reversible, they are seldom carried out in batch wise processes [26,28,36,70]. The reactor is usually a double jacket cylindrical flask fitted with a reflux condenser, magnetic stirrer, and thermometer connected with an ultrathermostat. The catalyst is added to the reaction mixture when the desired temperature has been reached [71,72]. A nitrogen atmosphere is used when the reactants are sensitive to atmospheric oxygen [36]. Dynamic methods require more complicated, but they have been widely used in preparative work as well as in kinetic studies of hydrolysis [72-74]. The reaction usually consists of a column packed with a layer of the resin and carrying a continuous flow of the reaction mixture. The equilibrium can... 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

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