Phase boundary area

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Hence, the sodium activity - and thus the cell potential - is related to the SO2 partial pressure. The auxiliary electrode must be in contact with both the electrolyte (with the mobile sodium ions) and the metal electrode (to measure the electrical signal), as well as the gas consequently, porous electrodes are typically used to provide a large three-phase-boundary area. It is also possible to mix the auxiliary electrode with the electrode either only near the surface (where it is needed) or throughout the electrolyte (which is sometimes easier to fabricate) this is referred to as a composite electrolyte. 

One such circumstance is when the phase boundary area is so large relative to the volume of the system that a substantial fraction of the total mass of the system is present at boundaries (e.g., in emulsions, foams, and dispersions of solids). In this circumstance, surfactants can always be expected to play a major role in the system. 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

In a porous catalyst particle, the reacting molecules must first diffuse through the fluid film surrounding the particle surface. They then diffuse into the pores of the catalyst, where the chemical reactions take place on active sites. The formed product molecules, of course, need to follow the opposite diffusion path. The phase boundary area is illustrated in Figure 9.8. 

Fig. 2.1-19 Concentration profile across the phase boundary area (Hatt number = 4). See text for discus-...

Balachandran et al. (2007) also showed several ways to increase the H2 production rate by H2O decomposition via MIEC membranes. Producing CGO/Ni membranes with a finer microstructure enlarged the triple-phase-boundary area and increased the H2 production rate to = 10 cm (STP)/min.cm. Using a single-phase, mixed-con-ducting membrane (SrFeCoo.50x(SFC2)) also produced higher H2 production rates relative to dual-phase cermet membranes. 

To shorten the equilibration time the most up-to-date headspace samplers have devices available for mixing the samples. For hquid samples, vigorous mixing strongly increases the phase boundary area and guarantees rapid delivery of analyte-rich sample material to the phase boundary. 

Catalyst Layer Morphology The microstructure of the catalyst has a strong effect on the overall effectiveness of the catalyst. From the generic fuel cell description of Chapter 2, the catalyst structure is highly three dimensional, and the potential reaction locations are limited to those with immediate access to ionic and electronic conductors, catalyst, and reactant gas. Maximization of this triple phase boundary area will reduce the activatiou polarization losses for a given current density. A catalyst layer with very low triple-phase boundary area density will have reduced number of available reaction sites and reduced performance. 

As discussed in Chapter 5, the pore size and hydrophobicity control the capillary pressure in the liquid. In a hydrophylic media, the smaller pore sizes have a liquid suction pressure, drawing liquid in. In the PAFC, the SiC matrix has uniformly small pores and is more hydrophilic than the catalyst layer or substrate reservoir, so that losses in electrolyte are readily replaced by suction from stored electrolyte these locations. The pressure and pore size distribution set up the electrolyte-catalyst-reactant interface area and thus are critical factors to control the performance of the electrode. The uniformity and control of the pore size in the matrix are extremely important to prevent local drainage spots with severe crossover. Control of the pore size distribution and hydrophobicity of the catalyst layer, substrate, and storage matrix are critical to ensure long life and maximized triple-phase boundary area for reaction in the catalyst layer. 

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