Effectiveness factor, porous electrode

Figure 15.3 Simulated effectiveness factor for porous carbon electrode as a function of the exchange current density jo and DCo for Ip] = 0.4 V for a 10wt% Pt/C catalyst layer with 7= 10, A = 140m g p = 2gcm, Nafion volume fraction 0.6, thickness p,m, and ionic conductivity 0.05 Scm See the text for details. (Reproduced from Gloaguen et al. [1994], with kind permission from Springer Science and Business Media.)...

The other approach is more complicated and requires a deeper knowledge of the agglomerate structure or yields more fitting parameters. In this approach, the porous-electrode equations are used, but now the effectiveness factor and the agglomerate model equations are incorporated. Hence, eq 64 is used to get the transfer current in each volume element. The gas composition and the overpotential... 

According to the definition, the effectiveness factor E of the porous electrode is given by... 

It is important to understand that increasing the roughness factor of a planar electrode increases the rate of charge transfer but has little effect on the rate of mass transport. On the other hand, the use of correctly designed porous electrodes can increase the rates of both processes. Thus tlie use of porous electrodes is essential whenever gaseous reactants (e.g., or O ) are employed, even if an ideal electrocatalyst could be found. 

It is to be noted (cf. Pajkossy [1994], Brug et al. [1984]) that the roughness-factor effect is really a Umiting case of the de Levie porous electrode effect The impedance spectrum for such systems can have a part that can be approximately modeled by a CPE having the 9 parameter equal to 0.5. 

This chapter will cover major topics of CL research, focusing on (i) electrocatalysis of the ORR, (ii) porous electrode theory, (iii) structure and properties of nanoporous composite media, and (iv) modern aspects in understanding CL operation. Porous electrode theory is a classical subject of applied electrochemistry. It is central to all electrochemical energy conversion and storage technologies, including batteries, fuel cell, supercapacitors, electrolyzers, and photoelectrochemi-cal cells, to name a few examples. Discussions will be on generic concepts of porous electrodes and their percolation properties, hierarchical porous structure and flow phenomena, and rationalization of their impact on reaction penetration depth and effectiveness factor. 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

In the Dilute Chemical Decontamination (DCD) process developed by Westing-house, a solution of citric acid, oxalic acid, EDTA and an inhibitor is applied, resulting in decontamination factors on the order of 10, as was determined in laboratory studies higher efficiencies were obtained when a pre-oxidation step was included (Murray et al., 1985). In order to improve the decontamination effectiveness, it was proposed that the spent solution be circulated over a porous DC electrode where dissolved ions are reduced. The Fe " ion generated in this manner is assumed to attack the contamination layer in a one-electron reaction, thus accelerating the dissolution of the oxides (Murray and Snyder, 1985). However, this process seems not yet to have been applied to operating light water reactor plants. 

Novel types of synthesis of modem electrocatalysts revealed that the properties of electrode materials can be affected by the controlled formation of nano-sized, finely dispersed, electrocatalyst particles. In the case of DSA, already the traditional preparation procedure involves the thermal decompositiOTi of the corresponding chlorides after dissolution in an appropriate solvent (usually a solvent of low viscosity, e.g., 2-propanol) [3], Recently, sol-gel synthesis was introduced for DSA preparatirMi, with the main effect being related to the increase in the real surface area of the anode [9,10], The effect is recognized as the geometric factor of increased electrocatalytic ability in addition to an electronic factor related to the chemical structure of electrocatalyst [2,4], which is essential for step (6). The geometric factor is important since the measure for the reaction rate is the current density, i.e., the current per surface area of the electrode available for the reaction. Thus, the reaction rate can be considerably increased by the application of nano-3D electrodes, which are porous systems with an extended real surface area. The polarization curves for the CER on... 

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