Contents Electrode Kinetics

The reactant gas must diffuse through the electrode structure which contains air (02, N2) and any products of reaction (CO2, N02, NO, H2O vapor, etc.). Response characteristics are dependent on electrode material, Teflon content, electrode porosity, thickness and diffusion/reaction kinetics of the reactant gas on the catalytic surface. By optimizing catalytic activity for a given reaction and controlling the potentiostatic voltage on the sensing electrode, the concentration of reactant gas can be maintained at essentially zero at the electrode/electrolyte interface. All reactant species arriving at the electrode/electrolyte interface will be readily reacted. Under these conditions, the rate of diffusion is proportional to C, where... 

As can be seen from the electrochemical journals and the literature, computers have not been applied to problems in fundamental electrochemistry in any significant way, although the problems are virtually identical to those encountered in the computerisation of spectroscopic techniques. A recent review of fundamental electrode kinetics and instrumentation, for example [6], is similar in content to a review of more than ten years earlier [7]. 

In this paper, we had employed binary carbon supports to fabricate thin film electrodes in DMFCs. The roles of binary carbon supports and an optimal mixing ratio will be evaluated and characterized through cyclic voltammetry measurements. It will be shown that with the usage of two carbon supports, electrochemical activities and loading contents of catalysts can be enhanced. This improvement is further exemplified by the enhanced electrode kinetics of methanol oxidation for a binary carbon support-electrode in comparison to a single support-electrode. 

Lithium additions to the electrolyte are important but not completely understood. Lithium hydroxide improves cell capacity and prevents capacity loss on cycling and also seems to facilitate nickel electrode kinetics. It expands the working plateau on charge and delays oxygen evolution. Some evidence exists for the formation of which improves electrode capacity. Lithium also decreases the carbonate content in the electrolyte since Li2C03 is not very soluble. It also decreases the tendency for swelling of the positive active material but increases the resistivity of the cell electrolyte. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

In Figure 6.5a it can be seen that the kinetic arc for the electrode with 30 wt% PTFE content in the gas diffusion layer has the smallest diameter. Indeed, the spectra for this electrode all have the minimum kinetic loop measured at all three cathode potentials, as seen in Figure 6.5b and c. This result is in agreement with that from the polarization curve measurements however, AC impedance spectra provide more information than polarization curves. This figure shows that the impedance arc due to mass transport in the low-frequency region grows with increasing electrode overpotential and is very sensitive to PTFE content in the gas diffusion layer. 

Song et al. [5] explained that for the electrode with 40 wt% PTFE content in the gas diffusion layer, the increase in the size of the kinetic arc was attributable to the substantial decrease in the active Pt area caused by low water content at the interface of the catalyst layer and the gas diffusion layer. This explanation has been verified by cyclic voltammetric results. A possible solution to improve the performance of this particular electrode is simply to raise the humidification temperature in order to increase the water content at the interface. The results at higher humidification temperatures are shown in Figures 6.6 and 6.7. 

Figure 6.6 proves that increasing the humidification temperature does improve fuel cell performance. Figure 6.7 also confirms that the size of the kinetic arc does decrease with increasing humidification temperature. From these results the authors concluded that it was the reduced water content at the interface that caused the increased charge-transfer resistance of the electrode with excessive PTFE content (40 wt%). 

W(CN)j /W(CN)j, and Fe3"/Fe2+ in acid solutions [90,91], Simple ETR at sodium tungsten bronzes, NaxW03, with the perovskite structure are fast and are influenced by the sodium bulk content of the electrode as can be seen in Table 2. Unfortunately, the kinetic pattern is not simple because the variation of ETR rate coefficients with sodium content is not the same for each couple [91]. A qualitative interpretation of the ETR kinetic results has been attempted in terms of the density of electronic states at the Fermi level of the oxide electrode [90]. 

Fortunately, the low enol content in simple ketone systems does not necessarily impose an obstacle to generating the corresponding enol radical cations in solution. As outlined in Sect. 2 the selective oxidation of the enol tautomer even in the presence of a vast excess of the ketone opens up an indirect, but quantitative access to enol radical cation intermediates for all systems, if an appropriate oxidant has been chosen. The first, albeit indirect evidence for this selective oxidation step stems from kinetic studies by Henry [109] and Littler [110-112] and will be discussed in more detail in Sect. 3.3. Direct evidence for a specific oxidation of enols was provided by Orliac-Le Moing and Simonet [108]. Using voltammetry at a rotating disc electrode they were able to establish a linear correlation between the anodic current and the enol content for various a-cyano ketones 11. In electrolysis experiments the corresponding 1,4-diketones 13 were obtained in high current yield (ca. 90%). 

The interaction of organic molecules with, and their subsequent reactivities at, electrode surfaces are among the more critical aspects of modem electrochemical surface science. However, the study of these processes is an exceedingly difficult proposition. In the past, experimental probes were limited to conventional electrochemical techniques. " But the information content of these methods is limited to the macroscopic properties of the electrodeelectrolyte interface. Consequently, results from surface studies based merely on ensemble thermodynamic and kinetic measurements can be rationalized only phenomenologically with little basis for interpretations at the molecular level." " Over the past few... 

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