RDE experiments

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

Figure 14 Schematic illustration of the composite electrode used in RDE experiments. (From A. J. Bard, L. E. Faulkner. Electrochemical Methods Fundamentals and Applications. 2d ed. John Wiley, New York, 2001, p. 335.)...

In the present case, the Levich equation describes the reaction rate for a mass-transfer limited reaction in an RDE experiment. The Levich equation is... 

Figure 39 Experimental and calculated diffusion-limited current densities for RDE experiment.

Equation (8) gives the limiting, potential-independent current density predicted for complete control of sequence (5b) -h (5c) by the dissociative chemisorption of H2 (process (5b)) at a catalyst surface with a small number of CO-free sites (see 18a). Such a limiting rate of hydrogen electro-oxidation at low anodic overpotentials has been observed recently in RDE experiments with H2/CO mixtures, performed with platinum and PtRu RDEs [18d,e]. This limiting current density (Eq. (8)) explains the PEFC characteristic observed with low CO levels in the fuel feed stream, depicted in Fig. 13. Under such conditions, the fuel cell will exhibit ordinary anode losses up to the current density defined by Eq. (8), but higher current demands would require a... 

Numerical simulations of these RDE experiments were carried out based on the sequence of reactions shown in Scheme 3.5, using values ofthe rate constants, k 1 k2 and k 2 in Equations 3.11 and 3.12 (which correspond to a and c in Scheme 3.3) determined by Endicott and coworkers by classical stop-flow techniques [47] ... 

The performance of the Pt-Au/C (2 1) and Pt/C catalysts toward ORR in the presence and absence of methanol was evaluated by RDE experiments and the results are shown in Fig. 5.6a. As can be observed, Pt-Au/C (2 1) and Pl/C possesses similar ORR half-wave potentials, but Pt-Au/C showed higher ORR activity in the presence of methanol. The electrocatalysts have also been tested in a DMFC [27]. The cell polarization data for Pt-Au/C catalysts with different Pt to Au atomic ratios in a methanol/02 DMFC are presented in Fig 5.6b. The results obtained for Pt/C were also included for comparison. These results show that the DMFCs with Pt-Au/C cathodes perform better than Pt/C. It was noted that the DMFC with Pt-Au/ C (2 1) in the cathode shows an enhanced peak power density of 120 mW cm , compared to 80 mW cm observed for the DMFC with a Pt/C cathode. As argued by the authors [27], since the anodes in both the DMFCs are identical, the enhanced performance for the DMFC with Pt-Au/C catalyst is clearly due to the synergistic promotion of ORR on the catalyst. 

This value corresponds to an RDE experiment, for example, with a rotating speed of about 500 rpm and therefore a diffusion layer thickness of about 30 pm. 

Pyrolysis of macrocyclic precursors has been shown in some cases to yield more active and stable catalysts for instance, pyrolysis of a mixture of Fe and Cu phthalocyanine-based complexes on carbon black produces a catalyst with 4-electron ORR activity higher than Pt/C at potentials lower than 0.84 V (RHE) in RDE experiments [13]. Such pyrolyzed macrocycle catalysts contain nitrogen coordinated to metal atoms, as well as nitrogen in CxNy compounds in which nitrogen is not coordinated to any metal. While the central importance of nitrogen to such catalysts is widely attested, the complex heterogeneous nature of these catalysts has thus far prevented any consensus on the identity of the ORR active site(s). Similarly, pyrolysis of Fe... 

The kinetic current and average number of electrons transferred per oxygen molecule (n) can be obtained through RDE experiments using the Koutecky-Levich equation (Eq. 1) ... 

The theoretical limiting current density in RDE experiments can be calculated using Levich Eq. 2 ... 

The values are calculated from the data of RDE experiments performed in 0.01 M phosphate buffer (pH 7.4) containing 0.1 M NaQ. DET indicates the fraction (in %) of the peroxidase molecules on the electrode, which possesses a direct (mediatorless) electronic coupling with the surface of the gold. Rate constant represents the rate of HjOj reaction with HRP(FeHT), i.e., the rate of the production of El. Constants and represent the rate of the reduction of El to HRP(Fein) by direct heterogeneous ET from the electrode or by soluble catechol (mediated ET), respectively. 

O2 RIE Resistance A polymer sample for the O2 RDE experiment was prepared in a similar way as stated above, viz, the film was exposed to an EB dose of 10 pC/cm at 100 pA, 20 kV followed by development in 4 °C water for 2 minutes. The plasma etching experiment was carried out in an oxygen atmosphere in a Model BP-1, from the Samco International Research Corporation. The radio frequency (RQ power and gas pressure were 100 W and 0.5 Torr, respectively. The etching time was varied from 5 to 60 minutes. The temperature of the substrate was maintained by heating the sample-plate at 40 °C during the etching. The thickness of the film was... 

Since RRDE experiments involve the examination of two potentials contributed by the disk electrode (Ed) and ring electrode (Er), the representation of the results involves more information than that of a single working electrode in RDE experiments. Thus, RRDE experiments are usually carried out with a bipotentiostat which is capable of controlling four electrodes and allowing an adjustment of Ed and Er separately [144]. 

FIGURE 9.6 (a) Variation of i with in the RDE experiment under a constant Ed for the slow reaction on the electrode, (b) Koutecky-Levich plots at potential Ei, where the rate of electron transfer is sufficiently slow to act as limiting factor, and at E2, where election transfer is rapid. 

The other studies of nonbulk metal HOR in alkaline come from Cabot et al. [25,44] who used a Pt-containing gas diffusion electrode (GDE) to closely represent the electrodes of a fuel cell in their RDE experiments. They concluded that at low overpotentials (near the OCV). the Tafel reaction is the rate-determining step in a Tafel-Volmer mechanism, with the diffusion of H2 becoming rate determining at higher overpotentials. These studies also showed that the exchange current density for HOR is lower in alkaline media for GDE. 

What sort of instrumentation would be needed for electrochemical experiments A potentiometry experiment requires little more than a pH meter. A potentiostat or galvanostat can be used for the controlling potential or current in an experiment. In a coulometric procedure, a device to integrate the current (i.e., a coulometer) would also be needed. A hydrodynamic voltammetry [e.g., a rotating disk electrode (RDE)] experiment would require an electrode rotor (to spin the electrode at a precisely known rotation speed), and the rotating ring-disk or RRDE refinement (see below) would necessitate the use of a bipotentiostat so that the disk and ring potentials can be independently controlled. An ac impedance measurement involves the use of a sine-wave oscillator and... 

RRDE voltammetry is designed to provide inherent analysis of the reaction selectivity instead of relying on complex calculations and data plotting techniques such as K-L plot. With electrode rotation, the electrolyte will flow the same way as in RDE experiment. Thus, while the reaction is occruring on the catalyst surface, reactants will continuously be swept away by hydrodynamic flow. This flow will traverse along the electrode surface, passing over the ring electrode. 

The strength of the analytical fitting model to quantify the kinetic parameters of an electrochemical reaction is shown. The coupling of statistically foxmded parameter estimation techniques with LSV/RDE experiments is an important innovative point of the modeling strategy. 

The application of the analytical modeling for the study of the ferri/ferrocyanide reaction with LSV/RDE experiments demonstrate that the modeling methodology is valid to extract the quantitative mechanism of an electrochemical reaction. In the case of the hexaammineruthenium (III)/(II) reaction, however, the results of the analytical modeling point out the importance of a correct formulation of the reaction mechanism. 

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