RRDE technique

Early work in our laboratory involved the examination of competitive photoanodic oxidation at a CdS electrode in electrolytes containing various reducing agents using the rotating ring-disk electrode (RRDE) technique.20 In the... 

The RRDE technique allows an estimation of the relative amounts of H202 (reaction [2]) and H20 (reaction [7]) produced during 02 electroreduction. Fig. 2 shows the Ti02 potential dependence of the H202 percent obtained as final product of 02 electroreduction. This percent slowly decrease from 40% down to zero as the bandbending diminishes. 

The RRDE technique can provide quantitative information for reactions having rate constants in the ranges given by Eqs. (96) and (97). The limits are dependent on the dimensions of the RRDE, in particular the size of the gap between the ring and the disk. 

However, based strictly on the standard reduction potentials of the two redox couples involved, the equilibrium constant of the reaction as written would be exceedingly small. To further complicate matters, the rates of disappearance of 02 in the presence of Fe(II)TMPyP, as monitored by RRDE techniques, and the rates of disappearance of 02 in the presence of Fe(III)TMPyP, as measured by spectrophotometric methods [52], yielded very similar values. The most likely resolution of this seeming quandary invokes formation of a macrocycle-dioxygen adduct as a short-lived, albeit yet to be detected, intermediate. 

Transition Metal Phthalocyanines Water-soluble tetrasulfonated phthalocyanines incorporating transition metals of the first row, particularly, FeTsPc and CoTsPc, may be regarded as among the first adsorbed macrocycles for which the reaction mechanisms for oxygen reduction in aqueous electrolytes have been studied in depth using RDE and RRDE techniques [60, 90]. 

In this case, the oxidized species of a redox couple produced at the semiconductor disc, can be collected at the Pt ring, provided that the ring is polarized negatively with respect to the standard potential of the redox system. (For details of the RRDE technique see Section 4.2.3). Usually, the competition between the redox process and the dissolution is quantitatively described by the stability factor, as defined by [64]... 

It is possible to investigate the mechanism using the rotating ring disc electrode (RRDE) technique. This has been described in detail by Albery and Hitchman [18]. Briefly the method is as follows. 

OH in alkaline solutions). However, there is still a small amount of intermediates, such as peroxide, which can be detected using RRDE technique. Therefore, the ORR catalyzed by a Pt catalyst has a complex reaction mechanism. In general, the first step of the mechanism is the adsorption of O2 on the Pt surface. The adsorbed oxygen can be electrochemically reduced through several elementary steps, as shown in Reactions 4-V)- 4-XI)3 ... 

In the exploring catalysts for fuel cell cathode ORR, besides new catalyst synthesis, catalyst characterization using electrochemical measurements such as RDE and RRDE techniques seems necessary in order to validate the catalytic ORR activity of the synthesized catalysts, and the down-selection of new catalyst designs. 

In Chapter 7, we will present other typical ORR examples measured by both RDE and RRDE techniques and their associated data analysis. 

ORR Apparent Electron-Transfer Number and the Formed Percentage of Peroxide Measured by RRDE Technique 221... 

In Chapter 5, the instrumentation of RDE technique including electrode design and electrochemical cell construction have been presented, which are also applicable to RRDE technique. 

ORR is probably one of the most widely studied processes due to its important applications. In literature, the ORR mechanisms have been proposed and extensively studied using RRDE technique. Here, we take the RRDE measurements of ORR as examples to illustrate the RRDE data analysis methods. 

Due to the production of H2O2 or HO2 through a 2-electron-transfer pathway, the overall electron-transfer number of the ORR process is always less than 4. This electron-transfer number is normally called the apparent number of electrons transferred per O2 molecule. Actually, this apparent number of electron transfer can be measured by the RRDE technique, from which the percentage of H2O2 formation in the ORR can also be calculated. Generally to say, the apparent number of electron transfer and the percentage of peroxide produced in the ORR process are two important pieces of information in evaluating the ORR catalyst s catalytic activity. 

Koutecky—Levich plot, measured at different electrode rotating rates, if the O2 concentration, the O2 diffusion coefficient, and the solution kinetic viscosity are known. Here, we give the expression of apparent electron-transfer number of ORR as functions of both disk and ring currents, measured using the RRDE technique. 

Pt-based rotating electrode smfaces include pure Pt and Pt-alloy surfaces. This kind of electrode surfaces are the mosdy used electrode surfaces for ORR by RDE/RRDE techniques in literature. In this section, we will give some typical examples in the following subsections. 

A Pt monolayer on other metal smface such as Pd(lll) could be prepared by the galvanic displacement and the ORR activity on such a Pt monolayer was studied using RDE/RRDE techniques. The RRDE results revealed that no peroxide could be detected at... 

The ORR catalytic activity of the electrochemically constructed Pt monolayer on the Au(l 11) was also investigated using the RRDE technique, as shown in Figure 7.3. Two Tafel slopes could be obtained on Pt(lll) surface to be 51 mV decade in the potential range of 0.89—0.93 V and 101 mV decade" in the potential range of 0.76—0.83 V, respectively. The Tafel slope of... 

The ORR behaviors observed by RRDE technique on these bare carbon surfaces in alkaline solution are slightly different although there are two distinguishable waves on their current—potential curves, as shown in Figure 7.5(A and B), respectively. 

One of the most practical applications of RDE/RRDE techniques is the evaluation of ORR electrocatalysts in terms of both their activity and stability for fuel cells or metal-air batteries. In order to increase the catalyst surface area and the catalyst utilization, all catalysts developed are normally supported on conductive support particles such as active carbon. In the... 

In addition to Pt—Au/C catalysts, several other Pt-based alloy catalysts, such as PtCo/C, PtNi/C, PtV/C, and PtPd/C were also reported, and the mechanism of enhancing ORR activity was investigated using both RDE and RRDE techniques. The mechanism of ORR improvement by alloying is ascribed to (1) increase in the catalyst surface roughness,(2) decrease in the coverage of surface oxides and an enrichment of the Pt-active sites of the catalyst surface,(3) increase in the d-orbital vacancy, which strengthened the Pt—O2 interaction, and (4) decrease in the Pt—Pt distance and the Pt—Pt coordination numbers. Table 7.4 lists the properties of PtCo/C and PtNi/C catalysts and their ORR performance parameters measured by RDE techniques for comparison. 

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

In basic solution, some carbon-supported metal oxides, such as Ti407/C, Lao.6Cao.4Co03/C, Carbon-Lao.6Cao.4Co03/C, MnOx/C, and Me-MnOx/C (Me = Ni, Mg), have been explored for ORR catalysts, evaluated by RDE/RRDE techniques. ... 

For example, carbon-supported magneli phase Ti407 was synthesized and characterized for ORR catalysts, and evaluated cyclic voltammetry (CV), RDE, and RRDE techniques in electrolyte solutions containing various concentrations of KOH. The electrochemical stability of the catalyst was also evaluated by cyclic voltammetry scans and chronopotentiometric tests. 

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