Disk electrodes reduction waves

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

The most interesting conclusion of the rotating ring-disk electrode experiment is the absence of a reduction wave due to any species other than Cul2" and Is". The implication is that no free Cus+ or any other reducible species with a lifetime beyond tens of milliseconds is produced when YBa2Cus07 dissolves. 

Figure 9.1 illustrates the electrochemical reduction of 02 at platinum electrodes in aqueous media (1.0 M NaC104). The top curve represents the cyclic voltammogram (0.1 V s-1) for 02 at 1 atm ( 1 mM), and the lower curve is the voltammogram with a rotated-disk electrode (900 rpm, 0.5 V min-1). Both processes are totally irreversible with two-electron stoichiometries and half-wave potentials (EU2) that are independent of pH. The mean of the Em values for the forward and reverse scans of the rotated-disk voltammograms for 02 is 0.0 V versus NHE. If the experiment is repeated in media at pH 12, the mean Em value also occurs at 0.0 V. 

The reduction of GcC.14 also proceeds easily. The polarizating curve for the reduction of GeCU (0.1 N BU4NCIO4 in absolute CH3CN, Pt rotating disk electrode) has a single two-electron cathodic wave with 1/2 = —0.35 V. The electron number was determined... 

Figure 56. SP microscope images of a potential wave on an Ag disk electrode during the reduction of peroxodisulfate. (After Flatgen et aZ. A color representation of this figure can be found following page 112.

The effect is to shift the reduction wave in a positive direction without a change in shape. For the rotating disk electrode, where mR = (1.5.24) becomes [assum-... 

Figure 2.19. Structures of substituted cobalt(II) porphyrins and O2 reduction waves at a rotating graphite disk electrode coated with the respective porphyrin ( 2 x 10 mol cm ). The four-electron reduction of O2 produces a significantly larger plateau current for II than for IV where only a two-electron reduction proceeds. From ref. [93], reproduced with permission of the American Chemical Society.

Jacquinot and Hauser reported preparation of an amalgamated Pb, Tl, Au-poly(tetrafluoroethylene) gas diffusion electrode and an internal electrolyte containing [Ni (cyclam)] +. This preparation involves the formation of various metal films by electrolytic deposition or in the case of Hg by dipping the Au disk electrode into elemental Hg. For concentrations between 0.1 and 1% the electrochemical cell showed a sensitivity of 3.58 mA% and the detection limit is 500 ppm. The optimum pH range was determined between 3.5 and 6 and a selectivity ratio of the catalyst for CO2/ H+ of 5 1 was found. The electrochemical process is limited by diffusion of CO2 demonstrated by rotating disk experiments it means that the relationship between reduction current and the square root of the angular speed was linearFigure 5.8 presents the response of a thin mercury film electrode (TMFE) when exposed to solutions equilibrated with CO2. As can be seen a well established reduetion wave for CO2 is only obtained in presence of the [Ni (cyclam)] + catalysts. 

For catalyst activity toward the ORR, the first step is normally to put the catalyzed electrode into the O2- or air-saturated electrolyte solution to record the cyclic votammograms. For example. Figure 5.13 shows the cyclic voltammograms recorded on a GC disk electrode (0.28 cm ) coated with Co-N-S/C catalyst layer in N2-saturated and 02-saturated 0.1 M KOH solutions, respectively. It can be seen that in the presence of O2, a large reduction wave appears with an onset potential close to that of surface Wave I/I in the presence of O2, suggesting that the surface redox process expressed by Wave I/I is probably responsible for the ORR catalytic activity. However, there is no significant ORR... 

Figure 6.11 presents the RRDE experimental results of ORR at various modified GC electrodes. At the bare GC-disk electrode (Figure 6.1 la), there are two reduction waves in the 02-saturated solution, both of which are ascribed to 2-electron reduction of O2 to H02- The first is mediated by the active native... 

The GC/Nafion/Co-(5-N02)TRP modified electrodes were stable in water/ethanol solution and presented only a reversible Ru(III/II) pair of waves at 0.8 V versus Ag/AgCl in the -1.0 to 1.0 V range. However, an irreversible reduction wave, whose intensity is linearly dependent on sulfite concentration in the 0.4 to 70 mg/L range, appeared at Epc = -0.52 V, confirming that tetramthenated porphyrins continue to be electrocatalytic active for reduction processes. The detection and quantification limits were estimated as 0.4 and 1.2 mg/L, respectively. The electrolysis at -0.65 V for 6 h produced thiosulfate and not dithionite, the conventional reduction product in acidic media. The modified electrodes exhibited high stability, and 4.1 % was the RSD measured for 10 independent electrodes stored for 90 days in open air. Rotating disk voltammetry experiments showed a linear Koutecky-Levich plot and the electrocatalytic oxidation of sulfite was limited by electron diffusion through the film. In fact, because of the presence of Co-(5-N02)TRP dispersed in nonelectroactive Nation, several processes such as (a) the mass transfer at film and electrode surface, (b) the electron and substrate diffusion... 

The half-wave potential of the catalytic wave for O2 reduction by rotating disk voltammetry. This value depends on the electrode rotation rate, bulk O2 concentration, pH, and in many cases on the scan rate, the amount of the adsorbed catalyst, and the nature of the supporting electrol5h e n.m. - not meaningful (no defined wave). 

Polarization curve of the preactivated iron disk of a HMRRD electrode in IM NaOH (dE/df = lOmV S ) with hydrogen oxidation, Fe(ll) and Fe(lll) formation at peak A1 (Epi), Fe(ll) oxidation at All (Epj), and oxide reduction at peak C. Simultaneous capacity measurements (dashed curves) (a,b) Fe(IlI) and (b,c) Fe(ll) dissolution as detected at the Pt half-ring electrodes. Frequency, W = 450min with square wave modulation amplitude, Aiv = 250min- and modulation frequency,/= 0.1 s. (From Haupt, S. and Strehblow, Langmuir, 3,873,1987)... 

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