Potential step electrolysis

El-Deab studied the influence of the electrodeposition time on the crystallographic orientation of Au nanoparticles electrodeposited on glassy carbon prepared by potential step electrolysis and on their electrocatalytic properties toward ORR in alkaline medium (0.5 M KOH) [73]. He observed that particles prepared in short time (5-60 s) had smaller size (10-50 nm) and showed a higher particle density (number of particles per unit area), as revealed by SEM images, than those prepared... 

A more useful chronoamperometric method involves a double potential step (somewhat reminiscent of the previously discussed reversal electrolysis). In this case, for instance for a reduction process, a first cathodic potential step is applied according to the preceding criteria (such that instantaneously COx(0,0 — 0), followed at a time x by the application of a second anodic potential step which causes the species Red previously generated at times < x to be instantaneously reoxidised (such that CRed(0,0 — 0). 

Current as a function of time is the system response as well as the monitored response in chronoamperometry. A typical double-potential-step chronoamperogram is shown by the solid line in Figure 3.3B. (The dashed line shows the background response to the excitation signal for a solution containing supporting electrolyte only. This current decays rapidly when the electrode has been charged to the applied potential.) The potential step initiates an instantaneous current as a result of the reduction of O to R. The current then drops as the electrolysis proceeds. 

The simulation of the ECE mechanism may also employ the double-potential-step technique, but a working curve can be constructed from single-potential-step data also. This is because some of the current that passes, as A is converted to B, is due to the electrolysis of C, the decomposition product of B. The greater the decomposition rate of B, the more current flows, approaching the rate of... 

A fast extraction rate was determined by the triple potentiai step electrolysis repeated in the single microdroplet/electrode system. Figure 9.5 shows the current (i) versus time (f) curve. The potential (E) was stepped from 0.45 V (no electrolysis of FcTMA" ") to 0.65 V. At 0.65 V, i rapidly decreased to a steady-state currerlt (I s) id FcTMA+ in the microdroplet was almost oxidized except for the solute entering the droplet during the steady-state. E was then stepped to 0.45 V and FcTMA+ was extracted into the microdroplet. E was changed again to 0.65 V and the i-t curve was measured for... 

The ingress of electrolyte cations into the MOF framework was confirmed by scanning electron microscopy/energy-dispersive x-ray (EDX) analysis of electrochemically treated deposits of Cu-MOF. Results obtained after application of a reductive potential step to Cu-MOF crystals in contact with acetate buffers are shown in Figure 5.3. Here, EDX spectra for (a) pristine Cu-MOF and (b) Cu-MOF after application of a constant potential of-1.0 V for 10 min are shown. EDX spectra of original Cu-MOF crystals exhibits prominent Cu peaks at 1.0, 8.0, and 8.4 keV accompanied by a Si signal at 1.9 keV. After the electrolysis step, an additional Na peak at 1.1 keV appears. 

This experiment, called double potential step chronoamperometry, is our first example of a reversal technique. Such methods comprise a large class of approaches, all featuring an initial generation of an electrolytic product, then a reversal of electrolysis so that the first product is examined electrolytically in a direct fashion. Reversal methods make up a powerful arsenal for studies of complex electrode reactions, and we will have much to say about them. 

Zinc deposits in the mercury during the potential steps thus a question arises about how the initial conditions are restored after each cycle in a sampled-current voltammetric experiment. Because the system is reversible, one can rely on reversed electrolysis at the base potential imposed before each step to restore the initial conditions in each cycle. This point is discussed in Section 7.2.3. 

The appropriate experiment involves a step at = 0 from an initial potential where electrolysis does not occur, to potential E, where it does. Let us consider the special case (45, 46) in which species O is initially present at concentration Cq and species R is initially absent. In Section 5.5.1, we found that the current transient for quasireversible electrode kinetics was given by (5.5.11). Integration from / = 0 provides the chronocoulo-metric response ... 

Although a major advantage of rotating disk electrode techniques, compared to stationary electrode methods, is the ability to make measurements at steady state without the need to consider the time of electrolysis, the observation of current transients at the disk or ring following a potential step can sometimes be of use in understanding an electrochemical system. For example the adsorption of a component. 

Chronocoulometry can also be applied to the cases discussed in Section 14.3.2 where only adsorbed species are electroactive (57). In this situation the potential step causes only double-layer charging and the electrolysis of the adsorbed species. One can estimate Qdi by steps between a potential at the foot of the adsorption wave, and potentials, Ef, beyond the adsorption wave. If Q is not a function of E in the region of the wave, the following equation results (57) ... 

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