Reverse electrolysis

At times it can be useful to carry out a sort of reversal electrolysis, i.e. reoxidize the reduced species (or reduce that oxidized) immediately after generating it to re-obtain the initial species. As shown in Table 3, the comparison between the amount of electricity (in C) consumed in the forward process, Qf, and that consumed in the return process, Qr, can be useful in the identification of the electrode mechanism involved in the process under study (even if in general it is less simple than appears). 

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). 

Figure 4.12 Variation of reverse transition time rr with the time of forward electrolysis tx. The quantity k represents the unimolecular homogeneous rate constant for the conversion of the product of the forward electrolysis to a nonelectroactive species (in terms of the reverse electrolysis).

William Grove (English) Grove discovered "reverse electrolysis" following electrolysis of water at platinum electrodes proposed the hydrogen-oxygen fuel cell. 

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. 

H. E. Armstrong held the view that chemical change is reversed electrolysis , and a circuit of change comprises three terms or components, one a conductor of electricity which is capable of forming with the two reacting substances a system analogous to a closed voltaic circuit. This third body or catalyst is usually electrically conducting (and therefore somewhat impure) water ... 

The discovery of the fuel cell followed soon after Faraday developed his laws of electrolysis. In 1839, Grove showed that the electrolysis of water was partially reversible. Hydrogen and oxygen formed by the electrolysis of water were allowed to recombine at the platinum electrodes to produce a current or what appeared to be reverse electrolysis. Using the same fundamental principles but somewhat more advanced technology, Bacon in 1959—after about 20 years of intensive effort—produced a 6 kW power unit that could drive a small truck. 

The grain size of coatings could be reduced when a reversal electrolysis was used (Figure 4.8.2). 

Profile studies (Figure 4.9.1) show that, as the deposit becomes thicker, it is transformed from a fine crystalline coating with grains of a uniform size into a coarser one. An attempt was made to reduce the grain size in the macrocrystalline stmcture by applying the reverse electrolysis mode. The ratio of the duration of the cathodic and anodic periods was varied in the range of 15-50, the duration of the anodic period was 0.5-3.0S, and the current density was 0.2-0.5 A/cm. ... 

The application of the reverse electrolysis mode allowed one to reduce the macrocrystalline stmcture and increase the coating thickness to 200 pm. The optimum parameters are as follows the ratio of t /t = 20-40, the anodic period duration was 0.5-2.0 s, and its current density was 0.15-0.50 A/cm. ... 

When the current between WE and CE approaches zero, or the corresponding charge reaches a plateau, electrolysis is completed and the absorption or emission spectrum of the electrogenerated species is recorded. It is also important to verify the reversibility of the process and, therefore, the experiment has to be repeated by applying a potential step opposite of the first one. For a fully reversible process the absorption or emission spectra obtained in the first (forward) and second (reverse) electrolysis have to coincide. If the recorded spectra are different, it means that the electron transfer is followed by a chemical reaction. As a consequence, the obtained spectra could be those of the products of the reaction and not those characteristic of the oxidized or reduced species. The spectroscopic characterization of such redox species with short lifetimes can be obtained by working at low temperatures under these conditions, indeed, the effect of the unwanted chemical reaction decreases because its rate is slowed down. 

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