Electroactive species cathodic reduction

If the cathode potential is sufficiently negative, all of the electroactive species undergo reduction to metal as soon as they reach its surface. The current at which this point is reached is known as the limiting... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Fig. 3.53 shows the effect of an electroactive species such as an acid or a more active depolarizer that undergoes cathodic reduction in one ac half-period and anodic oxidation in the next ac half-period (see also Fig. 3.54) x is the so-called transition time, well known from chronopotentiometry (see later), i.e., in Fig. 3.53 the transition time of reduction. 

The thin-layer configuration and its associated diffusion problems means that it is possible to oxidise (or reduce) all of the electroactive species in the thin layer before they can be replenished to any marked degree. Consider, for example, the 0"+/0 couple, with a standard redox potential well within the "electrochemical window of the solvent, so that the current in the absence of the couple is small and can easily be accounted for. With the electrode pushed against the window the potential is stepped cathodic enough to ensure the rapid reduction of the 0" + and the current measured as a function of time, the concentration such that the time for the current to reach zero, or a steady residual value, is small. If the area under the I ft curve is A ampere seconds, then the charge passed Q = A coulombs. Thus, the number of moles of 0"+ reduced, N0, is given by ... 

The resulting isopotential point confirms that H-Tg-H reacts to produce a new electroactive species without side reactions. The optical absorption of the electrochemically generated product is red-shifted and its cathodic peak potentials upon discharging (reduction) lie negative to those of the educt, indicating that the product consists of larger molecules with a more extended redox system. If experiments are carried out at higher sweep rates (v > 100 m V s ), broad waves are... 

Sometimes, we shall address an even simpler class of electrode reaction in which there is only a single electroactive species of a varible activity. The simplest instance of this class is the reduction of metal ions on a cathode composed of that metal, for example... 

S-shaped current potential curves emerge when a surface phase transition of an organic adsorbate is coupled with a faradaic reaction of some electroactive species. As a representative of such a system, the periodate reduction on Au(lll) single crystal electrodes in the presence of camphor was studied [160], Camphor adsorbed on Au(lll) electrodes exhibits two first-order phase transitions upon variation of the electrode potential [161]. In a cyclic voltammogram, the phase transition manifests itself in a pair of needle-like peaks (Fig. 28 (A)). Between the peak pairs, a condensed, well-ordered camphor film exists. At more negative potentials, the camphor coverage is low, while the state of the adsorbate at positive potentials beyond the second phase transition is not yet known. The small hystereses between the respective anodic and cathodic peaks are caused by the finite nucleation rate of the respective thermodynamically stable phase. 

The first type is connected with cathodic reduction in a protic medium, in which we can distinguish between the usual direct mechanism (73) in which the substrate molecule is the electroactive species and mechanism (74) involving formation of adsorbed hydrogen atoms. The latter reaction is obviously very similar to ordinary... 

Thus, in lithium chloride/HMPA the process responsible for the cathodic limit of the SSE is reduction of the HMPA molecule to give the HMPA radical anion = esolv in the normally used terminology. We then encounter exactly the same mechanistic problem with respect to the electroactive species as earlier for anodic processes is the process a direct reduction of the substrate (76) or an indirect one (77) mediated by the solvent (denoted S) radical anion Dealing with the semantic problem does not change the nature of the problem,... 

Ether solutions based on TAA salts are not reduced on noble metal electrodes. The major cathodic reaction of these solutions involves the cation reduction to trialkyl amine, alkane, and alkene (which are the stable disproportion products of the alkyl radical formed by the electron transfer to the cation) [3], Electrolysis of ethers such as THF or DME containing TBAP, formed in the catholyte tributyl amine, butane and butene, were unambiguously identified by NMR and GCMS analysis [3], In the presence of water (several hundred ppm and more), the electrolysis products were found to be tributyl amine and butene (butane was not detected) [3], The potential of this reduction reaction is higher than that of the dry solution, and it is clear that the initial electroactive species in this case is the... 

The Nernst equation takes into account the thermodynamics of the two redox reactions (fast in the sense of the kinetics of the heterogeneous electron transfer, but here in the absence of associated chemical reactions). The thermodynamic properties of a redox reaction are described by the standard potentials, E°, and the concentrations of the redox species at the electrode surface. At the anode, we have an electron extraction from the electroactive species, and at the cathode the electron is donated to an electroactive species to effect a reduction. These processes may be represented in the general form as ... 

Teeter and Rysselberghe reported that CO is reduced to HCOOH at a Hg cathode with the faradaic efficiency nearly 100%. They showed that the polarographic wave height depends on the CO2 concentration in the electrolyte solution, and concluded that the electroactive species of CO2 reduction is CO molecule, neither HCO. nor COj . " ... 

The first cathodic peak most probably results from the sulfur reduction. This is suggested by the similar results of this investigation and previous electrochemical studies (14) where the initial electroactive species was sulfur and by results from lithium-sulfur cell tests where sulfur is known to form from the electrochemical oxidation of Li2S. 

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