Cathodic current flows

Mishra et al. [198] discussed in an exemplary way the dark and photocorrosion behavior of their SnS-electrodeposited polycrystalline films on the basis of Pourbaix diagrams, by performing photoelectrochemical studies in aqueous electrolytes with various redox couples. Polarization curves for the SnS samples in a Fe(CN) redox electrolyte revealed partial rectification for cathodic current flow in the dark, establishing the SnS as p-type. The incomplete rectification was... 

If net cathodic current flows then this potential is shifted negatively. Concentration polarization (alternatively called -> mass-transport polarization or - concentration overpotential) is encountered if the rate of transport of the redox reactant to the electrode surface is lower than that of the -> charge-transfer reaction. Together with the charge-transfer or -> activation polarization (overpotential), q3, and the polarization (overpotential) due to a preceding chemical reaction, qrxn> (see... 

In fact, there is a small cathodic current flowing, which > mst be due to the reduction of some impurity, hut this is of no interest here. 

Figure 26. Electronic structure of an interface between an -type semiconductor and an electrolyte solution containing a simple redox system. The conduction band edge is shown as a function of the distance from the surface at the left side (solid line equilibrium, dashed line nett cathodic current flow).

Returning to the forward bias (cathodic) current flow, Eq. 17 bears some analogy to the famous Tafel expressions in electrochemical kinetics. Thus, ignoring the unity term within the square brackets, Eq. 17 predicts a Tafel slope of 60 mV per decade at 298 K. In many instances (see, e.g.. Refs. [157, 158]), such a slope indeed is observed. In many cases, however, the slopes are higher than the ideal value [14, 159-163]. 

The diffusion/recombination mechanism results in considerable overpotential for (cathodic) current flow in the dark (again assuming an n-type semiconductor for illustration). Such a rate-limiting process was found to describe the charge transfer at n-GaAs in 6 M HCl containing Cu as the hole-injecting species [159, 176]. 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

Whenever currents are passed, there is always a potential control error due to the uncompensated resistance. It was seen in Section 1.3.4 to be iR. If a cathodic current flows, the true working electrode potential is less negative than the nominal value by that amount. The opposite holds for an anodic current. Even small values of such as 1 to 10 n, can cause a large control error when substantial currents flow. This is one reason why large-scale electrosynthesis is not usually carried out potentiostatically. In that instance, controlling the current density is often more practical. 

What elementary processes occur before the formation of a PbS04 crystal layer These processes were studied using a rotating lead disc electrode at low anodic polarization. The obtained curves are presented in Fig. 2.9b [21]. When the electrode is rotated during the anodic polarization, the current increases rapidly. No cathodic current flows when the anodic polarization is stopped and the direction of the potential scan is reversed. There are no Pb + ions to be reduced as they have been thrown out into the bulk of the solution as a result of the rotation of the electrode. Pb ions survive in the solution for a certain period of time before reacting with ions to form the PbS04 phase. This indicates that the... 

Probably the first reported instance of observation of an APE was in 1977 for a n-Ti02-Na0H electrolyte interface [158]. The APE was observed in the saturation region of cathodic current flow and was induced by sub-band gap irradiation of the photoanode. A peak in the spectrum of the photoresponse at 800 nm (the corresponding photon energy being lower than the 3.0 eV band gap of Ti02) was used by the authors to invoke a surface... 

Three-dimen.si anal cathode Current flow direction Separator fir needed) Electrolyte flaw direction... 

A polarization of > 0 indicates an anodic and a polarization of f < 0 a cathodic current flow. Figure 4.1 schematically shows the difference between single and mixed electrodes. Corrosion of metals always involves mixed electrodes. 

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