Overpotentials decreasing

A correlation between the spacing of striae and convection downstream of protrusions does not fully describe the process. The initial protrusions arise far from transport control and cannot be attributed to a diffusive instability of the type described in the previous section. Jorne and Lee proposed that striations formed on rotating electrodes by deposition of zinc, copper and silver are generated by an instability that arises only in systems in which the current density at constant overpotential decreases with increasing concentration of metal ion at the interface [59]. 

The additional potential required to maintain a current flowing in a cell when the concentration of the electroactive species at the electrode surface is less than that in the bulk solution. In extreme cases, the cell current reaches a limiting value determined by the rate of transport of the electroactive species to the electrode surface from the bulk solution. The current is then independent of cell potential and the electrode or cell is said to be completely polarized. Concentration overpotential decreases with stirring and with increasing electrode area, temperature and ionic strength. 

Jiang et al. [44] also found that the anode overpotential decreased as the YSZ particle size decreased in the range of -0.1 to 1.5 pm, as shown in Figure 2.14(b). 

In the light of common parallelisms between catalysis and electrocatalysis, it is interesting to note that Co and Mo are used in catalytic processes of hydrodesulfurization [450], The activity of sulfide electrodes changes with time [439, 442, 446]. Normally, there is an initial period during which the overpotential decreases, then it increases again or levels off at a constant activity. The initial improvement is interpreted in various ways but it is generally attributed to some stabilization of the electrode surface (Fig. 23). It seems that a hydride phase is formed initially [442],... 

Figure 13 shows the potential and concentration distributions for different values of dimensionless potential under conditions when internal pore diffusion (s = 0.1) and local mass transport (y = 10) are a factor. As expected the concentration and relative overpotential decrease further away from the free electrolyte (or membrane) due to the combined effect of diffusion mass transport and the poor penetration of current into the electrode due to ionic conductivity limitations. The major difference in the data is with respect to the variation in reactant concentrations. In the case when an internal mass transport resistance occurs (y = 10) the fall in concentration, at a fixed value of electrode overpotential, is not as great as the case when no internal mass transport resistance occurs. This is due to the resistance causing a reduction in the consumption of reactant locally, and thereby increasing available reactant concentration the effect of which is more significant at higher electrode overpotentials. 

Hence the basic (reversible) thermodynamic standard potential ( therm) of decomposition (as also the overpotentials) decreases as the temperature increases. 

Figure 9 shows cyclic voltammograms for BDD and BDD/Ir02 electrodes (F = 6.4), obtained in 1 M HCIO4. The presence of Ir02 particles on the BDD surface results in a considerable overpotential decrease for the OER (about 1 V at... 

Carbons exhibit a low electrocatalytic activity for the hydrogen electrode reaction (HER). Structural characteristics have significant electrocatalytic effects on the HER as changes from 2 X 10 to 2.5 x 10 A/cm on going from the basal plane to the side face of pyrolytic graphite. On glassy carbon, the HER overpotential decreases as the pretreatment temperature is increased. This thermal treatment leads to stmctural and chemical transformations from carbonization, precrystaUization, and to graphitization. 

An overpotential contribution is required for both the anodic and the cathodic reaction and, in this case, it is mainly to oxidize water at the Pt anode. As copper is deposited and the concentration of Cu ions falls, both the ohmic potential drop IR across the electrolyte and the overpotential decrease. If we assume that the solution resistance R remains fairly constant, the ohmic potential drop is directly proportional to the net cell current. The overpotential increases exponentially with the rate of the electrode reaction. Once the Cu ions cannot reach the electrode fast enough, we say that concentration polarization has set in. An ideally polarized electrode is one at which no faradaic reactions ensue that is, there is no flow of electrons in either direction across the electrode-solution interface. When the potential of the cathode falls sufficiently to reduce the next available species in the solution (H" ions, or nitrate depolarizer), the copper deposition reaction is no longer 100% efficient. 

Interpolation of the data from Refs. 76, 77, and 108 indicates that, in the presence of 1M KI, the maximum overpotential decrease (in the lower Tafel region) is 0.16-0.20 V. If it is assumed that in the absence of specific ion adsorption il/, the overpotential shift, which can be identified with the shift of the il/i potential, is much closer to the variation in (0.14 V) than to the variation in (0.01 V). 

Parsons " has pointed out that, if we equate Ary to the -potential shift, we must assume that the concentration of ions in the double layer increases in the presence of iodide by several orders of magnitude. However, electrocapillary measurements give no indication of the H" ion adsorption increasing under these conditions. Therefore, Parsons assumed that the overpotential decrease is due to the influence of the iodide on the activity coefficient of the activated complex, rather than on the (/rj-potential shift. 

The above problem of the overpotential decrease, however, can be avoided if it is assumed that only those hydroxonium ions discharge which have penetratedt the first monolayer of water molecules adjacent to the electrode. The major part of cations, including ions, are in the... 

The H2O addition also decreased anodic overpotential [41], The anode gas was humidified with a bubbler that contained water at a certain temperature. Then the partial pressure of H2O was controlled by the water temperatiu-e in the bubbler. The anodic overpotential decreased monotonously with the increase in H2O content [41], although the H2O addition reduced Eqcv according to Eq. 8.13. In addition, Aymo rose with the anodic utilization, which was given larger overpotential by the low H2O flow rate. The positive order of H2O partial pressure in Eq. 8.8 is also the reason for the overpotential behavior. 

At large currents (150 and 200 mA cm ), near the membrane, the ORR overpotential increases (Figure 4.25b) and the MOR overpotential decreases. The distinct peaks of the ORR and MOR rate form and, with the growth of the cell current, these peaks shift from the membrane toward the GDL (Figure 4.26b,c). The peaks overlap, so that the virtual anode disappears. In other words, each proton produced in the MOR is converted in-place back to the neutral water molecule in the ORR (Figure 4.25a). 

With increasing x in Laj. Sr MnOj, the overpotential decreases with the onset of a minimum for the composition x 0.5 as shown in Fignre 12.11. As for electrical condnctivity, the best performance is achieved with Co-containing lanthanum manganite. Hanunouche has shown that the cathodic polarization curve, schematically drawn in Figure 12.12, can be divided into two domains separated by a specific transition potential At low cathodic polarization (E > E,), the oxide cathode behaves as a classic metallic electrode, i.e., platinum. In this region, the reactions involved are... 

Das D, Lee Y-M, Ohkubo K, Nam W, Karhn KD, Fukuzumi S (2013) Acid-induced mechanism change and overpotential decrease in dioxygen reduction catalysis with a dinuclear copper complex. J Am Chem Soc 135(10) 4018-4026... 

Contaminant adsorption (competitive in mixtures with preferential adsorption of the largest-affinity contaminant), contaminant decomposi-tion/electrochemical reaction intermediates production, O reduction reaction pathway modification (atop Oj adsorption favored rather than bridged Oj, electric double layer structure change induced by cation insertion in iono-mer, Pt oxide modification including kinetics, changes in proton activity) or contaminant deposition reduces the catalyst area, increases the reduction reaction overpotential, decreases faradaic efficiency, and increases product selectivity (increased HjO contaminant production) Pt particle dissolirtion acceleration by adsorbed S on Pt from SOj or other soirrces decreasing iono-mer ionic conductivity... 

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