Overpotential positive

The kinetics of MeOH oxidation of a 1 1 PfRu in an MEA has been well established by Vidakovic, Christov, and Sundmacher. At low overpotentials, the MeOH oxidation reaction was found to be zero order in MeOH concentration, indicating that CO oxidation is the rate-determining step. A Tafel slope of 50-60 mV dec was found at 60°C. At higher overpotentials, positive reaction orders were found, suggesting that MeOH adsorption becomes rate determining. An activation energy of 55 kj moP was found this agrees well with the values found for similar bulk PtRu electrodes. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

It is evident from these expressions that since in the Tafel region / (the current density actually determined) must be greater than /(, (the equilibrium exchange current density), the signs of the overpotentials will conform to equations 1.60 and 1.61, i.e. will be negative and will be positive. 

Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

The importance of the method in corrosion testing and research has stimulated other work, and since Stern s papers appeared there have been a number of publications many of which question the validity of the concept of linear polarisation. The derivation of linearity polarisation is based on an approximation involving the difference of two exponential terms, and a number of papers have appeared that have attempted to define the range of validity of polarisation resistance measurements. Barnartt" derived an analytical expression for the deviations from linearity and concluded that it varied widely between different systems. Leroy", using mathematical and graphical methods, concluded that linearity was sufficient for the technique to be valid in many practical corrosion systems. Most authors emphasise the importance of making polarisation resistance measurements at both positive and negative overpotentials. 

It can be seen that when tj = 0, / = / o and the rate of the cathodic process equals that of the anodic process, i.e. the reaction is at equilibrium. However, if jj is positive the first term increases exponentially, whilst the second term decreases exponentially, and at overpotentials > -I- 0-052 V the... 

The CH4 oxidation on Pd31 exhibits a very pronounced NEMCA behavior at much lower temperatures (380-440°C) compared with those on Pt catalysts (650-750°C). In this temperature range the reaction exhibits inverted volcano behavior.31 For positive overpotentials the p values are as high as 89, with A values up to 105.31 Negative overpotentials also enhance the rate31 with p values up to 8. 

Figure 8.41 shows the effect of positive overpotential, i.e. increasing work function, on the apparent activation energies E, and preexponential factors kf of the epoxidation (i=l) and deep oxidation (i=2) reactions. After... 

Figure 8.45. Effect of Pt catalyst overpotential on the kinetic constant of CH3OH oxidation to C02 on Pt/YSZ for positive (a) and negative (b) currents. Pch30h= 0.9 kPa, p02=19 kPa. T=, 698 K A, 650 K , 626 K Reprinted with permission from Academic Press.50...

Subsequently this classically promoted Rh(Na)/YSZ catalyst is subject to electrochemical promotion via application of positive (IV) and negative (-1V) overpotential. The classically promoted catalyst performance is further dramatically enhanced especially in terms of rN2, rN20 and SN2 (Figs. 2.3, 8.66 and 8.67) and particularly with positive overpotentials. The resulting pN2 and PN20 values are on the order of 10 in the temperature range 240° to 360°C. Figures 2.3, 8.66 and 8.67 demonstrate two important facts ... 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

Fig. 5.10 Relative band edge diagram for FeS2 and the energy position of some electron donor species. The thermodynamic reactions corresponding to corrosion processes at the anodic and cathodic sides are indicated as decomposition potentials due to holes, fip dec, and to electrons, n,dec> respectively. r]c and are the cathodic and anodic overpotentials, respectively, for the decomposition reaction of pyiite crystals in acid medium. (Reproduced from [159], Copyright 2009, with permission from Elsevier)...

Figure 10.11 Arrhenius plots of the ORR rate constants obtained at various electrodes. The symbols are the same as those in Fig. 10.10. Each solid line is the least squares fit of all the data at the constant applied potential. Since the standard potential E° and [RHE(r)] shift to less positive values in a different maimer, the corrected potential E is applied so as to keep a constant overpotential for the ORR at each temperature. The applied potentials of -0.485, -0.525, and -0.585 V vs. E° correspond to 0.80, 0.76, and 0.70 V vs. RHE, respectively, at 30 °C. (From Yano et al. [2006b], reproduced by permission of the PCCP Owner Societies.)...

On application of an overpotential the free energy of the reaction is changed, and so is the potential of mean force. A positive overpotential lowers the maximum and deepens the minimum the reverse is true for negative overpotentials (see Fig. 13). For very high overpotentials the two extrema disappear. Thus a very high positive overpoten-... 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

For large negative or positive overpotentials, i.e., for r)t P RT/nF, either the cathodic or the anodic partial current density predominates, so that according to eqn. 3.19... 

The high overpotential of hydrogen is an advantage of DME while this property can be exploited only with mercury-covered RDE and UME. On the other hand, the dissolution of mercury at rather low positive potentials is a disadvantage of DME which is not shared by RDE and UME made of nobler metals than mercury. 

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