Alkaline polarization curves

Typical polarization curves for alkaline fuel cells are shown in Fig, 27-63, It is apparent that the all aline fuel cell can operate at about 0,9 and 5()() rnA/cnr current density. This corresponds to an energy conversion efficiency of about 60 percent IIII, The space shuttle orbiter powder module consists of three separate units, each measuring 0,35 by 0,38 by I rn (14 by 15 by 40 in), weighing 119 kg (262 lb), and generating 15 kW of powder. The powder density is about 100 W/L and the specific powder, 100 W/kg,... 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

These reactions proceed very rapidly, so that the overall reaction corresponds to the transfer of two electrons. As reaction (5.7.9) is very slow in acid and neutral media, the electrode reaction is irreversible and the polarization curve does not depend on the concentration of hydrogen ions. In weakly alkaline media, reoxidation of H02 begins to occur. At pH > 11, the polarization curve at a dropping mercury electrode becomes reversible. In this way, the process proceeds in water and water-like solvents. On the other hand, for example in carbonate melts, the step following after the reaction (5.7.9) is the slow reaction 02 + e = 022-. 

Typical polarization curves for alkaline fuel cells are shown in Fig. 24-49. It is apparent that the alkaline fuel cell can operate at about 0.9 V and 500 mA/cm current density. This corresponds to an energy... 

Fig. 20 presents the data of a quantitative XPS evaluation for the passive layer formed in 1 M NaOH at different potentials on pure Fe [12,36], In alkaline solutions, all Fe oxides are insoluble, and thus may be examined without dissolution during emersion of the electrode. Fig. 20 contains the polarization curve in 1 M NaOH with the identical potential scale for comparison. Fe shows at high pH passivity already at very negative potentials without pronounced dissolution of Fe(II) ions. Fe(OH)2 is the dominating layer component starting at E = —1.00 V. Fe(III) starts at E =... 

Fig. 20. Composition (Fe(II) and Fe(III)) of the passive layer formed for 300 s on Fe in 1 M NaOH calculated from XPS measurements on the basis of a bilayer model including the potentiodynamic polarization curve with indication of formation of soluble Fe2+ and Fe3+ species. Hp and Epi are the passivation potentials in alkaline solution and acidic electrolytes (Flade potential) extrapolated to pH 12.9 [12],...

Ni is a frequent component for alloys as e.g. for stainless steels. Polarization curves of Fe53Ni and FelONi still show features known for pure Ni (Fig. 5). The current increase and the peaks in the transpassive range are suppressed to a large extent in acidic and alkaline solutions due to the influence of Fe [15, 48], Angular resolved XPS measurements indicate a bilayer structure of the passive film with an outer hydroxide and an inner oxide part. Circa 1 nm hydroxide is found with no change with the electrode potential. The oxide part increases linearly with the potential up to 5 nm and levels off to a constant value for the transpassive potential range at 0.70 V in 1 M NaOH and at 1.40 V for pH 2.9 [15, 48], At 0.70 V in 1 M NaOH one observes... 

Figure 28 Polarization curves on Pt and Fe in alkaline sulfide solution at 65°C illustrating danger of interpreting all anodic current as due to metal dissolution. Above approximately —0.7 V(SCE), the anodic current on both Pt and Fe is dominated by sulfide oxidation.

An additional interpretation issue involves the presence of oxidation reactions that are not metal dissolution. Figure 28 shows polarization curves generated for platinum and iron in an alkaline sulfide solution (21). The platinum data show the electrochemistry of the solution species sulfide is oxidized above -0.8 V(SCE). Sulfide is also oxidized on the iron surface, its oxidation dominating the anodic current density on iron above a potential of approximately -0.7 V(SCE). Without the data from the platinum polarization scan, the increase in current on the iron could be mistakenly interpreted as increased iron dissolution. The more complex the solution in which the corrosion occurs, the more likely that it contains one or more electroactive species. Polarization scans on platinum can be invaluable in this regard. 

Passivation potential — Figure 2. Evaluation of XPS data on the chemical structure of the passive layer on Fe formed for 300 s in 1M NaOH as a function of potential with a two-layer model Fe(II)/Fe(III). Insert shows the polarization curve with oxidation of Fe(II) to Fe(III) at the Flade potential EP2, indication of soluble corrosion products Fe2+ and Fe3+, and passivation potential EPi in alkaline solution [i, iii]... 

Polarization curves over a wide range of overpotentials, both anodic and cathodic, for the hydrogen reaction at 1 atm on rotating Pt(hkl) disk electrodes at 298 K are shown in Figure 12. At low positive overpotentials, the order of activity for the HOR increased in the sequence (111) (100) <<(110). These differences in activity with crystal face can be attributed to the different state of adsorbed hydrogen and to different effects of these states on the mechanism of the hydrogen reaction [52,53]. The HOR on Pt(lll) and (100) in alkaline solution is purely kinetically controlled... 

The reduction of dioxygen in aqueous 02 saturated alkaline solutions on Au single crystal electrodes exhibits a pronounced dependence on the surface microstructure. This behavior is clearly evidenced by the RDE dynamic polarization curves in 0.1 M NaOH shown in Figure 3.4, from which the electrocatalytic activity is found to follow the sequence Au(l 0 0) > Au(l 1 0) > Au(l 11). Rather startlingly, the activity of Au (111) for small T, even surpasses that of Pt(l 11) at the same pH (see dotted curve in this figure) under otherwise virtually the same experimental conditions [18, 19]. 

The kinetics of several well-known electrochemical reactions have been studied in the presence of an ultrasonic field by Altukhov et al. [142], The anodic polarization curves of Ag, Cu, Fe, Cd, and Zn in various solutions of HC1 and H2S04 and their salts were measured in an ultrasonic field at various intensities. The effect of the ultrasonic field on the reaction kinetics was found to be dependent on the mechanism of metal anodic dissolution, especially on the effect of this field on the rate-determining step of the reaction. The results showed that the limiting factor of the anodic dissolving of Cu and Ag is the diffusion of reaction products, while in the case of Fe it is the desorption of anions of solution from the anode surface, and at Cd the limiting factor is the rate of destruction of the crystal lattice. Similar results were obtained by Elliot et al. [ 143] who studied reaction geometry in the oxidation and reduction of an alkaline silver electrode. 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

The influence of Bi promotion on the chemisorption properties of Pt is shown in Fig. 3. The electrochemical polarization curves in aqueous alkaline solution indicate that Bi adatoms suppress the hydrogen sorption on Pt (0-0.45 V) and that the oxidation of the promoter by OH adsorption becomes considerable above 0.6 V [19],... 

The cathodic polarization curve has a trend similar to that observed in alkaHne concrete, and shows the same dependence on the moisture content in concrete. The curve, however, is shifted towards more positive potentials because the equiH-brium potential of oxygen reduction is approximately 200 mV higher at the pH of carbonated concrete than at the pH of alkaline concrete. 

As the half-wave potential, Ph = 8 for the oxidation of pyrocatechol monoorthophosphate is 1.16 V and that of pyrocatechol is 0.86 V, the polarization curve is displaced towards the cathode side. This is illustrated in Figure 34 showing data on the electrooxidation of pyrocatechol monoorthophosphate on a paste electrode as well as on this electrode modified by alkaline phosphatase. 

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