Cathodic water polarization curve

Fig. 5.8 Schematic representation of relative positions of anodic metal, cathodic hydrogen, and cathodic water polarization curves, pH = 1. Curve M, anodic polarization for metal (e.g., Fe-18% Cr) curve H, cathodic polarization for H+ curve W, cathodic polarization for H20 curve SC, sum of H+ and H20 polarization...

Fig. 5.12 Sum (SC) of cathodic oxygen, hydrogen, and water polarization curves of Fig. 5.11. Oxygen curve dominates above -300 mV (SHE) and hydrogen curve below-300 mV (SHE). Water reduction makes negligible contribution to the current density. pH = 1.Po2 =0.2 atm...

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

Figure 26 exhibits the polarization curves in terms of the cathode overpotential variation with current density for the CL27 obtained from the 3-D, single-phase DNS model prediction,25,27 the experimental observation25,27 and the liquid water transport corrected 1-D macrohomogeneous model.27 The polarization curve refers to the cathode overpotential vs. current density curve in the... 

Cathodic polarization curves for mild steel exposed to air-saturated tap water and tap water containing 200 ppm of CeCl3 [7] showed that oxygen reduction is the primary cathodic reaction in aerated solutions devoid of CeCl3 and the current density is considerably reduced by the addition of CeCb. A pale yellow film was also observed on the sample. 

FIGURE 27.6S Potential-current polarization curves of H2.O2 fuel cells using Nafion 117 (thickness 180 p,m and 30% water uptake) and STA with and without thiophene (termed NASTATH and NASTA, respectively) (thickness about 175 p,m and 60% water uptake). Anode and cathode based on 0.35 mg cm Pt from 20% Pt C catalyst pressure ratio H2.O2 3 5 atm gas flow rates 2 = 0.8 L min H2= 1.2 L min . (Reprinted from Tazi, B. and O. Savadogo, O., Electrochim. Acta, 45, 4329, 2000. With permission from Elsevier.)... 

Figure 8.28. Electnxie polarization curves for oxygen-containing solutions (a) in otherwise pure water and (b) in the presence (nonequilibrium) of some Fe. Curves are schematic but in accord with available data at significant points. Because the net current (a) is virtually zero over a considerable span of the electrode potentials, the exact location of the redox potential becomes difficult to determine or is determined by insidious redox impurities. A mixed potential (b) may be observed at the point where the anodic and cathodic currents are balanced but because the various redox partners are not in equilibrium with each other, it is not amenable to quantitative interpretation.

A separate chapter, Chapter 5, is used to introduce the corrosion behavior of active/passive type metals. This allows emphasis on the more complex anodic polarization behavior of these metals and the associated problems in interpreting their corrosion behavior. The chapter is introduced by discussing experimental observations on the anodic polarization of iron as a function of pH and how these observations can be related qualitatively to the iron-water Pourbaix diagram. Pedagogically, it would be desirable to analyze the corrosion behaviors of active/passive metals by relating their anodic polarization curves to curves for cathodic reactions as was done in Chapter 4 for nonpassive alloys. Because of the extreme sensitivity of an experimental curve to the environment, a reasonably complete curve usually can only be inferred. To do so requires understanding of the forms of experimental curves that can be derived from individual anodic and cathodic polar-... 

The electrode-electrolyte assembly was investigated in a single cell test station. After installing the MEA in the fuel cell housing, water was supplied to the anode and cathode backing layers and the cell was warmed-up step-wise from room temperature to 145°C. The polarization curves obtained for the fuel cells equipped with the Nafion-silica and Nafion-silica-PWA membranes, under same conditions in presence of oxygen feed at cathode and 2M methanol solution at anode, are reported in Fig. 6. 

If the concrete is completely saturated with water, and thus there is no oxygen, the only cathodic process possible is hydrogen evolution and the cathodic polarization curve is curve c. 

Fig. 7 Polarization curve of cathodic oxygen reduction with superimposed cathodic water dissociation at 18-8-CrNi steel in air-saturated, stirred NaOH/0.5 M NaCi solution, pH 11,25°C ( ) as well as in practically 02-free solution (o) potential referred to saturated calomel electrode [2].

Figure 6.15 Cathodic polarization curves of iron in a diluted strong acid and in water containing H2S, in both cases at pH = 4 (after Neumann and Carius [6.32]).

Figure 7.14 Polarization curves for the CFRP-metal cells in simulated sea water. 1 = Composite cathodic curve at 25°C, 2 = composite cathodic curve at 50°C, 3 = aluminium anodic curve at 25 °C, 4 = brass anodic curve at 25°C, 5 = stainless steel anodic curve at 50°C, 6 = stainless steel anodic curve at 25°C, 7 = titanium anodic curve at 25°C. (Reproduced from [114] by kind permission of TV Chukalovskaya and lAPC Nauka )...

When used for automotive applications, fuel cells must respond to changes in the load. Changing the load alters the water production, changing the balance between water produced and water removed, resulting in a change of the membrane water content. The effect of the load resistance on the water activity can be seen in the polarization curves for the STR PEM fuel cell shown in Figure 3.9A [23]. The STR PEM fuel cell was operated in the autohumidification mode. The STR PEM fuel cell was equilibrated at 80C for 12 h with a fixed load resistance (either 0.2 or 20 fi). After equilibration, the polarization curve was obtained by sweeping the load resistance between 0.2 and 20 ft in 100 s. The relative humidity in the anode and cathode streams... 

In Figure 7.5 (right), the water profiles of the anode and cathode sides of the membrane, and equilibrium values of membrane water content corresponding to the anode and cathode plenum relative humidities are shown for four points on the polarization curve depicted at left. At point A,Re = 5 Ohms, the membrane is well ignited and the gas plenums have relatively high humidity level. At point... 

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