Steady-State Anodic Polarization

Steady-state polarization curves have been obtained using two types of procedures  

When the potential changes its direction, a cyclic voltammogram is recorded during the same experiment useful qualitative information from the possible hysteresis is thus obtained. 

Despite the difficulties encountered in the interpretation of the stationary polarization curves, due to the interference of different processes occurring with rates that depend in an intricate way on time, potential, pH in the bulk of the solution, anions, and metal surface characteristics, and 

Consequently, different reaction schemes have been discussed based on the shape of the polarization curve, steady-state Tafel slope, and reaction orders with respect to pH, Vqh , and anions, v . Recent research has provided new information based on the dependence of the kinetic quantities on the time elapsed under open-circuit conditions prior to polarizaton, on the metal surface structure and morphology, controlled by chemical attack, heat treatment, or mechanical deformation, and also on the polarization range (in the active region) in which the E-4og i curve is recorded. 

Tafel-like regions in which the slope decreased with an increase in the solution pH were also found. For example, see Fig. 20, where the Tafel slope increased from 30 mV dec for pH 2.1 to 58 mV dec for pH 0.0. 

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Fig. 12. Steady-state anodic polarization curves (a), and potentiostatic transient curves (b), of a mild steel hemisphere in neutral Na2S04 solution. From [15].

Binary alloys whose phase diagram shows solid solubility over a wide concentration range are particularly well suited for the study of dealloying phenomena. Figure 7.28 schematically presents the pseudo-steady state anodic polarization curve of a binary single-phase alloy AB and of its two constituents, the element B being more noble than A. Two distinct potential regions are observed, separated by the critical potential,... 

Figure 23.14. Impact of ruthenium on oxygen reduction performance (a) CO stripping scans for the cathode and anode, (b) steady-state anode polarization plots before and alter contamination of the eathode, (c) H2-air steady-state polarization curves, and (d) DMFC steady-state polarization curves. Methanol concentration 0.3 M, anode potential during contamination 1.3 V vs. hydrogen counter/quasi-reference electrode, cell temperature 75 °C [65]. (Reprinted by permission of ECS— The Electrochemical Society, from Piela P, Eickes C, Brosha E, Garzon F, Zelenaya P. Ruthenium crossover in direct methanol fuel cell with Pt-Ru black anode.)...

Summary of the Initial and Steady-State Anodic Polarization Characteristics for the Limiting Cases Described in Table 2, When Only the Preponderant Terms ikaiTc x for U and for is Are Taken into Account"... 

Figure 35. Theoretical steady-state anodic polarization curves for iron dissolution in acid media, according to the consecutive mechanism, in the presence of different inhibitor concentrations, computed on the basis of ki-netic equations including the electromechanical desorption. (From Ref. 61, by courtesy of the authors.)...

FiG. 24. Anodic steady-state Tafel polarization relations for the OER at preoxidized Pt in 1 M aqueous HjSO (curve a) and 1 M aqueous NaOH (298 K.) (curve b) 257, 258). 

The galvanostatic-transient behavior of the catalyst potential and that of the reaction rate dne to anodic cnrrent steps, was investigated as a function of the applied cnrrent. The reaction was the combnstion of ethylene at 375°C over Ir02A"SZ catalyst at highly oxidative feed conditions (C2H4 O2 = 1 100). Transients under cmrent application (termed polarization) and after cnrrent intermption (relaxation) were recorded. In order to achieve well-established steady-states, long polarization of at least 100 min and relaxation of at least 200 min were applied. 

Anode Polarization-the difference between the potential of an anode passing current and the steady-state or equilibrium potential of the electrode with the same electrode reaction. 

D.Y. Wang, and A.S. Nowick, Cathodic and anodic polarization phenomena at platinum electrodes with doped Ce02 as electrolyte. I. Steady-state overpotential, J. Electrochem. Soc. 126(7), 1155-1165 (1979). 

It is basically irrelevant in steady-state measurements in which direction the polarization curves are recorded that is, whether the potential is moved in the direction of more positive (anodic scan) or more negative (cathodic scan) values. But sometimes the shape of the curves is seen to depend on scan direction that is, the curve recorded in the anodic direction does not coincide with that recorded in the cathodic direction (Eig. 12.3). This is due to changes occurring during the measurements in the properties of the electrode surface (e.g., surface oxidation at anodic potentials) and producing changes in the kinetic parameters. 

The intercept should reflect the unchanging activation polarization at the two interfaces, as well as some other effects (presence of a film before anodization, time lag in attainment of the steady state, etc.). Nevertheless, the fact that it is small or negligible indicates that charge transfer processes at the interfaces are fast and that the kinetics of the growth are entirely transport controlled. 

From a practical point of view, data obtained by CA methods are more useful. Figure 15.8 shows examples of the CA curves obtained in 0.5 M ethanol solution in 0.1 M HCIO4 at an anodic potential of 600 mV vs. SCE. In both of the CA curves there is a sharp initial current drop in the first 5 min, followed by a slower decay. The sharp decrease might be related to a double layer thus indicating that the catalysts differ mainly in their active area based on the above CV experiments in acid solution. In longer runs it was found that the current (j after 30 min polarization at 600 mV vs. SCE) obtained on PtSn-1 electrodes is higher than that on PtSn-2. The quasi-steady-state current density stabilized for both the catalysts within 0.5 h at the potential hold. The final current densities on PtSn-1 and PtSn-2 electrodes after holding the cell potential at 600 mV vs. SCE for 30 min were 3.5 and 0.3 mA, respectively. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

Such an equation represents the anodic current density that would be measured if a metal with a steady-state corrosion2 current of icon A cm-2 is polarized to a potential V, positive to the corrosionpotential, VcorT. 

Assuming negligible anode polarization, the steady-state polarization curves can be described by a semi-empirical equation ... 

Figure 68. The exchange current density as a function of oxygen partial pressure for different temperatures confirming the electrode kinetical model given in the text.256 (Reprinted from D. Y. Wang, A. S. Nowick, Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeC>2 as Electrolyte. I. Steady-State Overpotential. , J. Electrochem. Soc., 126, 1155-1165. Copyright 1979 with permission from The Electrochemical Society, Inc.)...

In an earlier study we had reported the XPS analysis of tungsten oxides formed during anodic polarization experiments. It was determined that even at high applied potentials, the oxide thickness values are less than the mean free path of electrons in the oxides (generally assumed to be between 30 to 50 A ). Clearly the oxide growth in tungsten is a slow process. However, despite the relatively small thickness vsilues, the steady state current density during anodic polarization is restricted to a few tens of microamperes. 

Figure 1 Effect of pH on steady state current density during anodic polarization at 3 V. 

Figure 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

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