Hydrogen electrode polarization

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

In an electrochemical system, gas supersaturation of the solution layer next to the electrode will produce a shift of equilibrium potential (as in diffusional concentration polarization). In the cathodic evolution of hydrogen, the shift is in the negative direction, in the anodic evolution of chlorine it is in the positive direction. When this step is rate determining and other causes of polarization do not exist, the value of electrode polarization will be related to solution supersaturation by... 

FIGURE 15.3 pH dependence of potential (1) and polarization (2) in cathodic hydrogen evolution at a mercury electrode (lOmA/cm ), and the pH dependence of equilibrium potential of the hydrogen electrode (3). 

Constant A in Eqs. (29.5) and (29.6) is about 4.4 eV when the standard hydrogen electrode is used as the reference electrode. This value has been determined from experimental values for the electron work function of mercury in vacuum, which is 4.48 eV, and for the Volta potential, between the solution and a mercury electrode polarized to = 0 V (SHE), which is -0.07 V (the work of electron transfer is 0.07 eV). The sum of these two values, according to Eq. (9.8), corresponds to the solution s electron work function at this potential (i.e., to the value of constant A with an inverted sign). 

For the reaction of hydrogen and oxygen to generate a current in a fuel cell, the anode needs to be polarized more positive than 0 V vs. NHE (Normal Hydrogen Electrode, the reference potential for all electrochemical reactions) for the oxidation of hydrogen, while the cathode needs to be polarized more negative than 1.229 V vs. NHE for the reduction of oxygen. 

Pretreatment of electrodes by cychc polarization needs special care because the surface structure depends on the number of cycles and the potential range of polarization. It was shown that during the polarization of Au and Pt electrodes, up to ca. 15V (vs. Normal Hydrogen Electrode) in lAf H2SO4, the quantities of dissolved metals corresponded to the... 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

Figure 3.9 Evans diagram showing effect of activation polarization on overpotential for a hydrogen electrode. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, p. 574, 5th ed. Copyright 2000 by John Wiley Sons, Inc.

Figure 15. Arrhenius plots for hydrogen oxidation and hydrogen oxidation in the presence of carbon monoxide. A results at 10 mV and B results at 25 mV polarization versus the reversible hydrogen electrode in the same electrolyte and at the same temperature. 0.5 mg Pt on carbon per square centimeter of electrode.

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