Oxygen, electrode potential

In alkaline solution, silver is a second option, but silver must be protected against anodic oxidation and partial dissolution by safely polarizing it to at least -200 mV vs reversible oxygen electrode potential. 

The metals with cell potentials that are more negative than the oxygen electrode potential are not thermodynamically stable when in contact with water and air, and a spontaneous reaction occurs in which oxygen will be converted into water. 

In the transition region, the current increases due to the oxygen evolution reaction. It is evident from Fig. 4.9 that the overvoltage for the oxygen evolution reaction decreases as the pH increases. This shift in the overvoltage is due to the change in the oxygen electrode potential with pH. 

The operating efficiency of direct methanol fuel cells is greatly lowered because of methanol crossover. This effect leads to unproductive methanol consumption and to a marked decrease in working voltage, caused by the action of methanol on the oxygen electrode potential. So far, only two options can be seen to lessen or completely... 

On further gradual change of the potential, the current density remains at the above low value, and the corrosion product is now Fe. At about 1.2 V, the equilibrium oxygen electrode potential is reached, but oxygen is not evolved appreciably until the potential exceeds the equilibrium value by several tenths of a volt (oxygen overpotential). The increased current densities in the region labeled transpassive represent O2 evolution plus Fe + formation. 

Actual cathode and anode potentials can be measured by a number of means relative to a reference electrode. For the fuel cell, the oxygen electrode, when measured against a standard hydrogen electrode, is assumed for this example to have a potential of 0.100 (LIO). For the electrolysis cell, the oxygen electrode potential is assumed to be 0.800 Yshe (Lll). The fuel cell anode potential and electrolysis cell cathode potentials are determined by Eq. 5 and have the values in Lll and LIO, respectively. 

It can similarly be shown that in add solution for a reaction, O2+2H2O+4C —> 2H2O, the oxygen electrode potential becomes ... 

Reductions. Hydrazine is a very strong reducing agent. In the presence of oxygen and peroxides, it yields primarily nitrogen and water with more or less ammonia and hydrazoic acid [7782-79-8]. Based on standard electrode potentials, hydrazine in alkaline solution is a stronger reductant than sulfite but weaker than hypophosphite in acid solution, it falls between and Ti ( 7). 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

The electrode potential behaviour of copper in various solutions has been investigated and discussed in considerable detail by Catty and Spooner . According to these workers a large part of the surface of copper electrodes in aerated aqueous solutions is normally covered with a film of cuprous oxide and the electrode potential is usually close to the potential of these film-covered areas. The filmed metal simulates a reversible oxygen electrode at... 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

In this equation is the standard electrode potential of the water/oxygen reaction, i.e. —AG% o/nF. Simplifying, equation 12.4 at 298K becomes... 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

---