Oxygen electrode thermodynamics

DR. DAVID STANBURY (Rice University) With regard to the problem of the electrocatalytic reduction of oxygen, I have attempted to formalize some ideas regarding the constraints of thermodynamics in order to elucidate the probable character of cobalt complexes which may catalyze the oxygen electrode via binuclear peroxo-bridged intermediates. The following gross mechanism is presupposed ... 

If one assumes that the reaction system is at thermodynamic equilibrium, the oxygen electrode method also allows one to infer the partial pressure of oxygen in equilibrium with the test solution. 

Although the thermodynamic potential for the oxygen electrode reaction [i.e. 1.229 V (RHE)] is lower than that of the chlorine electrode, in practice the kinetics of the chlorine electrode reactions are substantially faster than... 

Thermodynamic data have also been calculated for carbon—oxygen reactions in fused salts [7, 8], The oxidation of solid carbon principally yields carbon dioxide at low temperature and carbon monoxide at high temperature. In this case, at constant temperature, the CO/CO2 concentration ratio at solid carbon depends on pressure. The carbon—oxygen electrode is used as reference to investigate cryolite—alumina melts at c. 1000°C [9] and molten slags at higher temperatures. 

Fig. 2.4.1. -pO plots for gas oxygen electrode 1, according to equation (2.4.1) 2, peroxide function 3, thermodynamically favourable dependence 4, experimental... 

The method of isothermal saturation consists of the addition of an excess quantity of oxide to the melt studied. Such an addition results in the formation of a saturated solution which is in equilibrium with the oxide precipitate. The fact that the equilibrium conditions have been achieved is detected in different ways among them we should mention potentiometric measurements using different oxygen electrodes [238, 326] and titrimetric determination of the concentration of metal ions in a sample of the melt [327, 328]. The sum of the concentrations of ionic and non-dissociated forms of the oxide according to equation (3.6.5) is the main result of these determinations. Taking into account the thermodynamic parameters which describe equilibria in the saturated solutions, equation (3.6.5) may be rewritten as... 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

Advantages of oxygen (air) electrodes made of nickel-cobalt spinel are high electrochemical activity, relatively low cost and satisfactory thermodynamic stability. Life time reached 1000 hours at room temperature for air electrodes from nickel-cobalt spinel in alkaline electrolyte at a current density equal 100 mA/cm. Developed air (oxygen) electrodes ensure current density equal 100-150 mA/cm at polarization 0,2 V at room temperature. 

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. 

An example of an electrode having a nonequilibrium value of potential is the oxygen electrode. The thermodynamic value of potential E. e. of an oxygen electrode at which Reaction (16.3) takes place is 1.229 V (relative to the SHE). When supplying oxygen to a gas-diffusion electrode, the potential actually established at is 0.8-1.0 V, (i.e., 0.3-0.4 V less positive than the thermodynamic value) (steady-state potential). 

The reaction rates themselves strongly depend on the conditions under which the reactions are conducted. In particular, cathodic oxygen reduction that, at temperatures below 150°C, is far from equilibrium comes closer to the equilibrium state as the temperature is raised. The reasons why the real value of the oxygen electrode s potential at low temperatures is far from the thermodynamic value and why cathodic oxygen reduction is so slow are not clear so far, despite a large number of studies that have been conducted to examine it. 

Obviously, it will be possible to determine standard formation values of thermodynamic functions from standard potentials, and of course vice versa (see oxygen electrode). 

In Fig. 1.15 (bottom) curves /and m show the potential-pH relationships for the reversible hydrogen and oxygen electrodes at p 2 = Poi = f respectively. Within the area confined by the curves / and m, HjO is thermodynamically stable and Ph2 < i Poi < whereas below/and above m, Pn > 1 atm, and P02 > 1 atm, respectively see equations 1.11 and 1.12). Thus the diagram shows the solid phases of iron, the activities of metal ions and the pressures of hydrogen and oxygen gas that are at equilibrium at any given potential and pH when pure iron reacts with pure water. 

Many research efforts went into elucidating the behavior of different catalysts in the hydrazine oxidation reaction. Like the oxygen electrode, the open-circuit potential of a hydrazine electrode deviates from the thermodynamic value (—1.22 V) and in certain cases comes close to that of the hydrogen electrode... 

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