The Oxygen Electrode

With the development of plastic membranes which are selectively permeable to the molecules of oxygen and carbon dioxide, but not to ions and water molecules, it has become possible to construct specific electrodes for PO2 and PCO2 measurements. 

The silver wire is actually a silver-silver chloride reference electrode and is chloridized before the PO2 electrode is used. 

Electrode response time is relatively slow as one must wait for oxygen to diffuse through the membrane. The basic reaction at the cathode is probably 

TABLE 6.3. Gas Transmission Rates for Plastic Films 0.001 Inch Thick at 25°C (cc/24hr/100inVatm)  

Polyvinylidene chloride M onochlorotrifluoroethy lene Polyester (Mylar) 

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The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

The development of highly efficient methanol fnel cells depends on a nnmber of scientific aspects (1) the development of more highly active catalysts for methanol oxidation at temperatnres not over 60 to 70°C (desirable in cells without ruthenium, which is in short supply) (2) the development of selective catalysts for the oxygen electrode (i.e., of catalysts insensitive to the presence of methanol) and (3) the development of new membrane materials having a lower methanol permeability. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

In fact, the oxygen electrode is not irreversible, but the discharge potential S04 ion is so much on the high side that the OH ions are discharged preferentially, even in extremely dilute CuS04 solution. 

If the assumption is presently made that the oxygen electrode is not irreversible, it is possible to calculate from the Nemst equation the ratio of activities of OTT and S04 that must be present before S04 would begin to be discharged. In this context it is important to identify the numerical values that are obtained from the available data. These are presented below ... 

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 ... 

As the adsorption affinity of redox particles on the electrode interface increases, the hydrated redox particles is adsorbed in the dehydrated state (chemical adsorption, contact adsorption) rather than in the hydrated state (ph3 ical adsorption) as shown in Fig. 7-2 (b). Typical reactions of redox electron transfer of dehydrated and adsorbed redox particles on electrodes are the hydrogen and the oxygen electrode reactions in Eqns. 7-6 and 7-7 ... 

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. 

In addition, in a DAFC, the proton exchange membrane is not completely alcohol tight, so that some alcohol leakage to the cathodic compartment will lead to a mixed potential with the oxygen electrode. This mixed potential will decrease further the cell voltage by about 0.1-0.2 V. It turns out that new electrocatalysts insensitive to the presence of alcohols are needed for the DAFC. 

M Ag, in order to calculate the self-dissociation constants. Since alkali oxides are extremely difficult to prepare in the pure state and to handle, it was decided to produce the oxide ion coulometrically. Consequently, a known current of approximately 8 microamp. was run through the cell, the oxygen electrode acting as the cathode. The current was drawn from an electronically controlled constant current device and timed so that the number of equivalents of charge passing could be calculated. This was assumed to be the number of oxide ions produced. The variation in potential with oxide ion followed the Nernst expression very precisely. 

Next we considered the oxide ion electrode, and we were a little fearful of this one. There had been some work done in sulfates, and several hundred degrees warmer than this, in which there was some question as to whether or not the oxygen electrode was really acting reversibly on platinum. Furthermore, studies by Yeager and others have shown that whenever one uses oxygen in aqueous solution at electrodes there is a tendency to equilibrate with peroxide rather than with oxide or hydroxide. We were afraid that we might end up with some one-electron transfer reactions and get peroxide ions and the like but we had to try it because the other electrode just wouldn t work. As it turned out, it worked very nicely. 

Moreover, the conductivity, and hence the catalytic decomposition of hydrogen peroxide, has been observed to influence the stability of the oxygen electrode. The stability of phthalocyanine catalysts is a decisive factor for the practical applicability of organic catalysts in fuel cells operating in an acid medium. This is therefore a very important observation. The observed disturbance of the delocalization of the n electrons (rubiconjugation) in Fe-polyphthalocyanines, in addition to the correlation between conductivity on the one hand, and electrocatalysis and catalytic decomposition of hydrogen peroxide on the other, leads to a special model of the electroreduction of oxygen on phthalocyanines. The model... 

It is therefore almost impossible to establish the equilibrium potential of the oxygen electrode in aqueous electrolytes (+1.23 V vs rev. hydrogen electrode) and only in a very careful experiment of Bockris and Hug (112) has this been achieved. Usually at zero current density a lower potential of approximately 0.95 V is obtained, which is established by mixed reactions, with the main reaction being the reduction of oxygen to hydroperoxide. 

The oxygen electrode has been the subject of intensive study for many years. The electrode reaction is complex and is greatly affected by the electronic conductor and electrolyte used. In basic solution, it may be considered as a two stage process only the first of these is reversible. The two steps may be written as... 

The oxygen electrode suffers from considerable polarization losses on discharge, largely due to mass transport limitations. Metal-air cells have... 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

Unfortunately, -alumina cannot be used as a fuel cell electrolyte since its oxygen ions are immobile and so any oxygen electrodes in the corresponding galvanic cells will not be reversible. However, under open circuit conditions, /1-alumina has been successfully used as an oxygen potential sensor. In this mode of application, the oxygen electrode equilibrates with the Na+ ions of the electrolyte as follows... 

Second in popularity only to the pH electrode is the oxygen electrode. This device is a polarographic electrode system that can be used to measure oxygen concentration in a liquid sample or to monitor oxygen uptake or evolution by a chemical or biological system. 

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