Oxygen electrode, problems

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 a quantity of 1 to 50 jj.1 LOX solution does not yield a sufficient response using the oxygen electrode assay, larger quantities may be used to replace part of the buffer. In this case, pH adjustments may be necessary. Larger quantities may require time for temperature equilibration in the sample chamber, and possibly equilibration of dissolved oxygen (a stream of air bubbled into the solution can correct this problem). 

In the previous papers(12,13), we reported on the vessel access type, i.e. tubular type, glucose sensor. It consisted of a glucose electrode system with a GOX enzyme immobilized Nylon membrane and a glucose semipermeable membrane, and a reference oxygen electrode system. The sensor could directly measure up to 700 mg/dl of BGL in an arterial blood stream when it was placed into an external A-V shunt. This sensor, however, has some problems such as thrombus during in vivo testing without heparin and clinical complexity associated with implanting the sensor in a blood stream. 

The Oxygen Electrode.—The standard potential of the oxygen electrode cannot be determined directly from e.m.f. measurements on account of the irreversible behavior of this electrode (cf. p. 353) it is possible, however, to derive the value in an indirect manner. The problem is to determine the e.m.f. of the cell... 

The heart of this system is an yttria-stabilized zirconia, Zr(Y)0 film, which acts as a solid electrolyte, allowing high conductivity for 0 ions at about 1,000°C. The fuel electrode is a porous Ni/ZrO cermet, which serves both as the electrocatalyst and as the current collector. Actually, electrocalalysis is no longer a problem at such elevated temperatures, and different fuels, including H, regular fossil fuels, and even CO, can be used. The air (oxygen) electrode is a... 

The procedure just described essentially utilizes free energy values to calculate E.M.F. s, and another example, based on the same principle, is the determination of the standard potential of the oxygen electrode. The problem is to evaluate the E.M.F. of the cell... 

The response time of oxygen electrodes must be in the range of 2-3 s, if fetfl is to be measured to an accuracy of 10 s [560]. The concentration of oxygen in the air, which continuously decreases upon bubbling through and therefore for high values of k a (particularly for chemisorption ) has to be taken into consideration, can pose a problem. 

The term dynamic response method refers to methods in which oxygen electrodes in the liquid respond rapidly to changes in the composition of the gas phase. They are mainly employed in fermentation technology. The problems which arise either depend upon the response behavior of the electrodes or upon assumptions made with regard to the mixing of the gas cushion and the liquid. Many publications refer to the special features of the measuring technique [71, 97], the sources of error [265] and modeling [106, 372]. 

The problems of this system are the low specific power and efficiency, related with oxygen electrode, and short life, due to the change of zinc morphology during the cell charge [15]. 

The power available is limited by the relatively slow reduction of oxygen at the cathode surface, O2 + 4H" + 4e 2H2O this problem exists with any fuel cell that uses an oxygen electrode. At present, platinum seems to be the best catalyst, but even platinum is not nearly as good as we would like. The rate of the anodic reaction, H2 2H +2e, the oxidation of hydrogen at the platinum surface, is relatively rapid. However, it would be nice if we could use something less expensive than platinum as a catalyst. At higher temperatures, the reaction rates are faster and the cell performance is better. 

Chlor-alkali producers have been actively pursuing the development of oxygen electrodes for some time and it is unlikely that the full 1.23 V saving can be achieved at current densities typically used in chlor-alkali operations. Many problems remain to be solved before the oxygen cathode is ready for commercial... 

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