✍ Scribed by D.B. Šepa; A. Damjanovic; J.O'M. Bockris
No coin nor oath required. For personal study only.
ONLY a few metals and alloys can be used as electrodes for oxygen reduction in acid solution, and even then the electrodes are usually covered by their oxides,l+ except for oxygen reduction on some noble metals under certain conditions.4 Low exchange current densities on these metals (and alloys), and their unavailability for extensive technical use as components in practical electrochemical systems such as fuel cells, directed the search for new electrode materials, mostly of the compounds of transition metals with boron, carbon, nitrogen, oxygen, etc.&' Such a compound, in addition to availability, must satisfy three basic requirements: it must be highly conductive, corrosion-resistant at high positive electrode potentials in acid solutions, and must have good catalytic properties for the oxygen-reduction reaction.
g*Q Sodium tungsten bronzes of the general formula N%WO,, where x can varylo from O-2 to O-93, are known to have a high electrical conductivityll (of the order of 104 mho/cm), and a high resistance to chemical attack by strong acids.lO Little is known, however, of the catalytic activity of these bronzes, and that only for some organic reactions,u*B ortho-para-hydrogen conversion, and hydrogen-deuterium exchange.14 Certain, but not pronounced, catalytic effects were observed in some of these cases.
No evidence of any work on electrocatalysis by bronzes has been found in the literature. Chemical stability and the high metallic conductivity of sodium tungsten bronzes indicated their possible use as electrode materials for the oxygen reaction.
Electrodes were made from crystals, electrolytically grown from a molten mixtur&5 of N%WO, and WO* In oxygen-saturated sulphuric acid solutions, the electrodes were stable even at high potentials for the oxygen reaction. After 9 days under oxygen evolution at -1-7 V (he), only traces of tungsten (of the order of 1O-Q g/l) were detected? in the solution.
In Fig. 1 (crosses), the potential/log (current density) relationship for oxygen reduction is given for the sodium tungsten bronze Na,,.,WO,, in O-05 M H,S04 with pH N l-5. In the same figure, data for the same reaction on the oxide-frd6 and oxidacovereda*17 platinum electrode are also given for the same pH.
Sodium tungsten bronze (x N O-6) hence satisf?es the basic requirements for oxygen reduction in acid solutions : it is even more active (at least down to O-85 V (he)) than the platinum electrodes. On bare platinum electrodes, cathodic Tafel lines do not extend to a value more positive than 1 V (he), because above this potential the electrode rapidly oxidizes and becomes less active (circles in Fig. 1). However, potentials on bronzes can exceed this potential without de-activation of the bronze.
It seems probable that in so large a group of compounds as the bronzes, there exist others on which the catalysis of oxygen reduction may be better than on the * Manuscript received 1 November 1966. t By toluene_3,44ithiol as indicator.
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