| By the mid 1950s the commercial production of titanium was well advanced in the USSR, UK and USA, the principal market for the metal being the aerospace industry. The results of studies by J.B.Cotton indicated that titanium exhibited excellent corrosion and erosion resistance in sea water. The use of the metal as an inert electrode material was considered, particularly since titanium could be fabricated into a wide variety of shapes and sizes. However, when polarized anodically in aqueous electrolytes, the metal rapidly formed a passive film of titanium oxide. To effectively use the metal as an electrode it was necessary to develop a stable, conductive coating. To this end C.H.Angell, a coworker at the ICI laboratory, successfully electroplated an adherent coating of platinum on a titanium substrate and this electrode was quickly commercialized for cathodic protection systems.
The platinized titanium electrode appeared to offer an alternative anode structure to graphite for use in the commercial production of chlorine and sodium hydroxide. The Pt/Ti anode represented a dimensionally stable anode that would provide a fixed inter-electrode gap as well as eliminate the carbon sludge formed in the cells during operation. However, little enthusiasm for the anode was shown by the chlorine industry, which argued that both platinum and titanium were expensive and the latter metal was relatively scarce. In addition unexpectedly high rates of platinum corrosion in dilute brine solutions further limited acceptance of the Pt/Ti anode. It was also shown that the presence of organic compounds in the electrolyte, e.g., wetting agents and brighteners, shortened the lifetime of the anode, precluding use as an oxygen evolving electrode in electroplating.
It was later shown that the potentials at which chlorine and oxygen evolution occurred were dependent upon the concentration of ruthenium oxide in the coating. Hence, in 1967 a second patent (British Patent 1,195,871; 1967) was filed by Henri Beer, describing a coating containing a lower amount of ruthenium oxide, which became known as Beer 2. This second generation MMO electrode, costing less than the Beer 1 anode yet providing equivalent (if not better) performance, became the dominant coating used in commercial chlorine cells.
Since the 1970s MMO electrodes have transformed both technological and economic aspects for the production of chlorine, sodium chlorate and sodium hypochlorite. Modifications to the Beer 2 coating resulted in both longer lifetimes and reduced costs. The successful development of the membrane cell technology for large-scale production of chlorine and sodium hydroxide led to further modifications to the coating. RuO2 was partly replaced with IrO2 to counteract dissolution of the ruthenium oxide in the highly alkaline environment at the anode surface that can form in operating membrane cells. At high pH, oxygen evolution becomes the preferred anodic reaction and ruthenium dioxide is oxidized to a soluble ruthenium compound (RuO4) and lost from the electrode surface.
Parallel development programs focused upon the use of MMO electrodes for processes involving oxygen evolution. The stability of iridium oxide and itsâ€™ catalytic activity towards the oxygen evolution reaction made this platinum group metal oxide the preferred choice. The incorporation of tantalum oxide into the coating, rather than titanium oxide, provided superior extended performance by inhibiting the formation of TiO2 at the coating/substrate interface.
Many combinations of oxides, particularly oxides of the platinum group metals and valve metals, were prepared and characterized in efforts to identify patentable coatings with comparable or superior performance to the Beer coatings and modified Beer coatings. However presently, only mixtures of TiO2-RuO2, TiO2-RuO2-IrO2, TiO2-RuO2-SnO2 and TaO2-IrO2 are used commercially. The development of the MMO electrodes significantly increased the stability, current efficiency and operating voltages of dimensionally stable electrodes and enabled the introduction of new electrochemical processes.