Electrocatalytic Activities And Degradation Characteristics Of Ti/IrO₂ Electrodes In Methanol Aqueous Solutions

IrO₂-based DSAs were previously believed as one of the most successful electro-catalysts in the past decades due to their excellent electro-catalytic activities and extremely high durability for the OER. In recent years, as the increasing interests in environment and energy issues, these catalysts are supposed to open new applications in organic aqueous system. IrO₂ is less sensitive to poisoning due to simultaneous oxygen evolution and this is the reason why more and more authors have paid attentions to the study of these anodes on aqueous solutions containing organics substrates. However, most emphasis were laid on the electrochemical properties of these electrodes, studies on stability and degradation of electrodes in organic substrates are very rare, which to some extent impedes the practical application and the success of commercialization of these electrodes.Firstly, the influence of calcinations temperatures on the electrocatalytic activities of Ti/IrO₂electrodes in methanol aqueous solutions was studied. The results indicated that the electro-activity of Ti/IrO₂ electrodes generally decreases as the sintering temperature increases. The presence of methanol in testing electrolyte leads to the decrease in current response of CV curves and C_d1 values and to the negative shift in the occurrence potential for the OER. The activity decline in organics substrate was interpreted by "active dissolution" of oxide coatings and "organics adsorption" on electrode surface. However, in contrast to those prepared at lower temperatures the active oxides are abnormally activated by methanol component at electrodes prepared at higher temperatures (e. g. 600 ℃). Pronounced formation of metallic iridium component by the decomposition of iridium oxides was detected by the XRD measurements when sintering temperature went up to 600 ℃. The existence of metallic iridium might facilitate the organics oxidation as stated in previous literatures, which leads to the exceptional activation of oxide surface by methanol component.Secondly, the influence of the calcination procedure on electrocatalytic properties of Ti/IrO₂ electrodes in aqueous with and without the presence of methanol substrateswere investigated.Thermal decomposition at single 500 ℃ for each layer (conventional method) was adopted as the standard procedure. The standard procedure leads to compact oxide coatings, with large grain distributing on coatings and high resistance of obtained electrodes. Both applying the lower temperatures in first three under-layers (step-method) and the programming heating in each layer can, to some extent, overcome the disadvantages of the conventional procedure, and thereby improve the electrochemical activities of electrodes. When using the controlled programming calcination (heating rate), the coating becomes more porous and rugged. The highest global activities are found at electrodes prepared by programming heating (from 200 or 300 ℃ to 500 ℃) and first-300℃-then-500℃ procedures, while the conventional method and first-200℃-then-500℃ procedure fail to obtain electrodes with good eletrocatalytic properties. However, the presence of methanol in aqueous dramatically decreased the electrochemical properties of electrodes prepared by each calciantion procedures. Programming heating is also found to be the ones with highest activities and seamed to be least influened by methanol.Finally, we investigate the long-term stability and deactivation characteristics of IrO₂ type DSA in methanol aqueous solutions. Accelerated lifetime tests are used to identify the long-term stability of electrodes in Na₂SO₄ aqueous solution and CH₃OH aqueous solution respectively and emphasis on the influence of different thermolytic temperatures. It is found that the stability of electrodes increased as sintering temperature increasing firstly and then decreased after went highest value. The introduction of methanol dramatically decreased the lifetimes of electrodes. In lower calcinations temperatures, the direct reason leading to electrodes deactivation is dissolution of active layes in both solution systems. When sintering temperature increased, it seemed to be passivasion what lead to the deactivation of electrodes in blank solutions. However, in methanol aqueous solutions the deactivation of electrodes is the dissolution of oxides due to the presence of methanol. For higher sintering temperature, the oxidation of titaniuim became more serious, leading to the forming of isolating TiO₂ between Ti substrate and active layers. During electrolysisthe oxides is possibly detached from substrate, leading to deactivation of electrodes.