The IrO2-Ta2O5 Coated Titanium Anode for Oxygen Evolution Reaction in Sulfate Solution

Abstract

Titanium substrates coated with a conductive layer of metal oxides are well known gas evolving electrodes in the metal electrowinning industry, so-called dimensionally stable anodes (DSAs). DSA electrodes display better catalytic activity and durability towards oxygen evolution reaction (OER) than lead and graphite anodes. The IrO2 –Ta2O5 is considered to be the best catalytic coating for OER in acidic electrolyte. Further development regarding catalytic activity and durability of this type of anode is required in the perspective of long-term operation in the industry. This thesis is thus concerned with gaining a better understanding of the influence on the electrochemical properties of the specific IrO2 –Ta2O5 anodes produced by Permascand AB Sweden and used in copper pilot tankhouse at Glencore Nikkelverk AS Norway. The electrochemical properties including the electrochemically active surface area (ECSA) which was represented by voltammetric charge, catalytic activity, and durability of the commercial DSAs were studied in controlled lab experiments in a sulfate electrolyte at 60 ◦C. Firstly, the effect of the calcination temperature (400 - 550 ◦C) on the properties of the IrO2 –Ta2O5 anodes with a certain composition was investigated. The size of scattered IrO2 nanocrystallites formed in the flat areas of the "mud-crack" anode surface, which was found to be dependent on calcination temperature. The catalytic activity was influenced by the crystallinity and the preferred orientation of IrO2 crystallites of the IrO2 –Ta2O5 binary oxide. The (101) planes of the rutile IrO2 exhibited both a higher catalytic activity and a better stability. Secondly, the effect of applying sandblasting in titanium pretreatment during preparing of the IrO2 –Ta2O5 anodes was investigated. The titanium substrate appeared deeper with smaller etching pits with applying sandblasting in titanium pretreatment. In addition, the coating surface became more rough. The ECSA was slightly increased which is due to the increased outer ECSA since the inner ECSA was found to be independent of the sandblasting. Even though, the catalytic activity was only improved in a very limited range. Furthermore, the lifetime of the anode was found to be shortened while sandblasting was applied in titanium pretreatment. This is suggested to be due to the distance between the lowest spot of the outer coating surface and the highest spot of the outer substrate of the anode was shorter in the anode with sandblasting, and thus led to the failure of this anode prior to the another. Thirdly, the effect of the coating loading (2 - 15 layers) on the properties of the IrO2 –Ta2O5 anodes was studied. Both the ECSA and catalytic activity are found to be proportional to the coating loading but not the lifetime. However, improvement of the catalytic activity by increasing the coating loading was limited. The durability of the anodes was revealed to be independent of the coating loading, as the anode with 5 layers of coating was the most stable one and the anode with 15 layers was the second most stable. This was due to the variation of the iridium dissolution rate during accelerated lifetime test (ALT). It can be related to the preferred crystalline planes of the IrO2 phase of the referred anodes. As a result, a way to evaluate the stability of anodes with different coating loading was proposed by using the ratio of TC(101) / TC(110) of the anodes. In the end, the durability and deactivation of a series of IrO2 –Ta2O5 anodes were studied, regarding the effects of calcination temperature, coating loading, pretreatment of titanium substrate and coating method. The anode prepared at a moderate temperature in range of 350 - 550 ◦C exhibited an excellent lifetime of almost one year although its catalytic activity was not the best. Nevertheless, using the electrostatic spraying method to replace the hand-brush method in the coating process can prolong the service life even further and with less amount of coating loading. Moreover, it was revealed that the coating loss or combined with titanium substrate passivation resulted in the eventual deactivation of the anodes during ALT. No critical value of the amount of the residual iridium was found in this work to predict the eventual deactivation before forming the passive oxide film. Deactivation of the anodes was found to be more dependent on the calcination temperature than other manufacturing parameters.