Nanoscale Size Effects in Catalytic Activity of RuO2 for Oxygen Evolution

Abstract

The amorphous RuO2-Ta2O5 mixture obtained by thermal decomposition of the precursor solution containing Ru(III) and Ta(V) has been found to show an excellent catalytic property for oxygen evolution in acidic aqueous solutions by our group and the application to electrowinning anodes has been reported with a significant reduction in cell voltage compared to traditional lead alloy or other oxide coated titanium anodes [1]. The reason for such a high catalytic activity is mainly the generation of nanoscale RuO2 particles which are uniformly dispersed in amorphous Ta2O5 matrix. The nano particles give a high active surface area for oxygen evolution and our recent study has also revealed that the Tafel slope for oxygen evolution is decreased with the amorphization of RuO2-Ta2O5 mixture. However, there is little information on the nanoscale size effects of RuO2 on electron transfer and mass transfer processes of oxygen evolution. In this work, we tried to prepare RuO2 coating on titanium substrate by thermal decomposition so as to make the particle size distribution of RuO2 uniform in the coating and to change the particle size by changing the preparation conditions. Then, we investigated the effects of the particle size on double layer charge and polarization behaviors in a wide potential range. RuO2/Ti electrodes were prepared by thermal decomposition of the precursor solution containing RuCl3 or Ru(NO3)3 in 1-buthanol or ethanol, and thermal decomposition was carried out at a temperature of 250 oC to 360 oC. Some electrodes were re-calcined at 360 oC or 500oC to change the particle size of RuO2. XRD, SEM, and EDX were used for the surface analysis of the RuO2 coatings, and electrochemical measurements by cyclic voltammetry and linear sweep voltammetry were performed with a conventional three-electrode cell with KCl-saturated Ag/AgCl reference electrode and 2.0 mol/L H2SO4 solutions. The nanoscale particle size of RuO2 was examined by SEM observation and the results revealed that the electrode prepared in Condition A, in which the precursor solution was Ru(NO3)3 in ethanol and thermal decomposition was done at 260 oC for 5 min, comprised about 8 nm of RuO2 particles unformly formed on the coating surface and that the particle size was changed to about 20 nm by re-calcination at 500 oC for 30 min (Condition B). The polarization curves of the two electrodes A and B indicated that the Tafel slope depended on the particles size and decreased with the size change from 20 nm to 8 nm. On the other hand, the double layer charge increased with decreasing particle size, suggesting that the active surface area increases with the size down of RuO2 nano-particles. These results implies that the nanoscale particle size of RuO2 affects not only the active surface area for the potential region governed by mass transfer but also the overpotential region for electron transfer. More detailed results are shown in this paper. This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST).