Ruthenium oxide is one of the most active electrocatalytic material for oxygen evolution reaction (OER) and chlorine evolution reaction (CER). The attempts of improve its performance or to decrease the cost of the industrial electrodes resulted in many reports on synthesis of this conducting oxide with rutile structure with various dopants including Ti, Co, Sn, Ni, Ta and etc. In a series of those works it was found that transition elements in Ru site may alter the activity (and selectivity) of the electrocatalytic material in the two competing reactions – CER and OER. The nature of this phenomenon is not clearly understood.
This paper presents the results of the systematic studies of the effect of particle size on the activities of Ru1-xCoxO2 materials. Since it is well-known that presence of chloride ions may affect the activity of the RuO2 electrocatalysts in oxygen evolution reaction, the synthesis method which does not use chlorides was developed in this work. Pure RuO2 and Ru0.8Co0.2O2 nanocrystalline materials with the particles size of 10-60 nm were synthesized and their performance in the electrocatalytic OER and CER processes were compared. It was found that Co-doped single phase materials can be prepared only below 800OC and Co concentration can not exceed 20 mol% in Ru site. For higher concentration, the presence of a spinel phase was detected. The conventional cyclic voltamograms recorded in 0.1M HClO4 solution were complimented with the differential electrochemical mass spectroscopy (DEMS) data. The onset of oxygen evolution current was found to be independent of the particles size for pure RuO2, but it showed a systematic shift towards more positive potentials for the Co-doped materials. Analysis of the Tafel slopes showed their independence of the particles size for both types of materials. The analysis of DEMS data revealed that the OER on two types of electrocatalysts showed opposite trends. While in the case of RuO2 the activity decreased with increasing particle size, the same process on Co-doped RuO2 was enhanced on bigger particles. Such a behavior indicate that crystal edges represent the preferential reaction sites on pure RuO2 anodes, while in Ru1-xCoxO2 crystal surfaces act as major reaction active sites.
Synthesis of RuO2 and Ru0.8Co0.2O2 oxide materials in this paper were carried out by the co-precipitation method (although the authors refer to their approach as “sol-gel method”). Ruthenium (III) nitrosylnitrate (Aplpha Aesar, 98%) and cobalt (II) nitrate hexahydrate (Aldrich, 99.99%) powders were dissolved in propane-2-ol (Aldrich, ACS grade) and ethanol (1:1 ratio). Then, an amorphous precursor of (Ru,Co)O2●nH2O was precipitated from this solution by the reaction with aqueous solution of tetramethylamonium hydroxide (Alpha Aesar, 25%). The amorphous precipitate was aged in the PTFE lined autoclave at 105OC for 40h. After filtering, the powders were dried and heat treated at 400-900OC in the air for 4h. According to my experience, it is important to implement several modifications into the described synthesis procedures to achieve better and more consistent results.
- The obtained particles have typical size of 15-30 nm and it is difficult to achieve high yield by conventional filtration. Instead, the powders should be separated by centrifuge. Excellent results were achieved after four cycles of washing with distilled water using the ultrasonic bath and separation of the precipitate.
- The separated powders contain absorbed organic compounds even after many circles of washing. As a result, heat treatment of such precursors leads to the reduction of RuO2 and formation of metallic Ru as an impurity phase. Formation of metallic Ru phase was also confirmed by the authors in this work. In our work, we carried out the final washing of the powders by 1% H2O2 solution. CAUTION: RuO2 catalyzes decomposition of hydrogen peroxide. The reaction is vigorous and it is accompanied by heat release. Therefore, hydrogen peroxide should be added dropwise to the suspension of RuO2 in water, which is cooled down by the water bath.
- We deposited the RuO2 nanoparticles onto Ti mesh as well as onto graphite sheets. In both cases, to improve wetting, the substrate materials were washed with acetone, deionized water, 6M HNO3 again with deionized water followed by depositing the RuO2 suspension onto the wet substrate. After drying, the wetting of the substrate by the suspension of RuO2 does not present any difficulties.