{Low-temperature-synthesized RuO2 from acidic chloride solution for the electrode coating applications} J. Serb. Chem. Soc. 82 (6) 695–709 (2017) UDC 546.96–31+542.913:544.35+546.135– JSCS–4997 32+544.6.076.32 Original scientific paper 695 Low-temperature-synthesized RuO2 from acidic chloride solution for the electrode coating applications GAVRILO ŠEKULARAC1•#, SANJA ERAKOVIĆ1#, DUŠAN MIJIN2#, VESNA PAVELKIĆ1,3, JASMINA STEVANOVIĆ1 and VLADIMIR PANIĆ1*# 1Institute of Chemistry, Technology and Metallurgy, Department of Electrochemistry, University of Belgrade, Belgrade, Serbia, 2Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia and 3The Railway College of Vocational Studies, Zdravka Čelara 14, Belgrade, Serbia (Received 29 December 2016, revised 20 March, accepted 22 March 2017) Abstract: For the preparation of RuO2 coatings on Ti substrate, the RuO2 was synthesized in acidic aqueous medium by simple one-step low temperature-co- ntrolled microwave (MW) irradiation. The physical composition of synthesized solid phase was analysed through particle size distribution (PSD), whereas the coating was investigated for its capacitive response and activity in oxygen evolution reaction (OER). The oxide phase was found highly polydisperse, with overlapped fractions within rather narrow particle size range and clear ten- dency toward agglomeration. The smallest particles and their best resolved fractions were synthesized at the temperature just above the boiling point of the reaction medium, and quite below the chloride-to-oxide conversion tempera- ture. Consequently, the highest OER activity was registered for RuO2/Ti anodes prepared from this sample, with strong indication of different oxide structure, with respect to the electrodes prepared from samples synthesized at higher temperatures. However, the coatings from high temperature samples have considerably higher capacitance than those synthesized at lower tempera- tures. These findings can be rather correlated to the MW temperature-depend- ent oxide structure than to different morphology analysed through PSD. Keywords: electrocatalytic oxide materials; hydrothermal synthesis; microwave synthesis; electrochemical impedance spectroscopy; pseudocapacitance. INTRODUCTION Electrochemically active noble metal oxides found their application as coat- ings for industrial electrodes,1–5 due to the good electrocatalytic activity for chlor- * Corresponding author. E-mail: panic@ihtm.bg.ac.rs •Present address: Jožef Stefan Institute, Department of Physical and Organic Chemistry, Jamova cesta 39, Ljubljana, Slovenia. # Serbian Chemical Society member. doi: 10.2298/JSC161229040S 696 ŠEKULARAC et al. ine evolution reaction (CER) and oxygen evolution reaction (OER),6–9 but also for many other electrochemical reactions.10–12 These oxides show also the excel- lent supercapacitive performances.15–19 The main electrocatalytic coating compo- nent is RuO2.6,20–27 Typical procedure for the preparation of electrode oxide coat- ings on Ti substrate is by the thermal decomposition of the chloride salts.6,27–30 In order to increase the oxide electrocatalytic activity, selectivity and operational stability, the variety of methods, based on the separation of the oxide synthesis and coating formation, have been applied. The aim was to establish the defined coating structure, which should be as much as possible independent from coating formation procedure itself.31–39 Different types of hydrothermal oxide synthesis have been studied intensively, which promoted the sol–gel process due to imp- roved coating stability in comparison to typical thermally prepared coatings.35 However, the hydrothermal procedures have appeared rather complex and long- -lasting for the application in practice. The time required for the complete con- versions of chlorides to oxides of defined structure is rather long, since the tem- perature/pressure conditions are far from those required for controlled conversion.35 Microwave (MW)-assisted synthesis was proven to enhance many kinds of different processes, from organic synthesis40 to extraction procedures.41 It was shown39 that suitable conditions for the extremely short hydrothermal synthesis of RuO2 can be reached by highly controllable MW-assisted synthesis of the oxide from aqueous chloride solution. The corresponding coatings, prepared from the oxide synthesized at high pressures and MW temperatures above 200 °C, were found to be of high pseudocapacitance and activity for chlorine and oxygen evolution reaction. However, the applied MW conditions were rather extreme. The aim of this work is to investigate the physical properties of RuO2 synthesized at moderate MW conditions (low temperatures/pressure around boil- ing point of reaction medium) and consequently their influence on the electro- chemical behaviour of corresponding RuO2/Ti electrodes. The data were com- pared to those obtained in the investigation of MW-synthesized RuO2 at higher temperatures.39 EXPERIMENTAL Low temperature MW synthesis of RuO2 The precursor for the preparation of RuO2 coatings on Ti, the RuO2 colloidal dispersion, was synthesized by the simple one-step temperature-controlled microwave (MW) irradiation, starting from aqueous RuCl3 solution as described in previous work. 39 The reaction mixture (6 ml) was continuously mechanically stirred at 600 rpm inside a closed reactor (10 ml) and irradiated isothermally to 80, 120 and 150 °C in a MW oven (Monowave 300, Anton Paar, Ashland, VA, USA). The initial heating speed was set to the most rapid possible and the temperature was maintained constant for 5 min. The reaction mixture was cooled to 60 °C in the reactor and afterwards to ambient temperature. The obtained dispersions are denoted as LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 697 MWt80, MWt120 and MWt150. As a result of MW irradiation, initially brown precursor solution turned black with visible appearance of a solid phase. Physical characterization The reaction medium was subjected to dynamic light scattering (DLS) after the synthesis in order to analyse the solid phase presence. Laser-based particle size analyzer Zetasizer Nano ZS (Malvern Instruments, UK), operating at λ = 633 nm (produced by a He–Ne laser at scattering angle 173°) at 25±0.1 °C was employed. All samples were diluted with DI-water in volume ratio 1:100 and ultra-sonicated for 10 min before measurements. Coating preparation The reaction mediums, after MW irradiation at different temperatures, were used for the formation of a RuO2 coating on Ti. The sand-blasted titanium rods (commercially pure, Krupp AG, Essen, Germany), 3 mm in diameter, were thoroughly etched in hot aqueous 18 % HCl solution for 20 min, rinsed by water, and dried at 100 °C. Before the application of the react- ion medium onto Ti rods up to the height of 1 cm, the medium has been ultra-sonicated for 15 min. After deposition, the rod was dried at 120 °C for 30 min. These two steps were repeated and finally the coating was formed during the thermal treatment at 300 °C for 2 h. In the order to avoid possible detachment, the coating was deposited as rather thin layer in an amount of 0.11 mg cm-2. Thus prepared RuO2/Ti served as working electrodes in electrochemical testing. Electrochemical measurements The electrochemical experiments were carried out with 0.5 cm2 working surface area in a three-electrode cell, with Pt gauze electrode and saturated calomel electrode (SCE; all pot- entials are expressed vs. SCE) as counter and reference electrodes, respectively. Cyclic volt- ammetry (CV) at 50 mV/s, quasi-steady-state polarization at 5 mV/s and electrochemical impedance spectroscopy (EIS) measurements (at open circuit potential) in 1 M H2SO4 were performed with a Biologic SP-200 potentiostat/galvanostat (Bio-Logic SAS, Claix, France) at room temperature. Polarization measurements were performed in the potential range 0.90– –1.32 V with preceding anode potentiostatic conditioning at 0.90 V for 10 min. EIS data were recorded with ac potential amplitude of 10 mV (root mean square, rms) around open circuit potential in a multi-sine mode, within frequency range of 100 kHz–5 mHz with 10 points per decade. The fitting of EIS data was performed in ZView® software (Scribner Associates Inc., Southern Pine, NC, USA) with data-modulus type of data weighting in maximum 100 iter- ations. The fitting is assumed acceptable if the chi-squared and weighted sum of squares were of the orders of 10-5 and 10-3, respectively. With these fitting criteria, the relative error of the all equivalent circuit parameter values were below 10 % (the only exceptions was the value of R2 resistor for MWt150 sample, for which the fitting returned the error of 13 %). RESULTS AND DISCUSSION The changes in parameters, temperature (tMW), the MW power (P) and heat- ing rate ((dt/dτ)τ), of the MW reactor in the early stages of synthesis are shown in Fig. 1. The data were analyzed during the time required to reach the desired isothermal condition, i.e., during the early stages of MW irradiation. The time of about 1 min was required to reach the temperature plateau under characteristic heating conditions, which depend on the pre-set isothermal requirements. The 698 ŠEKULARAC et al. MW power was delivered continuously during ca. 6 s with no significant changes of temperature. When the power maxima, which considerably depend on required temperature (200, 400 and 650 W for 80, 120 and 150 °C, respectively), were reached after additional 5–6 s, the temperature started to increase with the differ- ent rate at the different pre-set temperatures. As in the case of power maxima, the peak heating rate can be reached faster if the required temperature is higher. In addition, the lower reaction temperature required finer power and heating rate tuning: there were two heating rate plateaus at 80 °C around 30 and 45 s, while the single one was seen at 120 °C around 30 s. No plateaus after heating rate peak could be observed at 150 °C. As a consequence, there was a minute over- throw of the pre-set temperature of 80 °C, as an indication of a hardly control- lable heating conditions, at temperatures below boiling point of the reaction medium. It should be noted that the reactor pressure above standard conditions was registered only at the heating to 150 °C, which reached the steady-state value of 5.5 bar. Fig. 1. The changes in MW reactor parameters: temperature (tMW), MW power (P) and heating rate ((dt/dτ)τ), in the early stages of synthesis. The considered changes in heating parameters which precede the conditions required to maintain the steady-state heating indicated that the synthesized solid phase should be of different physical properties, since the heating was expected to affect the particle nuclei formation and subsequent particle growth. This was LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 699 checked by investigation of the particle size distribution (PSD) by the dynamic light scattering (DLS) as presented in following section. PSD of synthesized RuO2 solid phase The PSDs by DLS in reaction media for RuO2 synthesized by MW irradi- ation at 80, 120 and 150 °C are shown in Fig. 2. The distribution was analysed during the ten consecutive runs of light irradiation in order to check the PSD stability. All samples appeared highly polydisperse, with distinct appearance of the grains within the diameter fractions between 20 nm and 1 µm. The agglo- merates of diameter above 3 µm were present in all samples, but their size distri- bution was becoming more pronounced upon increase in the pre-set temperature. The grains of the smallest diameter were obtained at 120 °C, grouped within well resolved fractions around 30 and 150 nm (Fig. 2B). The separation of the frac- tions was considerably less pronounced at lower and higher temperature (Fig. 2A and C). The initially registered peaks at higher MW temperatures (Run 1, Fig. 2B and C) were becoming better distinguished during the consecutive runs, which indicated the merging of the grains into larger ones. Consequently, in steady state Run 10 there were well resolved peaks around 50 and 300 nm for the sample synthesized at 120 °C (Fig. 2B) and around 100 and 500 nm if the synthesis tem- perature is increased to 150 °C (Fig. 2C). This effect was very weak, or even opposite at 80 °C (Fig. 2A). The sample at 80 °C contained the µm-sized agglo- merates, stable during the runs, of extremely narrow PSD in comparison to other two samples. Fig. 2. Particle size distribution by intensity for RuO2 suspension synthetized at: A) 80; B) 120 and C) 150 °C in an MW reactor. The run relates to the number of successively repeated DLS measurement. These considerations of DLS data indicate that the pre-set temperature below the boiling point favours the particle growth formation and generates the grains of low tendency to join into agglomerates of different size. On the other hand, the intensification of the heating condition induces the nucleation and the subsequent generation of the grains, tending to form the defined grouped structures and the agglomerates of different sizes between 3 and 5 µm. This tendency of grains is reflected into the fraction of agglomerates themselves too, since they appeared 700 ŠEKULARAC et al. joined between first and tenth run (Fig. 2B and C) into distinct size similar to those present in sample synthesised at 80 °C (Fig. 2A). Further increase in tem- perature up to 220 °C did not affect the main grain fracture around 300 nm,39 but the fractions of smaller grains (below 100 nm) was found transferred to the regions of lower diameter. These findings are a strong confirmation of nuclei formation, favoured over particle growth. Exclusively, the PSD of the sample synthesized at 200 °C did not show any presence of the agglomerates.39 How- ever, the particles appear to be of high surface energy, since they form stable grains and agglomerates. The electrochemical properties of the MW-synthesized RuO2/Ti electrodes Electrochemical properties of RuO2/Ti electrodes prepared from MWt80, MWt120 and MWt150 samples were investigated by the cyclic voltammetry (CV), the polarization measurement and the electrochemical impedance spectroscopy (EIS). The CV curves (as specific currents per mass of the coating) are shown in Fig. 3. Fig. 3. The Cyclic voltammograms of RuO2/Ti electrodes prepared from the oxide synthesized at the different temperatures. Sweep rate: 50 mV/s; electrolyte: 1 M H2SO4. Cyclic voltammograms had usual shape for RuO2/Ti electrodes obtained by the different synthesis procedures.35 In the most of CV potential region of CV anodic branch, the highest currents were registered for RuO2(MWt80)/Ti and the lowest for RuO2(MWt120)/Ti electrode, although the currents were quite similar for all samples. This indicated their almost equal capacitive ability. It should be noted that the order of currents was opposite right at the beginning of charging (–0.2 to 0.1 V). This part of the anodic charging branch follows the completion of the proton insertion into the hydrous structure of the oxide in the same pot- ential region of the cathodic branch. It appeared that the oxide synthesized at LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 701 higher temperatures was able to respond more efficiently to the redox transitions involving the lower oxidation states of Ru.42 Similarly, the transitions related to higher Ru oxidation states were improved when the synthesis temperature was below the boiling point of the reaction (RuO2(MWt80)/Ti). These findings indi- cate that the increasing temperature of MW synthesis to 120 and 150 °C produce more polycrystalline particles that create more accessible coatings with more defects sites (kink sites, defect sites, edges, etc.), which can be readily oxidized to higher oxidation states. Total capacitance of anodes was calculated by integration of CV curves. A bit higher value was obtained for RuO2(MWt80)/Ti, 108 F g–1, in comparison to RuO2(MWt120)/Ti and RuO2(MWt150)/Ti, 101 and 107 F g–1, respectively. In addition to small differences commented in previous paragraph, these small dif- ferences in CV capacitance could also originate from a bit more compact struc- ture of the RuO2(MWt120)/Ti and RuO2(MWt150)/Ti coatings, for which more pronounced presence of a fraction of smaller particles was registered (Fig. 2). The synthesis temperatures above 150 °C, which is assumed as optimal for good capacitive response of RuO2,43 caused considerable increase in CV currents.39 In order to investigate further coating structure-related pseudo-capacitive characteristics of synthesized RuO2 coatings, electrochemical impedance spectro- scopy (EIS) measurements at open circuit potential (OCP) were carried out. If it is assumed that OCP value corresponds to the fastest and the most convenient redox transition, the EIS analysis is expected to give more details, related to the differences in various potential regions indicated by CV curves (Fig. 3). Expe- rimental and fitted EIS data, presented in form of complex plane and Bode plots, along with the model of applied equivalent electrical circuit (EEC), are shown in Fig. 4. The registered EIS response and the EEC structures are typical for DSAs with a well-developed TiO2-rich interlayer.44 The semicircle-like dependence can be seen in the high frequency range down to 400 Hz (Fig. 4B), which precedes capacitive-like response in the intermediate frequency range, down to 0.5 Hz. The semicircle is assignable to TiO2-rich in the coating/Ti substrate interphase, created during annealing of the electrode as the product of the substrate oxid- ation. Its parameters found the equivalence in CPE1 and R2 EEC elements in parallel (Fig. 4A). The capacitive-like response is distributed through a transmis- sion line generated by the elements C1-C4 and R3-R5 and is well developed as a capacitive loop, Fig. 4C. In the low frequency range, there is an indication of finite diffusion limitations to the pseudo-capacitive response, which is repre- sented by the element CPE2. The mean values (± abs. error) of EEC parameters are shown in Fig. 5. EIS response which describes the properties of TiO2-rich int- erlayer appears similar for three samples (Fig. 4A), with corresponding resistance and capacitance around 0.6 Ω and 2,5 F g–1, respectively. These findings indicate that the corresponding TiO2-rich interlayer is of similar structure since the coat- 702 ŠEKULARAC et al. Fig. 4. The results of the fitting of measured EIS data to equivalent electrical circuit (EEC, A) presented as impedance (B) and capacitance (C) complex plane and Bode (D) plots of the RuO2/Ti electrodes synthetized at: 80; 120 and 150 °C in an MW reactor at OCP; symbols: measured data, lines: EEC data. ings should be of similar texture39 created from the particles of similar size dis- tribution around 200 nm (Fig. 2). Owing to more narrower and more stable par- ticle distribution in case of MWt80, the least compact coating structure was exp- ected, and hence the most compact TiO2 interlayer could be formed. This appears indicated by the larger interlayer capacitance for MWt80 in comparison to MWt120 and MWt150 (Fig. 4A, branch No. 1, CPE1). However, the interlayer thickness is similar for all samples and seems to be considerably thinner than the coating, since the corresponding resistance (0.6 Ω) is negligible in comparison to coating pore resistance around 100 Ω. The capacitance complex plane plots show the difference in EIS behaviour between investigated RuO2 coatings (Fig. 4C). Capacitive loop for RuO2(MWt80)/Ti is larger than those obtained for RuO2(MWt120)/Ti and RuO2(MWt150)/Ti coatings. LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 703 The fitting of EIS response with EEC model (Fig. 4A) showed that RuO2(MWt80)/Ti is of higher total capacitance of around 96,6 F g–1 in comparison to RuO2(MWt120)/Ti and RuO2(MWt150)/Ti coatings, which showed similar total capacitance of around 71.3 F g-1. This presents more pronounced difference between RuO2(MWt80)/Ti and other two samples, while the values are lower, with respect to CV data. This means that MWt80 has better pseudo-capacitive characteristics in comparison with RuO2(MWt120)/Ti and RuO2(MWt150)/Ti. Results obtained by fitting of experimental data are presented as capacitance and resistance through the branches of EEC, Fig. 5A and B, respectively. The more pronounced difference between the samples appears to be much higher pore res- istances of RuO2(MWt120)/Ti and RuO2(MWt150)/Ti due to the more compact coating structure, which consequently showed the lower capacitance value in the EEC branches 3–5. As it can be seen, the capacitance values in the second branch, associated to the most outer part of the coating, are quite similar with (A) (B) Fig. 5. A) Capacitance and B) resistance through the branches of the equivalent electrical circuit used for the fitting the impedance spectra of the RuO2/Ti electrodes prepared from MWt80, MWt120 and MWt150 samples. 704 ŠEKULARAC et al. even small opposite trend with respect to branches 3–5. These branches, accord- ing to the transmission line model,45 relates to coating inner structure. Hence, higher pore resistance values were obtained for the branches 3–5. On the other hand, branch 2 presented the negligible resistance difference between samples, apparently related to the interspace of a fraction of large particles of similar size of around 200 nm for all samples. Bearing in mind that the CV data where regis- tered with rather high potential sweep rate in comparison to EIS measurements, it could be noticed that registered CV data reflect fairly well the findings related to second EEC branches of the lower order. It appears that the pseudo-capacitive response is sensitive to charging/dis- charging potential limits, with wider limits improving the capacitance perform- ance (1.3 V in CV and only 10 mV rms amplitude in EIS), since the CV condi- tions double the values of those obtained by EIS. This sensitivity is less pro- nounced for RuO2(MWt80)/Ti (55 % of CV capacitance is seen by EIS) than for other two samples (45 %). This indicates that more defined structure is formed at higher synthesis temperatures, with the ability to promote electrocatalytic activity (Fig. 3). Figure 6 represents the total capacitance and total pore resistance for the samples synthesized at different MW temperatures. RuO2(MWt120)/Ti is of largest resistance because of the compact structure created by the particles belonging to PSD fractions of the smallest diameter in comparison to other samples (Fig. 2B). Consequently the total capacitance was small, as indicated by Fig. 6. Similarly, RuO2(MWt80)/Ti and RuO2(MWt150)/Ti were of lower resistance with correspond- ing higher values of total capacitance, owing to the structure created by larger particles (Figs. 2A and C). The increase in MW temperature to 200 °C brought Fig. 6. Total capacitance and resistance of the RuO2/Ti electrodes as a function of MW synthesis temperature. LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 705 about the compromise between pore resistance and coating capacitance, which resulted in highest capacitance for moderate pore resistance, but also to more defined crystalline structure as shown in previous work.39 PSD of RuO2(MWt200)/ /Ti39 was consisted of similar small particle from fractions as PSDs for RuO2(MWt150)/Ti and RuO2(MWt80)/Ti, but with the appearance of huge agglo- merates for these two samples. The intrinsic values of capacitance and resistance of RuO2(MWt200)/Ti can also be assigned to the more uniform coating texture due to absence of the agglomerates. Electrocatalytic properties of prepared RuO2/Ti electrodes in oxygen evol- ution reaction (OER) are demonstrated by polarization curves shown in Fig. 7. Recorded polarization curves showed typical OER behaviour of RuO2/Ti;39,45 all of the curves show regions of the two Tafel slopes: around 40 mV below 1.23 V, and larger ones above this potential value. The region of lower Tafel slopes is typical for polycrystalline, thermally prepared RuO2/Ti, assignable to the mech- anism with indirect water oxidation over simultaneous OH/O transition as rate- -determining step, mostly on (110) RuO2. The data from previous work39 show that the synthesis temperature just above 150 °C makes the oxide less active in this potential region. The data for 200 °C show the increase of a slope to around 55 mV and the consequent decrease in currents, Fig. 8A and B, respectively. Fig. 7. Quasi-steady-state polarization curves (5 mV s-1) for O2 evolution in 1 M H2SO4 at room temperature on the RuO2/Ti electrodes prepared from MWt80, MWt120 and MWt150 samples. 706 ŠEKULARAC et al. (A) (B) Fig. 8. A) Tafel slopes in the regions of lower and higher pot- entials and B) corresponding cur- rent densities at 1.26 and 1.17 V (Fig. 7) for the RuO2/Ti electrodes as a function of RuO2 synthesis temperature. Further increase in temperature to 220 °C recovers the slope (the electro- catalytic features of surface active sites) and the currents only partially, with respect to low-temperature samples. This indicates that considerable changes of oxide structure take place at and above 150 °C, as reported in literature.43 Similarly, the discontinuity is registered for large Tafel slopes, which could cor- respond to OER mechanism with a slowest step of direct water splitting on oxygen terminals at the oxide sites.44 However, the temperature of discontinuity is 120 °C, which relates to considerable promotion of the electroctalytic activity. The slope is considerably lower (Fig. 8A) and corresponding currents are the highest (Fig. 8B). It follows that substantial changes in the oxide structure during the nuclei growth are initiated at the synthesis temperature above the boiling LOW-TEMPERATURE-SYNTHESIZED RuO2 AS ELECTRODE COATING 707 point of the reaction mixture. It appears that MW temperature of 120 °C is able to generate a plenty of active sites at the surface of oxide particles, which is well preserved during the coating preparation and maintained as RuO2(MWt120)/Ti unique electrocatalytic feature. It even appears that mechanism is not changed in the whole investigated potential region for RuO2(MWt120)/Ti, and corresponds significantly to consecutive OER over OH/O transition of surface active sites. CONCLUSIONS RuO2 coatings on Ti were prepared by thermal treatment of the oxide solid phase synthesized from RuCl3 aqueous solution by the low temperature-con- trolled microwave (MW) heating. The minor difference in capacitance between coatings prepared from oxides synthesized in temperature range of 80–150 °C was registered from voltammetric responses. The difference can be associated with more compact structure of the oxide synthesized at the temperature of 120 °C in comparison to the other samples. The more compact structure was cor- related to the presence of the fractions of the smallest particles. The distribution of capacitance and pore resistance, as a result of impedance measurements, showed more pronounced difference between samples due to the same cause related to coating structure. The considerable differences in capacitance and pore resistance were found in inner parts of a coating, while similar behaviour was restricted only to the outer parts, as found by voltammetric measurements. The electrocatalytic activity of the coatings in oxygen evolution reaction (OER) was found considerably different and promoted the coatings prepared from the oxide synthesized at lower temperatures as more active. These dif- ferences were correlated to different oxide structure with characteristic structure transitions at the intrinsic temperatures just above the boiling point of reaction mixture and above the temperature recognized as crucial for optimum proton/ /electron conductivity of the oxide. Although low synthesis temperatures were found beneficial for OER, the synthesis temperatures above 150 °C promoted the coating capacitance. This opposite finding was supposed to be due to the differences in the structure of oxide surface active sites, which appears to be generated during the MW syn- thesis and preserved during the formation of the coating. Acknowledgments. Financial support for the reported investigation from the Ministry of Education, Science and Technological Development of the Republic of Serbia is acknow- ledged. The authors thank to Dr. Ivana Drvenica for DLS measurements. 708 ŠEKULARAC et al. И З В О Д НИСКО-ТЕМПЕРАТУРНА СИНТЕЗА RuO2 ИЗ КИСЕЛОГ ХЛОРИДНОГ РАСТВОРА ЗА ПРИПРЕМУ EЛЕКТРОДНЕ ПРЕВЛАКЕ ГАВРИЛО ШЕКУЛАРАЦ1, САЊА ЕРАКОВИЋ1, ДУШАН МИЈИН2, ВЕСНА ПАВЕЛКИЋ3, ЈАСМИНА СТЕВАНОВИЋ1 и ВЛАДИМИР ПАНИЋ1 1Институт за хемију, технологију и металургију, Центар за електрохемију, Универзитет у Београду, Београд, 2 Технолошко–металуршки факултет, Универзитет у Београду, Београд и 3Висока железничка школа струковних студија, Здравка Челара 14, Београд RuO2 је синтетисан у киселом воденом раствору једноставним микроталасним (MW) поступком у једном кораку на контролисаним ниским температурама и затим коришћен за припрему RuO2 превлаке на подлози од титана. Синтетисана чврста фаза је окарактерисана анализом расподеле величине честица (PSD), док су капацитивност превлаке и њена активност у реакцији издвајања кисеоника (OER) испитане електро- хемијским техникама. Нађено је да је оксидна фаза изражено полидисперзна, са фракцијама величине честица у уском опсегу пречника и тенденцијом ка укрупњавању. Најситније честице и изражено раздвојене фракције синтетисане су на температурама блиским температури кључања реакционе смеше, знатно испод температуре конверзије хлорида у оксид. На овим температурама је регистрована и највећа активност RuO2/Ti аноде за OER, вероватно због различите структуре оксида у односу на узорке припрем- љене на вишим температурама. Међутим, нађено је да превлаке формиране од оксида синтетисаног на вишим температурама имају већу кaпацитивност од оних синтетисаних на нижим темпераутрама. Овакви резултати су пре последица температурно-зависне структуре оксида, него промена у његовој морфологији које настају МW синтезом. (Примљено 29. децембра 2016, ревидирано 20. марта, прихваћено 22. марта 2017) REFERENCES 1. S. Trasatti, W. O’Grady, in: H. Gerisher, C.W. 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