B. Nikolić

Glassy carbon electrodes. I. Characterization and electrochemical activation

Abstract Electrochemical properties of glassy carbon electrodes of two types were examined, one thermally treated at 1000°C (sample K) and another thermally treated at 2500° (sample G). Mechanically polished or electrochemically polarized electrodes were characterized in NaOH, HClO4 and H2SO4 solutions by cyclic voltammetry (cv) at different sweep rates in the potential range between the hydrogen and oxygen evolution. The activity of the electrodes depended on the properties of the glassy carbon examined, as determined by both the temperature of thermal treatment and the mechanical or electrochemical pretreatment of the sample. It was noticed that both types of electrodes, when polished exhibited an increase in the double layer charge upon increasing the pH value of the solution. The cv charges, for both types of samples, increase upon anodic polarization. The higher the potential of oxidation, the more pronounced is the increase in charge, particularly in acidic solution. The increase in charge amounts from below 1 mC cm−2 for polished glassy carbon up to few hundreds of mC cm−2 for surfaces anodically polarized in acidic solution. Analysis of the dependence of voltammetric charge, as well as morphological changes of the electrode surface, on the time of oxidation suggests the existence of three stages in the electrochemical activation process. The first one occurs only once at the beginning of the activation, while the other two repeat themselves, reflecting a periodical activation and deactivation process. These stages were discussed and ascribed to a surface layer oxidation, graphite oxide layer growth and mechanical destruction of the surface. Independent surface analysis by AES, XPS and STM confirms the results obtained by electrochemical methods.

The properties of carbon-supported hydrous ruthenium oxide obtained from RuOxHy sol

Preparation of RuO x H v /carbon black nanocomposite material was performed by the impregnation method starting from RuO, H v sol as a precursor. Black Pearls 2000® (BP) and Vulcan® XC-72 R (XC) were used as supporting materials. Samples of the composite were calcined in nitrogen atmosphere at temperatures from 130 to 450 °C. Chemical and structural characterization of the precursor and the composites was performed by using energy-dispersive X-ray fluorescence spectroscopy (EDXRFS), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). These techniques showed that the oxide impregnated into carbon support is hydrous and amorphous. The amorphous structure is maintained if the composite is calcined up to 300 °C, while the low-intensity peak at 20 position that corresponds to the most intensive peak of the rutile structure was registered at 450 °C. The capacitance of the composite was determined using cyclic voltammetry. It was found that the capacitance is dependent on calcination temperature and surface area of the carbon black support. The highest specific capacitance of about 700 F g -1 of composite was registered for RuO x H y supported on BP and calcined at 300 °C while four times lower values was obtained for RuO x H y supported on XC. The capacitance measurements indicated that XC-supported RuO x H y composite is highly porous while BP-supported one is more compact.

RUO2-TIO2 COATED TITANIUM ANODES OBTAINED BY THE SOL-GEL PROCEDURE AND THEIR ELECTROCHEMICAL BEHAVIOUR IN THE CHLORINE EVOLUTION REACTION

Abstract TiO2 and RuO2 sols were obtained from RuCl3 and TiCl3 aqueous solutions by condensation and forced hydrolysis at elevated temperature. The Ti supported coatings were obtained by the sol–gel method using a sol mixture in which the metal content was Ru(40%)–Ti(60%). The crystal structure of the solid phase of the sols and their mixtures was examined by X-ray diffraction. The surface and morphology of the coatings were characterized by scanning electron microscopy and scanning tunneling microscopy. Polarization measurements and cyclic voltammetry were used to obtain information about the electrochemical properties of the obtained anodes. An accelerated corrosion test was applied to quantify the anode stability for the chlorine evolution reaction. A stability comparison of the anodes shows a considerably higher lifetime for sol–gel coated anodes than for those obtained by the usual thermal decomposition method.

The influence of the aging time of RuO2 and TiO2 sols on the electrochemical properties and behavior for the chlorine evolution reaction of activated titanium anodes obtained by the sol-gel procedure

Abstract The influence of the aging time of RuO 2 and TiO 2 sols used for the preparation of (40% RuO 2 +60% TiO 2 )/Ti anodes by the sol-gel procedure on the electrochemical properties and behavior for the chlorine evolution reaction of obtained anodes was studied. The electrochemical (active) surface area of the anode coatings was examined by cyclic voltammetry. The electrocatalytic activity and stability of obtained anodes for the chlorine evolution reaction were investigated by polarization measurements and accelerated stability test. The dependence of electrochemical properties of obtained activated titanium anodes on RuO 2 and TiO 2 sol particle size was established.

The roles of the ruthenium concentration profile, the stabilizing component and the substrate on the stability of oxide coatings

Abstract Electrocatalytic oxide coatings with variable concentration profiles of RuO2 as the active component were obtained through a combination of separately applied layers of RuO2, TiO2, IrO2, RuO2 + TiO2 and RuO2 + IrO2 on titanium and glassy carbon substrates. The stability of the samples was examined by accelerated tests performed at high anodic current densities. Electrochemical techniques, cyclic voltammetry for assessing the charge associated with the coating, polarization measurements for assessing the electrocatalytic activity, and Auger electron spectroscopy to register surface composition of the coatings, were applied to follow changes due to the stability experiments. The stability and the charge depended strongly on the sequence of layers in the RuO2−TiO2 coating, with the samples having the RuO2 + TiO2 layer facing the electrolyte exhibiting the highest values for both properties. In contrast to this, the stability of the RuO2−IrO2 coatings, besides being lower than the stability of RuO2−TiO2 coatings, showed no dependence on the sequence of the applied layers. Much lower stability was exhibited by the coatings applied on glassy carbon rather than on titanium. A mechanism of the stability of the coatings based on the interaction of lower than four valency state titanium with higher than four valency ruthenium, proposed for single-crystal surfaces, is corroborated. Finally, during the thermal treatment a diffusion of titanium originating in the titanium substrate through the coating was established.

Morphology of RuO2-TiO2 coatings and TEM characterization of oxide sols used for their preparation

Characterization of RuO(2) and TiO(2) sols of different aging times, obtained by forced hydrolysis of appropriate chloride salts, was performed by transmission electron microscopy (TEM). The aging time of TiO(2) sols was observed to affect the size of particles as well as the crystallinity of the solid phase of the sols. The surface morphology of RuO(2)-TiO(2) coatings on titanium, obtained by the sol-gel procedure using TiO(2) sols of different aging times and RuO(2) sol of fixed aging time, was investigated by scanning tunneling microscopy (STM) at three different scan sizes. The STM data indicated uniform microdistribution of the coating material (small microroughness) and an increase in nanoroughness with the aging time of the TiO(2) sol. The observed increase in real coating surface area with increasing TiO(2) particle size confirms the earlier cyclic voltammetry results.

Electrocatalytic activity of sol-gel-prepared RuO2/Ti anode in chlorine and oxygen evolution reactions

Electrocatalytic properties of RuO2/Ti anode with different coating masses, which are prepared by the alkoxide sol-gel procedure, are investigated in chlorine and oxygen evolution reactions by polarization measurements and electrochemical impedance spectroscopy in H2SO4 and NaCl electrolytes. According to polarization measurements, the activity of anodes at overpotentials below 100 mV is independent of coating mass. However, impedance measurements above 100 mV reveal changes in the activity of anodes in chlorine evolution reaction for different coating masses. The diffusion limitations related to the evolved chlorine are registered in low-frequency domain at 1.10 V (SCE), diminishing with the increase in potential to the 1.15 V (SCE). The observed impedance behavior is discussed with respect to the activity model for activated titanium anodes in chlorine evolution reaction involving formation of gas channels within porous coating structure. Gas channels enhance the mass transfer rate similarly to the forced convection, which also increases the activity of anode. This is more pronounced for the anode of greater coating mass due to its more compact surface structure. The more compact structure appears to be beneficial for gas channels formation.

The effect of the addition of colloidal iridium oxide into sol–gel obtained titanium and ruthenium oxide coatings on titanium on their electrochemical properties

Electrochemical properties of sol-gel processed Ti(0.6)Ir(0.4)O(2) and Ti(0.6)Ru(0.3)Ir(0.1)O(2) coatings on titanium substrate were investigated using cyclic voltammetry, polarization measurements and electrochemical impedance spectroscopy and compared to the properties of Ti(0.6)Ru(0.4)O(2) coating. The role of iridium oxide in the improvement of the electrocatalytic, capacitive and stability properties of titanium anodes activated by a RuO(2)-TiO(2) coating is discussed. The oxide sols were prepared by forced hydrolysis of the metal chlorides. The characterization by dynamic light scattering and X-ray diffraction showed that polydisperse oxide sols were obtained with the particles tending to form agglomerates. The presence of IrO(2) causes a suppression of the X-ray diffraction peaks of TiO(2) and RuO(2) in the sol-gel prepared Ti(0.6)Ir(0.4)O(2) and Ti(0.6)Ru(0.3)Ir(0.1)O(2) coatings. The IrO(2)-containing coatings had an enhanced charge storage ability and activity for the oxygen evolution reaction (OER) in comparison to Ti(0.6)Ru(0.4)O(2) coating. The voltammogram of the Ti(0.6)Ir(0.4)O(2)/Ti electrode showed well-resolved peaks related to Ir redox transitions, which are responsible for the enhanced charge storage ability of IrO(2)-containing coatings. Redox transitions of Ir were also registered in the high-frequency domain of the ac impedance spectra of the coatings as a semicircle with characteristics insensitive to the electrolyte composition and to the electrode potential prior to OER. However, the semicircle characteristics were different for the two IrO(2)-containing coatings, as well as at potentials outside the OER in comparison to those at which the OER occurs.

The Influence of the Aging Time of RuO2 Sol on the Electrochemical Properties of the Activated Titanium Anodes Obtained by Sol-Gel Procedure

The influence of the aging time of RuO 2 sol on the electrochemical properties and behaviour in chlorine evolution reaction of RuO 2 /Ti and (40%RuO 2 + 60%TiO 2 )/Ti anodes obtained by sol-gel procedure was studied. The electrochemically active surface area of the anode coatings was examined by cyclic voltammetry. The electrocatalytic activity and anode stability in chlorine evolution reaction were investigated by polarization measurements and accelerated stability test. The dependence of electrochemical properties of obtained activated titanium anodes on RuO 2 particle size was established.

THE ROLE OF SOL-GEL PROCEDURE CONDITIONS IN ELECTROCHEMICAL BEHAVIOR AND CORROSION STABILITY OF Ti/[RuO2-TiO2] ANODES

ABSTRACT Using a sol-gel procedure, RuO2-TiO2 active coatings were formed on a titanium support. The conditions of preparation of the RuO2 and TiO2 sols were investigated and the sols were characterized by transmission electron microscopy (TEM) and X-ray diffraction technique. Prepared anodes were investigated from the standpoint of morphology (scanning tunneling and electron microscopy) and electrochemical behavior (cyclic voltammetry, polarization measurements, and electrochemical impedance spectroscopy). The service life of the RuO2-TiO2 active coatings was examined by an accelerated corrosion stability test in 0.5 ;mol dm−3 NaCl solution. In order to resolve the role of TiO2 in the deactivation mechanism of the active coating, additional TiO2 layers were introduced in the support/coating interface or on the surface of the coating. The behavior of these anodes with the TiO2-enriched coating layer was compared with the behavior of anodes with regular RuO2-TiO2 coatings. The results show that the loss of coating electrocatalytic activity is caused by dissolution of active Ru species and TiO2 enrichment in the coating. It is also observed that the sol-gel preparation procedure favors the formation of the TiO2 layer at the surface of the RuO2-TiO2 coating.