Marco Blouin

Activation of Ruthenium Oxide, Iridium Oxide, and Mixed RuxIr1 − x Oxide Electrodes during Cathodic Polarization and Hydrogen Evolution

Ruthenium, iridium, and mixed ruthenium/iridium oxide layers on titanium substrates have been obtained by thermal decomposition of chloride solutions. The decomposition temperature of RuO 2 and IrO 2 was varied from 300 to 500°C and from 400 to 500°C, respectively. That on the mixed Ru x Ir 1-x 0 2 layer was kept constant at 400°C. For the mixed oxide electrode, the Ru content was varied over the whole compositional range. Current-potential curves and cyclic voltammetry measurements were performed in 1 M H 2 SO 4 . It is shown that such oxide layers can be activated through cathodic polarization, leading to an increase of the electrocatalytic activity for hydrogen evolution. The ratio between the current density at a given electrode potential before and after completion of the activation process or the ratio between the exchange current density before and after the completion of the activation process was used to quantify this activation phenomenon. Values as high as 100 have been observed in some cases, but typical values are around ten. Through a series of specific measurements and comparison with data taken from the literature, it is shown that this activation phenomenon is not related to an increase of the electrochemically active surface area as determined through cyclic voltammetry measurements. An explanation is proposed whereby H-chemisorption within the oxide layer is ultimately responsible for the increase of the electrocatalytic activity of the oxide layer.

High energy ball-milled Ti2RuFe electrocatalyst for hydrogen evolution in the chlorate industry

The high energy mechanical alloying of a Ti{endash}Ru{endash}Fe powder mixture (atomic ratio 2:1:1) has been performed by extensive ball-milling in a steel crucible. The structural evolution of the resulting materials has been studied by x-ray powder diffraction analysis. The identification of the various phases present in the materials, as well as the crystallite size and strain, has been performed by Rietveld refinement analysis. In the first stage of the material transformation, Ru or Fe atoms dissolved into Ti to yield to the formation of {beta}{minus}Ti. Upon further ball-milling, almost all the original constituents of the powder mixture have disappeared and a new simple cubic Ti{sub 2}RuFe phase is formed, with a crystallite size as small as 8 nm. The electrochemical properties of these materials have been tested in a typical chlorate electrolyte by cold-pressing the powders into disk electrodes. At 20 h of ball-milling, where the phase concentration of Ti{sub 2}RuFe reaches 96{percent}, a reduction of the activation overpotential at 250 mA cm{sup {minus}2} of nearly 250 mV is observed when compared to that of a pure iron electrode. {copyright} {ital 1997 Materials Research Society.}

Comparative study of nanocrystalline Ti2RuFe and Ti2RuFeO2 electrocatalysts for hydrogen evolution in long-term chlorate electrolysis conditions

Ti2RuFe and Ti2RuFeO2 nanocrystalline alloys were prepared by high energy ball-milling and used as cathodes for the hydrogen evolution reaction (HER) in the process of sodium chlorate synthesis. Ti2RuFe is almost single phase with the B2 structure. In contrast, Ti2RuFeO2 is made of a mixture of Ti2RuFe and TiOx phases. Tests in chlorate electrolysis conditions did not show any sign of degradation of Ti2RuFeO2 over a 300 h period, while Ti2RuFe breaks down after less than 100 h. The degradation of Ti2RuFe occurs because of hydrogen absorption and desorption during alternating hydrogen discharge and open-circuit conditions. Various hypotheses to explain the increase stability of the O containing alloy are considered.

Plasma-sprayed nanocrystalline Ti-Ru-Fe-O coatings for the electrocatalysis of hydrogen evolution reaction

Abstract Nanocrystalline Ti–Ru–Fe–O (2-1-1-2) was prepared by mechanical alloying in a ZOZ attritor. Vacuum plasma spray (VPS) was then used to deposit coatings of this material on a substrate. Energy dispersive X-ray fluorescence and X-ray diffraction analysis was used to follow the change in the chemical composition and crystalline structure of the powder upon deposition by VPS. Nanocrystalline Ti–Ru–Fe–O (2-1-1-2) prepared by the ZOZ attritor contains more than 50 wt.% of hexagonal Fe 2 Ti and a smaller amount ( 2 Ti and Ru results in the formation of several cubic phases with lattice parameters ranging from 2.96 to 3.02 A. This reflects a change in the Ru content on the 1 a ( 1 2 , 1 2 , 1 2 ) site of the cubic lattice. The deposition process also results in the formation of Ti 2 O 3 . This phase is present in excess at the surface of the coating but can be efficiently dissolved through etching in an acid solution. The cathodic overpotential for hydrogen evolution of such activated coatings in typical chlorate electrolysis conditions is η 250 =−550 mV.

Comparative Study of the Electrochemical Behavior of Polycrystalline and Nanocrystalline Ru Powder in NaOH Solution

In this study, a comparison was made of the microstructure and the electrochemical behavior of ruthenium in the polycrystalline (pc) and nanocrystalline (nc) states. The nc-ruthenium was prepared by high energy ballmilling. X-ray diffraction patterns show crystallite size ranging from 10 to 30 nm and larger than 100 nm for nc-Ru and pc-Ru, respectively. Scanning electron microscopy showed that the milled powder is made of agglomerates of crystallites. Evaluation of the accessible surface area by gas-phase adsorption (Brunauer-Emmett-Teller method measurements) and capacitance measurements through cyclic voltammetry shows an increase of about 150% for nc-Ru compared to pc-Ru. Ru in the nanocrystalline state is more prone to oxidation than its polycrystalline counterpart. Despite this, nc-ruthenium exhibits a better resistance to corrosion than pc-ruthenium. An explanation to reconcile these two observations is proposed.

Effect of oxygen on the structural and electrochemical properties of nanocrystalline Ti-Ru-Fe alloy prepared by mechanical alloying

Abstract The effect of the oxygen content on the structural and electrochemical properties of Ti-Ru-Fe-O alloys prepared by high energy ball milling was studied. The structural evolution of the materials has been analyzed by x-ray powder diffraction. The identification of the various crystalline phases, as well as the crystallite size, has been performed by Rietveld refinement analysis. In a first series of experiments, oxygen was added to pre-formed β 2 - Ti 2 RuFe . It was shown that this causes the β 2 phase to decompose into Ru, Fe and TiO. In a second series of experiments, various amounts of oxygen were added at the very beginning of the milling process by keeping the Ti:Ru ratio constant at 2:1 and varying the Ru:Ru0 2 ratio. It was shown that oxygen is only slightly soluble in the β 2 phase. X-ray photoelectron spectroscopy reveals that Ti and Fe at the surface of the material are highly oxidized while Ru is in the metallic state. The electrochemical properties of these materials have been tested in typical chlorate electrolysis conditions. The electrocatalytic activity of the cold-pressed powders did not show any marked variation with the oxygen content. This is most probably related to the fact that the surface composition of the material is almost independent of the bulk O content. A reduction of the activation overpotential at 250 mA cm −2 of 200–250 mV is observed when compared to that of a pure Fe cathode.

Kinetics of formation of nanocrystalline phases by mechanochemical reaction between Ti and RuO2

The structural evolution of a mixture of Ti and RuO2 mechanically alloyed over a period of 40 h was followed with respect to time by X-Ray diffraction. The structural parameters were extracted from the XRD traces by performing a Rietveld refinement analysis. The phase formation occurs in three distinct stages. In stage I (first 10 min of milling), RuO2 and Ti reacts to form RuTi, Ru, and TiO, with some other oxide phases of titanium (Ti2O3, and TiO2). In stage II (between 10 and 60 min), the reaction slows down and the titanium dioxides are being reduced. In stage III, decomposition of RuTi through reaction with Ti2O3 occurs, to yield to the formation of TiO and Ru. In stage I, the reaction rate is very high, for example, 34.7 wt % of RuTi is formed after only 10 min of milling. It is argued that such rapid reaction rate is due to the enthalpy of the redox reaction occurring between Ti and RuO2 which raised the local temperature and thus favors the atomic interdiffusion. Finally, a series of experiments performed as a function of the milling intensity points to the existence of an energy threshold below which the combustion reaction between Ti and RuO2 does not occur.

Large specific surface area nanocrystalline Ti-Ru-Fe cathode materials for sodium chlorate

Ball-milled nanocrystalline Ti3RuFe powders were mixed with 1, 2, 4, 10 and 20 equivalents of Al and the mixtures were milled again for 20 h. The amount of Al atoms dissolved into the B2 structure of Ti3RuFe does not exceed 8–9 at %, the remaining being present as elemental Al into the powder mixture. During a subsequent treatment of the composite powder in alkaline solutions, the elemental Al is leached out, while Al solutes in the B2 structure are not affected. An examination of the surface by scanning electron microscopy reveals that the leached powder has a highly porous surface structure. Surface area measurements performed by BET measurements show that there is a tenfold increase in the effective surface area. Activated electrodes made from these porous materials show a significant decrease of the cathodic overpotential for hydrogen evolution in typical chlorate electrolysis conditions of about 80 mV.

Metastable Ti-Ru-Fe-O nanocrystalline alloys for applications in the chlorate industry

Abstract Metastable Ti 2 FeRuO x alloys have been produced using three different methods: rapid melt-quenching, vapor phase condensation and high energy ball milling. Depending on the technique used and on the oxygen composition, metastable amorphous or nanocrystalline structures with various degrees of chemical order are produced. The structural properties have been studied using X-ray diffraction, scanning Auger microscopy and thermal analyses. The electrocatalytic properties for the hydrogen evolution reaction in conditions identical to that of the sodium chlorate industry are presented.