Effect of electrodeposition modes on ruthenium oxide electrodes for supercapacitors

With developments in energy storage devices, supercapacitors are gaining more attraction because of their potential to excel batteries shortly. In this work, ruthenium oxide (RuO₂) has been deposited on stainless steel and studied the influence of surface modification of solid electrodes on capacitance properties. Hydrous ruthenium oxide was plated by different modes such as potential sweep method (cyclic voltammetric), constant potential method (chronoamperometry) and optimised potential pulse method using a recently reported precursor material namely ruthenium nitrosylsulfate (RuNS). The structural information and morphology of electrodeposits were characterised by X-ray diffractometer and scanning electron microscope respectively. The XRD studies indicate a poor crystalline state for RuO₂ in all the modes of deposition but can contribute to a higher surface area when compared to a highly crystalline form. The SEM analysis revealed the formation of surface modification concerning the change of potential mode. Mud-cracked morphology, spherical particles and dendrimeric morphology observed on chronoamperometry, potential pulse and cyclic voltammetry respectively. Electrochemical studies were also conducted on the samples to assess their performance for supercapacitor applications. The spherical particles of hydrous RuO₂ show high performance of capacitance behaviour 1180 F/g in 0.5 M H₂SO₄ at the scan rate of 5 mV/s. Dendrimeric morphology and mud-cracked morphology shows 573 F/g and 546 F/g respectively in same 0.5 M H₂SO₄ at the scan rate of 5 mV/s. The studies reveal that RuO₂ electrodes can be exploited for their outstanding capacitive behaviour by properly controlling the morphology of the deposits.     

The growth mechanism and supercapacitor study of anodically deposited amorphous ruthenium oxide films

In the present study, ruthenium oxide (RuO₂) thin films were deposited on the stainless steel (s.s.) substrates by anodic deposition. The nucleation and growth mechanism of electrodeposited RuO₂ film has been studied by cyclic voltammetry (CV) and chronoamperometry (CA). The deposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive analysis by X-rays (EDAX) for structural, morphological, and compositional studies. The electrochemical supercapacitor study of ruthenium oxide thin films have been carried out for different film thicknesses in 0.5 M H₂SO₄ electrolyte. The highest specific capacitance was found to be 1190 F/g for 0.376 mg/cm² film thickness.     

X-ray K-absorption studies ofsome ruthenium compounds

X-ray K-absorption studies of ruthenium in ruthenium metal,RuO₂, K₂(RuCl₆) and K₄[Ru(CN)₆]: 3H₂O have been carried out using 400 mm bent crystal (mica) spectrograph. K-absorption edge of ruthenium in these compounds lies on the higher energy side with respect to that in the ruthenium metal; the divalent K₄[Ru(CN)₆]·3H₂O gives the shift in the range of tetravalent compounds RuO₂ and K₂[RuCl₆]. This discrepancy has been explained on the basis of molecular orbital picture.     

Behavior of Ru surfaces after ozonated water treatment

In order for the development of cleaning technology of extreme ultra violet lithography photomask, the behavior of Ru surfaces after treatment with ozonated deionized water (DIO₃) solution was studied using Ru and ruthenium oxide particles and 2 nm-thick Ru capping layers. No significant changes in crystalline structures or chemical states of the Ru surfaces, nor any similarities with the structures or states of ruthenium oxide, were observed after DIO₃ treatment. Oxidation of ruthenium to form RuO₂ or RuO₃ was not observed. Adsorption of H₂O molecules on the Ru layer increased the surface roughness, but the desorption of H₂O molecules recovered it. Local chemisorption of H₂O molecules on the Ru surface may be the reason why rougher Ru surfaces were observed after DIO₃ cleaning.     

Oxidation and reduction of thin Ru films by gas plasma

The oxidation and reduction of Ru thin films grown on a Si(1 0 0) surface were studied by X-ray photoemission spectroscopy (XPS). Ru thin films were oxidized with O₂ plasma generated by an rf discharge, and their XPS spectra were measured. The spectra were decomposed into several components for Ru suboxides attributable to different stages of oxidation. After sufficient exposure to oxygen, a stoichiometric rutile RuO₂ layer was found to have formed near the surface. Thermal annealing at 500 K resulted in a thicker RuO₂ layer. Experiments demonstrated that the Ru oxide layer can be removed by H(D) atoms via the desorption of water molecules.     

Atomistic computer simulation studies of Sr2RuO4 and Ca2RuO4

Atomistic static computer simulation techniques have been applied to investigate the energetics of defects and dopants in Sr₂RuO₄ (SRO) and Ca₂RuO₄ (CRO). Interatomic potentials have been derived which reproduced the crystal structures of these systems. Solution energies are calculated for different dopant ions to ascertain the site occupied by the dopant ion in the host lattice. Monovalent and divalent ions are predicted to substitute preferentially at the alkaline-earth site in both the systems. Trivalent cations of smaller ionic radii substitute at the Ru sites while those having larger ionic radii prefer to substitute at the Sr or Ca sites in SRO or CRO systems, respectively. In addition, there is a possibility of self-compensation, where a trivalent cation can substitute at both Sr(Ca) and Ru sites. Tetravalent dopants are found to substitute at the ruthenium sites in both systems.     

Thermal treatment effects on the Ru/CeO2 catalysts performance for partial hydrogenation of benzene

The aim of this work is to study the effect of the preparation conditions of Ru/CeO₂ catalyst (calcination temperature and/or reduction) over the performance in the partial hydrogenation of benzene reaction in the presence of TiCl₃. The catalysts were prepared through chlorinated precursors by incipient wetness impregnation method. The reaction occurred in three-phase reactional medium in presence of water at 373 K and 5.0 MPa. Temperature programmed reduction (TPR) profiles of calcinated catalysts indicate the presence of oxidated ruthenium. X-ray photoelectron spectroscopy (XPS) analysis confirms this supposition, showing that the ruthenium appears in the form of RuO₂ for the sample calcinated at 673 K, while for the reduced solid at 773 K, the Ru appears in the metallic state. However, the calcination step followed or not by reduction, strongly hinders the catalytic performance. In its turn, the direct reduction leads to a more active Ru/CeO₂ catalysts, as well as higher cyclohexene yields throughout all the reaction.     

Electrodeposited ruthenium oxide thin films for supercapacitor: Effect of surface treatments

Amorphous and porous ruthenium oxide thin films have been deposited from aqueous Ru(III)Cl₃ solution on stainless steel substrates using electrodeposition method. Cyclic voltammetry study of a film showed a maximum specific capacitance of 650 F g⁻¹ in 0.5 M H₂SO₄ electrolyte. The surface treatments such as air annealing, anodization and ultrasonic weltering affected surface morphology. The supercapacitance of ruthenium oxide electrode is found to be dependent on the surface morphology.     

Electrodeposition of mesoporous ruthenium oxide using an aqueous mixture of CTAB and SDS as a templating agent

Mesoporous RuO₂ films were electrochemically fabricated on ITO-coated glass substrate from aqueous ruthenium chloride (RuCl₃·nH₂O) solution. To achieve highly stable mesoporous structure, an aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) was used as a templating agent.The mesoporous structure was confirmed by small angle X-ray diffraction (SAXRD) and transmission electron microscopy (TEM). The addition of small amount (10wt%) of CTAB significantly improved the stability of porous structure. The crystallinity of synthesized RuO₂ thin film was confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Specific capacitance of the synthesized films was evaluated by measuring cyclic voltammetry (CV) and charge-discharge curves in 0.5 M H₂SO₄. Compared with non-porous electrode, mesoporous RuO₂ showed higher supercapacitor performance.     

A nanogravimmetric investigation of the charging processes on ruthenium oxide thin films and their effect on methanol oxidation

The charging processes and methanol oxidation that occur during the oxidation-reduction cycles in a ruthenium oxide thin film electrode (deposited by the sol-gel method on Pt covered quartz crystals) were investigated by using cyclic voltammetry, chronoamperometry and electrochemical quartz crystal nanobalance techniques. The ruthenium oxide rutile phase structure was determined by X-ray diffraction analysis. The results obtained during the charging of rutile ruthenium oxide films indicate that in the anodic sweep the transition from Ru(II) to Ru(VI) occurs followed by proton de-intercalation. In the cathodic sweep, electron injection occurs followed by proton intercalation, leading to Ru(II). The proton intercalation/de-intercalation processes can be inferred from the mass/charge relationship which gives a slope close to 1 g mol⁻¹ (multiplied by the Faraday constant) corresponding to the molar mass of hydrogen. From the chronoamperometric measurements, charge and mass saturation of the RuO₂ thin films was observed (440 ng cm⁻²) during the charging processes, which is related to the total number of active sites in these films. Using the electrochemical quartz crystal nanobalance technique to study the methanol oxidation reaction at these films was possible to demonstrate that bulk oxidation occurs without the formation of strongly adsorbed intermediates such as CO_ads, demonstrating that Pt electrodes modified by ruthenium oxide particles can be promising catalysts for the methanol oxidation as already shown in the literature.     

Free radical reactions at the Ru(0 0 0 1) surface: Atomic oxygen and dissociated NH3

X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0 0 0 1), and the effect of dissociated ammonia on RuO₂/Ru(0 0 0 1), in UHV conditions at ambient temperature. The Ru(0 0 0 1) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O₂ in a thermal catalytic cracker, with 45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO₂). No formation of higher oxides of Ru was observed. The formation of RuO₂ proceeded without saturation for total oxygen exposures of up to 10⁵ Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO₂ formed by the reaction with atomic oxygen was exposed to a flux of NH_x (x = 1, 2) + H generated by the cracker. The reduction of RuO₂ to Ru metal was observed by XPS. An exposure of 3.6 × 10² L of NH_x + H, resulted in the observation of adsorbed H₂O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NH_x + H exposure. No nitrogen adsorption was observed.     

Electronic state of ruthenium deposited onto oxide supports: An XPS study taking into account the final state effects

The electronic state of ruthenium in the supported Ru/EO_x (EO_x = MgO, Al₂O₃ or SiO₂) catalysts prepared by with the use of Ru(OH)Cl₃ or Ru(acac)₃ (acac = acetylacetonate) and reduced with H₂ at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d_5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EO_x samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d_5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5-279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al₂O₃ and SiO₂ (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EO_x systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid-base properties of the support, the mean size of supported Ru crystallites (within the range of 2-10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.     

Structure and spectroscopy of the supercapacitor material hydrous ruthenium oxide,RuO2·xH2o,by neutron scattering*

Hydrous ruthenium dioxide, RuO₂·xH₂O, is a material of active investigation as an electrode material for supercapacitors. A combination of elastic and inelastic neutron scattering together with thermal gravimetric studies and DFT calculations have provided new insight into the nature of the surface species present on RuO₂·xH₂O. Our results confirm that hydrous ruthenium oxide is a nanocrystalline material consisting of a core of RuO₂. We show that the surface consists largely of Ru–OH with small amounts of water hydrogen-bonded to the surface. The hydroxyls are stable up to ～200°C, i.e. over the composition range x?=?0.2–2. The optimal supercapacitor material has x?=?0.5–0.7, and in this range, the surface is fully hydroxylated. This provides a route for the proton transport: a proton can attach to a surface hydroxyl to generate coordinated water, proton transport then occurs along the hydrogen-bonded chain by a Grotthuss mechanism.     

Sonochemical effect on two new Ruthenium(II) complexes with ligand (E)-N-((6-bromopyridin-2-yl)methylene)-4-(methylthio)aniline precursors for synthesis of RuO2 nanoparticles

Two novel Ru(II) complexes [(η⁶-p-cymene)RuCl(L2)]PF₆ (R2) and [(η⁶-C₆H₆)RuCl(L2)]PF₆ (R4), with ligand (E)-N-((6-bromopyridin-2-yl)methylene)-4-(methylthio)aniline (L2), were synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR and IR spectroscopy. Based on X-ray crystallography studies, complexes R2 and R4 have coordination environments with formulated (η⁶-p-cymene)Ru(N₂Cl) and (η⁶-C₆H₆)Ru(N₂Cl), respectively. The thermal stabilities of compounds R2 and R4 were studied by thermal gravimetric (TG) and differential scanning calorimetry (DSC). Thermal decomposition of these complexes was at 280 °C and 260 °C under air atmosphere respectively. The interaction of these complexes with calf thymus DNA (CT-DNA) was explored through electronic absorption spectra, fluorescence and redox behavior studies. The results showed that the complexes bind to CT-DNA with electrostatic interactions. Nanoparticles of RuO₂ were prepared by calcination of R2 and R4. Also the role of the ultrasound waves on the characteristics of the RuO₂ nanoparticles was studied. The nanoparticles were characterized by IR spectroscopy and X-ray diffraction (XRD). Also size and morphology of nanoparticles were studied by scanning electron microscopy (SEM).     

Kinetic studies for the non-isothermal decomposition of un-irradiated and γ-irradiated ruthenium(III) acetylacetonate

Kinetic studies for the non-isothermal decomposition of un-irradiated and γ-irradiated ruthenium(III) acetylacetonate in air were carried out. The results show that the decomposition proceeds in one major step in the temperature range of 150–250 °C with the formation of RuO₂ as a final solid residue for un-irradiated Ru(acac)₃. For γ -irradiated Ru(acac)₃ with 10² KGy total γ-ray dose, the decomposition goes eventually to completion with almost 100% decomposition and proceeds in one major step, which contains four overlapping decomposition stages in the temperature range of 200–320 °C. The kinetics is shown to be non-isothermal, using both model-fitting and model-free approaches. Infrared (IR) spectroscopy and X-ray powder diffraction techniques were employed to follow the chemical composition of the solid residue obtained at different temperatures.     

Isomer shifts and changes of the nuclear charge radii in99Ru and101Ru

The recoilless nuclear gamma resonance of the 127 keV γ-rays of¹⁰¹Ru was observed in ruthenium metal, RuO₂ and [Ru(NH₃)₄(HSO₃)₂]. By comparison of the isomer shifts observed in these materials for the 127 keV absorption line with the corresponding shifts of the 90keV γ-rays of⁹⁹Ru one obtains δ〈r ²〉 [127 keV]/ δ〈r ²〉 [90 keV]=1.78±0.26 for the ratio of the changes of the mean square nuclear charge radii between the first excited and the ground states in these nuclei. An estimate of electron density differences based on free-ion relativistic self-consistent field calculations yields δ〈r ²〉[90keV]≈+1.4·10^?3 for⁹⁹Ru and δ〈r ²〉/〈r ²〉 [127 keV]≈+2.4·10^?3 for the¹⁰¹Ru case. These results are discussed in terms of the core excitation model.     

Ruthenium dioxide, a fascinating material for atomic scale surface chemistry

Over the past few years, RuO₂ has developed into one of the best-characterized late transition metal oxides in surface science, revealing unique and promising redox properties. The CO oxidation reaction over RuO₂ (110) was intensively studied by low-energy electron diffraction, scanning tunneling microscopy, high resolution core level spectroscopy, and density functional theory calculations, connecting structural and electronic properties with chemical properties. On the atomic scale the presence of one-fold coordinatively unsaturated Ru sites (1f-cus Ru) is the primary reason for the high activity of stoichiometric RuO₂ (110) towards the oxidation of CO and other small alcohols. On the stoichiometric RuO₂ (110) surface, CO molecules adsorb strongly (adsorption energy exceeding 1.2 eV) on top of the 1f-cus Ru atoms, from where the actual oxidation reaction step takes place via recombination with under-coordinated lattice oxygen to form CO₂ (the so-called Mars–van Krevelen mechanism); the conversion probability of this process is as high as 80%. This mechanism leads to a (partial) reduction of the RuO₂ (110) surface, producing two-fold coordinatively unsaturated Ru sites (2f-cus Ru) via the removal of bridging O atoms. Therefore, equally important for being a good catalyst is the facile re-oxidation of the mildly reduced RuO₂ (110) surface by oxygen supply from the gas phase. A weakly held oxygen species was found to adsorb on top of the 1f-cus Ru atoms and to actuate the restoration of the reduced RuO₂ (110) surface. On the reduced RuO₂ (110) surface, CO molecules adsorb in bridge sites above the 2f-cus Ru atoms by 1.85 eV, while the CO bond strength over 1f-cus Ru atoms is 1.61 eV. Received: 27 March 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Chemical synthesis of nano-porous ruthenium oxide (RuO2) thin films for supercapacitor application

Ruthenium oxide (RuO₂) thin films have been prepared using single step chemical method containing Ru(III) Cl₃ solution in an aqueous medium at low temperature. The structural, morphological and optical properties have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and optical absorption technique. The XRD study revealed the formation of amorphous RuO₂ thin film. The surface examination by SEM showed formation of nano-porous material on the substrate. The TEM study revealed the formation of nanostructured material. The optical absorption studies showed the presence of direct band transition with band gap equal to 2.2 eV. The RuO₂ has proved its applicability in supercapacitor showing 50 F/g specific capacitance in 0.5 M H₂SO₄ at 20 mV/s scan rate.     

Al2O3 supported Ru catalysts prepared by thermolysis of Ru3(CO)12 for catalytic wet air oxidation

Low loading catalysts Ru/γ-Al₂O₃ and Ru-Ce/γ-Al₂O₃ were prepared by thermolysis of Ru₃(CO)₁₂ on γ-Al₂O_3. The catalysts were characterized by XPS, XRD and SEM. Two new Ru species (Ru_A and Ru_B) were detected during the Ru₃(CO)₁₂ decomposition process due to chemical interaction with the active OH groups on the surface of Al₂O₃ support, and the reduction of them can lead to more dispersed metallic phases. The sample was completely decomposed at 673 K in H₂, and RuO₂ was formed with minor amounts of Ru⁰. When the temperature was increased to 773 K to heat the sample, the ratio of Ru⁰ to RuO₂ increased. However, after the addition of CeO₂, only RuO₂ was detected on surface. The catalysts exhibited high activities in Catalytic Wet Air Oxidation (CWAO) of different organic compounds at high concentration such as isopropyl alcohol, phenol, acetic acids and N,N-dimethylformamide, which is attributed to the better dispersion of Ru particles and the addition of CeO₂ further enhanced number of effectively active sites on the cluster-derived catalyst surface.     

The key Cl− ligand for metal‐to‐ligand charge transfer in mononuclear terpyridine ruthenium(II) and binuclear ruthenium(II) tetrapyridylpyrazine complexes

Electronic structures of binuclear ruthenium complexes [Ru₂(terpy)₂(tppz)]⁴⁺ ( 1A ) and [Ru₂Cl₂(L)₂(tppz)]²⁺ {L = bpy ( 2A ), phen ( 3A ), and dpphen ( 4A )} were studied by density functional theory calculations. Abbreviations of the ligands (Ls) are bpy = 2,2′‐bipyridine, phen = 1,10‐phenanthroline, dpphen = 4,7‐diphenyl‐1,10‐phenanthroline, terpy = 2,2′:6′,2″‐terpyridine, and tppz = tetrakis(2‐pyridyl)pyrazine. Their mononuclear reference complexes [Ru(terpy)₂]²⁺ ( 1B ) and [RuClL(terpy)]⁺ {L = bpy ( 2B ), phen ( 3B ), and dpphen ( 4B )} were also examined. Geometries of these mononuclear and binuclear Ru(II) complexes were fully optimized. Their geometric parameters are in good agreement with the experimental data. The binuclear complexes were characterized by electrospray ionization mass spectrometry, UV–Vis spectroscopy, and cyclic voltammograms. Hexafluorophosphate salts of binuclear ruthenium complexes of 3A and 4A were newly prepared. The crystal structure of binuclear complex 1A (PF₆)₄ was also determined. Orbital interactions were analyzed to characterize the metal‐to‐ligand charge‐transfer (MLCT) states in these complexes. The Cl^? ligand works to raise the orbital energy of the metal lone pair, which leads to the low MLCT state. Copyright © 2011 John Wiley & Sons, Ltd.