Microstructural and electrochemical characterization of RuO2/CNT composites synthesized in supercritical diethyl amine

A simple and efficient route to decorate carbon nanotubes (CNTs) with nanocrystalline RuO₂ has been developed. In this method, RuCl₃ · 3H₂O was oxidized into RuO₂ by oxygen in supercritical diethyl amine, and the produced RuO₂ deposited on CNTs, resulting in RuO₂/CNT nanocomposites. The as-prepared composites were structurally and morphologically characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM). TEM images showed that RuO₂ nanoparticles attached on CNTs had uniform shape and a narrow particle size distribution. The loading content and the size of RuO₂ particles on CNTs could be tuned by changing the mass ratio of RuCl₃ · 3H₂O/CNT. Electrochemical measurements by cyclic voltammetry demonstrated a substantial increment of the specific capacitance of CNTs due to a pseudocapacitance originated from the deposited RuO₂ nanoparticles.     

RuO2·xH2O/NiO composites as electrodes for electrochemical capacitors: Effect of the RuO2 content and the thermal treatment on the specific capacitance

RuO₂·xH₂O/NiO composites having RuO₂ contents in the range 0-100 wt.% have been prepared by a co-precipitation method. Structural, microstructural and textural transformations after heating the as-prepared composites at 200 and 600 °C have been followed by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms. At 200 °C the composites are made of micrometric particles in which nanometric crystallites of the two oxides are aggregated. The composites show microporosity (0.02-0.10 cm³/g), mesoporosity (0.07-0.12 cm³/g) and relatively high specific surface area (62-309 m²/g). At 600 °C the composites are fully dehydrated and RuO₂ has crystallized and segregated. Microporosity and mesoporosity as well as specific surface area are strongly decreased. Specific capacitance and specific surface area of the composites heated at 200 and 600 °C have been measured and discussed on the basis of the RuO₂ content. For comparison the specific capacitance and specific surface area of mixtures of NiO and RuO₂·xH₂O (or RuO₂) have been taken as references. The higher specific capacitance of the 200 °C-heated composites compared to the 600 °C-heated ones is due to the higher specific surface area of the former and the higher pseudocapacitance of RuO₂·xH₂O compared to RuO₂. The discussion reported in this work can be applied to other composites such as RuO₂·xH₂O/carbon and RuO₂·xH₂O/other oxides.     

Development of ceramic electrochemical sensor based on Bi2Ru2O7+x − RuO2 sub-micron oxide sensing electrode for water quality monitoring

Sub-micron Bi₂Ru₂O_7+x + RuO₂ oxide sensing electrodes (SE) for water quality sensors were prepared on platinised ceramic substrate of the sensor. Their morphology was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Sensing properties of the Bi₂Ru₂O_7+x + RuO₂-SE were investigated for potentiometric detection of pH and dissolved oxygen (DO) in water in the temperature range of 4–30 °C. Sensor was capable to measure DO from 0.5 to 8.0 ppm and pH from 2.0 to 13.0, respectively. The obtained results show acceptable linearity of the measuring characteristics. Long-term stability trial for Bi₂Ru₂O_7+x + RuO₂-SE revealed that bio-fouling can be one of the main destructive factors affecting the performance of the sensors in the long run. The screen-printing technology used in the multi-sensory implementation provides fundamental properties of miniaturization, reasonable accuracy and low cost.     

Effects of substrates on the capacitive performance of RuOx·nH2O and activated carbon-RuOx electrodes for supercapacitors

The electrochemical energy storage and delivery on the electrodes composed of hydrous ruthenium oxide (RuO_x·nH₂O) or activated carbon-hydrous ruthenium oxide (AC-RuO_x) composites are found to strongly depend on the substrate employed. The contact resistance at the active material-graphite interface is much lower than that at the active material-stainless steel (SS) mesh interface. Thin films of gold plus RuO_x·nH₂O deposited on SS meshes (RuO_x/Au/SS) are found to greatly improve the poor contact between SS meshes and electrode materials. The maximum specific capacitance (C_S,RuOx) of RuO_x·nH₂O, 1580 F g⁻¹ (measured at 1 mV s⁻¹), very close to the theoretic value, was obtained from an AC-RuO_x/RuO_x/Au/SS electrode with 10 wt.% sol-gel-derived RuO_x·nH₂O annealed in air at 200 °C for 2 h. The highly electrochemical reversibility, high-power characteristics, good stability, and improved frequency response of this AC-RuO_x/RuO_x/Au/SS electrode demonstrate its promising application potential in supercapacitors. The ultrahigh specific capacitance of RuO_x·nH₂O probably results from the uniform size distribution of RuO_x·nH₂O nanoparticles, ranged from 1.5 to 3 nm which is clearly observed from the high-resolution transmission electron microscopy (HRTEM).     

Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor

In this work, a novel type electrode based on RuO₂ nanoparticles-modified vertically aligned carbon nanotubes (RuO₂/MWCNTs) was prepared by magnetron sputtering deposition. This RuO₂/MWCNTs electrode not only shows a high capacity nature, but also possesses a good response to the pH value. The pH sensor based on the RuO₂/MWCNTs nanocomposite electrode exhibits some advantages over the conventional pH sensors. It shows good reproducibility, long-term storage stability (over 1 month) and linear response in the whole pH range (2-12) of Britton-Robinson (B-R) buffer solutions with near-Nernstian response (about −55 mV/pH). The hysteretic widths of the nanocomposite electrode are 6.4 mV, 5.1 mV and 10.2 mV in pH 7-4-7-10-7, pH 7-10-7-4-7 and pH 2-8-12-8-2 loop cycles, respectively. Moreover, the RuO₂/MWCNTs electrode displays an excellent anti-interference property and fast response time (less than 40 s). According to the electrochemical impedance measurements, the pH sensing properties of the RuO₂/MWCNTs electrode were also discussed.     

Highly dispersed hydrous ruthenium oxide in poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) for supercapacitor electrode

Hydrous RuO₂ particles were electrochemically loaded into poly(3,4-ethylenedioxythiophene) doped poly(styrene sulfonic acid), PEDOT-PSS, matrix by employing various potential cycles in cyclic voltammetry and to fabricate the PEDOT-PSS-RuO₂·xH₂O electrode. The amount of hydrous RuO₂ particles loaded into the PEDOT-PSS matrix was easily controlled by varying the number of potential cycles. Scanning electron microscopy photographs reveal a uniform dispersion of hydrous RuO₂ particles in the porous structure of PEDOT-PSS matrix. Raman spectrum confirms the incorporation of hydrous RuO₂ into PEDOT-PSS matrix. Chronopotentiometry and cyclic voltammetry were employed in 0.5 M H₂SO₄ to evaluate the capacitor properties. Specific capacitance values were determined by chronoamperometry. An increasing trend in specific capacitance with loaded amount of hydrous RuO₂ particles in PEDOT-PSS was noticed. A maximum specific capacitance of 653 F/g was achieved.     

Hydrothermal synthesis and electrochemical capacitance of RuO2·xH2O loaded on benzenesulfonic functionalized MWCNTs

Amorphous RuO₂·xH₂O was well coated on the benzenesulfonic functionalized multi-wall carbon nanotubes (f-MWCNTs) successfully via hydrothermal method. The decorated benzenesulfonic groups served as a bifunctional role both for solubilizing and dispersing MWCNTs into aqueous solution and for tethering Ru³⁺ precursor to facilitate the following uniform chemical deposition of RuO₂·xH₂O. The electrochemical performance of RuO₂/f-MWCNTs and utilization of RuO₂·xH₂O were evidenced by cyclic voltammetry and galvanostatic charge/discharge tests. The specific capacitance of 1143 Fg⁻¹ for RuO₂·xH₂O was obtained from RuO₂/f-MWCNTs with 32 wt.% RuO₂·xH₂O, which was much higher than that of just 798 Fg⁻¹ for the RuO₂/p-MWCNTs. Even though the RuO₂·xH₂O loading increases to 45 wt.%, the utilization of RuO₂·xH₂O still possesses as high as 844.4 Fg⁻¹, indicating a good energy capacity in the case of high loading.     

Textural and pseudocapacitive characteristics of sol-gel derived RuO2·xH2O: Hydrothermal annealing vs. annealing in air

Hydrous ruthenium dioxide (RuO₂·xH₂O) prepared in a modified sol-gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO₂·xH₂O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO₂·xH₂O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO₂·xH₂O are attributable to the high-pressured, water-enriched surroundings which restrain coalescence of RuO₂·xH₂O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO₂·xH₂O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO₂·xH₂O with hydrothermal annealing at 225 °C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 F g⁻¹.     

Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes

Composites are prepared by deposition of nanoparticles of RuO₂·xH₂O (1-4 nm) on two carbons: microporous carbon (1.3 nm of average micropore size) and mesoporous carbon (11 nm of average mesopore size). Two-preparation procedures are used: (i) procedure A consisting of repetitive impregnations of the carbons with RuCl₃·0.5H₂O solutions, and (ii) procedure B based on impregnation of the carbons with Ru(acac)₃ vapour. The procedure B leads to supported RuO₂·xH₂O particles that appear more crystalline than those obtained by the procedure A. Specific capacitance and specific surface area of the composites are discussed as functions of the RuO₂ content, and different dependences for the composites derived from the two carbons are found. Mesoporous carbon is better support than microporous carbon. Procedure A leads to supported RuO₂·xH₂O particles with higher specific capacitance than the particles deposited by procedure B.     

Improved antifouling resistance of electrochemical water quality sensors based on Cu2O-doped RuO2 sensing electrode

Improvement in both sensor's characteristics and antifouling resistance has been achieved by utilising Cu₂O as a dopant to the sub-micron-sized RuO₂ sensing electrode (SE) of the water quality sensors. 20 mol% Cu₂O-doped RuO₂-SE has exhibited a linear response to dissolved oxygen (DO) between 0.5 and 8.0 ppm at various temperatures with the sensitivity slope of −46 mV/decade. This paper describes the structural properties and characteristics of thin-film Cu₂O-doped RuO₂-SE subjected to a field trial in sewerage environment for three months. Selectivity measurements revealed that the presence of PO₄³⁻, SO₄²⁻, Ca²⁺, Mg²⁺, Li⁺, NO³⁻, F⁻, Na⁺, K⁺ and Cl⁻ in the test solution had no significant effect on the sensor's emf. Long-term stability test in harsh sewerage environment demonstrated that the Cu₂O-doped RuO₂-SE possesses improved antifouling resistance compared to RuO₂-SE. Morphology and crystalline structure of Cu₂O-doped RuO₂-SE were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) technique and Raman spectroscopy.     

Two-step hydrothermal synthesis of Ru-Sn oxide composites for electrochemical supercapacitors

A two-step hydrothermal process was developed to synthesize hydrous 30RuO₂-70SnO₂ composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol-gel procedure. A very high specific capacitance of RuO₂ (C_S,Ru), ca. 1150 F g⁻¹, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150 °C for 2 h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500 mV s⁻¹), demonstrating its excellent power property. From Raman, UV-vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (Ru_δSn_1−δ)O₂·xH₂O onto partially dissolved SnO₂·xH₂O and the formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second step. This effect significantly promotes the utilization of RuO₂ (i.e., very high C_S,Ru). The excellent capacitive performances, very similar to that of RuO₂·xH₂O, suggest the deposition of RuO₂-enriched (Ru_δSn_1−δ)O₂·xH₂O onto SnO₂·xH₂O seeds as well as the individual formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second hydrothermal step.     

Anodic deposition of hydrous ruthenium oxide for supercapaciors: Effects of the AcO concentration, plating temperature, and oxide loading

Effects of the sodium acetate (NaCH₃COO, denoted as NaAcO) concentration, plating temperature, and oxide loading on the pseudocapacitive characteristics of hydrous ruthenium oxide (denoted as RuO₂·xH₂O) films anodically plated from aqueous RuCl₃·xH₂O media were systematically investigated in this work. The electrochemical behavior of RuO₂·xH₂O with annealing in air at 200 °C for 2 h is approximately independent of the NaAcO concentration and plating temperature although a negative shift in the onset and peak potentials of deposition with rising the plating temperature is found. The morphologies and adhesion of RuO₂·xH₂O deposits strongly depend on the deposition rate which is obviously influenced by varying the above two deposition variables. The specific capacitance of RuO₂·xH₂O is monotonously decreased from 760 to 505 F g⁻¹ when the oxide loading is gradually increased from 0.34 to 1.0 mg cm⁻², due to the longer pathways of both electrons and protons during the redox transitions. The XRD and Raman spectroscopic analyses reveal the extremely localized crystalline nature of as-deposited RuO₂·xH₂O. All RuO₂·xH₂O deposits show the ideal pseudocapacitive characteristics, definitely illustrating the merits of RuO₂·xH₂O prepared by anodic deposition without considering the advantages of its simplicity, one-step, reliability, low cost, and versatility for electrode preparation.     

Evaluation of the pseudocapacitance in RuO2 with a RuO2/GC thin film electrode

The pseudocapacitance of nanocrystalline RuO₂ with BET surface area of 42 m² g⁻¹ was evaluated using a RuO₂ modified Glassy Carbon (RuO₂/GC) thin film electrode. The charge storage behavior of the RuO₂/GC thin film electrode was studied from fast to slow scan cyclic voltammetry between various potential windows. The utilization of the thin film electrode method for nanocrystalline RuO₂ with known specific surface area allowed a semi-quantitative understanding of the electric double-layer capacitance (C_dl), adsorption related charge (C_ad), and the irreversible redox related charge (C_irr) per unit mass and surface area of RuO₂. Comparison of the cyclic voltammograms between different voltage windows revealed that the contribution from C_irr is especially dominant below 0.4 V (versus RHE) at slow scan rates.     

Effect of lanthanide doping on crystal phase and near-infrared to near-infrared upconversion emission of Tm doped KF–YbF3 nanocrystals

Tm³⁺ doped KF–YbF₃ nanocrystals were synthesized by a hydrothermal method using oleic acid as a stabilizing agent at 190 °C. The influence of Gd³⁺ and Sm³⁺ content on the phase structure and upconversion (UC) emission of the final products was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. XRD analyses and TEM observations evidence that the phase and size of the as prepared Tm³⁺ doped KF–YbF₃ nanocrystals are closely related to the Gd³⁺ doping content. Without Gd³⁺ impurity, the undoped nanocrystals crystallize in orthorhombic KYb₂F₇ with an average diameter of 42 nm. When the Gd³⁺ doping is below 10 mol%, the orthorhombic KYb₂F₇ nanocrystals grow up. However, with Gd³⁺ addition beyond about 30 mol%, the complete phase transformation from orthorhombic KYb₂F₇ to cubic KGdF₄ occurs in the final products. Under the excitation of a 980 nm laser diode, the as prepared Tm³⁺ doped nanocrystals exhibit strong near-infrared UC emission at 800 nm. Particularly, the intensity of high energy UV and blue UC emissions of Tm³⁺ ions in Tm³⁺ doped KYb₂F₇ nanocrystals are selectively reduced compared to the NIR emission at 800 nm by co-doping a small amount of Sm³⁺ ions into the host matrix. Possible dynamic processes for UC emissions in Tm³⁺ doped nanocrystals are discussed in detail.     

Highly crystalline graphene oxide nano-platelets produced from helical-ribbon carbon nanofibers

Graphene oxide nano-platelets were produced from helical-ribbon carbon nanofibers by oxidation with KMnO₄/H₂SO₄ and further exfoliation by ultrasonication. The KMnO₄ to carbon nanofiber ratio is 1:1. TEM and AFM were used to characterize the samples. TEM shows individual nanocrystals with straight edges and SAED yields a hexagonal spot pattern arrangement, attributed to graphene layers. The thickness of the nanocrystals, measured by AFM, is approximately 1.7 nm, which corresponds to a single layer of hydrated graphene oxide.     

Tin dioxide/carbon nanotube composites with high uniform SnO2 loading as anode materials for lithium ion batteries

SnO₂/multi-walled carbon nanotube (MWCNT) composites were prepared by the solvothermal method and subsequent heat treatment at 360 °C. The samples were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results on the higher SnO₂ content composite sample indicate that a uniform layer of SnO₂ nanocrystals with crystal size around 5 nm was deposited on the surface of the carbon nanotubes. The composite demonstrates a reversible lithium storage capacity of 709.9 mAh g⁻¹ at the first cycle and excellent cyclic retention up to 100 cycles as anode for lithium ion batteries.     

Synthesis of new N, N-diphenylhydrazone dyes for solar cells: Effects of thiophene-derived π-conjugated bridge

Four novel D-π-A hydrazone dyes (HT, HM, HE, and HO) with an N, N-diphenylhydrazone moiety as the electron donor, different thiophene-derived π-conjugated bridges and a cyanoacrylic acid moiety as the electron acceptor have been designed and synthesized for the application in dye-sensitized solar cells. The influences of thiophene-derived bridges on the photoelectrochemical and photovoltaic performance of these hydrazone dyes were investigated. Results demonstrate that the introduction of 3,4-dialkyloxythiophene could red-shift the dye’s absorption spectrum due to the enhancement of the electron-donating ability of π-conjugated bridges. Importantly, electrochemical impedance spectroscopy analysis reveal that 3,4-dialkyloxythiophene bridge could change the charge recombination resistance at the TiO₂/dye/electrolyte interface and as a result to improve the open-circuit photovoltage. Among the four dyes, HO exhibits the maximum power conversion efficiency of 5.83% (V_oc = 0.65 V, J_sc = 12.69 mA/cm², FF = 0.707) under simulated AM 1.5 irradiation (100 mW/cm²).     

Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors

Co₃O₄/RuO₂·xH₂O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step co-precipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO₃)₂·6H₂O and RuCl₃·0.5H₂O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 °C, Brunauer-Emmet-Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m² g⁻¹. The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 °C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM).     

Study of carbon-supported IrO2 and RuO2 for use in the hydrogen evolution reaction in a solid polymer electrolyte electrolyzer

Carbon-supported IrO₂ and RuO₂ were prepared using an incipient wetness method and were then calcinated at various temperatures. IrO₂/C and RuO₂/C are less expensive than the conventional Pt/C material and more stable than metal Ni in an acidic electrolyte. Moreover, IrO₂/C and RuO₂/C are not influenced by under potential deposition (UPD) and show lower sensitivity to poisoning by Ni or Fe impurities. The physical properties of IrO₂/C and RuO₂/C were investigated via XRD and TEM. Cyclic voltammograms (CV) and Tafel plots were used to provide information regarding surface redox reaction and electrocatalytic activity. The activity and durability of IrO₂/C and RuO₂/C were studied after prolonged potential cycling between −0.3 and 0.3 V_SCE. After comparison of Tafel plots of Pt/C and IrO₂/C after activation, it was observed that they have similar electrocatalytic activities in a hydrogen evolution reaction (HER). A single cell test with solid polymer electrolyte (SPE) proved that the performance of IrO₂/C (0.5 mg cm⁻²) was similar to that of Pt/C (0.5 mg cm⁻²).     

Mesoporous RuO2 for the next generation supercapacitors with an ultrahigh power density

The 3D mesoporous, well crystalline RuO₂ film prepared via the evaporation-induced self-assembled method (EISA) successfully demonstrates the extremely high power performances (e.g., excellent capacitive behavior at 10,000 mV s⁻¹, ultrahigh-frequency capacitive responses (the absence of a knee point in the Nyquist plot), and 2.6 MW kg⁻¹ with an acceptable energy density of 4.6 Wh kg⁻¹). These excellent capacitive performances were identified by means of voltammetric and electrochemical impedance spectroscopic (EIS) analyses. The mesoporous (with mean pore spacing of 18.1 nm) and crystalline nature of this film was characterized by means of the field emission scanning electron microscopy (FE-SEM), Brunaur-Emmett-Teller (BET) method, small-angle X-ray scattering (SAXRS), high-resolution transmission electron microscopy (HR-TEM), electron diffraction (ED), and X-ray diffraction (XRD) analyses. This mesoporous, well crystalline RuO₂ film constrains the redox transition to the superficial region meanwhile the tailored mesoporous structure increases the electrochemically active centers, promotes the penetration of electrolytes, provides the “proton reservoirs”, and enhances the rate of electron transport simultaneously for the ultrahigh power application. The specific capacitance of this mesoporous RuO₂ can be enhanced from 84 to 185 F g⁻¹ after the microwave-assisted hydrothermal treatment.