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.     

Stable Ti/RuO2-Sb2O5-SnO2 electrodes for O2 evolution

RuO₂-based electrodes are generally known to be unstable for O₂ evolution. In this paper, a stable type of RuO₂-based electrode, Ti/RuO₂-Sb₂O₅-SnO₂, is demonstrated for O₂ evolution. In the ternary oxide coating, RuO₂ serves as the catalyst, SnO₂ as the dispersing agent, and Sb₂O₅ as the dopant. The accelerated life test showed that the Ti/RuO₂-Sb₂O₅-SnO₂ electrode containing 12.2 molar percent of RuO₂ nominally in the coating had a service life of 307 h in 3 M H₂SO₄ solution under a current density of 0.5 A cm⁻² at 25 °C, which is more than 15 times longer than other types of RuO₂-based electrodes. Instrumental analysis indicated that RuO₂-Sb₂O₅-SnO₂ was a solid solution with a compact structure, which contributed to the stable nature of the electrode.     

Application of Ti/RuO2-Ta2O5 electrodes in the electrooxidation of ethanol and derivants: Reactivity versus electrocatalytic efficiency

The influence of the preparation method on the performance of RuO₂-Ta₂O₅ electrodes was evaluated toward the ethanol oxidation reaction (EOR). Freshly prepared RuO₂-Ta₂O₅ thin films containing between 30 and 80 at.% Ru were prepared by two different methods: the modified Pechini-Adams method (DPP) and standard thermal decomposition (STD). Electrochemical investigation of the electrode containing RuO₂-Ta₂O₅ thin films was conducted as a function of electrode composition in a 0.5-mol dm⁻³ H₂SO₄ solution, in the presence and absence of ethanol and its derivants (acetaldehyde and acetic acid).At a low ethanol concentration (5 mmol dm⁻³), ethanol oxidation leads to high yields of acetic acid and CO₂. On the other hand, an increase in ethanol concentration (15-1000 mmol dm⁻³) favors acetaldehyde formation, so acetic acid and CO₂ production is hindered, in this case.Electrodes prepared by DPP provide higher current efficiency than STD electrodes for all the investigated ethanol concentrations. This may be explained by the increase in electrode area obtained with the DPP preparation method compared with STD.     

Li diffusion in LiNi0.5Mn0.5O2 thin film electrodes prepared by pulsed laser deposition

Kinetic and transport parameters of Li ion during its extraction/insertion into thin film LiNi_0.5Mn_0.5O₂ free of binder and conductive additive were provided in this work. LiNi_0.5Mn_0.5O₂ thin film electrodes were grown on Au substrates by pulsed laser deposition (PLD) and post-annealed. The annealed films exhibit a pure layered phase with a high degree of crystallinity. Surface morphology and thin film thickness were investigated by field emission scanning electron microscopy (FESEM). The charge/discharge behavior and rate capability of the thin film electrodes were investigated on Li/LiNi_0.5Mn_0.5O₂ cells at different current densities. The kinetics of Li diffusion in these thin film electrodes were investigated by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). CV was measured between 2.5 and 4.5 V at different scan rates from 0.1 to 2 mV/s. The apparent chemical diffusion coefficients of Li in the thin film electrode were calculated to be 3.13 × 10⁻¹³ cm²/s for Li intercalation and 7.44 × 10⁻¹⁴ cm²/s for Li deintercalation. The chemical diffusion coefficients of Li in the thin film electrode were determined to be in the range of 10⁻¹²-10⁻¹⁶ cm²/s at different cell potentials by GITT. It is found that the Li diffusivity is highly dependent on the cell potential.     

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.     

Planar ultracapacitors of miniature interdigital electrode loaded with hydrous RuO2 and RuO2 nanorods

Planar ultracapacitors of miniature interdigital electrode are prepared, using the standard technologies of photolithography and reactive sputtering. The ultracapacitor is denoted by its deposition sequence, for example, hRuO₂NRGT indicates pseudocapacitive hydrous RuO₂ (hRuO₂) and RuO₂ nanorods (NR) are grown on an interdigital stack layer of gold (G) and titania (T). The connection between structure and performance is studied through contrasting the hRuO₂NRGT capacitor with other capacitors built on a less conducting stack layer or without hydrous RuO₂ filling the gaps between the nanorods. The stack layer can be a major source of cell resistance. For instance, a buffer layer of titania could be utilized between the capacitive RuO₂ and the Au current collector to overcome the delamination problem. But the less conductive titania also makes its cyclic voltammograms (CV) elliptical and tilted, and causes a pronounced IR drop during the cell discharging. In contrast, CV of the hRuO₂NRGT capacitor on a conductive stack layer takes the shape of horizontal rectangle, and its discharge curve shows no sign of IR drop. Filling hydrous RuO₂ into the gap reduces the cell resistance between nanorods, improves the discharge performance as well. The power output of hRuO₂NRGT, with the two resistances minimized, is 30.6 μW at 75 μA and 1.23 μW at 5 μA.     

Electrochemical capacitance of nanoporous hydrous RuO2 templated by anionic surfactant

The nanoporous RuO₂·3.38H₂O was synthesized with a surfactant template using sodium dodecyl sulfate. The surface area of the material amounted to 220 m² g⁻¹ while the maximum specific capacitance obtained was 870 Fg⁻¹ at a scan rate of 10 mV s⁻¹. The specific capacitance of nanoporous RuO₂·3.38H₂O electrode exhibits enhancement, compared with other porous RuO₂ materials synthesized by different methods. The nanoporous RuO₂·3.38H₂O is a very promising material for high performance capacitance.     

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.     

NiO-based composite electrode with RuO2 for electrochemical capacitors

NiO/RuO₂ composite materials were prepared for use in electrochemical capacitors (ECs) by co-precipitation method followed by heat treatment. X-ray diffraction (XRD) spectra indicated that no new structural materials were formed and ruthenium oxide particles were coated by NiO particles. RuO₂ partly introduced into NiO-based electrode had improved its electrochemical performance and capacitive properties by using electrochemical measurements. A maximum specific capacitance of 210 F/g was obtained for NiO-based composite electrode with 10 wt.% RuO₂ in the voltage range from −0.4 to 0.5 V in 1 mol/l KOH solution. By comparison of effect of modified modes on the specific capacitance, chemically modified composite electrodes had more stable cycling properties than those of physically modified electrodes. After 200 cycles, specific capacitance of NiO-based chemical composite electrode with 5 wt.% RuO₂ kept 95% above, while that of physical electrode was only 79% of initial specific capacitance.     

Electrodeposited PbO2 thin film as positive electrode in PbO2/AC hybrid capacitor

Lead dioxide (PbO₂) thin films were prepared on Ti/SnO₂ substrates by means of electrodeposition method. Galvanostatic technique was applied in PbO₂ film formation process, and the effect of deposition current on morphology and crystalline form of the PbO₂ thin films was studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The energy storage capacity of the prepared PbO₂ electrode was investigated by means of cyclic voltammetry (CV) and charge/discharge cycles, and a rough surface structure PbO₂ film was selected as positive electrode in the construction of PbO₂/AC hybrid capacitor in a 1.28 g cm⁻³ H₂SO₄ solution. The electrochemical performance was determined by charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results showed that the PbO₂/AC hybrid capacitor exhibited high capacitance, good cycling stability and long cycle life. In the voltage range of 1.8-0.8 V during discharge process, considering the weight of all components of the hybrid capacitor, including the two electrodes, current collectors, H₂SO₄ electrolyte and separator, the specific energy and power of the device were 11.7 Wh kg⁻¹ and 22 W kg⁻¹ at 0.75 mA cm⁻², and 7.8 Wh kg⁻¹ and 258 W kg⁻¹ at 10 mA cm⁻² discharge currents, respectively. The capacity retains 83% of its initial value after 3000 deep cycles at the 4 C rate of charge/discharge.     

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.     

Supercapacitive properties of polyaniline/hydrous RuO2 composite electrode

A composite electrode based on polyaniline (PANI) and hydrous RuO₂ is prepared by electrochemical deposition of PANI onto hydrous RuO₂ (PANI/RuO₂) and its supercapacitive properties are investigated using cyclic voltammetry. The specific capacitances of PANI/RuO₂ and hydrous RuO₂ electrodes are determined to be 708 and 517 F g⁻¹ at 5 mV s⁻¹, respectively. Simple electrodeposition of PANI on the hydrous RuO₂ can achieve comparatively greater capacitance values.     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.     

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.     

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.     

Synthesis and characterization of highly stable optically passive CeO2-ZrO2 counter electrode

Transparent and adherent CeO₂-ZrO₂ thin films having film thicknesses ∼543-598 nm were spray deposited onto the conducting (fluorine doped tin oxide coated glass) substrates from a blend of equimolar concentrations of cerium nitrate hexahydrate and zirconium nitrate having different volumetric proportions (0-6 vol.% of Zr) in methanol. CeO₂-ZrO₂ films were polycrystalline with cubic fluorite crystal structure and the crystallinity was improved with increasing ZrO₂ content. Films were highly transparent (T ∼ 92%), showing decrease in band gap energy from 3.45 eV for pristine CeO₂ to 3.08-3.14 eV for CeO₂-ZrO₂ films. The different morphological features of the film obtained at various CeO₂-ZrO₂ compositions had pronounced effect on the ion storage capacity and electrochemical stability. CeO₂-ZrO₂ film prepared at 5 vol.% Zr concentration exhibited higher ion storage capacity of 24 mC cm⁻² and electrochemical stability of 10,000 cycles in 0.5 M LiClO₄ + PC electrolyte due to its film thickness (584 nm) coupled with relatively larger porosity (8%). The optically passive behavior of such CeO₂-ZrO₂ film (with 5 vol.% Zr) is affirmed by its negligible transmission modulation irrespective of repeated Li⁺ and electron insertion/extraction. The coloration efficiency of spray deposited WO₃ thin film is found to enhance from 47 to 107 cm² C⁻¹ when CeO₂-ZrO₂ is coupled as a counter electrode with WO₃ in an electrochromic device (ECD). These films can be used as stable ‘passive’ counter electrodes in electrochromic smart windows as they retain full transparency in both the oxidized and reduced states and ever-reported longevity.     

Investigations into capacity fading as a result of a Jahn-Teller distortion in 4 V LiMn2O4 thin film electrodes

This study reports the onset of the Jahn-Teller distortion in 4 V LiMn₂O₄ thin film electrodes that was investigated using an in situ bending beam method (BBM). The phase transformation during lithium insertion/extraction could be detected using the BBM technique. The phase transformation between the cubic and tetragonal phases was depicted by the larger value of the compressive or tensile differential strain, which is consistent with a well-known phase transformation between those phases in 3 V LiMn₂O₄. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously. The potential ranges responsible for the Jahn-Teller distortion in 4 V range, which takes place at the electrode surface, was determined by the charge versus. differential strain (dε/dQ) curve. The onset of the Jahn-Teller distortion was observed at the end of the cathodic scan, and the relaxation of the Jahn-Teller distortion was observed at the beginning of anodic scan. Furthermore, the onset of the Jahn-Teller distortion was found to be dependent on the lithium ion insertion rate, which was controlled by the scan rate.     

Determination of the potential range responsible for the replacement of surface film on LiMn2O4

This study determined the potential range where the dissolution of a surface film on a thin film LiMn₂O₄ electrode, which forms during electrode synthesis, and the formation of a new surface film during cycling at room temperature using an in situ bending beam method (BBM) and an in situ electrochemical quartz crystal microbalance (EQCM) technique with cyclic voltammetry and a galvanostatic charge/discharge cycle. The electrolytes used were LiClO₄/EC-DEC, LiClO₄/PC and LiPF₆/EC-DMC. The deflectogram and massogram showed large peaks during the anodic scan only in the first cycle. These phenomena, were observed regardless of the electrolytes and scan rate used. The tensile strain and the mass reduction in the early stage of the strain peak and the mass peak are related to the dissolution of the initial surface film. In addition, the compressive strain and the mass increase are related to the formation of a new surface film during cycling. The potential ranges where the formation of the new surface film begins ranged from 4.03 to 4.1 V, which appears to terminate at the end of the first anodic scan, and was also observed during the galvanostatic charge/discharge cycle in the same potential range.     

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.     

Electrochemical performances of poly(3,4-ethylenedioxythiophene)-NiFe2O4 nanocomposite as electrode for supercapacitor

Poly 3,4-ethylenedioxythiophene (PEDOT)-based NiFe₂O₄ conducting nanocomposites were synthesized and their electrochemical properties were studied in order to find out their suitability as electrode materials for supercapacitor. Nanocrystalline nickel ferrites (5-20 nm) have been synthesized by sol-gel method. Reverse microemulsion polymerization in n-hexane medium for PEDOT nanotube and aqueous miceller dispersion polymerization for bulk PEDOT formation using different surfactants have been adopted. Structural morphology and characterization were studied using XRD, SEM, TEM and IR spectroscopy. Electrochemical performances of these electrode materials were carried out using cyclic voltammetry at different scan rates (2-20 mV/s) and galvanostatic charge-discharge at different constant current densities (0.5-10 mA/cm²) in acetonitrile solvent containing 1 M LiClO₄ electrolyte. Nanocomposite electrode material shows high specific capacitance (251 F/g) in comparison to its constituents viz NiFe₂O₄ (127 F/g) and PEDOT (156 F/g) where morphology of the pore structure plays a significant role over the total surface area. Contribution of pseudocapacitance (C_FS) arising from the redox reactions over the electrical double layer capacitance (C_DL) in the composite materials have also been investigated through the measurement of AC impedance in the frequency range 10 kHz-10 mHz with a potential amplitude of 5 mV. The small attenuation (∼16%) in capacitance of PEDOT-NiFe₂O₄ composite over 500 continuous charging/discharging cycles suggests its excellent electrochemical stability.