Effect of surface tension gradient on gas hold-up enhancement in aqueous solutions of electrolytes

The effect of addition of electrolytes on gas hold-up of air/water system was investigated experimentally in a laboratory scale bubble column. The experiments were carried out with four electrolytes, namely, NaCl, MgSO₄·7H₂O, Na₂SO₄ and CaCl₂·2H₂O and the concentrations of the solutions were varied from 0 to 0.3 mol/l. Enhancement of gas hold-up was observed for all four electrolytes at concentrations less than 0.1 mol/l. With the increase in concentration, the gas hold-up showed two different trends; in Na₂SO₄ and CaCl₂·2H₂O solutions, gas hold-up formed a sharp peak after the enhancement and leveled off at a value somewhat higher than that in water, whereas in NaCl and MgSO₄·7H₂O solutions, gas hold-up leveled off immediately after the enhancement without forming any peak. Experiments were also conducted to measure the surface tensions of the solutions with special focus in the low concentration region. A strong relation between the gas hold-up enhancement and the change of surface tension with the addition of electrolyte was found. It was also observed that the concentration at which maximum value of C(dσ/dC)² i.e. (concentration × surface tension gradient with respect to concentration²) is obtained corresponds to the concentration at which maximum gas hold-up enhancement occurs.     

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.     

Hydrothermal synthesis of binary Ru-Ti oxides with excellent performances for supercapacitors

Hydrous, crystalline, binary (Ru-Ti)O₂·nH₂O with compositions equal to the ratios of metallic ions in the precursor solutions are successfully synthesized by a mild hydrothermal process. The maximum utilization of RuO₂·nH₂O (ca. 793 F/g) occurs at the composition of 60 M% TiO₂·nH₂O although phase separation is clearly found for this TiO₂-enriched binary oxides. The nano-structured architecture with a high BET surface area (ca. 253 m²/g) of the hydrothermal-derived (Ru-Ti)O₂·nH₂O with annealing at 200 °C favors the physical adsorption of water and maintains a high water content which is novel and never found before. Due to this novel nanostructure, the annealed (Ru-Ti)O₂·nH₂O synthesized by means of the hydrothermal process exhibits excellent performances (i.e., high utilization of RuO₂, high power property, and long cycle life) for supercapacitors.     

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.     

Novel attempts for the synthesis of calcium sulfate hydrates in calcium chloride solutions under atmospheric conditions

The medical grade calcium sulfate is widely used in clinical applications for treating bone defects. A high-purity and predictable calcium sulfate (CS) synthesis process is desirable in the medical industry. The objective of this study was to develop a one-pot method for the direct preparation of calcium sulfate hemihydrates (CSHs; such as the α- and β-forms) in a CaCl₂ solution. CS was synthesized by mixing K₂SO₄ and Ca(NO₃)₂·4H₂O under various CaCl_2(aq) concentrations and reaction temperatures under atmospheric pressure. The calcium sulfate dihydrate (CSD) was found to be an intermediate phase that converts to CSH during the synthesis process, and α-CSH was gradually transformed from β-CSH over time. Moreover, the kinetic of CSD conversion to CSH was strongly accelerated by increasing the CaCl₂ concentration. As the reaction temperature was fixed in 90 °C, the form of the CS reactant with an increase in the CaCl₂ concentration was in the following sequence: CSD → CSD + β-CSH → CSD + β-CSH + α-CSH → α-CSH. In this study, the synthesis processing window of the CS reactant was established according the test results, and it is worth noting that all phases of CS hydrate could be synthesized with this system and well predicted by the constructed processing window.     

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).     

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.     

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.     

Hydration of tricalcium aluminate in the presence of various amounts of calcium sulphite hemihydrate: Conductivity tests

Hydration of calcium aluminate C₃A (3CaO·Al₂O₃) in the presence of calcium sulphite hemihydrate (CaSO₃·0.5H₂O), with the molar ratio of substrates close to 1, produces the C₃A·CaSO₃·11H₂O calcium monosulphite aluminate phase. Small amounts of calcium sulphite added to calcium aluminate (the ratio of CaSO₃·0.5H₂O / C₃A equalling 0 : 1) change the rate of C₃A hydration and influence the whole reaction. Reaction processes for various ratios of the C₃A-CaSO₃·0.5H₂O mixture were examined in pure distilled water with a considerable amount of liquid W / S = 38-50 (constant W / C₃A). Processes in the liquid phase were monitored with conductivity equipment, and the XRD analysis was used to identify the phases precipitated during the examined reactions.     

Single crystal nanobelts of V3O7·H2O: A lithium intercalation host with a large capacity

In this study, single crystal V₃O₇·H₂O nanobelts were successfully synthesized using a simple hydrothermal route, in which templates or catalysts were absent. The synthesized V₃O₇·H₂O nanobelts are highly crystalline and have lengths up to several tens of micrometers. The width and thickness of the nanobelts are found to be about 30-50 and 30 nm, respectively. A lithium battery using V₃O₇·H₂O nanobelts as the positive electrode exhibits a high initial discharge capacity of 409 mAh g⁻¹, corresponding to the formation of Li_xV₃O₇·H₂O (x = 4.32). Such a high degree of electrochemical performance is attributed to the intrinsic properties of the single-crystalline V₃O₇·H₂O nanobelts.     

Kinetics of HCl emission from inorganic chlorides in simulated municipal wastes incineration conditions

Sulfation of inorganic chlorides (KCl, CaCl₂ and NaCl) of mean particles size 75-125 μm was done in a lab-scale tubular reactor. The reactor was supplied with a mixed gas consisting of SO₂:0.3-1.3 vol%, O₂:2.5-15 vol% and H₂O:5-20 vol% in the temperature range 623-1123 K. The rates of HCl emission from the particles samples were measured and compared.It was found that at 1023 K, an HCl of about 8000 ppm was emitted from CaCl₂ and this emission concentration was more than 3 times as high as those from NaCl and KCl. The reaction kinetic parameters such as rate constants and reaction orders with respect to SO₂, O₂ and H₂O partial pressures were determined for the sulfation of NaCl, KCl and CaCl₂. The rate of HCl emission from KCl, CaCl₂ and NaCl became increasingly high when the temperature was raised above 923 K and depend on SO₂, O₂, and H₂O partial pressures. It was considered that an increase in the rate of HCl emission from the inorganic chlorides at temperatures above 923 K was caused by a change in the reaction mode from a gas-solid to a gas-liquid and/or a gas phase reaction, owing to a partial melting and/or volatilization of the inorganic chlorides.     

Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor

Commercially available low cost exfoliated graphite (EG, nominal diameter 130 μm) was used as a conductive substrate for electrochemical capacitor of hydrated Mn(IV) oxide, MnO₂·nH₂O. The MnO₂·nH₂O-EG composites were prepared by addition of EG to potassium permanganate solution, followed by 1 h stirring and then slow addition of manganese(II) acetate solution. By this procedure submicrometer or smaller sized MnO₂·nH₂O particles having mesopores of 6-12 nm in diameter were formed on the graphite sheets of EG. Although EG alone showed only about 2 F g⁻¹, the composites showed good rectangular cyclic voltammograms at 2-20 mV s⁻¹ in 1 mol L⁻¹ Na₂SO₄. The capacitance per net amount of MnO₂ increased proportionally with EG content, that is, utilization ratio of MnO₂ increased with EG content. The composites of MnO₂·nH₂O and smaller diameter of EG (nominal diameter 45 μm) or artificial graphite powder (average diameter 3.7 μm) showed fairly good performance at 2 mV s⁻¹, but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors.     

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.     

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).     

Preparation of hydroxyapatite coating on smooth implant surface by electrodeposition

A novel process for the deposition of a hydroxyapatite (HA) coating on a smooth implant surface has been developed. Specimens were firstly subjected to electrodeposition at −1.8 V (versus Ag/AgCl) in a mixed solution of 0.042 M Ca(NO₃)₂·4H₂O and 0.025 M NH₄H₂PO₄ at 85 °C for 5 s, and then post-treated in 1 M NaOH solution for 30 min. The experimental results showed the specimens prepared by the designed process to have better adhesion properties than those prepared by the traditional electrodeposition process.     

Properties of sol–gel derived Al2O3–15 wt.% ZrO2 (3 mol% Y2O3) nanopowders using two different precursors

An alumina–15 wt.% zirconia (3 mol% yttria) nanopowder was synthesized by sol–gel method using salt and alkoxide as precursors. Al(NO₃)₃·9H₂O, ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O were used as salt precursors and Al(OC₄H₉)₃ and Zr(OC₄H₉)₄ were used as alkoxide precursors. The dried gels of two precursors were heat treated in the range of 450–1350 °C. The powders produced by alkoxide precursors calcined at 750 °C were in the range of 15–75 nm, while those prepared by salt precursors had the size in the range of 30–90 nm. The former powders had a higher surface area and smaller mean pore diameter compared with the later powder, i.e. 152 m²/g and 5.63 nm comparing with 121 m²/g and 9.79 nm, respectively. Therefore phase transformation in the former occurred in lower temperatures.     

Determination of the eutectic solubility lines of the ternary system NaHCO3–Na2CO3–H2O

A crystallizer was built and a procedure developed to accurately measure the eutectic solubility lines where ice and salt coexist in equilibrium with the solution, for potential application of Eutectic Freeze Crystallization. The eutectic solubility lines of the ternary system NaHCO₃–Na₂CO₃–H₂O were determined experimentally and calculated with the extended UNIQUAC model. The extended UNIQUAC model describes the experimental data quite well. Anhydrous NaHCO₃ and Na₂CO₃·10H₂O were the only two types of crystals present in equilibrium with ice crystals in the ternary system. At the quadruple point NaHCO₃ and Na₂CO₃·10H₂O are in equilibrium with a solution of about 4.34 wt% of Na₂CO₃ and 4.77 wt% of NaHCO₃ at −3.32 °C. The anhydrous NaHCO₃ crystals were needle shaped with lengths between 5 and 10 μm, that were agglomerated into particles of about 100–300 μm, while the Na₂CO₃·10H₂O crystals were hexagonally shaped with sizes between 100 and 500 μm.     

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.     

Large-scale synthesis of WO3 nanoplates by a microwave-hydrothermal method

Tungsten oxide (WO₃) nanoplates were synthesized by a 270 W microwave-hydrothermal reaction of Na₂WO₄·2H₂O and citric acid (C₆H₈O₇·H₂O) in deionized water. X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were used to reveal the synthesis of WO₃ complete rectangular nanoplates in the solution of 0.2 g citric acid for 180 min, with O-W-O FTIR stretching modes at 819 and 741 cm⁻¹, and two prominent O-W-O Raman stretching modes at 804 and 713 cm⁻¹. The 2.71 eV indirect energy gap, and 430-460 nm blue emission wavelength range of WO₃ complete rectangular nanoplates were determined using UV-visible and photoluminescence (PL) spectrometers. The formation mechanism was also proposed according to the experimental results.     

Thermodynamic modeling of the dissociation conditions of hydrogen sulfide clathrate hydrate in the presence of aqueous solution of inhibitor (alcohol,salt or ethylene glycol)

Hydrate dissociation conditions of hydrogen sulfide in the presence of aqueous solution of thermodynamic inhibitor (methanol, ethanol, ethylene glycol, NaCl, KCl and CaCl₂) is modeled in this communication. A thermodynamic model is developed to correlate the hydrate dissociation conditions for the systems of H₂S + water + salt (single and mixed salts of NaCl, KCl and CaCl₂), H₂S + water + alcohol (methanol or ethanol), H₂S + water + ethylene glycol and H₂S + water + mixed salt, and methanol/ethylene glycol. Extended-UNIQUAC (e-UNIQUAC) approach is used for modeling of the activity coefficient of water in aqueous phase. The structural parameters of e-UNIQUAC model are extracted from literature but interaction parameters of this model are obtained by fitting the model with experimental data. The results of the present model are in satisfactory agreement with experimental data.