CTAB facilitated spherical rutile TiO2 particles and their advantage in a dye-sensitized solar cell

Spherical rutile TiO₂ particles (14–20 nm) and their corresponding well-defined round clusters (500–600 nm) were obtained by using a cationic surfactant cetyltrimethylammonium bromide (CTAB). The surfactant was employed in two stages, i.e., in the hydrolysis of TiCl₄ and then in the precipitation of the corresponding Ti(IV) polymers at approximately 46 °C. On the other hand, without CTAB in the hydrolyzing solution, irregular clusters consisting of typical ellipsoidal TiO₂ particles were produced. The advantage of such spherical rutile TiO₂ particles and clusters was examined in terms of photovoltaic characteristics of a dye-sensitized solar cell (DSSC). Significantly higher overall solar energy conversion efficiency was obtained for a DSSC using the film of these spherical rutile TiO₂ particles, compared with that of a cell using a TiO₂ film of ellipsoidal particles. A mechanism for the formation of these spherical rutile particles and clusters is proposed.     

Hydrogen production for fuel cells through methane reforming at low temperatures

Hydrogen production for fuel cells through methane (CH₄) reforming at low temperatures has been investigated both thermodynamically and experimentally. From the thermodynamic equilibrium analysis, it is concluded that steam reforming of CH₄ (SRM) at low pressure and a high steam-to-CH₄ ratio can be achieved without significant loss of hydrogen yield at a low temperature such as 550 °C. A scheme for the production of hydrogen for fuel cells at low temperatures by burning the unconverted CH₄ to supply the heat for SRM is proposed and the calculated value of the heat-balanced temperature is 548 °C. SRM with and/or without the presence of oxygen at low temperatures is experimentally investigated over a Ni/Ce–ZrO₂/θ-Al₂O₃ catalyst. The catalyst shows high activity and stability towards SRM at temperatures from 400 to 650 °C. The effects of O₂:CH₄ and H₂O:CH₄ ratios on the conversion of CH₄, the hydrogen yield, the selectivity for carbon monoxide, and the H₂:CO ratio are investigated at 650 °C with a constant CH₄ space velocity. Results indicate that CH₄ conversion increases significantly with increasing O₂:CH₄ or H₂O:CH₄ ratio, and the hydrogen content in dry tail gas increases with the H₂O:CH₄ ratio.     

Steam reforming of methane over unsupported nickel catalysts

This paper describes a study of steam reforming of methane using unsupported nickel powder catalysts. The reaction yields were measured and the unsupported nickel powder surface was studied to explore its potential as a catalyst in internal or external reforming solid oxide fuel cells. The unsupported nickel catalyst used and presented in this paper is a pure micrometric nickel powder with an open filamentary structure, irregular ‘fractal-like’ surface and high external/internal surface ratio. CH₄ conversion increases and coke deposition decreases significantly with the decrease of CH₄:H₂O ratio. At a CH₄:H₂O ratio of 1:2 thermodynamic equilibrium is achieved, even with methane residence times of only ∼0.5 s. The CH₄ conversion is 98 ± 2% at 700 °C and no coke is generated during steam reforming which compares favorably with supported Ni catalyst systems. This ratio was used in further investigations to measure the hydrogen production, the CH₄ conversion, the H₂ yield and the selectivity of the CO, and CO₂ formation. Methane-rich fuel ratios cause significant deviations of the experimental results from the theoretical model, which has been partially correlated to the adsorption of carbon on the surface according to TEM, XPS and elemental analysis. At the fuel: water ratio of 1:2, the unsupported Ni catalyst exhibited high catalytic activity and stability during the steam reforming of methane at low-medium temperature range.     

Efficiency enhancement in DSSC using metal nanoparticles: A size dependent study

The photoelectrode of Eosin-Y sensitised DSSC was modified by incorporating Au-nanoparticles to enhance the power conversion efficiency via scattering from surface plasmon polaritons. Size dependence of Au nanoparticle on conversion efficiency was performed in DSSC for the first time by varying the particle size from 20 to 94 nm. It was found that, the conversion efficiency is highly dependent on the size of the Au nanoparticles. For larger particles (>50 nm), the efficiency was found to be increased due to constructive interference between the transmitted and scattered waves from the Au nanoparticle while for smaller particles, the efficiency decreases due to destructive interference. Also a reduction in the V_oc was observed in general, due to the negative shifting of the TiO₂ Fermi level on the adsorption of Au nanoparticle. This shift was negligible for larger particles. When 94 nm size particles were employed the conversion efficiency was doubled from 0.74% to 1.52%. This study points towards the application of the scattering effect of metal nanoparticle to enhance the conversion efficiency in DSSCs.     

A shock tube study of the rate constants of HO2 and CH3 reactions

HO₂ and CH₃ are major intermediate species presented during the oxidation of natural gas at intermediate temperatures and high pressures. Previous theoretical calculations have identified several product channels for HO₂ and CH₃ reactions, with CH₃ + HO₂ → CH₃O + OH and CH₃ + HO₂ → CH₄ + O₂ being the dominant reaction pathways. Both reaction pathways play an important role in the kinetics of CH₄ oxidation as CH₃ + HO₂ → CH₃O + OH is a chain-branching reaction whereas CH₃ + HO₂ → CH₄ + O₂ a chain termination reaction.H₂O₂/CH₄/Ar mixtures were shock-heated to a temperature between 1054 and 1249 K near 3.5 atm to initiate the reaction. OH radicals yielded from H₂O₂ thermal decomposition react with H₂O₂ and CH₄ respectively to produce HO₂ and CH₃ in the reacting system. Using laser absorption spectroscopy, time-histories of H₂O, OH and HO₂ were measured behind reflected shock waves. The rate constant of reaction CH₃ + HO₂ → CH₃O + OH was determined to be 6.8 × 10¹² cm³ mol^?1 s^?1 with an uncertainty factor of 1.4. The rate constant of the competing CH₃ + HO₂ → CH₄ + O₂ reaction was determined to be 4.4 × 10¹² cm³ mol^?1 s^?1, with an uncertainty factor of 2.1. In addition, the rate constants of two other major reactions of the reacting system, H₂O₂ (+M) → 2OH (+M) and OH + CH₄ → CH₃O + OH, were found to have excellent agreement with values recommended in literature.     

Thermodynamic evaluation of methanol steam reforming for hydrogen production

Thermodynamic equilibrium of methanol steam reforming (MeOH SR) was studied by Gibbs free minimization for hydrogen production as a function of steam-to-carbon ratio (S/C = 0–10), reforming temperature (25–1000 °C), pressure (0.5–3 atm), and product species. The chemical species considered were methanol, water, hydrogen, carbon dioxide, carbon monoxide, carbon (graphite), methane, ethane, propane, i-butane, n-butane, ethanol, propanol, i-butanol, n-butanol, and dimethyl ether (DME). Coke-formed and coke-free regions were also determined as a function of S/C ratio.Based upon a compound basis set MeOH, CO₂, CO, H₂ and H₂O, complete conversion of MeOH was attained at S/C = 1 when the temperature was higher than 200 °C at atmospheric pressure. The concentration and yield of hydrogen could be achieved at almost 75% on a dry basis and 100%, respectively. From the reforming efficiency, the operating condition was optimized for the temperature range of 100–225 °C, S/C range of 1.5–3, and pressure at 1 atm. The calculation indicated that the reforming condition required from sufficient CO concentration (<10 ppm) for polymer electrolyte fuel cell application is too severe for the existing catalysts (T_r = 50 °C and S/C = 4–5). Only methane and coke thermodynamically coexist with H₂O, H₂, CO, and CO₂, while C₂H₆, C₃H₈, i-C₄H₁₀, n-C₄H₁₀, CH₃OH, C₂H₅OH, C₃H₇OH, i-C₄H₉OH, n-C₄H₉OH, and C₂H₆O were suppressed at essentially zero. The temperatures for coke-free region decreased with increase in S/C ratios. The impact of pressure was negligible upon the complete conversion of MeOH.     

Large scale hydrothermal synthesis and electrochemistry of ammonium vanadium bronze nanobelts

Single crystalline ammonium vanadium oxide bronze NH₄V₄O₁₀ nanobelts were synthesized by the hydrothermal treatment of H₂C₂O₄·2H₂O and NH₄VO₃ at 140 °C for 48 h. The NH₄V₄O₁₀ nanobelts were characterized using a combination of techniques including X-ray diffraction, transmission electron microscopy, selected area electronic diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy techniques. The as-obtained nanobelts are several microns long, typically 30–40 nm wide, and 10–20 nm thick. The electrochemical properties of the nanobelts were tested in cells with metallic lithium as the negative electrode, the first discharge capacity of 171.8 mAh g⁻¹ was achieved.     

Elaboration of a new anode material for all-solid state Zn/MnO2 protonic cells

A new composite consisting of a mixture of zinc and hydrated ammonium zinc sulfate, Tutton salt, has been elaborated and studied as anode material for all-solid state protonic cells. The results obtained point out the ability of (NH₄)₂Zn(SO₄)₂·6H₂O double salt to concomitantly exchange protons with the solid proton conducting electrolyte and accommodate Zn²⁺ cations issued from oxidation. Such features have been evinced by thermal and crystallographic characterizations. The anode composition has been optimized through a kinetic study on a three-electrode type-cell, showing that a 35 wt.% salt-based composite displays the minimum polarization. Zn/MnO₂ cells prepared using such a composite anode achieve relatively high specific capacity and energy of, more than 30 Ah kg⁻¹ and 40 Wh kg⁻¹, respectively.     

Sol–gel template synthesis of highly ordered MnO2 nanowire arrays

In this paper, anodic aluminum oxide (AAO) templates with uniform pore diameter and periodicity were fabricated using a two-step oxidizating method at a constant current of 1.5 A dm⁻² in a mixed solution of 0.5 M sulfate acid and 5 g L⁻¹ oxalic acid at room temperature, pore-widening was done in 5 wt.% phosphoric acid. Then, arrays of manganese dioxide nanowires were prepared by combining the AAO template and a sol–gel solution containing Mn(CH₃COO)₂ and citric acid. SEM was used to characterize the structure of AAO and MnO₂ nanowires. The results show that uniform length and diameter of MnO₂ nanowires were obtained, and the length and diameter of MnO₂ nanowires are dependent on the pore diameter and the thickness of the applied AAO template. The pore diameter of the AAO was about 70 nm, the diameter of the MnO₂ nanowires was 70 nm and the length of the MnO₂ nanowires was about 500–700 nm. XRD analysis indicate the MnO₂ nanowires was α-MnO₂ polymorph. The results of cyclic voltammetry indicated that the α-MnO₂ nanowires are promising electrode materials for application in supercapacitors, the specific capacity of the electrode was 165 F g⁻¹.     

Thermal characteristics of manganese (II) nitrate hexahydrate as a phase change material for cooling systems

The imbalance of electrical demand in summer due to cooling system demand is a big problem in many countries. One promising solution is shifting peak demand from early afternoon to night by utilizing natural cold energy resources such as cool outside air during night or running a refrigerator driven by midnight power. In these cases, using the thermal energy storage (TES) of phase change material (PCM) which has a melting point from 15 to 25 °C is one of the most effective ideas. However, few suitable PCMs for this temperature range are at present commercially available. This study aims to evaluate the potential of Mn(NO₃)₂ · 6H₂O (manganese (II) nitrate hexahydrate) as a new PCM for the TES of cooling systems. First, experiments on the modulation of the melting point of Mn(NO₃)₂ · 6H₂O and reduction of supercooling were made by dissolving small amounts of salts in the material. Consequently, MnCl₂ · 4H₂O was found to have good performance with regard to both modulation of the melting temperature and the heat of fusion. Next, a thermal response test was carried out by using a small cylindrical vessel. Results showed that the required temperature levels for charging and discharging the heat of this mixture were clarified. In addition, the price and safety of this material as a PCM are discussed.     

Research on photocatalytic H2 production from acetic acid solution by Pt/TiO2 nanoparticles under UV irradiation

The Pt/TiO₂ particles have been prepared by the photodeposition of Pt on TiO₂ surface and characterized by X-ray diffraction. Photocatalytic H₂ production from acetic acid (HAc) over Pt/TiO₂ in aqueous solution has been studied at ambient temperature under UV irradiation. The effects of operational variables such as Pt loading, photocatalyst concentration, HAc concentration, and solution pH, have been systematically investigated. The optimum conditions for H₂ production from HAc by Pt/TiO₂ were Pt loading 1.0 wt.%, Pt/TiO₂ concentration 0.22 g/L, HAc concentration 6.52 g/L and pH 1.0. The H₂ yield is 0.27 mol-H₂/mol-HAc obtained under prolonged time irradiation. Experimental results showed that the photocatalytic H₂ production activity could be enhanced remarkably by depositing a suitable amount Pt on TiO₂ surface. Based on our results, a new process for H₂ production from biomass can be achieved by coupling fermentative H₂ production with photocatalytic H₂ production. The process also provides a method for degradation of organic pollutants with simultaneous H₂ production. A possible mechanism for photocatalytic decomposition of HAc over Pt/TiO₂ was also proposed.     

100 t/a-Scale demonstration of direct dimethyl ether synthesis from corncob-derived syngas

Direct conversion of biomass-derived syngas (bio-syngas) to dimethyl ether (DME) at pilot-scale (100 t/a) was carried out via pyrolysis/gasification of corncob. The yield rate of raw bio-syngas was 40–45 Nm³/h with less than 20 mg/Nm³ of tar content when the feedrate of dried corncob was 45–50 kg/h. After absorption of O₂, S, Cl by a series of absorbers and partial removal of CO₂ by the pressure-swing adsorption (PSA) unit sequentially, the obtained bio-syngas (H₂/CO≈1) was directly synthesized to DME over Cu/Zn/Al/HZSM-5 catalyst in the fixed-bed tubular reactor. CO conversion and DME space-time yield (STY) were 67.7% and 281.2 kg/m_cat³/h respectively at 260 °C, 4.3 MPa and 3000 h^?1(GHSV, syngas hourly space velocity). Synthesis performance would be increased if the tail gas (H₂/CO > 2) was recycled to the reactor when GHSV was 650–3000 h^?1.     

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La0.6Ba0.4Co0.2Fe0.8O3−δ (LBCF) cathodes prepared by combined citrate-EDTA method for IT-SOFCs

The potential candidates for IT-SOFCs cathode materials, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.6Ba_0.4Co_0.2Fe_0.8O_3−δ (LBCF), were synthesized by the combined citrate-EDTA method. The BSCF and LSCF aqueous precursors solutions were prepared from Sr(NO₃)₂, Ba(NO₃)₂, La(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, citric acid and EDTA-NH₃. BSCF precursor solutions with different pH values were dried at 130 °C and subsequently calcined at various temperatures. Symmetrical electrochemical cells consisting of porous BSCF or LBCF electrodes and a GDC electrolyte were fabricated by the screen-printing technique, and the cathode performance of the interfaces between the porous electrode (BSCF or LBCF) and GDC electrolyte was investigated at intermediate temperatures (500–700 °C) using AC impedance spectroscopy. The pH value of the precursor solution did not affect the phase evolution behavior of the BSCF powder. On the other hand, it appears that a low pH value results in the calcined BSCF powder having a more porous microstructure. The cathode performances of the BSCF and LBCF electrodes were sensitive to the powder preparation conditions. The BSCF electrode prepared from the precursor solution with a pH value of 8 showed low polarization resistance, and its area specific resistances (ASR) were 1.1, 0.15 and 0.035 Ω cm² at 500, 600 and 700 °C, respectively. On the other hand, the cathode polarization resistances of the LBCF electrode were slightly higher than those of the BSCF electrode.     

Deposition of ZrO2 film by liquid phase deposition

In this study, undoped ZrO₂ thin films were deposited on single-crystal silicon substrates using liquid phase deposition. The undoped films were formed by hydrolysis of zirconium sulfate (Zr(SO₄)₂·4H₂O) in the presence of H₂O. A continuous oxide film was obtained by controlling adequate (NH₄)₂S₂O₈ concentration. The deposited films were characterized by SEM, FT-IR, XRD and DTA. Typically, the films showed excellent adhesion to the substrate with uniform particle diameter about 150 nm. The thicknesses of ZrO₂ film were about 200 nm after 10 h deposition at 30 °C. These films shows single tetragonal phase after heat treated at 600 °C. High annealing temperature (e.g. 750 °C) may result in the phase transformation of (t)-ZrO₂ into (m)-ZrO₂.     

Syngas production by gasification of aquatic biomass with CO2/O2 and simultaneous removal of H2S and COS using char obtained in the gasification

Applicability of gulfweed as feedstock for a biomass-to-liquid (BTL) process was studied for both production of gas with high syngas (CO + H₂) content via gasification of gulfweed and removal of gaseous impurities using char obtained in the gasification. Gulfweed as aqueous biomass was gasified with He/CO₂/O₂ using a downdraft fixed-bed gasifier at ambient pressure and 900 °C at equivalence ratios (ER) of 0.1–0.3. The syngas content increased while the conversion to gas on a carbon basis decreased with decreasing ER. At an ER of 0.1 and He/CO₂/O₂ = 0/85/15%, the syngas content was maximized at 67.6% and conversion to gas on a carbon basis was 94.2%. The behavior of the desulfurization using char obtained during the gasification process at ER = 0.1 and He/CO₂/O₂ = 0/85/15% was investigated using a downdraft fixed-bed reactor at 250–550 °C under 3 atmospheres (H₂S/N₂, COS/N₂, and a mixture of gases composed of CO, CO₂, H₂, N₂, CH₄, H₂S, COS, and steam). The char had a higher COS removal capacity at 350 °C than commercial activated carbon because (Ca,Mg)S crystals were formed during desulfurization. The char simultaneously removed H₂S and COS from the mixture of gases at 450 °C more efficiently than did activated carbon. These results support this novel BTL process consisting of gasification of gulfweed with CO₂/O₂ and dry gas cleaning using self-supplied bed material.     

TiO2–Au plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance

Anatase TiO₂ nanoparticles dressed with gold nanoparticles were synthesized by hydrothermal process by using mixed precursor and controlled conditions. Diffused Reflectance Spectra (DRS) reveal that in addition to the expected TiO₂ interband absorption below 360 nm gold surface plasmon feature occurs near 564 nm. It is shown that the dye sensitized solar cells made using TiO₂–Au plasmonic nanocomposite yield superior performance with conversion efficiency (CE) of ～6% (no light harvesting), current density (J_SC) of ～13.2 mA/cm², open circuit voltage (V_oc) of ～0.74 V and fill factor (FF) 0.61; considerably better than that with only TiO₂ nanoparticles (CE ～ 5%, J_SC ～ 12.6 mA/cm², V_oc ～ 0.70 V, FF ～ 0.56).     

Characterization of RuO2·xH2O with various water contents

Hydrous ruthenium oxides (RuO₂·xH₂O) with different contents of water (x) were prepared by annealing commercial RuO₂·2.6H₂O powders at different temperatures. The morphologies and crystalline structures of RuO₂·xH₂O were investigated using transmission electron microscope (TEM) and selected area electron diffraction (SAED) techniques. From the TEM images, it was observed that the particle size of RuO₂·xH₂O increased with increasing annealing temperature. From the SAED patterns, it was observed that RuO₂·xH₂O powders became an amorphous phase at annealing temperatures <116 °C and became a crystalline phase at annealing temperatures above 116 °C. Amorphous RuO₂·xH₂O prepared at 116 °C reached its maximum specific capacitance as a result of proton insertion into the bulk of RuO₂ but with smaller Ru–Ru distance in the local structure. The more disordered structure induced by proton insertion was obtained by SAED pattern from a sample annealed at 116 °C. The possible connection between the microstructure and specific capacitance of RuO₂·xH₂O is discussed.     

Newly designed dye-sensitized solar cells fabricated with double-layered TiO2 (bottom layer)/Bx–TiO2 (top layer) combined electrodes for improved photocurrent

An unusual double-layered TiO₂ (bottom layer)/B_x–TiO₂ (top layer) combined electrode array was investigated to improve the photocurrent in dye-sensitized solar cells (DSSCs). A positive semiconductor, B_x–TiO₂, with nanometer-sized B (1.0, 5.0, and 10.0 mol%)-incorporated TiO₂ prepared using a solvothermal method, was utilized as the working electrode material by coating onto the second level above the TiO₂ electrode. The photocurrent and photovoltaic efficiency of the TiO₂ (bottom)/B_x–TiO₂ (top)-DSSC were 20.5% and 17.3% greater, respectively, than that of the double-layers of anatase TiO₂–DSSC in the photocurrent–voltage (I–V) curve of the optimal electrode. This result was attributed to their energy levels of reduction (LUMO)/oxidation (HOMO) as determined by cyclic voltammetry (CV). As the LUMO level of B_x–TiO₂ was located at a slightly higher level than that of pure anatase TiO₂, the electrons donated from the dye were easily transferred to the surface of the TiO₂ electrode without electron loss. Moreover, the recombination was also much slower in the TiO₂ (bottom)/B_x–TiO₂ (top)-based DSSCs than in the double-layered pure TiO₂ DSSC.     

Biodiesel from palm oil via loading KF/Ca–Al hydrotalcite catalyst

The solid base catalyst KF/Ca–Al hydrotalcite was obtained from Ca–Al layered double hydroxides and successfully used in the transesterification of methanol with palm oil to produce biodiesel. With the load of KF, the activity of Ca–Al mixed-oxides had been improved much. For the mass ratio 80 wt.%(KF·6H₂O to Ca–Al mixed-oxides) catalyst, under the optimal condition: 338 K, catalyst amount 5%(wt./wt. oil) and methanol/oil molar ratio 12:1, after 5 h reaction, the fatty acid methyl esters yield could reach 97.98%; for the mass ratio 100 wt.%(KF·6H₂O to Ca–Al mixed-oxides) ones, under the same reaction condition, only needed 3 h to get the FAME yield of 99.74%, and even only reacted 1 h, the FAME yield could obtain 97.14%.     

Nanocrystalline TiO2 (anatase) for Li-ion batteries

Nanocrystalline TiO₂ (anatase) was synthesized successfully by the direct conversion of TiO₂-sol at 85 °C. The as-prepared TiO₂ at 85 °C were calcined at different temperatures and time in order to optimize the system with best electrochemical performance. The particle sizes of the synthesized materials were found to be in the range of 15–20 nm as revealed by the HR-TEM studies. Commercial TiO₂ anatase (micron size) was also studied for its Li-insertion and deinsertion properties in order to compare with the nanocrystalline TiO₂. The full cell studies were performed with LiCoO₂ cathode with the best performing nano-TiO₂ as anode. The specific capacity of the nanocrystalline TiO₂ synthesized at 500 °C/2 h in a half-cell configuration was 169 mAh g⁻¹ while for the cell with LiCoO₂ cathode, it was 95 mAh g⁻¹ in the 2 V region. The specific reversible capacity and the cycling performance of the synthesized nano-TiO₂ anode in full cell configuration across LiCoO₂ cathode are superior to that reported in the literature. Cyclic voltammetry measurements showed a larger peak separation for the micro-TiO₂ than the nano-TiO₂, clearly indicating the influence of nano-particle size on the electrochemical performance.