Preparation of PtSnRh/C-Sb2O5·SnO2 electrocatalysts by an alcohol reduction process for direct ethanol fuel cell

PtSnRh/C-Sb₂O₅·SnO₂ electrocatalysts with different Pt/Sn/Rh atomic ratios (90:05:05, 70:25:05, and 50:45:05) were prepared by an alcohol reduction process using H₂PtCl₆·6H₂O, SnCl₂·2H₂O, RhCl₃·xH₂O as metal sources, ethylene glycol as solvent and reducing agent, and a physical mixture of Vulcan XC72 (85?wt%) and Sb₂O₅·SnO₂ (15?wt%) as support. The electrocatalysts were characterized by X-ray diffraction and transmission electron microscopy. The electro-oxidation of ethanol was studied by cyclic voltammetry and chronoamperometry at 25 and 50?°C and in single direct ethanol fuel cell (DEFC) at 100?°C. The diffractograms of PtSnRh/C-Sb₂O₅·SnO₂ electrocatalysts showed the peaks characteristic of Pt face-centered cubic structure and several others peaks associated with ·SnO₂ and Sb₂O₅·SnO₂. Transmission electron micrographs of PtSnRh/C-Sb₂O₅·SnO₂ electrocatalysts showed the metal nanoparticles distributed on the supports with particle sizes of about 2?C3?nm. The electrochemical measurements and the experiments in a single DEFC showed that PtSnRh/C-Sb₂O₅·SnO₂ (90:05:05) and PtSnRh/C-Sb₂O₅·SnO₂ (70:25:05) electrocatalysts exhibited higher performance for ethanol oxidation in comparison with PtSnRh/C electrocatalyst.     

Investigation of a Pt–Fe/C catalyst for oxygen reduction reaction in direct ethanol fuel cells

Three cathode catalysts (60% Pt/C, 30% Pt/C and 60% Pt–Fe/C), with a particle size of about 2–3 nm, were prepared to investigate the effect of ethanol cross-over on cathode surfaces. All samples were studied in terms of structure and morphology by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Their electrocatalytic behavior in terms of oxygen reduction reaction (ORR) was investigated and compared using a rotating disk electrode (RDE). The tolerance of cathode catalysts in the presence of ethanol was evaluated. The Pt–Fe/C catalyst showed both higher ORR activity and tolerance to ethanol cross-over than Pt/C catalysts. Moreover, the more promising catalysts were tested in 5 cm² DEFC single cells at 60 and 80 °C. An improvement in single cell performance was observed in the presence of the Pt–Fe catalyst, due to an enhancement in the oxygen reduction kinetics. The maximum power density was 53 mW cm⁻² at 2 bar rel. cathode pressure and 80 °C.     

Sonoelectrocatalytic decomposition of methylene blue using Ti/Ta2O5–SnO2 electrodes

Sonoelectrochemical decomposition of organic compounds is a developing technique among advanced oxidation processes (AOPs). It has the advantage over sonication alone that it increases the efficiency of the process in terms of a more rapid decrease in chemical oxygen demand (COD) and in total organic carbon (TOC) and accelerates electrochemical oxidation which normally requires a lengthy period of time to achieve significant mineralisation. Moreover the use of an electrocatalytic electrode in the process further accelerates the oxidation reaction rates. The aim of this study was to improve the decomposition efficiency of methylene blue (MB) dye by sonoelectrochemical decomposition using environmentally friendly and cost-effective Ti/Ta₂O₅–SnO₂ electrodes. Decolourisation was used to assess the initial stages of decomposition and COD together with TOC was used as a measure of total degradation. The effect of a range of sonication frequencies 20, 40, 380, 850, 1000 and 1176 kHz at different powers on the decolourisation efficiency of MB is reported. Frequencies of 850 and 380 kHz and the use of higher powers were found more effective towards dye decolourisation. The time for complete MB degradation was reduced from 180 min using electrolysis and from 90 min while carrying out sonolysis to 45 min when conducting a combined sonoelectrocatalytic experiments. The COD reduction of 85.4% was achieved after 2 h of combined sonication and electrolysis which is a slightly higher than after a single electrolysis (78.9%) and twice that of sonolysis (40.4%). A dramatic improvement of mineralisation values were observed within 2 h of sonoelectrocatalytic MB degradation. The TOC removal efficiency increased by a factor of 10.7 comparing to sonication alone and by a factor of 1.5 comparing to the electrolytic process. The energy consumption (kWh/m³) required for the complete degradation of MB was evaluated.     

I–V characteristics of a methanol concentration sensor for direct methanol fuel cell (DMFC) by using catalyst electrode of Pt dots

In this work, the methanol sensors were fabricated by using Pt dot catalyst electrode and the level of electrochemical response was analyzed. This kind of sensors can be applicable to sensing the methanol concentration in real-time. When we measured the methanol sensor with 5 nm of Pt dot, we could get 2.00 × 10⁻⁶, 3.06 × 10⁻⁶ and 6.25 × 10⁻⁶ A of electric current value for the methanol concentration of 1, 2 and 3 mole, respectively. The measured voltage was 1 V. To optimize the sensitivity level of Pt dot catalyst electrode, the electrodes were made in H-grid shape. The distance between electrode branches was designed to be 80, 150 and 300 μm, respectively. When we measured the electric current–voltage characteristics of methanol sensor with 2 M of methanol, it was 3.06 × 10⁻⁶, 2.02 × 10⁻⁶ and 1.50 × 10⁻⁶ A, for 80, 150 and 200 μm, respectively. Thus it is suggested that more efficient response of methanol sensing is possible when the distance between electrodes is reduced.     

Preparation of Pt–CeO2/MWNT nano-composites by reverse micellar method for methanol oxidation

We report the preparation of Pt–CeO₂ nanoparticles on the multi-walled carbon nanotubes (MWNTs) by a reverse micellar method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small Pt–CeO₂ nanoparticles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the Pt–CeO₂ nanoparticles on the MWNTs. Cyclic voltammetry (CV) results demonstrated that the Pt–CeO₂/MWNT exhibited a higher methanol oxidation than did the Pt/MWNT catalyst. The CO stripping test showed that CeO₂ can make CO stripped at a lower potential, which is helpful for CO and methanol electro-oxidation.     

Demonstration of medium temperature,one atmosphere reversible H2/O2 fuel and steam electrolysis cell operation using polycrystalline H3O+β/β″ -Al2O3

The development of a reversible fuel and steam electrolysis cell based on an H₃O⁺β/β″ -Al₂O₃ solid electrolyte is described. The unit has been operated between 100 and 300°C at one atmosphere steam pressure. The major limitations are discussed with reference to the solid electrolyte and the method of electrode preparation.     

Thermal and sonochemical synthesis of porous (Ce,Zr)O2 mixed oxides from metal β-diketonate precursors and their catalytic activity in wet air oxidation process of formic acid

Porous (Ce_0.5Zr_0.5)O₂ solid solutions were prepared by thermolysis (T = 285 °C) or sonolysis (20 kHz, I = 32 W cm⁻², P_ac = 0.46 W mL⁻¹, T = 200 °C) of Ce(III) and Zr(IV) acetylacetonates in oleylamine or hexadecylamine under argon followed by heat treatment of the precipitates obtained in air at 450 °C. Transmission Electron Microscopy images of the samples show nanoparticles of ca. 4–6 nm for the two synthetic approaches. The powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and μ-Raman spectroscopy of solids obtained after heat treatment indicate the formation of (Ce_0.5Zr_0.5)O₂ solid solutions with a metastable tetragonal crystal structure for the two synthetic routes. The specific surface area of the samples varies between 78 and 149 m² g⁻¹ depending on synthesis conditions. The use of Barrett–Joyner–Halenda and t-plot methods reveal the formation of mixed oxides with a hybrid morphology that combines mesoporosity and microporosity regardless of the method of preparation. Platinum nanoparticles were deposited on the surface of the mixed oxides by sonochemical reduction of Pt(IV). It was found that the materials prepared by sonochemistry exhibit better resistance to dissolution during the deposition process of platinum. X-ray photoelectron spectroscopy analysis shows the presence of Pt(0) and Pt(II) on the surface of mixed oxides. Porous (Ce_0.5Zr_0.5)O₂ mixed oxides loaded with 1.5 %wt. platinum exhibit high activity in catalytic wet air oxidation of formic acid at 40 °C.     

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Coexistence of one-electron- and monoprotonated two-electron-reduced species of the K5[PMo2VW9O40] · 24H2O heteropolyoxometalate identified by EPR

The simulation of the room-temperature experimental electron paramagnetic resonance spectrum of K₅[PMo₂VW₉O₄₀] · 24 H₂O heteropolyoxometalate indicates the presence of equal amounts of a one-electron-reduced species (g _∥ = 1.922,g _⊥ = 1.972,A _∥ = 181 G,A _⊥ = 63 G) and a monoprotonated two-electron-reduced species with mixed-valence V^IV, Mo^V and Mo^VI ions (g _iso = 1.972, ΔB _iso(p-p) = 450 G). Two unprotonated one-electron-reduced isomers are identified in dimethylsulfoxide-H₂O solution of the sample atT = 100 K (g _∥¹ = 1.929,g _⊥¹ = 1.990,A _∥¹ = 179 G,A _⊥¹ = 65 G andg _∥² = 1.918,g _⊥² = 2.000,A _∥² = 187 G,A _⊥² = 80 G, respectively). The values of the in-plane π(V−O) bond π₂² coefficient for the one-electron-reduced species (0.87 at room temperature and 0.83 and 0.74 for the species in frozen solution) suggests the delocalization of the vanadium unpaired electron towards the molybdenum ions via O_b atoms.     

Study of Au/SnO x –Al2O3 catalysts used in CO oxidation by in situ Mössbauer spectroscopy

The active catalytic components in tin oxide containing alumina-supported gold catalyst were examined by comparing and analysing the in situ Mössbauer spectra of the SnO _x –Al₂O₃ support and the 3 wt.% Au/SnO _x –Al₂O₃ catalyst (1.1 wt.% Sn, Au/Sn = 3:2 atomic ratio). Samples were prepared by using organometallic precursor of ¹¹⁹SnMe₄ (enriched). First tin was grafted to the alumina surface from the organometallic precursor compound. In the next step the grafted complexes were decomposed in flowing oxygen. Gold was deposited onto the SnO _x –Al₂O₃ support in the subsequent step. Analysis of in situ spectra shows that in Au/SnO _x –Al₂O₃ catalyst after activation in hydrogen at 620 K tin may occur in three different oxidation states [Sn (IV), Sn(II) and Sn(0)] simultaneously. The metallic tin is a component of the bimetallic AuSn alloy phase. Data presented provide the first evidence for the formation of alloy-type supported Sn–Au catalyst on alumina. Furthermore, from the spectra recorded at different temperatures, values of the Debye temperatures and recoilless fractions were also determined for the various species. The results show that in catalytic oxidation of carbon monoxide at room temperature the dominant part of Sn(II) and the AuSn alloy is oxidized.     

Hydrothermal synthesis and photocatalytic properties of WO3 nanorods by using capping agent SnCl4·5H2O

Hexagonal tungsten trioxide (h-WO₃) nano-rods of different sizes are prepared via hydrothermal synthesis using a capping agent of SnCl₄·5H₂O. The size of the synthesized WO₃ nanoparticles can be controlled by changing concentration of the capping agent SnCl₄·5H₂O alone. We also investigate microstructures and optical properties of the WO₃ nanorods and propose a synthesis mechanism for the nanorods. The photocatalytic activities of the h-WO₃ nanorods are evaluated by degradation of Rhodamine-B (RhB), revealing that these nanorods exhibit excellent photocatalytic properties. The capping agent SnCl₄·5H₂O is found to be critical to governing sizes and properties of the h-WO₃ nanorods. Our results demonstrate that functional nano-crystallites with tunable size and morphology can be synthesized via a facile hydrothermal synthesis process by adjusting the concentration of capping agent alone. Such a facile hydrothermal synthesis process should be applicable to other types of nanomaterials and relevant to a wide range of applications.     

Synthesis and characterization of (Bi2O3)1−x−y−z(Gd2O3)x (Sm2O3)y(Eu2O3)z quaternary solid solutions for solid oxide fuel cell

Sm₂O₃, Gd₂O₃, Eu₂O₃ triple-doped Bi₂O₃ based quaternary solid solutions were synthesized as a candidate electrolyte material using the solid-state reaction technique. The structural, thermal and electrical conductivity features of the ceramic samples were examined and compared by using X-ray powder diffraction (XRD), thermal gravimetry/differantial thermal analysis (TG/DTA) and the four-point probe technique (4PPT). The result of XRD measurements indicated that the (Bi₂O₃)_{(1−x−y−z)}(Gd₂O₃)_x(Sm₂O₃)_y(Eu₂O₃)_z (x = 10/y = 10/z = 5, 15, 20 mol % and x = 10/y = 5, 10, 15, 20/z = 10 mol %) samples have a stable face-centered cubic δ-phase and mixed phase crystallographic structure. The phase stability was also checked by the DTA evaluations results. The temperature dependent electrical conductivity measurements showed that the highest electrical conductivity was observed for the sample of the (Bi₂O₃)_0.75(Gd₂O₃)_0.10(Sm₂O₃)_0.05(Eu₂O₃)_0.10 system which has a stable and δ-phase was found as 6.67 × 10⁻³ (Ω cm)⁻¹ at 650 °C. This sample can be used as an electrolyte material in the solid oxide fuel cells (SOFCs) which is possible to operate at intermediate temperature ranges. The activation energy was also calculated at a low temperature range (350–650 °C) and high temperature range (above 650 °C). The values for the samples vary from 0.63 eV to 1.08 eV at low temperature and at high temperature they vary from 0.43 eV to 0.75 eV.     

Hydrothermal synthesis of Fe5(PO4)4(OH)3·2H2O microflowers for fabricating high-performance LiFePO4/C composites

Hierarchical Fe₅(PO₄)₄(OH)₃·2H₂O microflower was synthesized by a hydrothermal reaction with self-prepared β-FeOOH nanorod as raw material. The microflowers were self-assemblies of symmetric building blocks with deep grooves. The possible morphology evolution process was proposed. The microflowers morphology was retained when they were lithiated to prepare LiFePO₄/C composites through a carbothermal reduction method with citric acid as both reducing agent and carbonaceous coating conductor source. As cathode materials for lithium ion batteries, the as-obtained LiFePO₄/C composites deliver a high discharge capacity of 156 mAh g^?1 at 0.1 C rate and exhibit excellent cycling stability, which may be ascribed to the homogeneous coated carbon and the unique microflower structure with grooves.     

Li+ transportation kinetics of FeF3 · 0.33H2O/C nanocomposite synthesized by one-step solid state method

A new cathode material for lithium ion battery FeF₃?·?0.33H₂O/C was synthesized successfully by a simple one-step chemico-mechanical method. It showed a noticeable initial discharge capacity of 233.9 mAh g^?1 and corresponding charge capacity of 186.4 mAh g^?1. A reversible capacity of ca.157.4 mAh g^?1 at 20 mA g^?1 can be obtained after 50 charge/discharge cycles. To elucidate the lithium ion transportation in the cathode material, the methods of electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were applied to obtain the lithium diffusion coefficients of the material. Within the voltage level of 2.05–3.18 V, the method of EIS showed that \( {D}_{{\mathrm{Li}}^{+}} \) varied in the range of 1.2?×?10^?13?~?3.6?×?10^?14 cm² s^?1 with a maximum of 1.2?×?10^?13 cm² s^?1 at 2.5 V. The method of GITT gave a result of 8.1?×?10^?14?~?1.2?×?10^?15 cm² s^?1. The way and the range of the variation for lithium ion diffusion coefficients measured by the GITT method show close similarity with those obtained by the EIS method. Besides, they both reached their maximum at a voltage level of 2.5 V.     

Holographic recording by excitation of metastable electronic states in Na2[Fe(CN)5NO]·2H2O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na₂[Fe(CN)₅NO]·2H₂O, sodiumnitroprusside, by excitation of metastable electronic states in the blue–green spectral range. For light polarized parallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Δn≈2×10^-3 at λ=514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crystallographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depends mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that written gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is local. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavelengths within the excitation range as well as of the crystal thickness do not influence the maximum photorefractive response. Investigations on the grating vector of the written gratings show unambiguously that charge migration is not responsible for the photorefractive effect. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Preparation and characterization of polybithiophene/<Emphasis Type=BoldItalic>β</Emphasis>-MnO<Subscript>2</Subscript> composite electrode for oxygen reduction

A composite material of polybithiophene (PBTh) and β-MnO₂ was prepared by electrodeposition of organic conducting polymer on β-MnO₂ surface in 0.1 M LiClO₄/0.01 M BTh/CH₃CN. Synthesized material was characterized by using various techniques, i.e., X-ray diffractometry (XRD), scanning electron microscopy (SEM), and magnetic measurements (SQUID). Electrochemical features of oxygen reduction reaction were investigated using cyclic voltammetry on β-MnO₂ and PBTh/β-MnO₂ electrode, and chronopotentiometry tests were carried out at different currents. The results show that peak current and potential of oxygen reduction are changed for β-MnO₂ modified by polybithiophene.     

Ultrasound assisted co-precipitation of nanostructured CuO–ZnO–Al2O3 over HZSM-5: Effect of precursor and irradiation power on nanocatalyst properties and catalytic performance for direct syngas to DME

Nanostructured CuO–ZnO–Al₂O₃/HZSM-5 was synthesized from nitrate and acetate precursors using ultrasound assisted co-precipitation method under different irradiation powers. The CuO–ZnO–Al₂O₃/HZSM-5 nanocatalysts were characterized using XRD, FESEM, BET, FTIR and EDX Dot-mapping analyses. The results indicated precursor type and irradiation power have significant influences on phase structure, morphology, surface area and functional groups. It was observed that the acetate formulated CuO–ZnO–Al₂O₃/HZSM-5 nanocatalyst have smaller CuO crystals with better dispersion and stronger interaction between components in comparison to nitrate based nanocatalysts. Ultrasound assisted co-precipitation synthesis method resulted in nanocatalyst with more uniform morphology compared to conventional method and increasing irradiation power yields smaller particles with better dispersion and higher surface area. Additionally the crystallinity of CuO is lower at high irradiation powers leading to stronger interaction between metal oxides. The nanocatalysts performance were tested at 200–300 °C, 10–40 bar and space velocity of 18,000–36,000 cm³/g h with the inlet gas composition of H₂/CO = 2/1 in a stainless steel autoclave reactor. The acetate based nanocatalysts irradiated with higher levels of power exhibited better reactivity in terms of CO conversion and DME yield. While there is an optimal temperature for CO conversion and DME yield in direct synthesis of DME, CO conversion and DME yield both increase with the pressure increase. Furthermore ultrasound assisted co-precipitation method yields more stable CuO–ZnO–Al₂O₃/HZSM-5 nanocatalyst while conventional precipitated nanocatalyst lost their activity ca. 18% and 58% in terms of CO conversion and DME yield respectively in 24 h time on stream test.     

Preparation of honeycomb porous La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.2Ce0.8O2−δ composite cathodes by breath figures method for solid oxide fuel cells

Honeycomb porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-Gd_0.2Ce_0.8O_2−δ (LSCF-GDC) composite cathodes are prepared using the breath figures (BFs) method with nontoxic and easily available water droplets as templates. The fabrication of honeycomb porous membranes is realized in a relatively humid environment, using a volatile solvent. The microstructure and morphology of the membranes produced are investigated by scanning electron microscopy (SEM). The SEM micrographs suggest that experimental conditions, such as ambient temperature, relative humidity, and concentration of polymer and LSCF-GDC powder, which have direct influence on the solvent evaporation affects the pore structure of the porous membranes. Electrochemical impedance spectroscopy (EIS) is used to evaluate the polarization resistance of LSCF-GDC composite cathodes prepared at different experimental conditions. The honeycomb porous LSCF-GDC composite cathode showing average pore diameter of 10 μm illustrates the lowest polarization resistance.