Engineering study of continuous supercritical hydrothermal method using a T-shaped mixer: Experimental synthesis of NiO nanoparticles and CFD simulation

In the synthesis of metal oxide fine particles by continuous supercritical hydrothermal method, rapid mixing of starting solution with supercritical water is a key factor for producing nanoparticles that have a narrow size distribution. In this paper, continuous hydrothermal synthesis of NiO nanoparticles from Ni(NO₃)₂ aqueous solution at 400 °C and 30 MPa was carried out with T-shaped mixers and the effect of inner diameter, flow rate, and mixing directions on the particle size was examined. The computational fluid dynamics (CFD) simulation of the mixers was performed to evaluate the heating rate of the starting solution. When the inner diameter of the T-shaped mixer was decreased from 2.3 to 0.3 mm and the flow rate was increased from 30 to 60 g/min, the produced NiO particle size decreased remarkably from 54.3 to 20.1 nm. This trend of the decrease in particle size could be described as a function of the heating rate. The experimental and CFD results showed the detail regions of local heating that correlated with the NiO nanoparticle size.     

Synthesis and characterization of Sr2+ and Mg2+ doped LaGaO3 by co-precipitation method followed by hydrothermal treatment for solid oxide fuel cell applications

In the present study, LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8) powder has been synthesized using precipitation route followed by hydrothermal treatment. Quantitative phase analyses of different powders, have been done by Rietveld analyses of the XRD data and they reveal formation of single phase orthorhombic LSGM at 1400 °C, 8 h. Morphology of the calcined powder and microstructure of the sintered pellets are observed by transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. Thermal analysis has been carried out to find out the thermal expansion co-efficient. Successive electrical characterization of the 99% dense sintered pellet has been done by impedance spectroscopic analysis. The diffused semicircles observed in the Nyquist plots have been simulated as (RQ)(RQ) circuit and the total ionic conductivity obtained is found to be the highest for LSGM synthesized by similar routes.     

Synthesis of in situ functionalized iron oxide nanoparticles presenting alkyne groups via a continuous process using near-critical and supercritical water

The preparation of iron oxide nanoparticle dispersions of varying properties (e.g. color, crystal structure, particle size distribution) in a continuous hydrothermal pilot plant operating under near-critical and supercritical conditions with the aim of producing in situ functionalized nanoparticles suitable for secondary functionalization via click chemistry is reported. The effect of varying the mixing setup, reaction temperature and the starting material (iron salt) in the presence of different carboxylic acids on the resulting nanoparticle dispersions was investigated. The stability of the clickable ligands in the harsh hydrothermal environment was also tested and the clickability of the functionalized particles was demonstrated by means of XPS and fluorescence measurements after model click reactions.     

Imaging the continuous hydrothermal flow synthesis of nanoparticulate CeO2 at different supercritical water temperatures using in situ angle-dispersive diffraction

In situ high-energy synchrotron X-ray diffraction, a non-destructive synchrotron-based technique was employed to probe inside the steel tubing of a continuous hydrothermal flow synthesis (CHFS) mixer to spatially map, for the first time, the superheated water crystallisation of nanocrystalline ceria (CeO₂) at three different (superheated-water) temperatures representing three unique chemical environments within the reactor. Rapid hydrothermal co-precipitation at the three selected temperatures led to similarly sized ceria nanoparticles ranging from 3 to 7 nm. 2D maps of CeO₂ formation were constructed from the intensity and corresponding full width at half maximum (FWHM) values of the two most intense ceria reflections (111) and (002) for all three water inlet temperatures (350, 400 and 450 °C at 24 MPa) and subsequent changes in the particle size distribution were analysed. The accompanying high-resolution transmission electron microscopy (HRTEM) and tomographic particle size maps have confirmed that the mean ceria particle size slightly increases with temperature. This X-ray tomographic imaging study amounted to a formidable technical and engineering challenge, nevertheless one that has been met; this represents a significant achievement in imaging science, given the dynamic nature and hostile environment of a working CHFS reactor.     

The effect of NiO addition on the grain boundary behavior and electrochemical performance of Gd-doped ceria solid electrolyte under different sintering conditions

It has been reported that some transition metal oxides are effective aids both for the densification and the grain boundary behavior of ceria-based electrolytes. In the present work, NiO which is the most popular component of the anode of solid oxide fuel cells was added directly into the electrolyte ceramic, Ce_0.8Gd_0.2O_1.9, to investigate the effects of the presence of NiO on the properties of GDC electrolyte. All of the samples possess a single phase with cubic fluorite structure. The grain size is increased by the addition of NiO when the sintering temperature is 1400 °C. This modification in chemical composition also results in a decrease in activation energy and thus a tendency to enhance grain boundary mobility. The maximum power density of the composite electrolyte single cell is higher than that of a GDC single cell. Therefore, NiO can be used as an effective aid for ceria-based electrolytes.     

Synthesis and characterization of BaZrO3 nanoparticles by citrate-nitrate sol-gel auto-combustion technique: Systematic study for the formation of dense BaZrO3 ceramics

Citrate-nitrate sol-gel auto-combustion method, employing high purity (≈99.999%) precursors, is adopted for the synthesis of BaZrO₃ nanoparticles. TGA-DSC, PXRD, SEM, TEM and BET techniques are used to explain (1) thermolysis of gel, (2) optimization of thermal parameters for phase pure BaZrO₃ nanoparticles of unimodal size-distribution and (3) microstructural evolution at the sintering stage. Complexation of citrates with cations in the gel is confirmed with the help of FTIR and solid-state MAS-¹³C-NMR spectrometry. Dried gel undergoes self-sustained combustion and finally resulted in white colour ashes. Ashes on further calcination at 1000 °C-8 h resulted in BaZrO₃ powder with an average crystallite size of 69.5 nm, as calculated by microstrain-domain size analysis of XRD of BaZrO₃ using Rietveld refinement. The crystallite size is consistent with SEM and TEM particle size analysis. The powder is sintered without any sintering-aid to achieve the highest density (99%) ever reported, compared to similar processes reported earlier.     

Synthesis of iron nanoparticle: Challenge to determine the limit of hydrogen reduction in supercritical water

Supercritical hydrothermal syntheses of metal nanoparticles were investigated. Organic metal salt and hydrogen gas produced by water catalyzed decomposition of formic acid was employed as metal sources and reduction agent, respectively. The formation of iron was verified by measuring the magnetic property of the products by superconducting quantum interference device (SQUID) magnetometer as well as crystallographic analysis by X-ray diffraction (XRD). As predicted by the free energy calculation of reduction of metal oxides by hydrogen molecule, silver, palladium, copper, nickel and cobalt nanoparticles were synthesized without using surface modifier, whereas, iron could be synthesized at small yield. The main product was iron oxides (mainly magnetite). In order to increase the yield of iron, hexanoic acid was employed as an in situ surface modifier of the synthesis. The surface modification lessened the size of the synthesized nanoparticles and increased the yield of iron. The optimum condition for iron synthesis was also investigated, as a result, 7.6% yield of iron was achieved.     

Continuous hydrothermal gasification of glycerol mixtures: Effect of glycerol and its degradation products on the continuous salt separation and the enhancing effect of K3PO4 on the glycerol degradation

At the Paul Scherrer Institut (PSI) a continuous process for the catalytic hydrothermal gasification of wet biomass to synthetic natural gas (SNG) has been developed. The catalytic reactor is operated at temperatures of 400–450 °C and pressures of 25–30 MPa. Salts contained in the biomass and released during the liquefaction step are continuously withdrawn in the supercritical salt separation step and recovered as a concentrated brine upstream of the catalytic reactor. The recovered salts may be recycled as valuable nutrients or fertilizers after a certain work-up.Salt management was identified as critical issue in many different hydrothermal processes such as supercritical water oxidation (SCWO) and in catalyzed or non-catalyzed gasification technologies in near- and supercritical water. In this article we focus on the influence of organics, in this case glycerol and its hydrothermal degradation products, on the continuous salt separation performance. In the presence of organics higher temperatures are needed in the salt separator for an efficient salt separation and recovery due to a higher overall fluid density in the presence of glycerol compared to the density of pure water at the same conditions. Increasing temperatures in the salt separator lead to an increased degradation and, in particular, gasification of the glycerol. The salt studied, i.e. K₃PO₄, catalyzed the gasification of the glycerol to CO, H₂, CO₂, and CH₄ as well as the water gas shift reaction. Due to the increased glycerol gasification at 460 °C in the salt separator, the fluid mixture density was lowered to similar values of pure water under the same conditions. Hence, at the fluid temperature of 460 °C in the salt separator the same salt separation performance was observed for water–K₃PO₄ and for an aqueous mixture of 20 wt.% glycerol with K₃PO₄.     

Study of the lithium insertion–deinsertion mechanism in nanocrystalline γ-Fe2O3 electrodes by means of electrochemical impedance spectroscopy

Lithium intercalation hosts are a key point to the energy density of the largely used LiCoO₂ (even if of high cost and toxicity) as well as of manganese oxides which have been investigated most extensively. Iron oxides are attractive electrode materials for low-voltage rechargeable lithium batteries from both cost and environmental standpoints. However, search for iron oxides of conventional crystalline structures and micrometer particle sizes as lithium intercalation cathodes, has been greeted with disappointing results. Here we report on the synthesis, characterizations, electrochemical study and electrochemical impedance spectroscopy (EIS) of a nanocrystalline γ-Fe₂O₃ that simultaneously exhibits high lithium insertion capacity and good capacity retention upon cycling. These properties reveal thermodynamics of the nanocrystalline material inherently different from those of its microcrystalline counterpart. Moreover, EIS showed that the intercalation process of the lithium ion occurs according to two processes involving first the reduction of the surface Fe³⁺ with concomitant charge neutralization by Li⁺ ions onto the surface defects of the nanoparticle followed by the reduction of the core Fe³⁺ with insertion of the Li⁺ deeper in the particle.     

Synthesis of Zn2SnO4 anode material by using supercritical water in a batch reactor

Zn₂SnO₄ anode powders were successfully synthesized using supercritical water (SCW) and metal salt solutions with 10 min reaction time. Effect of NaOH concentration, Zn to Sn ratio, and synthesis temperature were studied with a SCW batch reactor. X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests were employed to characterize the physical properties and electrochemical performance of the as-prepared samples. Alkaline solution concentration and synthesis temperature played a key role in the production of single-phase Zn₂SnO₄ powders. At a solution concentration of 0.3 M NaOH and a molar ratio of Zn:Sn = 2:1 at 400 °C and 30 MPa, the average size range of the pure Zn₂SnO₄ powders was 0.5-1.0 μm, and the morphology was nearly uniform and cubic-like in shape. The initial specific discharge capacity of the Zn₂SnO₄ powders prepared at this condition was 1526 mAh/g at a current density of 0.75 mA/cm² in 0.05-3.0 V, and their irreversible capacity loss was 433 mAh/g. The discharge capacities of the Zn₂SnO₄ powders decreased with cycling and remained at 856 mAh/g after 50 cycles, which was 56% of the initial capacity.     

Characterisation of BaZr0.9Y0.1O3‐δ Prepared by Three Different Synthesis Methods: Study of the Sinterability and the Conductivity

BaZr_0.9Y_0.1O_3‐δ has been synthesised by three different methods: the solid‐state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid‐state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid‐state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σ_b <–3.6 at 300 °C) depending on the synthesis method and the sintering temperature. These variations of the bulk conductivity and also the activation energy are correlated with variations of the apparent lattice parameter. The influence of the preparation method on the electrical properties is discussed. The grain boundaries are more resistive than the bulk. The variation of the grain boundary conductivity could be correlated to the microstructure in terms of the grain size.     

Determination of the rate-determining step of the oxygen reduction reaction of La0.8Sr0.2MnO3(LSM)- 8mol% yttria-stabilized zirconia(YSZ): Composition and microstructure

In this study, LSM-YSZ composite cathodes were analyzed by changing the firing temperature, composition, and operating temperature to determine the contribution of each step of the oxygen reduction reaction (ORR). The overall resistance of the cathode reaction was characterized by fitting the AC impedance spectra with an equivalent circuit model. It was found that initial reactions of ORR (dissociative adsorption) are the main rate-determining step (RDS) regardless of operating or sintering temperature, while reactions on LSM surface become the main RDS when the ratio of LSM catalysts has a relatively small proportion. The [LSM-YSZ]_5:5 cathode fired at 1100 °C showed the best microstructure and lowest resistance in the ORR at an operating temperature of 700 °C (R_HF: 0.18 Ωcm², R_MF: 0.20 Ω cm², R_LF: 0.25 Ωcm², R_cathode: 1.14 Ωcm²). This demonstrates the potential use of LSM-YSZ cathodes for IT/LT-SOFC without the use of expensive materials, such as LSCF and BSCF.     

Synthesis, structural analysis and electrochemical performance of low-copper content La2Ni1−xCuxO4+δ materials as new cathodes for solid oxide fuel cells

In order to enhance the electrochemical performance of solid oxide fuel cells (SOFCs), La₂Ni_1−xCuO_4+δ (x = 0, 0.01, 0.02, 0.05 and 0.1) doped with copper in percentages, varying between 1% and 10%, were prepared following the modified Pechini method. The microstructure and morphology of the samples were analyzed by XRD and SEM. The electrochemical performance was followed by impedance spectroscopy. La₂Ni_0.99Cu_0.01O_4+δ samples showed good electrochemical and physicochemical properties with respect to the undoped material and is potentially a promising cathode. Indeed, doping with such small amounts of copper (1%) into the nickel site led to the formation of pure phases and stabilized the material before and after use at high temperature under air. In contrast, doping with higher amounts of copper (2%, 5% and 10%) led, after heating at 1000 °C for 48 h, to the formation of another phase resulting from the diffusion of copper into the YSZ electrolyte, limiting the interest to these materials as SOFC cathodes.     

Deposition of Al2O3 ceramic film on copper-based heterostructured coatings by aluminizing process: Study of the electrochemical responses and corrosion mechanism of the coating

The effect of deposition of the Al₂O₃ ceramic film by the aluminizing method on electrochemical responses and corrosion mechanism of copper-based heterostructured coatings was studied. The single layer coatings of Cu and Al₂O₃ and Cu/Al₂O₃ double layers were produced using reverse pulsed current electroplating process followed by powder cementation of aluminum on a substrate made of Inconel 600 superalloy. The produced coatings were then characterized using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD) methods. In order to evaluate the behavior and corrosion mechanism of the produced coatings, potentiodynamic polarization and electrochemical impedance spectroscopy methods were also used in 1 mol/L HCl solution at immersion times of 1, 12, 24, and 48 hours. The results of the study showed that the mechanism of the formation of Cu/Al₂O₃ copper-based coatings is that in the aluminizing step, first, the diffusion of Al from the surface layers to the interior occurs and then the diffusion of Cu from the plating layer to the exterior takes places. It was also found that the deposition of the Al₂O₃ ceramic film resulted in the formation of α-Al₂O₃ and CuAl₂O₄ phases and increased corrosion resistance in Cu/Al₂O₃ copper-based coatings at all immersion times and the corrosion mechanism has changed from uniform to localized state.     

Preparation and the physical/electrochemical properties of a Pt/C nanocatalyst stabilized by citric acid for polymer electrolyte fuel cells

Pt/C nanocatalysts were prepared by the reduction of chloroplatinic acid with sodium borohydride, with citric acid as a stabilizing agent in ammonium hydroxide solution. These nanocatalysts were obtained by altering the molar ratio of citric acid to chloroplatinic acid (CA/Pt) from 1:1, 2:1, 3:1 to 4:1. Transmission electron microscopy and X-ray diffraction analyses indicated that the well-dispersed Pt nanoparticles of around 3.82 nm in size were obtained when the CA/Pt ratio was maintained at 2:1. X-ray photoelectron spectroscopy measurements revealed that the 2:1, 3:1 and 4:1 molar ratio catalysts had a relatively higher amount of Pt in their metallic state than did the 1:1 molar ratio catalyst. Cyclic voltammetry results demonstrated that the Pt/C nanocatalysts annealed at 400 °C in an N₂ atm provided higher electrocatalytic activity. Among all the molar ratio catalysts, the 2:1 molar ratio catalyst exhibited the largest electrochemical active surface (EAS) area, and its methanol oxidation reaction current was superior to the E-TEK catalyst. The oxygen reduction reaction of the catalysts studied by linear sweep voltammetry and tested in a fuel cell indicated that the catalytic activity of the 2:1 molar ratio catalyst was comparable to that of an E-TEK catalyst.     

Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell

This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from −20 to −1 °C. It is found that the freezing-point depression of water in the cathode CL is 1.0 ± 0.5 °C and its effect on PEFC cold start under automotive conditions is negligible.     

超重力预混+动态水热法制备ZSM-5分子筛——水热过程影响机制

ZSM-5分子筛作为一种重要的催化剂,被广泛应用于石油化工行业,其制备工艺一直是分子筛研究的热点。传统静态水热法制备ZSM-5分子筛存在产物粒度分布不均、晶化时间长等问题,本文提出开展超重力预混+动态水热法制备ZSM-5分子筛,考察了晶化过程中晶化方式、搅拌釜转速、晶化时间和晶化凝胶体积对ZSM-5分子筛粒径的影响规律,获得了较优的操作条件;并针对较优条件下制备的产品进行了系统表征。结果表明,相比于超重力预混+静态水热,动态水热更有利于合成粒径小且分布均匀、酸量大、比表面积大的多级孔ZSM-5分子筛,搅拌釜转速越大、晶化凝胶体积越小所制备的产品平均粒径越小,粒径分布越均匀。本文研究结果表明,水热过程参数对分子筛的粒径、结构和酸性具有重要影响。     

Synthesis of a novel demulsifier by one-pot synthesis using two PEO-PPO demulsifiers as materials and the study of its demulsification performance

A novel demulsifier (noted as PSADP) having high molecular weight and branch structure was prepared via one-pot synthesis by using two PEO-PPO demulsifiers (SP-1 and DMEA-1), acrylic acid (AA), and pentaerythritol tetraacrylate (PET4A) as materials. The demulsification experiments showed that PSADP can achieve high water removal and clear separated water at the same time. Optimal preparation condition of PSADP was elected by orthogonal test. In addition, the study of partition coefficient and interfacial dilational moduli showed that the best PSADP had best capacity to displace the natural active agents and destroy the interface.     

Fabrication of nano-In2O3 phase junction by pulse alternating current synthesis for enhanced photoelectrochemical performance: Unravelling the role of synthetic conditions

Rational design and controllable synthesis of materials with improved activity and stability remains one of the key challenges to be addressed in the field of photoelectrochemical water splitting. In this work, a series of nano-In₂O₃ (c- and c/rh-In₂O₃) with tunable cubic-to-rhombohedral phase ratios was obtained via pulse alternating current electrosynthesis followed by air annealing. The physicochemical characterization of In₂O₃ using XRD, FESEM, TEM, XPS, Raman, UV–vis DRS and BET techniques along with photoelectrochemical test indicate that synthetic conditions greatly influence on structural, morphological and optical properties. The c/rh-phase ratio was feasibly controlled by altering the nature of the electrolyte (LiCl, KCl, NaCl and Na₂SO₄) without adding any organic additives. The c/rh-In₂O₃ synthesized using NaCl aqueous electrolyte shows enhanced photoelectrochemical activity (0.35 mA/cm² at 1.23 V vs RHE) and photoconversion efficiency (0.09%) as well as excellent long-term stability (over 7 h). The improved performance of the c/rh-In₂O₃ results from the constructing the well-defined phase-junction with optimal phase ratio (72% c-/28% rh-) and the high oxygen deficiency providing a synergistic effect between rh-In₂O₃ and c-In₂O₃ for more efficient separation and transfer of photogenerated charges. These results lead to a better understanding of designing metal oxide phase-junctions for photoelectrochemical applications.     

Organic template-directed indium phosphite-oxalate hybrid material: Synthesis and characterization of a novel 3D |C6H14N2|[In2(HPO3)3(C2O4)] compound with intersecting channels

A novel anionic three-dimensional indium phosphite-oxalate hybrid material, formulated as |C₆H₁₄N₂|[In₂(HPO₃)₃(C₂O₄)] (1) was prepared under hydrothermal conditions by using 1,4-diazabicyclo[2.2.2]octane (dabco) as a structure directing agent (SDA). Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the orthorhombic system space group Pna2₁ (No. 33) having unit cell parameters a = 12.4143(13) Å, b = 7.7166(8) Å, c = 18.327(2) Å, V = 1755.6(3) Å³, and Z = 4 with R₁ = 0.0282, wR₂ = 0.0632. The novel 3D open framework is constructed from InO₆ octahedra, HPO₃ pseudo-tetrahedra and C₂O₄ units. The assembly of these building units generates intersecting 8- and 12-membered ring (MR) channels along two different directions. To the best of our knowledge, it is the first reported indium phosphite-oxalate hybrid material. Further characterization of compound 1 was performed using X-ray powder diffraction (XRD), infrared (IR) spectra, thermal gravimetric analyses (TGA), inductively coupled plasma (ICP) and elemental analyses.