Pechini-type Polymerizable Complex Research Articles

Oxygen Species Involved in Complete Oxidation of CH4 by SrFeO3-δ in Chemical Looping Reforming of Methane.

Compared with conventional methane reforming technologies, chemical looping reforming (CLR) has the advantages of self-elimination of coke, a suitable syngas ratio for certain down-stream processes, and a pure H2 or CO stream. In the reduction step of CLR, methane combustion has to be inhibited, which could be achieved by designing appropriate oxygen carriers and/or optimizing the operating conditions. To gain a further understanding of the combustion reaction, methane oxidation by perovskite (SrFeO3-δ) at 900 °C and 1 atm in a pulse mode was investigated in this work. The oxygen non-stoichiometry of SrFeO3-δ prepared by a Pechini-type polymerizable complex method is 0.14 at ambient conditions, and it increases to 0.25 and subsequently to 0.5 when heating from 100 to 900 °C in argon that contains 2 ppmv of molecular oxygen. The activation energies of the first and second transitions are 294 and 177 kJ/mol, respectively. The presence of 0.99 vol.% hydrogen in argon significantly reduces the amount CO2 produced. At a pulse interval of 10 min, the amount of CO2 produced in the absence of hydrogen is one order of magnitude greater than that in the presence of hydrogen. In the former case, the amount of CO2 produced dramatically decreases first and then gradually approaches a constant, and the oxygen species involved in methane combustion can be partially replenished by extending the pulse interval, e.g., 82.5% of this type of oxygen species is replenished when the pulse interval is extended to 60 min. The restored species predominantly originate from those that reside in the surface layer or even in the bulk.

Ionic conduction and dielectric properties of yttrium doped LiZr2(PO4)3 obtained by a Pechini-type polymerizable complex route

We report on the ion transport properties of Li1+xZr2-xYx(PO4)3 (0.05 ≤ x ≤ 0.2) NASICON type nanocrystalline compounds prepared through a Pechini-type polymerizable complex method. Structural properties were characterized by means of powder X-ray diffraction, Raman spectroscopy and electron microscopy with selected area electron diffraction. Impedance spectroscopy was utilised to investigate the lithium ion transport properties. Y3+ doped LiZr2(PO4)3 compounds showed stabilized rhombohedral structure with enhanced total ionic conductivity at 30 °C from 2.87 × 10−7 S cm−1 to 0.65 × 10−5 S cm−1 for x=0.05 to 0.20 respectively. The activation energies of Li1+xZr2-xYx(PO4)3 show a decreasing trend from 0.45 eV to 0.35 eV with increasing x from 0.05 to 0.20. The total conductivity of these compounds is thermally activated, with activation energies and pre-exponential factors following the Meyer-Neldel rule. The tanδ peak position shifts to the high-frequency side with increasing yttrium content. Scaling in AC conductivity spectra shows that the electrical relaxation mechanisms are independent of temperature.

Solvothermal Synthesis of Nasicon-Type LiTi2(PO4)3/C and Their Application As Electrode Material in Lithium-Ion Battery

With the rapid growth in the use of portable electronics and electric vehicles (EVs), high performance energy storage systems are in great demand, such as high energy density lithium ion batteries (LIBs) and high power density lithium-ion hybrid supercapacitors (Li-HECs). Although the energy storage and conversion mechanisms are different, there are “electrochemical similarities” of these two systems[1, 2]. Common features are that the energy-providing processes take place at the phase boundary of the electrode/electrolyte interface and that electron and ion transport are separated. NASICON-structured LiTi2(PO4)3 is considered an outstanding electrode material for both LIBs and Li-HECs because of rapid Li+ diffusion in the bulk and on surface of LiTi2(PO4)3 [3]. LiTi2(PO4)3 can reversibly accommodate two additional lithium ions, operating on the Ti4+/Ti3+ at 2.48 V, via a two-phase mechanism that converts between LiTi2(PO4)3 and Li3Ti2(PO4)3. In addition, this ceramic electrode material can not only well preserve its framework during Li+ extraction what leads to long cycle life, but also can be used as an electrode for aqueous electrolytes to improve safety performance[4]. However, the vast majority of NASICON-type materials published in literature were synthesized by solid state process or sol-gel and high temperature post-calcination that resulted in polygonal or rounded particles and with relatively large particle size[5-7]. Aiming to decrease the synthesis temperature and further improve the performance of LiTi2(PO4)3/C by tailoring the particle size and morphology, we intended to prepare LiTi2(PO4)3/C by solvothermal method. In our recent research, the “bare” LiTi2(PO4)3 and LiTi2(PO4)3/C were synthesized from an intermediate precursor of Li-Ti-P-O or Li-Ti-P-O-C via one-pot solvothermal method and subsequent heat treatment under 350-650 oC to obtain carbon coating. Stoichiometric amounts of LiOH·H2O, Ti(OC2H5)4, and NH4H2PO4 were used as starting materials. Cetyltrimethylammonium bromide (CTAB) and oxalic acid (OA) are simultaneously used as surfactant and carbon source for the carbon coating on the surface of LiTi2(PO4)3. Ammonia solution was used to adjust the pH value to a certain range. All the chemicals in this work are purchased from Sigma-Aldrich and used as received. The reaction took place in a reactor with the starting pressure at 7.8 bar and constant stirring for 3 days under 210 oC. Pressure and temperature changes are recorded. Additionally, LiTi2(PO4)3/C was also prepared by using a Pechini-type polymerizable complex method for comparison. Crystalline phases, structures and morphology of the materials were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM) and Raman spectroscopy. The electrochemical experiments were performed using a coin cell incorporating metallic lithium foil as a counter electrode. The working electrode was prepared from a mixture of 90 wt% LiTi2(PO4)3/C, 5 wt% carbon black as conductive additive, and 5 wt% Polyvinylidene difluoride dissolved in N-methylpyrrolidone as binder. This slurry was spread onto copper foil by tape casting and then dried at 100 oC for 12 h. Galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a potentiostat from Biologic.

Synthesis and formation mechanism of submicrometer ZrB2 powders via the Pechini-type polymerizable complex route

Synthesis and formation mechanism of submicrometer ZrB2 powders via the Pechini-type route.

Novel Mo/TiN composites for an alkali metal thermal-to-electric converter (AMTEC) electrode

Core–shell composite powders composed of molybdenum (Mo) coated on titanium nitride (TiN) were synthesized using a Pechini-type polymerizable complex method to improve the microstructural stability and electrochemical performance of alkali metal thermal-to-electric converter (AMTEC) electrodes. The coverage of porous Mo shells was controlled by adjusting the concentration of resin-embedded molybdenum sources around TiN particles. Each composite powder was characterized by microstructural analysis (SEM, TEM) and X-ray spectroscopy (XRD, EDX and XPS). During the synthesis process, thin oxide layers (i.e., TiOx, with x=1.0–2.0) were formed on the TiN surface. Each Mo and TiN phase shares a common intermediate oxide layer, and there are strong interactions between oxygen anions and Mo atoms at the interface. As a result, there are cohesive networks between Mo and TiN at the core–shell interface, and these networks control the grain growth of Mo at high temperatures (>700°C). A single cell composed of Mo/TiN powder (Mo 1.0mol%) was observed to perform well (180mW/cm2) and exhibit a low area specific resistance (ASR at 0.7V) of 0.4Ωcm2 at 700°C.

Carbon‐Coated LiTi2(PO4)3: An Ideal Insertion Host for Lithium‐Ion and Sodium‐Ion Batteries

We report the extraordinary performance of carbon-coated sodium super ion conductor (NASICON)-type LiTi2 (PO4 )3 as an ideal host matrix for reversible insertion of both Li and Na ions. The NASICON-type compound was prepared by means of a Pechini-type polymerizable complex method and was subsequently carbon coated. Several characterization techniques such as XRD, thermogravimetric analysis (TGA), field-emission (FE) SEM, TEM, and Raman analysis were used to study the physicochemical properties. Both guest species underwent a two-phase insertion mechanism during the charge/discharge process that was clearly evidenced from galvanostatic and cyclic voltammetric studies. Unlike that of Li (≈1.5 moles of Li), Na insertion exhibits better reversibility (≈1.59 moles of Na) while experiencing a slightly higher capacity fade (≈8 % higher than Li) and polarization (780 mV) than Li. However, excellent rate capability profiles were noted for Na insertion relative to its counterpart Li. Overall, the Na insertion properties were found to be superior relative to Li insertion, which makes carbon-coated NASICON-type LiTi2 (PO4 )3 hosts attractive for the development of next-generation batteries.

ChemInform Abstract: Chemical Design of Highly Water-Soluble Ti, Nb, and Ta Precursors for Multi-Component Oxides.

Novel citric acid based Ti, Nb and Ta precursors that are highly stable in the presence of water were developed. No alkoxides of Ti, Nb and Ta were utilized in the preparation, instead much less moisture-sensitive metallic Ti, NbCl5 and TaCl5 were chosen as starting chemicals for Ti, Nb and Ta, respectively. The feasibility of these chemicals as precursors is demonstrated in the powder synthesis of BaTi 4O9, Y3NbO7 and LiTaO3. The waterresistant Ti precursor was employed as a new source of water-soluble Ti in the amorphous citrate method, and phase pure BaTi4O9 in powdered form was successfully synthesized at 800 °C. The Pechini-type polymerizable complex method using the water-resistant Nb and Ta precursors was applied to the synthesis of Y 3NbO7 and LiTaO3, and both the powder materials in their pure form were successfully synthesized at reduced temperatures, viz. 500-700 °C. The remarkable retardation of hydrolysis of these water-resistant precursors is explained in terms of the partial charge model theory.

Synthesis and studies of Zn-doped LaMnO3+d nanoperovskite by sol–gel processing

Nanoparticles with high surface area of LaMn1−x Zn x O3+d (LMZ) (0.0≤x≤0.38) were synthesized by the Pechini-type polymerizable complex technique based on in situ polyesterification between citric acid (CA) and ethylene glycol (EG). Heating at 90 °C of a mixed solution of CA, EG, H2O, and nitrates of La, Mn, and Zn, gave a polymeric resin. The polymeric resin or citrate-derived gel was preheated at 450 °C for 2 h to give a powdered precursor of LMZ. The final heat treatment of the powdered precursor was done in static air at a temperature range 700–800 °C for 6 h. After calcinations of the dried gels, the products were characterized by the X-ray powder diffraction technique, Fourier transform infrared analysis, and scanning electron microscopy. The products are of considerable interest both for utilization as catalysts and for their exceptional magnetotransport properties.

Ni-YSZ cermet anode fabricated from NiO-YSZ composite powder for high-performance and durability of solid oxide fuel cells

A functional composite powder, in which nano-sized NiO (40 vol.%) was crystallized on YSZ particles, was synthesized by the Pechini-type polymerizable complex method in an effort to improve the performance and durability of SOFC anodes. The pore-structure (e.g. porosity, pore size) and mechanical properties (e.g. strength, density) of the electrode were optimized by a combination of two different species of YSZ powder (Tosoh 0.3 μm and Millennium 10 μm). A single cell made from the composite powder exhibited high power density of 1.1 W cm − 2 and high durability under a load of 1.0 A cm − 2 during 530 h of operating time at 800 °C. It could be concluded that this functional composite would enhance the single cell performance and durability at elevated temperature.

Optimization of LaMO3 (M: Mn, Co, Fe) synthesis through the polymeric precursor route

Abstract LaMO3 (Me: Mn, Co, Fe) powders were synthesized through a Pechini-type polymerizable complex route, based on the ability of citric acid (CA) to form stable metal chelates and the polyesterification between CA and ethylene glycol (EG). The effect of the molar ratios of reactants (CA:total cations = 1–4 and EG:CA = 1–3) on the precursors and the final powders was studied. The gelation procedure was monitored through continuous conductivity measurements. The calcination reactions were recorded using TG/DTA measurements, while XRD and FTIR were used for the characterization of the precursors and the final products. The results indicated that the polymerizable complex method can be successfully applied for the preparation of LaMO3. The formation requires a 3-h calcination at 600 °C. The molar ratios citric acid:total cations = 2 and ethylene glycol:citric acid = 3 are the most favorable ratios for the synthesis of LaMO3.

Nano-composite materials for high-performance and durability of solid oxide fuel cells

A functional composite powder containing nano-sized NiO and YSZ (8 mol% Y 2O 3–ZrO 2) co-conjugated on YSZ powder is synthesized by a Pechini-type polymerizable complex method in an effort to yield improved performance and durability in solid oxide fuel cells (SOFCs). A SOFC single cell made from the dual composite powder exhibited high power density of 1.2 W cm −2 and showed excellent durability (zero degradation) under a load of 1.0 A cm −2 during 550 h of operation at 800 °C. The durability is attributed to the uniform and constrained cermet structure, and particularly from the interface between Ni and YSZ, which has been modified via nano-conjugation. The proposed composite powder is an attractive candidate material for making homogeneous, durable, and highly efficient electrode microstructures.

Synthesis of nanostructured LiTi 2(PO 4) 3 powder by a Pechini-type polymerizable complex method

The nanostructured NASICON-type LiTi 2(PO 4) 3 (LTP) material has been synthesized by Pechini-type polymerizable complex method. The use of water-soluble ammonium citratoperoxotitanate (IV) metal complex instead of alkoxides as precursor allows to prepare monophase material. Thermal analyses have been carried out on the powder precursor to check the weight loss and synthesis temperature. X-ray powder diffraction analysis (XRD) has been performed on the LTP powder obtained after heating the powder precursor over a temperature range from 550 to 1050 °C for 2 h. By varying the molar ratio of citric acid to metal ion (CA/Ti) and citric acid to ethylene glycol (CA/EG), the grain size of the LTP powder could be modified. The formation of small and well-crystalline grains, in the order of 50–125 nm in size, has been determined from the XRD patterns and confirmed by transmission electron microscopy.

Synthesis of La<sub>0.8</sub> Sr<sub>0.2</sub> MnO<sub>3±δ</sub> Powders by Amorphous Citrate and Pechini Techniques

La0.8Sr0.2MnO3±d (LSM) powders were prepared by a Pechini-type polymerizable complex process and by amorphous citrate technique. Both processes involve the complexation of the LSM cations from their nitrates or carbonates. The aim of this work is to determinate the differences between the powers obtained by these techniques. The powders synthesized by the Pechini-type process were calcined between 500 °C and 1100 °C. The powders prepared by amorphous citrate technique were calcined at 900 °C. No contamination of either of the powders was observed by X ray fluorescence analysis. X ray diffraction results showed that a perovskitetype structure phase was obtained. BET results showed that the specific surface areas of the powders prepared by the polymerizable complex process and the amorphous citrate route were 6.6 m2g-1 and 5.7 m2g-1, respectively.

Photoluminescence of Praseodymium-Doped Srn+1TinO3n+1 (n=1, 2, ∞)

Perovskite-related compounds, Srn+1TinO3n+1: Pr3+ (n=1, 2, ∞), were prepared by a Pechini-type polymerizable complex method using a citric metal complex and their photoluminescence properties were investigated. The photoluminescence spectra of these compounds show distinct peaks at approximately 490 nm and/or 610 nm, which correspond to the intra-4f transitions from the excited state 3P0 to the ground state 3H4, 1D2 to 3H4 of Pr3+, respectively. The excitation process is consistent with the transition between the valence band and the conduction band in host materials. The most intense red emission was observed for the compound with n=2 (Sr3Ti2O7: Pr3+), following by the compounds, n=1 and n=∞. The differences in luminescent properties among the compounds originate from the structural dependence of the energy transfer from the host to the activator Pr3+, and the site symmetry of Pr3+.

Polymerizable Complex Synthesis of Pure Sr2NbxTa2‐xO7 Solid Solutions with High Photocatalytic Activities for Water Decomposition into H2 and O2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Polymerizable Complex Synthesis of Pure Sr2NbxTa2-xO7 Solid Solutions with High Photocatalytic Activities for Water Decomposition into H2 and O2

Phase-pure solid solutions with the composition of Sr2NbxTa2-xO7 (SNT, x = 0−2) were prepared at 900 °C for 5 h by the Pechini-type polymerizable complex (PC) technique, based upon polymerization between citric acid and ethylene glycol. The two end compounds, Sr2Ta2O7 (x = 0) and Sr2Nb2O7 (x = 2), produced H2 and O2 in a stoichiometric ratio from pure water under UV light irradiation without a NiO cocatalyst. The photocatalytic activity of SNT for the water decomposition was greatly improved by loading NiO as a cocatalyst for a whole range of x. The photocatalytic activity was dramatically decreased approximately by 1 order of magnitude once Ta has been replaced by Nb, even when the amount of Nb was small. For all of the NiO-loaded SNT samples, water was stoichiometrically decomposed into H2 and O2. While samples prior to the complete crystallization showed very low activities despite their high surface area, the corresponding photocatalytic activities of well-crystallized samples depended primarily on th...

Synthesis of high surface area LaMnO3+d by a polymerizable complex method

Powders of LaMnO3+d were synthesized by the Pechini-type polymerizable complex (PC) technique based upon in situ polyesterification between citric acid (CA) and ethylene glycol (EG). Heating at 90°C of a mixed solution of CA, EG, H2O, La and Mn ions with a molar ratio of CA/EG/H2O/La/Mn=1.5/4/20/0.02/0.02 gave a transparent polymeric resin without any precipitation. The processing temperature of 90°C was critical for obtaining a clear transparent gel; i.e. above 90°C hydrolysis of La- and/or Mn–CA species seems to be accelerated to form insoluble precipitates before the polymerization was completed. The polymerization itself was also crucial for precluding formation of insoluble precipitates; i.e. the non-polymerizable amorphous citrate route without EG was always accompanied with formation of insoluble precipitates during evaporation of water from CA/H2O solutions irrespective of the processing temperature. The polymeric resin or citrate-derived gel was preheated at 450°C to give a powder precursor for LaMnO3+d. XRD analyses indicated that better mixing of La and Mn cations was achieved in the PC-derived precursor than in the precursor derived from the non-polymerizable amorphous citrate route. The formation of pure perovskite LaMnO3+d occurred when the powder precursor was heat-treated in a static air at 700°C for 6 h. The LaMnO3+d powder prepared by the PC method had a considerably large surface area of 20.2 m2/g, when compared e.g. with 8.3 m2/g for the powder of the same compound derived from the more widely used coprecipitation technique.

Spectroscopic Characterization of Precursors Used in the Pechini-Type Polymerizable Complex Processing of Barium Titanate

Precursor solutions containing citric acid (CA), ethylene glycol (EG), and barium and titanium ions, previously used in the Pechini-type polymerizable complex (PC) processing of BaTiO3, have been extensively characterized by Raman and 13C NMR spectroscopy. The simultaneous presence of Ba and Ti ions has brought about the unusual dissociation of proton from the alcohol OH group of CA, thereby creating an alkoxide oxygen atom with a strong nucleophilic nature, which prompts rearrangement of CA to form a barium−titanium mixed-metal CA complex with a stoichiometry close to Ba/Ti/CA = 1:1:3. The solution spectroscopy data have been compared with those obtained for an isolated mixed-metal CA complex from a solution with a chemical formula of BaTi(C6H6O7)3·4H2O. The Raman spectrum of BaTi(C6H6O7)3·4H2O has shown that the two terminal COOH groups of each CA coordinate in a monodentate fashion either to Ba or to Ti, while the central COOH group is not coordinating. The solid-state 13C NMR spectrum of the same complex has indicated that the alcohol OH in CA is fully deprotonated to form an alkoxide oxygen atom, in agreement with the solution 13C NMR data. A proposed model for the coordination structure of BaTi(C6H6O7)3 species, inferred from all these spectroscopic data, has displayed the presence of three fused six-memberd chelate rings arising from the full alkoxylation of CA ligands.

Polymerizable Complex Synthesis of RuO2/BaTi4O9 Photocatalysts at Reduced Temperatures: Factors Affecting the Photocatalytic Activity for Decomposition of Water

Phase-pure BaTi4O9 powders were prepared at temperatures higher than 700 °C by the Pechini-type polymerizable complex (PC) technique, based upon polyesterification between citric acid and ethylene glycol. BaTi4O9 was subsequently converted to nanocomposite materials by modifying its surface with ultrafine particles of RuO2, and the nanocomposite was used as photocatalyst for the decomposition of water into H2 and O2. The RuO2/PC-BaTi4O9 material prepared at 800 °C showed a photocatalytic activity ∼5 times higher than that of a sample prepared by a solid-state reaction at 1100 °C. The activity did not scale simply with the specific surface area of the samples; e.g. the sample having a surface area of 20.4 m2/g (prepared at 700 °C ) showed, unexpectedly, a ∼50% reduction in the activity when compared with that of a sample having a surface area of 7.6 m2/g (prepared at 800 °C ). The anomalous reduction in the activity was interpreted as a consequence of the increased number of lattice defects acting as inactivation centers. The average emission lifetime estimated from an emission decay curve decreased from 4.0 to 2.5 ns with a decrease in the preparation temperature from 900 to 700 °C, suggesting that the number of lattice defects is increased as the temperature is lowered.