Electrochemical properties of Cu6Sn5 alloy powders directly prepared by spray pyrolysis

Spherical shape Cu–Sn alloy powders with fine size for lithium secondary battery were directly prepared by spray pyrolysis. The mean size and geometric standard deviation of the Cu–Sn alloy powders prepared at a temperature of 1100 °C were 0.8 μm and 1.2, respectively. The powders prepared at a temperature of 1100 °C with low flow rate of carrier gas as 5 l min⁻¹ had main XRD peaks of Cu₆Sn₅ alloy and copper-rich Cu₃Sn alloy phases. Cu and Sn components were well dispersed inside the submicron-sized alloy powders. The discharge capacities of the Cu₆Sn₅ alloy powders prepared at a flow rate of 5 l min⁻¹ dropped from 485 to 313 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C. On the other hand, the discharge capacities of the Cu–Sn alloy powder prepared at a flow rate of 20 l min⁻¹ dropped from 498 to 169 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C.     

Experimental study on effect of compaction pressure on performance of SOFC anodes

NiO/yttria-stabilized zirconia (YSZ) anode substrates were fabricated at two compaction pressures of 200 and 1000 MPa, the particle size distributions of NiO and YSZ were investigated with powders treated under different conditions using a laser scattering technique (Mastersizer 2000, Malvern Instruments) and the effect of compaction pressure on the performance of solid oxide fuel cell (SOFC) anodes was investigated by studying the effect of compaction pressure on compaction density, sintered density, sintering shrinkage behavior, electronic and ionic conductivities. The results of investigation indicated that the SOFC with the anode compacted at a higher pressure exhibited a superior output performance, for example, a single cell with hydrogen as fuel and oxygen as oxidant exhibited excellent maximum power densities of 2.77 and 0.90 W cm⁻² at 800 and 650 °C, respectively, which suggested the development of an intermediate temperature SOFC through optimization of anode fabrication parameters.     

Effect of morphological properties of ionic liquid-templated mesoporous anatase TiO2 on performance of PEMFC with Nafion/TiO2 composite membrane at elevated temperature and low relative humidity

Three high-purity TiO₂ (anatase) powders (T_PF6, T_BF4, and T_conventional) were prepared by the sol–gel method with/without ionic liquid as template and calcinations at 450 °C. These powders were, then, characterized to investigate their differences in morphological properties. Electrochemical performances of the H₂/O₂ PEMFCs employing the Nafion composite membranes with these three TiO₂ powders as fillers were studied over 80–120 °C under 50% and 95% relative humidity (RH). The result showed that the order of the fillers effect on the performance at 80 and 90 °C was the same as that of the TiO₂ filler's specific surface area (i.e. T_PF6 > T_conventional > T_BF4 > P25, a commercially available nonporous TiO₂ powder). However, the order between T_conventional and T_BF4 was reversed at 110 and 120 °C under 50% RH. This indicates that the size and the amount of mesopores, which better confined the water molecules, were significant contributing factors to the performances at the higher temperatures. The best power density obtained under 50% RH at 120 °C and a voltage of 0.4 V was from the PEMFC with the T_PF6-containing Nafion composite membrane. It was about 5.7 times higher than the value obtained from that with the recast Nafion membrane.     

Preparation and performance of large-area La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte for intermediate temperature solid oxide fuel cell

Pre-treated LSGM starting powders decrease in particle size, leading to an increase in the LSGM relative density and electric conductivity. The starting powders are ball-milled with the assistance of absolute ethanol to reduce the particle size, dried ultrasonically to prevent the agglomeration of the powders and pre-calcined and re-balled to increase the rate of grain growth. These improvements make it possible to obtain single phase LSGM powders with small particle size at the calcination temperature of 1300 °C. A large-area (9 cm × 9 cm) LSGM electrolyte substrate has been prepared successfully by tape casting from these powders. The LSGM electrolyte exhibits a dense structure, the relative density reaches 96%, and the electrical conductivity is 0.08 S cm⁻¹ at 800 °C.     

Novel proton-exchange membrane based on single-step preparation of functionalized ceramic powder containing surface-anchored sulfonic acid

A novel approach to the synthesis of a low-cost proton-exchange membrane (PEM) based on the single-step preparation of a functionalized ceramic powder containing surface-anchored sulfonic acid (SASA) and a polymer binder, is presented for the first time. The added value of this technique, compared with earlier work published by our group, is the adoption of a direct, single-step synthesis, as opposed to a multiple-step synthesis. The latter requires an oxidation step, in order to convert the thiol group into a sulfonic group. SASA powders of different compositions have been prepared and characterized by means of Brunaur–Emmet–Teller (BET), thermogravimetric analysis–differential thermal analysis (TGA–DTG), differential scanning calorimeter (DSC), Fourier transformation infrared (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical techniques. The lowest equivalent weight measured for SASA powders is 1281 g equiv.⁻¹. The ionic conductivity of a 100-μm-thick membrane is measured ex situ at room temperature (25 ± 3 °C) and the highest proton conductivity is 48 mS cm⁻¹. The typical pore size, for the SASA powders is less than 10 nm and ranges from 2 to 50 nm for the SASA-based membranes. The membranes are thermally stable up to 250 °C.     

Significant impact of nitric acid treatment on the cathode performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite oxide via combined EDTA–citric complexing process

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) perovskite was synthesized by the sol–gel process based on EDTA–citrate (EC) complexing method, nitric acid modified EC route (NEC) and nitric acid aided EDTA–citrate combustion process (NECC). A crystallite size of 27, 38 and 42 nm, respectively, was observed for the powders of NECC-BSCF, NEC-BSCF and EC-BSCF calcined at 1000 °C, suggesting the suppression effect of nitric acid on the crystallite size growth of BSCF. The smaller crystallite size of the powders resulted in the higher degree of sintering of the cathode. Oxygen permeation study of the corresponding membranes demonstrated that in the powder synthesis, nitric acid also had a noticeable detrimental effect on the oxygen surface exchange kinetics and on the oxygen bulk diffusion rate of the BSCF oxides. The effect of powder synthesis route on the bulk properties of the oxide was validated by the oxygen temperature-programmed desorption technique. On the whole, a decreasing cathode performance in the sequence of EC-BSCF, NEC-BSCF and NECC-BSCF was observed. A peak power density of 693 mW cm⁻² was achieved for an anode-supported cell with an EC-BSCF cathode at 600 °C, which was significantly higher than that with an NEC-BSCF cathode (571 mW cm⁻²) or an NECC-BSCF cathode (543 mW cm⁻²) under similar operation conditions.     

Structural,morphological and electrical properties of Gd0.1Ce0.9O1.95 prepared by a citrate complexation method

A variant of the sol–gel technique known as cation complexation is used to prepare a nanocrystalline Gd_0.1Ce_0.9O_1.95 (GDC) solid solution. A range of techniques including thermal analysis (TGA/DTA), X-ray diffraction, specific surface area determination (BET) and electron microscopy (SEM and TEM) are employed to characterise the GDC powders. GDC calcined at 500 °C is found to have an average crystallite size of 11 nm. Specific surface areas are found to be 29.7 m² g⁻¹ for the as-calcined powder and 57.5 m² g⁻¹ after ball milling at 400 rpm. Dense ceramic pellets are prepared from unmilled and ball-milled GDC powders employing different thermal treatments. Their electrical properties are studied by impedance spectroscopy. Those samples sintered at 1300 °C for 30 h (starting from ball-milled powders) exhibit the highest density (96% of theoretical density) and the highest total ionic conductivity (1.91 × 10⁻² S cm⁻¹ at 600 °C).     

Synthesis of nano-crystalline Sr2MgMoO6−δ anode material by a sol–gel thermolysis method

Nano-crystalline Sr₂MgMoO_6−δ (SMMO) powders were synthesized successfully by a novel sol–gel thermolysis method using a unique combination of polyvinyl alcohol (PVA) and urea. The decomposition behavior of gel precursor was studied by thermogravimetric-differential thermal analysis (TG/DTA) and the results showed that the double-perovskite phase of SMMO began to form at 1000 °C. The microstructure of the samples had been investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns confirmed that well-crystalline double-perovskite SMMO powders were obtained by calcining at 1450 °C for 12 h. TEM morphological analysis showed that SMMO powders had a mean particle size around 50–100 nm. The SAED pattern and Raman spectroscopy showed that the SMMO powders were nano-polycrystalline well-developed A(B′_0.5B″_0.5)O₃ type perovskite material. The XPS results demonstrated that the Mo ions in SMMO had been reduced after exposure to H₂. The electric property was studied by four-probe method. The results showed that conductivity was 8.64 S cm⁻¹ in 5.0% H₂/Ar at 800 °C and the activation energies at low temperatures (400–640 °C) and high temperatures (640–800 °C) are about 21.43 and 6.59 kJ mol⁻¹, respectively.     

Ultra-thin nanocrystalline lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.8Fe0.2O3−δ) films synthesis by RF-sputtering and temperature-dependent conductivity studies

Nanocrystalline lanthanum strontium cobalt ferrite (LSCF) ultra-thin films with high in-plane electrical conductivity have been deposited by RF sputtering from composite targets. The films, with nominal thickness of 54 nm, are crystalline when annealed or deposited at temperatures above 450 °C. Effects of annealing temperature, annealing time, and substrate temperature on crystallization, microstructure, and room temperature lateral electrical conductivity have been systematically studied. No interfacial reaction products between the LSCF and single crystalline yttria-stabilized zirconia (YSZ) were observed from X-ray diffraction studies upon annealing until 750 °C. In-plane electrical conductivity as high as 580 S cm⁻¹ at 650 °C has been observed for LSCF thin films deposited on single crystalline YSZ substrates and sputtered nanocrystalline YSZ thin films; while activation energy for conductivity were determined to be 0.15 eV and 0.10 eV for the former and latter films, respectively, in 650–400 °C range. The high in-plane electrical conductivity for the nanocrystalline LSCF ultra-thin films is likely attributed to their low level of porosity. Micro-solid oxide fuels cells using 15 nm thick LSCF films as cathodes and sub-100 nm yttria-doped zirconia thin film electrolytes have been fabricated successfully and demonstrated to achieve peak power density of 60 mW cm⁻² at 500 °C. Our results demonstrate that RF sputtering provides a low-temperature synthesis route for realizing ultra-thin nanocrystalline LSCF films as cathodes for intermediate- or low-temperature solid oxide fuel cells.     

Mixed transport properties of Ce1−xSmxO2−x/2 system under fuel cell operating conditions

Ceria-samaria compositions were prepared by freeze-drying, yielding very homogeneous and fluorite single phase powders at temperatures as low as 375 °C. These powders were used to obtain dense ceramics by sintering at 1600 °C for 10 h. The mixed transport properties of these materials were characterized by impedance spectroscopy and the Hebb–Wagner ion-blocking method, at temperatures in the range 700–950 °C. The onset of n-type electronic conductivity was expressed as a function of oxygen partial pressure and temperature, and was found dependent on the Sm-content. Mixed transport properties were also analyzed for conditions in which the samples are exposed to fuels in a SOFC, either as electrolyte or component of cermet anodes. The degree of fuel conversion and corresponding changes in gas composition produce changes in the mixed transport properties of SOFC electrolytes, with emphasis on the fuel side. The mixed transport properties were also used to estimate open cell voltage as a function of fuel conversion.     

Thermally nitrided Cu–5.3Cr alloy for application as metallic separators in PEMFCs

Cr-nitride which offers good electrical conductivity and corrosion resistance was formed on the surface of Cu–5.3 (wt.%)Cr alloy and its characteristic properties including electrochemical behavior and electrical conductivity were evaluated. The sample was annealed for 12 h in a temperature range of 600–1000 °C in a nitrogen atmosphere. Nitridation of Cr in Cu–5.3Cr alloy occurred at about 600 °C and followed Cr → Cr₂N → CrN phase transformation sequence. A continuous Cr-nitride was formed at 1000 °C, but not below 900 °C. Corrosion behavior of the continuously nitrided sample was investigated in simulated PEMFC environments. Corrosion resistance of the nitrided sample was improved in an anode environment, but not in a cathode environment. This was attributed to the dissolution of Cu through pin-hole defects on the surface of the nitrided sample just in the cathode environment. Interfacial contact resistance of the nitrided Cu–5.3Cr alloy was satisfied the target value. Furthermore, there was no recession of electrical conductivity after polarization.     

Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3−δ

Perovskite oxide La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428), a wonderful electronic–ionic conductor could be used as cathode of solid oxide fuel cell (SOFC). Graded cathode with coarse layer and fine layer, could improve the diffusion rate and electrochemical reaction activity of oxidant. The fabrication and properties of graded LSCF6428 cathode were discussed in this paper. First, pure perovskite LSCF6428 powders were prepared by citrate–EDTA method (CEM), citrate method (CM) and solid phase synthesis (SPS). The powders with higher specific surface area and smaller grain size are easier to be sintered and densified. Single LSCF6428 cathode with thickness of 30 μm was prepared by SPS powders, the porosity of cathode was high about 30% and pore size was about 5 μm. Graded LSCF6428 cathode including 30 μm outer layer and 10 μm inner layer was prepared by SPS and CM powders, respectively. Clear double-layer cathode was observed by SEM, which combined tightly and transited gradually. Porosity of outer layer is high about 30% and pore size is about 1–5 μm; inner layer is finer and pore size is about 0.2–1 μm. Based on the above research, 300 μm yttria stabilized zirconia (YSZ) electrolyte supported cell with single LSCF6428 cathode and double-layer LSCF6428 cathode were prepared, and the properties of two type cells were tested in H₂. Power density of graded cell is 197 mW cm⁻² at 950 °C, and improved about 46% comparing that of single layer LSCF6428 cell (135 mW cm⁻²).     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

Effects of hot pressing conditions on the performances of MEAs for direct methanol fuel cells

The effects of hot pressing conditions (hot pressing temperature, pressure and time) on the performances of membrane electrode assemblies for direct methanol fuel cells were investigated. The performances of membrane electrode assemblies (MEAs) were characterized by the polarization curves and electrochemical impedance spectra (EIS). The surface morphologies of the electrodes were observed by scanning electron microscopy (SEM). The compression ratios of electrodes were determined by testing the thicknesses of the anodes and the cathodes before and after the hot pressing process. The MEA which was hot pressed at 135 °C under 80 kg cm⁻² for 90 s, showed the highest power density of 46.0 mW cm⁻² at 80 °C and ambient pressure. As the hot pressing temperature, pressure and time increased, the compression ratios of the anodes and cathodes increased, and the activating time required for MEA to reach optimum performance increased, too. The cell resistances of the MEAs hot pressed at higher hot pressing temperature (135 °C) and pressure (120 kg cm⁻²), or for longer time (90 s), decreased because of the good contact between the membrane and electrodes. The MEAs that were hot pressed under higher temperature (135 °C) and higher pressure (120 kg cm⁻²) benefited for long-time cell operating.     

Efficient catalytic properties of Co–Ni–P–B catalyst powders for hydrogen generation by hydrolysis of alkaline solution of NaBH4

The aim of the present work is to study the catalytic efficiency of amorphous Co–Ni–P–B catalyst powders in hydrogen generation by hydrolysis of alkaline sodium borohydride (NaBH₄). These catalyst powders have been synthesized by chemical reduction of cobalt and nickel salt at room temperature. The Co–Ni–P–B amorphous powder showed the highest hydrogen generation rate as compared to Co–B, Co–Ni–B, and Co–P–B catalyst powders. To understand the enhanced efficiency, the role of each chemical element in Co–Ni–P–B catalyst has been investigated by varying the B/P and Co/Ni molar ratio in the analyzed powders. The highest activity of the Co–Ni–P–B powder catalyst is mostly attributed to synergic effects caused by each chemical element in the catalyst when mixed in well defined proportion (molar ratio of B/P = 2.5 and of Co/(Co + Ni) = 0.85). Heat-treatment at 573 K in Ar atmosphere causes a decrease in hydrogen generation rate that we attributed to partial Co crystallization in the Co–Ni–P–B powder. The synergic effects previously observed with Co–Ni–B and Co–P–B, now act in a combined form in Co–Ni–P–B catalyst powder to lower the activation energy (29 kJ mol⁻¹) for hydrolysis of NaBH₄.     

Influence of through-lamella grain growth on ionic conductivity of plasma-sprayed yttria-stabilized zirconia as an electrolyte in solid oxide fuel cells

The development of a cost-effective fabrication method for stabilized zirconia electrolyte for the most advanced tubular solid oxide fuel cell (SOFC) remains the most important challenge for the commercialization of an SOFC power generation system. Atmospheric plasma spraying is expected to be a promising alternative to other costly electrolyte processing methods. The problem with the plasma-sprayed ceramic coating is the limited interface bonding of the lamellar structure, which reduces the ionic conductivity of stabilized zirconia deposits to one-fifth of the comparable bulk. Continuous growth of columnar grains across splat–splat interfaces has been achieved through control of the substrate surface temperature which affects spreading of molten droplets. These cross-splats columnar grains lead to improved bonding between lamellae. Measurements over the temperature range of 600–1000 °C have shown that the microstructural changes result in a significant increase of ionic conductivity of the yttria-stabilized zirconia deposit (by a factor of about 3). A change in activation energy at about 750 °C was observed for coatings deposited with two different sets of spray conditions. This change is associated with a switch of the predominant ion conduction path from grain boundary to intragrain with increasing temperature.     

Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte

La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd_0.1Ce_0.9O_1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm² at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm² was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm² at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm² at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm² at 650, 600, 550 and 500 °C, respectively.     

Electrical and microstructural characterization of two-step sintered ceria-based electrolytes

Gadolinium-doped ceria-based materials with and without Ga-additions were prepared following several firing schedules including one peak sintering temperature (up to 1300 °C) with or without subsequent dwell at lower temperature (at 1150 °C). Sintered disks with submicrometric grain size and densifications in the order of 92% or higher, were obtained in this manner, with the final result depending slightly on the sintering profile and presence of Ga as dopant. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. The grain boundary arcs were found slightly dependent on grain size and porosity but significantly on Ga-doping, due to the likely presence of large concentrations of Ga along the grain boundary region.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Hydrogen storage properties of 2Mg–Fe mixtures processed by hot extrusion: Influence of the extrusion ratio

Hot extrusion processing was employed to produce 2Mg–Fe bulk mixtures for hydrogen storage. 2Mg–Fe powder mixtures were prepared by high-energy ball milling. These mixtures were cold pressed into cylindrical pre-forms, which were then processed by hot extrusion (at 300 °C) to produce bulks. In this work, we analyzed the influence of the extrusion ratio (3/1, 5/1 and 7/1) on the sorption properties of the bulks. The nanometric grain size remained unaltered after all hot extrusion conditions. More porous bulks were produced at an extrusion ratio of 3/1. In the first cycle of hydrogenation, the sample processed at 3/1 absorbed more hydrogen (4 wt% of H) than the precursor powders (3 wt% of H). The results showed that the desorption temperature of bulks were very similar to that of 2Mg–Fe powders, which is good considering the lower surface area of bulks, and that samples with Fe in excess presented lower desorption temperatures.