NiO Grains Research Articles

Performance of NiO/MgFe2O4 composite cathode for a molten carbonate fuel cell

To improve the stability of the cathode in a molten carbonate (Li/Na) electrolyte an attempt has been made to fabricate a new cathode structure in with NiO grains are coated with an outer layer such as MgFe 2 O 4 . This is considered to offer a new cathode material for molten carbonate fuel cells. Metal (Ni) and ceramics (MgFe 2 O 4 ) composite particles, which consisted of a filament-shaped Ni grain coated with fine ceramic particles have been successfully prepared by a mechanical coating technique using a Theta Composer. The solubility of nickel from the NiO/MgFe 2 O 4 composite cathode is about one-fourth of that of NiO. A single cell with a NiO/MgFe 2 O 4 composite cathode displays a high performance and a good stability.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

Preparation and structural investigations of electrochromic nanosized NiO x films made via the sol–gel route

The main aim of this work was to study the influence of the NiSO 4 precursor and the annealing temperature on the electrochemical stability and electrochromic response of dip-coated Ni-oxide films. Nickel hydroxide was precipitated from the precursor solution using lithium hydroxide and the slurry was then peptized with acetic acid. Dynamic thermogravimetric measurements made on thin films showed that the decomposition and the formation of Ni-oxide phase differed for powders and films. The results showed that the films annealed at 270 °C (at which the acetate groups decomposed) consisted of cubic (space group Fm3m) NiO grains (2–3 nm, TEM measurements) having monodentately coordinated sulfate groups (IR spectrum). Electrochemical and in situ UV–visible spectroelectrochemical studies of the Ni-oxide films (NiSO 4 precursor, 270 °C) revealed that the photopic transmittance in the colored state (3rd cycle) is 66% and 89% in the bleached state. Cycling (100th cycle) reduced the transmittance of the films in the colored state to 50%, while the bleached films retained their high transmittance (89%). The coloration efficiency of the films was 35–41 cm 2 C −1 depending on the number of cycles. The evolution of the Ni 3+–O stretching vibration (transversal optical mode) of the films in the charged state was followed by the help of ex situ infrared spectroelectrochemical measurements.

Dependence of exchange coupling on NiO grain size in NiO/NiFe bilayers

The texture and grain-size effects on the exchange bias in sputtered polycrystalline NiO/NiFe bilayers were studied. Two oriented antiferromagnetic NiO layers along (111) and (200) planes were fabricated on SiO2/Si(100) substrates by varying the Ar/O2 ratio. An exchange anisotropy field Hex was detected in both NiO/NiFe bilayers with a NiO(111) plane where Ni moments are in ferromagnetic (FM) order, and with a NiO(200) plane where Ni moments are in fully compensated antiferromagnetic (AF) order. In order to clarify the presence of the Hex in a NiO(200)/NiFe bilayer, we prepared NiO(200) layers with different grain sizes by controlling a total pressure at a constant Ar/O2 ratio in a sputter chamber. We observed that the Hex of the bilayer films with small grains of NiO(200) is larger than the Hex with large grains. This observation is consistent with a model that the exchange interaction is caused by the reorientation of the moments with AF layer spins rotating, rather than FM layer spins rotating at the interface of the bilayer.

Synthesis of NiO–YSZ composite particles for an electrode of solid oxide fuel cells by spray pyrolysis

NiO–YSZ (YSZ: Y2O3-stabilized ZrO2) composite particles for a Ni–YSZ cermet anode in solid oxide fuel cells (SOFCs) were synthesized via spray pyrolysis (SP). The formation mechanism of the composite particles by this process was analyzed. The internal microstructure of the particles synthesized during SP processing was observed at each heating temperature of 200, 300, 400 and 1000 °C, and then the formation mechanism of the composite structure was discussed. As a result, it was found that NiO–YSZ composite particles were formed through the following steps. Firstly, during the evaporation stage up to 200 °C, a filled particle with Ni(CH3COO)2 and YSZ fine grains were formed from the atomized droplet containing Ni ion and dispersed YSZ sol by volume precipitation. Secondly, during the continuous thermolysis stage up to 400 °C, YSZ grains were formed and moved to the surface of the composite particle by the outgas and the oxidation of Ni(CH3COO)2. Finally, the NiO–YSZ composite particle that has NiO grains uniformly covered with fine YSZ grains was formed after the final sintering stage up to 1000 °C.

Performance and stability of SOFC anode fabricated from NiO/YSZ composite particles

Abstract NiO/YSZ composite particles with 20–35 mol% Y2O3 stabilized ZrO2 (YSZ) were synthesized by the spray pyrolysis (SP) method. The NiO/YSZ composite particles showed morphology of NiO grains partially or fully covered with fine YSZ grains. The composite particles also had good interface connection between nickel oxide and yttria stabilized zirconia (YSZ). The morphology of the Ni/YSZ cermet anode fabricated from composite particles is noticeably influenced by the YSZ content of the composite particles. Ni/YSZ cermet anodes with 25 mol% YSZ show the highest electrochemical activity and lowest internal resistance (IR). The electrochemical activity and IR of the anode in solid oxide fuel cells (SOFCs) highly depend on the three-phase boundary structure based on the metal (Ni) and ceramic (YSZ) interface as well as the network structure of each Ni grains. Moreover, the cell voltage of a single cell with the Ni/YSZ cermet anode kept almost constant value during 7200 h of operation. Consequently, it is concluded that the performance and stability of the Ni/YSZ cermet anode is improved by controlling the morphology of the NiO/YSZ composite particles.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

New Chemical Routes for Preparation of Ultrafine Nio-YSZ Powders for SOFC Anode Applications

Ultra-fine NiO-YSZ composite powders with a high dispersion level of NiO (around 40% molar) have been prepared. Two different chemical routes were used to obtain nanostructured powders as starting materials promising to get high performance anode for SOFC applications. Material characteristics such as porosity, NiO particle distribution, grain size and particle to particle contact were investigated. Different morphologies were observed after calcination processes. Very small grains of each phase, NiO and YSZ, have been obtained using both preparation methods.

High temperature growth kinetics and texture of surface-oxidised NiO for coated superconductor applications

Thick NiO films were grown in air, on biaxially textured (0 0 1) Ni and as-rolled Ni tapes, at temperatures from 1050 to 1350 °C. Ni diffusion through the NiO film mainly contributes to the growth since is much faster than oxygen diffusion and occurs by a vacancy diffusion mechanism in the lattice at high temperatures. Parabolic growth kinetics were found for both NiO film thickness and grain growth, and compared with the literature data. Competitive growth of (1 1 1) and (0 0 1) oriented grains establishes the final NiO orientation at temperatures below 1250 °C, while at higher temperatures leakage diffusion at/towards grain boundaries, grain coarsening and (1 1 0) oriented grains disrupt the (1 0 0) texture. Hence, development of epitaxy of NiO on textured Ni tapes was found to be largely due to growth kinetics depending on both, time and temperature. We report here a systematic study of the microstructure and kinetics of formation of textured NiO substrate for application as a buffer layer in coated conductor technology.

Vibrational properties of a Σ5(310)[001] NiO grain boundary as a function of temperature: a molecular dynamics simulation

Vibrational properties of a Σ5(310)[001] NiO grain boundary as a function of temperature: a molecular dynamics simulation

Vibrational properties of a Σ5(310)[001] NiO grain boundary as a function of temperature: A molecular dynamics simulation

In this communication we present results concerning the study of the vibrational properties of a Σ5(310)[001] NiO grain boundary by molecular dynamics simulations. The phonon density of states of the cation and anion sublattice in the boundary region is calculated as a function of temperature in the directions normal and parallel to the boundary plane. The obtained results are compared with those obtained for the bulk and the differences are discussed. We have also calculated the mean square displacements of the cations and anions in the grain boundary region as a function of temperature in the direction normal and parallel to the boundary plane and the results are compared with those of the bulk. The combination of the above results leads to the conclusion that the ions are more loosely coupled in the direction normal to the boundary plane than parallel to it and this anisotropy is persistent up to high temperatures. The increase of temperature also leads to the softening of the binding of the ions in both directions.

Ni–YSZ cermet anodes prepared by citrate/nitrate combustion synthesis

The synthesis of Ni–YSZ cermets with tailored particle package, shape and microstructural characteristics is essential when preparing anodes for solid oxide fuel cells (SOFC). These materials are generally prepared by sintering and subsequent reduction of the mixture of metal oxides. In order to obtain cermets with an adequate contact area between electrocatalyst (Ni) and ionic conductor (YSZ), an alternative route was used based on mixed gel combustion with the material synthesis, calcination and partial sintering achieved in one step. The precursor for the combustion synthesis was a mixed citrate/nitrate gel prepared from nickel, zirconium and yttrium nitrates and citric acid by vacuum evaporation of the solution. The combustion reaction of this gel produces submicrometer crystalline NiO–YSZ composite. The influence of the fuel/oxidant molar ratio of the precursor on the combustion rate and end product characteristics was investigated. The reaction period, phase composition, morphology and agglomerate formation were studied in detail. It was shown that the initial fuel/oxidant ratio strongly influences the characteristics of the powder mixtures thus obtained. The morphological properties of the prepared mixed oxides after the combustion synthesis reveal that the particle size distribution and the agglomerate formation in the voluminous intermediate mixed oxide product (green body) differ with the initial fuel/oxidant molar ratio. Narrower agglomerate and pore size distribution has a great influence on the subsequent sintering and reduction of the mixed material. If the particle and pore size distribution in the green body are narrow, the coarsening of the YSZ and NiO grains, and subsequently, YSZ and Ni grains are less pronounced.

Aging of the NiO-YSZ Working Electrode of the Electrochemical Cell for NO Decomposition.

The microstructure and chemical changes in the composite working electrode of electrochemical cells ((Composite working electrode)|Pt(Cathode)|YSZ|Pt(Anode)) for NO decomposition were observed and investigated. It was found that the NiO/YSZ interface is not stable at the electrochemical cell operating temperature of 600°C. Structure evolution was studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The formation of a small amount of Ni phase and of small size NiO grains with a size of about 10-100 nm in the NiO/YSZ interface region was observed and investigated. The reduction of NiO into Ni phase in the near surface region under the external voltage applied between the cathode and anode is proposed. The oxidation of small Ni grains into NiO grains as a result of NO and O2 gas adsorption at the NiO/YSZ interface is discussed.

Pulsed laser deposition of yttria stabilized zirconia for solid oxide fuel cell applications

Yttria stabilized zirconia (YSZ) films were grown by pulsed laser deposition (PLD) from a 8YSZ target using a KrF excimer laser source (248 nm). The films have been deposited under oxygen atmospheres on porous NiO/YSZ substrates heated from room temperature to 600 °C. YSZ films were obtained in the range of 1–2 μm thickness. The films have been investigated with respect to surface morphology, microstructure and film–substrate interface interaction. The film morphology varied from columnar to an irregular crystalline structure depending on the oxygen pressure and the substrate temperature. In all cases the films consisted of YSZ with the cubic fluorite structure. The formation of oxide layers under low oxygen pressures on the NiO/YSZ substrates is due to a film–substrate redox interaction. The NiO grains close to the coating interface are partially reduced and serve as an oxygen source for the oxidation of the film. The measured He leakage rates to analyze the gas tightness of the YSZ films have so far shown no improvements as compared with uncoated substrates.

Effect of La 2O 3 particles on microstructure and cracking-resistance of NiO scale on electrodeposited nickel films

The effect of La 2O 3 particles on the microstructure and cracking-resistance of thermally grown NiO scale, formed at 900 and 1000°C, on electrodeposited nickel films has been investigated through in situ acoustic emission (AE) testing in conjunction with SEM and TEM/EDX. NiO scale, formed on electrodeposited films of pure nickel, has a porous duplex layered structure, which exhibits a more brittle behavior than the scale on films with the addition of La 2O 3, particles. They played a profound role in the procrastination of the occurrence of scale cracking and reduction of the cracking events. Upon the comparison of the microstructure of NiO scale on the nickel film with the NiO scale on the LaO-containing film, several points can be concluded. For example, the stresses in NiO scale on the film with La 2O 3 were partly relaxed by a nondestructive route, such as scale plastic deformation due to La 2O 3-induced formation of a single-layered and fine-grained NiO scale. Also, it is believed that the particles reduced the size of the defects at the scale–metal interface and probably enhanced the cohesion between NiO grains and the adhesion between the scale–metal interface by retardation of the inward diffusion of nickel vacancies.

DC Sputtering of yttria-stabilised zirconia films for solid oxide fuel cell applications

Yttria-stabilised zirconia (YSZ) thin films were dc-sputtered and investigated with respect to surface morphology, microstructure and film-substrate interface interaction. The films were deposited under argon/oxygen atmospheres on NiO/YSZ substrates heated to between 500 and 700°C. Dense and crack-free coatings were obtained in the thickness range of 1 to 10 μm. The film morphology varied from columnar to crystalline structure depending on the oxygen pressure and the substrate temperature. Whereas the coated films consisted of YSZ with cubic and tetragonal crystal structure under low oxygen atmospheres, the same deposition experiments on Al2O3 substrates revealed highly disordered layers of cubic YSZ. The formation of oxide layers on the NiO/YSZ substrates is due to a film-substrate redox interaction. The NiO grains close to the coating interface are partially reduced and serve as an oxygen source for the oxidation of the film. An exponential decay of the gas leakage vs. coating thickness was found.

Computer simulation of the reactive element effect in NiO grain boundaries

Diffusion within the grain boundaries of ceramics is an important mechanism for the growth of oxide films at moderate temperatures. Experiments show that the addition of impurities can drastically reduce the rate at which the film grows. We have investigated this by atomistic computer simulation. Since the migration energies of grain boundary processes in ionic systems are too high for a conventional molecular dynamics simulation to be used, we have used a modified simulation. The structure of the boundary is equilibrated and a vacancy introduced. A migration trajectory is chosen and a small force pulling the ion along this trajectory added to the ion that is to hop. A counter-force is added to the remaining ions to prevent the whole cell moving and the velocities are scaled to remove the energy introduced by doing work on the moving ion. The effect of this is to enable the hopping trajectory to be plotted out and the migration energy found. We have investigated the effect of Mg 2+, Ca 2+, Sr 2+ and Ba 2+ impurities on the migration energies and diffusion pathways of cation vacancies in the {310}/[001] and {410}/[001] tilt grain boundaries of NiO at moderate temperatures. We show that there is a correlation between the size of the impurity and its favoured position within the boundary. The presence of impurities increases the migration energies and alters the diffusion pathways. We conclude that impurities will bind vacancies to themselves, reducing the rate of diffusion.

Cross-sectional high-resolution transmission electron microscopy of the microstructure of electrochromic nickel oxide

Electrochromic nickel oxide films on indium tin oxide (ITO) were investigated by cross-sectional high-resolution transmission electron microscopy and energy dispersive X-ray analysis. Microstructural features and the differences between high and poor-quality samples were studied and compared. The dominant phase of the NiO film has cubic structure, but selected area electron diffraction patterns revealed many extra diffraction spots for the high-performance samples which may result from additional hydrated nickel oxide phases. The NiO grains do not show clear shapes in the cross-sectional plane nor are there signs of a preferred orientation. The mean grain size is larger and there are more defects and superlattices in samples with good electrochromic properties. There was a clear correlation between grain size distribution and performance. The high-quality samples had a mean grain size of about 6.5 nm, whereas the poor-quality samples exhibited significantly smaller mean grain sizes of 5.0 and 3.8 nm.

Oxidation mechanism of Ni–20Cr thin strips and effect of a mechanical loading

The oxidation behaviour of Ni-20Cr foils of 200 μm thickness was studied in air at 600°C and 900°C. The oxide morphology and nature were determined by SEM and TEM. The scales formed at all temperatures were complex, with an outer NiO layer and an intermediate layer of equiaxed NiO, NiCr2O4 and Cr2O3 grains. At 600°C, internal oxidation of chromium occurred in the substrate while a continuous Cr2O3 layer was observed at 900°C. The effect of a tensile load on the oxidation kinetics and mechanism of Ni–20Cr alloy was also examined. The results indicated that applying a tensile load did not modify the oxide nature and morphology but induced an increase of the oxidation rate, promoting the formation of internal oxidation at 600°C, and of a NiCr2O4 layer at 900°C.

Properties of NiO cathode coated with lithiated Co and Ni solid solution oxide for MCFCs

The short circuit of cells due to the dissolution and the deposition of NiO in a molten carbonate is a big problem that has to be resolved if we want to use Molten Carbonate Fuel Cells (MCFCs) for long-term operation. To resolve this problem, we have proposed a new cathode structure in which NiO grains have been coated with LiCoO 2 grains by usual tape casting method, because LiCoO 2 is more stable in a carbonate melt than NiO. CoO/NiO composites with 5 to 20 mass% Co could be formed by using the CoO/Ni composite particles. As confirmed by SEM, these composites consisted of NiO cores covered with the outer layer of Li(Co, Ni) oxide. The stability of the lithiated CoO/NiO composite with 5 mass% CoO was significantly improved in comparison with that of conventional NiO for MCFCs. The lithiated CoO/NiO composite was also successfully formed by in-situ oxidation and lithiation process. We thought that this composite cathode material consisting of NiO core covered with Li(Co, Ni) oxide will be adopted as the new cathode for MCFCs.