•The oxidation kinetics of small NiFe particles were studied by using thermoanalysis.•Grain boundary, lattice, and phase conversion induced diffusions were recognized.•The activation energy of oxidation increases with the Fe content in the alloy.•Each diffusion process corresponds to a characteristic temperature in TPO analysis.•NiFe alloys with ~5–10 wt% Fe content have the lowest oxidation rates.The oxidation of permalloys is important to apply in a wide range. The oxidation and diffusion mechanisms of small permalloy particles with different Fe content are studied by using thermal gravimetric analysis (TGA) and microstructure characterizations. Fe2O3/(Ni, Fe)3O4 plays a key role in the morphology evolution and diffusion mechanisms of small NiFe particles upon oxidation. The activation energies of grain boundary diffusion for the NiFe alloys increase from 141 kJ/mol to 208 kJ/mol as the Fe content increases from 0 to ~50 wt%. We have developed a diffusion process resolved temperature programed oxidation (PR-TPO) analysis method. Three diffusion mechanisms have been recognized by using this method: In addition to the grain boundary diffusion and lattice diffusion, our TGA analysis suggests that the phase conversion from Fe2O3 to (Ni, Fe)3O4 induces diffusion change and affects the diffusion process at the intermediate temperature. Relevant oxidation kinetics and diffusion mechanisms are discussed.The oxidation mechanisms of small Permalloy particles with different Fe content is studied by using thermal gravimetric analysis (TGA) and microstructure characterizations. The activation energies of grain boundary diffusion for the NiFe alloys increases from 140 kJ/mol to 208 kJ/mol as the Fe content increases from 0 to 50 wt% as determined by TGA. We have developed a diffusion process resolved temperature programed oxidation (DPR-TPO) analysis method, and three diffusion mechanisms have been recognized by using this method: In addition to the well-known grain boundary diffusion and lattice diffusion, we found that the phase conversion from Fe2O3 to (Ni, Fe)3O4 will induce diffusion changes and affect the diffusion process at the intermediate temperature. The diffusion processes can be characterized by the corresponding characteristic peak temperatures in temperature programmed oxidation (TPO) analysis. This work not only give insight knowledge about the oxidation and diffusion processes of small permalloy particles, but also, provides a useful tool for analyzing solid-gas reactions of other materials.

We fabricated a high performance self-powered photoelectrochemical cell (PECC) type UV photodetector with ZnO nanorod arrays (NRs) as the photoanode, [Co(bpy)3]3+/2+ as the electrolyte and ITO glass coated by polymer poly(3,4-ethylenedioxythiophene) (PEDOT), PEDOT/ITO, as the counter electrode (CE). The UV photodetector shows a good photovoltaic performance (VOC = 0.5 V, ISC = 6.2 μA) and a high photosensitivity of 263 under the illumination of 365 nm UV light with an intensity of 2 mW cm−2. The device also shows a high response speed (response time < 0.2 s). The high photosensitivity and rapid response speed are attributed to the good electrocatalytic activity of PEDOT towards Co-complex redox shuttle. The high performance of the detector, together with the Pt-free low cost CE and the facile fabricating method, makes the device promising in optoelectronic applications.

Exploring low-cost catalysts with low carbon deposition and high activity for dry reforming of methane (DRM) is of great importance in both industrial and academic fields. La2(Ni1−xCux)O4 (x = 0.0–0.4) perovskites were synthesized by a sol–gel self-combustion method and evaluated as catalyst precursors for DRM. The reduced La2(Ni0.8Cu0.2)O4 shows optimal performance with negligible carbon deposition, and the conversions of CH4 and CO2 are 73% and 80%, respectively. The remarkably improved carbon resistance of the reduced La2(Ni0.8Cu0.2)O4 is attributed to the small metallic particles obtained from the reduced perovskite and the surface segregation of Cu in the metallic Ni–Cu particles.

In recent years, BiFeO3 has attracted significant attention as an interesting multiferroic material in the exploration of fundamental science and development of novel applications. Our previous study (Phys. Chem. Chem. Phys.18, 2016, 25409) highlighted the interesting physicochemical features of BiFeO3 of sub-5 nm dimension. The study also accentuated the existence of weak ferroelectricity at sub-5 nm dimensions in BiFeO3. Based on this feature, we have prepared thin films using sub-5 nm BiFeO3 nanoparticles and explored various physicochemical properties of the thin film. We report that during the formation of the thin film, the nanoparticles aggregated; particularly, annihilation of their nanotwinning nature was observed. Qualitatively, the Gibbs free energy change ΔG governed the abovementioned processes. The thin film exhibited an R3c phase and enhanced Bi–O–Fe coordination as compared to the sub-5 nm nanoparticles. Raman spectroscopy under the influence of a magnetic field shows a magnetoelectric effect, spin phonon coupling, and magnetic anisotropy. We report room-temperature ferroelectric behavior in the thin film, which enhances with the application of a magnetic field; this confirms the multiferroic nature of the thin film. The thin film shows polarization switching ability at multiple voltages and read–write operation at low bias (±0.5 V). Furthermore, the thin film shows negative differential-complementary resistive switching behavior in the nano-microampere current range. We report nearly stable 1-bit operation for 102 cycles, 105 voltage pulses, and 105 s, demonstrating the paradigm device applications. The observed results thus show that the thin films prepared using sub-5 nm BiFeO3 nanoparticles are a promising candidate for future spintronics and memory applications. The reported approach can also be pertinent to explore the physicochemical properties and develop potential applications of several other nanoparticles.

•UV photodetectors with large area can be operated at a self-powered mode.•The photoelectrical properties of the device can be tailored by hydrogen annealing.•Hydrogen annealing reduced the defects in the ZnO nanorod arrays.•The barrier height of the organic-inorganic heterojunction is increased.Exploring low cost and high performance self-powered UV photodetectors is very important for applications. In the present work, highly ordered and vertically oriented ZnO nanorod arrays on a glass substrate with an Al doped ZnO seed layer were synthesized by a hydrothermal method. It is shown that the point defects and Fermi level of the ZnO NRs can be effectively tuned by hydrogen annealing. The electrical properties of the ZnO/PSS:PEDOT heterojunctions using the hydrogen annealed ZnO nanorod arrays are improved significantly. A minimum ideality factor of 1.2 and a maximum rectification ratio of 255 at ± 2 V are achieved for the heterojunction with ZnO nanorod arrays annealed at 300 °C under hydrogen atmosphere. It is demonstrated that all the UV photodetectors based on the ZnO/PSS:PEDOT heterojunction can be operated at a self-powered mode with a response time < 1 s. The photodetector with the 300 °C hydrogen annealed ZnO nanorod arrays shows the highest photocurrent of 82 nA and the highest photosensitivity of ~ 1000 at zero bias under 365 nm UV light with an intensity of 2.0 mW/cm2. The improvement in photoelectrical properties of the photodetector is ascribed to the reduced point defects in the ZnO nanorod arrays and increased barrier height of the heterojunction, which is effectively tuned by hydrogen annealing.

•A two-step sintering method for the fabrication of half cells is reported.•The shrinkage of the NiO-YSZ anode material can be tailored by adding Al2O3.•Al2O3 transiently promote the grain growth of NiO at low temperature.•NiAl2O4 spinel suppresses the grain growth of NiO at high temperature.•High performance flat cells have been demonstrated by adding 0.2 wt% Al2O3 into the anode.The co-sintering process of half-cells has an important effect on the flatness and performance of solid oxide fuel cells. In this study, we report a two-step sintering method to fabricate flat three-layer half-cells. The first sintering step is a freestanding sintering process at a low temperature (1280 °C). The second sintering step is a constrained sintering process at 1400 °C. The shrinkage of the anode support layer (ASL) and the curvature of the half-cell can be adjusted by adding Al2O3 into the ASL in the first sintering step. Effects of Al2O3 addition on the NiO-YSZ anode material are also studied. We find that NiO reacts with Al2O3 to form NiAl2O4 spinel at the early sintering stage. This reaction transiently promotes the grain growth of NiO. Once the reaction terminates and the NiAl2O4 spinel is formed, the grain growth of NiO will be suppressed, even at higher sintering temperatures. Our results indicate that by a proper amount (approximately 0.2 wt%) of Al2O3 addition, smaller NiO grains can be obtained while the side effects of NiAl2O4 are negligible, which is favorable to increase the conductivity and stability of the ASL, and can enhance the performance of SOFC.

•Anodes with different NiO powders were imaged using nano-computed tomography.•During co-fired, anode substrate had an influence on anode functional layer.•0.142 wt.% MgO in anode −2 may be the main reason to cause the over-sintering.•The over-sintering can be avoided by lowering the sintering temperature by 50 °C.Understanding the impact of raw materials on electrode microstructures is important for improving the electrical performance of fuel cells. In this study, two similar NiO powders with different amounts of impurities were used to prepare an anode support for Ni–yttria-stabilized zirconia (Ni-YSZ) anodes. The anodes fabricated with high-purity NiO and with commercial grade NiO with a high impurity concentration are denoted anode-1 and anode-2, respectively. The nano-CT technique was used to reconstruct the anode support and AFL microstructures and to calculate several structural parameters, such as the triple phase boundary, Ni and YSZ particle sizes and pore sizes. In addition, the electrical properties of the cells fabricated with anode-1 and anode-2 were measured and compared. The electrical performance of the cell fabricated with anode-1 is higher than that of the cell fabricated with anode-2 because the anode-1 microstructure is more suitable for this application. Further investigations indicate that the differences in the microstructures and electrical properties of anode-1 and anode-2 are primarily due to anode-2 over-sintering. The anode-2 sintering temperature is lower than the anode-1 sintering temperature due to MgO impurities introduced by the anode-2 raw material, which results in the observed over-sintering phenomenon. When anode-2 is sintered at a lower temperature, its microstructure and electrical performance is similar to that of anode-1.

•An oxygen interstitial (Oi) mediated doping effect in ZnO:Al films is reported.•The concentration of Oi is tailored by varying the oxygen ratio in sputtering gas.•The doping efficiency is increased by ~10% due to the Oi-mediated doping effect.How to increase the doping efficiency in ZnO:Al (AZO) films is a longstanding question and a hard nut to crack. In the present work, we report an oxygen interstitial mediated doping effect for AZO films prepared by magnetron sputtering. The concentration of oxygen interstitials (Oi) in the as-grown films is tailored by changing the oxygen partial pressure during sputtering. Although the Al donors are temporarily passivated by Oi, they are easily reactivated through the removal of Oi by post-annealing in hydrogen at 500 °C. Our results show that the as-grown film which has the highest Oi concentration turns out to have the highest carrier concentration after hydrogen annealing, and the doping efficiency is increased by ~10% because of the oxygen interstitial mediated doping effect. We infer that although Oi deactivate Al donors in the as-grown films, they favor the distribution of the doped Al at the atomic level and the formation of Al substituting at Zn sites (AlZn), and thus, increase the effective Al donors after hydrogen annealing.

We report a potential way to enhance and tune the multiferroic and resistive switching properties of BiFeO3 nanoparticles through dilute aliovalent Li1+ doping (0.046 atomic percent) at the Fe3+ sites of BiFeO3. The high purity of the samples and the extent of doping were confirmed by different physical characterizations. Enhanced multiferroic properties with a magnetic moment per Fe atom ≈ 0.12 μB and electric polarization ≈ 49 μC cm−2 were observed in one of the Li1+ doped samples. A phenomenological model has been proposed to support the observed magnetic behavior of the doped samples. From a potential application point of view, we further report on the doping concentration and polarization coercivity dependent highly stable resistive switching behavior (endurance cycles >103 and stability >106 s) of Li-doped BiFeO3 nanoparticles. The stable complementary resistive switching behavior (1 bit operation) for >50 cycles and under voltage pulse for 103 cycles in the doped BiFeO3 at a low operating bias is reported. Thus, dilute aliovalent Li1+ doping enables tunability of the ferroic and resistive switching properties of BiFeO3and shows it to be a promising multiferroic material.

The effects of cobalt (Co) addition in the Ni-YSZ anode functional layer (AFL) on the structure and electrochemical performance of solid oxide fuel cells (SOFCs) are investigated. X-ray diffraction (XRD) analyses confirmed that the active metallic phase is a Ni1–xCox alloy under the operation conditions of the SOFC. Scanning electron microscopy (SEM) observations indicate that the grain size of Ni1–xCox increases with increasing Co content. Thermogravimetric analyses on the reduction of the Ni1–xCoxO-YSZ powders show that there are two processes: the chemical-reaction-controlled process and the diffusion-controlled process. It is found that the reduction peak corresponding to the chemical-reaction-controlled process in the DTG curves moves toward lower temperatures with increasing Co content, suggesting that the catalytic activity of Ni1–xCox is enhanced by the doping of Co. It is observed that the SOFC shows the best performance at x = 0.03, and the corresponding maximum power densities are 445, 651, and 815 mW cm–2 at 700, 750, and 800 °C, respectively. The dependence of the SOFC performance on the Co content can be attributed to the competing results between the decreased three-phase-boundary length in the AFL and the enhanced catalytic activity of the Ni1–xCox phase with increasing Co content.Keywords: anode functional layer; catalytic; nickel−cobalt alloy; solid oxide fuel cells; thermogravimetric analyses; three-phase boundary

•Gd2Ti2O7 is an effective sintering aid for GDC.•The relative density of Gd2Ti2O7-added GDC can reach over 97% by sintering at 1400 °C for 5 h.•The ionic conductivity decrease of the Gd2Ti2O7-added GDC is small.•The thermal expansion coefficient of GDC can be reduced by adding Gd2Ti2O7.•Gd2Ti2O7 is chemically stable and does not react with GDC.The effects of Gd2Ti2O7 (GT) as sintering aid on the densification, electrical properties and thermal expansion of Gd0.1Ce0.9O1.95 (GDC) are examined. Samples added with TiO2 sintering aid are also tested for comparison. It is found that by sintering at a moderate temperature of 1400 °C for 5 h, the relative density of the GT-added GDC can reach over 97% as the molar ratio of GT/GDC reaches 0.02 or higher. XRD analysis indicates that GT does not react with GDC, while TiO2 reacts with Gd in GDC to form GT. The ionic conductivities of the GT-added and the TiO2-added GDC are analyzed by AC impedance spectroscopy at 500–700 °C. The result shows that although the ionic conductivity of the GT-added GDC decreases as the GT/GDC molar ratio increases up to 0.05, it is still higher than that of 8YSZ and much higher than that of the GDC added with an equivalent amount of TiO2. It is also found that the thermal expansion coefficient of GDC decreases as the amount of GT increases. These results show that GT is an excellent sintering aid for GDC, and the optimal molar ratio of GT/GDC is 0.02 in terms of densification and ionic conductivity.

Application of La0.6Sr0.4Co0.2Fe0.8O3 perovskites cathode in solid oxide fuel cell (SOFC) can benefit from its high electrocatalytic activity at 600–800 °C. However, due to the chemical and mechanical incompatibility between the LSCF cathode and state-of-the-art yttria stabilized zirconia (YSZ) electrolyte, a ceria-based oxide barrier interlayer is usually introduced. In this work, gadolinia doped ceria (GDC) interlayers are prepared by screen printing (SP), electron beam evaporation (EB) and ion assisted deposition (IAD) methods. The microstructures of the GDC interlayers show great dependence on the deposition methods. The 1250 °C-sintered SP interlayer exhibits a porous microstructure. The EB method generates a thin and compact interlayer at a low substrate temperature of 250 °C. With the help of additional energetic argon and oxygen ions bombardment on the deposited species, the IAD method yields the densest GDC interlayer at the same substrate temperature, which leads to the best electrochemical performance of LSFC-based SOFC.Research highlights▶ Microstructure of GDC interlayer depends on the deposition method. ▶ Dense and thin GDC interlayer is deposited by IAD at low temperature. ▶ The IAD GDC interlayer promotes the performance of La0.6Sro.4Co0.2Fe0.8O3 cathode.

Understanding the mechanism of degradation in solid oxide fuel cells (SOFCs) using nickel/yttria-stabilized zirconia (Ni-YSZ) as the anode material is very important for the optimization of cell performance. In this work, the effects of thermal cycling on the microstructure of the Ni-YSZ anode are explored using the three-dimensional X-ray nano computed tomography (nano-CT) imaging technique. It is found that the average Ni particle size increased with thermal cycling, which is associated with the decreased connectivity of the Ni phase and the three-phase-boundary (TPB) length. Moreover, the conductivities of the anode samples are also reduced with the increase in thermal cycle times. The implication of these observations is discussed in terms of the relationship between the conductivity and connectivity of the Ni phase.Highlights► In this study we image the anode samples with thermal cycle using Nanotomography. ► We observe the Ni aggregation during thermal cycle. ► The microstructural degradation of anodes happens with thermal cycle. ► The link between anode microstructure and its performance is investigated.

Effects of a bismuth oxide (Bi2O3) sintering aid in the silver paste cathode current collectors on the electrochemical performance of solid oxide fuel cells with (La,Sr)MnO3 cathode is investigated. Anode-supported single cells are prepared and applied with pure and Bi2O3-added silver pastes for cathode current collecting. Cell performances are evaluated using a current–voltage test and electrochemical impedance spectroscopy. The results indicate that the Bi2O3-added silver paste cathode current collector artificially increases the power density and lowers the polarization resistance of single cell, which may be attributed to the observation of the improved cathode current collector surface morphology and enhanced contact at the cathode-current collector interface, as well as the migration of the Bi2O3 and silver into the cathode from the Bi2O3 contained silver paste cathode current collector.

The visualization of three-dimensional (3D) microstructures of solid oxide fuel cells helps to understand the efficiency of the electrochemical conversion process, study the device's reliability, and improve manufacturing processes. Here, we used X-ray nanotomography to investigate a porous nickel–yttria-stabilized zirconia (Ni–YSZ) composite anode. These results were used to characterize and quantify the key structural parameters, such as the volume ratio of the three phases (Ni, YSZ, and pore), connected porosity, surface area of each phase, interface of Ni/YSZ, volume-specific three-phase boundary length (TPB where the Ni, YSZ and fuel gas phases come together), and electrical conductivity of the anode.

YSZ films for anode-supported SOFCs were prepared by reactive sputtering method. It was found that the surface morphology of anode substrate has a very important effect on the quality of sputtered films. By applying an anode functional layer and making the anode surface smooth, dense and uniform YSZ films of 10 µm in thickness were successfully fabricated. The sintering behaviors of the sputtered YSZ films were also discussed. It is suggested that the optimized densification condition for the deposited YSZ films is sintering at 1250 °C for 4 h. Single cells with sputtered YSZ film as electrolyte and LSM–YSZ as active cathode materials were tested. 1.08 V open circuit voltage and a 700 mW/cm2 maximum power density were achieved at 750 °C by using humidified H2 as fuel and air as oxidant.

Niobium oxide (Nb2O5) films with thicknesses ranging from 200 to 1600 nm were deposited on fused silica at room temperature by low frequency reactive magnetron sputtering system. In order to study the optical losses resulting from the microstructures, the films with 500 nm thickness were annealed at temperatures between 600 and 1100 °C, and films with thicknesses from 200 to 1600 nm were annealed at 800 °C. Scanning electron microscopy and atomic force microscopy images show that the root mean square of surface roughness, the grain size, voids, microcracks, and grain boundaries increase with increasing both the annealing temperature and the thickness. Correspondingly, the optical transmittance and reflectance decrease, and the optical loss increases. The mechanisms of the optical losses are discussed. The results suggest that defects in the volume and the surface roughness should be the major source for the optical losses of the annealed films by causing pronounced scattering. For samples with a determined thickness, there is a critical annealing temperature, above which the surface scattering contributes to the major optical losses. In the experimental scope, for the films annealed at temperatures below 900 °C, the major optical losses resulted from volume scattering. However, surface roughness was the major source for the optical losses when the 500-nm films were annealed at temperatures above 900 °C.

•Mixed perovskite precursors with the nominal compositions of La2NiO4, LaNiO3, La2Ni0.5Fe0.5O4 and LaNi0.5Fe0.5O3 were prepared by a wet impregnation method.•LaNixFe1-xO3 phase enhances the metal-support interaction and suppresses the agglomeration of small Ni particles.•A stable Ni-based catalyst with high carbon resistance for dry reforming of methane is obtained through the LaNi0.5Fe0.5O3 precursor.Improving the carbon resistance of Ni-based catalysts for the dry reforming of methane (DRM) through the metal-perovskite interaction is an attractive strategy. Ni-based perovskite precursors with the nominal compositions of La2NiO4 and LaNiO3, as well as their Fe partially substituted counterparts (La2Ni0.5Fe0.5O4 and LaNi0.5Fe0.5O3), were prepared by a wet impregnation method. Perovskite structures in the samples without Fe partial substitution are unstable and completely reduced during the DRM test, forming catalysts composed of Ni as the active component and La2O3 as the support. The stability of the perovskite structure is significantly enhanced by the Fe partial substitution, and improved carbon resistance are observed in these catalysts, which is attributed to the smaller particle size and better dispersion of Ni resulted from the stronger metal-support interaction. The LaNixFe1-xO3 perovskite plays an important role in the structural stability of mixed perovskite catalysts in reducing atmosphere and the enhancement of metal-support interaction. Our results indicate that the LaNi0.5Fe0.5O3 precursor synthesized by wet impregnation method is feasible to obtain stable Ni-based catalysts with high carbon resistance for DRM.Download high-res image (61KB)Download full-size image