NiO Phase Research Articles

Low temperature dielectric study and photoconductive analysis of ZnO/NiO composite material

The sol-gel process was used to synthesize a ZnO–NiO composite material in this study. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the material, which confirmed the existence of both ZnO nanoparticles and NiO sheets in the composite. The composite material contained two distinct crystalline phases, the face-centered cubic phase of NiO and the hexagonal phase of ZnO. The composite's average crystallite size was determined to be 19.0 nm. The particle size of the composite material was calculated to be ∼121 nm. The band edge emission at 393 nm (3.15 eV) was measured using photoluminescence (PL) spectroscopy. The PL spectrum revealed other emissions as well. The dielectric properties of the material were studied at low temperatures in the frequency range of 20 Hz to 1 MHz, and it was found to have a low dielectric dissipation at high frequencies, making it suitable for use in high frequency microwave devices. Impedance data was fitted using Z-View software with a fitting error of <5%. Impedance spectroscopy revealed that the composite material had two dielectric relaxation processes related to the bulk and the grain boundary. Analysis of frequency dependent ac conductivity revealed Overlapping large-polaron hopping (OLPH) is responsible conduction mechanism in our case indicated by variation of temperature (200–280 K) dependent parameter “s” with temperature. Photoconductivity studies showed that the composite material had enhanced electrical conductivity and photoconductivity compared to pure ZnO and NiO. This makes it a promising candidate for use in optoelectronic applications such as solar cells, optical switches, and photodetectors. The responsivity of composite (0.86 mA/Watt) is found to be 10 times greater than that of the responsivity of NiO (0.085 mA/Watt).

Ni3+-Rich Ni/NiOx@C Nanocapsules Below 4nm Constructed by Low-Temperature Graphitization of Self-Assembled Few-Layer Coordination Polymers toward Efficient Alkaline Hydrogen Evolution Electrocatalysis.

Low-cost and eco-friendly Ni/NiO heterojunctions have been theoretically proven to be the ideal candidate for stepwise electrocatalysis of alkaline hydrogen evolution reaction, attributed to the preferred OHad adsorption by incompletely filled d orbitals of NiO phase and favorable Had adsorption energy of Ni phase. Nevertheless, most Ni/NiO compounds reported so far fail to exhibit excellent catalytic activity, possibly due to the lack of efficient electron transport, limited interfacial active sites, and unregulated Nin+ ratios. To address the above bottlenecks, herein, the ultrasmall Ni/NiOx@C nanocapsules (<5nm) are directly constructed by graphitization of four-layer Ni-based coordination polymers at record low temperatures of 400 °C. Ascribed to the accelerated electron and mass transfer by the carbon nano-onions coated around Ni/NiOx heterojunctions, the extreme rise in interfaces and Ni3+ defects with t6 2ge1 g electronic configuration owed to the ultrasmall size, the Ni/NiOx@C nanocapsules exhibit the highest catalytic activity and the lowest overpotential of η10 = 80mV among various Ni/NiO materials (measured on the glassy carbon electrode). This work not only constructs an industrialized high-efficiency electrocatalyst toward alkaline HER, but also provides a novel strategy for the constant-scale preparation of multicomponent transition metals-based nanocrystals below 4nm.

Electrochemical energy storage application of MOF-derived porous NiO thin films synthesized by solvothermal route

Metal oxides are the most suitable materials for supercapacitors because of their strong electrical characteristics and cheap prices. Owing to its large surface area, controlled pore structure, uniformity, the metal-organic frameworks are employed as inventive material for electrodes in the field of energy storage. In this report, Ni-MOFs have been prepared on conducting stainless steel substrates using a solvothermal technique with different Ni concentrations. Further, these films were annealed at 350 °C for getting NiO phase. The crystalline nature of MOF-derived NiO is confirmed by XRD. The surface morphology of the films has been studied with FE-SEM, showing a nanosheet-like structure. Consequently, electrochemical tests on NiO electrodes were performed in 1 M KOH with a potential window of -0.2–0.5 V. The NiO electrode deposited with 0.3 M concentration delivers a specific capacity of 680 C g−1 at a current density of 0.5 mA cm−2, an energy density of 22.40 Wh kg−1 at a power density of 2.4 kW kg−1 and good cyclic performance with 96% retention after 5000 cycles. The synthesized MOF derived NiO showed excellent specific capacity and good cycle stability due to their large surface area, porous and more durable structure. Hence, the synthesis approach designated here may provide an efficient way to develop MOF-derived NiO for effective supercapacitor applications.

A facile synthetic approach for TiO2@NiO core‐shell nanoparticles using TiO2@Ni(OH)2 precursors and their photocatalytic application

AbstractThe current study presents a simple and an efficient chemical method for the synthesis of TiO2@NiO core‐shell nanoparticles (CSNPs) as photocatalyst for effective degradation of rhodamine B (RhB) in an aqueous solution upon sunlight irradiation. First, TiO2 spheres were synthesized using a wet chemical method. Then, TiO2@Ni(OH)2 precursors were prepared via homogeneous precipitation and TiO2@NiO core‐shell nanoparticles were obtained on calcination of the precursors. The synthesized precursors and the TiO2@NiO CSNPs were analyzed using XRD, FT‐IR, FESEM and DRS. XRD data shows the presence of both TiO2 and NiO phases in the TiO2@NiO samples. FE‐SEM and TEM analyses confirm coating of NiO NPs on TiO2 spheres with varying shell thickness (40 nm to 170 nm). Optical studies reveal that TiO2@NiO CSNPs possess band gap of about 3.7 eV. Photoluminescence (PL) results show lower recombination rate of e− and h+ pairs in the TiO2@NiO CSNPs. XPS studies confirm the presence of different elements in the TiO2@NiO CSNPs. Magnetic investigations reveal ferromagnetic behavior of the core‐shell NPs at 5 K. The TiO2@NiO CSNPs were explored as photocatalyst for the degradation of rhodamine B in water under natural sunlight. The TiO2@NiO CSNPs exhibit better photocatalytic activity compared to TiO2 and NiO nanoparticles.

Composition dependent variation in structural, morphological, optical and magnetic properties of biogenic CuO/NiO mixed oxides nanoparticles

A series of CuO/NiO nanoparticles with different Cu and Ni precursor concentrations represented as N12 (1:2), N22 (1:1) and N21 (2:1) (Cu2+:Ni2+) were produced by modified green solution combustion route using Moringa oleifera leaves extract as an organic fuel. The prepared CuO/NiO nanoparticles were analytically characterized using UV-Vis diffuse reflectance spectroscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and vibrating sample magnetometry. Morphological studies showed uniform and nearly monodisperse nanoparticles having average grain size of 34 nm for N22 nanoparticles while the N12 and N21 compositions showed uniform nanoparticles with the grain size of 12 nm and 22 nm respectively. The existence of CuO and NiO phases in nanoparticles was verified by XPS analysis. N12, N22 and N21 had band gaps in the range of 2.19–2.49 eV and specific surface area in the range of 4.92–11.2 m2/g. The susceptibility vs. temperature plots for mixed oxides nanoparticles showed distinct magnetic behaviour for different ratio. The coercivity and saturation magnetization reached a maximum value for N22 (130 Oe and 0.133 emu/g). These biogenic nanoparticles could be a strong choice for biological applications due to their soft magnetic behaviour and biocompatibility.

Spectroscopic analysis of nanosized Zn(Ag, Ni)O systems and observation of superparamagnetism at low temperature.

To understand the impact of binary doping in ZnO, nanosized Zn(Ag, Ni)O systems were synthesized by the sol-gel method. The amount of Ag was fixed at 2 at%, and that of Ni was varied from 1 to 15 at%. Ni incorporation equal to or beyond 3 at% gave rise to the development of the NiO phase. The presence of Ag and Ni did not have much influence on the lattice constants of ZnO. However, a larger addition of Ni impacted the unit cell of NiO, as indicated by the reduction of the lattice constant of NiO. The increase in NiO and Ag contents in ZnO reduced the second and third harmonic intensities under non-linear investigations. X-ray photoelectron spectroscopy analysis indicated that initial Ni addition varied randomly along with Ag, and it stabilized itself at higher concentration. Field emission scanning electron microscopy revealed that interlinked particles and chains with tamarind shapes were formed, closely matching the rod-like structures under high resolution. Ag and Ni addition altered the structures slightly and randomly till 5 at% Ni; thereafter they deviated from the particle shape to flat disc-shapes. Interestingly, the magnetic response of the sample was determined by the NiO phase, and the effect of Ni and Ag substitution in the ZnO host matrix was almost irrelevant at low temperatures toward magnetic contribution. Weak ferromagnetism at low temperatures (≤50 K) with superparamagnetic-like behavior (cusp in ZFC magnetization) was observed in all the samples. This could be attributed to the finite nano-size effect and uncompensated spins at the surface of the particle.

Partially oxidized inter-doped RuNi alloy aerogel for the hydrogen evolution reaction in both alkaline and acidic media.

A facile reduction and doping process is employed with the supercritical ethanol drying method to form RuNi alloy aerogels. The optimized heterostructure comprising RuNi metal, RuO2, and NiO phases is synthesized through partial oxidation. When applied to the surface of Ni foam, the multiphase aerogels form a morphology of highly porous 0D colloidal aerogel networks on the surface. RuNi alloy-Ni foam oxidized at 350 °C (RuNi-350@NF) has an overpotential of 89 and 61 mV in 1 M KOH and 0.5 M H2SO4 media at 50 mA cm-2, as well as satisfactory long-term stability. Additionally, the Tafel slopes in alkaline and acidic media are found to be 34 and 30.9 mV dec-1, respectively. Furthermore, it exhibits long-term stability (35 h) in alkaline and acidic media at high current densities of 50 mA cm-2, respectively. This study presents a novel strategy for developing exceptionally efficient and free-standing 3D porous aerogel electrocatalysts with potential applications in hydrogen production.

Evolution of structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO in Ni doped ZnO thin films and their electronic structures

This study thoroughly examines the collective influence of compositional variation and annealing temperature on the electronic structure of sol–gel derived NixZn1−xO (x = 0 to 1) thin films annealed at different temperatures (700 ° C, 800 ° C, and 900 ° C) using x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy. A gradual structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO with increasing Ni concentration was revealed by x-ray diffraction (XRD). Grain growth was observed from scanning electron microscopy with increasing annealing temperature. Photoluminescence measurements indicate the presence of interstitial oxygen when Ni atoms are incorporated in the film. The Ni L 3,2 absorption edge shows an intensity enhancement in the white-line feature with increasing Ni concentration, evidencing the presence of higher oxidation states. Concurring results were observed by XPS where both Ni2+ and Ni3+ free ion multiplets are present in the Ni 2p core level spectrum for 20% and higher Ni concentration. O K and Zn L 3,2 XAS spectra demonstrated the e g-t 2g sub-band splitting at higher Ni concentration, triggered by band anti-crossing interaction and crystal field splitting. The extended x-ray absorption fine structure (EXAFS) simulation for the Zn K edge revealed a Zn–Zn/Ni bond length change for 60% Ni concentration. The thermal disorder factor increased up to 40% Ni concentration, and beyond that, it decreased due to stable NiO phase dominance in the alloy composite. Ni K edge EXAFS fitting indicated an insignificant change in the Ni-O and Ni–Ni/Zn bond lengths throughout the range of varying Ni concentrations. The thermal disorder factor increases with increasing annealing temperature, indicating a more disordered lattice. Such investigation is essential where the electronic properties of nanometer-sized materials determine the performance of functional devices. The present work critically elucidates the combined impact of compositional variations and annealing temperatures on electronic structures.

Facile Preparation of Porous Ni-YSZ Electrode Composite Material: From Highly Dense to Desirable Electrode Porosity Even without Pore Former

Ni-YSZ is a key electrode material for solid oxide electrochemical cells (SOC) applications, such as fuel or electrolysis cell applications. The number of active sites, specifically the triple-phase boundaries (TPB), strongly affects the electrode performance. Thus, in order to achieve good electrode performance, a desirable microstructure of the electrode is essential. This study investigated the effect of precursor particle size without using pore former in developing porous Ni-YSZ electrode materials. Precursors were prepared with different particle sizes using a planetary ball mill. In comparison, Ni-YSZ with carbon black as a pore former was also prepared. From the XRD patterns, major peaks can be attributed to the cubic phases of NiO and YSZ. SEM images revealed that a highly dense as-sintered NiO-YSZ electrode was achieved; however, a desirable porous microstructure was obtained after reduction to Ni-YSZ, even without a pore former. In comparison, the prepared electrode with carbon black pore former has a larger pore size than the sample prepared without pore former. The obtained total bulk conductivities for the reduced Ni-YSZ without pore former at 700 oC is 3.94x10-1 S/cm with Ea of 0.02 eV at 500-700 oC. Thus, a desirable porous Ni-YSZ electrode with more TPBs and high total conductivity can be achieved even without a pore former. Figure 1

(Digital Presentation) Facile Fabrication of Nickel and Sc-Doped Zirconia Cermet Electrode Thin Film on YSZ Substrate Via Screen-Printing for Solid Oxide Electrochemical Cells

Solid oxide electrochemical cells (SOCs) operating as either solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs) are widely known as promising advancements in green energy technologies for storage and high-efficiency energy generation. This study used a single screen-printing deposition method to fabricate a porous Nickel – scandia-stabilized zirconia (Ni-ScSZ) cermet electrode on a dense YSZ solid electrolyte substrate. The precursor powders were prepared via the glycine-nitrate combustion process. To investigate the effect of ink slurry solvent on the quality, structure, and morphology of the deposited films, ethanol, and alpha-terpineol were used. The screen-printed films were sintered at 1300°C for 6h and reduced under an Ar/5% H2 gas environment at 700°C for 2h to form porous electrodes. X-ray diffraction (XRD) analysis showed that all sintered NiO-ScSZ films exhibited cubic peak structures of both NiO and ScSZ. Scanning electron microscopy (SEM) micrographs and elemental mapping revealed finer morphology with good dispersion of NiO and ScSZ phases for sintered NiO-ScSZ films with alpha-terpineol as solvent. After exposure to Ar/5% H2 gas environment, a complete reduction of NiO to Ni was revealed by XRD for all Ni-ScSZ electrodes. Moreover, SEM micrographs of as-reduced samples showed an increase in porosity as oxygen was removed from NiO. Electrochemical impedance spectroscopy was used to determine the conductivity of the reduced Ni-ScSZ electrode films under Ar/5% H2 gas environment. The obtained total conductivity values upon reduction are 9.54x10-5 S/cm and 4.75x10-5 S/cm for ethanol and alpha-terpineol as solvents, respectively.

(Invited) High-Nickel Layered Oxide Cathodes: Crack Vs. Surface Reactivity

The appetite for long driving range and high energy density is pushing the nickel content in layered oxide cathodes for lithium-ion batteries. The lying of Ni3+/4+:3d band slightly above the Co3+/4+:3d band enables to extract more lithium during charge without encountering oxygen release from the lattice in high-nickel cathodes compared to that in LiCoO2. However, as the nickel content increases in the lattice, the cells suffer from cycle and thermal instability. The cycle instability has largely been attributed in the literature to the formation of cracks caused by the occurrence of H2-H3 phase transition with large lattice parameter changes during high degree of charge. Several approaches, such as doping with other cations and microstructural control of the primary and secondary particles, have been extensively pursued to overcome the challenges and improve the cycle life.With a systematic investigation of a number of cathode compositions and their characterization after extensive cycling, this presentation will illustrate that surface reactivity of high-nickel cathodes with electrolyte plays a dominant and decisive role on cycle life. Even cathodes like LiNiO2 with 100% Ni without any cation doping or surface coating could be cycled with high capacities over a large number of cycles with an appropriate choice of electrolyte that suppresses surface reactivity. Electrolytes that are stable to slightly higher voltages, even by as low as 100 mV, compared to the conventional electrolytes alter the surface reconstruction pathway on cathode surface in contact with electrolyte. Electrolytes with better high-voltage stability favor the formation of mildly reduced LiNi2O4 spinel on the surface without much oxygen release from the surface. In contrast, electrolytes that have inferior stability to higher voltages lead to a drastic reduction of the cathode surface to highly reduced Ni3O4 spinel and NiO rock salt phases with significant oxygen loss from and volume change on the surface, which could cause crack initiation on the surface, followed by their propagation into the bulk. Interestingly, the fast 3-dimensional lithium-ion and electron transport in spinel LiNi2O4 supports a facile charge transfer through the cathode-electrolyte interphase (CEI), resulting in better cycle life, in contrast to the poor lithium-ion transport in both spinel Ni3O4 and rock salt NiO phases, resulting in poor charge-transfer through the CEI and inferior cycle life. With robust electrolyte formulations with better high-voltage stability, long cycle life could be achieved even with the repeated occurrence of the H2-H3 phase transitions over a large number of cycles.

Revealing Aqueous-Processing Ni-Rich Cathode Cycling Performance in Term of Cathode Electrolyte Layer Formation and Phase Transition

Traditionally, Ni-rich cathodes for lithium ion batteries are produced by utilizing N-methyl-2-pyrrolidone (NMP) based casting. However, in order to prevent using the harmful solvent NMP, aqueous-processing method becomes one of the options.1 In this study, water-based electrodes have been produced to compare with the NMP-based electrodes. In addition to that, phosphoric acid addition was involved to modify the water-based electrode surface to improve the capacity retention.2 We used X-ray adsorption spectroscopy (XAS) to observe the existence of NiO phase in the aqueous processed electrodes due to Li/Ni disorder. The Galvanostatic cycling results shows that aqueous processed electrodes provide worse capacity retention compared with the NMP processed electrodes because of higher electron transfer resistance, which is caused by the cathode electrolyte layer (CEI) layer formation. Soft X-ray photoelectron spectroscopy (PES) results show that comparing to NMP-based electrodes, water-based electrodes contains more decomposition species from electrolyte in CEI layer after first battery cycle. From Operando X-ray diffraction, we observed the phase transition differences between these three types of electrodes. In this study we aim to answer if and/or how the additive phosphoric acid participates in the electrolyte degradation during battery cycling. A deeper understanding of the roles of acid can provide a new path to develop aqueous-processing electrodes method. Wu, F.; Kuenzel, M.; Diemant, T.; Mullaliu, A.; Fang, S.; Kim, J. K.; Kim, H. W.; Kim, G. T.; Passerini, S., Enabling High‐Stability of Aqueous‐Processed Nickel‐Rich Positive Electrodes in Lithium Metal Batteries. Small 2022, 2203874.Bichon, M.; Sotta, D.; De Vito, E.; Porcher, W.; Lestriez, B., Performance and ageing behavior of water-processed LiNi0.5Mn0.3Co0.2O2/Graphite lithium-ion cells. Journal of Power Sources 2021, 483.

Intolerance of Ruddlesden-Popper La2NiO4±δ Structure to a-Site Cation Deficiency

Mixed ionic-electronic conductors derived from Ruddlesden-Popper Ln2NiO4+δ (Ln = La, Pr, Nd) attract significant attention as oxygen electrode materials for solid oxide fuel and electrolysis cells. However, literature data on the impact of A-site deficiency on the phase relationships and functional properties of Ln2NiO4+δ-based phases are contradictory. While some works report an accommodation of A-site vacancies up to x = 0.15 in La2-x NiO4±δ, others indicate difficulties in the synthesis of single-phase La2-x NiO4+δ even for low x values. The present work explores the impact of the nominal A-site deficiency on the phase composition and electrical transport properties of La2-x NiO4±δ ceramics.Ceramics with the nominal composition La2-x NiO4±δ (x = 0, 0.02, 0.05, 0.10) were synthesized by the glycine-nitrate technique and sintered at 1350-1450°C. While XRD analysis suggested the formation of single-phase materials with orthorhombic K2NiF4-type structure, the lattice parameters remained independent of composition within the experimental uncertainty. Neutron diffraction studies of x = 0 and 0.05 samples revealed that the lattice parameters, La:Ni atomic ratio (2:1) and oxygen sites occupancy remain identical for both samples, while the presence of impurity NiO phase (0.4 wt.%) was detected for La1.95NiO4±δ. SEM/EDS inspection of ceramic samples confirmed the presence of nickel-rich inclusions for x ≥ 0.02, which could not be eliminated by increasing the temperature and time of sintering. The volume fraction of inclusions increased with x. The experimental observations are supported by the atomistic simulations showing that the coexistence of cation-stoichiometric La2NiO4+δ with nickel-rich La4Ni3O10 or NiO is more favorable compared to cation-deficient La1.95NiO4±δ. Molecular dynamics simulations of La1.95NiO4±δ lattice demonstrated that the introduction of La vacancies leads formation of instability zone in the rock-salt-type layer and destabilization of the crystal lattice. The intolerance of La2NiO4+δ lattice to A-site vacancies and coexistence of La2NiO4+δ with traces of Ni-rich secondary phase in the samples with the nominal composition La2-x NiO4±δ results in an insignificant variation of electrical transport properties with x. Acknowledgements This work was developed within the scope of the project CARBOSTEAM (POCI-01-0145-FEDER-032295) funded by FEDER through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT/MCTES, and the project CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020) financed by national funds through the FCT/MCTES (PIDDAC). A.B. acknowledges the PhD scholarship by the FCT (grant SFRH/BD/150704/2020). This work was partially supported by the National Science Centre, Poland, grant UMO-2020/37/B/ST8/02097.

Complex magnetic behaviour and photocatalytic response in narrowed band gap NiO nanoparticles synthesized by novel Lepidagathis ananthapuramensis leaf extract

The present work focussed on the detailed investigation on structural, morphological, optical, photocatalytic, and magnetic properties of NiO nanoparticles (NPs) derived by a novel Lepidagathis ananthapuramensis leaf extract. The structural information using X-ray powder diffraction and Raman spectroscopy method indicates a single phase NiO hexagonal crystal structure for the sample calcined at ≥ 600 °C. Field emission scanning electron microscopy and high-resolution transmission electron microscopy studies noticed the various morphologies. UV–Visible, photoluminescence, and X-ray photoelectron spectroscopy confirmed the presence of both Ni2+ and Ni3+ ions with the narrowed band gap. Thorough magnetic studies indicate the existence of antiferromagnetic, ferromagnetic and superparamagnetic nature of NiO NPs. The photocatalytic activity of NiO NPs for the photocatalytic degradation of methylene blue dye in the presence of sunlight showed considerable results. This study can help us to deliver the wide range of properties which can be utilised in the field of magnetism and photocatalysis.

Scalable Precursor-Assisted Synthesis of a High Voltage LiNiyCo1-yPO4 Cathode for Li-Ion Batteries.

A solid-solution cathode of LiCoPO4-LiNiPO4 was investigated as a potential candidate for use with the Li4Ti5O12 (LTO) anode in Li-ion batteries. A pre-synthesized nickel-cobalt hydroxide precursor is mixed with lithium and phosphate sources by wet ball milling, which results in the final product, LiNiyCo1-yPO4 (LNCP) by subsequent heat treatment. Crystal structure and morphology of the product were analyzed by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Its XRD patterns show that LNCP is primarily a single-phase compound and has olivine-type XRD patterns similar to its parent compounds, LiCoPO4 and LiNiPO4. Synchrotron X-ray absorption spectroscopy (XAS) analysis, however, indicates that Ni doping in LiCoPO4 is unfavorable because Ni2+ is not actively involved in the electrochemical reaction. Consequently, it reduces the charge storage capability of the LNCP cathode. Additionally, ex situ XRD analysis of cycled electrodes confirms the formation of the electrochemically inactive rock salt-type NiO phase. The discharge capacity of the LNCP cathode is entirely associated with the Co3+/Co2+ redox couple. The electrochemical evaluation demonstrated that the LNCP cathode paired with the LTO anode produced a 3.12 V battery with an energy density of 184 Wh kg-1 based on the cathode mass.

Liquid plasma spraying of NiO-YSZ anode layers applicable for SOFC

Plasma spraying (PS) from liquid feedstocks is a promising alternative for fabricating the NiO-YSZ anodes for Solid Oxide Fuel Cell (SOFC). However, despite the inherent advantages of the PS process, the plasma sprayed anodes are still inferior to the conventional ones prepared by wet-ceramic techniques due to the difficulty of achieving the optimal porosity level. This study shows that due to recent development, the NiO-YSZ anode layers with desirable porosity and microstructure can now be effectively fabricated in a one-step process using PS equipment with a water-argon stabilized plasma torch. As a liquid feedstock, we utilized Ni nitrate (hexahydrate) solutions and YSZ suspensions with different ratios, using ethanol or water as solvents. We show that the ethanol-based feedstock provides a favorable microstructure and porosity of the resulted layers over the water-based one. In addition, apart from NiO and YSZ phases, the layers deposited with ethanol solvent contained even metallic Ni. Different ratios of Ni nitrate to YSZ in ethanol-based feedstocks greatly affected the layer’s characteristics; when the content of Ni nitrate was high, the resulting layers had a non-uniform morphology with a very porous microstructure. On the contrary, the equal contents of Ni nitrate and YSZ in the feedstock resulted in the most optimal layer's microstructure, chemical composition, and thus sufficient electrical resistance, making such material potentially useful as anode for SOFC.

Monometallic interphase synergistic Ni-based catalysts prepared by facile magnetron sputtering for efficient alkaline hydrogen evolution

The development of affordable, efficient and long-lasting electrocatalysts plays a vital role in advancing clean and renewable energy technologies. Ni-based materials have emerged as highly promising alternatives to noble metal for HER. However, existing Ni-based catalysts are generally constrained by complex composition, structure and preparation process, as well as poor stability caused by the weak bonding with the electrode substrate. Here, we prepared monometallic interphase synergistic Ni/NiO self-supported electrodes on Cu foam. The as-prepared Ni/NiO/CF-110 shows excellent HER activity, displaying a lower overpotential of 22 mV at 10 mA cm−2 and achieving nearly 100% Faradaic efficiency. Meanwhile, Ni/NiO/CF-110 exhibits robust stability, withstanding a chronopotentiometric test at 100 mA cm−2 over 100 h. The Ni/NiO/CF-110 was assembled into an alkaline electrolytic cell for water splitting, achieving 10 mA cm−2 with a remarkably low cell voltage of 1.4 V. The outstanding HER performance can be attributed to the synergistic effect of the monometallic phase Ni and NiO, the structural benefits of foam substrate, the good combination with substrate, and the optimal Ni/NiO ratio. This study proposes a novel and simple preparation strategy and provides guidance for developing highly efficient non-precious metal catalysts.

Controlling phase transformation of La/Ni oxides by switching annealing conditions and their resultant pseudocapacitive behaviour

Annealing step is a critical step in the hydrothermal assisted synthesis of La/Ni oxides such as LaNiO3−δ (LNOA) and La2NiO4+δ (LNON). In the current study, we have discovered the interlink between the atmosphere and temperature conditions which dictate the product formed. La/Ni nitrate precursors were subjected to a hydrothermal synthesis followed by an annealing step at appropriate temperature and varying atmosphere resulting in the synthesis of the corresponding oxides. The annealing temperature was varied in the range between 650 °C and 800 °C and also the annealing was carried out either in pure N2 atmosphere or air. From the x-ray diffraction analysis, it was inferred that annealing in air invariably resulted in the rhombohedral LaNiO3 (LNOA) perovskite phase, while annealing in N2 atmosphere resulted in an orthorhombic Ruddlesden–Popper phase La2NiO4 (LNON), a layered oxide containing traces of NiO phase. Typically, iodometric titrations substantiate the presence of Ni (III) which further can be correlated to the presence of oxygen vacancies (δ). Iodometric test results demarcated the difference between the two phases with absolutely minimal I2 liberated from the LNON samples proving that negligible amount of Ni (III) was present in LNON Scanning electron microscopy (SEM) images showed an agglomeration of particles annealed at higher temperatures irrespective of the atmosphere. Temperature dependent oxygen non-stoichiometry (Δδ) was analyzed through thermogravimetric analysis, wherein Δδ was inversely proportional to the annealing temperature for all of the LNOA samples. Considering that large δ values favor pseudocapacitive behaviour, it was observed LNOA oxides showed excellent pseudocapacitive behaviour compared to the LNON oxides. Dunn deconvolution of the cyclic voltammograms of LNOA 800 °C at 5 m Vs−1 indicated that diffusive contribution (66%) was predominant over capacitive contributions. The LNOA sample annealed at 800 °C displayed the highest specific capacitance of 100.3 F g−1 at 1 A g−1 current density.

Role of power on physiochemical properties of Mo- and Ni-based nanoparticles manufactured by laser ablation

In this study, the influence of the laser energy on Molybdenum (Mo) and Nickel (Ni) based nanoparticles (NPs) produced by laser ablation is presented. The nanoparticles' physicochemical properties were studied for laser energy values from 70 to 300 mJ for Mo and from 100 to 250 mJ for Ni. X-ray Diffraction, Atomic Force Microscopy, Dynamic Light Scattering, Atomic Absorption, and UV–Vis Spectroscopy allowed for studying the crystalline structure, morphology, composition, and optical properties. Both samples Mo and Ni at low laser energies produce amorphous nanoparticles with sizes of 30 and 15 nm, respectively. When the laser energy increases, the crystalline structure, size, concentration, and optical properties of the molybdenum and nickel nanoparticles change. However, the shape of the nanoparticles (quasi-spherical) is independent of the laser energy. In Mo, NPs showed a monocrystalline MoO3 phase at intermediate laser energy. At high values of laser energy, Mo is the most stable structure. In Ni, NPs showed a polycrystalline NiO phase at medium laser energy. At high values of laser energy, a mixture of NiO and Ni is the most stable structure. The particle size remains constant for intermediate and high laser energy values. The main difference between the hydrodynamic diameter and the particle size was 7 nm. UV–vis spectra of Mo nanoparticles showed the formation of two compounds: (MoO4)2- and H2MoO4. UV–vis spectra of Ni nanoparticles showed that the absorbance decreased continuously above 200 nm, except for the sample prepared at 150 mJ. As the laser energy increases, the concentration of NPs increases in both cases. We concluded that the Ni sample at 150 mJ shows potential stability compared to the others due to the electrical charging of the material during the ablation process. As the most important result found in this research, it can be considered that the laser energy affects the structural, morphological, and compositional properties of Ni and Mo-based nanoparticles. Nevertheless, as expected, the most significant influence was observed in the nanoparticle concentration. The novelty of the work consists of the laser energy on the band gap and, precisely, the results for Ni-based nanoparticles. In this case, it was impossible to measure this parameter for almost all samples, possibly due to the magnetic influences, since the unoccupied d states atoms, such as Ni, are responsible for the ferromagnetism or high paramagnetism of the transition elements. There is a direct connection between the magnetic properties and the electrical conductivity. On the other hand, in the case of Mo-based nanoparticles, the influence of laser energy on the bandgap is almost negligible.

Bimetallic NiO/NiFe2O4 heterostructures with interfacial effects for boosting electrochemical water splitting applications

The present work reports the simple preparation of NiO/ NiFe2O4 heterostructures with interfacial effects as bifunctional electrocatalysts for overall water splitting. At increasing Ni/Fe precursor ratios, the evolution of NiO phases is observed in NFO heterostructures, which incorporate differential strain effects at the interface. The modulated heterojunction of NiO/NiFe2O4 heterostructure incorporates compressive strain at the interfaces enhancing its electrocatalytic performances. Notably, NiO/NiFe2O4 heterostructure prepared at equimolar Ni/Fe ratios (NFO-1) exhibits significant electrocatalytic performances due to modulated interfacial effects. As such, NFO-1 exhibits lower OER and HER overpotentials of about 242 mV and 106 mV for 10 mA cm−2 current density and exhibits improved water splitting activity, requiring 1.56 V to drive 10 mA cm−2 current density. The present work emphasizes the importance of modulating the heterostructures of transition metal oxides to enhance the interfacial effects as bifunctional electrocatalysts for water splitting applications.