Ni Core Research Articles

Bimetallic Cu–Ni core–shell nanoparticles anchored N-doped reduced graphene oxide as a high-performance bifunctional electrocatalyst for alkaline water splitting

Water electrolysis is regarded as the most promising method for developing sustainable energy technologies. However, low-cost bifunctional electrocatalysts with high efficacy and long-term stability are required to make this method economically viable. Core-shell nanoparticles, which comprise a thin layer of a catalytically active shell surrounding a subsurface core, have recently emerged as cutting-edge electrocatalysts for effective water electrolysis. Herein, we systematically fabricated distinct bimetallic Cu–Ni particles by tuning the Cu:Ni ratios, and then anchored them to an N-doped reduced graphene oxide (NRG) backbone for alkaline water splitting. A Cu:Ni molar ratio of 1:1 was determined to be optimal for forming an effective core–shell configuration, affording favorable adsorption energies toward reactants. The Cu–Ni(1:1) core–shell nanoparticles anchored NRG, termed Cu–Ni(1:1)@NRG, displayed excellent performance toward the H2 evolution reaction (HER) and oxygen evolution reaction (OER), with overpotentials at 10 mA cm−2 of 107 and 310 mV, respectively, versus a reversible hydrogen electrode (RHE). This current density (10 mA cm−2) was attained at a low cell voltage of 1.64 V when Cu–Ni(1:1)@NRG was used as the bifunctional electrocatalyst for alkaline water electrolysis. Furthermore, the Cu–Ni(1:1)@NRG electrocatalyst exhibited outstanding long-term stability in prolonged electrocatalytic studies at a constant current density.

In-situ Ni-oxidation-assisted coupling reduction of NiO and CO2 to synthesize core-shell Ni@octahedral carbon with energy storage properties

A bi-directional electrolysis method for synthesizing metal–carbon core–shell structure was reported, wherein a nickel-encapsulated octahedral carbon (Ni@OC) was synthesized in molten salt via the coupled electroreduction of CO2 and NiO formed by in-situ electrochemical oxidation of Ni. An in-situ generated NiO directs the self-template formation of Ni@OC. By etching the nickel core from Ni@OC with hydrochloric acid, difficult-to-prepare hollow octahedral carbon (HOC) is obtained. The plausible formation mechanism of Ni@OC was unraveled by a series of control experiments. When used as negative materials for lithium-ion batteries (LIBs), Ni@OC exhibits enhanced specific capacitance (433.6 mAh g−1 at 25 mA g−1) and stability (83.3% capacity retention after 4800 cycles at 2000 mA g−1) relative to HOC. The performance improvement can be attributed to the Ni core, which helps to improve the capacity and alleviates the structural collapse during the lithiation/delithiation process. This study not only develops new structures but also presents a unique technological pathway for the sustainable synthesis of functional metal-encapsulated carbons.

Compensation behavior in (Fe–Ni) core–shell nanostructures: Heisenberg Monte Carlo simulations

By performing atomistic simulations, we have studied some features of classical Heisenberg model using the statistical Monte Carlo method MC under the Hinzke–Nowak algorithm. First, we have deeply explored magnetic and thermal properties of a core–shell nanosphere model and investigate the behaviors of the temperature-dependent magnetization, magnetic susceptibility and phase diagrams for different possible exchange interactions. The obtained results show the existence of diverse -types behaviors in the Néel classification nomenclature. Then, we have applied the same computational method to the real (Fe, Ni) nanostructure using experimental values of magnetic parameters for iron and nickel. It is demonstrated that (Fe, Ni) nanoparticle exhibits a compensation phenomenon compatible with those found in the experimental studies.

Atomistic simulation of FCC and HCP Ni-Gd core–shell nanosystem

This research is focused on simulations of Gd, Ni, and Ni-Gd core–shell magnetic nanosystem using VAMPIRE software and the Landau-Lifshitz-Gilbert-Heun approach. The generation of temperature dependent magnetization curves allows the estimation of the corresponding Curie temperature. Simulations at various Ni and Gd particle diameters provided critical values below which the Tc value differs from the theoretical bulk Tc value, being 5 and 7 nm for Ni and Gd, respectively. It was demonstrated that both FCC and HCP structure configurations yield undistinguishable behaviors at large particle sizes. After fitting, the values of the phenomenological shift exponent and the microscopic length of Ni and Gd corresponded to spherical nanosystem. The Tc values have been tuned by varying the core diameter of Ni-Gd core–shell nanosystem. Moreover, the antiferromagnetic coupling was seen to play a significant role in the M-T curves and spin distributions of cores and shells. The hysteresis loops produced coercivity field values driven by inter-domain coupling, as well as anisotropy decreases caused by Ni% increments and rising temperatures. These magnetic properties disclosed here are promising for future biomedical and spintronic applications.

Ni3 S2 -Ni Hybrid Nanospheres with Intra-Core Void Structure Encapsulated in N-Doped Carbon Shells for Efficient and Stable K-ion Storage.

Iron group metals chalcogenides, especially NiS, are promising candidates for K-ion battery anodes due to their high theoretical specific capacity and abundant reserves. However, the practical application of NiS-based anodes is hindered by slow electrochemical kinetics and unstable structure. Herein, a novel structure of Ni3 S2 -Ni hybrid nanosphere with intra-core voids encapsulated by N-doped carbon shells (Ni3 S2 -Ni@NC-AE) is constructed, based on the first electrodeposited NiS nanosphere particles, dopamine coating outer layer, oxygen-free annealing treatment to form Ni3 S2 -Ni core and N-doped carbon shell, and selective etching of the Ni phase to form intra-core void. The electron/K+ transport and K+ storage reaction kinetics are enhanced due to shortened diffusion pathways, increased active sites, generation of built-in electric field, high K+ adsorption energies, and large electronic density of states at Fermi energy level, resulting from the multi-structures synergistic effect of Ni3 S2 -Ni@NC-AE. Simultaneously, the volume expansion is alleviated due to the sufficient buffer space and strong chemical bonding provided by intra-core void and yolk-shell structure. Consequently, the Ni3 S2 -Ni@NC-AE exhibits excellent specific capacity (438 mAh g-1 at 0.1 A g-1 up to 150 cycles), outstanding rate performances, and ultra-stable long-cycle performance (176.4 mAh g-1 at 1 A g-1 up to 5000 cycles) for K-ion storage.

Synergistic effect of porous carbon shell confinement and catalytic conversion of nickel nanoparticle cores for improved lithium-sulfur batteries.

Lithium-sulfur batteries (LSBs) are some of the most promising energy storage systems to break the ceiling of Li-ion batteries. However, the notorious shuttle effect and slow redox kinetics give rise to low sulfur utilization and discharge capacity, poor rate performance, and fast capacity decay. It is proved that the reasonable design of the electrocatalyst is one of the important ways to improve the electrochemical performance of LSBs. Here, a core-shell structure with gradient adsorption capacity for reactants and sulfur products was designed. The Ni nanoparticles core coated with graphite carbon shell was prepared by one-step pyrolysis of Ni-MOF precursors. The design takes advantage of the principle that the adsorption capacity decreases from the core to the shell, and the Ni core with strong adsorption capacity is easy to attract and capture soluble lithium polysulfide (LiPS) during the discharge/charging process. This trapping mechanism prevents the diffusion of LiPSs to the outer shell and effectively inhibits the shuttle effect. In addition, the Ni nanoparticles within the porous carbon, as the active center, expose most of the inherent active sites to the surface area, thus achieving a rapid transformation of LiPSs, significantly reducing the reaction polarization, and improving the cyclic stability and reaction kinetics of LSB. Therefore, the S/Ni@PC composites exhibited excellent cycle stability (a capacity of 417.4 mA h g-1 for 500 cycles at 1C with a fading rate of 0.11%) and outstanding rate performance (1014.6 mA h g-1 at 2C). This study provides a promising design solution of Ni nanoparticles embedded in porous carbon for high-performance, safe and reliable LSB.

Revealing the atomic-scale configuration modulation effect of boron dopant on carbon layers for H2O2 production.

This work reports an atomic-scale carbon layer configuration tuning strategy induced by a boron dopant. Through regulating the doping level of boron, it was found that the boron dopant not only favors carbon layer growth by strengthening the metallic state of the Ni core, but also enhances the abundance of pyrrolic N species and graphitization degree of carbon by tailoring the carbon/nitrogen atom configuration, thereby contributing to more active pyrrolic N/carbon sites and accelerated interface reaction dynamics. Consequently, the developed Ni@B,N-C catalyst achieves remarkable electrochemical H2O2 production performances with a high selectivity of 95.5% and a yield of 795 mmol g-1 h-1. In comparison with previous reports in which the boron dopant mainly acts as an electronic structure regulator, this study reveals the tuning effect of boron dopants on the atomic-scale carbon layer configuration, opening up a new avenue for the development of advanced catalysts.

The effect of tensile strain in Pd-Ni core-shell nanocubes with tuneable shell thickness on urea electrolysis selectivity.

Expanding our understanding of the structure-performance relationship in nanoscale electrocatalysts for urea electrolysis is crucial for efficient urea waste treatment and concomitant cathodic hydrogen production or CO2 reduction. Here, we elucidate the effect of the lattice strain in Pd-Ni core-shell nanocubes on the dominance of urea overoxidation pathway.

Lattice strain controlled Ni@NiCu efficient anode catalysts for direct borohydride fuel cells.

We successfully fabricated a novel tensile lattice strained Ni@NiCu catalyst with a popcorn-like morphology, which is composed of a crystalline Ni core and a NiCu alloy shell. It exhibits outstanding catalytic activity, selectivity, and stability towards borohydride electrooxidation. Moreover, a direct borohydride fuel cell (DBFC) with a Ni@NiCu anode can deliver a power density of 433 mW cm-2 and an open circuit voltage of 1.94 V, much better than the performances of DBFCs employing other anode catalysts reported in the literature. This could be attributed to the fact that the tensile lattice strain generated by the introduction of Cu leads to a rise in the d-band center of the Ni metal and promotes the final B-H decoupling, which is the rate-determining step in the borohydride oxidation reaction, thus improving remarkably the catalytic performances of Ni@NiCu.

Probing strongly exchange coupled magnetic behaviors in soft/hard Ni/CoFe2O4 core/shell nanoparticles.

Exchange coupling in a model core-shell system is demonstrated as a step on the path to 3d exchange spring magnets. Employing a model system of Ni@CoFe2O4, high quality core-shell nanoparticles were fabricated using a simple two-step method. The microstructural quality was validated using TEM, confirming a well-defined interface between the core and the shell. A strongly temperature dependent two-phase magnetic hysteresis loop was measured, wherein an analysis of step heights indicates coupling of roughly 50% between the core and the shell. Element-specific XMCD hysteresis confirms the presence of exchange coupling, suppressing the superparamagnetism of the Ni core at room temperature, and reaching a coercivity of >6 kOe at 80 K. These results provide a pathway to the development of heterostructured metal-oxide exchange coupled nanoparticles with improved maximum energy product.

One-Pot Synthesis of Nanostructured Ni@Ni(OH)2 and Co-Doped Ni@Ni(OH)2 via Chemical Reduction Method for Supercapacitor Applications.

Crystalline Ni@Ni(OH)2 (cNNH) and Co-doped cNNH were obtained via a simple one-pot hydrothermal synthesis using a modified chemical reduction method. The effect of each reagent on the synthesis of the nanostructures was investigated concerning the presence or absence of each reagent. The detailed morphology shows that both nanostructures consist of a Ni core and Ni(OH)2 shell layer (~5 nm). Co-doping influences the morphology and suppresses the particle agglomeration of cNNH. Co-doped cNNH showed a specific capacitance of 1238 F g-1 at 1 A g-1 and a capacitance retention of 76%, which are significantly higher than those of cNNH. The enhanced performance of the co-doped cNNH is attributed to the reduced path length of the electrons caused by the decrease in the size of the nanostructure and the increased conductivity due to Co ions substituting Ni ions. The reported synthesis method and electrochemical behaviors of cNNH and Co-doped cNNH affirm their potential as electrochemically active materials for supercapacitor applications.

Effect of locally-gradient Ni@NiTe2 inclusions on the Seebeck coefficient of Bi2Te3 + xNi composites

The Bi2Te3 + xNi composites with x = 0.00, 0.50, 0.85, 1.00, 1.25 and 1.50 wt% were prepared by spark plasma sintering method. Owing to high-temperature diffuse redistribution of matrix and filler atoms, filler inclusions in Bi2Te3 matrix are forming as locally-gradient Ni@NiTe2 inclusions. Since electrical conductivity of Ni is much higher as compared to that for Bi2Te3, the composites are electrically-inhomogeneous semiconductors. With increasing x, fraction of NiTe2 shell increases, and fraction of Ni core decreases, although all the composites with different x were sintered at the same temperature. This feature is originated from local overheating of the filler inclusions due to flowing pulse electrical current through the electrically-inhomogeneous medium. Diffusion coefficient of Ni inside Bi2Te3, estimated from analysis of Ni concentration profiles near the Ni@NiTe2 inclusions, enhances with increasing x. By using energy activation of the diffusion, local overheating temperatures of the filler inclusions were extracted. The x-dependent changes in electron concentration of the composites are related to competition of increasing in number of the filler inclusions and decreasing in fraction of Ni cores inside the inclusions. Effect of the filler content on the Seebeck coefficient of the composites was analysed by the Pisarenko relationship, plotted for simple parabolic band model. Magnetic electron scattering by ferromagnetic Ni cores in the Ni@NiTe2 inclusions can contribute to the Seebeck coefficient via relevant changes in scattering factor.

Self-assembled NiMn2O4 shell on nanoporous Ni(Mn) core for boosting alkaline hydrogen production

Alkaline hydrogen evolution reaction (HER) is more sluggish than that in acidic media because of the additional energy required for water dissociation. Herein, we report a scalable strategy (alloying-dealloying) to synthesize self-assembled shell-core porous configuration as active and cost-effective electrocatalyst for alkaline HER. Benefiting from the dual function of oxygen vacancies (OV) defects on NiMn2O4 shell and large specific surface area provided by Mn-doped nanoporous Ni core, the resultant OV-NMO/np-Ni30(Mn) electrode achieves a lower overpotential of 130 mV at 10 mA cm−2, a higher exchange current density of 0.5011 mA cm−2 and long-term operational durability in hostile basic environment. Density functional theory (DFT) calculations reveal that appropriate concentration of OV in NiMn2O4 could decreases the energy barrier of Volmer step from 1.59 eV to 1.49 eV, suggesting OV serves as a hot-spot for the dissociation of water molecules during alkali-HER. This work paves a novel approach to fabricate robust shell-core porous structure with abundant defects, dopants and active sites that are favorable for efficient and inexpensive alkaline hydrogen production.

Boosting borohydride oxidation by control lattice-strain of Ni@NiP electrocatalyst with core-shell structure

Here we fabricate an efficient Ni@NiP catalyst for borohydride oxidation by simple electrodeposition, which has a core-shell structure consisting of crystalline Ni core and amorphous NiP shell. The introduction P into the shell of Ni@NiP causes the expansion of Ni lattice, raising d-band center of Ni, strengthening adsorption to reactants, and improving intrinsic activity to BOR. Ni@NiP catalyst exhibits a superior intrinsic activity of 6.3 mAcmCat.-2 than most PGM and PGM-free catalysts. When using Ni@NiP catalyst anode, DBFC gives a peak power density of 474 mW cm−2, 2.5 times higher than that with commercial Pt/C anode. Density functional theory research reveals that tensile lattice strain effectively promoted BOR activity by reducing B* intermediate formation energy on Ni@NiP surface, which is the rate-determining step in BOR and verified by electrochemical measurement results. This work brings new insights into design efficient noble-metal-free catalysts to promote practice application of DBFC technology.

Conservation of Nickel Ion Single‐Active Site Character in a Bottom‐Up Constructed π‐Conjugated Molecular Network

AbstractOn‐surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building‐blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On‐surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π‐extended two‐dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single‐active site character of their single‐molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom‐up constructed covalent nanomeshes.

Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network.

On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes.

Facile preparation of hierarchical Ni@Mn-doped NiO hybrids for efficient and durable oxygen evolution reaction

Exploring highly efficient and non-noble-metal-based electrocatalysts for oxygen evolution reaction (OER) is of great importance not only for water splitting but also for rechargeable metal-air batteries and fuel cells. Herein, we describe a simple strategy to prepare hierarchical Ni@Mn-doped NiO hybrids using flower-like Ni-Mn layered double hydroxides (NiMn-LDHs) as a precursor. After calcination at 400 °C for an hour under N2 atmosphere, the flower-like NiMn-LDHs transform to porous microspheres consisting of nanoparticles, in which Ni cores are encapsulated by Mn-doped NiO shells (denoted as Ni@Mn-NiO-400). Benefiting to this unique porous, core-shell structures and element doping, the as-prepared Ni@Mn-NiO-400 hybrid shows a low overpotential of 178 mV at the current density of 10 mA/cm2 and Tafel slope of 52.7 mV/dec in 1 mol/L KOH solution. More significantly, the Ni@Mn-NiO-400 hybrid also demonstrates superior stability of 98.6% after 50 h continuously testing, much higher than pristine NiMn-LDHs and commercial IrO2 catalyst. In addition, theoretical simulation shows that Ni core and Mn doping greatly affect the electronic states and electronic structure of NiO. As a result, Ni@Mn-doped NiO hybrid possesses an optimal adsorption activity towards oxygen species than NiO and undoped Ni@NiO hybrid. Considering the compositional and structural flexibility of LDHs, this work may offer a simple method to prepare other non-noble metal-based electrocatalysts for OER.

Nickel and oxygen-containing functional groups co-decorated graphene-like shells as catalytic sites with excellent selective hydrogenation activity and robust stability

Encapsulation of metal nanoparticles is one of the promising strategies to overcome the instability of supported metal catalysts. However, improving the hydrogenation activity of multilayer graphene-like encapsulationremains a critical challenge. To address this issue, multilayer graphene-like shells (MGSs) doped with oxygen-containing functional groups (OFGs) encapsulated Ni nanoparticles catalysts (Ni@OC) were synthesized via calcining the calcination of Ni-based metal-organic framework. The Ni@OC catalysts exhibited 100 % nitroaromatics conversion, excellent product selectivity, and superior stability in harsh conditions. Experimental results indicate that the dissociation of H2 and the activation of nitrobenzene compounds can be achieved with the OFGs doped MGSs as the catalytic site in Ni@OC catalysts. In addition, OFGs promote the electron transfer from Ni core to graphene-like shells and result in structural defects of the MGSs to adsorb hydrogenation substrates. Thus, a reasonable design concept was provided to improve the hydrogenation activity of MGSs encapsulated catalysts with the stable existence of metal.

Charge Radii of ^{55,56}Ni Reveal a Surprisingly Similar Behavior at N=28 in Ca and Ni Isotopes.

Nuclear charge radii of ^{55,56}Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus ^{56}Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the ^{56}Ni core is supposed to be much softer than the ^{48}Ca core. The very low magnetic moment μ(^{55}Ni)=-1.108(20) μ_{N} indicates the impact of M1 excitations between spin-orbit partners across the N,Z=28 shell gaps. Our charge-radii results are compared to abinitio and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.

Chemically Induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles Deduced from Polarized Small-Angle Neutron Scattering.

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is not only detected but conclusively identified as a non-ferromagnetic Ni phosphide derived from the trioctylphosphine commonly used in hot-injection colloidal NP syntheses. The phosphide shell thickness is tunable via synthesis temperature, falling to as little as 0.5 nm at 170 °C. Temperature- and magnetic field-dependent polarized SANS measurements additionally reveal essentially bulk-like ferromagnetism in the Ni core and negligible interparticle magnetic interactions, quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a simple potential route to ligand-free stabilization, and highlight the power of the currently available suite of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.