Boosting thermoelectric efficiency of Ag_(2)Se through cold sintering process with Ag nano-precipitate formation

Silver selenide(Ag_(2)Se)stands out as a promising thermoelectric(TE)material,particularly for applications near room temper-atures.This research presents a novel approach for the fabrication of bulk Ag_(2)Se samples at a relatively low temperature(170℃)using the cold sintering process(CSP)with AgNO_(3)solution as a transient liquid agent.The effect of AgNO_(3)addition during CSP on the micro-structure and TE properties was investigated.The results from phase,composition and microstructure analyses showed that the introduc-tion of AgNO_(3)solution induced the formation of Ag nano-precipitates within the Ag_(2)Se matrix.Although the nano-precipitates do not af-fect the phase and crystal structure of orthorhombicβ-Ag_(2)Se,they suppressed crystal growth,leading to reduced crystallite sizes.The samples containing Ag nano-precipitates also exhibited high porosity and low bulk density.Consequently,these effects contributed to sig-nificantly enhanced electrical conductivity and a slight decrease in the Seebeck coefficient when small Ag concentrations were incorpor-ated.This resulted in an improved average power factor from~1540μW·m^(−1)·K^(−2)for pure Ag_(2)Se to~1670μW·m^(−1)·K^(−2)for Ag_(2)Se with additional Ag precipitates.However,excessive Ag addition had a detrimental effect on the power factor.Furthermore,thermal conductiv-ity was effectively suppressed in Ag_(2)Se fabricated using AgNO_(3)-assisted CSP,attributed to enhanced phonon scattering at crystal inter-faces,pores,and Ag nano-precipitates.The highest figure-of-merit(zT)of 0.92 at 300 K was achieved for the Ag_(2)Se with 0.5wt%Ag dur-ing CSP fabrication,equivalent to>20%improvement compared to the controlled Ag_(2)Se without extra Ag solution.Thus,the process outlined in this study presents an effective strategy to tailor the microstructure of bulk Ag_(2)Se and enhance its TE performance at room temperature.

Integration of Multiple Heterointerfaces in a Hierarchical 0D@2D@1D Structure for Lightweight,Flexible,and Hydrophobic Multifunctional Electromagnetic Protective Fabrics

The development of wearable multifunctional electromagnetic protective fabrics with multifunctional,low cost,and high efficiency remains a challenge.Here,inspired by the unique flower branch shape of“Thunberg’s meadowsweet”in nature,a nanofibrous composite membrane with hierarchical structure was constructed.Integrating sophisticated 0D@2D@1D hierarchical structures with multiple heterointerfaces can fully unleash the multifunctional application potential of composite membrane.The targeted induction method was used to precisely regulate the formation site and morphology of the metal–organic framework precursor,and intelligently integrate multiple heterostructures to enhance dielectric polarization,which improves the impedance matching and loss mechanisms of the electromagnetic wave absorbing materials.Due to the synergistic enhancement of electrospinning-derived carbon nanofiber“stems”,MOF-derived carbon nanosheet“petals”and transition metal selenide nano-particle“stamens”,the CoxSey/NiSe@CNSs@CNFs(CNCC)composite membrane obtains a minimum reflection loss value(RL_(min))of-68.40 dB at 2.6 mm and a maximum effective absorption bandwidth(EAB)of 8.88 GHz at a thin thickness of 2.0 mm with a filling amount of only 5 wt%.In addition,the multi-component and hierarchical heterostructure endow the fibrous membrane with excellent flexibility,water resistance,thermal management,and other multifunctional properties.This work provides unique perspectives for the precise design and rational application of multifunctional fabrics.

KOH-assisted aqueous synthesis of bimetallic metal-organic frameworks and their derived selenide composites for efficient lithium storage

To solve low efficiency,environmental pollution,and toxicity for synthesizing zeolitic imidazolate frameworks(ZIFs)in organic solvents,a KOH-assisted aqueous strategy is proposed to synthesize bimetallic ZIFs polyhedrons,which are used as precursors to prepare bimetallic selenide and N-doped carbon(NC)composites.Among them,Fe–Co–Se/NC retains the three-dimensional(3D)polyhedrons with mesoporous structure,and Fe–Co–Se nanoparticles are uniform in size and evenly distributed.When assessed as anode material for lithium-ion batteries,Fe–Co–Se/NC achieves an excellent initial specific capacity of 1165.9 m Ah·g^(-1)at 1.0 A·g^(-1),and the reversible capacity of Fe–Co–Se/NC anode is 1247.4 m Ah·g^(-1)after 550 cycles.It is attributed to that the uniform composite of bimetallic selenides and N-doped carbon can effectively tune redox active sites,the stable 3D structure of Fe–Co–Se/NCs guarantees the structural stability and wettability of the electrolyte,and the uniform distribution of Fe–Co–S nanoparticles in size esuppresses the volume expansion and accelerates the electrochemical reaction kinetics.

Hierarchical Encapsulation and Rich sp^(2)N Assist Sb_(2)Se_(3)-Based Conversion-Alloying Anode for Long-Life Sodium-and Potassium-Ion Storage

Sodium-and potassium-ion batteries have exhibited great application potential in grid-scale energy storage due to the abundant natural resources of Na and K.Conversion-alloying anodes with high theoretical capacity and low-operating voltage are ideal option for SIBs and PIBs but suffer the tremendous volume variations.Herein,a hierarchically structural design and sp^(2)N-doping assist a conversion-alloying material,Sb_(2)Se_(3),to achieve superior life span more than 1000 cycles.It is confirmed that the Sb_(2)Se_(3)evolves into nano grains that absorb on the sp^(2)N sites and in situ form chemical bonding of C-N-Sb after initial discharge.Simulation results indicate that sp^(2)N has more robust interaction with Sb and stronger adsorption capacities to Na^(+)and K^(+)than that of sp3 N,which contributes to the durable cycling ability and high electrochemical activity,respectively.The ex situ transmission electron microscopy and X-ray photoelectron spectroscopy results suggest that the Sb_(2)Se_(3)electrode experiences conversion-alloying dual mechanisms based on 12-electron transfer per formula unit.

Improving the charge kinetics through in-situ growth of NiSe nanoparticles on g-C_(3)N_(4)nanosheets for efficient hybrid supercapacitors

Nickel selenide(NiSe)has been a promising positive electrode for hybrid supercapacitors due to its multiple oxidation states,tunability,and high specific capacity.However,sluggish ion transfers and particle agglomeration hamper its electrochemical performance.In the present study,we have grown NiSe nanoparticles on two-dimensional(2D)graphitic carbon nitride(g-C_(3)N_(4))nanosheets to realize three-dimensional(3D)architecture.The 2D support,high nitrogen content,and features of g-C_(3)N_(4)enhanced the specific capacity of the NiSe/g-C_(3)N_(4)nanocomposite material.The resulting nanocomposite shows a specific capacity of 320 mA h g^(-1)at a current density of 1 A g^(-1),which is considerably higher than pristine NiSe.Later,the hybrid supercapacitor(HSC)device was fabricated using NiSe/g-C_(3)N_(4)composite as positive and activated carbon(AC)as negative electrodes.The cell delivered an energy density of 52.5 Wh kg^(-1)at a power density of 1488 W kg^(-1)with excellent cyclic stability of 84.9%over 8000 cycles.The electrochemical performance enhancement corresponds to a 3D structure,high electrochemical active sites,and improved charge transportation at the electrode/electrolyte interface.Thus,the present work offers an easy approach and architectural design for high-performance HSC.

Broadband and continuous wave pumped second-harmonic generation from microfiber coated with layered GaSe crystal

The conversion-efficiency for second-harmonic(SH)in optical fibers is significantly limited by extremely weak second-order nonlinearity of fused silica,and pulse pump lasers with high peak power are widely employed.Here,we propose a simple strategy to efficiently realize the broadband and continuous wave(CW)pumped SH,by transferring a crystalline GaSe coating onto a microfiber with phase-matching diameter.In the experiment,high efficiency up to 0.08%W-1mm-1 is reached for a C-band pump laser.The high enough efficiency not only guarantees SH at a single frequency pumped by a CW laser,but also multi-frequencies mixing supported by three CW light sources.Moreover,broadband SH spectrum is also achieved under the pump of a superluminescent light-emitting diode source with a 79.3 nm bandwidth.The proposed scheme provides a beneficial method to the enhancement of various nonlinear parameter processes,development of quasi-monochromatic or broadband CW light sources at new wavelength regions.

Effect of thickness of antimony selenide film on its photoelectric properties and microstructure

Antimony selenide(Sb2Se3) films are widely used in phase change memory and solar cells due to their stable switching effect and excellent photovoltaic properties. These properties of the films are affected by the film thickness. A method combining the advantages of Levenberg–Marquardt method and spectral fitting method(LM–SFM) is presented to study the dependence of refractive index(RI), absorption coefficient, optical band gap, Wemple–Di Domenico parameters, dielectric constant and optical electronegativity of the Sb2Se3films on their thickness. The results show that the RI and absorption coefficient of the Sb2Se3films increase with the increase of film thickness, while the optical band gap decreases with the increase of film thickness. Finally, the reasons why the optical and electrical properties of the film change with its thickness are explained by x-ray diffractometer(XRD), energy dispersive x-ray spectrometer(EDS), Mott–Davis state density model and Raman microstructure analysis.

Preparation of high pure tellurium from raw tellurium containing Cu and Se by chemical method

High pure tellurium was prepared from raw tellurium containing copper and selenium by chemical method containing oxidation with concentrated nitric acid, leaching with hydrochloric acid, reducing with sulfur dioxide and treating in hydrogen atmosphere at high temperature. Removal ratio of Cu in raw tellurium reaches 99% after raw tellurium is oxidized and leached with HNO3（69%） under the following conditions： 0.96 times stoichiometric quantity of concentrated nitric acid, 4：1 of ratio of liquid to solid, 20 °C of reaction temperature and 30 min of reaction time. Leaching ratio of Te reaches 99% after Te is leached with hydrochloric acid under the following conditions： 1.67 times stoichiometric quantity of hydrochloric acid, 4：1 of ratio of liquid to solid, 20 °C of reaction temperature and 30 min of reaction time. Tellurium powder（99.95%） is obtained when Te（IV） in leachate is reduced with sulfur dioxide. The purity of tellurium increases from 99.954% to 99.999 6% after tellurium（99.95%） is treated in hydrogen atmosphere at 723.15 K for 30 min.

In situ fabrication of Bi_(2)Se_(3)/g-C_(3)N_(4)S-scheme photocatalyst with improved photocatalytic activity

Bismuth selenide(Bi_(2)Se_(3))is an attractive visible-light-responsive semiconductor that can absorb a full range of visible and near-infrared light.However,its poor redox capacity and rapid carrier recombination limit its application in photocatalytic oxidation.In this study,we adopted Bi_(2)Se_(3)as the couple part of graphitic carbon nitride(g-C_(3)N_(4))to construct a Bi_(2)Se_(3)/g-C_(3)N_(4)composite photocatalyst.Through in situ fabrication,the self-developed Bi2O3/g-C_(3)N_(4)precursor was transformed into a Bi_(2)Se_(3)/g-C_(3)N_(4)heterojunction.The as-prepared Bi_(2)Se_(3)/g-C_(3)N_(4)composite exhibited much higher visible-light-driven photocatalytic activity than pristine Bi_(2)Se_(3)and g-C_(3)N_(4)in the removal of phenol.The enhanced photocatalytic activity was ascribed to the S-scheme configuration of Bi_(2)Se_(3)/g-C_(3)N_(4);this was confirmed by the energy-level shift,photoluminescence analysis,computational structure study,and reactive-radical testing.In the S-scheme heterojunction,photo-excited electrons in the conduction band of g-C_(3)N_(4)migrate to the valence band of Bi_(2)Se_(3)and combine with the excited holes therein.By consuming less reactive carriers,the S-scheme heterojunction can not only effectively promote charge separation,but also preserve more reactive photo-generated carriers.This property enhances the photocatalytic activity.

Tuning Metallic Co0.85Se Quantum Dots/Carbon Hollow Polyhedrons with Tertiary Hierarchical Structure for High-Performance Potassium Ion Batteries

Potassium-ion batteries(KIBs)are a potential candidate to lithium-ion batteries(LIBs)but possess unsatisfactory capacity and rate properties.Herein,the metallic cobalt selenide quantum dots(Co0.85Se-QDs)encapsulated in mesoporous carbon matrix were designed via a direct hydrothermal method.Specifically,the cobalt selenide/carbon composite(Co0.85Se-QDs/C)possesses tertiary hierarchical structure,which is the primary quantum dots,the secondary petals flake,and the tertiary hollow micropolyhedron framework.Co0.85Se-QDs are homogenously embedded into the carbon petals flake,which constitute the hollow polyhedral framework.This unique structure can take the advantages of both nanoscale and microscale features:Co0.85Se-QDs can expand in a multidimensional and ductile carbon matrix and reduce the K-intercalation stress in particle dimensions;the micropetals can restrain the agglomeration of active materials and promote the transportation of potassium ion and electron.In addition,the hollow carbon framework buffers volume expansion,maintains the structural integrity,and increases the electronic conductivity.Benefiting from this tertiary hierarchical structure,outstanding K-storage performance(402 mAh g?1 after 100 cycles at 50 mA g?1)is obtained when Co0.85Se-QDs/C is used as KIBs anode.More importantly,the selenization process in this work is newly reported and can be generally extended to prepare other quantum dots encapsulated in edge-limited frameworks for excellent energy storage.

Polar, catalytic, and conductive CoSe2/C frameworks for performance enhanced S cathode in Li–S batteries

Lithium-sulfur battery(Li-S)is considered as one of the new-generation rechargeable batteries with high performance because of its extremely high theoretical capacity,energy density,environmental harmony and low cost.However,low electrical and ionic conductivity of sulfur,safety concerns and parasitic reaction generated by the dissolved polysulfide species in electrolyte hinder the commercialization of Li-S battery.Herein,we report a polyhedral porous structure comprising of carbon coating metal selenide nanoparticles(CoSe2/C),which could not only host sulfur for Li-S battery owing to its porous and conductive structure,but also mitigate the shuttle phenomenon by polysulfides adsorption and catalytic acceleration of redox kinetics.As a result,a performance enhanced CoSe2/C-S electrode for Li-S battery is achieved.

MOF-derived molybdenum selenide on Ti_(3)C_(2)T_(x) with superior capacitive performance for lithium-ion capacitors

Two-dimensional Ti_(3)C_(2)T_(x) exhibits outstanding rate property and cycle performance in lithium-ion capacitors(LICs)due to its unique layered structure,excellent electronic conductivity,and high specific surface area.However,like graphene,Ti_(3)C_(2)T_(x) restacks during electrochemical cycling due to hydrogen bonding or van der Waals forces,leading to a decrease in the specific surface area and an increase in the diffusion distance of electrolyte ions between the interlayer of the material.Here,a transition metal selenide MoSe_(2) with a special three-stacked atomic layered structure,derived from metal-organic framework(MOF),is introduced into the Ti_(3)C_(2)T_(x) structure through a solvo-thermal method.The synergic effects of rapid Li+diffusion and pillaring effect from the MoSe_(2) and excellent conductivity from the Ti_(3)C_(2)T_(x) sheets endow the material with excellent electrochemical reaction kinetics and capacity.The composite Ti_(3)C_(2)T_(x)@MoSe_(2) material exhibits a high capacity over 300 mAh·g^(-1) at 150 mA·g^(-1) and excellent rate property with a specific capacity of 150 mAh·g^(-1) at 1500 mA·g^(-1).Addition-ally,the material shows a superior capacitive contribution of 86.0%at 2.0 mV·s^(-1) due to the fast electrochemical reactions.A Ti_(3)C_(2)T_(x)@MoSe_(2)//AC LIC device is also fabricated and exhibits stable cycle performance.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.

MOF‑Derived CoSe2@N‑Doped Carbon Matrix Confined in Hollow Mesoporous Carbon Nanospheres as High‑Performance Anodes for Potassium‑Ion Batteries

In this work,a novel vacuum-assisted strategy is proposed to homogenously form Metal-organic frameworks within hollow mesoporous carbon nanospheres(HMCSs)via a solid-state reaction.The method is applied to synthesize an ultrafine CoSe2 nanocrystal@N-doped carbon matrix confined within HMCSs(denoted as CoSe2@NC/HMCS)for use as advanced anodes in highperformance potassium-ion batteries(KIBs).The approach involves a solvent-free thermal treatment to form a Co-based zeolitic imidazolate framework(ZIF-67)within the HMCS templates under vacuum conditions and the subsequent selenization.Thermal treatment under vacuum facilitates the infiltration of the cobalt precursor and organic linker into the HMCS and simultaneously transforms them into stable ZIF-67 particles without any solvents.During the subsequent selenization process,the“dual confinement system”,composed of both the N-doped carbon matrix derived from the organic linker and the small-sized pores of HMCS,can effectively suppress the overgrowth of CoSe2 nanocrystals.Thus,the resulting uniquely structured composite exhibits a stable cycling performance(442 mAh g^−1 at 0.1 A g^−1 after 120 cycles)and excellent rate capability(263 mAh g^−1 at 2.0 A g^−1)as the anode material for KIBs.

Porous Microspheres Comprising CoSe2 Nanorods Coated with N-Doped Graphitic C and Polydopamine-Derived C as Anodes for Long-Lived Na-Ion Batteries

Metal–organic framework-templated nitrogen-doped graphitic carbon(NGC)and polydopaminederived carbon(PDA-derived C)-double coated one-dimensional CoSe_(2) nanorods supported highly porous threedimensional microspheres are introduced as anodes for excellent Na-ion batteries,particularly with long-lived cycle under carbonate-based electrolyte system.The microspheres uniformly composed of ZIF-67 polyhedrons and polystyrene nanobeads(φ=40 nm)are synthesized using the facile spray pyrolysis technique,followed by the selenization process(P-CoSe_(2)@NGC NR).Further,the PDA-derived C-coated microspheres are obtained using a solution-based coating approach and the subsequent carbonization process(P-CoSe_(2)@PDA-C NR).The rational synthesis approach benefited from the synergistic effects of dual carbon coating,resulting in a highly conductive and porous nanostructure that could facilitate rapid diffusion of charge species along with efficient electrolyte infiltration and effectively channelize the volume stress.Consequently,the prepared nanostructure exhibits extraordinary electrochemical performance,particularly the ultra-long cycle life stability.For instance,the advanced anode has a discharge capacity of 291(1000th cycle,average capacity decay of 0.017%)and 142 mAh g^(-1)(5000th cycle,average capacity decay of 0.011%)at a current density of 0.5 and 2.0 A g^(-1),respectively.

Phase transition and high temperature thermoelectric properties of copper selenide Cu_(2-x) Se (0 ≤ x ≤ 0.25)

With good electrical properties and an inherently complex crystal structure, Cu2-xSe is a potential ＂phonon glass electron crystal＂ thermoelectric material that has previously not attracted much interest. In this study, Cu2-xSe （0 ≤ x ≤0.25） compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu2-xSe were investigated in the temperature range of 300 K-750 K. The results of X-ray diffraction at room temperature show that Cu2-xSe compounds possess a cubic structure with a space group of Fm3m （#225） when 0.15 〈 x ≤ 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu2Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.

A Superaerophobic Bimetallic Selenides Heterostructure for Efficient Industrial-Level Oxygen Evolution at Ultra-High Current Densities

Cost-effective and stable electrocatalysts with ultra-high current densities for electrochemical oxygen evolution reaction(OER)are critical to the energy crisis and environmental pollution.Herein,we report a superaerophobic three dimensional(3D)heterostructured nanowrinkles of bimetallic selenides consisting of crystalline NiSe2 and NiFe2Se4 grown on NiFe alloy(NiSe2/NiFe2Se4@NiFe)prepared by a thermal selenization procedure.In this unique 3D heterostructure,numerous nanowrinkles of NiSe2/NiFe2Se4 hybrid with a thickness of ~100 nm are grown on NiFe alloy in a uniform manner.Profiting by the large active surface area and high electronic conductivity,the superaerophobic NiSe2/NiFe2Se4@NiFe heterostructure exhibits excellent electrocatalytic activity and durability towards OER in alkaline media,outputting the low potentials of 1.53 and 1.54 V to achieve ultra-high current densities of 500 and 1000 mA cm^−2,respectively,which is among the most active Ni/Fe-based selenides,and even superior to the benchmark Ir/C catalyst.The in-situ derived FeOOH and NiOOH species from NiSe2/NiFe2Se4@NiFe are deemed to be efficient active sites for OER.

Carbon-Coated Three-Dimensional MXene/Iron Selenide Ball with Core–Shell Structure for High-Performance Potassium-Ion Batteries

Two-dimensional(2D)MXenes are promising as electrode materials for energy storage,owing to their high electronic conductivity and low diffusion barrier.Unfortunately,similar to most 2D materials,MXene nanosheets easily restack during the electrode preparation,which degrades the electrochemical performance of MXene-based materials.A novel synthetic strategy is proposed for converting MXene into restacking-inhibited three-dimensional(3D)balls coated with iron selenides and carbon.This strategy involves the preparation of Fe_(2)O_(3)@carbon/MXene microspheres via a facile ultrasonic spray pyrolysis and subsequent selenization process.Such 3D structuring effectively prevents interlayer restacking,increases the surface area,and accelerates ion transport,while maintaining the attractive properties of MXene.Furthermore,combining iron selenides and carbon with 3D MXene balls offers many more sites for ion storage and enhances the structural robustness of the composite balls.The resultant 3D structured microspheres exhibit a high reversible capacity of 410 mAh g^(−1) after 200 cycles at 0.1 A g^(−1) in potassium-ion batteries,corresponding to the capacity retention of 97% as calculated based on 100 cycles.Even at a high current density of 5.0 A g^(−1),the composite exhibits a discharge capacity of 169 mAh g^(−1).

Nitrogen-Doped Carbon-Encased Bimetallic Selenide for High-Performance Water Electrolysis

Demand of highly efficient earth-abundant transition metal-based electrocatalysts to replace noble metal materials for boosting oxygen evolution reaction(OER)is rapidly growing.Herein,an electrochemically exfoliated graphite(EG)foil supported bimetallic selenide encased in N-doped carbon(EG/(Co,Ni)Se2-NC)hybrid is developed and synthesized by a vapor-phase hydrothermal strategy and subsequent selenization process.The as-prepared EG/(Co,Ni)Se2-NC hybrid exhibits a core-shell structure where the particle diameter of(Co,Ni)Se2 core is about 70 nm and the thickness of N-doped carbon shell is approximately 5 nm.Benefitting from the synergistic effects between the combination of highly active Co species and improved electron transfer from Ni species,and N-doped carbon,the EG/(Co,Ni)Se2-NC hybrid shows remarkable electrocatalytic activity toward OER with a comparatively low overpotential of 258 mV at an current density of 10 mA cm?2 and a small Tafel slope of 73.3 mV dec?1.The excellent OER catalysis performance of EG/(Co,Ni)Se2-NC hybrid is much better than that of commercial Ir/C(343 mV at 10 mA cm?2 and 98.1 mV dec?1),and even almost the best among all previously reported binary CoNi selenide-based OER electrocatalysts.Furthermore,in situ electrochemical Raman spectroscopy combined with ex situ X-ray photoelectron spectroscopy analysis indicates that the superb OER catalysis activity can be attributed to the highly active Co-OOH species and modified electron transfer process from Ni element.