Transition Metal Diselenides Research Articles

Equilibrium densities of intrinsic defects in transition metal diselenides of molybdenum and tungsten.

Point defects are thermodynamically stabilized in all crystalline materials, with increased densities negatively impacting the properties and performance of transition metal dichalcogenides (TMDs). While recent point defect reduction methods have led to considerable improvements in the optoelectronic properties of TMDs, there is a clear need for theoretical work to establish the lower limit of defect densities, as represented by thermal equilibrium. To that end, an abinitio and thermodynamic analysis of the equilibrium densities of intrinsic point defects in MoSe2 and WSe2 is presented. The intrinsic defect formation energies at the limits of the selenium and metal-rich regimes are determined by density functional theory (DFT) and then augmented with elemental chemical potential functions to determine temperature- and pressure-dependent formation energies. Equilibrium defect densities are determined for MSe, SeM, vM, and vSe, where M and v, respectively, represent the metal and the vacancy, as a function of synthesis temperature and pressure. The effects of vibrational free energy contributions and treatment of the DFT exchange-correlation potential are found to be non-negligible. Calculated equilibrium densities are several orders of magnitude below reported defect densities in TMDs made by chemical vapor deposition, chemical vapor transport, and flux methods, thereby establishing that current synthesis methods are either kinetically limited or impurity dominated.

Chalcogen-Based Precursors for Transition Metal (Co, Ni) Phosphides: (Di)chalcogenide-to-Phosphide Transformation via Chemical Extraction of Chalcogenides.

The chemical properties of polymorphic compounds are highly dependent on their stoichiometry and atomic arrangements, making certain phases technologically more important. Selective development of these phases is challenging. This study introduces a method where chalcogenide atoms from metal chalcogenides are chemically extracted by trioctylphosphine (TOP) and substituted with phosphide. Using this approach, dithiocarbamate/xanthate complexes of cobalt and nickel were employed for the selective synthesis of pure metal sulfides or phosphides. Optimization yielded either sulfur-deficient phases (Ni3S2, Co9S8) or a complete transformation to phosphides (Ni2P, CoP). Likewise, for the first time, selenobenzoate complexes of Ni and Co were used for the synthesis of transition metal diselenides (NiSe2, CoSe2), which could then be converted to metal phosphides (Ni2P, Co2P). The synthesis used solution-phase thermal decomposition with various precursors, surfactants, and temperatures. With TOP, different phases of metal chalcogenides, metal phosphides, and mixed metal phosphide nanomaterials (NiS, Ni3S2, Co9S8, NiSe2, CoSe2, Ni2P, Ni5P4, Co2P, CoP, and Ni2-xCoxP) were obtained by varying reaction conditions. The formation mechanism of nickel and cobalt phosphide nanoparticles from precursors is proposed, demonstrating that presynthesized metal chalcogenides can be transformed into phosphides, opening a new research avenue for dimensionally controlled metal chalcogenides as templates for metal phosphides.

Electronic structure and quantum capacitance analysis of transition metal doped cobalt diselenide for supercapacitors

The accelerating degradation of conventional fossil fuels and ever-growing consumption of energy have prompted the search for renewable energy sources. Renewable energy storage necessitates immediate attention toward the development of efficient storage technologies, such as supercapacitors. Among the various transition metal diselenides, CoSe2 is widely known for its improved supercapacitor applications. The present work uses density functional theory for quantum capacitance (CQ) and surface charge analysis of transition metal doped (Fe, Cu) CoSe2. Doping enhances the structural stability and increases the number of electronic states near the Fermi level. Fe-doping has a more prominent effect than Cu-doping, resulting in more redox active sites as compared to Cu-doping due to the lower redox potential of Fe. The optimized Co0.5Fe0.5Se2 material exhibits maximum a CQ of ∼1154 μF/cm2 compared to CoSe2 (906 μF/cm2) at −0.6 V. The CQ shows a higher value at negative potential, suggesting that pristine CoSe2 and Co0.5Fe0.5Se2 are suitable cathode materials for asymmetric supercapacitors. Surface charge analysis also suggests the asymmetric nature of CoSe2 and Co0.5Fe0.5Se2. In addition, the mechanical strength of CoSe2 and Co0.5Fe0.5Se2 was analyzed. The higher bulk modulus of Co0.5Fe0.5Se2 suggests its higher mechanical strength. All of our results suggest that doping a suitable transition metal can significantly boost the performance of supercapacitors.

Electrochemical performance investigation of different shaped transition metal diselenide materials based symmetric supercapacitor with theoretical investigation

Transition metal diselenide-based electrodes for hybrid symmetric supercapacitors appear as trending materials. Thereby in this paper, we report the preparation of different-shaped transition metal diselenides using a single-step hydrothermal route. The impact of the different morphology of the prepared transition metal diselenide material has been studied on their electrochemical performance. The nanoflower-shaped MoSe2 material was observed to deliver the highest electrochemical result than nanoneedles and nanospheres shape of CoSe2 and NiSe2 material respectively. The highest specific capacitance delivered by the MoSe2 material-based symmetric supercapacitor was 154 F g−1 at 10 mV s−1. It also exhibits a maximum energy density of 17 Wh kg−1 with 1267 W kg−1 power density. Further, the MoSe2-based symmetric supercapacitor has been utilized to burn different colors of light-emitting diodes along with a panel of 26 LEDs of red color. To make the working of the symmetric supercapacitor (MoSe2-based) easier to understand for the readers we have proposed a mechanism of charge storage associated with it. Additionally, the experimental finding has been supported by investigating the structural and electronic properties of MoSe2, CoSe2, and NiSe2 via density functional theory calculation.

Wrapping 2D layered VSe2 nanoplates in 3D carbon nanotube network for high-rate and long-cycling sodium storage capability in ether electrolytes

The layered transition metal diselenides are promising anode materials for sodium ion batteries, attributed to their large interlayer spacing and decent capacities. It is still a challenge to achieve long-term cycling and high-rate capability because of the serious volume effects and unsatisfied conductivity. In this work, the hierarchical architectures consisting of VSe2 nanoplates wrapped into the carbon nanotubes network (VSe2/CNT) are prepared through a simple hydrothermal method. Benefiting from the high conductivity and the structure protection of CNT network and the abundant Se vacancy in VSe2, the reaction kinetics and structure stability of the VSe2/CNT anode are significantly enhanced for sodium storage. Consequently, a discharge capacity of 170 mAh g−1 can be maintained over 10000 cycles at 2.0 A g−1 with a low capacity decay rate of 0.0048% per cycle, and a high capacity of 167 mAh g−1 is achieved at 100 A g−1 for the rate performance. Moreover, combinational modulations of electrolyte and electrode can take a synergetic influence on the electrochemical behaviors. The ether-based electrolyte has a lower stable overpotential, which promotes fast sodium storage. The CNT network passivation can suppress the “under-voltage failure” phenomenon, thereby providing superior cycling stability.

Fabrication of activated carbon coated MSe2 (M=Mo, Co, and Ni) nanocomposite electrode for high-performance aqueous asymmetric supercapacitor

Transition metal diselenides are trendy electrode materials for supercapacitors. The main reason behind it is their high specific capacitance. But, they suffer from poor electronic conductivity and agglomeration issues which hinder their practical use. Herein, we present the preparation of nanocomposites including transition metal diselenide MSe2 (M=Mo, Co, and Ni) with activated carbon using the facial hydrothermal route. Inspiring from the high electrochemical results of a hybrid nanocomposites symmetric cell, an aqueous asymmetric ultracapacitor cell (AUC) comprised of different shaped carbon-coated MSe2 (positive electrode) as well as activated carbon (AC) (negative electrode) has been fabricated. The NAC//6KOH//AC asymmetric cell displays the best outcomes among all fabricated devices. The specific capacitance observed is about 3740 F g−1 at 10 mV s−1. It delivers a high energy density of 69 Wh kg−1 along with a power density of 687 W kg−1 at 1 A g−1. The electrochemical results motivate us to explore the practical usability of the prepared NAC//6KOH//AC device via illuminating 26 red LED panels which glow for 26 min. The findings in this report indicate that MSe2-based nanocomposite has the scope and is a potential material in the energy storage field.

CO sensing properties of nanostructured WSe2/GaN and MoSe2/GaN based gas sensors

The sensing properties of GaN and hydrothermal synthesized transition metal di-selenides nanostructure decorated WSe 2 /GaN and MoSe 2 /GaN hybrid structures are reported for 100 ppm CO gas for temperature range of 27 °C–250 °C. The fabricated devices divulged that Schottky Barrier Height increases from 0.6 eV to 1.2 eV with increment in temperature while vice versa was perceived for series resistance, except for MoSe 2 /GaN structure. The gas sensing measurements revealed that the threshold temperature for CO sensing for GaN, WSe 2 /GaN and MoSe 2 /GaN based sensors was 100 °C, 100 °C and 150 °C along with a peak response value of 23.3 %, 38.8 % and 43.1 %, respectively. The gas sensing data displayed a highly competitive response & recovery time as all the fabricated devices were able to detect CO within 78 s of exposure. The recovery time of the devices varied in a similar fashion, demonstrating the promising potential of W(Mo)Se 2 /GaN hybrid structures for efficient CO gas detection. • GaN & TMDC hybrid structure based sensor for CO gas sensing. • Hydrothermally synthesized WSe 2 and MoSe 2 nanostructured on GaN for CO gas sensing. • Threshold temperature for CO sensing for GaN & its heterostructure varies between 100 o C- 150 °C. • MoSe 2 /GaN & WSe 2 /GaN devices with highly competitive response & recovery time is fabricated.

Theoretical Investigation of W(CO)6 and CO Selenization Process

Detailed atomic-level insight into the mechanism of W(CO)6 and CO selenization is essential for the fabrication of cheap and environmentally benign transition metal chalcogenides such as MoS2 and WSe2. Earlier discussions in literature have focused mainly on the CO methanation by sulfur and its derivatives but H2Se mediated CO methanation at the atomic level is yet to be explored. First-principles calculations and ReaxFF-based molecular dynamics simulations are conducted here to explore the relative stabilities of intermediates formed during the gas-phase interactions of W(CO)6 and H2Se, determined associated reaction energies and kinetic barriers. The methanation of CO, which is released from the organometal, by H2Se is further investigated. The results indicate that the chain reactions of W(CO)6 and H2Se lead to the formation of a thermodynamically stable end product of W(SeH)2Se2. Depending on the temperature, W(HSe)2Se2 is expected to go through a last uphill reaction by releasing H2Se into the environment and evolving into a WSe3 molecule. Additionally, the dehydrogenation of organometallic molecules is thermodynamically feasible but kinetically controlled, requiring a significant activation energy. When all CO groups are released from the W atom, the H2 release from W-compund becomes nearly barrierless. Since CO radical groups are dominant byproducts formed during the MOCVD chain reactions but in a chalcogen rich environment, this work also shed light into the CO selenization during the growth of transition metal diselenides (e.g., WSe2, MoSe2, CrSe2) and discusses the formation of potential products such as CSe2, CH4, H2Se, CO, H2O, Se2.

Recent progress on transition metal diselenides from formation and modification to applications.

The development of graphene promotes the research of similar two-dimensional (2D) materials, especially 2D transition metal dichalcogenides (TMDCs) with semiconductor properties. Monolayer or few-layer TMDCs have several advantages, such as direct band gap, weak interlayer van der Waals force, large interlayer spacing, and abundant marginal active sites, which make them widely used in catalysis, optoelectronics, as well as energy conversion and storage devices. In addition, transition metal diselenides (TMDSs) also possess many intriguing characteristics. For instance, transition metal diselenides (e.g., MoSe2) have a more stable 1T phase, larger interlayer spacing, smaller band gap, and more obvious metallic property of Se than TMDCs (e.g., MoS2). Thus, it has become one of the most attractive research topics branching out from TMDCs. Herein, this review unveils the structures, synthesis, properties, modifications, applications, and perspectives for TMDSs.

Optical pulse modulators based on layered vanadium diselenide nanosheets

The transition metal disulfides of VB group elements have gradually come into people’s field of vision owing to their two-dimensional structure and unique optical properties. Vanadium diselenide (VSe2) as a kind of transition metal diselenides, is competent for the applications of nonlinear saturable absorption. The dispersion of few-layer VSe2 is prepared by liquid phase exfoliation method. Clearly, it has an obvious layered structure, and the interlayer spacing is 0.31 nm. The VSe2 nanosheets are inserted into the Erbium-doped fiber laser through tapered deposition method and the measured modulation depth is 1.46%. A 1530.5 nm centered 851-fs pulse is observed with the 3.2 nm 3-dB spectral width. The experimental results show that the pulse is persistent under the power of 334 mW, with signal-to-noise ratio of 41 dB. And an up to 552.4 MHz modulation phenomenon is observed around 1560 nm, so is its frequency tunability. This is the first time that VSe2 is used to realize high frequency modulation in fiber laser. It is proved that VSe2 is expected to be a budding material of ultrafast optical modulation devices and widely used in the field of ultrafast photonics.

Structural and electronic properties of 2D chalcogenides

Two-dimensional (2D) materials and their heterostructures are useful in electronic applications, especially with the drive towards more and more miniaturization. Using first-principles calculations, we study the structural and electronic properties of some transition metal dichalcogenides and diselenides (MoS2, MoSe2, WS2, WSe2). Our studies were based on density functional theory (DFT) as implemented in the Quantum ESPRESSO package. 2D layered materials present a huge challenge for DFT as a result of interlayer van der Waals (vdW) interaction which is absent from the typical DFT exchange-correlation functional. In order to account for the van der Waals interactions between layers, we applied the semi-empirical DFT-D2 method of Grimme and the revised Vydrov-Van Voorhis non-local correlation functional (rVV10). We show that the two approaches give similar structural properties. We also found that the size of the electronic band gap of these structures changes with respect to the number of layers stacked.

Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies

We investigate the role of growth temperature and metal/chalcogen flux in atmospheric pressure chemical vapor deposition growth of MoSe2 and WSe2 on Si/SiO2 substrates. Using scanning electron microscopy and atomic force microscopy, we observe that the growth temperature and transition metal flux strongly influence the domain morphology, and the compact triangular or hexagonal domains ramify into branched structures as the growth temperature (metal flux) is decreased (increased). The competition between adatom attachment to the domain edges and diffusion of adatoms along the domain boundary determines the evolution of the observed growth morphology. Depending on the growth temperature and flux, two different branched structures-fractals and dendrites-grow. The fractals (with a dimension of ∼1.67) obey a diffusion-limited aggregation mechanism, whereas the dendrites with a higher fractal dimension of ∼1.80 exhibit preferential growth along the symmetry-governed directions. The effect of chalcogen environment is studied, where a Se-rich condition helps restrict Mo-rich nucleus formation, promoting lateral growth. For a Se-deficient environment, several multilayer islands cluster on two-dimensional domains, suggesting a transition from lateral to vertical growth because of insufficient Se passivation. X-ray photoelectron spectroscopy analysis shows a near perfect stoichiometry (Mo/Se = 1:1.98) of MoSe2 grown in a Se-rich environment, whereas in the Se-deficient condition, a ratio of Mo/Se = 1:1.68 is observed. This also supports the formation of metal-rich nuclei (Mo1+xSe2-x) under Se-deficient conditions, leading to three-dimensional clustering. Tuning the growth temperature and metal/chalcogen flux, we propose an optimized CVD growth window for synthesizing large-area Mo(W) selenide.

A comparative experimental and theoretical investigation on energy storage performance of CoSe2, NiSe2 and MnSe2 nanostructures

We have investigated a comparative supercapacitor performance of the transition metal (Co, Ni and Mn) diselenides nanostructures assembled in a symmetric electrode configuration. Supercapacitor electrodes based on the transition metal selenides fabricated on Ni-foam which delivered specific capacitance values of 148, 139 and 50 F/g respectively for CoSe2, NiSe2 and MnSe2 which demonstrates the better performance of CoSe2 as compared to NiSe2 and MnSe2. Moreover, CoSe2 electrode showed good cycling stability with capacitance retention of 98.46% over 3000 cycles, whereas NiSe2 and MnSe2 showed the cycling stability of 59.44% and 86.70% respectively. We have explored the origin of enhanced energy storage performance of CoSe2 both by supported experiments and density functional theory investigation. For theoretical studies, we have computed electronic and structural properties of CoSe2, NiSe2 and MnSe2, using state of art Density Functional Theory (DFT) simulations. Enhanced density of states around fermi level, largest tunnel size, highest surface area per atom and highest quantum capacitance for CoSe2 justifies superior supercapacitance behavior of CoSe2 compared to NiSe2 and MnSe2 as observed in the experiment. Quantum capacitance is highest for CoSe2 and it follows the trend CoSe2 > NiSe2 > MnSe2, similar to the trend for specific capacitance obtained in the experiment. Thus, the present work on comparative investigation on energy storage performance of transition metal diselenides demonstrate that CoSe2 can be further exploited as a potential material for energy storage application.

Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer

Many transition metal di-selenides such as MoSe2 and WSe2 show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the number of electrocatalytic active sites and in turn to improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, we have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe2 for HER and OER catalyst. The state-of-art density functional theory (DFT) based computations are performed for estimating the catalytic activity of the pristine and doped SnSe2 by means of evaluating the adsorption and Gibbs free energies subjected to hydrogen and oxygen adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, we have included the dispersion correction term to exchange-correlation functional. Results show that the pristine SnSe2 is not a good HER catalyst when hydrogen is adsorbed on its basal plane. However, edge-sites show the good hydrogen adsorption and indicates that the edges of SnSe2 are the most preferential site for hydrogen adsorption. As far as the catalytic activity of SnSe2 with dopants is concerned, the Na-doped SnSe2 among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe2 show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe2 can be improved by selective doping of alkali metals.

Transition-metal-doped NiSe2 nanosheets towards efficient hydrogen evolution reactions

Transition metal diselenides are promising electrocatalysts for hydrogen evolution and therefore different approaches have been proposed to enhance their catalytic activity. Herein, we describe systematic studies of the dependence of transition-metal doping on the catalytic activity of NiSe2 by first principles calculations, where Fe is demonstrated to be the best candidate element to tune the electrocatalytic activity of NiSe2 with lower ΔGH* values and increased electrical conductivity. To provide further experimental evidence, Fe-doped NiSe2 porous nanosheets grown on carbon cloth are successfully developed. These nanosheets show significantly improved efficiency for hydrogen evolution reactions compared to their un-doped counterpart. The optimized Ni0.8Fe0.2Se2 electrocatalyst gives rise to a current density of 10 mA·cm-2 at a very low overpotential of 64 mV with outstanding long-term stability. The present strategy of doping NiSe2 -based electrocatalysts with transition metals paves a new pathway for the design and synthesis of electrocatalysts for large-scale electrochemical energy applications.

Rational Design of Hierarchical Nanotubes through Encapsulating CoSe2 Nanoparticles into MoSe2/C Composite Shells with Enhanced Lithium and Sodium Storage Performance.

Transition-metal diselenides have been extensively studied as desirable anode candidates for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) because of their high theoretical capacities. However, it is of great challenge to achieve satisfactory cycling performance, especially for larger sodium ion storage, originated from electrode deterioration upon large volume change. Herein, we reported the construction of hierarchical tubular hybrid nanostructures through encapsulating CoSe2 nanoparticles into MoSe2/C composite shells via a simple two-step strategy including a hydrothermal method followed by vapor-phase selenization process. The unique tubular structure enables the highly reversible Li/Na storage with high specific capacity, enhanced cycling stability, and superior rate performance. It is indicated that the contribution of partial pseudocapacitive behavior greatly improves the rate capability for SIBs, where a high capacity retention of 81.5% can be obtained when the current densities range from 0.1 to 3 A g-1 (460 mA h g-1 at 0.1 A g-1 vs 379 mA h g-1 at 3 A g-1). This work provides an effective design rationale on transition-metal diselenide-based tubular nanostructures as superior hosts for both Li and Na ions, which could push forward the development of practical applications of transition-metal diselenide-based anodes in LIBs and SIBs.

Chemical vapor transport synthesis of a selenium-based two-dimensional material

Selenium-based layered materials, and in particular transition-metal diselenides (TMDSs), have intriguing properties in the monolayer limit. Materials such as MoSe$_{2}$, WSe$_{2}$, and NbSe$_{2}$ display striking features such as spin-valley coupling at the valence-band edges and offer great potential for optoelectronics applications. Although a dozen of other TMDSs have been realized or proposed, whether two-dimensional chalcogens are possible or not is still an open challenge. In this work, we show the chemical vapor transport synthesis of a novel, atomically thin selenium-based material on oxidized silicon substrates. This new member of the two-dimensional materials family has a unique Raman spectrum similar to that of bulk selenium and has an optical gap of $\sim $1.57 eV at room temperature determined by the photoluminescence. No transition metals are found in the stoichiometry of the crystals. Analysis of high-resolution transmission electron micrographs of the monolayers reveals a distinctive set of hexagonal spots indicating a sixfold symmetry of the lattice. Atomic force microscopy measurements show the monolayer thickness to be $\sim $0.75 nm.

Anomalous Corrosion of Bulk Transition Metal Diselenides Leading to Stable Monolayers.

In this paper we provide insight into an anomalous corrosion process, referred to as electroablation (EA), which converts multilayer flakes of transition metal diselenides like MoSe2 into their corresponding monolayers when micromechanically exfoliated on a conductive electrode and subsequently subjected to a high anodic potential inside a conventional electrochemical cell. Photoluminescence intensity maps and scanning transmission electron microscopy (STEM) images confirmed the single crystalline nature and 2H-hexagonal lattice structure of the remnant monolayer MoSe2 flakes, indicating the superior corrosion stability of the monolayers compared to that of the bulk counterpart. It is noted that the EA technique is a low-cost alternative for high-yield synthesis of single crystalline monolayer MoSe2 at room temperature. We also found that the dynamics of such an electro-oxidation-mediated and self-limiting corrosion process differs significantly for MoSe2 and WSe2. While we were able to engineer the corrosion conditions for the EA process to obtain monolayers of MoSe2, our attempts to obtain monolayers of WSe2 were largely unsuccessful. Finally, we constructed a phenomenological physical chemistry framework to explain such anomalous corrosion processes in transition metal diselenides.

Valley Zeeman Splitting and Valley Polarization of Neutral and Charged Excitons in Monolayer MoTe2 at High Magnetic Fields.

Semiconducting transition metal dichalcogenides (TMDCs) give rise to interesting new phenomena in external magnetic fields, such as valley Zeeman splitting and magnetic-field-induced valley polarization. These effects have been reported for monolayers (MLs) of the transition metal diselenides MoSe2 and WSe2 and, more recently, for disulfides MoS2 and WS2. Here, we present helicity-resolved magneto-photoluminescence and magneto-reflectance contrast measurements for MLs of the telluride member of the semiconducting TMDCs, 2H-MoTe2, in magnetic fields up to 29 T in Faraday geometry. Well-resolved valley Zeeman splittings for the neutral A and B excitons (XA0 and XB0) and the charged exciton X± are observed with effective g-factors of −4.6 ± 0.2, – 3.8 ± 0.6, and −4.5 ± 0.3, respectively. The magnetic field induced valley polarization of XA0 and X± reaches 78% and 36%, respectively, at a magnetic field of 29 T.

Three-dimensional-networked Ni-Co-Se nanosheet/nanowire arrays on carbon cloth: A flexible electrode for efficient hydrogen evolution

Transition metal diselenides have been considered as one of the most promising earth-abundant electrocatalysts as alternatives to noble metal for the hydrogen evolution reaction (HER). Here, hierarchically three-dimensional nickel cobalt diselenides (Ni1/3Co2/3Se2) with networked nanowires and nanosheets characteristics have been successfully synthesized. Such morphological features may not only provide large surface area allowing for exposure of more active sites, but also benefit deep electrolyte penetration for electrons and electrolyte ions. The resulting binder-free flexible electrodes with large production yield exhibit superior electrochemical HER activity and excellent durability after potential sweep tests for 1000 cycles. Besides their catalytic activity remain stable for electrolysis over 8h at mixed overpotential. The outstanding electrocatalytic performance makes the present nickel cobalt diselenide as a promising earth-abundant electrocatalyst for HER application.