Phase-Induced Strain Effect to Synthesize an Iron-Doped Orthogonal Cobalt Selenide Electrocatalyst for the Oxygen Evolution Reaction

The etching effect has the capability to control atom doping and trigger phase transformation, thereby enhancing the electrocatalytic reaction. Herein, iron-doped cobalt selenide (Fe-CoSe2) nanoparticle-decorated carbon nanofibers (Fe-CoSe2/CNFs) are synthesized by assembling an FeCo-Prussian blue analogue (FeCo-PBA) cube precursor with polyacrylonitrile fibers and then treating with hydrochloric acid, followed by gas phase selenization. The Fe-CoSe2/CNFs catalyst exhibits a large surface area and a porous structure, facilitating the permeation of electrolytes. Moreover, orthorhombic CoSe2 is obtained, which is in favor of improving the oxygen evolution reaction (OER). By modulating the etching time, the ideal crystal phase and the optimal amount of the dopant (Fe) can be achieved, thus showing favorable OER activity. Specifically, the Fe-CoSe2/CNFs electrocatalyst enables high electrocatalytic activity for the OER with a low overpotential of 263 mV to drive a current density of 10 mA cm-2 in 1 M KOH. A small Tafel slope of 51 mV dec-1 shows fast charge transfer kinetics. Density functional theory (DFT) calculations reveal that Fe-doped orthorhombic CoSe2(111) can modulate the electron structure, contributing to OH- adsorption ability. Given this, a strategy for phase transformation induced by etching technology is proposed to improve the intrinsic activity of the catalyst.

Fabricating a high-performance anode by coating a carbon layer on a yolk-shell bimetallic selenide microsphere for enhanced lithium storage

The rational synthesis of an electrode material with a highly active and stable architecture is very critical to achieving high-performance electrochemical energy storage. Herein, N-doped carbon restricting yolk-shell CoSe2/Ni3Se4 (CoSe2/Ni3Se4@NC) flower-like microspheres were successfully synthesized from solid CoNi-glycerate microspheres using a coating technology as an anode material for lithium-ion batteries (LIBs). The unique yolk-shell CoSe2/Ni3Se4@NC microspheres with hierarchical pores can increase the contact area with the electrolyte and provide enough transfer channels for the diffusion of Li+. The carbon layer on the surface of CoSe2/Ni3Se4@NC can not only improve the conductivity of the electrode but also provide the protective effect of active nanosheets during the process of synthesis, avoiding the overall structure collapse during the charge/discharge process of LIBs. Benefiting from the high conductivity, hollow structure, and elastic NC shell bestowed by the unique architecture, the yolk-shell CoSe2/Ni3Se4@NC anode shows excellent lithium storage performances, such as an excellent reversible specific capacity of 319 mA h g-1 at a current density of 1000 mA g-1 after 500 cycles and excellent cycling stability. This synthesis strategy provides a new way to optimize the lithium storage performance of transition metal compound electrode materials, which is helpful to the design of the next generation of high-performance LIBs.

Interfacial Se-O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation

Surface engineering is an effective strategy to improve the photoelectrochemical (PEC) catalytic activity of hematite, and the defect states with abundant coordinative unsaturation atoms can serve as anchoring sites for constructing intimate connections between semiconductors. On this basis, we anchored an ultrathin FeSe2 layer on Nb5+-doped Fe2O3 (FeSe2/Nb:Fe2O3) via interfacial Se-O chemical bonds to tune the surface potential. Density functional theory (DFT) calculations indicate that amorphous FeSe2 decoration could generate electron delocalization over the composite photoanodes so that the electron mobility was improved to a large extent. Furthermore, electrons could be transferred via the newly formed Se-O bonds at the interface and holes were collected at the surface of electrode for PEC water oxidation. The desired charge redistribution is in favor of suppressing charge recombination and extracting effective holes. Later, work function calculations and Mott-Schottky (M-S) plots demonstrate that a type-II heterojunction was formed in FeSe2/Nb:Fe2O3, which further expedited carrier separation. Except for spatial carrier modulation, the amorphous FeSe2 layer also provided abundant active sites for intermediates adsorption according to the d band center results. In consequence, the target photoanodes attained an improved photocurrent density of 2.42 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), 2.5 times as that of the bare Fe2O3. This study proposed a defect-anchoring method to grow a close-connected layer via interfacial chemical bonds and revealed the spatial charge distribution effects of FeSe2 on Nb:Fe2O3, giving insights into rational designation in composite photoanodes.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Oxygen evolution reaction on MoS2/C rods-robust and highly active electrocatalyst

Recently, water oxidation or oxygen evolution reaction (OER) in electrocatalysis has attracted huge attention due to its prime role in water splitting, rechargeable metal-air batteries, and fuel cells. Here, we demonstrate a facile and scalable fabrication method of a rod-like structure composed of molybdenum disulfide and carbon (MoS2/C) from parent 2D MoS2. This novel composite, induced via the chemical vapor deposition (CVD) process, exhibits superior oxygen evolution performance (overpotential = 132 mV at 10 mA cm-2and Tafel slope = 55.6 mV dec-1) in an alkaline medium. Additionally, stability tests of the obtained structures at 10 mA cm-2during 10 h followed by 20 mA cm-2during 5 h and 50 mA cm-2during 2.5 h have been performed and clearly prove that MoS2/C can be successfully used as robust noble-metal-free electrocatalysts. The promoted activity of the rods is ascribed to the abundance of active surface (ECSA) of the catalyst induced due to the curvature effect during the reshaping of the composite from 2D precursor (MoS2) in the CVD process. Moreover, the presence of Fe species contributes to the observed excellent OER performance. FeOOH, Fe2O3, and Fe3O4are known to possess favorable electrocatalytic properties, including high catalytic activity and stability, which facilitate the electrocatalytic reaction. Additionally, Fe-based species like Fe7C3and FeMo2S5offer synergistic effects with MoS2, leading to improved catalytic activity and durability due to their unique electronic structure and surface properties. Additionally, turnover frequency (TOF) (58 1/s at the current density of 10 mA cm-2), as a direct indicator of intrinsic activity, indicates the efficiency of this catalyst in OER. Based onex situanalyzes (XPS, XRD, Raman) of the electrocatalyst the possible reaction mechanism is explored and discussed in great detail showing that MoS2, carbon, and iron oxide are the main active species of the reaction.

Hierarchical Porous Tri-metallic NiCoFe-Se/CFP Derived from Ni-Co-Fe Prussian Blue Analogues as Efficient Electrocatalyst for Oxygen Evolution Reaction

The progress of inexpensive, high-efficiency, and steady oxygen evolution reaction (OER) electrocatalysts is of great importance to promoting water splitting for green hydrogen production. Herein, tri-metallic NiCoFe selenide catalyst backed up by carbon fiber paper (CFP) was synthesized by a facile selenization of NiCoFe Prussian blue analogues (PBAs) for OER in alkaline solutions. The NiCoFe-Se/CFP inherited the porous nanostructure of the metal-organic frameworks (MOFs) precursors prepared by rapid cyclic voltammetry electrodeposition. Benefiting from the 3D hierarchical porous structure, optimized electronic structure of NiCoFe selenides and high conductivity, the synthesized electrocatalyst exhibits outstanding catalytic activity to the corresponding mono-metallic or bi-metallic selenides. Specifically, the NiCoFe-Se/CFP electrode demands an overpotential of 221 mV to attain the 10 mA cm^-2 current density in 1.0 M KOH solution and a low Tafel slope of 38.6 mV dec^-1. The prepared catalyst also displays good stability and durability. These findings prove a feasible strategy to further improve the catalytic activities of non-precious metal based OER electrocatalysts by the cooperation of structure design and chemical component modification.

Composition and Morphology Modulation of Bimetallic Nitride Nanostructures on Nickel Foams for Efficient Oxygen Evolution Electrocatalysis

Metal-nitrides-based electrocatalysts for efficient oxygen-evolution have been extensively studied as one of the most promising candidates to fulfil the demand for future energy-conversion and storage. Herein, a series of NixCo1−xO- and NixCo1−xN-based nanostructures on nickel foams were reported to show excellent activities for oxygen-evolution reaction. The catalysts were prepared and modulated rationally via a facile-hydrothermal method, followed by high-temperature calcination under air or nitrogen atmosphere. The optimal bimetallic-nitride catalyst Ni0.3Co0.7N shows a small overpotential of 268 mV at 20 mA cm−2, and a Tafel slope of 66 mV dec−1 with good stability. The enhanced OER-performance is ascribed to the synergetic effect of the unique morphology and the intrinsic catalytic property of the nanostructure after nitridation.

NiSe2/FeSe2 heterostructured nanoparticles supported on rGO for efficient water electrolysis

Hybrid architectures composed of NiSe2/FeSe2 heterostructured nanoparticles supported on rGO were synthesized through a facile self-templating strategy, and exhibited excellent electrocatalytic performance for overall water splitting.

In situ construction of FeNi2Se4-FeNi LDH heterointerfaces with electron redistribution for enhanced overall water splitting

The abundant heterogeneous interfaces between the FeNi2Se4 and FeNi LDH can provide enriched active sites and accelerate reaction kinetics, which improves the overall water splitting performance.

Self-assembled CoSe2-FeSe2 heteronanoparticles along the carbon nanotube network for boosted oxygen evolution reaction

Water electrolysis is a significant alternative technique to produce clean hydrogen fuel in order to replace environmentally destructive fossil fuel combustion. However, the sluggish oxygen evolution kinetics makes this process vulnerable as it requires relatively high overpotentials. Hence, significantly effective electrocatalysts are necessary to access the water-oxidation process at a low overpotential to make this process industrially viable. Therefore, in order to reduce the energy barrier, we developed bimetallic CoSe2-FeSe2 heteronanoparticles along the carbon nanotube network (CoSe2-FeSe2/CNT) via a facile selenization strategy. Due to the unique assembly of highly conductive nanoparticles along the CNT network, the CoSe2-FeSe2/CNT displays an exceptionally good oxygen evolution (OER) activity; it requires 248 mV overpotential to reach a current density of 10 mA cm-2 (η10) with an ultra-low Tafel slope of 36 mV dec-1 and displays an overpotential of 1.59 V (η10) in the full water-splitting catalysis with the commercial Pt/C cathode. The high OER activity of CoSe2-FeSe2/CNT over the monometallic CoSe2/CNT and FeSe2/CNT electrocatalysts approve the synergistic interactions. Therefore, the superior performance is possibly ascribed to the unique porous nanoarchitecture and the strong coupling interactions between CoSe2 and FeSe2 heteronanoparticles on the conductive network. This study introduces an innovative approach to rationally design and fabricate cost-effective and highly proficient electrocatalysts for boosted OER performance.

Boosting Potassium Storage by Integration Advantageous of Defect Engineering and Spatial Confinement: A Case Study of Sb2 Se3

The potassium ion batteries (KIBs) based on conversion/alloying reaction mechanisms show high theoretical capacity. However, their applications are hampered by the poor cyclability resulting from the inherent large volume variations and the sluggish kinetics during K⁺ repeated insertion/extraction process. Herein, taken Sb₂ Se₃ as a model material, by rational design, nickel doped-carbon coated Sb₂ Se₃ nanorods (denoted as (Sb_0.99 Ni_0.01 )₂ Se₃ @C) are prepared through combined strategies of the conductive encapsulation and crystal structure modification. The carbon coating acts as an efficient buffer layer that maintains superior structural stability upon cycling. The introduction of Ni atoms can enhance electrical conductivity, leading to outstanding rate performance, which are confirmed by density functional theory calculation. The (Sb_0.99 Ni_0.01 )₂ Se₃ @C displays excellent reversible capacity (410 mAh g^-1 at 0.1 A g^-1 after 100 cycles) and unprecedented rate capability (140 mAh g^-1 at 10 A g^-1 ). Furthermore, KFeHCF//(Sb_0.99 Ni_0.01 )₂ Se₃ @C full cell exhibits a high specific capacity (408 mAh g^-1 at 0.1 A g^-1 ), superior rate capability (260 mAh g^-1 at 2 A g^-1 ). This work can open up a new avenue for the design of stable conversion/alloying-based anodes for high energy density KIBs.

Solid-solution hexagonal Ni0.5Co0.5Se nanoflakes toward boosted oxygen evolution reaction

The oxygen evolution reaction (OER) with sluggish kinetics is a bottleneck for the large-scale application of water electrolysis. Herein, solid-solution hexagonal Ni_0.5Co_0.5Se nanoflakes are designed and successfully synthesized via a facile hydrothermal method with a much lower overpotential of 216 mV at 10 mA cm^-2 and a Tafel slope of 37.08 mV dec^-1.

Electronic engineering of CoSe/FeSe2 hollow nanospheres for efficient water oxidation

First-row non-precious metal-based catalysts are widely studied and recognized as potential substitutes for precious metal-based catalysts in the oxygen evolution reaction (OER) for hydrogen generation but their application remains challenging. In this study, a unique class of Co-Fe selenide hollow nanospheres (CoSe@FeSe2) is well-designed through a facile hydrothermal method. The in situ formed hybrid composites possess numerous interfaces allowing partial electron transfer via O2- bridges to optimize the adsorption feature of the reaction intermediates, *OH, *O, and *OOH, on the catalysts. The collected surface valence band spectra evidence the optimization of the intermediate adsorption and active sites. The as-synthesized CoSe@FeSe2 exhibits excellent OER activity with a low overpotential of 281 mV to drive a current density of 10 mA cm-2 and a low Tafel slope of 34.3 mV dec-1 in an alkaline electrolyte. Additionally, the advanced catalyst also shows super stability with negligible current density decay after 12 h. This work presents a prototype for the fabrication of highly efficient electrocatalysts using an electronic engineering strategy.

Two-dimensional bimetallic CoFe selenite via metal-ion assisted self-assembly for enhanced oxygen evolution reaction

A 2D CoFe selenite crystals was used as an efficient OER catalyst, which was obtained in a high yield via a simple metal-ion self-assembly strategy under hydrothermal condition.

Facile doping of nickel into Co3O4 nanostructures to make them efficient for catalyzing the oxygen evolution reaction

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications. Herein, a simple and straightforward approach was developed to tune the electronic properties of cobalt oxide nanostructures by doping them with nickel and then using them to catalyze the oxygen evolution reaction (OER) in an aqueous solution of 1.0 M KOH. The addition of a nickel impurity improved the conductivity of the cobalt oxide, and further increased its activity towards the OER. Analytical techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and powder X-ray diffraction (XRD) were used to investigate, respectively, the morphology, composition and crystalline structure of the materials used. The nickel-doped cobalt oxide material showed randomly oriented nanowires and a high density of nanoparticles, exhibited the cubic phase, and contained cobalt, nickel and oxygen as its main elements. The nickel-doped cobalt oxide also yielded a Tafel slope of 82 mV dec⁻¹ and required an overpotential of 300 mV to reach a current density of 10 mA cm⁻². As an OER catalyst, it was shown to be durable for 40 h. Electrochemical impedance spectroscopy (EIS) analysis showed a low charge-transfer resistance of 177.5 ohms for the nickel-doped cobalt oxide, which provided a further example of its excellent OER performance. These results taken together indicated that nickel doping of cobalt oxide can be accomplished via a facile approach and that the product of this doping can be used for energy and environmental applications.

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications.

A facile sequential ion exchange strategy to synthesize CoSe2/FeSe2 double-shelled hollow nanocuboids for the highly active and stable oxygen evolution reaction

Transition metal-based nanostructures have been considered as promising substitutes for rare-earth metal oxide electrocatalysts toward the oxygen evolution reaction (OER). Herein, we report for the first time on a novel multicomponent metal selenide electrocatalyst based on CoSe2/FeSe2 double-shelled hollow nanocuboids (CoSe2/FeSe2 DS-HNCs) with the highly oxidative Co3+ species, which is synthesized via a facile sequential ion exchange strategy. The solid Co-precursor nanocuboids are first converted into the intermediate Co2[Fe(CN)6] with a mesoporous and double-shelled hollow structure produced through a facile ligand exchange at room temperature, and then the final CoSe2/FeSe2 DS-HNCs are obtained by a subsequent Se ion exchange reaction. The intermediate product of Co2[Fe(CN)6] plays an important role not only in constructing a double-shelled hollow structure but also in providing the Fe source for the growth of the final multicomponent metal selenides. Benefiting from the nanosized double-shelled hollow structure and mesoporous double-metal selenide shells with the highly oxidative Co3+ species, the as-prepared CoSe2/FeSe2 DS-HNCs exhibit superior OER performance to state-of-the-art metal selenides, including a small overpotential of 240 mV at a current density of 10 mA cm-2 and the excellent electrochemical durability over 50 h. This work opens up a new avenue towards developing highly active multicomponent noble-metal-free electrocatalysts.

Highly conductive and metallic cobalt-nickel selenide nanorods supported on Ni foam as an efficient electrocatalyst for alkaline water splitting

Water splitting has long been considered as a promising chemical reaction that can produce clean hydrogen fuel to relieve the energy crisis and environmental pollution. Herein, we report that Co0.75Ni0.25Se/NF formed by two-step growth of metallic cobalt-nickel selenide nanorods on porous nickel foam was used as a bifunctional electrocatalyst. Ni foam serves as a slow-releasing nickel source together with a Co source to form a special proportional cobalt-nickel selenide. Due to its unique rough nanostructure, bimetallic cooperative effects and intrinsic metallic character, the obtained Co0.75Ni0.25Se/NF electrode exhibits a low overpotential of 269 mV (50 mA cm-2) for the oxygen evolution reaction and an overpotential of 106 mV (10 mA cm-2) for the hydrogen evolution reaction. Furthermore, this bifunctional electrocatalyst requires a cell voltage of 1.60 V to achieve a current density of 10 mA cm-2. Besides, based on theoretical calculation, it is further shown that the synergy between Co and Ni elements is beneficial for improving the internal structure of the catalyst, resulting in a high electrical conductivity, and low HER Gibbs free-energy and water adsorption energy. The present results indicate that Co0.75Ni0.25Se/NF exhibits advanced electrocatalytic activity for overall water splitting. This work offers an appropriate methodology and theoretical guidance to synthesize a bimetal-selenide electrocatalyst for water splitting.

Synthesis from a layered double hydroxide precursor for a highly efficient oxygen evolution reaction

A hybrid of Co₃Se₄ and FeSe₂ prepared via a facile hydrothermal method achieves an efficient OER activity during water splitting.