NiCo 2 O 4 hexagonal nanoplates anchored on reduced graphene oxide sheets with enhanced electrocatalytic activity and stability for methanol and water oxidation

XPS Investigation of Co-Ni Oxidized Compounds Surface Using Peak-On-Satellite Ratio. Application to Co20Ni80 Passive Layer Structure and Composition

XPS data processing for cobalt and nickel core-level peaks can be complicated. This is especially true when analyzing a mixture of oxide/oxyhydroxide/hydroxide compounds of these metals. The objective of this study is to develop a method for decomposing XPS spectra of 2p core levels for nickel and cobalt-oxidized compounds. This methodology was then employed to study the passivation layer of the Co20Ni80 alloy. The analysis of Ni2p and Co2p photo peaks using a homemade code based on core-level peak structure and satellites enables us to determine the chemical composition of the surface layer, knowledge of which is particularly important because it is directly linked to the anticorrosive properties it confers on the surface of the oxidized alloy. The XPS analysis, coupled with sequential ion sputtering, revealed that the passive layer of the Co20Ni80 alloy is heterogeneously covered with either oxidized cobalt compounds or oxidized nickel compounds. The method used also demonstrates that the chemical heterogeneity is associated with the thickness heterogeneity of the passive layer.

Layer interfacing strategy to derive free standing CoFe@PANI bifunctional electrocatalyst towards oxygen evolution reaction and methanol oxidation reaction

Developing inexpensive, highly electrochemically active, and stable catalysts towards electrochemical studies remains challenge for researchers. In this regard, binder-free CoFe@PANI composite electrocatalyst is deposited on nickel foam (NF) substrate via successive electrodeposition of polyaniline (PANI) and CoFe-LDH at Room temperature (RT). As deposited binder-free CoFe@PANI electrocatalyst displays high electrocatalytic activity towards oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) in alkaline media. In CoFe@PANI structure, interfacing of high-electron conducting PANI establishes strong interconnection with CoFe-LDH by tuning electronic structures, which accelerates the electrochemical performance towards OER and MOR. For OER, CoFe@PANI requires low overpotential (η10) of 237 mV to reach current density (Id) of 10 mA cm-2 and displays low Tafel slope value of 46 mV dec-1 in 1 M KOH solution. Also, it displayed specific Id of 120 mA cm-2, when it was tested for MOR in 1 M KOH with 0.5 M methanol solution. The superior electrocatalytic activity of CoFe@PANI is mainly ascribed to high electrochemical active surface area (ECSA), abundant active sites and fast electron transfer between electrocatalyst and electrode surface. Of note, the current work may open new era for design and development of non-precious highly active and stable hybrid electrocatalysts at RT for various applications.

MnCo2O4/NiCo2O4/rGO as a Catalyst Based on Binary Transition Metal Oxide for the Methanol Oxidation Reaction

The demands for alternative energy have led researchers to find effective electrocatalysts in fuel cells and increase the efficiency of existing materials. This study presents new nanocatalysts based on two binary transition metal oxides (BTMOs) and their hybrid with reduced graphene oxide for methanol oxidation. Characterization of the introduced three-component composite, including cobalt manganese oxide (MnCo₂O₄), nickel cobalt oxide (NiCo₂O₄), and reduced graphene oxide (rGO) in the form of MnCo₂O₄/NiCo₂O₄/rGO (MNR), was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray (EDX) analyses. The alcohol oxidation capability of MnCo₂O₄/NiCo₂O₄ (MN) and MNR was evaluated in the methanol oxidation reaction (MOR) process. The crucial role of rGO in improving the electrocatalytic properties of catalysts stems from its large active surface area and high electrical conductivity. The alcohol oxidation tests of MN and MNR showed an adequate ability to oxidize methanol. The better performance of MNR was due to the synergistic effect of MnCo₂O₄/NiCo₂O₄ and rGO. MN and MNR nanocatalysts, with a maximum current density of 14.58 and 24.76 mA/cm² and overvoltage of 0.6 and 0.58 V, as well as cyclic stability of 98.3% and 99.7% (at optimal methanol concentration/scan rate of 20 mV/S), respectively, can be promising and inexpensive options in the field of efficient nanocatalysts for use in methanol fuel cell anodes.

Enhancement Strategy of Photoelectrocatalytic Activity of Cobalt-Copper Layer Double Hydroxide toward Methanol Oxidation: Cerium Doping and Modification with Porphyrin

Designing durable, high-active, and low-cost noble-metal-free photoelectrocatalysts for methanol electrooxidation is highly demanded but remains a challenge. Herein, the photoelectrocatalytic activity of cobalt-copper layer double hydroxide (CoCu-LDH) for methanol oxidation reaction (MOR) in the alkaline media under light was remarkably enhanced by cerium (Ce) doping and further by 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) modification. TCPP/Ce-CoCu-LDH exhibits a remarkable mass activity of 1788.2 mA mg^-1 in 1 mol L^-1 KOH with 1 mol L^-1 methanol under light, which is 2.3 and 1.8 times higher than that of CoCu-LDH (782.2 mA mg^-1) and Ce-CoCu-LDH (987.4 mA mg^-1). The UV-vis diffuse reflectance spectra and photoluminescence emission spectra reveal that TCPP/Ce-CoCu-LDH can effectively utilize the visible light and inhibit the electron-hole pairs' recombination because of the introduction of porphyrin. Furthermore, more active sites and the greater electrical conductivity of TCPP/Ce-CoCu-LDH also contributed to the high photoelectrocatalytic activity. Thus, TCPP/Ce-CoCu-LDH can be used as a low-cost alternative for Pt-based catalyst toward MOR.

Mesoporous K-doped NiCo2O4 derived from a Prussian blue analog: high-yielding synthesis and assessment as oxygen evolution reaction catalyst

The conversion and storage of clean renewable energy can be achieved using water splitting. However, water splitting exhibits sluggish kinetics because of the high overpotentials of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) and should therefore be promoted by OER and/or HER electrocatalysts. As the kinetic barrier of the former reaction exceeds that of the latter, high-performance OER catalysts are highly sought after. Herein, K-doped NiCo₂O₄ (HK-NCO) was hydrothermally prepared from a Prussian blue analog with a metal–organic framework structure and assessed as an OER catalyst. Extensive K doping increased the number of active oxygen vacancies and changed their intrinsic properties (e.g., binding energy), thus increasing conductivity. As a result, HK-NCO exhibited a Tafel slope of 49.9 mV dec⁻¹ and a low overpotential of 292 mV at 10 mA cm⁻², outperforming a commercial OER catalyst (Ir) and thus holding great promise as a component of high-performance electrode materials for metal-oxide batteries and supercapacitors.

OER characteristics of K-doped NiCo₂O₄ catalyst and K doping control through simple hydrothermal synthesis.

Foam Synthesis of Nickel/Nickel (II) Hydroxide Nanoflakes Using Double Templates of Surfactant Liquid Crystal and Hydrogen Bubbles: A High-Performance Catalyst for Methanol Electrooxidation in Alkaline Solution

This work demonstrates the chemical synthesis of two-dimensional nanoflakes of mesoporous nickel/nickel (II) hydroxide (Ni/Ni(OH)₂-NFs) using double templates of surfactant self-assembled thin-film and foam of hydrogen bubbles produced by sodium borohydride reducing agent. Physicochemical characterizations show the formation of amorphous mesoporous 2D nanoflakes with a Ni/Ni(OH)₂ structure and a high specific surface area (165 m²/g). Electrochemical studies show that the electrocatalytic activity of Ni/Ni(OH)₂ nanoflakes towards methanol oxidation in alkaline solution is significantly enhanced in comparison with that of parent bare-Ni(OH)₂ deposited from surfactant-free solution. Cyclic voltammetry shows that the methanol oxidation mass activity of Ni/Ni(OH)₂-NFs reaches 545 A/cm² g_cat at 0.6 V vs. Ag/AgCl, which is more than five times higher than that of bare-Ni(OH)₂. Moreover, Ni/Ni(OH)₂-NFs reveal less charge transfer resistance (10.4 Ω), stable oxidation current density (625 A/cm² g_cat at 0.7 V vs. Ag/AgCl), and resistance to the adsorption of reaction intermediates and products during three hours of constant-potential methanol oxidation electrolysis in alkaline solution. The high-performance electrocatalytic activity of Ni/Ni(OH)₂ nanoflakes is mainly derived from efficient charge transfer due to the high specific surface area of the 2D mesoporous architecture of the nanoflakes, as well as the mass transport of methanol to Ni²⁺/Ni³⁺ active sites throughout the catalyst layer.

Synthesis of a functionalized carbon supported platinum–iridium nanoparticle catalyst by the rapid chemical reduction method for the anodic reaction of direct methanol fuel cells

Direct methanol fuel cells (DMFCs) stand out among the most common technologies in energy storage and are environmentally friendly energy converters that convert chemical energy into electrical energy.

Multifunctionality exploration of NiCo2O4-rGO nanocomposites: photochemical water oxidation, methanol electro-oxidation and asymmetric supercapacitor applications

Different weight percentages of NiCo₂O₄-rGO nanocomposites were prepared via a facile hydrothermal method. The prepared nanocomposites were structurally and morphologically characterized by X-ray diffraction, Raman spectroscopy, and electron microscopy. The structural studies show the formation of rGO-NiCo₂O₄ nanocomposites by embedment of porous NiCo₂O₄ rods on rGO sheets. The effect of the NiCo₂O₄ content on photochemical water oxidation was investigated. It revealed that the catalysts NiCo₂O₄-rGO with 1 : 26 ratio (NCO26) and 1 : 13 ratio (NCO13) are efficient in generating oxygen under light illumination. It proves that NCO26 works far more effectively as a photocatalyst compared to NCO13. Methanol electro-oxidation of the NCO26 nanocomposite shows a current density of 24 mA cm^-2 at a potential of 0.45 V in cyclic voltammetry and maintains the current for 3600 s at 0.45 V in chronoamperometry. An onset potential of 0.344 V was observed for 0.5 M methanol oxidation. The specific capacitance values were found to be 354.75 F g^-1 and 375.32 F g^-1 at 1 mV s^-1 and 1 A g^-1, respectively, for NCO26 in supercapacitor studies. The charge stored via capacitive and diffusion-controlled processes was determined using Power's law and Trasatti plot. An asymmetric supercapacitor device shows a specific capacitance of 122.2 F g^-1 at a current density of 1 A g^-1 and exhibits a retention of 74.3% after 5000 cycles. An energy density of 67.89 W h kg^-1 and a power density of 1 kW kg^-1 at a current density of 1 A g^-1 are observed.

Electronic Structure Tuning of 2D Metal (Hydr)oxides Nanosheets for Electrocatalysis

2D metal (hydr)oxide nanosheets have captured increasing interest in electrocatalytic applications aroused by their high specific surface areas, enriched chemically active sites, tunable physiochemical properties, etc. In particular, the electrocatalytic reactivities of materials greatly rely on their surface electronic structures. Generally speaking, the electronic structures of catalysts can be well adjusted via controlling their morphologies, defects, and heterostructures. In this Review, the latest advances in 2D metal (hydr)oxide nanosheets are first reviewed, including the applications in electrocatalysis for the hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction. Then, the electronic structure-property relationships of 2D metal (hydr)oxide nanosheets are discussed to draw a picture of enhancing the electrocatalysis performances through a series of electronic structure tuning strategies. Finally, perspectives on the current challenges and the trends for the future design of 2D metal (hydr)oxide electrocatalysts with prominent catalytic activity are outlined. It is expected that this Review can shed some light on the design of next generation electrocatalysts.

Nickel Cobaltite Functionalized Silver Doped Carbon Xerogels as Efficient Electrode Materials for High Performance Symmetric Supercapacitor

Introducing new inexpensive materials for supercapacitors application with high energy density and stability, is the current research challenge. In this work, Silver doped carbon xerogels have been synthesized via a simple sol-gel method. The silver doped carbon xerogels are further surface functionalized with different loadings of nickel cobaltite (1 wt.%, 5 wt.%, and 10 wt.%) using a facile impregnation process. The morphology and textural properties of the obtained composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen physisorption analysis. The silver doped carbon xerogels display a higher surface area and larger mesopore volume compared to the un-doped carbon xerogels and hierarchically porous structure is obtained for all materials. The hybrid composites have been utilized as electrode materials for symmetric supercapacitors in 6 M KOH electrolyte. Among all the hybrid composites, silver doped carbon xerogel functionalized with 1 wt.% nickel cobaltite (NiCo1/Ag-CX) shows the best supercapacitor performance: high specific capacitance (368 F g⁻¹ at 0.1 A g⁻¹), low equivalent series resistance (1.9 Ω), high rate capability (99% capacitance retention after 2000 cycles at 1 A g⁻¹), and high energy and power densities (50 Wh/Kg, 200 W/Kg at 0.1 A g⁻¹). It is found that the specific capacitance does not only depend on surface area, but also on others factors such as particle size, uniform particle distribution, micro-mesoporous structure, which contribute to abundant active sites and fast charge, and ion transfer rates between the electrolyte and the active sites.

Stabilizing Oxygen Vacancy in Entropy-Engineered CoFe2O4-Type Catalysts for Co-prosperity of Efficiency and Stability in an Oxygen Evolution Reaction

We used entropy engineering to design a series of CoFe₂O₄-type spinels. Through microstructural characterization, electrochemical measurements, and X-ray photoelectron spectroscopy, we demonstrated that the entropy-stabilized oxide (Co_0.2Mn_0.2Ni_0.2Fe_0.2Zn_0.2)Fe₂O₄ has a single-phase spinel structure and exhibits both efficient and stable catalytic oxygen evolution. This is attributable to disordered occupation of multivalent cations, which induces severe lattice distortion and increases configurational entropy, thereby facilitating formation of structurally stable, high-density oxygen vacancies on the exposed surface of the spinel. Thus, more catalytic sites on the surface are activated and retained over the course of long-duration testing for oxygen evolution. Entropy engineering expands researchers' access to catalysts that link entropy-stabilized structures to useful properties.

High activity of NiCo2O4 promoted Pt on three-dimensional graphene-like carbon for glycerol electrooxidation in an alkaline medium

Spinel oxide NiCo₂O₄ supported on a three-dimensional hierarchically porous graphene-like carbon (3D HPG) material has been firstly used to enhance the activity of Pt for glycerol electrooxidation. The addition of NiCo₂O₄ into the Pt/HPG catalyst can significantly improve the catalytic performance for glycerol oxidation. When NiCo₂O₄ is added to the Pt/HPG catalyst, the onset potential is 25 mV more negative than that on the Pt/HPG catalyst without NiCo₂O₄. The current density at -0.3 V on the Pt-NiCo₂O₄ (wt 10 : 1)/HPG electrode is 1.3 times higher than that on the Pt (30 wt%)/HPG electrode. The Pt-NiCo₂O₄ electrode presented in this work shows great potential as an electrocatalyst for glycerol electrooxidation in an alkaline medium.

Plasma Generating—Chemical Looping Catalyst Synthesis by Microwave Plasma Shock for Nitrogen Fixation from Air and Hydrogen Production from Water for Agriculture and Energy Technologies in Global Warming Prevention

Simultaneous generation of plasma by microwave irradiation of perovskite or the spinel type of silica supported porous catalyst oxides and their reduction by nitrogen in the presence of oxygen is demonstrated. As a result of plasma generation in air, NOx generation is accompanied by the development of highly heterogeneous regions in terms of chemical and morphological variations within the catalyst. Regions of almost completely reduced catalyst are dispersed within the catalyst oxide, across micron-scale domains. The quantification of the catalyst heterogeneity and evaluation of catalyst structure are studied using Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and XRD. Plasma generating supported spinel catalysts are synthesized using the technique developed by the author (Catalysts; 2016; 6; 80) and BaTiO3 is used to exemplify perovskites. Silica supported catalyst systems are represented as M/Si = X (single catalysts) or as M(1)/M(2)/Si = X/Y/Z (binary catalysts) where M; M(1) M(2) = Cr; Mn; Fe; Co; Cu and X, Y, Z are the molar ratio of the catalysts and SiO2 support. Composite porous catalysts are synthesized using a mixture of Co and BaTiO3. In all the catalysts, structural heterogeneity manifests itself through defects, phase separation and increased porosity resulting in the creation of the high activity sites. The chemical heterogeneity results in reduced and oxidized domains and in very large changes in catalyst/support ratio. High electrical potential activity within BaTiO3 particles is observed through the formation of electrical treeing. Plasma generation starts as soon as the supported catalyst is synthesized. Two conditions for plasma generation are observed: Metal/Silica molar ratio should be > 1/2 and the resulting oxide should be spinel type; represented as MaOb (a = 3; b = 4 for single catalyst). Composite catalysts are represented as {M/Si = X}/BaTiO3 and obtained from the catalyst/silica precursor fluid with BaTiO3 particles which undergo fragmentation during microwave irradiation. Further irradiation causes plasma generation, NOx formation and lattice oxygen depletion. Partially reduced spinels are represented as MaOb–c. These reactions occur through a chemical looping process in micron-scale domains on the porous catalyst surface. Therefore; it is possible to scale-up this process to obtain NOx from MaOb for nitric acid production and H2 generation from MaOb–c by catalyst re-oxidized by water. Re-oxidation by CO2 delivers CO as fuel. These findings explain the mechanism of conversion of combustion gases (CO2 + N2) to CO and NOx via a chemical looping process. Mechanism of catalyst generation is proposed and the resulting structural inhomogeneity is characterized. Plasma generating catalysts also represent a new form of Radar Absorbing Material (RAM) for stealth and protection from radiation in which electromagnetic energy is dissipated by plasma generation and catalytic reactions. These catalytic RAMs can be expected to be more efficient in frequency independent microwave absorption.

Highly Active Core-Shell Carbon/NiCo2 O4 Double Microtubes for Efficient Oxygen Evolution Reaction: Ultralow Overpotential and Superior Cycling Stability

Developing highly efficient electrocatalysts with earth abundant elements for oxygen evolution reaction (OER) is a promising way to store light or electrical energy in the form of chemical energy. Here, a new type of electrocatalyst with core-shell carbon/NiCo₂ O₄ double microtubes architecture is successfully synthesized through a hydrothermal method combined with the calcination process with wet tissues as the template and carbon resource. The outer NiCo₂ O₄ nanosheet arrays contain abundant defects, which come from reduction of the carbon in wet tissues. This indicates that carbon is a very excellent defect inducer. The inner carbon microtubes can act as the robust structure skeleton and these core-shell double microtubes provide abundant diffusion channels for oxygen and electrolyte, both of which contribute to improving the stability by avoiding damage to the electrode from produced O₂ bubbles and the collapse of the outer NiCo₂ O₄ microtubes. Electrochemical results show that the electrode, core-shell carbon/NiCo₂ O₄ double microtubes loaded on carbon cloth, exhibits prominent electrocatalytic activity with an overpotential of only 168 mV at 10 mA cm^-2 and a Tafel slope as low as 57.6 mV dec^-1 in 1.0 mol L^-1 KOH. This new type of electrocatalyst possesses great potential in water electrolyzers and rechargeable metal-air batteries.

Facile efficient earth abundant NiO/C composite electrocatalyst for the oxygen evolution reaction

Due to the increasing energy consumption, designing efficient electrocatalysts for electrochemical water splitting is highly demanded. In this study, we provide a facile approach for the design and fabrication of efficient and stable electrocatalysts through wet chemical methods. The carbon material, obtained by the dehydration of sucrose sugar, provides high surface area for the deposition of NiO nanostructures and the resulting NiO/C catalysts show higher activity towards the OER in alkaline media. During the OER, a composite of NiO with 200 mg C can produce current densities of 10 and 20 mA cm-2 at a bias of 1.45 V and 1.47 V vs. RHE, respectively. Electrochemical impedance spectroscopy experiments showed the lowest charge transfer resistance and the highest double layer capacitance in the case of the NiO/C composite with 200 mg C. The presence of C for the deposition of NiO nanostructures increases the active centers and consequently a robust electrocatalytic activity is achieved. The obtained results in terms of the low overpotential and small Tafel slope of 55 mV dec-1 for non-precious catalysts are clear indications for the significant advancement in the field of electrocatalyst design for water splitting. This composite material based on NiO/C is simple and scalable for widespread use in various applications, especially in supercapacitors and lithium-ion batteries.

Ultra-efficient room-temperature H2S gas sensor based on NiCo2O4/r-GO nanocomposites

NiCo₂O₄/r-GO nanocomposites were synthesized successfully; the sensor based on these nanocomposites exhibited a fast response and high selectivity towards H₂S at room temperature.

Band gap tuning to improve the photoanodic activity of ZnInxSy for photoelectrochemical water oxidation

Elemental doping and band gap tuning of ZnIn_xS_y result in enhanced photoelectrochemical water splitting activity.

Synthesis of P-Doped and NiCo-Hybridized Graphene-Based Fibers for Flexible Asymmetrical Solid-State Micro-Energy Storage Device

Fiber supercapacitors (FSCs) are promising energy storage devices in portable and wearable smart electronics. Currently, a major challenge for FSCs is simultaneously achieving high volumetric energy and power densities. Herein, the microscale fiber electrode is designed by using carbon fibers as substrates and capillary channels as microreactors to space-confined hydrothermal assembling. As P-doped graphene oxide/carbon fiber (PGO/CF) and NiCo₂ O₄ -based graphene oxide/carbon fiber (NCGO/CF) electrodes are successfully prepared, their unique hybrid structures exhibit a satisfactory electrochemical performance. An all-solid-state PGO/CF//NCGO/CF flexible asymmetric fiber supercapacitor (AFSC) based on the PGO/CF as the negative electrode, NCGO/CF hybrid electrode as the positive electrode, and poly(vinyl alcohol)/potassium hydroxide as the electrolyte is successfully assembled. The AFSC device delivers a higher volumetric energy density of 36.77 mW h cm^-3 at a power density of 142.5 mW cm^-3 . In addition, a double reference electrode system is adopted to analyze and reduce the IR drop, as well as effectively matching negative and positive electrodes, which is conducive for the optimization and improvement of energy density. For the AFSC device, its better flexibility and electrochemical properties create a promising potential for high-performance micro-supercapacitors. Furthermore, the introduction of the double reference electrode system provides an interesting method for the study on the electrochemical performances of two-electrode systems.

A core@shell hollow heterostructure of Co3O4 and Co3S4: an efficient oxygen evolution catalyst

A hollow core@shell nanostructure of Co₃O₄ and Co₃S₄ helps to enhance the electrocatalytic activity for the OER.

3D plum candy-like NiCoMnO4@graphene as anodes for high-performance lithium-ion batteries

3D plum candy-like NiCoMnO4 microspheres have been prepared via ultrasonic spraying and subsequently wrapped by graphene through electrostatic self-assembly. The as-prepared NiCoMnO4 powders show hollow structures and NiCoMnO4@graphene exhibits excellent electrochemical performances in terms of rate performance and cycling stability, achieving a high reversible capacity of 844.6 mA h g-1 at a current density of 2000 mA g-1. After 50 cycles at 1000 mA g-1, NiCoMnO4@graphene delivers a reversible capacity of 1045.1 mA h g-1 while the pristine NiCoMnO4 only has a capacity of 143.4 mA h g-1. The hierarchical porous structure helps to facilitate electron transfer and Li-ion kinetic diffusion by shortening the Li-ion diffusion length, accommodating the mechanical stress and volume change during the Li-ion insertion/extraction processes. Analysis from the electrochemical performances reveals that the enhanced performances could be also attributed to the reduced charge-transfer resistance and enhanced Li-ion diffusion kinetics because of the graphene-coating. Moreover, Schottky electric field, due to the difference in work function between graphene and NiCoMnO4, might be favorable for the redox activity of the NiCoMnO4. In light of the excellent electrochemical performance and simple preparation, we believe that 3D plum candy-like NiCoMnO4@graphene composites are expected to be applied as a promising anode materials for high-performance lithium ion batteries.

CuO/Co(OH)2 Nanosheets: A Novel Kind of Electrocatalyst for Highly Efficient Electrochemical Oxidation of Methanol

With the booming of non-noble-metal electrocatalysts for efficient oxygen reduction reaction under alkaline conditions, corresponding anodic catalysts for methanol oxidation are urgently needed especially for direct methanol fuel cells with alkaline membranes. Here, we report the facile synthesis of a CuO/Co(OH)₂-nanosheet composite as a novel kind of high-performance electrochemical methanol oxidation reaction (MOR) catalyst. The obtained material with an optimized Cu/Co ratio shows much enhanced mass activity and area-specific activity, as well as excellent stability. The electronic structure interaction between Cu and Co, which results in the Co ion binding-energy elevation, is considered to be the origin of high MOR performance. This work promises the great potential of cobalt hydroxide as a novel kind of MOR catalyst and may arouse much interest in exploring more hydroxides as efficient nonprecious-metal electrocatalysts for MOR.

Nanotubes of NiCo2 S4 /Co9 S8 Heterostructure: Efficient Hydrogen Evolution Catalyst in Alkaline Medium

The most important issue in water splitting is the development of efficient, abundant, and cost-effective hydrogen and oxygen evolution catalysts. The development of an efficient electrocatalyst for the hydrogen evolution reaction (HER) under alkaline conditions is described here following a simple hydrothermal route. Here, a method for the synthesis of NiCo₂ S₄ /Co₉ S₈ , Co₉ S₈ , and NiCo₂ S₄ nanotubes has been developed. The NiCo₂ S₄ /Co₉ S₈ heterostructure has been introduced as an efficient electrocatalyst towards HER under alkaline conditions (1.0 m KOH). The vertically aligned nanotube heterostructure (NiCo₂ S₄ /Co₉ S₈ ) shows the most efficient activity as compared to bare Co₉ S₈ and NiCo₂ S₄ nanotubes. The heterostructure of NiCo₂ S₄ and Co₉ S₈ shows a significant anodic shift in the onset potential compared to the bare counterpart. NiCo₂ S₄ /Co₉ S₈ can generate a current density of 10 mA cm^-2 upon application of only -0.172 V vs. RHE, whereas Co₉ S₈ and NiCo₂ S₄ require -0.293 V and -0.239 V vs. RHE, respectively. The heterostructure formation and the nanotube morphology of Co₉ S₈ and NiCo₂ S₄ facilitates a fast charge transportation which results in higher electrocatalytic activity. The hydrogen gas evolution rate of the NiCo₂ S₄ /Co₉ S₈ heterostructure was determined to be 2.29 μmol min^-1 .