Bottom-up Design of Bimetallic Cobalt–Molybdenum Carbides/Oxides for Overall Water Splitting

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H₃PMo₁₂O₄₀], Co(OAc)₂ ⋅ 4 H₂O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co₆Mo₆C₂/Co₂Mo₃O₈) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO₂. Electrochemical and other materials analyses indicate that Co₆Mo₆C₂ is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.     

Heterostructured Inter-Doped Ruthenium–Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting

The development of transition-metal-oxides (TMOs)-based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter-doped ruthenium–cobalt oxide [(Ru–Co)O_x] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru–Co)O_x nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm⁻² and a small Tafel slope of 23.5 mV dec⁻¹, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm⁻². As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm⁻² for alkaline overall water splitting.     

Highly Dispersed TiO(6) Units in a Layered Double Hydroxide for Water Splitting

A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO(6) units in an MTi-layered double hydroxide matrix (M=Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H(2) -production rate of 31.4?μmol?h(-1) as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO(6) octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M(2+) -O-Ti network, rather different from the aggregation state of TiO(6) in the inorganic layered material K(2) Ti(4) O(9) . Both transient absorption and photoluminescence spectra demonstrate that the electron-hole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO(6) units was modified by the adjacent MO(6) octahedron by means of covalent interactions, with a much decreased bandgap of 2.1?eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO(6) units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.     

Ru-Doping Enhanced Electrocatalysis of Metal–Organic Framework Nanosheets toward Overall Water Splitting

An Ru-doping strategy is reported to substantially improve both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic activity of Ni/Fe-based metal–organic framework (MOF) for overall water splitting. As-synthesized Ru-doped Ni/Fe MIL-53 MOF nanosheets grown on nickel foam (MIL-53(Ru-NiFe)@NF) afford HER and OER current density of 50 mA cm⁻² at an overpotential of 62 and 210 mV, respectively, in alkaline solution with a nominal Ru loading of ≈110 μg cm⁻². When using as both anodic and cathodic (pre-)catalyst, MIL-53(Ru-NiFe)@NF enables overall water splitting at a current density of 50 mA cm⁻² for a cell voltage of 1.6 V without iR compensation, which is much superior to state-of-the-art RuO₂-Pt/C-based electrolyzer. It is discovered that the Ru-doping considerably modulates the growth of MOF to form thin nanosheets, and enhances the intrinsic HER electrocatalytic activity by accelerating the sluggish Volmer step and improving the intermediate oxygen adsorption for increased OER catalytic activity.     

Deciphering Iron-Dependent Activity in Oxygen Evolution Catalyzed by Nickel–Iron Layered Double Hydroxide

Nickel iron oxyhydroxide is the benchmark catalyst for the oxygen evolution reaction (OER) in alkaline medium. Whereas the presence of Fe ions is essential to the high activity, the functions of Fe are currently under debate. Using oxygen isotope labeling and operando Raman spectroscopic experiments, we obtain turnover frequencies (TOFs) of both Ni and Fe sites for a series of Ni and NiFe layered double hydroxides (LDHs), which are structurally defined samples of the corresponding oxyhydroxides. The Fe sites have TOFs 20–200 times higher than the Ni sites such that at an Fe content of 4.7 % and above the Fe sites dominate the catalysis. Higher Fe contents lead to larger structural disorder of the NiOOH host. A volcano-type correlation was found between the TOFs of Fe sites and the structural disorder of NiOOH. Our work elucidates the origin of the Fe-dependent activity of NiFe LDH, and suggests structural ordering as a strategy to improve OER catalysts.     

New Nickel–Cobalt–Manganese Spinels

Doklady Chemistry - Three types of solid solutions of new nickel–cobalt–manganese spinels were synthesized by substitution for manganese cations in oxide Mn3O4:...     

Hierarchical Nanoflower Arrays of Co9S8-Ni3S2 on Nickel Foam: A Highly Efficient Binder-Free Electrocatalyst for Overall Water Splitting

Hydrogen production is vital for meeting future energy demands and managing environmental sustainability. Electrolysis of water is considered as the suitable method for H₂ generation in a carbon-free pathway. Herein, the synthesis of highly efficient Co₉S₈-Ni₃S₂ based hierarchical nanoflower arrays on nickel foam (NF) is explored through the one-pot hydrothermal method (Co₉S₈-Ni₃S₂/NF) for overall water splitting applications. The nanoflower arrays are self-supported on the NF without any binder, possessing the required porosity and structural characteristics. The obtained Co₉S₈-Ni₃S₂/NF displays high hydrogen evolution reaction (HER), as well as oxygen evolution reaction (OER), activities in 1 m KOH solution. The overpotentials exhibited by this system at 25 mA cm⁻² are nearly 277 and 102 mV for HER and OER, respectively, in 1 m KOH solution. Subsequently, the overall water splitting was performed in 1 m KOH solution by employing Co₉S₈-Ni₃S₂/NF as both the anode and cathode, where the system required only 1.49, 1.60, and 1.69 V to deliver the current densities of 10, 25, and 50 mA cm⁻², respectively. Comparison of the activity of Co₉S₈-Ni₃S₂/NF with the state-of-the-art Pt/C and RuO₂ coated on NF displays an enhanced performance for Co₉S₈-Ni₃S₂/NF both in the half-cell as well as in the full cell, emphasizing the significance of the present work. The post analysis of the material after water electrolysis confirms that the surface Co(OH)₂ formed during the course of the reaction serves as the favorable active sites. Overall, the activity modulation achieved in the present case is attributed to the presence of the open-pore morphology of the as formed nanoflowers of Co₉S₈-Ni₃S₂ on NF and the simultaneous presence of the surface Co(OH)₂ along with the highly conducting Co₉S₈-Ni₃S₂ core, which facilitates the adsorption of the reactants and subsequently its conversion into the gaseous products during water electrolysis.     

Ultra-thin Co−Fe Layered Double Hydroxide Hollow Nanocubes for Efficient Electrocatalytic Water Oxidation

Hierarchical hollow nanocubes based on ultra-thin CoFe-layered double hydroxide (CoFe−LDH) nanosheets have been prepared. The obtained CoFe−LDH hollow nanocubes could effectively catalyze water oxidation at a low overpotential of 270 mV @ 10 mA cm⁻², low Tafel slope of 58.3 mV dec⁻¹ and show a long-term stability in alkali.     

Metal−Organic Precursors for Transition-Metal-Doped Ni2P via Pyrolysis for Efficient Overall Water Splitting and Supercapacitance

The synthesis of solvent-less bare-surface nickel phosphides is desired, considering their superior electrocatalytic properties and straightforward synthetic protocols compared to their analogues prepared from colloidal routes. Herein, we report the synthesis of [Ni{S₂P(OH)(4-CH₃OC₆H₄)}₂] (1), [Fe{S₂P(OH)(4-CH₃OC₆H₄)}₃] (2) and [Co{S₂P(OC₄H₉)(4-CH₃OC₆H₄)}₃] (3) and their utilization to form Ni₂P, Fe-Ni₂P and Co-Ni₂P in a solvent-less pyrolysis method. This solvent-free protocol involved the decomposition of complex ( 1 ) and the composites of complex ( 1 ) with ( 2 ) or ( 3 ) in the presence of triphenylphosphine (TPP) at 400 °C for one hour. The solvent-less decomposition of complex ( 1 ) without TPP formed nickel sulfide. A plausible explanation for this rare fabrication of pristine and doped Ni₂P in the absence of any solvent is suggested. All the transition metal doped phosphides improved the HER performance of pristine Ni₂P, with the 5 % Fe doped Ni₂P having the best performance, requiring 137 mV to reach a current density of 10 mA/cm². Similarly, the OER performance of un-doped Ni₂P was improved by all the doped Ni₂P catalysts, where 10 % Fe-doped Ni₂P showed the best performance requiring 326 mV to reach a current density of 10 mA/cm². Transition metal doping was also shown to improve the reaction kinetics, stability and durability of the solvent-free prepared Ni₂P.     

Modulating Hydroxyl-Rich Interfaces on Nickel–Copper Double Hydroxide Nanotyres to Pre-activate Alkaline Ammonia Oxidation Reactivity

The surface hydroxyl groups of Ni_xCu_1−x(OH)₂ play a crucial role in governing their conversion efficiency into Ni_xCu_1−xO_x(OH)_2−x during the electro-chemical pre-activation process, thus affecting the integral ammonia oxidation reaction (AOR) reactivity. Herein, the rational design of hierarchical porous NiCu double hydroxide nanotyres (NiCu DHTs) was reported for the first time by considering hydroxyl-rich interfaces to promote pre-activation efficiency and intrinsic structural superiority (i.e., annulus, porosity) to accelerate AOR kinetics. A systematic investigation of the structure–function relationship was conducted by manipulating a series of NiCu DHs with tunable intercalations and morphologies. Remarkably, the NiCu DHTs exhibit superior AOR activity (onset potential of 1.31 V with 7.52 mA cm⁻² at 1.5 V) and high ammonia sensitivity (detection limit of 9 μm ), manifesting one of the best non-noble metal AOR electrocatalysts and electro-analytical electrodetectors. This work deepens the understanding of the crucial role of surface hydroxyl groups on determining the catalytic performance in alkaline medium.     

Ru−FeNi Alloy Heterojunctions on Lignin-derived Carbon as Bifunctional Electrocatalysts for Efficient Overall Water Splitting

Rational design of efficient, stable, and inexpensive bifunctional electrocatalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) is a key challenge to realize green hydrogen production via electrolytic water splitting. Herein, Ru nanoparticles and FeNi alloy heterojunction catalyst (Ru−FeNi@NLC) encapsulated via lignin-derived carbon was prepared by self-assembly precipitation and in situ pyrolysis. The designed catalyst displays excellent performance at 10 mA cm⁻² with low overpotentials of 36 mV for HER and 198 mV for OER, and only needs 1.48 V for overall water splitting. Results and DFT calculations show the unique N-doped lignin-derived carbon layer and Ru−FeNi heterojunction contribute to optimized electronic structure for enhancing electron transfer, balanced free energy of reactants and intermediates in the sorption/desorption process, and significantly reduced reaction energy barrier for the HER and OER rate-determining steps, thus improved reaction kinetics. This work provides a new in situ pyrolysis doping strategy based on renewable biomass for the construction of highly active, stable and cost-effective catalysts.     

Metal–Organic Frameworks-Derived Self-Supported Carbon-Based Composites for Electrocatalytic Water Splitting

Electrocatalytic water splitting has been considered as a promising strategy for the sustainable evolution of hydrogen energy and storage of intermittent electric energy. Efficient catalysts for electrocatalytic water splitting are urgently demanded to decrease the overpotentials and promote the sluggish reaction kinetics. Carbon-based composites, including heteroatom-doped carbon materials, metals/alloys@carbon composites, metal compounds@carbon composites, and atomically dispersed metal sites@carbon composites have been widely used as the catalysts due to their fascinating properties. However, these electrocatalysts are almost powdery form, and should be cast on the current collector by using the polymeric binder, which would result in the unsatisfied electrocatalytic performance. In comparison, a self-supported electrode architecture is highly attractive. Recently, self-supported metal–organic frameworks (MOFs) constructed by coordination of metal centers and organic ligands have been considered as suitable templates/precursors to construct free-standing carbon-based composites grown on conductive substrate. MOFs-derived carbon-based composites have various merits, such as the well-aligned array architecture and evenly distributed active sites, and easy functionalization with other species, which make them suitable alternatives to non-noble metal-included electrocatalysts. In this review, we intend to show the research progresses by employment of MOFs as precursors to prepare self-supported carbon-based composites. Focusing on these MOFs-derived carbon-based nanomaterials, the latest advances in their controllable synthesis, composition regulation, electrocatalytic performances in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS) are presented. Finally, the challenges and perspectives are showed for the further developments of MOFs-derived self-supported carbon-based nanomaterials in electrocatalytic reactions.     

Voltammetric Study and Detection of Methane on Nickel Hydroxide–Modified Nickel Electrode

Abstract

The nickel hydroxide–modified nickel (NMN) electrode was prepared by cyclic voltammetry. The modified electrode exhibited better catalytic effect toward electrochemical oxidation of methane in 1.0 mol · L^?1 NaOH solution. The catalytic activation of nickel hydroxide on the nickel electrode surface was investigated in different supporting electrolyte solutions by the cyclic voltammetry method in detail, and the related electrochemical oxidation of methane at the NMN electrode was first proposed by amperometric i‐t curve method under the experiment conditions. The results indicated that in the 1.0 mol · L^?1 NaOH solution, the anodic peak current increased with the increased concentration of methane.     

Black Tungsten Nitride as a Metallic Photocatalyst for Overall Water Splitting Operable at up to 765 nm

Semiconductor photocatalysts are hardly employed for overall water splitting beyond 700 nm, which is due to both thermodynamic aspects and activation barriers. Metallic materials as photocatalysts are known to overcome this limitation through interband transitions for creating electron–hole pairs; however, the application of metallic photocatalysts for overall water splitting has never been fulfilled. Black tungsten nitride is now employed as a metallic photocatalyst for overall water splitting at wavelengths of up to 765 nm. Experimental and theoretical results together confirm that metallic properties play a substantial role in exhibiting photocatalytic activity under red-light irradiation for tungsten nitride. This work represents the first red-light responsive photocatalyst for overall water splitting, and may open a promising venue in searching of metallic materials as efficient photocatalysts for solar energy utilization.     

A Simple Ni-based Metal–organic Framework as Catalyst for Dye-sensitized Photocatalytic H2 Evolution from Water Reduction

Metal–organic frameworks (MOF) are recently developed coordination porous materials, and their unique structures are very conducive to catalytic reactions. In this paper, p-benzenedicarboxylic acid (PBA)-Ni²⁺ MOF materials (denoted as PBA-Ni-x, where x represents the initial ratio of PBA to Ni²⁺) were synthesized by a hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and N₂ gas adsorption. H₂ gas was produced using the synthesized MOF as a photocatalyst and Eosin Y as a photosensitizer. The dependence of the special surface area and thickness of the nanosheets of Ni-MOF on the initial ratio of PBA to Ni²⁺ (PBA/Ni²⁺) was investigated. The BET surface areas of PBA-Ni-1 PBA-Ni-2 and PBA-Ni-3 are 11.00, 24.61 and 13.04 m² g⁻¹, respectively. And the thicknesses of nanosheets are approximately 600–1000, 200–500 and 300–700 nm. Among the three materials, PBA-Ni-2 has the thinnest sheet-like structure and largest surface area. Thus, it displays the highest H₂ evolution rate of 20.0 μmol h⁻¹. The noble-metal-free hydrogen production system is valuable for the application of MOF materials in photocatalytic water splitting.     

Manganese-Modulated Cobalt-Based Layered Double Hydroxide Grown on Nickel Foam with 1D–2D–3D Heterostructure for Highly Efficient Oxygen Evolution Reaction and Urea Oxidation Reaction

Hydrogen production by energy-efficient water electrolysis is a green avenue for the development of contemporary society. However, the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR) occurring at the anode are impeded by the sluggish reaction kinetics during the water-splitting process. Consequently, it is promising to develop bifunctional anodic electrocatalysts consisting of nonprecious metals. Herein, a bifunctional CoMn layered double hydroxide (LDH) was grown on nickel foam (NF) with a 1D–2D–3D hierarchical structure for efficient OER and UOR performance in alkaline solution. Owing to the significant synergistic effect of Mn doping and heterostructure engineering, the obtained Co₁Mn₁ LDH/NF exhibits satisfactory OER activity with a low potential of 1.515 V to attain 10 mA cm⁻². Besides, the potential of the Co₁Mn₁ LDH/NF catalyst for UOR at the same current density is only 1.326 V, which is much lower than those of its counterparts and most reported electrocatalysts. An urea electrolytic cell with a Co₁Mn₁ LDH/NF anode and a Pt–C/NF cathode was established, and a low cell voltage of 1.354 V at 10 mA cm⁻² was acquired. The optimized strategy may result in promising candidates for developing a new generation of bifunctional electrocatalysts for clean energy production.     

Photocatalytic Water Splitting: How Far Away Are We from Being Able to Industrially Produce Solar Hydrogen?

Solar water splitting (SWS) has been researched for about five decades, but despite successes there has not been a big breakthrough advancement. While the three fundamental steps, light absorption, charge carrier separation and diffusion, and charge utilization at redox sites are given a great deal of attention either separately or simultaneously, practical considerations that can help to increase efficiency are rarely discussed or put into practice. Nevertheless, it is possible to increase the generation of solar hydrogen by making a few little but important adjustments. In this review, we talk about various methods for photocatalytic water splitting that have been documented in the literature and importance of the thin film approach to move closer to the large-scale photocatalytic hydrogen production. For instance, when comparing the film form of the identical catalyst to the particulate form, it was found that the solar hydrogen production increased by up to two orders of magnitude. The major topic of this review with thin-film forms is, discussion on several methods of increased hydrogen generation under direct solar and one-sun circumstances. The advantages and disadvantages of thin film and particle technologies are extensively discussed. In the current assessment, potential approaches and scalable success factors are also covered. As demonstrated by a film-based approach, the local charge utilization at a zero applied potential is an appealing characteristic for SWS. Furthermore, we compare the PEC-WS and SWS for solar hydrogen generation and discuss how far we are from producing solar hydrogen on an industrial scale. We believe that the currently employed variety of attempts may be condensed to fewer strategies such as film-based evaluation, which will create a path to address the SWS issue and achieve sustainable solar hydrogen generation.     

Participation of Water in the Secondary Transformations of Hydrocarbons on Cobalt–Zeolite Catalysts for the Fischer–Tropsch Synthesis

This review is dedicated to the effect of water as the main by-product of the Fischer–Tropsch synthesis on the process. The reasons for the negative effect of water are analyzed and the possible versions of the control of its participation in the process are considered. As an optimal solution to the problem, the use of zeolites in the H form as the constituents of cobalt catalysts for the Fischer–Tropsch synthesis is proposed. Bibliography: 148 references.     

Enhanced Metal–Semiconductor Interaction for Photocatalytic Hydrogen-Evolution Reaction

The selective immobilization of noble metals right at the place where photogenerated electrons migrate through the photodeposition approach is a unique strategy to load cocatalysts on semiconductors for solar hydrogen production. However, a poor metal–semiconductor interaction is often formed, which not only hinders the interfacial charge transfer, but also results in the easy aggregation and shedding of cocatalysts during photocatalytic reactions. Herein, it is demonstrated that the photodeposited ultrafine metals, such as nanosized Au, can be well stabilized on TiO₂ nanocrystallines without sintering by employing a sacrificial carbon coating annealing strategy to strengthen the metal-support interaction. Benefiting from the improved interfacial contact between Au and TiO₂ for fast charge transfer and the well-preserved size-dependent catalytic behavior of Au nanoparticles toward hydrogen evolution reaction, the annealed Au/TiO₂ exhibits a significant enhanced activity toward photocatalytic H₂ production with good durability.     

Intercalation of Dyes Containing SO3H Groups into Zn–Al Layered Double Hydroxide

The intercalates of Naphthol Yellow S, Tropaeolin 000, and Tropaeolin 00 were prepared by heating [Zn_0.67Al_0.33(OH)₂](CO₃)_0.165 · 0.5H₂O with acidic forms of the dye solutions in an open reaction vessel. The intercalates were characterized by chemical and thermal analysis, X-ray powder diffraction and UV–VIS spectroscopy. A possible arrangement of the dye molecules in the intercalates was suggested on the basis of their chemical compositions and interlayer distances, by taking into account van der Waals dimensions of the guest molecules and by assuming that the structure of the host layers is not changed during the intercalation process.This revised version was published online in July 2005 with a corrected issue number.