Hierarchical flower-like CoS2-MoS2 heterostructure spheres as efficient bifunctional electrocatalyst for overall water splitting

Electrochemical water splitting technique requires high-efficient bifunctional electrocatalysts to obtain large-scale hydrogen production for resolving the impending energy and environmental crisis. Herein, hierarchical flower-like CoS₂-MoS₂ heterostructure hybrid spheres grown on carbon cloth (CoS₂-MoS₂/CC) were prepared by sulfuring wheel-shaped polyoxometalate {Co₂₀Mo₁₆}. The as-prepared CoS₂-MoS₂/CC as bifunctional electrocatalyst manifests excellent alkaline oxygen evolution and hydrogen evolution activities with low overpotentials of 240 mV for OER and 60 mV for HER at 10 mA cm^?2, respectively. When assembled as two-electrode cell, CoS₂-MoS₂/CC delivers an extremely low cell-voltage of 1.52 V at 10 mA cm^?2 accompanied with remarkable long-term durability. Additionally, CoS₂-MoS₂/CC exhibits favorable overall-water-splitting performance in simulated seawater. The superior performance of CoS₂-MoS₂/CC should be ascribed to the optimized intrinsic electron structure via electron transfer from MoS₂ to CoS₂ along with the synergistic effect of well-exposed heterostructure interfaces and favorable diffusion channels. This work offers a practical strategy for exploring high-efficient bifunctional electrocatalysts for overall water splitting.     

CoSe2@NiSe2 nanoarray as better and efficient electrocatalyst for overall water splitting

Electrocatalytic water splitting is identified as one of the most promising solutions to energy crisis. The CoSe₂@NiSe₂ materials were first prepared and in situ grown on nickel foam by typical hydrothermal and selenification process at 120 °C. The results show that the CoSe₂@NiSe₂ material used as the 3D substrates electrode can maximize the synergy between the CoSe₂ and NiSe₂, and also exhibits high efficiency of water splitting reaction. The lower overpotential of only 235 mV is presented to attain 20 mA cm⁻² compared to the benchmark of RuO₂ electrodes (270 mV @ 20 mA cm⁻²). Besides, the CoSe₂@NiSe₂ material also shows a remarkable improved hydrogen evolution reaction activity compared to NiSe₂ (192 mV@10 mA cm⁻²) and Co precursor catalysts (208 mV@10 mA cm⁻²) individually, which a low overpotential of only 162 mV is achieved at 10 mA cm⁻². The CoSe₂@NiSe₂ catalysts exhibit excellent water splitting performance (cell voltage of 1.50 V@ 10 mA cm⁻²) under alkaline conditions. It was proved that the high water splitting performance of the catalyst is attributed to high electrochemical activity area and synergistic effect. The work offers new ideas for the exploitation of synergistic catalysis of composite catalysts and adds new examples for the exploitation of efficient, better and relatively non-toxic electrocatalysts.     

Facile construction of heterostructural Ni3(NO3)2(OH)4/CeO2 bifunctional catalysts for boosted overall water splitting

Interface engineering has aroused vitally widespread concern since it could be an effective strategy for exploring high-performance and low-cost water oxidation electrocatalysts. Herein, we report a hetero-structured Ni₃(NO₃)₂(OH)₄/CeO₂/NF (NNO/CeO₂/NF) electrode, exhibiting superior performance owning to the NO₃^? anion substitution for the OH^? in nickel hydroxide to form Ni₃(NO₃)₂(OH)₄, together with its interface synergy with ceria. In alkaline solution, the NNO/CeO₂/NF electrocatalyst could catalyze the OER with an overpotential of 330 mV to approach 50 mA cm^?2. Also, it needs only an overpotential of 120 mV to reach 10 mA cm^?2 for HER. Additionally, when a standard two-electrode water electrolyzer is fabricated by employing NNO/CeO₂/NF as both the cathode and anode, it can generate 10 mA cm^?2 at 1.64 V and operate steadily without performance degradation after 25 h. This research provides a novel perspective for reasonable design of advanced catalytic materials with improvements in the field of electrocatalysis.     

NiSe2@NixSy nanorod on nickel foam as efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic water splitting technology has become one of the most promising methods to solve the energy crisis, which can produce a large amount of high purity H₂ and O₂. It is necessary to develop efficient and stable water splitting catalyst for reducing the overpotential of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and accelerating their reaction kinetics. A series of NiSe₂@Ni_xS_y nanoarrays was firstly in situ grown on the nickel foam through the typical hydrothermal, selenylation and sulfuration pathways. The Na₂SeO₃ homogeneous solution is formed by hydrothermal and the selenization process is done at the temperature of 180^。C. Then the nickel foam (NF) is put into the Na₂SeO₃ solution to form NiSe₂ material at the temperature of 120^。C. After that, the NiSe₂ materials were sulfuretted with different amounts of sulfur to form NiSe₂@Ni_xS_y hybrid materials. The experimental results demonstrate that the NiSe₂@Ni_xS_y material as a 3D electrode can maximize the synergistic reaction between NiSe₂ and Ni_xS_y, thus exhibiting an efficient and comprehensive water splitting performance. The NiSe₂@Ni_xS_y-1 material presents a superior OER performance with requiring the overpotential of only 206 mV at 100 mA cm⁻². Moreover, the NiSe₂@Ni_xS_y-0.3 material presents a superior HER performance with requiring the overpotential of only 148 mV at 100 mA cm⁻². It is worth noting that when NiSe₂@Ni_xS_y-1 material and the NiSe₂@Ni_xS_y-0.3 material was used as cathode and anode, only 1.53 V cell voltage is needed to produce a current density of 10 mA cm⁻² throughout the water splitting process, which is one of the smallest values reported so far. Density functional theory calculations results show that the Ni₃S₂ has the best water adsorption energy, so it is an active species in the process of catalysis. However, NiSe₂ has more density distribution around the Fermi level, indicating that it exhibits better metallic properties, which makes the NiSe₂@Ni_xS_y-1 hybrid material exhibit better electronic conductivity.     

Crossed FeCo2S4 nanosheet arrays grown on 3D nickel foam as high-efficient electrocatalyst for overall water splitting

Great efforts in developing low-cost, highly efficient and stable electrocatalysts are to tune the chemical compositions and morphological characteristics for enhancing efficiency of water splitting. In this communication, FeCo₂S₄ nanosheet was grown in situ on nickel foam (FeCo₂S₄/NF) via a facile hydrothermal sulfidization method and served as a high-efficient bifunctional electrocatalyst for overall water splitting. As-synthesized FeCo₂S₄/NF self-supported electrode delivers 20 mA cm^?2 at an overpotential of 259 mV toward OER and 10 mA cm^?2 at an overpotential of 131 mV toward HER in alkaline media. Moreover, when used as both anode and cathode in a two-electrode electrolyzer, only a small cell voltage of 1.541 V is needed to afford a current density of 10 mA cm^?2 for overall water splitting. Bifunctional electrode FeCo₂S₄/NF also revealed a distinguished electrochemical durability during a 12 h stability test at 1.63 V, which would provide a promising water splitting installation for commercial hydrogen production.     

Design and synthesis of dispersed Ni2P/Co nano heterojunction as bifunctional electrocatalysis for boosting overall water splitting

The design of multi-components nanostructure with interface heterojunction is the cutting-edge research in recent years because the catalytic activity, stability, and durability of catalysts are highly affected by the strong electronic effects, geometric effects, and synergistic effects occurring at the interface. Based on this, an efficient bifunctional electrocatalyst embedding highly dispersed Ni₂P/Co nano heterojunction at the porous hollow-out carbon shell is developed for overall water splitting through evenly epitaxial growth of ultrathin Ni₂P nanosheets on Co-based ZIF-67. The distinct electron interaction between the interfacial Ni₂P (300) and Co (100) effectively lowers the overpotential of OER (316 mV vs. RHE) and HER (149 mV vs. RHE) at the current density of 10 mA cm^?2. Density functional theory (DFT) calculation further identifies that the Ni₂P and Co heterojunctions optimize the adsorption energy of intermediate products and lower the energy barrier of the rate-determining step of OER significantly. This work provides a rational design of a well-defined interface toward overall water splitting electrocatalysts and offers a scientific basis for an in-depth understanding of the mechanism of the catalysts with nano heterojunction.     

Ni2V2O7 dandelion microsphere for a high-performance electrocatalyst in water splitting

Water splitting is an effective way to produce hydrogen to solve the energy crisis problem, and inorganic metal compounds are widely used in electrocatalysis field due to efficient hydrogen evolution reaction (HER). Herein, we synthesize Ni₂V₂O₇ dandelion microsphere from nickel nitrate and vanadium pentoxide by “one-step hydrothermal” way, which exhibits large specific surface area of 102.74 m² g⁻¹. The as-prepared Ni₂V₂O₇ microsphere shows good electrocatalysis performances including OER overpotential of 358 mV and good stability, as well as HER overpotential of 195 mV. Furthermore, the Ni₂V₂O₇ microsphere electrode is assembled to Ni₂V₂O₇ microsphere//Ni₂V₂O₇ microsphere system, showing the water splitting voltage of 1.50 V at 10 mA cm⁻² by two-electrode method, which is much lower than those of commercial RuO₂//Pt/C system and most of spinel oxides electrocatalysts. Our work opens up a new and facile avenue for fabricating inorganic microsphere electrocatalyst in hydrogen production field.     

MoS2/NiS heterostructure grown on Nickel Foam as highly efficient bifunctional electrocatalyst for overall water splitting

It is great important to develop and explore a non-precious bifunctional electrocatalyst with high efficiency and good stability for Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in alkaline electrolyte. Herein, a three-dimensional (3D) needle-like MoS₂/NiS heterostructure supported on Nickel Foam (NF) (MoS₂/NiS/NF) is synthesized by a simple hydrothermal method for the first time, which can act as a good bifunctional electrocatalyst for overall water splitting. As expected, the optimal MoS₂/NiS/NF exhibits excellent catalytic performance with a low overpotential of 87 and 216 mV at 10 mA cm⁻² for HER and OER in 1 M KOH electrolyte, respectively, accompanied by good cycle stability. Furthermore, the MoS₂/NiS/NF as bifunctional electrocatalyst in an electrolyzer shows high efficiency with a cell voltage of 1.5 V at 10 mA cm⁻², as well as superior durability. The present work may open a new direction to design and develop a non-precious bifunctional electrocatalyst with excellent catalytic activity for water splitting in the future.     

Highly active Ni/Co-metal organic framework bifunctional electrocatalyst for water splitting reaction

Rational design of electrocatalycally active materials with excellent performance for renewable energy conversion is of great interest. We have developed a nanosheet array of Ni/Co metal-organic framework (MOF) grown on CoO modified Ni foam (CoO/NF) substrate via the solvothermal process. The high surface area and low resistance of Ni/Co-MOF@CoO/NF (NC@CoO/NF) catalyst contribute to efficient water splitting. We have prepared a series of NC-n/CoO/NF (n = 1–4) catalysts to optimize the molar ratio of the Co atom in Ni MOF-74. Among them, NC-2@CoO/NF shows an excellent electrochemical performance in alkaline medium, i.e., low overpotential of 290 and 139 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. For a two-electrode system with NC-2@CoO/NF, a low cell voltage of 1.54 V at 10 mA cm^?2 has been obtained for overall water splitting which is much smaller than that with commercial Ir/C– Pt/C pair. This excellent performance can be attributed to the synergistic effects of Ni/Co-MOF and CoO/NF. In addition, the as-prepared NC-2@CoO/NF exhibits excellent long-term stability. The computational simulation also supports experimental results.     

3D well-ordered wood substrate coupled transition metal boride as efficient electrode for water splitting

The choice of substrate is critical to the performance of binder-free electrode for water splitting. Here, carbonized wood is employed as 3D porous diffusion substrates to accommodate conductive transitional metal boride. The composite is efficiently and uniformly deposited on the inner wall of the carbonized wood by means of simple electroless plating technique. The well-defined 3D Ni-W-B/wood electrode requires overpotential of only 46 mV at 50 mA cm^?2 for hydrogen evolution reaction (HER), outperforming the Ni-W-B/NF electrode with nickel foam as the substrate. The ordered channel structure inherited from the carbonized wood endows the electrode with abundant active site density, fast mass and charge transportation, and efficient transport of evolved gas. The outstanding advantages of environmentally benign nature, low cost, favorable 3D channel structure, and anti-corrosion in harsh condition make wood as a promising substrate for binder-free electrode.     

A nano-spherical structure Ni3S2/Ni(OH)2 electrocatalyst prepared by one-step fast electrodeposition for efficient and durable water splitting

Developing non-noble metal catalysts with excellent electrocatalytic performance and stability is of great significance to hydrogen production by water electrolysis, but there are still problems of low activity, complex preparation and high cost. Herein, we fabricated a novel Ni₃S₂/Ni(OH)₂ dual-functional electrocatalyst by a one-step fast electrodeposition on nickel foam (NF). While maintaining the electrocatalytic performance of Ni₃S₂, the existence of heterostructure and Ni(OH)₂ co-catalyst function greatly improves the overall water splitting performance of Ni₃S₂/Ni(OH)₂–NF. Hence, It shows a low overpotential of 66 mV at 10 mA cm^?2 for HER and 249 mV at 20 mA cm^?2 for OER. The dual-functional electrocatalyst needs only 1.58 V at 20 mA cm^?2 when assembled two-electrode electrolytic cell. Impressively, the electrocatalyst also shows outstanding catalytic stability for about 800 h when 20 and 50 mA cm^?2 constant current was applied, respectively which demonstrates a potential electrocatalyst for overall water splitting.     

2D Triphosphides: SbP3 and GaP3 monolayer as promising photocatalysts for water splitting

Two-dimensional (2D) triphosphides, as an important family of 2D materials, have attracted more attention. Here, by using density functional theory (DFT) computations, we explored the remaining XP₃ monolayers (X = the metal of group IIIA-VA), and then proposed two novel 2D triphosphides, antimony triphosphide (SbP₃) and gallium triphosphide (GaP₃) monolayer. SbP₃ and GaP₃ monolayer are structurally stable semiconductors with indirect bandgaps of 2.36 eV and 1.45 eV, respectively. They not only show strong light absorption coefficients (up to 10⁵ cm⁻¹) in the visible and ultraviolet regions, but also their band edge positions straddle the redox potentials of water, making them promising catalysts for water splitting. Under strain, SbP₃ monolayer can effectively reduce the bandgap and GaP₃ monolayer can achieve the transition from indirect to direct bandgap, which can improve the photocatalytic efficiency accordingly. Our work would stimulate the fabrication of SbP₃ and GaP₃ monolayer and explore their potential applications in electronic and optoelectronic devices.     

The bifunctional 3D-on-2D FeCo/Ni(OH)2 hierarchical nanocatalyst for industrial-level electrocatalytic water splitting

Developing high-performing and cost-effective bifunctional electrocatalysts under the industrial conditions is significant for revolutionizing the hydrogen economy. Herein, we developed a bifunctional 3D-on-2D FeCo/Ni(OH)₂ hierarchical nanocatalyst on Ni mesh by a facile and low-cost method that can boost both the two half-reactions of water splitting all at once. The FeCo/Ni(OH)₂/Ni mesh showed an outstanding electrocatalytic performance under the industrial conditions (3 M KOH, 90 °C) with applied voltages of 1.47 and 1.91 V to drive the electrolyzer at 10 and 500 mA/cm², respectively, much better than the Ni(OH)₂/Ni mesh, FeCo/Ni mesh, and Raney Ni/Ni mesh samples. Furthermore, it can maintain good stability for more than 192 h under the working condition of 500 mA/cm². This promotion in electrocatalytic properties can be ascribed to the synergistic effects between 3D FeCo nanoparticles and 2D Ni(OH)₂ nanosheets, including the large electrochemically active surface areas, the intense electronic interplay between the two component and the elevated interfacial contact and charge transport. Eventually, this water electrolyzer yields a 14.7% solar-to-hydrogen efficiency when integrated with Si solar cells, which exceeds analogous solar-driven systems reported so far.     

The 3D ultra-thin Cu1-xNixS/NF nanosheet as a highly efficient and stable electrocatalyst for overall water splitting

In recent years, the exploration of efficient and stable noble-metal-free electrocatalysts is becoming increasingly important, used mainly for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this work, a new ultrathin porous Cu_1-xNi_xS/NF nanosheets array was constructed on the 3D nickel skeleton by two-step method: hydrothermal method and vulcanization method. Through these two processes, Cu_1-xNi_xS/NF has a larger specific surface area than that of foamed nickel (NF) and Cu_1-xNi_xO/NF. The Cu_1-xNi_xS/NF materials show excellent catalytic activity by accelerating the electron transfer rate and increase the amount of H₂ and O₂ produced. The lower overpotential was obtained only 350 mV at 20 mA cm⁻² for OER, not only that, but also the same phenomenon is pointed out in HER, optimal Cu_1-xNi_xS/NF presents low overpotentials of 189 mV to reach a current density of 10 mA cm⁻² in 1.0 M KOH for HER. Both OER and HER shows a lower Tafel slope: 51.2 mV dec⁻¹ and 127.2 mV dec⁻¹, subsequently, the overall water splitting activity of Cu_1-xNi_xS/NF was investigated, and the low cell voltage was 1.64 V (current density 10 mA cm⁻²). It can be stable for 14 h during the overall water splitting reaction. These results fully demonstrate that Cu_1-xNi_xS/NF non-precious metal materials can be invoked become one of the effective catalysts for overall water splitting, providing a richer resource for energy storage.     

Growth of nickel vacancy NiFe-LDHs on Ni(OH)2 nanosheets as highly efficient bifunctional electrocatalyst for overall water splitting

A bifunctional electrocatalyst was fabricated by in-situ vertical growth of Ni(OH)₂ nanosheets on nickel foam (NF), with subsequent accretion of nickel vacancy NiFe-LDHs (Ni^vacFe-LDHs) by two step hydrothermal method. It was exhibited to be a high-efficiency overall water splitting performance with good stability. The low over-potentials of 292, 330, and 376 mV were acquired when the current density was selected as 50, 100, and 200 mA/cm² for oxygen evolution reaction (OER) with a relatively low Tafel slope. It also achieved low over-potentials of 116 and 247 mV when the current densities were 10 and 200 mA/cm² for hydrogen evolution reaction (HER), and Tafel slope was estimated to be 95.87 mV/dec. For the overall water splitting, NF–Ni(OH)₂-Ni^vacFe-LDHs needed only a low overpotential (291 mV) to achieve 25 mA/cm² in 1 mol/L potassium hydroxide. The long-term testing of this electrode for 24 h chronopotentiometric test at 25 mA/cm² demonstrated very eminent stability.     

Self-grown one-dimensional nickel sulfo-selenide nanostructured electrocatalysts for water splitting reactions

Herein, one-dimensional (1-D) self-grown nickel sulfide (NiS), and nickel sulfo-selenide (NiSSe) nanostructures on Ni-foam are successfully prepared via a simple and low-cost hydrothermal synthesis route. The S²⁻ and Se²⁻ ions are obtained after decomposition of sulfur and selenium precursors, which on reacting with oxidized Ni²⁺ ions, from the surface of pristine Ni-foam, produces NiS, and NiSSe superstructures. The crystal phase, surface morphology and chemical states of as-grown electrocatalysts are monitored through various measurment tools. Due to synergistic effects, the NiSSe nanostructured electrode provides a superior over potentials as low as 154 and 75 mV for OERs and HERs activity in electrochemical water splitting measurement. Moreover, the NiSSe electrode exhibits an excellent chemical stability compared to the NiS and NiSe electrodes during the electrolysis process. Such an outstanding catalytic performance makes the NiSSe electrode as a potential candidate in water splitting applications.     

Metal-organic framework derived Co3O4/TiO2 heterostructure nanoarrays for promote photoelectrochemical water splitting

In this work, we proposed a simple and new method to fabricate Metal-organic frameworks (MOFs) derived Co₃O₄ modified TiO₂ nanorods (NRs) photoelectrode by immersion and anneal treatment. The positively charged Co-MOF (ZIF-67) was adsorbed on the negatively charged TiO₂ NRs by electrostatic interaction, and then annealed in air to obtain the Co₃O₄/TiO₂ photoelectrodes. The photoelectrochemical (PEC) performance of the Co₃O₄/TiO₂ photoelectrodes has been significantly improved compared with the pure TiO₂, the best photocurrent density of Co₃O₄/TiO₂ photoelectrode could reach 1.04 mA/cm² (1.23 V vs RHE) which was almost 1.65 times than that of pure TiO₂. On the Co₃O₄/TiO₂ photoelectrodes, the significant improvement in PEC performance could be attributed to the constructed p-n heterostructure, which can promote charge transfer within the system and improve the efficiency of electron/hole separation. Meanwhile, under the action of the MOFs-derived Co₃O₄, the number of active sites increases significantly and visibly improve the photoresponse performance.     

Coral reef structured cobalt-doped vanadate oxometalate nanoparticle for a high-performance electrocatalyst in water splitting

Facing the energy crisis in the whole world, it is important to decompose water to obtain high-clean hydrogen energy. However, water splitting by electrocatalysis is suffering from high voltage and poor stability. Herein, we synthesize Co₃V₂O₈ coral reef-like nanoparticles in a facile way, showing a low oxygen evolution reaction (OER) overpotential of 318 mV coupled with good stability, which is superior to commercial RuO₂. Besides, the Co₃V₂O₈ shows fast kinetics for hydrogen evolution reaction (HER) and small impedance. Furthermore, the Co₃V₂O₈ nanoparticles are assembled in symmetric two-electrode system, which has a very low overall water splitting voltage of 1.50 V at 10 mA cm^?2, this value surpasses the benchmark RuO₂//Pt/C assembling and most of the other oxometalate-based electrocatalysts. This work provides a novel and facile way of preparing oxometalates nanomaterial electrocatalyst for hydrogen energy.     

Coaxial Ni3S2@CoMoS4/NiFeOOH nanorods for energy-saving water splitting and urea electrolysis

High efficiency and energy-saving electrochemical hydrogen production has always been a research challenge, mainly due to the limitation of the sluggish kinetics of anodic reactions. Excellent performance depends largely on the clever design of nano-architectures and smart hybridization of active components. It is also very important to establish the relationship between structure and performance of materials. Form perspectives of chemical composition and nanostructure, we developed a novel heterostructure of Ni₃S₂@CoMoS₄/NiFeOOH coaxial nanorods on NF scaffold, in which the two-dimensional CoMoS₄ nanoplates and ultrathin NiFeOOH nanosheets vertically coil around the Ni₃S₂ nanorods by hydrothermal reaction combined with electrodepostion process. Such hierarchical nanorods can provide the heterointerface with highly open surface, ensuring the maximization of synergistic interaction. This heterostructure results in prominent bifunctional activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water electrolysis systems, and HER coupled with urea oxidation reaction in urea electrolysis devices. It exhibits very low cell voltages for alkaline water splitting (1.732 V) and urea electrolysis (1.660 V) to afford a current density of 100 mA cm^?2 in the two-electrode system as well as excellent long-term stability. Our work provides a new construction for combining various active materials to competitive bifunctional electrocatalysts applied in energy-relative electrochemical devices.     

NiS2@V2O5/VS2 ternary heterojunction for a high-performance electrocatalyst in overall water splitting

Water splitting is regarded as an effective way to produce hydrogen energy to solve the energy crisis all over the world. However, the electrocatalysts suffer from expensive prices, high voltage, and sluggish kinetics. The heterojunction is composed of two semiconductors and can accelerate electron transfer by relying on interface engineering. Herein, we first prepare NiS₂@V₂O₅/VS₂ ternary heterojunction electrocatalyst, showing the low OER overpotential of 333 mV and HER overpotential of 216 mV at 10 mA cm^?2, as well as good stability. Meanwhile, the NiS₂@V₂O₅/VS₂ heterojunction is assembled to the two-electrode system for overall water splitting, exhibiting a very low voltage value of 1.49 V, which is much superior to that of the benchmark RuO₂//Pt/C system. The energy band calculation reveals the mechanism that the NiS₂ and VS₂ lower the Fermi level of V₂O₅, thus promoting the electrons transfer in the electrocatalytic reactions. Our work opens up a novel route for heterojunction application in the electrocatalytic field.