Synthesis of CoMoO4/Co9S8 network arrays on nickel foam as efficient urea oxidation and hydrogen evolution catalyst

It is of high significance to design robust, low-cost and stable electrocatalysts for the urea splitting reaction under alkaline medium. In this communication, we present the exploitation of CoMoO₄/Co₉S₈ which directly grown on nickel foam (CoMoO₄/Co₉S₈/NF) as a robust and stable electrocatalyst for urea splitting. Such CoMoO₄/Co₉S₈/NF (C_Mo:C_S = 9:1) presents the lowest overpotential (172 mV@10 mAcm⁻²), which is better than that of CoMoO₄/NF (185 mV@10 mAcm⁻²), CoMoO₄/Co₉S₈/NF (C_Mo:C_S = 8:2) (208 mV@10 mAcm⁻²), CoMoO₄/Co₉S₈/NF (C_Mo:C_S = 7:3) (270 mV@10 mAcm⁻²) and Co₉S₈/NF (286 mV@10 mAcm⁻²) for hydrogen evolution. In addition, The CoMoO₄/Co₉S₈/NF (C_Mo:C_S = 8:2) presents a superior long-term electrocatalytic stability, keeping its activity at 40 mAcm⁻² for 13 h for urea oxidation.     

Three-dimensional Co3O4@NiCo2O4 nanoarrays with different morphologies as electrocatalysts for oxygen evolution reaction

Oxygen evolution reaction is one of the key factors restricting the whole process of electrolysis of water. In this paper, hydrothermal and calcination method are used to in situ grow Co₃O₄@NiCo₂O₄ on nickel foam (NF). The formation of Co₃O₄@NiCo₂O₄ nanostructures depends on the different hydrothermal time, which further results in the different growth mechanism of Co₃O₄@NiCo₂O₄ nanostructures. The result shows that Co₃O₄@NiCo₂O₄-8h, as a catalytic material, could play a synergistic role to largely accelerate the electron transfer process and could be efficiently and persistently used in oxygen evolution reaction. The oxygen evolution reaction activity of Co₃O₄@NiCo₂O₄-8h material is significantly improved compared with Co₃O₄, Co₃O₄@NiCo₂O₄-6h and Co₃O₄@NiCo₂O₄-10 h. When the current density is 50 mA cm⁻², the overpotential is only 290 mV for Co₃O₄@NiCo₂O₄-8h material. The enhanced activity Co₃O₄@NiCo₂O₄-8h is attributed to more active site exposure, rapid charge transfer and synergistic catalysis of Co₃O₄ and NiCo₂O₄. This work provides a new idea for the development of efficient, stable and environmentally friendly hybrid catalysts.     

Dual-functional Co3O4@Co2P4O12 nanoneedles supported on nickel foams with enhanced electrochemical performance and excellent stability for overall urea splitting

Urea splitting to produce hydrogen is one of the most promising solutions to the energy crisis in the future. A series of Co₃O₄ and cobalt phosphate composites on nickel foam were synthesized by hydrothermal and calcination process and firstly used as dual-functional electrode for the overall urea splitting. When the current density is 20 mA cm⁻², the required cell voltage is significantly lower than that of fully water splitting. The stability test results show that the composition and morphology of our catalyst do not change significantly before and after the reaction. By controlling the morphology under the same conditions, we concluded that the main factor affecting the activity of urea splitting was specific surface area and synergistic effect.     

Controlled synthesis of NiCo2O4@Ni-MOF on Ni foam as efficient electrocatalyst for urea oxidation reaction and oxygen evolution reaction

Heterostructured materials with special interfaces and features give a unique character for much electrocatalytic process. In this work, the introduction of exogenous modifier Ni-MOF improved the reaction kinetics and morphology of the NiCo₂O₄@Ni-MOF/NF catalyst. As-obtained NiCo₂O₄@Ni-MOF/NF has excellent oxygen evolution reaction (OER) performance and urea oxidation reaction (UOR) performance. The catalyst need overpotential of 340 mV at a current density of 100 mA cm^?2 for OER and a potential of 1.31 V at the same current density for UOR. The Tafel slopes of NiCo₂O₄@Ni-MOF/NF is 38.34 and 15.33 mV dec^?1 for OER and UOR respectively, which is more superior than 78.58 and 66.73 mV dec^?1 of NiCo₂O₄/NF. The nanosheets microstructure is beneficial to the adsorption and transport of electrolyte and the presence of a large number of mesoporous channels can also accelerate gas release, and then improves activity of the catalyst. Density functional theory calculation demonstrate that NiCo₂O₄ plays a role in absorbing water, while the existence of in situ generated NiOOH can promote the electron transfer efficiency. It is synergies of NiCo₂O₄ and in situ generated NiOOH that enhance the decomposition of water on the surface of the NiCo₂O₄@Ni-MOF/NF. This investigation provides a new strategy for the application of spinel oxide and MOF materials.     

Metal tungstate dominated NiCo2O4@NiWO4 nanorods arrays as an efficient electrocatalyst for water splitting

One of the most promising ways to produce high purity hydrogen is electrocatalytic water splitting, the slow rate of oxygen evolution reaction (OER) limited the whole water splitting process under alkaline conditions. Here, NiCo₂O₄@NiWO₄/NF is firstly acted as a metal tungstate dominated bifunctional water splitting catalyst, which a very low cell voltage of 1.59 V is obtained with 10 mA cm⁻² in alkaline media. Remarkably, the catalytic performance of NiCo₂O₄@NiWO₄/NF is also kept for at least 12 h, which shows long time electrochemical stability.     

Mo-doped Co9S8 nanorod array as a high performance electrochemical water splitting catalyst in alkaline solution

It is very important to exploit robust electrocatalysts for the water splitting in an alkaline medium. Hence, a series of Mo-doped Co₉S₈ nanorod array on Ni foam (Mo–Co₉S₈/NF) was successfully synthesized through hydrotherma and sulfuration processes for the first time and used as an efficient and stable difunctional electrocatalyst for the overall water splitting. Such Mo–Co₉S₈-3//Mo–Co₉S₈-2 electrodes couple display superior water splitting performance with the requirement of a cell voltage of 1.50 V to drive a catalytic current density of 10 mA cm⁻², which is lower than that of RuO₂//Pt/C (1.52 V). The activity of the catalyst is greatly enhanced by the molybdenum ion doping and the instability of the sulfide is resolved. The experiment result shows that the relationship between the current density and pH is different in neutral and alkaline media, which is most be likely assigned to the change of O–O formation by transforming the reactants from water molecule to the hydroxy ion.     

Construction of NiCo2S4/Ni3S2 nanoarrays on Ni foam substrate as an enhanced electrode for hydrogen evolution reaction and supercapacitors

Developing a multifunctional and sustainable electrode material for hydrogen evolution reaction and supercapacitors is a highly feasible avenue for producing the high energy density and renewable energies. In our study, nanostructured NiCo₂S₄/Ni₃S₂/NF nanoarrays are rational developed in experiments via a simple hydrothermal reaction. Ascribed to the 3D nanostructured NiCo₂S₄/Ni₃S₂ with numerous exposure active sites and large contact areas for the electrolyte, the binder-free feature of NiCo₂S₄/Ni₃S₂/NF facilitates a low charge transfer resistance, as well as the synergetic effect of NiCo₂S₄ and Ni₃S₂. The obtained electrocatalyst showed ultrahigh electrocatalytic activity with an overpotential of 111 mV at 10 mA cm⁻² and a Tafel slope of 57 mV dec⁻¹. In addition, the electrode showed an area specific capacity of 6.13 F cm⁻² at 10 mA cm⁻² and superior rate capability (2.72 F cm⁻² at 80 mA cm⁻²), accompanied by excellent cycling stability. This results presented in our work can provide an effective strategy for rational design of other hybrid materials with excellent electrochemical performance in the application of electrocatalysis and supercapacitors.     

3D MnCo2O4@CoS nanoarrays with different morphologies as an electrocatalyst for oxygen evolution reaction

The efficiency and stability of electrocatalysts are the key factors for measuring oxygen evolution reaction. In this work, the MnCo₂O₄ structure assembled from well-arranged nanowires or nanosheet arrays has been grown vertically on nickel foam by in-situ hydrothermal method. Interestingly, different morphology of MnCo₂O₄ can be easily regulated by adding NH₄F to a mixed solvent to achieve conversion from nanowires to nanosheets. In addition, further synthesis of unique three-dimensional hierarchical core/shell MnCo₂O₄@CoS nanowires or nanosheets arrays was performed primarily by electrochemical deposition. Both MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets exhibit high efficiency and long-lasting stability for the oxygen oxidation reaction. The lower overpotential of only 280 mV and 270 mV at 20 mA cm⁻² for the MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets were obtained with lower Tafel slopes of 139. 19 mV dec⁻¹ and 131.81 mV dec⁻¹ in 1.0 M potassium hydroxide respectively comparing with our other MnCo₂O₄@CoS catalysts. The results demonstrate that the crystal morphology of MnCo₂O₄@CoS does not significantly influence their electrocatalytic activity in water oxidation reactions by comparing nanostructured MnCo₂O₄@CoS nanowires and MnCo₂O₄@CoS nanosheets. The high catalytic activity of the MnCo₂O₄@CoS nanoarrays is attributed to the possession of more active sites, larger specific surface area, abundant oxygen vacancy, and fast electron transport rate. Not only that, the durability of the MnCo₂O₄@CoS nanoarrays is also excellent after continuous oxygen evolution test of 1000 cycles. The results of XRD, SEM and XPS show that MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets materials can be used as a highly efficient and stable catalyst for oxygen evolution reaction.     

A review of heteroatomic doped two-dimensional materials as electrocatalysts for hydrogen evolution reaction

Emerging two-dimensional (2D) materials, such as graphene, transition metal disulfide compounds (TMDCs), MXenes, layer double hydroxides (LDHs), black phosphorus (BP) and hexagonal boron nitride (h-BN), play an important role in speeding up hydrogen evolution reaction (HER) due to its large specific surface area as well as function of loading and efficient support. However, as an electrocatalyst, pure 2D materials cannot meet HER needs caused by their monotonous performance. Therefore, some nanoparticles are used to load and tune the 2D materials to develop efficient and inexpensive catalysts. Herein, we conduct a thorough analysis for materials based on heteroatoms, especially transition metal atoms and non-metal atoms (N, P, S, etc.) doped with graphene, TMDCs, MXenes, LDHs, BP and h-BN. It can be found that doping or coupling between 2D materials will affect the electronic structure, energy band, active area, conductivity and stability of the catalyst, which will induct a huge change in the catalytic performance. This review reveals the relationship between active centers, H₂O adsorption and chemical reaction processes. It also analyzes and summarizes the design principles and performance improvement mechanisms of hybrid catalysts. These discussions can provide references for other researchers to develop derivatives of related catalysts.     

Three-demensional NiCoNiCo2O4/NF as an efficient electrode for hydrogen evolution reaction

At present, there is an urgent need for plentiful non-noble metal catalyst to substitute for valuableness platinum based metal catalyst in electrochemical water splitting. Here, we fabricated a three-demensional (3D) NiCoNiCo₂O₄ nanosheets electrocatalyst that directly grew on Ni foam firstly and then were reduced in 0.1 mol dm⁻³ sodium borohydride solution. This electrode exhibited high activity in 1.0 mol dm⁻³ KOH solution with an onset potential of ∼40 mV and a tafel slope of 77 mV dec⁻¹. Furthermore, the NiCoNiCo₂O₄/NF electrode showed a splendid durability during long-playing electrochemical test. Our work may provide an inexpensive, easy-to-obtain and excellent catalyst candidate for future electrolytic water research and industry studies that may involve hydrogen applications in the future.     

Facile synthesis of Ni doped CoWO4 nanoarrays grown on nickel foam substrates for efficient urea oxidation

Urea electrolysis is a promising technology for hydrogen production, which can alleviate environmental pollution of urea-rich wastewater. It's worth noting that electrochemistry activity can be significantly improved by reasonably regulating the electron configuration around the active site for the doped materials. In this work, a series of well-tuned Ni doped CoWO₄ nanoarrays on Ni foam supports have been prepared through a typical hydrothermal approach for the first time. Moreover, the resulting Ni–CoWO₄-2 material significantly promotes urea oxidation performance with an applied potential of 1.35 V at 50 mA cm^?2, which is lower than that of water oxidation reaction (1.60 V). Density functional theory results suggest that the Ni doped CoWO₄ has larger urea adsorption energy compared with CoWO₄ and the CO(NH₂)₂ molecule is strongly adsorbed on surface of Ni doped CoWO₄, which is beneficial to accelerate the kinetics of the reaction and improve the electrocatalytic activity of the urea electrolysis.     

Facile fabrication of comb-like porous NiCo2O4 nanoneedles on Ni foam as an advanced electrode for high-performance supercapacitor

It is very desirable to develop the high-performance supercapacitors to meet the rapidly growing demands for energy-autonomous operation and miniaturization of devices. Herein, comb-like porous NiCo₂O₄ nanoneedles on the three-dimension (3D) nickel foam (NF) have been successfully synthesized through a facile pulsed laser ablation (PLA) approach without any post-treatments and surfactant (denoted as NiCo₂O₄-PLA). The influence of working solution during the fabricated process on the properties of NiCo₂O₄-PLA has been demonstrated in detail in terms of the crystalline structure, specific surface area, morphology, and electrochemical performance. Benefiting from the large specific surface (261.4 m² g⁻¹), abundant pores, and highly conductive scaffold, the NiCo₂O₄-PLA binder-free electrode exhibits an outstanding specific capacitance (1650 F g⁻¹ at a current density of 1 A g⁻¹) and eminent cycling performance (91.78% retention after a 12,000-cycle test at a current density of 10 A g⁻¹) compared with the control samples. The assembled asymmetric device (NiCo₂O₄-PLA//AC-ASCs) delivers the high specific capacitance of 126.9 F g⁻¹ at the current density of 1 A g⁻¹, the large energy density of 56.7 Wh kg⁻¹ at a power density of 756 W kg⁻¹, and the low internal resistance. The attractive results strongly prove that it is an ideal candidate for advanced supercapacitor application.     

Effect of cation substitution on the water splitting performance of spinel cobaltite MCo2S4 (M = Ni,Cu and Co)

The introduction of different ions is an effective method for regulating electron distribution and increasing the electrocatalytic activity of spinel cobalt sulfide (Co₃S₄). However, the effect of doping different ions on water splitting performance has not been systematically clarified. Therefore, a detailed research is done to illuminate the doping of different ions on the water splitting performance of spinel cobalt sulfide MCo₂S₄ (M = Ni, Cu and Co) nanorods grown on Ni foam. To drive the electrocatalytic current of 50 mA/cm² and 10 mA/cm², the CuCo₂S₄/NF material only requires an overpotential of 240 mV for oxygen evolution reaction (OER) and an overpotential of 142 mV for hydrogen evolution reaction (HER). The results of density functional theory and experiment demonstrate that the strong water adsorption energy and the large electrochemical activity area make CuCo₂S₄/NF show good catalytic activity. The CuCo₂S₄/NF nanorods material presents superior electrochemistry performance with a small voltage 1.53 V. The water oxidation activity increases linearly before nonlinearly improving with the increasing of pH, indicating that the substrate changes from water to hydroxyl. It is noteworthy that CuCo₂S₄/NF will be transformed into amorphous oxide active species, which will act as a stable catalyst during the reaction.     

Needle-like CoP/rGO growth on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

In this work, CoP/NF is synthesized at different temperature (250 °C, 300 °C, 350 °C) (denoted as CoP/NF-T, T = 250, 300, 350). Then, CoP/NF-300 with the best performance towards hydrogen evolution reaction (HER), is used to synthesize compounds with different ratio of reduced graphene oxide (rGO) (CoP/rGO/NF-X, X (quality ratio of rGO/CoP) = 1,3,5). In terms of morphology, under the synergistic effect of rGO, uniform and dense CoP provides the possibility to increase the electrochemical area. While CoP/rGO/NF-3 shows the minimum overpotential of 136 mV to drive 50 mA/cm, and the smallest Tafel slope 135 mV/dec among as-synthesized materials. Furthermore, CoP/rGO/NF-3 has good stability during at least 25 h. These result can be construed as the large electrochemical active area, high conductivity and long-time stability.     

Facile synthesis of molybdenum-based layered double hydroxide nanorods for boosting water oxidation reaction

Exploiting environmentally friendly and robust oxygen evolution reaction (OER) electrodes is still a great difficulty to promote the water oxidation for electrocatalytic water splitting. In the work, molybdenum-doped nickel copper hydrotalcite (NiCuMo_x LDH) nanoarrays have been firstly in situ grown on nickel foam (NF) via a typical hydrothermal process. When NiCuMo_0.2 LDH/NF was used as a OER electrocatalyst, it displays superior electrocatalytic performance with the need of small overpotentials of only 290 mV to drive 40 mA cm⁻² and a low Tafel slope of 39.8 mV dec⁻¹, which are almost one of the best water oxidation activities reported so far. The enhanced electrocatalytic performance is attributed to unique urchin-like structure, more exposure to active sites and enhanced charge transfer rate owing to the synergistic effect of Mo doping and NiCu LDH. The work put forward a new method for the development of efficient water oxidation electrocatalysts, which will fill the gap for the exploitation of trimetal LDH-based electrodes in large-scale water splitting applications in the future.     

3D hierarchical network NiCo2S4 nanoflakes grown on Ni foam as efficient bifunctional electrocatalysts for both hydrogen and oxygen evolution reaction in alkaline solution

The development of cost-effective and high-efficiency electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) still remains highly challenging. Exposing as many active sites as possible is the key method to improve activity of HER and OER performance. In this communication, we demonstrate a novel 3D hierarchical network NiCo₂S₄ nanoflake grown on Ni foam (NiCo₂S₄-NF) as a highly efficient and stable electrochemical catalyst. The NiCo₂S₄-NF exhibits overpotentials as low as 289 and 409 mV at 100 mA cm^?2, superior long-term durability during a 20 h measurement, and a low Tafel slope of 89 and 91 mV dec^?1 for HER and OER in 1.0 M NaOH solution. The outstanding performance is owe to the inherent activity of ultrathin NiCo₂S₄ nanoflakes and the special structure of NiCo₂S₄-NF that can provide a huge number of exposed active sites, accelerate the transfer of electrons, and facilitate the diffusion of electrolyte simultaneously.     

Ni3S2@Co(OH)2 heterostructures grown on Ni foam as an efficient electrocatalyst for water oxidation

It is of high significance to design robust, low-cost and stable electrocatalysts for the oxygen evolution reaction (OER) under alkaline medium. In this communication, we present the exploitation of Ni₃S₂@Co(OH)₂ which directly grown on nickel foam (Ni₃S₂@Co(OH)₂/NF) as a robust and stable electrocatalyst for OER. Such Ni₃S₂@Co(OH)₂/NF-5h demanding overpotential of only 290 mV is less than that of Ni₃S₂@Co(OH)₂/NF-10h (310 mV), Ni₃S₂@Co(OH)₂/NF-2h (320 mV) and Ni₃S₂/NF(350 mV), respectively, to drive a geometrical catalytic current density of 35 mA cm⁻², which is also better than that of noble metal catalyst IrO₂/NF (320 mV). In addition, the Ni₃S₂@Co(OH)₂/NF-5h presents a superior long-term electrocatalytic stability, keeping its activity at 26 mA cm⁻² for 40 h.     

Nickel nanocones as efficient and stable catalyst for electrochemical hydrogen evolution reaction

In this work, nickel nanocones (NNCs) were fabricated by single-step electrodeposition method. The NNCs were used as hydrogen evolution electrode and their electrocatalytic activity was compared with pure nickel film. Linear Sweep Voltammetry (LSV), Tafel polarization, Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV) and chronopotentiometry in 1 M KOH were used for study of the electrocatalytic activity for hydrogen evolution reaction (HER). The active surface area was increased by formation of NNCs and hence, the electrocatalytic activity of nickel electrode was improved. Results indicate that the current density corresponding to the amount of evolved hydrogen of NNCs is five times more than pure nickel film formed in the Watts bath.     

Fe-Cu coated nickel mesh usage as cathode catalyst for hydrogen evolution reaction

Nickel mesh electrodes were used as the working electrode. Iron and copper were electrochemically deposited on the nickel mesh in different amounts. When electrochemical coatings had been carried out, different currents were passed from the circuit at different times and coatings were accumulated at constant load. The prepared electrodes called as FexCux, FexCu3x and FexCu9x and these electrodes have been used for hydrogen evolution reaction (HER). The surface morphologies were investigated by scanning electron microscopy. The HER activity is assessed by recording cathodic current–potential curves, cyclic voltammetry, electrochemical impedance spectroscopy. The results show that FexCu9x catalysts have a compact and porous structure as well as good electrocatalytic activity for the HER in alkaline media.     

CuCo2O4 microflowers catalyst with oxygen evolution activity comparable to that of noble metal

It is of high significance to design efficient, low-cost and durable electrocatalysts for the reaction (OER) in alkaline solution. In this communication, we report the development of CuCo₂O₄ microflowers directly on nickel foam (CuCo₂O₄/NF) as an efficient and durable electrocatalyst for OER. Such CuCo₂O₄/NF demands overpotential of only 296 mV to drive a geometrical catalytic current density of 20 mA cm^?2, 73 mV and 145 mV less than that for Co₃O₄/NF and NF, respectively, which are better than that of RuO₂/NF. Furthermore, CuCo₂O₄/NF presents an excellent long-term electrochemical durability maintaining the activity at overpotential of 240 mV for 10 h.