Homogeneous core–shell NiCo2S4 nanorods as flexible electrode for overall water splitting

Exploring low-cost and highly efficient Water splitting electrocatalyst has been recognized as one of the most challenging and promising ways. NiCo₂S₄ core-shell nanorods supported on nickel foam (NF) have been fabricated by a facile hydrothermal method. The electrochemical performance of NiCo₂S₄@NiCo₂S₄ for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is studied. NiCo₂S₄@NiCo₂S₄/NF exhibits a significantly improved OER and HER performance with an overpotential of 200 mV at 40 mA/cm² and an overpotential of 190 mV at 10 mA/cm². The combination of low charge-transfer resistance, enhanced interaction and charge transport as well as large electrochemical double-layer capacitance enables superior OER and HER. The NiCo₂S₄@NiCo₂S₄/NF nanorod electrode shows excellent electrocatalytic activity with a low voltage 1.57 V and stability with long hour electrolysis, which is highly satisfactory for a prospective bifunctional electrocatalyst.     

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.     

Construction of unique NiCo2S4@Ni3V2O8 hierarchical heterostructures arrays on Ni foam as an efficient electrocatalyst with high stability for water oxidation

The development of high efficiency and stable electrocatalysts for the electrochemical water oxidation reaction (WOR) is a grand bottleneck in chemical energy storage and conversion. This article describes a simple co-precipitation route to deposit hierarchical NiCo₂S₄@Ni₃V₂O₈ core/shell hybrid on conductive nickel foam electrode by a simple two-step process. When it is firstly used as the 3D substrates electrode, the NiCo₂S₄@Ni₃V₂O₈ material makes use of both components and provides excellent water oxidation activity, 35 mA cm^?2 was achieved at a overpotential of 290 mV, which is better than the benchmark of IrO₂ electrodes (320 mV of overpotential at 35 mA cm^?2) and 13 mA cm^?2 at 1.47 V with excellent durability. The enhanced water oxidation performance of the NiCo₂S₄@Ni₃V₂O₈ materials is mainly benefiting from its particular core/shell structure, which exhibits big surface areas, and fast electron and ion transfer. Ni₃V₂O₈ shell protects NiCo₂S₄ core from oxidation in the in alkaline electrolytes and improves stability of NiCo₂S₄@Ni₃V₂O₈.This indicates that most metal vanadates oxides-based electrodes are promising as an efficient electrocatalyst and shows the advantages of the interfacial effect, which provide a new idea toward high-performance flexible water oxidation fabrication of robust and cheap catalyst sample.     

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.     

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.     

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.     

NiCoP coated on NiCo2S4 nanoarrays as electrode materials for hydrogen evolution reaction

It is very important to exploit robust electrocatalysts for the urea splitting in an alkaline medium. Hence, the NiCo₂S₄@NiCoP nanoarrays on Ni foam (NiCo₂S₄@NiCoP/NF) was successfully synthesized for the first time and used as an efficient and stable difunctional electrocatalyst for the overall urea splitting. As one of the most promising bifunctional electrocatalysts reported, NiCo₂S₄@NiCoP only needs 108 mV to reach current density of 10 mA cm⁻² for hydrogen evolution reaction. Moreover, such NiCo₂S₄@NiCoP//NiCo₂S₄@NiCoP electrodes couple display superior urea splitting performance with the requirement of a cell voltage of 1.53 V to drive a catalytic current density of 10 mA cm⁻². In addition, the NiCo₂S₄@NiCoP material presents high long-term electrocatalytic stability keeping its performance at 11 mA cm⁻² for 12 h. The experimental results demonstrate that the sluggish Volmer step has been improved by incorporating the NiCoP to the NiCo₂S₄.     

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.     

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.     

Plasmonic Ag nanoparticles decorated NaNbO3 nanorods for efficient photoelectrochemical water splitting

In the present work, photoanodes comprising of NaNbO₃ nanorods and Ag nanoparticles decorated NaNbO₃ nanorods have been fabricated by using hydrothermal and chemical solution method respectively. Photoelectrochemical water splitting performance of the fabricated photoanodes have been measured and it is found that Ag decorated NaNbO₃ nanorods exhibit ～4 fold enhancement in photocurrent as compared to bare NaNbO₃ nanorods. The enhancement in the photoelectrochemical water splitting activity of Ag decorated NaNbO₃ nanorods is attributed to efficient charge carrier separation and visible light sensitization due to Ag nanoparticles on NaNbO₃ nanorods.     

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.     

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.     

N,S co-doped NiCo2O4@CoMoO4/NF hierarchical heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic overall water splitting technology has received considerable attention in recent years. The fabrication of low-cost, earth-rich and potent bifunctional electrocatalysts is vital for hydrogen evolution (HER) and oxygen evolution reactions (OER). Herein, the N and S co-doped NiCo₂O₄@CoMoO₄ heterostructures (N, S–NCO@CMO₄₀₀) are fabricated by CVD and hydrothermal methods. N and S atoms as auxiliary active centers can increase the activity of Ni, Co and Mo atoms at the same time. Hierarchical heterostructures generate more interfaces to accelerate mass transfer and enlarge the electrochemical surface area, which greatly enhances the catalytic activity. The catalyst displays outstanding OER performance. The overpotentials of OER and HER are 165 and 100 mV at a current density of 10 mA cm^?2, respectively. More importantly, the N, S–NCO@CMO₄₀₀-based water splitting cell has a low voltage of 1.46 V at 10 mA cm^?2. Furthermore, the N, S–NCO@CMO₄₀₀ runs for 120 h in stable operation. This work provides new ideas for the design of hierarchical heterostructures with two-element incorporation.     

Interface engineering of NixSy@MoS2 heterostructured nanorods as high-efficient electrocatalysts for water splitting

Robust and low-cost oxygen evolution reaction (OER) electrocatalysts at low overpotentials play an increasingly pivotal role in clean energy storage and conversion systems. The emerging catalyst with core-shell heterostructure has excited the potentiality of non-noble-metal candidates. Herein, in order to enhance the sufficient exposure and utilization of actives sites and accelerate the electron transfer rate of catalyst, Ni_xS_y@MoS₂ core-shell nanorods decorated NiSe₂ framework (Ni_xS_y@MoS₂/NiSe₂) electrocatalyst has been successfully prepared via interface engineering. As expected, the as-prepared catalyst shows an outstanding OER activity with a small overpotential of 360 mV to drive 100 mA cm⁻² and a Tafel slope of 64 mV dec⁻¹. More importantly, a low cell voltage of 1.40 V is achieved for the Ni_xS_y@MoS₂/NiSe₂ based water splitting electrolyzer at 10 mA cm⁻², and it shows negligible decrement after continuous operation for 100 h. Furthermore, density functional theory (DFT) calculations further uncovered the synergetic catalytic effect between the Ni_xS_y@MoS₂ core-shell nanorods and the NiSe₂ framework played a key role in generating more charge carriers and declining the energy barriers in the process of forming intermediates.     

Porous sunflower plate-like NiFe2O4/CoNi–S heterostructure as efficient electrocatalyst for overall water splitting

Constructing high-efficient and nonprecious electrocatalysts is of primary importance for improving the efficiency of water splitting. Herein, a novel sunflower plate-like NiFe₂O₄/CoNi–S nanosheet heterostructure was fabricated via facile hydrothermal and electrodeposition methods. The as-fabricated NiFe₂O₄/CoNi–S heterostructure array exhibits remarkable bifunctional catalytic activity and stability toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. It presents a small overpotential of 219 mV and 149 mV for OER and HER, respectively, to produce a current density of 10 mA cm^?2. More significantly, when the obtained electrodes are used as both the cathode and anode in an electrolyzer, a voltage of 1.57 V is gained at 10 mA cm^?2, with superior stability for 72 h. Such outstanding properties are ascribed to: the 3D porous network structure, which exposes more active sites and accelerates mass transfer and gas bubble emission; the high conductivity of CoNi–S, which provides faster charge transport and thus promotes the electrocatalytic reaction of the composites; and the effective interface engineering between NiFe₂O₄ (excellent performance for OER) and CoNi–S (high activity for HER), which leads to a shorter transport pathway and thus expedites electron transfer. This work provides a new strategy for designing efficient and inexpensive electrocatalysts for water splitting.     

Controllable transformation of sheet-like CoMo-hydro(oxide) and phosphide arrays on nickel foam as efficient catalysts for alkali water splitting and Zn–H2O cell

Design and synthesis of cost-effective electrocatalysts with remarkable activity and stability is highly desirable for renewable energy devices. Herein, we have successfully constructed sheet-like CoMoO₄–Co(OH)₂ and CoMoP–CoP arrays on nickel foam (NF) through chemical etching ZIF-67 arrays and phosphorization in sequence. Series CoMoO₄–Co(OH)₂/NF as anode and CoMoP–CoP/NF as cathode showed excellent electrocatalytic activity and stability in alkali water splitting, where the combined catalysts only need 1.67 V cell voltage to drive 10 mA cm^?2 and obtain robust high current stability at 500 mA cm^?2 for 110 h with almost no attenuation. In addition, using CoMoP–CoP/NF as the cathode of a Zn–H₂O cell can provide a power density of 11.5 mW cm^?2 and a stable 170 h for simultaneous H₂ and electricity generation. The excellent performance of the system is attributed to the unique sheet-like array morphology of combined catalysts providing large surface area and rich pore structure conducive to electrolyte diffusion and gas emission, as well as the synergies between the different components providing more catalytic active sites.     

Facile synthesis of three-dimensional spherical Ni(OH)2/NiCo2O4 heterojunctions as efficient bifunctional electrocatalysts for water splitting

Synthesis of highly efficient, non-noble and bi-functional electrocatalysts is exceedingly challenging and necessary for water splitting devices. In this work, three-dimensional spherical Ni(OH)₂/NiCo₂O₄ heterojunctions are prepared by a one-step hydrothermal method and the hybrids are explored as efficient electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline electrolyte via tuning different Ni/Co atomic ratios of heterojunctions. The optimized Ni(OH)₂/NiCo₂O₄ (S (1:1)) exhibits high electrocatalytic activity with an ultralow over-potential of 189 mV at 10 mA cm⁻² for the HER. With regard to the OER, the over-potential of the as-synthesized S (1:1) heterojunction is only 224 mV at the current density of 10 mA cm⁻². The improved catalytic performance of the Ni(OH)₂/NiCo₂O₄ heterojunctions is attributed to the chemical synergic combining of Ni(OH)₂ and NiCo₂O₄, large specific surface area for exposing more accessible active sites, and heterointerface for activating the intermediates that facilitates electron/electrolyte transport. The prepared catalyst exhibits good durability and stability in HER and OER catalyzing conditions. This study provides a feasible approach for the building of highly efficient bifunctional water splitting electrocatalysts and stimulates the development of renewable energy conversion and storage devices.     

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.     

Facile fabrication of flower-like CuS/MnCO3 microspheres clusters on nickel foam as an efficient bifunctional catalyst for overall water splitting

Utilizing the abundant elements on earth to product inexpensive, high-active and stable catalysts for water splitting is very significant but still remains serious challenge to produce hydrogen. Herein, heterostructures of CuS/MnCO₃ on nickel foam substrate are firstly successfully synthesized via a facile one-step hydrothermal strategy. The as-prepared electrocatalyst displays an enhanced oxygen evolution reaction (OER) performance in alkaline conditions with a minimum overpotential of 70 mV and a small Tafel slope of 42.5 mV/dec to achieve 10 mA cm^?2. The catalyst also exhibits an excellent HER activity with a low overpotential of 143 mV and the Tafel slope of 51.4 mV/dec to acquire 10 mA cm^?2 in 1.0 M KOH. Moreover, when the CuS/MnCO₃//CuS/MnCO₃ electrode is applied for the overall water splitting, the electrolyzer cell device affords 10 mA cm^?2 at a relative low voltage of 1.43 V, which is one of the best catalysts ever reported. In stability test, its activity first decreases and then remains stable in 1 M KOH solution for about 10 h, indicating that the electrode has good electrochemical stability. Density functional theory calculations (DFT) show that MnCO₃ has a stronger adsorption energy for water than CuS does, indicating that MnCO₃ is a real active center and CuS plays a certain synergistic effect. This work not only provides a low-cost and efficient bifunctional catalyst for water splitting technology, but also extends the application of bifunctional catalyst based on transition metal sulfide and carbonate compound.     

Sulfuration of hierarchical Mn-doped NiCo LDH heterostructures as efficient electrocatalyst for overall water splitting

Constructing efficient bifunctional electrocatalysts for both cathode and anode is of great importance for obtaining green hydrogen by water splitting. Herein, sulfuration of hierarchical Mn-doped NiCo LDH heterostructures (Mn–NiCoS₂/NF) is constructed as a bifunctional electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) via a facile hydrothermal-annealing strategy. Mn–NiCoS₂/NF shows an overpotential of 310 mV at 50 mA cm⁻² for OER and 100 mV at 10 mA cm⁻² for HER in 1.0 M KOH. Moreover, only 1.496 V@10 mA cm⁻² is required for overall water splitting by using Mn–NiCoS₂/NF as catalyst dual electrodes in a two-electrode system. The excellent performance of Mn–NiCoS₂/NF should be attributed to the ameliorative energy barriers of adsorption/desorption for HO⁻/H₂O through the modification of electronic structure of NiCo basal plane by Mn-doping and the acceleration of water dissociation steps via rich delocalized electron inside sulfur vacancies. The construction of hierarchical Mn–NiCoS₂/NF heterostructures provides new prospects and visions into developing efficient-advanced electrocatalysts for overall water splitting.