Electrodeposition NiMoSe ternary nanoshperes on nickel foam as bifunctional electrocatalyst for urea electrolysis and hydrogen evolution reaction

Electrodeposition provides a simple but effective way to prepare advanced electrode for the application in electrochemical field. In this work, NiMoSe ternary nanospheres were deposited on nickel foam (NiMoSe/NF) by one-step electrodeposition. The morphology, phase and chemical composition of the electrode was characterized by using SEM, TEM, XRD and XPS. The electrode exhibited excellent performance for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). It only required 1.39 V and 81 mV (vs. RHE) to deliver a current density of 10 mA/cm² for UOR and HER, respectively. The electrolyzer constructed with NiMoSe/NF as both anode and cathode could deliver a current density of 10 mA/cm² at a driving potential of 1.44 V. The stability test showed that the electrode had good durability as electrode for both UOR and HER. Considering the easiness, simplicity and low cost, the NiMoSe/NF electrode could find wide application in urea electrolysis.     

Porous flower-like nickel nitride as highly efficient bifunctional electrocatalysts for less energy-intensive hydrogen evolution and urea oxidation

Finding a suitable replacement for the high potential of anodic water electrolysis (oxygen evolution reaction (OER)) is significant for hydrogen energy storage and conversion. In this work, a simple and scalable method synthesizes a structurally unique Ni₃N nanoarray on Ni foam, Ni₃N-350/NF, that provides efficient electrocatalysis for the urea oxidation reaction (UOR) that transports 10 mA cm⁻² at a low potential of 1.34 V. In addition, Ni₃N-350/NF exhibits electro-defense electrocatalytic performance for hydrogen evolution reaction, which provides a low overpotential of 128 mV at 10 mA cm⁻². As proof of concept, all-water-urea electrolysis measurement is carried out in 1 M KOH with 0.5 M Urea with Ni₃N-350/NF as cathode and anode respectively. Ni₃N-350/NF||Ni₃N-350/NF electrode can provide 100 mA cm⁻² at a voltage of only 1.51 V, 160 mV less than that of water electrolysis, which proves its commercial viability in energy-saving hydrogen production.     

Interlaced rosette-like MoS2/Ni3S2/NiFe-LDH grown on nickel foam: A bifunctional electrocatalyst for hydrogen production by urea-assisted electrolysis

In targeting the most important energy and environmental issues in current society, the development of low-cost, bifunctional electrocatalysts for urea-assisted electrocatalytic hydrogen (H₂) production is an urgent and challenging task. In this work, interlaced rosette-like MoS₂/Ni₃S₂/NiFe-layered double hydroxide/nickel foam (LDH/NF) is successfully synthesized by a two-step hydrothermal reaction. Due to its unique interlaced heterostructure, MoS₂/Ni₃S₂/NiFe-LDH/NF exhibits excellent bifunctional catalytic activity towards the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER) in 1.0 M KOH with 0.5 M urea. In a concurrent two-electrode electrolyser (MoS₂/Ni₃S₂/NiFe-LDH/NF(+,-)), only voltage of 1.343 V is required to reach 50 mA cm⁻², which is 216 mV lower than for pure water splitting. Furthermore, after 16 h of urea electrolysis in 1.0 M KOH with 0.5 M urea, the current density remains at 98% of the original value. Thus, the catalyst is not only favorable for H₂ production, but also has great significance for the problem of urea-rich wastewater treatment.     

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.     

Fe7Se8@Fe2O3 heterostructure nanosheets as bifunctional electrocatalyst for urea electrolysis

Water electrolysis for producing hydrogen is considered to be the most feasible means to develop new green energy. Compared with above, urea electrolysis can improve energy conversion efficiency by introducing urea, and can also be used for purification of wastewater rich in urea. In this paper, a bifunctional electrocatalyst with heterostructure, namely Fe₇Se₈@Fe₂O₃ nanosheets supported on nickel foam, were synthesized for the first time through typical hydrothermal and partial oxidation processes. Iron cation promotes electron transfer and adjusts electron structure under the synergistic action of selenium and oxygen anion, thus achieving excellent catalytic activity of urea electrolysis. In an alkaline solution of 1 M KOH with 0.5 M urea, the Fe₇Se₈@Fe₂O₃/NF catalyst can drive the current density of 10 mA cm^?2 with requiring only potential of 1.313 V and overpotential of 141 mV for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), respectively. What is noteworthy is that Fe₇Se₈@Fe₂O₃/NF heterostructure is used as bifunctional electrocatalyst to form urea electrolyzer device, which only needs potential of 1.55 V to drive current density of 10 mA cm^?2, which is one of the best catalytic activities reported so far, and the electrode couple showed remarkable stability for 15 h. Density functional theory shows that the Fe₇Se₈@Fe₂O₃/NF material exhibits the minimum Gibbs free energy for the adsorption of hydrogen. This work provides a new method for exploring novel and environmentally friendly bifunctional electrocatalysts for urea electrolysis.     

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-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

Ultrathin-layered MoS2 hollow nanospheres decorating Ni3S2 nanowires as high effective self-supporting electrode for hydrogen evolution reaction

High-activity and cost-effective transition metal sulfides (TMSs) have attracted tremendous attention as promising catalysts for hydrogen evolution reaction (HER). However, a significant challenge is the simultaneous construction of abundant electrochemical active sites and the fast electronic transmission path to boost a high-efficient HER. Herein, we demonstrate a facile one-step hydrothermal preparation of MoS₂ hollow nanospheres decorating Ni₃S₂ nanowires supported on Ni foam (NF), without any other additional template, surfactant or annealing. In this three-dimensional (3D) heterostructure, the ultrathin-layered MoS₂ hollow nanospheres contribute to the promotion of the total surface area and the electrochemical active sites. Meanwhile, the Ni₃S₂ nanowires are beneficial to the high electrical conductivity. Consequently, the optimized MoS₂/Ni₃S₂/NF-200-24 electrocatalyst presents an extremely superior HER activity to that of individual MoS₂/NF and Ni₃S₂/NF. The HER overpotentials are 85 mV at 10 mA cm⁻² and 189 mV at 100 mA cm⁻², which are also comparable with the state-of-the-art 20% Pt/C/NF electrode at both low and high current.     

Controlled synthesis of CrxPy-a/ComPn-b nanoarray on nickel foam as efficient electrocatalyst for overall urea splitting

Developing highly efficient bifunctional urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) catalysts for urea splitting to hydrogen are one of the strategies to cope with the energy crisis. Here, a series of Cr_xP_y-a/Co_mP_n-b composites were synthesized on Ni foam through hydrothermal and low-temperature phosphorization process for the first time. It is worth noting that Cr_xP_y-1/Co_mP_n-3@NF exhibited excellent UOR performance (1.331 V at 100 mA cm^?2) and HER performance (0.299 V at 100 mA cm^?2) in an electrolyte of 1 M KOH and 0.5 M urea due to the synergistic effect of Cr–Co. The Cr_xP_y-1/Co_mP_n-3@NF||Cr_xP_y-1/Co_mP_n-3@NF two-electrode system call for only 1.52 V to provide current density of 10 mA cm^?2, which is one of the best electrochemistry performances reported up to now. Experimental analysis show that the promoted electrochemistry performances is assigned to faster charge transfer rate, the exposure of more reaction site and better properties of metals. Density Functional theory (DFT) results demonstrate that the presence of the Co_mP_n material accelerates the kinetics of hydrogen production and the Cr_xP_y material improves the properties of metals for the electrode. The work provides a new idea to develop the environmentally friendly and low cost overall urea splitting catalyst with transition metals instead of noble metals.     

One-step synthesis of Ni3S2 nanowires at low temperature as efficient electrocatalyst for hydrogen evolution reaction

Ni₃S₂ is an emerging cost-effective catalyst for hydrogen generation. However, a large amount of reported Ni₃S₂ was synthesized via multi-step approaches and few were fabricated based on the one-step strategies. Herein, we report a facile one-step low-temperature synthesis of Ni₃S₂ nanowires (NWs). In this strategy, a resin containing sulfur element is recommended as a sulfur resource to form Ni₃S₂ NWs. It presents a plausible explanation on the vapor–solid–solid (VSS) growth mechanism according to the results of this experiment and reported in literature that has been published. The Ni₃S₂ NW exhibits a potential ∼199 mV at 10 mA cm⁻² and the long-term durability over 30 h at 20 mA cm⁻² HER operation, better than other reported Ni₃S₂. More importantly, according to replace transition metal foam as the initial metal, other transition metal sulfide can be readily synthesized via this original approach.     

Facile synthesis of MoS2/CuS nanoflakes as high performance electrocatalysts for hydrogen evolution reaction

The fabrication of metal sulfides heterostructure is a promising strategy for enhancing catalytic activity. Herein, the MoS₂/CuS heterostructure was successfully grown on carbon cloth (MoS₂/CuS/CC) through an efficient method. The SEM results confirmed that the fabricated MoS₂/CuS/CC composites have a flake morphology, which can not only improves the surface area but also offers ample surface catalytic active sites. Particularly, the optimized MoS₂/CuS/CC-2 electrocatalyst showed a small overpotential of 85 mV@10 mA cm^?2 and exceptional long-term cycling durability for hydrogen evolution in 1 M KOH. The outstanding catalytic activity is attributed to the fact that the combination of MoS₂ with CuS can greatly enhance the charge transport rate and improve the structural stability. These results suggest that the MoS₂/CuS/CC heterostructure is a potential electrocatalyst for hydrogen production.     

Growth of Ni/Mo/Cu on carbon fiber paper: An efficient electrocatalyst for hydrogen evolution reaction

The exploration of efficient catalysts toward hydrogen evolution reaction (HER) is still an urgent task. In this paper, Ni/Mo/Cu/C and Ni/Mo/C electrode were obtained by conventional pulse voltammetry, which acted as cathode in microbial electrolysis cells (MECs). The prepared samples are analyzed using SEM, XRD, XPS and electrochemical analysis techniques. Results indicated that the Ni/Mo/Cu coating has a rough and globular structure and presents high current density, a lower Tafel slope of 23.9 mV/dec than 30 mV/dec of Pt, which exceeds the electrochemical activity of Pt electrode. Its remarkably enhanced electrocatalytic activity is attributed to the high surface area, high conductivity as well as synergistic interaction among Ni, Mo and Cu.     

Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer

Co₃S₄ nanosheets on Ni foam (NS/NF) were prepared by sulfurization for various time after calcination of electrodeposited Co(OH)₂. In our FE-SEM images, we observed that Co₃S₄ NS was vertically, or obliquely, deposited on the Ni foam. As a result, the structure contained more active sites, and active sites were highly accessible to the electrolyte for the hydrogen evolution reaction (HER). Furthermore, results of XPS and XRD analysis confirmed S-conversion from Co₃O₄ to Co₃S₄ during sulfurization. 3-Co₃S₄ NS/NF with sulfurization for 3 h exhibited the highest sulfur content, while Co₃S₄ began to desulfurize to Co₉S₈ after sulfurization for 4 h. The 3-Co₃S₄ NS/NF electrocatalyst showed a lowest overpotential of 93 mV at −10 mA/cm², with a Tafel slope of −55.1 mV/dec in N₂-purged 1 M KOH. Also, the single cell anion exchange membrane water electrolyzer (AEMWE) showed a high current density of 431 mA/cm² with cell voltage 2.0 V_cell at 40–45 °C.     

Fabrication of P-doped Co9S8/g-C3N4 heterojunction for excellent photocatalytic hydrogen evolution

Developing lower-cost and higher-efficient photocatalysts is still a major challenge for the solar to hydrogen energy conversion by photocatalytic water splitting. Herein, P-doped Co₉S₈ (P–Co₉S₈) was synthesized by a hydrothermal process using low-cost RP as raw material, and then P–Co₉S₈ was employed to construct heterojunction with g-C₃N₄ via a mechanical-mixing method. Investigation shows that P–Co₉S₈ can not only improve the electrical conductivity and surface area of the composite, but also can lower the over-potential of H₂ evolution, leading to an enhanced H₂ evolution kinetics. The H₂ evolution rate of resultant 25% P–Co₉S₈/g-C₃N₄ reached 4362 μmol g⁻¹ h⁻¹ under UV and visible light, being nearly 121.2 times higher than that of g-C₃N₄. The charge transfer between P–Co₉S₈ and g-C₃N₄ follows the Type-I route based on the photoelectrochemical analysis, leading to more electrons on the conduction band of P–Co₉S₈ to participate the H₂ evolution processes. This work provides a new way for preparation of P-doped sulfides with potential applications in the field of photocatalysis.     

Synthesis of Co2FeAl alloys as highly efficient electrocatalysts for alkaline hydrogen evolution reaction

The development of cost-effective non-precious metal electrocatalysts is a major challenge for water splitting applications, but it is important for the realization of renewable energy systems. Alloying has proved an effective way to design metal-based electrocatalysts, and by controlling the annealing temperature, the surface morphology and crystallinity of the alloy can be tuned to control the hydrogen evolution reaction (HER) performance. In this work, with a simple coprecipitation method, we have prepared Co₂FeAl alloys at different annealing temperatures (550 °C–670 °C), which exhibit excellent crystallinity and electrocatalytic performance for HER in alkaline solution. Among all conditions, the Co₂FeAl alloys prepared at 620 °C shows the better crystallinity and the higher purity, and it could achieve a low overpotential of 149 mV at 10 mA cm^?2 in alkaline solution. The overpotential demonstrates persistent stability with only 3 mV change after over 1000 cycles. Both density functional theory (DFT) calculations and experimental results revealed that alloying optimizes the electronic structure near the Fermi surface of the system, improving the electron transport efficiency and enhancing the catalytic activity. These Co₂FeAl alloys are appealing candidates for high-performance alkaline HER electrocatalytic electrodes in water electrolysis due to their outstanding electrocatalytic properties.     

Novel peapod-like Ni2P@N-doped carbon nanorods array on carbon fiber for efficient electrocatalytic urea oxidation and hydrogen evolution reaction

Designing and optimizing structure is an effective method to enhance electrocatalytic performance of transition metal-based catalysts. In this work, an innovative nanostructured electrode, consisted of peapod-like Ni₂P@N-doped carbon nanorods array coating on carbon fiber (CF@p-Ni₂P@NC), is devised and synthesized. The N-doped carbon layer is crucial for maintaining the peapod-like nanostructure, which allows for multi-channel electrolyte transport and gas product release. And the carbon layer coating Ni₂P nanoparticles also enhance electrical conductivity and stability, thus ensuring fast electron transport from/to active sites and the long-term stability of catalyst during urea oxidation reaction (UOR)/hydrogen evolution reaction (HER). Benefit from the reasonable structure, CF@p-Ni₂P@NC present perfect performance with getting 100 mA cm^?2 at potential/overpotential of 1.417/0.194 V for UOR/HER in 1.0 M KOH containing 0.5 M urea. In addition, the overall urea-electrolysis system using CF@p-Ni₂P@NC bifunctional electrode only requires 1.590 V to obtain 100 mA cm^?2.     

Nano P zeolite modified with Au/Cu bimetallic nanoparticles for enhanced hydrogen evolution reaction

In the present study, the excellent catalytic performance of Au/Cu bimetallic nanoparticles based on nano P zeolite modified carbon paste electrode (Au/Cu-NPZ-CPE) as one of the most promising electrocatalyst toward hydrogen production is introduced. Herein, nano P zeolite is synthesized by using agriculture residues, stem sweep ash with purity approximately 80.205 wt% of SiO₂ which provides attractive economically silica source for the preparation of inexpensive zeolite. For the preparation of Au/Cu-NPZ-CPE, ion exchange protocol followed by galvanic replacement reaction was employed to result Au/Cu embedded zeolite framework. By evaluating the electrocatalytic activity of proposed catalyst with linear sweep voltammetry and Tafel polarization, a low overpotential of 100 mV and high exchange current density (2.51 mA cm⁻²) are demonstrated which compares favorably to most previously reported electrocatalysts for hydrogen evolution reaction. Owing to the inherent porosity of synthesized nano P zeolite, it successfully prevents the aggregation of bimetallic nanoparticles which promotes the hydrogen evolution reaction. Particularly, low Tafel slope for offered catalyst (33 mV dec⁻¹) demonstrates the acceleration of hydrogen evolution reaction kinetics owing to the increase in the number of accessible active sites. Tafel slope of Au/Cu-NPZ-CPE is 3, 5, 6, 6.5 and 7 times lower than that for Au-NPZ-CPE, Cu-NPZ-CPE, Au/Cu-CPE, NPZ-CPE and CPE, respectively, which shows the best electrocatalytic activity among other modified carbon paste electrodes. Furthermore, the corresponding long term stability test by chronoamperometry method indicates that the current density reaches to nearly 91% of its primary value (after 5500 s) which provides the favorable practical demands of the catalyst in hydrogen production.     

Hierarchical construction of hollow NiCo2S4 nanotube@NiCo2S4 nanosheet arrays on Ni foam as an efficient and durable electrocatalyst for hydrogen evolution reaction

Ternary transition metal chalcogenide (TTMC) with multicomponent, different phases and unique electronic structures have been studied in electrocatalytic hydrogen evolution reaction (HER). However, the strong interaction between adsorbed H (H1) and sulfur leads to the unfavorable hydrogen desorption properties of considerable TTMC. Herein, we constructed the hierarchical hollow NiCo₂S₄ nanotube@NiCo₂S₄ nanosheet arrays on Ni foam substrate (NT-NiCo₂S₄@NS-NiCo₂S₄/NF) by ion-exchange method. Homogeneous anion diffusion facilitates the formation of regular ultrathin nanosheets on hollow NiCo₂S₄ nanotube arrays, which presents hierarchical architecture with more surface area and channels to active site exposure, electrolyte diffusion, and gas desorption for HER. As-synthesized optimal NT-NiCo₂S₄@NS-NiCo₂S₄/NF electrode demonstrates an excellent HER activity, especially an overpotential of 221 mV, a Tafel slope of 108 mV dec^?1, and remarkable stability at current densities of 100 mA cm^?2 in 1.0 M NaOH electrolyte.     

Silicon monoxide assisted synthesis of Ru modified carbon nanocomposites as high mass activity electrocatalysts for hydrogen evolution

Extremely low content of Ruthenium (Ru) nanoparticles were loaded on the carbon black (Ru/C) via reducing Ru ions with silicon monoxide. The obtained Ru/C nanocomposites exhibit an exciting electrochemical catalytic activity for hydrogen evolution reaction (HER) in the oxygen-free 0.5 M H₂SO₄ medium. The optical one (Ru/C-2) with a low Ru amount of 2.34% shows higher activity than previously reported Ru-based catalysts. The overpotential at 10 mA cm⁻² is 114 mV and the Tafel slope is 67 mV·dec⁻¹. Ru/C-2 catalyst also has good stability. The overpotential that afford the current density of 10 mA cm⁻² of 20 wt% Pt/C increased 92 mV while that of Ru/C-2 only increased 50 mV after a 30,000 s chronopotentiometry test. Furthermore, the mass activity of Ru/C-2 catalyst is even better than that of the commercial 20 wt% Pt/C when the overpotential is larger than 0.18 V. This silicon monoxide-mediated strategy may open a new way for the fabrication of high performance electrocatalysts.     

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.