3D ternary NixCo2-xP/C nanoflower/nanourchin arrays grown on HCNs: a highly efficient bi-functional electrocatalyst for boosting hydrogen production via the urea electro-oxidation reaction

Synergistic modulation of the d-band center in Ni3S2 by selenium and iron for enhanced oxygen evolution reaction (OER) and urea oxidation reaction (UOR)

Efficient production of green hydrogen energy is crucial in addressing the energy crisis and environmental concerns. The oxygen evolution reaction (OER) poses a challenge in conventional overall water electrolysis due to its slow thermodynamically process. Urea oxidation reaction (UOR) offers an alternative anodic oxidation method that is highly efficient and cost-effective, with favorable thermodynamics and sustainability. Recently, there has been limited research on bifunctional catalysts that exhibit excellent activity for both OER and UOR reactions. In this study, we developed a selenium and iron co-doped nickel sulfide (SeFe-Ni3S2) catalyst that demonstrated excellent Tafel slopes of 53.9 mV dec-1 and 16.4 mV dec-1 for OER and UOR, respectively. Density Functional Theory (DFT) calculations revealed that the introduction of metal (iron) and nonmetallic elements (selenium) was found to coordinate the d-band center, resulting in improved adsorption/desorption energies of the catalysts and reduced the overpotentials and limiting potentials for OER and UOR, respectively. This activity enhancement can be attributed to the altered electronic coordination structure after the introduction of selenium (Se) and iron (Fe), leading to an increase in the intrinsic activity of the catalyst. This work offers a new strategy for bifunctional catalysts for OER and UOR, presenting new possibilities for the future development of hydrogen production and novel energy conversion technologies. It contributes towards the urgent search for technologies that efficiently produce green hydrogen energy, providing potential solutions to mitigate the energy crisis and protect the environment.

Engineering advanced noble-metal-free electrocatalysts for energy-saving hydrogen production from alkaline water via urea electrolysis

When the anodic oxygen evolution reaction (OER) of water splitting is replaced by the urea oxidation reaction (UOR), the electrolyzer can fulfill hydrogen generation in an energy-economic manner for urea electrolysis as well as sewage purification. However, owing to the sluggish kinetics from a six-electron process for UOR, it is in great demand to design and fabricate high-performance and affordable electrocatalysts. Over the past years, numerous non-precious materials (especially nickel-involved samples) have offered huge potential as catalysts for urea electrolysis under alkaline conditions, even in comparison with frequently used noble-metal ones. In this review, recent efforts and progress in these high-efficiency noble-metal-free electrocatalysts are comprehensively summarized. The fundamentals and principles of UOR are first described, followed by highlighting UOR mechanism progress, and then some discussion about density functional theory (DFT) calculations and operando investigations is given to disclose the real reaction mechanism. Afterward, aiming to improve or optimize UOR electrocatalytic properties, various noble-metal-free catalytic materials are introduced in detail and classified into different classes, highlighting the underlying activity-structure relationships. Furthermore, new design trends are also discussed, including targetedly designing nanostructured materials, manipulating anodic products, combining theory and in situ experiments, and constructing bifunctional catalysts. Ultimately, we point out the outlook and explore the possible future opportunities by analyzing the remaining challenges in this booming field.

M-Ni-Co MOF (M=Zn, Fe, Mn) for high-performance supercapacitors by adjusting its morphology

Metal-organic frameworks (MOF) have been wildly synthesised and studied as electrode materials for supercapacitors, and bimetallic MOF of Ni and Co has been broadly studied to enhance both specific capacitance and stability of supercapacitors. Herein, a best performance (about 320 F/g) of Ni-Co bimetallic MOF was found in a uniform preparation condition by adjusting the ratio of Ni to Co. Then tiny third metal ion was introduced, and we found that the morphology of material has a significant change on the original basis. Furthermore, certain ions (Zn, Fe, Mn) introduced make a huge improvement in capacitance based on Ni-Co MOF of 320 F/g. The result shows that Zn-Ni-Co MOF, Fe-Ni-Co MOF and Mn-Ni-Co MOF perform specific capacitance of 1135 F/g, 870 F/g and 760F/g at 1 A/g, respectively. Meanwhile, the asymmetric supercapacitor (ASC) was constructed by Zn-Ni-Co MOF as positive electrode and active carbon (AC) as negative electrode. The Zn-Ni-Co MOF//AC ASC possesses a energy density of 58 Wh/kg at a power density of 775 W/kg. This research provides a new methods to regulate the morphology of MOF and a novel viewpoint for assembling high-performance, low-price, and eco-friendly green energy storage devices.

Strongly Coupled Heterostructured CoP/MoO2 as an Advanced Electrocatalyst for Urea-Assisted Water Electrolysis

Developing low-cost electrocatalysts with excellent activity and durability in urea-assisted water splitting is urgently needed in order to achieve sustainable hydrogen production. Herein, we in situ synthesized a robust coupled heterostructured electrocatalyst (CoP/MoO2) on a nickel foam (NF) substrate and explored its electrocatalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and urea oxidation reaction (UOR). The overpotential of CoP/MoO2/NF is found to be only 11 mV at 10 mA cm-2 during the HER process, which is significantly lower than that of commercial Pt/C. Meanwhile, the UOR catalytic performance of CoP/MoO2/NF indicates fast reaction kinetics, along with a considerable low driving potential (1.26 V) compared to that of the OER (1.51 V). In situ and ex situ techniques demonstrate that these excellent electrocatalytic properties are mainly ascribed to the effective synergistic coupled effect and strong electronic interactions between single-component CoP and MoO2, which can tune electronic states of Co and Mo, expose more active sites, enhance intrinsic catalytic activity, and accelerate charge transfer. Moreover, when used in electrochemical overall water splitting and urea-assisted water electrolysis, CoP/MoO2/NF can reach a current density of 10 mA cm-2 at only 1.46 and 1.32 V. This outperforms Pt/C||RuO2 and numerous nonprecious metal electrocatalysts and maintains a stable long-term electrolytic operation for 84 h. This work provides a promising pathway for the development of efficient catalysts during urea-assisted water electrolysis for hydrogen production.

Regulating the electronic structure of Ni2P by one-step Co, N dual-doping for boosting electrocatalytic performance toward oxygen evolution reaction and urea oxidation reaction

The development of efficient bifunctional electrocatalysts for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) is critical for hydrogen production and wastewater purification. In this work, we propose a facile synthetic method for Co and N dual-doped Ni2P directly grown on Ni foam (Co-Ni2P-N/NF) using hydrothermal and annealing process. Simultaneous Co and N dual-doping into Ni2P not only modifies the surface electronic structure, but also generates a multitude of active sites with high valence states, which are beneficial for improving electrocatalytic kinetics for both OER and UOR. As a result, the Co-Ni2P-N/NF catalyst exhibits a low overpotential of 329 mV to deliver a current density of 100 mA cm-2 for OER in alkaline solution, which is much lower than that of the state-of-the-art RuO2 electrocatalyst. In addition, the urea-assisted water oxidation process exhibits a significant reduction of approximately 163 mV in the required potential at 100 mA cm-2 compared to that of the OER, which highlights the remarkable potential of the prepared Co-Ni2P-N/NF electrocatalyst in facilitating the purification of wastewater and hydrogen production with significantly lower energy consumption.

Biosynthesis of ternary NiCoFe2O4 nanoflowers: investigating their 3D structure and potential use in gene delivery

Multicomponent nanoparticle systems are known for their varied properties and functions, and have shown potential as gene nanocarriers. This study aims to synthesize and characterize ternary nickel-cobalt-ferrite (NiCoFe2O4) nanoparticles with the potential to serve as gene nanocarriers for cancer/gene therapy. The biogenic nanocarriers were prepared using a simple and eco-friendly method following green chemistry principles. The physicochemical properties of the nanoparticles were analyzed by X-ray diffraction, vibrating sample magnetometer, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller. To evaluate the morphology of the nanoparticles, the field emission scanning electron microscopy with energy dispersive X-Ray spectroscopy, high-resolution transmission electron microscopy imaging, and electron tomography were conducted. Results indicate the nanoparticles have a nanoflower morphology with a mesoporous nature and a cubic spinel structure, where the rod and spherical nanoparticles became rose-like with a specific orientation. These nanoparticles were found to have minimal toxicity in human embryonic kidney 293 (HEK-293 T) cells at concentrations of 1 to 250 µg·mL-1. We also demonstrated that the nanoparticles could be used as gene nanocarriers for delivering genes to HEK-293 T cells using an external magnetic field, with optimal transfection efficiency achieved at an N/P ratio of 2.5. The study suggests that biogenic multicomponent nanocarriers show potential for safe and efficient gene delivery in cancer/gene therapy.

P-Induced Permeation of Nickel into WO3 Octahedra to Form a Synergistic Catalyst for Urea Oxidation

Small-molecule induction can lead to the oriented migration of metal elements, which affords functional materials with synergistic components. In this study, phosphating nickel foam (NF)-supported octahedral WO₃ with phosphine affords P-WO₃ /NF electrocatalyst. Ni is found to form Ni-P bonds that migrate from NF to WO₃ under the induction of P, resulting in the complex oxides W_1.3 Ni_0.24 O₄ and Ni₂ P₂ O₇ in the particle interior and nickel phosphide on the octahedral grain surface. The catalytic activity of P-WO₃ /NF in the urea oxidation reaction (UOR) is improved by synergistic action of the components in the synthesized hybrid particles. A current density of 10 mA cm^-2 can be reached at a potential of 1.305 V, the double layer capacitance of the catalyst is significantly increased, and the electron transfer impedance in catalytic UOR is reduced. This work demonstrates that small-molecule induction is suitable for constructing co-catalysts with complex components in a simple protocol, which provides a new route for the design of highly efficient urea oxidation electrocatalysts.

Hollow NiCoP Nanoprisms Derived from Prussian Blue Analogues as Bifunctional Electrocatalysts for Urea-Assisted Hydrogen Production in Alkaline Media

Integrating the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) is an energy-saving approach for electrolytic H₂ production. Here, hollow NiCoP nanoprisms are derived from Prussian blue analogues by a combined self-template coordination reaction and gas-phase phosphorization strategy. Benefiting from the strong electron interaction, unique hollow nanostructure, and enhanced mass/charge transfer, NiCoP nanoprisms display outstanding alkaline HER and UOR performance. Specifically, low potentials of -0.052, -0.115, and -0.159 V for HER and ultralow potentials of 1.30, 1.36, and 1.42 V for UOR at current densities of 10, 50, and 100 mA cm^-2 are obtained. Moreover, in a urea-assisted water electrolysis system, NiCoP nanoprisms only require cell voltages of 1.36, 1.49, and 1.57 V to offer current densities of 10, 50, and 100 mA cm^-2 , about 170, 180, and 200 mV less than the traditional water electrolysis. Theoretical calculations indicate the Co substitution in Ni₂ P promotes the adsorption and dissociation of water molecules, optimizes the desorption energy of active hydrogen atoms, and enhances the adsorption of urea molecules, thus accelerating the kinetics of HER and UOR. This work facilitates the application of hollow bimetallic phosphides in electrochemical preparation of clean energy and provides a successful paradigm for urea-rich wastewater electrolysis.

Three-dimensional Ni4O4-cubane metal-organic framework as a high-performance electrocatalyst for urea oxidation

The urea oxidation reaction (UOR) is considered to be a replacement of the sluggish anodic oxygen evolution reaction (OER) in overall water-splitting. A three-dimensional (3D) nickel-containing metal-organic framework {[NiII 2(pdaa)(OH)2(H2O)] n (MOF 1) (where, H2pdaa = 1,4-phenylene diacetic acid) was investigated as a robust and highly efficient electrocatalyst for the UOR. MOF 1 comprised 1D nickel(ii) chains crosslinked through Ni4O4 cubane units to form a 3D extended network. Dangling Ni⋯OH- groups were exposed in the MOF 1 structure, and could act as active catalytic centers for the UOR. MOF 1 required a very small onset potential of 1.18 V for urea oxidation in KOH (1 M) and urea (0.33 M) and had a low Tafel slope of 38.8 mV dec-1 (in contrast to 1.84 V for the oxygen evolution reaction). The overpotential required to attain a catalytic current density of 10 mA cm-2 was 1.24 V, which is much lower than that for many materials. Controlled potential electrolysis, powder X-ray diffraction, and X-ray photoelectron spectroscopy affirmed the physicochemical integrity of the catalyst over a 17 h test reaction. This work not only addresses the problem of urea contamination, it also helps to utilize it in an energy-conversion process.

Heterostructured Co2P Nanocomposite Embedded in a N, P Co-Doped Carbon Layer as a High Performance Electrocatalyst for Overall Water Splitting

Hydrogen is the mainstream future energy source because of its high energy density and environmentally-friendly properties. In this study, Fe-Co2P/NPC materials were prepared by the wet chemical synthesis method, in which Fe-Co2P nanowires were wrapped by the N, P co-doped carbon layers (NPC) under aging and phosphorylation strategies. When Fe-Co2P/NPC/NF was subjected to hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the overpotential was only 73 mV and 217 mV to reach the current density of 10 mA cm−2, respectively. When the cathode and anode were both Fe-Co2P/NPC/NF, a current density of 10 mA cm−2 was achieved with only 1.56 V. This work provides a new idea for the design and preparation of non-precious metal-based transition metal phosphide catalysts.

Ruthenium/Ruthenium oxide hybrid nanoparticles anchored on hollow spherical Copper-Cobalt Nitride/Nitrogen doped carbon nanostructures to promote alkaline water splitting: Boosting catalytic performance via synergy between morphology engineering, electron transfer tuning and electronic behavior modulation

Exploring bi-functional electrocatalysts with excellent activity, good durability, and cost-effectiveness for electrochemical hydrogen and oxygen evolution reactions (HER and OER) in the same electrolyte is a critical step towards a sustainable hydrogen economy. Three main features such as high density of active sites, improved charge transfer, and optimized electronic configuration have positive effects on the electrocatalyst activity. In this context, understanding structure-composition-property relationships and catalyst activity is very important and highly desirable. Herein, for the first time, we present the design and fabrication of novel MOF-derived ultra-small Ru/RuO₂ nanoparticles doped in copper/cobalt nitride (CuCoN) encapsulated in nitrogen-doped nanoporous carbon framework (NC) (Ru/RuO₂/CuCoN@NC). For the synthesize of this nanocomposite, firstly bimetallic Cu-Co/MOF hollow nanospheres are prepared via a facile emulsion-based interfacial reaction method and used as the template for Ru ion doping (Ru-doped Cu-Co/MOF). Then, Ru-doped Cu-Co/MOF precursor during the carbonization/nitridation/cooling process converted to the Ru/RuO₂/CuCoN@NC nanocomposite. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, Ru/RuO₂/CuCoN@NC hollow nanosphere electrocatalyst demonstrates striking catalytic performances under alkaline conditions with a current density of 10 mA cm^-2at low overpotentials of 41 mV for HER and 231 mV for OER, respectively. Moreover, as a bifunctional electrocatalyst for overall water splitting, a two-electrode device needs a voltage of 1.51 V to reach a current density of 10 mA cm^-2. Comprehensive electrochemical studies show that the excellent electrocatalytic performance of the Ru/RuO₂/CuCoN@NC hollow nanosphere could be attributed to the improved physical and chemical properties such as desirable compositional, catalysts uniform dispersion, structural advantages of more exposed active sites, optimized electronic structure, high electrical conductivity, and interfacial synergy effect. This work paves a novel avenue for constructing robust bifunctional electrocatalyst for overall water splitting.

Advanced Nickel-Based Catalysts for Urea Oxidation Reaction: Challenges and Developments

The electrochemical urea oxidation reaction (UOR) is crucial for determining industrial and commercial applications of urea-based energy conversion devices. However, the performance of UOR is limited by the dynamic complex of the six-electron transfer process. To this end, it is essential to develop efficient UOR catalysts. Nickel-based materials have been extensively investigated owing to their high activity, easy modification, stable properties, and cheap and abundant reserves. Various material designs and strategies have been investigated in producing highly efficient UOR catalysts including alloying, doping, heterostructure construction, defect engineering, micro functionalization, conductivity modulation, etc. It is essential to promptly review the progress in this field to significantly inspire subsequent studies. In this review, we summarized a comprehensive investigation of the mechanisms of oxidation or poisoning and UOR processes on nickel-based catalysts as well as different approaches to prepare highly active catalysts. Moreover, challenges and prospects for future developments associated with issues of UOR in urea-based energy conversion applications were also discussed.

Dual-electrocatalysis behavior of star-like zinc-cobalt-sulfide decorated with cobalt-molybdenum-phosphide in hydrogen and oxygen evolution reactions

Although important advances have been acquired in the field of electrocatalysis, the design and fabrication of highly efficient and stable non-noble earth-abundant metal catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remain a significant challenge. In this study, we have designed a superior bifunctional catalyst for OER and HER in alkaline media based on the Co-Mo-P/Zn-Co-S multicomponent heterostructure. The as-prepared multicomponent heterostructure was successfully obtained via a simple three-step hydrothermal-sulfidation-electrodeposition process consisting of star-like Co-Zn-S covered with Co-Mo-P. The structure and morphology evaluation of the prepared catalysts were performed via Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and elemental mapping techniques. The electrochemical tests show that Co-Mo-P/Co-Zn-S exhibits outstanding activity toward both OER and HER with OER overpotentials of 273 mV and 312 mV to drive the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 41 mV dec^-1. In addition, the HER overpotentials of 120 mV and 165 mV were required to reach the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 61.7 mV dec^-1 that outperforms most other state-of-the-art catalysts. In the case of HER, the prepared catalyst required an overpotential of 202 mV to reach the current density of 200 mA cm^-2 that was much lower than the overpotential of Pt/C (286 mV) to achieve the same current density. Co-Mo-P/Co-Zn-S also exhibits a suitable stability length of 10 h for OER and HER during the chronoamperometric tests. The superior performance of the Co-Mo-P/Co-Zn-S multicomponent heterostructure toward OER and HER may be related to the large specific surface area, accelerated mass and electron transport, and synergistic effect of multiple hybrid materials. These merits suggest that Co-Mo-P/Co-Zn-S can be considered as a promising catalyst for bi-functional OER and HER, and can be offered a great promise for future applications.

An Mn-doped NiCoP flower-like structure as a highly efficient electrocatalyst for hydrogen evolution reaction in acidic and alkaline solutions with long duration

The exploration of efficient non-noble metal electrocatalysts for hydrogen evolution reaction has received considerable attention to replace commercial Pt catalyst. It is known that the cooperative coupling of appropriate non-noble metals exhibits excellent HER performance than a single component. Herein, an Mn-doped NiCoP flower-like electrocatalyst with self-assembled nanosheets on a nickel foam is synthesized via successive hydrothermal methods, followed by low temperature phosphidation. The as-synthesized Mn-NiCoP presents extraordinarily high catalytic activity and robust chemical stability towards the hydrogen evolution reaction in both acidic and alkaline electrolytes. Benefiting from the dual modulation of the morphology structure and chemical compositions, Mn-NiCoP/NF achieves a current density of 10 mA cm^-2 at a low overpotential of 37 mV for HER in a 0.5 M H₂SO₄ solution. Moreover, it only requires overpotentials of 67 mV and 142 mV to deliver current densities of 10 mA cm^-2 and 50 mA cm^-2 in a 1 M KOH solution, respectively. Remarkably, it holds enhanced stability in 1 M KOH, maintaining HER activity for at least 120 h with negligible overpotential decay. The highly efficient and stable Mn-NiCoP electrocatalyst is valuable in applications relevant to energy storage.

Hierarchical CoFe LDH/MOF nanorods array with strong coupling effect grown on carbon cloth enables efficient oxidation of water and urea

Oxygen evolution reaction (OER) and urea oxidation reaction (UOR) play important roles in the fields of hydrogen energy production and pollution treatment. Herein, a facile one-step chemical etching strategy is provided for fabricating one-dimensional hierarchical nanorods array composed of CoFe layered double hydroxide (LDH)/metal-organic frameworks (MOFs) supported on carbon cloth as efficient and stable OER and UOR catalysts. By precisely controlling the etching rate, the ligands from Co-MOFs are partially removed, the corresponding metal centers then coordinate with hydroxyl ions to generate ultrathin amorphous CoFe LDH nanosheets. The resultant CoFe LDH/MOFs catalyst possesses large active surface area, enhanced conductivity and extended electron/mass transfer channels, which are beneficial for catalytic reactions. Additionally, the intimate contact between CoFe LDH and MOFs modulates the local electronic structure of the catalytic active site, leading to enhanced adsorption of oxygen-containing intermediates to facilitate fast electrocatalytic reaction. As a result, the optimized CoFe LDH/MOF-0.06 exhibits superior OER activity with a low overpotential of 276 at a current density of 10 mA cm^-2with long-term durability. Additionally, it merely requires a voltage of 1.45 V to obtain 10 mA cm^-2in 1 M KOH solution with 0.33 urea and is 56 mV lower than the one in pure KOH. The work presented here may hew out a brand-new route to construct multi-functional electrocatalysts for water splitting, CO₂reduction, nitrogen reduction reactions and so on.

Cu/Cux S-Embedded N,S-Doped Porous Carbon Derived in Situ from a MOF Designed for Efficient Catalysis

The reasonable design of the precursor of a carbon-based nanocatalyst is an important pathway to improve catalytic performance. In this study, a simple solvothermal method was used to synthesize [Cu(TPT)(2,5-tdc)] ⋅ 2H₂ O (Cu-MOF), which contains N and S atoms, in one step. Further in-situ carbonization of the Cu-MOF as the precursor was used to synthesize Cu/Cu_x S-embedded N,S-doped porous carbon (Cu/Cu_x S/NSC) composites. The catalytic activities of the prepared Cu/Cu_x S/NSC were investigated through catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The results show that the designed Cu/Cu_x S/NSC has exceptional catalytic activity and recycling stability, with a reaction rate constant of 0.0256 s^-1 , and the conversion rate still exceeds 90 % after 15 cycles. Meanwhile, the efficient catalytic reduction of dyes (CR, MO, MB and RhB) confirmed its versatility. Finally, the active sites of the Cu/Cu_x S/NSC catalysts were analyzed, and a possible multicomponent synergistic catalytic mechanism was proposed.

Ultrathin 2D flower-like CoP@C with the active (211) facet for efficient electrocatalytic water splitting

Due to the more exposed (211) facet, the electrocatalytic water splitting activity of 2D CoP@C is superior to that of its counterpart 3D structure.