Morphology and electronic structure modulation induced by fluorine doping in nickel-based heterostructures for robust bifunctional electrocatalysis

Characterization of Iron Oxide Nanotubes Obtained by Anodic Oxidation for Biomedical Applications-In Vitro Studies

To improve the biocompatibility and bioactivity of biodegradable iron-based materials, nanostructured surfaces formed by metal oxides offer a promising strategy for surface functionalization. To explore this potential, iron oxide nanotubes were synthesized on pure iron (Fe) using an anodic oxidation process (50 V-30 min, using an ethylene glycol solution containing 0.3% NH4F and 3% H2O, at a speed of 100 rpm). A nanotube layer composed mainly of α-Fe2O3 with diameters between 60 and 70 nm was obtained. The effect of the Fe-oxide nanotube layer on cell viability and morphology was evaluated by in vitro studies using a human osteosarcoma cell line (SaOs-2 cells). The results showed that the presence of this layer did not harm the viability or morphology of the cells. Furthermore, cells cultured on anodized surfaces showed higher metabolic activity than those on non-anodized surfaces. This research suggests that growing a layer of Fe oxide nanotubes on pure Fe is a promising method for functionalizing and improving the cytocompatibility of iron substrates. This opens up new opportunities for biomedical applications, including the development of cardiovascular stents or osteosynthesis implants.

Durable Pulse-Electrodeposited Ni-Fe-S Nanosheets Supported on a Ni-S Three Dimensional Pattern as Robust Bifunctional Electrocatalysts for Hydrogen Evolution and Urea Oxidation Reactions

This study aims to establish easy-to-fabricate and novel structures for the synthesis of highly active and enduring electrocatalysts for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Gradient electrodeposition and four different time regimes were utilized to synthesize Ni-S 3D patterns with the optimization of electrodeposition time. Pulse electrodeposition was employed for the synthesis of Ni-Fe-S nanosheets at three different frequencies and duty cycles to optimize the pulse electrodeposition parameters. The sample synthesized at 13 min of gradient electrodeposition with a 1 Hz frequency and 0.7 duty cycle for pulse electrodeposition demonstrated the best electrocatalytic performance. The optimized electrode further showed remarkable performance for HER and UOR reactions, requiring only 54 mV and 1.25 V to deliver 10 mA cm-2 for HER and UOR, respectively. Moreover, the overall cell voltage of the two-electrode system in 1 M KOH and 0.5 M urea was measured at 1.313 V, delivering 10 mA cm-2. Constructing Ni-Fe-S nanosheets on 3D Ni-S significantly increased the electrochemical surface area from 51 to 278 for the Ni-S and Ni-Fe-S layers. Tafel slopes were measured as 138 and 182 mV dec-1 for the HER and UOR for the Ni-S coating layer and 97 mV dec-1 for the HER and 131 mV dec-1 for the UOR for the optimal Ni-Fe-S nanosheets on Ni-S. Minimal changes in the potential were observed at 100 mA cm-2 in 50 h regarding the HER and UOR, signifying exceptional electrocatalytic stability. This study provides economically viable, highly active, and long-lasting electrocatalysts suitable for HER and UOR applications.

Water-Stable Fluorous Metal-Organic Frameworks with Open Metal Sites and Amine Groups for Efficient Urea Electrocatalytic Oxidation

Urea oxidation reaction (UOR) is one of the promising alternative anodic reactions to water oxidation that has attracted extensive attention in green hydrogen production. The application of specifically designed electrocatalysts capable of declining energy consumption and environmental consequences is one of the major challenges in this field. Therefore, the goal is to achieve a resistant, low-cost, and environmentally friendly electrocatalyst. Herein, a water-stable fluorinated Cu(II) metalorganic framework (MOF) {[Cu2 (L)(H2 O)2 ]·(5DMF)(4H2 O)}n (Cu-FMOF-NH2 ; H4 L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) is developed utilizing an angular tetracarboxylic acid ligand that incorporates both trifluoromethyl (-CF3 ) and amine (-NH2 ) groups. The tailored structure of Cu-FMOF-NH2 where linkers are connected by fluoride bridges and surrounded by dicopper nodes reveals a 4,24T1 topology. When employed as electrocatalyst, Cu-FMOF-NH2 requires only 1.31 V versus reversible hydrogen electrode (RHE) to deliver 10 mA cm-2 current density in 1.0 m KOH with 0.33 m urea electrolyte and delivered an even higher current density (50 mA cm-2 ) at 1.47 V versus RHE. This performance is superior to several reported catalysts including commercial RuO2 catalyst with overpotential of 1.52 V versus RHE. This investigation opens new opportunities to develop and utilize pristine MOFs as a potential electrocatalyst for various catalytic reactions.

Lattice-disordered high-entropy metal hydroxide nanosheets as efficient precatalysts for bifunctional electro-oxidation

Electro-oxidation reactions (EORs) are important half reactions in overall and assisted water electrolysis, which are crucial in achieving economic and sustainable hydrogen production and realizing simultaneous wastewater treatment. Current studies indicate that the high-valence metal ions that are locally enriched in the catalysts or generated in situ during the anodic preoxidation process are active species for EORs. Hence, designing (pre)catalysts with enriched local active sites and boosted preoxidation is of great importance. In this work, with a focus on improving the EOR performance toward the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR), we fabricated a lattice-disordered high-entropy FeCuCoNiZn hydroxide nanoarray catalyst that exhibits robust bifunctional OER and UOR behavior. The high-entropy feature could bring in a unique catalytic ensemble effect and remarkably improve the intrinsic OER/UOR activity. The lattice-disordered structure could not only enrich the local high-valence metal ions as active sites but also provide abundant reactive surface sites to accelerate the preoxidation process, thus leading to enriched active sites for the OER and UOR. Benefitting from the structural merits, the lattice-disordered high-entropy catalyst exhibits excellent OER and UOR activity with low overpotential, large current density and enhanced intrinsic activity, and no performance degradation but dramatic 35.3% and 88.7% enhancement in activity can be achieved during the long-term OER and UOR tests, respectively. The robust OER and UOR performance makes the lattice-disordered high-entropy catalyst a promising candidate for overall and urea-assisted water electrolysis from industrial, agricultural and sanitary wastewater.

Co, Mn co-doped Fe9S11@Ni9S8 supported on nickel foam as a high efficiency electrocatalyst for the oxygen evolution reaction and urea oxidation reaction

The Earth's fossil resources will be exhausted soon, so it is urgent to find clean and efficient new energy for replacing fossil resources. Hydrogen energy is gradually attracting the attention of the public and electrolysis of water is considered to be one of the important means of hydrogen production because of its simplicity and convenience. In this paper, a hydrothermal method for the synthesis of a Co and Mn co-doped bimetallic sulfide Fe₉S₁₁@Ni₉S₈ electrocatalyst is proposed for the first time. The prepared Co-Mn-Fe₉S₁₁@Ni₉S₈/NF electrocatalyst exhibits excellent electrocatalytic activity for the oxygen evolution reaction (OER) and urea oxidation reaction (UOR). It can provide a current density of 10 mA cm^-2 with only 193 mV overpotential for the OER and a current density of 10 mA cm^-2 with only 1.33 V potential for the UOR, which are far superior to those of most reported electrocatalysts. What is noteworthy is that the unique nanoflower structure of Co-Mn-Fe₉S₁₁@Ni₉S₈/NF increases the specific surface area of the material and the introduction of Co and Mn ions promotes the formation of high valence state Ni and Fe and enhances the charge transfer rate. The density functional theory (DFT) calculation shows that the in situ generated Co-Mn-Fe-NiOOH material derived from Co-Mn-Fe₉S₁₁@Ni₉S₈ exhibits the best water adsorption energy and the best electrical conductivity, thus improving the catalytic performance of the material. This work provided a new idea for the development of bimetallic cation doped electrocatalysts with high efficiency and low cost.

Recent advances in the pre-oxidation process in electrocatalytic urea oxidation reactions

The electrocatalytic urea oxidation reaction (UOR) has attracted substantial research interests over the past few years owing to its critical role in coupled electrochemical systems for energy conversion, for example, coupling with the hydrogen evolution reaction (HER) to realize urea-assisted hydrogen production and assembling direct urea fuel cells (DUFC) by coupling with the oxygen reduction reaction (ORR). The UOR process has been proved to be a two-step process which involves an electrochemical pre-oxidation reaction of the metal sites and a subsequent chemical oxidation of the urea molecules on the as-formed high-valence metal sites. Hence, designing advanced (pre-)catalysts with a boosted pre-oxidation reaction is of great importance in improving the UOR performance and thus accelerating the coupled reactions. In this feature article, we discuss the significant role of the pre-oxidation process during the urea electro-oxidation reaction, and summarize detailed strategies and recent advances in promoting the pre-oxidation reaction, including the modulation of the crystallinity, active phase engineering, defect engineering, elemental incorporation and constructing hierarchical nanostructures. We anticipate that this feature article will offer helpful guidance for the design and optimization of advanced (pre-)catalysts for UOR and related energy conversion applications.

A Nickel–Salen as a Model for Bifunctional OER/UOR Electrocatalysts: Pyrolysis Temperature–Electrochemical Activity Interconnection

The nickel derivative of a salen-type Schiff base is pyrolyzed under controlled conditions to form a nano-size Ni/NiOx core-shell species that serves as a model for a proof-of-concept investigation of...

Rational Construction of a N, F Co-doped Mesoporous Cobalt Phosphate with Rich-Oxygen Vacancies for Oxygen Evolution Reaction and Supercapacitors

Transition-metal phosphates have been widely applied as promising candidates for electrochemical energy storage and conversion. In this study, we report a simple method to prepare a N, F co-doped mesoporous cobalt phosphate with rich-oxygen vacancies by in-situ pyrolysis of a Co-phosphate precursor with NH₄ ⁺ cations and F^- anions. Due to this heteroatom doping, it could achieve a current density of 10 mA/cm² at lower overpotential of 276 mV and smaller Tafel slope of 57.11 mV dec^-1 on glassy carbon. Moreover, it could keep 92 % of initial current density for 35 h, indicating it has an excellent stability and durability. Furthermore, the optimal material applied in supercapacitor displays specific capacitance of 206.3 F g^-1 at 1 A ⋅ g^-1 and maintains cycling stability with 80 % after 3000 cycles. The excellent electrochemical properties should be attributed to N, F co-doping into this Co-based phosphate, which effectively modulates its electronic structure. In addition, its amorphous structure provides more active sites; moreover, its mesoporous structure should be beneficial to mass transfer and electrolyte diffusion.

Coupling Interface Constructions of FeNi3-MoO2 Heterostructures for Efficient Urea Oxidation and Hydrogen Evolution Reaction

Urea electrolysis has prospects for urea-containing wastewater purification and hydrogen (H₂) production, but the shortage of cost-effective catalysts restricts its development. In this work, the tomentum-like FeNi₃-MoO₂ heterojunction nanosheets array self-supported on nickel foam (NF) as bifunctional catalyst is prepared by facile hydrothermal and annealing method. Only 1.29 V and -50.8 mV is required to obtain ±10 mA cm^-2 for urea oxidation and hydrogen evolution reaction (UOR and HER), respectively, showing great bifunctional catalytic activity. For overall urea electrolysis, it only needs 1.37 V to reach 10 mA cm^-2 and can last at 100 mA cm^-2 for 70 h without obvious activity attenuation, showing outstanding durability. Coupling interface constructions of FeNi₃-MoO₂ heterostructures, novel morphology with a mesoporous and self-supporting structure could be the reason for this good performance. This work thus proposes a promising catalyst for boosting UOR and HER to realize efficient overall urea electrolysis.

Lanthanum-incorporated β-Ni(OH)2 nanoarrays for robust urea electro-oxidation

Lanthanum-incorporated β-Ni(OH)₂ nanosheets display superior catalytic behavior and stability for urea electro-oxidation, which originates from the optimized electronic structure, the downshift of the d-band center and the increased number of exposed active sites.

Manganese-Modulated Cobalt-Based Layered Double Hydroxide Grown on Nickel Foam with 1D-2D-3D Heterostructure for Highly Efficient Oxygen Evolution Reaction and Urea Oxidation Reaction

Hydrogen production by energy-efficient water electrolysis is a green avenue for the development of contemporary society. However, the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR) occurring at the anode are impeded by the sluggish reaction kinetics during the water-splitting process. Consequently, it is promising to develop bifunctional anodic electrocatalysts consisting of nonprecious metals. Herein, a bifunctional CoMn layered double hydroxide (LDH) was grown on nickel foam (NF) with a 1D-2D-3D hierarchical structure for efficient OER and UOR performance in alkaline solution. Owing to the significant synergistic effect of Mn doping and heterostructure engineering, the obtained Co₁ Mn₁ LDH/NF exhibits satisfactory OER activity with a low potential of 1.515 V to attain 10 mA cm^-2 . Besides, the potential of the Co₁ Mn₁ LDH/NF catalyst for UOR at the same current density is only 1.326 V, which is much lower than those of its counterparts and most reported electrocatalysts. An urea electrolytic cell with a Co₁ Mn₁ LDH/NF anode and a Pt-C/NF cathode was established, and a low cell voltage of 1.354 V at 10 mA cm^-2 was acquired. The optimized strategy may result in promising candidates for developing a new generation of bifunctional electrocatalysts for clean energy production.

Spinel-type oxygen-incorporated Ni3+ self-doped Ni3S4 ultrathin nanosheets for highly efficient and stable oxygen evolution electrocatalysis

Spinel-type structured materials have attracted considerable attention and been regarded as promising alternative catalysts for oxygen evolution reaction (OER). However, the regulation of catalytically active octahedral sites in spinel structure to realize high activity and good stability for OER electrocatalysis is still a great challenge. Herein, we propose a self-doping strategy to boost OER performance of spinel-type Ni₃S₄ enriched high valence Ni³⁺ as active sites. By sacrificing Ni-based metal-organic framework, the ultrathin Ni₃S₄ manosheets are topologically grown on conductive Ni foam substrate and realize the simultaneous Ni³⁺ self-doping and surface oxygen incorporation during in situ sulfidation conversion process. These compositional and structural characteristics endow it with enhanced adsorption binding strength, enabling the highly efficient OER. As a result, the Ni₃S₄/NF exhibits excellent activity and outstanding stability toward OER electrocatalysis in alkaline medium, which only demands an ultralow overpotential of 266 mV to deliver a current density of 10 mA cm^-2 and manifests the stable OER process for at least 75 h. Moreover, when used as an effective overall water splitting electrolyzer, the Ni₃S₄/NF achieves a current density of 10 mA cm^-2 at only 1.638 V with good long-term stability.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

Recent progress with electrocatalysts for urea electrolysis in alkaline media for energy-saving hydrogen production

Urea electrolysis is a promising energy-saving avenue for hydrogen production owing to the low cell voltage, wastewater remediation and abundant electrocatalysts.

Confinement of fluorine anions in nickel-based catalysts for greatly enhancing oxygen evolution activity

A simple fluorine treatment was developed to confine abundant fluorine anions into Ni-based catalysts to greatly enhance OER catalytic activity.

Defective NiFe2 O4 Nanoparticles for Efficient Urea Electro-oxidation

Urea is an important organic pollutants in sewage and needs to be removed for environmental protection. Here, we report defective NiFe₂ O₄ (NFO) nanoparticles with excellent performance for urea electro-oxidation. The results show that defects can be effectively implanted at the surface of NFO nanoparticles by a facile and versatile lithium reduction method without affecting its main crystal structure and grain size. The defective NFO-5Li nanoparticles displayed a significantly improved urea electro-oxidation performance compared with NFO-Pristine nanoparticles. Particularly, the NFO-Pristine and NFO-5Li show a potential of 1.398 and 1.361 V at the current density of 10 mA cm^-2 and Tafel slope of 37.3 and 31.4 mV dec^-1 , respectively. In addition, the NFO-5Li nanoparticles also revealed outstanding electrocatalytic stability. The superior performance can be attributed to the designed tunable surface defect engineering. Furthermore, the defect engineering strategy as well as the defective NFO nanoparticles hold great potential for applications in other materials and areas.

An iron incorporation-induced nickel hydroxide multiphase with a 2D/3D hierarchical sheet-on-sheet structure for electrocatalytic water oxidation

An Fe-incorporated Ni(OH)₂ multiphase with a unique 2D/3D hierarchical sheet-on-sheet structure exhibits superior catalytic activity contributed by synergistic effects, enhanced electron transport and well-exposed active sites.