Modulating the Surface Electronic Structure of Active Ni Sites by Engineering Hierarchical NiFe-LDH/CuS over Cu Foam as an Efficient Electrocatalyst for Water Splitting

Design and Modification of Layered Double Hydroxides-Based Compounds in Electrocatalytic Water Splitting: a Review

Layered double hydroxides (LDHs) exhibit great potential in electrocatalytic water splitting due to the unique 2D feature and an adjustable structure composed of different metal centers. In addition, LDHs have the advantage of being inherently inexpensive compared to other catalysts and have good stability in electrocatalytic water splitting. Up to now, numerous methods have been put forward to improve the activity of LDHs in electrocatalytic water splitting, a comprehensive introduction and comb to the fabrication methods and modification strategies is helpful for the followers to get a clear vein to carry out efficient manipulation to the development of high promising LDHs catalysts. In this review, the basic principles of water electrolysis, and the evaluation indexes are introduced first, and then the basic properties and commonly utilized methods in the fabrication of LDHs are introduced. After that, the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting (OWS) performance of different LDHs-based catalysts and analyze the merits and shortcomings of LDHs in electrocatalytic water splitting is compared. Based on this, the advanced strategies for improving the performance of LDHs is introduced and give a brief prospect for the development of LDHs-based materials in electrocatalysis.

Ceria-Graphene Oxide Nanocomposite for Electro-oxidation of Urea: An Experimental and Theoretical Investigation

This study explores the potential of ceria-graphene oxide (CeO2-GO) nanocomposites as efficient electrocatalysts for urea electro-oxidation (UOR). This work combines experimental and theoretical investigations and characterization techniques confirm the successful formation of the CeO2 embedded on graphene oxide sheets. UOR activity was found to be dependent on both OH- and urea concentrations. The optimal UOR performance was achieved in a 0.1 M urea and 1.0 M KOH solution, as evidenced by the low Tafel slope of 60 mV/dec and high turnover frequency (TOF) of 1.690 s-1. DFT calculations revealed that the CeO2-GO nanocomposite exhibited strong urea adsorption due to its favorable bond lengths (Ce-O: 2.58 Å, O-H: 1.77 Å) and high adsorption energy (-1.05 eV). These findings revealed that the CeO2-GO nanocomposites are promising as efficient and durable electrocatalysts for urea conversion to valuable products like nitrogen and hydrogen gas, with potential applications in clean energy generation and ammonia synthesis.

NiO/RuO2 p-n Heterojunction Nanofoam as a High-Performance Electrocatalyst for Desulfurization and Concurrent Hydrogen Evolution

The development of bifunctional electrocatalysts with excellent performance in both the hydrogen evolution reaction (HER) and sulfide oxidation reaction (SOR) remains a formidable challenge. Herein, we experimentally synthesize a NiO/RuO2 p-n heterojunction nanofoam that exhibits highly desirable electrocatalytic properties for both the HER and the SOR. We further design an electrolytic cell by pairing alkaline HER with SOR utilizing the NiO/RuO2 heterojunction nanofoam as both the anode and the cathode, which demands a low applied voltage of 0.846 V to achieve a current density of 10 mA cm-2. Density functional theory calculations confirm that the formation of the NiO/RuO2 p-n heterojunction nanofoam effectively regulates the electronic structure, thereby boosting the electrocatalytic performances for both HER and SOR. This work not only provides a novel strategy to prepare an efficient and stable nanofoam electrocatalyst for hydrogen production but also highlights the potential application of oxide heterojunction electrocatalysts in treating sulfur-containing waste liquid.

Electronic Structure Modulation Via Iron-Incorporated NiO to Boost Urea Oxidation/Oxygen Evolution Reaction

The urea-assisted water splitting not only enables a reduction in energy consumption during hydrogen production but also addresses the issue of environmental pollution caused by urea. Doping heterogeneous atoms in Ni-based electrocatalysts is considered an efficient means for regulating the electronic structure of Ni sites in catalytic processes. However, the current methodologies for synthesizing heteroatom-doped Ni-based electrocatalysts exhibit certain limitations, including intricate experimental procedures, prolonged reaction durations, and low product yield. Herein, Fe-doped NiO electrocatalysts were successfully synthesized using a rapid and facile solution combustion method, enabling the synthesis of 1.1107 g within a mere 5 min. The incorporation of iron atoms facilitates the modulation of the electronic environment around Ni atoms, generating a substantial decrease in the Gibbs free energy of intermediate species for the Fe-NiO catalyst. This modification promotes efficient cleavage of C-N bonds and consequently enhances the catalytic performance of UOR. Benefiting from the tunability of the electronic environment around the active sites and its efficient electron transfer, Fe-NiO electrocatalysts only needs 1.334 V to achieve 50 mA cm-2 during UOR. Moreover, Fe-NiO catalysts were integrated into a dual electrode urea electrolytic system, requiring only 1.43 V of cell voltage at 10 mA cm-2.

Matrix-type bismuth-modulated copper-sulfur electrode using local photothermal effect strategy for efficient seawater splitting

Constructing catalytic electrodes with green economy, stability, and high efficiency is crucial for achieving overall economic water splitting. Herein, a matrix-type bismuth-modulated nickel-boron electrodes loaded on sulfurized copper foils (Bi-NiBx@CFS) is synthesized via in situ mild electroless plating. This electrode features a 2-dimensional (2D) matrix-type nanosheet structure with uniform, large pores, providing more active sites and ensuring a high gas transmission rate. Notably, the crystalline-amorphous structure constituted by the photothermal materials Bi and NiBx is loaded onto sulfide-based heterostructures. This enhances the catalytic activity through the "local photothermal effect" strategy. A performance enhancement of approximately 10 % is achieved for the Bi-NiBx@CFS at a current density of 10 mA cm-2 using this strategy at 298 K. This enhancement is equivalent to increasing the temperature of conventional electrolyte solutions by 321 K. In addition, the overpotential required to catalytically drive seawater splitting at the same current density is only 1.486 V. The Bi-NiBx@CFS electrode operates stably for 200 h without any performance degradation at industrial-grade current densities. The Bi-NiBx@CFS electrode under the "localized photothermal effect" strategy is expected to be a new type of electrocatalyst for overall seawater splitting.

Rapid and efficient removal of methylene blue dye from aqueous solutions using extract-modified Zn-Al LDH

Environmental pollution, particularly water pollution caused by organic substances like synthetic dyes, is a pressing global concern. This study focuses on enhancing the adsorption capacity of layered double hydroxides (LDHs) to remove methylene blue (MB) dye from water. The synthesized materials are characterized using techniques like FT-IR, XRD, SEM, TEM, TGA, EDS, BET, BJH, AFM, and UV-Vis DRS. Adsorption experiments show that Zn-Al LDH@ext exhibits a significant adsorption capacity for MB dye compared to pristine LDH. In addition, Zn-Al LDH@ext shows a significant increase in stability, which is attributed to the presence of phenolic compounds in the extract and the interactions between the functional groups of the extract and LDH. The pH and adsorbent dosage optimizations show that pH 7 and 0.7 g of Zn-Al LDH@ext are optimal conditions for efficient MB removal. The study assessed adsorption kinetics through the examination of Langmuir, Freundlich, and Temkin isotherms. Additionally, four kinetic models, namely pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich, were analyzed. The results indicated that the Temkin isotherm (R2 = 0.9927), and pseudo-second-order (R2 = 0.9999) kinetic provided the best fit to the experimental data. This study introduces a novel approach to enhance adsorption efficiency using modified LDHs, contributing to environmentally friendly and cost-effective water treatment methods.

Construction of Ni4Mo/MoO2 heterostructure on oxygen vacancy enriched NiMoO4 nanorods as an efficient bifunctional electrocatalyst for overall water splitting

Despite great efforts over the past decade, rational design of bifunctional electrocatalysts with low cost and high efficiency still remains a challenge to achieve industrial water splitting. Herein, we synthesized the nickel-molybdenum nanorod array catalyst supported on NF (NMO@NM/MO) by a two-step process of hydrothermal and reductive annealing. Partial reduction of the NiMoO4 induces the structural reconstruction and formation of the Ni4Mo/MoO2 heterostructure on oxygen vacancy enriched nanorod, which bring out sufficient active sites, large specific surface area and favorable interfacial charge transfer. Thanks to the unique core-shell structure with the heterostructured Ni4Mo/MoO2 surface and defect-rich NiMoO4 core, the obtained electrocatalyst shows greatly improved hydrogen evolution reaction (HER) activity with an ultralow overpotential of 63 mV at 100 mA cm-2 (vs. 314 mV for the NiMoO4). Density function theory calculations reveal that the construction of the Ni4Mo/MoO2 heterostructure effectively accelerates H2O dissociation kinetics, while the defective NiMoO4 facilitates H* adsorption/desorption. Moreover, the heterostructure catalyst also displays excellent oxygen evolution reaction (OER) performance with the low overpotential of 274 mV at 100 mA cm-2. When coupling HER and OER by using NMO@NM/MO as both the cathode and anode, the alkaline electrolyzer delivers a current density of 10 mA cm-2 at only 1.50 V as well as good robustness. The synergistic effect of the hetero-interface and the defect engineering endows the electrocatalyst with excellent bifunctional catalytic activity for HER and OER. This work may provide a route for rational design of heterostructure electrocatalysts with multiple active components.

Self-assembled c-oriented Ni(OH)2 films for enhanced electrocatalytic activity towards the urea oxidation reaction

This study investigates the electrocatalytic properties of the transparent c-oriented Ni(OH)2 films self-assembled from colloidal 2D Ni(OH)2 nanosheets for urea oxidation. The synthesis process yields highly uniform close-packed superlattices with a dominant c-axis orientation. The self-assembled c-oriented Ni(OH)2 films exhibit advantageous electrocatalytic performance in urea oxidation, presenting significantly lower overpotentials and higher current densities compared to randomly distributed Ni(OH)2 particles. In-depth in situ impedance analysis and Raman spectroscopy demonstrate that the c-oriented Ni(OH)2 films possess a higher propensity for a Ni valence transition from +2 to +3 during the urea oxidation process. This finding provides crucial insights into the catalytic behavior and electronic transformations of c-oriented Ni(OH)2 films, shedding light on their superior electrocatalytic activity for urea oxidation. Overall, this study advances our understanding of urea electrooxidation mechanisms and contributes to the design of efficient urea electrocatalysts.

Efficient oxygen evolution reaction from iron-molybdenum nitride/molybdenum oxide heterostructured composites

Integrating various active sites into a multi-component system might significantly enhance the oxygen evolution reaction (OER) performance. Herein, the as-prepared iron-molybdenum nitride/molybdenum oxide (Fe-Mo5N6/MoO3-550) composite electrocatalyst under optimum conditions demonstrates excellent electrocatalytic performance toward OER and reaches current densities of 10 and 20 mA cm-2 at overpotentials of 201 and 216 mV, respectively. The OER performance of Fe-Mo5N6/MoO3-550 exceeds that of most previously reported electrocatalytic systems. The significant improvement in the OER performance is ascribed to a combination of mechanisms. The strong electronic interactions among the Fe, Mo5N6 and MoO3 species can accelerate the OER reaction kinetics, which contributes to the OER performance. This work provides new insights into the construction of efficient electrocatalytic materials with inexpensive metals.

Tuning the Surface Electronic Structure of Amorphous NiWO4 by Doping Fe as an Electrocatalyst for OER

Water electrolysis is considered as one of the alternative potential approaches for producing renewable energy. Due to the sluggish kinetic nature of oxygen evolution reaction (OER), it encounters a significant overpotential to achieve water electrolysis. Hence, the advancement of cost-effective transition metal-based catalysts toward water splitting has gained global attention in recent years. In this work, the doping of Fe over amorphous NiWO₄ increased the OER activity effectively and achieved stable oxygen evolution in the alkaline medium, which show better electrocatalytic activity as compared to crystalline tungstate. As NiWO₄ has poor activity toward OER in the alkaline medium, the doping of Fe³⁺ will tune the electronic structure of Ni in NiWO₄ and boost the OER activity. The as-synthesized Fe-doped amorphous NiWO₄ exhibits a low overpotential of 230 mV to achieve a current density of 10 mA cm^-2 and a lower Tafel slope value of 48 mV dec^-1 toward OER in 1.0 M KOH solution. The catalyst also exhibits long-term static stability of 30 h during chronoamperometric study. The doping of Fe improves the electronic conductivity of Ni-3d states in NiWO₄ which play a dominant role for better catalytic activity via synergistic interaction between Fe and active Ni sites. In future, these results offer an alternative route for precious metal-free catalysts in alkaline medium and can be explicitly used in various tungstate-based materials to increase the synergism between the doped atom and metal ions in tungstate-based materials for further improvement in the electrocatalytic performance.