Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting

rBMSC osteogenic differentiation enhanced by graphene quantum dots loaded with immunomodulatory layered double hydroxide nanoparticles

Bone tissue defects caused by disease, trauma, aging or genetic factors emerged as one of the main factors that endanger human health. At present, advanced development of bone tissue engineering and regenerative medicine focused on the biomaterials regulated stem cell for responsive differentiation. In vivo transplantation of allogeneic bone materials has the needs of both osteogenic and immune regulation function. In this study, we utilized the extensively proved biocompatible layered double hydroxide (LDH) nanoparticles as the nanocarrier of graphene quantum dots (GQD), the functional loading was validated by characteristics analysis of scanning electron microscopy, surface zeta potential, X-ray diffraction and fourier transform infrared spectroscopy. Further, we investigated the cellular uptake of nanoparticles in rat bone marrow derived mesenchymal stem cells, the significant enhanced endocytosis was occurred in LDH-GQD treated groups. The enhanced osteogenic differentiation abilities of LDH-GQD were systematically investigated through alkaline phosphatase staining, alizarin red staining and qPCR analysis. In addition, the anti-inflammatory regulation of LDH facilitated the phenotypic transition of macrophage in LDH-GQD nanocomposites. Overall, the successful construction and functional validation of nanomaterials in this study will provide clinical therapeutic potential in bone defects regeneration.

Motivating borate doped FeNi layered double hydroxides by molten salt method toward efficient oxygen evolution

The incorporation of borate is a beneficial strategy to improve the catalytic activity of transition metal-based electrocatalyts for oxygen evolution reaction (OER). However, how to efficiently introduce borate has always been a challenge. Here, a facile and scalable molten salt method is developed to successfully dope borate into FeNi layered double hydroxides (FeB_i@FeNi LDH) for efficient OER. The molten salt method can not only promote the formation of evenly dispersed nano-pompous FeB_i precursor, thus providing the possibility to realize the direct doping of borate and the increase of mass, charge transfer and oxygen evolution active sites in FeNi LDH, but also promote the in-situ growth of FeB_i@FeNi LDH on the conductive iron foam, improvingconductivity and stability of the material. The results indicate that the synthesized FeB_i@FeNi LDH shows enhanced OER activity by delivering current densities of 10 and 100 mA cm^-2 at low overpotentials of 246 and 295 mV and showing a small Tafel slope of 56.48 mV dec^-1, benefiting from the optimization of geometric structure of active sites as well as the adjustment of electron density by borate doping especially in the case of molten salt. In addition, the sample can maintain durability at an industrial current density of 100 mA cm^-1 for 90 h. This work provides a new way for the construction of efficient catalysts using boron doping assisted by molten salt.

The Effect of Interlayer Anion Grafting on Water Oxidation Electrocatalysis: A Comparative Study of Ni- and Co-Based Brucite-Type Layered Hydroxides, Layered Double Hydroxides and Hydroxynitrate Salts

The urge for carbon‐neutral green energy conversion and storage technologies has invoked the resurgence of interest in applying brucite‐type materials as low‐cost oxygen evolution reaction (OER) electrocatalysts in basic media. Transition metal layered hydroxides belonging to the brucite‐type structure family have been shown to display remarkable electrochemical activity. Recent studies on the earth‐abundant Fe³⁺ containing mössbauerite and Fe³⁺ rich Co−Fe layered oxyhydroxide carbonates have suggested that grafted interlayer anions might play a key role in OER catalysis. To probe the effect of such interlayer anion grafting in brucite‐like layered hydroxides, we report here a systematic study on the electrocatalytic performance of three distinct Ni and Co brucite‐type layered structures, namely, (i) brucite‐type M(OH)₂ without any interlayer anions, (ii) LDHs with free interlayer anions, and (iii) hydroxynitrate salts with grafted interlayer anions. The electrochemical results indeed show that grafting has an evident impact on the electronic structure and the observed OER activity. Ni‐ and Co‐hydroxynitrate salts with grafted anions display notably earlier formations of the electrocatalytically active species. Particularly Co‐hydroxynitrate salts exhibit lower overpotentials at 10 mA cm⁻² (η=0.34 V) and medium current densities of 100 mA cm⁻² (η=0.40 V) compared to the corresponding brucite‐type hydroxides and LDH materials.

Structural Variations of Metal Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Electrocatalytic oxygen evolution reaction (OER), an important electrode reaction in electrocatalytic and photoelectrochemical cells for a carbon-free energy cycle, has attracted considerable attention in the last few years. Metal oxides have been considered as good candidates for electrocatalytic OER because they can be easily synthesized and are relatively stable during the OER process. However, inevitable structural variations still occur to them due to the complex reaction steps and harsh working conditions of OER, thus impending the further insight into the catalytic mechanism and rational design of highly efficient electrocatalysts. The aim of this review is to disclose the current research progress toward the structural variations of metal oxide-based OER electrocatalysts. The origin of structural variations of metal oxides is discussed. Based on some typical oxides performing OER activity, the external and internal factors that influence the structural stability are summarized and then some general approaches to regulate the structural variation process are provided. Some operando methods are also concluded to monitor the structural variation processes and to identify the final active structure. Additionally, the unresolved problems and challenges are presented in an attempt to get further insight into the mechanism of structural variations and establish a rational structure-catalysis relationship.

Iron and chromium co-doped cobalt phosphide porous nanosheets as robust bifunctional electrocatalyst for efficient water splitting

It is still a huge challenge to develop highly efficient and low-cost non-precious metal-based electrocatalysts for overall water splitting in alkaline electrolytes. Herein, Cr and Fe co-doped CoP porous mesh nanosheets (Mesh-CrFe-CoP NSs) were synthesized through hydrolysis reaction, ion exchange etching and subsequent low-temperature phosphating process. The Mesh-CrFe-CoP NSs provides overpotentials at a current density of 10 mA cm^-2under alkaline electrolyte of 103.7 mV and 256.4 mV for HER and OER, respectively. Furthermore, when using Mesh-CrFe-CoP NSs as anode and cathode, the water splitting system could afford a current density of 10 mA cm^-2at 1.55 V, which is better than an electrolytic cell composed of 20% Pt/C and RuO₂. The excellent electrocatalytic performance of Mesh-CrFe-CoP NSs is attributed to the co-doping and porous nanostructure. Specifically, the Cr and Fe co-doped porous CoP nanosheets electrocatalyst not only provided abundant exposure active sites, accelerated the entry of liquid and the diffusion of gas, but also regulated the electronic environment of active sites, and thus enhanced the electrochemical performance. This work proposes a strategy for the rational design of highly efficient and stable non-precious metal co-doped phosphide electrocatalysts in the of electrochemical water splitting.

Electronic Interaction between In Situ Formed RuO2 Clusters and a Nanoporous Zn3V3O8 Support and Its Use in the Oxygen Evolution Reaction

The catalytic activity and durability of RuO₂ clusters toward the oxygen evolution reaction (OER) are strongly associated with their support; however, how the electronic interaction would enhance the catalytic performance is still not quite clear. Herein, hierarchical nanoporous and single-crystal Zn₃V₃O₈ nanosheets are adopted to anchor in situ formed RuO₂ clusters. X-ray photoelectron analysis reveals significant binding energy changes of both Ru and V due to the creation of strong Ru-O-V bonding interaction, which would lead to the reconstruction of the electronic structure of the Zn₃V₃O₈ matrix and RuO₂ clusters. The ultrastrong electronic interaction also results in superior OER activity, indicated by a small overpotential at 10 mA cm^-2 (228 mV) and a shallow Tafel slope of 46 mV dec^-1. First-principles simulation further reveals the synergistic effect derived from the unique RuO₂@Zn₃V₃O₈ couple, which effectively regulates the electronic structure for the OER process. In addition, the created interfacial chemical bond and the confined microporous structure of the Zn₃V₃O₈ substrate could prevent the RuO₂ clusters from detachment and aggregation, making the nanocomposite a promising long-term stable OER electrocatalyst.

Two-dimensional Ti3C2 MXene-based nanostructures for emerging optoelectronic applications

Since the first discovery of Ti₃C₂ in 2011, two-dimensional (2D) transition-metal carbides, carbonitrides and nitrides, known as MXenes, have attracted significant attention. Due to their outstanding electronic, optical, mechanical, and thermal properties, versatile structures and surface chemistries, Ti₃C₂ MXenes have emerged as new candidates with great potential for applications in optoelectronic devices, such as photovoltaics, photodetectors and photoelectrochemical devices. The excellent metallic conductivity, high anisotropic carrier mobility, good structural and chemical stabilities, high optical transmittance, excellent mechanical strength, tunable work functions, and wide range of optical absorption properties of Ti₃C₂ MXene nanostructures are the key to their success in a number of electronic and photonic device applications. Herein, we summarize the fundamental properties and preparation of pure Ti₃C₂ MXenes, functionalized Ti₃C₂ MXenes and their hybrid nanocomposites, as well as their optoelectronic applications. In the end, the perspective and current challenges of Ti₃C₂ MXenes toward the development of advanced MXene-based nanostructures are briefly discussed for future optoelectronic applications.

Synergetic Advantages of Atomically Coupled 2D Inorganic and Graphene Nanosheets as Versatile Building Blocks for Diverse Functional Nanohybrids

2D nanostructured materials, including inorganic and graphene nanosheets, have evoked plenty of scientific research activity due to their intriguing properties and excellent functionalities. The complementary advantages and common 2D crystal shapes of inorganic and graphene nanosheets render their homogenous mixtures powerful building blocks for novel high-performance functional hybrid materials. The nanometer-level thickness of 2D inorganic/graphene nanosheets allows the achievement of unusually strong electronic couplings between sheets, leading to a remarkable improvement in preexisting functionalities and the creation of unexpected properties. The synergetic merits of atomically coupled 2D inorganic-graphene nanosheets are presented here in the exploration of novel heterogeneous functional materials, with an emphasis on their critical roles as hybridization building blocks, interstratified sheets, additives, substrates, and deposited monolayers. The great flexibility and controllability of the elemental compositions, defect structures, and surface natures of inorganic-graphene nanosheets provide valuable opportunities for exploring high-performance nanohybrids applicable as electrodes for supercapacitors and rechargeable batteries, electrocatalysts, photocatalysts, and water purification agents, to give some examples. An outlook on future research perspectives for the exploitation of emerging 2D nanosheet-based hybrid materials is also presented along with novel synthetic strategies to maximize the synergetic advantage of atomically mixed 2D inorganic-graphene nanosheets.

Boride-derived oxygen-evolution catalysts

Metal borides/borates have been considered promising as oxygen evolution reaction catalysts; however, to date, there is a dearth of evidence of long-term stability at practical current densities. Here we report a phase composition modulation approach to fabricate effective borides/borates-based catalysts. We find that metal borides in-situ formed metal borates are responsible for their high activity. This knowledge prompts us to synthesize NiFe-Boride, and to use it as a templating precursor to form an active NiFe-Borate catalyst. This boride-derived oxide catalyzes oxygen evolution with an overpotential of 167 mV at 10 mA/cm² in 1 M KOH electrolyte and requires a record-low overpotential of 460 mV to maintain water splitting performance for over 400 h at current density of 1 A/cm². We couple the catalyst with CO reduction in an alkaline membrane electrode assembly electrolyser, reporting stable C₂H₄ electrosynthesis at current density 200 mA/cm² for over 80 h.

Nanowire-structured FeP-CoP arrays as highly active and stable bifunctional electrocatalyst synergistically promoting high-current overall water splitting

The design and construction of highly efficient and durable non-noble metal bifunctional catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media is essential for developing the hydrogen economy. To achieve this goal, we have developed a bifunctional nanowire-structured FeP-CoP array catalyst on carbon cloth with uniform distribution through in-situ hydrothermal growth and phosphating treatment. The unique nanowire array structure and the strong electronic interaction between FeP and CoP species have been confirmed. Electrochemical studies have found that the designed Fe_0.14Co_0.86-P/CC catalyst appears excellent HER (130 mV@10 mA cm^-2)/OER (270 mV@10 mA cm^-2) activity and stability. Moreover, the bifunctional Fe_0.14Co_0.86-P/CC^(+/-) catalyst is also used in simulated industrial water splitting system, where the pair catalyst requires about 1.95 and 2.14 V to reach 500 and 1000 mA cm^-2, even superior to the control RuO₂⁽⁺⁾||Pt/C^(-) catalyst, showing good industrial application prospects. These excellent electrocatalytic properties are attributed to the synergy between FeP and CoP species as well as the unique microstructure, which can accelerate charge transfer, expose more active sites and enhance electrolyte diffusion and gas emissions.

Application of layered double hydroxide-biochar composites in wastewater treatment: Recent trends, modification strategies, and outlook

In recent years, layered double hydroxide-biochar (LDH-BC) composites as adsorbents and catalysts for contaminants removal (inorganic anions, heavy metals, and organics) have received increasing attention and became a new research point. It is because of the good chemical stability, abundant surface functional groups, excellent anion exchange ability, and good electronic properties of LDH-BC composites. Hence, we offer an overall review on the developments and processes in the synthesis of LDH-BC composites as adsorbents and catalysts. Special attention is devoted to the strategies for enhancing the properties of LDH-BC composites, including (1) magnetic treatment, (2) acid treatment, (3) alkali treatment, (4) controlling metal ion ratios, (5) LDHs intercalation, and (6) calcination. In addition, further studies are called for LDH-BC composites and potential areas for future application of LDH-BC composites are also proposed.

Layered Double Hydroxides as a Drug Delivery Vehicle for S-Allyl-Mercapto-Cysteine (SAMC)

The intercalations of anionic molecules and drugs in layered double hydroxides (LDHs) have been intensively investigated in recent years. Due to their properties, such as versatility in chemical composition, good biocompatibility, high density and protection of loaded drugs, LDHs seem very promising nanosized systems for drug delivery. In this work, we report the intercalation of S-allyl-mercapto-cysteine (SAMC), which is a component of garlic that is well-known for its anti-tumor properties, inside ZnAl-LDH (hereafter LDH) nanostructured crystals. In order to investigate the efficacy of the intercalation and drug delivery of SAMC, the intercalated compounds were characterized using X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The increase in the interlayer distance of LDH from 8.9 Å, typical of the nitrate phase, to 13.9 Å indicated the intercalation of SAMC, which was also confirmed using FT-IR spectra. Indeed, compared to that of the pristine LDH precursor, the spectrum of LDH-SAMC was richly structured in the fingerprint region below 1300 cm−1, whose peaks corresponded to those of the functional groups in the SAMC molecular anion. The LDH-SAMC empirical formula, obtained from UV-Vis spectrophotometry and thermogravimetric analysis, was [Zn0.67Al0.33(OH)2]SAMC0.15(NO3)0.18·0.6H2O. The morphology of the sample was investigated using SEM: LDH-SAMC exhibited a more irregular size and shape of the flake-like crystals in comparison with the pristine LDH, with a reduction in the average crystallite size from 3 µm to about 2 µm. In vitro drug release studies were performed in a phosphate buffer solution at pH 7.2 and 37 °C and were analyzed using UV-Vis spectrophotometry. The SAMC release from LDH-SAMC was initially characterized by a burst effect in the first four hours, during which, 32% of the SAMC is released. Subsequently, the release percentage increased at a slower rate until 42% after 48 h; then it stabilized at 43% and remained constant for the remaining period of the investigation. The LDH-SAMC complex that was developed in this study showed the improved efficacy of the action of SAMC in reducing the invasive capacity of a human hepatoma cell line.

Recent Advances in Layered-Double-Hydroxides Based Noble Metal Nanoparticles Efficient Electrocatalysts

With the energy crisis and environmental pollution becoming more and more serious, it is urgent to develop renewable and clean energy. Hydrogen production from electrolyzed water is of great significance to solve the energy crisis and environmental problems in the future. Recently, layered double hydroxides (LDHs) materials have been widely studied in the electrocatalysis field, due to their unique layered structure, tunable metal species and highly dispersed active sites. Moreover, the LDHs supporting noble metal catalysts obtained through the topotactic transformation of LDHs precursors significantly reduce the energy barrier of electrolyzing water, showing remarkable catalytic activity, good conductivity and excellent durability. In this review, we give an overview of recent advances on LDHs supporting noble metal catalysts, from a brief introduction, to their preparation and modification methods, to an overview of their application in the electrocatalysis field, as well as the challenges and outlooks in this promising field on the basis of current development.

Interface Engineering of Heterogeneous CeO2-CoO Nanofibers with Rich Oxygen Vacancies for Enhanced Electrocatalytic Oxygen Evolution Performance

The development of highly efficient and cheap electrocatalysts for the oxygen evolution reaction (OER) is highly desirable in typical water-splitting electrolyzers to achieve renewable energy production, yet it still remains a huge challenge. Herein, we have presented a simple procedure to construct a new nanofibrous hybrid structure with the interface connecting the surface of CeO₂ and CoO as a high-performance electrocatalyst toward the OER through an electrospinning-calcination-reduction process. The resultant CeO₂-CoO nanofibers exhibit excellent electrocatalytic properties with a small overpotential of 296 mV at 10 mA cm^-2 for the OER, which is superior to many previously reported nonprecious metal-based and commercial RuO₂ catalysts. Furthermore, the prepared CeO₂-CoO nanofibers display remarkable long-term stability, which can be maintained for 130 h with nearly no attenuation of OER activity in an alkaline electrolyte. A combined experimental and theoretical investigation reveals that the excellent OER properties of CeO₂-CoO nanofibers are due to the unique interfacial architecture between CeO₂ and CoO, where abundant oxygen vacancies can be generated due to the incomplete matching of atomic positions of two parts, leading to the formation of many low-coordinated Co sites with high OER catalytic activity. This research provides a practical and promising opportunity for the application of heterostructured nonprecious metal oxide catalysts for high-efficiency electrochemical water oxidation.

Accelerating Hydrogen Evolution by Anodic Electrosynthesis of Value-Added Chemicals in Water over Non-Precious Metal Electrocatalysts

Integrating electrolytic hydrogen production from water with thermodynamically more favorable aqueous organic oxidation reactions is highly desired, because it can enhance the energy conversion efficiency in relation to traditional water electrolysis, and produce value-added chemicals instead of oxygen at the anode. In this Minireview, we introduce some key considerations for anodic auxiliary electrosynthesis and outline three types of electrocatalytic organic reactions including biomass derivative, alcohol and amine oxidation reactions, which can boost cathodic hydrogen generation. Furthermore, frequently used noble-metal-free electrocatalysts are classified into nickel-based, cobalt-based, other transition-metal-based and bimetallic electrocatalysts. The preparation methods of these catalysts and their performance towards electrochemical oxidation reactions are also discussed in detail. We specifically highlight the importance of redox active sites on the surface of the electrocatalysts, which act as electron mediators to promote oxidation reactions. Finally, the current challenges and future developments in this emerging field are also discussed.

Synthesis and Electronic Modulation of Nanostructured Layered Double Hydroxides for Efficient Electrochemical Oxygen Evolution

Water electrolysis is considered to be one of the most promising technologies to produce clean fuels. However, its extensive realization critically depends on the progress in cost-effective and high-powered oxygen evolution reaction (OER) electrocatalysts. As a member of the big family of two-dimensional (2D) materials, nanostructured layered double hydroxides (nLDHs) have made significant processes and continuous breakthroughs for OER electrocatalysis. In this Review, the advancements in designing nLDHs for OER in recent years were discussed with a unique focus on their electronic modulations and in situ analysis on catalytic processes. After a brief discussion on different synthetic methodologies of nLDHs, including "bottom-up" and "top-down" approaches, the general strategies to enhance the catalytic performances of nLDHs reported so far were summarized, including compositional substitution, heteroatom doping, vacancy engineering, and amorphous/crystalline engineering. Furthermore, the in situ OER processes and mechanism analysis on engineering efficient nLDHs electrocatalysts were discussed. Finally, the research trends, perspectives, and challenges on designing nLDHs were also carefully outlined. This progress Review may offer enlightening experimental/theoretical guidance for designing highly catalytic active nLDHs and provide new directions to promote their future prosperity for practical utilization in water splitting.

Fabrication of Nanoflower-like MCoP (M = Fe and Ni) Composites for High-Performance Supercapacitors

Elaborating the development of functional materials with excellent performance for supercapacitors is important in energy storage devices. In the present study, nanoflower-like MCoP (M = Ni and Fe) composites were successfully fabricated on Ni foam (denoted as NF@MCoP) by a cost-effective hydrothermal and low-temperature phosphating method. Simultaneously, the unique three-dimensional structure, nanoflower morphology, and the conductive substrate provide a favorable large electroactive area, shorter electron transfer distance, and rapid electron conductivity. The as-synthesized nanoflower-like MCoP composites exhibit outstanding energy density, power density, and long-term cycling stability. These results show that the developed electrode materials with excellent performance have great application prospects in the field of supercapacitor applications.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

High-Performance Bifunctional Ni-Fe-S Catalyst in situ Synthesized within Graphite Intergranular Nanopores for Overall Water Splitting

Low-cost and efficient bifunctional catalysts are urgently needed for overall water splitting used in large-scale energy storage. In this study, we develop a nickel and iron (di)sulfide (Ni-Fe-S) composite catalyst that is in situ synthesized and fixed within the intergranular nanopores inside high pure polycrystalline graphite. Two precursor solutions (reactants) may permeate the graphite intergranular pores to a depth of more than 3.5 mm. The nanoscale pores serve as an array of nanoreactors for the synthesis of the Ni-Fe-S nanoparticles under conditions much milder than usual. The prepared catalyst efficiently catalyzes both the hydrogen and oxygen evolution reactions (HER and OER) in 1.0 M KOH. It delivers a current density of 400 mA cm^-2 at a full cell voltage of around 2.3 V without considerable activity decay over 24 h electrolysis. The active species of the catalyst are different for the HER and OER and discussed accordingly. The synthesis strategy based on the nanopores in a monolithic conductive substrate proves to be a simple, efficient, and promising way to prepare electrocatalysts that are cheap, abundant, and industrially attractive.

The impact of ultrasonic parameters on the exfoliation of NiFe LDH nanosheets as electrocatalysts for the oxygen evolution reaction in alkaline media

The ultrasonic process has been examined to exfoliate layered materials and upgrade their properties for a variety of applications in different media. Our previous studies have shown that the ultra-sonication treatment in water without chemicals has a positive influence on the physical and electrochemical performance of layered materials and nanoparticles. In this work, we have probed the impact of ultrasonication on the physical properties and the oxygen evolution reaction (OER) of the NiFe LDH materials under various conditions, including suspension concentration (2.5-12.5 mg mL-1), sonication times (3-20 min) and amplitudes (50-90%) in water, in particular, sonication times and amplitudes. We found that the concentration, amplitude and time play significant roles on the exfoliation of the NiFe LDH material. Firstly, the NiFe LDH nanosheets displayed the best OER performance under ultrasonic conditions with the concentration of 10 mg mL-1 (50% amplitude and 15 min). Secondly, it was revealed that the exfoliation of the NiFe LDH nanosheets in a short time (<10 min) or a higher amplitudes (≥80%) has left a cutdown on the OER activity. Comprehensively, the optimum OER activity was displayed on the exfoliated NiFe LDH materials under ultrasonic condition of 60% (amplitude), 10 mg mL-1 and 15 min. It demanded only 250 mV overpotentials to reach 10 mA cm-2 in 1 M KOH, which was 100 mV less than the starting NiFe LDH material. It was revealed from the mechanism of sonochemistry and the OER reaction that, after exfoliation, the promoted OER performance is ascribed to the enriched Fe3+ at the active sites, easier oxidation of Ni2+ to Ni3+, and the strong electrical coupling of the Ni2+ and Fe3+ during the OER process. This work provides a green strategy to improve the intrinsic activity of layered materials.

Preparation and application of layered double hydroxide nanosheets

Layered double hydroxides (LDH) with unique structure and excellent properties have been widely studied in recent years. LDH have found widespread applications in catalysts, polymer/LDH nanocomposites, anion exchange materials, supercapacitors, and fire retardants. The exfoliated LDH ultrathin nanosheets with a thickness of a few atomic layers enable a series of new opportunities in both fundamental research and applications. In this review, we mainly summarize the LDH exfoliation methods developed in recent years, the recent developments for the direct synthesis of LDH single-layer nanosheets, and the applications of LDH nanosheets in catalyzing oxygen evolution reactions, crosslinkers, supercapacitors and delivery carriers.

Transition Metal-Based 2D Layered Double Hydroxide Nanosheets: Design Strategies and Applications in Oxygen Evolution Reaction

Water splitting driven by renewable energy sources is considered a sustainable way of hydrogen production, an ideal fuel to overcome the energy issue and its environmental challenges. The rational design of electrocatalysts serves as a critical point to achieve efficient water splitting. Layered double hydroxides (LDHs) with two-dimensionally (2D) layered structures hold great potential in electrocatalysis owing to their ease of preparation, structural flexibility, and tenability. However, their application in catalysis is limited due to their low activity attributed to structural stacking with irrational electronic structures, and their sluggish mass transfers. To overcome this challenge, attempts have been made toward adjusting the morphological and electronic structure using appropriate design strategies. This review highlights the current progress made on design strategies of transition metal-based LDHs (TM-LDHs) and their application as novel catalysts for oxygen evolution reactions (OERs) in alkaline conditions. We describe various strategies employed to regulate the electronic structure and composition of TM-LDHs and we discuss their influence on OER performance. Finally, significant challenges and potential research directions are put forward to promote the possible future development of these novel TM-LDHs catalysts.

Electrochemical oxidation of boron-doped nickel-iron layered double hydroxide for facile charge transfer in oxygen evolution electrocatalysts

The oxygen evolution reaction (OER) is the key reaction in water splitting systems, but compared with the hydrogen evolution reaction (HER), the OER exhibits slow reaction kinetics. In this work, boron doping into nickel-iron layered double hydroxide (NiFe LDH) was evaluated for the enhancement of OER electrocatalytic activity. To fabricate boron-doped NiFe LDH (B:NiFe LDH), gaseous boronization, a gas-solid reaction between boron gas and NiFe LDH, was conducted at a relatively low temperature. Subsequently, catalyst activation was performed through electrochemical oxidation for maximization of boron doping and improved OER performance. As a result, it was possible to obtain a remarkably reduced overpotential of 229 mV at 10 mA cm-2 compared to that of pristine NiFe LDH (315 mV) due to the effect of facile charge-transfer resistance by boron doping and improved active sites by electrochemical oxidation.

Design of Local Atomic Environments in Single-Atom Electrocatalysts for Renewable Energy Conversions

Single-atom electrocatalysts (SAECs) have recently attracted tremendous research interest due to their often remarkable catalytic responses, unmatched by conventional catalysts. The electrocatalytic performance of SAECs is closely related to the specific metal species and their local atomic environments, including their coordination number, the determined structure of the coordination sites, and the chemical identity of nearest and second nearest neighboring atoms. The wide range of distinct chemical bonding configurations of a single-metal atom with its surrounding host atoms creates virtually limitless opportunities for the rational design and synthesis of SAECs with tunable local atomic environment for high-performance electrocatalysis. In this review, the authors first identify fundamental hurdles in electrochemical conversions and highlight the relevance of SAECs. They then critically examine the role of the local atomic structures, encompassing the first and second coordination spheres of the isolated metal atoms, on the design of high-performance SAECs. The relevance of single-atom dopants for host activation is also discussed. Insights into the correlation between local structures of SAECs and their catalytic response are analyzed and discussed. Finally, the authors summarize major challenges to be addressed in the field of SAECs and provide some perspectives in the rational construction of superior SAECs for a wide range of electrochemical conversions.

Formation of porous NiCoV-LTH nanosheet arrays by in situ etching of nickel foam for the hydrogen evolution reaction at large current density

We report the in situ generation of NiCoV-LTH on nickel foam for the HER. Interestingly, the introduction of Co into NiV-LDH can induce the formation of porous nanosheets to expose a large number of active sites and change the electron density around Ni and V to promote the absorption of hydrogen species and thus accelerate the HER kinetics.

Layered double hydroxides as heterogeneous catalyst systems in the cross-coupling reactions: an overview

Layered double hydroxides (LDHs) are recognized as two-dimensional (2D) clay materials, which comprise the interlayer anions and host layers with a positive charge (brucite-like M(OH)₆ octahedral). They have been used as effective and eco-friendly heterogeneous catalytic systems in cross-coupling reactions. In this review, we try to underscore the applications of (LDHs) as an efficient and green catalyst in some important name reactions, namely Suzuki, Heck, Sonogashira, and Ullmann cross-coupling reactions leading to carbon-carbon bond formations.

A hierarchical CuO@NiCo layered double hydroxide core–shell nanoarray as an efficient electrocatalyst for the oxygen evolution reaction

A hierarchical CuO@NiCo LDH core–shell nanoarray on copper foil (CuO@NiCo LDH/CF) acts as an efficient and durable oxygen-evolving electrocatalyst, capable of driving 20 mA cm⁻² at an overpotential of only 256 mV in 1.0 M KOH.

Interface engineering in the α-Co(OH)2/ZIF-67 heterostructure for enhanced oxygen evolution electrocatalysis

The interface coupling endows the heterostructural α-Co(OH)₂/ZIF-67-0.6 material with higher activity (η₁₀ = 320 mV), fast kinetics and excellent durability toward oxygen evolution reaction electrocatalysis.

In situ growth of an Fe-doped NiCo-MOF electrocatalyst from layered double hydroxide effectively enhances electrocatalytic oxygen evolution performance

The hierarchical book-like Fe-NiCo-MOF exhibits superior OER performance coupled with outstanding stability at a high current density.

Solvothermal synthesis of carbonate-type layered double hydroxide monolayer nanosheets: Solvent selection based on characteristic parameter matching criterion

Monolayer nanosheets of CO₃^2--type layered double hydroxides (LDHs) have many special applications, but their fabrication is challenging. Herein, Co₂Al-CO₃ and Co₂Fe-CO₃ LDH nanosheets were synthesized via a solvothermal method. 31 solvents with different characteristic parameters, including the surface free energy (γ) and solubility (δ) parameters were chosen, to explore the correlation between the formation of monolayer LDHs (ML-LDHs) and the characteristic parameters of solvents. The results reveal that when the solvents used have the characteristic parameters matching to those of the LDHs, CO₃^2--type ML-LDHs with a thickness of ca. 1 nm can be obtained. The mixed-solvent strategy can provide the effective solvents for the synthesis of ML-LDHs. The dispersions of CO₃^2--type ML-LDHs can be stable for at least six months without obvious precipitation. In addition, it is demonstrated that the δ parameters of LDHs can be calculated from the γ parameters via the molar volume-free γ-δ equations developed previously. Furthermore, a new parameter called "surface free energy distance" is introduced, which can be used for screening effective solvents for the synthesis of ML-LDHs. To the best of our knowledge, this is the first time to investigate the applicable of the characteristic parameter matching principle for the bottom-up synthesis of ML-LDHs. This work deepens the understanding on the feature of CO₃^2--type LDHs and provides a solvent selection strategy for the synthesis of CO₃^2--type ML-LDHs.

Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting

Single-component transition-metal oxide (TMO: FeO_x, NiO_x, or CoO_x) nanosheets grown on nickel foam (NF) were electrochemically optimized with Li ion (Na ion)-induced conversion reaction for bifunctional electrocatalysis. The optimum FeO_x/NF-Li electrocatalyst exhibits low overpotentials of 239 mV for hydrogen evolution reaction and 276 mV for oxygen evolution reaction at a current density of 100 mA cm^-2. A two-electrode water splitting cell using FeO_x/NF-Li as both anode and cathode requires only 1.60 V to achieve a current density of 10 mA cm^-2. The impressive water splitting performance of the FeO_x/NF-Li electrode is revealed to be attributed to Li-induced electrochemical conversion, which alters the crystal structure, creating more active sites for electrocatalytic reactions, as well as introduces O vacancies increasing the electron density and the intrinsic conductivity. More importantly, the atomic arrangement is regulated from tetrahedral Fe(Td) to octahedral Fe(Oh) coordination, which acts as catalytically active sites with reduced Gibbs free energy for the rate-determining steps. This electrochemical conversion reaction can be extended to other TMOs (i.e., NiO_x/NF and CoO_x/NF) for promoted electrocatalytic water splitting performances. This study provides an in-depth understanding on the nature of atomic and electronic structure evolution to promote the electrocatalytic activity.

Ni17 W3 -W Interconnected Hybrid Prepared by Atmosphere- and Thermal-Induced Phase Separation for Efficient Electrocatalysis of Alkaline Hydrogen Evolution

The development of efficient and stable noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) in alkaline media is still a challenge. Herein, a hybrid material formed by the interconnection of Ni₁₇ W₃ intermetallic compound with metallic W is demonstrated for HER. The Ni₁₇ W₃ -W hybrid is prepared by the atmosphere- and thermal-induced phase-separation strategy from a single-phase precursor (NiWO₄ ), which gives Ni₁₇ W₃ -W hybrid abundant and tight interfaces. The theoretical calculation manifests that Ni₁₇ W₃ shows more optimized energetics for adsorbed H atom, while W has lower energy barrier for water dissociation, and the synergistic effect between them is believed to facilitate the HER kinetics. Moreover, Ni₁₇ W₃ presents a proper adsorption strength for both adsorbed OH and H, and thus Ni₁₇ W₃ may also act as a high HER catalyst by itself. As a result, the Ni₁₇ W₃ -W hybrid demonstrates high activity and durability for HER in liquid alkaline electrolyte; the electrolyzer assembled by Ni₁₇ W₃ -W hybrid and Ni-Fe-layered double hydroxide (LDH) as, respectively, the cathode and anode electrocatalysts presents superior performance to Pt/C-IrO₂ benchmark. In addition, the Ni₁₇ W₃ -W hybrid also works well in the water electrolyzer based on solid hydroxide exchange membrane. The present work provides a promising pathway to the design of high-performance electrocatalysts.

Unveiling the Occurrence of Co(III) in NiCo Layered Electroactive Hydroxides: The Role of Distorted Environments

Co- and Ni-based layered hydroxides constitute a unique class of two-dimensional inorganic materials with exceptional chemical diversity, physicochemical properties and outstanding performance as supercapacitors and overall water splitting catalysts. Recently, the occurrence of Co(III) in these phases has been proposed as a key factor that enhance their electrochemical performance. However, the origin of this centers and control over its contents remains as an open question. We employed the Epoxide Route to synthesize a whole set of α-NiCo layered hydroxides. The PXRD and XAS characterization alert about the occurrence of Co(III) as a consequence of the increment in the Ni content. DFT+U simulation suggest that the shortening of the Co-O distance promotes a structural distortion in the Co environments, resulting in a double degeneration in the octahedral Co 3d orbitals. Hence, a strong modification of the electronic properties leaves the system prone to oxidation, by the appearance of Co localized electronic states on the Fermi level. This work combines a microscopic interpretation supported by a multiscale crystallochemical analysis, regarding the so-called synergistic redox behavior of Co and Ni, offering fundamental tools for the controllable design of highly efficient electroactive materials. To the best of our knowledge, this is the first computational-experimental investigation of the electronic and structural details of α-NiCo hydroxides, laying the foundation for the fine tuning of electronic properties in layered hydroxides.

Integrating hydrogen production with anodic selective oxidation of sulfides over a CoFe layered double hydroxide electrode

Replacing the sluggish oxygen evolution reaction (OER) with oxidation reactions for the synthesis of complex pharmaceutical molecules coupled with enhanced hydrogen evolution reaction (HER) is highly attractive, but it is rarely explored. Here, we report an electrochemical protocol for selective oxidation of sulfides to sulfoxides over a CoFe layered double hydroxide (CoFe-LDH) anode in an aqueous-MeCN electrolyte, coupled with 2-fold promoted cathodic H2 productivity. This protocol displays high activity (85-96% yields), catalyst stability (10 cycles), and generality (12 examples) in selective sulfide oxidation. We demonstrate its applicability in the synthesis of four important pharmaceutical related sulfoxide compounds with scalability (up to 1.79 g). X-ray spectroscopy investigations reveal that the CoFe-LDH material evolved into amorphous CoFe-oxyhydroxide under catalytic conditions. This work may pave the way towards sustainable organic synthesis of valuable pharmaceuticals coupled with H2 production.

Advanced electrocatalysts based on two-dimensional transition metal hydroxides and their composites for alkaline oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is a crucial part in developing high-efficiency fuel cells and metal-air batteries, which have been cherished as clean and sustainable energy conversion devices/systems to meet the ever-increasing energy demand. ORR electrocatalysts currently employed in the cathodes of fuel cells and metal-air batteries are mainly based on high-cost and scarce noble metal elements. It is thus of great importance to develop cheap and earth-abundant ORR electrocatalysts. In this aspect, redox-active transition metal hydroxides, a class of multifunctional inorganic layered materials, have been proposed as prospective candidates on account of their abundance and high ORR activities. In this article, the preparation and structural evolution of transition metal hydroxides, in particular their exfoliation into two-dimensional (2D) nanosheets, as well as compositing/integrating with catalytic active and/or conductive components to overcome the insulating nature of hydroxides in alkaline ORR, are summarized. Recent advances have demonstrated that 2D transition metal hydroxides with carefully tuned compositions and elaborately designed nanoarchitectures can achieve both high activity and high pathway selectivity, as well as excellent stability comparable to those of commercial Pt/C electrocatalysts. To realize the dream of renewable electrochemical energy conversion, new strategies and insights into rational designing of 2D hydroxide-based nanostructures with further enhanced electrocatalytic performance are still to be vigorously pursued.