Layered double hydroxides toward electrochemical energy storage and conversion: design, synthesis and applications

Adsorptive removal of heavy metals from aqueous solutions: Progress of adsorbents development and their effectiveness

Increased levels of heavy metals (HMs) in aquatic environments poses serious health and ecological concerns. Hence, several approaches have been proposed to eliminate/reduce the levels of HMs before the discharge/reuse of HMs-contaminated waters. Adsorption is one of the most attractive processes for water decontamination; however, the efficiency of this process greatly depends on the choice of adsorbent. Therefore, the key aim of this article is to review the progress in the development and application of different classes of conventional and emerging adsorbents for the abatement of HMs from contaminated waters. Adsorbents that are based on activated carbon, natural materials, microbial, clay minerals, layered double hydroxides (LDHs), nano-zerovalent iron (nZVI), graphene, carbon nanotubes (CNTs), metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) are critically reviewed, with more emphasis on the last four adsorbents and their nanocomposites since they have the potential to significantly boost the HMs removal efficiency from contaminated waters. Furthermore, the optimal process conditions to achieve efficient performance are discussed. Additionally, adsorption isotherm, kinetics, thermodynamics, mechanisms, and effects of varying adsorption process parameters have been introduced. Moreover, heavy metal removal driven by other processes such as oxidation, reduction, and precipitation that might concurrently occur in parallel with adsorption have been reviewed. The application of adsorption for the treatment of real wastewater has been also reviewed. Finally, challenges, limitations and potential areas for improvements in the adsorptive removal of HMs from contaminated waters are identified and discussed. Thus, this article serves as a comprehensive reference for the recent developments in the field of adsorptive removal of heavy metals from wastewater. The proposed future research work at the end of this review could help in addressing some of the key limitations facing this technology, and create a platform for boosting the efficiency of the adsorptive removal of heavy metals.

"On-The-Fly" Synthesis of Self-Supported LDH Hollow Structures Through Controlled Microfluidic Reaction-Diffusion Conditions

Layered double hydroxides (LDHs) are a class of functional materials that exhibit exceptional properties for diverse applications in areas such as heterogeneous catalysis, energy storage and conversion, and bio-medical applications, among others. Efforts have been devoted to produce millimeter-scale LDH structures for direct integration into functional devices. However, the controlled synthesis of self-supported continuous LDH materials with hierarchical structuring up to the millimeter scale through a straightforward one-pot reaction method remains unaddressed. Herein, it is shown that millimeter-scale self-supported LDH structures can be produced by means of a continuous flow microfluidic device in a rapid and reproducible one-pot process. Additionally, the microfluidic approach not only allows for an "on-the-fly" formation of unprecedented LDH composite structures, but also for the seamless integration of millimeter-scale LDH structures into functional devices. This method holds the potential to unlock the integrability of these materials, maintaining their performance and functionality, while diverging from conventional techniques like pelletization and densification that often compromise these aspects. This strategy will enable exciting advancements in LDH performance and functionality.

Uncovering thermally activated purple-to-blue luminescence in Co-modified MgAl-layered double hydroxide

Thermally activated blue-to-purple luminescence of Co-modified nano-sandrose MgAl-layered double hydroxides (LDHs) is concentration dependent, occurring only for MgCoAl-LDH with a molar metal cation concentration of 15% Co. Temperature sweep luminescence spectroscopy between 83 K and 298 K shows that the luminescence is strongest at room temperature, increasing with an activation energy of 1 kJ mol-1 between these temperatures. The luminescence occurs in a broad, but fine-structured band below the conduction band (CB) edge at 3.0 eV after excitation at 5.0 eV.

A superior catalyst for ozone decomposition: NiFe layered double hydroxide

Ground-level ozone is harmful to human beings and ecosystems, while room-temperature catalytic decomposition is the most effective technology for ozone abatement. However, solving the deactivation of existing metal oxide catalysts was caused by oxygen-containing intermediates is challenging. Here, we successfully prepared a two-dimensional NiFe layered double hydroxide (NiFe-LDH) catalyst via a facile co-precipitation method, which exhibited stable and highly efficient performance of ozone decomposition under harsh operating conditions (high space velocity and humidity). The NiFe-LDH catalyst with Ni/Fe = 3 and crystallization time over 5 hr (named Ni3Fe-5) exhibited the best catalytic performance, which was well beyond that of most existing manganese-based oxide catalysts. Specifically, under relative humidity of 65% and space velocity of 840 L/(g·hr), Ni3Fe-5 showed ozone conversion of 89% and 76% for 40 ppmV of O3 within 6 and 168 hr at room-temperature, respectively. We demonstrated that the layered structure of NiFe-LDH played a decisive role in its outstanding catalytic performance in terms of both activity and water resistance. The LDH catalysts fundamentally avoids the deactivation caused by the occupancy of oxygen vacancies by oxygen-containing species (H2O, O-, and O2-) in manganese-based oxide. This study indicated the promising application potential of LDHs than manganese-based oxide catalysts in removal of gaseous ozone.

NiTi-Layered Double Hydroxide@Carbon Nanotube as a Cathode Material for Chloride-Ion Batteries

Chloride-ion batteries (CIBs) are one of the promising candidates for energy storage due to their low cost, high theoretical energy density and high safety. However, the limited types of cathode materials in CIBs have hindered their development. In this work, a NiTi-LDH@CNT composite is prepared using a reverse microemulsion method and applied in CIBs for the first time. The specific surface area and the pore volume of the obtained NiTi-LDH@CNT composites can reach 266 m2 g-1 and 0.42 cm3 g-1, respectively. Electrochemical tests indicate that the composite electrode delivers a reversible specific capacity of 69 mAh g-1 after 150 cycles at a current density of 100 mA g-1 in 0.5 M PP14Cl/PC electrolyte. Ni2+/Ni3+ and Ti3+/Ti4+ valence changes during electrochemical cycling are demonstrated by X-ray photoelectron spectroscopy (XPS), while reversible migration of Cl- is revealed by ex-situ EDS and ex-situ XRD. The stable layered structure and abundant valence changes of the NiTi-LDH@CNT composite make it an exceptional candidate as a cathode material for CIBs.

Crystallographic and Geometrical Dependence of Water Oxidation Activity in Co-Based Layered Hydroxides

Cobalt-based layered hydroxides (LHs) stand out as one of the best families of electroactive materials for the alkaline oxygen evolution reaction (OER). However, fundamental aspects such as the influence of the crystalline structure and its connection with the geometry of the catalytic sites remain poorly understood. Thus, to address this topic, we have conducted a thorough experimental and in silico study on the most important divalent Co-based LHs (i.e., α-LH, β-LH, and LDH), which allows us to understand the role of the layered structure and coordination environment of divalent Co atoms on the OER performance. The α-LH, containing both octahedral and tetrahedral sites, behaves as the best OER catalyst in comparison to the other phases, pointing out the role of the chemical nature of the crystalline structure. Indeed, density functional theory (DFT) calculations confirm the experimental results, which can be explained in terms of the more favorable reconstruction into an active Co(III)-based oxyhydroxide-like phase (dehydrogenation process) as well as the significantly lower calculated overpotential across the OER mechanism for the α-LH structure (exhibiting lower Egap). Furthermore, ex situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of the α-LH phase into a highly reactive oxyhydroxide-like stable structure under ambient conditions. Hence, our findings highlight the key role of tetrahedral sites on the electronic properties of the LH structure as well as their inherent reactivity toward OER catalysis, paving the way for the rational design of more efficient and low-maintenance electrocatalysts.

Tunable Method for the Preparation of Layered Double Hydroxide Nanoparticles and Mesoporous Mixed Metal Oxide Electrocatalysts for the Oxygen Evolution Reaction

Preparation of high-performance and durable electrocatalysts for anion exchange membrane water electrolysis is a crucial step toward the broad implementation of this technology. Here, we present an easily tunable, one-step hydrothermal method for the preparation of Ni-based (NiX, X = Co, Fe) layered double hydroxide nanoparticles (LDHNPs) for the oxygen evolution reaction (OER), using tris(hydroxymethyl)aminomethane (Tris-NH₂) for particle growth control. The LDHNPs are used as building blocks of mesoporous mixed metal oxides (MMOs) with a block copolymer template (Pluronic F127), followed by thermal treatment at 250 °C. NiX MMOs have a significantly larger surface area compared to the analogous LDHNPs. NiX LDHNPs and MMOs exhibit excellent performance and long-term cycling stability, making them promising OER catalysts. Moreover, this versatile method can be easily tailored and scaled up for the preparation of platinum group metal-free electrocatalysts for other reactions of interest, which highlights the relevance of this work to the field of electrocatalysis.

Facile formation of Zn-incorporated NiFe layered double hydroxide as highly-efficient oxygen evolution catalyst

Electrochemical water splitting is the primary method to produce green hydrogen, which is considered an efficient alternative to fossil fuels for achieving carbon neutrality. For meeting the increasing market demand for green hydrogen, high-efficiency, low-cost, and large-scale electrocatalysts are crucial. In this study, we report a simple spontaneous corrosion and cyclic voltammetry (CV) activation method to fabricate Zn-incorporated NiFe layered double hydroxide (LDH) on commercial NiFe foam, which shows excellent oxygen evolution reaction (OER) performance. The electrocatalyst achieves an overpotential of 565 mV and outstanding stability of up to 112 h at 400 mA cm^-2. The active layer for OER is shown to be β-NiFeOOH according to the results of in-situ Raman. Our findings suggest that the NiFe foam treated by simple spontaneous corrosion has promising industrial applications as a highly efficient OER catalyst.

An enzyme-free Ti3C2/Ni/Sm-LDH-based screen-printed-electrode for real-time sweat detection of glucose

Here, we report the fabrication of an enzyme-free glucose sensor benefiting from nickel-samarium nanoparticles-decorated MXene layered double hydroxide (MXene/Ni/Sm-LDH). The electrochemical response of the MXene/Ni/Sm-LDH to glucose was studied via cyclic voltammetry (CV). The fabricated electrode has high electrocatalytic activity for glucose oxidation. The voltametric response of the MXene/Ni/Sm-LDH electrode to glucose was investigated by differential pulse voltammetry (DPV) that demonstrated an extended linear range of from 0.001 to 0.1 mM and 0.25-7.5 mM with a detection limit down to 0.24 μM (S/N = 3) and a sensitivity at 1673.54 μA mM^-1 cm^-2 1519.09 μA mM^-1 cm^-2 in concentrations of 0.01 mM and 1 mM respectively as well as good repeatability, high stability and applicability for the real sample analysis. Moreover, the as-fabricated sensor was applied to glucose detection in human sweat and showed promising results.

Defective multi-element hydroxides nanosheets for rapid removal of anionic organic dyes from water and oxygen evolution reaction

Water pollution by dyes is one of the biggest environmental problems. Adsorption technology has been widely used in wastewater treatment. In this work, high-entropy concept is used to design surface defective hydroxides realizing the rapid removal of dyes from water. Multi-element hydroxides (MEHs) containing three (CoMnNi, MEH-Ternary), four (CoMnNiZn, MEH-Quaternary), and five (CoMnNiZnFe, MEH-Quinary) metal elements are successfully synthesized through a polyol process. These as-synthesized MEHs are composed of nanosheets with a brucite-like structure. Along with the increase in compositional complexity (i.e., configurational entropy), the thickness of the nanosheets in these MEHs decreases, while the degree of surface defects increase. These surface defects are probably the active sites for anionic dyes adsorption, suggesting rapid adsorption kinetics with shortened diffusion path length. For MEH-Quinary in 0.2 mM Congo red (CR) and MEH-Ternary in 0.4 mM methyl orange (MO) aqueous solutions, respectively, high removal efficiency > 99.0% is achieved in the first 30 s. Their pseudo-second-order rate constants are two orders of magnitude higher than that of activated carbon and hydrotalcite. MEH-Quinary has maximum CR and MO adsorption quantity of 546.4 and 404.9 mg g^-1, respectively, by Langmuir model. The MEH-Quinary is also a potential electrocatalyst for oxygen evolution reaction.

2D Functionalized Germananes: Synthesis and Applications

In the realm of 2D layered materials, the monoelemental group 14 Xene, germanene, as the germanium analog of graphene, has emerged as the next prospective candidate. Preceded by silicon, germanium is widely used in the semiconductor industry; thus, germanene is deemed compatible with existing semiconductor technologies. Germanene consists of mixed sp² -sp³ -hybridized networks in a buckled hexagonal honeycomb structure. Chemical exfoliation of Zintl phases, such as CaGe₂ , specifically the topotactical deintercalation in acidic media, removes the alkaline earth metal ions Ca²⁺ , giving rise to layered germanane (germanene with the Ge centers covalently saturated with terminal hydrogen atoms). Diverse variants of functionalized germananes (with covalent group(s) termination) can be obtained by varying the topotactical deintercalation precursors, elevating the game with limitless functionalization possibilities for customizable properties or new functionalities. The preparation of Zintl phases to the details of functionalized and modified germananes and their properties, and the additional exfoliation step to achieve mono- or few-layer germananes, are comprehensively covered. The progress and challenges of 2D functionalized germananes in optoelectronics, catalysis, energy conversion and storage, sensors, and biomedical areas are reviewed. This review provides insight into designing and exploring this class of atomically thin semiconductors in realizing future nanoarchitectonics.

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

Water electrolysis encounters a challenging problem in designing a highly efficient, long durable, non-noble metal-free electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, in our work, a two-step hydrothermal reaction was performed to construct a hierarchal NiFe-layer double hydroxide (LDH)/CuS over copper foam for the overall water splitting reaction. While employed the same as an anode material, the designed heterostructure electrode NiFe-LDH/CuS/Cu exhibits excellent OER performance and it demands 249 mV overpotential to reach a current density of 50 mA cm^-2 with a lower Tafel slope value of 81.84 mV dec^-1. While as a cathode material, the NiFe-LDH/CuS/Cu shows superior HER performance and it demands just 28 mV of overpotential value to reach a current density of 10 mA cm^-2 and a lower Tafel slope value of 95.98 mV dec^-1. Hence, the NiFe-LDH/CuS/Cu outperforms the commercial Pt/C and RuO₂ in terms of activity in HER and OER, respectively. Moreover, when serving as both the cathode and anode catalysts in an electrolyzer for total water splitting, the synthesized electrode only needs a cell potential of 1.55 V versus RHE to reach a current density of 20 mA cm^-2 and long-term durability for 25 h in alkaline media. To study the interfacial electron transfer, Mott-Schottky experiments were performed, representing that the electron is transferred from n-type NiFe-LDH to p-type CuS as a result of creating the p-n junction in NiFe-LDH/CuS/Cu. The formation of this p-n junction allows the LDH layer to be more active toward the OH- adsorption and thereby could allow the OER or HER with a less energy input. This work affords another route to a cost effective, highly efficient catalyst toward producing clean energy across the globe.

Layer-by-Layer Materials for the Fabrication of Devices with Electrochemical Applications

The construction of nanostructured materials for their application in electrochemical processes, e.g., energy storage and conversion, or sensing, has undergone a spectacular development over the last decades as a consequence of their unique properties in comparison to those of their bulk counterparts, e.g., large surface area and facilitated charge/mass transport pathways. This has driven strong research on the optimization of nanostructured materials for the fabrication of electrochemical devices, which demands techniques allowing the assembly of hybrid materials with well-controlled structures and properties. The Layer-by-Layer (LbL) method is well suited for fulfilling the requirements associated with the fabrication of devices for electrochemical applications, enabling the fabrication of nanomaterials with tunable properties that can be exploited as candidates for their application in fuel cells, batteries, electrochromic devices, solar cells, and sensors. This review provides an updated discussion of some of the most recent advances on the application of the LbL method for the fabrication of nanomaterials that can be exploited in the design of novel electrochemical devices.

Layered double hydroxides functionalized by carbonaceous materials: from preparation to energy and environmental applications

Along with the exponential demand for energy and pollution-free-environment, layered double hydroxides (LDHs) have gained extensive explorations because of their diverse nanostructures and tunable elemental compositions. However, the applications of LDHs are hindered by their poor activity, sluggish mass transfer, and aggregation. LDHs functionalized by carbonaceous materials (CMs) (LDH-CM) are expected to overcome the above disadvantages and even generate more excellent performance. This review first analyzes the research evolvement of LDH-CM composites during the past 25 years. Next, the advantages of LDH-CM composites are highlighted, such as morphology optimization, high electrical conductivity, more stable, good heat, and mass transfer performance. Following the synthetic strategies, including chemical assembly of LDHs and CMs, direct growth of LDH on CMs (two-step nucleation and growth and surface-confined growth) and direct CM formation on LDHs are fully discussed. Then, the recent progress achieved in LDH-CM composites for the application of energy storage and environmental protection is summarized in detail. In particular, the review illustrates the reasons why these constructing strategies can improve the performance of LDH-CM composites. Finally, challenges and future research prospects of LDH-CM composites are highlighted.

The back-deposition of dissolved Mn2+ to MnO2 cathodes for stable cycling in aqueous zinc batteries

The Mn²⁺ dissolution of MnO₂ cathode materials causes rapid capacity decay in aqueous zinc batteries. We herein show that the dissolved Mn²⁺ can be deposited back to the cathode with the aid of a suitable conductive agent. The active material is thus retained for energy storage, and this MnO₂/Mn²⁺ redox process also provides capacity. In the Mn²⁺ free ZnSO₄ electrolyte, MnO₂ delivers 325 mA h g^-1 capacity at 0.1 A g^-1, and 90.4% capacity retention is achieved after 3000 cycles at 5 A g^-1. Our work demonstrates an effective strategy to realize stable cycling of MnO₂ cathodes in aqueous zinc batteries without Mn²⁺ additives.

Co-Ni Layered Double Hydroxide for the Electrocatalytic Oxidation of Organic Molecules: An Approach to Lowering the Overall Cell Voltage for the Water Splitting Process

Electrocatalytic oxidation of simple organic molecules offers a promising strategy to combat the sluggish kinetics of the water oxidation reaction (WOR). The low potential requirement, inhibition of the crossover of gases, and formation of value-added products at the anode are benefits of the electrocatalytic oxidation of organic molecules. Herein, we developed cobalt-nickel-based layered double hydroxide (LDH) as a robust material for the electrocatalytic oxidation of alcohols and urea at the anode, replacing the WOR. A facile synthesis protocol to form LDHs with different ratios of Co and Ni is adapted. It demonstrates that the reactants could be efficiently oxidized to concomitant chemical products at the anode. The half-cell study shows an onset potential of 1.30 V for benzyl alcohol oxidation reaction (BAOR), 1.36 V for glycerol oxidation reaction (GOR), 1.33 V for ethanol oxidation reaction (EOR), and 1.32 V for urea oxidation reaction (UOR) compared with 1.53 V for WOR. Notably, the hybrid electrolyzer in a full-cell configuration significantly reduces the overall cell voltage at a 20 mA cm^-2 current density by ∼15% while coupling with the BAOR, EOR, and GOR and ∼12% with the UOR as the anodic half-cell reaction. Furthermore, the efficiency of hydrogen generation remains unhampered with the types of oxidation reactions (alcohols and urea) occurring at the anode. This work demonstrates the prospects of lowering the overall cell voltage in the case of a water electrolyzer by integrating the hydrogen evolution reaction with suitable organic molecule oxidation.

Layered Double Hydroxide Catalysts Preparation, Characterization and Applications for Process Development: An Environmentally Green Approach

The adage of new generation of fine chemicals process is the best process applied in the absence of conventional methods. However, many methods use different reaction parameters, such as basic and acidic catalysts, for example oxidation, reduction, bromination, water splitting, cyanohydrin, ethoxylation, syngas, aldol condensation, Michael addition, asymmetric ring opening of epoxides, epoxidation, Wittig and Heck reaction, asymmetric ester epoxidation of fatty acids, combustion of methane, NOx reduction, biodiesel synthesis, propylene oxide polymerization. Layered Double Hydroxides (LDHs) have received considerable attention due their potential applications in flame retardant and has excellent medicinal property for reducing acidity. These catalysts are characterized using analytical techniques, such as: X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Raman spectroscopy, Thermogravimetric-Differential Thermal Analyzer (TG-DTA), Scanning electron microscope (SEM), Transmission electron microscopes (TEM), Brunauer-Emmett-Teller (BET) surface area, N2 Adsorption-desorption, Temperature programmed reduction (TPR), X-ray photoelectrons spectroscopy (XPS), which gives its overall picture of its structure, porosity, morphology, thermal stability, reusability, and activity of catalysts. LDHs catalysts have proven to be economic and environmentally friendly. The above discussed applications make these catalysts unique from Green Chemistry point of view since they are reusable, and eco-friendly catalysts. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Layered Double Hydroxide Catalysts for Ozone Decomposition: The Synergic Role of M2+ and M3

In previous work, we successfully prepared a NiFe-layered double hydroxide (LDH) with superior activity and stability for catalytic ozone decomposition, which fundamentally avoids deactivation under high-humidity conditions. However, the role of the metal elements (M²⁺ and M³⁺) in LDH catalysts is not clear. Here, LDH materials containing different metals (NiFe, NiAl, NiMn, CoFe, and MgFe) were prepared by a simple co-precipitation method. It was found that the LDHs containing Ni²⁺ exhibited catalytic performance far superior to that of Co²⁺ and Mg²⁺ for ozone elimination, and NiFe-LDH had the best activity and stability among LDH materials prepared in this study. The NiFe-LDH can maintain 78% catalytic activity within 144 h at room temperature, even under a relative humidity of 65% and a space velocity of 840 L·g^-1·h^-1. Physicochemical characterizations demonstrated that chemical stability in an oxidizing atmosphere and the synergic role of M²⁺ and M³⁺ ions are crucial. The result of density functional theory calculation showed that the synergic role of Ni²⁺ and Fe³⁺ weakens the interaction between O and H in the O-H bond, which effectively lowers the reaction barrier of ozone decomposition compared with MgFe-LDH.

Hydrogen production coupled with water and organic oxidation based on layered double hydroxides

Hydrogen production via electrochemical water splitting is one of the most green and promising ways to produce clean energy and address resource crisis, but still suffers from low efficiency and high cost mainly due to the sluggish oxygen evolution reaction (OER) process. Alternatively, electrochemical hydrogen-evolution coupled with alternative oxidation (EHCO) has been proposed as a considerable strategy to improve hydrogen production efficiency combined with the production of high value-added chemicals. Although with these merits, high-efficient electrocatalysts are always needed in practical operation. Typically, layered double hydroxides (LDHs) have been developed as a large class of advanced electrocatalysts toward both OER and EHCO with high efficiency and stability. In this review, we have summarized the latest progress of hydrogen production from the perspectives of designing efficient LDHs-based electrocatalysts for OER and EHCO. Particularly, the influence of structure design and component regulation on the efficiency of their electrocatalytic process have been discussed in detail. Finally, we look forward to the challenges in the field of hydrogen production via electrochemical water splitting coupled with organic oxidation, such as the mechanism, selected oxidation as well as system design, hoping to provide certain inspiration for the development of low-cost hydrogen production technology.

Fiber‐Shaped Electronic Devices

Textile electronics embedded in clothing represent an exciting new frontier for modern healthcare and communication systems. Fundamental to the development of these textile electronics is the development of the fibers forming the cloths into electronic devices. An electronic fiber must undergo diverse scrutiny for its selection for a multifunctional textile, viz., from the material selection to the device architecture, from the wearability to mechanical stresses, and from the environmental compatibility to the end‐use management. Herein, the performance requirements of fiber‐shaped electronics are reviewed considering the characteristics of single electronic fibers and their assemblies in smart clothing. Broadly, this article includes i) processing strategies of electronic fibers with required properties from precursor to material, ii) the state‐of‐art of current fiber‐shaped electronics emphasizing light‐emitting devices, solar cells, sensors, nanogenerators, supercapacitors storage, and chromatic devices, iii) mechanisms involved in the operation of the above devices, iv) limitations of the current materials and device manufacturing techniques to achieve the target performance, and v) the knowledge gap that must be minimized prior to their deployment. Lessons learned from this review with regard to the challenges and prospects for developing fiber‐shaped electronic components are presented as directions for future research on wearable electronics.

Integrated Battery-Capacitor Electrodes: Pyridinic N-Doped Porous Carbon-Coated Abundant Oxygen Vacancy Mn-Ni-Layered Double Oxide for Hybrid Supercapacitors

Integrating the battery behavior and supercapacitor behavior in a single electrode to obtain better electrochemical performance has been widely researched. However, there is still a lack of research studies on an integrated battery-capacitor supercapacitor electrode (BatCap electrode). In this work, an integrated BatCap electrode porous carbon-coated Mn-Ni-layered double oxide (Mn-Ni LDO-C) was fabricated successfully using controllable heat treatment of polypyrrole-precoated Mn-Ni-layered double hydroxide (Mn-Ni LDH@PPy). This Mn-Ni LDO-C electrode was grown on Ni foam directly and possessed a hierarchical structure that consisted of a pyridinic N (N-6)-doped porous carbon shell and a Mn-Ni LDO core within abundant oxygen vacancies. Benefiting from the synergistic effect of N-6-doped porous carbon and increased oxygen vacancies, Mn-Ni LDO-C exhibited excellent electrochemical performance. The capacity of Mn-Ni LDO-C reached 2.36 C cm^-2 (1478.1 C g^-1) at 1 mA cm^-2 and remained at 92.1% of the initial capacity after 5000 cycles at a current density of 20 mA cm^-2. The aqueous battery-supercapacitor hybrid device Mn-Ni LDO-C//active carbon (Mn-Ni LDO-C//AC) also presented superior cycle stability: it retained 85.3% of the original capacity after 5000 cycles at 2 A g^-1. Meanwhile, Mn-Ni LDO-C//AC could work normally under a wider potential window (2.0 V), so that the device held the highest energy density of 78.2 Wh kg^-1 at a power density of 499.7 W kg^-1 and retained 39.1 Wh kg^-1 at the highest power density of 31.3 kW kg^-1. Two Mn-Ni LDO-C//AC devices connected in series could light a light-emitting diode (LED) bulb easily and keep the LED brightly illuminated for more than 10 min. In general, this work synthesized an integrated BatCap electrode Mn-Ni LDO-C; the integrated electrode exhibited high electrochemical performance, thus has a promising application prospect in the field of energy storage.

Differences and Similarities of Photocatalysis and Electrocatalysis in Two-Dimensional Nanomaterials: Strategies, Traps, Applications and Challenges

Photocatalysis and electrocatalysis have been essential parts of electrochemical processes for over half a century. Recent progress in the controllable synthesis of 2D nanomaterials has exhibited enhanced catalytic performance compared to bulk materials. This has led to significant interest in the exploitation of 2D nanomaterials for catalysis. There have been a variety of excellent reviews on 2D nanomaterials for catalysis, but related issues of differences and similarities between photocatalysis and electrocatalysis in 2D nanomaterials are still vacant. Here, we provide a comprehensive overview on the differences and similarities of photocatalysis and electrocatalysis in the latest 2D nanomaterials. Strategies and traps for performance enhancement of 2D nanocatalysts are highlighted, which point out the differences and similarities of series issues for photocatalysis and electrocatalysis. In addition, 2D nanocatalysts and their catalytic applications are discussed. Finally, opportunities, challenges and development directions for 2D nanocatalysts are described. The intention of this review is to inspire and direct interest in this research realm for the creation of future 2D nanomaterials for photocatalysis and electrocatalysis.

Layered Double Hydroxide Quantum Dots for Use in a Bifunctional Separator of Lithium-Sulfur Batteries

Functional separators, which are chemically modified and coated with nanostructured materials, are considered an effective and economical approach to suppressing the shuttle effect of lithium polysulfide (LiPS) and promoting the conversion kinetics of sulfur cathodes. Herein, we report cobalt-aluminum-layered double hydroxide quantum dots (LDH-QDs) deposited with nitrogen-doped graphene (NG) as a bifunctional separator for lithium-sulfur batteries (LSBs). The mesoporous LDH-QDs/NG hybrids possess abundant active sites of Co²⁺ and hydroxide groups, which result in capturing LiPSs through strong chemical interactions and accelerating the redox kinetics of the conversion reaction, as confirmed through X-ray photoelectron spectroscopy, adsorption tests, Li₂S nucleation tests, and electrokinetic analyses of the LiPS conversion. The resulting LDH-QDs/NG hybrid-coated polypropylene (LDH-QDs/NG/PP) separator, with an average thickness of ∼17 μm, has a high ionic conductivity of 2.67 mS cm^-1. Consequently, the LSB cells with the LDH-QDs/NG/PP separator can deliver a high discharge capacity of 1227.48 mAh g^-1 at 0.1C along with a low capacity decay rate of 0.041% per cycle over 1200 cycles at 1.0C.

Toxicity of Zn-Fe Layered Double Hydroxide to Different Organisms in the Aquatic Environment

The application of layered double hydroxide (LDH) nanomaterials as catalysts has attracted great interest due to their unique structural features. It also triggered the need to study their fate and behavior in the aquatic environment. In the present study, Zn-Fe nanolayered double hydroxides (Zn-Fe LDHs) were synthesized using a co-precipitation method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and nitrogen adsorption-desorption analyses. The toxicity of the home-made Zn-Fe LDHs catalyst was examined by employing a variety of aquatic organisms from different trophic levels, namely the marine photobacterium Vibrio fischeri, the freshwater microalga Pseudokirchneriella subcapitata, the freshwater crustacean Daphnia magna, and the duckweed Spirodela polyrhiza. From the experimental results, it was evident that the acute toxicity of the catalyst depended on the exposure time and type of selected test organism. Zn-Fe LDHs toxicity was also affected by its physical state in suspension, chemical composition, as well as interaction with the bioassay test medium.

Role of Co in the Electrocatalytic Activity of Monolayer Ternary NiFeCo-Double Hydroxide Nanosheets for Oxygen Evolution Reaction

Monolayer nanosheets have gained significant attention as functional materials and also in photo/electrocatalysis due to their unique physical/chemical properties, abundance of highly exposed coordination sites, edges, and corner sites, motivating the pursuit of highly active monolayer nanosheets. NiFe-based layered double hydroxide (NiFe-LDH) nanosheets have been regarded as the most efficient electrocatalysis for oxygen evolution. However, the limited catalytic active site and the stacking layer limited the performance. Therefore, by introducing highly electroactive Co ions into monolayer NiFe-LDH, the obtained ternary NiFeCo-LDH monolayer structure possessed an increased concentration of defect (oxygen and metal vacancies), providing enough unsaturated coordination sites, benefitting the electrocatalytic water oxidation, as also explained by the density functional theory (DFT). This work reported an efficient strategy for the synthesis of ternary monolayer LDH in the application of energy conversion and storage.

VO2 Nanostructures for Batteries and Supercapacitors: A Review

Vanadium dioxide (VO₂ ) received tremendous interest lately due to its unique structural, electronic, and optoelectronic properties. VO₂ has been extensively used in electrochromic displays and memristors and its VO₂ (B) polymorph is extensively utilized as electrode material in energy storage applications. More studies are focused on VO₂ (B) nanostructures which displayed different energy storage behavior than the bulk VO₂ . The present review provides a systematic overview of the progress in VO₂ nanostructures syntheses and its application in energy storage devices. Herein, a general introduction, discussion about crystal structure, and syntheses of a variety of nanostructures such as nanowires, nanorods, nanobelts, nanotubes, carambola shaped, etc. are summarized. The energy storage application of VO₂ nanostructure and its composites are also described in detail and categorically, e.g. Li-ion battery, Na-ion battery, and supercapacitors. The current status and challenges associated with VO₂ nanostructures are reported. Finally, light has been shed for the overall performance improvement of VO₂ nanostructure as potential electrode material for future application.

A Novel Heterostructure Based on RuMo Nanoalloys and N-doped Carbon as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec^-1 ), and a high turnover frequency (3.57 H₂ s^-1 ) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon-metal heterostructures for highly efficient electrocatalysis.

Effect of LDHs and Other Clays on Polymer Composite in Adsorptive Removal of Contaminants: A Review

Recently, the development of a unique class of layered silicate nanomaterials has attracted considerable interest for treatment of wastewater. Clean water is an essential commodity for healthier life, agriculture and a safe environment at large. Layered double hydroxides (LDHs) and other clay hybrids are emerging as potential nanostructured adsorbents for water purification. These LDH hybrids are referred to as hydrotalcite-based materials or anionic clays and promising multifunctional two-dimensional (2D) nanomaterials. They are used in many applications including photocatalysis, energy storage, nanocomposites, adsorption, diffusion and water purification. The adsorption and diffusion capacities of various toxic contaminants heavy metal ions and dyes on different unmodified and modified LDH-samples are discussed comparatively with other types of nanoclays acting as adsorbents. This review focuses on the preparation methods, comparison of adsorption and diffusion capacities of LDH-hybrids and other nanoclay materials for the treatment of various contaminants such as heavy metal ions and dyes.

Layered Double Hydroxides Applications in the High-Performance Magnetic Nanomaterials

Layered double hydroxides (LDHs), which is related to magnetic nanomaterials’ have promising applications due to their unique structural and chemical properties. The easy tunability of cationic metals without changing the LDH structure as well as anion exchange features of LDH interlayer make them potential applications in supercapacitors, batteries, catalysis, water splitting, etc. Moreover, due to the high dispersion of active compounds in the matrix of LDH layers, LDHs have been used to construct various nanostructures such as nanoparticles, 2D monolayer nanosheets and 3D hierarchical’ which are valued in wide nanotechnological applications. Magnetic nanomaterials are an important research area because they have been applied to a wide range of disciplines such as biotechnology, data storage, magnetic fluids, magnetic resonance imaging, environmental remediation and catalysis. LDHs as starting materials including Ni, Fe or/and Co, can be used as magnetic nanomaterials. The combination between LDHs and magnetic nanostructures has improved the magnetic properties of those materials, hence can be used in more applications.

Development of Ti/Ni Nanolayered Structures to Be a New Candidate for Energy Storage Applications

Development of electrochemical supercapacitor electrode is the best way to improve the performance and conductivity of the alone materials and support energy storage devices. In this work, cyanate anions have used as building blocks to build series of nanolayered materials based on Ti/Ni layered double hydroxides (LDHs). The structural and morphological characteristics of the prepared Ti/Ni LDHs were examined using different techniques. The electrochemical supercapacitive behavior of the prepared LDHs was observed in the three-assembly electrochemical cell. These results showed that the optimized ratio of the nickel and titanium plays an important role to enhance the electrochemical performance of the LDHs. The optimized Ti/Ni LDHs, which has the highest content of titanium, showed the highest specific capacitance (675 F/g) value. In this trend, this LDH also retain a high percentage of the cyclic retention after long cyclic charging-discharging process. The enhanced performance could be due to the double layer structure, enough interplanar distance between the layer, and large number of exposed active site within the double layer structure of the LDHs. Finally, although there are no reports for the electrochemical supercapacitive performance of Ti/Ni LDHs in the literature, it is interesting to produce a new candidate for energy storage applications.

Anion-dependent topochemical conversion of CoAl-LDH microplates to hierarchical superstructures of CoOOH nanoplates with controllable orientation

Hierarchical superstructures of laterally or vertically oriented CoOOH nanoplates were prepared by topochemical conversion of CoAl-LDH microplates intercalated with CO₃^2- or SO₄^2- anions, respectively. The superstructure of vertically oriented nanoplates exhibited better electrocatalytic performance as compared to the lateral counterpart, attributable to the enlarged accessible surface area and promoted reaction kinetics.

Hierarchical Manganese–Iron-Layered Double Hydroxide Nanosheets for Asymmetric Supercapacitors

This work presents a synthesis of hierarchical manganese–iron-layered double hydroxide (MnFe-LDH) nanostructured electrodes using the hydrothermal synthesis route by varying the reaction time for electrochemical energy storage applications. The electrochemical behavior of the MnFe-LDH electrodes synthesized at different reaction times was analyzed in a three-electrode cell configuration using 2 M KOH electrolyte. The uniform and well-organized MnFe-LDH nanosheet electrode (MnFe-12h) showed the maximum areal capacitance of 2013 mFcm−2 at a 5 mVs−1 scan rate, and 1886 mFcm−2 at a 25 mA applied current. Furthermore, the electrochemical behavior of MnFe-12h was examined by assembling an asymmetric cell device using activated carbon (AC) as a negative electrode and MnFe-12h as a positive electrode and it was tested in a wide voltage window range of 0.0 to 1.6 V. This asymmetric cell device achieved an appropriate energy density of 44.9 µW h cm−2 (55.01 W h kg−1), with a power density of 16 mW cm−2 (5000 W kg−1) at an applied current of 10 mA, and had a long-term cycling stability (93% capacitance retention after 5000 cycles) within the 1.6 V operating voltage window.

Novel Two-Dimensional Porous Materials for Electrochemical Energy Storage: A Minireview

Two dimensional (2D) porous materials have great potential in electrochemical energy conversion and storage. Over the past five years, our research group has focused on Simple, Mass, Homogeneous and Repeatable Synthesis of various 2D porous materials and their applications for electrochemical energy storage especially for supercapacitors (SCs). During the experimental process, through precisely controlling the experimental parameters, such as reaction species, molar ratio of different ions, concentration, pH value of reaction solution, heating temperature, and reaction time, we have successfully achieved the control of crystal structure, composition, crystallinity, morphology, and size of these 2D porous materials including transition metal oxides (TMOs), transition metal hydroxides (TMHOs), transition metal oxalates (TMOXs), transition metal coordination complexes (TMCCs) and carbon materials, as well as their derivatives and composites. We have also named some of them with CQU-Chen (CQU is the initialism of Chongqing University, Chen is the last name of Lingyun Chen), such as CQU-Chen-Co-O-1, CQU-Chen-Ni-O-H-1, CQU-Chen-Zn-Co-O-1, CQU-Chen-Zn-Co-O-2, CQU-Chen-OA-Co-2-1, CQU-Chen-Co-OA-1, CQU-Chen-Ni-OA-1, CQU-Chen-Gly-Co-3-1, CQU-Chen-Gly-Ni-2-1, CQU-Chen-Gly-Co-Ni-1, etc. The introduction of 2D porous materials as electrode materials for SCs improves the energy storage performances. These materials provide a large number of active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity and facilitate electron transportation and ion penetration. The unique 2D porous structures review is mainly devoted to the introduction of our contribution in the 2D porous nanostructured materials for SC. Finally, the further directions about the preparation of 2D porous materials and electrochemical energy conversion and storage applications are also included.

Green Treatment of Phosphate from Wastewater Using a Porous Bio-Templated Graphene Oxide/MgMn-Layered Double Hydroxide Composite

Excessive phosphorus in water is the primary culprit for eutrophication, which causes approximately $2.2 billion annual economic loss in the United States. This study demonstrates a phosphate-selective sustainable method by adopting Garcinia subelliptica leaves as a natural bio-template, where MgMn-layered double hydroxide (MgMn-LDH) and graphene oxide (GO) can be grown in situ to obtain L-GO/MgMn-LDH. After calcination, the composite shows a hierarchical porous structure and selective recognition of phosphate, which achieves significantly high and recyclable selective phosphate adsorption capacity and desorption rate of 244.08 mg-P g-1 and 85.8%, respectively. The detail variation of LDHs during calcination has been observed via in situ transmission electron microscope (TEM). Moreover, the roles in facilitating phosphate adsorption and antimicrobial ability of chemical constituents in Garcinia subelliptica leaves, biflavonoids, and triterpenoids have been investigated. These results indicate the proposed bio-templated adsorbent is practical and eco-friendly for phosphorus sustainability in commercial wastewater treatment.