2D inorganic nanosheets as versatile building blocks for hybrid electrode materials for supercapacitor

Layered double hydroxide-based electrode materials derived from metal-organic frameworks: synthesis and applications in supercapacitors

Metal-organic frameworks (MOFs) have emerged as promising electrode materials for supercapacitors (SCs) due to their highly porous structures, tunable chemical compositions, and diverse morphologies. However, their applications are hindered by low conductivity and poor cycling performance. A novel approach for resolving this issue involves the growth of layered double hydroxides (LDHs) using MOFs as efficient templates or precursors for electrode material preparation. This method effectively enhances the stability, electrical conductivity, and mass transport ability of MOFs. The MOF-derived LDH exhibits a well-defined porous micro-/nano-structure, facilitating the dispersion of active sites and preventing the aggregation of LDHs. Firstly, this paper introduces synthesis strategies for converting MOFs into LDHs. Subsequently, recent research progress in MOF-derived LDHs encompassing pristine LDH powders, LDH composites, and LDH-based arrays, along with their applications in SCs, is overviewed. Finally, the challenges associated with MOF-derived LDH electrode materials and potential solutions are discussed.

Al-Ce co-doped BaTiO3 nanofibers as a high-performance bifunctional electrochemical supercapacitor and water-splitting electrocatalyst

Supercapacitors and water splitting cells have recently played a key role in offering green energy through converting renewable sources into electricity. Perovskite-type electrocatalysts such as BaTiO3, have been well-known for their ability to efficiently split water and serve as supercapacitors due to their high electrocatalytic activity. In this study, BaTiO3, Al-doped BaTiO3, Ce-doped BaTiO3, and Al-Ce co-doped BaTiO3 nanofibers were fabricated via a two-step hydrothermal method, which were then characterized and compared for their electrocatalytic performance. Based on the obtained results, Al-Ce co-doped BaTiO3 electrode exhibited a high capacitance of 224.18 Fg-1 at a scan rate of 10 mVs-1, high durability during over the 1000 CV cycles and 2000 charge-discharge cycles, proving effective energy storage properties. Additionally, the onset potentials for OER and HER processes were 11 and - 174 mV vs. RHE, respectively, demonstrating the high activity of the Al-Ce co-doped BaTiO3 electrode. Moreover, in overall water splitting, the amount of the overpotential was 0.820 mV at 10 mAcm-2, which confirmed the excellent efficiency of the electrode. Hence, the remarkable electrocatalytic performance of the Al-Ce co-doped BaTiO3 electrode make it a promising candidate for renewable energy technologies owing to its high conductivity and fast charge transfer.

Atomically-Thin Holey 2D Nanosheets of Defect-Engineered MoN-Mo5 N6 Composites as Effective Hybridization Matrices

The defect engineering of inorganic solids has received significant attention because of its high efficacy in optimizing energy-related functionalities. Consequently, this approach is effectively leveraged in the present study to synthesize atomically-thin holey 2D nanosheets of a MoN-Mo5 N6 composite. This is achieved by controlled nitridation of assembled MoS2 monolayers, which induced sequential cation/anion migration and a gradual decrease in the Mo valency. Precise control of the interlayer distance of the MoS2 monolayers via assembly with various tetraalkylammonium ions is found to be crucial for synthesizing sub-nanometer-thick holey MoN-Mo5 N6 nanosheets with a tunable anion/cation vacancy content. The holey MoN-Mo5 N6 nanosheets are employed as efficient immobilization matrices for Pt single atoms to achieve high electrocatalytic mass activity, decent durability, and low overpotential for the hydrogen evolution reaction (HER). In situ/ex situ spectroscopy and density functional theory (DFT) calculations reveal that the presence of cation-deficient Mo5 N6 domain is crucial for enhancing the interfacial interactions between the conductive molybdenum nitride substrate and Pt single atoms, leading to enhanced electron injection efficiency and electrochemical stability. The beneficial effects of the Pt-immobilizing holey MoN-Mo5 N6 nanosheets are associated with enhanced electronic coupling, resulting in improvements in HER kinetics and interfacial charge transfer.

Nanoarchitectonics of a Smectite with 4,4'-Diammonium-α-truxillic Acid and Its Methyl Ester for the Removal of o-Phenylphenol and Biphenyl from Water

Adsorbents with hydrophilic and hydrophobic natures were designed by intercalating a bioderived molecule; 4,4'-diammonium-α-truxillic acid (4ATA) and 4,4'-diammonium-α-truxillic acid dimethyl ester (E4ATA), which both are bioderived molecules, into a smectite (purified bentonite) to concentrate o-phenylphenol and biphenyl, respectively, from water. The adsorption isotherm showed high affinity between the 4ATA-smectite hybrid and o-phenylphenol with a high Langmuir constant (0.98 L mg-1). Meanwhile, the E4ATA-smectite hybrid adsorbed biphenyl with a high Langmuir constant (3.61 L mg-1). The adsorption properties of 4ATA- and E4ATA-smectite hybrid were contributed by the chemical characteristics of 4ATA and E4ATA in the interlayer space of the smectite.

Progress of 2D MXene as an Electrode Architecture for Advanced Supercapacitors: A Comprehensive Review

Supercapacitors, designed to store more energy and be proficient in accumulating more energy than conventional batteries with numerous charge-discharge cycles, have been developed in response to the growing demand for energy. Transition metal carbides/nitrides called MXenes have been the focus of researchers' cutting-edge research in energy storage. The 2D-layered MXenes are a hopeful contender for the electrode material due to their unique properties, such as high conductivity, hydrophilicity, tunable surface functional groups, better mechanical properties, and outstanding electrochemical performance. This newly developed pseudocapacitive substance benefits electrochemical energy storage because it is rich in interlayer ion diffusion pathways and ion storage sites. Making MXene involves etching the MAX phase precursor with suitable etchants, but different etching methods have distinct effects on the morphology and electrochemical properties. It is an overview of the recent progress of MXene and its structure, synthesis, and unique properties. There is a strong emphasis on the effects of shape, size, electrode design, electrolyte behavior, and other variables on the charge storage mechanism and electrochemical performance of MXene-based supercapacitors. The electrochemical application of MXene and the remarkable research achievements in MXene-based composites are an intense focus. Finally, in light of further research and potential applications, the challenges and future perspectives that MXenes face and the prospects that MXenes present have been highlighted.

High-Voltage MXene-Based Supercapacitors: Present Status and Future Perspectives

As an emerging class of 2D materials, MXene exhibits broad prospects in the field of supercapacitors (SCs). However, the working voltage of MXene-based SCs is relatively limited (typically ≤ 0.6 V) due to the oxidation of MXene electrode and the decomposition of electrolyte, ultimately leading to low energy density of the device. To solve this issue, high-voltage MXene-based electrodes and corresponding matchable electrolytes are developed urgently to extend the voltage window of MXene-based SCs. Herein, a comprehensive overview and systematic discussion regarding the effects of electrolytes (aqueous, organic, and ionic liquid electrolytes), asymmetric device configuration, and material modification on the operating voltage of MXene-based SCs, is presented. A deep dive is taken into the latest advances in electrolyte design, structure regulation, and high-voltage mechanism of MXene-based SCs. Last, the future perspectives on high-voltage MXene-based SCs and their possible development directions are outlined and discussed in depth, providing new insights for the rational design and realization of advanced next-generation MXene-based electrodes and high-voltage electrolytes.

The magnetic properties of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, and Cu2+ determined by a combined experimental and computational approach

The magnetic properties of the nickelalumite-type layered double hydroxides (LDH), MAl₄(OH)₁₂(SO₄)·3H₂O (MAl₄-LDH) with M = Co²⁺ (S = 3/2), Ni²⁺ (S = 1), or Cu²⁺ (S = 1/2) were determined by a combined experimental and computational approach. They represent three new inorganic, low-dimensional magnetic systems with a defect-free, structurally ordered magnetic lattice. They exhibit no sign of magnetic ordering down to 2 K in contrast to conventional hydrotalcite LDH. Detailed insight into the complex interplay between the choice of magnetic ion (M²⁺) and magnetic properties was obtained by a combination of magnetic susceptibility, heat capacity, neutron scattering, solid-state NMR spectroscopy, and first-principles calculations. The NiAl₄- and especially CoAl₄-LDH have pronounced zero-field splitting (ZFS, easy-axis and easy-plane, respectively) and weak ferromagnetic nearest-neighbour interactions. Thus, they are rare examples of predominantly zero-dimensional spin systems in dense, inorganic matrices. In contrast, CuAl₄-LDH (S = 1/2) consists of weakly ferromagnetic S = 1/2 spin chains. For all three MAl₄-LDH, good agreement is found between the experimental magnetic parameters (J, D, g) and first-principles quantum chemical calculations, which also predict that the interchain couplings are extremely weak (< 0.1 cm^-1). Thus, our approach will be valuable for evaluation and prediction of magnetic properties in other inorganic materials.

Multilayer Conductive Hybrid Nanosheets as Versatile Hybridization Matrices for Optimizing the Defect Structure, Structural Ordering, and Energy-Functionality of Nanostructured Materials

The hybridization of conductive nanospecies has garnered significant research interest because of its high efficacy in improving the diverse functionalities of nanostructured materials. In this study, a novel synthetic strategy is developed to optimize the defect structure, structural ordering, and energy-related functionality of nanostructured-materials by employing a multilayer multicomponent two-dimenstional (2D) graphene/metal oxide/graphene nanosheet (NS) as a versatile hybridization matrix. The hybridization of the robust trilayer, polydiallyldiammonium (PDDA)-anchored reduced-graphene oxide (prGO)/metal oxide/prGO NS effectively enhance the structural ordering and porosity of the hybridized MoS₂ /MnO₂ NS through suppression of defect formation and tight stacking. In comparison with monolayer rGO/RuO₂ NS-based homologs, the 2D superlattice trilayer prGO/RuO₂ /prGO NS hybrids deliver better functionalities as a hydrogen evolution electrocatalyst and as a supercapacitor electrode, demonstrating the merits of hybridization with multilayer NSs. The advantages of using multilayer multicomponent conductive NSs as hybridization matrices arise from the enhancement of charge and mass transport through the layer flattening or defect suppression of the hybridized NSs and the increase in porosity, as evidenced by density functional theory calculations. Finally, the universal utility of multilayer NSs is confirmed by investigating the strong effect of the stacking order on the electrocatalytic functionality of MoS₂ /rGO/RuO₂ films fabricated through layer-by-layer deposition.

In Situ Defect Engineering Route to Optimize the Cationic Redox Activity of Layered Double Hydroxide Nanosheet via Strong Electronic Coupling with Holey Substrate

A defect engineering of inorganic solids garners great deal of research activities because of its high efficacy to optimize diverse energy-related functionalities of nanostructured materials. In this study, a novel in situ defect engineering route to maximize electrocatalytic redox activity of inorganic nanosheet is developed by using holey nanostructured substrate with strong interfacial electronic coupling. Density functional theory calculations and in situ spectroscopic analyses confirm that efficient interfacial charge transfer takes place between holey TiN and Ni-Fe-layered double hydroxide (LDH), leading to the feedback formation of nitrogen vacancies and a maximization of cation redox activity. The holey TiN-LDH nanohybrid is found to exhibit a superior functionality as an oxygen electrocatalyst and electrode for Li-O2 batteries compared to its non-holey homologues. The great impact of hybridization-driven vacancy introduction on the electrochemical performance originates from an efficient electrochemical activation of both Fe and Ni ions during electrocatalytic process, a reinforcement of interfacial electronic coupling, an increase in electrochemical active sites, and an improvement in electrocatalysis/charge-transfer kinetics.

Three-Dimensional Porous Network Electrodes with Cu(OH)2 Nanosheet/Ni3S2 Nanowire 2D/1D Heterostructures for Remarkably Cycle-Stable Supercapacitors

Developing advanced electrode materials with highly improved charge and mass transfer is critical to obtain high specific capacities and long-term cycle life for energy storage. Herein, three-dimensionally (3D) porous network electrodes with Cu(OH)₂ nanosheets/Ni₃S₂ nanowire 2D/1D heterostructures are rationally fabricated. Different from traditional surface deposition, the 1D/2D heterostructure network is obtained by in situ hydrothermal chemical etching of the surface layer of nickel foam (NF) ligaments. The Cu(OH)₂/Ni₃S₂@NF electrode delivers a high specific capacity (1855 F g^-1 at 2 mA cm^-2) together with a remarkable stability. The capacity retention of the electrode is over 110% after 35,000 charge/discharge cycles at 20 mA cm^-2. The improved performance is attributed to the enhanced electron transfer between 1D Ni₃S₂ and 2D Cu(OH)₂, highly accessible sites of 3D network for electrolyte ions, and strong mechanical bonding and good electrical connection between Cu(OH)₂/Ni₃S₂ active materials and the conductive NF. Especially, the unique 1D/2D heterostructure alleviates structural pulverization during the ion insertion/desertion process. A symmetric device applying the Cu(OH)₂/Ni₃S₂@NF electrode exhibits a remarkable cycling stability with the capacitance retention maintaining over 98% after 30,000 cycles at 50 mA cm^-2. Therefore, the outstanding performance promises the architectural 1D/2D heterostructure to offer potential applications in future electrochemical energy storage.

Complementary combinative strategy of defect engineering and graphene coupling for efficient energy-functional materials

The synergetic combination of defect engineering and graphene coupling enables to develop an effective way of exploring efficient bifunctional electrocatalyst/electrode materials. Defect-engineered amorphous MoO₂ -reduced graphene oxide (rGO) nanohybrid was synthesized by soft-chemical reduction of K₂ MoO₄ in graphene oxide colloids. Mo K-edge X-ray absorption spectroscopy clearly demonstrates the rutile-type local atomic structure of amorphous MoO₂ with significant oxygen vacancies and intimate electronic coupling with rGO. The defect-introduced MoO₂ -rGO nanohybrid shows excellent bifunctionality as electrocatalyst for hydrogen evolution reaction and electrode for sodium-ion batteries, which are superior to those of crystalline MoO₂ -rGO homologue. The beneficial effect of simultaneous defect control and rGO coupling can be ascribed to the provision of oxygen vacancies acting as active sites, the increase of electrical conductivity, and the improvement of reaction kinetics.

Synthesis and Thermal Degradation of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, Cu2+, and Zn2

The synthesis and thermal degradation of MAl₄(OH)₁₂SO₄·3H₂O layered double hydroxides with M = Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ("MAl₄-LDH") were investigated by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, powder X-ray diffraction, Rietveld refinement, scanning electron microscopy, scanning tunnel electron microscopy, energy-dispersive X-ray spectroscopy, and solid-state ¹H and ²⁷Al NMR spectroscopy. Following extensive synthesis optimization, phase pure CoAl₄- and NiAl₄-LDH were obtained, whereas 10-12% unreacted bayerite (Al(OH)₃) remained for the CuAl₄-LDH. The optimum synthesis conditions are hydrothermal treatment at 120 °C for 14 days (NiAl₄-LDH only 9 days) with MSO₄(aq) concentrations of 1.4-2.8, 0.7-0.8, and 0.08 M for the CoAl₄-, NiAl₄-, and CuAl₄-LDH, respectively. A pH ≈ 2 for the metal sulfate solutions is required to prevent the formation of byproducts, which were Ni(OH)₂ and Cu₃(SO₄)(OH)₄ for NiAl₄- and CuAl₄-LDH, respectively. The thermal degradation of the three MAl₄-LDH and ZnAl₄-LDH in a nitrogen atmosphere proceeds in three steps: (i) dehydration and dehydroxylation between 200 and 600 °C, (ii) loss of sulfate between 600 and 900 °C, and (iii) formation of the end products at 900-1200 °C. For CoAl₄-LDH (ZnAl₄-LDH), these are α-Al₂O₃ and CoAl₂O₄ (ZnAl₂O₄) spinel. For NiAl₄-LDH, a spinel-like NiAl₄O₇ phase forms, whereas CuAl₄-LDH degrades by a redox reaction yielding a diamagnetic CuAlO₂ (delafossite structure) and α-Al₂O₃.

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.

The intergrated nanostructure of bimetallic CoNi-based zeolitic imidazolate framework and carbon nanotubes as high-performance electrochemical supercapacitors

In this study, a series of one-dimensional (1D)/two-dimensional (2D) heterostructure hybrids were fabricated through the in situ growth of a Co and Ni bimetallic zeolitic imidazolate framework (CoNi-ZIF) around N-doped carbon nanotubes (N-CNTs). The hybrids were further exploited as effective supercapacitor materials. The N-CNTs were prepared by carbonizing a mixture of glucose and the melamine-cyanuric acid complex at a high temperature (900 °C) under N₂ atmosphere and applied as the template for the in situ synthesis of CoNi-ZIF nanosheets (NSs). The 1D N-CNTs in the hybrids can act as the high-way for charge transfer to boost the faradaic reactions. Changing the usage of metal precursors not only provided abundant redox reaction sites in 2D CoNi-ZIF NSs but also modulated the microstructures and chemical components of the hybrids. The integration of the features of N-CNTs and CoNi-ZIF NSs can result in a synergistic effect between N-CNTs and CoNi-ZIF NSs. Therefore, the obtained CoNi-ZIFs and N-CNTs hybrid (CoNi-ZIF@N-CNT) exhibited superior electrochemical capacitive performance. Comparison revealed that the CoNi-ZIF@N-CNT-2 hybrid, which was prepared with a 1:1 mass ratio of Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O, displayed the largest specific capacitance of 1118F g^-1 at 1 A g^-1, which was higher than the capacitance of most reported metal-organic framework (MOF)-based supercapacitor electrodes. Moreover, the asymmetric supercapacitor based on the CoNi-ZIF@N-CNT-2 electrode exhibited a high energy density of 51.1 Wh kg^-1 at the power density of 860.1 W kg^-1 and good cycle stability. This work can provide a facile and effective way for the fabrication of heterostructured 1D/2D nanostructures based on 2D MOFs for advanced energy storage.

Wet-chemical synthesis of two-dimensional metal nanomaterials for electrocatalysis

Two-dimensional (2D) metal nanomaterials have gained ever-growing research interest owing to their fascinating physicochemical properties and promising application, especially in the field of electrocatalysis. In this review, we briefly introduce the recent advances in wet-chemical synthesis of 2D metal nanomaterials. Subsequently, the catalytic performances of 2D metal nanomaterials in a variety of electrochemical reactions are illustrated. Finally, we summarize current challenges and highlight our perspectives on preparing high-performance 2D metal electrocatalysts.

Lattice Engineering to Simultaneously Control the Defect/Stacking Structures of Layered Double Hydroxide Nanosheets to Optimize Their Energy Functionalities

An effective lattice engineering method to simultaneously control the defect structure and the porosity of layered double hydroxides (LDHs) was developed by adjusting the elastic deformation and chemical interactions of the nanosheets during the restacking process. The enlargement of the intercalant size and the lowering of the charge density were effective in increasing the content of oxygen vacancies and enhancing the porosity of the stacked nanosheets via layer thinning. The defect-rich Co-Al-LDH-NO₃^- nanohybrid with a small stacking number exhibited excellent performance as an oxygen evolution electrocatalyst and supercapacitor electrode with a large specific capacitance of ∼2230 F g^-1 at 1 A g^-1, which is the largest capacitance of carbon-free LDH-based electrodes reported to date. Combined with the results of density functional theory calculations, the observed excellent correlations between the overpotential/capacitance and the defect content/stacking number highlight the importance of defect/stacking structures in optimizing the energy functionalities. This was attributed to enhanced orbital interactions with water/hydroxide at an increased number of defect sites. The present cost-effective lattice engineering process can therefore provide an economically feasible methodology to explore high-performance electrocatalyst/electrode materials.

Hetero-nanostructures constructed by 2D porous metal oxide/hydroxide nanosheets supported on 1D hollow Co9S8 nanowires for hybrid supercapacitors with high areal capacity

Porous-hollow Co9S8/NiCo-Mo one-dimensional (1D) hetero-nanostructures are synthesized on carbon cloth using a stepwise fabrication process as battery-type electrodes.