A three-dimensional flower-like NiCo-layered double hydroxide grown on nickel foam with an MXene coating for enhanced oxygen evolution reaction electrocatalysis

Selective electrooxidation of 5-hydroxymethylfurfural at low working potentials promoted by 3D hierarchical Cu(OH)2@Ni3Co1-layered double hydroxide architecture with oxygen vacancies

Selective electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is of great significance in the manufacture of fine chemicals, liquid fuels, pharmaceuticals, plastics, etc., but still suffers from the high potential input, resulting in high electricity consumption. Developing active, low-cost and stable electrocatalysts is crucial for this electrochemical reaction at low working potentials. Herein, a three-dimensional (3D) hierarchical Cu(OH)2@Ni3Co1-layered double hydroxide architecture with abundant oxygen vacancies (Vo) was synthesized by facile electrodeposition of Ni3Co1-LDH nanosheets on copper foam (CF) supported-Cu(OH)2 nanorods (CF/Cu(OH)2@Ni3Co1-LDH) for the selective electrooxidation of HMF to FDCA. The 3D hierarchical architecture of the Cu(OH)2 nanorod core loaded with Ni3Co1-LDH nanosheet shell facilitates the rapid transfer of charges and exposes more active sites. The synergistic effect of the core-shell nanoarray structure, atomic level dispersion of Ni and Co on LDH laminates, and rich Vo gives 98.12% conversion of HMF, 98.64% yield and 91.71% selectivity for FDCA at a low working potential of 1.0 V vs. RHE. In addition, CF/Cu(OH)2@Ni3Co1-LDH exhibits superior stability by maintaining 93.26% conversion of HMF, 93.65% yield and 91.57% selectivity of FDCA after eight successive cycles, showing the immense potential of utilizing electrochemical conversion for biomass.

Subtle 2D/2D MXene-Based Heterostructures for High-Performance Electrocatalytic Water Splitting

Developing efficient electrocatalysts is significant for the commercial application of electrocatalytic water splitting. 2D materials have presented great prospects in electrocatalysis for their high surface-to-volume ratio and tunable electronic properties. Particularly, MXene emerges as one of the most promising candidates for electrocatalysts, exhibiting unique advantages of hydrophilicity, outstanding conductivity, and exceptional stability. However, it suffers from lacking catalytic active sites, poor oxidation resistance, and easy stacking, leading to a significant suppression of the catalytic performance. Combining MXene with other 2D materials is an effective way to tackle the aforementioned drawbacks. In this review, the focus is on the accurate synthesis of 2D/2D MXene-based catalysts toward electrocatalytic water splitting. First, the mechanisms of electrocatalytic water splitting and the relative properties and preparation methods of MXenes are introduced to offer the basis for accurate synthesis of 2D/2D MXene-based catalysts. Then, the accurate synthesis methods for various categories of 2D/2D MXene-based catalysts, such as wet-chemical, phase-transformation, electrodeposition, etc., are systematically elaborated. Furthermore, in-depth investigations are conducted into the internal interactions and structure-performance relationship of 2D/2D MXene-based catalysts. Finally, the current challenges and future opportunities are proposed for the development of 2D/2D MXene-based catalysts, aiming to enlighten these promising nanomaterials for electrocatalytic water splitting.

Recent advances in the role of MXene based hybrid architectures as electrocatalysts for water splitting

The development of non-noble metal based and cost-effective electrocatalysts for water splitting has attracted significant attention due to their potential in production of clean and green hydrogen fuel. Discovered in 2011, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, have demonstrated promising performance as electro catalysts in the water splitting process due to their high electrical conductivity, very large surface area and abundant catalytic active sites. However, their-long term stability and recyclability are limited due to restacking and agglomeration of MXene flakes. This problem can be solved by combining MXene with other materials to create their hybrid architectures which have demonstrated higher electrocatalytic performance than pristine MXenes. Electrolysis of water encompasses two half-cell reactions, hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. Firstly, this concise review explains the mechanism of water splitting. Then it provides an overview of the recent advances about applications of MXenes and their hybrid architectures as HER, OER and bifunctional electrocatalysts for overall water splitting. Finally, the recent challenges and potential outlook in the field have been presented. This concise review may provide further understanding about the role of MXene-based hybrid architectures to develop efficient electrocatalysts for water splitting.

Novel Strain Engineering Combined with a Microscopic Pore Synergistic Modulated Strategy for Designing Lattice Tensile-Strained Porous V2C-MXene for High-Performance Overall Water Splitting

Transition metal carbon/nitride (MXene) holds immense potential as an innovative electrocatalyst for enhancing the overall water splitting properties. Nevertheless, the re-stacking nature induced by van der Waals force remains a significant challenge. In this work, the lattice tensile-strained porous V₂C-MXene (named as TS₍₂₄₎-P₍₅₀₎-V₂C) is successfully constructed via the rapid spray freezing method and the following hydrothermal treatment. Besides, the influence of lattice strain degree and microscopic pores on the catalytic ability is reviewed and explored systematically. The lattice tensile strain within V₂C-MXene could widen the interlayer spacing and accelerate the ion transfer. The microscopic pores could change the ion transmission path and shorten the migration distance. As a consequence, the obtained TS₍₂₄₎-P₍₅₀₎-V₂C shows extraordinary hydrogen evolution reaction and oxygen evolution reaction activity with the overpotential of 154 and 269 mV, respectively, at the current density of 10 mA/cm², which is quite remarkable compared to the MXene-based electrocatalysts. Moreover, the overall water splitting device assembled using TS₍₂₄₎-P₍₅₀₎-V₂C as both anode and cathode demonstrates a low cell voltage requirement of 1.57 V to obtain 10 mA/cm². Overall, the implementation of this work could offer an exciting avenue to overcome the re-stacking issue of V₂C-MXene, affording a high-efficiency electrocatalyst with superior catalytic activity and desirable reaction kinetics.

Regulation of Structure and Anion-Exchange Performance of Layered Double Hydroxide: Function of the Metal Cation Composition of a Brucite-like Layer

As anion-exchange materials, layered double hydroxides (LDHs) have attracted increasing attention in the fields of selective adsorption and separation, controlled drug release, and environmental remediation. The metal cation composition of the laminate is the essential factor that determines the anion-exchange performance of LDHs. Herein, we review the regulating effects of the metal cation composition on the anion-exchange properties and LDH structure. Specifically, the internal factors affecting the anion-exchange performance of LDHs were analyzed and summarized. These include the intercalation driving force, interlayer domain environment, and LDH morphology, which significantly affect the anion selectivity, anion-exchange capacity, and anion arrangement. By changing the species, valence state, size, and mole ratio of the metal cations, the structural characteristics, charge density, and interlayer spacing of LDHs can be adjusted, which affect the anion-exchange performance of LDHs. The present challenges and future prospects of LDHs are also discussed. To the best of our knowledge, this is the first review to summarize the essential relationship between the metal ion composition and anion-exchange performance of laminates, providing important insights for regulating the anion-exchange performance of LDHs.

Facile Synthesis of NiCo2O4 Nanowire Arrays/Few-Layered Ti3C2-MXene Composite as Binder-Free Electrode for High-Performance Supercapacitors

Herein, a 3D hierarchical structure is constructed by growing NiCo₂O₄ nanowires on few-layer Ti₃C₂ nanosheets using Ni foam (NF) as substrate via simple vacuum filtration and solvothermal treatment. Ti₃C₂ nanosheets are directly anchored on NF surface without binders or surfactants, and NiCo₂O₄ nanowires composed of about 15 nm nanoparticles uniformly grow on Ti₃C₂/NF skeleton, which can provide abundant active sites and ion diffusion pathways for enhancing electrochemical performance. Benefiting from the unique structure feature and the synergistic effects of active materials, NiCo₂O₄/Ti₃C₂ exhibits a high specific capacitance of 2468 F g^-1 at a current density of 0.5 A g^-1 and a good rate performance. Based on this, an asymmetric supercapacitor (ASC) based on NiCo₂O₄/Ti₃C₂ as positive electrode and activated carbon (AC)/NF as negative electrode is assembled. The ASC achieves a high specific capacitance of 253 F g^-1 at 1 A g^-1 along with 91.5% retention over 10,000 cycles at 15 A g^-1. Furthermore, the ACS presents an outstanding energy density of 90 Wh kg^-1 at the power density of 2880 W kg^-1. This work provides promising guidance for the fabrication of binder-free, free-standing and hierarchical composites for energy storage application.