Two‐dimensional (2D) transition metal carbides and carbonitrides, called MXenes, with metallic conductivity and hydrophilic surfaces, show great promise as electrode materials for supercapacitors. A ...major drawback of 2D nanomaterials is the re‐stacking of the nanosheets, which prevents full utilization of surface area and blocks the access of the electrolyte. In this study, a free‐standing nanocomposite paper electrode is realized by combining Mo1.33C MXene and positively charged biopolymer lignin (the second most abundant biopolymer in nature, L‐DEA). The self‐assembled layered architecture with alternating polymer and MXene flakes increases the interlayer space to promote ion transport, and with combining charge storage capability of the lignin derivative and MXene in an interpenetrating MXene/L‐DEA nanocomposite, which offers an impressive capacitance of 503.7 F g−1. Moreover, we demonstrate flexible solid‐state asymmetric supercapacitors (ASCs) using Mo1.33C@L‐DEA as the negative electrode and electrochemically exfoliated graphene with ruthenium oxide (EG@RuOx) as the positive electrode. This asymmetric device operates at a voltage window of 1.35 V, which is about two times wider than that of a symmetric Mo1.33C@L‐DEA based supercapacitor. Finally, the ASCs can deliver an energy density of 51.9 Wh kg−1 at a power density of 338.5 W kg−1, with 86 % capacitance retention after 10000 charge‐discharge cycles.
Flexible supercap: Free‐standing nanocomposite paper electrodes with aligned layered architecture are achieved using MXene and biopolymer (L‐DEA), realizing high electrochemical performance, excellent cycle life, fast redox reactions, and improved ion transport due to short diffusion pathways. An asymmetric solid‐state supercapacitor is developed based on the Mo1.33C@L‐DEA and EG@RuOx electrode with elevated potential window and ultrahigh energy density.
The magnetic properties of the new phase (Cr0.5Mn0.5)2AuC are compared to the known MAX-phase (Cr0.5Mn0.5)2GaC, where the former was synthesized by thermally induced substitution reaction of Au for ...Ga in (Cr0.5Mn0.5)2GaC. The reaction introduced a lattice expansion of ∼3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr0.5Mn0.5)2AuC displays similar features in the magnetic field- and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr0.5Mn0.5)2GaC and (Mo0.5Mn0.5)2GaC. The work suggests a pathway for tuning the magnetic properties of MAX phases.
Ion intercalation materials are emerging as a highly attractive class of electrodes for efficient energy water desalination. Most materials and concepts so far have focused on the removal of cations ...(especially sodium). Anion intercalation, however, remains poorly investigated in water desalination. We present a study on the capability of Mo1.33C-MXene for removing cations and anions and demonstrate the desalination performance in brackish water and seawater concentrations. Mo1.33C-MXene was prepared via acid treatment of the transition metal carbide MAX phase (Mo2/3Sc1/3)2AlC. Binder-free electrodes were obtained by entangling MXene with carbon nanotubes and tested without the use of any ion exchange membrane at low (5 mM NaCl) and high (600 mM NaCl) salt concentrations. Such electrodes showed a promising desalination performance of 15 mg/g in 600 mM NaCl with high charge efficiency up to 95%. By employing chemical online monitoring of the effluent stream, we separated the cation and anion intercalation capacity of the electrode material.
Boridene: a 2D borideA range of two-dimensional (2D) materials, including graphene and hexagonal boron nitride, have been synthesized and studied because of the unusual properties that occur when one ...dimension becomes very small. MXenes are a family of materials made of layers of inorganic transition metal carbides and nitrides that are a few atoms thick and are manufactured by selective etching. Attempts to make similar boridene materials have been challenging because of the reactive nature of boride phases and because the parent materials tend to dissolve rather than selectively etch. Zhou et al. synthesized boridene in the form of single-layer 2D molybdenum boride sheets by selective etching in aqueous hydrofluoric acid to produce sheets with ordered metal vacancies, opening up an additional family of materials for study.Science, abf6239, this issue p. 801Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo4/3B2-xTz (where Tz is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2 in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.
The enigma of MAX phases and their hybrids prevails. We probe transition metal (
) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these ...MAX hybrids, namely, (V
Zr
)
AlC and (Mo
Y
)
AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagomé-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic
2/
. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials.
MXenes, a young family of 2D transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultrathin flexible solid‐state ...supercapacitor is demonstrated based on a Mo1.33C MXene with vacancy ordering in an aligned layer structure MXene/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film posttreated with concentrated H2SO4. The flexible solid‐state supercapacitor delivers a maximum capacitance of 568 F cm−3, an ultrahigh energy density of 33.2 mWh cm−3 and a power density of 19 470 mW cm−3. The Mo1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H2SO4 treatment, a higher capacitance (1310 F cm−3) and improved rate capabilities than both pristine Mo1.33C MXene and the nontreated Mo1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer spacing between Mo1.33C MXene layers due to insertion of conductive PEDOT, and surface redox processes of the PEDOT and the MXene.
A MXene‐based solution processable flexible solid‐state supercapacitor with high performance is developed from a MXene/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film. After posttreatment with concentrated H2SO4, the PEDOT nanofiber network is aligned between the MXene sheets, leading to highly improved flexibility and, most importantly, improved capacitances (1310 F cm−3), rate‐capabilities, and stability.
High-entropy (HE) ceramics, by analogy with HE metallic alloys, are an emerging family of multielemental solid solutions. These materials offer a large compositional space, with a corresponding large ...range of properties. Here, we report the experimental realization of a 3D HE MAX phase, Ti1.0V0.7Cr0.05Nb1.0Ta1.0AlC3, and a corresponding 2D HE MXene in the form of freestanding flakes of average composition Ti1.1V0.7Cr x Nb1.0Ta0.6C3T z (T z = −F, −O, −OH), as produced by selective removal of Al from the HE MAX phase in aqueous hydrofluoric acid (HF). Initial tests on HE MXene “paper” electrodes show their high potential as electrode materials in supercapacitors through volumetric and gravimetric capacitances of 1688 F/cm3 and 490 F/g, respectively, originating from a combination of diffusion- and surface-controlled charge storage processes. The introduction of the HE concept into the field of 2D materials suggests a wealth of future 2D materials and applications.
All atomically laminated MAB phases (M = transition metal, A = A-group element, and B = boron) exhibit orthorhombic or tetragonal symmetry, with the only exception being hexagonal Ti2InB2. Inspired ...by the recent discovery of chemically ordered hexagonal carbides, i-MAX phases, we perform an extensive first-principles study to explore chemical ordering upon metal alloying of M2AlB2 (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable novel phases with in-plane chemical ordering are identified, coined i-MAB, along with 16 disordered stable alloys. The predictions are verified through the powder synthesis of Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2 of space group R3̅m (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagomé ordering of the Al atoms, as evident from X-ray diffraction and electron microscopy. The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential two-dimensional derivatives.
Materials with tailored properties are crucial for high performance electronics applications. Hybrid materials composed of inorganic and organic components can possess unique merits for broad ...application by synergy between the advantages the respective material type offers. Here we demonstrate a novel electrochemical polymerization (EP) enabled by a 2D transition metal carbide MXene for obtaining conjugated polymer-MXene composite films deposited on conducting substrates without using traditional electrolytes, indispensable compounds for commonly electrochemical polymerization. The universality of the process provides a novel approach for EP allowing fast facile process for obtaining different new polymer/MXene composites with controlled thickness and micro-pattern. Furthermore, high performance microsupercapacitors and asymmetric microsupercapacitors are realized based on the excellent composites benefiting from higher areal capacitance, better rate capabilities and lower contact resistance than conventional electropolymerized polymers. The AMSCs exhibit a maximum areal capacitance of 69.5 mF cm−2, an ultrahigh volumetric energy density (250.1 mWh cm−3) at 1.6 V, and excellent cycling stability up to 10000 cycles. The excellent electrochemical properties of the composite polymerized with MXene suggest a great potential of the method for various energy storage applications.
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•Here we first realizing electrochemical polymerizing EDOT and Pyrrole using 2D MXenes to replace traditional electrolytes.•Patterned composite films with microstructures can be deposited on any conductive substrates by in-situ polymerization.•An all pseudocapacitive asymmetric microsupercapacitors were developed based on the PEDOT-MXene composite films and MnO2.•The AMSCs with an ultrahigh energy density and power density at a voltage window of 1.6 V.
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•HER activity on MoxCTz MXenes is strongly surface structural dependent.•Mo2CTz is more active for HER than Mo1.33CTz possessing order divacancies on the surface.•Oxidation of MoxCTz ...at cathodic conditions leads to decay in activity during accelerated durability testing.•MoxCTz MXenes have pH universal HER activity.
The exploration of non-precious catalysts for the hydrogen evolution reaction (HER) remains critical in the commercialization of electrochemical energy storage and conversion technologies. Two-dimensional transitional metal carbides called MXenes have been found to have great potential as electrocatalysts for HER. In this work, we synthesize two molybdenum-based MXenes: Mo1.33CTz and Mo2CTz and measure their HER activity and operational durability. The ordered divacancies on the basal planes of Mo1.33CTz cause a marked decrease in HER activity compared to Mo2CTz. The stoichiometry and atomic surface structure of MXenes is found to be critically important for catalytic activity while having less of an impact on operational durability. This work provides insight for the development of active 2D materials, in general and MXenes in particular for HER and other technologically relevant electrochemical reactions.
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