High-Performance All-Solid-State Supercapacitor Based on the Assembly of Graphene and Manganese(II) Phosphate Nanosheets

Manganese phosphate nanosheets (Mn₃(PO₄)₂·3H₂O NSs) with ∼2 nm thickness were prepared by exfoliating the bulk material in dimethylformamide (DMF) under ultrasonication. They can spontaneously form face-to-face stacked assemblies with exfoliated graphene NSs in DMF. The assemblied Mn₃(PO₄)₂·3H₂O and graphene NSs at the mass ratio of 1:10 (M₁G₁₀) revealed a specific capacitance of 2086 F g^–1 at 1 mV s^–1. These M₁G₁₀ assemblies were used to fabricate all-solid-state supercapacitor (M₁G₁₀-ASSS) on the basis of PVA/KOH solid polymer electrolytes, which exhibited a specific capacitance of 152 F g^–1 (or 40 mF cm^–2) at 0.5 A g^–1, an energy density of 0.17 μWh cm^–2 at 0.5 A g^–1 (1.3 A m^–2) and a power density of 46 μW cm^–2 at 2 A g^–1 (5.3 A m^–2). M₁G₁₀-ASSS also showed excellent cycling stability and nearly 100% capacitance retention was achieved after 2000 galvanostatic charge–discharge cycles at 2 A g^–1. Such extraordinary properties were attributed to the synergistic effect of high pseudocapacitance of Mn₃(PO₄)₂·3H₂O NSs, high conductivity and surface areas of graphene NSs

Amine-Capped Co Nanoparticles for Highly Efficient Dehydrogenation of Ammonia Borane

Highly efficient heterogeneous catalysts are desired for the development of new energy storage materials. The rational choice and use of capping ligands are of significant importance for performance optimization of metal nanoparticle (NP) catalysts. By exploiting amine-rich polyethylenimine (PEI) and graphene oxide (GO) as a NP support, we demonstrate that as a capping ligand, PEI deposited on GO provides a novel pathway able to simultaneously control the morphology, spatial distribution, surface active sites of cobalt (Co) NPs, and remarkably enhances their catalytic properties for the hydrolytic dehydrogenation of ammonia borane (AB). Such a synergistic effect enables the synthesized PEI-GO/Co catalysts to reveal extremely high dehydrogenation activities under atmosphere condition. A total turnover frequency of 39.9 mol_H2 min^–1 mol_Co^–1 and an apparent activation energy of 28.2 kJ mol^–1 make the catalytic performance of these PEI-GO/Co catalysts comparable to those of noble metal-based catalysts, including bimetallic and multimetallic catalysts

Reactivity-Controlled Preparation of Ultralarge Graphene Oxide by Chemical Expansion of Graphite

The production of ultralarge graphene oxide (ULGO) is hindered by sluggish diffusion process of the oxidizing agents into graphite layers, as well as sheet fracture resulting from inhomogeneous oxidation. Previous methods rely on an excess amount of oxidants or multiple oxidation to overcome large diffusion resistance, but at the cost of ULGO yield and environmental risk. Here, we discover the chemical expansion of graphite (CEG) with high solvent-accessible surface areas can effectively boost mass diffusion and facilitate exhaustive oxidation at low oxidant dosage (2 wt equiv). The oxidizing reaction is therefore controlled by the chemical reactivity of graphite with oxidant rather than the diffusion of oxidant, which results in a ∼100% yield of ULGO nanosheets with an area-average size of 128 μm. The worm-like structure of CEG and its oxide provides a chance to recover excess sulfuric acid using a 100-mesh filter, where subsequent exfoliation to ULGO nanosheets is achieved by mild agitation or shaking in several minutes. The ULGO paper prepared by blade casting exhibits superior mechanical properties (Young’s modulus of 11.9 GPa and tensile strength of 110.8 MPa) and electrical conductivity (∼613 S/cm after HI reduction)

Salicylideneanilines-Based Covalent Organic Frameworks as Chemoselective Molecular Sieves

Porous materials such as covalent organic frameworks (COFs) are good candidates for molecular sieves due to the chemical diversity of their building blocks, which allows fine-tuning of their chemical and physical properties by design. Tailored synthesis of inherently functional building blocks can generate framework materials with chemoresponsivity, leading to controllable functionalities such as switchable sorption and separation. Herein, we demonstrate a chemoselective, salicylideneanilines-based COF (SA-COF), which undergoes solvent-triggered tautomeric switching. This is unique compared to solid-state salicylideneanilines’ counterpart, which typically requires high energy input such as photo or thermal activation to trigger the enol–keto tautomerisim and <i>cis</i>–<i>trans</i> isomerization. Accompanying the tautomerization, the ionic properties of the COF can be tuned reversibly, thus forming the basis of size-exclusion, selective ionic binding or chemoseparation in SA-COF demonstrated in this work

Phase Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage

Achieving homogeneous phase transition and uniform charge distribution is essential for good cycle stability and high capacity when phase conversion materials are used as electrodes. Herein, we show that chemical lithiation of bulk 2H-MoS₂ distorts its crystalline domains in three primary directions to produce mosaic-like 1T′ nanocrystalline domains, which improve phase and charge uniformity during subsequent electrochemical phase conversion. 1T′-Li_<i>x</i>MoS₂, a macroscopic dense material with interconnected nanoscale grains, shows excellent cycle stability and rate capability in a lithium rechargeable battery compared to bulk or exfoliated-restacked MoS₂. Transmission electron microscopy studies reveal that the interconnected MoS₂ nanocrystals created during the phase change process are reformable even after multiple cycles of galvanostatic charging/discharging, which allows them to play important roles in the long term cycling performance of the chemically intercalated TMD materials. These studies shed light on how bulk TMDs can be processed into quasi-2D nanophase material for stable energy storage

Crystal Engineering of Naphthalenediimide-Based Metal–Organic Frameworks: Structure-Dependent Lithium Storage

Metal–organic frameworks (MOFs) possess great structural diversity because of the flexible design of linker groups and metal nodes. The structure–property correlation has been extensively investigated in areas like chiral catalysis, gas storage and absorption, water purification, energy storage, etc. However, the use of MOFs in lithium storage is hampered by stability issues, and how its porosity helps with battery performance is not well understood. Herein, through anion and thermodynamic control, we design a series of naphthalenediimide-based MOFs <b>1–4</b> that can be used for cathode materials in lithium-ion batteries (LIBs). Complexation of the <i>N</i>,<i>N</i>′-di­(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) ligand and CdX₂ (X = NO₃^– or ClO₄^–) produces complexes MOFs <b>1</b> and <b>2</b> with a one-dimensional (1D) nonporous network and a porous, noninterpenetrated two-dimensional (2D) square-grid structure, respectively. With the DPNDI ligand and Co­(NCS)₂, a porous 1D MOF <b>3</b> as a kinetic product is obtained, while a nonporous, noninterpenetrated 2D square-grid structure MOF <b>4</b> as a thermodynamic product is formed. The performance of LIBs is largely affected by the stability and porosity of these MOFs. For instance, the initial charge–discharge curves of MOFs <b>1</b> and <b>2</b> show a specific capacity of ∼47 mA h g^–1 with a capacity retention ratio of >70% during 50 cycles at 100 mA g^–1, which is much better than that of MOFs <b>3</b> and <b>4</b>. The better performances are assigned to the higher stability of Cd­(II) MOFs compared to that of Co­(II) MOFs during the electrochemical process, according to X-ray diffraction analysis. In addition, despite having the same Cd­(II) node in the framework, MOF <b>2</b> exhibits a lithium-ion diffusion coefficient (<i>D</i>_Li) larger than that of MOF <b>1</b> because of its higher porosity. X-ray photoelectron spectroscopy and Fourier transform infrared analysis indicate that metal nodes in these MOFs remain intact and only the DPNDI ligand undergoes the revisible redox reaction during the lithiation–delithiation process

Engineering the Electronic Structure of MoS₂ Nanorods by N and Mn Dopants for Ultra-Efficient Hydrogen Production

Developing economical and efficient electrocatalysts with nonprecious metals for the hydrogen evolution reaction (HER), especially in water-alkaline electrolyzers, is pivotal for large-scale hydrogen production. Recently, both density functional theory (DFT) calculations and experimental studies have demonstrated that earth-abundant MoS₂ is a promising HER electrocatalyst in acidic solution. However, the HER kinetics of MoS₂ in alkaline solution still suffer from a high overpotential (90–220 mV at a current density of 10 mA cm^–2). Herein, we report a combined experimental and first-principle approach toward achieving an economical and ultraefficient MoS₂-based electrocatalyst for the HER by fine-tuning the electronic structure of MoS₂ nanorods with N and Mn dopants. The developed N,Mn codoped MoS₂ catalyst exhibits an outstanding HER performance with overpotentials of 66 and 70 mV at 10 mA cm^–2 in alkaline and phosphate-buffered saline media, respectively, and corresponding Tafel slopes of 50 and 65 mV dec^–1. Moreover, the catalyst also exhibits long-term stability in HER tests. DFT calculations suggest that (1) the electrocatalytic performance can be attributed to the enhanced conductivity and optimized electronic structures for facilitating H* adsorption and desorption after N and Mn codoping and (2) N and Mn dopants can greatly activate the catalytic HER activity of the S-edge for MoS₂. The discovery of a simple approach toward the synthesis of highly active and low-cost MoS₂-based electrocatalysts in both alkaline and neutral electrolytes allows the premise of scalable production of hydrogen fuels

Two-Dimensional Polymer Synthesized <i>via</i> Solid-State Polymerization for High-Performance Supercapacitors

Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield <i>via</i> solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C–C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g^–1 at a current density of 1.0 A g^–1, and shows outstanding cycle performance

Molecular Engineering of Bandgaps in Covalent Organic Frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing the material for photovoltaic or photonic applications. Herein, we have developed an approach to narrow the bandgap of COFs by pairing triphenylamine and salicylideneaniline building units to construct an eclipsed stacked 2D COF. The ordered porous structure of 2D COF facilitates a unique moisture-triggered tautomerism. The combination of donor–acceptor charge transfer and tautomerization in the salicyclidineaniline unit imparts a large bandgap narrowing for the COF and turns it color to black. The synthesized COF with donor–acceptor dyad exhibits excellent nonlinear optical properties according to open aperture Z-scan measurements with 532 nm nanosecond laser pulses

Phase Transformations in TiS₂ during K Intercalation

The electrochemical performances of TiS₂ in potassium ion batteries (KIBs) are poor due to the large size of K ions, which induces irreversible structural changes and poor kinetics. To obtain detailed insights into the kinetics of phase changes, we investigated the electrochemical properties, phase transformations, and stability of potassium-intercalated TiS₂ (K_<i>x</i>TiS₂, 0 ≤ <i>x</i> ≤ 0.88). In situ XRD reveals staged transitions corresponding to distinct crystalline phases during K ion intercalation, which are distinct from those of Li and Na ions. Electrochemical (cyclic voltammetry and galvanostatic charge/discharge) studies show that the phase transitions among various intercalated stages slow down the kinetics of the discharge/charge in bulk TiS₂ hosts. By chemically prepotassiating the bulk TiS₂ (K_0.25TiS₂) to reduce the domain size of the crystal, these phase transitions are bypassed and more facile ion insertion kinetics can be obtained, which leads to improved Coulombic efficiency, rate capability, and cycling stability