Freestanding Three-Dimensional Graphene/MnO₂ Composite Networks As Ultralight and Flexible Supercapacitor Electrodes

A lightweight, flexible, and highly efficient energy management strategy is needed for flexible energy-storage devices to meet a rapidly growing demand. Graphene-based flexible supercapacitors are one of the most promising candidates because of their intriguing features. In this report, we describe the use of freestanding, lightweight (0.75 mg/cm²), ultrathin (<200 μm), highly conductive (55 S/cm), and flexible three-dimensional (3D) graphene networks, loaded with MnO₂ by electrodeposition, as the electrodes of a flexible supercapacitor. It was found that the 3D graphene networks showed an ideal supporter for active materials and permitted a large MnO₂ mass loading of 9.8 mg/cm² (∼92.9% of the mass of the entire electrode), leading to a high area capacitance of 1.42 F/cm² at a scan rate of 2 mV/s. With a view to practical applications, we have further optimized the MnO₂ content with respect to the entire electrode and achieved a maximum specific capacitance of 130 F/g. In addition, we have also explored the excellent electrochemical performance of a symmetrical supercapacitor (of weight less than 10 mg and thickness ∼0.8 mm) consisting of a sandwich structure of two pieces of 3D graphene/MnO₂ composite network separated by a membrane and encapsulated in polyethylene terephthalate (PET) membranes. This research might provide a method for flexible, lightweight, high-performance, low-cost, and environmentally friendly materials used in energy conversion and storage systems for the effective use of renewable energy

Constructed Uninterrupted Charge-Transfer Pathways in Three-Dimensional Micro/Nanointerconnected Carbon-Based Electrodes for High Energy-Density Ultralight Flexible Supercapacitors

A type of freestanding three-dimensional (3D) micro/nanointerconnected structure, with a conjunction of microsized 3D graphene networks, nanosized 3D carbon nanofiber (CNF) forests, and consequently loaded MnO₂ nanosheets, has been designed as the electrodes of an ultralight flexible supercapacitor. The resulting 3D graphene/CNFs/MnO₂ composite networks exhibit remarkable flexibility and highly mechanical properties due to good and intimate contacts among them, without current collectors and binders. Simultaneously, this designed 3D micro/nanointerconnected structure can provide an uninterrupted double charges freeway network for both electron and electrolyte ion to minimize electron accumulation and ion-diffusing resistance, leading to an excellent electrochemical performance. The ultrahigh specific capacitance of 946 F/g from cyclic voltammetry (CV) (or 920 F/g from galvanostatic charging/discharging (GCD)) were obtained, which is superior to that of the present electrode materials based on 3D graphene/MnO₂ hybrid structure (482 F/g). Furthermore, we have also investigated the superior electrochemical performances of an asymmetric supercapacitor device (weight of less than 12 mg/cm² and thickness of ∼0.8 mm), showing a total capacitance of 0.33 F/cm² at a window voltage of 1.8 V and a maximum energy density of 53.4 W h/kg for driving a digital clock for 42 min. These inspiring performances would make our designed supercapacitors become one of the most promising candidates for the future flexible and lightweight energy storage systems

Defects Engineered Monolayer MoS₂ for Improved Hydrogen Evolution Reaction

MoS₂ is a promising and low-cost material for electrochemical hydrogen production due to its high activity and stability during the reaction. However, the efficiency of hydrogen production is limited by the amount of active sites, for example, edges, in MoS₂. Here, we demonstrate that oxygen plasma exposure and hydrogen treatment on pristine monolayer MoS₂ could introduce more active sites via the formation of defects within the monolayer, leading to a high density of exposed edges and a significant improvement of the hydrogen evolution activity. These as-fabricated defects are characterized at the scale from macroscopic continuum to discrete atoms. Our work represents a facile method to increase the hydrogen production in electrochemical reaction of MoS₂ via defect engineering, and helps to understand the catalytic properties of MoS₂

Synthesis on Winged Graphene Nanofibers and Their Electrochemical Capacitive Performance

Assembly techniques of graphene have attracted intense attention since their performance strongly depends on the manners in which graphene nanosheets are arranged. In this work, we demonstrate a viable process to synthesize winged graphene nanofibers (G-NFs) which could generate optimized pore size distribution by the fiber-like feature of graphene. The G-NF frameworks were achieved by processing the precursor graphene oxide nanosheets with the following procedures: microwave (MW) irradiation, salt addition, freeze-drying, and chemical reduction. The resultant framework composed of winged G-NFs with a diameter of 200–500 nm and a length of 5–20 μm. Moreover, the crimp degree of G-NFs can be rationally controlled by MW irradiation time. A formation mechanism of such winged G-NFs based on the synergistic effects from MW irradiation and solution ionic strength change has been proposed. With a practice in flexible electrode, after decorated with amorphous MnO₂, the G-NF frameworks shows an enhanced specific capacitance compared to graphene nanosheets (G-NSs). This research has developed a controllable method to synthesis G-NFs, which can offer hierarchical pore structures, this kind of graphene nanostructure might enhance their performance in supercapacitor and related fields

Liquid Phase Exfoliation of Two-Dimensional Materials by Directly Probing and Matching Surface Tension Components

Exfoliation of two-dimensional (2D) materials into mono- or few layers is of significance for both fundamental studies and potential applications. In this report, for the first time surface tension components were directly probed and matched to predict solvents with effective liquid phase exfoliation (LPE) capability for 2D materials such as graphene, h-BN, WS₂, MoS₂, MoSe₂, Bi₂Se₃, TaS₂, and SnS₂. Exfoliation efficiency is enhanced when the ratios of the surface tension components of the applied solvent is close to that of the 2D material in question. We enlarged the library of low-toxic and common solvents for LPE. Our study provides distinctive insight into LPE and has pioneered a rational strategy for LPE of 2D materials with high yield

Controlled Synthesis of Atomically Thin 1T-TaS₂ for Tunable Charge Density Wave Phase Transitions

The charge density wave (CDW) in two-dimensional (2D) materials is attracting substantial interest because of its magnificent many-body collective phenomena. Various CDW phases have been observed in several 2D materials before they reach the phase of superconductivity. However, to date, the atomically thin CDW materials were mainly fabricated by mechanically exfoliating from their bulk counterparts, which leads to low production yield and small sample sizes. Here, we report the controlled synthesis of atomically thin 1T-TaS₂, a typical CDW material, by a chemical vapor deposition (CVD) method. The high quality of as-grown 1T-TaS₂ has been confirmed by complementary characterization technologies. Moreover, the thickness-dependent CDW phase transitions have been revealed in these ultrathin flakes by temperature-dependent Raman spectra. This work opens up a new window for the large-scale synthesis of ultrathin CDW materials and sheds light on the fabrication of next-generation electronic devices

Mitochondrial complex I bridges a connection between regulation of carbon flexibility and gastrointestinal commensalism in the human fungal pathogen <i>Candida albicans</i>

<div><p>Efficient assimilation of alternative carbon sources in glucose-limited host niches is critical for colonization of <i>Candida albicans</i>, a commensal yeast that frequently causes opportunistic infection in human. <i>C</i>. <i>albicans</i> evolved mechanistically to regulate alternative carbon assimilation for the promotion of fungal growth and commensalism in mammalian hosts. However, this highly adaptive mechanism that <i>C</i>. <i>albicans</i> employs to cope with alternative carbon assimilation has yet to be clearly understood. Here we identified a novel role of <i>C</i>. <i>albicans</i> mitochondrial complex I (CI) in regulating assimilation of alternative carbon sources such as mannitol. Our data demonstrate that CI dysfunction by deleting the subunit Nuo2 decreases the level of NAD⁺, downregulates the NAD⁺-dependent mannitol dehydrogenase activity, and consequently inhibits hyphal growth and biofilm formation in conditions when the carbon source is mannitol, but not fermentative sugars like glucose. Mannitol-dependent morphogenesis is controlled by a ROS-induced signaling pathway involving Hog1 activation and Brg1 repression. <i>In vivo</i> studies show that <i>nuo2</i>Δ/Δ mutant cells are severely compromised in gastrointestinal colonization and the defect can be rescued by a glucose-rich diet. Thus, our findings unravel a mechanism by which <i>C</i>. <i>albicans</i> regulates carbon flexibility and commensalism. Alternative carbon assimilation might represent a fitness advantage for commensal fungi in successful colonization of host niches.</p></div

Two-Step Growth of Two-Dimensional WSe₂/MoSe₂ Heterostructures

Two dimensional (2D) materials have attracted great attention due to their unique properties and atomic thickness. Although various 2D materials have been successfully synthesized with different optical and electrical properties, a strategy for fabricating 2D heterostructures must be developed in order to construct more complicated devices for practical applications. Here we demonstrate for the first time a two-step chemical vapor deposition (CVD) method for growing transition-metal dichalcogenide (TMD) heterostructures, where MoSe₂ was synthesized first and followed by an epitaxial growth of WSe₂ on the edge and on the top surface of MoSe₂. Compared to previously reported one-step growth methods, this two-step growth has the capability of spatial and size control of each 2D component, leading to much larger (up to 169 μm) heterostructure size, and cross-contamination can be effectively minimized. Furthermore, this two-step growth produces well-defined 2H and 3R stacking in the WSe₂/MoSe₂ bilayer regions and much sharper in-plane interfaces than the previously reported MoSe₂/WSe₂ heterojunctions obtained from one-step growth methods. The resultant heterostructures with WSe₂/MoSe₂ bilayer and the exposed MoSe₂ monolayer display rectification characteristics of a p–n junction, as revealed by optoelectronic tests, and an internal quantum efficiency of 91% when functioning as a photodetector. A photovoltaic effect without any external gates was observed, showing incident photon to converted electron (IPCE) efficiencies of approximately 0.12%, providing application potential in electronics and energy harvesting

Mannitol-dependent hyphal inhibition in the <i>nuo2</i>Δ/Δ mutant is not majorly due to defective proliferation and increased osmotic potential.

<p>(A) Vegetative growth of Wild type, <i>nuo2</i>Δ/Δ and NUO2 AB strains on YEP medium supplemented with 2% of mannitol, glucose or mannose. Cells were grown to mid-log stage (8h of growth; OD₆₀₀ = 1.0), serially diluted and displayed on plates. The colonies were photographed after incubation at 30°C for 3 days. (B) Hyphal comparison of indicated strains under normoxic and hypoxic conditions. Strains were initially plated for single colonies on Spider medium and incubated under normoxic or hypoxic (controlled CO₂ incubator; less than 0.2% oxygen) condition in the dark at 37°C. Pictures were taken after 3 days of incubation under normoxic condition and 7 days under hypoxic condition, respectively. (C) Phenotypic assay of Nuo2 including morphology under conditions that YEP medium is supplemented with different combinations of carbon sources and salt. The <i>nuo2</i>Δ/Δ mutant cells were inoculated on YEP medium containing different concentrations of carbon sources in the presence or absence of salt (1M NaCl) and incubated at 37°C. Pictures were taken after 3 h of incubation. (D) Effect of glucose or mannose supplementation on vegetative growth of <i>nuo2</i>Δ/Δ mutant under mannitol condition. Cell cultures (pre-diluted to OD₆₀₀ = 0.8) from wild type, <i>nuo2</i>Δ/Δ or NUO2 AB strains were diluted serially in 10 fold increments prior to being spotted onto YEP plates supplemented with different combinations of carbon sources. Plates were incubated at 30°C and pictures were taken after 2 days.</p

Nuo2 is required for mannitol-stimulated hyphal growth in <i>C</i>. <i>albicans</i>.

<p>(A to D) Cultures of wild type, <i>nuo2</i>Δ/Δ and NUO2 AB strains were grown overnight in a liquid YPD at 30°C, pelleted, washed in PBS, resuspended in an equal volume of PBS, and diluted 1:250 in either YEP or Spider medium supplemented with 2% of mannitol. Cells were continued to incubate at 37°C and hyphal morphologies were visualized under microscopy. Experiments were repeated in triplicates. Shown are representative images of <i>C</i>. <i>albicans</i> cells at initial overnight culture (A), 1.5 h (B), 3 h (C) and 6 h (D) after incubation in medium containing mannitol. Scale bars are 10μm. (E) Cells from Wild type, <i>nuo2</i>Δ/Δ and NUO2 AB strains were serially diluted and a representative dilution is displayed on Spider medium containing 2% mannitol at 37°C. Picture was taken after 2–3 days of growth.</p