Chemical Doping and O-Functionalization of Carbon-Based Electrode to Improve Vanadium Redox Flow Batteries.

The vanadium redox flow battery (VRFB) holds promise for large-scale energy storage applications, despite its lower energy and power densities compared to advanced secondary batteries available today. Carbon materials are considered suitable catalyst electrodes for improving many aspects of the VRFB. However, pristine graphite structures in carbon materials are catalytically inert and require modification to activate their catalytic activity. Among the various strategies developed so far, O-functionalization and chemical doping of carbon materials are considered some of the most promising pathways to regulate their electronic structures. Building on the catalytic mechanisms involved in the VRFB, this concise review discusses recent advancements in the O-functionalization and chemical doping of carbon materials. Furthermore, it explores how these materials can be tailored and highlights future directions for developing more promising VRFBs to guide future research.

Superior performance of oxygen vacancy-enriched Cu-Co3O4/urushiol-rGO/peroxymonosulfate for hypophosphite and phosphite removal by enhancing singlet oxygen.

The treatment of wastewater containing hypophosphite [P(I)] and phosphite [P(III)] is challenged by limitations of traditional Fenton oxidation such as low efficiency, secondary pollution and high costs. This study introduced a facile solvent-thermal method to synthesize Cu-Co3O4 nanoparticles uniformly loaded on graphene (Cu-Co3O4/U-rGO) through the reduction and coordination effects of urushiol (U). As prepared Cu-Co3O4/U-rGO exhibited excellent activity in activating peroxymonosulfate (PMS) for the oxidation of P(I)/P(III) to phosphate [P(V)] (0.229 min-1), along with high stability and reusability (91.5 % after 6 cycles), low metal leaching rate (Co: 0.2 mg/L, Cu: 0.05 mg/L), insensitivity to common anions in water and a wide pH range (3-11). The activation mechanism involved the synergistic effects from both urushiol and graphene, which promoted redox of Cu+/Cu2+ and Co2+/Co3+ and induced abundant oxygen vacancies for PMS activation to produce singlet oxygen. Furthermore, the Cu-Co3O4/U-rGO/PMS was also excellent in the oxidative removal of organic phosphorus. This study is expected to advance strategies for the treatment of P(I)/P(III)-rich wastewater and provide new insights for the development of low-cost, highly efficient heterogeneous catalysts with abundant oxygen vacancies.

Building hybrid structure of monolayered S-depleted Mo-S nanocrystals and 3D graphene towards promising aqueous supercapacitor applications.

Two-dimensional (2D) 1H molybdenum disulfide (1H-MoS2) is hard to be directly used in energy storage devices due to its inert basal plane and unfavorable 2D stacking. This work demonstrated how the basal plane of 1H MoS2nanocrystals (NCs) can be activated to offer doubled specific capacitance by simple surface S depletions. Building on the expanded graphene with three-dimensional (3D) structures, as-prepared NCs were chemically grafted on the graphene surface to deliver stable energy storage and high capacitance, which overcame above challenges of 1H-MoS2. Aside from the mostly focused metastable phase, this work confirmed that the stable 1H Mo-S material is also promising in energy storage applications.

Lattice Distortion and H-passivation in Pure Carbon Electrocatalysts for Efficient and Stable Two-electron Oxygen Reduction to H2 O2.

The exploration of inexpensive and efficient catalysts for oxygen reduction reaction (ORR) is crucial for chemical and energy industries. Carbon materials have been proved promising with different catalysts enabling 2 and 4e- ORR. Nevertheless, their ORR activity and selectivity is still complex and under debate in many cases. Many structures of these active carbon materials are also chemically unstable for practical implementations. Unlike the well-discussed structures, this work presents a strategy to promote efficient and stable 2e- ORR of carbon materials through the synergistic effect of lattice distortion and H-passivation (on the distorted structure). We show how these structures can be formed on carbon cloth, and how the reproducible chemical adsorption can be realized on these structures for efficient and stable H2 O2 production. The work here gives not only new understandings on the 2e- ORR catalysis, but also the robust catalyst which can be directly used in industry.

Electrolyte and Additive Engineering for Zn Anode Interfacial Regulation in Aqueous Zinc Batteries.

Aqueous Zn-metal batteries (AZMBs) have gained great interest due to their low cost, eco-friendliness, and inherent safety, which serve as a promising complement to the existing metal-based batteries, e.g., lithium-metal batteries and sodium-metal batteries. Although the utilization of aqueous electrolytes and Zn metal anode in AZMBs ensures their improved safety over other metal batteries meanwhile guaranteeing their decent energy density at the cell level, plenty of challenges involved with metallic Zn anode still await to be addressed, including dendrite growth, hydrogen evolution reaction, and zinc corrosion and passivation. In the past years, several attempts have been adopted to address these problems, among which engineering the aqueous electrolytes and additives is regarded as a facile and promising approach. In this review, a comprehensive summary of aqueous electrolytes and electrolyte additives will be given based on the recent literature, aiming at providing a fundamental understanding of the challenges associated with the metallic Zn anode in aqueous electrolytes, meanwhile offering a guideline for the electrolytes and additives engineering strategies toward stable AZMBs in the future.

A quasi-solid polymer electrolyte initiated by two-dimensional functional nanosheets for stable lithium metal batteries.

Lithium-metal batteries (LMBs) are expected to serve as next-generation energy storage systems due to their high theoretical energy density. However, their practical application is largely impeded due to the safety risks that arise from the uncontrollable Li dendrite growth and the high reactivity between high flammability liquid organic electrolytes and metallic lithium. Here, we report a highly safe quasi-solid gel polymer electrolyte (GPE) to achieve stable cycling of lithium metal with high coulombic efficiency, and it is prepared by in situ polymerization of 1,3-dioxolane (DOL) assisted by multi-functional H3Sb3P2O14 sheets. H3Sb3P2O14 acts as an initiator and a functional additive simultaneously that promotes the formation of a stable solid electrolyte interface (SEI) layer, thereby regulating the uniform deposition of Li and improving the Li plating/stripping efficiency. The obtained quasi-solid GPE exhibits high ionic conductivity and enhanced oxidative stability, favoring a stabilized electrode/electrolyte interface. Using the GPE, the electrochemical performance of the quasi-solid-state LMB with a LiFePO4 cathode and a lithium metal anode is significantly improved, delivering a discharge capacity of 125.7 mA h g-1 even after 1000 cycles. Therefore, the high reversibility and remarkable battery cyclability suggest that such a GPE is a promising choice of electrolyte for LMBs, while its facile preparation makes its large-scale application possible in the future.

The Dual Functions of Defect-Rich Carbon Nanotubes as Both Conductive Matrix and Efficient Mediator for LiS Batteries.

LiS batteries are considered a promising energy storage system owing to the great abundance of sulfur and its high specific capacity. Polysulfide shuttling and sluggish reaction kinetics in sulfur cathodes significantly degrade the cycle life of LiS batteries. A modified method is employed to create defects in carbon nanotubes (CNTs), anchoring polysulfides, and accelerating electrochemical reactions. The defect-rich CNTs (D-CNT) enable dramatic improvement in both cycling and rate performance. A specific capacity of 600 mAh g-1 with a current density of 0.5 C is achieved after 400 cycles, and even at a very high current density (5.0 C), a specific capacity of 434 mAh g-1 is observed. Cycling stability up to 1000 cycles is also achieved under the conditions of high sulfur loading and lean electrolyte. Theoretical calculations revealed that the improvement is mainly attributable to the electronic structure of defect-rich carbon, which has higher binding energy with polysulfides because of the upshift of the p-band center. Furthermore, rotating disk electrode measurements demonstrate that the defect-rich carbon can accelerate the polysulfide conversion process. It is anticipated that this new design strategy can be the starting point for mediator-like carbon materials with good conductivity and high catalytic activity for LiS batteries.

Mn-Fe3O4 nanoparticles anchored on the urushiol functionalized 3D-graphene for the electrochemical detection of 4-nitrophenol.

Preparation of highly active and cost-effective electrode materials is of great interest in electrochemical detection. In this study, a simple urushiol-templated solvothermal method combined with calcination was proposed to fabricate N-doped three-dimensional graphene (3D-G) with Mn-doped Fe3O4 nanoparticles loaded on the surface (Mn-Fe3O4/3D-G). Because of the large active surface area, porous channel and high loading ratio of Mn-Fe3O4 nanoparticles, as-prepared Mn-Fe3O4/3D-G sensor showed high activity on the determination of 4-nitrophenol (4-NP), which are much improved from the control un-modified samples. The wide linear concentration range (5-100 µM), low detection limit (19 nM) and satisfactory recovery of 4-NP in various water samples (98.38-100.41%) indicated that the Mn-Fe3O4/3D-G electrode can be potentially used for real-world applications. This study gives a simple but meaningful strategy for constructing transition metal oxide/graphene composite materials with high electrocatalytic activity.

Improved charge injection of edge aligned MoS2/MoO2 hybrid nanosheets for highly robust and efficient electrocatalysis of H2 production.

Molybdenum disulfide (MoS2) can be an efficient electro-catalyst for the hydrogen evolution reaction (HER) as an alternative to precious metals, but significant efforts are still needed to further improve its efficiency. Among various approaches, the formation of edge aligned MoS2 on an electrically conductive support is highly promising for cost-effective H2 production. Nevertheless, catalysis is highly impeded by the poor charge transport between the electrode materials and also between the multilayers of MoS2. This research presents a strategy to improve the HER catalysis by binding layers of metallic molybdenum dioxide (MoO2) and MoS2 to form hybrid MoS2/MoO2 nanosheets (attached and cross-linked to each other). Taking advantage of the hybrid structure and the mechanical strength of the carbon cloth, a catalyst with outstanding catalytic performance in the HER is demonstrated. This work shows not only a strategy to efficiently improve the electrochemical process, but also the preparation of a highly efficient catalyst for constant and robust H2 production.

Moss-Derived Mesoporous Carbon as Bi-Functional Electrode Materials for Lithium⁻Sulfur Batteries and Supercapacitors.

In this work, we reported a moss-derived biomass porous carbon (MPC) as a bi-functional electrode material for both the lithium⁻sulfur battery and the supercapacitor. The MPC was prepared from a high-temperature calcination procedure using the moss as the carbonaceous precursor. Using NaOH, the MPC was activated to give a mesoporous structure with a high specific surface area (1057.1 m² g-1) and large pore volume (0.72 cm³ g-1). When it was used as the cathode material in lithium⁻sulfur batteries, the MPC material realized a sulfur loading and exhibited a remarkably improved electrochemical performance, i.e., a high discharge capacity of 1070 mAh g-1 at 0.1 C. This activated MPC also worked well as a capacitive electrode in supercapacitors, demonstrating a high specific capacitance of 332 F g-1 (scan rate of 1.0 A g-1) and a high capacity retention > 97% in a long-term cycle of 1000 charge/discharges. This work demonstrated a facile method for the utilization of activated waste biomass material for future clean energy applications.

Biocompatibility and toxicity of graphene quantum dots for potential application in photodynamic therapy.

AIM: Achieving reliably high production of reactive oxygen species (ROS) in photodynamic therapy (PDT) remains challenging. Graphene quantum dots (GQDs) hold great promise for PDT. However, the photochemical processes leading to GQD-derived ROS generation have not yet been fully elucidated. MATERIALS & METHODS: Physicochemical characteristics of GQDs were comprehensively investigated, including electron paramagnetic resonance analysis of singlet oxygen production. Dark toxicity was assessed in vitro and in vivo. RESULTS: GQDs demonstrated excellent photoluminescent features, corrosion resistance, high water solubility, high photo/pH-stability, in vitro and in vivo biocompatibility and very efficient singlet oxygen/ROS generation. CONCLUSION: The enhanced ROS generation, combined with good biocompatibility and minimal toxicity in vitro and in vivo support the potential of GQDs for future PDT application.

Investigating the bioavailability of graphene quantum dots in lung tissues via Fourier transform infrared spectroscopy.

Biomolecular fractions affect the fate and behaviour of quantum dots (QDs) in living systems but how the interactions between biomolecules and QDs affect the bioavailability of QDs is a major knowledge gap in risk assessment analysis. The transport of QDs after release into a living organism is a complex process. The majority accumulate in the lungs where they can directly affect the inhalation process and lung architecture. Here, we investigate the bioavailability of graphene quantum dots (GQDs) to the lungs of rats by measuring the alterations in macromolecular fractions via Fourier transform infrared spectroscopy (FTIR). GQDs were intravenously injected into the rats in a dose-dependent manner (low (5 mg kg-1) and high (15 mg kg-1) doses of GQDs per body weight of rat) for 7 days. The lung tissues were isolated, processed and haematoxylin-eosin stained for histological analysis to identify cell death. Key biochemical differences were identified by spectral signatures: pronounced changes in cholesterol were found in two cases of low and high doses; a change in phosphorylation profile of substrate proteins in the tissues was observed in low dose at 24 h. This is the first time biomolecules have been measured in biological tissue using FTIR to investigate the biocompatibility of foreign material. We found that highly accurate toxicological changes can be investigated with FTIR measurements of tissue sections. As a result, FTIR could form the basis of a non-invasive pre-diagnostic tool for predicting the toxicity of GQDs.

Solvent-Assisted Oxygen Incorporation of Vertically Aligned MoS2 Ultrathin Nanosheets Decorated on Reduced Graphene Oxide for Improved Electrocatalytic Hydrogen Evolution.

Three-dimensional oxygen-incorporated MoS2 ultrathin nanosheets decorated on reduced graphene oxide (O-MoS2/rGO) had been successfully fabricated through a facile solvent-assisted hydrothermal method. The origin of the incorporated oxygen and its incorporation mechanism into MoS2 were carefully investigated. We found that the solvent N,N-dimethylformamide (DMF) was the key as the reducing agent and the oxygen donor, expanding interlayer spaces and improving intrinsic conductivity of MoS2 sheets (modulating its electronic structure and vertical edge sites). These O dopants, vertically aligned edges and decoration with rGO gave effectively improved double-layer capacitance and catalytic performance for hydrogen evolution reaction (HER) of MoS2. The prepared O-MoS2/rGO catalysts showed an exceptional small Tafel slope of 40 mV/decade, a high current density of 20 mA/cm(2) at the overpotential of 200 mV and remarkable stability even after 2000th continuous HER test in the acid media.

Sulfur-Depleted Monolayered Molybdenum Disulfide Nanocrystals for Superelectrochemical Hydrogen Evolution Reaction.

Catalytically driven electrochemical hydrogen evolution reaction (HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed by the scarcity of edges and low electrical conductivity. Here, we show how the catalytic performance of MoS2 monolayers can be improved dramatically by catalyst size reduction and surface sulfur (S) depletion. Monolayered MoS2 nanocrystals (NCs) (2-25 nm) produced via exfoliating and disintegrating their bulk counterparts showed improved catalysis rates over monolayer sheets because of their increased edge ratios and metallicity. Subsequent S depletion of these NCs further improved the metallicity and made Mo atoms on the basal plane become catalytically active. As a result, the S-depleted NCs with low mass (∼1.2 µg) showed super high catalytic performance on HER with a low Tafel slope of ∼29 mV/decade, overpotentials of 60-75 mV, and high current densities jx (where x is in mV) of j150 = 9.64 mA·cm(-2) and j200 = 52.13 mA·cm(-2). We have found that higher production rates of H2 could not be achieved by adding more NC layers since HER only happens on the topmost surface and the charge mobility decreases dramatically. These difficulties can be largely alleviated by creating a hybrid structure of NCs immobilized onto three-dimensional graphene to provide a very high surface exposure of the catalyst for electrochemical HER, resulting in very high current densities of j150 = 49.5 mA·cm(-2) and j200 = 232 mA·cm(-2) with ∼14.3 µg of NCs. Our experimental and theoretical studies show how careful design and modification of nanoscale materials/structures can result in highly efficient catalysis. There may be considerable opportunities in the broader family of transition metal dichalcogenides beyond just MoS2 to develop highly efficient atomically thin catalysts. These could offer cheap and effective replacement of precious metal catalysts in clean energy production.

Formation of tunable graphene oxide coating with high adhesion.

Graphene oxide (GO) can be applied as a coating on metals, but few of these coatings have an adhesion suitable for practical applications. We demonstrate here how to form a GO coating on metals with a high adhesion (∼ 10.6 MPa) and tuneable surface, which can be further applied using similar/modified techniques for special applications (e.g. anti-corrosion and anti-biofouling).

Fabrication and luminescence of monolayered boron nitride quantum dots.

Monolayered boron nitride (BN) quantum dots (QDs; lateral size ≈10 nm) are fabricated using a novel method. Unlike monolayered BN sheets, these BN QDs exhibit blue-green luminescence due to defects formed during preparation. This optical behavior adds significant functionality to a material that is already receiving much attention. It is further shown that the QDs are nontoxic to biological cells and well suited to bio-imaging.

Fabrication of luminescent monolayered tungsten dichalcogenides quantum dots with giant spin-valley coupling.

A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Creating high yield water soluble luminescent graphene quantum dots via exfoliating and disintegrating carbon nanotubes and graphite flakes.

We have developed an effective method to exfoliate and disintegrate multi-walled carbon nanotubes and graphite flakes. With this technique, high yield production of luminescent graphene quantum dots with high quantum yield and low oxidization can be achieved.