Electrocatalysis plays an essential role in diverse electrochemical energy conversion processes that are vital for improving energy utilization efficiency and mitigating the aggravating global ...warming challenge. The noble metals such as platinum are generally the most frequently used electrocatalysts to drive these reactions and facilitate the relevant energy conversion processes. The high cost and scarcity of these materials pose a serious challenge for the wide-spread adoption and the sustainability of these technologies in the long run, which have motivated considerable efforts in searching for alternative electrocatalysts with reduced loading of precious metals or based entirely on earth-abundant metals. Of particular interest are graphene-supported single atom catalysts (G-SACs) that integrate the merits of heterogeneous catalysts and homogeneous catalysts, such as high activity, selectivity, stability, maximized atom utilization efficiency and easy separation from reactants/products. The graphene support features a large surface area, high conductivity and excellent (electro)-chemical stability, making it a highly attractive substrate for supporting single atom electrocatalysts for various electrochemical energy conversion processes. In this review, we highlight the recent advancements in G-SACs for electrochemical energy conversion, from the synthetic strategies and identification of the atomistic structure to electrocatalytic applications in a variety of reactions, and finally conclude with a brief prospect on future challenges and opportunities.
The synthetic strategies, structural identification and electrocatalytic applications of single atom catalysts supported on graphene or graphene-like carbons are reviewed.
Ceramic aerogels are attractive for thermal insulation but plagued by poor mechanical stability and degradation under thermal shock. In this study, we designed and synthesized hyperbolic ...architectured ceramic aerogels with nanolayered double-pane walls with a negative Poisson's ratio (-0.25) and a negative linear thermal expansion coefficient (-1.8 × 10
per °C). Our aerogels display robust mechanical and thermal stability and feature ultralow densities down to ~0.1 milligram per cubic centimeter, superelasticity up to 95%, and near-zero strength loss after sharp thermal shocks (275°C per second) or intense thermal stress at 1400°C, as well as ultralow thermal conductivity in vacuum ~2.4 milliwatts per meter-kelvin (mW/m·K) and in air (~20 mW/m·K). This robust material system is ideal for thermal superinsulation under extreme conditions, such as those encountered by spacecraft.
2D layered materials typically feature strong in‐plane covalent chemical bonding within each atomic layer and weak out‐of‐plane van der Waals (vdW) interactions between adjacent layers. The ...non‐bonding nature between neighboring layers naturally results in a vdW gap, in which various foreign species may be inserted without breaking the in‐plane covalent bonds. By tailoring the composition, size, structure, and electronic properties of the intercalated guest species and the hosting layered materials, an expansive family of layered intercalation materials may be produced with highly variable compositional and structural features as well as widely tunable physical/chemical properties, invoking unprecedented opportunities in fundamental studies of property modulation and potential applications in diverse technologies, including electronics, optics, superconductors, thermoelectrics, catalysis, and energy storage. Here, the principles and protocols for various intercalation methods, including wet chemical intercalation, gas‐phase intercalation, electrochemical intercalation, and ion‐exchange intercalation, are comprehensively reviewed and how the intercalated species alter the crystal structure and the interlayer coupling of the host 2D layered materials, introducing unusual physical and chemical properties and enabling devices with superior performance or unique functions, is discussed. To conclude, a brief summary on future research opportunities and emerging challenges in the layered intercalation materials is given.
The intercalation of layered materials produces a rich class of materials with tunable structural and electronic properties for both fundamental studies and practical technologies. The key intercalation methods for preparing layered intercalation materials are summarized, the associated modulation of multiple properties is reviewed, and further prospects regarding the future challenges and opportunities in layered intercalation materials are considered.
The junctions formed at the contact between metallic electrodes and semiconductor materials are crucial components of electronic and optoelectronic devices
. Metal-semiconductor junctions are ...characterized by an energy barrier known as the Schottky barrier, whose height can, in the ideal case, be predicted by the Schottky-Mott rule
on the basis of the relative alignment of energy levels. Such ideal physics has rarely been experimentally realized, however, because of the inevitable chemical disorder and Fermi-level pinning at typical metal-semiconductor interfaces
. Here we report the creation of van der Waals metal-semiconductor junctions in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning. The Schottky barrier height, which approaches the Schottky-Mott limit, is dictated by the work function of the metal and is thus highly tunable. By transferring metal films (silver or platinum) with a work function that matches the conduction band or valence band edges of molybdenum sulfide, we achieve transistors with a two-terminal electron mobility at room temperature of 260 centimetres squared per volt per second and a hole mobility of 175 centimetres squared per volt per second. Furthermore, by using asymmetric contact pairs with different work functions, we demonstrate a silver/molybdenum sulfide/platinum photodiode with an open-circuit voltage of 1.02 volts. Our study not only experimentally validates the fundamental limit of ideal metal-semiconductor junctions but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics.
Display omitted
•ZnO, WO3, binary composite WO3-ZnO and ternary WO3-ZnO@rGO nanocomposites were prepared.•The minimum bandgap was 2.62 eV for WO3.•Maximum photocatalytic activity (i.e. > 90 %) was ...observed by rGO based composite.
Novel graphene supported ternary nanocomposites (WO3-ZnO@rGO) were prepared via simple ultrasound assisted fabrication of WO3-ZnO binary nanostructures over 2D rGO nanosheets. Structural, morphological and elemental characterization of as-synthesized samples were carried out through X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM) and Energy dispersive X-ray spectroscopy (EDX). Optical band gap analysis under UV–vis spectroscopy (UV–vis) revealed a decrement in the band gap energy value of WO3-ZnO@rGO ternary nanocomposites. Current-voltage (I–V) measurements evidenced the ohmic response of rGO supported WO3-ZnO nanocomposites as compared to bare samples. To evaluate the photocatalytic efficiency of WO3-ZnO@rGO nanocomposites in comparison to bare nanostructures, wastewater organic pollutant methylene blue (MB) dye was chosen as benchmark reaction. Results suggested that impregnation of WO3-ZnO over rGO sheets significantly improved the photocatalytic efficiency of WO3-ZnO@rGO with 94 % dye removal within 90 min under visible light irradiation. The enhanced photocatalytic efficiency is ascribed to synergistic effects originating at WO3-ZnO@rGO interfaces subsidizing high quantum yield and effective separation and transfer of photo induced electron-hole pairs. Moreover, WO3-ZnO@rGO also retained its remarkable dye degradation efficiency even after four recycling tests.
Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) ...because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb₂O₅) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.
Graphene‐supported single atomic metals (G‐SAMs) have recently attracted considerable research interest for their intriguing catalytic, electronic, and magnetic properties. The development of ...effective synthetic methodologies toward G‐SAMs with monodispersed metal atoms is vital for exploring their fundamental properties and potential applications. A convenient, rapid, and general strategy to synthesize a series of monodispersed atomic transition metals (for example, Co, Ni, Cu) embedded in nitrogen‐doped graphene by two‐second microwave (MW) heating the mixture of amine‐functionalized graphene oxide and metal salts is reported here. The MW heating is able to simultaneously induce the reduction of graphene oxide, the doping of nitrogen, and the incorporation of metal atoms into the graphene lattices in one simple step. The rapid MW process minimizes metal diffusion and aggregation to ensure exclusive single metal atom dispersion in graphene lattices. Electrochemical studies demonstrate that graphene‐supported Co atoms can function as highly active electrocatalysts toward the hydrogen evolution reaction. This MW‐assisted method provides a rapid and efficient avenue to supported metal atoms for wide ranges of applications.
Microwave (MW) heating is used for the rapid preparation of graphene‐supported single atomic metals. With MW heating, graphene oxide reduction, nitrogen doping, and metal incorporation are achieved in a single two‐second process, minimizing metal atom diffusion to achieve exclusive atomic dispersion. This offers a facile approach to wide ranges of single atomic metals with intriguing catalytic, magnetic and electronic properties.
3D electrodes with interconnected and interpenetrating pathways enable efficient electron and ion transport. In this Review, the design and synthesis of such 3D electrodes are discussed, along with ...their ability to address charge transport limitations at high areal mass loading and to enable composite electrodes with an unprecedented combination of energy and power densities in electrochemical energy storage devices.
Conducting polymers generally show high specific capacitance but suffer from poor rate capability and rapid capacitance decary which greatly limits their practical applications in supercapacitor ...electrodes. To this end, many studies have focused on improving the overall capacitive performance by synthesizing nanostructured conducting polymers or by depositing a range of coatings to increase the active surface area exposed to the electrolyte and enhance the charge transport efficiency and structural stability. Despite this, simultaneously achieving high specific capacitance, good rate performance, and long cycle life remains a considerable challenge. Among the various two-dimensional （2D） layered materials, octahedral （1T） phase molybdenum disulfide （MoS2） nano- sheets have high electrical conductivity, large specific surface areas, and unique surface chemical characteristics, making them an interesting substrate for the controlled growth of nanostructured conducting polymers. This paper reports the rational synthesis of carbon shell-coated polyaniline （PANI） grown on 1T MoS2 monolayers （MoS2/PANI@C）. The composite electrode comprised of MoS2/ PANI@C with a -3 nm carbon shell exhibited a remarkable specific capacitance of up to 678 F-g-1 （1 mV.s-1）, superior capacity retention of 80% after 10,000 cycles and good rate performance （81% at 10 mV.s-1） due to the multiple synergic effects between the PANI nanostructure and 1T MoS2 substrates as well as protection by the uniform thin carbon shell. These properties are comparable to the best overall capacitive performance achieved for conducting polymers-based supercapacitor electrodes reported thus far.
NiCo2O4 nanowire array on carbon cloth (NiCo2O4/CC) is proposed as a highly active electrocatalyst for ambient nitrate (NO3−) reduction to ammonia (NH3). In 0.1 m NaOH solution with 0.1 m NaNO3, such ...NiCo2O4/CC achieves a high Faradic efficiency of 99.0% and a large NH3 yield up to 973.2 µmol h−1 cm−2. The superior catalytic activity of NiCo2O4 comes from its half‐metal feature and optimized adsorption energy due to the existence of Ni in the crystal structure. A Zn‐NO3− battery with NiCo2O4/CC cathode also shows a record‐high battery performance.
NiCo2O4 nanowire array on carbon cloth is an efficient electrocatalyst for NO3− to NH3 conversion under ambient conditions, achieving a high Faradic efficiency of 99.0% and a large NH3 yield up to 973.2 µmol h−1 cm−2 in 0.1 m NaOH with 0.1 m NO3−. The catalytic mechanism is investigated by theoretical calculations.
PDF
