Carbon-based supercapacitors can provide high electrical power, but they do not have sufficient energy density to directly compete with batteries. We found that a nitrogen-doped ordered mesoporous few-layer carbon has a capacitance of 855 farads per gram in aqueous electrolytes and can be bipolarly charged or discharged at a fast, carbon-like speed. The improvement mostly stems from robust redox reactions at nitrogen-associated defects that transform inert graphene-like layered carbon into an electrochemically active substance without affecting its electric conductivity. These bipolar aqueous-electrolyte electrochemical cells offer power densities and lifetimes similar to those of carbon-based supercapacitors and can store a specific energy of 41 watt-hours per kilogram (19.5 watt-hours per liter).

No abstract

FeSe-derived superconductors show some unique behaviors relative to iron-pnictide superconductors, which are very helpful to understand the mechanism of superconductivity in high-T c iron-based superconductors. The low-energy electronic structure of the heavily electron-doped A x Fe 2 Se 2 (A=K, Rb, Cs) demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors 1,2 . The superconducting transition temperature (T c ) in the one-unit-cell FeSe on SrTiO 3 substrate can reach as high as ~65 K 3-6 , largely transcending the bulk T c of all known iron-based superconductors. However, in the case of A x Fe 2 Se 2 , the inter-grown antiferromagnetic insulating phase makes it difficult to study the underlying physics. Superconductors of alkali metal ions and NH 3 molecules or organic-molecules intercalated FeSe 7-9 and single layer or thin film FeSe on SrTiO 3 substrate 3-6 are extremely air-sensitive, which prevents the further investigation of their physical properties. Therefore, it is urgent to find a stable and accessible FeSe-derived superconductor for physical property measurements so as to study the underlying mechanism of superconductivity. Here, we report the air-stable superconductor (Li 0.8 Fe 0.2 )OHFeSe with high temperature superconductivity at ~40 K synthesized by a novel hydrothermal method. The crystal structure is unambiguously determined by the combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. It is also found that an antiferromagnetic order coexists with superconductivity in such new FeSe-derived superconductor. This novel synthetic route opens a new avenue for exploring other superconductors in the related systems. The combination of different structure characterization techniques helps to complementarily determine and understand the details of the complicated structures

The heterojunction semiconductors Bi 2 O 3 /BaTiO 3 were prepared by a milling-annealing method. The powders were characterized by X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) method, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and UV-vis diffuse reflection spectroscopy (DRS). Their UV-induced photocatalytic activities were evaluated by the degradations of methyl orange and methylene blue. The results generally show that the heterojunction semiconductors Bi 2 O 3 /BaTiO 3 exhibit better photocatalytic properties than the single-phase BaTiO 3 or Bi 2 O 3 . The obviously increased performance of Bi 2 O 3 /BaTiO 3 is ascribed mainly to the electric-field-driven electron-hole separations both at the interface and in the semiconductors. A strategy for the design of efficient heterojunction photocatalysts was proposed. That is, an electron-accepting semiconductor and a hole-accepting semiconductor with matching band potentials, which respectively possess high electron and hole conduction abilities, are tightly chemically bonded to construct the efficient heterojunction structure.

The direct growth of high-quality, large single-crystalline domains of graphene on a dielectric substrate is of vital importance for applications in electronics and optoelectronics. Traditionally, graphene domains grown on dielectrics are typically only ~1 μm with a growth rate of ~1 nm min−1 or less, the main reason is the lack of a catalyst. Here we show that silane, serving as a gaseous catalyst, is able to boost the graphene growth rate to ~1 μm min−1, thereby promoting graphene domains up to 20 μm in size to be synthesized via chemical vapour deposition (CVD) on hexagonal boron nitride (h-BN). Hall measurements show that the mobility of the sample reaches 20,000 cm2 V−1 s−1 at room temperature, which is among the best for CVD-grown graphene. Combining the advantages of both catalytic CVD and the ultra-flat dielectric substrate, gaseous catalyst-assisted CVD paves the way for synthesizing high-quality graphene for device applications while avoiding the transfer process.

TiO mesoporous crystal has been prepared by one-step corroding process via an oriented attachment (OA) mechanism with SrTiO as precursor. High resolution transmission electron microscopy (HRTEM) and nitrogen adsorption-desorption isotherms confirm its mesoporous crystal structure. Well-dispersed ruthenium (Ru) in the mesoporous nanocrystal TiO can be attained by the same process using Ru-doped precursor SrTiRu O. Ru is doped into lattice of TiO, which is identified by HRTEM and super energy dispersive spectrometer (super-EDS) elemental mapping. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) suggest the pentavalent Ru but not tetravalent, while partial Ti in TiO accept an electron from Ru and become Ti, which is observed for the first time. This Ru-doped TiO performs high activity for electrocatalytic hydrogen evolution reaction (HER) in alkaline solution. First-principles calculations simulate the HER process and prove TiO:Ru with Ru and Ti holds high HER activity with appropriate hydrogen-adsorption Gibbs free energies (Δ G).

Solar desalination driven by solar radiation as heat source is freely available, however, hindered by low efficiency. Herein, we first design and synthesize black titania with a unique nanocage structure simultaneously with light trapping effect to enhance light harvesting, well-crystallized interconnected nanograins to accelerate the heat transfer from titania to water and with opening mesopores (4-10 nm) to facilitate the permeation of water vapor. Furthermore, the coated self-floating black titania nanocages film localizes the temperature increase at the water-air interface rather than uniformly heating the bulk of the water, which ultimately results in a solar-thermal conversion efficiency as high as 70.9% under a simulated solar light with an intensity of 1 kW m (1 sun). This finding should inspire new black materials with rationally designed structure for superior solar desalination performance.

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.