Nitrogen-doped (N-doped) graphene has been prepared by a simple one-step hydrothermal approach using hexamethylenetetramine (HMTA) as single carbon and nitrogen source. In this hydrothermal process, HMTA pyrolyzes at high temperature and the N-doped graphene subsequently self-assembles on the surface of MgO particles (formed by the Mg powder reacting with H2O) during which graphene synthesis and nitrogen doping are simultaneously achieved. The as-synthesized graphene with incorporation of nitrogen groups possesses unique structure including thin layer thickness, high surface area, mesopores and vacancies. These structural features and their synergistic effects could not only improve ions and electrons transportation with nanometer-scale diffusion distances but also promote the penetration of electrolyte. The N-doped graphene exhibits high reversible capacity, superior rate capability as well as long-term cycling stability, which demonstrate that the N-doped graphene with great potential to be an efficient electrode material. The experimental results provide a new hydrothermal route to synthesize N-doped graphene with potential application for advanced energy storage, as well as useful information to design new graphene materials.
We
used entropy engineering to design a series of CoFe2O4-type spinels. Through microstructural characterization,
electrochemical measurements, and X-ray photoelectron spectroscopy,
we demonstrated that the entropy-stabilized oxide (Co0.2Mn0.2Ni0.2Fe0.2Zn0.2)Fe2O4 has a single-phase spinel structure and exhibits
both efficient and stable catalytic oxygen evolution. This is attributable
to disordered occupation of multivalent cations, which induces severe
lattice distortion and increases configurational entropy, thereby
facilitating formation of structurally stable, high-density oxygen
vacancies on the exposed surface of the spinel. Thus, more catalytic
sites on the surface are activated and retained over the course of
long-duration testing for oxygen evolution. Entropy engineering expands
researchers’ access to catalysts that link entropy-stabilized
structures to useful properties.
Recent numerical work has shown that highspeed confined granular flows down smooth inclines exhibit a rich variety of flow patterns, including dense unidirectional flows, flows with longitudinal vortices and supported flows characterized by a dense core surrounded by a dilute hot granular gas [1]. Here, we further analyzed the results obtained in [1]. More precisely, we characterize carefully the transition between the different flow regimes, including unidirectional, roll and supported flow regimes and propose for each transition an appropriate order parameter. Importantly, we also uncover that the effective friction at the basal and side walls can be described as a unique function of a dimensionless number which is the analog of a Froude number: F r = V / √ gH cos θ where V is the particle velocity at the walls, θ is the inclination angle and H the particle holdup (defined as the depth-integrated particle volume fraction). This universal function provides a boundary condition for granular flows running on smooth boundaries. Additionally, we show that there exists a similar universal law relating the local friction to a local Froude number F r loc = V loc / P loc /ρ (where V loc and P loc are the local velocity and pressure at the boundary, respectively, and ρ the particle density) and that the latter holds for unsteady flows.
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