Porous carbon framework decorated with carbon nanotubes encapsulating cobalt phosphide for efficient overall water splitting.

Exploration of catalysts for water splitting is critical for advancing the development of energy conversion field, but designing bifunctional catalysts remains a major challenge. Herein, we demonstrate the N-doped carbon nanotube (NCNT)-grafted N-doped carbon (NC) framework embedding CoP nanoparticles (CoP@NC/NCNT) as hydrogen and oxygen evolution reaction (HER and OER) catalysts for water splitting. As a result, the CoP@NC/NCNT electrode requires the overpotentials of 106 and 177 mV at 10 mA cm-2 in 0.5 M H2SO4 and 1.0 M KOH solutions for HER, respectively. Moreover, an overpotential of 324 mV for OER can drive 10 mA cm-2 in 1.0 KOH. The CoP@NC/NCNT-based electrolyzer derives a current density of 10 mA cm-2 at a low voltage of 1.72 V in 1.0 M KOH and remains stable for 10 h. The outstanding electrocatalytic performance is mainly attributed to the hierarchical structure with rich branches and highly active component of CoP. The intimate contacts between hierarchical porous NC frameworks by cross-linked NCNTs create a 3D conductive network, which facilitates electron or mass transfer and activates CoP. This work offers a novel route for preparing hierarchical carbon framework encapsulated metal phosphide particles for potential applications in energy conversion field.

Synthesis of MnO-Sn cubes embedding in nitrogen-doped carbon nanofibers with high lithium-ion storage performance.

In this paper, a carbon nanofiber (CNF) hybrid nanomaterial composed of MnO-Sn cubes embedding in nitrogen-doped CNF (MnO-Sn@CNF) is synthesized through electrospinning and post-thermal reduction processes. It exhibits good electrochemical lithium-ion storage performance as the anode, such as high reversible capacity, outstanding cycle performance (754 mAh g-1at 1 A g-1after 1000 cycles), and good rate capability (447 mAh g-1at 5 A g-1). The excellent electrochemical properties are derived from a unique nanostructure design. MnO-Sn@CNF has a three-dimensional conductive network with a stable core-shell structure, which improves the electrical conductivity and mechanical stability of the materials. In addition, the mesopores on the surface of carbon fibers can shorten the diffusion distance of lithium ions and promote the combination of active sites of the material with lithium ions. The internal MnO and Sn form a heterostructure, which enhances the stability of the physical structure of the electrode material. This material design method provides a reference strategy for the development of high-performance lithium-ion batteries anode.

Hollow Porous CoSnOx Nanocubes Encapsulated in One-Dimensional N-Doped Carbon Nanofibers as Anode Material for High-Performance Lithium Storage.

CoSnO3, as a high theoretical capacity electrode material (1235 mAh g-1) for lithium storage, has been limited due to its low rate performance, huge volume expansion, and an unstable solid electrolyte interface (SEI). A rational design of the material structure including carbon coating can effectively solve the problems. To buffer the volume change and achieve a superior rate capability, hollow CoSnOx nanocubes encapsulated in 1D N-doped carbon nanofibers (CNFs) were fabricated by electrospinning, showing a final discharge capacity of 733 mAh g-1 with a 96% capacity retention after 800 cycles at a current rate of 1 A g-1 and a brilliant rate performance (49% capacity maintenance with the current variation from 0.1 to 5 A g-1). Absolutely, these outstanding characteristics are ascribed to the unique structure. The N-doped carbon fibers outside not only prevent the volume expansion during Li+ intercalation/extraction but also improve the electron transport in the electrode. Most significantly, the hollow structure offers enough vacant space to buffer the internal strain, while the porous structure shortens the Li+ diffusion distance. Combined with electrospinning technology, this study shares a novel idea for designing various composites with rational structures and outstanding electrochemical properties.

The influence of wind in secondary settling tanks for wastewater treatment-A computational fluid dynamics study. Part I: Circular secondary settling tanks.

Computational fluid dynamics model is used to understand the impact of wind on the performance of a secondary settling tank (SST) in a wastewater treatment plant (WWTP). Unlike most of the previous modeling studies which evaluated the wind effect on the settling tank in a water treatment plant, this study evaluates a circular SST in a WWTP at different current velocities and flow conditions. Performance indicators, such as effluent suspended solids and sludge blanket height, and three-dimensional hydrodynamics profiles are compared among different windy conditions and the calm condition and under different wind directions and flow conditions. The simulation results show that the existence of wind has strong negative impacts on the overall performance of the circular SST. The prediction of ESS is doubled in the circular SST under the mild wind condition. Moreover, the circular SST is more sensitive to the wind along the inlet port direction. PRACTITIONER POINTS: This is the first comparison of wind effects on a circular secondary settling tank Detailed computational fluid dynamics solution procedures to simulate a secondary settling tank Wind effects are investigated under multiple flow conditions, current velocities, and wind directions The performance of a circular secondary settling tank is very sensitive to the wind Wind along the inlet port direction has stronger negative impacts than it along 45° to the inlet direction.

The influence of wind in secondary settling tanks for wastewater treatment - A computational fluid dynamics study. Part II: Rectangular secondary settling tanks.

Computational fluid dynamics (CFD) model is used to study the effect of wind on the performance of a rectangular secondary sedimentation tank (SST) in a wastewater treatment plant (WWTP). Unlike most of the previous CFD modeling studies which evaluated the wind effect on the sedimentation tank in only water treatment plants, this study evaluates a rectangular SST in a WWTP at different wind speeds and directions, and under different inflow loading conditions. The wind is qualitatively and quantitatively analyzed for a range of wind speeds and directions as well as loading rates. The net effect is to change the three-dimensional hydrodynamics profiles, effluent suspended solids, and sludge blanket height. The simulation results show that the wind deteriorates overall clarification performance of the rectangular SST and has little effect on the sludge thickening under mild wind conditions until the speed increases an extreme windy condition. These CFD simulation results suggest that in strong windy climates, covering SSTs or protecting them from strong winds may be justified. PRACTITIONER POINTS: This is the first comparison of wind effects on a rectangular secondary sedimentation tank The effects of varied flow conditions, wind directions, and velocities are compared Wind-induced currents in the settling tank negatively affect removal efficiency Winds have strong negative impact on the performance of sedimentation tanks.

Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review.

Secondary settling tanks (SSTs) are a crucial process that determines the performance of the activated sludge process. However, their performance is often far from satisfactory. In the last 30 years, computational fluid dynamics (CFD) has become a robust and cost-efficient tool for designing new SSTs, modifying the geometries of existing SSTs and improved control techniques in wastewater treatment plants. The first part of this review paper discusses the different approaches to model the motion of particles in SSTs. The applications of different multiphase approaches and the widely applied single-phase approach in different SST studies are reviewed. The second part reviews current CFD research and engineering practice, focusing on the formation and the effect of density currents, effects of different design variables, parameter uncertainties in modeling structures, and atmospheric conditions. Finally, challenges and future improvements of sub-models (sludge settling, rheology, turbulence, and flocculation) in the SST model framework are identified. PRACTITIONER POINTS: The first journal review for the CFD applications in SSTs over the last decade. The controversy over the relationship between SOR and SST performance can be largely explained by the prediction of the CFD model. Density decoupling in the turbulence model is possible for well-baffled SSTs. The relative importance of three modeling parameters is summarized. Recommendations for future data collection are provided.

Turbulence and interphase mass diffusion assumptions on the performance of secondary settling tanks.

Secondary settling tanks (SSTs), also known as secondary sedimentation tanks or secondary clarifiers, are a basic yet complicated process in a biological water resource recovery facility. In order to understand and improve SST performance, computational fluid dynamics methods have been employed over the last 30 years. In the present investigation, a Fluent-based two-dimensional axisymmetric numerical model is applied to understand the effects of the buoyancy term (Gb ) in the turbulent kinetic energy (TKE) equation and two model parameters (the coefficient of buoyancy term (C3 ) in the turbulent dissipation rate equation and the turbulent Schmidt number (σc ) in the sludge transport equation) on the performance of an SST. The results show that the hydrodynamics can only be correctly predicted by buoyancy-coupled TKE equation, unless the mixed liquor suspended solids is low and sludge settling velocity is extremely high. When the field observations show the SST is operating well, the buoyancy-decoupled TKE equation predicts the correct result, but the buoyancy-decoupled TKE equation may predict failure. Care is required in selecting the correct modeling technique for various conditions. This study provides guidance on how to avoid modeling problems and increase rates of convergence. PRACTITIONER POINTS: C3 can be set to zero to improve rate of convergence and reduce computing time. σc can be used to adjust SBH, when ESS and RAS concentrations are well calibrated to the field data, but the SBH does not fit field observation.

Evaluation of three turbulence models in predicting the steady state hydrodynamics of a secondary sedimentation tank.

The secondary sedimentation tank (SST) is a sensitive and complicated process in an activated sludge process. Due to the importance of its performance, computational fluid dynamics (CFD) methods have been employed to study the underflow hydrodynamics and solids distribution. Unlike most of the previous numerical studies, in the present investigation, the performance of three different types of turbulence models, standard k-ε, RNG k-ε and Realizable k-ε, are evaluated. Firstly, two-dimensional axisymmetric CFD models of two circular SSTs are validated with the field observations. Next, comprehensive comparisons are presented of the model predictions of the key physical quantities, such as the concentration of effluent suspended solids (ESS), and returned activated sludge (RAS), sludge blanket height (SBH), turbulent properties and flow and concentration patterns. A surprising result shows that the prediction of the ESS concentration is not sensitive to the change of turbulence models; while remarkable prediction difference can be observed in the inlet zone and near-field of sludge hopper and SBH. The results suggest that more observations inside the inlet zone are needed to achieve better model calibration and correct application of the turbulence model, which can be crucial to optimizing the geometry of inlet structure and sludge hopper as well as changing return solids concentration for the operation.

An urchin-like MgCo2O4@PPy core-shell composite grown on Ni foam for a high-performance all-solid-state asymmetric supercapacitor.

In recent years, the electrochemical properties of supercapacitors have been greatly improved due to continuous improvement in their composite materials. In this study, an urchin-like MgCo2O4@PPy/NF (MgCo2O4@polypyrrole/Ni foam) core-shell structure composite material was successfully developed as an electrode for supercapacitors. The MCP-2 composite material, obtained by a hydrothermal method and in situ chemical oxidative polymerization, shows a high specific capacitance of 1079.6 F g-1 at a current density of 1 A g-1, which is much higher than that of MC (783.6 F g-1) under the same conditions. Simultaneously, it has low resistance and an excellent cycling stability of 97.4% after 1000 cycles. Furthermore, an all-solid-state asymmetric supercapacitor (ASC) was assembled using MCP-2 as the positive electrode and activated carbon (AC) as the negative electrode. The MCP-2//AC ASC exhibits high specific capacitance (94 F g-1 at a current density of 0.4 A g-1), high energy density (33.4 W h kg-1 at a power density of 320 W kg-1), high volumetric energy density (17.18 mW h cm-3 at a volumetric power density of 0.16 W cm-3) and excellent cycling stability (retaining 91% of the initial value after 10 000 cycles). Simultaneously, the device has low leakage current and excellent self-discharge characteristics. All these results indicate that the MCP-2//AC ASC is a good energy storage device; it can support the function of two LEDs for 20 minutes. These results indicate that the MCP-2//AC ASC will play an important role in energy structures in the future.